calorimetry in particle physics experimentspersonalpages.to.infn.it/~arcidiac/calo_design.pdf ·...

Calorimetry in particle physicsexperiments

Unit n. 6Calorimeter design guidelines

Roberta Arcidiacono

R. Arcidiacono Calorimetria a LHC 2

Program recall

1. The relevance of Calorimetry in HEP2. The physics of calorimetry3. Detector response: energy resolution4. Electromagnetic calorimeters5. Hadron calorimeters6. Calorimeter design principles7. Front-end and Trigger electronics8. Calibration techniques9. Some examples


Lecture overview

● How to design a good calorimeter: some reminders

● Some Good TIPs from the wise...● Requirements for LHC calorimeters● !dead material!


MAIN IDEAs for EM

– Want to stop em particles NOT hadrons● small “radiation length” (X0) – to reduce

calorimeter size● large “interaction length” (I) – to have small

chance of hadron interacting

– Want to get good sampling of particles in shower to get good statistical sampling of energy


CALO Design Guidelines

- Size of the detector, in terms of depth, is dictated by X0 and is driven by the required resolution (% of back leakage at the target energy range)

- Cell size such that 70-80% of energy of a centrally incoming particle is deposited in the cell, while having energy in the neighboring cells large enough to measure the center of gravity coordinates

- High granularity is needed to separate showers induced by nearby particles. The angular separation achievable is limited by the lateral size of the shower and by the distance of the calorimeter from the interaction point



- The lateral segmentation determines the possibility to correlate calorimeter information with charged tracks upstream

- The ratio between I and X0, which can be up to a factor 30 for high-Z materials, can be fruitfully used to distinguish between EM and Hadronic showers



For “light” calorimeters:● Optimize light collection chain● Length of sensitive material has to be compatible with

light transmission properties and light attenuation length. Light output should be uniform along the length of the crystal


Some Good TIPS

Based on EM and HAD shower characteristics:● In the absorption processes, most of the energy is deposited by

very soft shower particles, yielding isotropic angular distribution of the shower particles. Does not matter the orientation of the active layers in sampling calo.

● Typical shower particle in EM showers is: 1 MeV electron. Range is very short, < 1 mm in typical absorber materials. This range set the scale for useful sampling frequency of em showers.

● Typical shower particles in Had showers are 50-100 MeV spallation protons and 3 MeV neutrons. Range of protons is ~ 1 cm. This set the scale for useful sampling frequency in hadron calorimeters. Neutrons travel several cm. Are important when active medium has a high probability of interaction.


Some Good TIPS

On fluctuations:● variety of fluctuations contribute to the energy resolution.

However one of these sources dominates. In the design of a calorimeter one should not waste money reducing fluctuations that do not dominate!

– It is very important to understand which contribution is dominant in the energy range under interest, to come out with the optimal calorimeter design


Some Good TIPS

On fluctuations:● Example: the light yield of quartz-fiber detectors is

typically so small that signal fluctuations (photoelectron statistics) are dominant → in this case, no much gain in increasing the sampling frequency by using more thinner fibers instead of fewer thick ones.


Some Good TIPS

On fluctuations:● The position of the supporting structure has a big impact

on the calorimeter performance. The performance is least affected when the structural elements are place directly upstream of the sensitive detector volume, perpendicular to the direction of the incoming particles.


LHC requirements for Calorimeters

● Fast response ( 50 ns or faster), high granularity, to reduce pileup induced noise (pileup r.m.s. → fluctuations in the average amount of energy deposited by N concurrent interactions in the same cell; the <Epileup> can be subtracted, fluctuations not!)

● Radiation-hard detectors and electronics, as well as quality control and radiation tests of every single piece of material installed in the experiment

● Angular coverage. Hermetic and cover the full azimuthal angle and the rapidity region |h|<5, to well measure the Missing ET



● Excellent electromagnetic energy resolution. Benchmark channel has been: H gg signal, to be extracted from the irreducible

background of gg events → a mass resolution of 1% is needed.

– Angle measurements. In order to reconstruct the two-photon invariant mass it is necessary to know the direction of both photons. ~50mrad/√E(GeV) is required to

achieve a gg mass resolution of 1%.

H ggm~2 GeV



● Large dynamic range. Electrons need to be measured with accuracy from a few GeV up to ;3 TeV (realized with multigain electronic chain).

● Jet energy resolution needs to be at the level of ~50%/√√EE(GeV)⊕3% , linearity @ 2% up to ~ 4 TeV

● Particle identification. Efficient rejection of jets faking electrons and photons is needed for several physics studies (jet can produce energetic p0) . This sets requirements on the granularity

of the electromagnetic calorimeter, which must provide adequate g/p0 discrimination.


On DEAD material...

Energy losses in the material (e.g., from tracking devices) that particles have to traverse before reaching the active part of the calorimeter are most important for electrons and photons

Inner detectors of LHC experiments are massive + coil + the calorimeter support structures and cables: <= 2X0 dead material in front of calorimeter.

Average energy lost by electrons and photons in the upstream material can be determined and corrected for; the event-by-event fluctuations cannot Additional contribution to the energy resolution.


On DEAD material...

Acceptable recovery of the energy resolution is possible by using massless gaps and presampler detectors ( if upstream material <2.5 - 3 X0)

Energy released by the incident particles in these devices is proportional to the energy lost upstream. By adding the energy collected (suitably weighted) to the energy measured in the calorimeter, it is possible to recover for energy losses event by event.

calorimetry in particle physics experimentspersonalpages.to.infn.it/~arcidiac/calo_design.pdf ·...

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