Particle Production and Collectivity in High Multiplicity pp and pPb at the LHC

Recent unexpected evidence for collectivity in high multiplicity pp and pPb collisions at LHC energies has challenged the notion that such small systems do not exhibit any of the properties that have been used to study the quark gluon plasma in heavy ion collisions. An overview of recent results concerning particle production and collectivity in such collisions using the CMS detector at the LHC is presented

Energy and centrality dependence of particle multiplicity in heavy ion collisions from sNN\sqrt{s_{_{NN}}} = 20 to 2760 GeV

The centrality dependence of midrapidity charged-particle multiplicities at a nucleon-nucleon center-of-mass energy of 2.76 TeV from CMS are compared to PHOBOS data at 200 and 19.6 GeV. The results are first fitted with a two-component model which parameterizes the separate contributions of nucleon participants and nucleon-nucleon collisions. A more direct comparison involves ratios of multiplicity densities per participant pair between the different collision energies. The results support and extend earlier indications that the influences of centrality and collision energy on midrapidity charged-particle multiplicities are to a large degree independent.Comment: 5 pages, 2 figures, 1 table, Replaced with published version, v3 has fixed typ

Centrality Dependence of Two-Particle Correlations in Heavy Ion Collisions

Data from the PHOBOS detector have been used to study two-particle correlations over a broad range of pseudorapidity. A simple cluster model parameterization has been applied to inclusive two-particle correlations over a range of centrality for both Cu+Cu and Au+Au collisions at sqrt(s_NN)=200GeV. Analysis of the data for Au+Au has recently been extended to more peripheral collisions showing that the previously-observed rise in cluster size with decreasing system size eventually reaches a maximum value. Model studies have been used to quantify the significant effect of limited detector acceptance on the extracted cluster parameters. In the case of Au+Au, correlations between a trigger particle with pT>2.5GeV and inclusive associated particles have also been studied. These reveal the presence of a `ridge' at small relative azimuthal angle which extends with roughly constant amplitude out to the largest relative pseudorapidity studied. The large phase-space coverage of the PHOBOS detector has enabled a quantitative understanding of the so-called `ZYAM' parameter used in the subtraction of the contribution of elliptic flow to these triggered correlations.Comment: 4 pages, 4 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee (version 2: No changes from version 1 other than removing line numbers, version 3: Added full author list and reformatted slightly to keep same number of pages

Evidence of Final-State Suppression of High-p_T Hadrons in Au + Au Collisions Using d + Au Measurements at RHIC

Transverse momentum spectra of charged hadrons with ${p_{T} <}$ 6 GeV/c have been measured near mid-rapidity (0.2 $< \eta <$ 1.4) by the PHOBOS experiment at RHIC in Au + Au and d + Au collisions at ${\sqrt{s_{_{NN}}} = \rm {200 GeV}}$. The spectra for different collision centralities are compared to ${p + \bar{p}}$ collisions at the same energy. The resulting nuclear modification factor for central Au + Au collisions shows evidence of strong suppression of charged hadrons in the high-$p_{T}$ region (${>2}$ GeV/c). In contrast, the d + Au nuclear modification factor exhibits no suppression of the high-$p_{T}$ yields. These measurements suggest a large energy loss of the high-$p_{T}$ particles in the highly interacting medium created in the central Au + Au collisions. The lack of suppression in d + Au collisions suggests that it is unlikely that initial state effects can explain the suppression in the central Au + Au collisions.Comment: 3 pages, 4 figures, International Europhysics Conference on High Energy Physics EPS (July 17th-23rd 2003) in Aachen, German

Centrality dependence of charged antiparticle to particle ratios near mid-rapidity in d+Au collisions at sqrt(s_NN)=200 GeV

The ratios of the yields of charged antiparticles to particles have been obtained for pions, kaons, and protons near mid-rapidity for d+Au collisions at sqrt(s_NN) = 200 GeV as a function of centrality. The reported values represent the ratio of the yields averaged over the rapidity range of 0.1<y_pi<1.3 and 0<y_(K,p)<0.8, where positive rapidity is in the deuteron direction, and for transverse momenta 0.1<p_(T)^(pi,K)<1.0 GeV/c and 0.3<p_(T)^(p)<1.0 GeV/c. Within the uncertainties, a lack of centrality dependence is observed in all three ratios. The data are compared to results from other systems and model calculations.Comment: 6 pages, 4 figures, submitted to PR

System Size, Energy, Pseudorapidity, and Centrality Dependence of Elliptic Flow

This paper presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events. For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects. A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system

Latest Results from PHOBOS

This manuscript contains a summary of the latest physics results from PHOBOS, as reported at Quark Matter 2006. Highlights include the first measurement from PHOBOS of dynamical elliptic flow fluctuations as well as an explanation of their possible origin, two-particle correlations, identified particle ratios, identified particle spectra and the latest results in global charged particle production.Comment: 9 pages, 7 figures, PHOBOS plenary proceedings for Quark Matter 200

System size, energy, centrality and pseudorapidity dependence of charged-particle density in Au+Au and Cu+Cu collisions at RHIC

Charged particle pseudorapidity distributions are presented from the PHOBOS experiment at RHIC, measured in Au+Au and Cu+Cu collisions at sqrt{s_NN}=19.6, 22.4, 62.4, 130 and 200 GeV, as a function of collision centrality. The presentation includes the recently analyzed Cu+Cu data at 22.4 GeV. The measurements were made by the same detector setup over a broad range in pseudorapidity, |eta|<5.4, allowing for a reliable systematic study of particle production as a function of energy, centrality and system size. Comparing Cu+Cu and Au+Au results, we find that the total number of produced charged particles and the overall shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants, N_part. Detailed comparisons reveal that the matching of the shape of the Cu+Cu and Au+Au pseudorapidity distributions over the full range of eta is better for the same N_part/2A value than for the same N_part value, where A denotes the mass number. In other words, it is the geometry of the nuclear overlap zone, rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence.Comment: 5 pages, 4 figures. Presented at the 20th International Conference on Nucleus-Nucleus Collisions (Quark Matter 2008), Jaipur, Rajasthan, India, 4-10 February 200

Centrality Dependence of Charged Particle Multiplicity at Mid-Rapidity in Au+Au Collisions at sqrt(s_NN) = 130 GeV

We present a measurement of the pseudorapidity density of primary charged particles near mid-rapidity in Au+Au collisions at sqrt(s_NN) = 130 GeV as a function of the number of participating nucleons. These results are compared to models in an attempt to discriminate between competing scenarios of particle production in heavy ion collisions.Comment: 5 pages, 4 figures, revtex (submitted to Phys. Rev. Letters

Recent Results from PHOBOS at RHIC

The PHOBOS experiment at RHIC has recorded measurements for Au-Au collisions spanning nucleon-nucleon center-of-mass energies from 19.6 GeV to 200 GeV. Global observables such as elliptic flow and charged particle multiplicity provide important constraints on model predictions that characterize the state of matter produced in these collisions. The nearly 4 pi acceptance of the PHOBOS experiment provides excellent coverage for complete flow and multiplicity measurements. Results including beam energy and centrality dependencies are presented and compared to elementary systems.Comment: 4 pages, 4 figures, proceedings from PANIC02 in Osaka, Japa