Closing the window on single leptoquark solutions to the B-physics anomalies

We examine various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both B-physics anomalies, viz. R_D(◦)^exp \u3e R_D(◦)^SM or/and R_K(◦)^exp \u3c R_K(◦)^SM. To do so we combine the constraints arising both from the low-energy observables and from direct searches at the LHC. We find that none of the scalar leptoquarks of mass mLQ ≃ 1 TeV can alone accommodate the above mentioned anomalies. The only single leptoquark scenario which can provide a viable solution for mLQ ≃ 1÷2 TeV is a vector leptoquark, known as U1, which we re-examine in its minimal form (letting only left-handed couplings to have non-zero values). We find that the limits deduced from direct searches are complementary to the low-energy physics constraints. In particular, we find a rather stable lower bound on the lepton flavor violating b → sℓ_1^±ℓ_2^± modes, such as B(B → K μΤ). Improving the experimental upper bound on B(B → K μΤ) by two orders of magnitude could compromise the viability of the minimal U1 model as well

Closing the window on single leptoquark solutions to the B-physics anomalies

We examine various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both B-physics anomalies, viz. R_D(◦)^exp \u3e R_D(◦)^SM or/and R_K(◦)^exp \u3c R_K(◦)^SM. To do so we combine the constraints arising both from the low-energy observables and from direct searches at the LHC. We find that none of the scalar leptoquarks of mass mLQ ≃ 1 TeV can alone accommodate the above mentioned anomalies. The only single leptoquark scenario which can provide a viable solution for mLQ ≃ 1÷2 TeV is a vector leptoquark, known as U1, which we re-examine in its minimal form (letting only left-handed couplings to have non-zero values). We find that the limits deduced from direct searches are complementary to the low-energy physics constraints. In particular, we find a rather stable lower bound on the lepton flavor violating b → sℓ_1^±ℓ_2^± modes, such as B(B → K μΤ). Improving the experimental upper bound on B(B → K μΤ) by two orders of magnitude could compromise the viability of the minimal U1 model as well

Cosmological Perturbations in a Universe with a Domain Wall Era

Topologically protected sheet-like surfaces, called domain walls, form when the potential of a field has a discrete symmetry that is spontaneously broken. Since this condition is commonplace in field theory, it is plausible that many of these walls were produced at some point in the early universe. Moreover, for potentials with a rich enough structure, the walls can join and form a (at large scales) homogeneous and isotropic network that dominates the energy density of the universe for some time before decaying. In this thesis, we study the faith of large scale perturbations in a cosmology with a short period of domain wall dominance. We start with some background material introducing the unperturbed universe, the physics of inflation and the basics on domain walls in cosmology. We also review the theory of cosmological perturbations in a medium with non-zero anisotropic stress. We then move on to our main result: treating the domain wall network as a relativistic elastic solid at large scales, we show that the perturbations that exited the horizon during inflation get suppressed during the domain wall era, before re-entering the horizon. This power suppression occurs because, unlike a fluid-like universe, a solid-like universe can support sizable anisotropic stress gradients across large scales which effectively act as mass for the scalar and tensor modes. Interestingly, the amplitude of the primordial scalar power spectrum can be closer to one in this cosmology and still give the observed value of 10^(-9) today. As a result, the usual bounds on the energy scale of inflation get relaxed to values closer to the (more natural) Planck scale. In the last part of this thesis, as an existence proof, we present a hybrid inflation model with N "waterfall" fields that can realize the proposed cosmology. In this model, a domain wall network forms when an approximate O(N) symmetry gets spontaneously broken at the end of inflation, and for N ≥ 5, we show that there is a region in parameter space where the network dominates the energy density for a few e-folds before decaying and reheating the universe

Closing the window on single leptoquark solutions to the B-physics anomalies

We examine various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both B-physics anomalies, viz. R_D(◦)^exp \u3e R_D(◦)^SM or/and R_K(◦)^exp \u3c R_K(◦)^SM. To do so we combine the constraints arising both from the low-energy observables and from direct searches at the LHC. We find that none of the scalar leptoquarks of mass mLQ ≃ 1 TeV can alone accommodate the above mentioned anomalies. The only single leptoquark scenario which can provide a viable solution for mLQ ≃ 1÷2 TeV is a vector leptoquark, known as U1, which we re-examine in its minimal form (letting only left-handed couplings to have non-zero values). We find that the limits deduced from direct searches are complementary to the low-energy physics constraints. In particular, we find a rather stable lower bound on the lepton flavor violating b → sℓ_1^±ℓ_2^± modes, such as B(B → K μΤ). Improving the experimental upper bound on B(B → K μΤ) by two orders of magnitude could compromise the viability of the minimal U1 model as well

Closing the window on single leptoquark solutions to the B-physics anomalies

We examine various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both B-physics anomalies, viz. R_D(◦)^exp \u3e R_D(◦)^SM or/and R_K(◦)^exp \u3c R_K(◦)^SM. To do so we combine the constraints arising both from the low-energy observables and from direct searches at the LHC. We find that none of the scalar leptoquarks of mass mLQ ≃ 1 TeV can alone accommodate the above mentioned anomalies. The only single leptoquark scenario which can provide a viable solution for mLQ ≃ 1÷2 TeV is a vector leptoquark, known as U1, which we re-examine in its minimal form (letting only left-handed couplings to have non-zero values). We find that the limits deduced from direct searches are complementary to the low-energy physics constraints. In particular, we find a rather stable lower bound on the lepton flavor violating b → sℓ_1^±ℓ_2^± modes, such as B(B → K μΤ). Improving the experimental upper bound on B(B → K μΤ) by two orders of magnitude could compromise the viability of the minimal U1 model as well

Learning the latent structure of collider events

We describe a technique to learn the underlying structure of collider events directly from the data, without having a particular theoretical model in mind. It allows to infer aspects of the theoretical model that may have given rise to this structure, and can be used to cluster or classify the events for analysis purposes. The unsupervised machine-learning technique is based on the probabilistic (Bayesian) generative model of Latent Dirichlet Allocation. We pair the model with an approximate inference algorithm called Variational Inference, which we then use to extract the latent probability distributions describing the learned underlying structure of collider events. We provide a detailed systematic study of the technique using two example scenarios to learn the latent structure of di-jet event samples made up of QCD background events and either tt¯.Fil: Dillon, B. M.. Institute Jo?ef Stefan; EsloveniaFil: Faroughy, D. A.. Universitat Zurich; SuizaFil: Kamenik, J. F.. Institute Jo?ef Stefan; Eslovenia. University of Ljubljana; EsloveniaFil: Szewc, Manuel. Universidad Nacional de San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

The Machine Learning Landscape of Top Taggers

Based on the established task of identifying boosted, hadronically decaying top quarks, we compare a wide range of modern machine learning approaches. Unlike most established methods they rely on low-level input, for instance calorimeter output. While their network architectures are vastly different, their performance is comparatively similar. In general, we find that these new approaches are extremely powerful and great fun.Comment: Yet another tagger included

Combined explanations of B-physics anomalies: the sterile neutrino solution

In this paper we provide a combined explanation of charged- and neutral-current B-physics anomalies assuming the presence of a light sterile neutrino NR which contributes to the B \u2192 D(*)\u3c4\u3bd processes. We focus in particular on two simplified models, where the mediator of the flavour anomalies is either a vector leptoquark U1\u3bc 3c (3, 1, 2/3) or a scalar leptoquark S1 3c (3\uaf , 1, 1/3). We find that U1\u3bc can successfully reproduce the required deviations from the Standard Model while being at the same time compatible with all other flavour and precision observables. The scalar leptoquark instead induces a tension between Bs mixing and the neutral-current anomalies. For both states we present the limits and future projections from direct searches at the LHC finding that, while at present both models are perfectly allowed, all the parameter space will be tested with more luminosity. Finally, we study in detail the cosmological constraints on the sterile neutrino NR and the conditions under which it can be a candidate for dark matter

Search for additional heavy neutral Higgs and gauge bosons in the ditau final state produced in 36 fb−1 of pp collisions at √s=13 TeV with the ATLAS detector

A search for heavy neutral Higgs bosons and Z′ bosons is performed using a data sample corresponding to an integrated luminosity of 36.1 fb−1 from proton-proton collisions at √s=13 TeV recorded by the ATLAS detector at the LHC during 2015 and 2016. The heavy resonance is assumed to decay to τ+τ− with at least one tau lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2-2.25 TeV for Higgs bosons and 0.2-4.0 TeV for Z′ bosons. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in benchmark scenarios. In the context of the hMSSM scenario, the data exclude tan β > 1.0 for mA= 0.25 TeV and tan β > 42 for mA=1.5 TeV at the 95% confidence level. For the Sequential Standard Model, ZSSM′ with mZ′< 2.42 TeV is excluded at 95% confidence level, while Z NU′ with mZ ′ < 2.25 TeV is excluded for the non-universal G(221) model that exhibits enhanced couplings to third-generation fermions