8,953 research outputs found

    Polarized parton distributions in perturbative QCD

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    We review the main results of next-to-leading order QCD analyses of polarized deep-inelastic scattering data, with special attention to the assessment of theoretical uncertainties.Comment: 4 pages Latex, 1 ps figure. Talk given at NUCLEON '99, Frascati(Italy), June 7-9 199

    Are Parton Distributions Positive?

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    We show that the naive positivity conditions on polarized parton distributions which follow from their probabilistic interpretation in the naive parton model are reproduced in perturbative QCD at the leading log level if the quark and gluon distribution are defined in terms of physical processes. We show how these conditions are modified at the next-to-leading level, and discuss their phenomenological implications, in particular in view of the determination of the polarized gluon distributionComment: 5 pages, 4 figures, latex with espcrc2 and epsfi

    Cancellation of power enhancements in non-spectator decays

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    Exclusive non-spectator decay rates of beauty hadrons contain power enhancements of the form (m_b/m)^2 and m_b/m, where m_b is the b-quark mass and m is a light quark mass. An implicit argument has been recently given according to which these singularities cancel in the totally inclusive decay width. We present in this note a completely explicit computation of the diagrams containing power enhancements. Our results agree with the previous general conclusion.Comment: 13 pages, LaTex version 2.0

    Neutrino Masses and Grand Unification

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    We discuss some models of neutrino masses and mixings in the context of fermion masses in Grand Unified Theories.Comment: Talk, 6th Int. Workshop on TAUP99, 6-10 September 1999, Paris,Franc

    Solving the Altarelli-Parisi equations with truncated moments

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    The technique of truncated moments of parton distributions allows us to study scaling violations without making any assumption on the shape of parton distributions. The numerical implementation of the method is however difficult, since the evolution equations for truncated moments are not diagonal. We present a simple way to improve the efficiency of the numerical solution of the evolution equations for truncated moments. As a result, the number of truncated moments needed to achieve the required precision in the evolution is significantly smaller than in the original formulation of the technique. The method presented here can also be used to obtain the value of parton distributions in terms of truncated moments, and therefore it can be viewed as a technique for the solution of the Altarelli-Parisi equations.Comment: 10 pages Late

    All Order Running Coupling BFKL Evolution from GLAP (and vice-versa)

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    We present a systematic formalism for the derivation of the kernel of the BFKL equation from that of the GLAP equation and conversely to any given order, with full inclusion of the running of the coupling. The running coupling is treated as an operator, reducing the inclusion of running coupling effects and their factorization to a purely algebraic problem. We show how the GLAP anomalous dimensions which resum large logs of x can be derived from the running-coupling BFKL kernel order by order, thereby obtaining a constructive all-order proof of small x factorization. We check this result by explicitly calculating the running coupling contributions to GLAP anomalous dimensions up to next-to-next-to leading order. We finally derive an explicit expression for BFKL kernels which resum large logs of Q^2 up to next-to-leading order from the corresponding GLAP kernels, thus making possible a consistent collinear improvement of the BFKL equation up to the same order

    NEW PHYSICS AND RECENT HIGH PRECISION ELECTROWEAK MEASUREMENTS

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    We analyze LEP and SLC data from the 1995 Winter Conferences for signals of new physics. We compare the data with the Standard Model (SM) as well as a number of test hypotheses concerning the nature of new physics: (i) nonstandard Zbb couplings, (ii) nonstandard Zff couplings for the entire third generation, (iii) nonstandard oblique corrections, (iv) nonstandard lepton couplings, (v) general nonstandard W and Z couplings to all fermions, as well as combinations of the above. In most of our analyses, we leave the SM variables αs\alpha_s and mtm_t as free parameters to see how the various types of new physics can affect their inferred values. We find that the best fit (χ2/d.o.f.=8.4/10\chi^2/d.o.f. = 8.4/10) is obtained for the nonstandard Zbb couplings, which also give a `low' value (0.112) for αs\alpha_s. The SM also gives a good description of the Z data, having χ2/d.o.f.=12.4/12\chi^2/d.o.f. = 12.4/12. If αs\alpha_s is held fixed to the low-energy value 0.112, then we find that a combination of the nonstandard Zbb couplings is fit to lie more than four standard deviations away from zero.Comment: Plain TeX, 9 pages, one figure (uuencoded), and 8 table

    The Pion Structure Function in a Constituent Model

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    Using the recent relatively precise experimental results on the pion structure function, obtained from Drell--Yan processes, we quantitatively test an old model where the structure function of any hadron is determined by that of its constituent quarks. In this model the pion structure function can be predicted from the known nucleon structure function. We find that the data support the model, at least as a good first approximation.Comment: 9 pages, 3 figure

    Status of Neutrino Masses and Mixing in 2010

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    We present a short summary of our present knowledge and understanding of neutrino masses and mixing.Comment: 15 pages, 2 figures. Talk given at the Quarks, Strings and the Cosmos - Hector Rubinstein Memorial Symposium, August 09-11, 2010, AlbaNova, Stockholm, Swede

    Status of the Standard Model at the LHC Start

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    I present a concise review of where we stand in particle physics today. First, I will discuss QCD, then the electroweak sector and finally the motivations and the avenues for new physics beyond the Standard Model.Comment: Plenary Talk at the 5th Italian Workshop on p-p Physics at the LHC. 13 pages, no figure
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