The “ridge“ in high energy nuclear collisions Jörn Putschke for the STAR Collaboration

The “ridge“ in high energy

nuclear collisions

Jörn Putschke for the STAR Collaboration

Yale University

Weisshorn (4505m), Switzerland

http://www-net.cs.yale.edu/images/yale_logo1.bmp

Jörn Putschke, WWND 2008 2

Outline

Near-side x correlations

Ridge/Jet characteristics in Au+Au

Current scenarios

Summary (and Outlook)

Can we test these scenarios?


Components:

Near-side jet peakcomparable to d+Au

Near-side independent ridge

Away-side (and v2)

3 < pt,trigger < 4 GeV

pt,assoc. > 2 GeV

Au+Au 0-10%STAR preliminary

New phenomenon inAu+Au “The ridge”:

• What is it ? ‘something’ coupling to long flow ? Can this quantify E-loss ?

• How to deal with it?Need to subtract for near-side studies?

associated

trigger

Di-hadron correlations

Ridge observation: near-side component picture

Jörn Putschke, WWND 2008

STAR preliminary

4

Near-side (ridge) shape in central Au+Au

3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV

pt,assoc. > 2 GeV

STAR preliminary

yiel

d(

i

n

win

dow

• Ridge approx. independent of and pt,trig

• Jet part increasing with pt,trig

Ridge

Jet

window center


Jet-like peak width in central Au+Au


pt,assoc. > 2 GeV

• Jet peak symmetric in and for pt,trig > 4 GeV and comparable to d+Au

• Jet peak asymmetric in for pt,trig < 4 GeV and significantly broader than d+Au

STAR preliminary


Ridge yield vs. pt,trig in Au+Au

pt,assoc. > 2 GeV

STAR preliminary

Ridge yield persists to highest trigger pt correlated to jet production

Ridge only in Au+Au (not present in p+p or d+Au or peripheral Au+Au)


Ridge/Jet pt,assoc spectrum in central Au+Au

• Jet pt-spectra harder and increasing with pt,trig, as expected from jet fragmentation

• Ridge pt-spectra are ‘bulk-like’ and approx. independent on pt,trig

STAR preliminary

Rid

ge/

Jet

yiel

d

Ridge, 6<pt,trig<10Jet, 6<pt,trig<10

Ridge, 4<pt,trig<6Jet, 4<pt,trig<6

inclusive

STAR preliminary


Subtracting the ridge: vacuum fragmentation (?)

Subtraction of -independent ‘ridge-yield’ recovers centrality-independent jet yield

p+p like (vacuum) fragmentation after energy loss?

zT = pT,assoc/pT,trigzT = pT,assoc/pT,trig

STAR preliminary

STAR preliminary

Jet+Ridge Jet in d+Au“Jet” in Au+Au


“Jet”/Ridge energy

STAR, Phys. Rev. Lett. 95 (2005) 15230

0.15 < pt,assoc < 4 GeV



Applying this “2-component picture” to lower pt,assoc

measurements: zt,jet(Au+Au) ~ zt,jet(d+Au)

subtracting p+p jet energy from Au+Au upper estimate of the energy deposit in the ridge ~ few GeV

“Direct” measure of energy loss ?

“Ridge energy”

“Ridge energy”

}

}


Ridge: B/M ratio closer to bulk

Jet : B/M ratio ~ p+p

Particle composition in Ridge/Jet vs. inclusive


Ridge characteristics

STAR Au+Au 0-10%, RHIC, US (~0m)

preliminary

Weisshorn (4505m), Switzerland

• ridge approx. independent on

• ridge persists up to highest trigger pt

correlated to jet production

• ridge spectrum ~ “bulk-like”

• ridge energy roughly a few GeV

• no significant PID trigger dependence (not shown)

• /K0S ratio in ridge ~ inclusive B/M ratio

• jet di-hadron fragmentation function

after subtracting the ridge contributions

comparable to d+Au

Are we seeing vacuum fragmentation after energy loss on the near-side in central Au+Au collisions with the lost energydeposited in the ridge ?


What is the ridge?

A. Majumder, B. Muller, S. Bass

hep-ph/0611135

Proposed explanations so far:

• Radiated gluons, broadened by– Longitudinal flow, Armesto et al, PRL 93 (2004)

– QCD magnetic fields, Majumder et al, hep-ph/0611035

– Anisotropic plasma, P. Romantschke, PRC,75014901 (2007)

• Medium heating + recombinationChiu & Hwa, PRC72, 034903

• Radial flow + trigger bias S. Voloshin, nucl-th/0312065, Nucl. Phys. A749, 287E. Shuryak, nucl-th/0706.3531

• Momentum-kick modelC.-Y. Wong , hep-ph/0707.2385

Armesto et al, nucl-ex/0405301

nucl-th/0706.3531

E. Shuryak


Discussion

pt,assoc.

ridge

/jet y

ield

h+,-

ridgejet

increasing

pt,trig

• ridge spectrum slightly harder (?)

than inclusive h+,- (tens of MeV)

consistent with medium heating

parton recombination (T~15 MeV) ?

• agreement with radial flow + jet quenching ?

• ridge spectrum qualitatively in agreement with

parton energy loss and coupling to longitudinal flow

Unfortunately all models are in qualitative agreement quantitative calculations needed !


What else can/have we measure(d) to test these scenarios …

• 3-particle near-side correlations test longitudinal flow picture

• Is there a ridge on the away-side ? di-hadron triggered correlations

(3-particle x correlations)

• Geometry effects (pathlength) ridge vs. reaction plane / Cu+Cu

• Energy dependence: 62 vs. 200 GeV similar medium but different

partonic spectrum

• How far does the ridge extend ? forward rapidity measurements

• Full jet reconstruction E-by-E jet-shape modification (radiation spectrum)

More ? Predictions ? Ideas ? …


3-particle near-side correlations

13

12

random

1

3

12

long. flow pictureenhancement

suppression

asymmetryin 13 x 12

Radial flow + trigger bias S. Voloshin, nucl-th/0312065, Nucl. Phys. A749, 287

Toy model

associated

trigger

associated

trigger

Long. flow pictureArmesto et al, PRL 93 (2004)


3-particle near-side correlation measurements

16

3<pTTrig<10 1<pT

Asso<3 ||<0.7

STAR Preliminary STAR Preliminary

d+Au Au+Au 0-12%

Caveats: STAR acceptance limits sensitivity inregions needed to distinguish between radial and long. flow picture. Non-trivial bkg. subtraction in 3-particle correlations …

The ridge appears to be uniform event by event within the STAR acceptance

STAR preliminary


Is there a ridge on the away-side ?

T1: “near-side trigger” > 6 (8) GeV

T2: “away-side trigger” > 4 (6) GeV(and < near-side trigger)within 180 deg. +- 30 deg.

A: assoc. particles > 2 GeV

disappearing bkg.

8<pt,trigger<15 GeV

Use “away-side trigger” for 2-particle correlations

Caveat: probably favor small energy loss of the surviving di-hadron pair


Au+Au

d+Au

-1-2 0 1 2 3 4 5

1

0

1

_d

N_

Ntr

ig d

)

STAR Preliminary

2

3

Di-jets in Au+Au

Surviving (di-jet) pairs at high pt seem to

favor conditions with small energy loss

ridge correlated with energy loss !(?)

Di-jet measurements suggest that neitherthe widths in and (ridge/mach cone) are modified nor the yields are suppressed and comparable to d+Au

Caveat: Non-trivial bkg. subtraction

projection projection

T1A & T2

T2A

T1A & T2

T2A

STAR preliminary STAR preliminary

T1: pT>5GeV/c

T2: pT>4GeV/c

A : pT>1.5GeV/c


Jet/ridge w.r.t. reaction plane

19

in-plane S=0 out-of-plane S=90o

3<pTtrig<4, 1.5<pT

trig<2.0 GeV/cRidge

Jet

In-plane

Out

-of-

plan

e

1

43

2

56

20-60% jet part, near-side

ridge part, near-side

20-60%

• Ridge yield decreases with φS. Smaller ridge yield at larger φS

• Jet yield approx. independent of φS and comparable with d+Au

Jet yield independent of φS, consistent with vacuum

fragmentation after energy loss and lost energy deposited

in ridge, if medium is “black” out-of-plane and more “gray”

in-plane for surviving jets.

STAR preliminary

STAR preliminary


Jet/Ridge in Cu+Cu

20

3.0 GeV/c < pT

trigger 6.0 GeV/c; 1.5 GeV/c < pT

associated < pT

trigger

Ridge

Jet

• Jet yield comparable in Cu+Cu and Au+Au at similar Npart

• Ridge yield comparable in Cu+Cu and Au+Au at similar Npart

(consistent with other jet-quenching variables like RAA)


Jet/Ridge 62 vs. 200 GeV

21

3.0 GeV/c < pT


associated < pT

trigger

Jet

200 GeV

62 GeV

Ridge200 GeV

62 GeV

• Jet yield significantly smaller in 62 GeV vs. 200 GeV

“trivial” kinematic effect due to steeper jet spectrum in 62 GeV (pQCD/Pythia)

• Ridge yield also suppressed in 62 GeV vs. 200 GeV (Ridge/Jet ratio comparable)

Ridge yield correlated with pQCD jet properties (?); radial flow and v2 ≈ in 62 and 200 GeV

(Does the ridge yield scale with RAA ? Less suppression in RAA at similar pt ?)


pTassoc = 0.2-2.0 GeV/c: no near-side peak within systematic errors

pTassoc > 1 GeV/c : non-zero correlation at near side (?)

Is there a ridge at forward rapidity ?

|trig| <1 and 2.7 < |assoc| < 3.9 PHOBOS (QM08) preliminary; central Au+Au

STAR and PHOBOS measurements suggestthe presence of a finite ridge yield at ~ 3 (?)

Caveat: no momentum measurement at high rapidity in PHOBOS, STAR data are not conclusive


Summary

Is the “ridge” due to energy loss ?If so, do we have a direct measurement of energy loss and medium response ?

Many measurements characterizing the ridge properties are already available. Quantitative calculations are needed now to understand the underlying mechanism of the ridge !

Additional measurements possible (on the way) to furtheraddress the origin of the ridge. Extend sensitivity withfull jet-reconstruction.


Outlook: Ridge/Jet v2; different event classes?

characteristics of the events yielding a “ridge pair” appear to be very different from those yielding a “jet pair”

• inferred v2 for events associated with “ridge” pairs is large• inferred v2 for events associated with “jet” pairs is small• this conclusion is a direct consequence of: zero-yield at minimum assumption and the 3-component model: (v2 modulated background + ridge + jet)

“ridge”

“jet”

STAR Preliminary

• the “ridge” is calculated by projecting ||>0.7 correlation to ||<0.7• the “jet” is the remaining correlation at ||<0.7 after subtracting the “ridge”


Heavy ions: large background from underlying event

Control background by limiting jet cone radius R, track pT cut measure a fraction of partonic energy

Cone radius R=sqrt(2+2)

pT

Unbiased jet reconstruction: parton kinematics hadrons

Outlook: Jet reconstruction in heavy ion collisions


Outlook: Full jet reconstruction in HI @ RHIC ?

STAR,hep-ex/0608030

Central Cu+Cu 200 GeV

Jets > 10-15 GeV should be reconstructable in Cu+Cu @ RHIC

Full jet reconstruction in p+p up to 40 GeV Back of the envelope calculation …

(should be sufficient statistic on tape …)

Armesto et al, nucl-ex/0405301


Backup slides


/K0S ratio:

in the ridge: ~ 1.0 similar to that from inclusive pT spectra

in the jet: ~ 0.5 consistent with p+p

|Δη|<2.0 |Δη|<0.7


Particle composition in Ridge/Jet

Outlook: similar measurement will be pursued using dE/dx ID to look at p/π ratio in ridge/jet


Ridge “observation”


Au+Au 0-10% d+Au minbias

Additional near-side long range corrl. in in central Au+Au (“ridge like” corrl.) observed

Dan Magestro, Hard Probes 2004, STAR, nucl-ex/0509030, Phys. Rev. C73 (2006) 064907 and P. Jacobs, nucl-ex/0503022



Ridge phenomenology

STAR preliminaryJet+Ridge ()

Jet ()

Jet )

yiel

d

,)

Npart


pt,assoc. > 2 GeV

STAR preliminary

window center

yiel

d(

i

n

win

dow

Definition of “ridge yield”:

ridge yield := Jet+Ridge( Jet()

• Ridge approx. independent of

• Jet yield in and independent on centrality and comparable to d+Au

• Jet peak symmetric in and for pt,trig > 4 GeV

Ridge

Jet


Extracting the ridge yield

relative ridge yield := ridge yield / Jet()

STAR preliminaryJet+Ridge ()

Jet ()

Jet )

yiel

d

,)

Npart


Jet yield independent of Npart and consistent with d+Au reference measurements !

Definition of “ridge yield”:

ridge yield := Jet+Ridge( Jet()


Fundamental quantity P(E)

~15 GeV

Renk, Eskola, hep-ph/0610059

Can we constrain this by experiment?

Salgado and Wiedemann, Phys. Rev. D68, 014008

Energy loss distribution P(E) depends on

Radiation spectrum Geometry, time evolution of matter

High-pt di-hadron correlations: favor cases with small/no energy loss

Low-pt di-hadron correlations: modification of near-side jet-part and complex away-side structure

Need an unbiased measure of the parton energy full jet reconstruction


Ridge/Jet ratio independent of energy, system

3.0 GeV/c < pT


associated < pT

trigger

Au+Au √sNN=200 GeV from J. Bielcikova (STAR), J.Phys.G34:S929-930,2007Cu+Cu √sNN=200 GeV from C. Nattrass (STAR), SQM2007

Ridge/Jet ratio 62 and 200 GeV


34

STAR Preliminary

3<pTtrig<4GeV/c 4<pT

trig<6GeV/c

Collision geometry? Gluon density?

At φS=0o: Ridge yields are similar in two centralities.

Ridge w.r.t reaction plane cont.


• ridge yield increases with centrality

(ridge for K0S trigger < ridge for Λ trigger ?)

• jet yield is independent of centrality and agrees with d+Au

Ridge JetJ. Bielcikova (STAR), QM’2006

Trigger PID Ridge/Jet


• Trigger PID ridge spectra comparable to h correlations and ~ bulk

• Trigger PID Jet spectra comparable to h correlations and slightly harder ~ 50 MeV

Trigger PID ridge/jet pt,assoc spectrum

J. Bielcikova (STAR), QM’2006


Ridge yield in Au+Au and Cu+Cu

Relative ridge yield comparable at same Npart in Au+Au and Cu+Cu

pt,assoc. > 2 GeV

STAR preliminary

STAR preliminary

relative ridge yield relative ridge yield

rela

tive

ridge

yie

ld

rela

tive

ridge

yie

ld

relative ridge yield := ridge yield / Jet()

Au+Au 200 GeV

Cu+Cu 200 GeV

3<pt,trigger<4 GeV

Au+Au 200 GeV (30-40 %)

Cu+Cu 200 GeV (0-10 %)


(J+R)

||<1.7

J = near-side jet-like corrl.

R = “ridge”-like corrl.

v2 modulated bkg. subtracted

(J+R)

||<1.7

flow (v2)corrected

Extracting near-side “jet-like” yields

1

Au+Au 20-30%

2

2

(J+R)- (R)

con

st b

kg.

sub

tra

cte

d

(J

)

||

<0.

7

(J)

no bkg. subtraction

const bkg. subtracted

(J)

||<0.7


Analysis methods cont.

preliminary

v2 and systematic error estimation in Au+Au:

a) Used v2 values = mean between v2 RP and v2{4}

b) Systematic errors mainly due to uncertainties

in v2; use v2 RP and v2{4} as upper and lower limit

1. Use event-mixing to account for pair acceptance and use eff. correction for ass. particles

2. Background:

a) Subtract constant background for (J)

b) Subtract v2 modulated background for (J+R)(ZYAM method)

3. Assume Gaussian correlation shape:yield() = gaus integral / bin counting () = gaus width

2.5 < pt,trigger < 3 GeV and 0.3 < pt,assoc. < 0.8 GeV

v2{4}<v2{RP}+v2{4}>

v2{RP}

ZYAM norm.


Di-hadron azimuthal correlations

low pt: 4 < pt,trigger < 6 and pt,assoc > 0.15 GeV high pt : 8 < pt,trigger < 15

increa

sing

pt,a

ss

oc

.

Away-side yield is suppressed but finite and measurable at high pt,trigger

and pt,assoc in central Au+Au collisions

Away-side (and near-side) yield is enhanced for low pt,assoc in Au+Au

Recoil distribution soft and broad, angular substructure,

Mach cone? (not topic of this talk)


Di-hadron associated spectra

low pt STAR, Phys. Rev. Lett. 95 (2005) 15230 high ptSTAR, Phys. Rev. Lett. 97 (2006) 162301

Surviving pairs at high pt seems to favor conditions with small energy loss:tangential (halo) emission, finite probability for no energy loss (energy loss fluctuations) or dilution due to the expanding

system (T. Renk, Hard Probes 06 and PQM: Dainese, Loizides and Paic)

limited sensitivity to energy loss in di-hadron fragmentation on the away-side ? study near-side modification

Low pt : Strong modification of near-side and away-side observed

High pt : Near-side unmodified, away-side suppressed but shape unmodified

The “ridge“ in high energy nuclear collisions Jörn Putschke for the STAR Collaboration

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