227 research outputs found
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Rare Z decays and new physics
Although the signatures for rare Z decays are often spectacular, the predicted standard model rates are usually extremely small. In many cases, however, rare decays are very sensitive to new phenomena and may lead to an observable rate. In this talk, I select some interesting rare decays and discuss how new physics might be identified. 25 refs., 4 figs., 2 tabs
Two-loop QCD corrections to gluon-gluon scattering
We present the virtual QCD corrections to gluon–gluon scattering due to the interference of tree- and two-loop amplitudes. We work in conventional dimensional regularisation and give analytic expressions renormalised in the scheme. The structure of the infrared divergences agrees with that predicted by Catani while formulae for the finite remainder are given in terms of logarithms and polylogarithms that are real in the physical region. These results, together with those previously obtained for quark–quark and quark–gluon scattering, complete the two-loop matrix elements needed for the next-to-next-to-leading order contribution to inclusive jet production at hadron colliders
Real-virtual corrections for gluon scattering at NNLO
We use the antenna subtraction method to isolate the mixed real-virtual infrared singularities present in gluonic scattering amplitudes at next-to-next-to-leading order. In a previous paper, we derived the subtraction term that rendered the double real radiation tree-level process finite in the single and double unresolved regions of phase space. Here, we show how to construct the real-virtual subtraction term using antenna functions with both initial- and final-state partons which removes the explicit infrared poles present in the one-loop amplitude, as well as the implicit singularities that occur in the soft and collinear limits. As an explicit example, we write down the subtraction term that describes the single unresolved contributions from the five-gluon one-loop process. The infrared poles are explicitly and locally cancelled in all regions of phase space prior to integration, leaving a finite remainder that can be safely evaluated numerically in four-dimensions. We show numerically that the subtraction term correctly approximates the matrix elements in the various single unresolved configurations
The singular behavior of massive QCD amplitudes
We discuss the structure of infrared singularities in on-shell QCD amplitudes
with massive partons and present a general factorization formula in the limit
of small parton masses. The factorization formula gives rise to an all-order
exponentiation of both, the soft poles in dimensional regularization and the
large collinear logarithms of the parton masses. Moreover, it provides a
universal relation between any on-shell amplitude with massive external partons
and its corresponding massless amplitude. For the form factor of a heavy quark
we present explicit results including the fixed-order expansion up to three
loops in the small mass limit. For general scattering processes we show how our
constructive method applies to the computation of all singularities as well as
the constant (mass-independent) terms of a generic massive n-parton QCD
amplitude up to the next-to-next-to-leading order corrections.Comment: version to appear in JHEP (sec. 3 with expanded discussion and
appendix with added results
Numerical Evaluation of One-Loop Diagrams Near Exceptional Momentum Configurations
One problem which plagues the numerical evaluation of one-loop Feynman
diagrams using recursive integration by part relations is a numerical
instability near exceptional momentum configurations. In this contribution we
will discuss a generic solution to this problem. As an example we consider the
case of forward light-by-light scattering.Comment: 5 pages, 5 figures, Talk given at the 7th DESY Workshop on Elementary
Particle Theory, Loops and Legs in Quantum Field Theory, Zinnowitz 200
NNLO QCD corrections in full colour for jet production observables at the LHC
Calculations for processes involving a high multiplicity of coloured particles often employ a leading colour approximation, where only the leading terms in the expansion of the number of colours Nc and the number of flavours nf are retained. This approximation of the full colour result is motivated by the 1/N2 c suppression of the first subleading terms and by the increasing complexity of including subleading colour contributions to the calculation. In this work, we present the calculations using the antenna subtraction method in the NNLOjet framework for the NNLO QCD corrections at full colour for several jet observables at the LHC. The single jet inclusive cross section is calculated doubly differential in transverse momentum and absolute rapidity and compared with the CMS measurement at 13 TeV. A calculation for dijet production doubly differential in dijet mass and rapidity difference is also performed and compared with the ATLAS 7 TeV data. Lastly, a triply differential dijet cross section in average transverse momentum, rapidity separation and dijet system boost is calculated and compared with the CMS 8 TeV data. The impact of the subleading colour contributions to the leading colour approximation is assessed in detail for all three types of observables and as a function of the jet cone size. The subleading colour contributions play a potentially sizable role in the description of the triply differential distributions, which probe kinematical configurations that are not easily accessed by any of the other observables
NNLO QCD corrections to event orientation in e+e- annihilation
We present a new implementation of the NNLO QCD corrections to three-jet final states and related event-shape observables in electron–positron annihilation. Our implementation is based on the antenna subtraction method, and is performed in the NNLOjet framework. The calculation improves upon earlier results by taking into account the full kinematical information on the initial state momenta, thereby allowing the event orientation to be computed to NNLO accuracy. We find the event-orientation distributions at LEP and SLC to be very robust under higher order QCD corrections
The two-loop scalar and tensor pentabox graph with light-like legs
We study the scalar and tensor integrals associated with the pentabox
topology: the class of two-loop box integrals with seven propagators - five in
one loop and three in the other. We focus on the case where the external legs
are light-like and use integration-by-parts identities to express the scalar
integral in terms of two master-topology integrals and present an explicit
analytic expression for the pentabox scalar integral as a series expansion in
ep = (4-D)/2. We also give an algorithm based on integration by parts for
relating the generic tensor integrals to the same two master integrals and
provide general formulae describing the master integrals in arbitrary dimension
and with general powers of propagators.Comment: Detailed expansions of intermediate results adde
Double virtual corrections for gluon scattering at NNLO
We use the antenna subtraction method to isolate the double virtual infrared singularities present in gluonic scattering amplitudes at next-to-next-to-leading order. In previous papers, we derived the subtraction terms that rendered (a) the double real radiation tree-level process finite in the single and double unresolved regions of phase space and (b) the mixed single real radiation one-loop process both finite and well behaved in the unresolved regions of phase space. Here, we show how to construct the double virtual subtraction term using antenna functions with both initial- and final-state partons which remove the explicit infrared poles present in the two-loop amplitude. As an explicit example, we write down the subtraction term for the four-gluon two-loop process. The infrared poles are explicitly and locally cancelled in all regions of phase space leaving a finite remainder that can be safely evaluated numerically in four-dimensions
Precise predictions for WH+jet production at the LHC
We present precise predictions for the production of a Higgs boson in association with a hadronic jet and a W boson at hadron colliders. The behaviour of QCD corrections are studied for fiducial cross sections and distributions of the charged gauge boson and jet-related observables. The inclusive process (at least one resolved jet) and the exclusive process (exactly one resolved jet) are contrasted and discussed. The inclusion of QCD corrections up to O(α3s)leads to a clear stabilisation of the predictions and contributes substantially to a reduction of remaining theoretical uncertaintie
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