2,638 research outputs found
Single soft gluon emission at two loops
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 . The results are presented as
Laurent expansions in in spacetime dimension. We
calculate the expansion to order 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
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 in
Super-Yang-Mills theory in the limit of large .Comment: typos corrected; journal versio
Diphoton excess at 750 GeV: gluon-gluon fusion or quark-antiquark annihilation?
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
We show that the normalized Higgs production and distributions
are sensitive probes of Higgs couplings to light quarks. For up and/or down
quark Yukawa couplings comparable to the SM quark Yukawa the or
fusion production of the Higgs could lead to appreciable softer
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 distribution at 8\,TeV. With 300 fb
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
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
Top-Quark Decay at Next-to-Next-to-Leading Order in QCD
We present the complete calculation of the top-quark decay width at
next-to-next-to-leading order in QCD, including next-to-leading electroweak
corrections as well as finite bottom quark mass and boson width effects. In
particular, we also show the first results of the fully differential decay
rates for top-quark semileptonic decay at
next-to-next-to-leading order in QCD. Our method is based on the understanding
of the invariant mass distribution of the final-state jet in the singular limit
from effective field theory. Our result can be used to study arbitrary
infrared-safe observables of top-quark decay with the highest perturbative
accuracy.Comment: 5 pages, 6 figures; version accepted for publication in Physical
Review Letter
Resummation prediction on top quark transverse momentum distribution at large pT
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.
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