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Highlights from PHENIX II: Photons and Leptons

By Terry Awes

The PHENIX experiment at RHIC was designed with emphasis on the measurement of leptons and photons. Photons and leptons are of special interest because they are penetrating probes that do not undergo strong interactions and therefore are unlikely to interact with the dense matter produced in heavy-ion collisions. Thus, they carry information about the system at the time of their production, throughout the entire evolution of the collision. Unfortunately, as a result of their weaker electromagnetic coupling, their production is a rare process that requires large data samples for precise measurements. As a consequence of their low production rate the measurements are also subject to large backgrounds. In the case of directly radiated photons, the backgrounds are mostly photons from radiative decays of long-lived neutral mesons, predominantly the abundantly produced neutral pions. In the case of the electron measurement the background is mostly the internal or external conversion of these radiative decay photons into electron-position pairs, and in the case of the muon measurements it is the weak decay of charged pions into muons. It is the study of these and other rare processes that drives the need for high luminosities at RHIC.

With the eighth RHIC running period currently coming to completion, there were many improved measurements of previously published results obtained from the smaller data sets of the earlier RHIC runs. This is particularly true of the lepton and photon measurements. As an example, due to the factor of 10 increase in the PHENIX Run 4 data sample compared to Run 2, the measurement of the neutral pion spectra in Au+Au collisions has been extended to nearly 20 GeV/c transverse momentum with decreased statistical and systematic errors [1]. This directly translates into improved direct photon and non-photonic electron measurements.

One of the most exciting early results from RHIC was the observed strong suppression of neutral pion production in central Au+Au collisions [2] compared to expectations from scaled p+p collisions, and the lack of suppression of the direct photon yield [3]. This strongly supported the conclusion that the initial collisions occurred at the expected rate, as evidenced by the expected direct photon yield, but that the neutral pions were suppressed due to strong interactions and energy loss of the initially scattered parton as they traversed the dense medium prior to fragmentation into particles, like the pion. The final results from the Run 4 analysis demonstrate that the neutral pion suppression is approximately constant with transverse momentum over the region from 5 to 20 GeV/c [1]. Other new PHENIX results indicate that the neutral pion suppression is similar for Cu+Cu and Au+Au collisions for centrality selections with the same number of participating nucleons [4]. The dependence of the pion suppression on collision energy (sqrt(sNN)) has also been investigated with the Cu+Cu measurements where it is seen that the suppression is quite similar at 200 and 62.4 GeV, but that the pion yield is instead enhanced compared to expectations from p+p collisions at 22.4 GeV [4]. On the other hand, the direct photon yields are consistent within errors with no suppression. The systematics of the measured suppression will provide important input to model descriptions from which information about the opacity of the produced matter may be deduced [5].

One of the earliest proposed signatures of the formation of dense deconfined matter (Quark Gluon Plasma) in relativistic heavy ion collisions was the predicted suppression of the J/ yield due to Debye screening of the c-cbar quark bound state in the dense partonic matter [6]. Such suppression was observed in measurements at the CERN SPS [7]. New results from PHENIX show that the J/ suppression is the same for Cu+Cu and Au+Au collisions for centrality selections with the same number of participating nucleons [8]. The amount of suppression increases with the number of participating nucleons and surprisingly is rather similar at RHIC and SPS energies [9]. At SPS energies the J/ yield is suppressed also in p+A collisions. This is interpreted as a “cold nuclear matter” effect as a result of modification of the nucleon parton momentum distributions in the nucleus and breakup of the J/ due to its interaction with the cold spectator nucleons. PHENIX has investigated the cold nuclear matter effects on J/ suppression at RHIC energies using d+Au collisions, with comparison to p+p collisions. A new analysis was recently completed using the Run 5 p+p data set, which is a factor of 10 larger than the Run 3 p+p data set together with the Run 3 d+Au data set. Nevertheless the new results still have large uncertainties that prevent making any firm quantitative statements on any additional suppression in Au+Au collisions beyond cold nuclear matter effects [10]. With the PHENIX Run 8 d+Au data sample just obtained it is estimated that the number of J/‘s accumulated is a factor of 50 greater than for Run 3 which should allow to constrain the contribution from cold nuclear matter effects more strongly.

The measurement of electron-positron pairs allows the study of direct virtual photon production with the advantage that the main background from neutral pion decays (due to internal or external photon conversion) is about two orders of magnitude smaller than for the measurement of real direct photons. An additional order of magnitude reduction of the background to the virtual photon measurement can be obtained by using pairs with mass above the pion mass. The final analysis of the Run 5 data set demonstrates that the measured electron-pair invariant yield mass spectrum for p+p collisions at 200 GeV is in very good agreement with the expected yield from hadronic decays, based on calculations using measured hadron yields [11]. A small but significant excess is observed in the mass region above the pion mass at transverse momenta above 1 GeV/c. This excess can be used to extract the virtual photon momentum spectrum with an error significantly smaller in the low transverse momentum region than obtained using the real photon measurements. The measured invariant photon yield by the virtual photon measurement is found to be consistent with expectations from pQCD predictions.

In the case of Au+Au collisions, the electron-pair mass spectrum shows a very large excess beyond expectations from hadronic decays in the low mass region between the pion and  meson masses, as shown in Figure 1 [12]. This excess is dominantly at transverse momenta below about 1 GeV/c, indicating that it is produced in the cooler late hadronic phase of the collision. Similar to the p+p case, a significant electron-pair excess also persists to higher transverse momenta. The virtual photon yield associated with this observed excess is greater than that expected from the p+p measurement suggesting that it is due to thermal radiation from the early phase of the collision. These measurements hold promise that the thermal photon spectrum may finally be extracted with sufficient precision to provide significant constraints on the initial temperature of the dense matter being created at RHIC.

References

  1. A.Adare et al., (PHENIX Collaboration), arXiv:0801.4020.
  2. K.Adcox et al., (PHENIX Collaboration), Phys. Rev. Lett. 88, 022301 (2002).
  3. S.S.Adler et al., (PHENIX Collaboration), Phys. Rev. Lett. 94, 232301 (2005).
  4. A.Adare et al., (PHENIX Collaboration), arXiv:0801.4555.
  5. A.Adare et al., (PHENIX Collaboration), arXiv:0801.1665.
  6. T.Matsui and H.Satz, Phys. Lett. B 178 (1985) 416.
  7. M.C.Abreu et al., (NA50 Collaboration), Phys. Lett. B 410 (1997) 337.
  8. A.Adare et al., (PHENIX Collaboration), arXiv:0801.0220.
  9. A.Adare et al., (PHENIX Collaboration), Phys. Rev. Lett. 98, 232301 (2007).
  10. A.Adare et al., (PHENIX Collaboration), arXiv:0711.3917.
  11. A.Adare et al., (PHENIX Collaboration), arXiv:0802.0050.
  12. S.Afanasiev et al., (PHENIX Collaboration), arXiv:0706.3034.

Figure 1. Invariant yield of electron-positron pairs compared to the yield expected from hadronic decays. Statistical (bars) and systematic (boxes) uncertainties are shown separately.