449 research outputs found

    A theoretical determination of the eta-eta' mixing

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    The systematic large N_c limit within chiral perturbation provides an optimal eta-eta' mixing angle of about -27 degrees at leading order in p^2. In this frame, agreement with the data can be reached with higher order corrections of about 20%.Comment: 12 pages, 2 figures; reference to the two-mixing-angle scheme added, minor corrections; to appear in JHE

    Single top polarisation as a window to new physics

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    We discuss the effect of heavy new physics, parameterised in terms of four-fermion operators, in the polarisation of single top (anti-)quarks in the tt-channel process at the LHC. It is found that for operators involving a right-handed top quark field the relative effect on the longitudinal polarisation is twice larger than the relative effect on the total cross section. This enhanced dependence on possible four-fermion contributions makes the polarisation measurements specially interesting, in particular at high momenta.Comment: LaTeX 10 pages. v2: comments and references added, journal versio

    Probing top-Higgs non-standard interactions at the LHC

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    Effective interactions involving both the top quark and the Higgs field are among the least constrained of all possible (gauge invariant) dimension-six operators in the Standard Model. Such a handful of operators, in particular the top quark chromomagnetic dipole moment, might encapsulate signs of the new physics responsible for electroweak symmetry breaking. In this work, we compute the contributions of these operators to inclusive Higgs and t tbar h production. We argue that: i) rather strong constraints on the overall size of these operators can already be obtained from the current limits/evidence on Higgs production at the LHC; ii) t tbar h production will provide further key information that is complementary to t tbar measurements, and the possibility of discriminating among different contributions by performing accurate measurements of total and differential rates.Comment: 8 pages, 9 figures. v2: Logarithmic contribution of the top quark chromomagnetic operator to Higgs production by gluon fusion correcte

    Baseline for assessing the impact of fairtrade certification on cocoa farmers and cooperatives in C么te d鈥橧voire

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    Report commissioned by Fairtrade Africa and Fairtrade International. World Agroforestry Centre, Nairobi, Kenya. Farms-Forests-LandscapesIn 2014, Fairtrade International, Fairtrade Africa, the World Agroforestry Centre (ICRAF) and Bioversity International initiated a collaboration for the development of a multidimensional baseline on small-scale cocoa farmers and their cooperatives in West Africa. The baseline is expected to provide a fuller understanding of the current situation for Fairtrade cocoa production and marketing as well as provide the foundation for rigorous assessment of outcomes and impacts of Fairtrade certification on cocoa cooperatives and smallholder households in West Africa in the future. C么te d鈥橧voire and Ghana, the two largest Fairtrade cocoa producers in West Africa, provide about 68 percent of the cocoa that is sold under Fairtrade terms in global markets. In 2013, the year this study was commissioned, the volume of Fairtrade cocoa sold from West Africa reached 133 400 tonnes, involving 71 cooperatives and producer associations and 138 800 farmers. Most of this cocoa originated from C么te d鈥橧voire and Ghana. The rapid growth in the number of cocoa-producing organizations joining the Fairtrade system in C么te d鈥橧voire and Ghana provides a unique opportunity to build a baseline on Fairtrade cocoa producers in West Africa for future monitoring and impact assessment. This report focuses on the Fairtrade cocoa baseline for C么te d鈥橧voire (a similar report is available for Ghana). It describes the conceptual framework and methods used in the design of the baseline, followed by an assessment of the context in C么te d鈥橧voire. Key features of the baseline data at the cooperative and household levels are covered in detail. The report concludes with some recommendations to Fairtrade for expanding Fairtrade International in C么te d鈥橧voire and for follow-up actions for future baseline work

    Virtual Compton Scattering and the Generalized Polarizabilities of the Proton at Q\u3csup\u3e2\u3c/sup\u3e = 0.92 and 1.76 GeV\u3csup\u3e2\u3c/sup\u3e

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    Virtual Compton scattering (VCS) on the proton has been studied at the Jefferson Laboratory using the exclusive photon electroproduction reaction ep 鈫 ep纬. This paper gives a detailed account of the analysis which has led to the determination of the structure functions PLL 鈭 PTT/蔚 and PLT and the electric and magnetic generalized polarizabilities (GPs) 伪E(Q2)and 尾M(Q2) at values of the four-momentum transfer squared Q2 = 0.92 and 1.76 GeV2. These data, together with the results of VCS experiments at lower momenta, help building a coherent picture of the electric and magnetic GPs of the proton over the full measured Q2 range and point to their nontrivial behavior

    A Fast Track towards the `Higgs' Spin and Parity

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    The LHC experiments ATLAS and CMS have discovered a new boson that resembles the long-sought Higgs boson: it cannot have spin one, and has couplings to other particles that increase with their masses, but the spin and parity remain to be determined. We show here that the `Higgs' + gauge boson invariant-mass distribution in `Higgs'-strahlung events at the Tevatron or the LHC would be very different under the J^P = 0+, 0- and 2+ hypotheses, and could provide a fast-track indicator of the `Higgs' spin and parity. Our analysis is based on simulations of the experimental event selections and cuts using PYTHIA and Delphes, and incorporates statistical samples of `toy' experiments.Comment: 18 pages, 9 pdf figure

    Towards testing a two-Higgs-doublet model with maximal CP symmetry at the LHC: construction of a Monte Carlo event generator

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    A Monte Carlo event generator is constructed for a two-Higgs-doublet model with maximal CP symmetry, the MCPM. The model contains five physical Higgs bosons; the \rho', behaving similarly to the standard-model Higgs boson, two extra neutral bosons hh' and h"h", and a charged pair HH^\pm. The special feature of the MCPM is that, concerning the Yukawa couplings, the bosons hh', h"h" and HH^\pm couple directly only to the second generation fermions but with strengths given by the third-generation-fermion masses. Our event generator allows the simulation of the Drell-Yan-type production processes of hh', h"h" and HH^\pm in proton-proton collisions at LHC energies. Also the subsequent leptonic decays of these bosons into the +\mu^+ \mu^-, +\mu^+ \nu_\mu and \mu^- \bar \nu_\mu channels are studied as well as the dominant background processes. We estimate the integrated luminosities needed in ppp p collisions at center-of-mass energies of 8 TeV and 14 TeV for significant observations of the Higgs bosons hh', h"h" and HH^\pm in these muonic channels
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