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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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Charge Fluctuations at Mid-Rapidity in Au+Au Collisions in the PHENIX Experiment at RHIC
for the PHENIX Collaboration
BBC – Beam-Beam Counters; charged ptcles in 3.0 || 3.9
ZDC– Zero-Degree Calorimeter; neutral beam fragments
Used for triggering and centrality selection
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
Charged particle multiplicity
nch = n+ + n–
Q = n+ - n–
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
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
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
|| 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.
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.
0 d 90º
for one tracking arm
130 A GeV - 10% most central events
(1 – p)
Band shows total
error bars show
Nearly linear decrease in v(Q) with d, reproduced by RQMD.
Stronger decrease than expected from charge conservation.
200 A GeV - 10% most central events
130 GeV data.
Similar trend and slope at 130 and 200 GeV
Test the scaling with p in the longitudinal direction,
0.3 pT 2.0 GeV/c
The scaling in is very similar to that in
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).
Poster by Jeff Mitchell
Event-by-event <pT> for data (+) and mixed event (+)
A small positive signal is seen in data at 200 GeV
Quantify the effect through the measure FpT
o AuAu 130 GeV
to appear in PRC
Maximum for semi-
FpT related to T:
The fluctuation magnitude tends to increase as the pT range used to calculate <pT> is extended to higher values.
FpT vs. PT range
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.