Experimental Review of Hard Processes (mostly at RHIC) W.A. Zajc Columbia University

09-Aug-04

Experimental Review of Hard Processes

(mostly at RHIC)

W.A. ZajcColumbia University

09-Aug-04

SummarySummary Hard processes:

One in which there exists some scale >> QCD Examples:

Large momentum transfer (jets) Heavy flavor production Direct photons

Preview as summary: RHIC is an ideal machine for using hard processes

to probe Deep interior of heavy ion collisions (quark-gluon

plasma?) Gluon, sea-quark contributions to proton spin (G. Bunce)

Measurements to date: Large momentum transfer (jets): calibrated,

basis for discovery Heavy flavor production: first measurements Direct photons: approaching first measurement

09-Aug-04

RHIC SpecificationsRHIC Specifications 3.83 km

circumference Two independent

rings 120 bunches/ring 106 ns crossing time

Capable of colliding ~any nuclear species on ~any other species

Energy:

500 GeV for p-p 200 GeV for Au-Au

(per N-N collision) Luminosity

Au-Au: 2 x 1026 cm-2 s-1

p-p : 2 x 1032 cm-2 s-1 (polarized)

09-Aug-04

RHIC’s ExperimentsRHIC’s Experiments

STARSTAR

09-Aug-04

RHIC Achievements to RHIC Achievements to DateDate

Machine : Runs 1-4:

Au+Au: operation at 4 energies (19, 62, 130, 200 GeV) d+Au comparison run (200 GeV) p+p baseline (200 GeV)

Routine operation in excess of twice design luminosity ! First polarized hadron collider !

Experimental Operations: Routine collection, analysis of 100 Tb datasets >50 publications in Physical Review Letters Excellent control of systematics and inter-experiment

comparisons Experimental Results:

Record densities created ~100 times normal nuclear density

New phenomena clearly observed (“jet” quenching) Strong suggestions of a new state of matter

09-Aug-04

Run Year Species s1/2 [GeV ] Ldt Ntot p-p Equivalent Data Size

01 2000 Au+Au 130 1 b-1 10M 0.04 pb-1 3 TB

02 2001/2002 Au+Au 200 24 b-1 170M 1.0 pb-1 10 TB

p+p 200 0.15 pb-1 3.7G 0.15 pb-1 20 TB

03 2002/2003 d+Au 200 2.74 nb-1 5.5G 1.1 pb-1 46 TB

p+p 200 0.35 pb-1 6.6G 0.35 pb-1 35 TB

04 2003/2004 Au+Au 200 241 b-1 1.5G 10.0 pb-1 270 TB Au+Au 62 9 b-1 58M 0.36 pb-1 10 TB

Run-1 to Run-4 Capsule History

RHIC Successes (to date) based on ability to deliver physics at ~all scales:

barn : Multiplicity (Entropy)

millibarn: Flavor yields (temperature)

microbarn: Charm (transport)

nanobarn: Jets (density)

picobarn: J/Psi (deconfinement ?)

}} This TalkThis Talk

09-Aug-04

Why RHIC ?Why RHIC ? Different from p-p, e-p colliders

Atomic number A introduces new scale Q2 ~ A1/3 Q02

Different from previous (fixed target) heavy ion facilities ECM increased by order-of-magnitude Accessible x (parton momentum fraction)

decreases by ~ same factor Access to perturbative phenomena

Jets Non-linear dE/dx

Its detectors are comprehensive ~All final state species measured with a suite of detectors

that nonetheless have significant overlap for comparisons It’s a dedicated facility

Able to Perform required baseline and control measurements Respond rapidly to new opportunities (e.g., 62 GeV Run)

s

p 2~x T

, , , , , K, K, , KK*0*0(892), K(892), Kss

00, , , ,

p, d, p, d, 00, , , , ,,, , *(1385), *(1385), *(1520),

±,,DD00, ,

DD±, J/, J/’s, ’s,

(+ anti-particles)(+ anti-particles) …

09-Aug-04

Access to Perturbative Access to Perturbative Phenomena?Phenomena?

Consider measurement of 0’s in p+p collisions at RHIC.

Compare to pQCD calculation

Phys. Rev. Lett. 91, 241803 (2003)

),(

)(

),(),(

2

22

/

//

hch

bBbaAa

zD

dcbad

xfxfd

•parton distribution functions, for partons a and b•measured in DIS, universality

•perturbative cross-section (NLO)•requires hard scale•factorization between pdf and cross section

•fragmentation function•measured in e+e-

09-Aug-04

RHIC Energy Reduces Scale DependenceRHIC Energy Reduces Scale Dependence The high √s of RHIC

makes contact with rigorous pQCD calculations minimizes “scale dependence”

A huge advantage in Spin program Providing calibrated probes in A+A

PHENIX p+p 0 + X-NLO pQCD F. Aversa et al. Nucl. Phys. B327, 105 (1989)

-CTEQ5M pdf/PKK frag

-Scales =pT/2, pT, 2pT

=pT/2

=2pT

09-Aug-04

Transverse DynamicsTransverse Dynamics The ability to

access “jet” physics also clearly anticipated in RHIC design manual (vintage: ISAJET) a new perturbative

probe of the colliding matter

Most studies to date have focused on single-particle“high pT” spectra Please keep in

mind:“High pT” is lower

than you think

09-Aug-04

Tremendous interest in hard scattering (and subsequent energy loss in QGP) at RHIC Production rate calculable in

pQCD a superb probe of density

But strong reduction predicted due to dE/dx ~ path-length (due to non-Abelian nature of medium)

However: “Traditional” jet methodology

very difficult at RHIC Dominated by the soft

background Investigate by (systematics

of) high-pT single particles

‘‘Jets’ at RHICJets’ at RHIC

RJet

Axis

09-Aug-04

Focus on some slice of collision: Assume 3 nucleons struck

in A, and 5 in B Do we weight this

contribution as Npart ( = 3 + 5) ? Ncoll ( = 3 x 5 ) ?

Answer is a function of pT

: Low pT large cross

sections yield ~Npart Soft, non-perturbative,

“wounded nucleons”, ... High pT small cross

sections yield ~Ncoll Hard, perturbative,

“binary scaling”, point-like, A*B, ...

Predicting pPredicting pTT Distributions at Distributions at RHICRHIC

09-Aug-04

Binary Collisions

Participants

b (fm)

Systematizing our KnowledgeSystematizing our Knowledge All four RHIC experiments

have carefully developed techniques for determining the number of participating

nucleons NPART in each collision(and thus the impact parameter)

The number of binary nucleon-nucleon collisions NCOLL as a function of impact parameter

This effort has been essential in making the QCD connection Soft physics ~ NPART

Hard physics ~ NCOLL

Often express impact parameter b in terms of “centrality”, e.g., 10-20% most central collisions

Participants

Spectators

Spectators

09-Aug-04

LuminosityLuminosity

Consider collision of ‘A’ ions per bunchwith ‘B’ ions per bunch:

Luminosity

A

A

B

B

Cross-sectional area ‘S’

S

BAL

~

09-Aug-04

Change scale by ~ 10Change scale by ~ 1099

Consider collision of ‘A’ nucleons per nucleuswith ‘B’ nucleons per nucleus:

‘Luminosity’

A

A

B

B

Cross-sectional area ‘S’

BANS

BAL Coll

~~

Provided:

No shadowing

Small cross-sections

BANnot Part

09-Aug-04

An example of Ncoll ~ A*B An example of Ncoll ~ A*B scalingscaling

Small cross section processes scale as though scattering occurs incoherently off nucleons in nucleus

scale as A1.0 in +A

scale as Ncoll ~A*B in A+B

7.2 GeV muons on various 7.2 GeV muons on various targets. targets.

M. May M. May et alet al., Phys. Rev. Lett. ., Phys. Rev. Lett. 3535, 407, , 407, (1975)(1975)

09-Aug-04

NNcollcoll Scaling in d+Au Scaling in d+Au

single electrons from non-photonic sources agree well with pp fit and binary scaling

PHENIX PRELIMINARY

1/T

ABE

dN/d

p3 [m

b G

eV-2]

PHENIX PRELIMINARYPHENIX PRELIMINARY

PHENIX PRELIMINARYPHENIX PRELIMINARY

1/T A

B1/

T AB

1/T A

B1/

T AB

1/T

ABE

dN/d

p3 [m

b G

eV-2]

1/T

ABE

dN/d

p3 [m

b G

eV-2]

1/T

ABE

dN/d

p3 [m

b G

eV-2]

1/T

ABE

dN/d

p3 [m

b G

eV-2]

09-Aug-04

1/

T AA

1/T A

A

1/T A

A

NNcollcoll Scaling in Au+Au Scaling in Au+Au

Again, good agreement of electrons from charm with Ncoll

1/T A

A

1/T A

A1

/TA

BE

dN/d

p3 [m

b G

eV-2]

1/T

ABE

dN/d

p3 [m

b G

eV-2]

1/T

ABE

dN/d

p3 [m

b G

eV-2]

1/T

ABE

dN/d

p3 [m

b G

eV-2]

1/T

ABE

dN/d

p3 [m

b G

eV-2]

1/T

ABE

dN/d

p3 [m

b G

eV-2]

09-Aug-04

NNcollcoll Scaling for Charm Scaling for Charm

0.906 < < 1.042

dN/dy = A (Ncoll)

binary collision scaling of pp result works VERY WELL for non-photonic electrons in d+Au, Au+Au open charm is a good CONTROL, similar to direct photons

09-Aug-04

NNcollcoll Scaling for Direct Scaling for Direct PhotonsPhotons

Ncoll scaling works to describe the direct photon yield in Au+Au, starting from NLO description of measured p+p yields

N.B. This method of analysis (double ratio of /0) shows Ncoll scaling after accounting for observed suppression of 0 yields in Au+Au collisions (to be discussed next)

PHENIX Preliminary

Vogelsang NLO

09-Aug-04

Another Example of NAnother Example of Ncollcoll Scaling Scaling

PHENIX (Run-2) data on 0 production in peripheral collisions:

Excellent agreement between PHENIX measured 0’s in p+p

and

PHENIX measured 0’s in Au-Au peripheralcollisions scaled by the number of collisions

over ~ 5 decades PHENIX Preliminary

09-Aug-04

Central Collisions Are Central Collisions Are Profoundly Profoundly DifferentDifferent

Q: Do all processes that should scale like A*B do just that?

A: No! Central collisions

are different .(Huge deficit at high pT)

This is a clear discoveryof new behavior at RHIC

Suppression of low-x gluons in the initial state?

Energy loss in a new state of matter? PHENIX Preliminary

09-Aug-04

no effect

Systematizing Our Systematizing Our ExpectationsExpectations

Describe in terms of scaled ratio RAA

= 1 for “baseline expectations”> 1 “Cronin” enhancements (as in proton-nucleus)< 1 (at high pT) “anomalous” suppression

Events pp in YieldBAEvents Au Auin Yield

09-Aug-04

Ratio: Au+Au / ( p+p Ratio: Au+Au / ( p+p Expectation )Expectation )

09-Aug-04

Is The Suppression Always Seen at Is The Suppression Always Seen at RHIC?RHIC?

NO! Run-3: a crucial control measurement via d-Au

collisions

d+Au results from

presented at a press conference at BNL on June, 18th, 2003

09-Aug-04

Is The Suppression Unique to Is The Suppression Unique to RHIC?RHIC?

Yes- all previous nucleus-nucleus measurements see enhancement, not suppression.

Effect at RHIC is

qualitatively new physics made accessible by RHIC’s ability to produce (copious) perturbative

probes (New states of matter?)

Run-2 results show that this effect persists (increases) to the highest available transverse momenta

Describe in terms of scaled ratio RAA

= 1 for “baseline expectations”

Events ppin YieldBA

EventsAu Auin Yield

Demonstrates importance of Demonstrates importance of

in situin situ measurement of measurement of requisite baseline physics!requisite baseline physics!

ISR 31 GeV

RHIC 200 GeV

SPS 17 GeV

09-Aug-04

First ConclusionFirst Conclusion The combined data from Runs 1-3 at

RHIC on p+p, Au+Au and d+Au collisions establish that a new effect (a new state of matter?) is produced in central Au-Au collisions

Au + Au Experiment d + Au Control Experiment

Preliminary DataFinal Data

09-Aug-04

c

ccch

c

c

bbBaaA

ba

bBbaAa

baabcd

baT

hAB

z

QzD

z

zPd

cdabtd

d

QxSQxS

gg

QxfQxf

dddxdxKpdyd

dN

),(

)(

)(ˆ

),(),(

)()(

),(),(

2*0/

1

0

*

22

2/

2/

222

kk

kk

Partonic Energy Loss

Parton Distribution Functions

Fragmentation Function

Hard-scattering cross-section

Intrinsic kT , Cronin EffectShadowing, EMC

Effect

High pHigh pTT Particle Production in Particle Production in A+AA+A

c

d

ab

(Slide courtesy of K. Filimonov)

09-Aug-04

09-Aug-04

Energy Loss of Fast Energy Loss of Fast PartonsPartons

Many approaches 1983: Bjorken

1991: Thoma and Gyulassy (1991)

1993: Brodsky and Hoyer (1993)

1997: BDMPS- depends on path length(!)

1998: BDMS

Numerical values range from ~ 0.1 GeV / fm (Bj, elastic scattering of partons) ~several GeV / fm (BDMPS, non-linear interactions of

gluons)

222

2

2 4ln~

4ln

4

303

M

ETT

M

ET

dx

dESS

D

SF

ETC

dx

dE

ln3

4 22

2

2Tk

dx

dE

gg

DRS

LL

C

dx

dE

ln

8

2

24

2TC

s

kN

dx

dE

09-Aug-04

Q When can a gluon be considered radiated?A When it’s about a wavelength away from source

(LPM = Landau-Pomeranchuk-Migdal) Coherent (reduced!) emission when

second scatter occurs “before” gluon is radiated

Gyulassy-Wang obtained for a parton crossing a series of static scattering centers

But! Baier-Dokshitzer-Mueller-Peigne-Schiff (BDMPS) noted: QCD non-Abelian Gluon FSI LESS destructive interference Non-linear energy loss

QCD Analog of the LPM QCD Analog of the LPM EffectEffect

dkk

lTT

coh

1

Tk

d

E0 E

0E

EE0

dC

ECq

C

dz

dE

R

As

ln22

22

~ LEd

Ldz

dE

(Slide courtesy of M. Chiu)

09-Aug-04

Exceedingly High Exceedingly High Densities?Densities?

Both Au+Au suppression (I. Vitev and M. Gyulassy,

hep-ph/0208108) d+Au enhancement (I. Vitev, nucl-th/0302002 )

understood in an approach that combines multiple scattering with absorption in a dense partonic medium

Our high pT probeshave been calibrated

dNg/dy ~ 1100

> 100 0 (!)

Au+Au

d+Au

25%25%

(50% ?)(50% ?)

09-Aug-04

Current Status (Runs 1-Current Status (Runs 1-3)3)

time

p+p

d+Au

Au+Au

Established:p+p:

Quantitative description of perturbative probes

d+Au: Role of initial state effects multiple scattering

Au+Au: (Strong) role of final state

effects Quantitative measure of

parton energy loss in a dense (expanding) medium

To do: Develop same quantitative understanding ofAngular correlations

of jet partnersFlavor composition of

“jets”

09-Aug-04

Further EvidenceFurther Evidence

C2(Au Au)C2(p p) A* (1 2v22 cos(2))

STAR azimuthal correlation function shows ~ complete absence of “away-side” jet

Surface emission only (?) That is, “partner” in hard

scatter is absorbed in the dense medium

GONE

GONE

Pedestal&flow subtracted

09-Aug-04

Path-Length (L) Effects? Path-Length (L) Effects?

pTtrigger=4-6 GeV/c, 2<pT

associated<pTtrigger, ||<1

y

x

in-plane

Out-of-plane

Effect of path length on suppression is experimentally accessible

-/4

/43/4

-3/4

Back-to-back suppression out-of-plane stronger than in-plane

09-Aug-04

Baryons Are DifferentBaryons Are Different Results from

PHENIX (protons and anti-protons) STAR (lambda’s and lambda-bars)

indicate little or no suppression of baryons in the range ~2 < pT < ~5 GeV/c

One explanation: quark recombination (next slide)

09-Aug-04

RecombinationRecombination The (normal) in vacuo

fragmentation of a high momentum quark to produce hadrons competes with the (new)in medium recombination of lower momentum quarks to produce hadrons

Example: Fragmentation: Dq→h(z)

produces a 6 GeV/c from a 10 GeV/c quark

Recombination: produces a 6 GeV/c

from two 3 GeV/c quarks produces a 6 GeV/c proton

from three 2 GeV/c quarks

Fries, et al, nucl-th/0301087

Greco, Ko, Levai, nucl-th/0301093

Lepez, Parikh, Siemens, PRL 53 (1984) 1216

...requires the assumption of a thermalized parton phase... (which) may be appropriately called a quark-gluon plasma

Fries et al., nucl-th/0301087

09-Aug-04

Recombination ExtendedRecombination ExtendedThe complicated observed flow pattern in v2(pT)

for hadrons d2n/dpTd ~ 1 + 2 v2(pT) cos (2 )

is predicted to be simple at the quark level under pT → pT / n , v2 → v2 / n , n = (2, 3) for (meson, baryon)

if the flow pattern is established at the quark level

Compilation courtesy of H. Huang

09-Aug-04

FurtherFurther Extending Extending RecombinationRecombination

New PHENIX Run-2 result on v2 of 0’s: New STAR Run-2 result on v2 for ’s: ALL hadrons measured to date

obey quark recombination systematics(!)PHENIX Preliminary

0

STAR Preliminary

09-Aug-04

Recombination Recombination ChallengedChallenged

Successes: Accounts for pT

dependence of baryon/meson yields

Unifies description of v2(pT) for baryons and mesons

Challenged by “Associated

emission” at high pT

Can the simple appeal of Thermal-Thermal correlations survive extension to Jet-Thermal ?

09-Aug-04

Does charm flow?Does charm flow?PHENIX PRELIMINARY

Is partonic flow realized?

v2 of non-photonic electrons indicates non-zero charm flow in Au+Au collisions

Uncertainties are large

Definite answer: Au+Au Run-04 at RHIC!(on tape)

09-Aug-04

J/J/ Measurements To Measurements To DateDate

p+p results: ~comparable

to other hadron facilities (especially at low pT)

Au+Au results: A limit only To be addressed in Run-4

(on tape) An entire program of

charmonium physics is just getting underway

09-Aug-04

Run-3 J/Run-3 J/’s in d+Au’s in d+Au

2.7 nb-1 d+Au Provides

clear J/ signals With modest

discrimination power to test shadowing models

A clear indication of the much greater x2 range made availableby RHIC

A clear need for 20 nb-1 : shadowing 200 nb-1 : ’, Drell-Yan >200 nb-1 : ’s

Measurements beyond ~20 nb-1 require RHIC II luminosities

SOUTH ARM

NORTH ARM

Vogt, PRL 91:142301,2003

Kopeliovich, NP A696:669,2001

09-Aug-04

80-92% Central AuAu 200 GeV

PHENIX Preliminary PbGl / PbSc Combined

1 + ( pQCD x Ncoll) / phenix backgrd Vogelsang NLO



















10-20% Central 200 GeV AuAu

PHENIX Preliminary PbGl / PbSc

1 + ( pQCD x Ncoll) / phenix backgrd Vogelsang

0-10% Central 200 GeV AuAu



Direct photons: centrality Direct photons: centrality dependencedependence

direct photons are not inhibited by hot/dense medium rather shine through consistent with pQCD

thermal photons: reduction of systematic uncertainties is essential

09-Aug-04

A beautiful example of the convergence between CGC as a description of the initial state Hydrodynamics as a description of the bulk matter evolution Jets as a probe of same

T. Hirano and Y. Nara, nucl-th/0404039:

What measurements can we perform at RHIC to test the assumptions of CGC initial state? (p+A measurements)

CGC + Hydro + JetsCGC + Hydro + Jets

09-Aug-04

A Striking ConnectionA Striking Connection We’ve yet to

understand the discrepancy between lattice results and Stefan-Boltzmann limit:

The success of naïve hydrodynamics requires very low viscosities

Both are predicted from ~gravitational phenomena in N=4 supersymmetric theories:

1.0~s

densityentropy

viscosity

4

34

1

SB

s

09-Aug-04

SummarySummary Evidence for bulk behavior (flow, thermalization): unequivocal Evidence for high densities (high pT suppression): unequivocal

(Control measurement of d+Au essential supporting piece of evidence)

The same initial state conditions time evolution of density

needed to explain hydrodynamic flow are obtained by measurements with perturbative probes

OUR FIRST TENTATIVE STEPS TOWARD “CONCORDANCE” Strong suggestions of recombination at the (bulk!) quark level:

scaling of v2 based on quark content pT dependence of meson/baryon ratios Again, perturbative probes may prove critical test of the model

What remains? (Much) more robust quantitative understanding Quantitative understanding of “failures” (e.g., HBT) Direct evidence for deconfiment leading to COMPLETE CHARACTERIZATION

OF THE NEW STATE OF MATTER FORMED AT RHIC

00 LL

??

Experimental Review of Hard Processes (mostly at RHIC) W.A. Zajc Columbia University

Documents

Transcript of Experimental Review of Hard Processes (mostly at RHIC) W.A. Zajc Columbia University