37 research outputs found
Photon to axion conversion during Big Bang Nucleosynthesis
We investigate how the resonant conversion at a temperature -
keV of a fraction of the CMB photons into an axion-like majoron affects BBN.
The scenario, that assumes the presence of a primordial magnetic field and the
subsequent decay of the majorons into neutrinos at eV, has been
proposed to solve the tension. We find two main effects. First, since we
lose photons to majorons at , the baryon to photon ratio is smaller at
the beginning of BBN than during decoupling and structure
formation (). This relaxes the mismatch between the
observed deuterium abundance and the one predicted by the standard CDM
model. Second, since the conversion implies a sudden drop in the temperature of
the CMB during the final phase of BBN, it interrupts the synthesis of lithium
and beryllium and reduces their final abundance, possibly alleviating the
lithium problem.Comment: 12 pages, 3 figures; new section added, matches version accepted by
JCA
Bounds on the Zγγ couplings from HERA
The possibility of testing trilinear neutral gauge boson couplings in radiative neutral current scattering at HERA is analyzed using a Monte Carlo program that includes the Standard Model at tree level and the anomalous vertices. Acceptance and isolation cuts are applied as well as optimized cuts to enhance the signal from new physics. The bounds on Z\gamma\gamma couplings that can be achieved are not so stringent as present bounds, even for high luminosities, but probe a different kinematical region almost unsensitive to form factors.This work has been partially supported by the CICYT and the European Commission under contract CHRX-CT-92-0004
Atmospheric lepton fluxes at very high energy
The observation of astrophysical neutrinos requires a detailed understanding
of the atmospheric neutrino background. Since neutrinos are produced in meson
decays together with a charged lepton, important constraints on this background
can be obtained from the measurement of the atmospheric muon flux. Muons,
however, can also be produced as mu+ mu- pairs by purely electromagnetic
processes. We use the Z-moment method to study and compare the contributions to
the atmospheric muon and neutrino fluxes from different sources (pi/K decay,
charmed and unflavored hadron decay, and photon conversion into a muon pair).
We pay special attention to the contribution from unflavored mesons (eta, eta',
rho0, omega and phi). These mesons are abundant in air showers, their lifetimes
are much shorter than those of charged pions or kaons, and they have decay
branching ratios of order 10^-4 into final states containing a muon pair. We
show that they may be the dominant source of muons at E_mu >10^3 TeV.Comment: Pdflatex, 28 pages, 6 figure
One-loop flavor change in Little Higgs models
The Little Higgs (LH) idea attempts to cure the little amount of fine-tuning
necessary to bridge the gap between the Higgs mass (electroweak scale) and the
new physics scale suggested by electroweak precision tests (~10 TeV). However,
we show that LH models do not survive the confrontation with experimental
limits on lepton flavor mixing, assuming the same naturalness arguments that
motivate their introduction. Two different LH models are analyzed and several
aspects of their one-loop predictions for lepton flavor-changing processes are
discussed.Comment: 6 pages, 1 figure; contribution to Loops and Legs in Quantum Field
Theory, Woerlitz, Germany, April 25-30, 201
TAU-PAIR PRODUCTION VIA PHOTON-PHOTON COLLISIONS AT LEP
We point out that the cross-section for the process at LEP is large enough to allow for a study of the anomalous
electromagnetic couplings of the lepton. We show that the present bounds
on the magnetic dipole moment can be improved and that competitive bounds can
be obtained for the electric dipole moment using the data taken from 1992 to
1994. Finally, we briefly discuss the improvements that can be obtained at LEP
II.Comment: 9 pages, latex, 2 figure
First -decay spectroscopy of and new -decay branches of
19 pags., 14 figs., 3 tabs.The decay of the neutron-rich and was investigated experimentally in order to provide new insights into the nuclear structure of the tin isotopes with magic proton number above the shell. The -delayed -ray spectroscopy measurement was performed at the ISOLDE facility at CERN, where indium isotopes were selectively laser-ionized and on-line mass separated. Three -decay branches of were established, two of which were observed for the first time. Population of neutron-unbound states decaying via rays was identified in the two daughter nuclei of and , at excitation energies exceeding the neutron separation energy by 1 MeV. The -delayed one- and two-neutron emission branching ratios of were determined and compared with theoretical calculations. The -delayed one-neutron decay was observed to be dominant -decay branch of even though the Gamow-Teller resonance is located substantially above the two-neutron separation energy of . Transitions following the decay of are reported for the first time, including rays tentatively attributed to . In total, six new levels were identified in on the basis of the coincidences observed in the and decays. A transition that might be a candidate for deexciting the missing neutron single-particle state in was observed in both decays and its assignment is discussed. Experimental level schemes of and are compared with shell-model predictions. Using the fast timing technique, half-lives of the , and levels in were determined. From the lifetime of the state measured for the first time, an unexpectedly large transition strength was deduced, which is not reproduced by the shell-model calculations.M.P.-S. acknowledges the funding support from the Polish National Science Center under Grants No.
2019/33/N/ST2/03023 and No. 2020/36/T/ST2/00547 (Doctoral scholarship ETIUDA). J.B. acknowledges
support from the Universidad Complutense de Madrid under the Predoctoral Grant No. CT27/16-
CT28/16. This work was partially funded by the Polish National Science Center under Grants No.
2020/39/B/ST2/02346, No. 2015/18/E/ST2/00217, and No. 2015/18/M/ST2/00523, by the Spanish
government via Projects No. FPA2017-87568-P, No. RTI2018-098868-B-I00, No. PID2019-104390GB-I00,
and No. PID2019-104714GB-C21, by the U.K. Science and Technology Facilities Council (STFC), the German BMBF
under Contract No. 05P18PKCIA, by the Portuguese FCT under the Projects No. CERN/FIS-PAR/0005/2017, and
No. CERN/FIS-TEC/0003/2019, and by the Romanian IFA Grant CERN/ISOLDE. The research leading to these
results has received funding from the European Union’s Horizon 2020 research and innovation programme under
Grant Agreement No. 654002. M.Str. acknowledges the funding from the European Union’s Horizon 2020 research
and innovation program under Grant Agreement No. 771036 (ERC CoG MAIDEN). J.P. acknowledges support from the
Academy of Finland (Finland) with Grant No. 307685. Work at the University of York was supported under STFC Grants
No. ST/L005727/1 and No. ST/P003885/1
Lepton flavor violation in the Simplest Little Higgs model
The flavor sector of Little Higgs models based on product groups, notably the
Littlest Higgs with T parity (LHT), has been extensively studied and some
amount of fine tuning was found to be required to meet the experimental
constraints. However, no such attention has been paid to other classes of
models. Here we analyze the phenomenology of flavor mixing in the lepton sector
of a simple group model, the Simplest Little Higgs (SLH). We obtain the Feynman
rules of the SLH in the 't Hooft-Feynman gauge up to the necessary order and
calculate the leading contributions to the rare processes mu -> e gamma, mu ->
eee and mu-e conversion in nuclei. We find results comparable to those of the
LHT model, because in both cases they arise at the one-loop level. These
require the flavor alignment of the Yukawa couplings of light and heavy leptons
at the per cent level or an effective scale of around 10 TeV.Comment: 41 pages, 8 figures; minor changes and one reference added, version
to appear in JHE