Formation Of Disoriented Chiral Condensates Research Articles

Modeling of charged kaon and neutral kaon fluctuations as a signature for the production of a disoriented chiral condensate in A−A collisions

Anomalous event-by-event fluctuations of the relative yields of neutral (K$^0_s$) and charged kaon (K$^\pm$) have been predicted to yield a signature for the formation of Disoriented Chiral Condensate (DCC) in relativistic heavy-ion collisions. In this work, we model the production and decay of DCCs in the context of heavy-ion collisions at the Large Hadron Collider, and estimate the sensitivity of large acceptance detectors, such as the ALICE detector, towards the identification of such anomalous decays. Our study is based on the robust statistical observable, $\nu_{\rm dyn}$, known for its sensitivity to dynamical fluctuations. We first present simulations without DCCs, based on the HIJING and AMPT models, in order to establish an approximate reference for the magnitude of $\nu_{\rm dyn}({\rm K}^{\pm},{\rm K}^{0}_{s})$ and its centrality evolution in Pb--Pb collisions at the TeV energy scale. We next introduce simple phenomenological models of K$^0_s$ vs. K$^\pm$ event-by-event yield fluctuations, which we use to study the feasibility and sensitivity of detection of the production of DCCs in heavy-ion collisions. Although the precision of models such as HIJING and AMPT limit their use as absolute references and thus render anomalous fluctuations difficult to define precisely, our studies demonstrate that the magnitude of $\nu_{\rm dyn}({\rm K}^{\pm},{\rm K}^{0}_{s})$ is in fact very sensitive to the presence of small admixture of DCCs in normal non-DCC events. Consequently, while large values of $\nu_{\rm dyn}({\rm K}^{\pm},{\rm K}^{0}_{s})$ may not be sufficient to identify the existence of DCCs, nonetheless they constitute a first and necessary condition to signal their possible production in heavy-ion collisions.

Dynamical formation of disoriented chiral condensates

We study the dynamical formation of disoriented chiral condensates in very high energy nucleus-nucleus collisions using Bjorken hydrodynamics and relativistic nucleation theory. It is the dynamics of the first order confinement phase transition which controls the evolution of the system. Every bubble or fluctuation of the new, hadronic, phase obtains its own chiral condensate with a probability determined by the Boltzmann weight of the finite temperature effective potential of the linear sigma model. We evaluate domain size and chiral angle distributions, which can be used as initial conditions for the solution of semiclassical field equations.

Photon production from non-equilibrium disoriented chiral condensates in a longitudinal expansion: A theoretic framework

A theoretical framework is developed for treating the quantization of the photons in a spacetime with a longitudinal expansion. This can be used to study the production of the photons through the non-equilibrium relaxation of a disoriented chiral condensate presumably formed in the expanding hot central region in ultra-relativistic heavy-ion collisions. These photons can be a signature of the formation of disoriented chiral condensates in the direct photon measurements of heavy-ion collisions.

STAR's measurement of correlations and fluctuations between photons and charged particles at forward rapidities at RHIC

The dynamical fluctuations measured between the multiplicities of the photons and the charged particles at forward pseudorapidities (−3.7 ⩽ η ⩽ −2.5) for Au+Au and Cu+Cu collisions at are presented. Correlation between photons and charged particle multiplicities provides a signal of the formation of disoriented chiral condensate (DCC). Photons and charged particles are measured with the photon multiplicity detector and the forward time projection chamber, respectively, at the STAR experiment. Two different quantities are used to study the fluctuations: one (νdynamics) measures the deviation from the Poissonian behavior of the correlated particle production and the other measures the width of the distribution of Nγ/Ncharge. Finite fluctuations with little pT dependence for different systems have been observed suggesting no significant presence of DCC within the given sensitivity.

Effects of mode-mode and isospin-isospin correlations on domain formation of disoriented chiral condensates

The effects of mode-mode and isospin-isospin correlations on nonequilibrium chiral dynamics are investigated by using the method of the time-dependent variational approach with squeezed states as trial states. Our numerical simulations show that large domains of the disoriented chiral condensate (DCC) are formed because of the combined effect of the mode-mode and isospin-isospin correlations. Moreover, it is found that, when the mode-mode correlation is included, the DCC domain formation is accompanied by the amplification of the quantum fluctuation, which implies the squeezing of the state. However, neither the DCC domain formation nor the amplification of the quantum fluctuation is observed if only the isospin-isospin correlation is included. This suggests that the mode-mode coupling plays a key role in the DCC domain formation.

Possible evidence of disoriented chiral condensates from the anomaly in Ω. and -Ω abundances at the super proton synchrotron

No conventional picture of nucleus-nucleus collisions has yet been able to explain the abundance of Ω and -Ω. in central collisions between Pb nuclei at 158 A GeV at the CERN SPS. We argue that such a deviation from predictions of statistical thermal models and numerical simulations is the evidence that they are produced as topological defects in the form of skyrmions arising from the formation of disoriented chiral condensates. The estimated domain size falls in the right range to be consistent with the so far non-observation of disoriented chiral condensate (DCC) from the distribution of neutral pions.

Centrality dependence of charged-neutral particle fluctuations in158AGeV208Pb+208Pbcollisions

Results on the study of localized fluctuations in the multiplicity of charged particles and photons produced in 158A GeV/c Pb+Pb collisions are presented for varying centralities. The charged versus neutral particle multiplicity correlations in common phase space regions of varying azimuthal sizes are analyzed by two different methods. Various types of mixed events are constructed to probe fluctuations arising from different sources. The measured results are compared to those from simulations and from mixed events. The comparison indicates the presence of nonstatistical fluctuations in both the charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charged-neutral fluctuations, a possible signature of formation of disoriented chiral condensates, are observed. An upper limit on the production of disoriented chiral condensates is set.

Φ-MEASURE AND DISORIENTED CHIRAL CONDENSATES

Fluctuations in the ratio of neutral to charged pions arising due to formation of disoriented chiral condensates (DCC) are discussed using the Φ-measure. The properties of the measure for various cases of DCC and non-DCC are discussed. The effect of detector efficiencies and other experimental factors are presented. Application of the Φ-measure to simulated data, within the context of a simple DCC model and to experimental data are also discussed.

Photon production from non-equilibrium disoriented chiral condensates in a spherical expansion

We study the production of photons through the non-equilibrium relaxation of a disoriented chiral condensate formed in the expanding hot central region in ultra-relativistic heavy-ion collisions. It is found that the expansion smoothes out the resonances in the process of parametric amplification such that the non-equilibrium photons are dominant to the thermal photons over the range 0.2–2 GeV. We propose that to search for non-equilibrium photons in the direct photon measurements of heavy-ion collisions can be a potential test of the formation of disoriented chiral condensates.

Fluctuation probe of disoriented chiral condensates

We show that an event-by-event fluctuation of the ratio of neutral pions or resulting photons to charged pions can be used as an effective probe for the formation of disoriented chiral condensates. The fact that the neutral pion fraction produced in case of disoriented chiral condensate formation has a characteristic extended non gaussian shape, is shown to be the key factor which forms the basis of the present analysis.

Anomalous production of Ω and is evidence of the formation of disoriented chiral condensates?

No conventional picture of nucleus–nucleus collisions has yet been able to explain the abundance of Ω and hadrons in central collisions between Pb nuclei at 158 A GeV at the CERN SPS. We argue that this is evidence that they are produced from topological defects in the form of skyrmions arising from the formation of disoriented chiral condensates. It is shown that the excess Ω or produced could not be easily washed out in the hadronic medium and therefore would survive in the final state.

Experimental signatures of anomaly induced disoriented chiral condensate formation

We discuss characteristic experimental signatures related to the formation of domains of disoriented chiral condensate (DCC) triggered by the axial anomaly in relativistic heavy ion collisions. We predict that the enhancement of the fraction of neutral pions compared to all pions depends on the angle of emission with respect to the scattering plane and is concentrated at small transverse momentum and small rapidity in the center-of-mass frame. The anisotropy with respect to the reaction plane is also observable in the inclusive photon distribution.

Dynamics of symmetry breaking and tachyonic preheating.

We reconsider the old problem of the dynamics of spontaneous symmetry breaking (SSB) using 3D lattice simulations. We develop a theory of tachyonic preheating, which occurs due to the spinodal instability of the scalar field. Tachyonic preheating is so efficient that SSB typically completes within a single oscillation as the field rolls towards the minimum of its effective potential. We show that, contrary to previous expectations, preheating in hybrid inflation is typically tachyonic. Our results may also be relevant for the theory of the formation of topological defects and of disoriented chiral condensates in heavy ion collisions.

Is anomalous production of omega and omega; evidence for disoriented chiral condensates?

No conventional picture of nucleus-nucleus collisions has yet been able to explain the abundance of Omega and Omega; hadrons in central collisions between Pb nuclei at 158A GeV at the CERN SPS. We argue that this is evidence that they are produced as topological defects arising from the formation of disoriented chiral condensates with an average domain size of about 2 fm.

Microscopic dynamics of disoriented chiral condensate formation

The dynamics of the pion field after a quench is studied in the framework of the linear sigma model. Our aim is to determine to what extent the amplified pion field resembles the disoriented chiral condensate (DCC) picture originally proposed in the early 1990s. We present the result of a computer experiment where, among other things, we study in detail the correlation between isospin orientations of the distinct modes of the field. We show that this correlation is absent. In a sense, the distinct modes behave as distinct DCCs. The implications of this observation are discussed.

Enhanced event-by-event fluctuations in pion multiplicity as a signal of disoriented chiral condensates in relativistic heavy-ion collisions

The factorial moments of the pion multiplicity distributions are calculated with HIJING and UrQMD and found to be independent of the ${p}_{T}$ range included, in contrast to recent simulations with the linear $\ensuremath{\sigma}$ model which leads to large enhancements for pions with transverse kinetic energies below 200 MeV. This supports the use of the ratio of the factorial moments of low and high ${p}_{T}$ pions as a signal of ``new'' physics at low momentum scales, such as the formation of disoriented chiral condensates.

Formation of disoriented chiral condensates in relativistic heavy-ion collisions

We present a brief review of the subject of disoriented chiral condensates (DCC). We describe the conventional scenarios for the formation of DCC which have been proposed in the literature. Observable signals, such as fluctuations in neutral to charged pion ratio, are discussed. We then discuss a novel scenario for DCC formation, recently proposed by us, where the entire region of hot partons can get converted into a single large DCC. Our arguments suggest that formation of such large DCC is unlikely in the collision of heavy nuclei, and ultra-high energy hadronic collisions may be better suited for this.

Dynamical simulation of disoriented chiral condensate formation in Bjorken rods

Using a semiclassical treatment of the linear $\ensuremath{\sigma}$ model, we simulate the dynamical evolution of an initially hot cylindrical rod endowed with a longitudinal Bjorken scaling expansion (a ``Bjorken rod''). The field equation is propagated until full decoupling has occurred and the asymptotic many-body state of free pions is then obtained by a suitable Fourier decomposition of the field and a subsequent stochastic determination of the number of quanta in each elementary mode. The resulting transverse pion spectrum exhibits visible enhancements below 200 MeV due to the parametric amplification caused by the oscillatory relaxation of the chiral order parameter. Ensembles of such final states are subjected to various event-by-event analyses. The factorial moments of the multiplicity distribution suggest that the soft pions are nonstatistical. Furthermore, their emission patterns exhibit azimuthal correlations that have a bearing on the domain size in the source. Finally, the distribution of the neutral pion fraction shows a significant broadening for the soft pions which grows steadily as the number of azimuthal segments is increased. All of these features are indicative of disoriented chiral condensates and it may be interesting to apply similar analyses to actual data from high-energy nuclear collision experiments.

Out-of-equilibrium photon production from disoriented chiral condensates

We study the production of photons through the nonequilibrium relaxation of a disoriented chiral condensate. We propose that to search for nonequilibrium photons in the direct photon measurements of heavy-ion collisions can be a potential test of the formation of disoriented chiral condensates.

Looking for disoriented chiral condensates from pion distributions

We suggest two methods for the detection of the formation of disoriented chiral condensates in heavy ion collisions. We show that the variance in the number of charged pions (in a suitable range of momentum space) provides a signature for the observation of a disoriented chiral condensate. The signal should be observable even if multiple domains of D$\chi$C form provided the average number of pions per domain is significantly larger than unity. The variance of the number charged pions alone provides a signal which can be used even if the number of neutral pions cannot be measured in a given detector. On the other hand, the probability distribution in $R$, the proportion of neutral pions to all pions emitted in heavy ion collisions in certain kinematic regions, has been suggested as a signal of a disoriented chiral condensate. Here we note that the signature can be greatly enhanced by making suitable cuts in the data. In particular, we consider reducing the data set such that the $k$ pions with lowest $p_T$ are all neutral. We find that, given such cuts, $<R>$ can be substantially different from 1/3. For example, for a single D$\chi$C domain without contamination due to incoherently emitted pions, $<R>$ is 3/5 given the pion with lowest $p_T$ is neutral, and 5/7 given the two pions with lowest $p_T$ are both neutral, {\it etc.}. The effects of multi-domain D$\chi$C formation and noise due to incoherent pion emission can be systematically incorporated. Potential applications to experiments and their limitations are briefly discussed.