Abstract
We show that the production of J/ψ or ϒ pairs in unpolarised pp collisions is currently the best process to measure the momentum distribution of linearly-polarised gluons inside unpolarised protons through the study of azimuthal asymmetries. Not only the short-distance coefficients for such reactions induce the largest possible cos4ϕ modulations, but analysed data are already available. Among the various final states previously studied in unpolarised pp collisions within the TMD approach, di-J/ψ production exhibits by far the largest asymmetries, in the region studied by the ATLAS and CMS experiments. In addition, we use the very recent LHCb data at 13 TeV to perform the first fit of the unpolarised transverse-momentum-dependent gluon distribution.
Highlights
One of the most striking phenomena arising from the extension of the collinear factorisation – inspired from Feynman’s and Bjorken’s parton model – to Transverse Momentum Dependent (TMD) factorisation [1,2,3,4] is the appearance of azimuthal modulations induced by the polarisation of partons with nonzero transverse momentum – even inside unpolarised hadrons
In this Letter, we show that di- J /ψ production, which among the quarkonium-associated-production processes has been the object of the largest number of experimental studies at the LHC and the Tevatron [8,9,10,11,12], is the ideal process to perform the first measurement of h⊥1 g (x, k2T, μ)
We have found out that the short-distance coefficients to the azimuthal modulations of J /ψ (Υ) pair yields equate the azimuthally independent terms, which renders these processes ideal probes of the linearly-polarised gluon distributions in an unpolarised proton, h⊥1 g
Summary
One of the most striking phenomena arising from the extension of the collinear factorisation – inspired from Feynman’s and Bjorken’s parton model – to Transverse Momentum Dependent (TMD) factorisation [1,2,3,4] is the appearance of azimuthal modulations induced by the polarisation of partons with nonzero transverse momentum – even inside unpolarised hadrons. In the case of gluons in a proton, which trigger most of the scatterings at high energies, this new dynamics is encoded in the distribution h⊥1 g (x, k2T , μ) of linearly-polarised gluons [5] They generate cos 2φ (cos 4φ) modulations in gluon-fusion scatterings where single (double) gluon-helicity flips occur. In this Letter, we show that di- J /ψ production, which among the quarkonium-associated-production processes has been the object of the largest number of experimental studies at the LHC and the Tevatron [8,9,10,11,12], is the ideal process to perform the first measurement of h⊥1 g (x, k2T , μ).
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