This Letter presents the first measurement of event-by-event fluctuations of the elliptic flow parameter v{sub 2} in Au+Au collisions at {radical}(s{sub NN})=200 GeV as a function of collision ...centrality. The relative nonstatistical fluctuations of the v{sub 2} parameter are found to be approximately 40%. The results, including contributions from event-by-event elliptic flow fluctuations and from azimuthal correlations that are unrelated to the reaction plane (nonflow correlations), establish an upper limit on the magnitude of underlying elliptic flow fluctuations. This limit is consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. These results provide important constraints on models of the initial state and hydrodynamic evolution of relativistic heavy ion collisions.
A measurement of two-particle correlations with a high transverse momentum trigger particle (p{sub T}{sup trig}>2.5 GeV/c) is presented for Au+Au collisions at sq root(s{sub NN})=200 GeV over the ...uniquely broad longitudinal acceptance of the PHOBOS detector (-4<{Delta}{eta}<2). A broadening of the away-side azimuthal correlation compared to elementary collisions is observed at all {Delta}{eta}. 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 {Delta}{eta} and known as the 'ridge' is found to reach at least |{Delta}{eta}|approx =4. The ridge yield is largely independent of {Delta}{eta} over the measured range, and it decreases towards more peripheral collisions. For the chosen p{sub T}{sup trig} cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.
A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of ...charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity (Δ
η
) and azimuthal angle (Δ
φ
) correlations between high transverse momentum charged hadrons (
p
T
>2.5 GeV/
c
) and all associated charged particles is presented at both short-range (small Δ
η
) and long-range (large Δ
η
) over a continuous detector acceptance covering −4<Δ
η
<2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au + Au collisions at
GeV. The results provide new information about the longitudinal (Δ
η
) extent of the near-side ‘ridge’ structure, first observed by the STAR Collaboration over a narrower
η
range. In central Au + Au collisions the ridge structure extends to at least Δ
η
=4, and its strength completely diminishes as collisions become more peripheral.
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 (sqrt(sNN)=19.6 GeV) and Cu+Cu (22.4 GeV) collisions at high pseudorapidity (\(\eta\)). The dominant multiply-charged fragment is the tightly bound Helium (\(\alpha\)), 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 \(\alpha\) 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 \(\alpha\) and Lithium fragments indicates that the centrality dependence of the deflections favors a scaling with the number of participants in the collision.
A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of ...charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity (Δη) and azimuthal angle (Δφ) correlations between high transverse momentum charged hadrons (pT>2.5 GeV/c) and all associated charged particles is presented at both short-range (small Δη) and long-range (large Δη) over a continuous detector acceptance covering −4<Δη<2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au + Au collisions at \(\sqrt{s_{NN}}=200\) GeV. The results provide new information about the longitudinal (Δη) extent of the near-side ‘ridge’ structure, first observed by the STAR Collaboration over a narrower η range. In central Au + Au collisions the ridge structure extends to at least Δη=4, and its strength completely diminishes as collisions become more peripheral.
Forward calorimetry in the PHOBOS detector has been used to study charged hadron production in d+Au, p+Au and n+Au collisions at sqrt(s_nn) = 200 GeV. The forward proton calorimeter detectors are ...described and a procedure for determining collision centrality with these detectors is detailed. The deposition of energy by deuteron spectator nucleons in the forward calorimeters is used to identify p+Au and n+Au collisions in the data. A weighted combination of the yield of p+Au and n+Au is constructed to build a reference for Au+Au collisions that better matches the isospin composition of the gold nucleus. The p_T and centrality dependence of the yield of this improved reference system is found to match that of d+Au. The shape of the charged particle transverse momentum distribution is observed to extrapolate smoothly from pbar+p to central d+Au as a function of the charged particle pseudorapidity density. The asymmetry of positively- and negatively-charged hadron production in p+Au is compared to that of n+Au. No significant asymmetry is observed at mid-rapidity. These studies augment recent results from experiments at the LHC and RHIC facilities to give a more complete description of particle production in p+A and d+A collisions, essential for the understanding the medium produced in high energy nucleus-nucleus collisions.
A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of ...charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity ({Delta}{eta}) and azimuthal angle ({Delta}{phi}) correlations between high transverse momentum charged hadrons (p{sub T} > 2.5 GeV/c) and all associated charged particles is presented at both short-range (small {Delta}{eta}) and long-range (large {Delta}{eta}) over a continuous detector acceptance covering -4 < {Delta}{eta} < 2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au+Au collisions at {radical}s{sub NN} = 200 GeV. The results provide new information about the longitudinal ({Delta}{eta}) extent of the near-side 'ridge' structure, first observed by the STAR Collaboration over a narrower {eta} range. In central Au+Au collisions the ridge structure extends to at least {Delta}{eta} = 4, and its strength completely diminishes as collisions become more peripheral.
Recently PHOBOS has focused on the study of fluctuations and correlations in particle production in heavy-ion collisions at the highest energies delivered by the Relativistic Heavy Ion Collider ...(RHIC). In this report, we present results on event-by-event elliptic flow fluctuations in Au + Au collisions at √s NN =200 GeV. A data-driven method was used to estimate the dominant contribution from non-flow correlations. Over the broad range of collision centralities, the observed large elliptic flow fluctuations are in agreement with the fluctuations in the initial source eccentricity.
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