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

Joakim NystrandLund University

for the PHENIX Collaboration

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

• Transverse momentum, pT

Detectors in PHENIX used for this analysis

Central tracking arms: •Drift chamber

BBC – Beam-Beam Counters; charged ptcles in 3.0 || 3.9 ZDC – Zero-Degree Calorimeter; neutral beam fragmentsUsed for triggering and centrality selection

• Pad Chambers (2, or 3 layers)

• EM Calorimeter (only position of hits used here)

Net charge fluctuations

”hadron-gas” ”quark-gluon plasma”

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

Proposed 2 years ago: Fluctuations in net charge and net baryon number significantly reduced if a QGP is formed in the collisionsAsakawa, Heinz, Müller PRL 85(2000)2072; Jeon&Koch PRL 85(2000)2076

Charged particle multiplicitynch = n+ + n–

Net chargeQ = 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

Measures of net charge fluctuations

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

Centrality SelectionSelect 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

v(Q) as a function of collision centrality

A small deviation from stochastic emission observed at 130 GeVK. 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.

|| 0.35, =/2, 0.3 pT 2.0 GeV/c

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.

Systematics for v(Q)

d

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.

0 d 90º

for one tracking arm

v(Q) vs. d

(1 – p)

130 A GeV - 10% most central events

Nearly linear decrease in v(Q) with d, reproduced by RQMD.Stronger decrease than expected from charge conservation.

Band shows totalstatistical error; error bars show errors bin-by-bin.

v(Q) vs. d

200 A GeV - 10% most central events

Similar trend and slope at 130 and 200 GeV

Band shows 130 GeV data.

v(Q) vs.

The scaling in is very similar to that in

= /2,0.3 pT 2.0 GeV/c

Test the scaling with p in the longitudinal direction,

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 (reconstructionefficiency and contribution from background tracks), and by comparing theresults for the 2 arms (200 GeV).

Result for v(Q)

<pT> fluctuationsPoster by Jeff Mitchell

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

A small positive signal is seen in data at 200 GeVQuantify the effect through the measure FpT

PHENIX Preliminary

Tp

Tp )(Var

random

randomdatapT

F

NrandomTpF

* AuAu 200 GeV

o AuAu 130 GeV nucl-ex/0203015to appear in PRC

Maximum for semi-central collisions.

FpT related to T:

PHENIX Preliminary

F pT (%

)

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)

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.

PHENIX Preliminary

F pT (%

)

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.