Browsing by Author "Horowitz, William"

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    Energy loss and theoretical uncertainties in small quark-gluon plasmas
    (2025) Faraday, Coleridge; Horowitz, William
    There is a wealth of evidence that a Quark Gluon Plasma (QGP) is formed in heavy-ion collisions at RHIC and the LHC. Recently, there have been observations of QGP signatures in much smaller collision systems—including proton proton, and proton heavy-ion collisions—wherein QGP was not expected to form. Experimentally measuring suppression in small systems is more difficult than in large systems, motivating the need for theoretical guidance on the problem. The goal of this thesis is to systematically improve current energy loss models, particularly in how they pertain to small systems. We present a perturbative Quantum Chromodynamics (pQCD) based energy loss model which receives small system size corrections to both the radiative and elastic energy loss, and which takes into account realistic collision geometry, production spectra, and fragmenta-tion. We use the Djordjevic-Gyulassy-Levai-Vitev (DGLV) radiative energy loss model, and include a small system size correction which adds back in previously neglected terms that are suppressed according to the system size. We find that the correction is extremely large for pions at high momenta, which leads us to question the validity of various approxima-tions in the model. We investigate the self- consistency of the various approximations used in the derivation of the Djordjevic-Gyulassy-Levai-Vitev (DGLV) radiative energy loss model, where we find that a particular approximation—the large formation time approximation—is not satisfied self-consistently within the model. We explore a kinematic cutoff on the trans-verse radiated gluon momentum, which restores the self-consistency of this approximation, but at the cost of an increased sensitivity to the exact cutoff chosen. The exploration and quantitative treatment of theoretical uncertainties in the energy loss model is a central theme of this thesis. In the same vein of uncertainty quantification, we investigate the common application of the central limit theorem to approximate the elastic energy loss as a Gaussian distribution. We find that all our results are remarkably insensitive to this approximation, not because we are in the regime of many scatters where the central limit theorem is applicable, but rather understood from an expansion of the RAA in terms of the moments of the underlying energy loss distributions. We also investigate the uncertainty in the elastic energy loss to the crossover between HTL and vacuum propagators. Finally, we perform a one-parameter fit of the strong coupling αs to available large system data from RHIC and LHC, keeping track of all of the aforementioned uncertainties. We find that the uncertainties may largely be absorbed into a different value of the strong coupling αs, but small uncertainty bands remain in any case. We explore differences in the energy loss models that remain even after the fit of the model to data, and find that the different elastic energy losses lead to different pT and system size dependencies. We also plot large-system constrained model results for small p/d/3He + A collisions, where we find that our results are quantitatively consistent with small system data from RHIC and inconsistent with small system data from LHC.
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    Finite kinematics in radiative energy loss
    (2023) Renecle, Antonio; Horowitz, William
    We investigate the behaviour of particle emission spectra in the largex region derived from a rigorous implementation of the exact kinematic constraints in the framework of a scalar model of radiative emissions. We find that the small-x kinematic constraints in the simpler theory are identical to those implemented in sophisticated QCD-based energy loss models, but that the exact large-x kinematics in the toy theory are more complicated than those implemented in those same QCD-based energy loss models. We compute the differential cross-section for various values of the parent parton energy and see that our spectra respect energy conservation by smoothly vanishing outside the classically allowed 0 < x < 1 region.
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    Fluctuating Open Heavy Flavour Energy Loss in a Strongly Coupled Plasma with Observables from RHIC and the LHC
    (2021) Ngwenya, Blessed Arthur; Horowitz, William
    We present predictions for the suppression (using the nuclear modification factor) of B-mesons using AdS/CFT techniques assuming a strongly coupled quark-gluon plasma (QGP). These energy loss predictions are presented for collision energies √ sNN = 2.76 TeV for central collisions and √ sNN = 5.5 TeV for various centrality classes. There are some uncertainties in terms of how we phenomenologicaly apply energy loss calculations computed in AdS/CFT. One uncertainty is related to how the diffusion coefficient behaves as a function of momentum in AdS/CFT. We make predictions for the two known limits, one where the diffusion coefficient depends on momentum and one where the diffusion coefficient is momentum independent. There also exists systematic theoretical uncertainties associated with the mapping of parameters in N = 4 SYM theory to QCD. We look at two reasonable sets of parameters to try and capture these uncertainties. We will also present results of the v2(pT ) for B-mesons describing the azimuthal anisotropy at √ sNN = 2.76 TeV for central collisions and √ sNN = 5.5 TeV for central, semi-central and peripheral collisions.
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    The Casimir effect in non-Abelian gauge theories on the lattice
    (2025) Ngwenya, Blessed Arthur; Horowitz, William; Rothkopf, Alexander
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    Towards a spacetime description of hard parton evolution in the quark gluon plasma
    (2015) Meiring, Benjamin Wallace; Horowitz, William
    Typical energy loss calcuations in AdS/CFT simulations use an initial condition of off-shell pairs of quarks placed back-to-back in the QGP, but a precise and theoretically motivated description of configuration does not exist. Quark virtuality can have noticeable effects on the rate of energy loss so a first principles calculation is needed for the early time behaviour of virtual particles soon after production. We use the Schwinger Keldysh formalism to calculate a perturbative expression for the Energy Momentum Tensor of hard partons created before the formation of the Quark Gluon Plasma. We propose this as a foundational model to use as an initial condition in jet energy loss calculations.