Model-independent precision constraints on dimension-6 operators

We discuss electroweak precision constraints on dimension-6 operators in the effective theory beyond the standard model. We identify the combinations of these operators that are constrained by the pole observables (the W and Z masses and on-shell decays) and by the W boson pair production. To this end, we define a set of effective couplings of W and Z bosons to fermions and to itself, which capture the effects of new physics corrections. This formalism clarifies which operators are constrained by which observable, independently of the adopted basis of operators. We obtain numerical constraints on the coefficients of dimension-6 operator in a form that can be easily adapted to any particular basis of operators, or any particular model with new heavy particles.

Electroweak and Higgs Physics at Very High Energy

Lorenzo Ricci

Effective Field Theories (EFTs) allow a description of low energy effects of heavy new physics Beyond the Standard Model (BSM) in terms of higher dimensional operators among the SM fields. EFTs are not only an elegant and consistent way to describe heavy new physics but they represent, at the same time, a valuable experimental tool for collider searches. The Standard Model Effective Field Theory naturally parametrizes the space of models BSM and measuring its interactions is, nowadays, substantial part of the theoretical and the experimental program at the (HL-)LHC and at future colliders. In this thesis we address the theoretical challenges of this Beyond the Standard Model precision program, following three different paths.Firstly, we present some results towards the so-called high-

p_T

program at the (HL-)LHC, targeting to measure energy growing effects of higher dimensional operators in the tail of kinematic distributions. Concretely, we focus on dilepton production and we study the sensitivity to flavor universal dimension-six operators interfering with the SM and enhanced by the energy. We produce theoretical predictions for the SM and the dim-6 EFT operators at NLO-QCD, including 1-loop EW logs. Our predictions are based on event reweighting of SM Montecarlo simulations and allow an easy scan of the multi-dimensional new physics parameter space on data. Furthermore we asses the impact of the various sources of theoretical uncertainties and we study the projected sensitivity of (HL-)LHC to the EFT interactions under consideration and to concrete BSM scenario.We then turn to future colliders and in particular to very high energy lepton colliders. In this context we study the potential of such machines with about 10 TeV center of mass energy to probe Higgs, ElectroWeak and Top physics at 100 TeV via precise measurements of EFT interactions. A peculiar aspect of so energetic ElectroWeak processes is the prominent phenomenon of the EW radiation. On one hand we find that consistent and sufficiently accurate predictions require resummations, that we perform at double logarithmic order. On the other hand we show how the study of the radiation pattern can enhance the sensitivity to new physics. We assess our results in Composite Higgs and Top scenarios and minimal Z' models.Finally, we move to a top-down perspective and we perform a phenomenological study of composite Higgs models with partially composite Standard Models quarks. Starting from maximally symmetric scenarios that realize minimal flavor violation, we test various assumptions for the flavor structure of the strong sector. Among the different models we consider, we find that there is an optimal amount of symmetries that protects from (chromo-)electric dipoles and reduces, at the same time, constraints from other flavor observables.

EPFL2022

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

The Large Hadron Collider (LHC) has been producing pp collisions at 7 and 8 TeV since 2010 and promises a new era of discoveries in particle physics. One of its experiments, the Large Hadron Collider beauty (LHCb) experiment, was constructed to study CP violation in the B meson system. In addition to B physics, new Physics beyond the Standard Model can also be searched for at this single-arm forward spectrometer. With the different sub-detectors and the high resolution of the tracking system, the LHCb detector has the ability to search for heavy, long-lived and charged particles, which are predicted by extensions of the Standard Model. One of these extensions, the minimal Gauge Mediated Supersymmetry Breaking (mGMSB), proposes such a particle, named stau (τ~) - the SUSY bosonic counterpart of the heavy lepton tau (τ). The theory proposes that the staus may be pair-produced in pp collisions or in the decays of heavier particles, and have only electromagnetic interactions with the atoms of the medium like the muons. Therefore, we expect that at the energy of the LHC these particles can be produced if they do exist and that we have a chance to discover them at LHCb, as well as at the other experiments of the LHC. This thesis is dedicated to the search for stau pairs produced in pp collisions at the centre-of-mass energies √s = 7 and 8 TeV in the LHCb detector. For this purpose, we generated the stau pairs with seven different particle masses ranging from 124 to 309 GeV/c2 and simulated their path through the LHCb detector, as well as their muon background from the decays Z0, γ∗ → μ+μ−. Based on the results from the simulation, a set of cuts are then defined to select the stau pairs. Some muon pairs at high energies will also pass the selection cuts. Thus, to separate the stau pairs from the muon pairs, the Neural Network technique has been used. A first Neural Network has been used to distinguish the stau tracks from the muon tracks using their signals left in the sub-detectors: the VELO silicon detector, the electromagnetic calorimeter, the hadron calorimeter and the RICH detectors. Then, two methods to select the stau pairs have been developed: the first one is based on the product of the two responses from the first Neural Network (NN1) for the two tracks, the second one employs a second Neural Network to separate the stau pairs from the muon pairs by using the above product of the two NN1 responses and the invariant mass of pair. Finally, a favourable region for the staus finding has been defined and the expected numbers of stau and muon pairs in this region have been evaluated. The training of the Neural Network has been achieved with the Monte Carlo variables, then the trained Neural Network has been used to classify the data. The data used in our work were collected by the LHCb experiment in 2011 and 2012 and correspond to integrated luminosities of 1 fb−1 at √s = 7 TeV and of 2 fb−1 at √s = 8 TeV. No significant excess of signal has been observed. Upper limits at 95% CL on the cross section for stau pair production in pp collisions at √s = 7 and 8 TeV have been computed by using the profile likelihood method, which is derived from the well known Feldman and Cousins method.

EPFL2013