412 research outputs found
Do p+p Collisions Flow at RHIC? Understanding One-Particle Distributions, Multiplicity Evolution, and Conservation Laws
Collective, explosive flow in central heavy ion collisions manifests itself
in the mass dependence of distributions and femtoscopic length scales,
measured in the soft sector ( GeV/c). Measured
distributions from proton-proton collisions differ significantly from those
from heavy ion collisions. This has been taken as evidence that p+p collisions
generate little collective flow, a conclusion in line with naive expectations.
We point out possible hazards of ignoring phase-space restrictions due to
conservation laws when comparing high- and low-multiplicity final states.
Already in two-particle correlation functions, we see clear signals of such
phase-space restrictions in low-multiplicity collisions at RHIC. We discuss how
these same effects, then, {\it must} appear in the single particle spectra. We
argue that the effects of energy and momentum conservation actually dominate
the observed systematics, and that collisions may be much more similar to
heavy ion collisions than generally thought.Comment: 4 pages, 3 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse
Pion Interferemetry from p+p to Au+Au in STAR
The geometric substructure of the particle-emitting source has been
characterized via two-particle interferometry by the STAR collaboration for all
energies and colliding systems at RHIC. We present systematic studies of
charged pion interferometry. The collective nature of the source is revealed
through the dependence of HBT radii for all particle types. Preliminary
results suggest a scaling in the pion HBT radii with overall system size, as
central Au+Au collisions are compared to peripheral collisions as well as with
Cu+Cu and even with d+Au and p+p collisions, naively suggesting comparable flow
strength in all systems. To probe this issue in greater detail,
multidimensional correlation functions are studied using a spherical
decomposition method. This allows clear identification of source anisotropy
and, for the light systems, the presence of significant long-range
non-femtoscopic correlations.Comment: Proceedings for WPCF, Kromeriz, Czech Republic, August 200
Global Conservation Laws and Femtoscopy of Small Systems
It is increasingly important to understand, in detail, two-pion correlations
measured in p+p and d+A collisions. In particular, one wishes to understand the
femtoscopic correlations, in order to compare to similar measurements in heavy
ion collisions. However, in the low-multiplicity final states of these systems,
global conservation laws generate significant N-body correlations which project
onto the two-pion space in non-trivial ways and complicate the femtoscopic
analysis. We discuss a model-independent formalism to calculate and account for
these correlations in measurements.Comment: 7 pages; 10 figures; Invited talk at the Second Workshop on Particle
Correlations and Femtoscopy (WPCF06), Sept 9-11 2006, Sao Paulo, Brazi
Recent HBT results in Au+Au and p+p collisions from PHENIX
We present Hanbury-Brown Twiss measurements from the PHENIX experiment at
RHIC for final results for charged kaon pairs from sqrt{s_{NN}} = 200 GeV Au+Au
collisions and preliminary results for charged pion pairs from sqrt{s} = 200
GeV p+p collisions. We find that for kaon pairs from Au+Au, each traditional 3D
Gaussian radius shows approximately the same linear increase as a function of
N^{1/3}_{part}. An imaging analysis reveals a significant non-Gaussian tail for
r \gtrsim 10 fm. The presence of a tail for kaon pairs demonstrates that
similar non-Gaussian tails observed in earlier pion measurements cannot be
fully explained by decays of long-lived resonances. The preliminary analysis of
pions from sqrt{s} = 200 GeV p+p minimum biased collisions show correlations
which are well suited to traditional 3D HBT radii extraction via the
Bowler-Sinyukov method, and we present R_out, R_side, and R_long as a function
of mean transverse pair mass.Comment: 4 pages, 3 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse
Elliptic flow in proton-proton collisions at 7 TeV
The angular correlations measured in proton-proton collisions at 7 TeV are
decomposed into contributions from back to back emission and elliptic flow.
Modeling the dominant term in the correlation functions as a momentum
conservation effect or as an effect of the initial transverse velocity of the
source, the remaining elliptic flow component can be estimated. The elliptic
flow coefficient extracted from the CMS Collaboration data is 0.04-0.08. No
additional small-angle, ridge-like correlations are needed to explain the
experimental data
Eccentricity and elliptic flow in proton-proton collisions from parton evolution
It has been argued that high-multiplicity proton-proton collisions at the LHC
may exhibit collective phenomena usually studied in the context of heavy-ion
collisions, such as elliptic flow. We study this issue using DIPSY - a Monte
Carlo event generator based on the QCD dipole model. We calculate the
eccentricity of the transverse area defined by the spatial distribution of
produced gluons. The resulting elliptic flow is estimated to be about 6%,
comparable to the value in nucleus-nucleus collisions at RHIC and the LHC.
Experimentally, elliptic flow is inferred from the azimuthal correlation
between hadrons, which receives contributions from collective flow, and from
various other effects referred to as "nonflow". We discuss how to identify in
experiments the signal of flow in the presence of large nonflow effects.Comment: v2: Four-particle correlation added, improved discussions on the
signatures of flow. v3: Improved treatment of fluctuations in the flow
analysis. v4: Minor changes for journal submissio
Probing elastic and inelastic breakup contributions to intermediate-energy two-proton removal reactions
The two-proton removal reaction from 28Mg projectiles has been studied at 93
MeV/u at the NSCL. First coincidence measurements of the heavy 26Ne projectile
residues, the removed protons and other light charged particles enabled the
relative cross sections from each of the three possible elastic and inelastic
proton removal mechanisms to be determined. These more final-state-exclusive
measurements are key for further interrogation of these reaction mechanisms and
use of the reaction channel for quantitative spectroscopy of very neutron-rich
nuclei. The relative and absolute yields of the three contributing mechanisms
are compared to reaction model expectations - based on the use of eikonal
dynamics and sd-shell-model structure amplitudes.Comment: Accepted for publication in Physical Review C (Rapid Communication
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