Search

We present a general purpose Monte Carlo program for the calculation of the radiative muon decay μ → e ν ν ¯ γ and the radiative decays τ → e ν ν ¯ γ and τ → μ ν ν ¯ γ at next-to-leading order in the Fermi theory. The full dependence on the lepton masses and polarization of the initial-sate lepton are kept. We study the branching ratios for these processes and show that fully-differential next-to-leading order corrections are important for addressing a tension between BaBar 's recent measurement of the branching ratio B ( τ → e ν ν ¯ γ ) and the Standard Model prediction. In addition, we study various distributions of the process μ → e ν ν ¯ γ and obtain precise predictions for the irreducible background to μ → e γ searches, tailored to the geometry of the MEG detector. [ABSTRACT FROM AUTHOR]

The differential cross sections for inclusive production of B + hadrons are measured as a function of the B + transverse momentum p T B and rapidity y B in pp collisions at a centre-of-mass energy of 13 TeV, using data collected by the CMS experiment that correspond to an integrated luminosity of 48.1 pb −1 . The measurement uses the exclusive decay channel B + → J / ψ K + , with J / ψ mesons that decay to a pair of muons. The results show a reasonable agreement with theoretical calculations within the uncertainties. [ABSTRACT FROM AUTHOR]

Neutrino mass hierarchy, CP-violation, and octant of θ are the fundamental unknowns in neutrino oscillations. In order to address all these three unknowns, we study the physics reach of a setup, where we replace the antineutrino run of T2HK with antineutrinos from muon decay at rest ( μ-DAR). This approach has the advantages of having higher statistics in both neutrino and antineutrino modes, and lower beam-on backgrounds for antineutrino run with reduced systematics. We find that a hybrid setup consisting of T2HK (ν) and μ-DAR $$ \left(\overline{\nu}\right) $$ in conjunction with full exposure from T2K and NO νA can resolve the issue of mass hierarchy at greater than 3 σ C.L. irrespective of the choices of hierarchy, δ , and θ . This hybrid setup can also establish the CP-violation at 5 σ C.L. for ∼ 55% choices of δ , whereas the same for conventional T2HK $$ \left(\nu + \overline{\nu}\right) $$ setup along with T2K and NOνA is around 30%. As far as the octant of θ is concerned, this hybrid setup can exclude the wrong octant at 5 σ C.L. if θ is at least 3° away from maximal mixing for any δ . [ABSTRACT FROM AUTHOR]

The measurement by LHCb suggests nonstandard Lepton-Universality Violation (LUV) to occur in decays with effects in muons rather than electrons. It is intriguing that a number of other measurements of transitions by LHCb and B-factories are consistent in magnitude and sign with the effect, and fit a coherent effective-theory picture. Further indications of nonstandard LUV are provided by the long-standing discrepancies in transitions via the ratios and . We review in detail the experimental situation and its rich outlook, the theoretical efforts - and their challenges - towards convincing dynamics beyond the effective-theory level, and discuss the many directions of further investigation that propagate from the current situation. [ABSTRACT FROM AUTHOR]

Using the automation program GoSam, fully differential NLO corrections were obtained for the rare decay of the muon μ → e ν ν ¯ e e . This process is an important Standard Model background to searches of the Mu3e Collaboration for lepton-flavor violation, as it becomes indistinguishable from the signal μ → 3 e if the neutrinos carry little energy. With our NLO program we are able to compute the branching ratio as well as custom-tailored observables for the experiment. With minor modifications, related decays of the tau can also be computed. [ABSTRACT FROM AUTHOR]

The production of triple Higgs (H+H-H0), (H+H-h0) and pairwise charged Higgs boson (H+H-) is studied in the context of future linear colliders within the two-Higgs-doublet model (2HDM) type II. The aim is to compare sources of charged Higgs pair through the above processes, that is, double and triple Higgs production. Cross sections are calculated at the leading order in 2HDM type II and Minimal Supersymmetric Standard Model (MSSM). Several orders of magnitude (~104) enhancement are observed in 2HDM compared to MSSM, while no sizable enhancement is seen in muon collider versus electron-positron collider. The analysis is based on a heavy charged Higgs with mass above 500 GeV. It is found that double charged Higgs production cross section (being the same in 2HDM and MSSM) is few femtobarns, while the triple Higgs production cannot exceed a fraction of femtobarn within the parameter space under study. [ABSTRACT FROM AUTHOR]

We find that the ratio r μe of the muon to the electromagnetic component of an extended air shower at the ground level provides an indirect measure of the depth X max  of the shower maximum. This result, obtained with the air-shower code AIRES, is independent of the hadronic model used in the simulation. We show that the value of r μe in a particular shower discriminates its proton or iron nature with a 98% efficiency. We also show that the eventual production of forward heavy quarks inside the shower may introduce anomalous values of r μe in isolated events. [ABSTRACT FROM AUTHOR]

The ALICE Collaboration studied the inclusive J/ ψ and ψ (2S) production in p-Pb collisions at the nucleon-nucleon centre of mass energy s NN = 5.02 TeV . The J/ ψ and the ψ (2S) are detected through their decays into muon pairs and the J/ ψ is also studied in the dielectron decay channel. The J/ ψ and ψ (2S) nuclear modification factors and the double ratio [ σ ψ ( 2 S ) / σ J / ψ ] pPb / [ σ ψ ( 2 S ) / σ J / ψ ] pp are presented as a function of the collision centrality. Our measurements are compared to theoretical calculations. [ABSTRACT FROM AUTHOR]

Abstract The installation of the Muon Telescope Detector (MTD) at STAR allows a measurement of the dimuon (μ + μ −) production in heavy-ion collisions over a large invariant mass range for the first time. Data has been collected with the MTD from Au+Au collisions at s N N = 200 GeV and from p+p collisions at s = 200 GeV. These two datasets allow for new opportunities to measure the dimuon invariant mass spectra at STAR. Before any dimuon measurements can be made, muons must be identified. This contribution presents muon identification employing deep neural networks (DNN) and compares it with other multi-variate techniques. Applications of the DNN technique for data-driven purity measurements are discussed. [ABSTRACT FROM AUTHOR]

The physics programme of the MEG II experiment can be extended with the search for new invisible particles produced in rare muon decays. The hunt for such elusive signals requires accurate simulations to characterise the detector response and estimate the experimental sensitivity. This work presents an improved simulation of muon decay in MEG II, based on McMule and Geant4. [ABSTRACT FROM AUTHOR]

We point out that when doubly-charged bileptons are pair produced at the LHC, kinematics dictate that they are both almost at rest in the lab frame and therefore their decays lead to final state muons in a characteristic X-shape with only very tiny track curvature because of the high muon energies. Such X-events have essentially no Standard Model background and provide a smoking gun for the 331 model. [ABSTRACT FROM AUTHOR]

The polarizations of the ϒ ( 1 S ) , ϒ ( 2 S ) , and ϒ ( 3 S ) mesons are measured as a function of the charged particle multiplicity in proton–proton collisions at s = 7 TeV . The measurements are performed with a dimuon data sample collected in 2011 by the CMS experiment, corresponding to an integrated luminosity of 4.9 fb −1 . The results are extracted from the dimuon decay angular distributions, in two ranges of ϒ ( n S ) transverse momentum (10–15 and 15–35 GeV), and in the rapidity interval | y | < 1.2 . The results do not show significant changes from low- to high-multiplicity pp collisions, although large uncertainties preclude definite statements in the ϒ ( 2 S ) and ϒ ( 3 S ) cases. [ABSTRACT FROM AUTHOR]

This paper describes a measurement of the inclusive top quark pair production cross-section ( σ t t ¯ ) with a data sample of 3.2 fb − 1 of proton–proton collisions at a centre-of-mass energy of s = 13 TeV , collected in 2015 by the ATLAS detector at the LHC. This measurement uses events with an opposite-charge electron–muon pair in the final state. Jets containing b -quarks are tagged using an algorithm based on track impact parameters and reconstructed secondary vertices. The numbers of events with exactly one and exactly two b -tagged jets are counted and used to determine simultaneously σ t t ¯ and the efficiency to reconstruct and b -tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section is measured to be: σ t t ¯ = 818 ± 8 ( stat ) ± 27 ( syst ) ± 19 ( lumi ) ± 12 ( beam ) pb , where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the integrated luminosity and the LHC beam energy, giving a total relative uncertainty of 4.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. A fiducial measurement corresponding to the experimental acceptance of the leptons is also presented. [ABSTRACT FROM AUTHOR]

The MEG experiment searches for the muon lepton flavor violating decay, μ + → e + γ . An upgrade of the experiment is ongoing, aiming at reaching a sensitivity of Br ( μ + → e + γ ) = 4 × 10 − 14 , an order of magnitude better than the sensitivity of the current MEG. To achieve this goal, all of the detectors are being upgraded. In MEG, the energy, position and timing of the gamma ray were measured by a liquid Xe calorimeter, which consists of 900 l of liquid Xe and 846 2-in. round-shaped photo-multiplier tubes (PMTs). In the upgrade, the granularity at the gamma ray incident face will be improved by replacing 216 PMTs with 4092 SiPMs (MPPCs) with an active area of 12×12 mm 2 each. The energy resolution for the gamma ray is expected to improve by a factor of 2, because the efficiency to collect scintillation light will become more uniform. The position resolution is also expected to improve by a factor of 2. In collaboration with Hamamatsu Photonics K.K., we have successfully developed a high performance MPPC for our detector. It has excellent photon detection efficiency for the liquid xenon scintillation light in VUV range. The size of the chips is large so that it can cover large area with a manageable number of readout channels. The characteristics of the MPPCs are being tested in liquid Xe, and also at the room temperature. The results of the tests will be presented, together with the expected performance of the upgraded detector. [ABSTRACT FROM AUTHOR]

In this paper the first study of the upward going events detected by the telescopes of the Extreme Energy Event (EEE) project is reported. The EEE project consists of a detector array of Multigap Resistive Plate Chambers located at selected sites on the Italian territory. During autumn 2014 the first coordinated data taking period took place and around one billion candidate tracks were collected. Among them, of particular interest is the sample of particles which cross the telescopes from below. The results obtained demonstrate that the EEE telescopes can distinguish the electrons produced as decay products of cosmic muons stopped in the ground, or in the last chamber of the telescopes themselves, confirming the excellent performance of the system for the investigation of intriguing cosmic phenomena. [ABSTRACT FROM AUTHOR]

We determine the O ( α ) correction to the energy spectrum of electrons produced in the decay of muons bound in atoms. We focus on the high-energy end of the spectrum that constitutes a background for the muon–electron conversion and will be precisely measured by the upcoming experiments Mu2e and COMET. The correction suppresses the background by about 15%. [ABSTRACT FROM AUTHOR]

The nuSTORM (neutrinos from STORed Muons) proposal presents a detailed design for a neutrino facility based on a muon storage ring, with muon decay in the production straight section of the ring providing well defined neutrino beams. The facility includes a primary high-energy proton beam line, a target station with pion production and collection, and a pion beamline for pion transportation and injection into a muon decay ring. The nuSTORM design uses “stochastic injection”, in which pions are directed by a chicane, referred to as the Orbit Combination Section (OCS), into the production straight section of the storage ring. Pions that decay within that straight section provide muons within the circulating acceptance of the ring. The design enables injection without kickers or a separate pion decay transport line. The beam line that the pions traverse before being extracted from the decay ring is referred to as the pion beamline. This paper describes the design and simulation of the pion beamline, and includes full beam dynamics simulations of the system. [ABSTRACT FROM AUTHOR]

We summarize the recent progress in evaluation of radiative corrections to bound muon decay spectrum. Calculated corrections reconcile the TWIST measurement with the theoretical prediction. Corrections in the endpoint region of the spectrum affect the sensitivity of the muon electron conversion searches. [ABSTRACT FROM AUTHOR]

The Large Hadron Collider (LHC) at CERN has allowed the ATLAS and CMS experiments to collect a large amount of proton-proton collision data at 7 TeV and 8 TeV centre-of-mass energies. This dataset was used to discover a Higgs boson with Standard Model-like properties at a mass of about 125 GeV. In parallel, an impressive number of searches for deviations from the Standard Model expectations have been carried out in various physics areas (BSM Higgs, Supersymmetry, and Exotics). Representative searches in all of these domains will be presented and their impact on theoretical models beyond the SM assessed. To date, there is no evidence in the LHC data of a signal for New Physics beyond the SM. However, a few legacy 2-3δ deviations remain from both experiments. After an 18-month shutdown, the LHC is about to deliver collision data at an increased centre-of-mass energy of 13 TeV. The large increase in collision energy will provide much improved sensitivity for various searches, in particular for high mass particles. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The forward acceptance of the LHCb detector allows it to probe proton-ion collisions in a unique kinematic range, complementary to the other LHC experiments. The production of J/Ψ and Υ-mesons decaying into two muons is studied at the LHCb experiment in proton-lead collisions at a proton-nucleon centre-of-mass energy √ sNN = 5 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 1.6 nb-1. The nuclear modification factor and the forward-backward production ratio are determined for J/Ψ and Υ(1S) mesons. Clear suppression of prompt J/Ψ production is observed with respect to the production in proton-proton collisions at large rapidity, while the suppression of J/Ψ from b-hadron decays is less pronounced. The nuclear modification factor for Υ(1S) mesons in the forward region is found to be similar to those for J/Ψ from b-hadron decays. Furthermore, a first observation of Z bosons at large rapidities in proton-lead collisions is reported. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The ATLAS Collaboration measures the inclusive production of Z bosons via their decays into electron and muon pairs in p + Pb collisions at √sNN = 5.02 TeV at the Large Hadron Collider. The measurements are made using data corresponding to integrated luminosities of 29.4 and 28.1 nb-1 for Z → ee and Z → µµ, respectively. The results from the two channels are consistent and combined to obtain a cross section times the Z → ℓ branching ratio, integrated over the rapidity region |y* Z| < 3.5, of 139.8 ± 4.8 (statistical) ± 6.2 (systematic) ± 3.8 (luminosity) nb. Differential cross sections are presented as functions of the Z boson rapidity and transverse momentum and compared with models based on parton distributions both with and without nuclear corrections. The centrality dependence of Z boson production in p + Pb collisions is measured and analyzed within the framework of a standard Glauber model and the model's extension for fluctuations of the underlying nucleon-nucleon scattering cross section. [ABSTRACT FROM AUTHOR]

The PHENIX Collaboration has measured ϕ meson production in d+Au collisions at √sNN=200 GeV using the dimuon and dielectron decay channels. The ϕ meson is measured in the forward (backward) d-going (Au-going) direction, 1.2

This paper describes the strategy for optimizing the magnetic horn for the neutrinos from STORed Muons (nuSTORM) facility. The nuSTORM magnetic horn is the primary collection device for the secondary particles generated by bombarding a solid target with 120 GeV protons. As a consequence of the non-conventional beamline designed for nuSTORM, the requirements on the horn are different from those for a conventional neutrino beamline. At nuSTORM, muons decay while circulating in the storage ring, and the detectors are placed downstream of the production straight so as to be exposed to the neutrinos from muon decay. nuSTORM aims at precisely measuring the neutrino cross sections, and providing a definitive statement about the existence of sterile neutrinos. The nuSTORM horn aims at focusing the pions into a certain phase space so that more muons from pion decay can be accepted by the decay ring. The paper demonstrates a numerical method that was developed to optimize the horn design to gain higher neutrino flux from the circulating muons. A Genetic Algorithm (GA) was applied to the simultaneous optimization of the two objectives in this study. The application of the technique discussed in this paper is not limited to either the nuSTORM facility or muon based facilities, but can be used for other neutrino facilities that use magnetic horns as collection devices. [ABSTRACT FROM AUTHOR]

A search is performed for heavy Majorana neutrinos (N) using an event signature defined by two muons of the same charge and two jets ( μ ± μ ± jj ). The data correspond to an integrated luminosity of 19.7 fb −1 of proton–proton collisions at a center-of-mass energy of 8 TeV, collected with the CMS detector at the CERN LHC. No excess of events is observed beyond the expected standard model background and upper limits are set on | V μ N | 2 as a function of Majorana neutrino mass m N for masses in the range of 40–500 GeV, where V μ N is the mixing element of the heavy neutrino with the standard model muon neutrino. The limits obtained are | V μ N | 2 < 0.00470 for m N = 90 GeV , | V μ N | 2 < 0.0123 for m N = 200 GeV , and | V μ N | 2 < 0.583 for m N = 500 GeV . These results extend considerably the regions excluded by previous direct searches. [ABSTRACT FROM AUTHOR]

Today, high position and timing resolutions can be simultaneously achieved using scintillating fibers coupled to silicon photomultipliers. In the framework of the MEGII experiment (MEG upgrade) which searches for the μ + → e + γ decay we are developing an active muon stopping target of 250 μ m square scintillating fibers coupled to silicon photomultipliers. This tool should provide an unique way to continuously monitor the beam (detecting the stopped muons) at the highest muon beam intensities in the world, and to measure the muon decay vertex (detecting the outgoing positron). A similar technology will also be applicable to the Mu3e experiment which searches for the μ + → e + e − e + decay. In this experiment a timing hodoscope, which complements the silicon tracker, will be made by few layers of 250 μ m square or round scintillating fibers, providing timing measurements with a resolution < 1 ns . In this work we report the results obtained with the current prototypes showing that spatial resolutions at a level of 70 μ m and timing resolutions of the order of 350 ps can be reached with a detection efficiency ≥ 90 % . [ABSTRACT FROM AUTHOR]

We derive the necessary conditions to build a class of invisible axion models with Flavor Changing Neutral Currents at tree-level controlled by the fermion mixing matrices and present an explicit model implementation. A horizontal Peccei–Quinn symmetry provides a solution to the strong CP problem via the Peccei–Quinn mechanism and predicts a cold dark mater candidate, the invisible axion or familon. The smallness of active neutrino masses can be explained via a type I seesaw mechanism, providing a dynamical origin for the heavy seesaw scale. The possibility to avoid the domain wall problem stands as one of the most interesting features of the type of models considered. Experimental limits relying on the axion–photon coupling, astrophysical considerations and familon searches in rare kaon and muon decays are discussed. [ABSTRACT FROM AUTHOR]

The μ → eγ decay is strictly forbidden in the Standard Model, where the lepton flavour is conserved as an accidental symmetry, but in extensions of the Standard Model the lepton flavour is generally no longer conserved, so that decay can take place. In this paper we calculate the amplitude, in an Rξ gauge, of muon decay in the small-neutrino-mass limit. The result, in which ξ-dependence is cancelled, is different from the amplitude and the technical points obtained in previous work. [ABSTRACT FROM AUTHOR]