William & Mary

Spring 2016

January 21, 2016 (Thursday) 4:00-5:00p.m. Small Hall 111
Speaker: Prof. Jozef Dudek , Jefferson Lab & Old Dominion University
Host: Prof. K. Orginos
Title: The elusive excited glue of QCD
Abstract: Quarks and gluons are believed to be the building blocks of hadrons, the strongly interacting particles of nuclear physics. Making predictions regarding the spectrum of hadrons within Quantum Chromodynamics (QCD), the field theory that describes quarks and gluons, has long been challenging. One particular mystery within QCD is whether the role of excitations of the gluon field, which is strongly coupled to both itself and to the quarks, can be observed in the excited spectrum of hadrons.  
I will present results from numerical calculations of QCD that indicate that exotic objects called hybrid hadrons, in which quarks are partnered with an excitation of the gluon field, are in fact a feature of the hadron spectrum and should be experimentally observable. Recent theoretical advances have allowed production and decay properties of excited hadrons to be calculated, opening up the possibility of providing predictions that offer guidance for current and near-future experimental exotic hadron searches.

January 28, 2016 (Thursday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Christoher Monahan, Rutgers University
Title: One quark, two quarks, three: How to build a proton with lattice QCD
Abstract: Protons and neutrons make up most of the mass of the visible Universe, but our knowledge of their internal structure is far from complete. Quantum chromodyamics (QCD), the theory of the strong force, provides the mathematical framework that connects protons and neutrons to their constituent quarks and gluons, but QCD cannot be solved analytically. 
Instead, we must use lattice QCD, in which we discretise spacetime and study QCD numerically, usually on large supercomputers. I will introduce lattice QCD, discuss our need for a numerical approach to QCD, and highlight some of the many ways in which lattice QCD contributes to our understanding of particle and nuclear physics.

February 4, 2016 (Thursday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Phiala Shanahan
Title: The strange proton: Why strange quarks are important in nucleon structure
Abstract: Protons and neutrons are the fundamental building blocks of atomic nuclei and constitute more than 99% of the visible mass in the universe. While understanding proton properties is clearly of fundamental importance, there are still many open questions. For example, the size of the proton has become a topic for debate as different experimental approaches give inconsistent values for its charge radius by 5 standard deviations. On the theory front, the modern picture of the proton is of a complex particle with a substructure of more basic constituents named quarks and gluons. Many proton properties are well-described within a model where only two ‘flavors' of quark appear: the up and down quarks. However, the theory of the strong interaction, Quantum Chromodynamics (QCD), describes a much more complicated dynamical structure in which quark-antiquark pairs of any flavor, such as strange quarks, can contribute. In this colloquium I will show why a quantitative understanding of the role of strange quarks in the proton is important in the context of physics issues as diverse as the experimental detection of dark matter particles, precision tests of the Standard Model, and the proton radius puzzle. I will describe how recent advances in numerical simulations of QCD have led to new benchmarks for experiments and new levels of precision in dark matter searches. 

February 11, 2016 (Thursday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Maxwell Hansen, (Institut für Kernphysik Johannes Gutenberg-Universität Mainz) Host: K. Orginos
Title:  From effective theories to quantum fields on the lattice: big ideas for femtoscale physics
Abstract: 
One of the most important ideas in physics is that of effective theories. This is based on the simple observation that different tools are required to describe systems of vastly different sizes. Starting here, I will introduce the Standard Model as the most effective known description of the smallest lengths we can measure. I will then turn attention to quantum chromodynamics (QCD), the part of the Standard Model that describes the strong force, responsible for protons, neutrons and the nuclei they form. QCD has a fascinating, rich structure that has proven highly challenging for theorists to understand. I will give motivation for using a space-time lattice to study QCD and will give various examples to illustrate the method, with particular focus on scattering predictions. 

February 12, 2016 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Seamus Riordan, Stony Brook University
Host: P. Vahle
Title: Studying the Dark Side of the Nucleus:  From Neutron Skins to Neutron Stars
Abstract: 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.

February 18, 2016 (Thursday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Matha Constantinou
Host: K. Orginos
Title: 
Simulating The Visible World
Abstract: Quantum Chromodynamics (QCD) is the theory of the strong interactions that binds quarks and gluons to form the nucleons, the fundamental constituents of visible matter.

Understanding the structure of the nucleon from first principles is a milestone of hadron physics and numerous experiments have been devoted to its study. Lattice QCD is a powerful approach for the ab initio calculation of the properties of hadrons and their interactions. 
Over the last five years, lattice QCD has made significant progress yielding results that can be compared to experimental measurements with controlled systematics.
We will review recent progress of lattice QCD results with emphasis on nucleon structure and, in particular, addressing questions, such as: “What is the size of a proton" and “What contributes to the spin of proton”? The quark content of the nucleon is also discussed, as an important quantity related to the explicit chiral symmetry breaking in QCD, as well as, entering the interpretation of experimental searches for dark matter.


February 24, 2016 (Wednesday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Simona Malace, Jefferson Lab
Host: P. Vahle
Title: Understanding the Structure of the Nucleon: Stepping Stone, Milestone
Abstract: 
The nucleon has been a laboratory for studying the strong interactions for decades. The investigation of its quark and gluon structure ultimately led to a fascinating and complicated picture that is still subject of vigorous, fundamental research. In this talk I will discuss the nucleon puzzle: from early explorations that brought us the knowledge of what nucleons are made of (stepping stone) to today's theoretical and experimental efforts > directed at acquiring exhaustive knowledge about the nucleon dynamics (milestone). I will present an upcoming experimental program at Jefferson Laboratory that will take measurements of electron-nucleon scattering cross sections to contribute to a precise characterization of nucleons in terms of their fundamental constituents.

February 26, 2016 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Paul Mattione, Jefferson Lab
Host: K. Griffioen
Title: 
Searches for Hybrid Mesons with GlueX
Abstract: QCD, the theory of the strong nuclear force, describes how quarks are bound together by gluons to form hadrons, which are particles such as the proton and neutron. Hybrid mesons are a type of hadron that are predicted to consist of a quark-antiquark pair, bound together by a gluonic field that is in an excited state. A rich spectrum of hybrid meson states has been predicted, but only a few experiments have reported evidence of their existence. Measuring the spectrum of these states will provide valuable information on the gluonic degrees of freedom of QCD in the quark-confinement regime.

The GlueX experiment at Jefferson Lab is specifically designed to search for and measure the spectrum of hybrid mesons. For the experiment, a 12 GeV electron beam incident on a diamond radiator is used to produce a linearly-polarized, coherent bremsstrahlung tagged-photon beam with a coherent peak at 9 GeV. The linearly-polarized photon beam is incident on a proton target located within the hermetic GlueX detector, which is capable of detecting many different final states to which the hybrid mesons are predicted to decay. Early physics data from the GlueX commissioning will be shown, demonstrating the readiness of GlueX for full physics running, which is scheduled to begin this Fall.  


March 2, 2016 (Wednesday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Justin Stevens
Host: W. Deconinck
Title: Exploring gluonic degrees of freedom in the meson spectrum
Abstract: 
The quark model has been remarkably successful in classifying the observed spectrum of bound states composed of quarks and gluons, consisting of simple quark-antiquark pairs (mesons) and collections of three quark (baryons).  The underlying theory of quark and gluon interactions, Quantum Chromodynamics (QCD), however, allows much more complex configurations, and first principles QCD calculations have recently predicted a rich spectrum of mesons containing an excited gluonic field in their wave functions.  The observation of these new states, known as hybrid mesons, would provide key insight into how the gluonic degrees of freedom manifest themselves in the bound states we observe in nature.  The GlueX experiment at Jefferson Lab is designed to search for and study the pattern of gluonic excitations in the meson spectrum using a high energy, polarized photon beam.  In this talk the status of the GlueX experiment will be presented, including results from recent commissioning data and planned upgrades to enhance the search for hybrid mesons.
March 3, 2016 (Thursday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Gernot Eichmann
Host: K. Orginos
Title: 
From quarks and gluons to the structure of hadrons
Abstract: Quarks and gluons are the fundamental building blocks of visible matter, yet we cannot observe them because they are confined inside hadrons. In light of ongoing experimental advances, the theoretical description of hadrons within Quantum Chromodynamics (QCD) still poses an enormous challenge. To name a few questions: what is the nature of baryon resonances? Do tetraquarks and pentaquarks exist and if yes, how should we interpret them? Can we describe nucleon form factors, polarizabilities, or electroproduction amplitudes from the level of quarks and gluons, and how is such a description related to effective field theories? What is the QCD contribution to the anomalous magnetic moment of the muon? And is it possible to understand nuclei from the fundamental interactions in QCD?
 

Here I will present an approach employing Dyson-Schwinger, Bethe-Salpeter and Faddeev equations, whose basic promise is to calculate hadron properties from the nonperturbative structure of the underlying Green functions in QCD – the quark and gluon propagators, quark-gluon vertex etc. I will discuss the basic ideas and highlight some recent progress that has been made regarding the spectrum of mesons and baryons, tetraquarks, nucleon and nucleon resonance form factors, Compton scattering, and the muon g-2 problem.


March 4, 2016 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Dr. Anselm Vossen, Indiana University
Host: W. Deconinck
Title: 
Exploring QCD Dynamics and Proton Structure in Polarized p-p Scattering and e⁺e⁻ Annihilation
Abstract: The discovery of transverse spin effects in e⁺e⁻→qq̅→jets in Belle has handed us a unique quark polarimeter that connects microscopic quark spin observables in high energy collisions with measurable angular distributions of final state hadrons reconstructed in the detector. The Belle discovery makes it possible for the first time, to extract the transverse spin distributions of quarks inside the proton from transverse spin observables measured in polarized proton-proton collisions and in lepton-nucleon scattering.  The net transverse polarization, the so-called tensor charge, is a fundamental property of the proton and can be computed ab initio QCD using lattice techniques. The theoretical effort aims at describing QCD at the nucleon mass scale and will shed light on the dynamics that leads to the creation of most of the visible mass in the Universe.  The tensor charge also can be used for constraining coupling constants for certain extension of the standard model. This colloquium will cover the measurement of quark spin effects in e⁺e⁻ annihilation in Belle and spin asymmetry measurements in polarized p+p collisions with the STAR detector at the Relativistic Heavy Ion Collider.  Together, these results are used to extract the transverse polarization of quarks in the proton. Finally, Belle is currently being upgraded to Belle II to take advantage of an increase of the delivered instantaneous luminosity by a factor of about 40.  The status of the upgrade and future possibilities it enables in conjunction with high precision data on transverse spin asymmetries in lepton-nucleon scattering to be collected at the JLab12 experiments will be discussed.
March 14, 2016 (Monday) 4:00-5:00p.m. Small Hall 111
Speaker:
Dr. David AbergelNordic Institute for Theoretical Physics
Host: E. Rossi
Title: 
2D materials and their heterostructures
Abstract: 2D materials are a large and growing area of study for both fundamental physics and device applications. We shall give an overview of some of the many different 2D materials and their properties, before focusing on systems where two or more materials are combined in a heterostructure. We shall discuss the essential physics of the inter-layer bonding for stacked layers, and the interesting properties of 1D interfaces between two side-by-side 2D layers.

March 25, 2016 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker:
Diana Vaman, University of Virginia, William & Mary (Spring 2016)
Host:
Prof. Sher
Title: 
String  Holograms
Abstract: In this talk I will review the holographic duality which relates strongly interacting quantum systems and weakly coupled strings.  In broad terms, the duality states that the information contained in the bulk of a 5-dimensional theory of gravity (which asymptotes to a space of constant negative curvature called anti de-Sitter or AdS) is mapped holographically onto the boundary of this space, in terms of a 4-dimensional QCD-like gauge theory, without gravity. I will sketch the holographic description of "quarks",  and of bound states like "mesons" and "baryons", which live in the boundary theory, in terms of bulk string excitations. I will conclude with a description of the duality at finite temperature, and describe some of its implications for the behavior of the QCD-like strongly coupled plasma.

April 1, 2016 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker:
Prof. Charles Perdrisat
Host: E. Tracy
Title:
The Elastic Form Factors of the Nucleon
Abstract: A series of experiments initiated at the then new CEBAF electron accelerator in Newport News Virginia, resulted in unexpected results, changing significantly our understanding of the structure of the proton. These experiments used a relatively new technique to obtain the two form factors of the proton, polarization. An intense beam of highly polarized electrons with energy up to 6 GeV was made to interact with protons in a hydrogen target, and the  resulting polarization of the recoiling protons was obtain from a second interaction in a polarimeter.

After a short introduction describing the path which brought me from the University of Geneva to the College of William and Mary in 1966, I will introduce the subject of elastic electron scattering, describe some of the apparatus required for such experiments, show results and then give a brief outline of several theoretical considerations to put the results in a proper perspective.  


April 8, 2016 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker:
Dr. William Wester, Fermilab
Host: 
M. Kordosky
Title: 
Exploring Inner Space with the Dark Energy Survey
Abstract: 
The Dark Energy Survey is a large astronomical survey to cover 5000 square degrees of the southern sky. Initial results include a large number of diverse astronomical and astrophysical measurements. The overall scientific goals are much more ambitious with deeper understanding being sought on the fundamental mysteries of dark matter and dark energy.
April 15, 2016 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker:
Prof. Sankar Das Sarma (University of Maryland)
Host: E. Rossi
Title: 
Quantum Many Body Localization
Abstract: How does an isolated quantum system come to thermal equilibrium due to interaction between its constituent subsystems?  Or does it?  What underlies the condition for quantum ‘ergodicity’?  These are some of the basic questions to be discussed in this talk.  The topic is of fundamental importance since it deals with the applicability of thermodynamics and statistical mechanics to isolated quantum systems, and asks the extent to which an isolated (macroscopic) quantum system can be considered to be acting as its own heat bath.
April 22, 2016 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker:
Prof. Utpal Chatterjee (UVA)
Host: E. Rossi
Title: 
Spectroscopic investigations of pseudogap state in CDW materials and cuprate High Temperature Superconductors (HTSCs) 
Abstract: 
Charge density waves (CDWs) and superconductivity are canonical examples of symmetry breaking in materials. Both are characterized by a complex order parameter –
namely an amplitude and a phase. In the limit of weak coupling and in the absence of disorder, the formation of pairs (electron-electron for superconductivity, electron-hole for
CDWs) and the establishment of macroscopic phase coherence both occur at the transition temperature Tc that marks the onset of long-range order. But, the situation may
be drastically different at strong coupling or in the presence of disorder. We have performed extensive experimental investigations on pristine and intercalated samples of
2H-NbSe2, a transition metal dichalcogenide CDW material with strong electron-phonon coupling, using a combination of structural (X-ray), spectroscopic (photoemission and
tunnelling) and transport probes. We find that Tc(?) is suppressed as a function of the intercalation-concentration ? and eventually vanishes at a critical value of ?=?c leading to
quantum phase transition (QPT). Our integrated approach provides clear signatures that the phase of the order parameter becomes incoherent at the quantum/ thermal phase
transition, although the amplitude remains finite over an extensive region above Tc or beyond ?c. This leads to the persistence of a gap in the electronic spectra in the absence of
long-range order, a phenomenon strikingly similar to the so-called pseudogap in completely different systems such as high temperature superconductors, disordered
superconducting thin films and cold atoms.