Nuclear/Particle Seminar Abstracts
Fall 2019
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LHCb is the dedicated heavy-quark flavour experiment at the Large Hadron Collider (LHC) at CERN. The experiment has been designed to select beauty and charm hadrons with the main purpose of studying matter-antimatter asymmetries to elucidate CP violation in the beauty and charm-quark sectors. As a consequence, LHCb has the largest available data-set to measure the spectroscopy of heavy hadrons. In this talk, I will highlight some recent results from LHCb in heavy-quark spectroscopy, including discovery of new beauty baryon states, the first observation of doubly-charm baryons and exploration of exotic states, such as tetraquark and pentaquark states observed at LHCb. |
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The anomalous magnetic moment of the muon is one of the most precisely measured quantities in particle physics and can therefore serve as a high-precision test of the Standard Model. The largest uncertainty in the Standard Model prediction comes from the hadronic sector, where the leading contribution is given by the Hadronic Vacuum Polarisation (HVP). A determination of the HVP from lattice QCD aiming at sub-percent level precision requires to include isospin breaking corrections in the computation. I will discuss how these corrections can be included in a lattice calculation and present results for the QED and strong isospin breaking corrections to the HVP at physical quark masses. |
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The discovery of two large mixing angles in neutrino mixing is in stark contrast with quarks where the largest mixing angle is small. Yet all standard model particles have similar gauge couplings. This tension has sparked much theoretical interest, couched in the language of crystal-like mixing matrices such as the prettifying Tri-Bi-Maximal (TBM) matrix with an ugly name! In this talk an asymmetric Yukawa texture is presented where the reactor angle is solely generated by electroweak breaking. SU(5) Grand-Unified patterns suggest close kinship between down quarks and charged leptons Yukawa mixings. Surprisingly a large Jarlskog-Greenberg CP violation, consistent with global fits emerges from this texture. The second half of the talk shows how to implement this asymmetric texture with a discrete family symmetry. |
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Squarks are a familiar example of scalar colored triplets from supersymmetry, but such fields can also play important roles in nonsupersymmetric models. In this talk I review three far-flung examples where they recently appeared. In the first, they allow for UV-completion of a low-energy effective theory of neutron-dark matter oscillations. In the second, they enable a model of pseudo-Goldstone boson dark matter to explain hints of excess collider signals around 96 GeV, with testable implications for the effective couplings of the Higgs boson and cosmic gamma rays. The final example is an economical new theory of inflation that simultaneously produces the baryon asymmetry, with possibly observable tensor and isocurvature perturbations. |
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Ultra-relativistic collisions of heavy nuclei provide a unique experimental window into the early universe and into extreme sectors of the QCD phase diagram by producing an exotic state of matter known as the quark-gluon plasma (QGP). The essential characteristic of the QGP, which is formed at temperatures sufficiently high to melt hadrons into a plasma of delocalized quarks and gluons, is a nearly vanishing fluid viscosity which makes the QGP among the most “perfect liquids” in nature. This nearly-ideal fluid flow, however, makes the final distributions of detected hadrons especially sensitive to the initial geometry of the fireball shortly after the collision. As such, tremendous effort has been expended to distinguish the features of the data arising from the final-state QGP evolution from the features reflecting the fluctuating initial state itself. At top RHIC and LHC energies, the initial energy density of a heavy-ion collision is composed almost entirely of gluons, with the hydrodynamic evolution reflecting energy-momentum conservation over the lifetime of the QGP fireball. However, quarks, which constitute a minority of the overall energy density, also carry other conserved charges such as baryon number and electric charge which are sensitive to entirely different transport properties of the QGP. In this talk, I will present a new model for reconstructing the initial distribution of quarks and antiquarks in a heavy-ion collision by sampling the (g → q qbar) splitting function over the initial energy density. In this way, we provide a new numerical tool which can be used to supplement any model for the initial energy density with the associated conserved charges. As a result, we find a strong flavor dependence of the initial geometries of different quarks, as characterized by their initial eccentricities. Importantly, we find that the strange quark geometry differs significantly from the geometry of the bulk energy density in an event, reflecting the geometry of the hot spots rather than the geometry of the bulk. This new tool for the initial conditions, when coupled to a charge-conserving viscous hydrodynamics code, will open the door to studying a wealth of new charge- and flavor-dependent correlations and transport parameters of the QGP. |
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Neutrino detectors measure the total event rate which is a convolution of flux and cross section effects. Because both detectors are exposed to neutrinos from the same beamline, the uncertainties in the neutrino flux prediction will be correlated. This fact combined with the different neutrino energy spectra seen at each detector will allow for some separation of flux and cross section effects, and presents an opportunity to study the neutrino cross section as a function of energy using the same neutrino beam. This analysis is the first cross section measurement on T2K to use samples from multiple detectors in the same beamline. |
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Coherent pion production by neutrinos is a rare process where the neutrino interacts with the nucleus as a whole, creating a W (Z) boson which decays into a charged pion (neutral pion) in charged (neutral) current interactions, by transferring a small four-momentum to the nucleus. The signature of the interaction consists of a lepton and a π meson, both in the forward direction with regard to the incident neutrino, and negligible vertex activity. This process has been observed in a variety of materials in a wide range of energies, from a few to hundreds of GeV. Experiments struggled to find evidence of the charged current channel at low neutrino energies (few GeV), and this puzzle was solved by the MINERvA collaboration in 2014 by fully containing the pion candidates and by only looking at model independent features of the interaction, using both muon neutrinos and anti-neutrinos in a CH target. This result also makes use of the MINERvA detector in the NuMI neutrino beam at Fermilab but with a more energetic and intense beam (2<Enu<20 GeV), to analyze the charged current channel using muon neutrinos to try to show the "A-dependence" of this interaction for the first time. Measurements were made using hydrocarbon, carbon graphite, iron (steel), and lead targets allowing a wide range of nuclear sizes. This talk aims to show the scaling of the interaction's cross section with the number of nucleons. Current models predict a scaling of A1/3, A2/3 or even an energy-dependent scaling. |
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The neutrino is a misfit in the Standard Model of particle physics. Initially predicted as a massless particle, experiments are currently exploring their fundamental properties through a mechanism called neutrino oscillations. One such experiment is called Tokai to Kamioka (T2K), which observes muon - neutrinos oscillate into electron-neutrinos. A subset of the neutrinos is first detected at the near detector complex, ND280, 280 meters from their production source. The neutrinos oscillate over a distance of 295 km and are observed at the Super-Kamiokande (Super-K) water Cherenkov detector. The purpose of ND280 is to provide the “near detector (ND) constraint”, which a measurement the neutrino flux directed towards Super-K and neutrino on water interactions that affect the oscillation analysis. This presentation describes the measurement of the ND constraint using the off-axis pi-zero detector (P0D) using a binned maximum likelihood estimator (MLE) technique and compares it to the nominal constraint. |
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Weak scale supersymmetry (SUSY) has been, for many years, the dominant paradigm for physics beyond the Standard Model and in fact the simplest model predicted the Higgs mass to lie exactly in the range where it was discovered. And yet the community seems discouraged as to the likelihood of SUSY due to lack of superpartners at LHC and the somewhat large value of the Higgs mass: these are thought to exacerbate the "naturalness" question. A more nuanced evaluation of electroweak naturalness points to a highly natural SUSY mass spectrum characterized by light Higgsino states with mass ~100-200 GeV while the SUSY breaking scale lies in the multi-TeV region. Such a spectrum of superparticles seems to emerge from simple statistical ideas about the string theory landscape of vacua. The natural SUSY particle mass spectrum 1. gives rise to distinctive SUSY signatures at LHC but 2. might also allow SUSY to escape LHC detection so that a high energy upgrade of LHC to 27 TeV may be needed. The required light higgsino states should appear at an e+e- collider operating at CM energy 400-600 GeV: the International Linear Collider or ILC. Requiring naturalness also in the QCD sector, one expects two dark matter particles: the axion and a higgsino-like neutralino where typically the axion makes up the bulk of dark matter. Detection of a higgsino-like WIMP is ultimately expected while the axion would have a suppressed coupling to photons which makes its detection more challenging than was previously considered. |