Quantity. Value. ALICE-USA Electromagnetic Calorimeter. . . Jet cone R= 0.3 , 0.5 , 0.7. Tower Size. Sampling Ratio. 5mm Pb/5mm Scintillator. 0.9. ~6.5 ~6.5 24.5 cm 3. Sampling Fraction. 13/1. 0. PHOS. Δη Δφ=0.0142 0.0146. Depth. ~25 rad. lengths. EMCAL. TPC. -0.9.
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ALICE-USA Electromagnetic Calorimeter
5mm Pb/5mm Scintillator
~25 rad. lengths
No. of Towers
No. of Modules
No. of Super-modules
Calorimeter response to 200 GeV (left) and 50 GeV (right) photons. The top panels show the tower response plotted as a percent of the full energy and the bottom panels show the reconstructed energies after clustering. The EMCal electronics will provide overlapping 4x4 tower sums that will produce the data seen in the lower panels at the trigger level.
~ 9 U.S. Tons
Some Physical parameter describing the proposed Electromagnetic Calorimeter for ALICE.
Jet fragmentation functions of PYTHIA jets reconstructed with the ALICE tracking system for jets of pT=50 to 600 GeV/c (black curves). The soft non-jet background found within the jet cone is indicated in red.
PYTHIA calculations of p0 spectra at SPS, RHIC, and LHC energies showing the dramatic growth of the cross section at high pTand with s.
This shows relative acceptance of the EMCAL for Jets as compared to the PHOS for the corresponding photon, for different sizes of “Jet-cones”. To a large extent, this sets the size of the EMCAL.
Response of the calorimeter to central HIJING events with (dN/dy)charged =8000. A mean tower energy of ~200MeV at 40% occupancy is observed. The lower right hand panel shows that this energy deposition is made up of 50% charged hadrons, 40% electromagnetic energy and ~10% long lived neutral hadrons.
End view of the EMCAL in a common support structure with the HMPID. A continuous arch of super-modules spanning ~120 degrees in azimuth is indicated. The EMCAL is positioned to provide almost complete back-to-back coverage with the PHOS calorimeter.
Calorimeter Resolution and Efficiencies
Anticipated Changes in Jet Fragmentation
Inclusive direct photon yield per ALICE year for minimum bias Pb+Pb collisions at sNN = 5.5 TeV into the PHOS (red), EMCAL (blue) and sum (black) acceptance. Calculations are based on PYTHIA scaled as A2 assuming a 30 day ALICE run with 100% efficiency. Similarly scaled yields for the sum acceptance from the direct photon measurements of CDF are plotted for comparison. (Note the suppressed scale.)
Predicted rates of g+jet events as a function of the invariant mass of the g-jet pair per ALICE year for minimum bias Pb+Pb collisions at sNN=5.5 TeV into the PHOS (red), EMCAL (blue) and sum (black) acceptance. Calculations are based on PYTHIA scaled as A2 assuming a 30 day ALICE run with 100% efficiency. Results are shown for quark (solid) and gluon (dashed) jets.
The ALICE EMCAL super-module (dimensions in mm). Four modules (one of which is highlighted) are installed in a single super-module.
Parton ET (GeV/c)
Simulated calorimeter response to mono-energetic photons from 25 GeV through 200 GeV (left) for a total active depth of 21 radiation lengths (right) and 25 radiation lengths. Note the asymmetric low energy tails for the 21 radiation length deep calorimeter, and how they are much more symmetric for the 25 radiation length deep calorimeter.
Parton ET (GeV/c)
Single electron yield from W and Z0 decays into the PHOS (red) or EMCAL (blue) acceptance per ALICE year for minimum bias Pb+Pb collisions at sNN = 5.5 TeV as predicted by PYTHIA scaled by A2. Electrons from Drell Yan pairs are also included.
Lead – Scintillator – Lead sandwich showing the fiber routing layer, the readout grooves, and the tower-to-tower isolation grooves.
η – η0
Energy and spatial resolution along with the reconstruction efficacy for reconstructed jets with a jet cone radius cut of R=0.2. The two upper right plots show the energy dependence of the energy resolution to be approximately constant at ~16%. The spatial resolution, s(h)/R~s(f)/R, is about ~10% and the number of improperly identified jets is low. The simulation was for PYTHIA (p-p) jets at s=5.5 TeV placed in a Pb-Pb background event.
Lego plots of 100 GeV/c and 200 GeV/c jets in ALICE. The energy deposited in pseudo cells is plotted versus h and f. The pseudo cell energy is deduced from the calorimeter tower response and from ITS/TPC/TRD tracking. In the right hand panel, the cell size is chosen equal to the calorimeter tower size.
Electron pair invariant mass yield from Drell Yan and Z0 decays into the PHOS (blue) or EMCAL (red) acceptance per ALICE year for minimum bias Pb+Pb collisions at sNN = 5.5 TeV as predicted by PYTHIA and scaled by A2.
A 100 GeV jet-jet PYTHIA simulated event. The tracks in the inner tracking detectors, ITS TPC and TRD, are shown along with signals from the PHOS below and EMCAL above.
General layout of the EMCAL optical system showing the scintillating mega tiles, fiber routing layer, fiber decoding, and APD matrix.
Collaborating US Institutions:
Creighton University,Kent State University, Lawrence Berkeley National Laboratory, Michigan State University, Oak Ridge National Laboratory, Ohio Supercomputer Center, Purdue University, The Ohio State University, University of California, Berkeley, University of California, Davis, University of California, Los Angeles, University of Houston, University of Tennessee, University of Washington, Vanderbilt University, Wayne State University
High PT Physics in Heavy Ions at the LHC
Physics Capabilities when EMCAL is added
Heavy ion collisions at LHC energies will allow us to explore regions of energies and particle densities which are significantly larger than those reachable at the SPS and RHIC. In addition, the expected lifetimes of the QGP will be significantly larger. The thermalization, the ratio of lifetime to interaction time, will be much larger.
Jet Inclusive Cross-Sections
To maximize the physics potential at a minimum cost, the ALICE-USA group, in consultations with the ALICE collaboration, is proposing to place a thick relatively large partial barrel Pb scintillator sampling electromagnetic calori-meter which will be coarsely segmented and placed opposite the much smaller but finer segmented PbWO4 crystal, PHOS, calorimeter. The EMCAL has the acceptance for jets while the PHOS has the fine energy resolution for the recoil photon. In this way the two detectors complement each other.
The ALICE EMCAL is based on the STAR calorimeter, and therefore, its response can be anticipated. Nevertheless, many simulations of the EMCAL have been done in order to optimize the design and to make sure that our physics goals can be met. The EMCAL has been fully integrated into the ALICE simulation and analysis framework. This has allowed us to do more complete studies of the EMCAL.
Calorimeter thickness Optimization
Soft interactions are not expected to produce results very different from those seen at the SPS or RHIC, whereas the rate of hard or high PT interactions are expected to be significantly enhanced. The inclusive jet rate at LHC energies is expected to be much larger than that at the SPS or even RHIC. This will allow for the first time, the effective use of jets as a probe of the QGP. Additionally, it is expected, from most theoretical calculations, that the changes induced by the QGP will be significantly enhanced over that expected at RHIC.
Photon Tagged Jets
Gamma-jet events will be produced with either a quark or gluon balancing the prompt direct photon. This quark or gluon will fragment into a jet of particles. The original direction of this quark or gluon can be inferred by summing all of the tracks within a predefined cone around a leading high PT particle. If the leading particle is neutral, π0 for instance, a large coverage electromagnetic calorimeter would be needed to find it. A high PT gamma, if it falls into the PHOS acceptance, can be used to indicate that there must be a jet in the opposite direction, or near enough. These photons can be used to determine the energy and momentum of all the particles that make up the jet.
Prompt Direct Photons
Prompt direct photons will not be affected by any QGP or subsequent hadron gas. They are therefore a good indicator of the initial state interactions which occur well before the QGP is formed. To the right is shown the rate of these direct photons anticipated to be measured in ALICE using the PHOS alone, the EMCAL alone, and the two detectors together. Also shown are extrapolations from CDF measurements for comparison.
The calorimeter will also give a small signal due to charged hadrons. These hadrons can be traced back to the EMCAL and the calorimeter’s
In addition to Jets, a large coverage EMCAL can aid in identifying heavy resonances. To the right and below are shown a few of these resonances and their anticipated rates at the LHC. In addition are shown the rates for prompt direct photons (with or without accompanying jets), W’s and Z bosons.
signals can be corrected for these hadrons. A similar correction can be estimated due to neutral hadrons.
By using the same readout electronics as that used for the PHOS, development costs can be kept to a minimum. This will allow both the PHOS and EMCAL to be used as Jet-trigger detectors.