One-loop multicollinear limits from 2-point amplitudes on self-dual backgrounds

For scattering amplitudes in strong background fields, it is — at least in principle — possible to perturbatively expand the background to obtain higher-point vacuum amplitudes. In the case of self-dual plane wave backgrounds we consider this expansion for two-point, one-loop amplitudes in pure Yang-Mills, QED and QCD. This enables us to obtain multicollinear limits of 1-loop vacuum amplitudes; the resulting helicity configurations are surprisingly restricted, with only the all-plus helicity amplitude surviving. These results are shown to be consistent with well-known vacuum amplitudes. We also show that for both abelian and non-abelian supersymmetric gauge theories, there is no helicity flip (and hence no vacuum birefringence) on any plane wave background, generalising a result previously known in the Euler-Heisenberg limit of super-QED.

On non-factorisable contributions to t-channel single-top production

We compute the non-factorisable contribution to the two-loop helicity amplitude for t-channel single-top production, the last missing piece of the two-loop virtual corrections to this process. Our calculation employs analytic reduction to master integrals and the auxiliary mass flow method for their fast numerical evaluation. We study the impact of these corrections on basic observables that are measured experimentally in the single-top production process.

Angular analysis of $$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ with $$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ decays

The first full angular analysis of the $$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ B 0 → D ∗ − D s ∗ + decay is performed using 6 fb−1 of pp collision data collected with the LHCb experiment at a centre-of-mass energy of 13 TeV. The $$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ D s ∗ + → D s + γ and D*− → $$ {\overline{D}}^0{\pi}^{-} $$ D ¯ 0 π − vector meson decays are used with the subsequent $$ {D}_s^{+} $$ D s + → K+K−π+ and $$ {\overline{D}}^0 $$ D ¯ 0 → K+π− decays. All helicity amplitudes and phases are measured, and the longitudinal polarisation fraction is determined to be fL = 0.578 ± 0.010 ± 0.011 with world-best precision, where the first uncertainty is statistical and the second is systematic. The pattern of helicity amplitude magnitudes is found to align with expectations from quark-helicity conservation in B decays. The ratio of branching fractions [ℬ($$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ B 0 → D ∗ − D s ∗ + ) × ℬ($$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ D s ∗ + → D s + γ )]/ℬ(B0 → D*−$$ {D}_s^{+} $$ D s + ) is measured to be 2.045 ± 0.022 ± 0.071 with world-best precision. In addition, the first observation of the Cabibbo-suppressed Bs → D*−$$ {D}_s^{+} $$ D s + decay is made with a significance of seven standard deviations. The branching fraction ratio ℬ(Bs → D*−$$ {D}_s^{+} $$ D s + )/ℬ(B0 → D*−$$ {D}_s^{+} $$ D s + ) is measured to be 0.049 ± 0.006 ± 0.003 ± 0.002, where the third uncertainty is due to limited knowledge of the ratio of fragmentation fractions.

Two-loop mixed QCD-EW corrections to gg → Hg

We compute the two-loop mixed QCD-Electroweak (QCD-EW) corrections to the production of a Higgs boson and a gluon in gluon fusion through a loop of light quarks. The relevant four-point functions with internal massive propagators are expressed as multiple polylogarithms with algebraic arguments. We perform the calculation by integration over Feynman parameters and, independently, by the method of differential equations. We compute the two independent helicity amplitudes for the process and we find that they are both finite. Moreover, we observe a weight drop when all gluons have the same helicity. We also provide a simplified expression for the all-plus helicity amplitude, which is optimised for fast and reliable numerical evaluation in the physical region.

Branching Ratio and CP Violation in B s 0 ¯ → ϕϕ Decay in the Framework of QCD Factorization

We present an analytic calculation of Branching Ratio (BR) and Charge-Parity (CP) violating asymmetries of the B s 0 ¯ meson decay into the two light vectors ϕ ϕ . In doing this we calculate the helicity amplitude of the present decay in the framework of QCD factorization approach. We find the BR of B s 0 ¯ → ϕ ϕ = ( 1.56 ± 0.23 ) × 10 − 5 . We also calculate the direct CP violation, CP violation in mixing and CP violation due to interference which are A C P dir = 0.00355 ± 0.00152 , A C P mix = − 0.00629 ± 0.03119 and A C P Δ Γ = 0.99997 ± 0.00019 , respectively. Our results are in agreement with the recent theoretical predictions and experimental measurements.