87 research outputs found
The colour evolution of the process q q -> q q g
We calculate the soft anomalous dimension matrix for a five-parton process,
qq -> qqg. Considering different bases we unveil some interesting properties of
this matrix.Comment: 11 pages; calculation extended to general 5-parton kinematic
Massive c\bar c g - Calculation in Diffractive DIS and Diffractive D^* - Production at HERA
We calculate the cross section for -production in diffractive DIS
with finite quark masses at zero momentum transfer . The calculation is done
in the leading log(1/) approximation and is valid in the region of high
diffractive masses (small ). We apply our cross section formula
including both - and in a Monte Carlo simulation to
diffractive meson production at HERA. We compare our predictions to
results of H1 using three parameterizations for the unintegrated gluon density.Comment: 11 pages, 5 figure
Breakdown of QCD coherence ?
We reconsider the calculation of a non-global QCD observable and find the
possible breakdown of QCD coherence. This breakdown arises as a result of wide
angle soft gluon emission developing a sensitivity to emission at small angles
and it leads to the appearance of super-leading logarithms. We use the `gaps
between jets' cross-section as a concrete example and illustrate that the new
logarithms are intimately connected with the presence of Coulomb gluon
contributions. Numerical estimates of their potential phenomenological
significance are presented.Comment: 7 pages, 3 figures; proceedings of Workshop on Diffraction in
High-Energy Physics - DIFFRACTION 2006, Sep. 5-10 2006, Adamantas, Milos
island, Greec
Precise Predictions for Higgs Production in Neutralino Decays in the Complex MSSM
Complete one-loop results are obtained for the class of processes
chi^0_i->chi^0_j h_a in the MSSM where all parameters entering this process
beyond lowest order are allowed to have arbitrary CP-violating phases. An
on-shell renormalisation scheme is worked out for the chargino-neutralino
sector that properly takes account of imaginary parts arising from complex
parameters and from absorptive parts of loop integrals. The genuine vertex
contributions to the neutralino decay amplitudes are combined with two-loop
propagator-type corrections for the outgoing Higgs boson. In this way the
currently most precise prediction for this class of processes is obtained. The
numerical impact of the genuine vertex corrections is studied for several
examples of CP-conserving and CP-violating scenarios. We find that significant
effects on the decay widths and branching ratios are possible even in the
CP-conserving MSSM. In the CP-violating CPX benchmark scenario the corrections
to the decay width are found to be particularly large, namely, of order 45% for
a Higgs mass of 40 GeV. This parameter region of the CPX scenario where a very
light Higgs boson is unexcluded by present data is analysed in detail. We find
that in this parameter region, which will be difficult to cover by standard
Higgs search channels at the LHC, the branching ratio for the decay
chi^0_2->chi^0_1 h_1 is large. This may offer good prospects to detect such a
light Higgs boson in cascade decays of supersymmetric particles.Comment: 24 pages, 6 figure
Super-leading logarithms in non-global observables in QCD: Colour basis independent calculation
In a previous paper we reported the discovery of super-leading logarithmic
terms in a non-global QCD observable. In this short update we recalculate the
first super-leading logarithmic contribution to the 'gaps between jets'
cross-section using a colour basis independent notation. This sheds light on
the structure and origin of the super-leading terms and allows them to be
calculated for gluon scattering processes for the first time.Comment: 21 page
Symmetry of anomalous dimension matrices explained
In a previous paper, one of us pointed out that the anomalous dimension
matrices for all physical processes that have been calculated to date are
complex symmetric, if stated in an orthonormal basis. In this paper we prove
this fact and show that it is only true in a subset of all possible orthonormal
bases, but that this subset is the natural one to use for physical
calculations.Comment: 4 page
Gaps between Jets in the High Energy Limit
We use perturbative QCD to calculate the parton level cross section for the
production of two jets that are far apart in rapidity, subject to a limitation
on the total transverse momentum Q0 in the interjet region. We specifically
address the question of how to combine the approach which sums all leading
logarithms in Q/Q0 (where Q is the jet transverse momentum) with the BFKL
approach, in which leading logarithms of the scattering energy are summed. This
paper constitutes progress towards the simultaneous summation of all important
logarithms. Using an "all orders" matching, we are able to obtain results for
the cross section which correctly reproduce the two approaches in the
appropriate limits.Comment: 29 pages, 9 figures, minor corrections to text and improved figure
Small x Phenomenology: summary of the 3rd Lund Small x Workshop in 2004
A third workshop on small-x physics, within the Small-x Collaboration, was held in Hamburg in May 2004 with the aim of overviewing recent theoretical progress in this area and summarizing the experimental status.A third workshop on small-x physics, within the Small-x Collaboration, was held in Hamburg in May 2004 with the aim of overviewing recent theoretical progress in this area and summarizing the experimental status
On the renormalization of multiparton webs
We consider the recently developed diagrammatic approach to soft-gluon
exponentiation in multiparton scattering amplitudes, where the exponent is
written as a sum of webs - closed sets of diagrams whose colour and kinematic
parts are entangled via mixing matrices. A complementary approach to
exponentiation is based on the multiplicative renormalizability of intersecting
Wilson lines, and their subsequent finite anomalous dimension. Relating this
framework to that of webs, we derive renormalization constraints expressing all
multiple poles of any given web in terms of lower-order webs. We examine these
constraints explicitly up to four loops, and find that they are realised
through the action of the web mixing matrices in conjunction with the fact that
multiple pole terms in each diagram reduce to sums of products of lower-loop
integrals. Relevant singularities of multi-eikonal amplitudes up to three loops
are calculated in dimensional regularization using an exponential infrared
regulator. Finally, we formulate a new conjecture for web mixing matrices,
involving a weighted sum over column entries. Our results form an important
step in understanding non-Abelian exponentiation in multiparton amplitudes, and
pave the way for higher-loop computations of the soft anomalous dimension.Comment: 60 pages, 15 figure
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