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.

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.

Pseudorapidity distributions of charged particles emitted in $Au+Au$, $Cu+Cu$, $d+Au$, and $p+p$ collisions over a wide energy range have been measured using the PHOBOS detector at RHIC. The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with $|\eta|<5.4$, which account for between 95% and 99% of the total charged-particle emission associated with collision participants, are presented for different collision centralities. Both the midrapidity density, $dN_{ch}/d\eta$, and the total charged-particle multiplicity, $N_{ch}$, are found to factorize into a product of independent functions of collision energy, $\sqrt{s_{_{NN}}}$, and centrality given in terms of the number of nucleons participating in the collision, $N_{part}$. The total charged particle multiplicity, observed in these experiments and those at lower energies, assumes a linear dependence of $(\ln s_{_{NN}})^2$ over the full range of collision energy of $\sqrt{s_{_{NN}}}$=2.7-200 GeV.

for the PHOBOS Collaboration: B.Alver4, B.B.Back1, M.D.Baker2, M.Ballintijn4, D.S.Barton2, R.R.Betts 6, A.A.Bickley7, R.Bindel 7, W.Busza 4, A.Carroll2, Z.Chai2, V.Chetluru6, M.P.Decowski 4, E.García6, T.Gburek3, N.George2, K.Gulbrandsen 4, C.Halliwell6, J.Hamblen8, I.Harnarine6, M.Hauer2, C.Henderson 4, D.J.Hofman6, R.S.Hollis6, R.Hołyński 3, B.Holzman2, A.Iordanova6, E.Johnson 8, J.L.Kane4, N.Khan8, P.Kulinich4, C.M.Kuo5, W.Li4, W.T.Lin5, C.Loizides4, S.Manly8, A.C.Mignerey7, R.Nouicer 2, A.Olszewski 3, R.Pak2, C.Reed4, E.Richardson 7, C.Roland4, G.Roland4, J.Sagerer 6, H.Seals2, I.Sedykh2, C.E.Smith6, M.A.Stankiewicz 2, P.Steinberg 2, G.S.F.Stephans 4, A.Sukhanov 2, A.Szostak 2, M.B.Tonjes7, A.Trzupek 3, C.Vale4, G.J.van Nieuwenhuizen 4, S.S.Vaurynovich 4, R.Verdier 4, G.I.Veres4, P.Walters8, E.Wenger 4, D.Willhelm7, F.L.H.Wolfs8, B.Wosiek 3, K.Woźniak3, S.Wyngaardt 2, B.Wysłouch 4

This Letter presents the first measurement of event-by-event fluctuations of the elliptic flow parameter v(2) in Au+Au collisions at square root(s(NN))=200 GeV as a function of collision centrality. The relative nonstatistical fluctuations of the v(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.

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)

Low temperature magnetic properties of Cd-doped Ce2MIn8 (M = Rh and Ir) single crystals are investigated. Experiments of temperature dependent magnetic susceptibility, heat capacity and electrical resistivity measurements revealed that Cd-doping enhances the antiferromagnetic (AFM) ordering temperature from TN = 2.8 K (x = 0) to TN = 4.8 K (x = 0.21) for Ce2RhIn8-xCdx and induces long range AFM ordering with TN = 3.8 K (x = 0.21) for Ce2IrIn8-xCdx. Additionally, X-ray and neutron magnetic scattering studies showed that Cd-doped samples present below TN a commensurate antiferromagnetic structure with a propagation vector (1/2,1/2,0). The resolved magnetic structures for both compounds indicate that the Cd-doping tends to rotate the direction of the ordered magnetic moments toward the ab-plane. This result suggests that the Cd-doping affects the Ce3+ ground state single ion anisotropy modifying the crystalline electrical field (CEF) parameters at the Ce3+ site. Indications of CEF evolution induced by Cd-doping were also found in the electrical resistivity measurements. Comparisons between our results and the general effects of Cd-doping on the related compounds CeMIn5 (M = Co, Rh and Ir) confirms the claims that the Cd-doping induced electronic tuning is the main effect favoring AFM ordering in these compounds.

United States Department of Energy (grants DE-AC02-98CH10886, DE-FG02-93ER40802, DEFG02- 94ER40818, DE-FG02-94ER40865, DE-FG02- 99ER41099, and DE-AC02-06CH11357)

B. Alver, B. B. Back, M.D. Baker, M. Ballintijn, D. S. Barton, R. R. Betts, A.A. Bickley, R. Bindel, W. Busza, A. Carroll, Z. Chai, V. Chetluru, M. P. Decowski, E. Garcı́a, T. Gburek, N. George, K. Gulbrandsen, C. Halliwell, J. Hamblen, M. Hauer, C. Henderson, D. J. Hofman, R. S. Hollis, R. Hołyński, B. Holzman, A. Iordanova, E. Johnson, J. L. Kane, N. Khan, P. Kulinich, C.M. Kuo, W. Li, W. T. Lin, C. Loizides, S. Manly, A. C. Mignerey, R. Nouicer, A. Olszewski, R. Pak, C. Reed, C. Roland, G. Roland, J. Sagerer, H. Seals, I. Sedykh, C. E. Smith, M.A. Stankiewicz, P. Steinberg, G. S. F. Stephans, A. Sukhanov, M.B. Tonjes, A. Trzupek, C. Vale, G. J. van Nieuwenhuizen, S. S. Vaurynovich, R. Verdier, G. I. Veres, P. Walters, E. Wenger, F. L. H. Wolfs, B. Wosiek, K. Woźniak, and B. Wysłouch Argonne National Laboratory, Argonne, Illinois 60439-4843, USA Brookhaven National Laboratory, Upton, New York 11973-5000, USA Institute of Nuclear Physics PAN, Kraków, Poland Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA National Central University, Chung-Li, Taiwan University of Illinois at Chicago, Chicago, Illinois 60607-7059, USA University of Maryland, College Park, Maryland 20742, USA University of Rochester, Rochester, New York 14627, USA (Received 16 March 2009; revised manuscript received 24 December 2009; published 11 February 2010)

This paper presents results on event-by-event elliptic flow fluctuations in Au+Au collisions at sqrt(s_NN)=200Gev, 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 (Delta eta < 2). Assuming that non-flow correlations are of the order that is observed in p+p collisions for long range correlations (Delta eta > 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 stronger compared to the p+p data to lead to the observed azimuthal anisotropy fluctuations with no intrinsic elliptic flow fluctuations.

A measurement of two-particle correlations with a high transverse momentum trigger particle (p(T)(trig) > 2.5 GeV/c) is presented for Au+Au collisions at square root(s(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| approximately = 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(T)(trig) cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.

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) values as a function of collision centrality. The PHOBOS detector at the Relativistic Heavy Ion Collider 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 semicentral Cu + Cu and Au + Au collisions is comparable to that found in proton-proton collisions but a nontrivial decrease in size with increasing centrality is observed. Moreover, a comparison of results from Cu + Cu versus 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 similar to 180 degrees drops more rapidly toward central collisions than the size at Delta phi similar to 0 degrees. 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.

A measurement of two-particle correlations with a high transverse momentum trigger particle (p trig T > 2.5 GeV/c) is presented for Au+Au collisions at √ s N N =200 GeV over the uniquely broad longitudinal acceptance of the PHOBOS detector (-4 < ∆η < 2).A broadening of the away-side azimuthal correlation compared to elementary collisions is observed at all ∆η.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 ∆η and known as the 'ridge' is found to reach at least |∆η| ≈ 4. The ridge yield is largely independent of ∆η over the measured range, and it decreases towards more peripheral collisions.For the chosen p trig T cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.

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 root 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.

We present the first measurements of the pseudorapidity distribution of primary charged particles in Cu+Cu collisions as a function of collision centrality and energy, sqrt[s_{NN}]=22.4, 62.4, and 200 GeV, over a wide range of pseudorapidity, using the PHOBOS detector. A comparison of Cu+Cu and Au+Au results shows that the total number of produced charged particles and the rough shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants. More detailed studies reveal that a more precise matching of the shape of the Cu+Cu and Au+Au pseudorapidity distributions over the full range of pseudorapidity occurs for the same N{part}/2A rather than the same N_{part}. In other words, it is the collision geometry rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence at RHIC energies.

The centrality dependence of the midrapidity charged-particle multiplicity density (vertical bar eta vertical bar < 1) is presented for Au+Au and Cu+Cu collisions at RHIC over a broad range of collision energies. The multiplicity measured in the Cu+Cu system is found to be similar to that measured in the Au+Au system, for an equivalent N-part, with the observed factorization in energy and centrality still persistent in the smaller Cu+Cu system. The extent of the similarities observed for bulk particle production is tested by a comparative analysis of the inclusive transverse momentum distributions for Au+Au and Cu+Cu collisions near midrapidity. It is found that, within the uncertainties of the data, the ratio of yields between the various energies for both Au+Au and Cu+Cu systems are similar and constant with centrality, both in the bulk yields and as a function of p(T), up to at least 4 GeV/c. The effects of multiple nucleon collisions that strongly increase with centrality and energy appear to only play a minor role in bulk and intermediate transverse momentum particle production.