25 research outputs found
Bootstrapping Multi-Parton Loop Amplitudes in QCD
We present a new method for computing complete one-loop amplitudes, including
their rational parts, in non-supersymmetric gauge theory. This method merges
the unitarity method with on-shell recursion relations. It systematizes a
unitarity-factorization bootstrap approach previously applied by the authors to
the one-loop amplitudes required for next-to-leading order QCD corrections to
the processes e^+e^- -> Z,\gamma^* -> 4 jets and pp -> W + 2 jets. We
illustrate the method by reproducing the one-loop color-ordered five-gluon
helicity amplitudes in QCD that interfere with the tree amplitude, namely
A_{5;1}(1^-,2^-,3^+,4^+,5^+) and A_{5;1}(1^-,2^+,3^-,4^+,5^+). Then we describe
the construction of the six- and seven-gluon amplitudes with two adjacent
negative-helicity gluons, A_{6;1}(1^-,2^-,3^+,4^+,5^+,6^+) and
A_{7;1}(1^-,2^-,3^+,4^+,5^+,6^+,7^+), which uses the previously-computed
logarithmic parts of the amplitudes as input. We present a compact expression
for the six-gluon amplitude. No loop integrals are required to obtain the
rational parts.Comment: 43 pages, 8 figures, RevTeX, v2-v4 clarifications and minor
correction
Higher-Order Corrections to Timelike Jets
We present a simple formalism for the evolution of timelike jets in which
tree-level matrix element corrections can be systematically incorporated, up to
arbitrary parton multiplicities and over all of phase space, in a way that
exponentiates the matching corrections. The scheme is cast as a shower Markov
chain which generates one single unweighted event sample, that can be passed to
standard hadronization models. Remaining perturbative uncertainties are
estimated by providing several alternative weight sets for the same events, at
a relatively modest additional overhead. As an explicit example, we consider Z
-> q qbar evolution with unpolarized, massless quarks and include several
formally subleading improvements as well as matching to tree-level matrix
elements through alpha_s^4. The resulting algorithm is implemented in the
publicly available VINCIA plugin to the PYTHIA 8 event generator.Comment: 72 pages, 78 figure
Left-Handed W Bosons at the LHC
The production of W bosons in association with jets is an important
background to new physics at the LHC. Events in which the W carries large
transverse momentum and decays leptonically lead to large missing energy and
are of particular importance. We show that the left-handed nature of the W
coupling, combined with valence quark domination at a pp machine, leads to a
large left-handed polarization for both W^+ and W^- bosons at large transverse
momenta. The polarization fractions are very stable with respect to QCD
corrections. The leptonic decay of the W bosons translates the common
left-handed polarization into a strong asymmetry in transverse momentum
distributions between positrons and electrons, and between neutrinos and
anti-neutrinos (missing transverse energy). Such asymmetries may provide an
effective experimental handle on separating W + jets from top quark production,
which exhibits very little asymmetry due to C invariance, and from various
types of new physics.Comment: 32 pages, revtex, 17 figures, 3 tables, v2 minor corrections to ME+PS
results, no changes to conclusions, added reference
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
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On Yang--Mills Theories with Chiral Matter at Strong Coupling
Strong coupling dynamics of Yang-Mills theories with chiral fermion content remained largely elusive despite much effort over the years. In this work, we propose a dynamical framework in which we can address non-perturbative properties of chiral, non-supersymmetric gauge theories, in particular, chiral quiver theories on S{sub 1} x R{sub 3}. Double-trace deformations are used to stabilize the center-symmetric vacuum. This allows one to smoothly connect smaller(S{sub 1}) to larger(S{sub 1}) physics (R{sub 4} is the limiting case) where the double-trace deformations are switched off. In particular, occurrence of the mass gap in the gauge sector and linear confinement due to bions are analytically demonstrated. We find the pattern of the chiral symmetry realization which depends on the structure of the ring operators, a novel class of topological excitations. The deformed chiral theory, unlike the undeformed one, satisfies volume independence down to arbitrarily small volumes (a working Eguchi-Kawai reduction) in the large N limit. This equivalence, may open new perspectives on strong coupling chiral gauge theories on R{sub 4}
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One-Loop Multi-Parton Amplitudes with a Vector Boson for the LHC
In this talk, we present the first, numerically stable, results for the one-loop amplitudes needed for computing W; Z + 3 jet cross sections at the LHC to next-to-leading order in the QCD coupling. We implemented these processes in BlackHat, an automated program based on on-shell methods. These methods scale very well with increasing numbers of external partons, and are applicable to a wide variety of problems of phenomenological interest at the LHC
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Vector Boson Jets with BlackHat and Sherpa
We review recent NLO QCD results for W, Z + 3-jet production at hadron colliders, computing using BlackHat and SHERPA, and including also some new results for Z + 3-jet production for the LHC at 7 TeV. We report new progress towards the NLO cross section for W + 4-jet production. In particular, we show that the virtual matrix elements produced by BlackHat are numerically stable. We also show that with an improved integrator and tree-level matrix elements from BlackHat, SHERPA produces well-behaved real-emission contributions. As an illustration, we present the real-emission contributions - including dipole-subtraction terms - to the p{sub T} distribution of the fourth jet, for a single subprocess with the maximum number of gluons