Studying QCD Under Extreme Conditions at the LHC with the

ATLAS Detector

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?

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 ?

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

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

??

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

Pb+Pb Jet Rates (2003)

Jet Modifications: Vitev

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

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

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

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??

Slide from QM 2005 Talk by K. Itakura

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, …

ATLAS Cavern as of Today

ATLAS Calorimetery

EM Long. Segmentation

Hadronic Barrel

Hadronic EndCap

EM EndCap

EM Barrel

Forward

ATLAS EM Calorimeter Close up

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

Installation: LAr Barrel

•In final position and aligned•Electronics installation nearly complete•Cool-down April 2006!

ATLAS (Inner) Tracker

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)

ATLAS Tracker

•Pixel + SCT mated with TRT barrel in clean assembly area.

ATLAS ZDC

Global Event Properties

+ Pb-Pb

Not EM cal first layer!

180 GeV Jet + HIJING Pb+Pb, b=0

60 GeV Jet in Central Pb+Pb Event

Probably a -jet event!

Jet Reconstruction Performance:LOI

ATLAS Tracking Performance

Results ~ 1 ½ years old, new ATLAS algorithm that can be easily optimized for heavy ion collisions now near completion.

b Jets

Essential test of quantitative understanding of quenching

Jet Fragmentation Observables

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

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

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

Upsilon Screening

•Physics in behavior of different states: doable

Atlas J/ Reconstruction

•J/ acceptance limited by momentum required for muons to penetrate to muon spectrometers.

ATLAS: Simulated Hijing p-Pb Event

Jet at forward (actually backward) rapidity

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

Heavy Ion Participants

+ SUNY Stonybrook (chemistry) + BNL Brahms group + others in the process of joining …

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

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

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

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”

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!

Calorimeter Occupany in Pb+Pb Events