Session FG: Heavy Ion Collisions: HBT and Friends

New Physics at Zero Degrees with Heavy Ions

Photons and neutrons from spectator matter from Pb $+$ Pb at 5.5 TeV per nucleon center of mass energy are concentrated mostly into a 10 cm diameter circle at 140 m from the interaction point. A spectator is that part of the Pb nucleus that is sheared from the rest of the nucleus by the oncoming Pb nucleus. The shearing breaks up tightly bound nucleon pairs with the result that energetic nucleons are emitted from the sheared face of the spectator. The resulting asymmetric neutron distribution shows not only the orientation of the reaction plane but also the direction of the overall angular momentum vector. This additional information opens a new window to heavy-ion reactions, including polarization effects and the angles between jet pairs. The spectators are exposed to an enormous electric field of the order 2 x 10$^{\mathrm{28}}$ V/m. In such a large electric field, quarks in the spectator radiate virtually all of the transferred energy. Studying these photons should produce new information about effects of extremely large electromagnetic fields. The necessary measurements could be made at CMS by upgrading the electromagnetic part of the Zero Degree Calorimeter to allow position resolution in both transverse and longitudinal directions. A suggested design is 5 W-metal plates interleaved between 6 layers of pixilated silicon detectors. The thickness of the W should be enough to cause 50{\%} of the neutrons to initiate showers (.693 $\Lambda _{\mathrm{int.}})$.   

Conformal anomaly and photon anisotropy in heavy ion collisions

I introduce a novel mechanism for anisotropic photon production in heavy ion collisions, stemming from the interplay between anomalies of QCDxQED and the existence of strong (electro)magnetic fields. For the case of conformal anomaly, using the hydrodynamical description of the bulk modes of QCD plasma, I show that this mechanism leads to the pho- ton production yield that is comparable to the yield from conventional sources. Furthermore, this mechanism provides a a significant positive contribution to the azimuthal anisotropy of photons (v2) and shows agree- ment with the PHENIX data.   

P- and CP-odd observables in photons and di-leptons from axially charged plasma

Axial charge in a QCD plasma is P- and CP-odd. We propose and study P- and CP-odd observables in photon and di-lepton emissions from an axially charged QCD plasma, which may provide possible experimental evidences of axial charge fluctuations in the plasma created in heavy-ion collisions. Our observables measure spin polarization asymmetries of the emitted photons and di-leptons, and are shown to be related to the imaginary part of chiral magnetic conductivity at finite frequency-momentum, which ultimately arises from the underlying triangle anomaly of the QCD plasma with a finite axial charge density.   

Kinetic Evolution and Bose-Einstein Condensation in the Glasma

We study the evolution of a dense system of gluons, such as those produced in the early stages (the Glasma) of ultra-relativistic heavy ion collisions. We describe the approach to thermal equilibrium using the small angle approximation for gluon scattering in a Boltzmann equation that includes the effects of Bose statistics. Simple power counting arguments indicate that the gluon system as in the Glasma is over-occupied and driven towards the formation of a Bose--Einstein condensate. We derive and solve the transport equation for initial conditions that correspond to the overpopulated Glasma and present numerical evidence that such over-populated systems reach the onset of Bose-Einstein condensation in a finite time. The approach to condensation is characterized by a scaling behavior that we briefly analyze. Finally we analyze the effects of the inelastic, number changing, processes on the dynamical formation of the Bose-Einstein condensate by analytically deriving the $2\leftrightarrow 3$ kernel under the collinear and small angle approximations and numerically solving it. References: J. Blaizot, J. Liao and L. McLerran, arXiv:1305.2119; X. Huang and J. Liao, arXiv:1303.7214; J. Blaizot, F. Gelis, J. Liao, L. McLerran and R. Venugopalan, arXiv:1107.5296.   

Femtoscopy of pp and Pb-Pb collisions with the ALICE experiment at the LHC

Femtoscopy is unique among all analysis techniques utilized in subatomic collision experiments as it directly addresses the space-time structure of the evolving system at the femtometer scale. We report on the results of two-particle Bose-Einstein correlation analyses in $pp$ and Pb-Pb collisions at $\sqrt{s}=7$~TeV and $\sqrt{s_{NN}}=2.76$~TeV, respectively, recorded by the ALICE experiment at the LHC. We discuss femtoscopic correlations for pions, kaons, and protons as a function of event multiplicity and total pair momentum.   

Energy Dependence of HBT Measurements for Charged Pions at RHIC-PHENIX

HBT measurements are a sensitive probe for the space-time extent of the hot and dense systems produced in heavy-ion collisions. New HBT measurements have been recently obtained for charged pion pairs in the Au+Au collisions at $\sqrt {{s_{NN}}} $ = 39, 62.4 and 200 GeV with the PHENIX detector. These measurements will be presented with an emphasis on the scaling of HBT measurements with charged particle multiplicity. The results will also be compared to the results from hydrodynamical models.   

Measurement of HBT radii in 200 GeV d+Au collisions at RHIC

Recent analysis of data from d+Au collisions at RHIC have given rise to the speculation that the QGP is formed even in such dilute systems. Measurement of Hanbury Brown-Twiss (HBT) effect can give an estimate of the spatial and temporal extent of the region of homogeneity of the medium. Results are presented for the 3-dimensional HBT radii for charged pions in 200 GeV d+Au collisions as measured by the PHENIX detector.   

Precision Measurement of the Neutron $d_{2}$: A Probe of the Color Force

The $g_2$ nucleon spin-dependent structure function measured in electron deep inelastic scattering contains information beyond the simple parton model description of the nucleon. It provides insight into quark-gluon correlations and a path to access the confining local color force a struck quark experiences just as it is hit by the virtual photon due to the remnant di-quark. The quantity $d_2$, a measure of this local color force, has its information encoded in an $x^2$ weighted integral of a linear combination of spin structure functions $g_1$ and $g_2$ and thus is dominated by the valence-quark region at large momentum fraction $x$. To date, theoretical calculations and experimental measurements of the neutron $d_2$ differ by about two standard deviations. Therefore, JLab experiment E06-014, performed in Hall A, made a precision measurement of this quantity. Double spin asymmetries and absolute cross-sections were measured in both DIS and resonance regions by scattering longitudinally polarized electrons at beam energies of 4.74 and 5.89 GeV from a longitudinally and transversely polarized $^3$He target. Results for the absolute cross-sections and spin structure functions on $^3$He will be presented as well as results for the neutron $d_2$.