Nuclear/Particle Seminar Abstracts
Spring 2016
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The neutron densities in atomic nuclei are notoriously difficult to observe
with high precision: the standard tool of electromagnetic interactions which
has been used to map out the nuclear charge distributions simply doesn't see
them. In fact, it has only recently been experimentally confirmed that the
neutron-rich lead nucleus even has a neutron skin, and is only a fraction of a
neutron radius thick. Encoded in these distributions is a wealth of
important information about how the strong nuclear force builds systems
where the number of protons and neutrons are unequal. This information has
bearing not only for our understanding of asymmetric nuclei, but also in the
construction of extreme systems like neutron stars. Fortunately, nature gives
us a novel way to image this side of the nucleus: through fundamental weak
force interactions, which interact primarily to neutrons rather than protons.
In this colloquium I will discuss why these neutron distributions play an
important part in our understanding of nuclear physics and astrophysics, how
one images such tiny systems with electron beams, and the recent and upcoming
experimental efforts for such measurements.
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In the context of the gauge/gravity duality, the flux tube between a quark-antiquark pair in strongly coupled Yang-Mills theories is described by a Nambu-Goto string hanging in AdS spacetime from its boundary. I will talk on time evolution and nonlinear dynamics of the string in AdS_5 dual to N=4 supersymmetric Yang-Mills theory when the string is perturbed with finite amplitudes. I will firstly show that waves on the string show turbulent behaviors, and as a result cusps form on the string. I will then show if the perturbations are very strong, the initially connected string stretches infinitely, forms effective event horizons, and transitions to two separated strings dynamically.
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Quantum ChromoDynamics (QCD) describes the interaction between the elementary constituents of hadronic matter, the quarks and gluons. Quarks and gluons are not observed as free particles, but are confined inside color-singlet hadrons. QCD, however, predicts the existence of a phase of matter at high temperature (T > 10^12 K) where quarks and gluons are no longer confined, called the Quark-Gluon Plasma (QGP). The QGP can be created and studied in the laboratory colliding heavy nuclei at ultra-relativistic energies. The ALICE experiment at the CERN LHC is pursuing this program at the energy frontier (\sqrt{s_NN} > 2.76 TeV). After a brief introduction on the physics of confinement and heavy-ion experiments, I will discuss recent results from the ALICE experiment on the collective properties and hadronization mechanism of the system created in the collision. Measurements of light-flavor hadrons obtained in lead-lead, proton-proton and proton-lead collisions will be compared and contrasted. These studies revealed unexpected features, which are currently not clarified and could lead to a better understanding of high-energy hadronic interactions.
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When large nuclei are accelerated to relativistic energies and brought into collision at the CERN Large
Hadron Collider (LHC), the enormous temperature and energy density in the nuclear system trigger a phase
transition into an exotic form of matter. The resulting quark-gluon plasma (QGP) evolves in an expanding,
flowing fireball, and investigating its remarkable properties allows us to study how the theory of the
strong nuclear force (quantum chromodynamics, or QCD) manifests itself in a novel high-density,
high-temperature regime. Since the first nucleus-nucleus collisions in late 2010, a rich experimental
program with the large, state-of-the-art detectors at the LHC has flourished. In this talk, I will focus on
two valuable experimental tools developed to explore the QGP: reconstructed jets which arise from cascades
of strongly-interacting quarks and gluons passing through the plasma; and high-energy photons which escape
the interaction zone without further interaction.
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Relativistic heavy-ion collisions produce matter at the highest temperatures accessible in the laboratory. Results from the RHIC experiments show that the system produced in these collisions behaves like a near-perfect fluid. These observations indicate that strong-coupling dynamics dominate the long wavelength behavior of the system and are in surprising contrast to initial expectations that system would form a weakly-coupled plasma. This exciting discovery has led to a number of questions about the nature of strongly coupled quantum systems and how such behavior can emerge from a theory possessing asymptotic freedom. Another discovery of the RHIC program was the observation of jet quenching, where energetic partons produced in the early stages of the collisions lose energy though their interactions with the medium. Jets probe the medium at a variety of length scales and are thus sensitive to both its microscopic structure and the onset of the strong coupling. In this colloquium I will present the latest LHC jet measurements performed with the ATLAS experiment, in which precision techniques developed in high energy physics have been adapted to the heavy-ion environment. I will discuss the rapidly improving theoretical picture of jet quenching, the implications of new measurement capabilities during the next lead ion runs at the LHC, and the role of a future detector to measure jets at RHIC.
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High energy heavy ion collisions, like those performed at the Relativistic Heavy Ion Collider (RHIC), allow us
to test our understanding of nuclear physics at the highest temperatures accessible in the laboratory. At
these high temperatures, a new phase of nuclear matter called the Quark Gluon Plasma (QGP), where the
fundamental quarks and gluons are no longer confined within hadrons, is created. The RHIC experiments
discovered that the QGP is strongly coupled and behaves as a nearly perfect fluid, contrary to initial
expectations. Understanding the microscopic structure of the QGP, and its strongly coupled nature, requires
calibrated probes at varying length scales. Heavy quarks, namely charm and bottom, are produced in initial
hard scatterings, and therefore provide an excellent tool for these studies as they experience the full
evolution of the produced medium. In this colloquium, I will discuss the use of heavy quarks as a probe of the
medium produced in heavy ion collisions including recent experimental advances made possible by detector
upgrades of the PHENIX experiment at RHIC.
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The discovery of neutrino oscillations has demonstrated that neutrinos have small, but non-zero masses. These
masses are at least 10^6 times smaller than those of the other fundamental particles, suggesting that neutrinos
could be getting their masses through a different physical mechanism. Understanding the fundamental nature of
neutrinos and the mechanism by which they acquire mass are key goals of nuclear and particle physics in the coming
years.
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No abstract to be posted.
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No abstract yet provided.
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