155 research outputs found

### Constraining a CP-violating WWV coupling from the W^+W^- threshold cross section at LEP2

The most general form of the $WWZ$ and $WW\gamma$ interaction contains a
$CP$-violating term which has the same threshold behaviour as the Standard
Model \ee \to \ww cross section. We calculate the cross section as a function
of the corresponding anomalous coupling, and estimate the bounds which can be
obtained from a measurement of the threshold cross section at LEP2. We show how
the effect of the coupling is most pronounced in the angular distributions of
the final-state fermions.Comment: 7 pages, LaTeX file, 5 ps figure

### BFKL Physics in Dijet Production at the LHC

The production in hadron-hadron collisions of jet pairs with large rapidity
separation and comparable modest transverse momentum is, in principle,
described by the perturbative QCD BFKL equation. The measurement of such jet
pairs appears well suited to the LHC detectors, with their ability to detect
forward jets. We present predictions for dijet cross sections and correlations
obtained using a BFKL Monte Carlo which allows kinematic and other subleading
effects to be incorporated. The enhanced phase space for gluon emission at the
LHC makes the BFKL behavior somewhat easier to observe than at the Tevatron,
although kinematic effects are still important. The production of forward jets
in association with heavy Higgs bosons via the gauge boson fusion mechanism is
also studied and compared with QCD dijet production.Comment: 13 pages including 5 figures, LaTeX, uses eps

### Uncertainties of predictions from parton distributions I: experimental errors

We determine the uncertainties on observables arising from the errors on the experimental data that are fitted in the global MRST2001 parton analysis. By diagonalizing the error matrix we produce sets of partons suitable for use within the framework of linear propagation of errors, which is the most convenient method for calculating the uncertainties. Despite the potential limitations of this approach we find that it can be made to work well in practice. This is confirmed by our alternative approach of using the more rigorous Lagrange multiplier method to determine the errors on physical quantities directly. As particular examples we determine the uncertainties on the predictions of the charged-current deep-inelastic structure functions, on the cross-sections for W production and for Higgs boson production via gluon-gluon fusion at the Tevatron and the LHC, on the ratio of W- to W+ production at the LHC and on the moments of the non-singlet quark distributions. We discuss the corresponding uncertainties on the parton distributions in the relevant x,Q2 domains. Finally, we briefly look at uncertainties related to the fit procedure, stressing their importance and using sW, sH and extractions of aS(MZ2) as examples. As a by-product of this last point we present a slightly updated set of parton distributions, MRST2002

### MRST2001: partons and α <SUB>S</SUB> from precise deep inelastic scattering and Tevatron jet data

We use all the available new precise data for deep inelastic and related hard scattering processes to perform NLO global parton analyses. These new data allow an improved determination of partons and, in particular, the inclusion of the recent measurements of the structure functions at HERA and of the inclusive jets at the Tevatron help to determine the gluon distribution and aS better than ever before. We find a somewhat smaller gluon at low x than previous determinations and that aS (MZ2) = 0.119 ±0.002 (expt.) ±0.003 (theory)

### Production of Jet Pairs at Large Relative Rapidity in Hadron-Hadron Collisions as a Probe of the Perturbative Pomeron

The production of jet pairs with small transverse momentum and large relative
rapidity in high energy hadron-hadron collisions is studied. The rise of the
parton-level cross section with increasing rapidity gap is a fundamental
prediction of the BFKL `perturbative pomeron' equation of Quantum
Chromodynamics. However, at fixed collider energy it is difficult to
disentangle this effect from variations in the cross section due to the parton
distributions. It is proposed to study instead the distribution in the
azimuthal angle difference of the jets as a function of the rapidity gap. The
flattening of this distribution with increasing dijet rapidity gap is shown to
be a characteristic feature of the BFKL behaviour. Predictions for the Fermilab
proton-antiproton collider are presented.Comment: 17 pages, 11 figures, preprint DTP/94/0

### J/psi production at the Tevatron and HERA: the effect of k_T smearing

We study the effects of intrinsic transverse momentum smearing on J/psi
production both at the Tevatron and at HERA. For the case of large-p_T J/psi
production at the Tevatron, the effects due to k_T smearing are mild. On the
other hand, inelastic J/psi photoproduction at HERA is very sensitive to the
k_T smearing and, in fact, with a reasonable value of it is possible to
resolve the large-z discrepancy seen by comparing non-relativistic QCD (NRQCD)
predictions with the HERA data. We conclude that, with the present kinematic
cuts, photoproduction at HERA is not a good test of NRQCD.Comment: 11 pages, LaTeX, 2 figures, uses epsfi

### Analytic Approaches to the Evolution of Polarised Parton Distributions at Small $x$

The $Q^2$ evolution of polarised parton distributions at small $x$ is
studied. Various analytic approximations are critically discussed. We compare
the full evolution with that obtained from the leading-pole approximation to
the splitting functions, and show that the validity of this approximation
depends critically on the $x \to 0$ behaviour of the starting distributions. A
new analytic solution which is valid at small $x$ is obtained, and its domain
of applicability is discussed.Comment: 14 pages, LATeX, 4 figures availabe as .uu-fil

### Rare decay Z --> neutrino antineutrino photon photon via quartic gauge boson couplings

We present a detailed calculation of the rare decay Z --> neutrino
antineutrino photon photon via the quartic neutral gauge boson coupling
Z-Z-photon-photon in the framework of the effective Lagrangian approach. The
current experimental bound on this decay mode is then used to constrain the
coefficients of this coupling. It is found that the bounds obtained in this
way, of the order of $10^{-1}$, are weaker than the ones obtained from the
analysis of triple-boson production at LEP-2Comment: 5 pages, 2 figures, to appear in Physical Review D Brief Report

### The aT distribution of the Z boson at hadron colliders

We provide the first theoretical study of a novel variable, $a_T$, proposed
in Ref.[1] as a more accurate probe of the region of low transverse momentum
$p_T$, for the $Z$ boson $p_T$ distribution at hadron colliders. The $a_T$ is
the component of $p_T$ transverse to a suitably defined axis. Our study
involves resummation of large logarithms in $a_T$ up to the next-to--leading
logarithmic accuracy and we compare the results to those for the well-known
$p_T$ distribution, identifying important physical differences between the two
cases. We also test our resummed result at the two-loop level by comparing its
expansion to order $\alpha_s^2$ with the corresponding fixed-order results and
find agreement with our expectations.Comment: 30 pages, 3 figures, JHEP class included. Final version published in
JHE

### Bosonic Quartic Couplings at LHC

We analyze the potential of the CERN Large Hadron Collider (LHC) to study
anomalous quartic vector-boson interactions Z Z gamma gamma, Z Z Z gamma, W+ W-
gamma gamma, and W+ W- Z gamma through the weak boson fusion processes q q -> q
q gamma gamma and q q -> q q gamma Z(-> l+ l-) with l = electron or muon. After
a careful study of the backgrounds and how to extract them from the data, we
show that the process p p -> j j gamma l+ l- is potentially the most sensitive
to deviations from the Standard Model, improving the sensitivity to anomalous
couplings by up to a factor 10^4 (10^2) with respect to the present direct
(indirect) limits.Comment: 18 pages, 2 figures, revised versio

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