Bounds on Heavy-to-Heavy Weak Decay Form Factors

We provide upper and lower bounds on the semileptonic weak decay form factors for $B \to D^(*)$ and $\Lambda_b \to \Lambda_c$ decays by utilizing inclusive heavy quark effective theory sum rules. These bounds are calculated to second order in $\Lambda_{QCD}/m_Q$ and first order in $\alpha_s$. The $O(\alpha_s^2 \beta_0)$ corrections to the bounds at zero recoil are also presented.Comment: 3 pages, talk given at DPF 2000, Columbus, OH, August 9, 2000; reference adde

SU(3) symmetry breaking and CP violation in D -> PP decays

Evidence of CP violation in the charm sector has been observed recently by the LHCb and CDF Collaborations. Adopting the topological diagram approach, we study flavor SU(3) symmetry breaking effects in the weak decay tree amplitudes of singly Cabibbo-suppressed $D\to PP$ decays. The symmetry breaking in the color-allowed and color-suppressed amplitudes is estimated with the help of the factorization ansatz, while that in the $W$-exchange amplitude is done by fitting to related branching fraction data. We find that the $W$-exchange amplitudes stay in the second quadrant relative to the color-allowed tree amplitude, albeit there are two possibilities for one type of $W$-exchange amplitude. The weak decay penguin amplitudes, on the other hand, are evaluated within the framework of QCD factorization. Using the input of topological tree amplitudes extracted from the Cabibbo-favored decay modes and the perturbative results for QCD penguin amplitudes, we make predictions for the branching fractions and CP asymmetries of singly Cabibbo-suppressed modes. The predictions of branching fractions are generally improved from those in the SU(3) limit. We conclude that the direct CP asymmetry difference between $D^0 \to K^+ K^-$ and $D^0 \to \pi^+ \pi^-$ is about $-(0.139\pm 0.004)$ and $-(0.151\pm 0.004)$ for the two solutions of $W$-exchange amplitudes, respectively. We also find that the CP asymmetry of D^0\to K^0\ov K^0 dominated by the interference between $W$-exchange amplitudes ranges from $-0.62\times 10^{-3}$ to $-1.82\times 10^{-3}$. We study phenomenological implications of two new physics scenarios for explaining the observed CP asymmetry in the charm sector, one with large penguin amplitudes and the other with a large chromomagnetic dipole operator. We find that the two scenarios can be discriminated by the measurements of CP asymmetries of a set of decay modes.Comment: 23 pages, three new paragraphs added in the beginning of Sec. III. Version to appear in PRD. arXiv admin note: substantial text overlap with arXiv:1201.078

3.5-keV X-ray line from nearly-degenerate WIMP dark matter decays

The unidentified emission line at the energy of $\sim$3.5~keV observed in X-rays from galaxy clusters may originate from a process involving a dark matter particle. On the other hand, a weakly interacting massive particle (WIMP) has been an attractive dark matter candidate, due to its well-understood thermal production mechanism and its connection to physics at the TeV scale. In this paper, we pursue the possibility that the 3.5-keV X-ray arises from a late time decay of a WIMP dark matter into another WIMP dark matter, both of which have the mass of $O(100)$~GeV and whose mass splitting is about 3.5~keV. We focus on the simplest case where there are two Majorana dark matter particles and two charged scalars that couple with a standard model matter particle. By assuming a hierarchical structure in the couplings of the two dark matter particles and two charged scalars, it is possible to explain the 3.5-keV line and realize the WIMP dark matter scenario at the same time. Since the effective coupling of the two different Majorana dark matter particles and one photon violates CP symmetry, the model always contains a new source of CP violation, so the model's connection to the physics of electric dipole moments is discussed. The model's peculiar signatures at the LHC are also studied. We show the prospect of detecting the charged scalars through a detailed collider simulation

Electroweak Phase Transition in Georgi-Machacek Model

The Georgi-Machacek model extends the standard model Higgs sector by adding two isospin triplet scalar fields and imposing global SU(2)$_R$ symmetry on them. A feature of the model is that the triplets can acquire a large vacuum expectation value without conflicting with the current experimental bound on the $\rho$ parameter. We investigate the electroweak phase transition in the Georgi-Machacek model by evaluating the finite-temperature effective potential of the Higgs sector. The electroweak phase transition can be sufficiently strong in a large parameter space when the triplets acquire a vacuum expectation value of $O(10)$ GeV, opening a possibility to realize successful electroweak baryogenesis

On gauge dependence of gravitational waves from a first-order phase transition in classical scale-invariant U(1)′U(1)' models

We study gauge dependence of gravitational waves produced from a first-order phase transition in classical scale-invariant $U(1)'$ models. Accidental gauge independence of the one-loop effective potential in this class of models is spoiled by including thermal resummation. The gauge artifact in the resummed effective potential propagates to the gravitational wave spectrum and results in one order of magnitude uncertainties in the prediction under a specific gauge choice.Comment: 12 pages, 1 figure; v2: updated to match version published in PL