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 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 dN g /dy?

LHC Physics Program: Key Questions(2) Quarkonium screening –Will primordial J/  be screened by LHC?  Will we know? –Will  states be screened by LHC? –Will ,  ’, , … production/suppression provide quantitative insight on medium properties? EM Probes & “exotica” –Will we observe  symmetry 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/fm 3 ?

Multiplicity, E T Multiplicity data provided first evidence for RHIC Measurements of dN/d , dE T /d , … will provide crucial test of saturation –and/or our understanding of particle multiplicities dN/d  also relevant to jet RHIC –Expect to be LHC ZDC will be important in centrality determination  N part and N coll

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

Jets & Jet Quenching (2) Why such uncertainty? –Fluctuations in energy loss –Fluctuations in fragmentation –“Trigger bias” effect –Statistics/limited p T reach –No direct measure of (e.g.) –No photon-jet data yet Full jet LHC solves these problems! –Directly measure the modified frag. func. from energy loss –No trigger bias effect * –Statistics a non-issue –k T dist. directly sensitive to –Photon-jet much easier  Rate  Acceptance Hadrons, not jets but close enough

Pb+Pb Jet Rates (2003)

Jet Modifications: Vitev

Jet LHC (SW) Modification of radiated gluon k T distribution – Crucial point of the figure is that the large k T 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. Note that in Nucl. Phys. A747: 51, SW estimate > 100 GeV 2 based on RHIC data

LHC: Hard Radiation For high energy jets, copious production of hard radiation in final-state parton shower.  Complicated structure of observed jets  Large k T 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 k T 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 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 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 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: x –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, 1 st EM layer –E 01 = energy in pre-sampler, E tot = total EM energy  60% of background stopped before/in 1 st 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 2 nd + 3 rd layers –Very preliminary: ~ 50% improvement in (jet + BG) / BG  =0.1 x 0.1 cells

Longitudinal Segmentation: Going Further 1 st EM layer consists of  = x  =0.1 strips Expect ~ 100 MeV/strip soft BG in central Pb+Pb  Very low BG/strip even in heavy ion collisions  Occupancy in jet determine by jet structure not BG. Photons deposit ~ 50% of their energy in 1 st layer  Can easily detect individual photons even in Pb+Pb  Can 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 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 Program

ATLAS QGP Physics Program

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 E t  R 2  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 –d 2 Et/d  d  –Hadron j T distribution –Fragmentation function Look for other “structure”

Jet Structure Sub-jet measurements will be critical for HI physics –Energy scale for initial gluon LHC ~ 4 GeV  Proper time  ~ 0.05 fm for “medium” to be present –Initial parton splittings occur at  ~ 1/  Q 2  Hard (k T > 4 GeV/c) radiation independent parent parton. “Holy Grail” of quenching studies –Direct measurement of gluon radiation spectrum (E, k T ) 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