1 / 17

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

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.

raja-mckay
Télécharger la présentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. 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

  3. 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”

  4. 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

  5. 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

  6. 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

  7. 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.

  8. 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.

  9. 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

  10. 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.

  11. 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

  12. 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 

  13. 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).

  14. <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

  15. * 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 (%)

  16. 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.

  17. 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.

More Related