Helicity Suppression Research Articles

InvisibleKLdecays as a probe of new physics

The decay $K_L \to invisible$ has never been experimentally tested. In the Standard Model (SM) its branching ratio for the decay into two neutrinos is helicity suppressed and predicted to be $Br(K_L \to \nu \bar{\nu}) \lesssim 10^{-10}$. We consider several natural extensions of the SM, such as two-Higgs-doublet (2HDM), 2HDM and light scalar, and mirror dark matter models, those main feature is that they allow to avoid the helicity suppression factor and lead to an enhanced $Br(K_L \to invisible)$. For the decay $K_L \to \nu \bar{\nu}$ the smallness of the neutrino mass in the considered 2HDM model is explained by the smallness of the second Higgs doublet vacuum expectation value. The small nonzero value of the second Higgs isodoublet can arise as a consequence of nonzero quark condensate. We show that taking into account the most stringent constraints from the $K \to \pi +invisible$ decay, this process could be in the region of $Br(K_L \to invisible) \simeq 10^{-8}-10^{-6}$, which is experimentally accessible. In some scenarios the $K_L \to invisible$ decay could still be allowed while the $K \to \pi +invisible$ decay is forbidden. The results obtained show that the $K_L \to invisible$ decay is a clean probe of new physics scales well above 100 TeV, that is complementary to rare $K \to \pi+invisible$ decay, and provide a strong motivation for its sensitive search in a near future experiment.

Electroweak fragmentation functions for dark matter annihilation

Electroweak corrections can play a crucial role in dark matter annihilation. The emission of gauge bosons, in particular, leads to a secondary flux consisting of all Standard Model particles, and may be described by electroweak fragmentation functions. To assess the quality of the fragmentation function approximation to electroweak radiation in dark matter annihilation, we have calculated the flux of secondary particles from gauge-boson emission in models with Majorana fermion and vector dark matter, respectively. For both models, we have compared cross sections and energy spectra of positrons and antiprotons after propagation through the galactic halo in the fragmentation function approximation and in the full calculation. Fragmentation functions fail to describe the particle fluxes in the case of Majorana fermion annihilation into light fermions: the helicity suppression of the lowest-order cross section in such models cannot be lifted by the leading logarithmic contributions included in the fragmentation function approach. However, for other classes of models like vector dark matter, where the lowest-order cross section is not suppressed, electroweak fragmentation functions provide a simple, model-independent and accurate description of secondary particle fluxes.

Enhancement of Majorana dark matter annihilation through Higgs bremsstrahlung

For Majorana dark matter, gauge boson bremsstrahlung plays an important role in enhancing an otherwise helicity-suppressed s-wave annihilation cross-section. This is well known for processes involving a radiated photon or gluon together with a Standard Model fermion-antifermion pair, and the case of massive electroweak gauge bosons has also recently been studied. Here we show that internal Higgs bremsstrahlung also lifts helicity suppression and could be the dominant contribution to the annihilation rate in the late Universe for dark matter masses below ∼ 1 TeV. Using a toy model of leptophilic dark matter, we calculate the annihilation cross-section into a lepton-antilepton pair with a Higgs boson and investigate the energy spectra of the final stable particles at the annihilation point.

Supersymmetry confrontsBs→μ+μ−: Present and future status

The purely leptonic rare decay ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ is very sensitive to supersymmetric contributions which are free from the helicity suppression of its Standard Model diagrams. The recent observation of the decay by the LHCb experiment and the first determination of its branching fraction motivate a review of their impact on the viable parameter space of supersymmetry (SUSY). In this paper we discuss the implications of the present and expected future accuracy on $\mathrm{BR}({B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})$ for constrained and unconstrained scenarios of the minimal supersymmetric extension of the Standard Model, in relation to the results from direct SUSY searches and the Higgs data at the LHC. While the constraints from $\mathrm{BR}({B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})$ can be very important in specific SUSY regions, we show that the current result, and even foreseen future improvements in its accuracy, will leave a major fraction of the SUSY parameter space, compatible with the results of direct searches, unconstrained. We also highlight the complementarity of the ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decay with direct SUSY searches.

On the effective operators for Dark Matter annihilations

We consider effective operators describing Dark Matter (DM) interactions with Standard Model fermions. In the non-relativistic limit of the DM field, the operators can be organized according to their mass dimension and their velocity behaviour, i.e. whether they describe s- or p-wave annihilations. The analysis is carried out for self-conjugate DM (real scalar or Majorana fermion). In this case, the helicity suppression at work in the annihilation into fermions is lifted by electroweak bremsstrahlung. We construct and study all dimension-8 operators encoding such an effect. These results are of interest in indirect DM searches.

B→D*τν¯τsensitivity to new physics

B physics has played a prominent role in investigations of new physics effects at low-energies. Presently, the largest discrepancy between a standard model prediction and experimental measurements appears in the branching ratio of the charged current mediated B to tau nu decay, where the large tau mass lifts the helicity suppression arising in leptonic B decays. Less significant systematic deviations are also observed in the semileptonic B to D(*) tau nu rates. Due to the rich spin structure of the final state, the decay mode B to D* tau nu offers a number of tests of such possible standard model deviations. We investigate the most general set of lowest dimensional effective operators leading to helicity suppressed modifications of b to c (semi)leptonic transitions. We explore such contributions to the B to D* tau nu decay amplitudes by determining the differential decay rate, longitudinal D* polarization fraction, D* - tau opening angle asymmetry and the tau helicity asymmetry. We identify the size of possible new physics contributions constrained by the present B to D(*) tau nu rate measurements and find significant modifications are still possible in all of them. In particular, the opening angle asymmetry can be shifted by almost 30%, relative to the standard model prediction, while the tau helicity asymmetry can still deviate by as much as 80%.

Dark matter annihilations into two light fermions and one gauge boson: general analysis and antiproton constraints

We study in this paper the scenario where the dark matter is constituted by Majorana particles which couple to a light Standard Model fermion and an extra scalar via a Yukawa coupling. In this scenario, the annihilation rate into the light fermions with the mediation of the scalar particle is strongly suppressed by the mass of the fermion. Nevertheless, the helicity suppression is lifted by the associated emission of a gauge boson, yielding annihilation rates which could be large enough to allow the indirect detection of the dark matter particles. We perform a general analysis of this scenario, calculating the annihilation cross section of the processes χχ→ff̄V when the dark matter particle is a SU(2)L singlet or doublet, f is a lepton or a quark, and V is a photon, a weak gauge boson or a gluon. We point out that the annihilation rate is particularly enhanced when the dark matter particle is degenerate in mass to the intermediate scalar particle, which is a scenario barely constrained by collider searches of exotic charged or colored particles. Lastly, we derive upper limits on the relevant cross sections from the non-observation of an excess in the cosmic antiproton-to-proton ratio measured by PAMELA.

Formula omitted] bremsstrahlung as the dominant annihilation channel for dark matter, revisited

We revisit the calculation of electroweak bremsstrahlung contributions to dark matter annihilation. Dark matter annihilation to leptons is necessarily accompanied by electroweak radiative corrections, in which a W or Z boson is also radiated. Significantly, while many dark matter models feature a helicity suppressed annihilation rate to fermions, bremsstrahlung process can remove this helicity suppression such that the branching ratios Br(ℓνW), Br(ℓ+ℓ−Z), and Br(ν¯νZ) dominate over Br(ℓ+ℓ−) and Br(ν¯ν). We find this is most significant in the limit where the dark matter mass is nearly degenerate with the mass of the boson which mediates the annihilation process. Electroweak bremsstrahlung has important phenomenological consequences both for the magnitude of the total dark matter annihilation cross section and for the character of the astrophysical signals for indirect detection. Given that the W and Z gauge bosons decay dominantly via hadronic channels, it is impossible to produce final state leptons without accompanying protons, antiprotons, and gamma rays.

W/Zbremsstrahlung as the dominant annihilation channel for dark matter

Dark matter annihilation to leptons, $\chi \chi \rightarrow l \bar{l}$, is necessarily accompanied by electroweak radiative corrections, in which a W or Z boson is radiated from a final state particle. Given that the W and Z gauge bosons decay dominantly via hadronic channels, it is thus impossible to produce final state leptons without accompanying protons, antiprotons, and gamma rays. Significantly, while many dark matter models feature a helicity suppressed annihilation rate to fermions, radiating a massive gauge boson from a final state fermion removes this helicity suppression, such that the branching ratios Br(l \nu W), Br(l^+l^- Z), and Br(\nu\nubar Z) dominate over Br(l\bar{l}). W/Z-bremsstrahlung thus allows indirect detection of many WIMP models that would otherwise be helicity-suppressed, or v^2 suppressed. Antiprotons and even anti-deuterons become consequential final state particles. This is an important result for future DM searches. We discuss the implications of W/Z-bremsstrahlung for "leptonic" DM models which aim to fit recent cosmic ray positron and antiproton data.

TWO DARK MATTER COMPONENTS IN NDMMSSM AND PAMELA DATA

PAMELA has observed the significant excess of the positron flux fraction. We explain it in the context of the dark matter annihilation process in our Galaxy. Since there is the helicity suppression, a majorana spin-1/2 (e.g. a neutralino in MSSM) and spin-0 dark matter cannot explain the data. We extend the MSSM minimally to incorporate 2 component dark matter and to produce positron flux from their co-annihilation.

Search for B s → µµγ at large hadron collider

The branching ratio for B s → l+l− γ mode is of the same order as B s → l+l−, since there is no helicity suppression in the 3-body decay mode. New Physics beyond Standard Model may affect these rates favourably for experimental observation at LHC and simultaneous measurements of the modes B s → µ+µ− and B s → µ+µ− γ at LHC experiment will indicate the basic nature of the interaction at play. A simulation study has been performed to evaluate the potential of CMS detector to observe the more difficult mode of B s → µ+µ−γ. An upper limit of 2.08 × 10−7 on the branching ratio is expected to be achieved corresponding to an integrated luminosity of 10 fb−1.

Generic dark matter signature for gamma-ray telescopes

We describe a characteristic signature of dark matter (DM) annihilation or decay into gamma rays. We show that if the total angular momentum of the initial DM particle(s) vanishes, and helicity suppression operates to prevent annihilation/decay into light fermion pairs, then the amplitude for the dominant 3-body final state ${f}^{+}{f}^{\ensuremath{-}}\ensuremath{\gamma}$ has a unique form dictated by gauge invariance. This amplitude and the corresponding energy spectra hold for annihilation of DM Majorana fermions or self-conjugate scalars, and for decay of DM scalars, thus encompassing a variety of possibilities. Within this scenario, we analyze Fermi LAT, PAMELA, and HESS data, and predict a hint in future Fermi gamma-ray data that portends a striking signal at atmospheric Cherenkov telescopes.

Radiative leptonic decay:B−→μ−ν¯μγin a relativistic independent quark model

We study the radiative leptonic decay, ${B}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\gamma}$, in the framework of a relativistic independent quark model, based on the confining potential in the scalar-vector harmonic form. As expected, we find that the photon emission in this decay overcomes the so-called helicity suppression inevitable in the case of pure leptonic decay $({B}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}})$. Our result for the branching ratio is $\mathcal{B}r({B}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\gamma})=1.70\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, which is comparable with other model predictions within the current experimental upper limit. The photon energy spectrum predicted in the model is slightly asymmetric with the peak value around 1.45 GeV, which should render it quite accessible to experimental analysis.

LHC program for very rare B decays

This paper gives an overview of ATLAS, CMS and LHCb potential in measurement of di-muonic rare B-decays B s 0 → μ + μ − and B d 0 → μ + μ − . Branching ratios (BR) of these decays are small due to helicity suppression and Flavour Changing Neutral Currents (FCNC) forbidden at tree level. Accounting clear theoretical picture for the BR prediction and simple experimental signature, measurement of the BR provides excellent probe of New Physics effects. While present experimental upper limits from Tevatron are roughly 50× above Standard Model (SM) prediction, LHC experiments will reach the SM branching already around the end of initial low-luminosity stage of LHC ( 30 fb −1 ) . In the paper, offline analysis and the expected number of signal and background events are presented. Because of small BR, background coming from misidentification effects and rare exclusive and exotic decays can be important. Therefore part of this paper is also devoted to study of these background sources.

Photon polarization from helicity suppression in radiative decays of polarized [formula omitted] to spin-3/2 baryons

We give a general parameterization of the Λ b → Λ ( 1520 ) γ decay amplitude, applicable to any strange isosinglet spin-3/2 baryon, and calculate the branching fraction and helicity amplitudes. Large-energy form factor relations are worked out, and it is shown that the helicity-3/2 amplitudes vanish at lowest order in soft-collinear effective theory (SCET). The suppression can be tested experimentally at the LHC and elsewhere, thus providing a benchmark for SCET. We apply the results to assess the experimental reach for a possible wrong-helicity b → s γ dipole coupling in Λ b → Λ ( 1520 ) γ → p K γ decays. Furthermore we revisit Λ b -polarization at hadron colliders and update the prediction from heavy-quark effective theory. Opportunities associated with b → d γ afforded by high-statistics Λ b samples are briefly discussed in the general context of CP and flavour violation.

Short-distance analysis of [formula omitted] mixing

We study the Standard Model short-distance prediction for the mass and lifetime differences between the two neutral D meson mass eigenstates. We find that, despite α s / 4 π suppression, next-to-leading order (NLO) short-distance QCD corrections exceed the corresponding leading order (LO) amplitudes. For the lifetime difference, this stems from the lifting of helicity suppression of a light-quark intermediate state. We find y D is given by y NLO to a reasonable approximation but x D is greatly affected by destructive interference between x LO and x NLO . The net effect is to render y D ∼ x D ≃ 6 × 10 −7 . Our NLO short-distance results, still smaller than most long-distance estimates, depend on the same two nonperturbative matrix elements of four-quark operators as in leading order.

MesonBcannihilation to leptons and inclusive light hadrons

The annihilation of the ${B}_{c}$ meson to leptons and inclusive light hadrons is analyzed in the framework of nonrelativistic QCD (NRQCD) factorization. We find that the decay mode, which escapes from the helicity suppression, contributes a sizable fraction width. The branching ratio due to the contributions from the color-singlet component of the ${B}_{c}$ meson can be of the order of ${10}^{\ensuremath{-}2}.$ The contributions from the color-octet components are estimated. With the consideration of the NRQCD velocity scaling rule, we find that the color-octet contributions are sizable too, especially in a certain phase space of the annihilation. They are greater than (or comparable to) those from the color-singlet component. A few observables on the spectrum of charged leptons $(e,\ensuremath{\mu})$ relevant to the contributions are suggested that may be used as measurements of the evidence of the color-octet components in future ${B}_{c}$ experiments. A kind of typical ``long-distance contributions'' in the annihilation is also estimated.

Nonperturbative effects in semileptonicBcdecays

We discuss the impact of the soft degrees of freedom inside the B_c meson on its rate in the semi-leptonic decay B_c -> X l nu_l where X denotes light hadrons below the D^0 threshold. In particular we identify contributions involving soft hadrons which are non-vanishing in the limit of massless leptons. These contributions become relevant for a measurement of the purely leptonic B_c decay rate, which due to helicity suppression involves a factor m_l^2 and thus is much smaller than the contributions involving soft hadrons.

Analysis ofB→φKdecays in QCD factorization

We analyze the decay $B\to \phi K$ within the framework of QCD-improved factorization. We found that although the twist-3 kaon distribution amplitude dominates the spectator interactions, it will suppress the decay rates slightly. The weak annihilation diagrams induced by $(S-P)(S+P)$ penguin operators, which are formally power-suppressed by order $(\Lambda/m_b)^2$, are chirally and logarithmically enhanced. Therefore, these annihilation contributions are not subject to helicity suppression and can be sizable. The predicted branching ratio of $B^-\to\phi K^-$ is $(3.8\pm0.6)\times 10^{-6}$ in the absence of annihilation contributions and it becomes $(4.3^{+3.0}_{-1.4})\times 10^{-6}$ when annihilation effects are taken into account. The prediction is consistent with CLEO and BaBar data but smaller than the BELLE result.

PURE LEPTONIC RADIATIVE DECAYS B±, Ds→ℓνγ AND THE ANNIHILATION GRAPH

Pure leptonic radiative decays of heavy-light mesons are calculated using a very simple nonrelativistic model. For B± the dominant contribution originates from photon emission from light initial quark. Emission of the photon overcomes the helicity suppression and leads to Br (B±→ℓνγ)~3.5×10–6 (which is about 16 times more than Br (B±→µ+ν)). Also, we find Br (Ds→ℓνγ)~2.6×10–5. The importance of these reactions in clarifying the dynamics of the annihilation graph is emphasized.