Next-to-leading-order QCD corrections to a vector bottomonium radiative decay into a charmonium

Within the framework of nonrelativistic QCD (NRQCD) factorization, we calculate the next-to-leading-order (NLO) perturbative corrections to the radiative decay Υ → ηc(χcJ) + γ. Both the helicity amplitudes and the helicity decay widths are obtained. It is the first computation for the processes involving both bottomonium and charmonium at two-loop accuracy. By employing the Cheng-Wu theorem, we are able to convert most of complex-valued master integrals (MIs) into real-valued MIs, which makes the numerical integration much efficient. Our results indicate the $$ \mathcal{O}\left({\alpha}_s\right) $$ O α s corrections are moderate for ηc and χc2 production, and are quite marginal for χc0 and χc1 production. It is impressive to note the NLO corrections considerably reduce the renormalization scale dependence in both the decay widths and the branching fractions for χcJ, and slightly improve that for ηc. With the NRQCD matrix elements evaluated via the Buchmüller-Tye potential model, we find the decay width for ηc production is one-order-of-magnitude larger than χcJ production, which may provide a good opportunity to search for Υ → ηc + γ in experiment. In addition, the decay width for χc1 production is several times larger than those for χc0,2. Finally, we find the NLO NRQCD prediction for the branching fraction of Υ → χc1 + γ is only half of the lower bound of the experimental data measured recently by Belle. Moreover, there exists serious contradiction between theory and experiment for Υ → ηc + γ. The discrepancies between theory and experiment deserve further research efforts.

Next-to-leading-order corrections to the Higgs strahlung process from electron–positron collisions in extended Higgs models

We present the cross section for $$e^{+}e^{-}\rightarrow hZ$$ e + e - → h Z with arbitrary sets of electron and Z boson polarizations at the full next-to-leading order in various extended Higgs models, such as the Higgs singlet model (HSM), the inert doublet model (IDM) and the two Higgs doublet model (2HDM). We systematically perform complete one-loop calculations to the helicity amplitudes in the on-shell renormalization scheme, and present the full analytic results as well as numerical evaluations. The deviation $$\Delta R^{hZ}$$ Δ R hZ in the total cross section from its standard model (SM) prediction is comprehensively analyzed, and the differences among these models are discussed in details. We find that new physics effects appearing in the renormalized hZZ vertex almost govern the behavior of $$\Delta R^{hZ}$$ Δ R hZ , and it takes a negative value in most cases. The possible size of $$\Delta R^{hZ}$$ Δ R hZ reaches several percent under the theoretical and experimental bounds. We also analyze the deviation $$\Delta R^{hZ}_{XY}$$ Δ R XY hZ in the total cross section times decay branching ratios of the discovered Higgs boson by utilizing the program. It is found that the four types of 2HDMs can be discriminated by analyzing the correlation between $$\Delta R^{hZ}_{\tau \tau }$$ Δ R τ τ hZ and $$\Delta R^{hZ}_{bb}$$ Δ R bb hZ and those between $$\Delta R^{hZ}_{\tau \tau }$$ Δ R τ τ hZ and $$\Delta R^{hZ}_{cc}$$ Δ R cc hZ . Furthermore, the HSM and the IDM can be discriminated from the 2HDMs by measuring $$\Delta R^{hZ}_{WW}$$ Δ R WW hZ . These signatures can be tested by precision measurements at future Higgs factories such as the International Linear Collider.

Scattering amplitudes for all masses and spins

We introduce a formalism for describing four-dimensional scattering amplitudes for particles of any mass and spin. This naturally extends the familiar spinor-helicity formalism for massless particles to one where these variables carry an extra SU(2) little group index for massive particles, with the amplitudes for spin S particles transforming as symmetric rank 2S tensors. We systematically characterise all possible three particle amplitudes compatible with Poincare symmetry. Unitarity, in the form of consistent factorization, imposes algebraic conditions that can be used to construct all possible four-particle tree amplitudes. This also gives us a convenient basis in which to expand all possible four-particle amplitudes in terms of what can be called “spinning polynomials”. Many general results of quantum field theory follow the analysis of four-particle scattering, ranging from the set of all possible consistent theories for massless particles, to spin-statistics, and the Weinberg-Witten theorem. We also find a transparent understanding for why massive particles of sufficiently high spin cannot be “elementary”. The Higgs and Super-Higgs mechanisms are naturally discovered as an infrared unification of many disparate helicity amplitudes into a smaller number of massive amplitudes, with a simple understanding for why this can’t be extended to Higgsing for gravitons. We illustrate a number of applications of the formalism at one-loop, giving few-line computations of the electron (g − 2) as well as the beta function and rational terms in QCD. “Off-shell” observables like correlation functions and form-factors can be thought of as scattering amplitudes with external “probe” particles of general mass and spin, so all these objects — amplitudes, form factors and correlators, can be studied from a common on-shell perspective.

Two-loop leading-colour QCD helicity amplitudes for Higgs boson production in association with a bottom-quark pair at the LHC

We compute the two-loop QCD helicity amplitudes for the production of a Higgs boson in association with a bottom quark pair at a hadron collider. We take the approximations of leading colour and work in the five flavour scheme, where the bottom quarks are massless while the Yukawa coupling is non-zero. We extract analytic expressions from multiple numerical evaluations over finite fields and present the results in terms of an independent set of special functions that can be reliably evaluated over the full phase space.

Time-reversal asymmetries and angular distributions in Λb → ΛV

We study the spin correlations to probe time-reversal (T) asymmetries in the decays of Λb→ ΛV (V = ϕ, ρ0, ω, K∗0). The eigenstates of the T-odd operators are obtained along with definite angular momenta. We obtain the T-odd spin correlations from the complex phases among the helicity amplitudes. We give the angular distributions of Λb→ Λ(→ pπ−)V (→ PP′) and show the corresponding spin correlations, where P(′) are the pseudoscalar mesons. Due to the helicity conservation of the s quark in Λ, we deduce that the polarization asymmetries of Λ are close to −1. Since the decay of Λb→ Λϕ in the standard model (SM) is dictated by the single weak phase from the product of CKM elements, $$ {V}_{tb}{V}_{ts}^{\ast } $$ V tb V ts ∗ , the true T and CP asymmetries are suppressed, providing a clean background to test the SM and search for new physics. In the factorization approach, as the helicity amplitudes in the SM share the same complex phase, T-violating effects are absent. Nonetheless, the experimental branching ratio of Br(Λb→ Λϕ) = (5.18 ± 1.29) × 10−6 suggests that the nonfactorizable effects or some new physics play an important role. By parametrizing the nonfactorizable contributions with the effective color number, we calculate the branching ratios and direct CP asymmetries. We also explore the possible T-violating effects from new physics.

Probing anomalous γγγZ couplings through γZ production in γγ collisions at the CLIC

We have estimated the sensitivity to the anomalous couplings of the γγγZ vertex in the γγ → γZ scattering of the Compton backscattered photons at the CLIC. Both polarized and unpolarized collisions at the e+e− energies 1500 GeV and 3000 GeV are addressed, and anomalous contributions to helicity amplitudes are derived. The differential and total cross sections are calculated. We have obtained 95% C.L. exclusion limits on the anomalous quartic gauge couplings (QGCs). They are compared with corresponding bounds derived for the γγγZ couplings via γZ production at the LHC. The constraints on the anomalous QGCs are one to two orders of magnitude more stringent that at the HL-LHC. The partial-wave unitarity constraints on the anomalous couplings are examined. It is shown that the unitarity is not violated in the region of the anomalous QGCs studied in the paper.

Calculation of doubly heavy baryons decay

We calculated a subclass of four nonleptonic two-body weak decays of the double charm baryon ground states Ξcc++. Nonleptonic decays can be dividedinto two group: factorizable and nonfactorizable decays. The first one can be easily calculated from first principles. Therefore, its good example understands all pros and cons of a model. We focused on a weak two-body nonleptonic decay consists only fromthe factorizing contribution precluding a contamination from W-exchange. We use the covariant confined quark model previously developed by us to calculate the various helicity amplitudes which describe the dynamics of the transition induced by the Cabibbo-favored effective currents. Achievement of CCQM is that only size parameter Λvarying can describe all tree diagrams of nonleptonic decay. We then proceed to calculate the rates of the decay. The rates, branching ratios and helicity amplitude were calculated using dimensionless invariant form factors. Also, we calculated leptonic constant for scalar and vector mesons which has good agreement with experimental data. There isn’t any experimental data about the decay so we waiting for new experimental observation in the heavy baryon sector.

Lattice QCD study of the elastic and transition form factors of charmed baryons

Composite nature of a particle can be probed by electromagnetic interactions and information about their structure is embedded in form factors. Most of the experimental and theoretical efforts on baryon electromagnetic form factors have been focused on nucleon while the data on charmed sector are limited to spectroscopy, and weak and strong decays. Forthcoming experiments with a heavy-hadron physics program at major experimental facilities are expected to provide a wealth of information on charmed baryons, which calls for a better understanding of the heavy-sector dynamics from theoretical grounds. We review the progress in calculating the elastic and transition form factors of charmed baryons in lattice QCD. A collection of static observables, e.g. charge radii, multipole moments, are presented along with the elastic form factors up to [Formula: see text]. As one would expect the charmed baryons are compact in comparison to nucleon and this is due to the presence of valence charm quark(s). The elastic and transition magnetic moments are both suppressed. The lattice results provide predictions for the transition magnetic moments, transition and helicity amplitudes and consequentially the decay widths of some singly and doubly charmed baryons. In general, lattice results are consonant with the qualitative expectations of quark model and heavy-quark symmetry, although there are apparent quantitative differences up to two orders of magnitude in some cases. There are, however, indications that the lattice results can be utilized to improve the model predictions. Nevertheless, discrepancies between the lattice and nonlattice calculations need to be understood better to have a solid insight into the dynamics of the heavy sector. Furthermore, reliably determined charmed baryon observables would be invaluable input to investigate the nature of exotic states, which further emphasizes the importance of rigorous, first-principles calculations to advance our understanding of the dynamics of the heavy quarks and strong interactions.