High-pt triggered dihadron correlations with vn background subtraction by STAR

5.1 MB·English

by  Fuqiang Wang

High-p triggered dihadron correlations with v
T n
background subtraction by STAR
Fuqiang Wang (for the STAR Collaboration)
2
Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA
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0
2

n
a STARmeasurementsofdihadroncorrelationswithhigh-p triggerpart-
√ T
J cles in Au+Au collisions at s =200 GeV are presented with subtrac-
NN
4 tion of v , v , and v backgrounds. The v azimuthal anisotropies were
2 3 4 n
2 measured by the two-particle cumulant method with η gap to reduce non-
flow. Thedihadroncorrelationsrelativetothev2 aswellasthev3 harmonic
]
x planes are also presented. Implications of the results are discussed.
e
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l 1. Introduction
c
u
n Relativistic heavy ion collisions create a hot and dense medium ex-
[ hibiting hydrodynamic properties. The particle azimuthal distributions are

anisotropic resulting from hydrodynamic conversion of the initial spatial
1
v anisotropy. Hydrodynamics describe well the expansion and anisotropies of
6 particles at transverse momentum p < 2 GeV/c. Hard-scattering probes,
T
0
on the other hand, interact with the medium and lose energy, resulting in
0
depletion of high-p hadrons in the final state. This jet-quenching phe-
5 T
. nomenon provides an exclusive channel to study QCD interactions and de-
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0 duce the properties of the QCD matter at high energy densities.
2 Dihadron correlations with high-p trigger particles have proven valu-
T
1
able in jet-quenching studies. One of the difficulties in dihadron correlation
:
v measurements is the subtraction of combinatoric background. The back-
Xi ground is not uniform but modulated by flow harmonics (correlation to the
eventplane). Triggeredcorrelationshavebeenmeasuredwithonlyv andv
r 2 4
a subtraction, assuming vanishing triangular flow (v ). Novel structures were
3
observed constituting a near-side long-range speudo-rapidity (∆η) correla-
tion (‘ridge’) [1, 2] and a away-side double-peak azimuthal (∆φ) correlation
(‘Mach-cone’) [1, 3, 4, 5].
Recent model studies suggest, however, that triangular and other odd
harmonicsdonotvanishbecauseofinitialgeometryfluctuations[6]. Anon-
zero v {2} from two-particle cumulant method has been measured [7, 8];
3
(1)
2
√
v {2} = V , V ≡ (cid:104)cos(n∆φ)(cid:105) where ∆φ is the two-particle opening
n n∆ n∆
azimuthal angle. While a non-zero v {2} itself is not a proof of finite hydro-
3
dynamictriangularflowbecausenonflowalsocontributes,thecentralityand
p dependences of the measured v {2} do suggest that part of the v {2} is
T 3 3
of hydrodynamic origin, and this part should be subtracted from dihadron
correlations. Two questions arise: (1) How much does nonflow (correlation
unrelated to the reaction plane) contribute to the measured v {2}? (2)
n
What are the effects of jet-medium interactions?
This talk does not provide a complete answer to these questions, but
makes progress towards that end. We report STAR measurements ofv and
√ 2
v in Au+Au collisions at s = 200 GeV. We subtract v backgrounds
3 NN n
and present the obtained dihadron correlations integrated over as well as
relative to the v and v harmonic planes. We discuss possible implications
2 3
of our results.
2. Dihadron correlation with v subtraction
n
Figure 1 shows the v {2} and v {2} measurements by the two-particle
2 3
cumulant method [8]. The four-particle cumulant v {4} is also shown for
2
comparison. The v {2} has a weak centrality dependence, consistent with
3
its fluctuation origin. At large p in central collisions, v is comparable to
T 3
v . An η-gap, |∆η| > 1, was applied in the analysis to reduce the nonflow
2
contributionsfromsmall-anglecorrelations,suchasjet-likecorrelations,res-
onance decays, etc. Nonflow correlations beyond |∆η| > 1 still remain. One
likely contribution comes from away-side jet-like correlations because the
away-side jet partner is uncorrelated to the near-side jet in η.
Fig.1. (Color online) Two- and four-particle anisotropy measurements v {2},
√ 2
v {4}, v {2} as a function of centrality in Au+Au collisions at s = 200 GeV
2 3 NN
by STAR. The abscissa bins 0-6 stand for 10%-size centralities from 80% to 10%,
and 7-8 for 5-10% and top 5% centralities. Three p ranges are shown, 0-2 GeV/c
T
(left), 1-2 GeV/c (middle), and 3-4 GeV/c (right).
Two-particlecumulantv {2}measurestheneteffectofflow, flowfluctu-
n
ations, and nonflow. Except the nonflow contamination, v {2} is the most
n
truthful background to dihadron correlation. However, if v {2} is measured
n
3
from the same pairs used in dihadron correlation analysis, then v {2} will
n
precisely describe the correlation function and the v {2}-subtracted signal
n
will by definition be zero. It is thus important to measure v {2} using pairs
n
farremovedinphase-spacefromthoseusedindihadroncorrelationanalysis.
Dihadron correlations at low-to-intermediate p with ∆η gap in Pb+Pb
T
collisions at LHC were shown to be completely described by the sum of
a few Fourier harmonics [7]. The particle anisotropies at a given p were
T
measured by the Fourier coefficients of their correlations to reference par-
ticles from 0.2 < pref < 5 GeV/c with |∆η| > 1 in ALICE [7]: v (p ) =
T n T
(cid:113)
V (p ,pref)/ V (pref,pref). The dihadron correlation result of Ref. [7],
n∆ T T n∆ T T
with trigger ptrig=2-3 GeV/c and associated passoc=1-2 GeV/c, thus indi-
T T
catesV (ptrig,pref)V (passoc,pref) ≈ V (ptrig,passoc)V (pref,pref),which
n∆ T T n∆ T T n∆ T T n∆ T T
is not surprising given the relatively small nonflow contributions (as gauged
by the v {2} and v {4} measurements). Quantitatively how much nonflow
2 2
contributes to the measured V will require further studies.
n∆
Fig.2. (Color online) v -subtracted dihadron correlations at |∆η| > 1 in Au+Au
√ n
collisions at s = 200 GeV by STAR. Three centrality ranges are shown: 50-
NN
80%(left),20-50%(middle),andZDCtop12%(right). Thetriggerandassociated
p ranges are 3-6 GeV/c and 2-3 GeV/c, respectively. The subtracted v {2} were
T n
measuredbythetwo-particlecumulantmethodwith|∆η|>1andreferenceparticle
pref <2GeV/c. Thebackgroundisnormalizedsuchthattheaveragesignaliszero.
T
Figure 2 shows the dihadron correlation signals in Au+Au collisions at
√
s = 200 GeV by STAR after v subtraction. The trigger and asso-
NN n
ciated p ranges are 3-6 GeV/c and 2-3 GeV/c, respectively. The v {2}
T n
were measured with reference particles from pref < 2 GeV/c, which is some-
T
what removed from the trigger and associated p regions. The blue data
T
points show the results with v , v , and v subtraction, while the black ones
2 3 4
show those with only v and v subtraction for comparison. In peripheral
2 4
collisions, no visible signal remains besides a negative dipole. This may
suggest that the subtracted v contain nonflow contributions comparable
n
to the jet-like correlation between the trigger-associated pairs. In medium-
central collisions, there seems finite correlation amplitude on the near side
4
which may indicate a ridge contribution. In other words, the previously
observed ridge may not be completely explained by v . On the away side, a
3
broad correlation signal is still observed, however, the double-hump struc-
ture is weaker. In the most central collisions, no away-side correlation is
visible, consistent with the “disapparence” of away-side high-p hadrons.
T
The near-side correlation seems to indicate a finite peak consistent with
remaining ridge contributions.
3. Dihadron correlation relative to the event plane
Dihadron correlations as a function of the trigger particle azimuth rel-
ative to the event plane, φ = φ − ψ , have been analyzed in 20-60%
s t 2
Au+Au collisions by STAR [9]. The v and v backgrounds were sub-
2 4
tracted. The v was obtained from the average of two- and four-particle
2
cumulant methods [10]. The v was measured with respect to the v har-
4 2
monic event plane [10]. The effect of v background was estimated and
3
found to be insignificant [9]. In this talk we use for background subtrac-
tion the measured v {2} (|∆η| > 0.7) and the parameterized v {ψ } =
2 4 2
1.15v {2}2 for v correlated to ψ . The resultant dihadron correlations at
2 4 2
|∆η| > 0.7 are shown in the black histograms in Fig. 3 and are consistent
with the results in Ref. [9]. We further subtract the additional v {2} and
3
(cid:112)
v {uncorr.} = v {2}2−v {ψ }2 backgrounds (both uncorrelated to ψ ):
4 4 4 2 2
2vtrig{2}vassoc{2}cos3∆φ+2vtrig{uncorr.}vassoc{uncorr.}cos4∆φ. The re-
3 3 4 4
sults are shown in Fig. 3 in the red histograms. The results again show
that the effect of v is insignificant; however, the reduction in the near-side
3
correlation magnitude is noticeable.
The event-plane dependent dihadron correlation analysis can be carried
outwithrespecttothev harmonicplaneψ . Inreconstructingψ (andψ ),
3 3 3 2
particleswithin|∆η| < 0.5ofthetriggerparticleareexcluded. Analogousto
correlationsrelativetoψ ,thev backgrounddependsonthetriggerparticle
2 3
azimuth φ = φ − ψ . The v and v backgrounds are independent of
s,3 t 3 2 4
φ . Figure 4 shows in the right panels the dihadron correlation for trigger
s,3
particles in-phase of ψ (|φ | < π/6) and out-of-phase (|φ −π/3| < π/6).
3 s,3 s,3
For comparison those for trigger particles in-plane of ψ (|φ | < π/4) and
2 s
out-of-plane (|φ −π/2| < π/4) are shown in the left panels. The cartoon
s
inserts help visulize the geometry. The correlation results seem to indicate
a near-side ridge for trigger particles both in-plane of ψ and in-phase of
2
ψ . For away side, the correlation is broader for triggers out-of-plane than
3
in-plane of ψ . To the contrary, the correlation is broader for triggers in-
2
phase than out-of-phase of ψ . These results seem to suggest path-length or
3
geometry effects on the away-side jet-like correlations. However, the exact
nature of the effects need further investigations.
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Fig.3. (Color online) v -subtracted dihadron correlations at |∆η|>0.7 in 20-60%
√n
Au+Aucollisionsat s =200GeVbySTAR.Thecorrelationsareshown,from
NN
upper left (in-plane) to lower right (out-of-plane) panel, in 15◦ steps in trigger
particle azimuth relative to the event plane, φ . The trigger and associated p
s T
ranges are 3-4 GeV/c and 1-2 GeV/c, respectively. The subtracted v were mea-
n
sured by the two-particle cumulant method with |∆η|>0.7 and reference particle
pref < 2 GeV/c. The subtracted v background (uncorrelated to the event plane)
T 3
and the uncorrelated portion of v are shown in the green and pink histograms,
4
respectively. The background is normalized by the ZYAM prescription.
4. Summary
We have presented dihadron correlations with trigger ptrig=3-6 GeV/c
√T
and associated passoc=2-3 GeV/c in Au+Au collisions at s = 200 GeV
T NN
by STAR, with v , v , and v background subtraction. The v anisotropy
2 3 4 n
were measured by two-particle cumulant method with reference particles
from 0.2 < pref < 2 GeV/c and with a relatively large η gap. A near-
T
side ‘ridge’ peak seems to remain in non-peripheral collisions. The away-
side correlation, while strongly suppressed in central collisions, is broad in
medium-central collisions.
Wehavealsopresenteddihadroncorrelationsrelativetothev harmonic
2
event plane ψ as well as the v harmonic plane ψ , also with v , v , and
2 3 3 2 3
v background subtraction, in 20-60% Au+Au collisions. The effect of v
4 3
is small. A near-side ridge seems to be present in-plane of ψ (and in-
2
phase of ψ ), and disappear out-of-plane of ψ (and out-of-phase of ψ ).
3 2 3
The away-side correlation is single peaked for triggers in-plane of ψ and
2
broadens out-of-plane. This trend seems reversed relative to ψ –The away-
3
sidecorrelationissingle-peakedfortriggersout-of-phaseofψ andbroadens
3
6
Fig.4. (Color online) v -subtracted dihadron correlations at |∆η|>0.7 in 20-60%
√n
Au+Au collisions at s = 200 GeV by STAR. The correlations are shown for
NN
trigger particles in-plane (upper-left) and out-of-plane (lower-left) of the v event
2
plane(ψ ),andfortriggerparticlesin-phase(upper-right)andout-of-phase(lower-
2
right) of the v harmonic plane (ψ ). The trigger and associated p ranges are 3-4
3 3 T
GeV/cand1-1.5GeV/c,respectively. Thesubtractedv weremeasuredbythetwo-
n
particle cumulant method with |∆η| > 0.7 and reference particle pref < 2 GeV/c.
T
The background is normalized such that the average signal is zero.
in-phase. These results seem to indicate nontrivial path-length or geometry
effects of the underlying physics.
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