Browsing by Subject "Particle Physics"

Now showing 1 - 12 of 12
  • Thumbnail Image
    Item
    Open Access
    Alpha decay of excited states of carbon-12
    (1970) Shackleton, David; Brooks, F D
    A crystal of the carbon-rich phosphor, anthracene, has been bombarded by monoenergetic fast neutrons. Alpha-particle decays of carbon nuclei have been separated from other events by a specialised application of the pulse shape discrimination technique. A numerical analysis of the data has yielded alpha-particle energy spectra over a wide energy range. A counter has been assembled to detect neutrons scattered by carbon nuclei in the crystal, and the measurements repeated in coincidence with these neutrons. The energy range of particle resolving power has been extended. Structure in the resulting alpha-particle and proton energy spectra is attributed to particle decays of specific levels in carbon-12. It is shown how the experiment may be modified and extended to measure decay parameters in absolute terms.
  • Thumbnail Image
    Item
    Open Access
    Analysis of a deep neural network for missing transverse momentum reconstruction in ATLAS
    (2020) Leigh, Matthew; Yacoob, Sahal; Young, Christopher
    The ATLAS detector is a multipurpose particle detector built to record almost all possible decay products of the high energy proton-proton collisions provided by the Large Hadron Collider. The presence and combined kinematics of unobserved particles can be inferred by the observed momentum imbalance in the transverse plane. In this work, a deep neural network was trained using supervised learning to measure this imbalance. The performance of this network was evaluated in MC simulation and in 43 fb⁻¹ of data recorded at ATLAS. The network offered superior resolution and significantly better pileup resistance than all other pre-existing algorithms in every tested topology. The network also provided the best discriminator between events that did and did not contain neutrinos. The potential gain insensitivity to new physics was demonstrated by using this network in a search for the electroweak production of supersymmetric particles. The expected sensitivity to observe the production of said particles was increased by up to 26%.
  • Thumbnail Image
    Item
    Open Access
    The anomalous magnetic moment of the nucleon in cavity QCD
    (1991) O'Connor, M S
    Perturbative quantum chromodynamics is developed in a spherical cavity using a symmetric form of the Gell-Mann and Low theorem. This formalism allows one to generate any desired term in the perturbation series, in a manner which is similar to the familiar Feynman rules in free space. All corrections to order eg² in the electromagnetic and strong coupling constants which contribute to the magnetic moment of a baryon are generated using this formalism. The O(eg²) radiative corrections to the magnetic moment of the nucleon are calculated here in an arbitrary covariant gauge. The gauge-dependent parts are found to vanish identically, and the divergences arising from the loop diagrams cancel amongst each other, making renormalization unnecessary. However, it is shown here that one can, if it is necessary, remove the divergences from the cavity diagrams by subtracting from them a singular factor which is found using dimensional regularization in the analogous free-space diagrams.
  • Thumbnail Image
    Item
    Open Access
    Canonical quantization and quantum chromodynamics in a cavity
    (1986) Zimak, Petr; Viollier, Raoul D
    The canonical quantization formalism is applied to the Lagrange density of chromodynamics in a general covariant gauge. The physical states are characterized by their BRS-invariance. We develop the quantum theory of the interacting fields in the Dirac picture, based on the Gell-Mann and Low Theorem and the Dyson expansion of the time evolution operator. Subsequently, confinement is introduced phenomenologically by imposing, on the quark, gluon and ghost field operators, the linear boundary conditions of the M.I.T. bag model at the surface of a spherically symmetric and static cavity. Based on this formalism, we calculate, in the Feynman gauge, all non-divergent Feynman diagrams of second order in the strong coupling constant g. Explicit values of the matrix elements are given for low-lying quark and gluon cavity modes.
  • Thumbnail Image
    Item
    Open Access
    Charged particle beam transport for a cyclotron facility
    (1980) Merry, Corinne Margaret; Cornell, John
    We develop a number of new techniques and systems to be used in the design of beamlines for charged particle beam transport. A few of these refer specifically to beamlines to or from a cyclotron, while others may be used in beamlines from any accelerator. In the former category, we develop a method for determining the eigen-ellipsoid in all six dimensions of phase space, when the beam under consideration is to be (a) extracted from a cyclotron, or (b) injected into a cyclotron. We also develop an alternative method to (a) above, which uses the data derived from tracking (i) central momentum particles through the extraction elements of an accelerator in 4-dimensional (x, x´, y, y´) phase space and (ii) a single particle with higher momentum. For this purpose we expand the convenient E-matrix formalism from a 2-dimensional treatment to a 6-dimensional treatment, and relate this to the more usual σ-matrix formalism. We describe the eight possible symmetry types of beams transport systems and examine their group properties. We also examine the second-order aberrations in these systems. We use the symmetry properties to examine various configurations of two quadrupole triplets. This system may be used to achieve unit magnification, as is well known: or variable magnification in one or both of the horizontal or vertical planes, independently of the beam parameters, as we describe. We also develop a system of quadrupoles which may be used for independent horizontal and vertical beam control. We calculate the optimum spacing and field strength of these quadrupoles. Dipole systems which are used to control the dispersed rays are discussed. In particular we consider a system of two quadrupoles between two dipoles: this system has the least number of beamline elements necessary to control the position and direction of the dispersed ray while simultaneously permitting momentum-selection. We discuss the principles of transfer beamline design and illustrate these (and the techniques described above) by reference to the design of a specific transfer beamline between cyclotrons. The design of a specific external beamline is also described and used to illustrate the techniques developed.
  • Thumbnail Image
    Item
    Open Access
    The effects of resonance decays on particle ratios and momentum spectra
    (1997) Brookes, David Thomas
    A Hadron Gas model, including resonance decays, is used to predict momentum spectra and particle ratios. The model, including all significant two and three body resonance decays, is compared with transverse momentum spectra from the CERN-SPS NA44 Pbr Pb experiment and the implications of the fits are discussed. It is shown that resonance decays fail to explain the low mT anomaly in the pion transverse momentum spectrum. The effects of resonance decays are then considered in the calculation of various particle ratios. The width of the rapidity window is varied and its effect on the particle ratios is examined.
  • Thumbnail Image
    Item
    Open Access
    Finite-temperature quantum field theory and the structure functions of the nucleon
    (1992) Joubert, Jean; Cleymans, Jean
    The deep inelastic scattering of leptons off a proton in the statistical model is considered. The interior of the nucleon is viewed as a thermalized assembly of up and down quarks and gluons. This enables the incorporation of features which are absent in the parton model. These include the presence of identical quarks and gluons in initial and final states and of quantum statistical correlations which have a role to play in the propagation of particles when considering Feynman diagrams containing internal lines in next-to-leading-order calculations. These features are incorporated through the use of Fermi-Dirac and Bose-Einstein distributions for quarks and gluons, respectively. Stimulated emission factors for final-state gluons and Pauli-blocking factors for final-state quarks are incorporated. The propagation of particles through a many-body medium is taken into account by using thermal Feynman rules for propagators and vertices. The statistical model could also be seen as an attempt to describe the interior of the nucleon at a more fundamental level than that attained through the use of arbitrary parton distributions containing many parameters in the parton model.
  • Thumbnail Image
    Item
    Open Access
    Hadronic gas models in particle production and phase transitions
    (1992) Weber, Götz Mathias
    In this work the methods of exact quantum number conservation in statistical mechanics are discussed and applied to the field of high energy nucleus-nucleus collisions. Various types of hadronic gas models are discussed as well as their merits and restrictions. Attempts to construct a phenomenological equation of state for nuclear matter are discussed in the context of the phase transition from hadronic matter to the quark-gluon plasma (QGP).
  • Thumbnail Image
    Item
    Open Access
    The (p, n) charge-exchange reaction on ⁹⁰Zr at intermediate energies
    (1992) Kabutz, Rudolf T; Aschman, David G
    Using the Time-of-Flight facility at the National Accelerator Centre at Faure, the (p, n) charge-exchange reaction has been studied at intermediate energies of 120, 160 and 200 MeV, and at angles of 0°, 2° and 4°. In this work the data collected for the ⁹⁰Zr target will be presented. The influence on the data from slow neutrons due to previous pulses is discussed and the best manner of removing them from the spectra is recommended. It is shown how the background cosmic rays can be utilised to measure the intrinsic resolution of the detectors and to obtain an estimate of the neutron energy threshold. The differential cross-sections for the states corresponding to Fermi and Gamow-Teller transitions were extracted from the time spectra. The sum of the strength of all the discrete Gamow-Teller states was determined and compared to the Ikeda Sum Rule. It was found that only 50% of the sum could be accounted for in the discrete states. An overview of the theory that has been developed to extract Gamow-Teller strengths from the (p, n) cross-sections is given. Some of the theoretical models that have been used to describe the ⁹⁰Zr(p, n)⁹⁰ Nb reaction and account for the missing Gamow-Teller strength are briefly discussed.
  • Thumbnail Image
    Item
    Open Access
    The study of Gamow-Teller strength in the 51V(p,n)51 Cr reaction at intermediate energies
    (1992) Tshivhase, Victor Makondelele; Aschman, David G
    The ⁵¹V(p,n)⁵¹Cr reaction was studied at four different proton beam energies namely 90, 120, 160, and 200 MeV. The experiment was performed in the angular range between 0 and 4°, using the beam swinger facility at National Accelerator Centre(NAC) at Faure, near Cape Town. The cross sections of the ground state, Isobaric Analog State, and the giant Gamow-Teller resonance were calculated at 0° for each beam energy. The ratio of the cross section of the giant Gamow-Teller resonance to the cross section of the Isobaric Analog State were studied at beam energies 90, 120, 160 and 200 Me V, and beam angles 0, 2 and 4 °. The Gamow-Teller strength was estimated for energies 120, 160 and 200 MeV. The fractions of Fermi and Gamow-Teller strength in the Isobaric Analog State were also estimated.
  • Thumbnail Image
    Item
    Open Access
    Suppression of the fake lepton background in same-sign W-boson scattering with the ATLAS experiment
    (2017) McConnell, Lucas Henry; Hamilton, Andrew; Yacoob, Sahal
    Same-sign W-boson scattering is a rare Standard Model process that is useful for probing the nature of electroweak symmetry breaking and the Higgs mechanism. Analysis is currently underway to measure the cross-section to a significance of 5σ or higher using √s = 13 TeV data from the ATLAS detector's Run 2. The two scattered W-bosons decay leptonically leaving a distinctive experimental signature of two same-sign leptons, two forward jets, and missing transverse energy carried away by two neutrinos. Non-prompt leptons are defined as leptons coming from the decay of hadrons. Such leptons, together with jets misreconstructed as leptons, contribute to the background processes in same-sign W-boson scattering; making up the so-called fake lepton background. In this thesis the fake lepton background is suppressed using two strategies: 1) implementing an optimised veto on events found to contain a b-jet; and 2) optimising the isolation requirements set on signal lepton candidates using the cumulative significance quantity. The approach using the cumulative significance is then extended to optimise additional analysis cuts on the lepton invariant mass mₗₗ, jet invariant mass mⱼⱼ , and the jet separation rapidity Δyⱼⱼ.
  • Thumbnail Image
    Item
    Open Access
    Vector boson production with the ALICE detector
    (2017) Senosi, Kgotlaesele Johnson; Cleymans, Jean; Buthelezi, E
    The main objective of this thesis is to study and investigate the production of massive vector bosons (W+ and W−). This a priori mentioned production is not sensitive to hot nuclear matter effects because of the weak coupling nature of these vector bosons. Thus, in heavy ion collisions they provide a good reference for the medium-induced effects on other probes. The production mechanism of these vector bosons is highly isospin dependent and thus they are affected by the initial state effects. Initial state effects include isospin, Fermi motion, EMC effect, shadowing and nuclear absorption. Hence their production in lead-lead (Pb-Pb) and proton-lead (p-Pb) collisions can be used to test some of these initial state effects. In this thesis only two of these initial state effects will be considered namely, isospin and shadowing (referring to shadowing and anti-shadowing). Eke, these vector bosons can be used to provide a non-arbitrary reference to the probes affected by the medium. Traditionally, in heavy ion collisions, hard processes are expected to scale with the number of binary collision thus a precise study of these vector bosons can be used to test the factorisation assumed in models used to determine centrality. This unique property of electroweak (W) bosons makes them essential probes to study the possible inherent bias in centrality determination. In proton-proton (pp) collisions, their production can be used to obtain information on quark parton distribution functions (PDF). The data used in the analysis was collected by A Large Ion Collider Experiment (ALICE) at the Large Hadron Collider (LHC). The ALICE detector is designed to study ultrarelativistic heavy-ion collisions, in which a hot and dense, strongly-interacting medium is created. The production of W bosons is studied in p-Pb, p-p and Pb-Pb collisions at 5.023, 8 and 5.023 TeV centre-of-mass energies, respectively. The forward muon spectrometer with the pseudorapidity acceptance −4.0 < ŋ < −2.5 is used. W bosons are studied via the inclusive single muon differential pT spectrum.