Observation of Feynman scaling violations and evidence for a new resonance at RHIC
L. C. Bland, E. J. Brash, H. J. Crawford, A. A. Derevschikov, K. A. Drees, J. Engelage, C. Folz, E. G. Judd, X. Li, N. G. Minaev, R. N. Munroe, L. Nogach, A. Og… 展开作者
L. C. Bland, E. J. Brash, H. J. Crawford, A. A. Derevschikov, K. A. Drees, J. Engelage, C. Folz, E. G. Judd, X. Li, N. G. Minaev, R. N. Munroe, L. Nogach, A. Ogawa, C. Perkins, M. Planinic, A. Quintero, G. Schnell, P. V. Shanmuganathan, G. Simatovic, B. Surrow, T. G. Throwe, A. N. Vasiliev
We report measurements of forward jets and dijets produced in Cu+Au collisions at $\sqrt{s_{NN}}=200$ GeV at the Relativistic Heavy Ion Collider. We also report dijet production cross sections in p+p collisions at $\sqrt{s}=510$ GeV. We use the invariant dijet mass to search for indications of new particles. The p+p dijet results are compatible with string fragmentation models tuned to fit LHC data. The Cu+Au jet results far exceed Feynman scaling limits, and are compatible with models that incorporate string fusion to increase parton energy, acting as a QCD accelerator. The Cu+Au dijet results can be mostly explained by double parton scattering due to a parton flux from multiple p+p interactions with $\sqrt{s}>>\sqrt{s_{NN}}$. Further indication of the increased parton energy is obtained from evidence of single- and double-$Υ$(1S) production in the forward direction in Cu+Au collisions. Finally, we report evidence for the production of a new resonance, reconstructed from its dijet decay.
We report the measurement of azimuthal anisotropy $v_{n}$ for n = 1-5 as a function of transverse momentum $p_{T}$ and centrality in U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV, recorded with the STAR detector at RHIC. We also present results on $v_{2}$ and azimuthal correlator related to charge separation across the reaction plane due to local parity violation(LPV) in the ultra-central collisions.
It has been suggested that local parity violation in QCD would lead to charge separation of quarks by the Chiral Magnetic Effect (CME) in heavy ion collisions. Charge separation could yield a dynamical charge multiplicity asymmetry with respect to the reaction plane. In this talk, we report results on charge multiplicity asymmetry correlations in $\sqrt{s_{NN}}$ = 200 GeV Au+Au and d+Au collisions by the STAR experiment, as well as from the RHIC beam energy scan. We found that the correlation results could not be explained by CME alone. To gain further insights, we study our results as a funct…
The large values of the transverse single spin asymmetry, A_N, seen in forward pi0 production from polarized proton collisions have stimulated important questions and have been studied in QCD based transverse spin models. We report the first RHIC measurement of A_N for the two photon mass region of the eta meson. The eta peak for meson energies greater than 50 GeV (Feynman x_F >0.5) was observed in the STAR Forward Pion Detectors, along with pi0 mesons, at pseudo-rapidity of 3.65 in sqrt{s}=200 GeV pp collisions. The pi0 transverse asymmetry, which has already been reported in detail by the ST…
We study effects of the hadronic rescattering on final observables especially for multi-strange hadrons such as $ϕ$, $Ξ$ and $Ω$ in high-energy heavy-ion collisions within an integrated dynamical approach. In this approach, (3+1)-dimensional ideal hydrodynamics is combined with a microscopic transport model, JAM. We simulate the collisions with or without hadronic rescatterings and compare observables between these two options so that we quantify the effects of the hadronic rescattering. We find that the mean transverse momentum and the elliptic flow parameter of multi-strange hadrons are less…
The STAR collaboration has previously reported a large transverse single spin asymmetry, A_N, for forward pi0 meson production. The cross-section in this region was measured up to Feynman-x (xF) of 0.55, and found to be consistent with perturbative Quantum Chromo-Dynamics predictions. During RHIC running in the year 2006 (6.8 inverse-pb, 56 % average polarization), an even larger A_N was observed for the eta meson at forward rapidity for xF > 0.5. All data were collected by the STAR Forward Pion Detector during sqrt(s) = 200 GeV polarized p+p collisions at RHIC. Understanding these large spin …
Highlights of news from Brookhaven National Laboratory (BNL) and results from the Relativistic Heavy Ion Collider (RHIC) in the period July 2015-2016 are presented. Transverse single spin asymmetries from polarized p+p collisions are presented for $π^0$ and jets as a function of Feynman $x_F$. An energy scan to study the $\sqrt{s}$ dependence of collectivity and flow for small systems was performed as well as a high luminosity Au$+$Au run. The failure of a quench protection diode resulted in a pause in the run to replace it, but otherwise performance of RHIC was the best ever. Experimental res…
One of the most interesting and important phenomena predicted to occur in heavy-ion collisions is the local strong parity violation. In non-central collisions, it is expected to result in charge separation of produced particles along the system's orbital momentum. I report on results of the charge separation measurement in Au+Au and Cu+Cu collisions at sqrt(s_NN)= 200 and 62 GeV with the STAR detector at RHIC based on three-particle mixed harmonic azimuthal correlations. Systematic study of parity conserving (background) effects with existing heavy-ion event generators, and their possible cont…
We present highlights of recent results from the STAR Collaboration at RHIC, focusing on the properties of the early medium created in heavy ion collisions. We emphasize the strangeness production including the observation of a hypernucleus (the hypertriton), the observation of reaction-plane-dependent angular correlation of charged particles searching for local strong parity violation effects in heavy ion collisions, and the evaluation of the medium viscosity from measurements of elliptic flow. We discuss STAR's plan for the "Critical Point Search" program at RHIC.
We propose the measurement of net $Λ$ and $\barΛ$ helicity, correlated event-by-event with the magnitude and sign of charge separation along the event's magnetic field direction, as a probe to investigate the Chiral Magnetic Effect in Heavy-Ion Collisions. With a simple simulation model of heavy-ion events that includes effects of Local Parity Violation, we estimate the experimental correlation signal that could be expected at RHIC given the results of previous measurements that are sensitive to the CME.
In terms of the hadron-string-dynamics (HSD) approach we investigate the correlation function in the azimuthal angle $ψ$ of charged hadrons that is expected to be sensitive to a signal of local strong parity violation. Our analysis of Au+Au collisions is based on the recent STAR data within the RHIC Beam-Energy-Scan (BES) program. The HSD model reasonably reproduces STAR data for $\sqrt{s_{NN}}=$7.7 GeV, while there are some deviations from the experiment at the collision energy of 11.5 GeV and an increase of deviations between theory and experiment at $\sqrt{s_{NN}}=$39 GeV. For reference, th…
Quark interactions with topological gluon configurations can induce chirality imbalance and local parity violation in quantum chromodynamics. This can lead to electric charge separation along the strong magnetic field in relativistic heavy-ion collisions -- the chiral magnetic effect (CME). We report measurements by the STAR collaboration of a CME-sensitive observable in $p$+Au and $d$+Au collisions at 200 GeV, where the CME is not expected, using charge-dependent pair correlations relative to a third particle. We observe strong charge-dependent correlations similar to those measured in heavy-…
In this paper, we propose an experiment for the precise measurements of very forward particle production at RHIC. The proposal is to install a LHCf-like calorimeter in the ZDC installation slot at one of the RHIC interaction points. By installing a high-resolution electromagnetic calorimeter at this location we measure the spectra of photons, neutrons and pi0 at pseudo rapidity eta above 6. The new measurements at 500 GeV p-p collisions contribute to improve the hadronic interaction models used in the cosmic-ray air shower simulations. Using a similar kinematic coverage at RHIC to that of the …
The Monte Carlo HYDJET++ model, that contains both hydrodynamic state and jets, is applied to study the influence of the interplay between soft and hard processes on the formation of the elliptic flow in heavy-ion collisions at RHIC and LHC energies. Jets are found to cease the hydro-like increase of the elliptic flow with rising p_T. Since jets are more influential at LHC than at RHIC, the v_2 at LHC should be weaker than that at RHIC. Violation of the number-of-constituent-quark (NCQ) scaling is predicted. The decays of resonances are found to enhance the low-p_T part of the elliptic flow of…
Multiplicity and pseudorapidity ($η$) density ($dN_{\rm ch}/dη$) distributions of charged hadrons provide key information towards understanding the particle production mechanisms and initial conditions of high-energy heavy-ion collisions. However, detector constraints limit the $η$-range across which charged particle measurements can be carried out. Extrapolating the measured distributions to large $η$-range by parameterizing measured distributions and by using calculations from event generators, we characterize the production of charged particles over the full kinematic range. In the present …
Separation of charges along the extreme magnetic field created in non-central relativistic heavy--ion collisions is predicted to be a signature of local parity violation in strong interactions. We report on results for charge dependent two particle azimuthal correlations with respect to the reaction plane for Pb--Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV recorded in 2010 with ALICE at the LHC. The results are compared with measurements at RHIC energies and against currently available model predictions for LHC. Systematic studies of possible background effects including comparison with conven…
We provide a phenomenological analysis of present experimental searches for local parity violation manifested through the Chiral Magnetic Effect. We introduce and discuss the relevant correlation functions used for the measurements. Our analysis of the available data from both RHIC and LHC shows that the present experimental evidence for the Chiral Magnetic Effect is rather ambiguous. We further discuss in some detail various background contributions due to conventional physics, which need to be understood quantitatively in order to draw a definitive conclusion about the existence of local par…
Heavy quark observables are applied to probe the initial energy density distribution with violation of longitudinal boost invariance produced in relativistic heavy-ion collisions. Using an improved Langevin model coupled to a (3+1)-dimensional viscous hydrodynamic model, we study the nuclear modification factor ($R_\text{AA}$), directed flow ($v_1$) and elliptic flow ($v_2$) coefficients of heavy mesons and their decayed electrons at the RHIC energy. We find that the counter-clockwise tilt of the nuclear matter in the reaction plane results in a positive (negative) heavy flavor $v_1$ in the ba…
Metastable domains of fluctuating topological charges can change the chirality of quarks and induce local parity violation in quantum chromodynamics. This can lead to observable charge separation along the direction of the strong magnetic field produced by spectator protons in relativistic heavy-ion collisions, a phenomenon called the chiral magnetic effect (CME). A major background source for CME measurements using the charge-dependent azimuthal correlator ($Δγ$) is the intrinsic particle correlations (such as resonance decays) coupled with the azimuthal elliptical anisotropy ($v_{2}$). In he…
We present measurements of pion elliptic flow ($v_2$) in Au+Au collisions at $\sNN =$ 200, 62.4, 39, 27 and 19.6 GeV, as a function of event-by-event charge asymmetry ($A_{\pm}$), based on data from the STAR experiment at RHIC. We find that $π^-$ ($π^+$) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins and for all the beam energies under study. The slope parameter ($r$) from $v_2(A_\pm)$ difference between $π^-$ and $π^+$ shows a centrality dependency similar with the calculations with Chiral Magnetic Wave. The measurements of charge separation with r…
Previous experimental results based on data (15 million events) collected by the STAR detector at RHIC suggest event-by-event charge separation fluctuations perpendicular to the event plane in non-central heavy-ion collisions. Here we present the correlator previously used split into its two component parts to reveal correlations parallel and perpendicular to the event plane. The results are from a high statistics 200 GeV Au+Au collisions data set (57 million events) collected by the STAR experiment. We explicitly count units of charge separation from which we find clear evidence for more char…
For heavy-ion collisions at RHIC a scaling behavior is found in the dependencies on azimuthal angle $ϕ$ and impact parameter $b$ for pion production at high $p_T$ essentially independent of the hadronization process. The scaling variable is in terms of a dynamical path length $ξ$ that takes into account detailed properties of geometry, medium density and probability of hard scattering. It is shown in the recombination model how the nuclear modification factor depends on the average $\barξ(ϕ,b)$. The data for $π^0$ production at $p_T=$ 4-5 and 7-8 GeV/c at RHIC are shown to exhibit the same sca…
We argue that the local violation of P and CP invariance in heavy ion collisions and the universal thermal aspects observed in high energy collisions are in fact two sides of the same coin, and both are related to quantum anomalies of QCD. We argue that the low energy relations representing the quantum anomalies of QCD are saturated by coherent low dimensional vacuum configurations as observed in Monte Carlo lattice studies. The thermal spectrum and approximate universality of the temperature with no dependence on energy of colliding particles in this framework is due to the fact that the emis…
We present the measurement of light neutral mesons, $π^{0}$ and $η$, in pp collisions at different center-of-mass energies obtained with the ALICE experiment at the LHC. The $π^{0}$ and $η$ mesons are measured via photons reconstructed by the electromagnetic calorimeters and the central tracking system. The invariant cross-section of $π^{0}$ and $η$ mesons are measured in a broad $p_{\rm T}$ range at $\sqrt{s} = 0.9, 2.76, 7, 5.02$ and 8 TeV. The spectra of $π^{0}$ and $η$ mesons measured in pp collisions at different collision energies show $x_{\rm T}$-scaling at high $p_{\rm T}$ and violatio…
