Heavy%20Ion%20Physics%20at%20the%20LHC%20with%20the%20Compact%20Muon%20Solenoid%20Detector

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Heavy Ion Physics at the LHC with the Compact
Muon Solenoid Detector

• Matthew Searle
• UC Davis Nuclear Physics Group
• 25 Aug 2004

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CMS Geometry
Transverse view of the CMS Detector.
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CMS Geometry
Longitudinal view of a quadrant of the CMS
detector.
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CMS Geometry

• CMS is designed to identify and precisely measure
  muons, electrons, photons, and jets over a broad
  energy and rapidity range.
• Main CMS detecting systems
• Tracker
• Electromagnetic Calorimeter (ECAL)
• Hadronic Calorimeter (HCAL)
• Muon Chambers
• CASTOR and the Forward detectors
• Zero-Degree Calorimeters (ZDCs)

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Tracker

• The Tracker is made up of two types of detectors
  the pixel layers and the silicon strip counters.
• Pixel detector 3 pixel barrel layers at 4.5,
  7.5, and 10 cm from beam axis and 2 endcap disks
  in forward and backward directions. Barrel layers
  cover up to ? lt 2.1.
• The barrel layers contain 9.6, 16, and 22.4
  million pixels, respectively. Pixel dimensions
  100x150 µm2.

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Tracker

• continued
• The inner Si strip counter consists of 4 barrel
  layers an 3 disks in each endcap.
• The outer MSGCs has 6 barrel layers 9 disks in
  each endcap.

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Tracker
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Tracker
Longitudinal view of the Tracker.
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Calorimeters

• ECAL
• Made up of 76,000 scintillating crystals of
  PbWO4. Light is detected with avalanche
  photodiodes (barrel) and vacuum phototriodes
  (endcap).
• EB covers up to ? lt 1.48.
• Crystals are 23 cm long, corresponding to 25.8 X0.

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Calorimeters
Longitudinal view of the ECAL.
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Calorimeters

• HCAL
• Split into two parts HB and HE.
• HB ? lt 3
• HE 3 lt ? lt 5

• The HCAL is a sampling calorimeter made up of
  scintillator/copper
• plates. The copper absorber plates are 5(8) cm
  think in the
• barrel(endcaps). The scintillator is 4mm thick.
• The barrel calorimeter is 79cm thick
  corresponding to 5.15 nuclear
• interaction lengths.

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Calorimeters
Longitudinal view of the HCAL.
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Muon Chambers

• The muon system uses three different detecting
  elements drift tubes (DTs), cathode strip
  chambers (CSCs), and resistive plate chambers
  (RPCs) dedicated to triggering. The muon system
  covers up to
• ? lt 2.4.

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Muon Chambers
Longitudinal view of the Muon Chambers
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CASTOR and the Forward Region

• CMS will have a suite of detectors beyond
• ? 5 that are unique at the LHC CASTOR and
  T2 at 5 lt ? lt 7, the TOTEM Roman pots 7 lt ? lt 10,
  and the ZDCs.
• The ZDCs will be able to measure neutrons and
  photons at 0 degrees.
• The ZDCs are vital for beam tuning and make
  possible or enhance measurements of
• Cross section measurements
• Centrality, through number of spectators
• UPCs through detection of mutual Coulomb
  dissociation
• Energy flow
• pA collisions and calibration of cosmic ray
  experiments

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Intro to CMS

• LHC provides a unique opportunity to study the
  strong interaction.
• Studies at RHIC at vSNN200 GeV strongly suggest
  that an equilibriated, strongly-coupled partonic
  system exists.
• Extrapolation to LHC energies hint at new
  discoveries at the TeV scale.

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Intro to CMS

• The Compact Muon Solenoid is an ideal Heavy Ion
  detector large acceptance for tracking
  calorimetry, high granularity resolution, fast
  detector technologies, sophisticated
  triggering.
• The Heavy Ion community can gain access to CMS
  for a small fraction of developing a new collider.

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Intro to CMS

• The LHC CMS will be able to explore the dense
  matter formed in a heavy ion collision at higher
  density, higher temperatures, and for longer
  lifetimes of the fireball.
• LHC energies will provide qualitatively new
  probes high pT jets, y, Z0, Y family, D and B
  mesons, high-mass dileptons.
• CMS has a dedicated amount of beam time for Heavy
  Ion physics (1 month per year).

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Intro to CMS

• Why CMS is currently the ideal Heavy Ion
  detector
• High rate pp collisions at L1034 cm-2 s-1 , or
  pp collision rate of 40 MHz, and 25 pp collisions
  per bunch at full pp luminosity.
• High resolution and granularity
• 4T field and pixel layers give ?pT/pT lt 1.5 up
  to pT 100 GeV/c
• b resolution lt 50 µm (lt 20 µm at pT gt 10 GeV/c)
• Calorimetry 16 jet energy resolution for 100
  GeV jets w/ dN/dy 5000
• ECAL spatial resolution in ? and f of .028 and
  .032, respectively.

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Intro to CMS

• Continued
• Large acceptance
• CASTOR, 5 lt ? lt 7
• T2 silicon detector, 5 lt ? lt 7
• TOTEM Roman Pots, 7 lt ? lt 10

• CMS performance in most categories far exceeds
  capabilities
• of existing or planned heavy-ion detectors.
• CMS will have significant but not excellent
  performance in low pT
• spectra and 2-particle correlations.

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Intro to CMS

• Whereas RHIC can approach x 0.02,
• LHC RHIC x 30 (energy),
• and will approach x 10-6-10-7.
• Nonlinear evolution of gluon density with Q2 and
  gluon saturation is expected to be seen.
• The Final State will be hotter and longer lived
• LHC RHIC x 20 (energy density),
• with the initial temperature T0 doubling, and
  the fireball living roughly 3 times longer.

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Physics Studies

• Primary focus of heavy ion physics at CMS will
  center around such processes as quarkonia, jets,
  and gauge bosons.
• Experience at RHIC has shown that global
  variables are essential for event categorization
  in various analysesand for placing important
  constraints on fundamental properties of particle
  production.
• Breaking of energy scaling is expected at LHC
  energies (14 TeV pp, 5.5 TeV Pb-Pb).

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Global Observables

• Global Observables
• Charged particle multiplicity, dN/dy
• Transverse energy, ET d ET/dy
• Azimuthal anisotropy, v2
• Zero-degree energy of neutral spectators, E0

• The dependence of these observables on collision
  geometry allows for characterization of the
  events.
• They will play an important role in testing
  models of particle and energy production and may
  allow selection of rare exotic events with high
  dN/dy or ET.

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Global Observables

• On the left is an example of the charged particle
  multiplicity for a single event reconstructed
  from pixel layer hits for a central Pb-Pb event.
• The largest contribution to the systematic error
  comes from the uncertainty in the yield of
  secondaries from the surrounding material and
  weakly decaying particles. This is expected to be
  reduced as experimental data becomes available at
  the LHC.

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Flow Azimuthal Anisotropy

• Measurement of azimuthal flow provides info about
  the initial spatial geometry of the collision
  region, and is sensitive to the early conditions
  and thermalization of the system.
• In hydrodynamic models, v2 arises from
  anisotropic pressure gradients in the transverse
  plane due to the almond shape of the overlap
  region of the nuclei.
• Deviation from linear dependence on centrality
  may indicate a phase transition or thermal
  disequilibrium.

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Measurement of Flow

• CMS proposes a new calorimetry-based analysis of
  flow using the azimuthal distribution of
  reconstructed jets. The proposed method is based
  on correlations between the azimuthal position of
  a jet axis the angles of particles not
  incorporated in the jet. Reconstuction of the
  reaction plane is avoided estimation of the jet
  energy is not necessary.

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Impact Parameter

• Many phenomena in a heavy ion collision depend
  crucially on the event centrality. Thus, the
  measurement of the impact parameter, b, is
  essential to characterize the event. CMS can do
  this by measuring the transverse energy, ET, with
  its hadronic and electromagnetic calorimeters.
• The strong correlation between ET and b makes a
  measurement of the impact parameter to less than
  1 fm possible.

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Impact Parameter

• Simulations of the calorimeter system during
  PbPb collisions indicate that over 80 of ET
  will be detected.
• Shown on the right, the upper plot shows the
  relationship between the summed transverse energy
  in the calorimeter and the impact parameter. The
  lower plot shows the accuracy of the impact
  parameter measured with ET flow in the HF
  calorimeter ( 3 lt eta lt 5 ) for ArAr collisions.

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Quarkonia ( c/c-bar, b/b-bar)

• CMS will focus on detecting quarkonia through
  their decay into muon pairs.
• Excellent momentum resolution for muons leads to
  a Y mass resolution of 50 MeV/c2. This provides a
  clean separation between the members of the Y
  family.
• On the right, Y detection in the CMS detector for
  different species with hadronic background
  calculated with the highest multiplicity
  estimates for each system.

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Quarkonia

• Signal in PbPb collisions after background
  subtraction in both the J/psi (left) and Y mass
  regions (right).

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Jet Physics at CMS

• CMS is ideally suited to study high pT jets. This
  makes it possible to study a wide range of
  observables key for understanding the
  modification of jet properties due to parton
  energy loss in a dense nuclear medium.
• Dijet quenching monojets may be signals of
  dense matter formation in a relativistic nuclear
  collision.
• At leading order, hard jets are produced with
  p1-p2. A monojet occurs if one of the members of
  a dijet loses so much evergy traversing the
  medium that only a single jet cone is observable.

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Jet Physics

• The dijet rate in AA relative to pp collisions
  can be studied by using a reference process,
  unaffected by energy loss and with a rate
  proportional to the number of nucleon-nucleon
  collisions , such as Drell-Yan production, or Z0
  production.
• This normalization is necessary to remove
  systematic errors in the luminosity. A
  measurement relative to a reference process
  requires pp and PbPb runs at the same energy.
  However, pp runs at 5.5 TeV will count against
  the heavy ion beam time. Therefore, this data
  will not be available when heavy ion data taking
  starts at the LHC.

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Jet Physics

• In the figure on the left, the monojet/dijet
  ratio as a function of the threshold jet energy
  ET in central Pb-Pb collisions for different
  quenching scenarios
• The scaled PYTHIA result for the dijet spectrum
  is shown as the solid curve at a ratio of 2.

Solid line no quenching Dashed line ideal
plasma Dotted line maximally viscous plasma
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Tagged Heavy quark jets

• Since there is an expected difference in the
  quenching mechanism for heavy quarks, b-quark
  jets will yield important information regarding
  energy loss in the medium.
• Dead cone effect for heavy quarks, gluon
  bremsstralung at small angles is suppressed,
  leading to significantly smaller energy loss.
• Heavy quark jets are tagged by reconstructing
  secondary vertices leading to D and B mesons.
• CMS will provide good tagging efficiency with low
  contamination of light quark/gluon jets.

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Three jet events

• Three jet events offer an interesting opportunity
  to study energy loss by comparing gluon and quark
  jets.

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Z0-jet and y-jet channels

• Processes where a hard parton jet is tagged by an
  unquenched ( not strongly interacting )
  particle such as a Z0 or y are ideal to measure
  jet energy loss of the corresponding away-side
  jet.
• Given the high granularity hadronic and
  electromagnetic calorimeters of CMS, a good
  rejection factor can be achieved against
  misidentified p0s ( vs. y-jet events).

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Z0-bosons

• The Z0 provides a unique opportunity to study
  quark distributions in the nucleus at high
  Q2m2z0, and CMS should be able to measure
  nuclear modifications as a function of Z0
  rapidity.

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pA collisions

• pA collisions provide the cleanest measure of the
  initial state for AA collisions.
• It is postualted that when viewed a fast probe, a
  nucleus may appear to resemble a sheet (or
  pancake) of highly correlated gluons known as a
  Color Glass Condensate (CGC).
• In a CGC, two soft gluons can merge to form a
  harder gluon. This can lead to a suppression of
  low pT hadrons in dAu collisions compared to pp
  collisions. These effects should be stronger at
  forward rapidities where x is smaller but gluon
  density is higher.

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pA collisions

• At CMS
• Central rapidities at ?lt 2.4, silicon
  tracking, photon, jet, muon measurements can
  give very detailed descriptions of the collision.
• Forward rapidities CMS is able to reconstruct
  jets up to ? lt 5.
• The CASTOR and T2 detectors may push jet
  reconstruction up to ? 7.

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pA collisions

• Almost hermetic calorimeter coverage of CMS will
  allow very precise measurements of energy
  stopping in pA collisions. This data will provide
  strong constraints on AA collisions.
• pA measurements will also serve to calibrate
  the energy scale of ultra-high energy cosmic ray
  experiments which currently rely on
  extrapolations of GeV measurements to the PeV
  range to simulate their detector response.

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Forward physics

• The forward detectors, CASTOR the ZDCs will
  play an important role at CMS. Forward coverage
  is essential for measuring parton (especially
  gluon) distribution functions in protons and
  nuclei.
• CMS will be able to study x as low as
• 10-6-10-7 . Nonlinear evolution of parton
  densities saturation effects will be able to be
  mapped out in x and Q2.

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Forward Physics

• Near hermetic coverage is also important for the
  study of diffractive processes. Diffractive
  events are characterized by large rapidity gaps
  in collision products. Hard diffractive
  production of heavy quarks jets may lead to a
  further understanding of the Pomeron.

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Forward Physics

• PHOBOS and BRAHMS have shown the value of
  studying Au-Au dAu collisions over a large
  pseudorapidity range. Based on extrapolation from
  BRAHMS, it is expected that CASTOR will cover the
  region of maximum baryon density at CMS. Thus,
  CMS will be able to study partonic matter over a
  very large range of baryo-chemical potential.

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Ultra-peripheral collisions

• UPCs can shed light on a number of physics
  topics including nuclear parton distributions and
  meson spectroscopy.
• Gluon distribution functions can be measured by
  studying photoproduction (yg ? q/q-bar) of heavy
  quarks (usually c or b, t being available at LHC
  energies).
• It is important to determine if/how the nuclei
  break up after a UPC. The ZDC system is therefore
  essential to measure neutral particle flux very
  near the beam.

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Summary

• LHC will push energies up to 5.5 TeV for Pb-Pb
  collisions, and 14 TeV for pp collisions. Many
  new phenomena are expected at these energies.
• CMS is the ideal Heavy Ion detector for studying
  these new phenomena at these energies.

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Acknowledgements

• It should be noted that this presentation was
  meant to inform the UC Davis Nuclear Physics
  Group and in no way represents original research
  of my own.
• I would like to thank Dr. Daniel Cebra, Dr. Juan
  Romero, Roppon Picha, and David Cherney for
  useful discussions related to this presentation.
• This presentation is based of Heavy Ion Physics
  at the LHC with the Compact Muon Solenoid
  detector by
• R. Arcidiacono et al.
• Pictures were obtained from http//cmsinfo.cern.
  ch/Welcome.html/CMSdetectorInfo/CMSdetectorInfo.ht
  ml and its subpages.