60 research outputs found
Simultaneous Extraction of the Fermi constant and PMNS matrix elements in the presence of a fourth generation
Several recent studies performed on constraints of a fourth generation of
quarks and leptons suffer from the ad-hoc assumption that 3 x 3 unitarity holds
for the first three generations in the neutrino sector. Only under this
assumption one is able to determine the Fermi constant G_F from the muon
lifetime measurement with the claimed precision of G_F = 1.16637 (1) x 10^-5
GeV^-2. We study how well G_F can be extracted within the framework of four
generations from leptonic and radiative mu and tau decays, as well as from K_l3
decays and leptonic decays of charged pions, and we discuss the role of lepton
universality tests in this context. We emphasize that constraints on a fourth
generation from quark and lepton flavour observables and from electroweak
precision observables can only be obtained in a consistent way if these three
sectors are considered simultaneously. In the combined fit to leptonic and
radiative mu and tau decays, K_l3 decays and leptonic decays of charged pions
we find a p-value of 2.6% for the fourth generation matrix element |U_{e 4}|=0
of the neutrino mixing matrix.Comment: 19 pages, 3 figures with 16 subfigures, references and text added
refering to earlier related work, figures and text in discussion section
added, results and conclusions unchange
Experimental bounds on sterile neutrino mixing angles
We derive bounds on the mixing between the left-chiral ("active") and the
right-chiral ("sterile") neutrinos, provided from the combination of neutrino
oscillation data and direct experimental searches for sterile neutrinos. We
demonstrate that the mixing of sterile neutrinos with any flavour can be
significantly suppressed, provided that the angle theta_13 is non-zero. This
means that the lower bounds on sterile neutrino lifetime, coming from the
negative results of direct experimental searches can be relaxed (by as much as
the order of magnitude at some masses). We also demonstrate that the results of
the negative searches of sterile neutrinos with PS191 and CHARM experiments are
not applicable directly to the see-saw models. The reinterpretation of these
results provides up to the order of magnitude stronger bounds on sterile
neutrino lifetime than previously discussed in the literature. We discuss the
implications of our results for the Neutrino Minimal Standard Model (the
NuMSM).Comment: 18 pages + Appendices. Journal version with updated figure
Measuring the Invisible Higgs Width at the 7 and 8 TeV LHC
The LHC is well on track toward the discovery or exclusion of a light
Standard Model (SM)-like Higgs boson. Such a Higgs has a very small SM width
and can easily have large branching fractions to physics beyond the SM, making
Higgs decays an excellent opportunity to observe new physics. Decays into
collider-invisible particles are particularly interesting as they are
theoretically well motivated and relatively clean experimentally. In this work
we estimate the potential of the 7 and 8 TeV LHC to observe an invisible Higgs
branching fraction. We analyze three channels that can be used to directly
study the invisible Higgs branching ratio at the 7 TeV LHC: an invisible Higgs
produced in association with (i) a hard jet; (ii) a leptonic Z; and (iii)
forward tagging jets. We find that the last channel, where the Higgs is
produced via weak boson fusion, is the most sensitive, allowing branching
fractions as small as 40% to be probed at 20 inverse fb for masses in the range
between 120 and 170 GeV, including in particular the interesting region around
125 GeV. We provide an estimate of the 8 TeV LHC sensitivity to an
invisibly-decaying Higgs produced via weak boson fusion and find that the reach
is comparable to but not better than the reach at the 7 TeV LHC. We further
estimate the discovery potential at the 8 TeV LHC for cases where the Higgs has
substantial branching fractions to both visible and invisible final states.Comment: 23 pages, 7 figures. v2: version published in JHEP. 8 TeV analysis
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Invisible Higgs and Dark Matter
We investigate the possibility that a massive weakly interacting fermion
simultaneously provides for a dominant component of the dark matter relic
density and an invisible decay width of the Higgs boson at the LHC. As a
concrete model realizing such dynamics we consider the minimal walking
technicolor, although our results apply more generally. Taking into account the
constraints from the electroweak precision measurements and current direct
searches for dark matter particles, we find that such scenario is heavily
constrained, and large portions of the parameter space are excluded.Comment: arXiv admin note: text overlap with arXiv:0912.229
Mixing of Active and Sterile Neutrinos
We investigate mixing of neutrinos in the MSM (neutrino Minimal Standard
Model), which is the MSM extended by three right-handed neutrinos. Especially,
we study elements of the mixing matrix between three
left-handed neutrinos () and two sterile
neutrinos () which are responsible to the seesaw mechanism
generating the suppressed masses of active neutrinos as well as the generation
of the baryon asymmetry of the universe (BAU). It is shown that
can be suppressed by many orders of magnitude compared with
and , when the Chooz angle is large in the
normal hierarchy of active neutrino masses. We then discuss the neutrinoless
double beta decay in this framework by taking into account the contributions
not only from active neutrinos but also from all the three sterile neutrinos.
It is shown that and give substantial, destructive contributions
when their masses are smaller than a few 100 MeV, and as a results receive no stringent constraint from the current bounds on such decay.
Finally, we discuss the impacts of the obtained results on the direct searches
of in meson decays for the case when are lighter than pion
mass. We show that there exists the allowed region for with such
small masses in the normal hierarchy case even if the current bound on the
lifetimes of from the big bang nucleosynthesis is imposed. It is also
pointed out that the direct search by using and might miss such since the branching ratios can be
extremely small due to the cancellation in , but the search by
can cover the whole allowed region by improving the
measurement of the branching ratio by a factor of 5.Comment: 30 pages, 32 figure
Dichromatic dark matter
Both the robust INTEGRAL 511 keV gamma-ray line and the recent tentative hint of the 135 GeV gamma-ray line from Fermi-LAT have similar signal morphologies, and may be produced from the same dark matter annihilation. Motivated by this observation, we construct a dark matter model to explain both signals and to accommodate the two required annihilation cross sections that are different by more than six orders of magnitude. In our model, to generate the low-energy positrons for INTEGRAL, dark matter particles annihilate into a complex scalar that couples to photon via a charge-radius operator. The complex scalar contains an excited state decaying into the ground state plus an off-shell photon to generate a pair of positron and electron. Two charged particles with non-degenerate masses are necessary for generating this charge-radius operator. One charged particle is predicted to be long-lived and have a mass around 3.8 TeV to explain the dark matter thermal relic abundance from its late decay. The other charged particle is predicted to have a mass below 1 TeV given the ratio of the two signal cross sections. The 14 TeV LHC will concretely test the main parameter space of this lighter charged particle.University of Wisconsin--Madison (Start-up funds)SLAC National Accelerator Laboratory (US DOE contract DE-AC02-76SF00515)Aspen Center for Physics (NSF Grant No. 1066293)United States. National Aeronautics and Space Administration (Einstein Postdoctoral Fellowship grant number PF2-130102)Smithsonian Astrophysical Observatory (Chandra X-ray Center, NASA under contract NAS8-03060
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