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Charge Fluctuations at Mid-Rapidity in Au+Au Collisions in the PHENIX Experiment at RHIC Joakim Nystrand Lund University. for the PHENIX Collaboration. Two studies of event-by-event fluctuations: Net charge, Q = n + – n – Transverse momentum, p T.

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slide1

Charge Fluctuations at Mid-Rapidity in Au+Au Collisions in the PHENIX Experiment at RHIC

Joakim Nystrand

Lund University

for the PHENIX Collaboration

  • Two studies of event-by-event fluctuations:
  • Net charge, Q = n+– n–
  • Transverse momentum, pT
slide2

Detectors in PHENIX used for this analysis

  • Central tracking
  • arms:
  • Drift chamber
  • Pad Chambers (2,
  • or 3 layers)
  • EM Calorimeter
  • (only position of hits
  • used here)

BBC – Beam-Beam Counters; charged ptcles in 3.0  ||  3.9

ZDC– Zero-Degree Calorimeter; neutral beam fragments

Used for triggering and centrality selection

slide3

Net charge fluctuations

Proposed  2 years ago: Fluctuations in net charge and net baryon

number significantly reduced if a QGP is formed in the collisions

Asakawa, Heinz, Müller PRL 85(2000)2072; Jeon&Koch PRL 85(2000)2076

Fractional electric charges (q =  1/3, 2/3) of the quarks ==>

Charges more evenly spread in a plasma ==> Reduced net charge

fluctuations in a small region of phase-space

”hadron-gas”

”quark-gluon plasma”

slide4

Measures of net charge fluctuations

Charged particle multiplicity

nch = n+ + n–

Net charge

Q = n+ - n–

Define:

v(Q)  Var(Q)/<nch>

For stochastic emission, v(Q) = 1

Globally, one expects v(Q) = 0 – charge conservation

If we observe a fraction p of all produced particles 

v(Q)  (1 – p ) from global charge conservation

slide5

Additional reduction of v(Q) in hadron gas (decay of neutral resonances) and QGP. For large acceptances y  1:

Hadron gas: v(Q)  0.7 QGP: v(Q)  0.25

Other measures have been proposed:

v(R) = <nch> Var(R) , where R = n+ / n–

 = < (Q –  nch)2 > / <nch>

 = 4 < (n+ /(1 +  ) – n– /(1 –  ))2 > / <nch>2

 is the charge asymmetry,  = <Q>/<nch>

v(R) not suitable for small acceptances

,  similar to v(Q), for =0

v(Q) =  = <nch>  / 4

slide6

Centrality Selection

Select events based on ZDC and BBC information.

nch and Q distributions for centrality classes (5% bins).

||  0.35, 0.3  pT  2.0 GeV/c, =/2

slide7

v(Q) as a function of collision centrality

||  0.35, =/2,

0.3  pT  2.0 GeV/c

A small deviation from stochastic emission observed at 130 GeV

K. Adcox et al. (PHENIX) nucl-ex/0203014 to appear in PRL

No dramatic change at 200 GeV - the upward shift of 0.01 units can be explained by harder track quality cuts leading to a reduced acceptance.

slide8

Systematics for v(Q)

Expected reduction in v(Q) from global charge conservation:

v(Q)  (1 – p)

where p is the fraction of the produced particles inside the acceptance.

Is this enough to explain the measured value of v(Q)?

The reduction in v(Q) from decay of neutral resonances should also scale with the geometrical acceptance (need a certain opening angle to catch both decay products).

The scaling with p for a QGP is not known. A theoretical model would

be desirable to be able to do exact experimental comparisons.

 Study how v(Q) varies with the geometrical acceptance to understand the origin of the effect.

slide9

Systematics for v(Q)

  • Vary p by using only part of the detector, p d
  • p can be calculated from the global dnch/d
  • Resonance contribution estimated from hadronic model (RQMD)
  • Any additional suppression would indicate a positive signal.

d

0  d  90º

for one tracking arm

slide10

v(Q) vs. d

130 A GeV - 10% most central events

(1 – p)

Band shows total

statistical error;

error bars show

errors bin-by-bin.

Nearly linear decrease in v(Q) with d, reproduced by RQMD.

Stronger decrease than expected from charge conservation.

slide11

v(Q) vs. d

200 A GeV - 10% most central events

Band shows

130 GeV data.

Similar trend and slope at 130 and 200 GeV

slide12

v(Q) vs. 

Test the scaling with p in the longitudinal direction, 

 = /2,

0.3  pT  2.0 GeV/c

The scaling in  is very similar to that in 

slide13

Result for v(Q)

10% most central collisions. For ||<0.35, pT > 200 MeV/c, =/2:

v(Q) = 0.965 ± 0.007(stat.) – 0.019 (syst.) snn =130 GeV

v(Q) = 0.969 ± 0.006(stat.) ± 0.020 (syst.) snn =200 GeV (PRELIMINARY)

Systematical error estimated from geant simulations (reconstruction

efficiency and contribution from background tracks), and by comparing the

results for the 2 arms (200 GeV).

slide14

<pT> fluctuations

Poster by Jeff Mitchell

Event-by-event <pT> for data (+) and mixed event (+)

PHENIX Preliminary

A small positive signal is seen in data at 200 GeV

Quantify the effect through the measure FpT

slide15

* AuAu 200 GeV

o AuAu 130 GeV

nucl-ex/0203015

to appear in PRC

Maximum for semi-

central collisions.

FpT related to T:

PHENIX Preliminary

FpT (%)

slide16

The fluctuation magnitude tends to increase as the pT range used to calculate <pT> is extended to higher values.

FpT vs. PT range

(0.2<pT<pT, max)

FpT (%)

PHENIX Preliminary

Centrality and pT dependence similar to elliptic flow.

Simulations using PHENIX preliminary pT-dependent v2

measurements wrt to the reaction plane can, however, not

reproduce the signal.

slide17

Conclusions

  • Net charge and <pT> fluctuations in Au+Au interactions have been studied with the PHENIX detector.
  • A small reduction in v(Q) from what is expected for stochastic emission is observed. The reduction is consistent with hadronic models containing global charge conservation and neutral resonances.
  • Similar trends for v(Q) at snn =130 and 200 GeV.
  • A positive signal, with a maximum in semi-central collisions, is seen in FpT at snn =200 GeV.