Calorimetry at LHC

1. Why should I want a calorimeter ? Interaction relevant for Electromagnetic Calorimeters Calorimeter characteristics: linearity and resolution The ATLAS and CMS em calorimeters: different choices Hadronic interactions and issues relevant to hadron calorimeters ATLAS and CMS hadronic calorimeters Calorimetry at LHC C.Roda INFN & Universita` di Pisa C.Roda University and INFN Pisa

2. R. Wigmans, “Calorimetry, Energy Measurements in Particle Physics” all figures without other cited source are from this book Priscilla B.Cushman, “Electromagnetic and Hadronic Calorimeters” D.Prieur, “Etalonnage du calorimetre electromagnetique du detector ATLAS”, PhD Thesis M.Diemoz, “Calorimetri elettromagnetici a cristalli per la fisica delle alte energie” Lezioni Villa Gualino 3.2.2005 U.Amaldi, “Fluctuations in Calorimetry measurements” 1981 Phys.Scr.23 409 C.W.Fabjan and F.Gianotti, “Calorimetry for particle physics”, Reviews of Modern Physics, Vol.75, October 2003 R.Wigmans et al., “On the energy measurement of hadron jets” ATLAS & CMS TDRs References C.Roda University and INFN Pisa

3. The Calorimeter concepts originates from thermodynamics: thermally isolated box containing a substance under study of which we want to measure the temperature. “Our” calorimeters also measure temperature as an energy measurement. The very basic concept is thus taken from thermodynamics but the sensitivity we need is much higher, the effect of 1 TeV (1 eV = 10-19 J) in 1 liter of water (cwater = 4.19 J g-1K-1) at 20o is: T = E / cwater M = 1.6 10-7 / 103 4.19 = 3.9 10-7 K the sensitivity that we need is much higher. …also calorimeter in particle and nuclear physics are invasive devices: Calorimeters are detectors able to measure the particle energy through total absorption. The first idea to use calorimeter was originated by the need to measure not only charged particles (bending magnetic field) but also neutral particles: 0  …there were born around the 1970 … What is a Calorimeter ? C.Roda University and INFN Pisa

4. Wigmans - Calorimetry C.Roda University and INFN Pisa

5. Key role in the past UA2 measurement of W → jj invariant mass before and after background subtraction, the wider is the peak the more difficult it is to see the signal on the QCD background. The sigma of the signal peak is 8 GeV of which 5 GeV are attributed to calorimeter resolution. Here the resolution is not enough to separate the W and Z peaks. C.Roda University and INFN Pisa

6. Sensitivity both to neutral and charged particles; Energy measurement precision (more or less)  with  E spectrometer measurement precision  with p; Do not need magnetic field (infact it is easier without); Shower length  ln(E) thus dimension are compact; Particle identification; Not only E but also spatial measurement through segmentation; They can have a fast response, useable at high rate and for trigger signals. General charcteristics CALO E C.Roda University and INFN Pisa

7. Higgs discovery: H →  H → ZZ → 4 e (particle identification against jets) SUSY discovery: easiest event signature is given by excess of events high ET miss e high pT jets; top mass measurement tt → WWbb → ljjbb,W→ jj; precise ET miss measurement requires precise and hermitic calorimetries. forward jet tagging … I hope I have convinced you that there are numerous reasons why we need a calorimeter… Also we will see how some of the mentioned physics channels will be used to define the design requirements of the LHC calorimeters. Key role in LHC C.Roda University and INFN Pisa

8. A few concepts on Electromagnetic interactions C.Roda University and INFN Pisa

9. e  interaction with matter First issure is to understand the mean energy deposit/interaction How does a EM shower forms ? Electrons and Positrons Photon PDG 2004 PDG 2004 C.Roda University and INFN Pisa

10. Since the cross section of these processes depends on the particle energy the relevance of each process changes as the shower develops. The cross section depends on Z of the material thus the characteristics of the signal depends strongly on the type of material we use to build the calorimeter. Now we try to better understand the relevant points of this processes for what concerns shower formation. What do we need to understand … C.Roda University and INFN Pisa

11. Electron and positron Bremsstralung • E > 100 MeV it is the most important process for energy loss for e+e- Radiation of real photons in Coloumb field nuclei. QED • mean energy loss per unit length (per gr-1cm2) proportional to energy of the particle; • Scaling factor for high energy ele in one X0 the particle reduces its energy by 63%. • X0 can be multiplied by the density to measure in cm C.Roda University and INFN Pisa

12. Ionization loss • Interactions of electrons with the atomscharacterized by many interactions with a small release of energy. Two regimes of energy loss → the border is set by the critical energy EC: Sol Liq C.Roda University and INFN Pisa

13. Electrons vs photons • There is a main difference between the interactions of electrons (and positrons) and photons with matter at high energy. Electrons loose energy but they do not disappear, photons as they interact they are destroyed. electrons photons C.Roda University and INFN Pisa

14. Pair production • Interaction of photons with the field of the nucleus (or of the electrons): • nucleus → nucleus + e+ e- Threshold: E ≥ 2 me High energy approssimation, E independent Reduction of photon beam intensity. Photons scaling factor is of the same order of electrons: pair = 9/7 X0  1.3 X0 C.Roda University and INFN Pisa

15. Photoelectric and Compton interaction  e → ’ e’  A → A+ e-  shows strong dependence on Eshell In both interactions secondaries do not follow the direction of the incident electron, almost no reminder of initial particle direction. C.Roda University and INFN Pisa

16. Direction of particles that release energy ? C.Roda University and INFN Pisa

17. Carbon Z = 6 Lead Z = 82 Photon cross sections Particle Data Book 2004 C.Roda University and INFN Pisa

18. C.Roda University and INFN Pisa

19. The shower is formed through a process of particle multiplication that degrades the particle energy; Interplay between different interaction processes depends on Z of material; As the energy of the particles reaches very low energies  eV,KeV, electrons and positrons are absorbed by the material which is “heated” by the released energy. In summary how the shower is formed C.Roda University and INFN Pisa

20. Scaling of shower profile with E and X0 • The position of the shower maximum XMaximum is approximatly described as a function of X0 – since both gamma pair and brem scale with it - and the particle initial energy by the simple formula: to= - 0.5 for electrons 0.5 for photons C.Roda University and INFN Pisa

21. Electron longitudinal shower profile Electron longitudinal shower profile in copper [Wigmans – Text Book] Shower maximum moves with energy as log(E) C.Roda University and INFN Pisa

22. Photon/electron difference Few photons do not interact at all [Wigmans – Text Book] Almost no electrons do not release C.Roda University and INFN Pisa

23. X0 scaling is approximate 10 GeV e- [Wigmans – Text Book] Shower Mx is deeper in Lead than in Aluminium: multiplication continues for longer since critical energy is lower in Lead than in Aluminum (7.4 MeV vs 43 MeV). C.Roda University and INFN Pisa

24. X0 scaling is approximate 10 GeV e-EGS4 [Wigmans – Text Book] Shower “decade” slowlier in lead than in aluminum since the total number of particle created is 3 times higher than in Aluminium. C.Roda University and INFN Pisa

25. Consequence on longitidunal Shower containment Percentual shower containment More radiation lengths of U than of Al needed to absorb 95% of em showers. C.Roda University and INFN Pisa

26. Calorimeter dimension Material needed for 95% shower containment: Calorimeters of 25X0 allows to contain electron showers at 1% up to 300 GeV. 25X0 25-50 cm C.Roda University and INFN Pisa

27. Transverse profile • The lateral shower development is dominated by two effects: • multiple scattering at the early phase of the shower; • long free path for low energy photons in Compton energy range. • The measurement of the transverse size, integrated over the full longitudinal range, is given by the Molière radius (same units as X0): On average 90% of the shower is contained in 1 RM. C.Roda University and INFN Pisa

28. Transverse profile Transverse profile at various depths. Two regimes: multiples scattering and Compton photons travelling away from the axis. 10 GeV e- in copper C.Roda University and INFN Pisa

29. What are the particle that deposit energy Fraction of energy deposited to the material by a 10 GeV electron: The low energy particles are responsible for most of the energy deposition. C.Roda University and INFN Pisa

30. What is the range of the particle that release energy C.Roda University and INFN Pisa

31. From the energy deposit we have to generate the signal. Two calorimeter design possibilities: Homogeneous: the calorimeter consists of a single material which acts both as absorber and active device that transform all e+ e- energy deposit in signal. Sampling: absorber and active device are made of different materials and signal is generated from a sample of the total e+ e- energy deposit. From energy deposit to signal C.Roda University and INFN Pisa

32. The most used techniques to generate the signal in calorimeters are: Cerenkov radiation from e+ e- Scintillation signals Ionization of the detection medium All these tecniques are characterized by a threshold energy which is the minimum detectable energy Es. Signal generation Light collection Charge collection C.Roda University and INFN Pisa

33. Linearity in a given energy range: Signal = a E The larger the range the more difficult it is for example range @ LHC [MIP → TeV] What I need from the calorimeter a = Signal/Energy Signal/Energy: pC/GeV, ADC count/MeV … The request might seem easy but many different source might spoil the calorimeter response (a). C.Roda University and INFN Pisa

34. Spoiling the linearity: Saturation effects: of electronics, of energy deposition … Leakage (transverse or lateral) noise, this at low side Factors that affect the linearity Example: PMT saturation. 1.02 Linearity within 2% 1 PMT signal (a.u.) PMT signal/injected charge 0.98 Injected charge (a.u.) C.Roda University and INFN Pisa Injected charge (a.u.)

35. What else I need from a calorimeter Response to monochromatic source of energy E H   bad resolution H   good resolution Perfect good bad background mgg Signal = constant integrated B  → S/B  1/   … but  = f(calo) Calorimeter signal (calo) defines the energy resolution for energy E. C.Roda University and INFN Pisa

36. What affects the resolution • Up to this moment we have described the <mean> behaviour of the calorimeter, fluctuations around this value are the sources of the calorimeter resolution. • The sources of fluctuations are various: • Signal quantum fluctuations (i.e.: photoelectric statistics …) • Sampling fluctuations • Shower leakage • Instrumental effects (i.e.: structural non-uniformity, electronic noise, light attenuation, …) • Usually in each calorimeter, and in each energy range, one of these sources dominates. C.Roda University and INFN Pisa

37. Resolution and signal quanta fluctuation • Simple model: a particle of energy E will produce N signal quanta: • N  E/Ec • N is the number of e+ e- that realese energy by ionization and excitation. The signal S is proportional to the total track length (T) : • T  X0 E/Ec • The measured energy EM is proportional to the particle energy E: • EM= k T C.Roda University and INFN Pisa

38. Resolution and signal quanta fluctuation EM= k T Fluctuation of number of track segments is poissonian → gaussian for large number of track segment Assuming (for the moment) that k = 0 Stochastic term C.Roda University and INFN Pisa

39. Resolution and signal quanta fluctuations The intrisic limit to the energy resolution is given by the maximum detectable track length which depends on the signal threshold energy: Tdetectable = fs T  fs X0 E/Ec fs fraction of N particles over energy threshold Es. Thus: EM = k Tdetectable k fs X0 E/Ec • Low energy threshold for detecting → high fs C.Roda University and INFN Pisa

40. Crystal calorimeters have best intrinsic limit on energy resolution Compare processes with different energy threshold Scintillating crystals Cherenkov radiators Real Resolution with all contributions: C.Roda University and INFN Pisa

41. In sampling calorimeters there is a further contribution to fluctuations which is due to the sampling procedure and usually dominates other stochastic fluctuations: Resolution in sampling calorimeters Active mean Electron shower in a cloud chamber with lead absorber Absorber plates Rossi gave a semi-emipirical expression for the sampling fluctuations considering the fluctuation of the number of particles crossing “a set of active layers equally spaced at distance x”: Emip = energy lost by a mip on a sampling layer (Active + absorber) C.Roda University and INFN Pisa

42. Resolution in sampling calorimeters The higher the number of planes the smaller the Emip → the better the energy resolution However this is clearly only part of the story … C.Roda University and INFN Pisa

43. The previous formula however fails to describe the correct dependence of the resolution with the active layer thickness … it goes in the opposite direction. Sampling fluctuations C.Roda University and INFN Pisa

44. Sampling fluctuations • We have seen that the calorimeter signal is given by many low energetic ( MeV) e+ and e-: • e+ e- created in active layers • e+e- created in absorber that reach the active layers • the pathlength of particles with E  1MeV is fraction of the distance between active layers thus increasing the number of boundary surfaces between layers increases the contribution to the signal • The fluctuations depend on: • Sampling fraction • Sampling frequency d d/2 C.Roda University and INFN Pisa

45. Sampling fluctuations • fsamp ↓ resolution ↓d ↓ resolution C.Roda University and INFN Pisa

46. Many other sources affect the energy resolution which can be parametrized as the sum of three terms added in quadrature assuming independent sources: Other contribution to energy resolution a = stochastic term, fluctuations in signal quanta b = noise term (Stot = Sparticle + Snoise): electronic noise but also contribution from pile-up c = smearing of the calorimeter response due any structure non uniformity that cause variation in the signal generation, non hermetic coverage (cracks) C.Roda University and INFN Pisa

47. It is the leading term at high energies. It is affected by non uniform response of the detector as a function of the impact point position (equalization), temperature… It is mainly related to the precision and stability of setting working conditions … EM = kTdetectable where now we are considering the variation of k: Resolution constant term Very hard work to have a low constant term in order not to spoil resolution at high energy expecially if the stochastic term is low…. C.Roda University and INFN Pisa

48. These fluctuations are non poissonian since are due to fluctuations in numbers of interactions in first calo layer …and increase with ln(E) lateral shower leakage much less fluctuating the longitudinal one Usefull parametrization for longitudinal fraction energy lost f < 10%: Resolution and shower leakage i.e. for f = 5% → 13% degradation in energy resolution C.Roda University and INFN Pisa

49. Resolution and shower leakage pair = 9/7X0 CHARM Collaboration NIM 1980 178,27 Longitudinal dominated by first interaction, lateral by fluctuations of many low energy particles Percentual energy loss C.Roda University and INFN Pisa

50. Which is the source I should take care of … 0.7% Es.: ATLAS EM barrel Calorimeter C.Roda University and INFN Pisa