13 research outputs found
Seven parton amplitudes from recursion relations
We present the first calculation of two-quark and five-gluon tree amplitudes
using on-shell recursion relations. These amplitudes are needed for tree level
5-jet cross-section and an essential ingredient for next-to-leading order 4-jet
and next-to-next-to-leading order 3-jet production at hadronic colliders. Very
compact expressions for all possible helicity configurations are provided,
allowing for direct implementation in Monte-Carlo codes.Comment: 11 page
Scattering amplitudes with massive fermions using BCFW recursion
We study the QCD scattering amplitudes for \bar{q}q \to gg and \bar{q}q \to
ggg where q is a massive fermion. Using a particular choice of massive fermion
spinor we are able to derive very compact expressions for the partial spin
amplitudes for the 2 \to 2 process. We then investigate the corresponding 2 \to
3 amplitudes using the BCFW recursion technique. For the helicity conserving
partial amplitudes we again derive very compact expressions, but were unable to
treat the helicity-flip amplitudes recursively, except for the case where all
the gluon helicities are the same. We therefore evaluate the remaining partial
amplitudes using standard Feynman diagram techniques.Comment: 21 page
Next-to-Leading Order W + 5-Jet Production at the LHC
We present next-to-leading order QCD predictions for the total cross section
and for a comprehensive set of transverse-momentum distributions in W + 5-jet
production at the Large Hadron Collider. We neglect the small contributions
from subleading-color virtual terms, top quarks and some terms containing four
quark pairs. We also present ratios of total cross sections, and use them to
obtain an extrapolation formula to an even larger number of jets. We include
the decay of the boson into leptons. This is the first such computation
with six final-state vector bosons or jets. We use BlackHat together with
SHERPA to carry out the computation.Comment: RevTex, 27 pages, 7 figures, v2 minor corrections and corrected
reference
From Yang-Mills Lagrangian to MHV Diagrams
We prove the equivalence of a recently suggested MHV-formalism to the
standard Yang-Mills theory. This is achieved by a formally non-local change of
variables. In this note we present the explicit formulas while the detailed
proofs are postponed to a future publication.Comment: Latex,11 pages, minor changes, reference added, version to appear in
JHE
Color-dressed recursive relations for multi-parton amplitudes
Remarkable progress inspired by twistors has lead to very simple analytic
expressions and to new recursive relations for multi-parton color-ordered
amplitudes. We show how such relations can be extended to include color and
present the corresponding color-dressed formulation for the Berends-Giele, BCF
and a new kind of CSW recursive relations. A detailed comparison of the
numerical efficiency of the different approaches to the calculation of
multi-parton cross sections is performed.Comment: 31 pages, 4 figures, 6 table
On-shell recursion relations for all Born QCD amplitudes
We consider on-shell recursion relations for all Born QCD amplitudes. This
includes amplitudes with several pairs of quarks and massive quarks. We give a
detailed description on how to shift the external particles in spinor space and
clarify the allowed helicities of the shifted legs. We proof that the
corresponding meromorphic functions vanish at z --> infinity. As an application
we obtain compact expressions for helicity amplitudes including a pair of
massive quarks, one negative helicity gluon and an arbitrary number of positive
helicity gluons.Comment: 30 pages, minor change
Driving Missing Data at Next-to-Leading Order
The prediction of backgrounds to new physics signals in topologies with large
missing transverse energy and jets is important to new physics searches at the
LHC. Following a CMS study, we investigate theoretical issues in using
measurements of gamma + 2-jet production to predict the irreducible background
to searches for missing energy plus two jets that originates from Z + 2-jet
production where the Z boson decays to neutrinos. We compute ratios of gamma +
2-jet to Z + 2-jet production cross sections and kinematic distributions at
next-to-leading order in alpha_s, as well as using a parton shower matched to
leading-order matrix elements. We find that the ratios obtained in the two
approximations are quite similar, making gamma + 2-jet production a
theoretically reliable estimator for the missing energy plus two jets
background. We employ a Frixione-style photon isolation, but we also show that
for isolated prompt photon production at high transverse momentum the
difference between this criterion and the standard cone isolation used by CMS
is small.Comment: 27 pages, 9 figures, 3 tables, RevTex, v2 minor corrections and added
reference
Missing Energy and Jets for Supersymmetry Searches
We extend our investigation of backgrounds to new physics signals, following
CMS's data-driven search for supersymmetry at the LHC. The aim is to use
different sets of cuts in gamma + 3-jet production to predict the irreducible Z
+ 3-jet background (with the Z boson decaying to neutrinos) to searches with
missing transverse energy + 3-jet signal topologies. We compute ratios of Z +
3-jet to gamma + 3-jet production cross sections and kinematic distributions at
next-to-leading order (NLO) in alpha_s. We compare these ratios with those
obtained using a parton shower matched to leading-order matrix elements
(ME+PS). This study extends our previous work [arXiv:1106.1423 [hep-ph]] on the
Z + 2-jet to gamma + 2-jet ratio. We find excellent agreement with the ratio
determined from the earlier NLO results involving two instead of three jets,
and agreement to within 10% between the NLO and ME+PS results for the ratios.
We also examine the possibility of large QCD logarithms in these processes.
Ratios of Z + n-jet to gamma + n-jet cross sections are plausibly less
sensitive to such corrections than the cross sections themselves. Their effect
on estimates of Z + 3-jet to gamma + 3-jet ratios can be assessed
experimentally by measuring the gamma + 3-jet to gamma + 2-jet production ratio
in search regions. We partially address the question of potentially large
electroweak logarithms by computing the real-emission part of the electroweak
corrections to the ratio using ME+PS, and find that it is 1% or less. Our
estimate of the remaining theoretical uncertainties in the Z to gamma ratio is
in agreement with our earlier study.Comment: 32 pages, 10 figures, 8 tables, RevTe
From Trees to Loops and Back
We argue that generic one-loop scattering amplitudes in supersymmetric
Yang-Mills theories can be computed equivalently with MHV diagrams or with
Feynman diagrams. We first present a general proof of the covariance of
one-loop non-MHV amplitudes obtained from MHV diagrams. This proof relies only
on the local character in Minkowski space of MHV vertices and on an application
of the Feynman Tree Theorem. We then show that the discontinuities of one-loop
scattering amplitudes computed with MHV diagrams are precisely the same as
those computed with standard methods. Furthermore, we analyse collinear limits
and soft limits of generic non-MHV amplitudes in supersymmetric Yang-Mills
theories with one-loop MHV diagrams. In particular, we find a simple explicit
derivation of the universal one-loop splitting functions in supersymmetric
Yang-Mills theories to all orders in the dimensional regularisation parameter,
which is in complete agreement with known results. Finally, we present concrete
and illustrative applications of Feynman's Tree Theorem to one-loop MHV
diagrams as well as to one-loop Feynman diagrams.Comment: 52 pages, 17 figures. Some typos in Appendix A correcte
Multigluon tree amplitudes with a pair of massive fermions
We consider the calculation of n-point multigluon tree amplitudes with a pair
of massive fermions in QCD. We give the explicit transformation rules of this
kind of massive fermion-pair amplitudes with respect to different reference
momenta and check the correctness of them by SUSY Ward identities. Using these
rules and onshell BCFW recursion relation, we calculate the analytic results of
several n-point multigluon amplitudes.Comment: 15page