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Register a new userA supernova constraint on bulk majorons
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In models with large extra dimensions all gauge singlet fields can in principle propagate in the extra dimensional space. We have investigated possible constraints on majoron models of neutrino masses in which the majorons propagate in extra dimensions. It is found that astrophysical constraints from supernovae are many orders of magnitude stronger than previous accelerator bounds. Our findings suggest that unnatural types of the see-saw mechanism for neutrino masses are unlikely to occur in nature, even in the presence of extra dimensions.
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The Bulk Randall-Sundrum model, where all Standard Model particles except the Higgs are free to propagate in the bulk, predicts the existence of Kaluza-Klein (KK) modes of the gluon with a large branching into top-antitop pairs. We study the production of the lowest KK gluon mode at the Tevatron energy and use the data on the top cross-section from the Run II of Tevatron to put a bound on the mass of the KK gluon. The resulting bound of 800 GeV, while being much smaller than the constraints obtained on the KK gluon mass from flavour-changing neutral currents, is the first, direct collider bound which is independent of the specificities of the model.
We derive a general expression for Delta rho (or, equivalently, for the oblique parameter T) in the SU(2) x U(1) electroweak model with an arbitrary number of scalar SU(2) doublets, with hypercharge +-1/2, and an arbitrary number of scalar SU(2) singlets. The experimental bound on Delta rho constitutes a strong constraint on the masses and mixings of the scalar particles in that model.
We present a critical assessment of the SN1987A supernova cooling bound on axions and other light particles. Core-collapse simulations used in the literature to substantiate the bound omitted from the calculation the envelope exterior to the proto-neutron star (PNS). As a result, the only source of neutrinos in these simulations was, by construction, a cooling PNS. We show that if the canonical delayed neutrino mechanism failed to explode SN1987A, and if the pre-collapse star was rotating, then an accretion disk would form that could explain the late-time ($tgtrsim5$ sec) neutrino events. Such accretion disk would be a natural feature if SN1987A was a collapse-induced thermonuclear explosion. Axions do not cool the disk and do not affect its neutrino output, provided the disk is optically-thin to neutrinos, as it naturally is. These considerations cast doubt on the supernova cooling bound.
We present a new constraint on a lepton mixing matrix $V$ from lepton-flavor violating (LFV) processes in supersymmetric standard models with massive neutrinos. Here, we assume Yukawa-coupling unification $f_{ u 3}simeq f_{rm top}$, in which $tau$-neutrino Yukawa coupling $f_{ u 3}$ is unified into top-quark Yukawa coupling $f_{rm top}$ at the unification scale $M_*simeq 3times 10^{16}$ GeV. We show that the present experimental bound on $mu to e gamma$ decay already gives a stringent limit on the lepton mixing (typically $V_{13}<0.02$ for $V_{23}=1/sqrt{2}$). Therefore, many existing neutrino-mass models are strongly constrained. Future improvement of bounds on LFV processes will provide a more significant impact on the models with the Yukawa-coupling unification. We also stress that a precise measurement of a neutrino mixing $(V_{MNS})_{e3}$ in future neutrino experiments would be very important, since the observation of non-zero $(V_{MNS})_{e3}$, together with negative experimental results for the LFV processes, have a robust potential to exclude a large class of SUSY standard models with the Yukawa-coupling unification.
General lowest order perturbations to hermitian squared mass matrices of leptons are considered away from the tribimaximal (TBM) limit in which a weak flavor basis with mass diagonal charged leptons is chosen. The three measurable TBM deviants are expressed linearly in terms of perturbation induced dimensionless coefficients appearing in the charged lepton and neutrino flavor eigenstates. With unnatural cancellations assumed to be absent and the charged lepton perturbation contributions to their flavor eigenstates argued to be small, we analytically derive the following result. Within lowest order perturbations, a deviation from maximal atmospheric neutrino mixing and the amount of CP violation in neutrino oscillations cannot both be large (i.e. $12$-$17 % $), posing the challenge of verification to forthcoming experiments at the intensity frontier.
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