Initial state radiation effects in inclusive J/ψJ/\psi production at B factories

Based on Monte Carlo techniques, we analyze the initial state radiation (ISR) effects in prompt \jpsi inclusive production at B factories. ISR enhances cross section \sigma(e^-e^+\to\jpsi+gg+X) by about $15-25\%$, which is almost the same size as the QCD and relativistic correction. Moreover, ISR slightly changes \sigma(e^-e^+\to\jpsi+c\bar{c}+X). The \jpsi momentum spectrum in e^-e^+\to\jpsi+gg+X and in e^-e^+\to\jpsi+c\bar{c}+X is softer after the photon showering from the initial $e^{\pm}$ beam radiation. After combining the QCD,relativistic, and ISR corrections,a more precise theoretical result is obtained. The new result provides a more stringent constraint of the color-octet contribution to \sigma(e^-e^+\to\jpsi+X_{\rm{non-}c\bar{c}}).Comment: journal version;16 pages, 9 figures, 4 table

Colored Particle Production in New Physics at NLO QCD and Its Matching to Parton Showers

In this talk, I show the automated Monte Carlo simulations at next-to-leading order in QCD as well as its matching to parton showers are already feasible within the framework of \MG5aMC. I briefly overview the recent activities and take the colored particle production at the LHC as examples. The tools and the models are ready for using by both phenomenologists and experimentalists.Comment: 12 pages, 3 figures, 2 tables, contribution to proceedings of XXIII Cracow EPIPHANY Conference, 9-12 January 2017, IFJ PAN, Cracow, Polan

Boosting perturbative QCD stability in quarkonium production

The aim of this paper is to introduce a general way to stabilize the perturbative QCD computations of heavy quarkonium production in the boosted or high-momentum transferring region with tree-level generators only. Such an approach is possible by properly taking into account the power-enhanced perturbative contributions in a soft and collinear safe manner without requiring any complete higher-order computations. The complicated NLO results for inclusive quarkonium hadroproduction can be well reproduced within our approach based on a tree-level generator {\sc\small HELAC-Onia}. We have applied it to estimate the last missing leading-twist contribution from the spin-triplet color-singlet S-wave production at $\mathcal{O}(\alpha_s^5)$, which is a NNLO term in the $\alpha_s$ expansion for the quarkonium $P_T$ spectrum. We conclude that the missing NNLO contribution will not change the order of the magnitude of the short-distance coefficient. Such an approach is also quite appealing as it foresees broad applications in quarkonium-associated production processes, which are mostly absent of complete higher-order computations and fragmentation functions.Comment: 40 pages, 26 figures, 3 tables; v2: journal version, fix a few typos fixed, add two figures, reorganize the figure

HELAC-Onia: an automatic matrix element generator for heavy quarkonium physics

By the virtues of the Dyson-Schwinger equations, we upgrade the published code \mtt{HELAC} to be capable to calculate the heavy quarkonium helicity amplitudes in the framework of NRQCD factorization, which we dub \mtt{HELAC-Onia}. We rewrote the original \mtt{HELAC} to make the new program be able to calculate helicity amplitudes of multi P-wave quarkonium states production at hadron colliders and electron-positron colliders by including new P-wave off-shell currents. Therefore, besides the high efficiencies in computation of multi-leg processes within the Standard Model, \mtt{HELAC-Onia} is also sufficiently numerical stable in dealing with P-wave quarkonia (e.g. $h_{c,b},\chi_{c,b}$) and P-wave color-octet intermediate states. To the best of our knowledge, it is a first general-purpose automatic quarkonium matrix elements generator based on recursion relations on the market.Comment: Published version. 24 pages,1 figure, 7 tables, HELAC-Onia is available from http://helac-phegas.web.cern.ch/helac-phega