Next-to-leading-logarithm threshold resummation for exclusive B meson decays

We extend the threshold resummation of the large logarithms $$\ln x$$ ln x which appear in factorization formulas for exclusive B meson decays, x being a spectator momentum fraction, to the next-to-leading-logarithm (NLL) accuracy. It is shown that the NLL resummation effect provides suppression in the end-point region with $$x\sim 0$$ x ∼ 0 stronger than the leading-logarithm (LL) one, and thus improves perturbative analyses of the above processes. We revisit the $$B\rightarrow K\pi $$ B → K π decays under the NLL resummation in the perturbative QCD approach, and find that it induces up to 20% variation of the direct CP asymmetries in various modes relative to the LL results. Our method to avoid the Landau singularity in the inverse Mellin transformation causes at most 10% theoretical uncertainty.

Improvements on dipole shower colour

The dipole formalism provides a powerful framework from which parton showers can be constructed. In a recent paper (Forshaw et al. 2020), we proposed a dipole shower with improved colour accuracy and in this paper we show how it can be further improved. After an explicit check at $${\mathcal {O}}(\alpha _{\mathrm {s}}^{2})$$ O ( α s 2 ) we confirm that our original shower performs as it was designed to, i.e. inheriting its handling of angular-ordered radiation from a coherent branching algorithm. We also show how other dipole shower algorithms fail to achieve this. Nevertheless, there is an $${\mathcal {O}}(\alpha _{\mathrm {s}}^{2})$$ O ( α s 2 ) topology where it differs at sub-leading $$N_{\mathrm {c}}$$ N c from a coherent branching algorithm. This erroneous topology can contribute a leading logarithm to some observables and corresponds to emissions that are ordered in $$k_t$$ k t but not angle. We propose a simple, computationally efficient way to correct this and assign colour factors in accordance with the coherence properties of QCD to all orders in $$\alpha _{\mathrm {s}}$$ α s .

Precision QCD phenomenology of exotic spin-2 search at the LHC

The complete next-to-next-to leading order (NNLO) QCD correction matched with next-to-next-to leading logarithm (NNLL) has been studied for Drell-Yan production via spin-2 particle at the Large hadron collider (LHC). We consider generic spin-2 particle which couples differently to quarks and gluons (non-universal scenario). The threshold enhanced analytical coefficient has been obtained up to third order using the universal soft function and the process dependent form factors at the same order. We performed a detailed phenomenological analysis and gave a prediction for the 13 TeV LHC for the search of such BSM signature. We found that the resummed result gives sizeable corrections over a wide range of invariant mass of the lepton pair. The scale variation also stabilizes at this order and reduces to 4%. As a by-product, we also provide ingredients for third-order soft-virtual (SV) prediction as well as resummation and study the impact on LHC searches.

Transverse Momentum Resummation for Z boson Plus Jet Production at the LHC

We study the transverse momentum resummation effects for the Z boson plus jet associated production at the LHC within the transverse momentum dependent (TMD) factorization formalism. The large logarithm ln (Q2/q⊥2) with Q ≫ q⊥ has been resumed to Next-to-Leading Logarithm (NLL) accuracy. We compare our numerical results to the CMS data by reweighting method. It shows that our prediction agree with the data very well.

On spin asymmetry in muon and tau decays

The angular asymmetry in decays of polarized muons and tau leptons is discussed. Both the standard [Formula: see text] Fermi model and the general parametrization via Michel parameters are considered. Numerical importance of contributions suppressed by charged lepton mass ratio is underlined. Contribution of the second order QED correction is estimated in the leading logarithm approximation.

Saturation in inclusive production beyond leading logarithm accuracy

We review the recent progress on the calculations on the inclusive forward hadron production within the saturation formalism. After introducing the concept of perturbative parton saturation and nonlinear evolution we discuss the formalism for the forward hadron production at high energy in the leading and next-to-leading order. Numerical results are presented and compared with the experimental data on forward hadron production in [Formula: see text] and [Formula: see text]. We discuss the problem of the negativity of the NLO cross-section at high transverse momenta, study its origin in detail and present possible improvements which include the corrected kinematics and the suitable choice of the rapidity cutoff.