Neutral pion production with respect to reaction plane in Au+Au collisions at RHIC-PHENIX

It has been observed that the yield of neutral pions at high transverse momentum (pT $>$ 5 GeV/c) region is strongly suppressed in central Au+Au collisions at Relativistic Heavy Ion Collider (RHIC), compared to the one expected in p+p collisions. This suppression may be due to an energy loss of hard scattered partons in the medium (jet quenching), that results in a decrease of the yield at a given pT. The magnitude of the suppression would depend on the path length of scattering partons in the medium, and therefore is associated with azimuthal angle from reaction plane in non-central collisions. Studying the path length dependence of energy loss would give additional information on understanding the energy loss mechanism. We discuss the parton energy loss mechanism using the nuclear modification factor ($R_{AA}$) of neutral pion with respect to reaction plane. A new reaction plane detector was installed in the PHENIX detector in RHIC Year-7 run, and improved the reaction plane resolution. More precise measurement of the hadron suppression with respect to path length is expected using the detector. I will report about analysis status of neutral pion production in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.Comment: 4 pages, 5 figures, Proceedings of the 20th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, "Quark Matter 2008", Jaipur, India, February 4-10, 200

Low-mass dark matter search with CDMSlite

The SuperCDMS experiment is designed to directly detect weakly interacting massive particles (WIMPs) that may constitute the dark matter in our Galaxy. During its operation at the Soudan Underground Laboratory, germanium detectors were run in the CDMSlite mode to gather data sets with sensitivity specifically for WIMPs with masses < 10 GeV/c^2. In this mode, a higher detector-bias voltage is applied to amplify the phonon signals produced by drifting charges. This paper presents studies of the experimental noise and its effect on the achievable energy threshold, which is demonstrated to be as low as 56 eV_ee (electron equivalent energy). The detector-biasing configuration is described in detail, with analysis corrections for voltage variations to the level of a few percent. Detailed studies of the electric-field geometry, and the resulting successful development of a fiducial parameter, eliminate poorly measured events, yielding an energy resolution ranging from ∼9 e V_(ee) at 0 keV to 101 e V_(ee) at ∼ 10 ke V_(ee). New results are derived for astrophysical uncertainties relevant to the WIMP-search limits, specifically examining how they are affected by variations in the most probable WIMP velocity and the Galactic escape velocity. These variations become more mportant for WIMP masses below 10 Ge V /c^2. Finally, new limits on spin-dependent low-mass WIMP-nucleon interactions are derived, with new parameter space excluded for WIMP masses ≲ 3 Ge V /c^2