2,087 research outputs found

    Single soft gluon emission at two loops

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    We study the single soft-gluon current at two loops with two energetic partons in massless perturbative QCD, which describes, for example, the soft limit of the two-loop amplitude for ggHggg\to Hg. The results are presented as Laurent expansions in ϵ\epsilon in D=42ϵD=4-2\epsilon spacetime dimension. We calculate the expansion to order ϵ2\epsilon^2 analytically, which is a necessary ingredient for Higgs production at hadron colliders at next-to-next-to-next-to-leading order in the soft-virtual approximation. We also give two-loop results of the single soft-gluon current in N=4{\cal N}=4 Super-Yang-Mills theory, and find that it has uniform transcendentality. By iteration relation of splitting amplitudes, our calculations can determine the three-loop single soft-gluon current to order ϵ0\epsilon^0 in N=4{\cal N}=4 Super-Yang-Mills theory in the limit of large NcN_c.Comment: typos corrected; journal versio

    Diphoton excess at 750 GeV: gluon-gluon fusion or quark-antiquark annihilation?

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    Recently, ATLAS and CMS collaboration reported an excess in the diphoton events, which can be explained by a new resonance with mass around 750 GeV. In this work, we explored the possibility of identifying if the hypothetical new resonance is produced through gluon-gluon fusion or quark-antiquark annihilation, or tagging the beam. Three different observables for beam tagging, namely the rapidity and transverse momentum distribution of the diphoton, and one tagged bottom-jet cross section, are proposed. Combining the information gained from these observables, a clear distinction of the production mechanism for the diphoton resonance is promising.Comment: 20 pages, 7 figure

    Light quark Yukawa couplings from Higgs kinematics

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    We show that the normalized Higgs production pTp_T and yhy_h distributions are sensitive probes of Higgs couplings to light quarks. For up and/or down quark Yukawa couplings comparable to the SM bb quark Yukawa the uˉu\bar u u or dˉd\bar d d fusion production of the Higgs could lead to appreciable softer pTp_T distribution than in the SM. The rapidity distribution, on the other hand, becomes more forward. We find that, owing partially to a downward fluctuation, one can derive competitive bounds on the two couplings using ATLAS measurements of normalized pTp_T distribution at 8\,TeV. With 300 fb1{}^{-1} at 13\,TeV LHC one could establish flavor non-universality of the Yukawa couplings in the down sector.Comment: 15 pages, 7 figures; v2: add clarifications, plot and refs. conclusion unchanged; v3: matched to the published versio

    Collinear factorization violation and effective field theory

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    The factorization of amplitudes into hard, soft and collinear parts is known to be violated in situations where incoming particles are collinear to outgoing ones. This result was first derived by studying limits where non-collinear particles become collinear. We show that through an effective field theory framework with Glauber operators, these factorization-violating effects can be reproduced from an amplitude that is factorized before the splitting occurs. We confirm results at one-loop, through single Glauber exchange, and at two-loops, through double Glauber exchange. To approach the calculation, we begin by reviewing the importance of Glauber scaling for factorization. We show that for any situation where initial state and final state particles are not collinear, the Glauber contribution is entirely contained in the soft contribution. The contributions coming from Glauber operators are necessarily non-analytic functions of external momentum, with the non-analyticity arising from the rapidity regulator. The non-analyticity is critical so that Glauber operators can both preserve factorization when it holds and produce factorization-violating effects when they are present.Comment: 55 Pages, 5 figure

    Resummation prediction on top quark transverse momentum distribution at large pT

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    We study the factorization and resummation of t-channel top quark transverse momentum distribution at large pT in the SM at both the Tevatron and the LHC with soft-collinear effective theory. The cross section in the threshold region can be factorized into a convolution of hard, jet and soft functions. In particular, we first calculate the NLO soft functions for this process, and give a RG improved cross section by evolving the different functions to a common scale. Our results show that the resummation effects increase the NLO results by about 9%-13% and 4%-9% when the top quark pT is larger than 50 and 70 GeV at the Tevatron and the 8 TeV LHC, respectively. Also, we discuss the scale independence of the cross section analytically, and show how to choose the proper scales at which the perturbative expansion can converge fast.Comment: 32 pages, 10 figures, version published in Phys.Rev.