Proton number fluctuation as a signal of the QCD critical end-point

We argue that the event-by-event fluctuation of the proton number is a meaningful and promising observable for the purpose of detecting the QCD critical end-point in heavy-ion collision experiments. The long range fluctuation of the order parameter induces a characteristic correlation between protons which can be measured. The proton fluctuation also manifests itself as anomalous enhancement of charge fluctuations near the end-point, which might be already seen in existing data.Comment: 4 pages, version accepted in PR

Sub-threshold J/ψ and ϒ production in γA collisions

We study sub-threshold heavy quarkonium (J/ψ and ϒ) photo-productions in γA collisions as an independent test of the universality of the nucleon-nucleon short range correlation (SRC) in nuclear scattering processes. Just below the γp threshold, the cross section is dominated by the mean field contribution of nucleons inside the nucleus. The SRC contributions start to dominate at lower photon energies, depending on the fraction of the SRC pairs in the target nucleus. We give an estimate of the cross sections in the sub-threshold region both for J/ψ and ϒ. This may be helpful for future measurements at JLab as well as at the Electron-Ion Collider in the U.S., and especially in China

Parton picture for the strongly coupled SYM plasma

Deep inelastic scattering off the strongly coupled N=4 supersymmetric Yang-Mills plasma at finite temperature can be computed within the AdS/CFT correspondence, with results which are suggestive of a parton picture for the plasma. Via successive branchings, essentially all partons cascade down to very small values of the longitudinal momentum fraction x and to transverse momenta smaller than the saturation momentum Q_s\sim T/x. This scale Q_s controls the plasma interactions with a hard probe, in particular, the jet energy loss and its transverse momentum broadening.Comment: 4 pages, Talk given at Quark Matter 2008: 20th International Conference on Ultra-Relativistic Nucleus Nucleus Collisions (QM 2008), Jaipur, India, 4-10 Feb 200

Phase Fluctuations near the Chiral Critical Point

The Helmholtz free energy density is parametrized as a function of temperature and baryon density near the chiral critical point of QCD. The parametrization incorporates the expected critical exponents and amplitudes. An expansion away from equilibrium states is achieved with Landau theory. This is used to calculate the probability that the system is found at a density other than the equilibrium one. Such fluctuations are predicted to be very large in heavy ion collisions.Comment: 7 pages, 8 figures, Winter Workshop on Nuclear Dynamics 201

Holographic current correlators at finite coupling and scattering off a supersymmetric plasma

By studying the effect of the order(\alpha'^3) string theory corrections to type IIB supergravity, including those corrections involving the Ramond-Ramond five-form field strength, we obtain the corrected equations of motion of an Abelian perturbation of the AdS_5-Schwarzschild black hole. We then use the gauge theory/string theory duality to examine the coupling-constant dependence of vector current correlators associated to a gauged U(1) sub-group of the global R-symmetry group of strongly-coupled N=4 supersymmetric Yang-Mills theory at finite temperature. The corrections induce a set of higher-derivative operators for the U(1) gauge field, but their effect is highly suppressed. We thus find that the order(\alpha'^3) corrections affect the vector correlators only indirectly, through the corrected metric. We apply our results to investigate scattering off a supersymmetric Yang-Mills plasma at low and high energy. In the latter regime, where Deep Inelastic Scattering is expected to occur, we find an enhancement of the plasma structure functions in comparison with the infinite 't Hooft coupling result.Comment: 38 pages, 6 figures, minor clarifications added, typos corrected, references adde

Search for the QCD Critical Point: Higher Moments of Net-proton Multiplicity Distributions

Higher moments of event-by-event net-proton multiplicity distributions have been applied to search for the QCD critical point. Model results are used to provide a baseline for this search. The measured moment products, $\kappa\sigma^2$ and $S\sigma$ of net-proton distributions, which are directly connected to the thermodynamical baryon number susceptibility ratio in Lattice QCD and Hadron Resonance Gas (HRG) model, are compared to the transport and thermal model results. We argue that a non-monotonic dependence of $\kappa \sigma^2$ and $S \sigma$ as a function of beam energy can be used to search for the QCD critical point.Comment: 7 pages, 3 figures. CPOD 2010 Proceeding

Virtual Compton Scattering off a Spinless Target in AdS/QCD

We study the doubly virtual Compton scattering off a spinless target $\gamma^*P\to\gamma^*P'$ within the Anti-de Sitter(AdS)/QCD formalism. We find that the general structure allowed by the Lorentz invariance and gauge invariance of the Compton amplitude is not easily reproduced with the standard recipes of the AdS/QCD correspondence. In the soft-photon regime, where the semi-classical approximation is supposed to apply best, we show that the measurements of the electric and magnetic polarizabilities of a target like the charged pion in real Compton scattering, can already serve as stringent tests.Comment: 21 pages, version to be published in JHEP

Exploring the proton spin structure

Understanding the spin structure of the proton is one of the main challenges in hadronic physics. While the concepts of spin and orbital angular momentum are pretty clear in the context of non-relativistic quantum mechanics, the generalization of these concepts to quantum field theory encounters serious difficulties. It is however possible to define meaningful decompositions of the proton spin that are (in principle) measurable. We propose a summary of the present situation including recent developments and prospects of future developments.Comment: 8 pages, 1 figure, 2 tables, contribution to the proceedings of the DAE-BRNS High Energy Physics Symposium 2014, Dec 8-12, Guwahati, Indi

Polarized DIS in N=4 SYM: Where is spin at strong coupling?

Using the AdS/CFT correspondence, we calculate the polarized structure functions in strongly coupled N=4 supersymmetric Yang-Mills theory deformed in the infrared. We find that the flavor singlet contribution to the g_1 structure function is vanishingly small, while the flavor non-singlet contribution shows the Regge behavior at small-x with an intercept slightly less than 1. We explicitly check that the latter satisfies the moment sum rule. We discuss the `spin crisis' problem and suggest that at strong coupling the spin of a hadron entirely comes from the orbital angular momentum.Comment: 24 page

On the Beaming of Gluonic Fields at Strong Coupling

We examine the conditions for beaming of the gluonic field sourced by a heavy quark in strongly-coupled conformal field theories, using the AdS/CFT correspondence. Previous works have found that, contrary to naive expectations, it is possible to set up collimated beams of gluonic radiation despite the strong coupling. We show that, on the gravity side of the correspondence, this follows directly (for arbitrary quark motion, and independently of any approximations) from the fact that the string dual to the quark remains unexpectedly close to the AdS boundary whenever the quark moves ultra-relativistically. We also work out the validity conditions for a related approximation scheme that proposed to explain the beaming effect though the formation of shock waves in the bulk fields emitted by the string. We find that these conditions are fulfilled in the case of ultra-relativistic uniform circular motion that motivated the proposal, but unfortunately do not hold for much more general quark trajectories.Comment: 1+33 pages, 2 figure