Measurement of [phi] meson production in Cu+Cu collisions at 200 GeV using the PHOBOS detector at RHIC

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.In title on title-page "[phi]" appears as the lower-case Greek letter. Cataloged from PDF version of thesis.Includes bibliographical references (p. 245-271).Strong enhancement of production of strange particles, and in particular of [phi] mesons, in heavy ion collisions of sufficiently high energies has been predicted to be an indication of a formation of a new state of matter, composed of deconfined quarks and gluons and having a property of chiral symmetry, called Quark Gluon Plasma (QGP). Studying production of [phi] mesons is of special interest due to their small cross-section of interaction with non-strange hadrons and due to their long lifetime, which should allow [phi] mesons to decouple from the strongly interacting medium produced in heavy ion collisions early in time and to escape the medium before decaying, thereby preserving information about the conditions in which the mesons were produced. In addition, the decay properties of [phi] mesons have been predicted to be modified in a hadronic gas medium. The [phi] -> K+K~ decay is of particular interest since the mass of a [phi] meson in vacuum is very close to the mass of two charged kaons, and consequently, even a small change in the mass or the width of [phi] mesons or in the mass of kaons would have a strong effect on the decay properties. Measurement of [phi] meson production using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC) has proven to be especially challenging due to a small acceptance of the PHOBOS spectrometer and due to a much lower than predicted yield of [phi] mesons in heavy ion collisions at the highest RHIC energy. The measurement required a development of a new tracking algorithm, specifically tailored to reconstruct charged kaons with a high efficiency in a high hit density environment, keeping at the same time the necessary computing time within feasible limits. Results of a measurement of [phi] meson invariant yield in the rapidity interval 0 390 MeV/c), however, since a full analysis of the systematic errors was not performed for the measurement, the results cannot be considered conclusive. The V dependence of the yield of [phi] mesons at mid-rapidity was studied separately and with respect to the charged particle multiplicity, showing no indication of any special interval of js values which could be associated with such a transition (of a QCD matter into a new state) that would produce a noticeable sudden increase in the number of produced [phi] mesons. No evidence of any change in the mean or the width of the [phi] meson invariant mass peak with respect to the vacuum values was observed.by Siarhei Vaurynovich.Ph.D

System size dependence of cluster properties from two-particle angular correlations in Cu+Cu and Au+Au collisions at sNN\sqrt{s_{_{NN}}} = 200 GeV

We present results on two-particle angular correlations in Cu+Cu and Au+Au collisions at a center of mass energy per nucleon pair of 200 GeV over a broad range of pseudorapidity ($\eta$) and azimuthal angle ($\phi$) as a function of collision centrality. The PHOBOS detector at RHIC has a uniquely-large angular coverage for inclusive charged particles, which allows for the study of correlations on both long- and short-range scales. A complex two-dimensional correlation structure in $\Delta \eta$ and $\Delta \phi$ emerges, which is interpreted in the context of a cluster model. The effective cluster size and decay width are extracted from the two-particle pseudorapidity correlation functions. The effective cluster size found in semi-central Cu+Cu and Au+Au collisions is comparable to that found in proton-proton collisions but a non-trivial decrease of the size with increasing centrality is observed. Moreover, a comparison between results from Cu+Cu and Au+Au collisions shows an interesting scaling of the effective cluster size with the measured fraction of total cross section (which is related to the ratio of the impact parameter to the nuclear radius, $b/2R$), suggesting a geometric origin. Further analysis for pairs from restricted azimuthal regions shows that the effective cluster size at $\Delta\phi \sim 180^{\circ}$ drops more rapidly toward central collisions than the size at $\Delta\phi \sim 0^{\circ}$. The effect of limited $\eta$ acceptance on the cluster parameters is also addressed, and a correction is applied to present cluster parameters for full $\eta$ coverage, leading to much larger effective cluster sizes and widths than previously noted in the literature. These results should provide insight into the hot and dense medium created in heavy ion collisions.Comment: 9 pages, 8 figures, Published in Phys. Rev.

Participant and spectator scaling of spectator fragments in Au + Au and Cu + Cu collisions at √sNN = 19.6 and 22.4 GeV

Spectator fragments resulting from relativistic heavy ion collisions, consisting of single protons and neutrons along with groups of stable nuclear fragments up to nitrogen (Z=7), are measured in PHOBOS. These fragments are observed in Au+Au (√sNN =19.6GeV) and Cu+Cu (22.4 GeV) collisions at high pseudorapidity (η). The dominant multiply-charged fragment is the tightly bound helium (α), with lithium, beryllium, and boron all clearly seen as a function of collision centrality and pseudorapidity. We observe that in Cu+Cu collisions, it becomes much more favorable for the α fragments to be released than lithium. The yields of fragments approximately scale with the number of spectator nucleons, independent of the colliding ion. The shapes of the pseudorapidity distributions of fragments indicate that the average deflection of the fragments away from the beam direction increases for more central collisions. A detailed comparison of the shapes for α and lithium fragments indicates that the centrality dependence of the deflections favors a scaling with the number of participants in the collision.United States. Department of Energy (Grant DE-AC02-98CH10886)United States. Department of Energy (Grant DE-FG02-93ER40802)United States. Department of Energy (Grant DE-FG02-94ER40818)United States. Department of Energy (Grant DE-FG02-94ER40865)United States. Department of Energy (Grant DE-FG02- 99ER41099)United States. Department of Energy (Grant DE-AC02-06CH11357)National Science Foundation (U.S.) (Grant 9603486)National Science Foundation (U.S.) (Grant 0072204)National Science Foundation (U.S.) (Grant 0245011

Non-flow correlations and elliptic flow fluctuations in Au+Au collisions at √sNN=200 GeV

This article presents results on event-by-event elliptic flow fluctuations in Au+Au collisions at [sqrt]sNN= 200 GeV, where the contribution from non-flow correlations has been subtracted. An analysis method is introduced to measure non-flow correlations, relying on the assumption that non-flow correlations are most prominent at short ranges (|Δeta|2), relative elliptic flow fluctuations of approximately 30–40% are observed. These results are consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. It is found that the long-range non-flow correlations in Au+Au collisions would have to be more than an order of magnitude strongercompared to the p+p data to lead to the observed azimuthal anisotropy fluctuations with no intrinsic elliptic flow fluctuations.United States. Department of Energy (Grants DE-AC02-98CH10886, DE-FG02-93ER40802, DE-FG02- 94ER40818, DE-FG02-94ER40865, DE-FG02-99ER41099, and DE-AC02-06CH11357)National Science Foundation (Grants 9603486, 0072204, and 0245011)Polish Ministry of Science and Higher Education (Grant N N202 282234 (2008-2010))National Science Council of Taiwan (Contract NSC 89-2112-M-008-024)Hungarian Scientific Research Foundation (grant F 049823

High transverse momentum triggered correlations over a large pseudorapidity acceptance in Au+Au collisions at sqrt(s_NN)=200 GeV

A measurement of two-particle correlations with a high transverse momentum trigger particle (p[subscript T][superscript trig]>2.5  GeV/c) is presented for Au+Au collisions at √sNN=200   GeV over the uniquely broad longitudinal acceptance of the PHOBOS detector (-4<Δη<2). A broadening of the away-side azimuthal correlation compared to elementary collisions is observed at all Δη. As in p+p collisions, the near side is characterized by a peak of correlated partners at small angle relative to the trigger particle. However, in central Au+Au collisions an additional correlation extended in Δη and known as the “ridge” is found to reach at least |Δη|≈4. The ridge yield is largely independent of Δη over the measured range, and it decreases towards more peripheral collisions. For the chosen p[subscript T][superscript trig] cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.United States. Dept. of Energy (Grants No. DE-AC02-98CH10886, No. DE-FG02-93ER40802, No. DE-FG02-94ER40818, No. DE-FG02-94ER40865, No. DE-FG02-99ER41099, and No. DE-AC02-06CH11357)National Science Foundation (Grants No. 9603486, No. 0072204, and No. 0245011)Polish MNiSW (Grant No. N202 282234 (2008–2010)NSC of Taiwan (Contract No. NSC 89-2112-M-008-024)Hungarian OTKA (Grant No. F 049823

Commissioning and performance of the CMS silicon strip tracker with cosmic ray muons

During autumn 2008, the Silicon Strip Tracker was operated with the full CMS experiment in a comprehensive test, in the presence of the 3.8 T magnetic field produced by the CMS superconducting solenoid. Cosmic ray muons were detected in the muon chambers and used to trigger the readout of all CMS sub-detectors. About 15 million events with a muon in the tracker were collected. The efficiency of hit and track reconstruction were measured to be higher than 99% and consistent with expectations from Monte Carlo simulation. This article details the commissioning and performance of the Silicon Strip Tracker with cosmic ray muons

Alignment of the CMS muon system with cosmic-ray and beam-halo muons

The CMS muon system has been aligned using cosmic-ray muons collected in 2008 and beam-halo muons from the 2008 LHC circulating beam tests. After alignment, the resolution of the most sensitive coordinate is 80 microns for the relative positions of superlayers in the same barrel chamber and 270 microns for the relative positions of endcap chambers in the same ring structure. The resolution on the position of the central barrel chambers relative to the tracker is comprised between two extreme estimates, 200 and 700 microns, provided by two complementary studies. With minor modifications, the alignment procedures can be applied using muons from LHC collisions, leading to additional significant improvements

Alignment of the CMS silicon tracker during commissioning with cosmic rays

The CMS silicon tracker, consisting of 1440 silicon pixel and 15 148 silicon strip detector modules, has been aligned using more than three million cosmic ray charged particles, with additional information from optical surveys. The positions of the modules were determined with respect to cosmic ray trajectories to an average precision of 3-4 microns RMS in the barrel and 3-14 microns RMS in the endcap in the most sensitive coordinate. The results have been validated by several studies, including laser beam cross-checks, track fit self-consistency, track residuals in overlapping module regions, and track parameter resolution, and are compared with predictions obtained from simulation. Correlated systematic effects have been investigated. The track parameter resolutions obtained with this alignment are close to the design performance

Performance of CMS muon reconstruction in cosmic-ray events

The performance of muon reconstruction in CMS is evaluated using a large data sample of cosmic-ray muons recorded in 2008. Efficiencies of various high-level trigger, identification, and reconstruction algorithms have been measured for a broad range of muon momenta, and were found to be in good agreement with expectations from Monte Carlo simulation. The relative momentum resolution for muons crossing the barrel part of the detector is better than 1% at 10 GeV/c and is about 8% at 500 GeV/c, the latter being only a factor of two worse than expected with ideal alignment conditions. Muon charge misassignment ranges from less than 0.01% at 10GeV/c to about 1% at 500 GeV/c

Commissioning and performance of the CMS pixel tracker with cosmic ray muons

The pixel detector of the Compact Muon Solenoid experiment consists of three barrel layers and two disks for each endcap. The detector was installed in summer 2008, commissioned with charge injections, and operated in the 3.8 T magnetic field during cosmic ray data taking. This paper reports on the first running experience and presents results on the pixel tracker performance, which are found to be in line with the design specifications of this detector. The transverse impact parameter resolution measured in a sample of high momentum muons is 18 microns