Abstract

The Standard Model (SM) when extended with a Leptoquark (LQ) and right handed neutrinos, can have interesting new implications for Higgs physics. We show that the sterile neutrinos can induce a significant boost to the down-type quark Yukawa interactions through a diagonal coupling associated with the quarks and a scalar LQ of electromagnetic charge 1/3. The relative change is much more pronounced in case of the first two generations of quarks as they have vanishingly small Yukawa couplings in the SM. The enhancement in the couplings would also lead to a non-negligible contribution of the quark fusion process to the production of the 125 GeV Higgs scalar in the SM, though the gluon fusion always dominates. However, this may not be true for a general scalar. As an example, we consider a scenario with a SM-gauge-singlet scalar φ where an O(1) coupling between φ and the LQ is allowed. The φqq¯ Yukawa couplings can only be generated radiatively through a loop of LQ and sterile neutrinos. Here, the quark fusion process can have significant cross section, specially for a light φ. It can even supersede the normally dominant gluon fusion process for a moderate to large value of the LQ mass. This model can be tested/constrained at the high luminosity run of the LHC through a potentially large branching fraction of the scalar to two-jets.

Abstract

At the LHC, a TeV-scale leptoquark (LQ) that decays dominantly to a top quark (t) and a light charged lepton (` = e, µ) would form a resonance system of boosted-t + high-pT-`. We consider all possible vector LQ models within the Buchmüller-Rückl-Wyler classifications with the desired decay. We propose simple phenomenological Lagrangians that are suitable for bottom-up/experimental studies and, at the same time, can cover the relevant parameter spaces of these models. In this simplified framework, we study the pair and single production channels of vector LQs at the LHC. Interestingly, we find that, like the pair production, the cross sections of some single production processes also depend on the parameter κ that appears in the gluon-vector LQ coupling. We adopt a search strategy of selecting events with at least one boosted hadronic top quark and exactly two high-pT leptons of the same flavor and opposite sign. This combines events from the pair and single production processes and, therefore, can enhance the discovery potential beyond that of the pair-production-only searches. For 5σ discovery we find that vector LQs can be probed up to 2.55 TeV for 100% branching ratio in the t` decay mode and O(1) new couplings at the 14 TeV LHC with 3 ab−1 of integrated luminosity.

Abstract

A search is presented for a charged Higgs boson heavier than the top quark, produced in association with a top quark, or with a top and a bottom quark, and decaying into a top-bottom quark-antiquark pair. The search is performed using proton-proton collision data collected by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb(-1). Events are selected by the presence of a single isolated charged lepton (electron or muon) or an opposite-sign dilepton (electron or muon) pair, categorized according to the jet multiplicity and the number of jets identified as originating from b quarks. Multivariate analysis techniques are used to enhance the discrimination between signal and background in each category. The data are compatible with the standard model, and 95% confidence level upper limits of 9.6-0.01 pb are set on the charged Higgs boson production cross section times branching fraction to a top-bottom quark-antiquark pair, for charged Higgs boson mass hypotheses ranging from 200 GeV to 3 TeV. The upper limits are interpreted in different minimal supersymmetric extensions of the standard model.

Abstract

Motivated by the persistent anomalies in the semileptonic B-meson decays, we investigate the competency of LHC data to constrain the RDðÞ -favored parameter space in a charge −1=3 scalar leptoquark (S1) model. We consider some scenarios with one large free coupling to accommodate the RDðÞ anomalies. As a result, some of them dominantly yield nonresonant ττ and τν events at the LHC through the t-channel S1 exchange. So far, no experiment has searched for leptoquarks using these signatures and the relevant resonant leptoquark searches are yet to put any strong exclusion limit on the parameter space. We recast the latest ττ and τν resonance search data to obtain new exclusion limits. The nonresonant processes strongly interfere (destructively in our case) with the Standard Model background and play the determining role in setting the exclusion limits. To obtain precise limits, we include non-negligible effects coming from the subdominant (resonant) pair and inclusive single leptoquark productions systematically in our analysis. To deal with large destructive interference, we make use of the transverse mass distributions from the experiments in our statistical analysis. In addition, we also recast the relevant direct search results to obtain the most stringent collider bounds on these scenarios to date. These are independent bounds and are competitive to other known bounds. Finally, we indicate how one can adopt these bounds to a wide class of models with S1 that are proposed to accommodate the RDðÞ anomalies.

Abstract

The LHC search strategies for leptoquarks that couple dominantly to a top quark are different than for the ones that couple mostly to the light quarks. We consider charge 1=3 (ϕ1) and 5=3 (ϕ5) scalar leptoquarks that can decay to a top quark and a charged lepton (tl) giving rise to a resonance system of a boosted top and a high-pT lepton. We introduce simple phenomenological models suitable for bottom-up studies and explicitly map them to all possible scalar leptoquark models within the Buchmüller-Rückl-Wyler classifications that can have the desired decays. We study pair and single productions of these leptoquarks. Contrary to the common perception, we find that the single production of top-philic leptoquarks ϕ ¼ fϕ1; ϕ5g in association with a lepton and jets could be significant for order one ϕtl coupling in certain scenarios. We propose a strategy of selecting events with at least one hadronic-top and two high-pT same flavor opposite sign leptons. This captures events from both pair and single productions. Our strategy can significantly enhance the LHC discovery potential especially in the high-mass region where single productions become more prominent. Our estimation shows that a scalar leptoquark as heavy as ∼1.7 TeV can be discovered at the 14 TeV LHC with 3 ab−1 of integrated luminosity in the tll þ X channel for 100% branching ratio in the ϕ → tl decay mode. However, in some scenarios, the discovery reach can increase beyond 2 TeV even though the branching ratio comes down to about 50%

Abstract

A search for exotic decays of the Higgs boson to a pair of light pseudoscalar particles a 1 is performed under the hypothesis that one of the pseudoscalars decays to a pair of opposite sign muons and the other decays to b b‾. Such signatures are predicted in a number of extensions of the standard model (SM), including next-to-minimal supersymmetry and two-Higgs-doublet models with an additional scalar singlet. The results are based on a data set of proton-proton collisions corresponding to an integrated luminosity of 35.9 fb− 1, accumulated with the CMS experiment at the CERN LHC in 2016 at a centre-of-mass energy of 13 TeV. No statistically significant excess is observed with respect to the SM backgrounds in the search region for pseudoscalar masses from 20 GeV to half of the Higgs boson mass. Upper limits at 95% confidence level are set on the product of the production cross section and branching …

Abstract

We compute the top quark threshold contributions to the γγ production at the LHC. They appear when the invariant mass of the photon pair, Mγ γ just exceeds two times the mass of the top quark and induce some feature in the Mγ γ distribution. We determine the magnitude of this threshold effect and characterize this feature with a simple empirical fitting function to show that it is possible to observe this effect at the LHC in future. We also explore some possible improvements that may enhance its significance.

Abstract

We consider a generalization of the Standard Model whose action displays conformal invariance in d dimensions. The model contains a strongly coupled dark matter sector which breaks conformal symmetry dynamically. The model evades conformal anomaly and leads to identically zero vacuum energy in flat space-time. Hence it does not suffer from the problem of fine tuning of the cosmological constant. We determine the contribution of non-relativistic matter to the energy-momentum tensor and determine a parameter regime in which it approximately reduces to the standard result. We show how dark energy and dark matter arises in this model. We discuss the parameter range for which the model reduces to the ΛCDM model and hence is consistent with observations.

Abstract

We present the scalar leptoquark pair production process at the LHC computed at the next-to-leading order in QCD, matched to the Pythia parton shower using the MC@ NLO formalism. We consider the leading order decay of a leptoquark to a lepton (e, μ, τ or ν e, ν μ, ν τ) and a jet and observe the effects of parton shower on the final states. For demonstration, we display the kinametical distributions of a selection of observables along with their scale uncertainties for the 13 TeV LHC. We also present pair production cross sections and K-factors using a massless five-quark flavor scheme for different LHC center-of-mass energies. The complete stand-alone code is available online.

Abstract

Quark-lepton compositeness is a well-known beyond the Standard Model (SM) scenario with heavy exotic particles like leptoquarks (LQs) and leptogluons (LGs) etc. These particles can couple to leptons and jets simultaneously. In this letter, we use the recent CMS scalar LQ search data in the e e j j and eej channels to probe this scenario. We recast the data in terms of a color octet partner of the SM electron (or a first generation spin-1/2 LG) that couples to an electron and a gluon via a dimension five operator suppressed by the quark–lepton compositeness scale (Λ). By combining different production processes of the color octet electron (e 8) at the LHC, we use the CMS 8 TeV data to obtain a simultaneous bound on Λ and the mass of the e 8 (M e 8). We also study the reach of the 13 TeV LHC to discover the e 8 and interpret the required luminosity in terms of M e 8 and Λ.