Calorimetry at LHC Davide Pinci INFN Sezione di Roma. Calorimetry at LHC 4th Summer School on THE PHYSICS OF LHC

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1 Calorimetry at LHC

2 What is a calorimeter? In High Energy Physics a calorimeter is each detector able to measure the energy of a particle; It is often based on the total absorption of the particle to be measured; The aim is to transform all the particle energy in visible and detectable signals; A suitable calibration is always needed to translate the measured quantities into the particle energy; It provides several, and sometimes unique, information: Energy of charged and neutral particles and jets; Direction of charged and neutral particles and jets; Missing transverse energy; Particle ID; Usually provides fast signals useful for triggering; Sometimes the only detector as in cosmic ray experiments (Auger) or neutrino experiments.

3 Two kinds of calorimeter In the world of the HEP calorimetry, there are two main classes of particles: Electromagnetic particles: electrons, positrons and photons; Hadronic particles: pions, kaons, protons, neutrons... Because of the very different interactions they have with the matter different detectors have to be used and very different performance are expected; CMS electromagnetic calorimeter ATLAS hadronic calorimeter

4 Outline First lecture Calorimetry for electromagnetic particles Interactions of photons, electrons and positrons with matter; The development of electromagnetic showers; Homogeneous and sampling calorimeters; Examples of e.m. calorimeters operating at LHC. Second lecture Calorimetry for hadronic particles Interactions of hadrons with matter; Development of the hadronic showers; Invisible energy and the e.m. fraction; Examples of hadronic calorimeters operating at LHC.

5 Calorimetry for e.m. particles

6 Electromagnetic particles: Photons in the matter make: Conversion or pair production is a three body process with a cross section weakly dependent on the energy σpair X0 is defined as the mean free path of the process;

7 The radiation length X0 Several parametrization have been proposed for the radiation length; The best and simple description is: As expected it decreases with the square of Z; Al = 8.9 cm U = 3.2 mm The radiation length in a mixture or compound may be approximated by: where wj is the fraction by weight of the jth element;

8 Electromagnetic particles: e± Electrons and positrons lose energy in the matter mainly by ionization and bremmstrahlung (radiation); Bremsstrahlung is a 3 body process with a σ weakly dependent on the electron energy; Mean free path is 9/7 of X0;

9 Electromagnetic showers For energies above the MeV range bremmstrahlung and pair creation are the main processes electrons/positrons and photons make in the matter; The interesting thing is that they can give rise to an avalanche process called electromagnetic shower; Rossi's model A very simple model was made by Bruno Rossi: Each process happens exactly after an X0; The energy is exactly subdivided into thetwo daughter particles; The shower stops once the electron energy is lesser than Ec; At the jth step we'll have: 2j particles with E0/2j energy. The process stops after N steps, if E0/2N = Ec N = ln(e0/ec)/ln2; The length of the shower NX0=X0ln(E0/Ec)/ln2 increases logarithmically with the primary energy, while the total number of produced particles 2N=E0/Ec linearly;

10 The shower tail The very simple Rossi's model describes pretty well the average development of the shower up to the critical energy; From that point the main process becomes: Photons are in the energy range where Compton is the main effect. Their interaction probability is at its minimum and they can run through the Compton windows ; The slow photon absorption determines a tail with an exponential decay of the shower; Electrons and positrons release energy to the matter via ionization processes; Total length of the charged tracks is proportional to E0/(dE/dx)ioniz.;

11 Longitudinal development The Rossi's model predicts that the position of the maximum number of produced particles shifts logarithmically with the energy; A more accurate parametrization of the longitudinal development is due to Longo-Sestili: where t is x/x0; This parametrization is able to describe showers in a large variety of material; The shower maximum occurs for b 0.5;

12 ± Differences between and e Because of the continuous ionization produced by the charged particles, in the first stages the longitudinal profile of the showers produced by photons and electrons/positrons are different: There is a non-negligible probability that a photon doesn't interact in the first 5 X 0; This information is often used to distinguish between showers induced by photons or electrons/positrons;

13 Real longitudinal distribution The Longo-Sestili parametrization is a good approximation but: The longitudinal containment doesn't scale linearly with the particle energy; X0 is not the only scale of the all processes. The dependence of ionization and Compton effects on Z are different from the X0 one; Materials with high Z have shorter showers = 6 cm 13 = 115 cm

14 Lateral development The lateral development of an electromagnetic shower is determined by two parameters: The angles of emission of the photon in the bremsstrahlung process ( = me/pe) and of the e± in the pair production ( e+e- = me/e ); The multiple scattering of the electrons and positrons in the material; The lateral development can be described by means of the Moliere radius: where the scale energy Es is 21.2 MeV and the Moliere radius can be evaluated as: Once calculated in terms of Moliere radii the lateral development of a shower is almost independent on the material;

15 Lateral development 99% of the shower is contained in 3 RM; Scaling is not perfect with the Moliere radius Tails can be very different

16 Electromagnetic calorimeters The principle of operation of the electromagnetic calorimeters is to make the particle shower within them and measure the signals provided by the produced charged particles; Sampling Calorimeters Homogeneous Calorimeters Made of a single material that is at the same time the radiator and the detector of the particles produced in the shower. A high density material, with a high Z and good performance as a detector is needed. The signals provided by charged particles in the shower can be: scintillation, Cherenkov, ionization, bolometry. A high density and high Z material is used as a radiator and it is interleaved with layers of detectors such as: scintillating material, Cherenkov radiators, gas detectors... Usually simple to build and less expensive. Poorer energy resolution because of the fluctuations introduced by the sampling. Higher energy resolution, but much expensive and usually lower granularity.

17 Energy resolution: the statistics The signal provided by an electromagnetic calorimeter is proportional to the total length of the charged tracks; In the Rossi's model: Let's suppose that the only quantity that fluctuates is the number of produced particles that has a Poissonian distribution and fluctuates as its own square root; The resolution on the energy is: and thus the relative resolution on the energy will be: Inversely proportional to the square root of the energy; Directly proportional to the square root of the critical energy of the material; The statistics of the shower development imposes this lower limit.

18 Energy resolution More in general: a: stochastic term due to the fluctuations in the shower development b: constant term. The absolute resolution get worst as the energy increases. It is due to: - mis-calibration of the detector; - inhomogeneities of the different parts; - leakages of the calorimeter; c: noise term. The absolute resolution is independent from the energy. It is due to: - noise of the readout electronics; - pile-up of different events within the apparatus

19 Examples of sampling calorimeters Absorber/Cherenkov radiator Absorber/scintillator Absorber/liquid noble gas Absorber/proportional gas detector

20 Effects of sampling The sampling calorimeters have other terms that downgrade the energy resolution: Sampling fluctuations Path length fluctuations Landau fluctuations. In case of using a gas detector as sensitive part.

21 EM calorimeters at LHC All the four LHC main experiments are equipped with large and performing electromagnetic calorimeters; Depending on the physics program, different solutions were adopted and different performances are expected: ALICE: Photon Spectrometer (PHOS) homogeneous based on PbWO4 crystals + sampling EM Pb/Scintillator; CMS: homogeneous calorimeter with PbWO4 crystals; ATLAS: sampling calorimeter based on liquid Argon; LHCb: sampling calorimeter Pb/Scintillator with WLS fibres in Shashlik config

22 ALICE: EM Calorimeter The EMCal is located back to back with the PHOS inside the L3 solenoid; The aim is the study of jet physics: Large coverage: -0.7 < η < ΔΦ = 100 Good granularity: towers with size: Δφ ~ ; Sampling calorimeter: 20.1 X0; sandwich, 1.44 mm Pb / 1.76 mm Scint; sampling fraction 1/10.5; density 5.86 g/cm3; RM = 3.20 cm; X0 = 12.3 mm;

23 EM Cal structure

24 EM Cal read out The readout is based Wave-Length-Shifting (WLS) fibers read with avalanche photo-diodes; The aluminization of one side of the fibers increases the amount of light collected by the APD;

25 EM Cal calibration By using cosmic muons all towers were pre-calibrated; The APD supply voltages were moved iteratively to get the same response from each tower;

26 EM Cal performance (test beam) Linearity better than 1% above 20 GeV

27 EM Cal performance (LHC) With the first interactions of LHC it was possible to evaluate the performance of the EMCAL with π0; For different Pt bin the π0 mass was reconstructed; A slight dependence of the central value and width on the PT was found

28 ALICE: PHOS To measure, π0 and η ( GeV) PWO crystals readout with APD distance to IP: 4.6m coverage Δη < 0.12 and ΔΦ< 100 Detector depth 20X0 operating temperature: -25 oc Not pre-calibrated, calibrating now

29 The CMS EM cal Compact crystal electromagnetic calorimeter inside the 4T solenoid; Barrel based on PWO+APD;

30 Performance on test beam and cosmics On test beam measured performance; ECAL was pre-calibrated prior to LHC collisions with a mixture of testbeams, cosmics, beam splashes and lab data; 0.5%-2% Barrel and 5% End-Cap 2004 TB 0.5%

31 Performance with LHC: timing Detector synchronization crucial for triggering; Used 2009 LHC beam splashes for the online synchronization (black) of the trigger towers (5x5 channels); Residual channel timing within a trigger tower is further improved offline (red shaded): ~ 0.3ns RMS spread;

32 Performance with LHC: resonances With first LHC collisions it was already possible to reconstruct first signals from resonances:

33 ATLAS EM cal - Sampling Calorimeter Pb-LAr (87 K); - Fine Granularity and multi-layer ensure accurate e/ id and energy reconstruction. - Grosser Granularity at High η region sufficient for Jet/ETmiss measurements

34 ATLAS EM Cal Very important construction accuracy and operation stability; <d>= mm σ = 100 m High mechanical precision 1% Pb variation 0.6% drop High temperature stability -2%/K signal variation achieved 59 mk LAr purity Electronegative impurities would reduce the measured signal Require purity better than 1ppm 30 purity monitors in the three cryostats Measured impurity: Barrel ~ 200ppb, EndCap ~ 140ppb

35 ATLAS EM Cal: Timing The drift velocity of electrons in the LAr has to be taken under control to have a well timed detector; 2009 SPLASH

36 ATLAS EM Cal performance TEST-BEAM Global uniformity agrees to within 1% in the Barrel ATLAS has started data taking with well pre-calibrated calorimeter ( ~ 1%) π0 and η resonances already well seen!

37 LHCb EM Cal Fast trigger on energetic e/ /πº Distance to i.p. ~13 m Solid angle coverage 300x250 mrad Shashlik technology: Pb/Scint volume ratio 2/4 readout with Photomultipliers via WSF; Moliere radius 3.5 cm; Longitudinal size equivalent to 25 X0; Average light yield: 3000 p.e./gev;

38 Performance: timing Time alignment: on the general level a precise synchronization of calorimeters with each other and with accelerator cycle is needed for efficient triggering; special time-alignment events (TAE) containing up to 7 consecutive time slots around the one under interest; cosmic particles + special injection events in 2009: relative time alignment of different detectors and their sub-parts; Fine absolute synchronization with accelerator cycle: 450x450 GeV collisions in the end of 2009 / crosschecked in the end of March 2010 with 3.5x3.5 TeV collisions

39 Performance: energy resolution A pre-calibration of the PTMs gains was performed with a LED; The gain dependence of all PMTs on the HV was measured; An inter-calibration at the order of 10% was expected; Clear π0 signal was observed immediately after LHC start-up in the end of 2009 By using first LHC data (with π0 ανδ ) a more detailed calibration and inter- calibration was performed;

40 Performance: resonances

41 Calorimeters for hadronic particles

42 Outline First lecture Calorimetry for electromagnetic particles Interactions of photons, electrons and positrons with matter; The development of electromagnetic showers; Homogeneous and sampling calorimeters; Examples of e.m. calorimeters operating at LHC. Second lecture Calorimetry for hadronic particles Interactions of hadrons with matter; Development of the hadronic showers; Invisible energy and the e.m. fraction; Examples of hadronic calorimeters operating at LHC.

43 Hadronic showers As seen for the electromagnetic particles, hadrons can give rise to an avalanche production of secondary particles when interacting with the nuclei of a material; Main differences w.r.t the e.m. showers are: possibility of strong interactions; large variety of secondary particles: π, K, p, n, ν... A very much complicate phenomenon...

44 Hadron interactions: spallation When an incoming high-energy hadron strikes an atomic nucleus, the most likely process to occur is spallation: The incoming hadron makes quasi-free collisions with nucleons inside the struck nucleus. The affected nucleons start traveling themselves through the nucleus and collide with other nucleons. In this way, a cascade of fast nucleons develops; Some of the particles taking part in this cascade reach the nuclear boundary and escape (high energy); Then a de-excitation of the nucleus takes place: a certain number of particles (low energy free nucleons, α or even heavier nucleon aggregates) evaporates, until the excitation energy is less than the binding energy of one nucleon. The remaining energy, typically a few MeV, is released in the form of -rays. In very heavy nuclei, e.g. uranium, the intermediate nucleus may also fission.

45 Hadron interaction: spallation Example of a spallation reaction produced by a 30 GeV proton in nuclear emulsions. Much less dense ionization tracks emerge from the collision roughly following the direction of the incoming projectile. Incoming proton Most likely, these tracks represent pions and fast spallation protons. Neutrons are not visible. About 20 densely ionizing particles, presumably all protons, are produced in this reaction. These particles are more or less isotropically emitted from the struck nucleus.

46 Interaction length: λint Let's define int the mean free path between two nuclear interactions; In a very simple approximation, the cross section for nuclear interaction is proportional to the area of the nucleus itself; If A3 is the volume of the nucleus, their section will be A2/3: The mean free path is: While X0 was the mean free path for the bremmstrahlung and the creation pair, the nuclear interaction cross sections are different for different particles: σ(pp) = 38 barn while σ(pπ) = 24 barn and different (of about 40%) are the mean free paths.

47 Summary for various materials 40 cm 16 cm 10 cm 17 cm 10 cm

48 Summary of various materials For Tungsten, Lead and Uranium there is a factor almost 30 between X0 and λint; This explains the big dimension needed for a hadronic calorimeter; Moreover, 10 λint for protons are less than 7 λint for pions; Very different containments are expected.

49 Hadron interactions: ionization The ionization process is well described by the Bethe-Block formula; The main characteristics are that: - the de/dx is proportional to the square of the particle charge z; - slow particles have a high energy release per path unit; - after a minimum (mip), there is a small increase of the de/dx with the particle momentum;

50 Hadron interactions: ionization Charged hadrons produced in the shower give rise to Coulomb interactions with the electrons of the material and ionize the atoms; Because of their high mass, low energy and high electric charge (for α or nuclear fragments) they are not mip and they give a very dense ionization: 300 MeV, 150 MeV and 100 MeV are released int in one in U, Fe, Al.

51 Hadron interactions: invisible energy A annoying part of the hadron interactions, is the possibility of having a large fraction of energy lost in the so called invisible energy : Energy spent in breaking the nuclear bindings in the spallation processes; The kinetic energy of the recoiling nuclei undergoing elastic interactions with the particles of the shower. The order of magnitude of this energy is a fraction m/m of the particle energy; Neutrinos and muons. In the decays of pions and kaons inside the shower these particles can escape the detector releasing no or few energy; The very annoying part are the large fluctuations of the invisible energy: A neutron giving rise to spallation processes can hide up to 60% of their total energy; In an elastic scattering a neutron gives all its energy to a proton that can release it via ionization in a completely visible way; To conclude, the signal provided by the hadrons in the shower has very large variations from event to event.

52 Fluctuations (I) The energy released to break nuclear bindings and the number of neutrons produced can have very large fluctuations

53 Electromagnetic fraction (fem) Suppose a π0 is produced in the shower. It decays in less than s into 2 ; The photons will transfer all the π0 energy to a completely electromagnetic shower; Processes like this are one way ones: They can happen several times in a hadronic shower; It is not possible to produce a hadronic shower within an electromagnetic one; Depending on the energy of the π0 (or ) the amount of energy going into the electromagnetic mode has large fluctuations from event to event; In a very simple model, only pions are produced in the nuclear interactions, 1/3 of them being π0. The fem at the nth generation of the avalanche is: The electromagnetic fraction increases while the shower develops;

54 Electromagnetic fraction (fem) The higher the energy, the larger the total number of generations and thus the fem The electromagnetic fraction depends also on the material. Is higher in copper than in lead.

55 Fluctuations (II) The electromagnetic fraction of a shower, also for a fixed energy of the primary, can have large variations: Moreover, the fem has a not symmetric and not gaussian distribution which is then visible in the detector response

56 Hadronic shower summary The shower initiated by a hadron is called hadronic, but: A large amount of the energy is carried by the various electromagnetic sub- showers; The real hadronic component comprises a large variety of particles that undergo different interactions: Charged particles heavily ionizing; Hadrons with nuclear interactions (spallation or fission); Hadrons with elastic interactions; Muons and neutrinos escaping with a very small or null energy release; Evaporation photons with a large energy release; The fluctuations of various components make difficult the precise measurement of the energy of an hadron.

57 The longitudinal development As in the e.m. case the longitudinal development has two phases: A fast increase of the number of particles; A slow tail due to the absorption of them; 300 GeV pions on Uranium; 95% of the energy is contained in 8 λint 85 cm; The shower maximum is reached after 2 λint ; 300 GeV electrons would be contained in 10 cm of Uranium;

58 Longitudinal containment Longitudinal containment The longitudinal containment scales linearly with the energy. For 200 GeV pions, at least 8 λint are needed to contain in average the 95% of a shower;

59 Longitudinal containment Because of the large variety of processes possible within an hadronic shower, different showers can have very different shapes; The longitudinal dimensions can vary very much too! The fluctuations in the containment can be very high;

60 The lateral development The radial development of an hadronic shower shows typically two components. The narrow core is due to the electromagnetic shower component, caused by π0s produced in the shower development. The halo, which has an exponentially decreasing intensity, is caused by the non-electromagnetic shower component.

61 Lateral containment Lateral containment The lateral containment is almost independent from the energy because the hadronic halo carries a small fraction of the energy.

62 Fluctuations (III) The variation in the shower shape and thus in the longitudinal and lateral containment is another important source of fluctuations in the calorimeter response:

63 The response of a calorimeter In a very simple model the response of a hadronic calorimeter to a pion is: where e and h are the calibration constants for the e.m and the not-e.m. components; By using electrons it is possible to make fem = 1 and to measure e and express everything as a function of h/e. In general: fem < 1 and usually e/h > 1 e/π > 1: non-compensate calorimeter: Because fem depends on the energy (increases), the response per unit of energy deposited of the calorimeter increases with the energy. Moreover, because fem varies event by event, the are are big fluctuations in the detector response.

64 The non-compensate calorimeter In the case of e/h = 1 e/π = 1 for any value of the fem: compensate calorimeter

65 How to make a compensate calorimeter Homogeneous calorimeters are intrinsically non-compensate; Sampling calorimeters can be designed to be as much compensate as possible; Increase h: exploit the high ionization density of the spallation protons using a high ionization sensitive medium; Transfer the kinetic energy of the neutrons to protons in elastic scattering; Use material as Uranium that can Decrease e: fission and increase the hadronic response; the use of high Z absorber can contain low energy photons and low Z sensitive medium can suppress the photoelectric effect;

66 Measure fem In order to get rid of the fem fluctuation it is also possible to measure event by event fem: By studying off-line the shower shape (e.m. component is more contained, a segmented calorimeter is needed); By mixing two different sensitive media with different e/h ratios (Dual Readout Method); the fem can be evaluated event by event and the energy corrected for the fem fluctuations is: where is a constant of the calorimeter

67 Dual Readout Method The DRM showed to work well and to make the calorimeter response more accurate and less sensitive to the fem fluctuations; The total and not-symmetric response distribution of a calorimeter is a convolution of gaussian distribution obtained for different e.m. fraction.

68 Hadronic Calorimeters: requirements Physics channels with jets good energy and position resolution for high energy jets (11 λ for Ejet~1TeV); Searches jets measurements, hermeticity, good resolution on missing ET ; Jets are made of an em component as well, so good combined Hadronic and Electromagnetic Calorimeter resolution; Compactness to fit inside the whole detector; We'll focus on the ATLAS and CMS hadronic calorimeters

69 ATLAS hadronic calorimeter The hadron calorimeter is sampling one: steel/plastic scintillator in the barrel; Cu/LAr in the end caps. Barrel HCAL (TileCal): Steel/Plastic scintillator. Tiles perpendicular to beam axis. Wavelength shifting fibers carry light to PMT. It covers η <1.7 Endcap HCAL (HEC): Cu-LAr 4 wheels 10 λ 4 longitudinal samplings Δη x Δφ = 0.1 x x 0.2 for η >2.5

70 Calibration EM scale calibration: Set with a beam of electrons on 11% of the modules and propagated to all the others with the calibration systems. Used 3 calibration systems: 137Cesium: allow to equalize cell response (precision 0.3%) Laser: Monitor the PMT gain, and the timing of the channels Charge injection: ADC counts to pc monitoring

71 Timing The LHC splash events were very useful to check and adjust the timing of the detector Cell times are corrected for the Time Of Flight (TOF) of the particles through the 12 m calorimeter: Very good inter-calibration in all the calorimeter cells. Time inter-calibration with splash events within 450ps

72 Performance Atlas HCAL ensures at least 10 interaction lengths for all particles; Missing energy resolution

73 CMS hadronic calorimeter Hadronic Barrel and Endcap calorimeters are sampling brass/scintillator tiles calorimeters. A forward calorimeter made of steel and quartz fiber covers up to η <5.2 Detector is well timed and already used in the trigger

74 CMS hadronic calorimeter Scintillator tiles readout via WLS fibers

75 Calibration The calibration of the single modules was performed by means of a mix of test beam and radioactive source measurement; The whole detector was used for an intense cosmic ray campaign in the pit after the installation (more than 20 million good events used for analysis); Distribution of the signals for cosmic muons in the HCAL barrel; Better than 5% agreement between source and cosmics.

76 Synchronization By means of the beam splashes in 2009 the responses of the calorimeter modules were synchronized;

77 CMS HCAL performance First measurement on jet energy performed and good agreement with the MC expectations were found; Events with very high Di-jet mass already seen! In the barrel less than average 8 interaction length depth; 5% of 300 GeV pion energy escapes!

78 CMS HCAL non-compensation The CMS hadronic calorimeter is non-compensated (e/h) = 1.4; An increasing response is then expected as the energy increases with a consequent loss of linearity: Once corrected for this effect, a better resolution is obtained;

79 Conclusion The choices made for the hadronic central section by ATLAS and CMS are similar: sampling calorimeters with scintillator as active material. In both cases the dominant factor on resolution and linearity is the non compensation e/h = 1 but rather e/h=1.4; CMS HCAL space limitations have conditioned the design and performance; ATLAS higher segmentation and containment gives better total resolution;