Enhanced lepton flavour violation in the supersymmetric inverse seesaw

In minimal supersymmetric seesaw models, the contribution to lepton flavour violation from Z-penguins is usually negligible. In this study, we consider the supersymmetric inverse seesaw and show that, in this case, the Z-penguin contribution dominates in several lepton flavour violating observables due to the low scale of the inverse seesaw mechanism. Among the observables considered, we find that the most constraining one is the muon to electron conversion rate which is already restricting the otherwise allowed parameter space of the model. Moreover, in this framework, the Z-penguins exhibit a non-decoupling behaviour, which has previously been noticed in lepton flavour violating Higgs decays.Comment: 8 pages, 4 figures, Proceedings of Discrete 2012, Published under licence in Journal of Physics: Conference Series (JPCS) by IOP Publishing Ltd, v2 includes a preprint numbe

Lepton Universality in Kaon Decays

In the Standard Model extended by sterile neutrinos, modified W-l-nu couplings arise, which are able to induce a tree-level enhancement to lepton flavour universality violation in kaon decays. The additional mixing between the active neutrinos and the sterile ones can generate deviations from unitarity in the leptonic mixing matrix for charged currents. We reconsidered this idea in the context of the inverse seesaw and evaluated its impact on the well measured ratio $R_K$. We show that the current experimental bound can be saturated in agreement with the different experimental and observational constraints. Similar results can be obtained when considering the ratio $R_\pi$.Comment: 4 pages, 1 figure (2 plots), Proceedings of the Moriond 2013 EW session, v2 includes a preprint numbe

Effets des neutrinos singulets fermioniques sur les observables de haute et basse énergie

In this doctoral thesis, we study both low- and high-energy observables related to massive neutrinos. Neutrino oscillations have provided indisputable evidence in favour of non-zero neutrino masses and mixings. However, the original formulation of the Standard Model cannot account for these observations, which calls for the introduction of new Physics. Among many possibilities, we focus here on the inverse seesaw, a neutrino mass generation mechanism in which the Standard Model is extended with fermionic gauge singlets. This model offers an attractive alternative to the usual seesaw realisations since it can potentially have natural Yukawa couplings (O(1)) while keeping the new Physics scale at energies within reach of the LHC. Among the many possible effects, this scenario can lead to deviations from lepton flavour universality. We have investigated these signatures and found that the ratios R_K and R_π provide new, additional constraints on the inverse seesaw. We have also considered the embedding of the inverse seesaw in supersymmetric models. This leads to increased rates for various lepton flavour violating processes, due to enhanced contributions from penguin diagrams mediated by the Higgs and Z bosons. Finally, we also found that the new invisible decay channels associated with the sterile neutrinos present in the supersymmetric inverse seesaw could significantly weaken the constraints on the mass and couplings of a light CP-odd Higgs boson.Dans cette thèse, nous étudions à la fois des observables de basse et de haute énergie liée à la présence de neutrinos massifs. Les oscillations de neutrinos ont apporté des preuves indiscutables en faveur de l'existence de masses non-nulles et de mélanges. Néanmoins, la formulation originale du Modèle Standard ne permet pas d'expliquer ces observations, d'où la nécessité d'introduire de nouveaux modèles. Parmi de nombreuses possibilités, nous nous concentrons ici sur le seesaw inverse, un mécanisme générant des neutrinos massifs par l'ajout de fermions singulets de jauge au Modèle Standard. Ce modèle offre une alternative attractive aux réalisations habituelles du seesaw puisqu'il a des couplages de Yukawa potentiellement naturels (O(1)) tout en conservant l'échelle de la nouvelle physique à des énergies accessibles au LHC. Parmi de nombreux effets, ce scénario peut générer de larges écarts à l'universalité leptonique. Nous avons étudié ces signatures et trouvé que les rapports R_K et R_π constituent de nouvelles contraintes pour le seesaw inverse. Nous nous sommes aussi intéressé à l'intégration de l'inverse seesaw dans différents modèles supersymétriques. Ceci conduit à une augmentation de la section efficace de divers processus violant la saveur leptonique du fait de contributions plus importantes venant des diagrammes pingouins comportant un boson de Higgs ou Z. Finalement, nous avons aussi trouvé que les nouveaux canaux de désintégration ouverts par la présence de neutrinos stériles dans les modèles de seesaw inverse supersymétriques peuvent significativement relaxer les contraintes sur la masse et les couplages d'un boson de Higgs CP-impair

Lepton Universality in Kaon Decays

4 pages, 1 figure (2 plots), Proceedings of the Moriond 2013 EW sessionIn the Standard Model extended by sterile neutrinos, modified W-l-nu couplings arise, which are able to induce a tree-level enhancement to lepton flavour universality violation in kaon decays. The additional mixing between the active neutrinos and the sterile ones can generate deviations from unitarity in the leptonic mixing matrix for charged currents. We reconsidered this idea in the context of the inverse seesaw and evaluated its impact on the well measured ratio $R_K$. We show that the current experimental bound can be saturated in agreement with the different experimental and observational constraints. Similar results can be obtained when considering the ratio $R_\pi$

The triple Higgs coupling: a new probe of low-scale seesaw models

The measure of the triple Higgs coupling is one of the major goals of the high-luminosity run of the CERN Large Hadron Collider (HL-LHC) as well as the future colliders, either leptonic such as the International Linear Collider (ILC) or hadronic such as the 100 TeV Future Circular Collider in hadron-hadron mode (FCC-hh). We have recently proposed this observable as a test of neutrino mass generating mechanisms in a regime where heavy sterile neutrino masses are hard to be probed otherwise. We present in this article a study of the one-loop corrected triple Higgs coupling in the inverse seesaw model, taking into account all relevant constraints on the model. This is the first study of the impact on the triple Higgs coupling of heavy neutrinos in a realistic, renormalizable neutrino mass model. We obtain deviations from the Standard Model as large as to ∼ +30% that are at the current limit of the HL-LHC sensitivity, but would be clearly visible at the ILC or at the FCC-hh

Effets des neutrinos singulets fermioniques sur les observables de haute et basse énergie

Dans cette thèse, nous étudions à la fois des observables de basse et de haute énergie liée à la présence de neutrinos massifs. Les oscillations de neutrinos ont apporté des preuves indiscutables en faveur de l'existence de masses non-nulles et de mélanges. Néanmoins, la formulation originale du Modèle Standard ne permet pas d'expliquer ces observations, d'où la nécessité d'introduire de nouveaux modèles. Parmi de nombreuses possibilités, nous nous concentrons ici sur le seesaw inverse, un mécanisme générant des neutrinos massifs par l'ajout de fermions singulets de jauge au Modèle Standard. Ce modèle offre une alternative attractive aux réalisations habituelles du seesaw puisqu'il a des couplages de Yukawa potentiellement naturels (O(1)) tout en conservant l'échelle de la nouvelle physique à des énergies accessibles au LHC. Parmi de nombreux effets, ce scénario peut générer de larges écarts à l'universalité leptonique. Nous avons étudié ces signatures et trouvé que les rapports R_K et R_p constituent de nouvelles contraintes pour le seesaw inverse. Nous nous sommes aussi intéressé à l'intégration de l'inverse seesaw dans différents modèles supersymétriques. Ceci conduit à une augmentation de la section efficace de divers processus violant la saveur leptonique du fait de contributions plus importantes venant des diagrammes pingouins comportant un boson de Higgs ou Z. Finalement, nous avons aussi trouvé que les nouveaux canaux de désintégration ouverts par la présence de neutrinos stériles dans les modèles de seesaw inverse supersymétriques peuvent significativement relaxer les contraintes sur la masse et les couplages d'un boson de Higgs CP-impair.In this doctoral thesis, we study both low- and high-energy observables related to massive neutrinos. Neutrino oscillations have provided indisputable evidence in favour of non-zero neutrino masses and mixings. However, the original formulation of the Standard Model cannot account for these observations, which calls for the introduction of new Physics. Among many possibilities, we focus here on the inverse seesaw, a neutrino mass generation mechanism in which the Standard Model is extended with fermionic gauge singlets. This model offers an attractive alternative to the usual seesaw realisations since it can potentially have natural Yukawa couplings (O(1)) while keeping the new Physics scale at energies within reach of the LHC. Among the many possible effects, this scenario can lead to deviations from lepton flavour universality. We have investigated these signatures and found that the ratios R_K and R_p provide new, additional constraints on the inverse seesaw. We have also considered the embedding of the inverse seesaw in supersymmetric models. This leads to increased rates for various lepton flavour violating processes, due to enhanced contributions from penguin diagrams mediated by the Higgs and Z bosons. Finally, we also found that the new invisible decay channels associated with the sterile neutrinos present in the supersymmetric inverse seesaw could significantly weaken the constraints on the mass and couplings of a light CP-odd Higgs boson.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Heavy neutrinos with dynamic jet vetoes: multilepton searches at √s = 14, 27, and 100 TeV

Heavy neutrinos (N) remain one of most promising explanations for the ori-gin of neutrinos’ tiny masses and large mixing angles. In light of broad advances in un-derstanding and modeling of hadron collisions at large momentum transfers, we revisit the long-standard search strategy for heavy N decaying to multiple charged leptons (ℓ), pp → NℓX → 3ℓνX. For electroweak and TeV-scale N, we propose a qualitatively new collider analysis premised on a dynamic jet veto and discriminating, on an event-by-event basis, according to the relative amount of hadronic and leptonic activity. We report that the sensitivity to heavy neutrinos at the CERN Large Hadron Collider (LHC) can be improved by roughly an order of magnitude at both ℒ = 300 fb−1 and 3 ab−1. At s√=14 TeV with ℒ = 3 ab−1, we find active-sterile mixing as small as |VℓN|2 = 10− 2(10− 3) [5 × 10− 4] can be probed at 95% CL for heavy Dirac neutrinos masses mN ≲ 1200 (300) [200] GeV, well beyond the present |VℓN|2 ≲ 10− 3 − 10− 1 constraints for such heavy states set by indirect searches and precision measurements. The improvement holds also for Majorana N, and is largely independent of whether charged lepton flavor is conserved or violated. The analysis, built almost entirely from inclusive, transverse observables, is designed to be robust across increasing collider energies, and hence serves as a basis for searches at future colliders: with ℒ = 15ab− 1 at s√=27 TeV,onecanprobemixingbelow |VℓN|2 = 10− 2(10− 3)[2 × 10− 4] for mN ≲ 3500 (700) [200] GeV. At a hypothetical 100 TeV pp collider with ℒ = 30 ab− 1, one can probe mixing down to 9 × 10−5 for mN ≲ 200 GeV, below 10−3 for mN ≲ 4 TeV, and below 10−2 for mN ≲ 15 TeV. We anticipate these results can be further improved with detector-specific tuning and application of multi-variant / machines learning techniques. To facilitate such investigations, we make publicly available Monte Carlo libraries needed for the precision computations/simulations used in our study

Safe jet vetoes

Central jet vetoes are powerful tools for reducing QCD background in measurements and searches for electroweak and colorless, new physics processes in hadron collisions. In this letter, we report the key findings of a new philosophy to designing searches for such phenomena at hadron colliders, one designed and centered around a dynamical jet veto instead of a static veto applied independently of other selection criteria. Specifically, we investigate the theoretical and phenomenological consequences of setting the jet veto scale to the transverse momentum of the leading charged lepton ℓ in multi-lepton processes on an event-by-event basis. We consider the case of a TeV-scale heavy neutrino N decaying to the trilepton final state and find the following: (i) Perturbative uncertainties associated with the veto greatly reduce due to tying the veto scale to the hard process scale. (ii) The signal efficiency for passing the veto jumps to and exhibits little-to-no dependence on the neutrino mass scale. (iii) Top quark and “fake” lepton rejection capabilities also improve compared to only vetoing heavy flavor-tagged jets above a fixed . This results in an increased sensitivity to active–sterile neutrino mixing by approximately an order of magnitude over the LHC's lifetime. For a Dirac neutrino with mass GeV and the representative active–sterile mixing hypothesis with , we find that LHC experiments can probe , surpassing the global upper limit for GeV, with ab−1 of data at TeV. Due to the color structures of the heavy N production mechanisms considered, we argue that our results hold broadly for other color-singlet processes

A natural connection between neutrino mass generation and the lightness of a next-to-minimal supersymmetric Standard Model pseudoscalar

One of the attractive properties of the NMSSM is that it can accommodate a light pseudoscalar of order 10 GeV. However, such scenarios are constrained by several experimental results, especially those related to the fermionic decays of the pseudoscalar. In this work, extending the NMSSM field content by two gauge singlets, with lepton number + 1 and − 1, we generate neutrino masses via the inverse see-saw mechanism at one hand and on the other hand a very light pseudoscalar becomes experimentally viable by having dominant invisible decay channels which help it to evade the existing bounds

Report from Working Group 3: Beyond the Standard Model physics at the HL-LHC and HE-LHC

This is the third out of five chapters of the final report [1] of the Workshop on Physics at HL-LHC, and perspectives on HE-LHC [2]. It is devoted to the study of the potential, in the search for Beyond the Standard Model (BSM) physics, of the High Luminosity (HL) phase of the LHC, defined as $3$ ab$^{-1}$ of data taken at a centre-of-mass energy of 14 TeV, and of a possible future upgrade, the High Energy (HE) LHC, defined as $15$ ab$^{-1}$ of data at a centre-of-mass energy of 27 TeV. We consider a large variety of new physics models, both in a simplified model fashion and in a more model-dependent one. A long list of contributions from the theory and experimental (ATLAS, CMS, LHCb) communities have been collected and merged together to give a complete, wide, and consistent view of future prospects for BSM physics at the considered colliders. On top of the usual standard candles, such as supersymmetric simplified models and resonances, considered for the evaluation of future collider potentials, this report contains results on dark matter and dark sectors, long lived particles, leptoquarks, sterile neutrinos, axion-like particles, heavy scalars, vector-like quarks, and more. Particular attention is placed, especially in the study of the HL-LHC prospects, to the detector upgrades, the assessment of the future systematic uncertainties, and new experimental techniques. The general conclusion is that the HL-LHC, on top of allowing to extend the present LHC mass and coupling reach by $20-50\%$ on most new physics scenarios, will also be able to constrain, and potentially discover, new physics that is presently unconstrained. Moreover, compared to the HL-LHC, the reach in most observables will, generally more than double at the HE-LHC, which may represent a good candidate future facility for a final test of TeV-scale new physics