Guevara, A., Lopez Castro, G., & Roig, P. (2022). Improved description of dilepton production in tau(-) -> nu(tau)P(- )decays. Phys. Rev. D, 105(7), 076007–15pp.
Abstract: Recently, the Belle Collaboration reported the first measurements of the tau(-) -> nu(tau)pi(-) e(+) e(-) branching fraction and the spectrum of the pion-dielectron system. In an analysis previous to Belle's results, we evaluated this branching fraction which turned out to be compatible with that reported by Belle, although with a large uncertainty. This is the motivation to seek for improvement on our previous evaluation of tau(-) -> nu(tau)pi(-) l(+) l(-) decays (l = e, mu). In this paper we improve our calculation of the WP-gamma* vertex by including flavor-symmetry breaking effects in the framework of the resonance chiral theory. We impose QCD short-distance behavior to constrain most parameters and data on the pi(-) e(+) e(-) spectrum reported by the Belle Collaboration to fix the remaining free ones. As a result, improved predictions for the branching ratios and hadronic/leptonic spectra are reported, which are in good agreement with observations. Analogous calculations for the strangeness-changing tau(-) -> nu(tau) K- l(+) l(-) transitions are reported for the first time. Albeit one expects the m(pi mu+ mu- )spectrum to be measured in Belle-II and the observables with l = e can be improved, it is rather unlikely that the K channels can be measured due to the suppression factor vertical bar V-ud/V-us vertical bar(2) = 0.05.
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Hagedorn, C., Kriewald, J., Orloff, J., & Teixeira, A. M. (2022). Flavour and CP symmetries in the inverse seesaw. Eur. Phys. J. C, 82(3), 194–32pp.
Abstract: We consider an inverse seesaw mechanism of neutrino mass generation in which the Standard Model is extended by 3 + 3 (heavy) sterile states, and endowed with a flavour symmetry G(f), G(f) = Delta(3n(2)) or G(f) = Delta(6n(2)), and a CP symmetry. These symmetries are broken in a peculiar way, so that in the charged lepton sector a residual symmetry G(l) is preserved, while the neutral fermion sector remains invariant under the residual symmetry G(nu) = Z(2) x CP. We study the concrete setup, where the Majorana mass term for three of the sterile states conserves G(nu), while the remaining mass terms (i.e. couplings of left-handed leptons and heavy sterile states, as well as the Dirac-type couplings among the latter) do not break the flavour or CP symmetry. We perform a comprehensive analysis of lepton mixing for different classes of residual symmetries, giving examples for each of these, and study in detail the impact of the additional sterile states on the predictions for lepton mixing. We further confront our results with those obtained in the model-independent scenario, in which the light neutrino mass matrix leaves the residual symmetry G(nu) intact. We consider the phenomenological impact of the inverse seesaw mechanism endowed with flavour and CP symmetries, in particular concerning effects of non-unitarity of the lepton mixing matrix (which strongly constrain the parameter space of the scenario), prospects for neutrinoless double beta decay and for charged lepton flavour violating processes.
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Han, C., Lopez-Ibañez, M. L., Melis, A., Vives, O., & Yang, J. M. (2022). Anomaly-free ALP from non-Abelian flavor symmetry. J. High Energy Phys., 08(8), 306–21pp.
Abstract: Motivated by the XENON1T excess in electron-recoil measurements, we investigate the prospects of probing axion-like particles (ALP) in lepton flavor violation experiments. In particular, we identify such ALP as a pseudo-Goldstone from the spontaneous breaking of the flavor symmetries that explain the mixing structure of the Standard Model leptons. We present the case of the flavor symmetries being a non-Abelian U(2) and the ALP originating from its U(1) subgroup, which is anomaly-free with the Standard Model group. We build two explicit realistic examples that reproduce leptonic masses and mixings and show that the ALP which is consistent with XENON1T anomaly could be probed by the proposed LFV experiments.
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HAWC Collaboration(Albert, A. et al), & Salesa Greus, F. (2022). HAWC Study of the Ultra-high-energy Spectrum of MGRO J1908+06. Astrophys. J., 928(2), 116–13pp.
Abstract: We report TeV gamma-ray observations of the ultra-high-energy source MGRO J1908+06 using data from the High Altitude Water Cherenkov Observatory. This source is one of the highest-energy known gamma-ray sources, with emission extending past 200 TeV. Modeling suggests that the bulk of the TeV gamma-ray emission is leptonic in nature, driven by the energetic radio-faint pulsar PSR J1907+0602. Depending on what assumptions are included in the model, a hadronic component may also be allowed. Using the results of the modeling, we discuss implications for detection prospects by multi-messenger campaigns.
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HAWC Collaboration(Albert, A. et al), & Salesa Greus, F. (2022). Long-term Spectra of the Blazars Mrk 421 and Mrk 501 at TeV Energies Seen by HAWC. Astrophys. J., 929(2), 125–12pp.
Abstract: The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory surveys the very high-energy sky in the 300 GeV to >100 TeV energy range. HAWC has detected two blazars above 11 sigma, Markarian 421 (Mrk 421) and Markarian 501 (Mrk 501). The observations are comprised of data taken in the period between 2015 June and 2018 July, resulting in similar to 1038 days of exposure. In this work, we report the time-averaged spectral analyses for both sources, above 0.5 TeV. Taking into account the flux attenuation due to the extragalactic background light, the intrinsic spectrum of Mrk 421 is described by a power law with an exponential energy cutoff with index alpha = 2.26 +/- (0.12)(stat)((+0.17)(-0.2))(sys) and energy cutoff E-c = 5.1 +/- (1.6)(stat)((+1.4)(-2.5))(sys) TeV, while the intrinsic spectrum of Mrk 501 is better described by a simple power law with index alpha = 2.61 +/- (0.11)(stat)((+)(0.01)(-0.07))(sys). The maximum energies at which the Mrk 421 and Mrk 501 signals are detected are 9 and 12 TeV, respectively. This makes these some of the highest energy detections to date for spectra averaged over years-long timescales. Since the observation of gamma radiation from blazars provides information about the physical processes that take place in their relativistic jets, it is important to study the broadband spectral energy distributions (SEDs) of these objects. For this purpose, contemporaneous data in the gamma-ray band to the X-ray range, and literature data in the radio to UV range, were used to build time-averaged SEDs that were modeled within a synchrotron-self Compton leptonic scenario.
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