67 research outputs found
Top pair threshold production at a linear collider with WHIZARD
We briefly describe how the Monte Carlo generator WHIZARD 2.2 can be employed
to study large QCD effects enhancing the top-antitop production threshold at a
next-generation lepton collider. While present state-of-the-art predictions at
NNLL order are confined to inclusive total cross sections, our tool can be used
to simulate differential distributions including NLL threshold resummation in
the production, and with off-shell decaying tops. The new model will be shipped
with WHIZARD from version 2.2.3 onwards, to be released along with this
article.Comment: 4 pages, 1 figure. Proceedings of TOP2014, 7th International Workshop
on Top Quark Physics, Cannes, France, September 29 - October 3 201
The Top-Antitop Threshold at the ILC: NNLL QCD Uncertainties
We discuss the top-antitop production cross section near threshold at a
future linear collider accounting for the NNLL QCD corrections to the anomalous
dimension of the leading S-wave production current computed recently within
renormalization-group-improved NRQCD perturbation theory. We argue that the
still unknown soft NNLL mixing corrections are negligible so that the NNLL QCD
corrections to the total cross section can be considered complete for practical
purposes. Based on combined variations of the renormalization and matching
scales and the overall size of the perturbative corrections we estimate that
the NNLL QCD total cross section has a normalization uncertainty of 5% at
threshold. We present results for the total cross section and also for the
experimentally more relevant case, when moderate cuts are imposed on the
reconstructed top and antitop invariant masses.Comment: 23 pages, 7 figures; v2: plots and appendix added, journal versio
The Fully-Differential Quark Beam Function at NNLO
We present the first calculation of a fully-unintegrated parton distribution
(beam function) at next-to-next-to-leading order (NNLO). We obtain the
fully-differential beam function for quark-initiated processes by matching it
onto standard parton distribution functions (PDFs) at two loops. The
fully-differential beam function is a universal ingredient in resummed
predictions of observables probing both the virtuality as well as the
transverse momentum of the incoming quark in addition to its usual longitudinal
momentum fraction. For such double-differential observables our result provides
the part of the NNLO singular cross section related to collinear initial-state
radiation (ISR), and is important for the resummation of large logarithms
through N3LL.Comment: 17 pages, 1 figure; v2: journal versio
Rapidity-Dependent Jet Vetoes
Jet vetoes are a prominent part of the signal selection in various analyses
at the LHC. We discuss jet vetoes for which the transverse momentum of a jet is
weighted by a smooth function of the jet rapidity. With a suitable choice of
the rapidity-weighting function, such jet-veto variables can be factorized and
resummed allowing for precise theory predictions. They thus provide a
complementary way to divide phase space into exclusive jet bins. In particular,
they provide a natural and theoretically clean way to implement a tight veto on
central jets with the veto constraint getting looser for jets at increasingly
forward rapidities. We mainly focus our discussion on the 0-jet case in
color-singlet processes, using Higgs production through gluon fusion as a
concrete example. For one of our jet-veto variables we compare the resummed
theory prediction at NLL'+NLO with the recent differential cross section
measurement by the ATLAS experiment in the channel, finding
good agreement. We also propose that these jet-veto variables can be measured
and tested against theory predictions in other SM processes, such as Drell-Yan,
diphoton, and weak diboson production.Comment: 17 pages, 8 figure
On the Casimir scaling violation in the cusp anomalous dimension at small angle
We compute the four-loop contribution proportional to the quartic
Casimir of the QCD cusp anomalous dimension as an expansion for small cusp
angle . This piece is gauge invariant, violates Casimir scaling, and
first appears at four loops. It requires the evaluation of genuine non-planar
four-loop Feynman integrals. We present results up to .
One motivation for our calculation is to probe a recent conjecture on the
all-order structure of the cusp anomalous dimension. As a byproduct we obtain
the four-loop HQET wave function anomalous dimension for this color structure.Comment: 13 pages, 2 figures, 1 ancillary file; v2: journal versio
Two-Loop Massive Quark Jet Functions in SCET
We calculate the corrections to the primary massive
quark jet functions in Soft-Collinear Effective Theory (SCET). They are an
important ingredient in factorized predictions for inclusive jet mass cross
sections initiated by massive quarks emerging from a hard interaction with
smooth quark mass dependence. Due to the effects coming from the secondary
production of massive quark-antiquark pairs there are two options to define the
SCET jet function, which we call universal and mass mode jet functions. They
are related to whether or not a soft mass mode (zero) bin subtraction is
applied for the secondary massive quark contributions and differ in particular
concerning the infrared behavior for vanishing quark mass. We advocate that a
useful alternative to the common zero-bin subtraction concept is to define the
SCET jet functions through subtractions related to collinear-soft matrix
elements. This avoids the need to impose additional power counting arguments as
required for zero-bin subtractions. We demonstrate how the two SCET jet
function definitions may be used in the context of two recently developed
factorization approaches to treat secondary massive quark effects. We clarify
the relation between these approaches and in which way they are equivalent. Our
two-loop calculation involves interesting technical subtleties related to
spurious rapidity divergences and infrared regularization in the presence of
massive quarks.Comment: 51 pages + appendices, 8 figures, v2: journal versio
The Gluon Beam Function at Two Loops
The virtuality-dependent beam function is a universal ingredient in the
resummation for observables probing the virtuality of incoming partons,
including N-jettiness and beam thrust. We compute the gluon beam function at
two-loop order. Together with our previous results for the two-loop quark beam
function, this completes the full set of virtuality-dependent beam functions at
next-to-next-to-leading order (NNLO). Our results are required to account for
all collinear ISR effects to the N-jettiness event shape through N^3LL order.
We present numerical results for both the quark and gluon beam functions up to
NNLO and N^3LL order. Numerically, the NNLO matching corrections are important.
They reduce the residual matching scale dependence in the resummed beam
function by about a factor of two.Comment: 21 pages, 6 figures; v2: journal versio
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