A Longitudinal Property of the Charge Balance Function and its Transverse Momentum Dependence in Au-Au Collisions at GeV. LI Na, LI Zhiming, LIU Lianshou and WU Yuanfang (for the STAR Collaboration) Institute of Particle Physics (IOPP) Huazhong Normal university Wuhan.
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A Longitudinal Property of the Charge Balance Function and its Transverse Momentum Dependence in Au-Au Collisions at GeV
LI Na, LI Zhiming, LIU Lianshou and WU Yuanfang
(for the STAR Collaboration)
Institute of Particle Physics (IOPP)
Huazhong Normal university
All the particlesare within the rapidity window
counting the pairs that satisfy the criteria in the rapidity window
1.Introduction: ☞ What does Balance Function measure
The narrowing of the width of BF with centrality
in Au+Au collisions at√sNN=130GeV, and
in lighter nuclei collisions at√sNN=17.2GeV,
and with multiplicity in π+p and K+p Collisions at√s = 22GeV
J. Adams et al., (STAR Coll.), PRL90, 172301(2003); C. Alt et al., (NA49 Coll.), PRC71, 034903(2005);
M. R. Atayan et al., (NA22 Coll.), PLB, 034903(2006).
acceptances in ηor y
STAR: [-1.3, 1.3]
NA49: [ 2.6, 5.0]
NA22: whole y
We want to measureB (δ η|ηw) as a function of δη
but this function depends on the observed windowηw!
Study the dependence of BF on ηwand try to find
a measure independent of observed window.
π+ p and K + p at 22GeV
M.R. Atayan, et al., Phys. Lett. B 637 (2006) 39-42;
Gradient of density leads to non-zero collective velocities
Hydrodynamic expansion: gradient pressure acts
Yu M. S., et al., PRL 89 052301 (2002)
J. Adams et al., (STAR Coll.), NPA757, 102(2005)
3. Since a subset of particles can behave collectively only if they are produced at the same proper-time, therefore, examine the pT dependence will provide direct experimental access on this subject.
2.Experiment and data sample: STAR experiment
Relativistic Heavy Ion Collider (RHIC), BNL, Upton, NY, USA
Au-Au collision at √sNN = 200 GeV on STAR of Run 4
STAR / TPC acceptance:
η : [-1.3, 1.3]
φ : full
Event cuts:|Vertexx|<1, |Vertexy|<1, |Vertexz|<25, MinTrack≥5
Track cuts:0.15 < pT < 2.0, DCA<2, Fitpts>15,
Number of events:5.7 million minimum bias eventsare selected!
3. Results and discussions:☞ Longitudinal property of BF in different subsamples
The balance function is expected to be narrower for a scenario with delayed hadronization ,and is therefore sensitive to the conjecture that a quark-gluon plasma may be produced. [S. A. Bass et al., PRL 85, 2689 (2000) ].
Associated particle density:
the density of particles of charge Q at rapidity yunder the condition that
a particle of charge Qs is detected at the rapidity ys.
Associated net charge density:
the net charge density at rapidity yunder the condition that there exists
a particle of charge Qs at the rapidity ys.
Associated charge density balance:
A measure of the change of the associated net charge density, when the
charge of the selected particle(s) is changed from negative to positive.
D. Drijard, et al., (ACCDHW Coll.) , Nucl. Phys. B166(1980)233-242;
D. Drijard, et al., (CCHK Coll.) , Nucl. Phys. B155(1979)269.
[S. Jeon and S. Pratt, PRC 65, 044902 (2002)]
If B(δη|ηw) is replaced by B(δη|∞) in,
is wider than that of
decreases with centrality
faster than that of
The width of all centralities is 0.553±0.003, which is smaller than 0.683±0.015 for pp collision at √s =200GeV and
0.991±0.008 for π+p and K+p collisions at √s =22GeV.