Protons consist of three valence quarks, two up-quarks and one down-quark,
held together by gluons and a sea of quark-antiquark pairs. Collectively,
quarks and gluons are referred to as partons. In a proton-proton collision,
typically only one parton of each proton undergoes a hard scattering –
referred to as single-parton scattering – leaving the remainder of each
proton only slightly disturbed. Here, we report the study of double- and
triple-parton scatterings through the simultaneous production of three J/ψ
mesons, which consist of a charm quark-antiquark pair, in proton-proton
collisions recorded with the CMS experiment at the Large Hadron Collider.
We observed this process – reconstructed through the decays of J/ψ mesons
into pairs of oppositely charged muons – with a statistical signifcance above
fve standard deviations. We measured the inclusive fducial cross-section to
be 272+141
−104 (stat) ± 17 (syst)fb, and compared it to theoretical expectations
for triple-J/ψ meson production in single-, double- and triple-parton
scattering scenarios. Assuming factorization of multiple hard-scattering
probabilities in terms of single-parton scattering cross-sections, double-and triple-parton scattering are the dominant contributions for the
measured process.