PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 2009 1

Search for Antihelium with the BESS-Polar Spectrometer

M. Sasaki∗,1, K. Abe†,2, H. Fuke‡, S. Haino§,3, T. Hams∗,4, M. Hasegawa§, A. Horikoshi§, A. Itazaki†,K. C. Kim ¶, T. Kumazawa§, M. H. Lee¶, Y. Makida §, S. Matsuda§, Y. Matsukawa†,

K. Matsumoto§, J. W. Mitchell ∗, A. A. Moiseev∗, Z. Myers¶,5, J. Nishimura‖, M. Nozaki§,R. Orito †,6, J. F. Ormes∗∗, K. Sakai‖, E. S. Seo¶, Y. Shikaze†,7, R. Shinoda‖, R. E. Streitmatter∗,

J. Suzuki§, Y. Takasugi†, K. Takeuchi†, K. Tanaka§, N. Thakur ∗∗, T. Yamagami‡,A. Yamamoto§, T. Yoshida‡ and K. Yoshimura§

∗National Aeronautics and Space Administration, Goddard Space Flight Center (NASA/GSFC),Greenbelt, MD 20771, USA

†Kobe University, Kobe, Hyogo 657-8501, Japan‡Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA),

Sagamihara, Kanagawa 229-8510, Japan§High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

¶University of Maryland, College Park, MD20742, USA‖The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan

∗∗University of Denver, Denver, CO 80208, USA

Abstract. We have searched for antihelium incosmic rays since 1993 using a series of the BESSflights and set the upper limit of 6.8 × 10−7 forthe abundance ratio of antihelium/helium at thetop of the atmosphere in the rigidity range of 1-14 GV. Recently, two long duration balloon flightsover Antarctica were carried out by the BESS-Polar collaboration to search for antihelium withhigher statistics. The BESS-Polar spectrometer isan evolutionary development of the previous BESSinstruments, improved to adapt to long-durationflights. The first flight, BESS-Polar I, was carriedout at Antarctica in the 2004-2005 season, observingthe cosmic rays for about 8.5 days. No antiheliumcandidate was found in the rigidity ranges of 0.6-20 GV among 8 × 106 helium nuclei events in thedata. The resultant upper limit of 2.7 × 10−7 wasset by combining with all the BESS data and theBESS-Polar I data, which is the most stringent limitobtained to date. The second flight, BESS-Polar II,was carried out in the season of 2007-2008, observingthe cosmic rays for about 24.5 days, recording 4.7billion events. The analysis is ongoing and we will

1Also at CRESST/UMCP: Department of Astronomy, University ofMaryland, College Park, MD 20742, USA

2Present address: Kamioka Observatory, ICRR, The University ofTokyo, Kamioka, Gifu 506-1205, Japan

3Present address: Istituto Nazionale di Fisica Nucleare (INFN),Perugia 06123, Italy

4Also at CRESST/UMBC: Department of Physics, University ofMaryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD21250, USA

5Present address: Physics Department, Technion Israel Institute ofTechnology, Technion City, Haifa 32000, Israel

6Present address: Max-Planck-Institut für Physik, München 80805,Germany

7Present address: Japan Atomic Energy Agency (JAEA), Tokaimura,Naka-gun, Ibaraki 319-1195, Japan

search for antihelium with an anticipated sensitivityin the antihelium/helium ratio of ∼ 3×10−8. For thispresentation, we will show the current status of theantihelium search using the BESS-Polar II data.

Keywords: BESS-Polar, Antimatter, Antihelium

I. I NTRODUCTION

The existence of antimatter was predicted by Diracas a consequence of the Dirac equation and confirmedby Anderson through the discovery of the positron,antiparticle of the electron, in the cosmic rays. It wasfollowed by the discovery of the antiproton by Segrèand Chamberlain. However, in spite of many effortsto find antiparticles with|Z| ≥ 2 in the cosmic rays,there is no evidence that those antiparticles do exist.This asymmetry of matter and antimatter in the cosmicrays is not obvious and is one of the fundamentalquestions in cosmology. Many cosmologists considerthat this asymmetry was caused by the symmetry-breaking between matter and antimatter just after BigBang and cosmological antimatter vanished at the earlystage of the universe. However, the existence of|Z| ≥ 2antiparticles is not excluded by theory. There might beremnant antiparticle domains from the Big Bang.We have searched for those antinuclei in cosmic rayssince 1993 using the BESS spectrometer and set themost stringent upper limit obtained to date. Table Ishows the summary of the results. From 1993 to 2000,we had seven conventional one day balloon flight fromnorthern Canada. In 2002, we had upgraded the centraltracker and installed a new detector (ODC) to obtainhigher rigidity resolution, so that we could search forantihelium up to 500 GV. In order to search for Anti-helium with more sensitivity, we realized long-durationballoon flights over Antarctica in the 2004-2005 season(BESS-Polar I) and 2007-2008 season (BESS-Polar II).

2 M. SASAKI et al. SEARCH FOR ANTIHELIUM ...

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M. Sasaki et al. (2002)

BESS combined

J. F. Ormes et al. (1997)

T. Saeki et al. (1998)

J. Alcaraz et al. (1999)

Buffington et al. (1981)

Golden et al. (1997)

Badhwar et al. (1978)

/He Limit (95% C.L.)He

Fig. 1. The upper limit ofHe/He together with other experimentalresults. ([2],[3],[4],[5],[6],[7],[8])

The result of the BESS-Polar I flight was reported in[1]. Figure 1 shows the upper limit ofHe/He togetherwith other experimental results of several BESS flightsthrough BESS-Polar I. The result of the BESS-Polar IIflight is reported in this paper.

TABLE ISUMMARY OF THE RIGIDITY REGION AT THE TOA AND

RESULTANT UPPER LIMIT FOR EACHBESSEXPERIMENT AND THEOVERALL COMBINED LIMIT

Rigidity range (GV) Upper limit(1993-2000) 1-14 6.8 × 10−7

TeV(2002) 5-500 1.4 × 10−4

TeV(2002) 1-14 2.1 × 10−5

Polar I(2004) 1-20 4.4 × 10−7

Combined 1-14 2.7 × 10−7

II. T HE BESS-POLAR SPECTROMETER

The BESS-Polar spectrometer (Figure 2) is an evo-lutionary development of the previous BESS experi-ments improved to adapt to a long duration flight inAntarctica [9]. To measure the lowest energy particlesdown to 100MeV at the top of the atmosphere, thematerial thickness along the incident particle trajecto-ries is minimized. In the BESS-Polar experiment, therewas no pressure vessel outside the detector. The Time-of-Flight counters and Aerogel Cherenkov counter aswell as their related front-electronics were operatedin vacuum. The cryostat of the solenoid magnet wasused as the pressure vessel for the central trackerwhich worked successfully in the BESS-TeV experi-ment. The basic spectrometer configuration was the samefor the BESS-Polar I and BESS-Polar II flight. For theBESS-Polar II flight, the spectrometer was improved in

performance[10][11][12][13] and achieved an extendedlife-time during the flight[14]. A newly constructedmagnet with a larger liquid He reservoir tank and a datastorage system with larger capacity of hard disk drives(HDDs) enabled longer observation time.

III. FLIGHT CONDITIONS

The BESS-Polar spectrometer was successfully flownover Antarctica twice. First BESS-Polar flight waslaunched on December 13th, 2004. The spectrome-ter was flown 8.5 days around Antarctica successfullyrecording 900 million cosmic-ray events. During theflight, there were some issues for the TOF PMT andseveral PMTs had to be turned off. However, we couldsustain 66% of full geometrical acceptance by control-ling the trigger algorism through telemetry. The secondBESS-Polar flight was launched on December 22nd,2007. The spectrometer was flown for 29.5 days overAntarctica and observed cosmic-ray events for about24.5 days at float altitude with the magnet energized,recording 4.7 billion events on the HDD. Figure 3shows the flight trajectory of BESS-Polar II. The fullgeometrical acceptance could be kept during the entireflight, though two TOF PMTs had to be turned off dueto the HV control issue. However, the drift chamber HVsystem had an issue, and we could not apply full HVfor the JET chamber. We adjusted the gas pressure andHV value so that we could keep taking the cosmic-rayevents.

IV. DATA ANALYSIS

As described in the previous section, we had experi-enced some issues for the drift chamber HV system. Weconfirmed that the performance of the drift chamber wasas good as that of the BESS-Polar I flight, however weshould check carefully for each physics parameter. Forthis paper, we present the search for antihelium using thelatter 1/3 of the BESS-Polar II flight data which wererelatively stable. The remaining flight data are now beinganalyzed.

A. Event Selection

During the flights, we recorded all events whichpenetrated the spectrometer. It is possible that more thanone particle passed through the spectrometer at the sametime. First, we have chosen events with a single goodtrack. The criterion is that there be only one track in thedrift chamber, and one track each passing thorough theupper and lower TOF counters. In the TOFs, it is allowedthat a single track may consist of a particle whosetrajectory passes through two immediately adjacent TOFpaddles and whose dE/dx signal is consequently splitbetween those two paddles. Then, we have applied trackquality selection such as hit data consistency betweenTOF and drift chambers, smallχ2 in trajectory fitting,fiducial selection, etc. None of these selections dependon the sign of the particle charge.

PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 2009 3

Fig. 2. The BESS-Polar Instrument cross section view.

Fig. 3. The BESS-Polar II flight trajectory.

B. Particle identification

As with the BESS spectrometer, the BESS-Polar spec-trometers were designed as general purpose detectors. Sowe have measured not only the helium (antihelium) nu-clei but also measured the protons, deuterons, etc. duringthe flights. After selecting the events, we identified thehelium (antihelium) nuclei from selected events by theirmass. Particle massM is related to rigidityR, velocityβ and chargeZ (dE/dx) as

M2 = R2Z2(1

β2− 1). (1)

The β, dE/dx and rigidity were measured by the TOFcounter and the drift chamber. We applied a1/β bandcut and dE/dx band cut instead of selecting particle massdirectly. Figure 4 shows the1/β band cut and dE/dxband cut for the TOF counters.

V. RESULTS

Figure 5 shows the 1/rigidity distribution after allselections applied. No antihelium candidates were foundin the rigidity region 1-14 GV, among1.1× 107 heliumnuclei events. If we assume that the energy spectrum of

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Fig. 4. The upper figure shows theβ−1 vs absolute rigidity of theBESS-Polar II flight data. The lower two figures show the dE/dx vsabsolute rigidity measured by top and bottom TOF counters. The eventcandidates are between the red lines.

antihelium was same as the energy spectrum of helium,we can set the upper limit using the following formula:

RHe/He <

∫3.1 dE

NObs,He × η × ǫsngl/(η × ǫsngl). (2)

Here, we take 3.1 as the number of antiheliums(NObs,He) for the calculation of the 95 % confidencelevel upper limit([15]).NObs is the number of observedHe, η (η) is a survival probability of He(He) totraverse the atmosphere above the spectrometer,ǫsngl(ǫsngl) is single track efficiency for He(He). The η

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Fig. 5. R−1 distribution of helium events from latter 1/3 of theBESS-Polar II flight data.

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[BESS ’93 ~ ’00] M. Sasaki et al. (2002)

[BESS ~ BESS-Polar I ] M. Sasaki et al. (2008)

This work ( with 1/3 of the BESS-Polar II flight data )

with all BESS-Polar II flight data (Sensitivity)

[BESS ’95] J. F. Ormes et al. (1997)

[BESS ’93 ’94 ’95] T. Saeki et al. (1998)

[AMS] J. Alcaraz et al. (1999)

/He Limit (95% C.L.)He

Fig. 6. The new upper limit ofHe/He and the BESS-Polar IIsensitivity.

(η) and ǫsngl (ǫsngl) are determined by the MonteCarlo simulation which has been developed based onthe GEANT/GHEISHA code for the spectrometers. Byusing the 1/3 of BESS-Polar II flight data, we set thenew upper limit for the antihelium to helium ratio of1.5×10−7 in the 1-14 GV rigidity range. We can searchfor antihelium with sensitivity in theHe/He ratio of3×10−8 by using all BESS-Polar flight data. The resultsare shown in the figure 6.

ACKNOWLEDGEMENTS

The authors thank NASA Headquarters for the con-tinuous encouragement in this U.S.-Japan cooperativeproject. Sincere thanks are expressed to the NASA Bal-loon Program Office at GSFC/WFF and CSBF for their

experienced support. They also thank ISAS/JAXA andKEK for their continuous support and encouragement.Special thanks go to the National Science Foundation(NSF), U.S.A., and Raytheon Polar Service Company fortheir professional support in U.S.A. and in Antarctica.The BESS-Polar experiment is being carried out as aJapan-U.S. collaboration, and is supported by a KAK-ENHI(13001004 and 18104006) in Japan, and by NASAin U.S.A.

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