High-pTp_T dilepton tails and flavour physics

We investigate the impact of flavour-conserving, non-universal quark-lepton contact interactions on the dilepton invariant mass distribution in $p~p \to \ell^+ \ell^-$ processes at the LHC. After recasting the recent ATLAS search performed at 13 TeV with 36.1 fb$^{-1}$ of data, we derive the best up-to-date limits on the full set of 36 relevant four-fermion operators, as well as estimate the sensitivity achievable at the HL-LHC. We discuss how these high-$p_T$ measurements can provide complementary information to the low-$p_T$ rare meson decays. In particular, we find that the recent hints on lepton flavour universality violation in $b \to s \mu^+ \mu^-$ transitions are already in mild tension with the dimuon spectrum at high-$p_T$ if the flavour structure follows minimal flavour violation. Even if the mass scale of New Physics is well beyond the kinematical reach for on-shell production, the signal in the high-$p_T$ dilepton tail might still be observed, a fact that has been often overlooked in the present literature. In scenarios where new physics couples predominantly to third generation quarks, instead, the HL-LHC phase is necessary in order to provide valuable information.Comment: 9 pages, 5 figures, 1 tabl

Third Family Quark-Lepton Unification at the TeV Scale

We construct a model of quark-lepton unification at the TeV scale based on an $SU(4)$ gauge symmetry, while still having acceptable neutrino masses and enough suppression in flavor changing neutral currents. An approximate $U(2)$ flavor symmetry is an artifact of family-dependent gauge charges leading to a natural realization of the CKM mixing matrix. The model predicts sizeable violation of PMNS unitarity as well as a gauge vector leptoquark $U_1^\mu = ({\bf 3}, {\bf 1}, 2/3)$ which can be produced at the LHC -- both effects within the reach of future measurements. In addition, recently reported experimental anomalies in semi-leptonic $B$-meson decays, both in charged $b \to c \tau \nu$ and neutral $b \to s \mu \mu$ currents, can be accommodated.Comment: 9 pages, 5 tables. Version accepted for publication in PL

On the breaking of Lepton Flavor Universality in B decays

In view of recent experimental indications of violations of Lepton Flavor Universality (LFU) in $B$ decays, we analyze constraints and implications of LFU interactions, both using an effective theory approach, and an explicit dynamical model. We show that a simple dynamical model based on a $SU(2)_L$ triplet of massive vector bosons, coupled predominantly to third generation fermions (both quarks and leptons), can significantly improve the description of present data. In particular, the model decreases the tension between data and SM predictions concerning: i) the breaking of $\tau$-$\mu$ universality in $B\to D^{(*)} \ell \nu$ decays; ii) the breaking of $\mu$-$e$ universality in $B \to K \ell^+\ell^-$ decays; iii) the difference between exclusive and inclusive determinations of $|V_{cb}|$ and $|V_{ub}|$. The minimal version of the model is in tension with ATLAS and CMS direct searches for the new massive vectors (decaying into $\tau^+\tau^-$ pairs), but this tension can be decreased with additional non-standard degrees of freedom. Further predictions of the model both at low- and high-energies, in view of future high-statistics data, are discussed.Comment: 20 pages, 7 figures. v2: discussion on flavor structure clarified; added discussion on the associated production of the heavy vectors and a few refs. Published versio

Knocking on New Physics' door with a Scalar Resonance

We speculate about the origin of the recent excess at ~750 GeV in diphoton resonance searches observed by the ATLAS and CMS experiments using the first 13 TeV data. Its interpretation as a new scalar resonance produced in gluon fusion and decaying to photons is consistent with all relevant exclusion bounds from the 8 TeV LHC run. We provide a simple phenomenological framework to parametrize the properties of the new resonance and show in a model-independent way that, if the scalar is produced in gluon fusion, additional new colored and charged particles are required. Finally, we discuss some interpretations in various concrete setups, such as a singlet (pseudo-) scalar, composite Higgs, and the MSSM.Comment: 20 pages, 4 figures, 1 table. Extraction of the ATLAS 13 TeV diphoton signal and 8 TeV ZZ bound corrected, no sizable change in the final combination. Presentation improved, references added, and conclusions unchanged. Version to appear in EPJ

Disentangling Flavor Violation in the Top-Higgs Sector at the LHC

We study the LHC phenomenology of flavor changing Yukawa couplings between top quark, Higgs boson, and either an up or charm quark. Such $tuh$ or $tch$ couplings arise for instance in models in which Higgs sector is extended by the existence of additional Higgs bosons or by higher dimensional operators. We emphasize the importance of anomalous single top plus Higgs production in these scenarios, in addition to the more widely studied $t \to h j$ decays. By recasting existing CMS searches in multilepton and diphoton plus lepton final states, we show that bounds on $tuh$ couplings are improved by a factor of 1.5 when single top plus Higgs production is accounted for. We also recast the CMS search for vector boson plus Higgs production into new, competitive constraints on $tuh$ and $tch$ couplings, setting the limits of $BR(t\to hu) < 0.7$ and $BR(t\to hc) <1.2$. We then investigate the sensitivity of future searches in multilepton channel and in fully hadronic channel. In multilepton searches, studying the lepton rapidity distributions and charge assignments can be used to discriminate between $tuh$ couplings, for which anomalous single top production is relevant, and $tch$ couplings, for which it is suppressed by the parton distribution function of the charm quark. An analysis of fully hadronic $t+h$ production and $t\to h j$ decay can be competitive with the multilepton search at 100 fb$^{-1}$ of 13 TeV data if jet substructure techniques are employed to reconstruct boosted top quarks and Higgs bosons. To show this we develop a modified version of the HEPTopTagger algorithm, optimized for tagging $t \to h j$ decays. Our sensitivity estimates on $BR(t\to hu)$ ($BR(t\to hc)$) at 100 fb$^{-1}$ of 13 TeV data for multilepton searches, vector boson plus Higgs search and fully hadronic search are $0.22$ ($0.33$), $0.15$ ($0.19$) and $0.36$ ($0.48$), respectively.Comment: Version published in JHE

Leptoquark toolbox for precision collider studies

We implement scalar and vector leptoquark (LQ) models in the universal FeynRules output (UFO) format assuming the Standard Model fermion content and conservation of baryon and lepton numbers. Scalar LQ implementations include next-to-leading order (NLO) QCD corrections. We report the NLO QCD inclusive cross sections in proton-proton collisions at 13 TeV, 14 TeV, and 27 TeV for all on-shell LQ production processes. These comprise (i) LQ pair production ($p p \to \Phi \Phi$) and (ii) single LQ + lepton production ($p p \to \Phi \ell$) for all initial quark flavours ($u$, $d$, $s$, $c$, and $b$). Vector LQ implementation includes adjustable non-minimal QCD coupling. We discuss several aspects of LQ searches at a hadron collider, emphasising the implications of $SU(2)$ gauge invariance, electroweak and flavour constraints, on the possible signatures. Finally, we outline the high-$p_T$ search strategy for LQs recently proposed in the literature to resolve experimental anomalies in $B$-meson decays. In this context, we stress the importance of complementarity of the three LQ related processes, namely, $p p \to \Phi \Phi$, $p p \to \Phi \ell$, and $p p \to \ell \ell$.Comment: 18 pages, 5 figures, 3 tables. Version published in JHE

Toward a coherent solution of diphoton and flavor anomalies

We propose a coherent explanation for the 750 GeV diphoton anomaly and the hints of deviations from Lepton Flavor Universality in B decays in terms a new strongly interacting sector with vectorlike confinement. The diphoton excess arises from the decay of one of the pseudo-Nambu-Goldstone bosons of the new sector, while the flavor anomalies are a manifestation of the exchange of the corresponding vector resonances (with masses in the 1.5-2.5 TeV range). We provide explicit examples (with detailed particle content and group structure) of the new sector, discussing both the low-energy flavor-physics phenomenology and the signatures at high $p_T$. We show that specific models can provide an excellent fit to all available data. A key feature of all realizations is a sizable broad excess in the tails of $\tau^+ \tau^-$ invariant mass distribution in $p p \to \tau^+ \tau^-$, that should be accessible at the LHC in the near future.Comment: v2: 32 pages, 9 figures, 2 tables. Published version. Extended discussion about the flavor structure of the model and high-PT phenomenology, typos corrected. Added note about the relevance of the paper in light of the absence of the diphoton signal at the LH

B-physics anomalies: a guide to combined explanations

Motivated by additional experimental hints of Lepton Flavour Universality violation in B decays, both in charged- and in neutral-current processes, we analyse the ingredients necessary to provide a combined description of these phenomena. By means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons, we show how this is possible. We demonstrate, in particular, how to solve the problems posed by electroweak precision tests and direct searches with a rather natural choice of model parameters, within the context of a $U(2)_q \times U(2)_\ell$ flavour symmetry. We further exemplify the general EFT findings by means of simplified models with explicit mediators in the TeV range: coloured scalar or vector leptoquarks and colour-less vectors. Among these, the case of an $SU(2)_L$-singlet vector leptoquark emerges as a particularly simple and successful framework.Comment: 33 pages, 7 figures, 2 tables. Extended discussion and one plot added on single production of leptoquarks, typos corrected, references adde

Lepton Flavor Non-Universality in B decays from Dynamical Yukawas

The basic features of quark and lepton mass matrices can be successfully explained by natural minima of a generic potential with dynamical Yukawa fields invariant under the $[\mathrm{SU(3)}]^5\times \mathcal{O}(3)$ flavor symmetry. If this symmetry is gauged, in order to avoid potentially dangerous Goldstone bosons, and small perturbations are added to exactly fit the observed pattern of fermion masses, the spectrum of massive flavor gauge bosons can naturally explain the hints for new physics in $b\to s \ell^+\ell^-$ transitions, including $R_K$. In particular, the desired pattern of the Standard Model Yukawa couplings is compatible with a gauged $\mathrm{U(1)}_q$ in the quark sector, and $\mathrm{U(1)}_{\mu-\tau}$ in the lepton sector spontaneously broken around the TeV scale. In order to explain the aforementioned experimental hints, the corresponding neutral gauge bosons are required to mix, yielding to potentially observable signals in dimuon resonance searches at the LHC.Comment: 11 pages, 9 figures. v2: version published in Phys.Lett.

Global constraints on anomalous triple gauge couplings in effective field theory approach

We present a combined analysis of LHC Higgs data (signal strengths) together with LEP-2 WW production measurements. To characterize possible deviations from the Standard Model (SM) predictions, we employ the framework of an Effective Field Theory (EFT) where the SM is extended by higher-dimensional operators suppressed by the mass scale of new physics $\Lambda$. The analysis is performed consistently at the order $\Lambda^{-2}$ in the EFT expansion keeping all the relevant operators. While the two data sets suffer from flat directions, together they impose stringent model-independent constraints on the anomalous triple gauge couplings. As a side product, we provide the results of the combined fit in different EFT bases.Comment: 7 pages. v2: References and clarifications adde