Leptonic Processes Research Articles

Cosmic ray models of the ridge-like excess of gamma rays in the Galactic Centre

The High-Energy Stereoscopic System (HESS) has detected diffuse TeV emission correlated with the distribution of molecular gas along the Ridge at the Galactic Center. Diffuse, non-thermal emission is also seen by the Fermi large area telescope (Fermi-LAT) in the GeV range and by radio telescopes in the GHz range. Additionally, there is a distinct, spherically symmetric excess of gamma rays seen by Fermi-LAT in the GeV range. A cosmic ray flare, occurring in the Galactic Center, $10^4$ years ago has been proposed to explain the TeV Ridge. An alternative, steady-state model explaining all three data sets (TeV, GeV, and radio) invokes purely leptonic processes. We show that the flare model from the Galactic Center also provides an acceptable fit to the GeV and radio data, provided the diffusion coefficient is energy independent. However, if Kolmogorov-type turbulence is assumed for the diffusion coefficient, we find that two flares are needed, one for the TeV data (occurring approximately $10^4 $ years ago) and an older one for the GeV data (approximately $10^5$ years old). We find that the flare models we investigate do not fit the spherically symmetric GeV excess as well as the usual generalized Navarro-Frenk-White spatial profile, but are better suited to explaining the Ridge. We also show that a range of single-zone, steady-state models are able to explain all three spectral data sets. Large gas densities equal to the volumetric average in the region can be accommodated by an energy independent diffusion or streaming based steady-state model. Additionally, we investigate how the flare and steady-state models may be distinguished with future gamma-ray data looking for a spatial dependence of the gamma-ray spectral index.

Exploring X-ray lines as scotogenic signals

We consider some implications of X-ray lines from certain astronomical objects as potential effects of dark matter decay in the context of the scotogenic model, where neutrinos acquire mass radiatively via one-loop interactions with dark matter. As an example, we focus on the 3.5 keV line recently detected in the X-ray spectra of galaxy clusters, assuming that it stands future scrutiny. We explore the scenario in which the line originates from the slow decay of fermionic dark matter in the model. After obtaining a number of benchmark points representing the parameter space consistent with the new data and various other constraints, we make predictions on several observables in leptonic processes. They include the effective Majorana mass in neutrinoless double-beta decay, the sum of neutrino masses, and the rate of flavor-changing decay μ→eγ, as well as the cross sections of e+e− collisions into final states containing nonstandard particles in the model. These are testable in ongoing or future experiments and thus offer means to probe the scotogenic scenario studied.

Describing weak leptonic processes on the basis of currents with complex coupling constants. II

A spectrum of recoil electrons in an experiment on neutrino scattering off a polarized electron tar- get is obtained through the interaction between leptonic neutral currents with complex coupling constants. A possible violation of T-invariance in the lepton sector is considered. A quantitative analysis and a graphic description of the spectrum are presented for the scattering of beryllium solar neutrinos.

Double lepton pair production with electron capture in relativistic heavy-ion collisions

We present a theoretical study of a double lepton pair production in ultra--relativistic collision between two bare ions. Special emphasis is placed to processes in which creation of (at least one) $e^+ e^-$ pair is accompanied by the capture of an electron into a bound ionic state. To evaluate the probability and cross section of these processes we employ two approaches based on (i) the first--order perturbation theory and multipole expansion of Dirac wavefunctions, and (ii) the equivalent photon approximation. With the help of such approaches, detailed calculations are made for the creation of two bound--free $e^+ e^-$ pairs as well as of bound--free $e^+ e^-$ and free--free $\mu^+ \mu^-$ pairs in collisions of bare lead ions Pb$^{82+}$. The results of the calculations indicate that observation of the double lepton processes may become feasible at the LHC facility.

Testing the Standard Model and beyond with the LENA proposal

We discuss the possibility of a precision measurement of the electroweak mixing angle and a probe for new physics in the leptonic process of neutrino electron scattering. In the new physics schemes we explore the case of non standard neutrino interactions (NSI). The LENA proposal, currently under discussion, considers a large detector and the use of an artificial, 51Cr, radioactive neutrino source with of 5 MCi intensity. We also discuss the possible use of the solar neutrino flux as a neutrino source, in particular the case of the 7Be emission line.

A New Aspect of Two-Loop Neutrino Mass Generation

We propose a class of models which the neutrino masses are generated at two-loop level. In this mechanism, only the right-handed leptonic Yukawa interaction is involved without imposing any new symmetry but the Standard Model gauge symmetry. Neutrino mass spectrum is predicted to be normal hierarchy. The novel feature of the direct relation between the neutrino masses and the Yukawa couplings enable us pin down the neutrino parameters such as the absolute scale, the Dirac CP phase via the leptonic processes. The dynamics of lepton number breaking and some collider phenomenology of this model are studied.

Acceleration of cosmic rays and gamma-ray emission from supernova remnants in the Galaxy

Galactic cosmic rays are believed to be accelerated at supernova remnant shocks. Though very popular and robust, this conjecture still needs a conclusive proof. The strongest support to this idea is probably the fact that supernova remnants are observed in gamma-rays, which are indeed expected as the result of the hadronic interactions between the cosmic rays accelerated at the shock and the ambient gas. However, also leptonic processes can, in most cases, explain the observed gamma-ray emission. This implies that the detections in gamma rays do not necessarily mean that supernova remnants accelerate cosmic ray protons. To overcome this degeneracy, the multi-wavelength emission (from radio to gamma rays) from individual supernova remnants has been studied and in a few cases it has been possible to ascribe the gamma-ray emission to one of the two processes (hadronic or leptonic). Here we adopt a different approach and, instead of a case-by-case study we aim for a population study and we compute the number of supernova remnants which are expected to be seen in TeV gamma rays above a given flux under the assumption that these objects indeed are the sources of cosmic rays. The predictions found here match well with current observational results, thus providing a novel consistency check for the supernova remnant paradigm for the origin of galactic cosmic rays. Moreover, hints are presented for the fact that particle spectra significantly steeper than E^-2 are produced at supernova remnants. Finally, we expect that several of the supernova remnants detected by H.E.S.S. in the survey of the galactic plane should exhibit a gamma-ray emission dominated by hadronic processes (i.e. neutral pion decay). The fraction of the detected remnants for which the leptonic emission dominates over the hadronic one depends on the assumed values of the physical parameters and can be as high as roughly a half.

Phenomenology of scalar leptoquarks

We study the simplest renormalizable scalar leptoquark models where the standard model is augmented only by one additional scalar representation of $SU(3)\ifmmode\times\else\texttimes\fi{}SU(2)\ifmmode\times\else\texttimes\fi{}U(1)$. The requirement that there be no proton decay from renormalizable interactions singles out two such models, one of which exhibits an unusual top mass enhancement of the $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ decay rate. We analyze the phenomenology of the model with the unusual top mass enhancement of loop level chirality changing charged lepton processes in the light of existing and upcoming experiments. Both of the models that do not allow proton decay from renormalizable interactions have dimension-5 operators that, even if suppressed by the Planck scale, can give rise to an unacceptably high level of baryon number violation. We discuss symmetries that can forbid these dimension-5 operators.

Describing weak leptonic processes on the basis of currents with complex coupling constants

Describing weak leptonic processes on the basis of currents with complex coupling constants

Search for very-high-energyγ-ray emission from Galactic globular clusters with H.E.S.S.

Globular clusters (GCs) are established emitters of high-energy (HE, 100 MeV<E<100 GeV) \gamma-ray radiation which could originate from the cumulative emission of the numerous millisecond pulsars (msPSRs) in the clusters' cores or from inverse Compton (IC) scattering of relativistic leptons accelerated in the GC environment. These stellar clusters could also constitute a new class of sources in the very-high-energy (VHE, E>100 GeV) \gamma-ray regime, judging from the recent detection of a signal from the direction of Terzan 5 with the H.E.S.S. telescope array. We searched for point-like and extended VHE \gamma-ray emission from 15 GCs serendipitously covered by H.E.S.S observations and also performed a stacking analysis combining the data from all GCs to investigate the hypothesis of a population of faint emitters. Assuming IC emission as the origin of the VHE \gamma-ray signal from the direction of Terzan 5, we calculated the expected \gamma-ray flux from each of the 15 GCs, based on their number of millisecond pulsars, their optical brightness and the energy density of background photon fields. We did not detect significant VHE \gamma-ray emission from any of the 15 GCs in either of the two analyses. Given the uncertainties related to the parameter determinations, the obtained flux upper limits allow to rule out the simple IC/msPSR scaling model for NGC 6388 and NGC 7078. The upper limits derived from the stacking analyses are factors between 2 and 50 below the flux predicted by the simple leptonic scaling model, depending on the assumed source extent and the dominant target photon fields. Therefore, Terzan 5 still remains exceptional among all GCs, as the VHE \gamma-ray emission either arises from extra-ordinarily efficient leptonic processes, or from a recent catastrophic event, or is even unrelated to the GC itself.

Invisible decay of muonium: Tests of the standard model and searches for new physics

In the Standard Model there are several canonical examples of pure leptonic processes involving the muon, the electron and the corresponding neutrinos which are connected by the crossing symmetry: i) the decay of muon, ii) the inverse muon decay, and iii) the annihilation of a muon and an electron into two neutrinos. Although the first two reactions have been observed and measured since long ago, the third process, resulting in the invisible final state, has never been experimentally tested. It may go either directly, or, at low energies, via the annihilation of a muon and an electron from an atomic bound state, called muonium (M=\mu^+e^-). The M\to \nu_\mu \nu_e decay is expected to be a very rare process, with the branching fraction predicted to be Br(M\to \nu_\mu\nu_e) = 6.6 10^{-12} with respect to the ordinary muon decay rate. Using the reported experimental results on precision measurements of the positive muon lifetime by the MuLan Collaboration, we set the first limit Br(M \to invisible) < 5.7 10^{-6}, while still leaving a big gap of about six orders of magnitude between this bound and the predictions. To improve substantially the limit, we proposed to perform an experiment dedicated to the sensitive search for the M\to invisible decay. A feasibility study of the experimental setup shows that the sensitivity of the search for this decay mode in branching fraction Br(M\to invisible) at the level of 10^{-12} could be achieved. If the proposed search results in a substantially higher branching fraction than predicted, say Br(M \to invisible) < 10^{-10}, this would unambiguously indicate the presence of new physics. We point out that such a possibility may occur due the muonium-mirror muonium conversion in the mirror matter model. A result in agreement with the Standard Model prediction would be a clean check of the pure leptonic bound state annihilation.

Precise measurement of thee+e−→π+π−(γ)cross section with the initial-state radiation method atBABAR

A precise measurement of the cross section of the process $e^+e^-\to\pi^+\pi^-(\gamma)$ from threshold to an energy of 3GeV is obtained with the initial-state radiation (ISR) method using 232fb$^{-1}$ of data collected with the BaBar detector at $e^+e^-$ center-of-mass energies near 10.6GeV. The ISR luminosity is determined from a study of the leptonic process $e^+e^-\to\mu^+\mu^-(\gamma)\gamma_{\rm ISR}$, which is found to agree with the next-to-leading-order QED prediction to within 1.1%. The cross section for the process $e^+e^-\to\pi^+\pi^-(\gamma)$ is obtained with a systematic uncertainty of 0.5% in the dominant $\rho$ resonance region. The leading-order hadronic contribution to the muon magnetic anomaly calculated using the measured $\pi\pi$ cross section from threshold to 1.8GeV is $(514.1 \pm 2.2({\rm stat}) \pm 3.1({\rm syst}))\times 10^{-10}$.

Universal constraints on low-energy flavour models

Abstract It is pointed out that in a general class of flavour models one can identify certain universally present FCNC operators, induced by the exchange of heavy flavour messengers. Their coefficients depend on the rotation angles that connect flavour and fermion mass basis. The lower bounds on the messenger scale are derived using updated experimental constraints on the FCNC operators. The obtained bounds are different for different operators and in addition they depend on the chosen set of rotations. Given the sensitivity expected in the forthcoming experiments, the present analysis suggests interesting room for discovering new physics. As the highlights emerge the leptonic processes, μ → eγ, μ → eee and μ → e conversion in nuclei.

HOW MANY ULTRA-HIGH ENERGY COSMIC RAYS COULD WE EXPECT FROM CENTAURUS A?

The Pierre Auger Observatory has associated a few ultra high energy cosmic rays with the direction of Centaurus A. This source has been deeply studied in radio, infrared, X-ray and $\gamma$-rays (MeV-TeV) because it is the nearest radio-loud active galactic nuclei. Its spectral energy distribution or spectrum shows two main peaks, the low energy peak, at an energy of $10^{-2}$ eV, and the high energy peak, at about 150 keV. There is also a faint very high energy (E $\geq$ 100 GeV) $\gamma$-ray emission fully detected by the High Energy Stereoscopic System experiment. In this work we describe the entire spectrum, the two main peaks with a Synchrotron/Self-Synchrotron Compton model and, the Very High Energy emission with a hadronic model. We consider p$\gamma$ and $pp$ interactions. For the p$\gamma$ interaction, we assume that the target photons are those produced at 150 keV in the leptonic processes. On the other hand, for the pp interaction we consider as targets the thermal particle densities in the lobes. Requiring a satisfactory description of the spectra at very high energies with p$\gamma$ interaction we obtain an excessive luminosity in ultra high energy cosmic rays (even exceeding the Eddington luminosity). However, when considering pp interaction to describe the $\gamma$-spectrum, the obtained number of ultra high energy cosmic rays are in agreement with Pierre Auger observations. Moreover, we calculate the possible neutrino signal from pp interactions on a Km$^3 $ neutrino telescope using Monte Carlo simulations.

Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants

Context. Energetic gamma rays (GeV to TeV photon energy) have been detected toward several supernova remnants (SNRs) that are associated with molecular clouds. If the gamma rays are produced mainly by hadronic processes rather than leptonic processes like bremsstrahlung, then the flux of energetic cosmic ray nuclei (>1 GeV) required to produce the gamma rays can be inferred at the site where the particles are accelerated in SNR shocks. It is of great interest to understand the acceleration of the cosmic rays of lower energy (<1 GeV) that accompany the energetic component. These particles of lower energy are most effective in ionizing interstellar gas, which leaves an observable imprint on the interstellar ion chemistry. A correlation of energetic gamma radiation with enhanced interstellar ionization can thus be used to support the hadronic origin of the gamma rays and to constrain the acceleration of ionizing cosmic rays in SNR.

Constraints on a noncommutative physics scale with neutrino-electron scattering

Neutrino-electron scatterings ($\ensuremath{\nu}\ensuremath{-}e$) are purely leptonic processes with robust standard model predictions. Their measurements can therefore provide constraints to physics beyond the standard model. Noncommutative (NC) field theories modify space-time commutation relations, and allow neutrino electromagnetic couplings at the tree level. Their contribution to the neutrino-electron scattering cross section was derived. Constraints were placed on the NC scale parameter ${\ensuremath{\Lambda}}_{\mathrm{NC}}$ from $\ensuremath{\nu}\ensuremath{-}e$ experiments with reactor and accelerator neutrinos. The most stringent limit of ${\ensuremath{\Lambda}}_{\mathrm{NC}}&gt;3.3\text{ }\text{ }\mathrm{TeV}$ at 95% confidence level improves over the direct bounds from collider experiments.

Positron annihilation as a cosmic ray probe

ABSTRACT Recently, the gamma-ray telescopes AGILE and Fermi observed several middle-aged supernova remnants (SNRs) interacting with molecular clouds. A plausible emission mechanism of the gamma-rays is the decay of neutral pions produced by cosmic ray (CR) nuclei (hadronic processes). However, observations do not rule out contributions from bremsstrahlung emission due to CR electrons. TeV gamma-ray telescopes also observed many SNRs and discovered many unidentified sources. It is still unclear whether the TeV gamma-ray emission is produced via leptonic processes or hadronic processes. In this Letter, we propose that annihilation emission of secondary positrons produced by CR nuclei is a diagnostic tool of the hadronic processes. We investigate MeV emissions from secondary positrons and electrons produced by CR protons in molecular clouds. The annihilation emission of the secondary positrons from SNRs can be robustly estimated from the observed gamma-ray flux. The expected flux of the annihilation line from SNRs observed by AGILE and Fermi is sufficient for the future Advanced Compton Telescope to detect. Moreover, synchrotron emission from secondary positrons and electrons and bremsstrahlung emission from CR protons can be also observed by the future X-ray telescope NuSTAR and Astro-H.

CANGAROO-III OBSERVATION OF TeV GAMMA RAYS FROM THE UNIDENTIFIED GAMMA-RAY SOURCE HESS J1614–518

We report the detection, with the CANGAROO-III imaging atmospheric Cherenkov telescope array, of a very high energy gamma-ray signal from the unidentified gamma-ray source HESS J1614-518, which was discovered in the H.E.S.S. Galactic plane survey. Diffuse gamma-ray emission was detected above 760 GeV at the 8.9 sigma level during an effective exposure of 54 hr from 2008 May to August. The spectrum can be represented by a power-law: 8.2+-2.2_{stat}+-2.5_{sys}x10^{-12}x (E/1TeV)^{-Gamma} cm^{-2} s^{-1} TeV^{-1} with a photon index Gamma of 2.4+-0.3_{stat}+-0.2_{sys}, which is compatible with that of the H.E.S.S. observations. By combining our result with multi-wavelength data, we discuss the possible counterparts for HESS J1614-518 and consider radiation mechanisms based on hadronic and leptonic processes for a supernova remnant, stellar winds from massive stars, and a pulsar wind nebula. Although a leptonic origin from a pulsar wind nebula driven by an unknown pulsar remains possible, hadronic-origin emission from an unknown supernova remnant is preferred.

TWIST: precision measurement of the muon decay parameters

The TRIUMF Weak Interaction Symmetry Test (TWIST) collaboration has nearly completed the world's most precise measurement of the energy-angle spectrum of positrons from the decay of highly polarized muons. A simultaneous measurement of the muon decay parameters ρ, δ, and \U0001d4abμπξ tests the Standard Model (SM) in a purely leptonic process and provides improved limits for relevant extensions to the SM. Since the completion of data taking in 2007, the analysis has focused on reducing systematic uncertainties, estimating residual biases, and evaluating consistency checks. The analysis was blind with respect to the central values of the parameters, so the results were unknown until the values of the hidden parameters were revealed. The total uncertainties for the parameters are dominated by systematic uncertainties. While there are still some subtle systematic effects that preclude the determination of final values of the decay parameters, the goal of an order-of-magnitude improvement on pre-TWIST precisions has been achieved.

Measurement of the [formula omitted] hadrons cross-section at low energy with ISR events at BABAR

The precise measurement of the cross section e+e- --> pi+ pi-(gamma) from threshold to an energy of 3 GeV, using events with Initial State Radiation (ISR) collected with the BABAR detector, is presented. The ISR luminosity is determined from a study of the leptonic process e+e- --> mu+ mu- gamma(gamma), and the method is tested by the comparison with the next-to-leading order (NLO) QED prediction. The leading-order hadronic contribution to the muon magnetic anomaly calculated using the BABAR pi pi cross section measured from threshold to 1.8 GeV is (514.1 +- 2.2(stat) +- 3.1(syst)) * 10^{-10}. Other results on ISR multihadronic cross sections from BABAR are presented.