Leptonic Processes Research Articles

Galactic Stellar Black Hole Binaries: Spin Effects on Jet Emissions of High-Energy Gamma-Rays

In the last few decades, galactic stellar black hole X-ray binary systems (BHXRBs) have aroused intense observational and theoretical research efforts specifically focusing on their multi-messenger emissions (radio waves, X-rays, γ-rays, neutrinos, etc.). In this work, we investigate jet emissions of high-energy neutrinos and gamma-rays created through several hadronic and leptonic processes taking place within the jets. We pay special attention to the effect of the black hole’s spin (Kerr black holes) on the differential fluxes of photons originating from synchrotron emission and inverse Compton scattering and specifically on their absorption due to the accretion disk’s black-body radiation. The black hole’s spin (dimensionless spin parameter a*) enters into the calculations through the radius of the innermost circular orbit around the black hole, the RISCO parameter, assumed to be the inner radius of the accretion disk, which determines its optical depth τdisk. In our results, the differential photon fluxes after the absorption effect are depicted as a function of the photon energy in the range 1GeV ≤E≤103GeV. It is worth noting that when the black holes’ spin (α*) increases, the differential photon flux becomes significantly lower.

Analysis of γ-ray emission above 30 GeV from the LMC pulsar wind nebula N 157B with Fermi-LAT

ABSTRACT N 157B located in the Large Magellanic Cloud is the first pulsar wind nebula detected outside of the Galaxy in γ-rays. In this paper, we analyse the emission above 30 GeV from N 157B using ∼14.3 yr of Fermi-LAT data. The γ-ray spectrum between 30 and 500 GeV is well described by a single power-law function with a photon index of 1.83 ± 0.26, and its integral photon flux is (4.10 ± 0.83) × 10−11 photons cm−2 s−1. We adopt a one-zone leptonic model to investigate whether the multiband non-thermal emission of the target source can be generated by synchrotron radiation and inverse-Compton scattering of the electrons/positrons. Assuming the electrons/positrons in the nebula have a broken power-law spectrum with two breaks, the model can reproduce the observed fluxes in the radio, X-ray, and γ-ray bands. This result indicates that the γ-ray emission from N 157B can be explained by the leptonic process of electrons/positrons via inverse-Compton scattering.

Predictions of the ratio and asymmetry probes of the invisible Z-boson decay

Higher-order predictions through the combined accuracy including next-to-leading order (NLO) electroweak (EW) and next-to-NLO (NNLO) quantum chromodynamics (QCD) corrections in underlying perturbation theories are presented thoroughly for the invisible decay of the Z boson into neutrino pair relative to its decay into charged-lepton pair (leptonic decay). The combined NNLO QCD+NLO EW predictions are achieved based on the fully-differential calculations of cross sections of both the invisible and leptonic processes in proton-proton (pp) collisions at 13 TeV center-of-mass energy. Differential distributions of cross-section ratios of the invisible process to the leptonic process are presented as a function of the transverse momentum of the Z boson . For the first time, the predictions for differential distributions of cross-section asymmetries between the invisible process and the leptonic process are presented in bins of the . The cross-section ratio and asymmetry distributions, which are referred to as the invisible probes, are considered to be important for controlling the invisible process by the leptonic process of the Z boson and probing deviation from the Standard Model (SM) for new-physics searches. The predictions are extensively presented beyond the Z-boson mass resonance region to assess the potential of the invisible ratio and asymmetry probes for new-phenomena searches in high-invariant mass region of the lepton-pair final states. Various tests with threshold requirements of transverse momenta of neutrino pair and leptons are performed to assess the impact on the combined predictions. The invisible ratio and asymmetry probes are proposed to be important probes for indirect searches of new-physics scenarios.

Rare leptonic processes induced by massless dark photon

We introduce a dark photon considering a U(1) gauge extension of the standard model in particle physics. Provided that the extra U(1) symmetry is unbroken, the dark photon is massless and has no coupling to the standard electromagnetic current. Higher-dimensional operators describe interactions of the massless dark photon with particles in the standard model. We investigate the interactions of the massless dark photon with charged leptons via dipole operators, mainly focusing on the lepton family-violating processes. We present an improved constraint in the polarized two-body muon decay and a set of new bounds in tau decays. We also examine possible lepton family-violating signals of the massless dark photon in future lepton colliders.

Charged Higgs induced 5 and 6 lepton signatures from heavy neutrinos at the LHC

We propose an anomaly free gauged U(1) extension of the SM where three right handed heavy neutrinos, being charged under the general U(1) gauge group, are introduced to explain the origin of the tiny neutrino mass through the seesaw mechanism after the general U(1) symmetry is broken. Due to the breaking of the general U(1) symmetry a neutral beyond the standard model gauge boson Z^prime acquires mass. There are two Higgs doublets in this model where one interacts with the SM fermions and the other one interacts with the right handed heavy neutrinos and charged leptons. The charged multiplet of the second Higgs can completely decay into the heavy neutrinos and charged lepton in the neutrinophilic limit of the model parameters. The charged Higgs pair production can be influenced due to presence of the Z^prime boson at the High Luminosity LHC (HL-LHC) in addition to the neutral SM gauge bosons. The pair produced charged Higgs bosons decay into SM charged leptons and heavy neutrinos. Following the leading decay modes of the heavy neutrinos into charged leptons and W boson we study the 5 and 6 lepton final states after the leptonic and hadronic decay of the W bosons considering solely muons and electrons in the final state. Combining the electron and muon final states we estimate the significance of the 5 and 6 charged lepton processes in the m_{H^pm }-m_N plane for different benchmark points of m_{Z^prime }. It is found that the 5 (6) charged lepton processes could be probed at the High Luminosity LHC (HL-LHC) with at least 5 (3) sigma significance and there are parameter regions where the significance could be larger.

Multiwavelength analysis of Galactic Supernova Remnants

The origin of Galactic Cosmic Rays (CRs) and the possibility of Supernova Remnants (SNRs) being potential CR accelerators is still an open debate. The charged CRs can be detected indirectly by the γ-ray observatories through the π 0 production and consequent decay, leading to the generation of high-energy γ-rays. The goal of the study is to identify qualitative and quantitative trends in favour of hadronic scenario and search for SNRs which could be potential accelerators up to PeV energies (PeVatrons).We have performed a Multiwavelength (MWL) study using different radiative models to evaluate the hadronic contribution. The spectral energy distributions (SEDs) of selected SNRs are modeled using the Naima [1] package. Two different radiative scenarios are considered, pure leptonic and lepto-hadronic scenarios and different methods are used to evaluate their importance.This study shows that the lepto-hadronic scenario is favored for most SNRs. Two particular indicators of hadronic contribution come from the data around the π 0 production threshold and the data above a few TeV. The hard rise at the π 0 production threshold cannot be explained by leptonic processes. More data in this region would be valuable for these studies. For some SNRs, an important hadronic contribution is observed up to a few TeV, thus making them promising PeVatron candidates. In this high-energy region where the leptonic processes are expected to be suppressed, more data is required to help distinguish between the leptonic and hadronic origin of γ-ray emission. In the future, we intend to use the obtained model parameters to simulate data for CTA and assess its capability to identify PeVatrons.

Radio and infrared study of the supernova remnant candidate HESS J1912+101

Aims.We provide new insights into theγ-ray emission from HESS J1912+101, a TeV supernova remnant candidate probably associated with the radio pulsar PSR J1913+1011.Methods.We obtained new observations at 1.5 GHz using the VLA in the D configuration, with the purpose of detecting the radio shell of the putative remnant. In addition, we observed a single pointing at 6.0 GHz toward PSR J1913+1011 to look for a radio pulsar wind nebula. We also studied the properties of the surrounding interstellar medium using data of the13CO, HI, and infrared emissions, obtained from public surveys.Results.We do not find evidence of a radio shell down to the sensitivity of the new image at 1.5 GHz. We detect faint diffuse emission around PSR J1913+1011 at 6.0 GHz, which could represent a radio pulsar wind nebula powered by the pulsar. We find dense ambient gas at ~60 km s−1, which shows a good spatial correspondence with the TeV emission only in the western and eastern directions. There is also dense gas near the center of HESS J1912+101, where the TeV emission is weak. Using infrared data, we identify an active star-forming region in the western part of the shell.Conclusions.Based on the poor spatial match between the ambient gas and the TeV emission (which shows a good correlation in the western and eastern directions and an anticorrelation in the other directions), we conclude that the hadronic mechanism alone does not give a satisfactory explanation of theγrays from HESS J1912+101. Additional contributions may come from leptonic processes in the shell of the supernova remnant, together with contributions from PSR J1913+1011 and its pulsar wind nebula and/or from the star-forming region. A confident determination of the distance to the putative remnant is necessary to determine whether these sources are associated or just appear superimposed in the line of sight.

Constraints on lepton universality violation from rare B decays

The LHCb Collaboration has recently released a new study of ${B}^{+}\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ and $B\ensuremath{\rightarrow}{K}^{*0}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ ($\ensuremath{\ell}=e$, $\ensuremath{\mu}$) decays, testing lepton universality with unprecedented accuracy using the whole Run 1 and 2 dataset. In addition, the CMS Collaboration has reported an improved analysis of the branching ratios ${B}_{(d,s)}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$. While these measurements offer, per se, a powerful probe of new physics, global analyses of $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ transitions also rely on the assumptions about nonperturbative contributions to the decay matrix elements. In this work, we perform a global Bayesian analysis of new physics in (semi)leptonic rare $B$ decays, paying attention to the role of charming penguins which are difficult to evaluate from first principles. We find data to be consistent with the Standard Model once rescattering from intermediate hadronic states is included. Consequently, we derive stringent bounds on lepton universality violation in $|\mathrm{\ensuremath{\Delta}}B|=|\mathrm{\ensuremath{\Delta}}S|=1$ (semi)leptonic processes.

W±Z production at NNLO QCD and NLO EW matched to parton showers with MiNNLOPS

We consider W±Z production in hadronic collisions and present high-precision predictions in QCD and electroweak (EW) perturbation theory matched to parton showers. To this end, we match next-to-next-to-leading order QCD corrections to parton showers using the MiNNLOPS method and consistently combine them with next-to-leading order EW corrections matched to parton showers. This is the first time such accuracy in the event generation is achieved for any collider process, and we study in detail the impact of different choices in the combination of QCD and EW corrections as well as QCD and QED showers. Spin correlations, interferences and off-shell effects are retained by considering the full leptonic processes ppto {ell}^{+}{ell}^{-}{ell}^{prime pm }{nu}_{ell}^{prime } with ℓ′ ≠ ℓ and ℓ′ = ℓ without approximations, and the matching to QED radiation is performed preserving the resonance structure of the process. We find that NNLO QCD predictions including QCD and QED shower effects provide a very good approximation in the bulk-region of the phase space, while EW effects become increasingly important in the high-energy tails of kinematic distributions. Our default predictions are in excellent agreement with recent ATLAS data.

Magnetic moments of leptons, charged lepton flavor violations and dark matter phenomenology of a minimal radiative Dirac neutrino mass model

In a simple extension of the standard model (SM), a pair of vector like lepton doublets (L1 and L2) and a SU(2)L scalar doublet (η) have been introduced to help in accommodating the discrepancy in determination of the anomalous magnetic moments of the light leptons, namely, e and μ. Moreover, to make our scenario friendly to a Dirac like neutrino and also for a consistent dark matter phenomenology, we specifically add a singlet scalar (S) and a singlet fermion (ψ) in the set-up. However, the singlet states also induce a meaningful contribution in other charged lepton processes. A discrete symmetry {mathcal{Z}}_2times {mathcal{Z}}_2^{prime } has been imposed under which all the SM particles are even while the new particles may be assumed to have odd charges. In a bottom-up approach, with a minimal particle content, we systematically explore the available parameter space in terms of couplings and masses of the new particles. Here a number of observables associated with the SM leptons have been considered, e.g., masses and mixings of neutrinos, (g − 2) anomalies of e, μ, charged lepton flavor violating (cLFV) observables and the dark matter (DM) phenomenology of a singlet-doublet dark matter. Neutrinos, promoted as the Dirac type states, acquire mass at one loop level after the discrete {mathcal{Z}}_2^{prime } symmetry gets softly broken, while the unbroken {mathcal{Z}}_2 keeps the dark matter stable. The mixing between the singlet ψ and the doublet vector lepton can be constrained to satisfy the electroweak precision observables and the spin independent (SI) direct detection (DD) cross section of the dark matter. In this analysis, potentially important LHC bounds have also been discussed.

Emission from hadronic and leptonic processes in galactic jet-driven bubbles

ABSTRACT We investigate the multiwavelength emission from hadronic and leptonic cosmic rays (CRs) in bubbles around galaxies, analogous to the Fermi bubbles of the Milky Way. The bubbles are modelled using 3D magnetohydrodynamical simulations, and are driven by a 0.3 Myr intense explosive outburst from the nucleus of Milky Way-like galaxies. We compute their non-thermal emission properties at different stages throughout their evolution, up to 7 Myr, by post-processing the simulations. We compare the spectral and spatial signatures of bubbles with hadronic, leptonic, and hybrid hadro-leptonic CR compositions. These each show broadly similar emission spectra, comprised of radio synchrotron, inverse Compton, and non-thermal bremsstrahlung components. However, hadronic and hybrid bubbles were found to be brighter than leptonic bubbles in X-rays, and marginally less bright at radio frequencies, and in γ-rays between ∼0.1 and a few 10s of GeV, with a large part of their emission being driven by secondary electrons formed in hadronic interactions. Hadronic systems were also found to be slightly brighter in high-energy γ-rays than their leptonic counterparts, owing to the π0 decay emission that dominates their emission between energies of 100s of GeV and a few TeV.

Probing the Origin of Cosmic Rays in Cygnus Cocoon Using Ultrahigh-energy Gamma-Ray and Neutrino Observations

Recent ultrahigh-energy gamma-ray observations by the High Altitude Water Cherenkov Observatory up to 100 TeV and LHAASO observatories up to 1.4 PeV energies from the direction of Fermi Large Area Telescope 4FGL source 4FGL J2028.6 + 4110e (Cygnus Cocoon) are indicative of a hadronic origin over a leptonic process for their creation. The IceCube Neutrino Observatory has reported IceCube-201120A, a neutrino event coming from the same direction, suggesting that the Cygnus Cocoon may correspond to one of the most plausible sources of high-energy cosmic rays. The apparent relationship of the neutrino event with the observed ultrahigh-energy gamma rays from the Cygnus Cocoon is investigated in this work to study if it can be explained consistently in hadronic interactions of accelerated cosmic rays with ambient matter. Our findings reveal that leptonic mechanisms, together with pure hadronic mechanisms, make a considerable contribution to the understanding of the total electromagnetic spectrum as well as the observed neutrino event. The estimate of expected muon neutrino events from the Cygnus Cocoon agrees with the one muon neutrino event detected so far in IceCube multiyear observations. Thus, our results are indicative of the potential of the Cygnus Cocoon to be a Galactic cosmic-ray source capable of accelerating at least up to PeV energies.

An asymmetrical model for high-energy radiation of Cassiopeia A

ABSTRACT The supernova remnant Cassiopeia A (Cas A) shows strong radiation from radio to gamma-ray bands. The mechanism of gamma-ray radiation in Cas A and its possible contribution to PeV cosmic rays are still under debate. The X-ray imaging reveals an asymmetric profile of Cas A, suggesting the existence of a jet-like structure. In this paper, we propose an asymmetrical model for Cas A, consisting of a fast moving jet-like structure and a slowly expanding isotropic shell. This model can account for the multiwavelength spectra of Cas A, especially for the power-law hard X-ray spectrum from ∼60 to 220 keV. The GeV to TeV emission from Cas A should be contributed by both hadronic and leptonic processes. Moreover, the jet-like structure may produce a gamma-ray flux of $\sim 10^{-13}\rm erg\, cm^{-2}\, s^{-1}$ at ∼100 TeV, to be examined by the Large High Altitude Air Shower Observatory (LHAASO) and the Cherenkov Telescope Array (CTA).

Flavor specific [formula omitted] gauge model for muon g − 2 and [formula omitted] anomalies

The muon (g−2)μ and b→sμ¯μ induced B anomalies as hints of new physics beyond the standard model (SM) have attracted much attention. These two anomalies indicate that there may exist new interaction specifically related to muon. A lot of theoretical ideas have been proposed to explain these anomalies. Gauged flavor specific U(1)Bq−Lμ is among the promising ones. The new gauge boson Z′ from U(1)Bq−Lμ interacts with muon and provides necessary ingredient to solve the (g−2)μ anomaly. The Z′-quark coupling can generate flavor changing interactions after diagonalization of quark mass matrix between weak eigen-state and mass eigen-state basis. We revisit challenges for such models attempting to explain the (g−2)μ and B anomalies separately or simultaneously. We find although for U(1)Bq−Lμ models there is still parameter space to provide solutions for separately explaining the (g−2)μ and B anomalies, there exists no parameter space for such models to solve both the anomalies simultaneously, after taking into account existing constraints from τ→μγ, τ→3μ, neutrino trident and Bs−B¯s data. Among them leptonic processes restrict Z′ mass to be less than a few hundred MeV if required to solve the (g−2)μ anomaly, which causes conflict between data from Bs−B¯s, D0−D¯0 mixing and also hadron decays with Z′ in the final states. The effects of U(1)Y and U(1)Bq−Lμ kinetic mixing on these anomalies are also studied. We find that neither can these effects do much to bring the two anomalies together to be solved simultaneously.

RGE effects on the LFV scale from meson decays

We consider the lepton-flavor violating (LFV) lepton-quark dimension-6 operators and analyze their contributions to the LFV leptonic decays of vector, pseudoscalar, and scalar neutral mesons Mrightarrow ell _1 ell _2 as well as to mu (tau ) rightarrow ell ee, ell gamma gamma decays. These operators contribute to the purely leptonic processes via quark loop. On the basis of quark-hadron duality, we relate these loops to the appropriate meson-exchange contributions. In this way, we extract lower bounds on the individual scales of the studied LFV operators from the experimental and phenomenological limits on the leptonic decays of mesons and leptons. As a byproduct, we shall obtain new limits on the LFV leptonic decays of flavored mesons from the experimental bounds on the three-body lepton decays. We study the effects of QED and QCD radiative corrections to the LFV lepton-quark operators in question. We derive for them the one-loop matrix of the RGE evolution and examine its effect on the previously derived tree-level limits on these operators. We show that the QED corrections are particularly relevant due to operator mixing. Specifically, for some of them the limits on their individual LFV scales improve by up to 3 orders of magnitude.

Multiwavelength emission from leptonic processes in ageing galaxy bubbles

ABSTRACT The evolutionary behaviour and multiwavelength emission properties of bubbles around galaxies, such as the Fermi bubbles of the Milky Way, is unsettled. We perform 3D magneto-hydrodynamical simulations to investigate the evolution of leptonic galaxy bubbles driven by a 0.3-Myr intense explosive outburst from the nucleus of Milky-Way-like galaxies. Adopting an ageing model for their leptonic cosmic rays, we post-process our simulations to compute the multiwavelength emission properties of these bubbles. We calculate the resulting spectra emitted from the bubbles from radio frequencies to γ-rays, and construct emission maps in four energy bands to show the the development of the spatial emission structure of the bubbles. The simulated bubbles show a progression in their spectral properties as they age. In particular, the TeV γ-ray emission is initially strong and dominated by inverse Compton scattering, but falls rapidly after ∼1 Myr. In contrast, the radio synchrotron emission remains relatively stable and fades slowly over the lifetime of the bubble. Based on the emission properties of our post-processed simulations, we demonstrate that γ-ray observations will be limited in their ability to detect galaxy bubbles, with only young bubbles around nearby galaxies being within reach. However, radio observations with, e.g. the upcoming Square Kilometer Array, would be able to detect substantially older bubbles at much greater distances, and would be better placed to capture the evolutionary progression and diversity of galaxy bubble populations.

Bulk viscosity from Urca processes: npeμ matter in the neutrino-trapped regime

In this work, we extend our previous study of the bulk viscosity of hot and dense $npe$ matter induced by the Urca processes in the neutrino trapped regime to $npe\ensuremath{\mu}$ matter by adding the muonic Urca processes as well as the purely leptonic electroweak processes involving electron-muon transition. The nuclear matter is modeled in a relativistic density functional approach with two different parametrizations which predict neutrino dominated matter (DDME2 model) and antineutrino dominated matter (NL3 model) at temperatures for which neutrinos/antineutrinos are trapped. In the case of neutrino-dominated matter, the main equilibration mechanism is lepton capture, whereas in the case of antineutrino-dominated matter this is due to neutron decay. We find that the equilibration rates of Urca processes are higher than that of the pure leptonic processes, which implies that the Urca-process-driven bulk viscosity can be computed with the leptonic reactions assumed to be frozen. We find that the bulk viscosity decreases with temperature as $\ensuremath{\zeta}\ensuremath{\sim}{T}^{\ensuremath{-}2}$ at moderate temperatures. At high temperatures this scaling breaks down by sharp drops of the bulk viscosity close to the temperature where the proton fraction is density-independent and the matter becomes scale-invariant. This occurs also when the matter undergoes a transition from the antineutrino-dominated regime to the neutrino-dominated regime where the bulk viscosity attains a local maximum. We also estimate the bulk viscous dissipation timescales and find that these are in the $\mathrm{range}\ensuremath{\gtrsim}1\text{ }\text{ }\mathrm{s}$ for temperatures above the neutrino trapping temperature. These timescales would be relevant only for long-lived objects formed in binary neutron star mergers and hot proto-neutron stars formed in core-collapse supernovas.

Bounds on charged-lepton flavor violations via resonant scattering

We explore the possibility of probing flavor violations in the charged-lepton sector by means of high-luminosity lepton-photon and electron-muon collisions, by inverting initial and final states in a variety of decay channels presently used to bound such violations. In particular, we analyze the resonant lepton, γℓ→ℓ′, and neutral-meson, e−μ+→ϕ,η,π0, scattering channels, whose cross sections are critically dependent on the colliding-beams energy spread, being particularly demanding in the case of leptonic processes. For these processes, we compute upper bounds to the cross-section corresponding to present limits on the inverse decay channel rates. In order to circumvent the beam energy spread limitations we extend the analysis to processes in which a photon accompanies the resonance in the final state, compensating the off-shellness effects by radiative return. These processes might be studied at future facilities with moderate energies, in case lepton-photon and electron-muon collisions with sufficiently high luminosity will be available.

Determination of collinear and TMD photon densities using the Parton Branching method

We present the first determination of transverse momentum dependent (TMD) photon densities with the Parton Branching method. The photon distribution is generated perturbatively without intrinsic photon component. The input parameters for quarks and gluons are determined from fits to precision measurements of deep inelastic scattering cross sections at HERA. The TMD densities are used to predict the mass and transverse momentum spectra of very high mass lepton pairs from both Drell-Yan production and Photon-Initiated lepton processes at the LHC.

TeV Cosmic-Ray Nucleus Acceleration in Shell-type Supernova Remnants with Hard γ-Ray Spectra

Abstract The emission mechanism for hard γ-ray spectra from supernova remnants (SNRs) is still a matter of debate. Recent multiwavelength observations of the TeV source HESS J1912+101 show that it is associated with an SNR with an age of ∼100 kyr, making it unlikely produce the TeV γ-ray emission via leptonic processes. We analyzed Fermi observations of it and found an extended source with a hard spectrum. HESS J1912+101 may represent a peculiar stage of SNR evolution that dominates the acceleration of TeV cosmic rays. By fitting the multiwavelength spectra of 13 SNRs with hard GeV γ-ray spectra with simple emission models with a density ratio of GeV electrons to protons of ∼10−2, we obtain reasonable mean densities and magnetic fields with a total energy of ∼1050 erg for relativistic ions in each SNR. Among these sources, only two of them, namely SN 1006 and RCW 86, favor a leptonic origin for the γ-ray emission. The magnetic field energy is found to be comparable to that of accelerated relativistic ions and their ratio has a tendency to increase with the age of SNRs. These results suggest that TeV cosmic rays mainly originate from SNRs with hard γ-ray spectra.