CMS ECAL performance and upgrade Anton Karneyeu (INR, Moscow) CMS Collaboration INSTR14,...

CMS ECAL performance and upgrade

Anton Karneyeu (INR, Moscow)

CMS Collaboration

INSTR14, Novosibirsk, Russia, 27 February 2014

INSTR14, 27/02/2014, Anton Karneyeu 2

CMS experiment

HCAL: hadronic calorimeter

Muon chambers (white) and return yoke

Solenoid: 3.8 T

Compact Muon SolenoidA General purpose experiment at LHC for Higgs boson and New Physics searches•Weight: 14000 tonnes•External diameter: 15.0 m•Length: 21.5 m•Magnetic field: 3.8 T• ECAL inside the coil• Tracker coverage |η|<2.5

Tracker: charged-particle tracks and momentum measurements

ECAL: photon and electron energy and position


Pb/Si preshowerPb/Si preshower1.65 < | 1.65 < | | < 2.6| < 2.6

Barrel (EB)|| < 1.4861200 crystals

Endcaps (EE)1.48 < || < 3.014648 crystals

%3.0)(

%12

)(

%8.2)(

GeVEGeVEE

EENERGY RESOLUTION (BARREL, test-beam results)

The energy resolution for photons from H→γγ in EB is 1.1% to 2.6% and in EE 2.2% to 5%. Timing resolution is 190 ps and 280 ps in EB and EE.

Electromagnetic calorimeterWeight: 90 t

Length: 7.8 m

Diameter: 3.5 m


Crystals (PbWO4)Reasons for choice:Homogeneous medium 8.3 g/cm3

Fast light emission ~ 80% in 25 nsShort radiation length 0.89 cmSmall Molière radius 2.20 cmEmission peak 420 nm

Caveats:• Low light yield (LY ~ 100 γ/MeV ) Need photodetectors with gain in magnetic field• LY temperature dependence ~ −2.2%/C Need to stabilise to few 0.05 CObtained stability: ~ 0.03 C• Formation/decay of colour centres under irradiation altering crystal transparencyNeed precise monitoring system• LY spread between crystals ~ 10% Need intercalibration

Producers:• Bogoroditsk Techno-Chemical Plant, Russia (> 95% of crystals)• Shanghai Institute of Ceramic, China


Photodetectors

Barrel: Avalanche photo-diodes (APD, Hamamatsu)Two 5x5 mm2 APDs/crystal, ~ 4.5 p.e./MeVGain 50QE ~ 75% at 420 nmTemperature dependence 1/G ΔG/ΔT = −2.4%/C High-Voltage dependence 1/G ΔG/ΔV = 3.1%/VNeed to stabilize HV at 30 mVMeasured HV fluctuation: ~30 mV

Endcaps: Vacuum photo-triodes (VPT, Research Institute “Electron”, Russia)More radiation resistant than Si diodesUV glass windowActive area ~ 280 mm2/crystal, ~ 4.5 p.e./MeVGain 8 -10 (B=4T)Q.E. ~ 20% at 420 nmGain spread among VPTs ~ 25% Need intercalibration


Response monitoring system

Sources of response variations under irradiation:• Crystal transparency loss and recovery• VPT ageing (not yet disentangle from transparency loss)

Monitoring with LASER:• Laser: 447 nm (main laser), green and infra-red• Laser light injection in each crystal every ~ 40 minutes• Light also injected in PN diodes • ECAL signals compared event by event to PN reference• Energy corrections derived

Also LED (blue/orange) in endcaps.

Laser


Response monitoring system

Observed response change is from 6% (barrel) to 70% (endcap).

These measurements are used to correct the physics data.


Calibration1. π0/η → γγ method: equalizes measured π0/η peaks for individual crystals.2. φ-symmetry: invariance around the beam axis of the energy flow in zero-bias

events.3. single-electrons from W decays: use E/p ratio where p is measured in the

tracker and E in the ECAL. In addition to single-crystal intercalibration, this method also intercalibrates the average response of 248 φ-rings.

4. di-electrons from Z decays: use measured invariant mass to obtain the global scale corrections and study the ECAL resolution.

• Precalibration in 2000-2009 performed using test beams, cosmic rays, radiation source and "beam splashes" during the first LHC runs.

• ~30% of the Barrel and 400 crystals in the endcaps were calibrated in the test beams to the design-goal single-crystal precision of 0.5%.


Calibration

The precision of the phi-symmetry and photon calibrations is at the level of thesystematic errors. The precision of the electron calibration is still dominatedby the statistical errors for |η|>1.


ECAL energy calibration

Impact on the Z → e+e- energy scale and resolution of corrections which account for the intrinsic spread in crystal and photo-detector response, and compensate crystal response lost.


ECAL supercluster energy

Impact on the Z → e+e- energy scale and resolution from the incorporation of more sophisticated clustering and cluster correction algorithms.


Electron energy resolution

Relative energy resolution of electron (from Z→e+e- decays)

=> Affected by the amount of material in front of the ECAL

=> Degraded near eta cracks between ECAL modules (vertical lines in the plot)

=> Dedicated calibration improves resolution significantly


Z → e+e- mass resolution

The mass resolution of the Z → e+e- as a function of time.=> Mass resolution is stable at the per mill level.


Physics results: H →γγ

High energy resolution of the ECAL was critical for the discovery of the Higgs

=> Latest results on Moriond 2014 conference (March 15th - 22nd)


LHC schedule

ECM=13 TeVL=1 ·1034 cm-2s-1

50 fb-1 per year3 years

L=2 ·1034 cm-2s-1

≥50 fb-1 per year3 years

~ 300 fb-1

HL-LHC:

L=5 ·1034 cm-2s-1

250 fb-1 per year ~140 events per bunch-crossing

Phase1Phase1 Phase2Phase2

A new machine, for high luminosity, to measure the H couplings, H rare decays, HH, Vector boson scattering, other searches and difficult SUSY benchmarks, measure properties of other particles eventually discovered in Phase1.

LS2LS1 LS3

~ 3000 fb-1


Detector challenges at HL-LHC

• Number of events per bunch crossing (pile-up): ~ 140

• Radiation levels will be 6 times higher than for the nominal LHC design.Strong η dependence in the endcaps (at η=2.6 hadron fluence 2·1014/cm2, gamma radiation: 30 Gy/h, total: 300kGy) MARS calculations,

P.C. Bhat et al., CERN-CMS-NOTE-2013-001


Crystal radiation damage

Gamma irradiation damage is spontaneously recovered at room temperature.

Hadron damage creates clusters of defects which cause light transmission loss. The damage is permanent and cumulative at room temperature.

Plot shows “external” optical transmission of crystal after diffent irradiation doses.

- - - PbWO4 emission spectrum


ECAL endcap response evolution

Simulation50 GeV e-

Extensive test-beam studies with proton-irradiated crystals

Reduction of light output causes:•Worsening of stochastic term•Amplification of the noise•light collection non-uniformity (impact on the constant term)

Progressive deterioration of energy resolution and trigger efficiency, with strong η dependencePerformance for e/y is acceptable up to 500 fb-1

ECAL endcaps should be replaced during LS3

Frac

tion

of E

CAL

resp

onse


Endcap calorimetry options for HL-LHC

CMS plan is to replace the Endcap calorimeters in LS3

HEEE

Two possible scenarios:

1. HE absorber is left, only active material is replaced. New EE will be a standalone calorimeter

2. HE is fully replaced. This opens the possibility of a more coherent redesign of the endcap calorimeters.

Aim: radiation hardness, high granularity, precision timing


Scenario 1: standalone EE• Shashlik Calorimeter in a

sampling configuration with inorganic scintillator (LYSO or CeF3) and W as absorber.

• Light readout via wavelength shifting fibers (WLS)

• Transverse dimensions are 14x14mm → fine grained

• Light path is short → rad-hard

• Challenges: rad-hard fibers, photo-detectors, mechanical mounting (tolerances)


Scenario 2: Combined Forward Calorimeter

• Dual Readout: simultaneous measurements of Cherenkov and scintillation signal (2 types of fibers) enables event-by-event compensation resulting in superior hadron and jet measurement (DREAM/RD52 collaboration)

• Challenges: rad-hard fibers, photo-detectors, mechanical design, triggering

Scintillation fiber

Cherenkov fiber

Brass absorber


Scenario 2: High Granularity Calorimeter

• Layers of silicon “pads” (1 cm2) separated by absorber (lead/Cu)

• Measure charged particle momentum with the inner tracker, and neutrals in the calorimeter.

• Key point: resolving/separating showers through a finely granulated and longitudinally segmented calorimeter.

• Challenges: number of channels, compact and inexpensive electronics, trigger, cooling, performance in high pile-up, linearity


Pile-up Mitigation

Within tracker acceptance, charged particles are well reconstructed (Zvertex resolution ~ 0.1 mm)

Pile-up is critical in the forward region:

1. Increased granularity and segmentation.

2. High precision (pico second) timing.Studies are ongoing for pile-up mitigation through time-of-flight.Desired resolution is 20-30 ps.

HL-LHC expected luminosity region:Gaussian width of ~ 10 cmNumber of events per bunch crossing (pile-up) ~ 140


Conclusions

• The excellent ECAL performance was crucial for the Higgs boson discovery by CMS

• ECAL will continue to be highly performant throughout all Phase 1

• The HL-LHC poses severe requirements to detectors in terms of performance and radiation hardness.

• ECAL endcaps should be replaced at the end of the LHC phase1 (after 500 fb-1).

• New calorimeter options are being studied. Key points are radiation hardness, granularity and segmentation.

• Timing resolution may add important information for pile-up mitigation.

CMS ECAL performance and upgrade Anton Karneyeu (INR, Moscow) CMS Collaboration INSTR14,...

Documents

Transcript of CMS ECAL performance and upgrade Anton Karneyeu (INR, Moscow) CMS Collaboration INSTR14,...