Parton Shower and NLO-Matching uncertainties in Higgs Boson Pair Production

We perform a detailed study of NLO parton shower matching uncertainties in Higgs boson pair production through gluon fusion at the LHC based on a generic and process independent implementation of NLO subtraction and parton shower matching schemes for loop-induced processes in the Sherpa event generator. We take into account the full top-quark mass dependence in the two-loop virtual corrections and compare the results to an effective theory approximation. In the full calculation, our findings suggest large parton shower matching uncertainties that are absent in the effective theory approximation. We observe large uncertainties even in regions of phase space where fixed-order calculations are theoretically well motivated and parton shower effects expected to be small. We compare our results to NLO matched parton shower simulations and analytic resummation results that are available in the literature

Monte Carlo Simulations for BSM Physics and Precision Higgs Physics at the LHC

Monte Carlo event generators are indispensable tools for the interpretation of data taken at particle collider experiments like the Large Hadron Collider (LHC), the most powerful particle collider to date. In this thesis, the general purpose Monte Carlo event generator Sherpa is used to implement a new simulation framework for models that go beyond the Standard Model of particle physics. This is achieved by means of an newly designed interface to a universal format for generic models and by extending existing functionalities in such a way as to handle a generic class of coupling structures that appear in many extensions of the Standard Model. Furthermore, an improved modeling of the dominant LHC Higgs pro- duction mechanism in the Standard Model is described and the effects of the improvements are quantified. The improved simulation that is implemented in Sherpa supplements the description of Higgs production at the LHC in terms of an effective Higgs-gluon interaction with finite top quark mass effects that restore a reliable description of the kinematics in events with large momentum transfers. Using this improved description of Higgs production at the LHC, this work demonstrates how the transverse momentum spectrum of the Higgs boson can be used to constrain models that modify the Higgs-gluon coupling. In addition, state-of-the-art Monte Carlo event generation techniques are used in order to assess the sensitivity of analysis strategies in the search for invisibly decaying Higgs bosons. In this analysis, it was found that previously neglected loop-induced contributions have a significant impact and it is demonstrated how multi-jet merging techniques can be used to obtain a reliable description of these contributions. Furthermore, the work presented in the last chapter of this thesis shows how jet substructure techniques can be used in order to search for rare Higgs decays into light resonances that decay further into hadrons. This analysis closes with a demonstration on how such an analysis can be used to constrain extensions of the Standard Model that feature multiple Higgs bosons

Monojet signatures from heavy colored particles: future collider sensitivities and theoretical uncertainties

In models with colored particle Q that can decay into a dark matter candidate X, the relevant collider process pp → QQ¯ → X X¯ + jets gives rise to events with significant transverse momentum imbalance. When the masses of Q and X are very close, the relevant signature becomes monojetlike, and Large Hadron Collider (LHC) search limits become much less constraining. In this paper, we study the current and anticipated experimental sensitivity to such particles at the High-Luminosity LHC at √s = 14 TeV with L = 3 ab−1 of data and the proposed High-Energy LHC at √s = 27 TeV with L = 15 ab−1 of data. We estimate the reach for various Lorentz and QCD color representations of Q. Identifying the nature of Q is very important to understanding the physics behind the monojet signature. Therefore, we also study the dependence of the observables built from the pp → QQ¯ + j process on Q itself. Using the state-of-theart Monte Carlo suites MadGraph5_aMC@NLO+Pythia8 and Sherpa, we find that when these observables are calculated at NLO in QCD with parton shower matching and multijet merging, the residual theoretical uncertainties are comparable to differences observed when varying the quantum numbers of Q itself. We find, however, that the precision achievable with NNLO calculations, where available, can resolve this dilemma

Event Generation with Sherpa 2.2

Sherpa is a general-purpose Monte Carlo event generator for the simulation of particle collisions in high-energy collider experiments. We summarize essential features and improvements of the Sherpa 2.2 release series, which is heavily used for event generation in the analysis and interpretation of LHC Run 1 and Run 2 data. We highlight a decade of developments towards ever higher precision in the simulation of particle-collision events

Jet cross sections at the LHC and the quest for higher precision

We perform a phenomenological study of $Z$ plus jet, Higgs plus jet and di-jet production at the Large Hadron Collider. We investigate in particular the dependence of the leading jet cross section on the jet radius as a function of the jet transverse momentum. Theoretical predictions are obtained using perturbative QCD calculations at the next-to and next-to-next-to-leading order, using a range of renormalization and factorization scales. The fixed order predictions are compared to results obtained from matching next-to-leading order calculations to parton showers. A study of the scale dependence as a function of the jet radius is used to provide a better estimate of the scale uncertainty for small jet sizes. The non-perturbative corrections as a function of jet radius are estimated from different generators.Comment: 23 pages, 19 figure