Showing papers on "Lepton published in 2020"

Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations

Abstract: The charge-conjugation and parity-reversal (CP) symmetry of fundamental particles is a symmetry between matter and antimatter. Violation of this CP symmetry was first observed in 19641, and CP violation in the weak interactions of quarks was soon established2. Sakharov proposed3 that CP violation is necessary to explain the observed imbalance of matter and antimatter abundance in the Universe. However, CP violation in quarks is too small to support this explanation. So far, CP violation has not been observed in non-quark elementary particle systems. It has been shown that CP violation in leptons could generate the matter–antimatter disparity through a process called leptogenesis4. Leptonic mixing, which appears in the standard model’s charged current interactions5,6, provides a potential source of CP violation through a complex phase δCP, which is required by some theoretical models of leptogenesis7,8,9. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible using accelerator-produced beams as established by the Tokai-to-Kamioka (T2K) and NOvA experiments10,11. Until now, the value of δCP has not been substantially constrained by neutrino oscillation experiments. Here we report a measurement using long-baseline neutrino and antineutrino oscillations observed by the T2K experiment that shows a large increase in the neutrino oscillation probability, excluding values of δCP that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δCP, which is cyclic and repeats every 2π, is [−3.41, −0.03] for the so-called normal mass ordering and [−2.54, −0.32] for the inverted mass ordering. Our results indicate CP violation in leptons and our method enables sensitive searches for matter–antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger datasets will test whether leptonic CP violation is larger than the CP violation in quarks.

Search for Heavy Higgs Bosons Decaying into Two Tau Leptons with the ATLAS Detector Using pp Collisions at s =13 TeV

Abstract: A search for heavy neutral Higgs bosons is performed using the LHC Run 2 data, corresponding to an integrated luminosity of 139 fb^{-1} of proton-proton collisions at sqrt[s]=13 TeV recorded with the ATLAS detector. The search for heavy resonances is performed over the mass range 0.2-2.5 TeV for the τ^{+}τ^{-} decay with at least one τ-lepton decaying into final states with hadrons. The data are in good agreement with the background prediction of the standard model. In the M_{h}^{125} scenario of the minimal supersymmetric standard model, values of tanβ>8 and tanβ>21 are excluded at the 95% confidence level for neutral Higgs boson masses of 1.0 and 1.5 TeV, respectively, where tanβ is the ratio of the vacuum expectation values of the two Higgs doublets.

Axionlike Particles, Lepton-Flavor Violation, and a New Explanation of a μ and a e

Abstract: Axionlike particles (ALPs) with lepton-flavor-violating couplings can be probed in exotic muon and tau decays. The sensitivity of different experiments depends strongly on the ALP mass and its couplings to leptons and photons. For ALPs that can be resonantly produced, the sensitivity of three-body decays such as μ→3e and τ→3μ exceeds by many orders of magnitude that of radiative decays like μ→eγ and τ→μγ. Searches for these two types of processes are therefore highly complementary. We discuss experimental constraints on ALPs with a single dominant lepton-flavor-violating coupling. Allowing for one or more such couplings offers qualitatively new ways to explain the anomalies related to the magnetic moments of the muon or the electron. The explanation of both anomalies requires lepton-flavor-nonuniversal or lepton-flavor-violating ALP couplings.

Drell-Yan Cross Section to Third Order in the Strong Coupling Constant.

Abstract: We present phenomenological results for the inclusive cross section for the production of a lepton pair via virtual photon exchange at next-to-next-to-next-to-leading order in perturbative QCD. In line with the case of Higgs production, we find that the hadronic cross section receives corrections at the percent level, and the residual dependence on the perturbative scales is reduced. However, unlike in the Higgs case, we observe that the uncertainty band derived from scale variation is no longer contained in the band of the previous order.

Anomalous magnetic moments from asymptotic safety

Abstract: The measurements of the muon and electron anomalous magnetic moments hint at physics beyond the standard model. We show why and how models inspired by asymptotic safety can explain deviations from standard model predictions naturally. Our setup features an enlarged scalar sector and Yukawa couplings between leptons and new vectorlike fermions. Using the complete two-loop running of couplings, we observe a well-behaved high-energy limit of models including a stabilization of the Higgs. We find that a manifest breaking of lepton universality beyond standard model Yukawas is not necessary to explain the muon and electron anomalies. We further predict the tau anomalous magnetic moment and new particles in the TeV energy range, whose signatures at colliders are indicated. With small $CP$ phases, the electron EDM can be as large as the present bound.

Global Fit to Modified Neutrino Couplings and the Cabibbo-Angle Anomaly.

Abstract: Recently, discrepancies of up to $4\ensuremath{\sigma}$ between the different determinations of the Cabibbo angle were observed. In this context, we point out that this ``Cabibbo-angle anomaly'' can be explained by lepton flavor universality violating new physics in the neutrino sector. However, modified neutrino couplings to standard model gauge bosons also affect many other observables sensitive to lepton flavor universality violation, which have to be taken into account in order to assess the viability of this explanation. Therefore, we perform a model-independent global analysis in a Bayesian approach and find that the tension in the Cabibbo angle is significantly reduced, while the agreement with other data is also mostly improved. In fact, nonzero modifications of electron and muon neutrino couplings are preferred at more than 99.99% C.L. (corresponding to more than $4\ensuremath{\sigma}$). Still, since constructive effects in the muon sector are necessary, simple models with right-handed neutrinos (whose global fit we update as a by-product) cannot fully explain data, pointing towards more sophisticated new physics models.

Search for pairs of scalar leptoquarks decaying into quarks and electrons or muons in √s = 13 TeV pp collisions with the ATLAS detector

Abstract: A search for new-physics resonances decaying into a lepton and a jet performed by the ATLAS experiment is presented. Scalar leptoquarks pair-produced in pp collisions at $ \sqrt{s} $ = 13 TeV at the Large Hadron Collider are considered using an integrated luminosity of 139 fb$^{−1}$, corresponding to the full Run 2 dataset. They are searched for in events with two electrons or two muons and two or more jets, including jets identified as arising from the fragmentation of c- or b-quarks. The observed yield in each channel is consistent with the Standard Model background expectation. Leptoquarks with masses below 1.8 TeV and 1.7 TeV are excluded in the electron and muon channels, respectively, assuming a branching ratio into a charged lepton and a quark of 100%, with minimal dependence on the quark flavour. Upper limits on the aforementioned branching ratio are also given as a function of the leptoquark mass.[graphic not available: see fulltext]

Modular invariant models of lepton masses at levels 4 and 5

Abstract: We explore alternative descriptions of the charged lepton sector in modular invariant models of lepton masses and mixing angles. In addition to the modulus, the symmetry breaking sector of our models includes ordinary flavons. Neutrino mass terms depend only on the modulus and are tailored to minimize the number of free parameters. The charged lepton Yukawa couplings rely upon the flavons alone. We build modular invariant models at levels 4 and 5, where neutrino masses are described both in terms of the Weinberg operator or through a type I seesaw mechanism. At level 4, our models reproduce the hierarchy among electron, muon and tau masses by letting the weights play the role of Froggatt-Nielsen charges. At level 5, our setup allows the treatment of left and right handed charged leptons on the same footing. We have optimized the free parameters of our models in order to match the experimental data, obtaining a good degree of compatibility and predictions for the absolute neutrino masses and the C P violating phases. At a more fundamental level, the whole lepton sector could be correctly described by the simultaneous presence of several moduli. Our examples are meant to make a first step in this direction.

Type II seesaw models with modular A 4 symmetry

Abstract: We discuss type II seesaw models adopting modular ${A}_{4}$ symmetry in a supersymmetric framework. In our approach, the models are classified by the assignment of ${A}_{4}$ representations and modular weights for leptons and triplet Higgs fields. Then neutrino mass matrix is characterized by modulus $\ensuremath{\tau}$ and two free parameters. Carrying out numerical analysis, we find allowed parameter sets which can fit the neutrino oscillation data. For the allowed parameter sets, we obtain the predictions in the neutrino sector such as $CP$ violating phases and the lightest neutrino mass. Finally, we also show the predictions for the branching ratios of a doubly charged scalar boson, focusing on the case where the doubly charged scalar boson dominantly decays into charged leptons.

Long-baseline neutrino oscillation physics potential of the DUNE experiment

Abstract: The sensitivity of the Deep Underground Neutrino Experiment (DUNE) to neutrino oscillation is determined, based on a full simulation, reconstruction, and event selection of the far detector and a full simulation and parameterized analysis of the near detector. Detailed uncertainties due to the flux prediction, neutrino interaction model, and detector effects are included. DUNE will resolve the neutrino mass ordering to a precision of 5σ, for all δCP values, after 2 years of running with the nominal detector design and beam configuration. It has the potential to observe charge-parity violation in the neutrino sector to a precision of 3σ (5σ) after an exposure of 5 (10) years, for 50% of all δCP values. It will also make precise measurements of other parameters governing long-baseline neutrino oscillation, and after an exposure of 15 years will achieve a similar sensitivity to sin 22 θ13 to current reactor experiments.

Dark matter assisted lepton anomalous magnetic moments and neutrino masses

Abstract: We propose a framework that addresses the origin of neutrino mass, explains the observed discrepancies in the electron and the muon anomalous magnetic moments (AMMs) data, and incorporates the dark matter (DM) relic abundance. Both the neutrino mass and the lepton AMMs are generated at one-loop level mediated by a common set of beyond the Standard Model (SM) states. In this class of models, the SM is extended with vectorlike charged fermion and scalar multiplets, all odd under an imposed ${\mathcal{Z}}_{2}$ symmetry, which stabilizes the fermionic or scalar DM candidate residing in one of them. Two scalar multiplets appear in the AMM loops, thus allowing for different signs of their contributions, in agreement with the observed discrepancies which are of opposite sign for electron and muon. The vectorlike fermions give rise to large new physics contributions to the lepton AMMs via chirally enhanced terms that are proportional to their mass. To demonstrate the viability of this framework, we perform a detailed study of a particular model for which a fit to the neutrino masses and mixing together with lepton AMMs are provided. Furthermore, DM phenomenology and collider signatures are explored.

Heavy Neutral Leptons from low-scale seesaws at the DUNE Near Detector

Abstract: Heavy nearly-sterile neutrinos are a common ingredient in extensions of the Standard Model which aim to explain neutrino masses, like for instance in Type I seesaw models, or one of its variants. If the scale of the new Heavy Neutral Leptons (HNLs) is sufficiently low, observable signatures can arise in a range of current and upcoming experiments, from the LHC to neutrino experiments. In this article, we discuss the phenomenology of sterile neutrinos in the MeV to GeV mass range, focusing on their decays. We embed our discussion in a realistic mass model and consider the resulting implications. We focus in particular on the impact on the signal of the strong polarisation effects in the beam for Majorana and (pseudo-)Dirac states, providing formulae to incorporate these in both production and decay. We study how the Near Detector of the upcoming Deep Underground Neutrino Experiment can constrain HNL states by searching for their de- cay products inside the detector. We conduct a Monte Carlo background analysis for the most promising signatures, incorporating the detector’s particle identification capabilities, and estimate the experimental sensitivity of DUNE to these particles. We also present an estimate of the ντ -derived HNL flux at DUNE, currently missing in the literature, which allows us to discuss searches for HNLs at higher masses.

New A(4) lepton flavor model from S-4 modular symmetry

Abstract: We study a flavor model with A4 symmetry which originates from S4 modular group. In S4 symmetry, Z2 subgroup can be anomalous, and then S4 can be violated to A4. Starting with a S4 symmetric Lagrangian at the tree level, the Lagrangian at the quantum level has only A4 symmetry when Z2 in S4 is anomalous. We obtain modular forms of two singlets and a triplet representations of A4 by decomposing S4 modular forms into A4 representations. We propose a new A4 flavor model of leptons by using those A4 modular forms. We succeed in constructing a viable neutrino mass matrix through the Weinberg operator for both normal hierarchy (NH) and inverted hierarchy (IH) of neutrino masses. Our predictions of the CP violating Dirac phase δCP and the mixing sin2 θ23 depend on the sum of neutrino masses for NH.

Search for heavy Higgs bosons decaying to a top quark pair in proton-proton collisions at √s= 13 TeV

Abstract: A search is presented for additional scalar (H) or pseudoscalar (A) Higgs bosons decaying to a top quark pair in proton-proton collisions at a center-of-mass energy of 13 TeV. The data set analyzed corresponds to an integrated luminosity of 35.9 fb−1 collected by the CMS experiment at the LHC. Final states with one or two charged leptons are considered. The invariant mass of the reconstructed top quark pair system and variables that are sensitive to the spin of the particles decaying into the top quark pair are used to search for signatures of the H or A bosons. The interference with the standard model top quark pair background is taken into account. A moderate signal-like deviation compatible with an A boson with a mass of 400 GeV is observed with a global significance of 1.9 standard deviations. New stringent constraints are reported on the strength of the coupling of the hypothetical bosons to the top quark, with the mass of the bosons ranging from 400 to 750 GeV and their total relative width from 0.5 to 25%. The results of the search are also interpreted in a minimal supersymmetric standard model scenario. Values of mA from 400 to 700 GeV are probed, and a region with values of tan β below 1.0 to 1.5, depending on mA, is excluded at 95% confidence level.

Vector boson fusion at multi-TeV muon colliders

Abstract: High-energy lepton colliders with a centre-of-mass energy in the multi-TeV range are currently considered among the most challenging and far-reaching future accelerator projects. Studies performed so far have mostly focused on the reach for new phenomena in lepton-antilepton annihilation channels. In this work we observe that starting from collider energies of a few TeV, electroweak (EW) vector boson fusion/scattering (VBF) at lepton colliders becomes the dominant production mode for all Standard Model processes relevant to studying the EW sector. In many cases we find that this also holds for new physics. We quantify the size and the growth of VBF cross sections with collider energy for a number of SM and new physics processes. By considering luminosity scenarios achievable at a muon collider, we conclude that such a machine would effectively be a “high-luminosity weak boson collider,” and subsequently offer a wide range of opportunities to precisely measure EW and Higgs couplings as well as discover new particles.

The CKM unitarity problem: a trace of new physics at the TeV scale?

Abstract: After the recent high precision determinations of $$V_{us}$$ and $$V_{ud}$$, the first row of the CKM matrix shows more than $$4\sigma $$ deviation from unitarity. Two possible scenarios beyond the Standard Model can be investigated in order to fill the gap. If a 4th non-sequential quark $$b'$$ (a vector-like weak isosinglet) participates in the mixing, with $$\vert V_{ub'} \vert \sim 0.04$$, then its mass should be no more than 6 TeV or so. A different solution can come from the introduction of the gauge horizontal family symmetry $$SU(3)_\ell $$ acting between the lepton families and spontaneously broken at the scale of about 6 TeV. Since the gauge bosons of this symmetry contribute to muon decay in interference with Standard Model, the Fermi constant is slightly smaller than the muon decay constant so that unitarity is recovered. Also the neutron lifetime problem, that is about $$4\sigma $$ discrepancy between the neutron lifetimes measured in beam and trap experiments, is discussed in the light of the these determinations of the CKM matrix elements.

Hunting for ALPs with lepton flavor violation

Abstract: We examine the low-energy signatures of axion-like particles (ALPs) in lep- ton flavor violating (LFV) processes. By using a dimension-5 effective Lagrangian, we compute the most general ALP contributions to LFV decays of leptons and mesons. The provided expressions are valid for any choice of ALP mass and couplings. We explore the complementarity of different processes, identifying specific patterns to be experimentally tested. Constraints on LFV couplings are derived from existing data and prospects for forthcoming experiments are also discussed. As a by-product, we revisit the possibility of a simultaneous explanation of the observed discrepancies in the muon and electron g − 2 through ALP interactions.

Is it possible to explain the muon and electron g − 2 in a Z ′ model?

Abstract: In order to address this question, we consider a simple renormalizable and gauge invariant model in which the ${Z}^{\ensuremath{'}}$ only has couplings to the electron and muon and their associated neutrinos, arising from mixing with a heavy vectorlike fourth family of leptons. Within this model we discuss the contributions to the electron and muon anomalous magnetic moments from ${Z}^{\ensuremath{'}}$ exchange, subject to the constraints from $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ and neutrino trident production. Using analytic and numerical arguments, we find that such a ${Z}^{\ensuremath{'}}$ model can account for either the electron or the muon $g\ensuremath{-}2$ anomalies, but not both, while remaining consistent with the experimental constraints from $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ and neutrino trident production.

Probing dark sectors with long-lived particles at Belle II

Abstract: We propose a new search for light scalar singlets in rare meson decays. For couplings well below the electroweak interaction strength, the scalar is long-lived at detector scales and decays into displaced pairs of leptons or light mesons. We show that Belle II has a remarkable potential to probe scalars in the GeV range with couplings as small as ${10}^{\ensuremath{-}5}$. The predicted sensitivity is higher than at the long-baseline experiments NA62 and FASER and comparable with projections for FASER 2. We also investigate signatures of invisibly decaying scalars in rare meson decays with missing energy.

Search for direct stau production in events with two hadronic τ-leptons in s =13 TeV pp collisions with the ATLAS detector

Abstract: A search for the direct production of the supersymmetric partners of τ -leptons (staus) in final states with two hadronically decaying τ -leptons is presented. The analysis uses a dataset of p p collisions corresponding to an integrated luminosity of 139 fb − 1 , recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of 13 TeV. No significant deviation from the expected Standard Model background is observed. Limits are derived in scenarios of direct production of stau pairs with each stau decaying into the stable lightest neutralino and one τ -lepton in simplified models where the two stau mass eigenstates are degenerate. Stau masses from 120 GeV to 390 GeV are excluded at 95% confidence level for a massless lightest neutralino.

An extended analysis of Heavy Neutral Leptons during Big Bang Nucleosynthesis

Abstract: Heavy Neutral Leptons (HNLs) are strongly motivated by theory due to their capability of simultaneously explaining the observed phenomena of dark matter, neutrino oscillations and the baryon asymmetry of the Universe. The existence of such particles would affect the expansion history of the Universe and the synthesis of primordial abundances of light elements. In this work we review, revise and extend the phenomenology of HNLs during the Big Bang Nucleosynthesis (BBN) epoch for masses up to 1 GeV. This is of great importance, as BBN is able to provide complementary bounds to those from upcoming and proposed laboratory experiments. To this end we have developed a high-precision Boltzmann code that simulates BBN in the presence of HNLs and takes into account all relevant HNL decay channels, as well as subsequent interactions of decay products (thermalization and decay showers), dilution due to QCD phase transition, active neutrino oscillations and corrections to the weak reaction rates. We present robust bounds on the lifetime and mixing angles of HNLs for masses $3\,\mathrm{MeV}\leq m_N \leq 1\,\mathrm{GeV}$ and show that BBN is able to constrain HNL lifetimes down to $0.03 - 0.05$ s, depending on the mixing pattern. Moreover, combining our results with current experimental searches, we can exclude HNLs that mix purely with electron neutrinos up to ${\sim}$450 MeV and those that mix purely with muon neutrinos up to ${\sim}$360 MeV, in both cases for lifetimes up to at least a few tens of seconds. Finally, we compare the BBN constraints with those obtained from Cosmic Microwave Background observations and explore how our results will be improved by a number of upcoming and proposed laboratory experiments.

Lepton-Quark Collisions at the Large Hadron Collider.

Abstract: Processes commonly studied at the Large Hadron Collider (LHC) are induced by quarks and gluons inside the protons of the LHC beams. In this Letter, we demonstrate that, since protons also contain leptons, it is possible to target lepton-induced processes at the LHC as well. In particular, by picking a lepton from one beam and a quark from the other beam, we present for the first time a comprehensive analysis of resonant single leptoquark (LQ) production at a hadron collider. In the case of minimal scalar LQs, we derive novel bounds that arise from the LHC Run II considering all possible flavor combinations of an electron or a muon and an up (u), a down (d), a strange, or a charm quark. For the flavor combinations with a u or a d quark, the obtained limits represent the most stringent constraints to date on LQs of this type. The prospects of our method at future LHC runs are also explored. Given the discovery reach of the proposed LQ signature, we argue that dedicated resonance searches in final states featuring a single light lepton and a single light-flavor jet should be added to the exotics search canon of both the ATLAS and the CMS Collaborations.