The Seminars are normally held Wednesdays at 2:00 PM in the Theory Lounge

- Wednesday, 05-26-2010

**Topics on Small-x Physics**- Kirill Tuchin (Iowa State University)
- Abstract:

- Friday, 05-21-2010

**Multiple scattering in Drell-Yan process**- Hong-xi Xing (LBNL)
- Abstract: With the high twist calculation, we studied the Drell-Yan dilepton pair transverse momentum distribution in PA collisions. We found that both the single and double scatterings appears a 1/PT2 or 1/Q2 suppression, it is a typical behavior of the PQCD calculation. There are three kinds of processes in double scattering: soft-hard and hard-hard and their interference, in the large PT region, the interference term gives a small contribution to the final result. We also show that the double scattering have a strong nuclear size dependence.

- Tuesday, 04-06-2010

**STAR azimuthal correlations of forward di-pions in d+Au collisions in the Color Glass Condensate**- Cyrille Marquet (Saclay, France)
- Abstract: STAR measured the azimuthal correlations of forward pion pairs in d+Au collisions. The disappearance of the away-side peak in central collisions, compared to peripheral and p+p collisions, is consistent with Color Glass Condensate predictions. Such monojets are produced because at forward rapidities, the Gold nucleus wave function is probed at small-x and the non-linear saturation regime of QCD is reached.

- Wednesday, 12-09-2009

**Equilibration of QGP from gravitational collapse and shock wave collision**- Shu Lin (Stony Brook)
- Abstract: In this talk, I will focus on the equilibiration of matter and formation of QGP in the gauge/gravity duality approach. I will discuss a specific gravitational collapse model, which is dual to the thermalization of gauge field on the boundary. We found universal behavior of the spectral densities as the gauge fields approaches local equilibrium. The spectral densities differ from their thermal counterpart by general oscillations. I will also talk about the recent works on entropy estimation in heavy ion collision from gravitational shock wave collision. We found the existence of critical impact parameter, beyond which no horizon can form. In the dual field theory, it means matter produced in the collision cannot thermalize.

- Wednesday, 11-18-2009

**QCD thermodynamics and hadron fluctuations on lattice**- Chuan Miao (Brookhaven National Lab)
- Abstract: The thermodynamics of QCD is of great interest for the heavy ion collision experiments. In this talk, I will introduce the equation of state calculations on lattice by the HotQCD collaboration. Using the improved staggered fermion, the calculations are carried out at almost physical quark masses. Pressure, energy density as well as the speed of sound are analyzed throughout the temperature range from 150MeV to 400 MeV. I will also show the calculations on the hadron fluctuations, which can give us insights on the effective degrees of freedom in the system. I will compare these results withthe Hardon resonance gas model in the lower temperature phase.

- Wednesday, 11-04-2009

**"SUSY electroweak baryogenesis, EDM experiments, and ILC signature"**- Yingchuan Li (University of Wisconsin, Madison)
- Abstract: Among all the baryogenesis mechanisms, electroweak baryogenesis (EWB) remains to be the most testable one. It can not happen in SM, but possibly in SUSY models. However, the CP violation needed in electroweak baryogenesis is constrained by electric dipole moment experiments. We study the interplay of electroweak baryogenesis and EDM experiments in MSSM. We point out that, among all the possible scenarios, bino-driven baryogenesis is the one that is least constrained by current EDM bounds. We found the lower bound on EDMs from SUSY EWB is within the reach of next generation of EDM experiments. We show that SUSY EWB can also be probed by the CP violating observable at future linear collider.

- Tuesday, 10-27-2009

**Threshold Resummation in Pair Production**- Leandro Almeida (Stony Brook University)
- Abstract: I will discuss the resummation of large logarithmic perturbative corrections to the partonic cross sections relevant for dihadron production in hadronic collisions, H1 H2 ->h1 h2 X, at high invariant mass of the produced hadron pair. These corrections arise near the threshold for the partonic reaction and are associated with soft-gluon emission. We perform the resummation to next-to-leading logarithmic accuracy, and show how to incorporate consistently cuts in rapidity and transverse momentum of the observed particles. We present numerical results for ﬁxed-target and ISR regimes and ﬁnd enhancements over the next-to-leading order cross section, which signiﬁcantly improve the agreement between theoretical predictions and data.

- Wednesday, 10-14-2009

**Some recent progress on single transverse-spin asymmetry**- Zhong-Bo Kang (RIKEN BNL Research Center)
- Abstract: Single transverse-spin asymmetries (SSAs) have received much attention in recent years, both experimentally and theoretically. In this talk, I will discuss some recent theoretical developments, including the twist-3 correlation functions, their evolution and their appearance in experiments. I will also outline some ideas in understanding the recent experimental puzzles, including the pt behavior of SSA and the anomalous large SSA for eta meson. Finally I discuss the SSA of W/Z bosons at RHIC.

- Friday, 09-18-2009

**"Optimized Perturbation Theory at Finite Temperatures and Densities"**- Marcus Benghi Pinto (Universidade Federal de Santa Catarina, Brazil )
- Abstract: The alternative non perturbative analytical method know as Optimized Perturbation Theory will be presented. The way the method works will be illustrated with applications to obtain the phase diagram of the Gross-Neveu model in 2+1 d and the thermodynamical potential for Yukawa theory in 3+1 d.

- Wednesday, 06-17-2009

**"GPDs & TMDs - an overview"**- Marc Schlegel (JLab)
- Abstract: The information content of ordinary, collinear parton distributions about the partonic substucture of nucleons is somewhat limited. Straight-forward extensions of the collinear picture can be defined through Generalized Parton Distributions (GPDs) and Transverse-Momentum Dependent parton distributions (TMDs). An overview for these two classes of parton distributions is given - how they are defined, where they can be measured, what we can learn from them. In the end, also proposed relations between these a priori different classes of parton distributions are discussed.

- Friday, 01-23-2009

**QCD factorization and its role in understanding high energy nuclear collisions**- Zhong-Bo Kang (Iowa State University)
- Abstract: Nuclear dependence is an excellent probe of nuclear structure and dynamical properties of strong interacting medium produced in high energy nuclear collisions. Understanding the features of different sources of nuclear dependence is extremely important for discovering and learning new properties of QCD and strong interaction physics. In this talk, I discuss the close connection between QCD partonic multiple scattering and nuclear dependence in high energy nuclear collisions within perturbative QCD factorization approach. I will present a systematic calculation of the universal nuclear dependence to nuclear parton distribution functions(nPDFs), and show that coherent multiple scattering significantly modifies the gluon distribution and its evolution at small-$x$ and low $Q^2$. I will then show that low mass lepton pair production at large transverse momentum will be a very sensitive probe for gluon distribution. Finally I will present a calculation for transverse momentum broadening of heavy vector boson production and discuss its role in probing the property of the strongly interacting medium in nuclear collisions.

- Wednesday, 01-21-2009

**Medium Response to Hard Probes in the Quark-Gluon Plasma: Mach Cones**- R. Bryon Neufeld (Duke University)
- Abstract:

- Tuesday, 01-20-2009

**"Extracting the QGP viscosity from RHIC data, -- a status report from viscous hydrodynamics"**- Huichao Song (Ohio State University)
- Abstract:

- Wednesday, 12-03-2008

**Heavy-quark energy loss and thermalization in a strongly coupled plasma**- Cyrille Marquet (Columbia University)
- Abstract: I will discuss the problem of heavy-quark energy loss and $p_\perp-$broadening in a strongly-coupled $N=4$ SYM plasma. Using the AdS/CFT correspondence, the strong coupling problem has been reduced by Herzog et al. to solving the classical dynamics of a trailing string in AdS space. I will propose an interpretation of the results in terms of the saturation momentum $Q_s$ at strong coupling. Interestingly, the energy loss is parametrically the same than for a weakly-coupled QCD plasma, when expressed in terms of $Q_s,$ while $p_\perp-$broadening is radiation dominated in SYM theory and multiple scattering dominated in pQCD. Finally, I will consider the situation where the heavy quark is slowing down in the medium and estimate the thermalization time.

- Monday, 08-25-2008

**Medium dependence of multiplicity distributions in Modified Leading Logarithmic Approximation**- Paloma Quiroga Arias (University Santiago de Compostela)
- Abstract: We study the medium dependence of the multiplicity distributions in MLLA. We introduce the medium (f_{med}) by enhancing the soft infrared parts of the kernels of QCD equations. Being our ansatz to the quarks and gluons mean multiplicities < n_G>=e^{\gamma y} and
=r^{-1}e^{\gamma y}, we study two cases: we fix \gamma and abtain the medium dependence of \alpha_s and $r$ and we fix \alpha_s to abtain the medium dependence of \gamma and r. With these results we find the behavior of the dispersion of the multiplicity distributions with the medium. For fix \gamma a decrease of \alpha_s and the dispersion of the distributions are found. When we fix \alpha_s we find an increase in the dispersion of both quarks and gluons distributions with the increasing of the medium contribution.

- Wednesday, 02-20-2008

**GenEvA: A new framework for event generation**- Christian Bauer (Lawrence Berkeley National Laboratory)
- Abstract: I show how many contemporary issues in event generation can be understood in a new framework is called \GenEvA. This framework can be used to smoothly merge any partonic calculation with a parton shower. The best partonic calculation can be determined through physics considerations alone, independent of the algorithmic details of the merging. I will illustrate the approach by presenting results for e e -> jets at NLO with LL resummation.

- Friday, 02-15-2008

**Measuring QGP thermalization time with dileptons**- Michael Strickland (Johann Wolfgang Goethe University)
- Abstract: I show how to calculate the medium dilepton yield from a quark-gluon plasma which has a time-dependent momentum-space anisotropy. A phenomenological model for the hard momentum scale, p_hard(tau), and plasma anisotropy parameter, xi(tau), is constructed which interpolates between free streaming behavior at early times (tau << tau_iso) and ideal hydrodynamic behavior at late times (tau >> tau_iso). I show that high-energy dilepton production is sensitive to the assumed plasma isotropization time, tau_iso, and can therefore be used to experimentally determine the time of onset for hydrodynamic expansion of a quark-gluon plasma and the magnitude of expected early-time momentum-space anisotropies.

- Friday, 01-25-2008

**Aspects of Gluon Saturation in Heavy Ion Collisions**- Javier Albacete (Ohio State University)
- Abstract: At high energies QCD enters a novel regime characterized by high partonic densities. Such regime is governed by coherence phenomena and non-linear saturation effects, both required by unitarity of the theory. I will review the basic features of the formalism used to describe this high density regime of QCD, making emphasis in the latest developments involving running coupling corrections to the BK-JIMWLK evolution equations. Next I will concentrate on the phenomenological applications to the physics of ultra-relativistic heavy ion collisions that arise from our enhanced knowledge of the small-x dynamics, focusing in the expectations for the LHC. I will discuss the energy and rapidity dependence of multiparticle production in A-A collisions as well as of the nuclear modification factors in d-A collisions.

- Wednesday, 01-23-2008

**Simulating elliptic flow with viscous hydrodynamics**- Kevin Dusling (Stony Brook)
- Abstract: I present simulations of a viscous hydrodynamical model of non-central Au-Au collisions in 2+1 dimensions, assuming longitudinal boost invariance. The model fluid equations were proposed by \"{O}ttinger and Grmela \cite{OG}. Freezeout is signaled when the viscous corrections become large relative to the ideal terms. Then viscous corrections to the transverse momentum and differential elliptic flow spectra are calculated. When viscous corrections to the thermal distribution function are not included, the effects of viscosity on elliptic flow are modest. However, when these corrections are included, the elliptic flow is strongly modified at large $p_T$. We also investigate the stability of the viscous results by comparing the non-ideal components of the stress tensor ($\pi^{ij}$) and their influence on the $v_2$ spectrum to the expectation of the Navier-Stokes equations ($\pi^{ij} = -\eta \llangle \partial_i u_j \rrangle$). We argue that when the stress tensor deviates from the Navier-Stokes form the dissipative corrections to spectra are too large for a hydrodynamic description to be reliable. For typical RHIC initial conditions this happens for $\eta/s \gsim 0.3$.

- Friday, 01-18-2008

**Small-x physics in high energy QCD**- Bo-wen Xiao (Columbia University)
- Abstract: High energy hard scattering in large $N_{c}$ limit can be described by the QCD dipole model. In this seminar, I will first give a brief introduction to BFKL Pomeron, and Mueller's dipole model as well as Balitsky-Kovchegov-JIMWLK equation. Furthermore, the new Langevin type evolution equation which includes Pomeron loops will be introduced. Then, their solutions and applications in phenomenology also will be discussed. Later, I would like to focus on my recent work on the anomalous dimensions of the multiple Pomeron exchanges. In this work, single, double and triple BFKL pomeron exchange amplitudes are computed explicitly within the dipole model. Based on the calculation, a general formula which governs the anomalous dimension of the k-Pomeron exchanges amplitude in dipole-nucleus scattering is conjectured. In the end, if time allows, I would like to give a brief discussion on the Ads/CFT approach to small-x physics.

- Wednesday, 01-09-2008

**Magnetic Quasiparticles in sQGP Explain Low Viscosity and Static Potentials**- Jinfeng Liao (Department of Physics & Astronomy, SUNY Stony Brook)
- Abstract: Although ``dual superconductor'' nature of confinement was suggested by 't Hooft and Mandelstam long ago, the importance of magnetic quasiparticles near deconfinement transition and their relation to the "perfect liquid" observed at RHIC were realized only last year. In the RHIC region T=1--2T_c both electric and magnetic quasiparticles are important degrees of freedom. They are about equally massive and abundant at 1.4$T_c$ and below this T the magnetic ones get lighter, more weakly coupled, and become the dominant component. One evidence of this phenomenon is magnetic screening mass which exceeds electric one. Another is the persistence of ``flux tube''-related linear potential, seen in potential energy and entropy associated with static quarks on the lattice. In a model of electric-magnetic plasma the origin of such tension --- which is few times larger than at T=0 and is seen till about 1.3T_c --- is qualitatively explained. The condition for electric flux tube's existence/disappearance is derived: it led to magnetic particle density n~4.4-6.6/fm^3 at 1.3T_c, consistent with several lattice results. It has been found that a plasma made of electric and magnetic quasiparticles leads to mutual trapping, similar to ``magnetic bottle effect'', and explains low viscosity and small diffusion observed at RHIC. Molecular Dynamics (MD) simulation has been carried out for a strongly coupled plasma consisting of totally ~1000 charges of both types with variable density ratio of electric/magnetic components. The measured shear viscosity and diffusion constant are the lowest in the maximally mixed plasma (i.e. 50\%-50\%) which takes the most advantage of trapping effect, and their values after suitable mapping to sQGP system's parameter regions fall very close to those suggested by RHIC experiments. The importance of this effect for understanding such short mean free path as indicated by RHIC is further elucidated by studying the monopole motion in a cubic cell with alternating electric charges sitting at corners. If this mechanism is the main one responsible for short mean free path, and since it is operative only not too far from T_c, then viscosity/diffusion will grow away from "perfect liquid" regime above 2T_c and LHC would show significantly different picture than RHIC.

- Wednesday, 12-12-2007

**Transport in Strongly Coupled Plasmas**- Michael S. Murillo (Los Alamos National Laboratory)
- Abstract: Strongly coupled plasmas represent a diverse set of plasmas that span orders of magnitude in density and temperature. In part, their behavior is determined by their transport properties. In this talk, I will begin with a broad overview of strongly coupled plasmas. Then, I will focus on two scenarios, sedimentation in white dwarfs and turbulence in inertial confinement fusion, where diffusion and viscosity play important roles. I will describe some of the theoretical challenges to computing such transport coefficients, and how accurate determinations can be made with molecular dynamics simulation. Such simulations, and the various computational issues that arise, will be described in detail. Finally, I will discuss how viscosity may be measured experimentally in a dense plasma using x-ray Thomson scattering on next-generation light sources.

- Wednesday, 10-24-2007

**A test of statistical hadronization with exclusive rates in e+e-**- Lorenzo Ferroni (LBNL)
- Abstract: We have calculated the probability of exclusive channels by using a formulation of the statistical hadronization model in its fundamental microcanonical framework, i.e. enforcing the maximal set of conservation laws relevant to strong interaction and space-time symmetries. A test has been made on e+e- collisions at \sqrt{s}=2.1, 2.2 and 2.4 GeV. The analysis has been repeated assuming the phase space dominance model. Both models were shown to be in a fair agreement with the data.

- Wednesday, 10-17-2007

**Perfect Fluidity in Atomic Physics**- Thomas Schaefer (North Carolina State University)
- Abstract: It has been argued that the quark gluon plasma produced at RHIC is a ``perfect fluid'', i.e. that it saturates a proposed lower bound on the viscosity-to-entropy density ratio. In order to better understand what this means, and to get a feeling for how reliably one can extract the viscosity of a strongly coupled quantum fluid, I have studied the transport properties of cold trapped fermions. In this talk I will focus, in particular, on transport in the ``unitarity limit'', that is the limit in which the low energy s-wave cross section between the atoms saturates the quantum mechanical unitarity bound.

- Wednesday, 09-05-2007

**String theory and RHIC physics: The fundamental story**- David Mateos (UC Santa Barbara)
- Abstract: I will describe recent progress on the connection between string theory and the physics of the deconfined quark-gluon plasma created in heavy ion collisions, with special emphasis on the physics of matter in the fundamental representation. ***NOTE: TALK WILL BE AT 2:00***

- Friday, 03-16-2007

**Electromagnetic excitation of the Delta(1232)-resonance**- Shin-Nan Yang (National Taiwan University)
- Abstract: The electromagnetic excitation of the ∆ (1232) provides crucial information on the hadron structure and has been studied intensively, both experimentally and theoretically, in the past quarter of the century. It is one of the the main sources to probe color magnetism and hadron deformation.. I ll review recent advances in the theoretical study of * N->∆ (1232) transition form factors, which include dynamical models, effective field theory, and lattice gauge calculations and compare their predictions with the recent precision measurements from MAMI, BATES, and Jlab. Reference: V. Pascalutsa, M. Vanderhaeghen, and S.N. Yang, Physics Reports 437 (2007) 125.

- Thursday, 03-15-2007

**What can we learn from a U+U program at RHIC**- Anthony Kuhlman (Ohio State University)
- Abstract: Full overlap collisions between large, deformed nuclei, such as U, provide an opportunity to examine a number of unresolved issues from RHIC. These collisions are capable of producing energy densities up to 60\% larger than those in central Au+Au collisions, yielding an outstanding laboratory to test the ideal hydrodynamic behavior of $v_2$. In addition, in edge-on-edge collisions, the 25\% deformation allows the path length dependence of parton energy loss to be tested to large transverse momenta. We illustrate the advantages to be gained from the implementation of a U+U program at RHIC with a number of quantitative calculations and provide a Monte Carlo simulation which demonstrates the results to be expected from such a program.

- Tuesday, 03-13-2007

**Finite Creation Time Effects on Collisional Energy Loss of Non-Asymptotic Jets in a QGP**- Azfar Adil (Columbia University)
- Abstract: We calculate the collisional energy loss suffered by a heavy (charm) quark created at a finite time within a Quark Gluon Plasma (QGP) in the classical linear response formalism as in Peigne et al. [1]. We pay close attention to the problem of formulating a conserved current and accounting for both binding and radiative energy loss effects. We find that binding effects in particular are large and need to be accounted for in the analysis. We find that the finite time correction is on the order of a Debye length as expected and the overall energy loss is similar in magnitude to the energy loss suffered by a charge created in the asymptotic past. This result has significant implications for the relative contribution to energy loss from collisional and radiative sources and will have ramifications for the single electron puzzle at RHIC as well as other experimental observables. The presentation is based on work detailed in [1]. References [1] A. Adil, M. Gyulassy, W. A. Horowitz, and S. Wicks, arXiv:nucl-th/0606010.

- Friday, 03-09-2007

**Deconfinement heating in neutron stars**- Morten Stejner Pedersen (San Diego State University)
- Abstract: The conditions found in neutron star cores provide unique opportunities to study strong interaction phenomenology at densities beyond beyond the nuclear saturation density. Quarks may become deconfined and form a new phase of matter consisting of almost equal numbers of up, down and strange quarks (strange quark matter), but the existence of this phase in neutron stars depends on poorly constrained strong interaction properties and remains to be decided by observation or experiment. One possible observable from such a transition would be the heat released as hadronic matter crosses the the phase boundary and becomes deconfined. It has been hypothesized that strange quark matter may be the ground state of the strong interaction in which case neutron stars would consist almost exclusively of strange quark matter, and such deconfinement heating would have to power the thermal emission from quiescent soft X-ray transients - but even if strange quark matter is not absolutely stable heating associated with a transition in the core may still have consequences for the thermal evolution of isolated neutron stars.

- Wednesday, 03-07-2007

**Perturbative Shear Viscosity in Super Yang Mills**- Sangyong Jeon (McGill University)
- Abstract: It is by now well known that the shear viscosity in N=4 Super Yang Mills theory can be solve exactly in the strong coupling limit and it is small: $\eta/s = 1/4\pi$. This is small enough for the `perfect fluid' at RHIC and it is conjectured to be the physical lower bound. However, there remains some doubt how close this SYM theory is to the real QCD. To test this, we have calculated the weak-coupling limit shear viscosity in the SYM theory and compared it with the known QCD shear viscosity calculation. It turned out that in this limit, the QCD $\eta/s$ is about 7 times larger than the SYM$\eta/s$. I will talk about the theoretical issues and implications of this result.

- Monday, 10-09-2006

**Some Recent Results on Chaos and Regularity in Nuclei**- Thomas Guhr (Lund Universitet, Lund, Sweden)
- Abstract: To study regular and chaotic motion in nuclei, one often uses statistical approaches. Recent experimental progress makes it now possible to analyse levels which belong to individual excitation modes. Employing such an statistical analysis, it is shown directly from experimental data that certain low-lying magnetic dipole excitations are regular and collective, thereby outruling the commonly believed doorway interpretation. The analysis is extended into the regime of electrical pygmy excitations. Some concluding general comments on statistical observables are presented.

- Wednesday, 05-31-2006

**Superconductors with asymmetric spin populations**- Massimo Mannarelli (MIT)
- Abstract: We will discuss the properties of systems with imbalanced fermionic populations interacting via a short-range S-wave attractive interaction. We determine the energetically favored state as a function of the mismatch between the two Fermi spheres in the weak and strong coupling regime considering both homogeneous and non-homogeneous superconductive states. We will mainly consider non-relativistic systems, relevant for cold atoms experiments, but results for relativistic systems (relevant for high density QCD) will also be presented.

- Thursday, 05-25-2006

**Calculating the Jet Quenching Parameter from AdS/CFT**- Krishna Rajagopal (MIT/LBNL)
- Abstract: Models of medium-induced radiative parton energy loss account for the suppression of high-pT hadron spectra at RHIC in terms of a single jet quenching parameter q-hat. The available suite of jet quenching measurements make q-hat one of the experimentally better constrained properties of the hot fluid produced in RHIC collisions. We observe that q-hat can be given a model-independent, nonperturbative, quantum field-theoretic definition in terms of the short-distance behavior of a particular light-like Wilson loop. We then use the AdS/CFT correspondence to obtain a strong-coupling calculation of q-hat in hot N=4 supersymmetric QCD, finding q-hat = 18.87 \sqrt{ alpha Nc } T^3 in the limit in which both Nc and 4 pi alpha Nc are large. We thus learn that q-hat is not proportional to the entropy density or to some number density of scatterers since, unlike the number of degrees of freedom, q-hat does not grow like Nc^2. We close with an attempt to compare to values of q-hat extracted from RHIC data.