Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session Q7: Mini-Symposium: Exotic Phenomena in High Energy Nuclear Collisions |
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Sponsoring Units: DNP Chair: Dmitri Kharzeev, Brookhaven National Laboratory Room: Delaware A |
Monday, February 15, 2010 1:30PM - 2:06PM |
Q7.00001: Observation of charge-dependent azimuthal correlations and possible local strong parity violation in heavy ion collisions Invited Speaker: Parity-odd domains, corresponding to non-trivial topological solutions of the QCD vacuum, might be created during relativistic heavy ion collisions. These domains are predicted to lead to charge separation of quarks along the orbital momentum of the system created in non-central collisions. To study this effect, we investigate a three particle mixed harmonics azimuthal correlator which is a \P~even observable, but directly sensitive to the charge separation effect. We report measurements of this observable using the STAR detector in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$=200 and 62~GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. A signal consistent with several of the theoretical expectations is detected in all four data sets. We compare our results to the predictions of existing event generators, and discuss in detail possible contributions from other effects that are not related to parity violation. [Preview Abstract] |
Monday, February 15, 2010 2:06PM - 2:18PM |
Q7.00002: Hadronic and partonic mechanisms for a reaction plane charge asymmetry Berndt Mueller, Masayuki Asakawa, Abhijit Majumder We discuss several mechanisms for the creation of a charge asymmetry with respect to the reaction plane among hadrons emitted in relativistic heavy-ion collisions. We show that such mechanisms exist in both, the hadron gas and the quark-gluon plasma phase. All such mechanisms have in common that they require the presence of a strong magnetic field (the ``chiral magnetic effect''), but they do not involve P or PC symmetry violations. We analyze how a transient local electric current generated by the chiral magnetic effect can dynamically evolve into a charge asymmetry of the final-state hadron distribution in momentum space. We estimate the magnitude of the event-by-event fluctuations of the final-state charge asymmetry due to the partonic and hadronic mechanisms. [Preview Abstract] |
Monday, February 15, 2010 2:18PM - 2:30PM |
Q7.00003: Effect of Cluster Particle Correlation on Local Parity Violation Observables Fuqiang Wang It has been suggested that metastable domains may form in chiral symmetry restored quark gluon plasma where the parity and time-reversal symmetries are locally violated. Such violation would lead to back-to-back opposite-charge massless quark pairs by the chiral magnetic effect along the system's orbital angular momentum. This may lead to observable consequences of final state particle correlations. The recent measurements of particle azimuth correlator by the STAR experiment are consistent with some of the expectations from local parity violation~[1]. However, the correlator observable is parity-even and is subject to background correlations, some of which are discussed in Ref.~[1]. In this talk, we will investigate effects of particle correlations due to clusters on the azimuth correlator. We use two-particle angular correlation measurements as input. We found that the correlator measurements by STAR can be accounted for by cluster particle correlations together with cluster anisotropies in medium-central to central Au+Au collisions. Our results suggest that local strong parity violation may not be required to explain the correlator data. \\[4pt] [1] B.I.~Abelev et al.~(STAR Collaboration), arXiv:0909.1717, arXiv:0909.1739. [Preview Abstract] |
Monday, February 15, 2010 2:30PM - 2:42PM |
Q7.00004: Search for Local Strong Parity Violation Using Reaction Plane Determined by Spectator Neutrons in STAR Gang Wang Parity-odd domains [1] are predicted to lead to charge separation of quarks along the orbital momentum of the system created in non-central relativistic heavy ion collisions [2]. A signal consistent with several of the theoretical expectations has been detected by STAR [3], with a three particle azimuthal correlator, a P-even observable, but sensitive to the charge separation effect. In this correlator, the first two particles reveal the physics of interest, and the third particle serves as a reference for the reaction plane. To minimize the non-parity correlation between the three particles, we utilize the spectator neutrons detected by STAR ZDC-SMDs to define the event plane. The 1st- order event plane thus obtained can also be used to study the global strong parity violation effects. In this work, we report measurements of both P-even and P-odd observables using the STAR ZDC-SMDs in Au+Au at 200 GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. Good consistency has been observed between current and previous results [3]. Systematic checks on the impact from the directed flow will also be presented. [1] T. D. Lee, Phys. Rev. D 8, 1226 (1973). [2] D. Kharzeev, Phys. Lett. B 633, 260 (2006). [3] B. I. Abelev et al., accepted by Phys. Rev. Lett. [arXiv:0909.1739]. [Preview Abstract] |
Monday, February 15, 2010 2:42PM - 2:54PM |
Q7.00005: Measurement of multiplicity asymmetry correlation between positive and negative particles in Au+Au and d+Au collisions by STAR Quan Wang It has been suggested that metastable domains may form in chiral symmetry restored quark qluon plasma where the parity and time-reversal symmetries are locally violated. Such violation would lead to separation of positive and negative particles by the chiral magnetic effect into the two hemispheres along the system's orbital angular momentum direction. A direct consequence of this separation is a reduction in correlation present in the event between the multiplicity asymmetry of positive particles in the up and down hemispheres separated by the reaction plane ($A_{+}^{UD}$) and that of negative particles ($A_{-}^{UD}$). Such a reduction can be gauged by comparing the up-down asymmetry correlation ($\langle A_{+}^{UD}A_{-}^{UD}\rangle$) to that in the left and right hemispheres separated by the plane perpendicular to the reaction plane ($\langle A_{+}^{LR}A_{-}^{LR}\rangle$). The latter is our null-reference because local parity violation does not contribute to this asymmetry correlation. In this talk, we will report results on both $\langle A_{+}^{UD}A_{-}^{UD}\rangle$ and $\langle A_{+}^{LR}A_{-}^{LR}\rangle$ in Au+Au as well as d+Au collisions at 200 GeV at RHIC by the STAR experiment. The physics implications of our results will be discussed. [Preview Abstract] |
Monday, February 15, 2010 2:54PM - 3:06PM |
Q7.00006: A fundamental test of the Higgs Yukawa coupling in Relativistic Heavy Ion Collisions Michael Tannenbaum Hard-scattering of point-like constituents (or partons) in p-p collisions was discovered in 1972 by experiments utilizing inclusive single or pairs of hadrons with large transverse momentum $p_T > 2-5$ GeV/c. Similar measurements in Au+Au collisions at RHIC revealed suppression by a factor of 5 relative to point-like scattering for nearly all measured identified hadrons such as $\pi^0$ and $\eta$ with $p_T>2-5$ GeV/c but no-suppression for direct-$\gamma$ production. This indicated a strong medium effect on outgoing partons suggestive of the predicted LPM radiative energy loss in a QGP. Evidence for such energy loss was provided by the away side conditional yield of associated particles, with $p_{T_a}$, from a trigger $\pi^0$, with $p_{T_t}$, the $x_E\sim p_{T_a}/p_{T_t}$ distribution. The ratio of the $x_E$ distributions for a given $p_{T_t}$ in A+A to p-p collisions, $I_{AA}(x_E)$, shows an exponential drop at low $x_E$, indicating energy loss. However, observation that direct-electrons from the decay of heavy quarks are also suppressed by a factor of 5 for $p_{T_e}>5$ GeV/c in Au+Au raised doubts about radiative energy loss and has attracted more exotic explanations. For instance if the Higgs mechanism gives mass to gauge bosons but not to fermions then a proposal that all 6 quarks are nearly massless in a QGP could explain the apparent equal suppression of light and heavy quarks. This proposal can be tested with future measurements of $b-\bar{b}$ correlations in Au+Au collisions. [Preview Abstract] |
Monday, February 15, 2010 3:06PM - 3:18PM |
Q7.00007: Strangelets at the LHC E. Norbeck, Y. Onel The strangelets considered are nuclei containing several strange quarks. In high-energy heavy-ion collisions, the reaction region consists of a quark-gluon plasma (QGP) in the overlap part and spectators in the non-overlap part. The QGP contains an abundance of strange quarks but is much too hot to produce bound nuclear systems. With a sufficiently large impact parameter, spectators can end up as bound nuclear systems. It can be expected that some strange quarks would cross the boundary between the QGP and the spectators to make spectators into strangelets. These strangelets will follow the outgoing beam. Eventually, dipole magnets will deflect them out of the beam because of their different Z/A ratio. Strangelets, or their decay products, with Z/A sufficiently less than the beam would be seen by the CMS Zero Degree Calorimeter, which is divided into five sections in the horizontal direction. Objects, thought to be strangelets, are seen in cosmic ray showers. These could be formed by cosmic iron nuclei on atmospheric nitrogen at center of mass energies comparable to LHC energies. For cosmic events the A value of the strangelets would be some fraction of $\sim $55. For PbPb the A would be some fraction of 208. [Preview Abstract] |
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