## Nuclear Theory Seminars

## Spring Semester 2015

Located at Columbia University in the seminar room in 1209 Pupin Hall, 538 W 120th St, New York, NY 10027

#### 1/19/15

#### 2:00

##### A new exact solution to the Boltzmann equation and its hydrodynamical limit

##### Mauricio Martinez (The Ohio State University)

We present an exact solution to the Boltzmann equation which describes a system undergoing boostinvariant longitudinal and azimuthally symmetric radial expansion for arbitrary shear viscosity to entropy density ratio. This new solution is constructed by considering the conformal map between Minkowski space and the direct product of three dimensional de Sitter space with a line. The resulting solution respects SO(3)_q x SO(1,1) x Z_2 symmetry. We compare the exact kinetic solution with exact solutions of the corresponding macroscopic equations that were obtained from the kinetic theory in ideal and second-order viscous hydrodynamic approximations. The macroscopic solutions are obtained in de Sitter space and are subject to the same symmetries used to obtain the exact kinetic solution.

#### 1/21/15

#### 2:15

##### Strong coupling QGP thermalization with longitudinal dynamics

Wilke van der Schee (Massachusetts Institute of Technology)

This talk will give an overview of recent attempts to use holography to understand the thermalization of heavy-ion collisions. For this I will argue that the AdS/CFT duality may improve our understanding of some aspects of these collisions, in particular focusing on the longitudinal dynamics. This may provide a distinctive way to probe the results from these strong coupled methods. In the end I will show some preliminary results allowing to make a direct comparison with experimental data.

#### 2/6/15

#### 2:30

##### Elliptic flow of thermal photons in heavy-ion collisions

##### Akihiko Monnai (Brookhaven National Laboratory)

Large azimuthal momentum anisotropy of hadronic spectra, characterized by elliptic flow v2, is considered as an evidence for the existence of a strongly-coupled QGP fluid in heavy-ion collisions. Direct photon elliptic flow, on the other hand, is found to be a few times larger than naive hydrodynamic expectations. This is now recognized as the "photon v2 puzzle". In this talk I discuss possible modifications of the quantity due to (i) incomplete quark chemical equilibration [1,2] and (ii) medium refraction [3]. Numerical estimations based on a hydrodynamic model indicate that thermal photon elliptic flow can be enhanced. I will also discuss implications of the distortion of phase-space distributions due to shear and bulk viscosities.

[1] A. Monnai and B. Mueller, arXiv:1403.7310 [hep-ph]

[2] A. Monnai, Phys. Rev. C 90, 021901(R) (2014)

[3] A. Monnai, arXiv:1408.1410 [nucl-th]

#### 2/13/15

##### Initial-state angular asymmetries in pA collisions

##### Andre Giannini (Baruch College and University of Sao Paulo)

We present a simple model for generating initial-state azimuthal asymmetries in pA collisions due to scattering on an anisotropic dense target. Here the scenario where rotational symmetry is broken due to the formation of a condensate for the electric field $\vec{E}$ in the target is explored. This leads to an angular correlation with the direction of $\vec E$; the parity even angular harmonics are generated through the C-even real part of the dipole S-matrix. Parity odd harmonics are generated by the C-odd imaginary part (odderon) due to coupling to coherent target fluctuations which again break rotational invariance. We perform a first qualitative extraction of the amplitude and cutoff of C-odd fluctuations in the dense target and show how the existence of multiple $\vec{E}$-field domains in the target allows us to study two-, three- and four-particle cumulants.

A. Dumitru and A. V. Giannini, Nucl. Phys. A 933, 212 (2014) [arXiv:1406.5781 [hep-ph]].

#### 2/20/15

#### 2:30

##### TBA

##### Andrej Ficnar (Oxford)

TBA

#### 3/9/15

#### 2:00

##### Anisotropy of the semi-classical gluon field of a large nucleus at high energy

##### Vladi Skokov (Western Michigan Univeristy)

Motivated by the recent measurements of the hadronic azimuthal anisotropy in pA collisions, we investigated the McLerran-Venugopalan model describing a highly boosted hadron/nucleus as a sheet of random color charges which source soft classical Weizsacker-Williams gluon fields. During this talk, I will demonstrate that the S-matrix in the MV model has high azimuthal anisotropy and it is preserved by small-x evolution.

#### 4/2/15

#### 1:00

##### Jet transport and medium excitation in high-energy heavy-ion collisions.

##### Xin-Nian Wang (LBNL)

Jet quenching is a useful tool for studying properties of the strongly coupled quark-gluon plasma in heavy-ion collisions. Recent progresses in experimental and theoretical studies and phenomenological extraction of jet transport coefficient will be reviewed. Development of a Linear Boltzmann jet transport model coupled to 3+1D hydrodynamic evolution will be discussed. Jet modification and jet-induced medium excitation will be illustrated and their implication on future experimental studies will be discussed.

#### 5/4/15

#### 2:30

##### Spin-Orbit Coupling in an Unpolarized Heavy Nucleus

##### Matt Sievert (Brookhaven National Laboratory)

The next-generation Electron-Ion Collider (EIC) will make high precision measurements of spin-dependent observables at high energies on nuclear targets. This unique nuclear physics laboratory will bring together access to the multitude of spin-spin and spin-orbit structures which can exist in hadronic targets, and the high color-charge densities which generate the most intense gluon fields permitted by quantum mechanics. The interplay between those two features gives rise to new physical mechanisms which translate these spin-orbit structures into the observed cross-sections, and it makes these mechanisms amenable to first-principles calculation. In this talk, I will discuss the spin-orbit structure of quarks within an unpolarized heavy nucleus in the quasi-classical approximation. The possibility of polarized nucleons with orbital motion inside the unpolarized nucleus generates nontrivial mixing between the spin-orbit structures of the nucleons, and the corresponding structures in the nucleus. This generic feature of a dense quasi-classical system leads to direct predictions testable at an EIC, and in principle allows direct access to the orbital angular momentum in the nucleus.

#### 5/8/15

#### 2:30

##### Photon and dilepton production in semi-quark gluon plasma

##### Shu Lin (Brookhaven National Laboratory)

The RHIC and LHC measurement of photon shows puzzling results on both yield and flow, which most theoretical models fail to describe. Direct photon is known to be produced from different sources in heavy ion collisions, including prompt photon, QGP photon, hadronic photon etc. In this talk, I will focus on photon production in QGP in temperature window between 1~2T_c. This is called semi-QGP, which is characterized by a non-pertubrtive Polyakov loop. I will present results on the modification of photon production due to non-perturbative Polyakov loop. We found the photon production from different processes are modified differently: photon rate from 2 to 2 processes is suppressed by loop factors as compare to perturbative QGP, while the collinear photon rate has additional 1/N suppression factor in SU(N) gauge theory. The latter also indicates suppression of LPM effect. I will also present results on dilepton production in semi-QGP, which shows qualitatively very different behavior: Instead of suppression, we found a mild enhancement as compared to perurbative QGP.

#### 5/11/15

#### TBA

##### Collective phenomena in proton collisions

##### Kevin Dusling (American Physical Society)

There has been a resurgence of interest in the relativistic collisions of small

systems after the surprising discovery of a long-range rapidity correlation in

high-multiplicity proton-proton collisions at the LHC. The same signal was

also seen in p+Pb collisions and d+Au collisions at RHIC which will follow up

with p+Au and $^3$He-Au in the near future. Multi-particle correlation

measurements suggest the presence of collective phenomena in these systems.

One possible explanation is hydrodynamic flow. Another is intrinsic

correlations generated from saturated gluonic matter within the proton. This

talk will provide an overview of the experimental findings and interpretations.

#### 6/1/15

#### 2:00

#### 6/3/15

#### 2:00

##### Elliptic flow from anisotropic escape (room 425)

##### Denes Molnar (Purdue)

While hydrodynamics is regarded as the dominant paradigm for describing heavy-ion collisions at RHIC and LHC energies, its applicability to nuclear reactions is not very well understood. Open question remain about the mechanism of rapid thermalization, initial conditions, treatment of decoupling (conversion of the fluid to particles), finite system effects, and quantum corrections in very small systems, for example. In a recent work (arXiv:1502.05572) we showed that in the AMPT transport model elliptic flow is generated quite differently from hydrodynamics, mainly through anisotropic escape from the collision zone. I will demonstrate that this is, in fact, a general feature of kinetic theory, originating in the modest opacities <Ncoll> \sim 4-5 in AMPT calculations. Implications of the escape effect will be discussed together with connections to other hydro related problems such as proper particle distributions (arXiv:1404.8750) and anisotropic flow from quantum mechanics (arXiv:1404.4119).

#### 6/17/15

#### 2:00

##### Weak and Strong Coupling Energy Loss at RHIC and LHC (room 425)

##### W. A. Horowitz (Cape Town)

Experiments at RHIC and LHC are teasing out the properties of the phase diagram of quantum chromodynamics, one of the four fundamental forces of Nature. At temperatures a hundred thousand times hotter than the center of the Sun, the structure of nuclear matter changes abruptly, transitioning from the usual protons and neutrons into quark-gluon plasma. Since quark-gluon plasma permeated all of space a microsecond after the Big Bang, no other experiments probe an earlier time in the history of the Universe.

Jet tomography, the study of the highest momentum particles observed from collisions at RHIC and LHC, provides the best femtoscope for examining the non-trivial, non-Abelian, many-body dynamics of quark-gluon plasma. We compare our theoretical predictions to measured data for the distribution of these high momentum particles resulting from dramatically different hypotheses for the dynamics of the quark-gluon plasma: on the one hand, assuming the quark-gluon plasma is best approximated by a weakly-coupled gas of nearly-free quarks and gluons in which the methods of perturbative QCD are applicable and, on the other, assuming the quark-gluon plasma is dominated by the physics of a strongly-coupled liquid, for which the techniques of 5-dimensional classical gravity from the AdS/CFT correspondence best capture the relevant physics. Neither story provides a fully compelling narrative, and we suggest future theoretical and experimental advances that should shed further light on the physics of quark-gluon plasma.

#### 6/22/15

#### 2:00

##### Hadronization Scheme Dependence of Azimuthal Harmonics in p+A (room 705)

##### Angelo Esposito (Columbia)

We compare the distortion effects of three popular final-state hadronization schemes. We show how hadronization modifies the initial-state gluon correlations in high energy p+A collisions. The three models considered are (1) LPH: local parton-hadron duality, (2) CPR: collinear parton- hadron resonance independent fragmentation, and (3) LUND: color string hadronization. The strong initial-state azimuthal asymmetries are generated using the GLVB model for non-abelian gluon bremsstrahlung, assuming a saturation scale Qsat = 2 GeV. Long-range elliptic and triangu- lar harmonics for the final hadron pairs are compared based on the three hadronization schemes. Our analysis shows that the process of hadronization causes major distortions of the partonic az- imuthalharmonicsfortransversemomentaatleastuptopT =3GeV.Inparticular,theyappear tobegreatlyreducedforpT <1Ã·2GeV.