arXiv subjects

R. Pak

Publications and source records attributed to R. Pak.

At least 19 recordsLinked to original sources

arXiv2022

Measurement of forward photon production cross-section in pp collisions at $\sqrt{s}$ = 510 GeV with RHICf detector

This study reported the inclusive differential production cross-section of photons in six pseudorapidity regions: 6.1 < $\eta$ < 6.5, 6.5 < $\eta$ < 7.0, 7.0 < $\eta$ <7.5, 7.5 < $\eta$ <8.0, 8.0 < $\eta$ < 8.5, and $\eta$ > 8.5, measured through the RHICf experiment with pp collisions at $\sqrt{s}$ = 510 GeV conducted in June 2017. In addition, the cross-sections in the three regions of the $x_F$-$p_T$ phase space coverage that are same as those of the LHCf results at $\sqrt{s}$ = 7 and 13 TeV were obtained and compared. Considering the uncertainties, the results were observed to be consistent with both the Feynman scaling law and the model predictions of EPOS-LHC, QGSJET-II-04, Sibyll 2.3d, and DPMjet-III 2019.1, although certain models exhibited weak collision energy dependencies.

hep-ex↗

arXiv2016

Scaling properties of the mean multiplicity and pseudorapidity density in $e^{-}+e^{+}$, $e^{\pm}$+p, p($\bar{\mathrm{p}}$)+p, p+A and A+A(B) collisions

The pseudorapidity density (dN/deta) for p+p, p+A and A+A(B) collisions, and the mean multiplicity for ee, ep, and p+p collisions, are studied for an inclusive range of beam energies (Root_s). Characteristic scaling patterns are observed for both dN/deta and , consistent with a thermal particle production mechanism for the bulk of the soft particles produced in all of these systems. They also validate an essential role for quark participants in these collisions. The scaled values for dN/deta and are observed to factorize into contributions which depend on log(Root_s) and the number of nucleon or quark participant pairs (Npp). Quantification of these contributions give expressions which serve to systematize dN/deta and measurements spanning nearly four orders of magnitude in Root_s, and to predict their values as a function of Root_s and Npp.

nucl-ex↗

arXiv2015

Participant and spectator scaling of spectator fragments in Au+Au and Cu+Cu collisions at sqrt(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 (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.

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arXiv2015

A Study of a Mini-drift GEM Tracking Detector

A GEM tracking detector with an extended drift region has been studied as part of an effort to develop new tracking detectors for future experiments at RHIC and for the Electron Ion Collider that is being planned for BNL or JLAB. The detector consists of a triple GEM stack with a small drift region that was operated in a mini TPC type configuration. Both the position and arrival time of the charge deposited in the drift region were measured on the readout plane which allowed the reconstruction of a short vector for the track traversing the chamber. The resulting position and angle information from the vector could then be used to improve the position resolution of the detector for larger angle tracks, which deteriorates rapidly with increasing angle for conventional GEM tracking detectors using only charge centroid information. Two types of readout planes were studied. One was a COMPASS style readout plane with 400 micron pitch XY strips and the other consisted of 2x10mm2 chevron pads. The detector was studied in test beams at Fermilab and CERN, along with additional measurements in the lab, in order to determine its position and angular resolution for incident track angles up to 45 degrees. Several algorithms were studied for reconstructing the vector using the position and timing information in order to optimize the position and angular resolution of the detector for the different readout planes. Applications for large angle tracking detectors at RHIC and EIC are also discussed.

physics.ins-det↗

arXiv2015

Nucleon-Gold Collisions at 200 AGeV Using Tagged d+Au Interactions in PHOBOS

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.

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arXiv2013

The PHENIX Forward Silicon Vertex Detector

A new silicon detector has been developed to provide the PHENIX experiment with precise charged particle tracking at forward and backward rapidity. The Forward Silicon Vertex Tracker (FVTX) was installed in PHENIX prior to the 2012 run period of the Relativistic Heavy Ion Collider (RHIC). The FVTX is composed of two annular endcaps, each with four stations of silicon mini-strip sensors, covering a rapidity range of $1.2<|\eta|<2.2$ that closely matches the two existing PHENIX muon arms. Each station consists of 48 individual silicon sensors, each of which contains two columns of mini-strips with 75 $\mu$m pitch in the radial direction and lengths in the $\phi$ direction varying from 3.4 mm at the inner radius to 11.5 mm at the outer radius. The FVTX has approximately 0.54 million strips in each endcap. These are read out with FPHX chips, developed in collaboration with Fermilab, which are wire bonded directly to the mini-strips. The maximum strip occupancy reached in central Au-Au collisions is approximately 2.8%. The precision tracking provided by this device makes the identification of muons from secondary vertices away from the primary event vertex possible. The expected distance of closest approach (DCA) resolution of 200 $\mu$m or better for particles with a transverse momentum of 5 GeV/$c$ will allow identification of muons from relatively long-lived particles, such as $D$ and $B$ mesons, through their broader DCA distributions.

physics.ins-det↗

arXiv2010

A Reaction Plane Detector for PHENIX at RHIC

A plastic scintillator paddle detector with embedded fiber light guides and photomultiplier tube readout, referred to as the Reaction Plane Detector (RXNP), was designed and installed in the PHENIX experiment prior to the 2007 run of the Relativistic Heavy Ion Collider (RHIC). The RXNP's design is optimized to accurately measure the reaction plane (RP) angle of heavy-ion collisions, where, for mid-central $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions, it achieved a $2^{nd}$ harmonic RP resolution of $\sim$0.75, which is a factor of $\sim$2 greater than PHENIX's previous capabilities. This improvement was accomplished by locating the RXNP in the central region of the PHENIX experiment, where, due to its large coverage in pseudorapidity ($1.0<|\eta|<2.8$) and $\phi$ (2$\pi$), it is exposed to the high particle multiplicities needed for an accurate RP measurement. To enhance the observed signal, a 2-cm Pb converter is located between the nominal collision region and the scintillator paddles, allowing neutral particles produced in the heavy-ion collisions to contribute to the signal through conversion electrons. This paper discusses the design, operation and performance of the RXNP during the 2007 RHIC run.

nucl-ex↗

arXiv2010

Phobos results on charged particle multiplicity and pseudorapidity distributions in Au+Au, Cu+Cu, d+Au, and p+p collisions at ultra-relativistic energies

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.

nucl-ex↗

arXiv2010

Azimuthal anisotropy: transition from hydrodynamic flow to jet suppression

Measured 2nd and 4th azimuthal anisotropy coefficients v_{2,4}(N_{part}), p_T) are scaled with the initial eccentricity \varepsilon_{2,4}(N_{part}) of the collision zone and studied as a function of the number of participants N_{part} and the transverse momenta p_T. Scaling violations are observed for $p_T \alt 3$ GeV/c, consistent with a $p_T^2$ dependence of viscous corrections and a linear increase of the relaxation time with $p_T$. These empirical viscous corrections to flow and the thermal distribution function at freeze-out constrain estimates of the specific viscosity and the freeze-out temperature for two different models for the initial collision geometry. The apparent viscous corrections exhibit a sharp maximum for $p_T \agt 3$ GeV/c, suggesting a breakdown of the hydrodynamic ansatz and the onset of a change from flow-driven to suppression-driven anisotropy.

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arXiv2010

Constraints on models for the initial collision geometry in ultra relativistic heavy ion collisions

Monte Carlo (MC) simulations are used to compute the centrality dependence of the collision zone eccentricities ($\epsilon_{2,4}$), for both spherical and deformed ground state nuclei, for different model scenarios. Sizable model dependent differences are observed. They indicate that measurements of the $2^{\text{nd}}$ and $4^{\text{th}}$ order Fourier flow coefficients $v_{2,4}$, expressed as the ratio $\frac{v_4}{(v_2)^2}$, can provide robust constraints for distinguishing between different theoretical models for the initial-state eccentricity. Such constraints could remove one of the largest impediments to a more precise determination of the specific viscosity from precision $v_{2,4}$ measurements at the Relativistic Heavy Ion Collider (RHIC).

nucl-ex↗

arXiv2008

Scaling properties in bulk and p$_{\rm T}$-dependent particle production near midrapidity in relativistic heavy ion collisions

The centrality dependence of the midrapidity charged-particle multiplicity density ($|η|$$<$1) is presented for Au+Au and Cu+Cu collisions at RHIC over a broad range of collision energies. The multiplicity measured in the Cu+Cu system is found to be similar to that measured in the Au+Au system, for an equivalent N$_{\rm part}$, with the observed factorization in energy and centrality still persistent in the smaller Cu+Cu system. The extent of the similarities observed for bulk particle production is tested by a comparative analysis of the inclusive transverse momentum distributions for Au+Au and Cu+Cu collisions near midrapidity. It is found that, within the uncertainties of the data, the ratio of yields between the various energies for both Au+Au and Cu+Cu systems are similar and constant with centrality, both in the bulk yields as well as a function of p$_{\rm 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.

nucl-ex↗

arXiv2008

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.

nucl-ex↗

arXiv2008

Non-flow correlations and elliptic flow fluctuations in Au+Au collisions at sqrt(s_NN)=200GeV

We present first results on event-by-event elliptic flow fluctuations in nucleus-nucleus collisions corrected for effects of non-flow correlations where the magnitude of non-flow correlations has been independently measured in data. Over the measured range in centrality, we see large relative fluctuations of 25-50%. The results are consistent with predictions from both color glass condensate and Glauber type initial condition calculations of the event-by-event participant eccentricity fluctuations.

nucl-ex↗

arXiv2007

Radiation Damage Study for PHENIX Silicon Stripixel Sensors

Silicon stripixel sensors which were developed at BNL will be installed as part of the RHIC-PHENIX silicon vertex tracker (VTX). RHIC II operations provide luminosity up to 2x10^32 /cm2/s so the silicon stripixel sensors will be exposed to a significant amount of radiation. The most problematic radiation effect for VTX is the increase of leakage current, which degrades the signal to noise ratio and may saturate the readout electronics. We studied the radiation damage using the same diodes as CERN-RD48. First, the proportionality between the irradiation fluence and the increase of leakage current of CERN-RD48 was reproduced. Then beam experiments with stripixel sensor were done in which leakage current was found to increase in the same way as that of thereference diode. A stripixel sensor was also irradiated at the PHENIX interaction region (IR) during the 2006 run. We found the same relation between the integrated luminosity and determined fluence from increase of leakage current. The expected fluence is 3-6x10^12 Neq/cm2 (1 MeV neutron equivalent) in RHIC II operations for 10 years. Due to this expected exposure, setting the operating temperature in PHENIX to T< 0 deg. C to suppress leakage current is needed to avoid saturation of preamplifiers.

nucl-ex↗

arXiv2007

System Size, Energy and Centrality Dependence of Pseudorapidity Distributions of Charged Particles in Relativistic Heavy Ion Collisions

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, \sqrtsnn = 22.4, 62.4 and 200 GeV, over a wide range of pseudorapidity, using the PHOBOS detector. Making a global comparison of Cu+Cu and Au+Au results, we find 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 Npart/2A value rather than the same Npart value. 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.

nucl-ex↗

arXiv2006

Identified hadron transverse momentum spectra in Au+Au collisions at sqrt(s_(NN))=62.4 GeV

Transverse momentum spectra of pions, kaons, protons and antiprotons from Au+Au collisions at sqrt(s_(NN)) = 62.4 GeV have been measured by the PHOBOS experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The identification of particles relies on three different methods: low momentum particles stopping in the first detector layers; the specific energy loss (dE/dx) in the silicon Spectrometer, and Time-of-Flight measurement. These methods cover the transverse momentum ranges 0.03-0.2, 0.2-1.0 and 0.5-3.0 GeV/c, respectively. Baryons are found to have substantially harder transverse momentum spectra than mesons. The pT region in which the proton to pion ratio reaches unity in central Au+Au collisions at sqrt(s_(NN)) = 62.4 GeV fits into a smooth trend as a function of collision energy. At low transverse momenta, the spectra exhibit a significant deviation from transverse mass scaling, and when the observed particle yields at very low pT are compared to extrapolations from higher pT, no significant excess is found. By comparing our results to Au+Au collisions at sqrt(s_(NN)) = 200 GeV, we conclude that the net proton yield at midrapidity is proportional to the number of participant nucleons in the collision.

nucl-ex↗

arXiv2003

Production of phi Mesons in Au+Au Collisions at 11.7 A GeV/c

We report on a measurement of phi-meson production in Au+Au collisions at a beam momentum of 11.7 A GeV/c by Experiment E917 at the AGS. The measurement covers the midrapidity region 1.2 < y < 1.6. Transverse-mass spectra and the rapidity distribution are presented as functions of centrality characterized by the number of participant projectile nucleons. The yield of phi's per participant projectile nucleon increases strongly in central collisions in a manner similar to that observed for kaons.

nucl-ex↗

arXiv2001

Antilambda production in Au+Au collisions at 11.7 A GeV/c

We present results from Experiment E917 for antilambda and antiproton production in Au+Au collisions at 11.7 A GeV. We have measured invariant spectra and yields for both species in central and peripheral collisions. We find that the antilambda/antiproton ratio near mid-rapidity increases from 0.26+0.19-0.15 in peripheral collisions to 3.6+4.7-1.8 in central collisions, a value that is substantially larger than current theoretical estimates.

nucl-ex↗