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Studying QCD Under Extreme Conditions at the LHC with the
ATLAS Detector
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LHC Physics Program: Key Questions
•Saturation/CGC – Correct description of heavy ion I.C. at RHIC, LHC?– Can we test/constrain saturation in p-p, p-A?
•Bulk phenomena– Do we understand particle production?– Do we understand elliptic flow?– What are QGP properties @ LHC vs RHIC?
•Hard probes– What is correct physical picture for jet quenching?– Can we truly perform jet tomography?– How do hard partons interact with medium?– Can we learn more about the medium than
dNg/dy?
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LHC Physics Program: Key Questions(2)
•Quarkonium screening– Will primordial J/ be screened by QGP @ LHC?
Will we know?– Will states be screened by QGP @ LHC?– Will , ’, , … production/suppression provide
quantitative insight on medium properties?
•EM Probes & “exotica”– Will we observe symmetry restoration @ LHC?
Under background of heavy quark decays?– Will we observe radiation from plasma?– Does strong CP violation occur? Will we see it?– Are there surprises in store at ~ 40 GeV/fm3 ?
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Multiplicity, ET
•Multiplicity data provided first evidence for saturation @ 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 @ RHIC– Expect to be true @ LHC
•ZDC will be important in centrality determination
Npart and Ncoll
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Elliptic Flow sQGP?
•Study flow vs collision energy/centrality
•Compare w/ hydrodynamic calculations of flow. – Reach hydro limit @
RHIC(?)
•But what if we went further in (1/S)dN/dy?– @ LHC, more than x2
•Is strong coupling due to plasma instabilities?– Stronger @ LHC(?)
)2cos(21 2
vd
dN
??
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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 @ 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
q̂
q̂
Hadrons, not jets but close enough
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Pb+Pb Jet Rates (2003)
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Jet Modifications: Vitev
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Jet Modifications @ 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.
2GeV4ˆ64 Lqc
2GeV4ˆ64 Lqc2GeV8ˆ132 Lqc
Lq̂Note that in Nucl. Phys. A747: 51, SW estimate > 100 GeV2
based on RHIC dataLq̂
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Jets @ LHC: Hard Radiation
•For high energy jets, copious production of hard radiation in final-state parton shower.
Complicated structure of observed jets
Large kT gluon radiation will lose energy independently
Not sufficient to measure “full jets” – need sub-jets
Ivan’s slide showing both vacuum and medium-induced radiation
kT and energy of vacuum radiation ranges up to hard scattering scale
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di-jet, -jet, …
•Imagine making such a plot with > x1000 counts– With full jets and not hadrons– With ~ full acceptance for second jet– With the ability to tag b’s– With prompt photon isolation– While simultaneously studying single jet structure
Mach cone(?), flow distortion(?), RP dependence, …
After full run at or above design luminosity @ RHIC
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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
•@ LHC measurable via di- – c/b decay background needs study– But at low mass, c/b decay
background suppressed. Sufficiently??
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Slide from QM 2005 Talk by K. Itakura
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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 =5, low background
•Synergy– Technical/physics overlap with high-
energy ATLAS groups @ BNL, Columbia, …
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ATLAS Cavern as of Today
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ATLAS Calorimetery
EM Long. Segmentation
Hadronic Barrel
Hadronic EndCap
EM EndCap
EM Barrel
Forward
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ATLAS EM Calorimeter Close up
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ATLAS Calorimeters•Large acceptance:
– hadronic || < 4.9, EM || < 3.2*
– Three sections: barrel (|| < 1.5) endcap (1.5 < || < 3.2) and forward (3.2 < || < 4.9)
– Full azimuthal coverage in all sections.
•Segmentation (typical)– EM calorimeter: 0.025 x 0.025
– Hadronic calorimeter: 0.1x0.1
•Both barrel and calorimeters longitudinally segmented with three sections
•Forward calorimeter has two longitudinal sections
•Barrel EM first layer has fine segmentation– Strips with width 0.003
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Installation: LAr Barrel
•In final position and aligned•Electronics installation nearly complete•Cool-down April 2006!
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ATLAS (Inner) Tracker
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ATLAS Inner Tracker (2)Pixel Detector•3 layers (1 partially staged), 50 m x 400 m•R = 4.7, 10.5, 13.7 cm•~ 50 MeV cutoff in pixels alone•Few % occupany in central Pb+Pb (Hijing)
Silicon strips•4 layers, 2 stereo measurements each•80 m x 12 cm strips (barrel)•From stereo: r = 16 m, z = 580 m•Occupancy 5-7% in central Pb+Pb (Hijing)
TRT•~not usable for HI collisions (under investigation)
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ATLAS Tracker
•Pixel + SCT mated with TRT barrel in clean assembly area.
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ATLAS ZDC
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Global Event Properties
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+ Pb-Pb
Not EM cal first layer!
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180 GeV Jet + HIJING Pb+Pb, b=0
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60 GeV Jet in Central Pb+Pb Event
Probably a -jet event!
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Jet Reconstruction Performance:LOI
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ATLAS Tracking Performance
Results ~ 1 ½ years old, new ATLAS algorithm that can be easily optimized for heavy ion collisions now near completion.
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b Jets
Essential test of quantitative understanding of quenching
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Jet Fragmentation Observables
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Using Longitudinal Segmentation
•Much of the soft background stops in material before the EM calorimeter, pre-sampler, 1st EM layer– E01 = energy in pre-sampler, Etot = total EM energy
60% of background stopped before/in 1st EM layer
=0.1 x 0.1 cells
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Using Longitudinal Segmentation(2)
•Use pre-sampler plus first EM layer as “absorber”•Compare (jet + BG) / BG using full EM calorimeter and using 2nd + 3rd layers– Very preliminary: ~ 50% improvement in (jet + BG) /
BG
=0.1 x 0.1 cells
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Longitudinal Segmentation: Going Further
•1st EM layer consists of =0.0031 x =0.1 strips•Expect ~ 100 MeV/strip soft BG in central Pb+Pb
Very low BG/strip even in heavy ion collisionsOccupancy in jet determine by jet structure not
BG.
•Photons deposit ~ 50% of their energy in 1st layerCan easily detect individual photons even in Pb+PbCan use ATLAS tools for photon isolation, etc.
Jet Region
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Upsilon Screening
•Physics in behavior of different states: doable
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Atlas J/ Reconstruction
•J/ acceptance limited by momentum required for muons to penetrate to muon spectrometers.
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ATLAS: Simulated Hijing p-Pb Event
Jet at forward (actually backward) rapidity
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Summary•ATLAS was last of three experiments @ LHC to pursue a heavy ion program.–LOI submitted to LHCC in April 2004
–Heavy ion program is an official part of ATLAS
–Soon to become official part of USATLAS project
•ATLAS HI effort in US & outside growing
•ATLAS has clear strengths in:– Jet, , *, Z, …measurements
–Global event properties
– single, di-muon measurements
–DAQ/trigger system
–Synergy with components of p-p program
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Heavy Ion Participants
+ SUNY Stonybrook (chemistry) + BNL Brahms group + others in the process of joining …
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LHC QGP Physics ProgramInitial conditions
Thermalization,Pressure, bulk motion
Freeze-out
Medium properties (other than E/parton density)
“Deconfinement”
Radiation from Plasma
Chiral Symmetry
Anomalous Events
Minimum bias (Day-1)
Event Properties vs centrality
Low-pT particle production, particle ID
Single high-pT particle production
Single jet production, “fragmentation
di-jet, -jet, multi-jets
Quarkonia
Low mass di-leptons
Single leptons
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ATLAS QGP Physics ProgramInitial conditions
Thermalization,Pressure, bulk motion
Freeze-out
Parton probesof medium
“Deconfinement”
Radiation from Plasma
Chiral Symmetry
Anomalous Events
Minimum bias (Day-1)
Event Properties vs centrality
Single high-pT particle production
Single jet production, “fragmentation
di-jet, -jet, multi-jets
Quarkonia
Low mass di-leptons
Single leptons
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ATLAS vs CMS Jet Resolution•CMS Pb+Pb Jet resolution (Nov 2005)
– @ 75 GeV, CMS~16%, ATLAS~13%– @ 125 GeV, CMS~15%, ATLAS~10%– @ 175 GeV, CMS~12%, ATLAS~8%– ATLAS better than CMS even in p-p
•CMS sees degradation in jet resolution in Pb+Pb even at very high energy
•In ATLAS, no degradation for E>150
•Note: ATLAS numbers from 2003
From Bolek’s talk at the PANIC LHC HI workshop
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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 conesMaybe as small/smaller than 0.2!
– Small cones are also better for measuring jet direction
•Then measure statistically– d2Et/dd– 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 @ LHC ~ 4 GeV
Proper time ~ 0.05 fm for “medium” to be present
– Initial parton splittings occur at ~ 1/Q2
Hard (kT > 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 Pb+Pb Events
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