arXiv subjects

J. M. Campbell

Publications and source records attributed to J. M. Campbell.

63 records · Page 4Linked to original sources

arXiv2006

Les Houches Physics at TeV Colliders 2005, Standard Model and Higgs working group: Summary report

This Report summarises the activities of the "SM and Higgs" working group for the Workshop "Physics at TeV Colliders", Les Houches, France, 2-20 May, 2005. On the one hand, we performed a variety of experimental and theoretical studies on standard candles (such as W, Z, and ttbar production), treating them either as proper signals of known physics, or as backgrounds to unknown physics; we also addressed issues relevant to those non-perturbative or semi-perturbative ingredients, such as Parton Density Functions and Underlying Events, whose understanding will be crucial for a proper simulation of the actual events taking place in the detectors. On the other hand, several channels for the production of the Higgs, or involving the Higgs, have been considered in some detail. The report is structured into four main parts. The first one deals with Standard Model physics, except the Higgs. A variety of arguments are treated here, from full simulation of processes constituting a background to Higgs production, to studies of uncertainties due to PDFs and to extrapolations of models for underlying events, from small-$x$ issues to electroweak corrections which may play a role in vector boson physics. The second part of the report treats Higgs physics from the point of view of the signal. In the third part, reviews are presented on the current status of multi-leg, next-to-leading order and of next-to-next-to-leading order QCD computations. Finally, the fourth part deals with the use of Monte Carlos for simulation of LHC physics.

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arXiv2001

W/Z + b,bbar/jets at NLO using the Monte Carlo MCFM

We summarize recent progress in next-to-leading order QCD calculations made using the Monte Carlo MCFM. In particular, we focus on the calculations of p anti-p -> W b-bbar, Z b-bbar and highlight the significant corrections to background estimates for Higgs searches in the channels WH and ZH at the Tevatron. We also report on the current progress of, and strategies for, the calculation of the process p anti-p -> W/Z + 2 jets.

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arXiv2000

Radiative corrections to Zb{\bar b} production

We report on QCD radiative corrections to the process, p\bar{p} to Zb\bar{b} in the approximation in which the b-quark is considered massless. The implementation of this process in the general purpose Monte Carlo program MCFM is discussed in some detail. These results are used to investigate backgrounds to Higgs boson production in the ZH channel. We investigate the Higgs mass range (100 GeV < m_H < 130 GeV) for the Tevatron running at sqrt{s}=2 TeV.

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arXiv1999

An update on vector boson pair production at hadron colliders

We present numerical results (including full one-loop QCD corrections) for the processes p p-bar and pp -> W+ W-, W+/- Z/gamma* and Z/gamma* Z/gamma* followed by the decay of the massive vector bosons into leptons. In addition to their intrinsic importance as tests of the standard model, these processes are also backgrounds to conjectured non-standard model processes. Because of the small cross sections at the Tevatron, full experimental control of these backgrounds will be hard to achieve. This accentuates the need for up-to-date theoretical information. A comparison is made with earlier work and cross section results are presented for p p-bar collisions at sqrt{s}=2 TeV and pp collisions at sqrt{s}=14 TeV. Practical examples of the use of our calculations are presented.

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arXiv1998

Four jet event shapes in electron-positron annihilation

We report next-to-leading order perturbative QCD predictions of 4 jet event shape variables for the process e+e- \to 4 jets obtained using the general purpose Monte Carlo EERAD2. This program is based on the known `squared' one loop matrix elements for the virtual γ^* \to 4 parton contribution and squared matrix elements for 5 parton production. To combine the two distinct final states numerically we present a hybrid of the commonly used subtraction and slicing schemes based on the colour antenna structure of the final state which can be readily applied to other processes. We have checked that the numerical results obtained with EERAD2 are consistent with next-to-leading order estimates of the distributions of previously determined four jet-like event variables. We also report the next-to-leading order scale independent coefficients for some previously uncalculated observables; the light hemisphere mass, narrow jet broadening, Aplanarity and the 4 jet transition variables with respect to the JADE and Geneva jet finding algorithms. For each of these observables, the next-to-leading order corrections calculated at the physical scale significantly increase the rate compared to leading order (the K factor is approximately 1.5 -- 2). With the exception of the 4 jet transition variables, the published DELPHI data lies well above the O(α_s^3) predictions. The renormalisation scale uncertainty is still large and in most cases the data prefers a scale significantly smaller than the physical scale. This situation is reminiscent of that for three jet shape variables, and should be improved by the inclusion of power corrections and resummation of large infrared logarithms.

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arXiv1998

Double Unresolved Approximations to Multiparton Scattering Amplitudes

We present approximations to tree level multiparton scattering amplitudes which are appropriate when two partons are unresolved. These approximations are required for the analytic isolation of infrared singularities of n+2 parton scattering processes contributing to the next-to-next-to-leading order corrections to n jet cross sections. In each case the colour ordered matrix elements factorise and yield a function containing the singular factors multiplying the n parton amplitudes. When the unresolved particles are not colour connected, the approximations are simple products of the familar eikonal and Altarelli-Parisi splitting functions used to describe single unresolved emission. However, when the unresolved particles are colour connected the factorisation is more complicated and we introduce new and general functions to describe the triple collinear and soft/collinear limits in addition to the known double soft gluon limits of Berends and Giele. As expected the triple collinear splitting functions obey an N=1 SUSY identity. To illustrate the use of these double unresolved approximations, we have examined the singular limits of the tree level matrix elements for e+e- to 5 partons when only three partons are resolved. When integrated over the unresolved regions of phase space, these expressions will be of use in evaluating the O(alpha_s^3) corrections to the three jet rate in electron positron annihilation.

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arXiv1998

The One-loop QCD Corrections for gamma^* to q qbar gg

We compute the one-loop QCD amplitudes for the decay of an off-shell vector boson with vector couplings into a quark-antiquark pair accompanied by two gluons keeping, for the first time, all orders in the number of colours. Together with previous work this completes the calculation of the necessary one-loop amplitudes needed for the calculation of the next-to-leading order O(alpha_s^3) corrections to four jet production in electron-positron annihilation, the production of a gauge boson accompanied by two jets in hadron-hadron collisions and three jet production in deep inelastic scattering.

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arXiv1998

Determination of Lambda_QCD from the measured energy dependence of <1-Thrust>

We directly fit the experimentally measured energy dependence of the average value of 1-Thrust, <1-T>, over the e^+e^- centre-of-mass energy range Q=14 - 172 GeV to the QCD expectation obtained by integrating up the evolution equation for d<1-T>/dlog Q in terms of <1-T>. We fit for Lambda_QCD, uncalculated O(alpha_S^3) and higher perturbative corrections parameterized by the scheme invariant rho_2, and the parameter lambda which characterizes the magnitude of the leading 1/Q power corrections anticipated for <1-T>. A 3-parameter fit yields Lambda_QCD=245^{+20}_{-17} MeV, rho_2=-16\pm 11 and lambda=0.27^{+0.12}_{-0.10} GeV, equivalent to alpha_S(M_Z)=0.1194 \pm 0.0014. In this approach, there is no error associated with the choice of the renormalization scale mu.

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arXiv1998

One-Loop Tensor Integrals in Dimensional Regularisation

We show how to evaluate tensor one-loop integrals in momentum space avoiding the usual plague of Gram determinants. We do this by constructing combinations of $n$- and $(n-1)$-point scalar integrals that are finite in the limit of vanishing Gram determinant. These non-trivial combinations of dilogarithms, logarithms and constants are systematically obtained by either differentiating with respect to the external parameters - essentially yielding scalar integrals with Feynman parameters in the numerator - or by developing the scalar integral in $D=6-2\e$ or higher dimensions. As an explicit example, we develop the tensor integrals and associated scalar integral combinations for processes where the internal particles are massless and where up to five (four massless and one massive) external particles are involved. For more general processes, we present the equations needed for deriving the relevant combinations of scalar integrals.

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