cosmic-ray physics with icecube: - department of...

LUX and High Energy Muon Induced Backgrounds in DUSEL

Project Description

I. Introduction

As part of the effort of pursuing large deep underground facilities by the particle physics community to solve some fundamental problems in physics, several experiments have been planned or proposed in the Deep Underground Science and Engineering Lab (DUSEL, [1]) in Homestake, Lead, South Dakota to address topics such as dark matter [2, 3], neutrino-less double beta decay [4,5], neutrino physics, and proton decay [6,7] (such as the Long-Baseline Neutrino Experiment, LBNE). All these underground experiments will benefit from the low background environment in deep underground caverns in DUSEL. Nevertheless, various backgrounds, such as gamma rays, neutrons from fissions, cosmic ray induced high energy muons and particles produced by them when they interact in the rocks and the detector materials still exist at a level that affects not only the experiment design but also the explanation of the experimental data. For most underground experiments that run at very low energy threshold, these particles form a complicated radiation environment for which we have to fit experimental data taken at the particular experiment sites in order to give precise predictions and estimate the backgrounds in the experiments.

Being the first dark matter experiment currently scheduled in DUSEL, the LUX experiment is primarily designed to look for signals from dark matter particles, such as the weakly interacting massive particles (WIMPs) predicted in supersymmetric and extra-dimensional extensions of the standard model [8]. The LUX detector consists of a 350 kg liquid xenon detector that is submerged in a 300-m3 water tank, see Figure 1. The water tank is designed to shield gamma rays and attenuate neutrons that are produced in the rocks and surrounding materials. In addition, the water inside the tank is monitored by 20 photomultiplier tubes (PMTs) so that muons and other charged particles with tracks of sufficient length in the water can be tagged. Due to its low threshold, capability of electron and nuclear recoil discrimination and very low internal radioactivity level in the LUX inner detector, systematic study of the data from the liquid Xenon (LXe) detector and the water tank will provide a benchmark in describing and characterizing various backgrounds in DUSEL, which will further benefit all other future experiments in DUSEL. The LUX collaboration is currently assembling its inner detector in the surface lab at Homestake Lead. Underground deployment to Davis Cavern at 4850 ft level (~ 4300 m.w.e. meter-water-equivalent overburden) is scheduled in the summer of 2011. A 1.5-ton scale two-phase Xenon dark matter experiment LZS using the same water shield technique is under planning, as is a 20-ton experiment, LZD.

Muons with energy above several TeV on the surface can reach labs at 4850 ft level in DUSEL. These multi-TeV muons originate from the interaction of high-energy cosmic ray particles in the Earth’s atmosphere. Our understanding of their production and propagation plays a critical role in the underground background estimation, and its association with other large scale environmental conditions, such as the seasonal changes in the Earth’s atmosphere, the position and moving direction of the Earth in space, etc. The Comic-ray Working Group in the IceCube Collaboration has a long history in cosmic ray physics, and it is currently studying high-energy muon signals using data collected with IceTop and the in-ice array in coincidence. Such data provides unique insight into not only cosmic rays from 300 TeV (1 TeV=1012 eV) to 1 EeV (1 EeV=1018 eV), but also high-energy muon and muon bundle production in the atmosphere [9, 10, 11, 12, 13]. Rapid progress on ultra-high energy cosmic rays and muon production are being made in Pierre Auger Project [14] as well. Since the muons or muon bundles produced by comic rays in this energy range are the dominant muon component at the DUSEL depths, to apply the expertise and progress accumulated in IceCube to the study of underground muons at DUSEL is of great interest to both projects.

The proposed research work in this proposal will focus on the LUX experiment and the use of its data for a systematic study of underground muons and muon-induced background signals in


Davis Cavern. Being the first dark matter experiment at DUSEL, LUX will provide a benchmark for improving and verifying the simulation of various backgrounds in DUSEL. The research also includes building a simulation scheme to describe the production of high-energy muons and the backgrounds they induce in DUSEL. Such a full Monte Carlo will not only produce more realistic fluctuations in muon-induced backgrounds, but also enable us to study possible correlations between muon-induced background effects and large-scale/long-term modulations in the Earth atmosphere or in cosmic ray flux. These two elements are essential in the explanation of experimental data; however, both are ignored to a large extent in the data analysis of many underground experiments.

Figure 1. LUX water shield conceptual diagram. The water shield is a cylinder, 8 meters in diameter and 6 meters high, filled with purified water. 20 PMTs around the sidewall and at the bottom monitor Cherenkov light produced by relativistic charged particles in the tank. The cylinder hung on the frame at the center is the two-phase Xenon detector. The cryogenic and circulating systems are on the upper level that is not shown in this diagram.

The PI on this proposal, Xinhua Bai, became a faculty member in the physics department at South Dakota School of Mines and Technology (SDSMT) in August of 2009. Since then, SDSMT has become an institutional member in the LUX Collaboration and an associate institutional member in the IceCube Collaboration. As a new member in LUX, SDSMT missed all project funding opportunities for the LUX S4 experiment R&D, hardware design, and fabrication. Funding requested in this proposal will support our work with LUX water shield calibration and simulation, LUX deployment, operation, data analysis and other service work including education outreach activities in South Dakota. The PI anticipates hiring one postdoctoral research fellow and supporting two master graduate students and one undergraduate summer student in the next three years. Funds for adding a computer cluster at SDSMT will enable the group to carry out the proposed analysis and simulation work more effectively. Since the LUX Collaboration does not have a centralized computing facility for data storage and analysis, the cluster will also be shared with the other LUX groups and visitors from IceCube. A new digital oscilloscope, a fast laser diode pulser (two wavelengths), an optical attenuator, two beam splitters and seven multimode fibers are needed in the PI’s lab and in Davis Cavern for forthcoming testing, calibration, and education outreach (EO) activities. More details about the personnel plan and the usage of these equipments/devices are given in the “Budget Justification”. Current resources in PI’s lab are summarized in “Facilities, Equipment, and Other Resources”. In addition to research work, the PI’s lab will become the only base on the SDSMT campus for physics major student’s training and education outreach activities related to DUSEL physics programs.

II. Results From Prior NSF Supported Research

While working as a post doctoral research fellow and then as a research scientist in Fermilab, University of Wisconsin-Madison, Bartol Research Institute and in the Department of Physics and Astronomy at the University of Delaware, the PI, Dr. Xinhua Bai was supported by several NSF grants listed in Table 1; however, being a non-faculty member, Dr. Bai was not a PI or Co-PI on these prior grants. This section only summarizes the major results of these grants in which the PI of current proposal played a significant role.


Table 1. Projects the PI of this proposal joined since 1998. Other projects funded by the Chinese National Science Foundation the PI participated before 1998 are not listed.

Title & Grant No. of the Projects

Support Period Total Grant PI/Award No. Co-PIs

South Pole Air Shower Experiment -2No. 9615101

May 01, 1997 - January 31, 2001

$665,000 Thomas Gaisser Todor Stanev, Paul Evenson

Continued Operation of the South Pole Air Shower Experiment -2No. 9980801

July 01, 2001 - June 30, 2004

$719,855 Thomas Gaisser Todor Stanev

IceCube Startup and Construction Project No. 0236449

August 1, 2002 - March 31, 2011

$201,914,198 Francis Halzen

Air Showers in IceCubeNo. 0602679

June 01, 2006 - May 31, 2010

$750,000 Thomas Gaisser Todor Stanev,David Seckel

(1) SPASE2 experiment (Grants No. 9615101, No. 9980801): The SPASE2 experiment [15] was an air shower experiment located at the South Pole. Dr. Bai first worked as a winter-over scientist at the South Pole for this experiment and AMANDA (Antarctica Muon and Neutrino Detector Array) during 1998-1999. In later years, he worked in almost all aspects in this experiment, including detector calibration, experiment operation, improving air shower reconstruction techniques, experiment simulation and physics analysis. He did the muon survey for AMANDA optical modules (OMs) with SPASE2-AMANDA coincident events [16]. In the cosmic ray composition study [17] and point source search work [18] with SPASE2 data, he made significant contributions by reconstructing air shower events and building up a Monte Carlo library at Bartol Research Institute during those two funding period. (2) IceCube experiment (Grants No. 0236449, No. 0602679): Dr. Bai joined IceCube Project in its conceptual design phase. By carrying out tests at the South Pole and in the lab he set up on the campus at the University of Delaware, Dr. Bai made important contributions in the IceTop ice Cherenkov detector design, early R&D and simulation work [19] and IceCube DOM calibration work [20]. After the first IceCube string was successfully deployed, Dr. Bai carried out and led on various calibrations and performance verifications using IceTop and in-ice coincident data. Some of the results were reported in journal publications or international professional conference proceedings [10, 21, 22, 23]. Particularly and more relevant to the research work in this proposal, Dr. Bai was the first who carefully measured the muon flux on the surface at the South Pole [24]. He did the measurement using scintillate detectors together with a large ice Cherenkov detector. The Cherenkov detector was used as an absorber and as a detector in coincidence. The experiment measured muons with zenith angle from vertical to nearly horizontal. To eliminate the severe background for the horizontal events, various techniques were used, such as coincidence and anti-coincidence, time-of-flight (TOF), and waveform discrimination. In this work, the measured flux was also compared with muon flux from Monte Carlo simulations. Recently, in order to understand the energy loss of high-energy muons or muon bundles in deep under ice, Dr. Bai and his colleagues carried out a Monte Carlo study [25], in which several questions regarding muon bundle energy losses through different interaction channels and the reconstruction of muon bundle energy loss were addressed.

Dr. Bai is also well experienced in using computer clusters. He has been using clusters for his data analysis and simulation work for many years in IceCube. He helped manage the cluster from the science side at Bartol Research Institute for nearly two years. He also worked closely with the computer system administer at Bartol Research Institute in the extension and upgrade of the Bartol IceCube cluster in 2008.

In addition to his research activities, Dr. Bai has participated and led several educational outreach activities for SPASE2, AMANDA and IceCube Project; for example, the presentation


for IceCube at the “Antarctic Treaty Meeting Displays” at the Maryland Science Center in April 2009. At “2010 Engineer's Week” on February 19th in Rapid City, SD, the PI led the physics department effort introducing dark matter detection in DUSEL in five sessions to an audience of about 80 students from regional middle schools including Spearfish Middle School, Newell Middle School and Dakota Middle School. Dr. Bai has also served as a member on the IceCube publication committee between 2007 and early 2010.

Located in Rapid City, 50 miles from the DUSEL, SDSMT is expanding its research interest into experimental astro-particle physics, dark matter search, and neutrino physics. Using the startup fund (total $100K) the PI received from SDSMT, the group has transformed three storage rooms into a particle physics lab that consists of an analysis room, an optical lab and a general electronics/assembly room. We are playing an active role in LUX, such as LXe PMT internal review, water shield PMT unit assembly, testing and calibration. In addition to a master degree student, Mark Hanhardt, currently dedicated to the LUX experiment, the PI is also tutoring an undergraduate student, Douglas Tiedt, in GEANT4 detector simulation using IceTop ice Cherenkov detector simulation package as an example. This student is planning to participate in LUX water shield simulations and to become a graduate student here. The PI and the current graduate student also support and participate in the LUX detector integration in the surface lab. The requested funding in this proposal will allow this local group to continue all the work already started with our university funds and to make more contributions to the LUX experiment deployment, operation and physics analysis.

III. Proposed Work

The proposed work for this project includes: A) LUX water shield calibration, monitoring, simulation and operation; B) analysis of the data from LUX experiment to systematically characterize underground muons and muon-induced background signals in the Davis Cavern; C) building up a full Monte Carlo scheme to simulate muon induced backgrounds in DUSEL starting from cosmic ray primary fluxes, and improving the simulation by applying progress in high energy muon production anticipated in IceCube. As part of this proposal, we will also participate in educational outreach programs through the administration of the Education Outreach (EO) Office at Sanford Lab.

A. Work toward the first dark matter experiment at DUSEL: LUX water shield calibration, simulation and operation.

The water shield used in the LUX experiment is a 300-ton water Cherenkov detector that is designed to shield gamma rays and attenuated neutrons in the Davis Cavern. With the water volume monitored by 20 PMTs (10” Hamamatsu R7081), the water shield can also provide a veto to the LUX inner detector to be free from particles (and their secondary particles) that trigger the water shield. In order to estimate the precise veto efficiency and extract more physics results using the signal in the water shield, one has to carry out very careful calibration and simulation of the PMTs and the integrated water Cherenkov detector. Given the similarity between the LUX water shield and the Ice Cherenkov detector of IceTop and the water Cherenkov detector in Pierre Auger surface array, the experience and knowledge the PI has with these two cosmic ray experiments are unique among LUX member institutions in developing a research and service plan for the LUX experiment.

Currently, we are doing the PMT testing and calibration in the PI’s lab at SDSMT. This work includes taking the single photon electron (SPE) spectrum and waveform data for all 20 PMTs to be used in the water shield, the calibration of their gains, linearity, timing and dark noise rates in the PI’s Particle Physics Lab at SDSMT. The PI’s lab now has the equipment needed to perform most of the calibration work: a data acquisition system, a dark box that can test three PMTs at a time, high voltage supplies, necessary NIM electronics. The water shield signal/ HV splitter box and pre-amplifier box were designed and fabricated by another LUX collaborator UC Davis. Supported by the PI’s start-up fund, a programmable LED pulser recently fabricated at


SDSMT (modified from the design used in IceCube PMT calibration work [26]) is under testing. It can be used to study the PMT response to extremely bright light pulses. A fast (~ tens of pico- second) laser diode (such as Hamamatsu PLP-10 or PicoQuant PDL-800 plus diode heads) is requested in the proposal for the timing calibration.

At the time of proposal writing, one Ph.D. student, Nick Walsh, from UC Davis works part time with one master degree student, Mark Handardt, of SDSMT on the PMT calibration in the PI’s lab. A calibration database will be created for each of the PMTs. This database is crucial in signal simulation, veto trigger and data acquisition (DAQ) system design, and the estimation of the energy loss by particles in the water. To optimize the operation of the water shield and use the data from it for physics analysis described in Section B below, a lot more calibration and simulation work are required in the lab and at the experiment site in Davis Cavern. Having a local team will provide great convenience during the experiment. Since the funding for the physics Ph.D. program at SDSMT was delayed, both UC Davis and SDSMT agreed to cooperate on this project closely (letter of agreement from LUX group at UC Davis is attached). In the following years, Mark Hanhardt will continue working together with Nick Walsh from UC Davis on the LUX water shield calibration, monitoring and operation, with Douglas Tiedt to join in the LUX water shield simulation and analysis in year 2011.

A-1: Water shield calibration, simulation, and monitoring using various sources: In the design, the water shield should have 100% trigger efficiency for particles that have

their Cherenkov radiation path length greater than one meter in water [27]. This trigger can be used as a veto to the background produced by particles such as muons that hit the water shield. Signals from the water shield can also be used to veto muons that stop and decay in the water volume (see analysis in Section B-1). To determine the background in the final dark matter search results, one has to quantitize the actual veto efficiency and its dependence on the energy and the type of the incoming particles. This cannot be done without precise Monte Carlo simulations. Along with the PMT testing, the SDSMT group will participate in building the water shield simulation package.

At present, many other parameters in the simulation, such as the reflectivity of the Tyvek liner and water property are taken from published results [28]. In reality, even with the properties of individual detector components measured and incorporated in the simulation, the integrated detector’s performance may still be different from what the simulation predicts. One main reason for the discrepancy is the chance in the interface between the detector components after they are integrated into a system under operation condition. A practical way to overcome this difficulty is to calibrate the detector during its operation. Unlike water Cherenkov detectors in air shower experiment on the surface where background muons suffice for effective calibration and monitoring purposes [19, 29], the water shield for LUX has to be calibrated with artificial sources due to the very low muon flux at 4850 ft level. We plan to calibrate and monitor the water shield with optical light. The optical light will be produced with a LED pulser (large pulses for linearity, saturation and after-pulse study) or laser diode (fast pulses for timing calibration) and guided into the tank through several optical fibers (to be added under this proposal). We can make several measurements at different wavelengths by choosing different LEDs or laser diodes. Calibration run using the optical light can happen periodically to monitor the long-term stability of the water shield without interrupting LUX dark matter data taking.

An additional R&D project with the large water shield is to study how to use it for the study of muon-generated small showers in Davis Cavern. The physics of those sub-MeV showers is poorly known due to the lack of our knowledge about muon photonuclear cross sections in the range of materials constituting the overburden in underground laboratories. Gamma ray with energy higher than several MeV (like those in muon induced local showers) can also be detected after Compton scattering or pair production in the water. In the present design, since the water shield is one continuous volume, except the pulse size and time, little other information can be used to identify different particles. On the other hand, since the water volume is a lot larger than the radiation length (6 m – 8 m versus ~ 40 cm) and Moliere radius (6 m - 8 m versus ~10 cm),


small electromagnetic showers can be well contained in a small portion of the water volume. One improvement in the study of muon-induced underground showers may be achieved by adding several more layers or sections in the water volume [30] in future experiments. The R&D work during this funding period will only focus on optimizing the design with full Monte Carlo simulation.

A-2: Work toward the first dark matter search experiment in DUSEL and prepared to make more contributions in the future:

Since the LUX integration campaign started in Lead at the end of 2009, SDSMT group has contributed shifts and provided support to the LUX surface integration with resources based on SDSMT campus, such as sharing electronics and equipment in the PI’s lab, and cleaning LUX internal pieces using facilities in our chemistry department. The SDSMT group will participate in the LUX detector deployment to the underground lab, the water shield maintenance, operation, monitoring, and LUX physics data analysis.

After reliable calibration and simulation tools are developed for the water shield, we will carry out measurements using this 300-ton water shield alone and in coincidence with the inner LXe detector (see proposed research work in Section B below). As described in the “LZ Governance Structure” [31], SDSMT is expected to take the responsibility for the water shield in LUX future development such as LZS (1.5 or 3.0-ton liquid Xenon) and LZD (20-ton liquid Xenon). It also became clear at the last “Fall Workshop on DUSEL Science and Development of the MREFC” that the water shield at present size or bigger will be used in all dark matter experiments being planned for DUSEL [32]. Systematic analysis and simulations of the water shield and LUX experiment fits in DUSEL dark matter search plan. To grow a strong local group through the actual research activities with LUX will benefit the forthcoming projects in DUSEL.

B. Study of the muons and muon induced backgrounds in DUSEL using LUX data. In addition to the low energy radiation from surrounding rocks and the construction

material, high-energy muons produced by cosmic rays are an important background source at 4850 ft level. The muon flux at 4850 ft level was estimated to be ~ 4.4×10-9 cm-2sec-1, with an expected mean energy of 320 GeV [33] on a spectrum spread over several orders of magnitude. These muons can create complicated backgrounds that may affect the experiment design, data acquisition design, detector performance, and data analysis. For example, high-energy neutrons produced by these muons can penetrate the attenuator and mimic dark matter signal in the LXe detector. Muons with kinetic energy less than 55 MeV (muon Cherenkov threshold energy in water) can be stopped in the water shield without producing Cherenkov light to trigger the water shield. Depending on their incident angle and position, muons with energy up to 400 MeV ~ 600 MeV can also stop in the inner detector after losing their energy in the water shield around it. These stopped muons can either decay (μ+/μ- to positron/electron and neutrinos) or be captured through the semileptonic weak interaction by a proton μ- + p → νμ + n or by a nucleus μ- + (Z, N) → νμ + (Z−1, N+1)*. Since the μ- -capture cross-section increases rapidly with Z of the target elements [34], besides neutrons produced in the water, over long time μ--capture processes can make more significant contribution in cosmogenic production [35] in the detector construction materials and in Xenon than in water and scintillation materials used for active vetoing.

For many years, different experiments have been done in major underground laboratories to measure muon and neutron flux and/or spectrum [see references in 33, 36, 37]. With these data, great progress has also been made in the flux parameterization and Monte Carlo simulation [see for example 36, 38, 39, 40, 41]. For the muon flux calculation, it is important to know the average rock composition and density between the lab and the surface, while for evaluation of background from radioactivity and muons one has to know the rock composition around the lab, which can be very different for different underground sites. At present, for certain experiment sites, assuming that the knowledge of rock composition and surface muon profile gives accurate predictions for muon flux and spectrum, reports show Monte Carlo simulations of muon-induced neutron background using GEANT4 and FLUKA can predict the neutron event rate with an accuracy of


about a factor of two [ex. 39, 40, 41, 42, 43]. Nevertheless, big uncertainties still exist at higher energies. For example, at 3650 m.w.e. level, the calculated neutron yield starts to be lower than the LVD data at about 100 MeV. It becomes lower by nearly one order of magnitude at about 400 MeV [39]. Ideally any simulation of muon propagation (with MUSIC, MMC, etc.) should be normalized using experimental data; however, comparing with other major underground sites, few measurements of muon and neutron fluxes were done at DUSEL site except several early measurements done in the Davis Cavern [44, 45].

With its sensitivity goal of a cross-section of 7×10-46 cm2 (for WIMPs with mass M=100 GeV), LUX is expected to be sensitive to WIMP signals at a rate of about 6.5×10-6 drur (1 drur=1 evt/keVr/kg/day). To reach this goal, given the identification capability between electron recoils and nuclear recoils, the LUX inner detector is designed to ensure background level at or below 8.3×10-4 druee for gammas and 3.7×10-6 drur for neutrons prior to applying the charge to light ratio-based discrimination. Water radioactivity goal of U/Th/K less than 2 ppt/3 ppt/4 ppb (~106 lower than the rock) is attained by a commercial purifier. Radon in the purified water will be at the level of about 2 mBq/m3, and will be further reduced with an N2 purge blanket over time [46]. Given such low internal backgrounds and the LXe (350 kg) and water (300 Ton) mass in a single detector system, the LUX experiment is also an ideal instrument to study in-situ backgrounds in addition to its primary goal of dark matter search. The background results may affect all forth-coming projects in DUSEL. In this proposal we will focus on the following three different, but closely related, measurements and analysis:

B-1. Full particle spectrum, through-going and decay muons in the water shield Muon flux and multiplicity were measured in several early experiments in the Davis

Cavern [44, 45], with a focus on their implications for cosmic ray physics and high-energy interactions. In this proposal we will measure the signal in the water shield with a focus on their connection with the backgrounds in a modern dark matter search experiments.

One interesting measurement is the energy deposition in the water shield together with identified muon component. Based upon the absolute calibration proposed in Section A above, together with detailed simulation of water shield response function to different particles, one can measure the energy deposition of local sub-MeV showers that hit the water shield. By separating different particles (only possible to some extent and on a statistical basis with the current water shield configuration), one can estimate the contributions from muons and other electromagnetic particles on the spectrum. This can be used to further estimate the spectrum and properties of local small showers in the Davis Cavern. In order to identify the muon component from the electromagnetic component in water shield signal, we propose the following two steps: (1) Add a layer of plastic scintillator at the bottom of the water tank to identify muons that can penetrate the 6-meter water depth from those stopped in the water volume or small shower events. The electronics and eight plastic scintillator detectors (0.2 m2 each) are from the South Pole Air Shower Experiment II (SPASE2) [15], which was decommissioned in 2006. Nearly vertical muons of energy Eμ≥~1.6 GeV passing through these scintillator detectors will trigger them at an estimated rate of about 1~5 events per day according to a previous measurement [45] and estimation [33]. To reject the dark noise rate in a single scintillate detector, a coincidence with signal by energy deposition between 1.6 GeV±2σ in the water shield will be first tried. Detailed trigger criteria will be determined by more study of the actual data. (2) Identify low energy muons by looking for muon decay signature in the water. The energy loss of muon in water can be described as dE/dX ≈ - a - b×E, with a ~ 0.260 GeV/m.w.e. and b ~ 0.360×10-3/m.w.e. [47]. Local muons with energy up to ~1.5 GeV may stop in the water shield. The maximum and average energies of Michel electron from muon decay are 53 MeV and 37 MeV. Given the experience in Cherenkov detectors used in IceTop and Pierre Auger projects summarized in Table 2 on the next page, we expect to see an average of no less than 10 PEs for Michel electrons on each individual PMT in the LUX water shield. Using data from other experiments [36, p. 558 and 48] and the parameterization in [38, p.80], the estimated muon decay event rate is between 1 and10 per day; however, with unknown systematic uncertainties related to


the local rock composition and other environmental parameters. This measurement is very interesting because the stopped μ- can be captured by the nuclei of the detector materials and produces various isotopes, some of which are radioactive [35]. Since the μ- capture cross-section depends on the atomic mass of the target elements, successfully tagging muon decay events in water, together with detailed detector simulation, one can estimate the μ- -capture rate in the dark matter detector construction materials and liquid Xenon that have much larger atomic numbers.

The through-going and decay muons provide an alternative ways to calibrate the water shield and crosscheck the simulation. If the water shield is stable enough (monitored by the periodic runs using optical pulses as described in Section A), after 1~2 months the penetrating muons will be statistically enough to provide an additional calibration point at a mean energy deposition of about 1.5 GeV in the water shield and with decay muons of about 37 MeV. Based on the results, together with measurements done with different techniques, such as the recent gamma spectrum measurement [51], we can further check our understanding and the accuracy of simulations of local muon-induced backgrounds in DUSEL.

Table 2. Summary of the Cherenkov detectors used in IceTop, Auger and LUX water shield. IceTop and Auger calibration results are also included [49, 50]. For IceTop and Auger tanks, photoelectron (PE) numbers in the last column are the mean values from the fit to the observed spectrum. They are listed here for the estimation of the signal size in LUX water shield (in italic). The lower PE yield (150PEs) in some IceTop tanks is due to the lower reflectivity of the Zirconium coating used in those tanks. “M.E.” stands for Michel electron from muon decay. “VEM” stands for Vertical Equivalent Muon.

Project Tank Size (m),Target Medium,

Liner

PMT N. of PMTs

Photocathode Coverage (%)

Muon Cal.Michel ele. Cal.

IceTop Φ2.0-H0.9, Clear ice,Tyvek (Zirconium)

10”Hamamatsu R7081-02

2 0.80 240PEs/PMT/VEM(150PEs/ PMT/VEM)45PEs/PMT/M.E.

Auger Φ3.6-H1.2,Filtered Water,Tyvek

9” Photonis XP1805

3 0.36 90PEs/ PMT/VEM11PEs/PMT/M.E.

LUX Φ8.0-H6.0,Filtered Water,Tyvek (?, TBD)

10”Hamamatsu R7081

20 0.40 ~100 PEs/ PMT/VEM~12PEs/PMT/M.E.

B-2. Neutrons produced by high-energy muonsNeutrons are probably among the most important backgrounds that affect almost all

underground experiments. For double-beta decay experiments, high-energy neutrons (a few MeV and above) produce background gamma rays via inelastic scattering while thermal neutrons contribute to the gamma ray background through neutron capture. Neutrons at several MeV and above also pose a threat to neutrino detection in low-energy neutrino experiments via inverse beta decay. Neutrons at sub-GeV and GeV energies, although rare, constitute a background for proton decay and atmospheric neutrino experiments. In WIMP dark matter detectors, nuclear recoils of several keV energies by neutron elastic scattering mimic WIMP-nucleus interactions.

The LUX experiment provides an opportunity to study the muon-induced neutrons around and in a dark matter detector. Immersed in the water shield, the LXe detector is a two-phase liquid xenon detector. It can achieve high sensitivity, low threshold and electron recoil versus nucleon recoil discrimination down to a few keV recoil energy. The way it works is shown in Figure 2. Events in the liquid Xenon target create direct scintillation light (“S1”, measured largely by the bottom PMT array). Electrons that survive electron-ion recombination and are extracted into the gas phase by the electric fields (~ 5-10 kV/cm) will create scintillation light (“S2”, measured largely by the top PMT array). Since the top PMT array images the x-y location of the


S2 signal while the drift time between S1 and S2 gives the depth of the recoil, this technique provides a 3-dimensional imaging of the recoil location. The position resolution is expected to be better than 5 mm in the x-y plane and 2 mm in the z-direction for events down to several keVee (keVee: energy of electron recoil event). For a given event in the liquid Xenon, the nuclear or electron recoil energy can be determined based on the scintillation signal S1. In addition, this technique also provides the discrimination of electron recoils (caused by gammas and betas) from nuclear recoils from neutrons or WIMPs. This can be explained by an example shown in Figure 3 [52], in which the ratio of charge to scintillation light (S2/S1) versus S1-based energy is shown for electron recoils and nuclear recoils. The separation between these two distributions is clearly visible. The electron recoil events in LUX may be rejected at a level higher than 99% above the analysis threshold of 5 KeVr (keVr: energy of electron recoil event). In a typical two-phase Xenon experiment, the electron recoils are expected to yield a detectable signal of about 5 PEs/keVee and the nuclear recoils of about 2 PEs/keVr [3, 46]. With the PMTs sensitive to single photoelectrons, the threshold can be in the range of a few keVr, which is the crucial energy region in many underground experiments.

Figure 2. Signals of interactions in the LUX detector. Color circles at the top and bottom of the volume represent two arrays of PMTs (Hamamatsu R8778). The hit pattern in the bottom array provides the x-y localization of an event, while the time between the primary (S1) and the secondary (S2) scintillation signals provides the z-localization of the collision.

Figure 3. Calibration data for the LUX prototype detector in Case [52] showing the ratio of charge to light (S2/S1) versus S1-based energy for electron recoils (left) and nuclear recoils (right). In both plots, the red line and the green line represent the mean of distribution, along with 99% and 99.9% ER discrimination levels.

EDELWEISS-II (4,800 m.w.e. level, Frejus site, muon flux ~4×10-6 /m2/d, fast neutron flux ~1.6×10-6 /cm2/s [53]) for the first time claimed several coincidences between its muon veto and muon-induced neutrons in its Ge crystals (consisting of 330 g Ge/NTD, 400 g Ge/NbSi and new 400 g Ge/NTD) [54]. However, the rather small rate of ~0.04 coincidences/kg/d for both E≤250 keV neutron- and electron-type recoils limits a detailed investigation of μ-induced neutrons in


this experiment. The measurement of muon-induced neutrons reported by the ZEPLIN-II group [55] has not detected any coincidences between low-energy (< 100 keV) events in its xenon vessel and high-energy events (> 0.5 MeV) in its liquid scintillator [43]. Comparing to the 730 kg liquid scintillator shield and 7.2 kg of liquid xenon used in ZEPLIN-II, LUX (300-tons of water and 350 kg of LXe) should have a better chance to see the coincidence between high-energy muons (triggering the water shield) and nuclear recoil in the inner LXe detector. Successful observation of the coincidence between muons and the nuclear recoil signals in the LUX inner detector will give us enormous confidence on muon-induced neutron level in the LUX experiment, which will further provide an anchoring point in the simulation of neutron background level in the Davis Cavern. Accumulating more coincidence events will eventually help to characterize the true fluctuations of muon-induced backgrounds in a real detector, which cannot be done by either calibration with radioactive sources or in present simulation.

Given the importance of neutrons on all underground experiments, we also propose to have more study on muon-induced neutrons in the LUX water shield. Two approaches for tagging neutrons in a water Cherenkov detector were proposed. One is to use the 2.2 MeV gamma from the n + p d + γ reaction [56]. The second is by adding gadolinium trichloride GdCl3, which is highly soluble and transparent in solution [57]. Neutron capture on gadolinium yields a 7.9 MeV (80.5%) and an 8.5 MeV (19.3%) gamma cascade. Using these techniques, progress was made recently in measuring high-energy muon induced neutrons in the ZEPLIN-II liquid scintillator [55] and at Super-Kamiokande [58]. The maximum kinetic energy of a Compton-scattered electron by a 2.2 MeV gamma is 1.97 MeV, the so-called “Compton edge”. Super-Kamiokande reported the 8 MeV gamma cascades from neutron capture on gadolinium to have a mean measurable energy of 4.3±0.1 MeV. Taking those numbers in Table 2 for vertical equivalent muons (VEM) or Michel electrons in the Auger tank for example, one expects an average of 0.5 PEs and 1.1 PEs for a 1.97 MeV electron and a 4.3 MeV cascade in the LUX water shield. Without changing its current configuration, tagging neutrons in the LUX water shield by either of these two methods seem marginally doable.

Because of the very low light yield in either of the two processes, neutron tagging efficiency strongly depends on Cherenkov light collection. We will use the verified water shield simulation package to investigate how to improve neutron tagging techniques and Cherenkov light collection efficiency by optimizing PMT locations and using various reflective liners and/or wavelength shifter additives. We may propose, for example, a fully instrumented water shield with greater PMT cathode coverage and/or Gd doping for next generation dark matter experiments. We will also simulate neutron-tagging efficiency in the liquid scintillator proposed for active vetoing in our next projects LZS and LZD and compare all these techniques to optimize the design for next generation dark matter experiment.

B-3. Study the long-term behavior in LUX data and compare with IceCube results Because the expected event rate is extremely low, most underground experiments have to

take years of data to have enough statistics to reach their physics goals. Therefore, monitoring the long-term behavior of their signals (both background signals and “event” signals) is essential in pursuing any physics results. Despite the anisotropy of cosmic ray arrival directions observed by surface array [59] and deep under-ice muon detector [60], one well-known long-term behavior is the annual modulations observed in different experiments at different depths. Two representatives are, (a) the rate in the DAMA dark matter search experiment, (b) the underground muon rate. (a) DAMA: The DAMA Collaboration reported preliminary results of a positive annual modulation indication in late 1997 [61]. The modulation was later confirmed by the DAMA experiment that consists of an array of nine NaI(Tl) crystals with a total mass of 100 kg operated for a continuous seven-year period that ended in July 2002 [62]. The annual modulated variation in the signal was reported at 6.3σ CL. The newly combined 11 years from data of both experiments (with an exposure of 0.83 ton×year) show an 8.2σ annual modulation signal [63], Figure 4.


DAMA is the only experiment that has claimed to observe an annual modulation in their data compatible with the signal expected from dark matter particles bound to our galactic halo, contrary to the negative results of all other direct DM searches. DAMA statement of detection of dark matter signals has raised many discussions on the WIMP scattering mechanism with nuclei and various halo models [3, 63, 64]. Nevertheless, given various assumptions in the explanation of their signal, it is clear that more analysis is needed to reach a consensus.

Figure 4. Model-independent residual rate of the single-hit scintillation events, measured by the DAMA/LIBRA experiment in the 2-6 keV energy intervals as a function of time, from [63].

Figure 5. Top: Annual muon flux modulation for energies great than 1.3 TeV observed by the LVD experiment [69]. Bottom: The superposition of the mean monthly variations in the muon rate (in percentage on left Y axis), and the mean monthly variation in the effective temperature (in percentage on right Y axis), from [65]. A total of 5.33×106 single muons collected in 1992-1994 were used.

(b) Annual variation of muon flux: The muon flux modulation has been reported by several underground or under-ice experiments, such as the MACRO experiment [65], MINOS [66, 67], IceCube [68] and, very recently, LVD [69, see the top entry in Figure 5]. The muon flux measured by MACRO shows a nearly sinusoidal time behavior with one-year period and a maximum in summer, (see the bottom entry in Figure 5). The plotted effective temperature


depends on both the atmosphere temperature profile at different depths and the atmospheric attenuation length for pions and nucleons as described by the formula below,

in which the Λπ and ΛN are the atmosphere attenuation lengths for pions and nucleons. The integral is from the surface to the top of the atmosphere. This analysis also showed that muons from the decay of pions and kaons are 77% and 23% in MACRO data. The annual modulation in the IceCube in-ice muon rate was associated with the temperature profile of the stratosphere at pressure layers from 20 hPa to 100 hPa where the first cosmic ray interactions happen [68]. Authors in IceCube also explained the modulation as the result of the seasonal change of the Antarctic atmosphere and the characteristics of cosmic ray interactions in the atmosphere. Given the strong evidence of the correlation of muon flux with the atmospheric temperature, the scale of the modulation also depends on the depth of the experiment and the threshold in the experiment. Studying these effects in LUX data is important because it helps determine the background due to non-dark matter contributions.

It is very much worth pointing out that, using the details provided in [63], the annual modulation amplitude in DAMA experiments with recoil energy between 2-6 keV is (0.0129±0.0016) cpd/kg/keV. This corresponds to relative amplitude of about 1.3±0.1 % against an overall background counting rate of about 1 cpd/kg/keV. The phase is 144±8 days, with the maximum in early June. Meanwhile, in the LVD muon result, the amplitude is 1.5±0.1 % with a phase of 185±15 days and the maximum in early July [69]. Before any solid conclusion can be made whether the similarity is a true effect or just a coincidence, detailed studies with both simulation and new experiment data are needed, especially using data from different experiment such as LUX.

One important lesson from the arguments around the DAMA results is that eliminating any modulation due to cosmic rays or other terrestrial sources by normal matter or processes, annual or not, is essential to understand any observed modulation in dark matter data. In this proposal, we will carry out the following systematic study of modulations in the water shield and LXe data: (1) Look for modulations in signals in the water shield and in the LXe detector. The total muon rate in the LUX water shield is high enough to see monthly modulation of several percent after 2~3 years of data taking. (2) Estimate the annual modulation amplitude in muon flux at the 4850 ft level in the Davis Cavern by simulation. In order to include the annual change in the atmosphere, the simulation will be done using cosmic ray primary particle flux. We will also update the atmospheric parameters to include the local atmosphere monitoring data. See more details about this work in Section C below. (3) If any modulations are measured in the LUX water shield, we will compare them with simulations to cross check the expectations. We will compare them with the muon modulation recorded in IceCube data taken during the same time period to look for any anti-correlations between them. Since LUX and IceCube have different overburdens and are located in different hemispheres and under very different weather and environmental conditions, such combined analysis will provide a unique view in the sense that the two data sets represent different over-burden depths, different locations on the Earth and different view directions in space.

C. Building a full Monte Carlo simulation of muon induced backgrounds in DUSEL, starting from cosmic ray primary particles and improving the simulation by applying up-to-date progress on high-energy muons or muon bundles made in IceCube and Auger.

Most current underground background simulations start from an average muon flux on the surface. The advantage of this approach is that one can alter parameters and make sure the surface muon profile gives accurate predictions for muon flux and spectrum. Nevertheless, this approach


does not include several important features that are equally important for underground experiments searching for exotic events. One of them is the fluctuation (in both energy and multiplicity) related to the muon production in air shower development. This fluctuation is much larger than the fluctuations introduced in the muon propagation through the overburden, and can affect the dispersion of muon-induced backgrounds in the LXe detector. The other is the modulation in air shower development in the Earth atmosphere. Both of these have features visible in underground labs at DUSEL level.

The IceCube and Auger collaborations are making significant effort in the study of high-energy muon production by high-energy cosmic rays. In particular, the IceCube Cosmic-ray Working Group led by Bartol Research Institute and the University of Delaware is working on several topics using IceTop and in-ice coincident events that may eventually improve the calculation of muon production in the TeV region and above. One topic is the production of muons above 100 GeV. Despite the progress made through many years of both experimental and theoretical work, discrepancies still exist among different model predictions. See summary in Figure 6 [70, 71].

Figure 6. Left: Comparison of inclusive muon flux predictions to L3 data [70]. Shown are calculations using QGSJET, SIBYLL and TARGET as high-energy hadronic interaction model. Right: Vertical atmospheric muon (and neutrino) fluxes. Conventional muon and neutrino fluxes by solid and dashed lines marked “conven.” Other curves and shaded areas are the prompt muon flux predictions from different model calculations together with two experimental bounds from LVD and AKENO. See [71] and references therein for more details.

Another relevant topic is the high-energy muon bundle structure. For a long time, people have known high-energy air showers can produce multiple high energy muons in the form of muon bundle in which muons are highly collimated and close to each other in space [38]. An empirical description of the integral muon energy spectrum in air shower was given by the Elbert formula [72]:

in which A, E0 and are the mass, total energy, and zenith angle of the primary nucleus. p1=0.757 and p2=5.25 [38]. Nevertheless, most underground muon flux measurements have assumed single, uncorrelated muons. In the LUX water shield, the ratio of multiple-muon events to single muon events is about few percent. Study has shown that giving the energy carried by all muons in the bundle to a single muon makes a huge difference in IceCube [73].

Some efforts have been made to improve the IceTop-InIce coincidence data analysis [25, 74]. As IceCube completes its deployment in 2011, within several years, the 1-km2 IceTop array will accumulate significant amount of air shower data from several hundred TeV up to ~ 1 EeV,


among which there will be unprecedented coincident events between IceTop and the in-ice array. Progress is expected in the calculation of muon production and muon bundle characteristics in the energy region that dominates the muon flux in DUSEL. The PI of this proposal has been joining the cosmic-ray working group phone conferences and will follow the progress in IceCube and manage to apply it in the development of a full Monte Carlo simulation scheme to simulate muon-induced backgrounds in DUSEL.

With the advantage of working on LUX experiment, we will be able to cross check the simulation to great detail with the systematic measurements done with LUX data described in Section B, which is the only reliable way of developing a precise background models for a certain site.

IV. Broader Impacts

Benefits to science: The results of the proposed work will provide the underground physics community with the first systematic experimental muon background profile at the 4850 ft level in DUSEL and their signatures in a sensitive underground dark matter detector, which will provide a benchmark for all forth-coming underground experiments in DUSEL. The modulation study will help the physics community understand better this long debated phenomenon in dark matter experiment and its influence in the search for extremely rare events. By developing a full Monte Carlo background simulation scheme starting from cosmic ray flux, we expect to eventually build up a full Monte Carlo background library for DUSEL that also includes the fluctuations in air shower development and seasonal effect in the Earth atmosphere. After being improved over time, such a library can eventually serve all planned underground experiments in DUSEL. Before dark matter particles are detected, the search for them is basically to increase the detector sensitivity by increasing the detector size and understand/eliminate the backgrounds. It is worth to point out that significant uncertainties still exist in our understanding of various backgrounds in present dark matter search experiments [75]. Based on new results and ideas that may come out of our research, with the full Monte Carlo background simulation to be built, we will be able to re-evaluate more quantitatively the necessity of having an appropriate surface array for DUSEL [76].

Prepare for future DUSEL programs by integrating research and education at SDSMT: During the course of the proposed work, the postdoctoral fellow and graduate students will be expected to play major roles in all areas of the research, and will have the chance to be trained in many aspects of research including data analysis, simulation and some service tasks. Through participation in the large international collaborations LUX and IceCube, they will also have the opportunity to experience working with large teams of diverse scientists, helping them to develop important skills for their future careers. All group members will be encouraged to attend professional conferences to present their work, and to mentor undergraduate interns, thus improving their own communication skills.

Similarly, advanced undergraduate students will gain valuable research experience by working closely with the postdoctoral fellow and graduate students on subtasks such as maintaining and upgrading the software and the study of simpler problems in major tasks. They will also accompany the group to the DUSEL site to participate in experiment deployment, calibration, maintenance and operation. They will be encouraged to present research results at undergraduate research symposia at professional conferences.

Moreover, since water Cherenkov detector is one of the favored options in the Long-Baseline Neutrino Experiment design, to grow a local group with experience on the LUX 300-ton water shield will benefit DUSEL science programs in the future.

Formal education: The Education Department at Sanford Laboratory is in the planning stages for a major science education center, the Sanford Center for Science Education (SCSE), to be built as part of the DUSEL Laboratory. The mission for the SCSE is, in part, to draw upon the


science and engineering to be pursued at DUSEL to inspire and prepare the next generation of scientists, engineers and educators. They are in the process of developing prototype programs that would transition to and build capacity for the programs to be run by the SCSE, both onsite, offsite and virtual. The programs will be grounded in education research best practices and employ rigorous evaluation.

The planning team for the SCSE has need of content experts in designing these programs. From scientific point of view, the detection of dark matter would open a new window to the hidden portion in the Universe that is nearly ten times heavier than what we know today. It would be a major scientific discovery with wide-ranging implications for all of particle physics, cosmology and astrophysics. The search for dark matter has gained broad public attention. To help impose more positive repercussions for the role of science in society, the PI will work in partnership with the planning team for the SCSE to help devise and prototype models for hands-on or virtual delivery of astroparticle physics content. The PI is beginning that partnership in the summer of 2010 by being a lecturer for the Davis-Bahcall and Fermilab-BNL-Homestake summer scholars programs, which will bring twenty of the brightest science students (rising freshmen and sophomore undergraduates) in South Dakota to Sanford Lab for one week each. While there, the students will learn modern physics, participate in tours and hand-on activities, and perform an experiment of their choosing underground. These students will be tracked and mentored through the rest of their undergraduate careers and will be prime candidates for internships at SDSMT, Sanford Lab and elsewhere (students from 2009 have secured DUSEL-related internships for 2010 at Princeton University, Brookhaven National Laboratory, Colorado School of Mines and Sanford Lab)

Involvement of underrepresented groups: Located in the Great Plains, residents in South Dakota have a long tradition of mining and farming, the population of 800,000 being 67% rural. South Dakota is also the home to nine Native American tribes, comprising 9% of the population. From family structures to spirituality, people in South Dakota have a rich and colorful culture. In general the perception among all groups is that if one is interested in a career in Science, Technology, Engineering or Math (STEM) fields, one must move out of the state.

The reservations located in South Dakota are among the poorest counties in the country. American Indian students have high drop-out rates and only small numbers typically attend college; even fewer finish college, and only a very few pursue degrees in STEM fields. The presence of DUSEL in western South Dakota provides an opportunity to involve more American Indians in the design, construction, science and engineering of the facility.

SDSMT only has about 2.5% Native Americans on campus and many of them are the first generation to attend college in their family history. In order to improve this, SDSMT hosts several programs, which include NSF Tiospaye in Engineering Program, and the SD GEAR UP Honors Program, etc. Some of those programs (such as the SD GEAR UP Honors Program [77]) are designed to prepare Native American students to be successful in the science and college setting. Being the first astroparticle physicist in the history of the SDSMT, the PI will participate in local educational outreach programs for all groups with different cultural backgrounds. The PI and other group members will develop mini-lectures related to the research work with the IceCube and LUX experiments for interested SD GEAR UP students during their six week residential program on campus every summer. Tours will be provided of the PI’s lab on campus. This work will be coordinated with the work that Sanford Lab Education Department is also doing with GEAR-UP students, in order to provide an integrated approach to activities and lectures based on DUSEL and the science and engineering to take place there. This integrated approach will touch the GEAR-UP students at every grade level, and prepare them to graduate into other enrichment activities such as the Davis-Bahcall Scholars program, which would give the students an opportunity to visit Gran Sasso Laboratory in Italy and to study physics at Princeton for three weeks during the summer after their senior year of high school.

cosmic-ray physics with icecube: - department of...

Documents