Rapidity Dependence of Elliptic Flow at RHIC

0.04 MB·English

by  Erik Johnson

Rapidity Dependence of Elliptic Flow at RHIC
E. B. Johnson for the BRAHMS Collaboration
UniversityofKansas,Lawrence,KS66045
6 Abstract. Themeasuredellipticflow(v )ofidentifiedparticlesasafunctionofp andcentrality
2 T
0 atRHICsuggeststhecreatedmediuminAu+Aucollisionsachievesearlylocalthermalequilibrium
0 thatisfollowedbyhydrodynamicexpansion.Itisnotknowniftheh dependenceonv [1,2]isa
2
2
generalfeatureofellipticflow orreflectsotherchangesin theparticlespectrain goingfrommid-

n rapiditytofowardrapidities.TheBRAHMSexperimentprovidesauniquecapabilitycomparedto
a theotherRHICexperimentstomeasurev foridentifiedparticlesoverawiderapidityrange.From
2
J Run4Au+Aucollisionat√sNN =200GeV,identifiedellipticflowisstudiedusingtheBRAHMS
4 spectrometers,whichcover0<h <3.4.TheBRAHMSmultiplicityarrayisusedtodeterminethev
2
eventplanefortheidentifiedparticleellipticflowandtomeasurethep -integratedflowforcharged
T
1 hadrons.
v
0
1
0 The Relativisitic Heavy-Ion Collider has produced Au-Au collisions at √sNN
1 = 200GeV in order to create a novel state of matter, the quark-gluon plasma (QGP),
0
in which quarks and gluons are no longer confined [3]. Hydrodynamical models that
6
0 assume the formation of a QGP have been used to model the behavior of the created
/ medium [4]. The data presented by the STAR, Phobos, and PHENIX collaborations in
x
e ref. [1, 2, 5] show strong evidence that the created medium behaves as a fluid. These
-
l studieshavebeen limited,however,to themid-rapidityregionofthecollision.
c
u BRAHMSstudiesellipticflowasafunctionofthelongitudinalexpansionofthecre-
n ated medium. The STAR and Phobos collaborations have shown that the p -integrated
T
:
v elliptic flow is monotonically dependent on pseudorapidity [1, 2]. Phobos reports on a
Xi “limiting framentation” behavior [6], showing a universal dependence of elliptic flow
on h ’ (= h - y ) as h ’ goes to zero for various collision energies. We seek to better
r beam
a
understandthisbehaviorbystudyingitsrapidityandtransverse-momentumdependence.
Ellipticflowisdirectlyinfluencedbytheinitialstateofthesystembutisalsoaffected
by final state dynamics. The p -integrated elliptic flow shows a centrality dependence
T
that is attributed to the inital collision geometry. The mid-rapidity elliptic flow depen-
dence on p has a number of signals suggesting an interplay between final and initial
T
state effects. The elliptic flow for charged-hadrons shows a strong dependence on p ,
T
and initial state hydrodynamical models are in good agreement with the measurement
up to about 1.5 GeV/c [2, 5]. At this point, the signal seems to saturate for the higher
p particles. The elliptic flow at intermediate p is found to scale with the number of
T T
constituent quarks, n, (i.e. v /n vs. p /n has a universal dependence), which is consis-
2 T
tent with a final state effect of quark coalescence [7]. This dependence holds well for
baryons and kaons for p /n > 0.75 GeV/c, as shown in ref. [2, 5], but the pions behave
T
somewhatdifferently.Thedeviationseen forpionscould bedueto resonancedecays or
thedifferencein massbetween thepion andtheconstituentquarks.
The BRAHMS experiment [8] consists of two spectrometers, one covering angles
near mid-rapidity(MRS) and onecovering forward rapidities (FS). A numberof global
detector arrays characterize the overall charged-particle production. The collision re-
gion is surrounded by arrays of silicon detectors (SMA) and scintillating tile detectors
(TMA). Three azimuthally symmetric rings of Si detectors, and one ring of tile detec-
tors are used in the reaction-plane analysis. The BRAHMS azimuthally symmetric, left
large-tubebeam-beamcounters (BBL) arealso usedforthisanalysis.
Thebasicprocedure to measuretheellipticflow signalis outlinedin ref. [9]. Oncea
reaction planeis determined for the fivedetector rings (3Si, 1 Tile, 1BBL), thef of the
particles measured in one detector is correlated to thereaction plane in another. Several
combinations of the detector rings were used for the integrated v analysis, while p
2 T
dependentv analysisonlycorrelatedthetilereactionplanewiththeparticlesidentified
2
inthespectrometers.
To obtain the correct v value, signal distortions resulting from the reaction-plane
2
resolution, the background, and other non-flow effects, need to be removed. Since the
BRAHMS experiment is not symmetric about h = 0, the reaction-plane resolution
measurement requires using three independent reaction planes. Auto-correlations are
removedbycorrelatingdetectorsthatareseperatedbyatleast0.2unitsinh .Normalized
weightsbasedonaveragesovermanyeventsareusedtoremoveanyanisotropiceffects.
These weights are determined using minimum biased events for the intergated flow
analysis,butalleventsareusedinthespectrometeranalysis.Tofurtherremoveanyfirst
order effects, the S wsin(2f ) and S w cos(2f ) terms in the reaction plane calculation
i i i i
are centered to zero on average. From ref. [10], the final reaction plane distribution
can be completely flattened by taking a Fourier decomposition of the distribution over
minimum bias events. Background and geometrical effects are removed using GEANT
simulationswithaknownellipticflow signal.
The BRAHMS integrated v versus centrality and h dependencies are consistent
2
with the Phobos results [1] (see Fig 1a and 1b). The p dependent elliptic flow signal
T
is determined for charged-hadrons and protons at mid-rapidity, and these results agree
with the data presented by the STAR [2] and PHENIX [5] collaborations. The charged-
(a) 0.08 PBBhRRoAAbHHoMMs SSv2 :vv 221::5 MS-2pu5elt%c At rCrormaeyne:t tr1ea5rl:- 2150%-3 0C%e nCtreanltral (b) 0.1 BBRRAAHHMMSS vv22:: MSpueltc Atrrormayeter
0.07 3SDT AHRy dvr2o: Mbyin H-Biriaanso Phobos v2
0.06 0.08
0.05
2 20.06
v v
0.04
0.03 0.04
0.02
0.02
BRAHMS Preliminary
0.01 BRAHMS Preliminary
0 0 1 2 h 3 4 5 00 50 100 150 200 250 300 350
N
part
FIGURE 1. (a) Integratedv versus h for chargedhadrons. The BRAHMS spectrometer results are
2
consistent, but systemmatic errors with the spectra may distort the shape. For comparison, a 3D hydro
modelresults[11]areplotted.(b)Integratedv versusN forchargedhadronswithin-1.0<h <1.0.
2 Part
0.3 BRAHMS Preliminary
0.2
0.1 h » 0
0.3
V2 0.2
0.1 h » 1
0.3 3D Hydro (15-25%), T. Hirano
0.2
0.1 h » 3.2
00 0.5 1 1.5 2 2.5 3 3.5 4
p (GeV/c)
T
FIGURE2. p dependentelliptic flow forchargedhadronsforthe 10%to 30%mostcentralevents.
T
Thetoppanelisforh 0,themiddleshowsh 1,andthebottompaneldisplaysh 3.2.The3Dhydro
≈ ≈ ≈
calculationsarealsogivenforthe15%to25%mostcentralevents.
hadron spectrum at mid-rapidity is used to determine the integrated v as a function
2
of centrality for the spectrometer measurements, with results that agree within errors
with the integrated v determined from the BRAHMS global detectors (see Fig. 1a and
2
1b). The measured charged-hadron v dependence on p for centralities from 10% to
2 T
30% does not seem to change much over rapidity, as shown in Fig. 2. This rapidity
independent behavior is consistent with a 3D hydrodynamical model [11]. The result
suggeststhattheh dependenceoftheintegratedv isnotaninherentfeaturebutreflects
2
thecharged-particle p spectrum withrapidity.
T
The BRAHMS experiment is able to reproduce the elliptic flow signitures seen in
the other RHIC experiments and expand the study of identified-particle elliptic flow to
forward rapidity. The study of the elliptic flow over a large rapidity range will allow
a better understanding of the longitudinal expansion of the created medium. Current
work is focussed on establishingthe ellipticflow for protons, pions, and kaons over the
rapidityacceptance ofBRAHMS.
Work supported in part by the Office of Nuclear Physics of US DOE under contract
DE-FG03-96ER40981andDOE EPSCoR DE-FG02-04ER46113.
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