Session DA: Invited Session on Nuclear and Partical Physics

Neutrinoless Double Beta Decay: A Search for Lepton-Number Violation

The search for neutrinoless double-beta decay of certain even-even nuclei is likely to have a significant impact on our understanding of some of the remaining unknown fundamental neutrino characteristics within the next few decades. Observation of this decay would establish the Majorana nature of the neutrino and set the absolute mass scale of neutrinos. I will present the physics case for the current and next generation searches. I will also review the status of the field, focusing primarily on current generation experiments that are running, or have recently produced results, such as the EXO, GERDA and Kamland-Zen experiments.   

Direct Observation of the Second \boldmath$J^\pi=2^+$ State in \boldmath${}^{12}\mathrm{C}$ and New Triple-\boldmath$\alpha$ Thermonuclear Reaction Rates

During core-collapse supernovae, the triple-$\alpha$ thermonuclear reaction rates at high temperatures can affect the outcome of explosive nucleo\-synthesis and the production of heavy elements. The question of the existence of a second $J^\pi=2^+$ state in ${}^{12}$C has led to a long-standing disagreement in the triple-$\alpha$ thermonuclear reaction rates at high temperatures. This $2^+_2$ state has been directly observed in the ${}^{12}\mathrm{C}(\gamma,\alpha){}^8\mathrm{Be}$ reaction using the intense, nearly mono\-energetic $\gamma$-ray beams available at the High Intensity $\gamma$ Source (HI$\gamma$S) facility. The $\alpha$ particles produced by the photo\-disintegration of ${}^{12}\mathrm{C}$ were detected using an optical time projection chamber (OTPC)\@. This allowed for the measurement of complete angular distributions which were used to determine the $E1$ and $E2$ amplitudes and their relative phases. The $2^+_2$ state was observed in the $E2$ cross section and confirmed in the behavior of the relative phases. This unique combination of a Compton-backscattered $\gamma$-ray beam and an active-target system made possible the first unambiguous identification of this $2^+$ state. New triple-$\alpha$ thermonuclear reaction rates have been calculated based on the results of this experiment, and simulations based on the $\nu p$ process~[1] have been performed illustrating the effect of the second $2^+$ state in ${}^{12}\mathrm{C}$ on the outcome of explosive nucleo\-synthesis.\\[4pt] [1] A.~Arcones, C.~Fr\"ohlich, and G.~Mart\'inez-Pinedo, ApJ \textbf{750}, 18 (2012)   

Modification of Nucleon Structure in the Nuclear Medium - Recent Insight from Jefferson Lab

One of the primary goals of the experimental program at Jefferson Lab is to study whether or not, and to what extent, protons and neutrons (nucleons) are modified in a nucleus. Since the 1980's, it has been known that the quark distributions in a nucleus are different than in a free proton and neutron. This observation, dubbed the EMC effect, created an industry of experiments dedicated to fully quantifying this effect and looking for other signatures of nucleon modification in nuclei. Recent results from Jefferson Lab have provided unique insight into the nuclear dependence of the EMC effect, indicating that the effect depends not on atomic mass number, $A$, or average nuclear density. Instead, it appears to depend on the local density of nucleons probed by the electron scattering reaction. Even more surprisingly, it has been observed that the detailed nuclear dependence of the EMC effect is shared by another, seemingly minimally related quantity - the relative number of protons and neutrons to be found in a correlated pair in the nucleus. In this presentation, I will discuss the Jefferson Lab program of electron scattering measurements targeted at understanding the modification of nucleons in nuclei, elucidating the short-range structure of the nuclear wave function, and the connection between the two. Results from the completed 6~GeV program, as well as future measurements planned for execution after the completion of the JLab 12~GeV Upgrade will be discussed.   

Probing the Quark Gluon Plasma

High energy collisions of heavy nuclei permit the study of nuclear matter at temperatures and energy densities so high the fundamental theory for strong interactions, QCD, predicts a phase transition to a plasma of quarks and gluons. This matter, called a Quark Gluon Plasma (QGP), has been studied experimentally for the last decade and has been observed to be a strongly interacting liquid with a low viscosity. High energy partons (quarks and gluons) created early in the collision interact with the QGP and provide unique probes of its properties. Studies of these partons through full jet reconstruction and through the studies of high-momentum particles have demonstrated that the QGP is a strongly interacting, dense medium.   

Exploring Collective Behavior in the Quark-Gluon Plasma

Measurements of heavy-ion collisions carried out at the Relativistic Heavy Ion Collider (RHIC) indicate that a new state of matter, called the quark-gluon plasma, with an energy density similar to that achieved in the early universe shortly after the Big-Bang is created. This medium exhibits collective behavior characteristic of a strongly coupled and nearly perfect fluid. Much of the same liquid is created at the Large Hadron Collider (LHC). LHC explores the evolution of the quark-gluon plasma state over more than an order of magnitude in collision energy. Recent results of charged-hadron correlations in high multiplicity pPb collisions reveal signals suggestive of collective flow, which are very similar to those observed in heavy-ion collisions. In this context, measurements of two- and four-particle angular correlations for charged particles emitted in pPb collisions will be presented over a wide range of pseudorapidity and full azimuth from CMS. These results will be compared to 2.76 TeV semi-peripheral PbPb collision data, covering a similar range of particle multiplicity.   

Three-Dimensional nucleon structure: Jefferson Lab 12 and beyond

.