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 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 Univesity

    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 have 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 are 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 which 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 axion minicluster, which have 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 are 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.