Calorimetry at LHC - INFN – Sezione di Lecce

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Calorimetry at LHC
Davide Pinci – INFN Sezione di Roma.
“Calorimetry at LHC”
4th Summer School on THE PHYSICS OF LHC
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.
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
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.
Calorimetry for e.m. particles
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;
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;
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;
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;
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.;
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;
±
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;
Real longitudinal distribution
✗ The Longo-Sestili parametrization is a good approximation but:
9 12 15 19.5
✗ 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.
17.5 = 6 cm
13 = 115 cm
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;
Lateral development
✗ 99% of the shower is contained in 3 RM;
Scaling is not
perfect with
the Moliere
radius
Tails can be very different
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.
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.
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
Examples of sampling calorimeters
Absorber/Cherenkov radiator
Absorber/scintillator
Absorber/liquid noble gas
Absorber/proportional gas detector
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.
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
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 < 0.7 - ΔΦ = 100°
✗ Good granularity: 11520 towers with
size: Δφ° ~ 0.0143°;
✗ 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;
EM Cal structure
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;
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;
EM Cal performance (test beam)
✗ Linearity better than 1% above 20 GeV
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
ALICE: PHOS
To measure , π0 and η (0.5-10 GeV)
✗ 18000 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
The CMS EM cal
✗ Compact crystal electromagnetic calorimeter inside the 4T solenoid;
✗ Barrel based on PWO+APD;
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%
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;
Performance with LHC: resonances
✗ With first LHC collisions it was already possible to reconstruct first signals from
resonances:
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
ATLAS EM Cal
✗ Very important construction accuracy and operation stability;
<d>= 2.211 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
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
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!
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;
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
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;
Performance: resonances
Calorimeters for hadronic particles
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.
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...
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.
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.
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.
Summary for various materials
40 cm
16 cm
10 cm
17 cm
10 cm
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.
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;
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.
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.
Fluctuations (I)
✗ The energy released to break nuclear bindings and the number of neutrons
produced can have very large fluctuations
Electromagnetic fraction (fem)
✗ Suppose a π0 is produced in the shower. It decays in less than 10-16 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;
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.
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
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.
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;
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;
✗ 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;
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.
Lateral containment
Lateral containment
The lateral containment is almost independent from the energy because
the hadronic halo carries a small fraction of the energy.
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:
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.
The non-compensate calorimeter
✗ In the case of e/h = 1 → e/π = 1 for any value of the fem: “compensate” calorimeter
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;
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
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.
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
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 0.1
0.2 x 0.2 for |η|>2.5
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
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
Performance
Atlas HCAL ensures at least 10 interaction lengths for all particles;
Missing energy resolution
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
CMS hadronic calorimeter
Scintillator tiles readout
via WLS fibers
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.
Synchronization
✗ By means of the beam splashes in 2009 the responses of the calorimeter
modules were synchronized;
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!
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;
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;

Calorimetry at LHC - INFN – Sezione di Lecce

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