Standard Model Leptons Research Articles

Inclusive nonresonant multilepton probes of new phenomena at s=13 TeV

An inclusive search for nonresonant signatures of beyond the standard model (SM) phenomena in events with three or more charged leptons, including hadronically decaying $\tau$ leptons, is presented. The analysis is based on a data sample corresponding to an integrated luminosity of 138 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} =$ 13 TeV, collected by the CMS experiment at the LHC in 2016-2018. Events are categorized based on the lepton and b-tagged jet multiplicities and various kinematic variables. Three scenarios of physics beyond the SM are probed, and signal-specific boosted decision trees are used for enhancing sensitivity. No significant deviations from the background expectations are observed. Lower limits are set at 95% confidence level on the mass of type-III seesaw heavy fermions in the range 845-1065 GeV for various decay branching fraction combinations to SM leptons. Doublet and singlet vector-like $\tau$ lepton extensions of the SM are excluded for masses below 1045 GeV and in the mass range 125-150 GeV, respectively. Scalar leptoquarks decaying exclusively to a top quark and a lepton are excluded below 1.12-1.42 TeV, depending on the lepton flavor. For the type-III seesaw as well as the vector-like doublet model, these constraints are the most stringent to date. For the vector-like singlet model, these are the first constraints from the LHC experiments. Detailed results are also presented to facilitate alternative theoretical interpretations.

Scalar triplet flavor leptogenesis with dark matter

We investigate a simple variant of type-II seesaw, responsible for neutrino mass generation, where the particle spectrum is extended with one singlet right-handed neutrino and an inert Higgs doublet, both odd under an additional $Z_2$ symmetry. While the role of the dark matter is played by the lightest neutral component of the inert Higgs doublet, its interaction with the Standard Model lepton doublets and the right-handed neutrino turns out to be crucial in generating the correct baryon abundance of the Universe through flavored leptogenesis from the decay of the $SU(2)_L$ scalar triplet, involved in type-II framework.

Freeze-in leptogenesis via dark-matter oscillations

We study the cosmology and phenomenology of freeze-in baryogenesis via dark-matter oscillations, taking the dark matter to couple to Standard Model leptons. We investigate viable models both with and without a $Z_2$ symmetry under which all new fields are charged. Lepton flavor effects are important for leptogenesis in these models, and we identify scenarios in which the baryon asymmetry is parametrically distinct from and enhanced relative to leptogenesis from sterile neutrino oscillations. The models we study predict the existence of new, electroweak-charged fields, and can be tested by a combination of collider searches, structure-formation studies, X-ray observations, and terrestrial low-energy tests.

Low scale Dirac leptogenesis and dark matter with observable Delta N_{mathrm{eff}}

We propose a gauged B-L extension of the standard model (SM) where light neutrinos are of Dirac type by virtue of tiny Yukawa couplings. To achieve leptogenesis, we include additional heavy Majorana fermions without introducing any B-L violation by two units. An additional scalar doublet with appropriate B-L charge can allow such heavy fermion coupling with SM leptons so that out-of-equilibrium decay of the former can lead to generation of lepton asymmetry. Due to the B-L gauge interactions of the decaying fermion, the criteria of successful Dirac leptogenesis can also constrain the gauge sector couplings. The same B-L gauge sector parameter space can also be constrained from dark matter requirements if the latter is assumed to consist of SM singlet particles with non-zero B-L charges, which also keep the model anomaly free. The same B-L gauge interactions also lead to additional thermalised relativistic degrees of freedom Delta N_{mathrm{eff}} from light Dirac neutrinos which are tightly constrained by Planck 2018 data. While there exists parameter space from the criteria of successful leptogenesis, dark matter and Delta N_{mathrm{eff}} even after incorporating the latest collider bounds, the currently allowed parameter space can be probed by future measurements of Delta N_{mathrm{eff}}.

Search for exotic leptons in final states with two or three leptons and fat-jets at 13 TeV LHC

Exotic leptons in large gauge multiplets, appearing in many scenarios beyond the Standard Model (SM), can be produced at the LHC in pairs or association. Owing to their large masses, their eventual decay products — SM leptons and bosons — tend to be highly boosted, with the jets stemming from the SM bosons more likely to manifest themselves as a single fat-jet rather than two resolved ones. With the corresponding SM backgrounds being suppressed, final states with two or three leptons and one or two fat-jets are expected to be sensitive in probing exotic fermions much heavier than 1 TeV, and we propose and investigate an appropriate search strategy. To concentrate on the essential, we consider extensions of the SM by leptonic multiplets of a single kind (triplets, quadruplets or quintuplets), bearing in mind that such simplified models typically arise as low-energy limits of more ambitious scenarios addressing various lacunae of the SM. Performing a systematic and comprehensive study of nine such scenarios at the 13 TeV LHC, we find that the corresponding 5σ discovery reaches a range from 985 GeV to 1650 GeV (1345 GeV to 2020 GeV) for 300 (3000) fb−1.

Effective Leptophilic WIMPs at the e+e− collider

We consider higher-dimensional effective (EFT) operators consisting of fermion dark matter (DM) connecting to Standard Model (SM) leptons upto dimension six. Considering all operators together and assuming the DM to undergo thermal freeze-out, we find out relic density allowed parameter space in terms of DM mass (mχ) and New Physics (NP) scale (Λ) with one loop direct search constraints from XENON1T experiment. Allowed parameter space of the model is probed at the proposed International Linear Collider (ILC) via monophoton signal for both Dirac and Majorana cases, limited by the centre-of-mass energy sqrt{s} =1 TeV, where DM mass can be probed within {m}_{chi }<frac{sqrt{s}}{2} for the pair production to occur and Λ > sqrt{s} for the validity of EFT framework.

Charged B mesogenesis

We leverage the CP violation in charged $B$ meson decays to generate the observed baryon asymmetry and dark matter at $\mathcal{O}(10 \text{ MeV})$ temperatures. We realize this in two scenarios: $B_c^+$ Mesogenesis and $B^+$ Mesogenesis. In the first, CP violating $B_c^\pm$ decays to $B^\pm$ mesons are followed by decays to dark and Standard Model baryons. In the second, CP violating $B^\pm$ decays to lighter charged mesons are accompanied by the latter's decays to dark and Standard Model leptons, which then scatter into the baryon asymmetry. $B_c^+$ Mesogenesis is actively being probed at Belle and LHCb, while $B^+$ Mesogenesis can be tested at colliders and sterile neutrino searches.

Fermiophobic Z′ model for simultaneously explaining the muon anomalies RK(*) and (g−2)μ

We discuss a simple renormalisable, gauge invariant model with a fermiophobic $Z'$ boson: it has no couplings to the three Standard Model (SM) chiral families, but does couple to a fourth vector-like (VL) family. The SM Higgs couples to the fourth VL lepton, leading to an enhanced contribution to the muon anomalous magnetic moment $\left(g-2\right)_{\mu}$. The latter contribution requires a non-vanishing coupling of $Z'$ to right-handed muons, which arises within this model due to mixing effects between the SM and VL fermions, along with $Z'$ couplings to the second generation SM lepton doublet and third generation SM quark doublet. This model can simultaneously account for the measured $B$-decay ratios $R_{K^{(*)}}$ and $\left(g-2\right)_{\mu}$. We identify the parameter space where this explanation is consistent with existing experimental constraints coming from $B_{s}-\bar{B}_{s}$ mixing, neutrino trident production and collider searches. We also check that the SM Higgs coupling to the fourth VL lepton does not produce a dangerous contribution to the Higgs diphoton decay.

Cosmic evolution of lepton flavor charges

In the early Universe above the weak scale, both baryon $B$ and lepton $L$ numbers are violated by nonperturabive effects in the Standard Model while $B\ensuremath{-}L$ remains conserved. Introducing new physics which violates perturbatively $L$ and/or $B$, one can generate dynamically a nonzero $B\ensuremath{-}L$ charge and hence a nonzero $B$ charge. In this work, we focus on the former scenario, which is also known as leptogenesis. We show how to describe the evolutions of lepton flavor charges taking into account the complete Standard Model lepton flavor and spectator effects in a unified and lepton flavor basis--independent way. The recipe we develop can be applied to any leptogenesis model with arbitrary number of new scalars carrying nonzero hypercharges and is valid for cosmic temperature ranging from ${10}^{15}\text{ }\text{ }\mathrm{GeV}$ down to the weak scale. We demonstrate that in order to describe the physics in a basis-independent manner and to include lepton flavor effect consistently it is necessary to describe both left-handed and right-handed lepton charges in terms of density matrices. This is a crucial point since physics should be basis independent. As examples, we apply the formalism to type-I and type-II leptogenesis models where in the latter case a flavor-covariant formalism is indispensable.

Dark photon bounds in the dark EFT

Dark photons are massive abelian gauge bosons that interact with ordinary photons via a kinetic mixing with the hypercharge field strength tensor. This theory is probed by a variety of different experiments and limits are set on a combination of the dark photon mass and kinetic mixing parameter. These limits can however be strongly modified by the presence of additional heavy degrees of freedom. Using the framework of dark effective field theory, we study how robust are the current experimental bounds when these new states are present. We focus in particular on the possible existence of a dark dipole interaction between the Standard Model leptons and the dark photon. We show that, under certain assumptions, the presence of a dark dipole modifies existing supernovæ bounds for cut-off scales up to mathcal{O} (10–100 TeV). On the other hand, terrestrial experiments, such as LSND and E137, can probe cut-off scales up to mathcal{O} (3 TeV). For the latter experiment we highlight that the bound may extend down to vanishing kinetic mixing.

Di-Higgs and tri-Higgs boson signals of muon g−2 at a muon collider

We show that new physics explanations of the muon $g-2$ anomaly by the contributions of new leptons mediated by the standard model Higgs boson necessarily lead to large rates for $\mu^+ \mu^- \to hh$ and $\mu^+ \mu^- \to hhh$ irrespectively of details of the model or the scale of new physics. For new leptons with the same quantum numbers as the standard model leptons, cross sections are expected to be about 240 ab for $\mu^+ \mu^- \to hh$ independently of the center of mass energy, $\sqrt{s}$, and about 2.7 ab for $\mu^+ \mu^- \to hhh$ for $\sqrt{s} = 1$ TeV and growing quadratically with $\sqrt{s}$. Predictions for models featuring new leptons with different quantum numbers and for a type-II two Higgs doublet model, where additional Higgs bosons can contribute to muon $g-2$, are also presented.

Non-Abelian vector dark matter and lepton g-2

The mystery of dark matter remains an unsettled problem of particle physics. On top of that, experiments show a persistent contention of the muon anomalous magnetic moment (AMM) relative to the Standard Model (SM) prediction. In this work, we consider the possibility of extending the SM with a non-Abelian gauge symmetry SU(2) X , under which SM leptons transform non-trivially. SM leptons receive corrections to their AMMs of right order via one-loop processes mediated by beyond SM (BSM) fermions required to cancel anomalies, and BSM gauge bosons that play the role of dark matter. We show that simultaneous explanation of the muon AMM along with reproducing correct relic abundance allows rather a narrow range of 0.5–2 TeV dark matter mass, consistent with current experimental constraints. However, a concurrent description that also includes electron AMM is challenging in this set-up.

Double type-II Dirac seesaw accompanied by Dirac fermionic dark matter

A TeV-scale Higgs doublet with a small mixing to the standard model Higgs doublet can have the sizable Yukawa couplings to several right-handed neutrinos and the standard model lepton doublets. This provides a testable Dirac neutrino mass generation. We further consider a seesaw mechanism involving a U(1)B−L gauge symmetry, which predicts the existence of two right-handed neutrinos and a stable Dirac fermionic dark matter, to simultaneously explain the small mixing between the two Higgs doublets and the generation of the cosmic baryon asymmetry.

Leptophilic composite asymmetric dark matter and its detection

We propose a model which explains the baryon asymmetry of the universe and dark matter relic density at the same time. In this model, dark matter candidate is the dark baryon composed by dark quarks. A scalar mediator, which couples to the standard model leptons and dark quarks, is introduced to generate the asymmetry of baryon and dark baryon simultaneously. Direct detection and collider detection of this model are studied. We find that current underground direct detection experiments and LHC can hardly detect this model. But future lepton colliders, such as CEPC, have great potential to detect a large portion of the model parameter space by "displaced lepton jet" signal.

Leptophilic fermion WIMP: Role of future lepton colliders

The leptophilic weakly interacting massive particle (WIMP) is realized in a minimal renormalizable model scenario where scalar mediators with lepton number establish the WIMP interaction with the standard model (SM) leptons. We perform a comprehensive analysis for such a WIMP scenario for two distinct cases with an SU(2) doublet or singlet mediator considering all the relevant theoretical, cosmological and experimental constraints at present. We show that the mono-photon search at near-future lepton collider experiments (ILC, FCC-ee, CEPC, etc.) can play a significant role to probe the yet unexplored parameter range allowed by the WIMP relic density constraint. This will complement the search prospect at the near-future hadron collider experiment (HL-LHC). Furthermore, we discuss the combined model scenario including both the doublet and singlet mediator. The combined model is capable of explaining the long-standing muon (g-2) anomaly which is an additional advantage. We demonstrate that the allowed region for anomalous muon (g-2) explanation, which has been updated very recently at Fermi National Accelerator Laboratory, can also be probed at the future colliders which will thus be a simultaneous authentication of the model scenario.

Forbidden dark matter annihilations into Standard Model particles

We present kinematically forbidden dark matter annihilations into Standard Model leptons. This mechanism precisely selects the dark matter mass that gives the observed relic abundance. This is qualitatively different from existing models of thermal dark matter, where fixing the relic density typically leaves open orders of magnitude of viable dark matter masses. Forbidden annihilations require the dark matter to be close in mass to the particles that dominate its annihilation rate. We show examples where the dark matter mass is close to the muon mass, the tau mass, or the average of the tau and muon masses. We find that most of the relevant parameter space can be covered by the next generation of proposed beam-dump experiments and future high-luminosity electron positron colliders. Forbidden dark matter predicts large couplings to the Standard Model that can explain the observed value of (g − 2)μ.

Nonperturbative effects on electroweakly interacting massive particles at hadron collider

Electroweakly Interacting Massive Particles (EWIMPs), in other words, new massive particles that are charged under the electroweak interaction of the Standard Model (SM), are often predicted in various new physics models. EWIMPs are probed at hadron collider experiments not only by observing their direct productions but also by measuring their quantum effects on Drell-Yan processes for SM lepton pair productions. Such effects are known to be enhanced especially when the di-lepton invariant mass of the final state is close to the EWIMP threshold, namely twice the EWIMP mass. In such a mass region, however, we have to carefully take non-perturbative effects into account, because the EWIMPs become non-relativistic and the prediction may be significantly affected by e.g., bound states of the EWIMPs caused by the electroweak interaction. We study such non-perturbative effects using the non-relativistic effective field theory of the EWIMPs, and found that those indeed affect the differential cross section of the Drell-Yan processes significantly, though the effects are smeared due to the finite energy resolution of the lepton measurement at the Large Hadron Collider experiment.

A Review of Life Science Studies with Muons

Positive muon is a spin half elementary particle lies in the second generation leptons in standard model of particles. It has been used as a sensitive magnetic (spin) probe for study of life and materials. Due to its special characteristics – 100% polarization and asymmetric decay to positron, it provides information about local electronic and spin states of material in which it stops. The asymmetry, relaxation of muon and its charge states in materials provide information about the interested phenomena. For life sciences study, muon can probe the dynamics of electron, proton, ions, H, O2, reaction dynamics, catalytic processes, concentration of molecules, magnetic behaviors, etc. in the biosamples. Here, the applications of positive muon to understand the life related phenomena are reviewed.

Search for the singlet vector-like lepton at future e^+ e^- colliders

Searching for the new particles beyond the standard model (SM) is an important way to probe new physics beyond the SM. In one class of new particles there are nonchiral color singlet fermions that couple to the SM leptons, that is vector-like leptons (VLLs), which have been widely concerned in experiment and theory. The existing literature shows that the singlet VLLs suffer from a big difficult challenge for discovery at future proton-proton colliders. In this paper, we study the prospects of searching for the singlet VLLs in pure leptonic and fully hadronic channels at e^+ e^- colliders. We find that there is an opportunity for excluding the region m_{tau ^{prime pm }} in [180 GeV, 240 GeV] with {mathcal {L}}in [3.0hbox { fb}^{-1}, 14.9hbox { fb}^{-1}] ([0.1hbox { fb}^{-1}, 0.3hbox { fb}^{-1}]) at ILC of sqrt{s} = 500 hbox { GeV}, the region m_{tau ^{prime pm }} in [240 GeV, 450 GeV] with {mathcal {L}} in [9.9hbox { fb}^{-1}, 23.1hbox { fb}^{-1}] ([0.2hbox { fb}^{-1}, 0.3hbox { fb}^{-1}]) at ILC of sqrt{s} = 1000 hbox { GeV} and the region m_{tau ^{prime pm }} in [450 GeV, 700 GeV] with {mathcal {L}} in [52.1hbox { fb}^{-1}, 197.9hbox { fb}^{-1}] ([2.21hbox { fb}^{-1}, 4.5hbox { fb}^{-1}]) at CLIC of sqrt{s} = 1500 hbox {GeV} in the pure leptonic (fully hadronic) channel. It is more optimistic for excluding and discovering the singlet VLL through the fully hadronic channel at future high energy e^+ e^- colliders.

Constraints on long range force from perihelion precession of planets in a gauged L_e-L_{mu ,tau } scenario

The standard model leptons can be gauged in an anomaly free way by three possible gauge symmetries namely {L_e-L_mu }, {L_e-L_tau }, and {L_mu -L_tau }. Of these, {L_e-L_mu } and {L_e-L_tau } forces can mediate between the Sun and the planets and change the perihelion precession of planetary orbits. It is well known that a deviation from the 1/r^2 Newtonian force can give rise to a perihelion advancement in the planetary orbit, for instance, as in the well known case of Einstein’s gravity (GR) which was tested from the observation of the perihelion advancement of the Mercury. We consider the long range Yukawa potential which arises between the Sun and the planets if the mass of the gauge boson is M_{Z^{prime }}le mathcal {O}(10^{-19})mathrm {eV}. We derive the formula of perihelion advancement for Yukawa type fifth force due to the mediation of such U(1)_{L_e-L_{mu ,tau }} gauge bosons. The perihelion advancement for Yukawa potential is proportional to the square of the semi major axis of the orbit for small M_{Z^{prime }}, unlike GR where it is largest for the nearest planet. For higher values of M_{Z^{prime }}, an exponential suppression of the perihelion advancement occurs. We take the observational limits for all planets for which the perihelion advancement is measured and we obtain the upper bound on the gauge boson coupling g for all the planets. The Mars gives the stronger bound on g for the mass range le 10^{-19}mathrm {eV} and we obtain the exclusion plot. This mass range of gauge boson can be a possible candidate of fuzzy dark matter whose effect can therefore be observed in the precession measurement of the planetary orbits.