Ivan Begic - Honours Project 2019

A Coherent Treatment of QCD Radiation from Supersymmetric Quarks in the Antenna Shower Formalism

In the phenomenology of high energy physics, Monte Carlo event generators are used extensively to simulate real-life particle collisions. Since analytical solutions of the Standard Model (SM), in particular Quantum Chromodynamics (QCD), are not known, various perturbative methods are implemented to simulate the complex quantum mechanics of fundamental particles at subatomic scales.

This thesis presents two such models; the parton shower which is a monopole based treatment of QCD radiation, and the antenna shower which describes the radiation produced by dipoles of partons in a coherent manner. We apply the antenna shower to study the decay of a Supersymmetric top quark, a hypothetical Beyond the Standard Model (BSM) process.

We make a comparitive study under a regime where all BSM masses are the same as their SM counterparts, and we find that the radiation pattern of a stop antenna is more sensitive to destructive interference than that of the top. Destructive interference effects begin to manifest beyond a ∼45◦ emission angle of gluons from the (radiating) daughter of decay. This culminates in an extreme suppression where almost no radiation is emitted at 180◦. Furthermore we compare the perturbative results underpinning the antenna shower to that of the parton shower, and observe that the latter ampli- tude begins to diverge from the former at an emission angle of ∼30◦. Finally we investigate the radiation pattern in the context of shower evolution following an e+e− collision and observe that the angular distribution of gluon energies is everywhere smaller for the BSM process than the SM one, by an approximately constant amount for a large range of angles. We study as well the effect of the mass of a radiating particle on its collinear radiation properties, and observe suppression in agreement with the ‘dead-cone’ angle cited in the literature as roughly the mass/energy ratio of the radiating particle.

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