83 research outputs found
Complete off-shell effects in top quark pair hadroproduction with leptonic decay at next-to-leading order
Results for next-to-leading order QCD corrections to the pp(p\bar{p}) -> t
\bar{t} -> W^+W^- b\bar{b} -> e^{+} \nu_{e} \mu^{-} \bar{\nu}_{\mu} b \bar{b}
+X processes with complete off-shell effects are presented for the first time.
Double-, single- and non-resonant top contributions of the order
{\cal{O}}(\alpha_{s}^3 \alpha^4) are consistently taken into account, which
requires the introduction of a complex-mass scheme for unstable top quarks.
Moreover, the intermediate W bosons are treated off-shell. Comparison to the
narrow width approximation for top quarks, where non-factorizable corrections
are not accounted for is performed. Besides the total cross section and its
scale dependence, several differential distributions at the TeVatron run II and
the LHC are given. In case of the TeVatron the forward-backward asymmetry of
the top is recalculated afresh. With inclusive selection cuts, the
forward-backward asymmetry amounts to A^{t}_{FB} = 0.051 +/- 0.0013.
Furthermore, the corrections with respect to leading order are positive and of
the order 2.3% for the TeVatron and 47% for the LHC. A study of the scale
dependence of our NLO predictions indicates that the residual theoretical
uncertainty due to higher order corrections is 8% for the TeVatron and 9% for
the LHC.Comment: 35 pages, 39 figures, 3 tables. References and note added, version to
appear in JHE
Improved TMD factorization for forward dijet production in dilute-dense hadronic collisions
Polarizing the Dipoles
We extend the massless dipole formalism of Catani and Seymour, as well as its
massive version as developed by Catani, Dittmaier, Seymour and Trocsanyi, to
arbitrary helicity eigenstates of the external partons. We modify the real
radiation subtraction terms only, the primary aim being an improved efficiency
of the numerical Monte Carlo integration of this contribution as part of a
complete next-to-leading order calculation. In consequence, our extension is
only applicable to unpolarized scattering. Upon summation over the helicities
of the emitter pairs, our formulae trivially reduce to their original form. We
implement our extension within the framework of Helac-Phegas, and give some
examples of results pertinent to recent studies of backgrounds for the LHC. The
code is publicly available. Since the integrated dipole contributions do not
require any modifications, we do not discuss them, but they are implemented in
the software.Comment: 20 pages, 4 figures, Integrated dipoles implemented for massless and
massive case
Gauge invariant sub-structures of tree-level double-emission exact QCD spin amplitudes
In this note we discuss possible separations of exact, massive, tree-level
spin amplitudes into gauge invariant parts. We concentrate our attention on
processes involving two quarks entering a color- neutral current and, thanks to
the QCD interactions, two extra external gluons. We will search for forms
compatible with parton shower languages, without applying approximations or
restrictions on phase space regions. Special emphasis will be put on the
isolation of parts necessary for the construction of evolution kernels for
individual splittings and to some degree for the running coupling constant as
well. Our aim is to better understand the environment necessary to optimally
match hard matrix elements with partons shower algorithms. To avoid
complications and ambiguities related to regularization schemes, we ignore, at
this point, virtual corrections. Our representation is quite universal: any
color-neutral current can be used, in particular our approach is not restricted
to vector currents only.Comment: 27 pages, formula in section 5 correcte
Precision QCD, Hadronic Structure & Forward QCD, Heavy Ions: Report of Energy Frontier Topical Groups 5, 6, 7 submitted to Snowmass 2021
This report was prepared on behalf of three Energy Frontier Topical Groups of
the Snowmass 2021 Community Planning Exercise. It summarizes the status and
implications of studies of strong interactions in high-energy experiments and
QCD theory. We emphasize the rich landscape and broad impact of these studies
in the decade ahead. Hadronic interactions play a central role in the
high-luminosity Large Hadron Collider (LHC) physics program, and strong
synergies exist between the (HL-)LHC and planned or proposed experiments at the
U.S. Electron-Ion Collider, CERN forward physics experiments, high-intensity
facilities, and future TeV-range lepton and hadron colliders. Prospects for
precision determinations of the strong coupling and a variety of
nonperturbative distribution and fragmentation functions are examined. We also
review the potential of envisioned tests of new dynamical regimes of QCD in
high-energy and high-density scattering processes with nucleon, ion, and photon
initial states. The important role of the high-energy heavy-ion program in
studies of nuclear structure and the nuclear medium, and its connections with
QCD involving nucleons are summarized. We address ongoing and future
theoretical advancements in multi-loop QCD computations, lattice QCD, jet
substructure, and event generators. Cross-cutting connections between
experimental measurements, theoretical predictions, large-scale data analysis,
and high-performance computing are emphasized.Comment: 95 pages (bibliography 30 pages), 28 figures; v.2: minor changes,
authors and references adde
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