Experimental Studies of QCD in p/d/e-A Collisions at RHIC, the LHC, and e-RHIC

Prof. B.A. ColeColumbia University

1. High (moderate) - pT physics in p-p @ RHIC 2. High (moderate) - pT physics in d-A @ RHIC 3. Theoretical analyses of d-A forward suppression 4. Future: p-A @ LHC (with a focus on ATLAS) 5. Further future: e-A, p-A with e-RHIC detector 6. Summary

p-A Physics Goals• Nuclear effects (hard)

– Shadowing / saturation @ low xA.

– Jet structure / mono-jets @ low xA.

– pT broadening / energy loss.– Modifications of baryon production.– Tests of pQCD: factorization / universality.

• Nucleus as a filter (soft)– Diffraction.– Proton break-up, color transparency.– Baryon junction excitation.– Soft phenomenology.

• In this talk: focus on “hard” effects (?)

Why p-A (d-A) Collisions ?• Probe Initial-State Effects at RHIC

– Shadowing of Nuclear PDF’s– Parton saturation– Cronin effect

p broadening of hard processes

– It’s becoming clear that these are all due to or reflect the same underlying physics

• Unique feature of RHIC measurements– Ability to constrain “centrality” – i.e. impact

parameter range of d-A collisions.

peripheral

central

Hard Scattering in p-p Collisions

–Factorization: separation of into•Short-distance physics: •Long-distance physics: ’s

p-p di-jet Event

STARSTAR

a/A

b/B

A

B

ab̂

From Collins, Soper, Sterman Phys. Lett. B438:184-192, 1998

Single High-pt Hadron Production

–NLO calculation agrees well with PHENIX 0 spectrum (!?)• BUT, FF dependence ?• Lore: KKP better for gluons • Includes soft-gluon resummation!

KKP

Kretzer

data vs pQCDdt

d

z

QzD

QxQxdxdxdp

dE

c

abcaBbaAaba

ˆ),,(

),,(),,(

2

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2/

2/3

3

0

Phys. Rev. Lett. 91, 241803 (2003)a/A

b/B

A

B

ab̂

D(z)

But QCD is not Nearly So Simple …

• Initial and final state radiation leads to QCD evolution– Parton distributions– Fragmentation func’s

• Well-controlled (infrared safe) evolution depends on cancellation of real and virtual radiation.

• Why does this matter?– Radiation broadening of transverse momenta– Phase space restrictions inhibit the real/virtual cancellation.

• High pT hadron production at large xT (low s).• Heavy quark production at low transverse momenta.

• “Re-summation” of large logarithms needed.

Application of pQCD vs s

– How well does NLO pQCD work as we go down in energy from RHIC ?

– Clearly describes data more poorly for decreasing s.

• And for more forward production.

– Also, sensitivity to factorization scale also grows.

Soffer and Bourrely, Eur. Phys. J. C36:371-374,2004

Threshold re-summation• Threshold & soft

gluon resummation (NLL) improves agreement with data at lower s.

• Much smaller effect for RHIC at mid-rapidity.– But still a factor of

~2!

– pT dependence ??

• What about at forward rapidity??

Forward Production at RHIC

• NLO pQCD works at RHIC @ large xF

• But ~40% scale error (=pT vs =pT/2)

• Re-summed NLO (Vogelsang) also agrees with data.

• But, scale error in non-resummed NLO:– Strong sensitivity to

“nuclear” effects???

Soffer and Bourrely, Eur. Phys. J. C36:371-374,2004

PHENIX: 200 GeV p-p Prompt • Background removed

via combination of:– (Jet) isolation cuts 0 decay tag– Statistical subtraction

• Spectrum and yield well-described by NLO pQCD (w/ threshold & recoil resummation).

• ~ 15% scale error above 5 GeV/c.

• More work needed to go below 5 GeV/c.

A-A Hard Scattering Rates–Parton flux density “thickness”

•

–For point-like interactions:

• dNhard / dA product of nuclear T’s

• Integrate over transverse area

• Then

• Nbinary (also known as Ncoll) is fiction

– no successive nucleon-nucleon scattering !

– Just a convenience (pure number not fm-2)

),()(

rzdzrT nucleon

A

rT

b

|)(||)(|)( rbTrTrdbT BAAB

binary

NNhardNN

inelAB

NNhard

AB

NNhard

ABhard N

dp

dnbT

dp

dnbT

dp

d

dp

dn2222

)()(

Coherence in p/d-A @ RHIC• View in nucleus rest frame

– For mid-rapidity jet with MT

• Relative to nucleus, y=5.4

• E pL = MT cosh(y) 100 MT

• Lorentz boost: = cosh(y) 100

– Also, Jet formation time: ~1/ mT

– Giving (jet) formation length (LF )

• LF = 20 GeV fm / mT

• From this simple analysis we can conclude:– All for the “action” for mid-rapidity particle production (and

forward) occurs along the straight path of the incoming nucleon.

– Even high-pT and heavy quark production processes may be affected by coherence in the multiple scattering process.

• New at RHIC:– Ability to select on “centrality” (poor man’s impact parameter)

text

d-Au “Centrality” • # soft scatters of n/p:

–

• Parameterize multiplicity at large vs n, p.

– Cut data according to fraction of total dA.

– For each, determine TdAu

– e.g for PHENIX (in %)• 0-20, 20-40, 40-60, 60-88

• Define:

NNpnAupn bT )( //

22

22

/

/1

dp

dn

dp

dn

T

TR

dp

d

dp

dn

TR

periphhard

centhard

centdA

periphdA

cp

NNhard

dAhard

dAdA

STAR d-Au @ High-pT

– Beware: • Top plot is RdA

• Bottom plot is Rcp

– Strong enhancement in charged hadron production at =0.

– Enhancement larger for baryons than for mesons.• Ks similar to

similar to

PHENIX: d-Au Neutral Mesons

• Now evaluate consistency with pQCD:– TAB scaling (factorization)

0 production vs centrality production vs centrality

PHENIX d-Au 0 vs Centrality

–Small Cronin effect (not expected to be large)–It is now known that preliminary data suffer from small trigger bias (central will go peripheral ).

PHENIX d-Au Production

–PHENIX sees small Cronin effect• Approx. consistent within errors with STAR Ks result • Enhancement seen in charged (baryons) all the more striking!

PHOBOS: d-Au h RdA

• Clearly the “enhancement” of charged hadron production in d-Au depends on rapidity ().

• dependence suggests suppression for >1

nucl-ex/0406017, PRC in press nucl-ex/0406017, PRC in press

PHENIX d-Au Forward/Backward h

– PHENIX observes similar trend in hadron spectra• Suppression relative to “expected” TAB scaling

• Suppression greater for more central collisions• Suppression NOT confined to large only!

BRAHMS: d-Au RdA or Rcp vs

–BRAHMS also sees suppression of (h-) yields at larger (beware “isospin” effect for =2.2, =3.2)

–Suppression increases for more central collisions.

BRAHMS – A closer look– Rcp shows suppression

increases with TAu

• Clearer than RdA (pp data?)

– Suppression smooth in – But see h+/h- difference !

• Reflects Z=+1 of d ??

– Rcp with (h++h-)/2 still shows suppression.

Rcp

=3

Forward Suppression (CGC ??)• Kharzeev, Kovchegov, Tuchin (Phys.Lett.B599:23-31,2004)

• Evolution from enhancement (Cronin effect) at mid-rapidity to suppression at forward rapidity.

• h- RdA modified by charge bias in p-p coll’s.

• Rcp less sensitive.

Model Comparisons (I)• Vitev(nucl-th/0302002)

– pQCD w/ shadowing

– Include self-consistent p broadening, dE/dx

• Both elastic & radiative

correct enhancement at mid-rapidity

• But EKS anti-shadowing overestimates RdA

– Predict RdA >1 at y = 3

• dE/dx small effect.

– But significant dE/dx effects at y = -3.

Vitev and Qiu: Higher Twist

• “Higher Twist”: – multiple exchanges

between projectile & target.

• Vitev & Qiu: coherent multiple scattering

• Effective rescaling of x of parton from deuteron.

Model Comparisons (II)• Describe hard scattering

in nuclear rest frame.– “Cronin effect” from multiple

semi-hard scattering

• With unitarity corrections:

• Fit to p-p + Fermilab p-A– Reproduces y=0 0 Rcp

– But not y=3 ***– Even if opacity increased x3

• BRAHMS data changed– But p dependence wrong …

A. Accardi nucl-th/0402101

(Semi) Hard Scattering in d-A @ RHIC

• We don’t have to look very hard to see the effects of coherence.

• Effects near mid- disappear by pT ~ 6 (?)

• @ = 3.2 kinematic limit: pT 8 GeV/c.

• Limited phase space for truly high-pT physics

Brahms

d-A J/ Production (from M. Leitch)

Klein,Vogt, PRL 91:142301,2003 Kopeliovich, NP A696:669,2001

RdA Low x2 ~ 0.003(shadowing region)

(in gold)

•Not universal versus X2 : not shadowing !??

– BUT does scale with xF ! - why?– Initial-state gluon energy loss depends

on x1~xF - weak at RHIC energy?

•But Kopeliovich: – Effect can be due to “energy loss”

compared to lower sE866: PRL 84, 3256 (2000)NA3: ZP C20, 101 (1983)

xF = xd - xAu

•Data favors (weak) shadowing + (weak) absorption ( > 0.92)

•With current statistics hard to separate different nuclear effects

•Will need more d-Au data!

Summary of d-A @ RHIC• Observe clear suppression of forward

hadron production at pT >~ 4 GeV/c.– Continuation of trend over large y range.– Does not fit within pQCD calculations

• Issue: EMC (0.2 < x < 0.9) suppression of gluons typically included in calculations, valid???

• weak Cronin effect at mid-y for mesons.– But, also clearly depends on rapidity.

• Some crucial aspect of physics is missing in “pQCD” calculations.– Kopeliovich: factorization breaking?

• “Sudakov suppression” – but at low/negative xF ?

Centrality in d(p)-A• The ability to select on centrality in d(p)-A

collisions is NEW and very important.• Potentially the first opportunity to measure

the impact parameter dependence of:– Initial-state broadening, Shadowing, …

• Observations of centrality dependence have already been important.

• But, there are some limitations:– Rely on Glauber model to indirectly relate

“centrality” observables to impact parameter.– Kopeliovich: Flaw in Glauber models due to

neglect of diffraction – which I think is a real issue.

– May be important for understanding RCP.

Di-hadron Azimuthal () Correlations

• jT represents hadron pT relative to jet

• kT represents the di-jet momentum imbalance

• “y” implies projection onto transverse plane.

yTj

yTj

Jet

yTk

PHENIX d-Au/p-p, - h, Correlations

–“Trigger” pion pT > 5 GeV/c

–Four different associated hadron pT bins

–Clearly see role of constant jT, contribution from kT

0.4-1 GeV/c1-2 GeV/c

2-3 GeV/c 3-5 GeV/c

PHENIX preliminary

p-pd-Au

PHENIX: Di-jet KT• No jet reconstruction

in PHENIX (yet)• But can measure KT

via two-hadron correlations.

• Additional broadening from fragmentation.– But can be measured

in single jet.

• Then:

• Study vs ph1

• KT Same in p-p, d-Au?– Sensitivity ??– More work needed.

1jetp

2jetp 1hp

2hp

1jetp

2jetp

2hp

1hp

22jetjetdiTT pk

y

Studying Jet Properties @ RHIC

– Use hadron pairs to study jet properties

– pout dist. has both non-pert. (Gaussian) + hard (power) contributions.

pp

Radiative tails

PHENIX, From J. Jia, DNP’04 Talk

PHENIX Preliminary

JetPout

Pout

Jet Properties in d-Au• Compare pout dist’s in

p-p and d-Au.• Evidence for effects of

re-scattering, modified radiation, … ?– Not so far!– But this is just the

beginning!

• Such measurements w/ one jet @ > 2 would be very interesting!!– But not possible yet

Radiative Effects on (di)Jets

– Conclude: large radiative component to di-jet kT

• Also see Vitev, Qiu : Phys.Lett.B570:161-170,2003.

– Without accounting for radiation initial parton intrinsic kT ~ 2 GeV/c (RMS).

– After accounting for radiation ~ 1 GeV/c

Analysis of STAR di-hadron distribution by Boer & Vogelsang,

Phys. Rev. D69 094025, 2004

Hard Scattering – IS/FS Radiation• Radiative contributions from initial & final state• Initial state radiation due to parton shower prior

to the hard scattering

• The development of the initial-state shower must be different in nucleus (?).– “Quantum evolution” an

important part of CGC– Treatment of soft radiation in

co-linear vs kT factorization?

• “Model-independent approach”• Measure di-jet acoplanarity • Better: -jet and - (hard) processes

Direct Photon Production• kT broadening and

evolution of parton distributions will modify production.

• If there are mono-jets, are there mono-photons??

-jet angular correlations more sensitive because less broadening from jet.

• Di- production even more interesting – kinematics completely determined.

• Need good photon/0 separation.

J. Jalilian-Marian, hep-ph/0501222

p-A Collisions @ LHC

• Summary of LHC “Yellow Report” on p-A

Physics Motivation / Goals

•From DOE LHC Heavy Ion Review (2002)

p-A @ LHC

• p-A @ LHC can reach low x at high Q2

• Rates for high-pT processes are enormous

• Concerns– No p-p measurements at same s (?).– Centrality selection will require care.– Little particle (baryon) identification away from mid-rapidity

Parameters from LHC Yellow Report

Rates for pT > 100 GeV/c

2QQ

Low-x Effects @ LHC

•Measurable shadowing even at 100 GeV.

• Modest effects at mid-rapidity (but going away slowly)

Q=100 GeVQ=10 GeVQ=2 GeV

Frankfurt, Strikman: Shadowing Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 94:022002 (2005)

Di-jet / -jet / - Acoplanarity (2)

• d-A measurements @ RHIC limited by– Luminosity and Acceptance

• Both of these limitations are removed in (e.g.) ATLAS @ LHC

• Isolate initial-state radiation effects (modified in p-A) by comparing:– Di-jets, (isolated) -jets, (hard) di-photon

• Prediction from saturation:– “disappearance” of di-jet signal at pT ~ Qs

– But, presumably measurable (calculable?) effects at higher pT?? (precision vs “discovery”)

Example: from CDF

p-A in ATLAS (CMS)

• p-p detectors @ LHC ideal for studying high-pT physics in p-A collisions.

Hadronic CalorimeterElectromagnetic Calorimeter

Inner Detectors Silicon Pixels Silicon Strips Transition Radiation Tracker

SuperconductingSolenoid

Muon chambers

Superconducting Coils for Toroidal Field for Muon System

For CMS:EMCal covers ||<5

Had. Cal: ||<5

TOTEM: ||<7

ATLAS Calorimeter System (1)

Hadronic TileCalorimeters

EM AccordionCalorimeters

Silicon Tracker in Inner Detector

Forw

ard

LAr

Calo

rimete

rsH

adro

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LA

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p-A Collisions: Soft “Background”

• Some numerology:– @ LHC energies, p-Pb collisions ~ 7– Due to coherence (wounded-nucleon scaling) ~ 7 4 times soft multiplicity (on average)

– In p-p @ high-, ~ 25 collisions/bunch crossing– Typical p-Pb collision has 1/6 the soft

background of high- p-p collision.

• Conclusion: for high-pT measurements ATLAS p-Pb performance better than p-p.

• Beware: this argument neglects rapidity dependence of soft p-Pb/p-p.– Observe: best performance in low XA direction.

Simulated (& Recon) Hijing p-Pb Event

Simulated (& Recon) Hijing p-Pb Event #2

•Jet at forward (actually backward) rapidity

Detecting Forward jets (from Takai)

Event Characteristics• Use Hijing to simulate (central) p-Pb

events– Apply ideal ATLAS acceptance cuts to

particles.– Study what ATLAS “sees” in typical events

• e.g. charged multiplicity

Fra

ctio

n o

f eve

nts

Charged part. multiplicity

ATLAS does not measure a large fraction of charged particles

1) coverage

2) Magnetic bend (minimum pT ~ 0.5)

Pseudo-rapidity ()

dN

chg/d

ALL

ATLAS

Event Characteristics (2)• Instead, look at ET (electromagnetic)

Electromagnetic ET (GeV)

Fra

ctio

n o

f Eve

nts

Need serious analysis of effects of noise on ET measurement

pseudo-rapidity ()

dE

T/d

(Ge

V)

ALL

ATLAS

Centrality Measurement• The ability to measure centrality has

been an important feature of RHIC d-Au.

From Mark Baker (BNL), Talk @ RHIC pA/eA workshop

Centrality Measurement (2)• Problem w/ centrality measurement:

– Measurements at mid-rapidity are biased• By hard processes• By the very low-x physics we want to study

– At RHIC, measurements @ || > 3 are “safe”• Hard processes suppressed by phase space.

– How far out in is “safe” at LHC (6, 7, 9?)

• Zero-degree calorimeter(s) are useful– But evaporation neutron yield saturates.– Can distinguish peripheral from central but

…

• p-A Centrality determination @ LHC needs careful study by all experiments.

p-A in ATLAS: Studies Needed• Basically everything ! But specifically:

– Real simulations of mult. and ET measurement

– Centrality determination.

– Forward jet measurement @ moderate pT

– Measurement of < 20 GeV jets at mid-rapidity. isolation efficiency and rejection vs pT

– Analysis of -jet kinematic (x1, x2) reconstruction

– Sensitivity to changes in di-jet/ -jet/ … acoplanarity.

– Double b-tag efficiency, rate (moderate pT).

– Jet overlap, double parton scattering events.– …

p-A @ LHC: Plans• p-A is considered an “upgrade” @ LHC

– Straight-forward but needs second timing system.

– Cabling will be in place but the $ are not yet committed (but small change: few 100k$)

• Meeting on p-A @ LHC May 25-28.– We will know much more then.

• “Guaranteed” that there will be p-A @ the LHC but when?– Presumably all three experiments will run.

• LHC p-A complementary to RHIC p-A and e-RHIC.

e-A in Target Rest Frame• q q-bar pair + …

(evolution) interacts with target

• In many ways similar to p-A collision.

• But:– Transverse size

controlled by Q2

– Kinematics much better determined

• For moderate Q2 get multiple scattering– Shadowing– “Centrality” !!?

• Connection between structure function & unintegrated PDFs

• But only at leading twist!• Can we directly measure

violation using p-A/e-A ?

dxdk

NdQxxG

Qd

d

T2

22

2),(

log

p-A: What is Unique to RHIC• At LHC we will not be able to measure

into the fragmentation region.• At RHIC, we could in principle cover a

large part of the fragmentation region • Simultaneously measure

– Proton break-up– Hard/semi-hard processes with good

efficiency for capturing di-jet, -jet, -– Nuclear break-up

• In a detector that would look much like an e-A detector w/ similar requirements (?)

Semi-inclusive DIS• HERMES has very

interesting results on modifications of quark fragmentation in nuclei.

• In target rest-frame:– Nucleus as filter of

different dipole+… configurations

Nuclear Modification Factor RAA

evt 2AB AB T

AB 2AB pp T

1 N d N /dydpR

T d /dydp

strongsuppression

Phys. Rev. Lett. 91, 072301 (2003)

integrated 0 yield above pT = 4 GeV/c