STFC RAL An introduction to calorimeters for particle physics Bob Brown STFC/PPD Graduate lectures 2009/10 R M Brown - RAL 1 Overview STFC RAL Introduction Electromagnetic cascades Hadronic cascades Calorimeter types Energy resolution e/h ratio and compensation Measuring jets Energy flow calorimetry DREAM approach Graduate lectures 2009/10 CMS as an illustration of practical calorimeters EM calorimeter (ECAL) Hadron calorimeter (HCAL) Summary General principles Items not covered R M Brown - RAL 2 STFC RAL General principles Calorimeter: A device that measures the energy of a particle by absorbing ‘all’ the initial energy and producing a signal proportional to this energy. There is an absorber and a detection medium (may be one and the same) Absorption of the incident energy is via a cascade process leading to n secondary particles, where n EINC The charged secondary particles deposit ionisation that is detected in the active elements, for example as a current pulse in Si or light pulse in scintillator. The energy resolution is limited by statistical fluctuations on the detected signal, and therefore grows as n, hence the relative energy resolution: sE / E 1/n 1/ E The depth required to contain the secondary shower grows only logarithmically. In contrast, the length of a magnetic spectrometer scales as p in order to maintain sp /p constant Charged and neutral particles, and collimated jets of particles can be measured. Hermetic calorimeters provide inferred measurements of missing (transverse) energy in collider experiments and are thus sensitive to , o etc Graduate lectures 2009/10 R M Brown - RAL 3 The electromagnetic cascade STFC RAL A high energy e or g incident on an absorber initiates a shower of secondary e and g via pair production and bremsstrahlung Absorber 1 X0 Graduate lectures 2009/10 R M Brown - RAL 4 STFC RAL Depth and radial extent of em showers Longitudinal development in a given medium is characterised by radiation length: The distance over which, on average, an electron loses all but 1/e of its energy. X0 180 A / Z2 g.cm-2 For photons, the mean free path for pair production is: Lpair = (9 / 7) X0 The critical energy is defined as the energy at which energy losses by an electron through ionisation and radiation are, on average, equal: eC 560 / Z (MeV) The lateral spread of an em shower arises mainly from the multiple scattering of non-radiating electrons and is characterised by the Molière radius: RM = 21X0 /eC 7A / Z g.cm-2 For an absorber of sufficient depth, 90% of the shower energy is contained within a cylinder of radius 1 RM Graduate lectures 2009/10 R M Brown - RAL 5 STFC RAL Average rate of Bremsstrahlung energy loss E E(x) = Ei exp(-x/X0) dE/dx (x=0) = - Ei/X0 Ei Ei/e X0 Graduate lectures 2009/10 R M Brown - RAL x 6 EM shower development in liquid krypton STFC RAL EM shower development in krypton (Z=36, A=84) GEANT simulation of a 100 GeV electron shower in the NA48 liquid Krypton calorimeter (D.Schinzel) Graduate lectures 2009/10 R M Brown - RAL 7 STFC RAL Hadronic cascades High energy hadrons interact with nuclei producing secondary particles (mostly p ,p ) ± 0 The interaction cross section depends on the nature of the incident particle, its energy and the struck nucleus. Shower development is determined by the mean free path between inelastic collisions, the nuclear interaction length, given (in g.cm-2) by: lI = (NAsI / A)-1 (where NA is Avogadro’s number) In a simple geometric model, one would expect sI A2/3 and thus lI A1/3. In practice: lI 35 A1/3 g.cm-2 The lateral spread of a hadronic showers arises from the transverse energy of the secondary particles which is typically <pT>~ 350 MeV/c. Approximately 1/3 of the pions produced are p0 which decay p0 gg in ~10-16 s Thus the cascades have two distinct components: hadronic and electromagnetic Graduate lectures 2009/10 R M Brown - RAL 8 Hadronic cascade development STFC RAL lI eg Cu: X0 = 12.9 g.cm-2 lI = 135 g.cm-2 In dense materials: X0 180 A / Z << lI 35 A and the em component develops more rapidly than the hadronic component. 2 1/3 Thus the average longitudinal energy deposition profile is characterised by a peak close to the first interaction, followed by an exponential fall off with scale lI Graduate lectures 2009/10 R M Brown - RAL 9 STFC RAL Depth profile of hadronic cascades Average energy deposition as a function of depth for pions incident on copper Individual showers show large variations from the mean profile, arising from fluctuations in the electromagnetic fraction Graduate lectures 2009/10 R M Brown - RAL 10 STFC RAL Calorimeter types There are two general classes of calorimeter: Sampling calorimeters: Layers of passive absorber (such as Pb, or Cu) alternate with active detector layers such as Si, scintillator or liquid argon Homogeneous calorimeters: A single medium serves as both absorber and detector, eg: liquified Xe or Kr, dense crystal scintillators (BGO, PbWO4 …….), lead loaded glass. Si photodiode or PMT Graduate lectures 2009/10 R M Brown - RAL 11 STFC RAL Energy Resolution The energy resolution of a calorimeter is often parameterised as: sE / E = a /E b / E c (where denotes a quadratic sum) The first term, with coefficient a, is the stochastic term arising from fluctuations in the number of signal generating processes (and any further limiting process, such as photo-electron statistics in a photodetector) The second term, with coefficient b, is the noise term and includes: - noise in the readout electronics - fluctuations in ‘pile-up’ (simultaneous energy deposition by uncorrelated particles) The third term with coefficient c, is the constant term and arises from: - imperfections in calorimeter construction (dimensional variations, etc.) - non-uniformities in signal collection - channel to channel inter-calibration errors - fluctuations in longitudinal energy containment - fluctuations in energy lost in dead material before or within the calorimeter For em calorimeters, energy resolution at high energy is usually dominated by c The goal of calorimeter design is to find, for a given application, the best compromise between the contributions from the three terms Graduate lectures 2009/10 R M Brown - RAL 12 STFC RAL Intrinsic Energy Resolution of em calorimeters Homogeneous calorimeters: The signal amplitude is proportional to the total track length of charged particles above threshold for detection. The total track length is the sum of track lengths of all the secondary particles. Effectively, the incident electron behaves as would a single ionising particle of the same energy, losing an energy equal to the critical energy per radiation length. Thus: T=S N T i=1 i = (E /eC) X0 If W is the mean energy required to produce a ‘signal quantum’ (eg an electron-ion pair in a noble liquid or a ‘visible’ photon in a crystal), then the mean number of such ‘quanta’ produced is n = E / W . Alternatively n = T / L where L is the average track length between the production of such quanta. The intrinsic energy resolution is given by the fluctuations on n. At first sight: sE / E = n / n = (L / T) However, T is constrained by the initial energy E (see above). Thus fluctuations on n are reduced: sE / E = (FL / T) = (FW / E) where F is the Fano Factor Graduate lectures 2009/10 R M Brown - RAL 13 STFC RAL Resolution of crystal em calorimeters A widely used class of homogeneous em calorimeter employs large, dense, monocrystals of inorganic scintillator. Eg the CMS crystal calorimeter which uses PbWO4, instrumented (Barrel section) with Avalanche Photodiodes. Since scintillation emission accounts for only a small fraction of the total energy loss in the crystal, F ~ 1 (Compared with a GeLi g detector, where F ~ 0.1) Furthermore, fluctuations in the avalanche multiplication process of an APD give rise to a gain noise (‘excess noise factor’) leading to F ~ 2 for the crystal /APD combination. PbWO4 is a relatively weak scintillator. In CMS, ~ 4500 photo-electrons are released in the APD for 1 GeV of energy deposited in the crystal. Thus the coefficient of the stochastic term is expected to be: ape = (F / Npe) = (2 / 4500) = 2.1% However, so far we have assumed perfect lateral containment of showers. In practice, energy is summed over limited clusters of crystals to minimise electronic noise and pile up. Thus lateral leakage contributes to the stochastic term. The expected contributions are: aleak = 1.5% (S(5x5)) and aleak =2% (S(3x3)) Thus for the S(3x3) case one expects a = ape aleak = 2.9% This is to be compared with the measured value: ameas = 2.8% Graduate lectures 2009/10 R M Brown - RAL 14 Resolution of sampling calorimeters STFC RAL In sampling calorimeters, an important contribution to the stochastic term comes from sampling fluctuations. These arise from variations in the number of charged particles crossing the active layers. This number increases linearly with the incident energy and (up to some limit) with the fineness of the sampling. Thus: nch E / t (t is the thickness of each absorber layer) If each sampling is statistically independent (which is true if the absorber layers are not too thin), the sampling contribution to the stochastic term is: ssamp / E 1/ nch (t / E) Thus the resolution improves as t is decreased. However, for an em calorimeter, of order 100 samplings would be required to approach the resolution of typical homogeneous devices, which is impractical. Typically: ssamp / E ~ 10%/ E A relevant parameter for sampling calorimeters is the sampling fraction, which bears on the noise term: Fsamp = s.dE/dx(samp) / [s.dE/dx(samp) + t.dE/dx(abs) ] (s is the thickness of the sampling layers) Graduate lectures 2009/10 R M Brown - RAL 15 STFC RAL Resolution of hadronic calorimeters The absorber depth required to contain hadron showers is 10lI (150 cm for Cu), thus hadron calorimeters are almost all sampling calorimeters Several processes contribute to hadron energy dissipation, eg in Pb: Thus in general, the hadronic component of a hadron shower produces a smaller signal than the em component: e/h > 1 Nuclear break-up (invisible) Charged particle ionisation Neutrons with TN ~ 1 MeV Photons with Eg ~ 1 MeV Fp° ~ 1/3 at low energies, increasing with energy Fp° ~ a log(E) If e/h 1 :- response with energy is non-linear (since em component ‘freezes out’) 42% 43% 12% 3% - fluctuations on Fp° contribute to sE /E Furthermore, since the fluctuations are non-Gaussian, sE /E scales more weakly than 1/ E Constant term: Deviations from e/h = 1 also contribute to the constant term. In addition calorimeter imperfections contribute: inter-calibration errors, response non-uniformity (both laterally and in depth), energy leakage and cracks . Graduate lectures 2009/10 R M Brown - RAL 16 STFC RAL Compensating calorimeters ‘Compensation’ ie obtaining e/h =1, can be achieved in several ways: Increase the contribution to the signal from neutrons, relative to the contribution from charged particles: Plastic scintillators contain H2, thus are sensitive to n via n-p elastic scattering Compensation can be achieved by using scintillator as the detection medium and tuning the ratio of absorber thickness to scintillator thickness Use 238U as the absorber: 238U fission is exothermic, releasing neutrons that contribute to the signal Sample energy versus depth and correct event-by-event for fluctuations on Fp° p0 production produces large local energy deposits that can be suppressed by weighting: E*i = Ei (1- c.Ei ) Using one or more of these methods one can obtain an intrinsic resolution sintr / E 20%/ E Graduate lectures 2009/10 R M Brown - RAL 17 STFC RAL Compensating calorimeters ZEUS at HERA had an intrinsically compensated 238U/scintillator calorimeter The ratio of 238U thickness (3.3 mm) to scintillator thickness (2.6 mm) was tuned such that e/p = 1.00 ± 0.03 (implying e/h = 1.00 ± 0.045) For this calorimeter the intrinsic energy resolution was: sintr / E = 26%/ E However, Sampling fluctuations also degrade the energy resolution. As for electromagnetic calorimeters calorimeters: ssamp / E t where t is the absorber thickness For the ZEUS calorimeter: ssamp / E = 23%/ E Giving a nonetheless excellent overall energy resolution for hadrons: shad / E ~ 35%/ E The downside is that the 238U thickness required for compensation (~ 1X0) led to a rather modest EM energy resolution: sem / E ~ 18%/ E Graduate lectures 2009/10 R M Brown - RAL 18 STFC RAL Dual Readout Module (DREAM) approach From W. Vandelli, HEP2007, Manchester Measure electromagnetic component of shower independently event-by-event Independent measurements of the scintillation and Cerenkov light yields allow an estimation of the two components, thus measuring Fp° Graduate lectures 2009/10 R M Brown - RAL 19 STFC RAL Graduate lectures 2009/10 DREAM test results From W. Vandelli, HEP2007, Manchester R M Brown - RAL 20 Jet energy resolution STFC RAL At colliders, hadron calorimeters serve primarily to measure jets and missing ET: For a single particle: sE / E = a / E c At low energy the resolution is dominated by a, at high energy by c Consider a jet containing N particles, each carrying an energy ei = zi EJ S zi = 1, S ei = EJ d ei = a ei and: d EJ = S (d ei )2 = S a2ei d EJ / EJ = a / EJ If the stochastic term dominates: Thus: the error on Jet energy is the same as for a single particle of the same energy d EJ S (cei )2 = cEJ S (zi )2 d EJ / EJ = c S (zi )2 and since S (zi )2 < S zi = 1 If the constant term dominates: Thus: the error on Jet energy is less than for a single particle of the same energy For example, in a calorimeter with sE / E = 0.3 / E 0.05 a 1 TeV jet composed of four hadrons of equal energy has d EJ = 25 GeV, compared to d E = 50 GeV, for a single 1 TeV hadron Graduate lectures 2009/10 R M Brown - RAL 21 STFC RAL Graduate lectures 2009/10 Particle flow calorimetry From M. Thomson, HEP2007, Manchester R M Brown - RAL 22 Compact Muon Solenoid STFC RAL 4 TTmagnetic field 3.8 Length ~ 22 m Diameter ~ 15 m Weight ~ 14.5 kt Current data suggest a light Higgs Favoured discovery channel H gg Intrinsic width very small Measured width, hence S/B given by experimental resolution High resolution electromagnetic calorimetry is a hallmark of CMS Objectives: • Higgs discovery • Physics beyond the Standard Model Graduate lectures 2009/10 R M Brown - RAL Target ECAL energy resolution for photons: ≤ 0.5% above 100 GeV 120 GeV SM Higgs discovery (5s) with 10 fb-1 (100 d at 1033 cm-2s-1) 23 Measuring particles in CMS STFC RAL Muon Electron Hadron Photon Electromagnetic Calorimeter Silicon Tracker Hadron Calorimeter Superconducting Solenoid Iron field return yoke interleaved with Tracking Detectors Cross section through CMS Graduate lectures 2009/10 R M Brown - RAL 24 STFC The Electromagnetic Calorimeter RAL Barrel: 36 Supermodules (18 per half-barrel) 61200 Crystals (34 types) – total mass 67.4 t Endcaps: 4 Dees (2 per Endcap) 14648 Crystals (1 type) – total mass 22.9 t Full Barrel ECAL installed in CMS The crystals are slightly tapered and point towards the collision region 22 cm Pb/Si Preshowers: Each crystal weighs ~ 1.5 kg 4 Dees (2/Endcap) Graduate lectures 2009/10 R M Brown - RAL 25 STFC RAL Energy resolution: random impact Series of runs at 120 GeV centred on many points within S(3x3) Results averaged to simulate the effect of random impact positions 22 mm Graduate lectures 2009/10 Resolution goal of 0.5% at high energy achieved R M Brown - RAL 26 STFC RAL Hadron calorimeter Light produced in the scintillators is tranported through optical fibres to photodetectors The HCAL being inserted into the solenoid Graduate lectures 2009/10 The brass absorber under construction R M Brown - RAL 27 STFC RAL Hadron calorimetry in CMS Compensated hadron calorimetry & high precision em calorimetry are incompatible In CMS, hadron measurement combines HCAL (Brass/scint) and ECAL(PbWO4) data This effectively gives a hadron calorimeter divided in depth into two compartments Neither compartment is ‘compensating’: e/h ~ 1.6 for ECAL and e/h ~ 1.4 for HCAL Hadron energy resolution is degraded and response is energy-dependent (ECAL+HCAL) raw response to pions vs energy (red line is MC simulation) Graduate lectures 2009/10 R M Brown - RAL 28 STFC RAL Particle-Flow Event Reconstruction in CMS The design of CMS detector is almost ideally suited to particle-flow reconstruction at LHC: - Strong magnetic field, - High tracking efficiency with low fake rate, - Fine granularity electromagnetic calorimeter - Reconstruction of muons with high purity Particle-flow reconstruction improves jet energy resolution dramatically below 100 GeV/c Graduate lectures 2009/10 R M Brown - RAL Particle-flow reconstruction improves the measurement of Missing Transverse Energy by almost a factor of 2, compared to a measurement based on calorimetry alone. 29 Search for heavy gauge bosons STFC RAL I Z (1000 GeV) m+mI Z (800 GeV) e+e- Calorimetry is a powerful tool at very high energy Graduate lectures 2009/10 R M Brown - RAL 30 STFC RAL Summary Design optimisation is dictated by physics goals and experiment conditions Compromises may be necessary: eg high resolution hadron calorimetry vs high resolution em calorimetry A variety of mature technologies are available for their implementation Calorimeters will play a crucial role in discovery physics at LHC: I eg: H g g , Z e+e- , SUSY (ET) Calorimeters are key elements of almost all particle physics experiments Not covered: Triggering with calorimeters Particle identification Di-jet mass resolution ………………………… Some useful references Particle Detectors, Claus Grupen, Cambridge University Press. Calorimetry for Particle Physics, C.W. Fabian and F. Gianotti, Rev Mod Phys, 75, 1243 (2003). Graduate lectures 2009/10 R M Brown - RAL 31 STFC RAL Graduate lectures 2009/10 Spare slides R M Brown - RAL 32 STFC RAL ECAL design benchmark High resolution electromagnetic calorimetry is central to the CMS design Benchmark process: H g g sm / m = 0.5 [sE1/E1 sE2/E2 s / tan( / 2 )] Where: sE / E = a / E b/ E c (d is small – measurement relies on interaction vertex identification) Coloured histograms are separate contributing backgrounds for 1fb-1 Graduate lectures 2009/10 R M Brown - RAL 33 STFC RAL Lead tungstate properties Fast light emission: ~80% in 25 ns Peak emission ~425 nm (visible region) Short radiation length: X0 = 0.89 cm Small Molière radius: RM = 2.10 cm Radiation resistant to very high doses But: Temperature dependence ~2.2%/OC Stabilise to 0.1OC Formation and decay of colour centres in dynamic equilibrium under irradiation Precise light monitoring system Low light yield (1.3% NaI) Photodetectors with gain in mag field Graduate lectures 2009/10 R M Brown - RAL 34 Photodetectors STFC RAL Barrel - Avalanche photodiodes (APD) Two 5x5 mm2 APDs/crystal - Gain: 50 QE: ~75% - Temperature dependence: -2.4%/OC 20 = 26.5 mm MESH ANODE 40mm Graduate lectures 2009/10 Endcaps: - Vacuum phototriodes (VPT) More radiation resistant than Si diodes (with UV glass window) - Active area ~ 280 mm2/crystal - Gain 8 -10 (B=4T) Q.E.~20% at 420nm R M Brown - RAL 35 Hadron calorimeters in CMS STFC RAL Had Barrel: HB Had Endcaps: HE Had Forward: HF Had Outer: HO Hadron Barrel 16 scintillator planes ~4 mm Interleaved with Brass ~50 mm plus scintillator plane immediately after ECAL ~ 9mm plus Scintillator planes outside coil HO Coil HB HB ECAL HE HF Graduate lectures 2009/10 R M Brown - RAL 36 Cluster-based response compensation STFC RAL Use test beam data to fit for e/h (ECAL) , e/h (HCAL) and Fp° as a function of the raw total calorimeter energy (eE + eH ). Then: E = (e/p)E . eE + (e/p)H . eH Where: (e/p)E,H = (e/h)E,H / [1 + ((e/h)E,H -1) . Fp°)] (ECAL+HCAL) For single pions with clusterbased weighting Graduate lectures 2009/10 R M Brown - RAL 37 STFC RAL Jet energy resolution ‘Active’ weighting cannot be used for jets, since several particles may deposit energy in the same calorimeter cell. Passive weighting is applied in the hardware: the first HCAL scintillator plane, immediately behind the ECAL, is ~2.5 x thicker than the rest. One expects: d EJ / EJ = 125% / EJ + 5% However, at LHC, the energy resolution for jets is dominated by fluctuations inherent to the jets and not instrumental effects Graduate lectures 2009/10 R M Brown - RAL 38