Gamma Decay Rate Research Articles

Four-body contributions to B ¯ → X s γ $$ \overline{B}\to {X}_s\gamma $$ at NLO

Ongoing efforts to reduce the perturbative uncertainty in the B -> Xs gamma decay rate have resulted in a theory estimate to NNLO in QCD. However, a few contributions from multi-parton final states which are formally NLO are still unknown. These are parametrically small and included in the estimated error from higher order corrections, but must be computed if one is to claim complete knowledge of the B -> Xs gamma rate to NLO. A major part of these unknown pieces are four-body contributions corresponding to the partonic process b -> s qbar q gamma. We compute these NLO four-body contributions to B -> Xs gamma, and confirm the corresponding tree-level leading-order results. While the NLO contributions arise from tree-level and one-loop Feynman diagrams, the four-body phase-space integrations make the computation non-trivial. The decay rate contains collinear logarithms arising from the mass regularization of collinear divergences. We perform an exhaustive numerical analysis, and find that these contributions are positive and amount to no more than 1% of the total rate in the Standard Model, thus confirming previous estimates of the perturbative uncertainty.

Higgs decays in gauge extensions of the standard model

We explore the phenomenology of virtual spin-1 contributions to the h to gamma gamma and h to Z gamma decay rates in gauge extensions of the standard model. We consider generic lorentz and gauge invariant vector self-interactions, which can have non-trivial structure after diagonalizing the quadratic part of the action. Such features are phenomenologically relevant in models where the electroweak gauge bosons mix with additional spin-1 fields, such as occurs in little higgs models, extra dimensional models, strongly coupled variants of electroweak symmetry breaking, and other gauge extensions of the standard model. In models where non-renormalizable operators mix field strengths of gauge groups, the one-loop higgs decay amplitudes can be logarithmically divergent, and we provide power counting for the size of the relevant counter-term. We provide an example calculation in a 4-site moose model that contains degrees of freedom that model the effects of vector and axial vector resonances arising from TeV scale strong dynamics.

Exotic leptons: Higgs, flavor and collider phenomenology

We study extensions of the standard model by one generation of vector-like leptons with non-standard hypercharges, which allow for a sizable modification of the h -> gamma gamma decay rate for new lepton masses in the 300 GeV - 1 TeV range. We analyze vaccum stability implications for different hypercharges. Effects in h -> Z gamma are typically much smaller than in h -> gamma gamma, but distinct among the considered hypercharge assignments. Non-standard hypercharges constrain or entirely forbid possible mixing operators with standard model leptons. As a consequence, the leading contributions to the experimentally strongly constrained electric dipole moments of standard model fermions are only generated at the two loop level by the new CP violating sources of the considered setups. We derive the bounds from dipole moments, electro-weak precision observables and lepton flavor violating processes, and discuss their implications. Finally, we examine the production and decay channels of the vector-like leptons at the LHC, and find that signatures with multiple light leptons or taus are already probing interesting regions of parameter space.

Higgs boson decays to quarkonia and theHc¯ccoupling

In this paper we discuss decays of the Higgs boson to quarkonia in association with a photon. We identify a new mechanism for producing such final states in Higgs decays that leads to predictions for the decay rates that differ by an order of magnitude from previous estimates. Although the branching ratios for these processes are still small, the processes are experimentally clean, and the H \to J/\psi+gamma decay should be observable at a 14 TeV LHC. We point out that quantum interference between two different production mechanisms makes the decay rates sensitive to the HQQbar couplings. Consequently, measurements of the H \to J/\psi+gamma decay rate would allow one to probe the Higgs-charm coupling directly at the LHC. We discuss the experimental prospects for the observation of these decays and for the direct measurement of the Hccbar coupling.

Vector meson dominance and theπ0transition form factor

It is shown that the pi^0 transition form factor F(Q_1^2,Q_2^2) differs substantially from its one-real-photon limit F(Q_1^2,0) even for rather small values of Q_2^2 (approx 0.1 GeV^2), which cannot be excluded in experiments with one "untagged" electron. It indicates that the comparison of data with theoretical calculations, which usually assume Q_2^2=0, may be untrustworthy. Our phenomenological model of the pi^0 transition form factor is based on the vector-meson-dominance hypothesis and all its parameters are fixed by using the experimental data on the decays of vector mesons. The model soundness is checked in the two-real-photon limit, where it provides a good parameter-free description of the pi^0 -> 2 gamma decay rate, and in the pi^0 Dalitz decay. The dependence of F(Q_1^2,Q_2^2) on Q_1^2 at several fixed values of Q_2^2 is presented and the comparison with existing data performed.

QCD corrections to flavor changing neutral coupling mediated rare top quark decays

Recently we have presented an analysis of flavor changing neutral coupling mediated radiative top quark decays at next-to-leading order in QCD. In the present paper we provide the details of the calculation of QCD corrections to t-> q gamma and t-> q Z decays within the effective theory approach including operator mixing. In particular, we calculate virtual matrix element corrections and the corresponding bremsstrahlung contributions. In the case of t-> q gamma we study the effects of kinematic cuts on the extracted branching ratios. Analytical formulae are given at all stages of the calculation. We find that the t-> q gamma decay can be used to probe also the effective operators mediating t-> q g processes, since these can naturally contribute 10% or more to the radiative decay, given typical experimental cuts on the decay kinematics at hadron colliders. Conversely, we argue that any positive experimental signal of the t-> q g process would indicate a natural lower bound on t-> q gamma decay rate.

Advanced predictions for moments of theB¯→Xsγphoton spectrum

Based on a new, exact QCD factorization formula for the partial B->X_s+gamma decay rate with a restriction on large photon energy, improved predictions are presented for the partial moments <E_gamma> and <E_gamma^2>-<E_gamma>^2 of the photon spectrum defined with a cut E_gamma>E_0. In the region where Delta=m_b-2E_0 is large compared with Lambda_{QCD}, a theoretical description without recourse to shape functions can be obtained. However, for Delta<<m_b it is important to separate short-distance contributions arising from different scales. The leading terms in the heavy-quark expansion of the moments receive contributions from the scales Delta and \sqrt{m_b Delta} only, but not from the hard scale m_b. For these terms, a complete scale separation is achieved at next-to-next-to-leading order in renormalization-group improved perturbation theory, including two-loop matching contributions and three-loop running. The results presented here can be used to extract the b-quark mass and the quantity mu_pi^2 with excellent theoretical precision. A fit to experimental data reported by the Belle Collaboration yields m_b^{SF}=(4.62+-0.10_{exp}+-0.03_{th})GeV and mu_pi^{2,SF}=(0.11+-0.19_{exp}+-0.08_{th})GeV^2 in the shape-function scheme at a scale mu_f=1.5GeV, while m_b^{kin}=(4.54+-0.11_{exp}+-0.04_{th})GeV and mu_pi^{2,kin}=(0.49+-0.18_{exp}+-0.09_{th})GeV^2 in the kinetic scheme at a scale mu_f=1GeV.

Electromagnetic dipole operator effect onB¯→XsγatO(αs2)

The flavor-changing electromagnetic dipole operator O_7 gives the dominant contribution to the B -> Xs gamma decay rate. We calculate two-loop QCD corrections to its matrix element together with the corresponding bremsstrahlung contributions. The optical theorem is applied, and the relevant imaginary parts of three-loop diagrams are computed following the lines of our recent t -> Xb W calculation. The complete result allows us to test the validity of the naive non-abelianization (NNA) approximation that has been previously applied to estimate the NNLO QCD correction to Gamma(B -> Xs gamma)/Gamma(B -> Xu e nu). When both decay widths are normalized to m^5_{b,R} in the same renormalization scheme R, the calculated O(alpha_s^2) correction is sizeable (~ 6%), and the NNA estimate is about 1/3 too large. On the other hand, when the ratio of the decay widths is written as S*(m_b(m_b)/m_{b,pole})^2, the calculated O(alpha_s^2) correction to S is at the level of 1% for both the complete and the NNA results.

Updated reach of CERN LHC and constraints from relic density,b→sγ andaμin the mSUGRA model

{We present an updated assessment of the reach of the CERN LHC pp collider for supersymmetric matter in the context of the minimal supergravity (mSUGRA) model. In addition to previously examined channels, we also include signals with an isolated photon or with a leptonically decaying Z boson. For an integrated luminosity of 100 fb^{-1}, values of m_{1/2}\sim 1400 GeV can be probed for small m_0, corresponding to a gluino mass of m_{\tg}\sim 3 TeV. For large m_0, in the hyperbolic branch/focus point region, m_{1/2}\sim 700 GeV can be probed, corresponding to m_{\tg}\sim 1800 GeV. We also map out parameter space regions preferred by the measured values of the dark matter relic density, the b\to s\gamma decay rate, and the muon anomalous magnetic moment a_\mu, and discuss how SUSY might reveal itself in these regions. We find the CERN LHC can probe the entire stau co-annihilation region and also most of the heavy Higgs annihilation funnel allowed by WMAP data, except for some range of large m_0 and m_{1/2} if \tan\beta \agt 50.

Supersymmetric seesaw mechanism without singlet neutrinos: Neutrino masses and lepton-flavor violation

We consider the supersymmetric seesaw mechanism induced by the exchange of heavy SU(2)_W triplet states, rather than `right-handed' neutrino singlets, to generate neutrino masses. We show that in this scenario the neutrino flavour structure tested at low-energy in the atmospheric and solar neutrino experiments is directly inherited from the neutrino Yukawa couplings to the triplets. This allows us to predict the ratio of the tau --> mu gamma (or tau --> e gamma) and mu --> e gamma decay rates in terms of the low-energy neutrino parameters. Moreover, once the model is embedded in a grand unified model, quark-flavour violation can be linked to lepton-flavour violation.

Phenomenological constraints on SUSY SU(5) GUTs with nonuniversal gaugino masses

We study phenomenological aspects of supersymmetric SU(5) grand unified theories with non-universal gaugino masses. For large tan beta, we investigate constraints from the requirement of successful electroweak symmetry breaking, the positivity of stau mass squared and the b to s gamma decay rate. In the allowed region, the nature of the lightest supersymmetric particle is determined. Examples of mass spectra are given. We also calculate loop corrections to the bottom mass due to superpartners.

Remarks on nonperturbativeO(1/mc2)corrections toΓ(B¯→Xsγ)

We present an estimate of certain higher order corrections to the contribution of the charm triangle loop in the inclusive B -> X_s gamma decay rate, recently discussed by Voloshin. We find that these corrections are minute and hence the result found by Voloshin, although small, is quite robust.

Energy levels, electromagnetic transitions and annihilation of positronium in strong magnetic fields

As a further application of the authors previous discussion of the general two-body problem in the presence of a homogeneous magnetic field, a quantitative investigation of the physical properties of positronium in super-strong magnetic fields is presented. They show that, in magnetic fields, the energy values and electromagnetic transitions rates of positronium can also be reduced by a scaling transformation to those obtained for the hydrogen atom in the limit of an infinite proton mass. Additionally, the annihilation process is considered, and the B dependence of the gamma decay rates is discussed. For magnetic fields B=2.35*1011 G and 4.70*1012 G these quantities are calculated in the adiabatic approximation and are presented explicitly for the lowest-lying states.

Radiative weak decays of uncharmed hyperons

By assuming that the anomalous electromagnetic (EM) Hamiltonian transforms like the T11+bT22 component of an octet, the authors explain the large asymmetry parameter for Sigma + to p gamma decay. By employing the non-exoticity of intermediate states and s-u-channel symmetry of the weak Hamiltonian, they obtain several new sum rules relating decay amplitudes in different charge sectors. Branching fractions and asymmetry parameters for these decays are also predicted. The calculated Xi - to sigma - gamma decay rate is in agreement with the measured upper limit.

The slope of the form factor in pseudoscalar Dalitz decays

It is pointed out that a negative slope for the form factor in pseudoscalar Dalitz decays can be explained by vector-meson dominance with a subtracted dispersion relation. Calculations are made for the pi 0 and eta using the experimental omega to pi 0 gamma , phi to pi 0 gamma and phi to eta gamma decay rates and SU(3) symmetry. An incidental consequence of these calculations is that excitable colour gluons do not significantly affect eta Dalitz decay.

Inelastic Electron Scattering Studies:Cu63,In115,Ta181,Pb208, andBi209

Measurements were made of the cross sections for scattering of electrons of primary energies from 183 MeV to 600 MeV from ${\mathrm{Cu}}^{63}$, ${\mathrm{In}}^{115}$, ${\mathrm{Ta}}^{181}$, ${\mathrm{Pb}}^{208}$, and ${\mathrm{Bi}}^{209}$, using high-resolution detectors. A number of $E2$, $E3$, $E4$, and two $E5$ transitions were induced with nuclear excitations up to 7 MeV. The levels excited have enhanced gamma-decay rates to the nuclear ground states. In the odd-$A$ nuclei most of the observed transitions were to clusters of near-similar states. The results are compared in detail to studies of ($p,{p}^{\ensuremath{'}}$), ($d,{d}^{\ensuremath{'}}$), ($\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}$), and ($n,{n}^{\ensuremath{'}}$) reactions in the same nuclei. Great similarities between these and the ($e,{e}^{\ensuremath{'}}$) are established. It is shown how the combined results can be used to determine the disposition and intensities of the levels within the clusters and how these are related to the discrete-energy core excitations in neighboring even-even nuclei. Data taken at high momentum transfer show that inelastic electron scattering continues to be dominated by excitation of collective states in disagreement with predictions based on present understanding of the process. Excitations seen in both ${\mathrm{Bi}}^{209}$ and ${\mathrm{Pb}}^{208}$ at about 5.8 MeV are identified as leading to collective states in these nuclei. They are probably E5 transitions although $E4$ assignments cannot be excluded. The collective states are not seen in ${\mathrm{Ta}}^{181}$ and ${\mathrm{Au}}^{197}$ in ($e,{e}^{\ensuremath{'}}$).

XVIII. The decay of potassium 40

Abstract The decay of 40K has been reinvestigated and values of the branching ratio, defined as the ratio of gamma or electron capture to beta transitions, have been determined by two methods, giving the values 0·124±0·002 and 0·121±0·004 respectively. The absolute beta and gamma decay rates and the half-life have been measured, the value of the latter being 1·28±0±02 × 109 years. A fresh determination of the gamma energy is described, and evidence as to the mode of decay is discussed. Finally, the results are given of experiments on the reflection of electrons by solid source supports.