Net Charge Fluctuations Research Articles

Modeling net-charge fluctuations in heavy-ion collisions at the LHC

Abstract The analysis of Pb+Pb data for net-charge fluctuations at LHC energies
within the HYDJET++ model is presented. The strongly intensive
quantities $D$ and $\Sigma$ are used to get rid of the effects related
to volume fluctuations of the system. We employ two versions of the
HYDJET++ for the analysis. The first one is the standard or default
version, whereas the second one represents its further modification,
which takes into account explicit event-by-event conservation of the
electric net-charge of produced particles. Inclusion of the canonical
net-charge conservation in the model allows one for better description
of the experimental data obtained by the ALICE and the CMS
Collaborations. Comparison with calculations of other models is also
presented.

Cumulants of conserved charges in hydrodynamic simulations

We introduce a fast and simple method of computing cumulants of net-proton or net-charge fluctuations in event-by-event hydrodynamic simulations of heavy-ion collisions. One evaluates the mean numbers of particles in every hydrodynamic event. Cumulants are then expressed as a function of these mean numbers. We implement the corrections due to global conservation laws. The method is tested using ideal hydrodynamic simulations of $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200A$ GeV with the NeXSPheRIO code. Results are in good agreement with experimental data on net-proton and net-charge fluctuations by the STAR Collaboration.

Highlights from the NA61/SHINE Experiment

The NA61/SHINE experiment is a fixed-target, broad acceptance facility at the CERN SPS. This contribution summarizes the most recent results from the strong interactions NA61/SHINE programme and presents news on the detector upgrade in preparation for the future data taking. The strong interactions programme consists in a two-dimensional scan in beam momentum (from 13A to 150A/158A GeV/c, \sqrt{s_{NN}} from 5.1 to 17.3 GeV) and system size (p + p, Be + Be, Ar + Sc, Xe + La reactions). The experiment searches for the second-order critical end-point in the temperature versus baryo-chemical potential phase diagram and studies the properties of the onset of deconfinement discovered by its predecessor, NA49 at the CERN SPS. The presented results include K/{\pi} multiplicity ratios as a function of energy and system size, singly and multi-strange hadron production in p + p reactions, multiplicity and net-charge fluctuations measured by higher order moments in p + p, Be + Be and Ar + Sc collisions, proton and charged hadron intermittency in Ar + Sc and Pb + Pb reactions, HBT measurements in Ar + Sc and collective electromagnetic effects in Ar + Sc collisions.

Conserved number fluctuations in interacting hadron resonance gas model

Fluctuations of net-baryon, net-charge and net-strangeness numbers are studied within the framework of ideal hadron resonance gas (IHRG), including attractive and repulsive van der Waals interactions. The binding energy and thermodynamic parameters are calculated for different extended versions of the HRG (E-HRG) model. The susceptibility ratios of conserved numbers are estimated in full phase-space using various HRG models that show a strong dependence on collision energies. The effect of kinematic cuts (transverse momentum (pT) and pseudo-rapidity (η)) on those ratios of conserved numbers are also discussed. It is observed that, the normalized susceptibility ratios as a function of collision energies strongly depend on lower pT cut-off and Δη window. The E-HRG models are very sensitive to kinematic acceptance even for net-proton fluctuations. The susceptibility ratios calculated from different HRG models with proper kinematic acceptances are compared with the experimental measurements, which will help to better understand the phase transition and existence of critical point in the QCD phase diagram.

Multiplicity and Net-Charge Fluctuations in Ion $$\mathbf{+}$$ Ion Collisions at the SPS Energies

Multiplicity and Net-Charge Fluctuations in Ion $$\mathbf{+}$$ Ion Collisions at the SPS Energies

Particlization of an interacting hadron resonance gas with global conservation laws for event-by-event fluctuations in heavy-ion collisions

We revisit the problem of particlization of a QCD fluid into hadrons and resonances at the end of the fluid dynamical stage in relativistic heavy-ion collisions in a context of fluctuation measurements. The existing methods sample an ideal hadron resonance gas, therefore, they do not capture the non-Poissonian nature of the grand-canonical fluctuations, expected due to QCD dynamics such as the chiral transition or QCD critical point. We address the issue by partitioning the particlization hypersurface into locally grand-canonical fireballs populating the space-time rapidity axis that are constrained by global conservation laws. The procedure allows to quantify the effect of global conservation laws, volume fluctuations, thermal smearing and resonance decays on fluctuation measurements in various rapidity acceptances, and can be used in fluid dynamical simulations of heavy-ion collisions. As a first application, we study event-by-event fluctuations in heavy-ion collisions at the LHC using an excluded volume hadron resonance gas model matched to lattice QCD susceptibilities, with a focus on (pseudo)rapidity acceptance dependence of net baryon, net proton, and net charge cumulants. We point out large differences between net proton and net baryon cumulant ratios that make direct comparisons between the two unjustified. We observe that the existing experimental data on net-charge fluctuations at the LHC shows a strong suppression relative to a hadronic description.

Isospin Effect on Baryon and Charge Fluctuations from the pNJL Model

We have studied the possible isospin corrections on the skewness and kurtosis of net-baryon and net-charge fluctuations in the isospin asymmetric matter formed in relativistic heavy-ion collisions at RHIC-BES energies, based on a 3-flavor Polyakov-looped Nambu–Jona–Lasinio model. With typical scalar–isovector and vector–isovector couplings leading to the splitting of u and d quark chiral phase transition boundaries and critical points, we have observed considerable isospin effects on the susceptibilities, especially those of net-charge fluctuations. Reliable experimental measurements at even lower collision energies are encouraged to confirm the observed isospin effects.

Charge conservation and higher moments of charge fluctuations

Higher moments of distributions of net charge and baryon number in heavy-ion collisions have been proposed as signals of fundamental QCD phase transitions. In order to better understand background processes for these observables, models are presented which enable one to gauge the effects of local charge conservation, decays of resonances and clusters, Bose symmetrization, and volume fluctuations. Monte Carlo methods for generating samplings of particles consistent with local charge conservation are presented, and are followed by a review of simple analytic models involving a single type of charge with a constant experimental efficiency. The main model consists of thermal emission superimposed onto a simple parameterization of collective flow, known as a blast-wave, with emission being consistent with individual canonical ensembles. The spatial extent of local charge conservation is parameterized by the size and extent over which charge is conserved. The sensitivity of third and fourth order moments, skewness and kurtosis, to these parameters, and to beam energy and baryon density is explored. Comparisons with STAR data show that a significant part of the observed non-Poissonian fluctuations in net-proton fluctuations are explained by charge and baryon-number conservation, but that measurements of the STAR collaboration for fluctuations of net electric charge significantly differ from expectations of the models presented here.

Chemical freeze-out parameters of net-kaons in heavy-ion collisions

We study chemical freeze-out parameters for heavy-ion collisions by performing two different thermal analyses. We analyze results from thermal fits for particle yields, as well as, net-charge fluctuations in order to characterize the chemical freeze-out. The Hadron Resonance Gas (HRG) model is employed for both methods. By separating the light hadrons from the strange hadrons in thermal fits, we study the proposed flavor hierarchy. For the net-charge fluctuations, we calculate the mean-over-variance ratio of the net-kaon fluctuations in the HRG model at the five highest energies of the RHIC Beam Energy Scan (BES) for different particle data lists. We compare these results with recent experimental data from the STAR collaboration in order to extract sets of chemical freeze-out parameters for each list. We focused on particle lists which differ largely in the number of resonant states. By doing so, our analysis determines the effect of the amount of resonances included in the HRG model on the freeze-out conditions. Our findings have potential impact on various other models in the field of relativistic heavy ion collisions.

NA61/SHINE results on fluctuations and correlations at CERN SPS energies

The aim of the NA61/SHINE strong interaction programme is to explore the phase diagram of strongly interacting matter. The main physics goals are the study of the onset of deconfinement and the search for the critical point of strongly interacting matter. These goals are pursued by performing a beam momentum (13A – 150/158A GeV/c) and system size (p+p, p+Pb, Be+Be, Ar+Sc, Xe+La, Pb+Pb) scan. This contribution presents new results from NA61/SHINE on fluctuations and correlations which include in particular quantum correlations, as well as multiplicity and net-charge fluctuations, proton density fluctuations and anisotropic collective flow. Obtained results are compared with other experiments and with model predictions.

Cumulants of Net-charge Distribution from Particle–Antiparticle Sources

It is shown how high-order cumulants of net-charge distribution in hadronic collisions at LHC energies can be expressed via lower-order terms under the assumption that particle-antiparticle pairs are produced in independent local processes. It is argued and tested with HIJING model that this assumption is typically valid for net-proton fluctuations in the case when no critical behaviour is present in the system. Values estimated in such a way can be considered as baselines for direct measurements of high-order net-charge fluctuations in real data.

Beam energy dependence of cumulants of the net-baryon, net-charge, and deuteron multiplicity distributions in Au + Au collisions at sNN=3.0–5.0 GeV

Within the ultra-relativistic quantum molecular dynamics (UrQMD) model, in which the Lorentz-covariant treatment of nuclear mean-field potential is considered, the fluctuations of net-baryon, net-charge and deuterons multiplicity distributions in Au+Au head-on collisions at $\sqrt{s_{NN}}=3.0-5.0$ GeV are calculated. The results show that the nuclear mean-field potential can significantly enhance the magnitude of baryon number fluctuations in narrow rapidity windows, and this enhancement rapidly weakens with increasing beam energy. However, for proton and net-charge number fluctuations, the mean-field effects are less noticeable than that for baryon number. In addition, for net-charge number fluctuations, the negative binomial distribution agrees well with the calculated results at mid-pseudorapidity window. Finally, the event-by-event fluctuations of deuteron number in the coalescence production picture are calculated as well, it is found that its cumulant ratios decrease linearly with increasing the average multiplicity of deuterons per event, i.e., increase with increasing beam energy.

Fluctuations of identified particle yields using the νdyn variable at energies available at the BNL Relativistic Heavy Ion Collider

We study the fluctuations of net-charge, net-pion, net-kaon and net-proton using $D$-measure and $\nu_{dyn}$ variables in heavy-ion jet interaction generator (HIJING), ultra-relativistic quantum molecular dynamics (UrQMD), and hadron resonance gas (HRG) model at different collision energies \sqsn. It has been observed that, the values of $D$ strongly dependent on $\Delta \eta$ in HIJING and UrQMD models and independent in HRG model. The diffusion coefficients ($\sigma$) of identified particles are estimated at various \sqsn. It is observed that, the $\sigma$ values are independent of collision energies but emphasizes the particle species dependence of diffusion coefficient in the QGP medium. This study provides a realistic baseline for comparison with the experimental data.

System size dependence of net-charge fluctuation in nucleus-nucleus collisions at FAIR energies

Dynamical net-charge fluctuations are studied in symmetric nuclear collisions of different system size and at two different collision energies that in the near future will be available at the Facility for Antiproton and Ion Research (FAIR). Multiparticle emission data simulated by the ultra-relativistic quantum molecular dynamics (UrQMD) are analyzed for this purpose. Our primary objective is to examine the system size dependence of dynamical fluctuation of conserved charge at typical FAIR conditions. The centrality, pseudorapidity interval size and azimuthal angle interval size dependence of the dynamical fluctuations are studied too. The effects of global charge conservation (GCC) and finite multiplicity bin width correction are taken into account. To a large extent the systematics followed by the dynamical fluctuations at FAIR conditions are found to be a lower energy extrapolation of the observations made in this regard in the Relativistic Heavy-ion Collider (RHIC) experiments.

Contributions of Jets in Net Charge Fluctuations from the Beam Energy Scan at RHIC and LHC

Dynamical net charge fluctuations have been studied in ultrarelativistic heavy-ion collisions from the beam energy scan at RHIC and LHC energies by carrying out the hadronic model simulation. Monte Carlo model, HIJING, is used to generate events in two different modes, HIJING-default with jet quenching switched off and jet/minijet production switched off. A popular variable, ν[+-,dyn], is used to study the net charge fluctuations in different centrality bins and the findings are compared with the available experimental values reported earlier. Although the broad features of net charge fluctuations are reproduced by the HIJING, the model predicts the larger magnitude of fluctuations as compared to the one observed in experiments. The role of jets/minijets production in reducing the net charge fluctuations is, however, distinctly visible from the analysis of the two types of HIJING events. Furthermore, dNch/dη and 1/N scaling is partially exhibited, which is due to the fact that, in HIJING, nucleus-nucleus collisions are treated as multiple independent nucleon-nucleon collisions.

Coherent Radio Emission from a Twisted Magnetosphere after a Magnetar-quake

Abstract Magnetars are a class of highly magnetized, slowly rotating neutron stars, only a small fraction of which exhibit radio emission. We propose that the coherent radio curvature emission is generated by net charge fluctuations from a twist-current-carrying bundle (the j-bundle) in the scenario of magnetar-quake. Two-photon pair production is triggered, which requires a threshold voltage not too much higher than 109 V in the current-carrying bundle, and which can be regarded as the “open field lines” of a magnetar. Continued untwisting of the magnetosphere maintains change fluctuations, and hence coherent radio emission, in the progressively shrinking j-bundle, which lasts for years until the radio beam is too small to be detected. The modeled peak flux of radio emission and the flat spectrum are generally consistent with the observations. We show that this time-dependent, conal-beam, radiative model can interpret the variable radio pulsation behaviors and the evolution of the X-ray hot spot of the radio-transient magnetar XTE J1810−197 and the high-B pulsar/anomalous X-ray pulsar PSR J1622−4950. Radio emission with luminosity of and high-frequency oscillations are expected to be detected for a magnetar after an X-ray outburst. Differences of radio emission between magnetars and ordinary pulsars are discussed.

Isolating dynamical net-charge fluctuations

We modify the usual definitions of cumulants of net-charge fluctuations in a way that isolates dynamical fluctuations. The new observables, which we call dynamical cumulants, are robust with respect to trivial correlations induced by volume fluctuations and global charge conservation. We illustrate the potential of dynamical cumulants by carrying out Monte Carlo simulations where all correlations are trivial. The results of our simulations agree well with Relativistic Heavy Ion Collider (RHIC) data, and are used to illustrate that dynamical cumulants consistently isolate dynamical fluctuations.

Freeze-out conditions from strangeness observables at RHIC

We determine chemical freeze-out conditions from strangeness observables measured at RHIC beam energies. Based on a combined analysis of lowest-order net-kaon fluctuations and strange anti-baryon over baryon yield ratios we obtain visibly enhanced freeze-out conditions at high beam energies compared to previous studies which analyzed net-proton and net-charge fluctuations. Our findings are in qualitative agreement with the recent study (Bellwied et al. in arXiv:1805.00088 [hep-ph], 2018) which utilizes the net-kaon fluctuation data in combination with information from lattice QCD. Our complementary approach shows that also strange hadron yield ratios are described by such enhanced freeze-out conditions.

Poisson baseline of net-charge fluctuations in relativistic heavy ion collisions**Supported by Fund Project of National Natural Science Foundation of China (11647093, 11405088, 11521064), Fund Project of Chengdu Technological University (2016RC004), the Major State Basic Research Development Program of China (2014CB845402) and the Ministry of Science and Technology (MoST) (2016YFE0104800)

Taking doubly charged particles, positive-negative charge pair production and the effects of volume fluctuations into account, the Poisson baseline of the fluctuations of net-charge is studied. Within the Poisson baseline, the cumulants of net-charge are derived. Comparing to the Skellam baseline of net-charge, we infer that doubly charged particles broaden the distributions of net-charge, while positive-negative charge pairs narrow the distributions. Using the ratios of doubly charged particles and positive-negative charge pairs from neutral resonance decays to the total positive charges from THERMINATOR 2, the first four orders of moments and the corresponding moment products are calculated in the Poisson baseline for Au + Au collisions at at RHIC/STAR. We find that the standard deviation is mainly influenced by the resonance decay, while the third and fourth order moments and corresponding moment products are mainly modified and fit the data of RHIC/STAR much better after including the effects of volume fluctuations.

Higher moments of multiplicity fluctuations in a hadron-resonance gas with exact conservation laws

Higher moments of multiplicity fluctuations of hadrons produced in central nucleus-nucleus collisions are studied within the hadron-resonance gas model in the canonical ensemble. Exact conservation of three charges, baryon number, electric charge, and strangeness, is enforced in the large volume limit. Moments up to the forth order of various particles are calculated at SPS, RHIC and LHC energies. The asymptotic fluctuations within a simplified model with only one conserved charge in the canonical ensemble are discussed where simple analytical expressions for moments of multiplicity distributions can be obtained. Moments products of net-proton, net-kaon, and net-charge distributions in Au + Au collisions at RHIC energies are calculated. The pseudo-rapidity coverage dependence of net-charge fluctuation is discussed.