Discovering Higgs boson pair production through rare final states at a 100 TeV collider

We consider Higgs boson pair production at a future proton collider with centre-of-mass energy of 100 TeV, focusing on rare final states that include a bottom-anti-bottom quark pair and multiple isolated leptons: $hh \rightarrow (b\bar{b}) + n \ell + X$, $n = \{2,4\}$, $X = \{ E_T^\mathrm{miss}, \gamma, -\}$. We construct experimental search strategies for observing the process through these channels and make suggestions on the desired requirements for the detector design of the future collider.Comment: 9 pages, 2 figures, 7 tables. Matches published versio

Higgs Boson pair production merged to one jet

We develop a Monte Carlo event generator for Higgs Boson pair production merged to exact one-jet matrix elements. The matrix elements are generated with OpenLoops and event generation is performed with the HERWIG++ general-purpose event generator. This allows us to simulate fully-exclusive hadronic final states with accurate description of the kinematics of the leading jet in conjunction with a parton shower. We use the implementation to examine in detail the systematic uncertainties which result from the merging procedure. We assess the magnitude of the impact of the merging on experimental searches of Standard Model di-Higgs production that aim to constrain the Higgs boson self-coupling. We find that the use of a merged sample can reduce theoretical systematic uncertainties in the efficiencies of cuts on certain observables. This constitutes the most accurate simulation of the process available to date. The Monte Carlo event generator developed for this project is available as an add-on to the HERWIG++ event generator at http://www.itp.uzh.ch/~andreasp/hhComment: 19 pages, 10 figures, updated UR

Rare top quark decays at a 100 TeV proton-proton collider: t→bWZt \rightarrow bWZ and t→hct\rightarrow hc

We investigate extremely rare top quark decays at a future proton-proton collider with centre-of-mass energy of 100 TeV. We focus on two decay modes: radiative decay with a $Z$ boson, $t \rightarrow b WZ$, and flavour-changing neutral decay with a Higgs boson, $t \rightarrow h c$, the former being kinematically suppressed with a branching ratio of $\mathcal{O}(10^{-6})$, and the latter highly loop-suppresed, with a branching ratio of $\mathcal{O}(10^{-15})$. We find that $t \rightarrow b WZ$ will be very challenging to observe in top quark pair production, even within well-motivated beyond-the-Standard Model scenarios. For the mode $t\rightarrow h c$ we find a stronger sensitivity than that obtained by any future LHC measurement by at least one order of magnitude.Comment: Matches published version, two column, 8 pages, 4 figure

Uncovering the relation of a scalar resonance to the Higgs boson

We consider the associated production of a scalar resonance with the standard model Higgs boson. We demonstrate via a realistic phenomenological analysis that couplings of such a resonance to the Higgs boson can be constrained in a meaningful way in future runs of the LHC, providing insights on its origin and its relation to the electroweak symmetry breaking sector. Moreover, the final state can provide a direct way to determine whether the new resonance is produced predominantly in gluon fusion or quark-anti-quark annihilation. The analysis focusses on a resonance coming from a scalar field with vanishing vacuum expectation value and its decay to a photon pair. It can however be straightforwardly generalised to other scenarios.Comment: 17 pages, 29 figures. Version matches published versio

Higgs boson to di-tau channel in Chargino-Neutralino searches at the LHC

We consider chargino-neutralino production, $\tilde{\chi}_2^0 \tilde{\chi}_1^\pm \to (h \tilde{\chi}_1^0)(W^\pm \tilde{\chi}_1^0)$, which results in Higgs boson final states that subsequently decay (inclusively) to leptons (either $h\rightarrow \tau^+\tau^-$ or $h\rightarrow W^+W^- \to (e^+e^-, \mu^+\mu^-, \tau^+\tau^-)+E_T^\mathrm{miss}$). Such channels are dominant in large regions of the allowed supersymmetric parameter space for many concrete supersymmetric models. The existence of leptons allows for good control over the backgrounds, rendering this channel competitive to the conventional $h\rightarrow b\bar{b}$ channel that has been previously used to impose constraints. We include hadronic decays of the $\tau$ leptons in our analysis through a $\tau$-identification algorithm. We consider integrated luminosities of 100 fb$^{-1}$, 300 fb$^{-1}$ and 3000 fb$^{-1}$, for an LHC running at $pp$ centre-of-mass energy of 14 TeV and provide the expected constraints on the $M_2$-$M_1$ plane.Comment: 28 pages, 14 figures. Additional efficiency files for signal regions of analysis included in "efficiencies" director

Phenomenological aspects of new physics at high energy hadron colliders

This thesis contains studies of phenomenological aspects of new physics at hadron colliders, such as the Large Hadron Collider (LHC). After a general introduction in chap- ter 1, in chapter 2 we outline the main features of the Standard Model (SM) of particle physics and the theoretical motivations for going beyond it. We subsequently provide brief descriptions of a few popular models that aim to solve the issues that arise within the SM. In chapter 3 we describe the general Monte Carlo method for evaluating multidimen- sional integrals and show how it can be used to construct a class of computational tools called Monte Carlo event generators. We describe the main generic features of event generators and how these are implemented in the HERWIG++ event generator. By applying resummation techniques, we provide, in chapter 4, analytical calcula- tions of two types of hadron collider observables. The first, global inclusive variables, are observables that make use of all measured particle momenta and can provide useful information on the scale of new physics. The second observable is the transverse energy of the QCD initial state radiation (ET ), associated with the either Drell-Yan gauge boson production or Higgs boson production. In both cases we provide comparisons to results obtained from Monte Carlo event generators. In chapter 5 we examine two well-motivated models for new physics: one of new heavy charged vector bosons (W prime), similar to the SM W gauge bosons, and a model moti- vated by strong dynamics electroweak symmetry breaking that contains new resonances, leptoquarks, that couple primarily to quarks and leptons of the third generation. In the prior model, we improve the current treatment of the W′ by considering interference ef- fects with the SM W and construct an event generator accurate to next-to-leading order which we use to conduct a phenomenological analysis. For the leptoquark model, starting from an effective Lagrangian for production and decay, we provide an implementation in the HERWIG++ event generator and use it to form a strategy for mass reconstruction. The thesis ends with some conclusions and suggestions for extensions of the work presented. Further details and useful formulæ are given in the appendices.This work was supported by the STFC

Higgs boson pair production at the LHC in the bbˉW+W−b \bar{b} W^+ W^- channel

We consider Higgs boson pair production at the LHC in the $b \bar{b} W^+ W^-$ channel, with subsequent decay of the $W^+W^-$ pair into $\ell \nu j j$. Employing jet substructure and event reconstruction techniques, we show that strong evidence for this channel can be found at the 14 TeV LHC with 600 fb$^{-1}$ of integrated luminosity, thus improving the current reach for the production of Higgs boson pairs. This measurement will allow to probe the trilinear Higgs boson coupling $\lambda$.Comment: 5 pages, 3 figures, references added, accepted for publication by PR