Single Leptoquark Solutions to the BB-physics Anomalies

We examine various scenarios in which the Standard Model is extended by a light leptoquark state to explain one or both $B$-physics anomalies. Combining low-energy constraints and direct searches at the LHC, we confirm that the only single leptoquark model that can explain both anomalies at the same time is a vector leptoquark, known as $U_1$. Focusing on $U_1$, we highlight the complementarity between LHC and low--energy constraints, and argue that improving the experimental bound on $\mathcal{B}(B\to K\mu\tau)$ by two orders of magnitude could compromise its viability as a solution to the $B$-physics anomalies.Comment: 6 pages, 1 figure, 1 table; contribution to the 2019 EW session of the 54th Rencontres de Moriond; based on arXiv:1808.0817

Dark Matter Phenomenology of SM and Enlarged Higgs Sectors Extended with Vector Like Leptons

We will investigate the scenario in which the Standard Model (SM) Higgs sector and its 2-doublet extension (called the Two Higgs Doublet Model or 2HDM) are the "portal" for the interactions between the Standard Model and a fermionic Dark Matter (DM) candidate. The latter is the lightest stable neutral particle of a family of vector-like leptons (VLLs). We will provide an extensive overview of this scenario combining the constraints purely coming from DM phenomenology with more general constraints like Electro-weak Precision Tests (EWPT) as well as with collider searches. In the case that the new fermionic sector interacts with the SM Higgs sector, constraints from DM phenomenology force the new states to lie above the TeV scale. This requirement is relaxed in the case of 2HDM. Nevertheless, strong constraints coming from Electroweak Precision Tests (EWPT) and the Renormalization Group Equations (RGEs) limit the impact of VLFs on collider phenomenology.Comment: 32 pages, 12 figure

Vector-like top/bottom quark partners and Higgs physics at the LHC

Vector-like quarks (VLQs) that are partners of the heavy top and bottom quarks are predicted in many extensions of the Standard Model (SM). We explore the possibility that these states could explain not only the longstanding anomaly in the forward-backward asymmetry in $b$-quark production at LEP, $A_{\rm FB}^b$, but also the more recent $\sim 2\sigma$ deviation of the cross section for the associated Higgs production with top quark pairs at the LHC, $\sigma(pp\to t\bar t H)$. Introducing three illustrative models for VLQs with different representations under the SM gauge group, we show that the two anomalies can be resolved while satisfying all other theoretical and experimental constraints. In this case, the three different models predict VLQ states in the $1-2$ TeV mass range that can be soon probed at the LHC. In a second step, we discuss the sensitivity on the VLQ masses and couplings that could be obtained by means of a percent level accuracy in the measurement of ratios of partial Higgs decay widths, in particular $\Gamma(H \! \to\! \gamma\gamma)/\Gamma(H \! \to\! ZZ^*)$ and $\Gamma(H \! \to \! b\bar b)/\Gamma(H \! \to \! WW^*)$. We show that top and bottom VL partners with masses up to $\sim 5$ TeV and exotic VLQs with masses in the $10$ TeV range can be probed at the high-luminosity LHC.Comment: 27 pages, 6 figures; v2: added reference

Scenarii for interpretations of the LHC diphoton excess: two Higgs doublets and vector-like quarks and leptons

An evidence for a diphoton resonance at a mass of 750 GeV has been observed in the data collected at the LHC run at a center of mass energy of 13 TeV. We explore several interpretations of this signal in terms of Higgs-like resonances in a two-Higgs doublet model and its supersymmetric incarnation, in which the heavier CP-even and CP-odd states present in the model are produced in gluon fusion and decay into two photons through top quark loops. We show that one cannot accommodate the observed signal in the minimal versions of these models and that an additional particle content is necessary. We then consider the possibility that vector-like quarks or leptons strongly enhance the heavy Higgs couplings to photons and eventually gluons, without altering those of the already observed 125 GeV state.Comment: 13 pages, 2 figures. v3: Discussion in section 3 extended and inconsistency in the pseudoscalar coupling corrected; conclusions unchange

Diboson resonances within a custodially protected warped extra-dimensional scenario

We propose an interpretation of the diboson excess recently observed by the ATLAS and CMS collaborations in terms of Kaluza-Klein excitations of electroweak gauge bosons stemming from a realization of a warped extra-dimensional model that is protected by a custodial symmetry. Besides accounting for the LHC diboson data, this scenario also leads to an explanation of the anomalies that have been observed in the measurements of the forward-backward asymmetries for bottom quarks at LEP and top quarks at the Tevatron.Comment: 10 pages, 2 figure

Integrating out new fermions at one loop

We present the fermionic universal one-loop effective action obtained by integrating out heavy vector-like fermions at one loop using functional techniques. Even though previous approaches are able to handle integrating out heavy fermions with non-chiral interactions, i.e. vanishing γ5 interaction terms, the computations proceed in a tedious manner that obscures a physical interpretation. We show how directly tackling the fermionic functional determinant not only allows for a much simpler and transparent computation, but is also able to account for chiral interaction terms in a simple, algorithmic way. Finally, we apply the obtained results to integrate out at one loop the vector-like fermions appearing in a toy model and in a fermionic model that exhibits strong cosmological phase transitions

Integrating Out New Fermions at One Loop

We present the fermionic universal one--loop effective action obtained by integrating out heavy vector--like fermions at one loop using functional techniques. Even though previous approaches are able to handle integrating out heavy fermions with non--chiral interactions, i.e. vanishing $\gamma^5$ interaction terms, the computations proceed in a tedious manner that obscures a physical interpretation. We show how directly tackling the fermionic functional determinant not only allows for a much simpler and transparent computation, but is also able to account for chiral interaction terms in a simple, algorithmic way. Finally, we apply the obtained results to integrate out at one loop the vector--like fermions appearing in a toy model and in a fermionic model that exhibits strong cosmological phase transitions.Comment: 36 pages, no figure

Lepton Flavor Violation and Dilepton Tails at the LHC

Starting from a general effective Lagrangian for lepton flavor violation (LFV) in quark-lepton transitions, we derive constraints on the effective coefficients from the high-mass tails of the dilepton processes $pp \to \ell_k \ell_l$ (with $k\neq l$). The current (projected) limits derived in this paper from LHC data with $36~\mathrm{fb}^{-1}$ ($3~\mathrm{ab}^{-1}$) can be applied to generic new physics scenarios, including the ones with scalar, vector and tensor effective operators. For purely left-handed operators, we explicitly compare these LHC constraints with the ones derived from flavor-physics observables, illustrating the complementarity of these different probes. While flavor physics is typically more constraining for quark-flavor violating operators, we find that LHC provides the most stringent limits on several flavor-conserving ones. Furthermore, we show that dilepton tails offer the best probes for charm-quark transitions at current luminosities and that they provide competitive limits for tauonic $b\to d$ transitions at the high-luminosity LHC phase. As a by-product, we also provide general numerical expressions for several low-energy LFV processes, such as the semi-leptonic decays $K\to \pi \ell^{\pm}_k \ell^{\mp}_l$, $B\to \pi \ell^{\pm}_k \ell^{\mp}_l$ and $B\to K^{(\ast)} \ell^{\pm}_k \ell^{\mp}_l$.Comment: 14 pages, 2 figures, 5 tables, published versio