Table of contents

The motivation for this workshop began with the discovery of the Higgs boson four years ago, and the realisation that many problems remain in particle physics, such as why there is more matter than anti-matter, better determining the still poorly measured parameters of the strong force, explaining possible sources for dark matter, naturalness etc. While the newly discovered Higgs boson seems to be compatible with the Standard Model, current experimental accuracy is far from providing a definitive statement with regards to the nature of this new particle. There is a lot of room for physics beyond the Standard Model to emerge in the exploration of the Higgs boson. Recent measurements in high-energy heavy ion collisions at the LHC have shed light on the complex dynamics that govern high-density quark-gluon interactions. An array of results from the ALICE collaboration has been highlighted in a recent issue of CERN courier. The physics program of high-energy heavy ion collisions promises to further unveil the intricacies of high-density quark-gluon plasma physics.

The great topicality of high energy physics research has also seen a rapid increase in the number of researchers in South Africa pursuing such studies, both experimentally through the ATLAS and ALICE colliders at CERN, and theoretically. Young researchers and graduate students largely populate these research groups, with little experience in presenting their work, and few support structures (to their knowledge) to share experiences with. Whilst many schools and workshops have sought to educate these students on the theories and tools they will need to pursue their research, few have provided them with a platform to present their work. As such, this workshop discussed the various projects being pursued by graduate students and young researchers in South Africa, enabling them to develop networks for future collaboration and discussion.

The workshop took place at the iThemba Laboratories - North facility, in Gauteng, from the 8^th to the 10^th of February 2016, where excellent conference facilities with outdoors and indoor tea areas for discussions and interactions were provided, along with a state-of-the-art remote access to the conference venue such that those who were unable to attend the workshop in person could also be present. The laboratory is located next door to the Wits Professional Development Hub (on the corner of Jan Smuts Avenue and Empire Road), which provided the catering for this workshop. A morning plenary session, followed 15+10 minute presentations, was the format across our three days. The topics covered being in high-energy theory and phenomenology (heavy ions, pp, ep, ee collisions), ATLAS physics and ALICE physics. The workshop website is http://hep.wits.ac.za/HEPPW2016.php

Alan S. Cornell and Bruce Mellado

Details of the conference sponsors, organising committee and list of attendees can be found in the PDF.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Run 1 of the LHC saw many interesting and surprising results, hinting at physics beyond the Standard Model. One of these is the search for lepton pairs in association with high missing energy, which produced a sizeable excess in its rate from both the ATLAS and CMS collaborations. While ATLAS and CMS interpret this using a supersymmetric model, we have shown that the excess can partly be explained by the existence of a new heavy scalar boson, H. This heavy scalar can decay to weak vector bosons and also Standard Model Higgs bosons and missing energy, making it a prime candidate in the search for high p_T di-lepton pairs. An analysis is performed to determine the rate of di-lepton production with large missing energy for opposite sign pairs. We show that the heavy scalar produces a non-negligible rate, but cannot fully explain the excesses. For this reason, we look also to incorporating a full two Higgs doublet model to enhance this rate, where the pseudo-scalar A in the spectrum of new particles is considered to be a candidate for the Run 2 di-photon excess at 750 GeV.

In June 2015 the Large Hadron Collider was able to produce collisions with an energy of 13TeV, where collisions at these energy levels may allow for the formation of higher dimensional black holes. In order to detect these higher dimensional black holes we require an understanding of their emission spectra. One way of determining this is by looking at the absorption probabilities associated with the black hole. In this proceedings we will look at the absorption probability for spin-3/2 particles near N-dimensional Schwarzschild black holes. We will show how the Unruh method is used to determine these probabilities for low energy particles. We then use the Wentzel-Kramers-Brillouin approximation in order to determine these absorption probabilities for the entire possible energy range.

We provide a very brief introduction to n-dimensional scalar field theory, with an eye to renormalization and expectation values of operators. We assume the audience already has some experience with QFT.

We calculate the elastic scattering cross section for an electron off of a classical point source in weak-coupling perturbative quantum electrodynamics at next-to-leading order accuracy in the renormalization scheme. Since we use the $\overline {MS}$ renormalization scheme, our result is valid up to arbitrary large momentum transfers between the source and the scattered electron.

We test in a simplified 5-dimensional model with SU(3) gauge symmetry, the evolution equations of the gauge couplings of a model containing bulk fields, gauge fields and one pair of fermions. In this model we assume that the fermion doublet and two singlet fields are located at fixed points of the extra-dimension compactified on an S¹/Z₂ orbifold. The gauge coupling evolution is derived at one-loop in 5-dimensions, for the gauge group G = SU(3), and used to test the impact on lower energy observables, in particular the Weinberg angle. The gauge bosons and the Higgs field arise from the gauge bosons in 5 dimensions, as in a gauge- Higgs model. The model is used as a testing ground as it is not a complete and realistic model for electroweak interactions.

Recent surprising discoveries of collective behaviour of low-p_T particles in pA collisions at LHC hint at the creation of a hot, fluid-like QGP medium. The seemingly conflicting measurements of non-zero particle correlations and R_pA that appears to be consistent with unity demand a more careful analysis of the mechanisms at work in such ostensibly minuscule systems. We study the way in which energy is dissipated in the QGP created in pA collisions by calculating, in pQCD, the short separation distance corrections to the well-known DGLV energy loss formulae that have produced excellent predictions for AA collisions. We find that, shockingly, the large formation time (compared to the 1/μ Debye screening length) assumption that was used in the original DGLV calculation, results in a highly non-trivial cancellation of correction terms. We investigate the effect of relaxing the large formation time assumption in the final stages of the calculation and find that, not only is the effect of the small separation distance correction important even in large (~ 5 fm) systems, but also that the correction term dominates over the leading term at high energies.

In this study we consider an effective model by introducing two hypothetical real scalars, H and χ - a dark matter candidate, where the masses of these scalars are 2m_h < m_H < 2m_t and m_χ ≈ m_t/2 with m_h and m_t being the Standard Model Higgs boson and top quark masses, respectively. A distortion in the transverse momentum distributions of h in the intermediate region of the spectrum through the processes pp → H → h_χχ could be observed in this model. An additional scalar, S, has been postulated to explain large H → h_χχ branching ratios, assuming m_h ≲ m_S ≲ m_H — m_h and m_S > 2m_x. Furthermore, a scenario of a two Higgs doublet model (2HDM) is introduced and a detailed proposal at the present energies of the Large Hadron Collider to study the extra CP-even (h, H), CP-odd (A) and charged (H^±) scalars has been pursued. With possible phenomenological implications, production and decay modes for these scalars are discussed. Based on the mass spectrum of H, A and H^±, the production of multi-leptons and Z+jets+missing-energy events are predicted. A specific, Type-II 2HDM model is discussed in detail.

Following the comparative study of proton induced radiation damage on various plastic scintillator samples from the ATLAS-CERN detector, a study on neutron irradiation and damage assessment on the same type of samples will be conducted. The samples will be irradiated with different dose rates of neutrons produced in favourable nuclear reactions using a radiofrequency linear particle accelerator as well as from the SAFARI nuclear reactor at NECSA. The MCNP 5 code will be utilized in simulating the neutron transport for determining the dose rate. Light transmission and light yield tests will be performed in order to assess the radiation damage on the scintillators. In addition, Raman spectroscopy and Electron Paramagnetic Resonance (EPR) analysis will be used to characterize the samples after irradiation. The project aims to extent these studies to include radiation assessment damage of any component that processes the scintillating light and deteriorates the quantum efficiency of the Tilecal detector, namely, photomultiplier tubes, wavelength shifting optical fibres and the readout electronics. They will also be exposed to neutron irradiation and the damage assessed in the same manner.

We study the energy loss rate of light quarks via the AdS/CFT correspondence in both a static and an expanding plasma. Unlike heavy quarks, light quark energy loss in AdS/CFT is surprisingly dependent on both the string initial conditions and the very definition of the jet itself in the gravity theory. We aim to more closely match the string initial conditions to those expected from perturbative quantum chromodyanics (pQCD)-the theory known to describe the physics of high-momentum particles at early times in heavy ion collisions-by computing the energy-momentum tensor associated with the propagation of the classical string solution. The jet energy-momentum tensor in a strongly-coupled calculation can be found by a superposition of contributions from a collection of point particles whose paths approximate the evolution of the string world-sheet. My results show that some times after creation the pair of quark-anti quark, the energy density is not time dependent. This means that the corresponding jet does not lose energy and the associated nuclear modification factor would be one as expected. Also, the results reveal the virtuality dependency of energy density distribution over space. As expected, the energy of a more virtual jet is spread over wider angles.

We present a technique that we are developing to compute the non-abelian effects to the well-known poisson distribution from QED results to obtain the QCD momentum distribution. We first introduce the MHV method used compute scattering amplitudes in gauge theory, especially = 4 SYM, then we discuss the case of massless QCD. In second part we present why the irreducible representation of the symmetric group is the natural framework to obtain the non-abelian correction to the QED distribution.

This proceedings summarises the search for resonances decaying into two photons with a mass greater than 200 GeV. Models, which have scalars, such as the extended Higgs sector are used to optimise the analysis. The results presented are for a dataset containing 3.2 fb⁻¹ of pp collisions at $\sqrt{s}=13$ TeV using the ATLAS detector. The data is consistent with the expected background with the most significant deviation appearing in the 750 GeV region with a global significance of 2σ. A detailed description of the analysis will be presented.

The Intermediate missing transverse analysis group are investigating the production of Higgs with missing transverse energy. This analysis is based on missing transverse momentum $\left({E}_{T}^{miss}\right)$ in ATLAS detector and Jets are important objects in reconstruction of missing transverse momentum in ATLAS experiment. In run I, the Jet ${E}_{T}^{miss}$ group used the jet vertex faction method to discriminate pileup jet from hard-scatter. Due to increase in collusion energy in the ATLAS run II a new method called the Jet vertex tagger has been developed. This document presents the performance of Jet Vertex Tagger (JVT) working points using ATLAS simulated Monte Carlo samples. Comparing the fraction and efficiency of jets passing JVT cuts or run I and run II, we see about 20% difference in run II and run I.

The measurement of neutral mesons, particularly π^0, s and η's, plays an important role in the study of the Quark-Gluon Plasma (QGP), the hot and dense medium created in high-energy heavy-ion collisions. Parton energy loss in the QGP, often called jet quenching, can be assessed via measuring the suppression of high-p_T π^0, s in heavy-ion collisions, when compared to pp collisions using the nuclear modification factor (Raa). Furthermore, neutral mesons are the dominant source of photons in pp and Pb-Pb collisions, and their precise measurement is required to measure direct photons that are produced thermally within the QGP or in hard initial scatterings in the earliest phases of the collision. In both cases, high- quality measurements in pp collisions are required as a reference for Pb-Pb collisions. ALICE measurements of neutral meson spectra cover a large p_T range, with the Photon Conversion Method - which requires measurements from the ITS and TPC - covering low to intermediate p_T and the PHOS and EMCal electromagnetic calorimeters covering an intermediate to high p_T range. In this presentation, measurements of π^0, s and η's obtained from the ALICE experiment, for pp collisions at several collisional center of mass energies $\left(\sqrt{{s}_{NN}}\right),$ from 0.9 TeV to 8 TeV and in Pb-Pb collisions at $\sqrt{{s}_{NN}}=2.76\,\,{\rm{TeV}},$ will be presented. The reconstruction of neutral mesons using the Photon Conversion Method (PCM) will also be discussed.