Particle-Astro-Nuclear (PAN) Physics seminars

This seminar series presents research covering topics high energy physics, quantum field theory, astroparticle, particle, and nuclear and hadronic physics.

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  • Winter 2021

    May 7, 2021

    Prof. Dr. Mattia Dalla Brida, CERN

    The energy-momentum tensor of QCD from a moving frame


    The energy-momentum tensor (EMT) is a fundamental quantity of any quantum field theory. In QCD, in particular, its correlation functions encode information on the thermodynamic properties of the quark-gluon plasma, as well as on the internal structure of hadrons. Quantitative determinations of these properties require a non-perturbative solution of the theory. At present, lattice QCD is the only known method that allows for first-principles non-perturbative calculations. The application of these techniques, however, necessitates a proper non-perturbative definition of the EMT. The problem is in fact non-trivial due to the explicit breaking of (Euclidean) Poincare' symmetry by the lattice regularization. In this talk, we review the difficulties in devising a practical definition of the EMT in lattice QCD, and present a novel framework to address the problem. The latter is based on studying QCD at finite temperature from the point of view of an observer in a moving reference frame. We shall present how within this framework it is possible to find many compelling identities through which the EMT can be defined. In addition, new ways to compute thermodynamic quantities are possible.



    April 30, 2021

    Prof. Dr. Jean-Yves Ollitrault, Institut de Physique Théorique, CEA Saclay

    Effective viscosities for anisotropic flow


    Determining the transport coefficients of the quark-gluon plasma, such as its shear (η) and bulk (ζ) viscosities, is one of the goals of heavy-ion physics. An important question is how their temperature dependence can be constrained using experimental data. The observable that is most sensitive to the viscosity is anisotropic flow. I show that in relativistic hydrodynamics, the damping of elliptic and triangular flows is determined by an effective viscosity, which is independent of the system size and centrality. This in turn means that this effective viscosity is the only quantity that can be extracted from data at a given collision energy. It is the sum of effective shear and bulk viscosities, each of which is a weighted average of the temperature-dependent viscosity. The effective shear viscosity is driven by the lowest temperatures, just above freeze-out, while the effective bulk viscosity is driven by somewhat higher temperatures.




    April 23, 2021

    Prof. Michael Lisa, The Ohio State University

    Subatomic Smoke Rings: Polarization and Toroidal Vorticity in the QGP


    Since the discovery of global hyperon polarization in Au+Au collisions at RHIC about five years ago, there has been intense theoretical and experimental focus on the topic. After a brief review, I will discuss novel vortex structures that may be generated in two situations. The first is a p+A collision, which may produce droplets of QGP that develop toroidal vortex ("smoke ring") structure. Experimental observation of such a structure would provide compelling evidence supporting the hydrodynamic nature of this tiny system, a much-debated topic today. The other is an idealized "hot moving spot" that may result from thermalization of a jet in an expanding QGP; in this case, the "smoke ring" centers on the jet direction. In both cases, we suggest an experimental observable to measure the toroidal vortex structure, and present present full hydrodynamical simulations to make quantitative predictions. I will also discus next steps for more realistic calculations.




    April 16, 2021

    Prof. Dr. Xin-Nian Wang, Lawrence Berkeley National Laboratory

    Surfing the wake of the jet-induced shock-wave in a quark-gluon plasma


    Jets in QCD have become a powerful tool in high-energy particle and nuclear physics since the concept was developed in the 70's. They have been used to study properties of the quark-gluon plasma (QGP) in high-energy heavy-ion collisions through the jet quenching phenomenon. Jet quenching is caused by parton energy loss when an energetic parton propagates through the QGP medium. Since the parton travels at the speed of light which is much faster than the velocity of sound inside a QGP, such energy loss to the medium will generate a supersonic shock wave. In this talk, I will discuss recent renewed efforts to search for such a shock wake and how to use it to study properties of the QGP in high-energy heavy-ion collisions.


    April 9, 2021

    Dr. Christopher Plumberg, University of Illinois at Urbana-Champaign

    Constraining Relativistic Hydrodynamic Behavior in Nuclear Collisions


    Hydrodynamics is a ubiquitous phenomenon in nature.  It also provides a remarkably powerful description of the space-time evolution of relativistic nuclear collisions.  Nevertheless, understanding its success in such systems is far from trivial.  To date, several indicators have been developed to quantitatively assess the validity of relativistic hydrodynamics quite generally, including systematic expansions in deviations from local equilibrium, expansions in thermodynamic gradients, and constraints imposed by relativistic causality.  In this talk, I will discuss some recent progress in exploring the applicability of relativistic hydrodynamics to both small and large nuclear collisions, using a number of these quantitative indicators to assess its validity.  I will focus in particular on recent findings concerning large violations of causality in state-of-the-art numerical simulations of p+Pb, O+O, and Pb+Pb collisions at the LHC.  I will also discuss some of the implications of these findings for further constraining our understanding of nuclear collision evolution.



    April 2, 2021

    Dr. Daniel Tapia Takaki, University of Kansas

    Entanglement and Quantum tomography for collider physics


    Quantum mechanics is experiencing an experimental and theoretical renaissance. In this talk, we will discuss novel ways to use quantum mechanics and provide several experimental applications of quantum tomography for proton-proton and heavy-ion collision experiments at the CERN Large Hadron Collider. We will discuss application of this model-independent analysis technique for Z bosons, dijets and quarkonia. The first observation of an unexpected correlation of spin and momentum in the experimental data will also be presented.


    March 26, 2021

    Dr. Giuliano Giacalone, ITP Heidelberg

    Correlating isotropic flow with anisotropic flow in heavy-ion collisions: nuclear phenomenology at high energy beyond the quark-gluon plasma


    Experiments conducted at the Relativistic Heavy Ion Collider and at the Large Hadron Collider have established that the interaction of two nuclei at relativistic energy creates a small droplet of quark-gluon plasma (QGP), whose dynamics, from its formation till its fragmentation into hadrons, can be effectively described with the laws of relativistic (viscous)hydrodynamics. I argue that the great success of this hydrodynamic description permits us to search for exciting nuclearphenomena beyond the original purposes of the heavy-ion program. Experimentally, this can be achieved by looking at somewhat unconventional multi-particle correlation observables, where the measures of the anisotropy of the emission of hadrons, the so-called flow coefficients, v_n, are correlated with the magnitude of the isotropic emission, i.e., the average transverse momentum, , of the hadron spectrum. I show that these v_n- correlations provide in particular: i) a novel means to observe deformations in the geometric shape (structure) of the colliding ions; ii) a handle to manipulate the strong electromagnetic fields produced in the interaction of nuclei at high energy; iii) a way to trigger manifestations of the correlation of gluon fields in the pre-QGP stages of the collision process. I point out a few additional phenomena that can be analyzed with the same techniques, and emphasize that the upcoming experimental advances (LHC 3&4, sPHENIX, STAR forward upgrade) will enable us to pursue such studies with success over the next decade.



    March 19, 2021

    Dr. Jasmine Therese Brewer, CERN

    Slow modes in the rapidly-expanding quark–gluon plasma


    A crucial question in understanding the onset of hydrodynamic behavior in the quark–gluon plasma is whether hydrodynamics is “unreasonably effective” in describing systems far-from-equilibrium. This is motivated by the observation that many models of expanding systems exhibit an apparent simplification in their description while gradients are large and hydrodynamics is not expected to apply. We provide a new conceptual approach to understanding this simplification based on far-from-equilibrium slow modes. For a class of collision integrals in kinetic theory, these slow modes can be explicitly identified as the instantaneous ground states of an effective Hamiltonian describing the evolution of moments of the distribution function. They are qualitatively distinct from hydrodynamic modes when the system is far from equilibrium, suggesting that the simplification is not directly related to the onset of hydrodynamics. Importantly, these slow modes dominate the evolution when gradients are small compared to the energy gap, which occurs both at early times and in the hydrodynamic limit.



    March 5, 2021

    Prof. Alakabha Datta, University of Mississippi

    Light Particles and Flavor Anomalies.


    Weakly coupled light states like the dark photon or the dark Higgs  have been discussed extensively. I will discuss a few flavor anomalies that could indicate the presence of light  particles. These anomalies are in B and K decays, g-2 measurements of the charged leptons and in neutrino oscillation experiments. While most of the discussion will employ a general description of the interactions of these light states I will also present a specific model of a dark Higgs that mixes with a 2HDM Higgs sector.



    February 26, 2021

    Prof. Dr. Raju Venugopalan, Brookhaven National Laboratory, Stony Brook University

    The chiral anomaly and the proton's spin: axion-like dynamics and spin diffusion via topological transitions


    The proton's spin arises fundamentally from the complex many-body dynamics of its spinning  quark and gluon constituents. In Quantum Chromodynamics, this dynamics is strongly influenced by the chiral anomaly, a property of the QCD vacuum not manifest in the equations of motion. We will outline the key elements of a powerful formalism where the role of the anomaly is made manifest and show that the appropriate treatment of the anomaly is deeply tied to the resolution of the so-called U_A(1) problem in QCD. At high energies, we conjecture that the corresponding emergent dynamics is analogous to that of a popular dark matter candidate, the axion, with spin diffusion occurring via topological "sphaleron" transitions. These ideas have strong bearing on searches for the Chiral Magnetic Effect at RHIC and can be cleanly tested at a future Electron-Ion Collider.

    February 19, 2021

    Dr. Sören Schlichting, Universität Bielefeld

    Jet quenching and equilibration in Heavy-Ion collisions


    We investigate the medium induced fragmentation of jets in a high-temperature QCD plasma. Based on an effective kinetic theory of QCD, we study the non-equilibrium evolution of the jet shower [1] and the chemical equilibration of jet fragments in the medium [2]. By including radiative emissions as well as elastic interactions, our approach extends all the way from the jet energy scale to the temperature of the medium and includes important effects such as the recoil of the medium. We present results for the in-medium fragmentation, including chemical and kinetic equilibration of the soft fragments [1,2] and discuss implications of our result to jet quenching physics and the problem of thermalization of the quark-gluon plasma in heavy ion collisions.

    [1] Y.Methar-Tani, S.Schlichting JHEP 09(2018) 144 
    [2] S.Schlichting, I. Soudi arXiv:2008.04928[hep-ph]


    Recording (Link will become inactive 6 months after February 19, 2021)

    February 12, 2021

    Dr. Konrad Tywoniuk, University of Bergen

    Energy loss of QCD jets in heavy-ion collisions


    The quark-gluon plasma (QGP) created in high-energy heavy-ion collisions at RHIC and LHC is opaque to energetic and heavy particles that are created in short-distance particle scattering. Jets, aligned collections of energetic hadrons resulting from the fragmentation of fundamental quarks and gluons that are collected within a cone of radius R, are of special interest since they develop on time-scales comparable to the lifetime of the plasma. Jet ``quenching'', or the suppression of the jet yield at large transverse momentum, is therefore a probe not only of the elastic and inelastic interactions with the medium, but also of the medium's capability to resolve correlated QCD color charges. The energy removed from the jet is redistributed in modes that span hard collinear gluon radiation (bremsstrahlung) to softer excitations that ultimately thermalize with the surrounding medium.

    The radius dependence of the jet spectrum is particularly sensitive to the rich physics outlined above. In the first part of the talk, I will present a recent calculation of the jet spectrum in heavy-ion collisions where the medium parameters are sampled from a realistic hydrodynamic evolution of the QGP. Up to relatively large radii R~0.6, the suppression is dominated by perturbative physics while non-perturbative effects, related to the details of thermalization, start to dominate at R~1. This provides, for the first time, a solid basis for higher-order precision calculations of the jet spectrum that are paramount to realize the potential of hard probes as precision tools to extract the properties of the QGP.

    However, jets are rare events and their spectrum drops rapidly with transverse momentum. This induces a strong bias on any process happening in the medium and makes it hard to dig out rare events where heavy-ion jets were substantially modified. In the second part of my talk, I will report on a recent attempt to extract the jet energy before quenching using machine learning. This allows to reduce the biases and enhance the signal of medium modifications. It also allows to better constrain jets as tomographic tools of the medium.


    Recording (Link will become inactive 6 months after February 12, 2021)

    February 5, 2021

    Prof. Rainer J. Fries, Texas A&M University

    Constraining the Effective Shear Viscosity of Hot Hadron Matter


    Transport properties of exotic QCD matter have received much attention since it was discovered that very low shear viscosity-over-entropy ratios eta/s are realized in quark gluon plasma just above the pseudo-critical temperature Tc that separates this phase from hot hadron matter. Both theoretical calculations and estimates from data collected in high-energy nuclear collisions have indicated values of eta/s close to the conjectured lower bound, implying that quark gluon plasma is an almost perfect liquid. Moving our attention below Tc, the transport properties of hot hadron matter are less clear. Theoretical predictions differ by an order of magnitude. We argue that constraints on hadronic eta/s can be derived from the same high-energy nuclear data sets, if the deviations of final freeze-out particle distributions from thermal equilibrium are analyzed. In this talk we describe our method, our attempts to quantify uncertainties, our final results, and the remaining open questions.     


    January 29, 2021

    Dr. Aihong Tang, Brookhaven National Laboratory

    Search for the Chiral Magnetic Effect in Heavy Ion Collisions: Challenges and Status


    The Chiral Magnetic Effect (CME) is the generation of electric current induced by local chirality-imbalance in the presence of magnetic field, and the existence of CME in relativistic heavy ion collisions is an indication of local parity violation in strong interactions. The experimental search for the CME has been carried out extensively in the past decade at the Relativistic Heavy Ion Collider and the Large Hadron Collider. In this talk, I will review the current progress of experimental studies of CME, with particular emphasize on the challenge associated with background separation, as well as special considerations on isobaric collisions.


    January 22, 2021

    Dr. Peter M. Jacobs, Lawrence Berkeley National Laboratory

    Jet quenching at RHIC and LHC: a status report


    Jet quenching, the interaction of energetic jets with the Quark Gluon Plasma (QGP), was discovered 20 years ago. However, its underlying mechanisms, and what jet quenching measurements tell us about the QGP, are still only partly understood. There has been an explosion of new jet quenching observables and measurements in the past few years, whose connections and overall physics message are sometimes difficult to discern. I will suggest a framework to think about them in a unified way, and discuss selected measurements of inclusive jet suppression and in-medium jet scattering by STAR@RHIC and ALICE@LHC. I will also discuss prospects of a comprehensive understanding of jet quenching using the tools developed by the JETSCAPE Collaboration.


    January 15, 2021

    Prof. Scott Pratt, Michigan State University

    Extracting the Diffusivity of Light Quarks in the Quark Gluon Plasma from Heavy-Ion Collisions


    The diffusivity and viscosity are the two most fundamental transport coefficients of the quark-gluon plasma. Whereas viscosity has attracted enormous attention, the diffusivity has garnered less attention because it has been seen as less experimentally accessible. In this talk, experimental measurements of correlations based on local charge conservation, especially those involving kaons, will be shown to be sensitive to the diffusivity. It will be demonstrated that with careful modeling and analysis the diffusivity might be extracted at the 50% or factor of 2 level, which is similar to the degree to which the shear viscosity has been constrained.

  • Fall 2020

    December 11, 2020

    Dr. Chun Shen, Wayne State University

    Observable signatures of initial state momentum anisotropies in nuclear collisions


    We show that the correlation between the elliptic momentum anisotropy, v2, and the average transverse momentum, [pT ], at fixed multiplicity in nuclear collisions carries information on the origin of the observed momentum anisotropy. We develop the hybrid IP-Glasma+Music+UrQMD model that includes contributions from final state response to the initial geometry as well as initial state momentum anisotropies of the Color Glass Condensate. This model predicts a characteristic sign change of the correlator ρ_2(v2, [pT ]) as a function of charged particle multiplicity in p+Au and d+Au collisions at s = 200 GeV, and p+Pb collisions at s = 5.02 TeV. This sign change is absent in calculations without initial state momentum anisotropies. The model further predicts a qualitative difference between the centrality dependence of ρ_2(v2,[pT]) in Au+Au collisions at s = 200GeV and Pb+Pb collisions at s = 5.02TeV, with only the latter showing a sign change in peripheral events. Experimental observation of these distinct qualitative features of ρ_2(v2, [pT ]) in small and large systems would constitute strong evidence for the presence and importance of initial state momentum anisotropies predicted by the Color Glass Condensate effective theory.

    December 4, 2020

    Dr. Matthew Kelsey, University of Cincinnati

    Heavy Flavor in Nuclear Science


    Measurements of hadrons containing heavy flavor quarks has emerged as one of the pillars of heavy-ion physics due to the nature of their production, which allows them to be treated as “external” probes of the de-confined medium produced in heavy-ion collisions. Their utility in probing the various stages of the heavy-ion collisions, spanning from the initial conditions to hadronization, has been exploited at both the Relativistic Heavy-Ion Collider (RHIC) and Large Hadron Collider (LHC), with some interesting results emerging in recent years. In this seminar I will give an experimental review of (open) heavy flavor hadron measurements in heavy-ion collisions, with a focus on recent results from the STAR collaboration which has extended the measurements into the bottom quark sector.  Additionally, with the Electron-Ion Collider (EIC) emerging as the future US-based precision QCD machine set to supersede RHIC, I will highlight potential experimental opportunities with heavy flavor with a precision silicon-based vertex detector.

    November 20, 2020

    Dr. Elena Gramellini, Fermi National Accelerator Laboratory

    The What, the Why and the How of Testbeam Experiments for Neutrinos


    As we enter the era of neutrino precision physics, the need arises for detectors with highly accurate response models, for full scale proto-types, and for a detailed understanding of the interplay between the products of neutrino interactions and the target nucleus. Testbeam experiments can provide all of that. This talk will present a (biased) overview of recent testbeam efforts for neutrino physics, focusing on experiments with high Fermilab involvement. By discussing a few selected examples, we will explore why and how testbeam experiments are key players in the current neutrino physics panorama.

    November 13, 2020

    Dr. Joachim Moritz Brod, University of Cincinnati

    Precision Standard-Model Prediction of epsilon_K


    The parameter epsilon_K describes CP violation in the neutral kaon system and is one of the most sensitive probes of new physics. The large uncertainties related to the charm-quark contribution to epsilon_K have so far prevented a reliable standard-model prediction. In this talk, I will review mixing in the neutral kaon system, and then show that CKM unitarity suggests a unique form of the weak effective Hamiltonian in which the short-distance theory uncertainty of the imaginary part is dramatically reduced. The uncertainty related to the charm-quark contribution is now at the percent level. I will conclude with an outlook on further calculations that have the potential to render epsilon_K a precision observable on par with the rare kaon decays.

    November 6, 2020

    Dr. James Mulligan, Lawrence Berkeley National Laboratory, University of California-Berkeley

    Simulating real-time dynamics of hard probes in nuclear matter on a quantum computer


    Recent advances in quantum devices have raised the prospect for widespread scientific applications of quantum computing within the next few years. We present a framework to simulate the real-time dynamics of hard probes such as heavy quarks or jets in a hot, strongly-coupled quark-gluon plasma (QGP) on a quantum computer. Hard probes in the QGP can be treated as open quantum systems, in which a subsystem interacts with an external environment. Due to large computational costs, most current phenomenological calculations of hard probes evolving in the QGP use semiclassical approximations of the quantum evolution. Quantum computation can mitigate these costs, and offers the potential for a fully quantum treatment with exponential speedup over classical techniques. We report a simplified demonstration of our framework on IBM Q quantum devices, and apply recently developed error mitigation techniques. Our work demonstrates the feasibility of simulating open quantum systems on current and near-term quantum devices, which is of broad relevance to applications in both hot and cold nuclear matter.


    October 30, 2020

    Dr. Robert Bernstein, Fermi National Accelerator Laboratory

    Searching for muon to electron conversion: The Mu2e experiment at Fermilab


    The Mu2e experiment will measure the charged-lepton flavor violating (CLFV) neutrino-less conversion of a negative muon into an electron in the field of a nucleus. The conversion process results in a monochromatic electron with an energy slightly below the muon rest mass. Mu2e will improve the previous measurement by four orders of magnitude using a new technique, reaching a SES (single event sensitivity) of 3 x 10^{-17} on the conversion rate, and a discovery at 2 x 10^{-16}. The experiment will reach mass scales of nearly 10^4 TeV, far beyond the direct reach of colliders. The experiment is sensitive to a wide range of new physics, complementing and extending other CLFV searches. Mu2e is under design and construction at the Muon Campus of Fermilab with our first physics run in early 2025.

    October 23, 2020

    Dr. Linda Carpenter, Ohio State University

    Indirect Detection in Next Generation Dark Matter Models


    Next Generation DM models go beyond simplified models to offer a full theoretical picture of the mediating particle between DM and the Standard Model. These models often have multiple mediating particles, and multiple coupling between Dark Matter and the Standard Model fields. As such indirect detection spectra are very complex. I introduce a new calculational tool to automatically compute all 2-2 DM annihilation process to and any and all gauge invariant pairs of SM particles, in generic user generated models. This tool computes and visualized the partial annihilation rates and photon flux spectrum, then compares the spectrum to known photon flux measurements to generate constraints on the Dark Sector.

    October 16, 2020

    Dr. David Curtin, University of Toronto

    Cosmology and Astrophysics of Dark Complexity


    Dark matter could be part of a complex dark sector that is distinct from but in some ways analogous to the visible sector. This is motivated, for example, by the Twin Higgs model, which solves the little Hierarchy Problem without colored top partners, but also generically by analogy to the richness and non-minimality of the Standard Model. The Twin Higgs model provides a useful benchmark to explore the vast signature space of dark complexity in a theoretically motivated framework. Dark complexity is the flipside of hidden valleys phenomenology at colliders, and depending on model details, the phenomenology looks very different: either spectacular long-lived particle signals at colliders, or a plethora of unusual cosmological and astrophysical signatures via the existence of a predictive hidden sector. I will focus on the latter possibility, and describe how the asymmetrically reheated Twin Higgs realization of dark complexity predicts correlated signals in the CMB, Large Scale Structure, and direct detection searches, as well as higgs precision measurements at colliders. The existence of complex interacting DM relics also predicts highly exotic stellar objects called Mirror Stars with signatures that are remarkably robust with respect to theoretical uncertainties in the hidden sector. Mirror stars could be the first and highly spectacular harbinger of dark complexity and may show up in optical and X-ray searches, LIGO gravitational wave signals, or microlensing surveys. This provides a vivid, multidisciplinary example of the complementarity between collider experiments and cosmological/astrophysical observations to discover the solution to fundamental mysteries like dark matter and the hierarchy problem.

    October 9, 2020

    Dr. Brian Shuve, Harvey Mudd College

    Baryogenesis and Dark Matter from Freeze-In


    I propose a simple, unified, and testable framework for the generation of the baryon asymmetry and dark matter. The baryon asymmetry and dark matter are created via freeze-in through the the decays and inverse decays of QCD-triplet scalars, at least one of which must be in the TeV mass range. I will show that the out-of-equilibrium baryogenesis condition and the dark matter density constraint typically require the lightest scalar to be long-lived, giving good prospects for detection or exclusion in current and upcoming colliders. I will also motivate other astrophysical, cosmological, and collider signatures.

    October 2, 2020

    Dr. Robert Schabinger, Michigan State University

    Cusp and Collinear Anomalous Dimensions in Four-Loop QCD from Form Factors


    In this talk, I present a recent ab initio calculation of the four-loop cusp anomalous dimensions of massless QCD. Novel computational technology developed by us over the course of several years was critical to our success. Our methods and approach to the calculation open the door to the calculation of the four-loop collinear anomalous dimensions of massless QCD, for which we have already obtained precise numerical results superior to the estimates currently available in the literature.

    September 18, 2020

    Dr. Miguel Arratia, University of California-Riverside

    Jet tomography of the proton at the future Electron-Ion Collider


    The future Electron-Ion Collider (EIC) will use electrons to image the quarks and gluons inside protons and neutrons with unprecedented precision. This will allow us to reveal the origin of their mass, spin, radius, and other properties. The EIC will not only allow us to extend traditional studies from fixed-target experiments to uncharted kinematic regions, but also will give us a novel tool: “jets” of particles.  In this talk, I will focus on the prospects of using jets to perform a “quantum 3D tomography” of the proton at the EIC. I will discuss the experimental feasibility of key measurements that will exploit the unprecedented combination of hermetic tracking, particle identification, and calorimetry of the future EIC detectors. I will argue that a jet program at the EIC could unleash a new era in the field of 3D imaging of the nucleon.

    September 11, 2020

    Dr. Lia Corrales, University of Michigan

    New Frontiers of X-ray Exploration: From Astromineralogy to Supermassive Black Holes


    X-ray astronomy as a field has only existed for about 60 years, yet has led to some of the most breathtaking discoveries in the last century of astrophysics. In this talk, I will discuss two science frontiers of high resolution X-ray imaging and spectroscopy: (1) the astromineralogy of interstellar dust grains, and (2) the accretion flow of Sgr A*, the supermassive black hole at the center of the Milky Way. The X-ray energy band is sensitive to absorption and emission by all abundant metals in the interstellar medium (ISM), both in gas and dust form, enabling us to answer key questions in dust grain growth and processing. X-ray photoabsorption features observed in high resolution spectra of Galactic X-ray binaries directly reveal the mineral composition of interstellar dust. I will review my work on the heavily obscured Galactic Center sight line, where dust scattering significantly alters the apparent size, spectral shape, and timing of X-ray variable sources. And finally, after 20 years of the Chandra X-ray Observatory, we have released new high resolution spectral data for Sgr A* that will be irreplaceable for the next 10-20 years.  I will describe how future X-ray missions — XRISM, ARCUS, Athena, and Lynx  — will contribute to each of these topics.

    September 4, 2020

    Dr. Prithwish Tribedy, Brookhaven National Laboratory

    Status of CME Search Before Isobar Collisions and Methods of Blind Analysis From STAR


    The STAR collaboration is currently pursuing the blind analysis of the data for isobar collisions that was performed at RHIC in the year 2018 to make a decisive test of the Chiral Magnetic Effect (CME). Why is it so difficult to detect signals of CME in the experiment? Do we  really understand different sources of background? Why observing similar charge separation between p/d + A and A + A does not stop us from pursuing the search for CME? In my talk, I‘ll attempt to address some of these questions and briefly outline a few recent STAR analyses based on new methods and observables to isolate the possible CME-driven signal and non-CME background contributions at the top RHIC energy. Finally, I describe the procedure for the blind analysis of the isobar data. An outstanding question remains – what happens if we go down in energy? I address this by discussing how the new event-plane detector (EPD) upgrade provides a new capability at STAR towards CME search using the data from the RHIC BES-II program.

  • Winter 2019

    January 18, 2019

    Yacine Mehtar-Tani, Brookhaven National Laboratory

    Higher order corrections to jet quenching in heavy ion collisions

    In standard analytic approaches to energy loss in heavy ion collisions, jets are approximated by single partons and thus higher-order effects in the strong coupling constant are neglected. This may prove insufficient to reliably extract QGP properties at high pT, where a significant jet suppression was recently reported by the ATLAS collaboration in PbPb collisions at the LHC. In this work, we explore higher-order contributions to the inclusive jet spectrum which may be sizable owing to the fact that the probability for a highly virtual parton to split in the medium increases with the jet pT. As the effective number of jet constituents increases, jets are expected to lose more energy than a single color charge. This translates into a logarithmic enhancement of higher orders in the perturbative series that need to be resumed. As a result, we obtain a Sudakov-like suppression factor which we investigate in the leading logarithmic approximation. We note, however, that the phase space for higher-order corrections is mitigated by coherence effects that relate to the fact that, below a characteristic angular scale, the medium does not resolve the inner jet structure. In this case, the jet loses energy coherently as a single color charge, namely, the primary parton.


    February 1, 2019

    Jorge Noronha, Rutgers University

    High-density matter out of equilibrium: from the lab to the sky

    In this seminar, I will discuss some of the main challenges concerning the description of quantum chromodynamics (QCD) in the baryon-rich regime in heavy-ion collisions and also in neutron star mergers. After presenting a realistic prediction for the location of the critical point in the QCD phase diagram, I will present the first systematic study of the emergence of hydrodynamics in a far-from-equilibrium relativistic fluid with a critical point. For rapidly expanding systems such as the matter formed in heavy-ion collisions, the onset of hydrodynamic behavior is shown to be significantly delayed by the presence of critical phenomena. We then switch gears to consider the out-of-equilibrium behavior of the extremely dense matter formed in neutron star mergers. We solved a long-standing open problem in the field of viscous hydrodynamics and its coupling to general relativity by proving causality, existence, and uniqueness of the solutions of the highly nonlinear equations of motion of viscous hydrodynamics in curved spacetime. These results pave the way for the inclusion of viscous effects in state-of-the-art simulations of gravitational-wave signals coming from neutron star mergers.


    February 8, 2019

    Elias Kammoun, University of Michigan

    X-Ray obscuration in active galactic nuclei

    It is commonly thought that "type 2" active galactic nuclei (AGN) are usually obscured in X-rays by material with high column densities (NH), whose exact location and distribution remain an open question. The obscuring material is generally identified with dusty molecular "torus" at the parsec scale, within the "Unification Scenarios" of AGN. However, several pieces of evidence challenge this interpretation. In particular, a handful of AGN has shown rapid changes in column density that are in favor of a clumpy distribution of optically thick clouds rather than a homogeneous structure. The NH-variability timescale suggests that the material is located closer distances to the supermassive black hole (SMBH) which is consistent with the broad-line region (BLR). In this context, the passage of a BLR-gas cloud in our line of sight, that is orbiting the SMBH, will not affect only the AGN's light curve but it shows also a strong impact on its spectroscopic and polarimetric properties. In fact, as the cloud moves in our line of sight it will shade different regions of the accretion disc, which will allow us to probe the innermost regions close to the SMBH. In my talk, I will present the X-ray spectral and polarimetric effects of such eclipsing events. Then, I will present the first results from a survey carried with the Nuclear Spectroscopic Telescope Array (NuSTAR) that aims to characterize the hard X-ray properties of obscured AGN in the local Universe. I will also discuss the role that future high-resolution X-ray observatories such as Athena and XARM will play in identifying and studying obscured AGN.


    March 1, 2019

    Abigail Stevens, MSU

    Mapping Matter in Strong Gravity: Spectral-Timing of Black Holes and Neutron Stars

    One of the best laboratories to study strong-field gravity is the inner 100s of kilometers around black holes and neutron stars in binary systems with low-mass stars like our Sun. The light curves of low-mass X-ray binaries show variability on timescales from milliseconds to months — the shorter (sub-second) variability is particularly interesting because it probes the inner region of the accretion disk and compact object. My research looks at X-ray quasi-periodic oscillations (QPOs) from black holes and neutron stars (as well as coherent X-ray pulsations from neutron stars) by fitting the phase-resolved energy spectra of these signals to constrain their physical origin and track their evolution in time. In this talk, I will present a state-of-the-art "spectral-timing" analysis of QPOs from different classes of sources and different accretion states, and I will discuss how this sets the stage for future research.


    April 5, 2019

    Amber Stuver, Villanova University

  • Fall 2018

    September 14, 2018

    Joe Osborne, University of Michigan

    Effects from color flow in proton-proton and proton-nucleus collisions

    In the last two decades, the study of nucleon structure has shifted from a one-dimensional picture to exploring the dynamic three-dimensional structure of partons within the nucleon. In the transverse-momentum-dependent framework, non-perturbative parton distribution functions and fragmentation functions explicitly carry dependence on partonic transverse momentum rather than only the collinear momentum of the parton with respect to the hadron. The recent interest in the transverse structure of the proton has largely been motivated by the novel phenomenological consequences that have been predicted for these nonperturbative functions, in particular regarding the role that color charge plays in hard scattering processes. For example, factorization breaking has been predicted in hadronic collisions where a final-state hadron is measured and the observable is sensitive to nonperturbative transverse momentum. This prediction has the interesting quantum mechanical consequence that partons are entangled via their color across colliding protons. In this talk, I will discuss the role of color flow in addition to recent results sensitive to these predicted effects in proton-proton and proton-nucleus collisions.


    September 28, 2018

    Gojko Vujanovic, Ohio State University

    Understanding the QGP through the lens of electromagnetic radiation

    Recent viscous hydrodynamical studies [1,2] at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC), show that bulk viscosity plays an important role in their phenomenological description. A temperature-dependent bulk viscosity in the hydrodynamical evolution of the medium can modify the development of the hydrodynamic momentum anisotropy differently in the high- and low-temperature regions. Thus, anisotropic flow coefficients of various observables are affected differently depending where their surface of last scattering lies. For the case of hadronic observables, they are predominantly sensitive to low-temperature regions, while electromagnetic radiation is emitted at all temperatures. Therefore, bulk viscosity should affect electromagnetic radiation differently than hadron emission. The effects of bulk viscosity on one of the electromagnetic probes, namely photons, has already been investigated [1]. The same statement holds true for hadrons [2]. The goal of this presentation is to study how thermal dilepton production, the other source of electromagnetic radiation, gets modified owing to the presence of bulk viscosity at RHIC and LHC energies. With calculations at different collision energies, comparisons in the dilepton signal can be made and more robust conclusions regarding the role of bulk viscosity in high energy heavy-ion collisions can be drawn. Dilepton radiation from the dilute hadronic phase of the medium will also be included to ascertain whether these modifications may be observable in experimental data. [1] Jean-François Paquet et al., Phys. Rev. C 93 no. 4, 044906 (2016) [2] S. Ryu et al., Phys. Rev. Lett. 115 no. 13, 132301(2015).


    October 5, 2018

    Joshua Berger, University of Pittsburgh

    Detecting boosted dark matter at large volume neutrino detectors

    We study novel scenarios where thermal dark matter (DM) can be efficiently captured in the Sun and annihilate into boosted dark matter. In models with semi-annihilating DM, where DM has a non-minimal stabilization symmetry, or in models with a multi-component DM sector, the annihilation of DM can give rise to stable dark sector particles with moderate Lorentz boosts. Taking advantage of the energetic proton recoils that arise when the boosted DM scatters off matter, we propose a detection strategy that uses large volume terrestrial detectors, such as those designed to detect neutrinos or proton decays. In particular, we propose a search for proton tracks pointing away from the Sun. We present bounds and sensitivity at Super-Kamikande and Hyper-Kamiokande respectively. We then discuss the possibilities for enhanced sensitivity at DUNE and other liquid argon TPC detectors. We present the first full Monte Carlo event generation for boosted dark matter and discuss the modeling challenges. We use this tool to discuss the first fully realized analysis of boosted dark matter at DUNE.


    October 9, 2018

    Jake Bennett, University of Mississippi

    First collisions and plans for the future at Belle II

    The Belle II experiment, currently under construction at the KEK laboratory in Tsukuba, Japan, is the next generation of the highly successful B-factories. A substantial upgrade of both the Belle detector and the KEKB accelerator represent an essentially new experiment. The ultimate goal of Belle II is to collect about 50 times as much data as its predecessor, opening the path to measuring rare decays that may give hints on the hunt for new physics. Commissioning of the accelerator was recently completed and preparations are ongoing for data taking with the full Belle II detector in the spring of 2019. With this early data, it is possible to start getting an idea of the expected performance of the detector as well as to better understand beam backgrounds. In this talk, I will introduce the experiment and present some of the first results coming from Belle II.


    October 19, 2018

    Chun Shen, WSU

    Dynamical modeling of relativistic heavy-ion collision: correlations from flows and beyond

    In this talk, I will discuss that the universal hydrodynamic response can lead to a consistent description of the flow observables in heavy-ion collisions over more than 2 orders of magnitude in measured hadron multiplicity. This standard theoretical framework helps us to elucidate the origin of the collective behavior in small collision systems, such as high multiplicity p+Pb collisions. Next, by going beyond the long-range flow correlations, we implement local charge conservation during the particlization stage. It introduces non-trivial short-range correlations. A first study gives promising results compared to the Au+Au measurements at the top RHIC energy. This can provide the most realistic background calculations for the RHIC isobar runs for the search of the Chiral Magnetic Effects. 


    October 26, 2018

    Luke Pickering, University of Michigan

    Neutrino interactions: Effects on T2K oscillation analyses

    Neutrino oscillation experiments are chasing the answer to a number of fundamental questions, most importantly: Is there enough Charge-Parity violation in the lepton sector to explain the deficit of anti-matter in the observable universe? In the 1–5 GeV neutrino energy region—where current and planned accelerator-based beams peak—neutrino–matter interaction theory is non-perturbative, and currently available models fail to predict the extant data well. This is problematic for oscillation analyses that depend strongly on interpreting observations through such models. Over the next decade, interaction-model uncertainties will set the limit on the precision of oscillation experiments. The Tokai-to-Kamioka (T2K) experiment, which has been taking data since 2010, is one such oscillation experiment. The latest analysis shows tantalizing hints of differences in the oscillation of neutrinos and antineutrinos—i.e. lepton sector CP violation. This talk will discuss what we are currently learning with the T2K data, how the neutrino interaction uncertainties are parameterized, and what the future holds for the field. 


    November 2, 2018

    Malte Buschmann, University of Michigan

    Simulation of a cosmological axion through the QCD phase transition

    We perform a full (3+1)-dimensional numerical simulation of the axion field around the QCD epoch.
    Our aim is to fully resolve large dynamical non-linear effects in the inhomogenous axion field. These effects are important as they lead to large overdensities in the field at late times. Those overdensities will eventually evolve into an axion mini-cluster, which has various phenomenological implications like microlensing events. It is therefore important to have a reliable estimate of the number of overdensities and their mass relation.


    November 9, 2018

    Felix Ringer, LBNL

    Jet substructure in high-energy hadron collisions

    Collimated jets of hadrons serve as precision tests of the standard model and in particular QCD. For example, inclusive jet and jet substructure observables have been applied extensively to constrain Parton distribution functions and to probe the hot and dense medium created in heavy-ion collisions, as well as to the search for physics beyond the standard model. In this talk, I will mainly focus on recent theoretical developments based on the effective field theory approach to jet physics. This newly established framework allows for precision calculations of jet cross-sections and enables a direct comparison of experimental data and first-principles calculations of jet substructure observables. In addition, I will discuss a new approach to QCD factorization of jet cross-sections in heavy-ion collisions. 


    November 16, 2018

    Takafuni Niida, WSU    

    Vorticity and polarization in heavy-ion collisions

    The matter created in non-central heavy-ion collisions is expected to possess a significant fraction of the initial angular momentum carried by the two colliding nuclei.  This angular momentum can lead to vorticity of the system and be partially transferred to the spin of produced particles due to the spin-orbit coupling, leading to the phenomenon of global polarization. The STAR Collaboration observed finite signals in Au+Au collisions at √sNN = 7.7 - 39 GeV and later at  √sNN = 200 GeV, indicating non-zero vorticity of the system. The global polarization might differ between particles and antiparticles, due to the opposite sign of the magnetic moments, which could be a direct tool to study the magnetic field in heavy-ion collisions. Furthermore, a possible local vortical structure along the beam direction might be caused by azimuthal anisotropic flow. In this talk, I will present recent results on the polarizations of Λ hyperons in heavy-ion collisions.


    November 30, 2018

    Rebecca Coles, Ohio State University

    DESI commissioning instrument and metrology

    The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers, in turn, feed ten broad-band spectrographs. We will describe the methods and results for the commissioning instrument and metrology program, the products of which will be used to determine absolute astrometric reference data for subsequent telescope commissioning. We will use the commissioning instrument and metrology program to measure the absolute three-axis Cartesian coordinates of the optical devices on the commissioning instrument which will be needed to ensure that we accurately map the DESI correctors optical distortions.


    December 7, 2018

    Francis Halzen, Wisconsin IceCube Particle Astrophysics Center and Department of Physics, University of Wisconsin-Madison

    IceCube: Beyond Astronomy

    An informal seminar on the other physics done with IceCube.

    This special seminar is a follow-up to Professor Halzen's colloquium on Thursday, December 6, 2018.


    December 7, 2018

    Chris Monahan

    Institute of Nuclear Theory

    Semileptonic decays of B mesons from lattice QCD

    Where is all the new physics hiding? In spite of enormous successes, such as the discovery of the Higgs boson, and a mountain of data, direct searches at the LHC--the energy frontier--have so far failed to turn up new fundamental particles. An alternative approach is to hunt for answers at the intensity frontier: the possibility of new physics lurks in tantalizing discrepancies between theoretical predictions and experimental observations in the flavor physics sector. The Cabibbo-Kobayashi-Maskawa (CKM) matrix, which describes the mixing of quark mass eigenstates under the weak force, is unitary in the Standard Model. By constraining CKM matrix parameters through measurements of multiple different processes, any observations of deviations from unitarity may hint at new physics effects. Key to this program is precise theoretical predictions, which require lattice QCD calculations of hadronic contributions to Standard Model processes. I review the latest lattice calculations of B-meson decays from the HPQCD collaboration and highlight the role of results from LHCb, which are expected in the near future.