arXiv subjects

A. Iordanova

Publications and source records attributed to A. Iordanova.

At least 19 recordsLinked to original sources

arXiv2020

Nuclear-modification factor of charged hadrons at forward and backward rapidity in $p$$+$Al and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

The PHENIX experiment has studied nuclear effects in $p$$+$Al and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV on charged hadron production at forward rapidity ($1.4<η<2.4$, $p$-going direction) and backward rapidity ($-2.2<η<-1.2$, $A$-going direction). Such effects are quantified by measuring nuclear modification factors as a function of transverse momentum and pseudorapidity in various collision multiplicity selections. In central $p$$+$Al and $p$$+$Au collisions, a suppression (enhancement) is observed at forward (backward) rapidity compared to the binary scaled yields in $p$+$p$ collisions. The magnitude of enhancement at backward rapidity is larger in $p$$+$Au collisions than in $p$$+$Al collisions, which have a smaller number of participating nucleons. However, the results at forward rapidity show a similar suppression within uncertainties. The results in the integrated centrality are compared with calculations using nuclear parton distribution functions, which show a reasonable agreement at the forward rapidity but fail to describe the backward rapidity enhancement.

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arXiv2019

Nuclear dependence of the transverse single-spin asymmetry in the production of charged hadrons at forward rapidity in polarized $p+p$, $p+$Al, and $p+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

We report on the nuclear dependence of transverse single-spin asymmetries (TSSAs) in the production of positively-charged hadrons in polarized $p^{\uparrow}+p$, $p^{\uparrow}+$Al and $p^{\uparrow}+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The measurements have been performed at forward rapidity ($1.4<η<2.4$) over the range of $1.8<p_{T}<7.0$ GeV$/c$ and $0.1<x_{F}<0.2$. We observed a positive asymmetry $A_{N}$ for positively-charged hadrons in \polpp collisions, and a significantly reduced asymmetry in $p^{\uparrow}$+$A$ collisions. These results reveal a nuclear dependence of charged hadron $A_N$ in a regime where perturbative techniques are relevant. These results provide new opportunities to use \polpA collisions as a tool to investigate the rich phenomena behind TSSAs in hadronic collisions and to use TSSA as a new handle in studying small-system collisions.

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arXiv2019

Measurement of charm and bottom production from semileptonic hadron decays in $p$$+$$p$ collisions at $\sqrt{s_{NN}}=200$ GeV

Measurements of the differential production of electrons from open-heavy-flavor hadrons with charm- and bottom-quark content in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV are presented. The measurements proceed through displaced-vertex analyses of electron tracks from the semileptonic decay of charm and bottom hadrons using the PHENIX silicon-vertex detector. The relative contribution of electrons from bottom decays to inclusive heavy-flavor-electron production is found to be consistent with fixed-order-plus-next-to-leading-log perturbative-QCD calculations within experimental and theoretical uncertainties. These new measurements in $p$$+$$p$ collisions provide a precision baseline for comparable forthcoming measurements in A$+$A collisions.

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arXiv2019

Nonperturbative transverse momentum broadening in dihadron angular correlations in $\sqrt{s_{NN}}=200$ GeV proton-nucleus collisions

The PHENIX collaboration has measured high-$p_T$ dihadron correlations in $p$$+$$p$, $p$$+$Al, and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The correlations arise from inter- and intra-jet correlations and thus have sensitivity to nonperturbative effects in both the initial and final states. The distributions of $p_{\rm out}$, the transverse momentum component of the associated hadron perpendicular to the trigger hadron, are sensitive to initial and final state transverse momenta. These distributions are measured multi-differentially as a function of $x_E$, the longitudinal momentum fraction of the associated hadron with respect to the trigger hadron. The near-side $p_{\rm out}$ widths, sensitive to fragmentation transverse momentum, show no significant broadening between $p$$+$Au, $p$$+$Al, and $p$$+$$p$. The away-side nonperturbative $p_{\rm out}$ widths are found to be broadened in $p$$+$Au when compared to $p$$+$$p$; however, there is no significant broadening in $p$$+$Al compared to $p$$+$$p$ collisions. The data also suggest that the away-side $p_{\rm out}$ broadening is a function of $N_{\rm coll}$, the number of binary nucleon-nucleon collisions, in the interaction. The potential implications of these results with regard to initial and final state transverse momentum broadening and energy loss of partons in a nucleus, among other nuclear effects, are discussed.

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arXiv2019

Measurements of $μμ$ pairs from open heavy flavor and Drell-Yan in $p+p$ collisions at $\sqrt{s}=200$ GeV

PHENIX reports differential cross sections of $μμ$ pairs from semileptonic heavy-flavor decays and the Drell-Yan production mechanism measured in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV at forward and backward rapidity ($1.2<|η|<2.2$). The $μμ$ pairs from $c\bar{c}$, $b\bar{b}$, and Drell-Yan are separated using a template fit to unlike- and like-sign muon pair spectra in mass and $p_T$. The azimuthal opening angle correlation between the muons from $c\bar{c}$ and $b\bar{b}$ decays and the pair-$p_T$ distributions are compared to distributions generated using {\sc pythia} and {\sc powheg} models, which both include next-to-leading order processes. The measured distributions for pairs from $c\bar{c}$ are consistent with {\sc pythia} calculations. The $c\bar{c}$ data presents narrower azimuthal correlations and softer $p_T$ distributions compared to distributions generated from {\sc powheg}. The $b\bar{b}$ data are well described by both models. The extrapolated total cross section for bottom production is $3.75{\pm}0.24({\rm stat}){\pm}^{0.35}_{0.50}({\rm syst}){\pm}0.45({\rm global})$[$μ$b], which is consistent with previous measurements at the Relativistic Heavy Ion Collider in the same system at the same collision energy, and is approximately a factor of two higher than the central value calculated with theoretical models. The measured Drell-Yan cross section is in good agreement with next-to-leading-order quantum-chromodynamics calculations.

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arXiv2018

Nonperturbative transverse-momentum-dependent effects in dihadron and direct photon-hadron angular correlations in $p+p$ collisions at $\sqrt{s}=200$ GeV

Dihadron and isolated direct photon-hadron angular correlations are measured in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV. The correlations are sensitive to nonperturbative initial-state and final-state transverse momentum $k_T$ and $j_T$ in the azimuthal nearly back-to-back region $Δϕ\simπ$. In this region, transverse-momentum-dependent evolution can be studied when several different hard scales are measured. To have sensitivity to small transverse momentum scales, nonperturbative momentum widths of $p_{\rm out}$, the out-of-plane transverse momentum component perpendicular to the trigger particle, are measured. These widths are used to investigate possible effects from transverse-momentum-dependent factorization breaking. When accounting for the longitudinal momentum fraction of the away-side hadron with respect to the near-side trigger particle, the widths are found to increase with the hard scale; this is qualitatively similar to the observed behavior in Drell-Yan and semi-inclusive deep-inelastic scattering interactions. The momentum widths are also studied as a function of center-of-mass energy by comparing to previous measurements at $\sqrt{s}=510$ GeV. The nonperturbative jet widths also appear to increase with $\sqrt{s}$ at a similar $x_T$, which is qualitatively consistent to similar measurements in Drell-Yan interactions. To quantify the magnitude of any transverse-momentum-dependent factorization breaking effects, calculations will need to be performed to compare to these measurements.

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arXiv2018

Single-spin asymmetry of $J/ψ$ production in $p$$+$$p$, $p$$+$Al, and $p$$+$Au collisions with transversely polarized proton beams at $\sqrt{s_{_{NN}}}=200$ GeV

We report the transverse single-spin asymmetries of $J/ψ$ production at forward and backward rapidity, $1.2<|y|<2.2$, as a function of $J/ψ$ transverse momentum ($p_T$) and Feynman-$x$ ($x_F$). The data analyzed were recorded by the PHENIX experiment at the Relativistic Heavy Ion Collider in 2015 from $p$$+$$p$, $p$$+$Al, and $p$$+$Au collisions with transversely polarized proton beams at $\sqrt{s_{_{NN}}}=200$ GeV. At this collision energy, single-spin asymmetries for heavy-flavor particle production of $p$$+$$p$ collisions provide access to the spin-dependent gluon distribution and higher-twist correlation functions inside the nucleon, such as the gluon Qiu-Sterman and trigluon correlation functions. Proton+nucleus collisions offer an excellent opportunity to study nuclear effects on the correlation functions. The data indicate negative asymmetries at the two-standard-deviation level in the $p$$+$Au data for $p_T<2$ GeV/$c$ at both forward and backward rapidity, while in $p$$+$$p$ and $p$$+$Al collisions the asymmetries are consistent with zero within the range of experimental uncertainties.

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arXiv2018

Correlations of $μμ$, $eμ$, and $ee$ pairs in $p$+$p$ collisions at $\sqrt{s}=200$ GeV and implications for $c\bar{c}$ and $b\bar{b}$ production mechanisms

PHENIX has measured the azimuthal correlations of muon pairs from charm and bottom semi-leptonic decays in $p$+$p$ collisions at $\sqrt{s}=200$ GeV, using a novel analysis technique utilizing both unlike- and like-sign muon pairs to separate charm, bottom and Drell-Yan contributions. The dimuon measurements combined with the previous electron-muon and dielectron measurements span a wide range in rapidity, and are well described by PYTHIA Tune A. Through a Bayesian analysis based on PYTHIA Tune A, we show that leading order pair creation is the dominant $(76\%\pm^{14}_{19}\%)$ contribution for $b\bar{b}$ production, whereas the data favor the scenario in which next-to-leading-order processes dominate $c\bar{c}$ production. The small contribution of next-to-leading-order processes in $b\bar{b}$ production at the collision energies of the Relativistic Heavy Ion Collider contrasts with the case at Large-Hadron-Collider energies, where next-to-leading-order processes are expected to dominate.

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arXiv2017

Measurement of long-range angular correlations and azimuthal anisotropies in high-multiplicity $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

We present the first measurements of long-range angular correlations and the transverse momentum dependence of elliptic flow $v_2$ in high-multiplicity $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. A comparison of these results with previous measurements in high-multiplicity $d$$+$Au and $^3{\rm He}$$+$Au collisions demonstrates a relation between $v_2$ and the initial collision eccentricity $\varepsilon_2$, suggesting that the observed momentum-space azimuthal anisotropies in these small systems have a collective origin and reflect the initial geometry. Good agreement is observed between the measured $v_2$ and hydrodynamic calculations for all systems, and an argument disfavoring theoretical explanations based on momentum-space domain correlations is presented. The set of measurements presented here allows us to leverage the distinct intrinsic geometry of each of these systems to distinguish between different theoretical descriptions of the long-range correlations observed in small collision systems.

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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 ($η$). The dominant multiply-charged fragment is the tightly bound Helium ($α$), 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 $α$ 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 $α$ 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

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

A Proposal for the Muon Piston Calorimeter Extension (MPC-EX) to the PHENIX Experiment at RHIC

The PHENIX MPC-EX detector is a Si-W preshower extension to the existing PHENIX Muon Piston Calorimeters (MPC). The MPC-EX will consist of eight layers of alternating W absorber and Si mini-pad sensors and will be installed in time for RHIC Run-15. Covering a large pseudorapidity range, 3.1 < eta < 3.8, the MPC-EX and MPC access high-x partons in the projectile nucleon (and low-x partons in the target nucleon) in p+A and transversely polarized proton-proton collisions at 200 GeV. With the addition of the MPC-EX, the neutral pion reconstruction range extends to energies > 80 GeV, a factor of four improvement over current capabilities. Not only will the MPC-EX strengthen PHENIX's existing forward neutral pion and jet measurements, it also provides the necessary neutral pion rejection to make a prompt photon measurement feasible in both p+A and p+p collisions. With this neutral pion rejection, prompt (direct + fragmentation) photon yields at high p_T, p_T > 3 GeV, can be statistically extracted using a double ratio method. In p+A collisions direct photons at forward rapidities are optimally sensitive to the gluon distribution because, unlike pions, direct photons are only produced by processes that are directly sensitive to the gluon distribution at leading order. A measurement of the forward prompt photon R_pA will cleanly access and greatly expand our understanding of the gluon nuclear parton distribution functions and provide important information about the initial state in heavy ion collisions. In transversely polarized p+p collisions the MPC-EX will make possible a measurement of the prompt photon single spin asymmetry A_N, and will help elucidate the correlation of valence partons in the proton with the proton spin.

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arXiv2011

Participant-like scaling behavior of multiplicity and charged hadron spectra in relativistic heavy ion collisions

We present a method for parameterizing the charged particle multiplicity and charged hadron spectra from heavy ion data with a simple, Glauber-inspired, model. The basis of this model is derived from the observation of leading hadrons in $pp$ collisions and the number of interactions calculated by a Glauber model. Singly hit and multiply hit nucleons are treated as different sub-components of the same collision. With this scheme, for fixed sub-component yields, we find that the multiplicity and charged hadron spectra can be reproduced, without the need for large suppression at high-p$_T$. Suppression is still observed, in the low- to intermediate-p$_T$ region, but this is confined to suppression of surface emission partons. At high-p$_T$, the suppression disappears entirely, leaving only a possible Cronin-type enhancement. The suppression observed in 200GeV collisions is found to be the same for 62.4GeV data.

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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 $|η|<5.4$, 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, $dN_{ch}/dη$, 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.

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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.

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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.

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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.

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arXiv2007

The Importance of Correlations and Fluctuations on the Initial Source Eccentricity in High-Energy Nucleus-Nucleus Collisions

In this paper, we investigate various ways of defining the initial source eccentricity using the Monte Carlo Glauber (MCG) approach. In particular, we examine the participant eccentricity, which quantifies the eccentricity of the initial source shape by the major axes of the ellipse formed by the interaction points of the participating nucleons. We show that reasonable variation of the density parameters in the Glauber calculation, as well as variations in how matter production is modeled, do not significantly modify the already established behavior of the participant eccentricity as a function of collision centrality. Focusing on event-by-event fluctuations and correlations of the distributions of participating nucleons we demonstrate that, depending on the achieved event-plane resolution, fluctuations in the elliptic flow magnitude $v_2$ lead to most measurements being sensitive to the root-mean-square, rather than the mean of the $v_2$ distribution. Neglecting correlations among participants, we derive analytical expressions for the participant eccentricity cumulants as a function of the number of participating nucleons, $\Npart$,keeping non-negligible contributions up to $\ordof{1/\Npart^3}$. We find that the derived expressions yield the same results as obtained from mixed-event MCG calculations which remove the correlations stemming from the nuclear collision process. Most importantly, we conclude from the comparison with MCG calculations that the fourth order participant eccentricity cumulant does not approach the spatial anisotropy obtained assuming a smooth nuclear matter distribution. In particular, for the Cu+Cu system, these quantities deviate from each other by almost a factor of two over a wide range in centrality.

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