49,488 research outputs found

    Conservation and persistence of spin currents and their relation to the Lieb-Schulz-Mattis twist operators

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    Systems with spin-orbit coupling do not conserve "bare" spin current j\bf{j}. A recent proposal for a conserved spin current J\bf{J} [J. Shi {\it et.al} Phys. Rev. Lett. {\bf 96}, 076604 (2006)] does not flow persistently in equilibrium. We suggest another conserved spin current Jˉ\bar{\bf{J}} that may flow persistently in equilibrium. We give two arguments for the instability of persistent current of the form J\bf{J}: one based on the equations of motions and another based on a variational construction using Lieb-Schulz-Mattis twist operators. In the absence of spin-orbit coupling, the three forms of spin current coincide.Comment: 5 pages; added references, simplified notation, clearer introductio

    The impact of the LHCb B±B^{\pm} production cross-section on parton distribution functions and determining the mass of heavy quarks

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    In this work, the impact of recent measurements of heavy-flavour production in pppp collisions on parton distribution functions (PDFs) and their uncertainties is studied. In this regard, the absolute and normalised cross sections of beauty hadron production measured by LHCb at center-of-mass energy of 7 TeV and 13 TeV are separately included in the next-to-leading order (NLO) global QCD analysis together with the measurements of the inclusive and heavy-flavour production cross sections at HERA. It is illustrated that the heavy-flavour data of the LHCb experiment impose additional constraints on the PDFs, especially on the gluon distribution at low partonic fractions xx of the proton momentum. One of the most important results of the present analysis is the significant reduction of the gluon uncertainties in the region less than x=10−4x=10^{-4} that can play a crucial rule in many areas of high energy physics investigations.Comment: 36 pages, 8 figures, has been accepted for publication in Nuclear Physics
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