the “ridge“ in high energy nuclear collisions jörn putschke for the star collaboration
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The “ridge“ in high energy
nuclear collisions
Jörn Putschke for the STAR Collaboration
Yale University
Weisshorn (4505m), Switzerland
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?
Jörn Putschke, WWND 2008 3
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
Jörn Putschke, WWND 2008 5
Jet-like peak width in central Au+Au
4 < pt,trigger < 5 GeV and pt,assoc. > 2 GeV
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
Jörn Putschke, WWND 2008 6
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)
Jörn Putschke, WWND 2008 7
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
Jörn Putschke, WWND 2008 8
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
Jörn Putschke, WWND 2008 9
“Jet”/Ridge energy
STAR, Phys. Rev. Lett. 95 (2005) 15230
0.15 < pt,assoc < 4 GeV
4 < pt,trigger < 6 GeV
6 < pt,trigger < 10 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”
}
}
Jörn Putschke, WWND 2008 10
Ridge: B/M ratio closer to bulk
Jet : B/M ratio ~ p+p
Particle composition in Ridge/Jet vs. inclusive
Jörn Putschke, WWND 2008 11
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 ?
Jörn Putschke, WWND 2008 12
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
Jörn Putschke, WWND 2008 13
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 !
Jörn Putschke, WWND 2008 14
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 ? …
Jörn Putschke, WWND 2008 15
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)
Jörn Putschke, WWND 2008
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
Jörn Putschke, WWND 2008 17
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
Jörn Putschke, WWND 200818
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
Jörn Putschke, WWND 2008
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
Jörn Putschke, WWND 2008
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)
Jörn Putschke, WWND 2008
Jet/Ridge 62 vs. 200 GeV
21
3.0 GeV/c < pT
trigger 6.0 GeV/c; 1.5 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 ?)
Jörn Putschke, WWND 2008 22
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
Jörn Putschke, WWND 2008 23
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.
Jörn Putschke, WWND 2008 24
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”
Jörn Putschke, WWND 2008 25
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
Jörn Putschke, WWND 2008 26
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
Jörn Putschke, WWND 2008 27
Backup slides
Jörn Putschke, WWND 2008 28
/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
STAR preliminary STAR preliminary
Particle composition in Ridge/Jet
Outlook: similar measurement will be pursued using dE/dx ID to look at p/π ratio in ridge/jet
Jörn Putschke, WWND 2008 29
Ridge “observation”
3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV
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
STAR preliminary STAR preliminary
Jörn Putschke, WWND 2008 30
Ridge phenomenology
STAR preliminaryJet+Ridge ()
Jet ()
Jet )
yiel
d
,)
Npart
3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV
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
Jörn Putschke, WWND 2008 31
Extracting the ridge yield
relative ridge yield := ridge yield / Jet()
STAR preliminaryJet+Ridge ()
Jet ()
Jet )
yiel
d
,)
Npart
3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV
Jet yield independent of Npart and consistent with d+Au reference measurements !
Definition of “ridge yield”:
ridge yield := Jet+Ridge( Jet()
Jörn Putschke, WWND 2008 32
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
Jörn Putschke, WWND 2008
Ridge/Jet ratio independent of energy, system
3.0 GeV/c < pT
trigger 6.0 GeV/c; 1.5 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
Jörn Putschke, WWND 2008
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.
Jörn Putschke, WWND 2008 35
• 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
Jörn Putschke, WWND 200836
• 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
Jörn Putschke, WWND 2008 37
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 %)
Jörn Putschke, WWND 2008 38
(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
Jörn Putschke, WWND 2008 39
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.
Jörn Putschke, WWND 2008 40
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)
Jörn Putschke, WWND 2008 41
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
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