The state of art Towards the next Collider

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M. LivanThe Art of Calorimetry

Lecture III

The next Collider✦ Discussions about the post-LHC era have mainly

concentrated on a high-energy electron-positron collider with a center-of-mass energy that would allow this machine to become a factory for t-tbar and Higgs boson production.

✦ Both linear colliders (ILC, CLIC) and circular ones (FCC) are being considered in this context.

✦ A sufficiently large circular collider could additionally be used to further push the energy frontier for hadron collisions beyond the LHC limit

✦ Calorimetry R&D in the past 15 years has evolved looking forward to experiments at these machines.

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Calorimetry for the next Collider✦ An often mentioned design criterion for calorimeters at a

future e+e- collider is the need to distinguish between hadronically decaying W and Z bosons.

✦ The requirement that the di-jet masses of events are separable by at least one Rayleigh criterion implies that one should be able to detect 80 - 90 GeV jets with a resolution of 3 -3.5 GeV.

✦ This goal can be, and has been achieved for single hadrons but not for jets.

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W ) qq̄ and Z ) qq̄

The importance of energy resolution

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Calorimetry for the next Collider

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✦ Different approaches are followed to develop calorimeter systems that are up to this task✦ Compensating calorimeters

✦ Proven technology, current holders of all performance records

✦ Dual-readout calorimeters

✦ Try to improve on the performance of compensating calorimeters by eliminating the weak points of the latter

✦ Systems based on Particle Flow Analysis (PFA)

✦ Combine the information from a tracking system and a fine-grained calorimeter

Compensating calorimeters

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✦ Reasons for poor hadronic performance of non-compensating calorimeters understood

✦ Experimentally demonstrated with Pb/scintillator calorimeters (ZEUS, SPACAL)

Hadronic signal distributions in a compensating calorimeter

SPACAL NIM A308 (1991) 481

Pros and Cons of Compensating Calorimeters

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✦ Pros✦ Same energy scale for electrons, hadrons and jets. No ifs, ands or buts.

✦ Calibrate with electrons and you are done

✦ Excellent hadronic energy resolution (SPACAL 30%/√E)

✦ Linearity, Gaussian response function

✦ Compensation fully understood. (We know how to built these things, even though GEANT doesn’t)

✦ Cons✦ Small sampling fraction (2.4%in Pb/plastic) ⇒ em resolution limited (SPACAL:

13%/√E, ZEUS: 18%/√E)

✦ Compensation relies on detecting neutrons

✦ ⇒ Large integration volume

✦ ⇒ Long integration time (> 50 ns)

✦ Jet energy resolution not as good as for single hadrons in Pb, U calorimeters

What is the problem with the jet energy resolution ?

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Signal non-linearities at low energy (< 5 GeV) due to non-showering hadrons.Many jet fragments fall in this categoryA copper or iron calorimeter would be much better in that respect

How to improve the excellent ZEUS/SPACAL performance ?

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✦ Reduce the contributions of the sampling fluctuations to energy resolution

✦ (THE limiting factor in SPACAL/ZEUS)

✦ Use lower-Z absorber material

✦ to eliminate / reduce the jet problem

✦ Maintain advantages of compensation

✦ (eliminate / reduce effects of fluctuations in fem and invisible energy)

➔ Dual -Readout Calorimetry

The DREAM principle✦ Quartz fibers are only sensitive to em shower component !

✦ Production of Čerenkov light ⇒ Signal dominated by electromagnetic component

✦ Non-electromagnetic component suppressed by a factor 5 ⇒ e/h=5 (CMS)

✦ Hadronic component mainly spallation protons Ek ∼ few hundred MeV ⇒non relativistic ⇒ no Čerenkov light

✦ Electron and positrons emit Čerenkov light up to a portion of MeV

✦ Use dual-readout system:

✦ Regular readout (scintillator, LAr, ...) measures visible energy

✦ Quartz fibers measure em shower component Eem

✦ Combining both results makes it possible to determine fem and the energy E of the showering hadron

✦ Eliminates dominant source of fluctuations10

Quartz fibers calorimetry

Radial shower profiles in: SPACAL (scintillating fibers)QCAL (quartz fibers)

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Radial hadron shower profiles (DREAM)

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DREAM structure

Some characteristics of the DREAM detector:Depth 200 cm (10 λint)16.2 cm effective radius (0.81 λint, 8.0 ρM)Mass instrumented volume:1030 KgNumber of fibers 35910, diameter 0.8 mmX0 = 20.10 mm, ρM =20.35 mm19 hexagonal towers, 270 rods eachEach tower read-out by 2 PMs (1 for Q and 1 for S fibers) 13

The (energy independent) Q/S method

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S = E

�fem +

1(e/h)S

(1� fem)⇥

Q = E

�fem +

1(e/h)Q

(1� fem)⇥

If e/h = 1.3(S), 4.7(Q)

Q

S=

fem + 0.21(1� fem)

fem + 0.77(1� fem)

E =S �XQ

1�X

with X =1� (h/e)S1� (h/e)Q

' 0.3

Effect of selection based on fem

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DREAM: Effect of corrections (200 GeV “jets”)

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Uncorrected

Q/S Method

Hadronic response:Effect Q/S correction

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CONCLUSIONS from tests✦ DREAM offers a powerful technique to

improve hadronic calorimeter performance:✦ Correct hadronic energy reconstruction, in an

instrument calibrated with electrons !

✦ Linearity for hadrons and jets

✦ Gaussian response functions

✦ Energy resolution scales with √E

✦ σ/E < 5% for high-energy “jets”, in a detector with a mass of only 1 ton ! (dominated by fluctuations in shower leakage)

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How to improve DREAM performance✦ Build a larger detector → reduce effects side leakage

✦ Increase Čerenkov light yield

✦ DREAM: 8 p.e./GeV → fluctuations contribute 35%/√E

✦ No reason why DREAM principle is limited to fiber calorimeters

✦ Homogeneous detector ?!

✦ ⇒ Need to separate the light into its Č, S components

✦ Increase sampling fraction

✦ For ultimate hadron calorimetry (15%/√E) Measure Ekin (neutrons).

✦ Is correlated to nuclear binding energy loss (invisible energy)

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Expected effect of full shower containment

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Time structure of the DREAM signals: the neutron tail (anticorrelated with fem)

✦ High resolution hadron calorimetry also requires efficient detection of the “nuclear” shower component

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Probing the total signal distribution with the neutron fraction

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Homogeneous calorimeters (crystals)✦ No reason why DREAM principle should be limited to fiber

calorimeters

✦ Crystals have the potential to solve light yield + sampling fluctuations problem

✦ HOWEVER: Need to separate the light into its Č, S components

✦ OPTIONS:

✦ Directionality: S light is isotropic, Č light directional

✦ Time structure: Č light is prompt, S light has decay constant(s)

✦ Spectral characteristics: Č light λ-2, S depend on scintillator

✦ Polarization: Č light polarized, S light not23

Čerenkov and Scintillation information from one signal !

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BGO crystalUG 11 (UV) filterfrom NIM A595 (2008) 359

Conclusions on DREAM✦ The DREAM approach combines the advantages of compensating calorimetry with

a reasonable amount of design flexibility

✦ The dominating factors that limited the hadronic resolution of compensating calorimeters (ZEUS; SPACAL) to 30 - 35%/√E can be eliminated

✦ The theoretical resolution limit for hadron calorimeters (15%/√E) seems within reach

✦ The DREAM project holds the promise of high-quality calorimetry for all types of particles, with an instrument that can be calibrated with electrons

✦ Open issues:

✦ Projective geometry

✦ Read-out

✦ Copper fabrication

✦ Better fibre quality (attenuation length)

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Particle Flow Analysis (PFA)✦ The basic idea: combine the information of the tracker and the

calorimeter system to determine the jet energy. Momenta of charged jet fragments are determined with the tracker, energies of the neutral jet fragments come from the calorimeter

✦ This principle has been used successfully to improve the performance of experiments with modest hadronic calorimetry. However, the improvements are fundamentally limited. In particular, no one ha sever come close to separating W/Z this way

✦ The problem: the calorimeters do not know that the charged fragments have already measured by the tracker: These fragments are also absorbed in the calorimeter. Confusion: Which part of the calorimeter signals comes from the neutral jet fragments ?

✦ Advocates of this method claim that a fine detector granularity will help solve this problem. Other believe it would only create more confusion. Like with all other issues in calorimetry, this issue has to settled by means of experiments, NOT by Monte Carlo simulations !

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PFA in CDF

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Improvement of energy resolution expected with PFA

28from NIM A495 (2002) 107

PFA at CMS

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PFA method: “π” jet

✦ The circles indicate the characteristic size of the showers initiated by the jet fragments, i.e. ρM for em showers and λint for the hadronic ones

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from NIM A495 (2002) 107

CALICE: the zoo of PFA calorimeters

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PFA: High Density + Fine Granularity✦ In order to reduce problems of shower overlap, PFA R&D focuses on

reducing the shower dimensions and decreasing the calorimeter cell size

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X0 = 1.8 cm, λI=17 cm X0 = 0.35 cm, λI=9.6 cm

Iron Tungsten

PFA: High Density + Fine Granularity✦ Unfortunately the calorimeters do not see colors !

✦ How about calibration ?

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Iron Tungsten

On calibration problems and its non-solutions✦ Proposed PFA systems consist of millions of readout channels

(fine granularity)

✦ Question: how does one want to calibrate these calorimeters ?

✦ Answer (CALICE): DIGITAL calorimetry (energy ∝ # of channels that fired)

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This was tried and abandoned in 1983, for good reasons: Particle density in the core of em showers is very high

⇒ Non linearity & Signal saturation

The extremely narrow electromagnetic shower profile

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from NIM A735 (2014) 130

CALICE hadronic digital prototypes

36from Rev. Mod. Phys, 2016 vol.88, 015300

Fe or W absorber and RPC readout

CALICE W/Si + Fe/plastic Hadronic prototype

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rms90 is referred to as “the resolution”

from Rev. Mod. Phys, 2016 vol.88, 015300

CALICE W/Si ECAL

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data from Rev. Mod. Phys, 2016 vol.88, 015300 and NIM A608, 372

Courtesy of R. Wigmans, to be published

Emeas = Emean + α

The CALICE Philosophy

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from Rev. Mod. Phys, 2016 vol.88, 015300

“Detailed simulation studies in the framework ofpreparing detector concepts for future electron positron linear colliders such as ILC and CLIC have shown that resolutions around 3.5%–4% can be realized for jet energies from 50 up to1500 GeV. Such a resolution was shown to provide a separation of W and Z bosons decaying into pairs of jets on the basis of the dijet invariant mass, as shown in Fig. 83.”

“It was suggested to directly test the jet energy performance in test beams by creating bundles of particles from a primary beam hadron impinging on a thin target. Leaving aside the differences in particle momenta and multiplicity, or energy density, between these “jets” and those generated in quark fragmentation at the same primary energy, such an experiment would have prohibitive cost, as simulation studies have shown (Morgunov, 2009). For particle flow reconstruction magnetic momentum spectroscopy and large acceptance are indispensable.Consequently the experimental strategy must be to validate the critical ingredients of particle flow calorimetry individually.”

Conclusions I✦ In the past 25 years calorimetry has become a mature art

✦ High quality energy measurements will be an important tool for collider experiments at the TeV scale

✦ There are no fundamental reasons why the four vectors of all elementary could not be measured with a precision of 1% or better at these energies

✦ However this goal is far from trivial, especially for the hadronic constituents of matter and unfortunately little guidance is provided by hadronic Montecarlo shower simulations in this respect.

✦ In the past 30 years, all progress in this domani has been achieved trough dedicated R&D projects

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Conclusions II✦ Two major R&D efforts are underway to improve the quality of

hadronic energy measurements.

✦ R&D in the PFA framework is more concentrated on the technicalities of detector design, despite the fact that some fundamental questions about applicability of this concept ( e.g. calibration) are still not fully clarified.

✦ The potential of the PFA concept may be real, but will need to be experimentally demonstrated

✦ R&D on dual-readout (DREAM) project concentrates strongly on experimental tests of the validity of the principles on which improvements of hadronic calorimetry with little attention to a 4π detector design and simulations in general.

✦ Until now DREAM has been remarkably successful. It combines the advantages of compensating calorimetry with a reasonable amount of design flexibility, with an instrument that can be calibrated with electrons 41