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A. M. Cooper-Sarkar

Publications and source records attributed to A. M. Cooper-Sarkar.

At least 55 records · Page 3Linked to original sources
arXiv2012

Scaled momentum distributions for K0s and Lambda/bar Lambda in DIS at HERA

Scaled momentum distributions for the strange hadrons K0s and Lambda/bar Lambda were measured in deep inelastic ep scattering with the ZEUS detector at HERA using an integrated luminosity of 330 pb-1. The evolution of these distributions with the photon virtuality, Q2, was studied in the kinematic region 10<Q2<40000 GeV2 and 0.001<x<0.75, where x is the Bjorken scaling variable. Clear scaling violations are observed. Predictions based on different approaches to fragmentation were compared to the measurements. Leading-logarithm parton-shower Monte Carlo calculations interfaced to the Lund string fragmentation model describe the data reasonably well in the whole range measured. Next-to-leading-order QCD calculations based on fragmentation functions, FFs, extracted from e+e- data alone, fail to describe the measurements. The calculations based on FFs extracted from a global analysis including e+e-, ep and pp data give an improved description. The measurements presented in this paper have the potential to further constrain the FFs of quarks, anti-quarks and gluons yielding K0s and Lambda/bar Lambda strange hadrons.

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arXiv2012

Search for single-top production in ep collisions at HERA

A search for single-top production, $ep \rightarrow etX$, has been performed with the ZEUS detector at HERA using data corresponding to an integrated luminosity of $0.37\fbi$. No evidence for top production was found, consistent with the expectation from the Standard Model. Limits were computed for single-top production via flavour changing neutral current transitions. The result was combined with a previous ZEUS result yielding a total luminosity of 0.50fb-1. A 95% credibility level upper limit of 0.13 pb was obtained for the cross section at the centre-of-mass energy of $\sqrt{s}=315\gev$.

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arXiv2012

Handbook of LHC Higgs Cross Sections: 2. Differential Distributions

This Report summarises the results of the second year's activities of the LHC Higgs Cross Section Working Group. The main goal of the working group was to present the state of the art of Higgs Physics at the LHC, integrating all new results that have appeared in the last few years. The first working group report Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables (CERN-2011-002) focuses on predictions (central values and errors) for total Higgs production cross sections and Higgs branching ratios in the Standard Model and its minimal supersymmetric extension, covering also related issues such as Monte Carlo generators, parton distribution functions, and pseudo-observables. This second Report represents the next natural step towards realistic predictions upon providing results on cross sections with benchmark cuts, differential distributions, details of specific decay channels, and further recent developments.

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arXiv2011

Exclusive electroproduction of two pions at HERA

The exclusive electroproduction of two pions in the mass range 0.4 < Mππ < 2.5 GeV has been studied with the ZEUS detector at HERA using an integrated luminosity of 82 pb-1. The analysis was carried out in the kinematic range of 2 < Q2 < 80 GeV2, 32 < W < 180 GeV and |t| < 0.6 GeV2, where Q2 is the photon virtuality, W is the photon-proton centre-of-mass energy and t is the squared four-momentum transfer at the proton vertex. The two-pion invariant-mass distribution is interpreted in terms of the pion electromagnetic form factor, |F(Mππ)|, assuming that the studied mass range includes the contributions of the ρ, ρ' and ρ" vector-meson states. The masses and widths of the resonances were obtained and the Q2 dependence of the cross-section ratios σ(ρ' \rightarrow ππ)/σ(ρ) and σ(ρ" \rightarrow ππ)/σ(ρ) was extracted. The pion form factor obtained in the present analysis is compared to that obtained in e+e- \rightarrow π+π-.

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arXiv2011

Measurement of heavy-quark jet photoproduction at HERA

Photoproduction of beauty and charm quarks in events with at least two jets has been measured with the ZEUS detector at HERA using an integrated luminosity of 133 $pb^{-1}$. The fractions of jets containing b and c quarks were extracted using the invariant mass of charged tracks associated with secondary vertices and the decay-length significance of these vertices. Differential cross sections as a function of jet transverse momentum, $p_{T}^{\text{jet}}$, and pseudorapidity, $η^{\text{jet}}$, were measured. The data are compared with previous measurements and are well described by next-to-leading-order QCD predictions.

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arXiv2011

Measurement of beauty production in deep inelastic scattering at HERA using decays into electrons

The production of beauty quarks in ep interactions has been studied with the ZEUS detector at HERA for exchanged four-momentum squared Q^2 > 10 GeV^2, using an integrated luminosity of 363 pb^{-1}. The beauty events were identified using electrons from semileptonic b decays with a transverse momentum 0.9 < p_T^e < 8 GeV and pseudorapidity |eta^e| < 1.5. Cross sections for beauty production were measured and compared with next-to-leading-order QCD calculations. The beauty contribution to the proton structure function F_2 was extracted from the double-differential cross section as a function of Bjorken-x and Q^2.

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arXiv2010

Measurement of beauty production in DIS and F_2^bbbar extraction at ZEUS

Beauty production in deep inelastic scattering with events in which a muon and a jet are observed in the final state has been measured with the ZEUS detector at HERA using an integrated luminosity of 114 pb^-1. The fraction of events with beauty quarks in the data was determined using the distribution of the transverse momentum of the muon relative to the jet. The cross section for beauty production was measured in the kinematic range of photon virtuality, Q^2 > 2 Gev^2, and inelasticity, 0.05 < y < 0.7, with the requirement of a muon and a jet. Total and differential cross sections are presented and compared to QCD predictions. The beauty contribution to the structure function F_2 was extracted and is compared to theoretical predictions.

hep-ex↗
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.

hep-ph↗
arXiv2005

Uncertainties on $W$ and $Z$ production at the LHC

Uncertainties on low-$x$ PDFs are crucial for the standard model benchmark processes of $W$ and $Z$ production at the LHC. The current level of PDF uncertainty is critically reviewed and the possibility of reducing this uncertainty using early LHC data is investigated taking into account realistic expectations for measurement accuracy, kinematic cuts and backgrounds.

hep-ex↗
arXiv2004

An NLO QCD Analysis of Inclusive Cross-section Data and Jet Production Data from the ZEUS Experiment at HERA-I

The ZEUS inclusive cross-section data from HERA-I, for charged and neutral current processes, taken with $e^+$ and $e^-$ beams, have been used in a new NLO QCD analysis to extract the parton distribution functions (PDFs) of the proton within a single experiment. The precision of the high-$x$ gluon determination is improved by using ZEUS differential cross-section data on inclusive jet production in $e^+p$ scattering and di-jet production in $γ-p$ scattering

hep-ph↗
arXiv2002

Questions on Uncertainties in Parton Distributions

A discussion is presented of the manner in which uncertainties in parton distributions and related quantities are determined. One of the central problems is the criteria used to judge what variation of the parameters describing a set of partons is acceptable within the context of a global fit. Various ways of addressing this question are outlined.

hep-ph↗
arXiv2002

Uncertainties on parton distribution functions from the ZEUS NLO QCD fit to data on deep inelastic scattering

An NLO QCD analysis of the ZEUS data on $e^+ p$ deep inelastic scattering together with fixed-target data has been performed from which the gluon and quark densities of the proton and the value of the strong coupling parameter, $α_s(M_Z^2)$, have been extracted. The study includes a full treatment of the experimental systematic uncertainties, including point-to-point correlations. Different ways of incorporating correlated systematic uncertainties into the fit are discussed and compared.

hep-ph↗
arXiv1999

High Q^2 Deep Inelastic Scattering at HERA

High Q^2 NC and CC cross-sections as measured at HERA can give information on two distinct areas of current interest. Firstly, supposing that all the electroweak parameters are well known, these cross-sections may be used to give information on parton distributions at high x and high Q^2. Secondly, supposing that parton distributions are well known, after evolution in Q^2 from the kinematic regime where they are already measured, these cross-sections can be used to give information on electroweak parameters in a process where the exchanged boson is `spacelike' rather than `timelike'. WG1 addressed itself to clarifying the limits of our present and possible future knowledge on both these areas.

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