86 research outputs found
Form Factors in N=4 Super Yang-Mills and Periodic Wilson Loops
We calculate form factors of half-BPS operators in N=4 super Yang-Mills
theory at tree level and one loop using novel applications of recursion
relations and unitarity. In particular, we determine the expression of the
one-loop form factors with two scalars and an arbitrary number of
positive-helicity gluons. These quantities resemble closely the MHV scattering
amplitudes, including holomorphicity of the tree-level form factor, and the
expansion in terms of two-mass easy box functions of the one-loop result. Next,
we compare our result for these form factors to the calculation of a particular
periodic Wilson loop at one loop, finding agreement. This suggests a novel
duality relating form factors to periodic Wilson loops.Comment: 26 pages, 10 figures. v2: typos fixed, comments adde
Strong Double Higgs Production at the LHC
The hierarchy problem and the electroweak data, together, provide a plausible
motivation for considering a light Higgs emerging as a pseudo-Goldstone boson
from a strongly-coupled sector. In that scenario, the rates for Higgs
production and decay differ significantly from those in the Standard Model.
However, one genuine strong coupling signature is the growth with energy of the
scattering amplitudes among the Goldstone bosons, the longitudinally polarized
vector bosons as well as the Higgs boson itself. The rate for double Higgs
production in vector boson fusion is thus enhanced with respect to its
negligible rate in the SM. We study that reaction in pp collisions, where the
production of two Higgs bosons at high pT is associated with the emission of
two forward jets. We concentrate on the decay mode hh -> WW^(*)WW^(*) and study
the semi-leptonic decay chains of the W's with 2, 3 or 4 leptons in the final
states. While the 3 lepton final states are the most relevant and can lead to a
3 sigma signal significance with 300 fb^{-1} collected at a 14 TeV LHC, the two
same-sign lepton final states provide complementary information. We also
comment on the prospects for improving the detectability of double Higgs
production at the foreseen LHC energy and luminosity upgrades.Comment: 54 pages, 26 figures. v2: typos corrected, a few comments and one
table added. Version published in JHE
Analytic two-loop form factors in N=4 SYM
The original publication is available at www.springerlink.co
Space-like (vs. time-like) collinear limits in QCD: is factorization violated?
We consider the singular behaviour of QCD scattering amplitudes in
kinematical configurations where two or more momenta of the external partons
become collinear. At the tree level, this behaviour is known to be controlled
by factorization formulae in which the singular collinear factor is universal
(process independent). We show that this strict (process-independent)
factorization is not valid at one-loop and higher-loop orders in the case of
the collinear limit in space-like regions (e.g., collinear radiation from
initial-state partons). We introduce a generalized version of all-order
collinear factorization, in which the space-like singular factors retain some
dependence on the momentum and colour charge of the non-collinear partons. We
present explicit results on one-loop and two-loop amplitudes for both the
two-parton and multiparton collinear limits. At the level of square amplitudes
and, more generally, cross sections in hadron--hadron collisions, the violation
of strict collinear factorization has implications on the non-abelian structure
of logarithmically-enhanced terms in perturbative calculations (starting from
the next-to-next-to-leading order) and on various factorization issues of mass
singularities (starting from the next-to-next-to-next-to-leading order).Comment: 81 pages, 5 figures, typos corrected in the text, few comments added
and inclusion of NOTE ADDED on recent development
Linear relations between N >= 4 supergravity and subleading-color SYM amplitudes
The IR divergences of supergravity amplitudes are less severe than those of
planar SYM amplitudes, and are comparable to those subleading-color SYM
amplitudes that are most subleading in the 1/N expansion, namely O(1/epsilon^L)
for L-loop amplitudes. We derive linear relations between one- and two-loop
four-point amplitudes and one-loop five-point amplitudes of N = 4, 5, and 6
supergravity and the most-subleading-color contributions of the analogous
amplitudes of N = 0, 1, and 2 SYM theory, extending earlier results for N = 8
supergravity amplitudes. Our work relies on linear relations between N >= 4
supergravity and planar SYM amplitudes that were recently derived using the
double-copy property of gravity, and color-kinematic duality of gauge theories.Comment: 21 pages, 1 figur
Factorization Properties of Soft Graviton Amplitudes
We apply recently developed path integral resummation methods to perturbative
quantum gravity. In particular, we provide supporting evidence that eikonal
graviton amplitudes factorize into hard and soft parts, and confirm a recent
hypothesis that soft gravitons are modelled by vacuum expectation values of
products of certain Wilson line operators, which differ for massless and
massive particles. We also investigate terms which break this factorization,
and find that they are subleading with respect to the eikonal amplitude. The
results may help in understanding the connections between gravity and gauge
theories in more detail, as well as in studying gravitational radiation beyond
the eikonal approximation.Comment: 35 pages, 5 figure
RELATIONSHIP OF POLYPHARMACY AND POLYPATHOLOGY WITH FALLS AMONG INSTITUTIONALIZED ELDERLY
General properties of multiparton webs: proofs from combinatorics
Recently, the diagrammatic description of soft-gluon exponentiation in
scattering amplitudes has been generalized to the multiparton case. It was
shown that the exponent of Wilson-line correlators is a sum of webs, where each
web is formed through mixing between the kinematic factors and colour factors
of a closed set of diagrams which are mutually related by permuting the gluon
attachments to the Wilson lines. In this paper we use replica trick methods, as
well as results from enumerative combinatorics, to prove that web mixing
matrices are always: (a) idempotent, thus acting as projection operators; and
(b) have zero sum rows: the elements in each row in these matrices sum up to
zero, thus removing components that are symmetric under permutation of gluon
attachments. Furthermore, in webs containing both planar and non-planar
diagrams we show that the zero sum property holds separately for these two
sets. The properties we establish here are completely general and form an
important step in elucidating the structure of exponentiation in non-Abelian
gauge theories.Comment: 38 pages, 10 figure
- …