A Longitudinal Property of the Charge Balance Function and its Transverse Momentum Dependence in Au-...
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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 Wuhan.

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LI Na, LI Zhiming, LIU Lianshou and WU Yuanfang (for the STAR Collaboration)

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Li na li zhiming liu lianshou and wu yuanfang for the star collaboration

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


Li na li zhiming liu lianshou and wu yuanfang for the star collaboration

Outline:

  • Introduction

  • ●what does BF measure●current status and problems

  • ● motivations

  • 2. Experiment and data sample

  • 3. Results and discussions

  • ● Dependence of BFon size and position ofηwindow

  • ● Independence of Bs on size and position of η window

  • ●Transverse momentum dependence of Bs

  • 4. Summary and conclusions

李治明


Li na li zhiming liu lianshou and wu yuanfang for the star collaboration

Relative rapidity:

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

Definition :

  • BF measures how the conserved electric charges compensate in the phase space, i.e., how the surrounding electric charges are rearranged if the charge at any selected point changes.

  • It tells how the production of charged particles in the whole phase space is constrained by global charge balance, thus provides a direct access to collision dynamics.

李治明


1 introduction motivation difficulty in heavy ion

1.Introduction:☞ Motivation: Difficulty in heavy ion

Current status:

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).

Problem:

acceptances in ηor y

STAR: [-1.3, 1.3]

NA49: [ 2.6, 5.0]

NA22: whole y

Incomparable results!

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.

Aim:

李治明


Li na li zhiming liu lianshou and wu yuanfang for the star collaboration

  • NA22 experiment results

π+ p and K + p at 22GeV

  • full 4π acceptance

  • 0.001GeV/c < pT< 10 GeV/c

M.R. Atayan, et al., Phys. Lett. B 637 (2006) 39-42;

  • BF in different finite rapidity windows can be obtained from BF in whole phase space due to boost invariance.

  • Boost invariance of BF is valid over the whole rapidity space.

李治明


1 introduction motivation why p t dependence

1.Introduction:☞ Motivation: why pT dependence

  • The longitudinal property of BF comes from the special longitudinal interaction of charged particles under the constraint of global electric charge balance.

  • The range of pT of final state particles is supposed to be related to its emitted proper time.

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.

李治明


Li na li zhiming liu lianshou and wu yuanfang for the star collaboration

2.Experiment and data sample: STAR experiment

Relativistic Heavy Ion Collider (RHIC), BNL, Upton, NY, USA

李治明


2 experiment and data sample data sample

2.Experiment and data sample: data sample

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,

FitPts/FitMax>0.52

Number of events:5.7 million minimum bias eventsare selected!

李治明


3 results and discussions dependence of bf on the size and position of w

3. Results and discussions:☞ dependence of BF on the size and position ofηw

STAR Preliminary

  • BF strongly depends on the size of ηw and becomes wider with increasing size of ηw.

  • BF’s in five different positions with same size are equal to each other, i.e., BF is invariant under a longitudinal translation within (-1,1).

  • Bs for different widths and positions of ηw coincide with each other, indicating that Bs is independent of the size and place of observed window.

李治明


Li na li zhiming liu lianshou and wu yuanfang for the star collaboration

3. Results and discussions:☞ Longitudinal property of BF in different subsamples

STAR Preliminary

  • In the left two subsamples, the global net charge is out of balance, and the longitudinal charge correlation is uniformly changed.

  • The farther the charge is out of balance, the more seriously the longitudinal property of BF is violated.

  • In the right subsample, the longitudinal property of BF keeps well since the global charge balance maintains in each event in this case. So,the global charge balance is a necessary condition for the observed longitudinal property of BF!

李治明


3 results and discussions transverse momentum dependence of b s

STAR Preliminary

3. Results and discussions:☞ Transverse momentum dependence of Bs

  • The points at a given relative psudorarapidity in a restrict pT interval are close to each other.

  • It shows that both the special property of longitudinal interaction of charged particles and the global charge conservation are kept in the subset of particles within a give pT range.

  • This is in consistent with the scenario that particles in same restricted pT range are produced at the same proper-time of expansion with well balanced electric charge.

李治明


3 results and discussions p t dependence of the width of b s

QGP Signal

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) ].

3. Results and discussions:☞ pTdependence of the width of Bs

  • The width of Bs becomes narrower with the increase of pT, which is consistent to the expected delayed hadronization of high pT particles in the thermal model [S. Jeon et al., PRC 65, 044902(2002)].

李治明


4 summary conclusions

4. Summary & conclusions:

  • Within the pseudorepidity range (-1,1), the balance function is independent of the position of the pseudorapidity window with the same size.

  • The scaled BF Bs, i.e., B (δη|ηw) measured in an observed window ηwdivided by (1-δη|ηw) is found to be independent of the choice of observed pseudorapidity window.

  • The longitudinal property of the balance function is further found to be valid for the particles in same restricted transverse-momentum range, which is in consistent with the scenario that particles in same restricted transverse-momentum range are produced simultaneously with well balanced electric charge, and provides for the first time an experimental support for the successful study of the pT dependence of elliptic flow.

  • Moreover, the higher the pT of the particles, the narrower the scaled balance function, as expected from the thermal model.

谢谢/Thanks!

李治明


Appendix i on the measure of charge balance function

Appendix I● on the measure of charge balance function

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.

李治明


Li na li zhiming liu lianshou and wu yuanfang for the star collaboration

[S. Jeon and S. Pratt, PRC 65, 044902 (2002)]

李治明


2 results and discussions centrality dependence of the width of bf

21.5%

19.9%

2. Results and discussions:☞ Centrality dependence of the width of BF

If B(δη|ηw) is replaced by B(δη|∞) in,

  • The width of B(δη|∞).

    is wider than that of

    B(δη|ηw)

  • The width of B(δη|∞)

    decreases with centrality

    faster than that of

    B(δη|ηw)

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.

李治明


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