Sγ Branching Ratio Research Articles

Top quark anomalous couplings from top-pair tagged photoproduction at pe− colliders

We summarize the quantitative results of our analysis [1] of top-pair photoproduction in semileptonic mode in pe collisions at the LHeC and FCC-he. We define three photoproduction regions, based on the rapidity acceptance range of the electron tagger, that provide different degrees of sensitivity to top-quark effective couplings. We focus on the ttγ dipole couplings and the left-handed vector tbW coupling, for which we determine limits at both energies in the different photoproduction regions. Furthermore, we find that the LHeC and FCC-he will yield tight direct bounds on top dipole moments, greatly improving on current direct limits from hadron colliders, and direct limits on the tbW coupling as restrictive as those expected from the HL-LHC. We also consider indirect limits from b → sγ branching ratio and CP asymmetry, that are well known to be very sensitive probes of top electromagnetic dipole moments.

Adiabatic compression and indirect detection of supersymmetric dark matter

Recent developments in the modelling of the dark matter distribution in our Galaxy pointout the necessity to consider some physical processes to satisfy observational data. Inparticular, models with adiabatic compression, which include the effect of the baryonic gasin the halo, increase significantly the dark matter density in the central region of the MilkyWay. On the other hand, the non-universality in the scalar and gaugino sectors ofsupergravity models can also increase significantly the neutralino annihilation cross section.We show that the combination of both effects gives rise to a gamma-ray flux arising fromthe galactic centre largely reachable by future experiments like GLAST. We also analyse inthis framework the EGRET excess data above 1 GeV, as well as the recent data fromCANGAROO and HESS. The analysis has been carried out imposing the most recentexperimental constraints, such as the lower bound on the Higgs mass, the b → sγ branching ratio, and the muong − 2. In addition, the recently improved upper bound on B(Bs → μ+μ-) has also been taken into account. The astrophysical (WMAP) bounds on the dark matterdensity have also been imposed on the theoretical computation of the relic neutralinodensity through thermal production.

Three-loop matching of the dipole operators for b→ sγ and b→ sg

We evaluate the three-loop matching conditions for the dimension-five operators that are relevant for the b→ sγ decay. Our calculation completes the first out of three steps (matching, mixing and matrix elements) that are necessary for finding the next-to-next-to-leading QCD corrections to this process. All such corrections must be calculated in view of the ongoing accurate measurements of the B ̄ →X sγ branching ratio.

WMAPing out supersymmetric dark matter and phenomenology

The recent WMAP data provide a rather restricted range of the Cold Dark Matter (CDM) density ΩCDMh2 of unprecedented accuracy. We combine these new data along with data from BNL E821 experiment measuring (gμ−2), the b→sγ branching ratio and the light Higgs boson mass bound from LEP, to update our analysis of the allowed boundaries in the parameter space of the Constrained Minimal Supersymmetric Standard Model (CMSSM). The prospects of measuring Supersymmetry at LHC look like a very safe bet, and the potential of discovering SUSY particles at a s=1.1 TeV linear collider is enhanced considerably. The implications for dark matter direct searches are also discussed.

The Radiative rare decays B ? K**? in the light-cone QCD sum rule approach

We predict contributions of higher K-resonances to the radiative rare decays b → sγ, in the framework of the QCD sum rules on the light cone, which combines the traditional QCD sum rule method with the description of final state mesons in terms of the light-cone wave functions of increasing twist. Our calculations are restricted to the leading twist-two operators for K*(892) and to the asymptotic wave function for the other K**-mesons.Using experimental data on the semileptonic τ → K**ντ decays, we extract the corresponding decay constants for vector and axial-vector K**-mesons. Based on our estimate of the transition form factor F1K*(892)(0) = 0.32 ± 0.06 and the K2*(1430) decay constant fK2*(1430) = (160 ± 20) MeV, we find good agreement with experimental results. The two largest fractions of the inclusive b → sγ branching ratio are found to be (10.0 ± 4.0)% for B → K*(892)γ and (5.0 ± 2.0)% for B → K2*(1430)γ decays. We also compare our results with the existing theoretical predictions.

Anomalous Wt̄b Couplings andb→sγ in the Topcolour-Assisted Technicolour Models

Using topcolour-assisted technicolour models, we give the anomalousWt̄b couplings arising from the exchange of the new gaugebosons (ETC gauge boson, colorons and Z') and calculatetheir contributions to the b→sγ branchingratio. We find that the contributions mainly come from coloronsand the recent measured value of the b→sγbranching ratio gives a strong constraint on the masses ofcolorons.

A measurement of the inclusive b→sγ branching ratio

Abstract The flavour changing neutral current decay b→sγ has been detected in hadronic Z decays collected by ALEPH at LEP. The signal is isolated in lifetime-tagged b b events by the presence of a hard photon associated with a system of high momentum and high rapidity hadrons. The background processes are normalised from the data themselves. The inclusive branching ratio is measured to be (3.11±0.80 stat ±0.72 syst )×10 −4 , consistent with the Standard Model expectation via penguin processes.

Multiscale technicolor and b → sγ

Correction to the b → sγ branching ratio in the multiscale walking technicolor model (MWTCM) is examined. For the original MWTCM, the correction is too large to explain the recent CLEO data. We show that if topcolor is further introduced, the branching ratio in the topcolor assisted MWTCM can be in agreement with the CLEO data for a certain range of the parameters.

Mutual consideration of b → sγ and μ → eγ in supersymmetric SO(10) grand unification

We compare the branching ratios for b → sγ and μ → eγ in terms of constraining the parameter space in supersymmetric SO(10) grand unification models where supersymmetry is broken softly near the Planck scale by generationally symmetric operators. We observe two general cases. One with small tan β = 2 and the other one with large tan β having third generation Yukawa coupling unification at the GUT scale. We show that for small tan β the branching ratio constraints allow only a smaller region of parameter space for μ > 0 compared to μ < 0 for gluino mass ≲ 500 GeV. With large tan β, we find acceptable regions of parameter space with |μ| ≲ 1 TeV only for μ < 0. The dominant constraint on large tan β with μ > 0 parameter space is found to be given by the b → sγ branching ratio, while for large tan β with μ < 0 it is found to be given by the μ → eγ branching ratio. In many of these acceptable regions, we find that the μ → eγ branching ratio is predicted to be within one order of magnitude of its current experimental bound. We also show that the usually neglected gluino mediated diagrams in b → sγ can not be ignored in some regions of parameter space, especially for large tan β scenarios when the gluino mass is near its lower experimental bound.

EXPLANATION OF THE DATA ON $\Gamma (Z^0\to b\bar b) /\Gamma (Z^0\to {\rm hadrons})$ AND THE INCLUSIVE b→sγ BRANCHING RATIO

Calculations show that the data on the Z0 hadronic branching ratio fraction to [Formula: see text] at the Z0 pole as well as the inclusive b→sγ branching ratio can be explained by an anomalous (γ-Z0)WW coupling having ∆κ=−1 and λ=0. Such a vertex was derived from a representation in spinor space some time ago. The agreement is analyzed and possible implication of that basic spinor symmetry are described. Finally, the recent data on diboson production and their interpretation are examined critically. The used and shortly presented electroweak unification scheme of “structureless compositeness” provides two natural explanations of the increase of the inclusive jet cross-section above 200 GeV jet transverse energy observed at FNAL recently.

Radiative rare B decays revisited

We reexamine contributions of higher K-resonances to the radiative rare decays b → sγ in the limit where both the b- and s-quark are considered heavy. Using form factor definitions consistent with the HQET covariant trace formalism, we find significant disagreement with previous work and excellent agreement with experimental results. In particular, the two largest fractions of the inclusive b → sγ branching ratio are found to be (16.8 |+- 6.4)% for B → K ∗ (892)γ and (6.2 ± 2.9)% for B → K 2 ∗ (1430)γ decays. We also compare the contribution from the radiative decays into the eight K-meson states to the inclusive experimental b → sγ mass distribution.