QCD @ The Next Frontier

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

Detector

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 that’s how we will really learn something new about QCD

•How? One way:– Study experimental observables

under controlled variations of initial conditions

Why the LHC? (2)•CGC initial conditions

– Unique QCD physics

– Crucial for controlled QGP initial conditions

•QGP @ (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

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

Strongly Interacting “Matter” @ RHIC

•“Pressure” converts spatial anisotropy to momentum anisotropy.

•Requires early thermalization.•Unique to heavy ion collisions•Answer: yes

dN

/d

x

yz

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

•We have beautiful data on jet quenching @ RHIC•But severe theoretical disagreement on interpretationBut 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 GLV+Huovinen!

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

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

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.

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??

Gluon Saturation @ LHC•Gluon saturation already plays a role @ RHIC

•Expected to completely determine A+A initial conditions @ LHC

Will be studied inWill be studied in p-A collisionsp-A collisions

•p-A @ 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

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 @ BNL, Columbia, …

Why ATLAS? Calorimetery!

Low Energy Jet in Central Pb+Pb Event

Very likely a -jet event

From Ketevi’s 2003 studies

High Energy Jet in Central Pb+Pb Event

2 Pythia jets plus a Hijing Jet

Jet splitting (sub-jets) typical of high Q2 processes

Copious hard gluon radiation.

Jet Reconstruction Performance:LOI

Past Studies of HI Jet Analysis (2)

Yes, but WHY ATLAS????•Answer: EMEM calorimeter segmentationcalorimeter segmentation

– In particular, longitudial segmentationlongitudial segmentation

•No other experiment @ 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 1layer 2 likelihood analysis likely to be better at handling background than any algorithm studied so far.

–Plus we have the pre-sampler

+ Pb-Pb

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)

Yes, but WHY ATLAS???? (3)•We don’t really know how jets will be modified @ 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 don’t 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 ATLAS EM-calorimeter long. segmentation is the most potent tool available in any of 3 most potent tool available in any of 3 experimentsexperiments

ATLAS EM Calorimeter Structure

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

ATLAS: Simulated Hijing p-Pb Event

Jet at forward (actually backward) rapidity

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 can’t 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

Why Should BNL Participate? (2)Physics Argument•In spite of RHIC successes, we’re still missing firm conclusions on important physics issues – Why is the QGP strongly coupled?– How opaque (to jets) is the matter created @ RHIC?– Why is J/Psi suppression so small?– Is forward d+Au 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

CMS EM Calorimeter Segmentation

•CMS has marginally better transverse segmentation than ATLAS (0.0175 vs 0.025) for ||<1.5

•But ATLAS much better for 1.5 < || < 2.5 (0.025 vs 0.05)

•More important: CMS has not longitudinal segmentation.

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

ATLAS

“Jet Quenching” @ 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

@RHIC measure using:(Leading) high-p hadronsDi-jet correlations

STAR: Jet “Re-emergence” @ 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 doesn’t go away.

(di)Jet Angular Correlations (PHENIX)

•PHENIX (nucl-ex/0507004): moderate pT

“Perfect Fluid?”

My view: Perfect fluid is reasonable interpretation of available data but there is room for skepticism.

Why Heavy Ions @ 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”!

Gluon “Saturation” @ low x•@ LHC, nuclei are Lorentz contracted by > 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 Qs>>QCD, perturbative calculations possible.

Large occupation #s for kT<Qs

Classical gluon fields

– Related to low-x @ HERA but QsA1/3

Qs ~ 4 GeV/c for Pb at LHC**

Simulated Pb+Pb Event in ATLAS (No Jet)

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 ideal opportunity to start new research effortsnew 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

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