QCD @ The Next Frontier

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QCD _at_ The Next Frontier

• Studying the QGP and the CGC at the LHC using the
  ATLAS Detector

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Why the LHC?

• The QGP discovery phase of heavy ion
  physics is nearly over.
• Due to the success of RHIC
• But we have wisely always looked past QGP
  discovery to the goal of
• Understanding its properties.
• Because thats how we will really learn
  something new about QCD
• How? One way
• Study experimental observables under controlled
  variations of initial conditions

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Why the LHC? (2)

• CGC initial conditions
• Unique QCD physics
• Crucial for controlled QGP initial conditions
• QGP _at_ (more) extreme conditions
• Longer-lived QGP state
• sQGP? Quarkonium screening?
• Copious production of hard probes
• True jet measurements from Day 1(.1)
• Detector(s)!
• State of the art detector for free

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Multiplicity, ET

• Multiplicity data provided first evidence for
  saturation _at_ RHIC
• Measurements of dN/d?, dET/d?, will provide
  crucial test of saturation
• and/or our understanding of particle
  multiplicities
• dN/d? also relevant to jet quenching _at_ RHIC
• Expect to be true _at_ LHC
• ZDC will be important in centrality determination
• Npart and Ncoll

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Strongly Interacting Matter _at_ RHIC
dN/d?

• Pressure converts spatial anisotropy to
  momentum anisotropy.
• Requires early thermalization.
• Unique to heavy ion collisions
• Answer yes

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Elliptic Flow ? sQGP

• Study flow vs collision energy/centrality
• Compare w/ hydrodynamic calculations of flow.
• Reach hydro limit _at_ RHIC(?)
• But what if we went further in (1/S)dN/dy?
• _at_ LHC, more than x2
• Is strong coupling due to plasma instabilities?
• Stronger _at_ LHC(?)

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Jets Jet Quenching

• We have beautiful data on jet quenching _at_ RHIC
• But severe theoretical disagreement on
  interpretation
• GLV, AMY consistent w/ expected parton
  densities
• BDMS SW, PQM need 5x expected parton
  density??
• Or just strong transverse flow effects on energy
  loss?
• But, not found by GLVHuovinen!

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Jets Jet Quenching (2)

• Why such uncertainty?
• Fluctuations in energy loss
• Fluctuations in fragmentation
• Trigger bias effect
• Statistics/limited pT reach
• No direct measure of (e.g.)
• No photon-jet data yet
• Full jet measurements _at_ LHC solves these
  problems!
• Directly measure the modified frag. func. from
  energy loss
• No trigger bias effect
• Statistics a non-issue
• kT dist. directly sensitive to
• Photon-jet much easier
• Rate
• Acceptance

Hadrons, not jets but close enough
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Jet Modifications _at_ LHC (SW)

• Modification of radiated gluon kT distribution
• Crucial point of the figure is that the large kT
  spectrum is unaffected by energy cut
• Can measure with particles well above background
• Can measure in small cone
• Angular distribution is characteristic of
• For gluons, not hadrons!
• If (newer) SW estimate is correct, we will see
  radiation as sub-jets measureable.

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Quenching as Modified Parton Shower

• New work on analyzing quenching as modified
  parton shower
• Change in shape of the MLLA hump-back plateau
• Promising approach that takes advantage of pQCD
  methods
• But, for now very ad hoc modification of
  splitting kernel
• In particular, relies on angular ordering
• Relevant for high energy jets with extensive
  gluon emission

Jet quenching started as a QGP signal But now
starting to address fundamental QCD
physics (e.g.) Baier new scale (R) in angular
dist.
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di-? Probes of the QGP

• At RHIC we are hot on trail of new source of hard
  photons
• Jet conversion photons
• Direct probe of QGP
• _at_ LHC measurable via di-?
• c/b decay background needs study
• But at low mass, c/b decay background suppressed.
  Sufficiently??

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Gluon Saturation _at_ LHC

• Gluon saturation already plays a role _at_ RHIC
• Expected to completely determine AA initial
  conditions _at_ LHC
• Will be studied in p-A collisions
• p-A _at_ LHC will provide most complete tests of
• LT Shadowing
• Saturation
• Factorization (violation)
• For hadron interations in nuclei (compl. to e-A)

Broadening Only
Including Quantum Evolution
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Why ATLAS?

• Calorimeters
• High granularity EM hadronic calorimetry
• With longitudinal segmentation
• Large acceptance
• ??10 coverage w/ calorimetry
• ?? 6.4 coverage w/ tracking
• Muon spectrometers
• Large acceptance, low background
• Synergy
• Technical/physics overlap with high-energy ATLAS
  groups _at_ BNL, Columbia,

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Why ATLAS? Calorimetery!
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Low Energy Jet in Central PbPb Event
From Ketevis 2003 studies
Very likely a ?-jet event
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High Energy Jet in Central PbPb Event
2 Pythia jets plus a Hijing Jet Jet splitting
(sub-jets) typical of high Q2 processes Copious
hard gluon radiation.
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Jet Reconstruction PerformanceLOI
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Past Studies of HI Jet Analysis (2)
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Yes, but WHY ATLAS????

• Answer EM calorimeter segmentation
• In particular, longitudial segmentation
• No other experiment _at_ LHC has longitudinally
  segmented EM calorimeter
• Why does this matter?
• First longitudinal layer dominated by soft
  particles.
• Removing first layer removes significant
  background
• In principle, predictor of soft component in 2nd
  layer
• Albeit w/ fluctuations but layer 1?layer 2
  likelihood analysis likely to be better at
  handling background than any algorithm studied so
  far.
• Plus we have the pre-sampler

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Pb-Pb
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Yes, but WHY ATLAS???? (2)

• Longitudial segmentation was essential for
  analysis done for LOI
• finding isolated neutral clusters in jets
• Large z neutral hadrons
• Rare but precise probe of energy loss
• Long. segmentation will surely be important in
  prompt photon isolation.
• Same technique as for the isolated clusters
  (which are a reducible background)

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Yes, but WHY ATLAS???? (3)

• We dont really know how jets will be modified _at_
  LHC
• But surely will manifest as change in jet
  structure
• Which will require detailed measurements of
• Jet energy flow, sub-jets (hard radiation),
• Photon-jet measurements will be important
• Background THE most important issue w/ jet
  analysis
• We dont know how large the backgrounds will be
• I want every tool at my disposal to
• reduce background fluctuations,
• measure jet energy profile
• isolate photons
• ATLAS EM-calorimeter long. segmentation is the
  most potent tool available in any of 3 experiments

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ATLAS EM Calorimeter Structure

• We have not yet attempted (but we will) to use
  fine ?? segmentation of first layer for ? - ?/?
  separation.

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ATLAS Simulated Hijing p-Pb Event

• Jet at forward (actually backward) rapidity

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Why Should BNL Participate?

• Programmatic argument
• BNL has declared its intent to lead the
  field of strong interaction physics for the
  foreseeable future QCD Lab.
• BNL cant afford to not participate in the
  next major program in QCD physics.
• But, with a modest manpower investment in ATLAS,
  BNL can play a significant role in the LHC
  physics program
• That complements RHIC and e-RHIC
• That leverages BNL investment in ATLAS

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Why Should BNL Participate? (2)

• Physics Argument
• In spite of RHIC successes, were still missing
  firm conclusions on important physics issues
• Why is the QGP strongly coupled?
• How opaque (to jets) is the matter created _at_
  RHIC?
• Why is J/Psi suppression so small?
• Is forward dAu suppression due to CGC
  evolution ?
• It is unlikely these will all be solved by LHC
  startup.
• It is likely that LHC measurements will provide
  new insight on these (and other) questions
  relevant to RHIC, RHIC II, e-RHIC.
• ? no substitute for direct involvement

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CMS EM Calorimeter Segmentation

• CMS has marginally better transverse segmentation
  than ATLAS (0.0175 vs 0.025) for
  ?lt1.5
• But ATLAS much better for 1.5 lt ? lt 2.5 (0.025
  vs 0.05)
• More important CMS has not longitudinal
  segmentation.

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ATLAS vs CMS Jet Resolution

• CMS PbPb Jet resolution (Nov 2005)
• _at_ 75 GeV, CMS16, ATLAS13
• _at_ 125 GeV, CMS15, ATLAS10
• _at_ 175 GeV, CMS12, ATLAS8
• ATLAS better than CMS even in p-p
• CMS sees degradation in jet resolution in PbPb
  even at very high energy
• In ATLAS, no degradation for Egt150
• Note ATLAS numbers from 2003

From Boleks talk at the PANIC LHC HI workshop
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ATLAS
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Jet Quenching _at_ RHIC

• Use quarks gluons from high-Q2 scattering
• Sensitive to earliest times, highest
  temperatures.
• (QCD) Energy loss of (color) charged particle
• Until recently thought to be dominated by
  radiation
• Strong coherence effects for high-pT jets
• Virtual gluon(s) of high-pT quark/gluon multiple
  scatter in the medium and are emitted as real
  radiation

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STAR Jet Re-emergence _at_ High-pT

• Keeping hadron momentum cuts fixed, change size
  of the colliding system.
• Strength of the jet signal constant (surface
  bias)
• Strength of di-jet signal decreases but doesnt
  go away.

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(di)Jet Angular Correlations (PHENIX)

• PHENIX (nucl-ex/0507004) moderate pT

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Perfect Fluid?
My view Perfect fluid is reasonable
interpretation of available data but there is
room for skepticism.
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Why Heavy Ions _at_ LHC?

• Low x Gluon production from saturated initial
  state
• Energy density 50 GeV/fm3 (?)
• Rate copious jet production above 100 GeV
• Jets Full jet reconstruction
• Detector (nearly) perfect detector for free!

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Gluon Saturation _at_ low x

• _at_ LHC, nuclei are Lorentz contracted by ? gt 2000
• Except for soft gluons
• Which overlap longitudinally
• Gluons combine coherently
• Broadening gluon kT distribution
• Generates a new scale Qs
• Typical kT of gluons
• When Qsgtgt?QCD, perturbative calculations
  possible.
• Large occupation s for kTltQs
• Classical gluon fields
• Related to low-x _at_ HERA but Qs?A1/3
• Qs 4 GeV/c for Pb at LHC

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Simulated PbPb Event in ATLAS (No Jet)
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ATLAS Heavy Ion Program

• Heavy Ion physics is part of the ATLAS program.
• Currently a modest effort
• 30 part time physicists
• The ATLAS heavy ion program provides an ideal
  opportunity to start new research efforts
• Using high-energy physics techniques
• To study the only non-Abelian matter that we
  can create in the lab.
• To better understand consequences of QCD

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Jet Definition in HI Collisions

• For now, take a purely practical approach
• Develop an algorithm that is least sensitive to
  bkgd
• That takes into account what we know about
  quenching
• And calibrate using p-p data
• Some practicalities (R ? cone size)
• Bkgd Et ?R2
• For jet energy measurement use small cones
• Maybe as small/smaller than 0.2!
• Small cones are also better for measuring jet
  direction
• Then measure statistically
• d2Et/d?d?
• Hadron jT distribution
• Fragmentation function
• Look for other structure

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Jet Structure

• Sub-jet measurements will be critical for HI
  physics
• Energy scale for initial gluon production _at_ LHC
  4 GeV
• Proper time ? 0.05 fm for medium to be
  present
• Initial parton splittings occur at ? 1/?Q2
• Hard (kT gt 4 GeV/c) radiation independent parent
  parton.
• Holy Grail of quenching studies
• Direct measurement of gluon radiation spectrum
  (E, kT)
• How best to measure jet structure/sub-jets?
• kT algorithm (modified to handle bkgd)?
• Cone w/ splitting?
• Would small cone algorithm work?
• Something else?

Advice from the experts would be
helpful/appreciated!
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Calorimeter Occupany in PbPb Events