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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2025). Three-pion Bose-Einstein correlations measured in proton-proton collisions. J. High Energy Phys., 08(8), 174–27pp.
Abstract: A study on the Bose-Einstein correlations for triplets of same-sign pions is presented. The analysis is performed using proton-proton collisions at a centre-of-mass energy of root s = 7TeV, recorded by the LHCb experiment, corresponding to an integrated luminosity of 1.0 fb(-1). For the first time, the results are interpreted in the core-halo model. The parameters of the model are determined in regions of charged-particle multiplicity. This measurement provides insight into the nature of hadronisation in terms of coherence, being consistent with the presence of coherent emission of pions.
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LHCb Collaboration(Aaij, R. et al), Jaimes Elles, S. J., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Rebollo De Miguel, M., et al. (2024). Measurement of associated Jψ-ψ(2S) production cross-section in pp collisions at √s=13 TeV. J. High Energy Phys., 05(5), 259–27pp.
Abstract: The cross-section of associated J/psi-psi(2S) production in proton-proton collisions at a centre-of-mass energy of root s = 13 TeV is measured using a data sample corresponding to an integrated luminosity of 4.2 fb(-1), collected by the LHCb experiment. The measurement is performed for both J/psi and psi(2S) mesons having transverse momentum p(T) < 14 GeV/c and rapidity 2.0 < y < 4.5, assuming negligible polarisation of the J/psi and psi(2S) mesons. The production cross-section is measured to be 4.5 +/- 0.7 +/- 0.3 nb, where the first uncertainty is statistical and the second systematic. The differential cross-sections are measured as functions of several kinematic variables of the J/psi-psi(2S) candidates. The results are combined with a measurement of J/psi- J/psi production, giving a cross-section ratio between J/psi-psi(2S) and J/psi- J/psi production of 0.274 +/- 0.044 +/- 0.008, where the first uncertainty is statistical and the second systematic.
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LHCb Collaboration(Aaij, R. et al), Jaimes Elles, S. J., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Rebollo De Miguel, M., et al. (2024). Measurement of J/ψ-pair production in pp collisions at √s=13 TeV and study of gluon transverse-momentum dependent PDFs. J. High Energy Phys., 03(3), 088–40pp.
Abstract: The production cross-section of J/psi pairs in proton-proton collisions at a centre-of-mass energy of root s = 13TeV is measured using a data sample corresponding to an integrated luminosity of 4.2 fb(-1) collected by the LHCb experiment. The measurement is performed with both J/psi mesons in the transverse momentum range 0 < p(T) < 14 GeV/c and rapidity range 2.0 < y < 4.5. The cross-section of this process is measured to be 16.36 +/- 0.28 (stat) +/- 0.88 (syst) nb. The contributions from single-parton scattering and double-parton scattering are separated based on the dependence of the cross-section on the absolute rapidity difference Delta y between the two J/psi mesons. The effective cross-section of double-parton scattering is measured to be sigma(eff) = 13.1 +/- 1.8 (stat) +/- 2.3 (syst) mb. The distribution of the azimuthal angle phi(CS) of one of the J/psi mesons in the Collins-Soper frame and the p(T)-spectrum of the J/psi pairs are also measured for the study of the gluon transverse-momentum dependent distributions inside protons. The extracted values of < cos4 phi(CS)> and < cos2 phi(CS)> are consistent with zero, but the presence of azimuthal asymmetry at a few percent level is allowed.
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