Thermalization of charm quarks in infinite and finite QGP matter

We study the thermalization process of charm quarks in hot and dense matter. The diffusion of heavy quarks is calculated via a Langevin equation, both for a static medium as well as a QGP medium generated by a (3+1)D hydrodynamic model. We define two criteria for the thermalization of the heavy quarks, and observe thermalization times that are longer than the lifetime of the QGP phase for reasonable values of the diffusion constant.Comment: 10 pages, 12 figure

Erraticity of Rapidity Gaps

The use of rapidity gaps is proposed as a measure of the spatial pattern of an event. When the event multiplicity is low, the gaps between neighboring particles carry far more information about an event than multiplicity spikes, which may occur very rarely. Two moments of the gap distrubiton are suggested for characterizing an event. The fluctuations of those moments from event to event are then quantified by an entropy-like measure, which serves to describe erraticity. We use ECOMB to simulate the exclusive rapidity distribution of each event, from which the erraticity measures are calculated. The dependences of those measures on the order of $q$ of the moments provide single-parameter characterizations of erraticity.Comment: 10 pages LaTeX + 5 figures p

Destruction of the Mott Insulating Ground State of Ca_2RuO_4 by a Structural Transition

We report a first-order phase transition at T_M=357 K in single crystal Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous decrease in electrical resistivity signals the near destruction of the Mott insulating phase and is triggered by a structural transition from the low temperature orthorhombic to a high temperature tetragonal phase. The magnetic susceptibility, which is temperature dependent but not Curie-like decreases abruptly at TM and becomes less temperature dependent. Unlike most insulator to metal transitions, the system is not magnetically ordered in either phase, though the Mott insulator phase is antiferromagnetic below T_N=110 K.Comment: Accepted for publication in Phys. Rev. B (Rapid Communications

Pareto Boundary of the Rate Region for Single-Stream MIMO Interference Channels: Linear Transceiver Design

We consider a multiple-input multiple-output (MIMO) interference channel (IC), where a single data stream per user is transmitted and each receiver treats interference as noise. The paper focuses on the open problem of computing the outermost boundary (so-called Pareto boundary-PB) of the achievable rate region under linear transceiver design. The Pareto boundary consists of the strict PB and non-strict PB. For the two user case, we compute the non-strict PB and the two ending points of the strict PB exactly. For the strict PB, we formulate the problem to maximize one rate while the other rate is fixed such that a strict PB point is reached. To solve this non-convex optimization problem which results from the hard-coupled two transmit beamformers, we propose an alternating optimization algorithm. Furthermore, we extend the algorithm to the multi-user scenario and show convergence. Numerical simulations illustrate that the proposed algorithm computes a sequence of well-distributed operating points that serve as a reasonable and complete inner bound of the strict PB compared with existing methods.Comment: 16 pages, 9 figures. Accepted for publication in IEEE Tans. Signal Process. June. 201

Baryonic Signatures in Large-Scale Structure

We investigate the consequences of a non-negligible baryon fraction for models of structure formation in Cold Dark Matter dominated cosmologies, emphasizing in particular the existence of oscillations in the present-day matter power spectrum. These oscillations are the remnants of acoustic oscillations in the photon-baryon fluid before last scattering. For acceptable values of the cosmological and baryon densities, the oscillations modulate the power by up to 10%, with a `period' in spatial wavenumber which is close to Delta k approximately 0.05/ Mpc. We study the effects of nonlinear evolution on these features, and show that they are erased for k > 0.2 h/ Mpc. At larger scales, the features evolve as expected from second-order perturbation theory: the visibility of the oscillations is affected only weakly by nonlinear evolution. No realistic CDM parameter combination is able to account for the claimed feature near k = 0.1 h/ Mpc in the APM power spectrum, or the excess power at 100 Mpc/h wavelengths quoted by several recent surveys. Thus baryonic oscillations are not predicted to dominate existing measurements of clustering. We examine several effects which may mask the features which are predicted, and conclude that future galaxy surveys may be able to detect the oscillatory features in the power spectrum provided baryons comprise more than 15% of the total density, but that it will be a technically challenging achievement.Comment: 16 pages, 13 Figures, to be published in MNRA

On the dust tori in Palomar-Green quasars

The dust clouds in the torus of the quasar are irradiated by the central source, and the clouds at the inner radius of the torus re-radiate mostly in the near-infrared (NIR) wavebands. The ratio of the near-infrared luminosity to the bolometric luminosity L_NIR/L_bol can therefore reflect the torus geometry to some extent. We find a significant correlation between the ratio of the near-infrared luminosity to the bolometric luminosity L_NIR/L_bol and the central black hole mass M_bh for the Palomar-Green(PG) quasars, whereas no correlation is found between the Eddington ratio L_bol/L_Edd and the ratio L_NIR/L_bol. Similar correlations are found for the mid-infrared and far-infrared cases. It may imply that the torus geometry, i.e., the solid angle subtended by the dust torus as seen from the central source, does not evolve with the accretion rate. The correlation of the solid angle subtended by the torus with the central black hole mass M_bh implies that the formation of the dust torus is likely regulated by the central black hole mass. We find that the torus thickness H increases with quasar bolometric luminosities, which is different from the constant torus thickness H with luminosity assumed in the receding torus model. The mean covering factor of the dust clouds at the inner radius of the torus derived from the IR emission data is ~0.39 for PG quasars. The average relative thickness H/R of the tori in the PG quasars derived from the ratios of the infrared to bolometric luminosities is ~0.9. We suggest that the further IR observations on a larger quasar sample including more fainter quasars by the Spitzer Space Telescope will help understand the physics of the dust tori in quasars.Comment: The incorrect V-magnitude used for 1351+640 is fixed, the main conclusions are not changed, accepted for publication in Ap

The flavour asymmetry of polarized anti-quarks in the nucleon

We present a study of the flavour asymmetry of polarized anti-quarks in the nucleon using the meson cloud model. We include contributions both from the vector mesons and the interference terms of pseudoscalar and vector mesons. Employing the bag model, we first give the polarized valence quark distribution of the $\rho$ meson and the interference distributions. Our calculations show that the interference effect mildly increases the prediction for \Delta \dbar(x)-\Delta \ubar(x) at intermediate $x$ region. We also discuss the contribution of `Pauli blocking' to the asymmetry.Comment: 22 pages, LaTex, 5 PS figures. Version to appear in Eur. Phys. J. C. An appendix is added for expressions for the helicity dependent fluctuation functions. An error in the programme for fluctuation function f_{(\pi\rho)\Delta /N} is corrected, which increases numerical results by about 10%. Unchanged conclusion

RKKY Interactions in Graphene Landau Levels

We study RKKY interactions for magnetic impurities on graphene in situations where the electronic spectrum is in the form of Landau levels. Two such situations are considered: non-uniformly strained graphene, and graphene in a real magnetic field. RKKY interactions are enhanced by the lowest Landau level, which is shown to form electron states binding with the spin impurities and add a strong non-perturbative contribution to pairwise impurity spin interactions when their separation $R$ no more than the magnetic length. Beyond this interactions are found to fall off as $1/R^3$ due to perturbative effects of the negative energy Landau levels. Based on these results, we develop simple mean-field theories for both systems, taking into account the fact that typically the density of states in the lowest Landau level is much smaller than the density of spin impurities. For the strain field case, we find that the system is formally ferrimagnetic, but with very small net moment due to the relatively low density of impurities binding electrons. The transition temperature is nevertheless enhanced by them. For real fields, the system forms a canted antiferromagnet if the field is not so strong as to pin the impurity spins along the field. The possibility that the system in this latter case supports a Kosterlitz-Thouless transition is discussed

SUSY Dark Matter In Light Of CDMS/XENON Limits

In this talk we briefly review the current CDMS/XENON constraints on the neutralino dark matter in three popular supersymmetric models: the minimal (MSSM), the next-to-minimal (NMSSM) and the nearly minimal (nMSSM). The constraints from the dark matter relic density and various collider experiments are also taken into account. The conclusion is that for each model the current CDMS/XENON limits can readily exclude a large part of the parameter space allowed by other constraints and the future SuperCDMS or XENON100 can cover most of the allowed parameter space. The implication for the Higgs search at the LHC is also discussed. It is found that in the currently allowed parameter space the MSSM charged Higgs boson is quite unlikely to be discovered at the LHC while the neutral Higgs bosons $H$ and $A$ may be accessible at the LHC in the parameter space with a large $\mu$ parameter.Comment: talk given at 2nd International Workshop on Dark Matter, Dark Energy and Matter-Antimatter Asymmetry, Nov 5-6, 2010, Hsinchu, Taiwan (to appear in Int. J. Mod. Phys. D