1,116 research outputs found
Jet vetoing and Herwig++
We investigate the simulation of events with gaps between jets with a veto on
additional radiation in the gap in Herwig++. We discover that the
currently-used random treatment of radiation in the parton shower is generating
some unphysical behaviour for wide-angle gluon emission in QCD 2 to 2
scatterings. We explore this behaviour quantitatively by making the same
assumptions as the parton shower in the analytical calculation. We then modify
the parton shower algorithm in order to correct the simulation of QCD
radiation.Comment: 18 pages, 11 figure
Consistent perturbations in an imperfect fluid
We present a new prescription for analysing cosmological perturbations in a
more-general class of scalar-field dark-energy models where the energy-momentum
tensor has an imperfect-fluid form. This class includes Brans-Dicke models,
f(R) gravity, theories with kinetic gravity braiding and generalised galileons.
We employ the intuitive language of fluids, allowing us to explicitly maintain
a dependence on physical and potentially measurable properties. We demonstrate
that hydrodynamics is not always a valid description for describing
cosmological perturbations in general scalar-field theories and present a
consistent alternative that nonetheless utilises the fluid language. We apply
this approach explicitly to a worked example: k-essence non-minimally coupled
to gravity. This is the simplest case which captures the essential new features
of these imperfect-fluid models. We demonstrate the generic existence of a new
scale separating regimes where the fluid is perfect and imperfect. We obtain
the equations for the evolution of dark-energy density perturbations in both
these regimes. The model also features two other known scales: the Compton
scale related to the breaking of shift symmetry and the Jeans scale which we
show is determined by the speed of propagation of small scalar-field
perturbations, i.e. causality, as opposed to the frequently used definition of
the ratio of the pressure and energy-density perturbations.Comment: 40 pages plus appendices. v2 reflects version accepted for
publication in JCAP (new summary of notation, extra commentary on choice of
gauge and frame, extra references to literature
Jet vetoing at the LHC
We study the effect of a veto on additional jets in the rapidity region
between a pair of high transverse momentum jets at the LHC. We aim to sum the
most important logarithms in the ratio of the jet transverse momentum to the
veto scale and to that end we attempt to assess the significance of the
super-leading logarithms that appear at high orders in the perturbative
expansion. We also compare our results to those of HERWIG++, in an attempt to
ascertain the accuracy of the angular ordered parton shower. We find that there
are large corrections that arise for large enough jet transverse momenta as a
consequence of Coulomb gluon exchanges.Comment: 25 page
Emerging themes to support ambitious UK marine biodiversity conservation
Healthy marine ecosystems provide a wide range of resources and services that support life on Earth and contribute to human wellbeing. Marine Protected Areas (MPAs) are accepted as an important tool for the restoration and maintenance of marine ecosystem structure, function, health and ecosystem integrity through the conservation of significant species, habitats, or entire ecosystems. In recent years there has been a rapid expansion in the area of ocean designated as an MPA. Despite this progress in spatial protection targets and the progressive knowledge of the essential interdependence between the human and the ocean system, marine biodiversity continues to decline, placing in jeopardy the range of ecosystem services benefits humans rely on. There is a need to address this shortcoming. Ambitious marine conservation:âą Requires a shift from managing individual marine features within MPAs to whole-sites to enable repair and renewal of marine systems;âą Reflects an ambition for sustainable livelihoods by fully integrating fisheries management with conservation (Ecosystem Based Fisheries Management) as the two are critically interdependent;âą Establishes a world class and cost effective ecological and socio-economic monitoring and evaluation framework that includes the use of controls and sentinel sites to improve sustainability in marine management; andâą Challenges policy makers and practitioners to be progressive by integrating MPAs into the wider seascape as critical functional components rather than a competing interest and move beyond MPAs as the only tool to underpin the benefits derived from marine ecosystems by identifying other effective area-based conservation measures (OECMs) to establish synergies with wider governance frameworks
Measurement of the ratio of branching fractions BR(B0 -> K*0 gamma)/BR(Bs0 -> phi gamma)
The ratio of branching fractions of the radiative B decays B0 -> K*0 gamma
and Bs0 -> phi gamma has been measured using 0.37 fb-1 of pp collisions at a
centre of mass energy of sqrt(s) = 7 TeV, collected by the LHCb experiment. The
value obtained is BR(B0 -> K*0 gamma)/BR(Bs0 -> phi gamma) = 1.12 +/- 0.08
^{+0.06}_{-0.04} ^{+0.09}_{-0.08}, where the first uncertainty is statistical,
the second systematic and the third is associated to the ratio of fragmentation
fractions fs/fd. Using the world average for BR(B0 -> K*0 gamma) = (4.33 +/-
0.15) x 10^{-5}, the branching fraction BR(Bs0 -> phi gamma) is measured to be
(3.9 +/- 0.5) x 10^{-5}, which is the most precise measurement to date.Comment: 15 pages, 1 figure, 2 table
Differential branching fraction and angular analysis of the decay B0âKâ0ÎŒ+ÎŒâ
The angular distribution and differential branching fraction of the decay B 0â K â0 ÎŒ + ÎŒ â are studied using a data sample, collected by the LHCb experiment in pp collisions at sâ=7 TeV, corresponding to an integrated luminosity of 1.0 fbâ1. Several angular observables are measured in bins of the dimuon invariant mass squared, q 2. A first measurement of the zero-crossing point of the forward-backward asymmetry of the dimuon system is also presented. The zero-crossing point is measured to be q20=4.9±0.9GeV2/c4 , where the uncertainty is the sum of statistical and systematic uncertainties. The results are consistent with the Standard Model predictions
Bose-Einstein correlations of same-sign charged pions in the forward region in pp collisions at âs=7 TeV
Bose-Einstein correlations of same-sign charged pions, produced in protonproton collisions at a 7 TeV centre-of-mass energy, are studied using a data sample collected
by the LHCb experiment. The signature for Bose-Einstein correlations is observed in the
form of an enhancement of pairs of like-sign charged pions with small four-momentum
difference squared. The charged-particle multiplicity dependence of the Bose-Einstein correlation parameters describing the correlation strength and the size of the emitting source
is investigated, determining both the correlation radius and the chaoticity parameter. The
measured correlation radius is found to increase as a function of increasing charged-particle
multiplicity, while the chaoticity parameter is seen to decreas
Measurements of the branching fractions of B+âppK+ decays
The branching fractions of the decay B+ â ppÌK+ for different intermediate states are measured using data, corresponding to an integrated luminosity of 1.0 fb-1, collected by the LHCb experiment. The total branching fraction, its charmless component MppÌ < 2.85 GeV/c2 and the branching fractions via the resonant ccÌ states η c(1S) and Ï(2S) relative to the decay via a J/Ï intermediate state are [Equation not available: see fulltext.] Upper limits on the B + branching fractions into the η c(2S) meson and into the charmonium-like states X(3872) and X(3915) are also obtained
Search for the decay Bs0âD*âϱ
A search for the decay Bs0âD*âϱ is presented using a data sample corresponding to an integrated luminosity of 1.0ââfb-1 of pp collisions collected by LHCb. This decay is expected to be mediated by a W-exchange diagram, with little contribution from rescattering processes, and therefore a measurement of the branching fraction will help us to understand the mechanism behind related decays such as Bs0âÏ+Ï- and Bs0âDD- . Systematic uncertainties are minimized by using B0âD*âϱ as a normalization channel. We find no evidence for a signal, and set an upper limit on the branching fraction of B(Bs0âD*âϱ)<6.1(7.8)Ă10-6 at 90% (95%) confidence level
Study of charmonium production in b -hadron decays and first evidence for the decay Bs0
Using decays to Ï-meson pairs, the inclusive production of charmonium states in b-hadron decays is studied with pp collision data corresponding to an integrated luminosity of 3.0 fbâ1, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. Denoting byBC ⥠B(b â C X) Ă B(C â ÏÏ) the inclusive branching fraction of a b hadron to a charmonium state C that decays into a pair of Ï mesons, ratios RC1C2 ⥠BC1 /BC2 are determined as RÏc0ηc(1S) = 0.147 ± 0.023 ± 0.011, RÏc1ηc(1S) =0.073 ± 0.016 ± 0.006, RÏc2ηc(1S) = 0.081 ± 0.013 ± 0.005,RÏc1 Ïc0 = 0.50 ± 0.11 ± 0.01, RÏc2 Ïc0 = 0.56 ± 0.10 ± 0.01and Rηc(2S)ηc(1S) = 0.040 ± 0.011 ± 0.004. Here and below the first uncertainties are statistical and the second systematic.Upper limits at 90% confidence level for the inclusive production of X(3872), X(3915) and Ïc2(2P) states are obtained as RX(3872)Ïc1 < 0.34, RX(3915)Ïc0 < 0.12 andRÏc2(2P)Ïc2 < 0.16. Differential cross-sections as a function of transverse momentum are measured for the ηc(1S) andÏc states. The branching fraction of the decay B0s â ÏÏÏ is measured for the first time, B(B0s â ÏÏÏ) = (2.15±0.54±0.28±0.21B)Ă10â6. Here the third uncertainty is due to the branching fraction of the decay B0s â ÏÏ, which is used for normalization. No evidence for intermediate resonances is seen. A preferentially transverse Ï polarization is observed.The measurements allow the determination of the ratio of the branching fractions for the ηc(1S) decays to ÏÏ and p p asB(ηc(1S)â ÏÏ)/B(ηc(1S)â p p) = 1.79 ± 0.14 ± 0.32
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