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/$\psi$ 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/$\psi$ mesons into pairs of oppositely charged muons - with a statistical significance above five standard deviations. We measured the inclusive fiducial cross section to be 272 $^{+141}_{-104}$ (stat) $\pm$ 17 (syst) fb, and compared it to theoretical expectations for triple-J/$\psi$ 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.
Kinematic properties of each one of the three \JPsi mesons selected in the 5? 6? signal events.
Dimuon invariant mass ($m$), proper decay-length ($L$), transverse momentum ($p_{T}$), rapidity ($y$), and azimuthal angle ($\phi$) of each of the three $J/\psi$ candidates measured in the six triple-$J/\psi$ events passing our selection criteria.
DPS effective cross section
The nuclear modification factors of J/$\psi$ and $\psi$(2S) mesons are measured in PbPb collisions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}} =$ 5.02 TeV. The analysis is based on PbPb and pp data samples collected by CMS at the LHC in 2015, corresponding to integrated luminosities of 464 $\mu$b$^{-1}$ and 28 pb$^{-1}$, respectively. The measurements are performed in the dimuon rapidity range of $|y| <$ 2.4 as a function of centrality, rapidity, and transverse momentum (p$_\mathrm{T}$) from p$_\mathrm{T}=$ 3 GeV/$c$ in the most forward region and up to 50 GeV/$c$. Both prompt and nonprompt (coming from b hadron decays) mesons are observed to be increasingly suppressed with centrality, with a magnitude similar to the one observed at $\sqrt{s_{\mathrm{NN}}}=$ 2.76 TeV for the two J/$\psi$ meson components. No dependence on rapidity is observed for either prompt or nonprompt J/$\psi$ mesons. An indication of a lower prompt J/$\psi$ meson suppression at p$_\mathrm{T} >$ 25 GeV/$c$ is seen with respect to that observed at intermediate p$_\mathrm{T}$. The prompt $\psi$(2S) meson yield is found to be more suppressed than that of the prompt J/$\psi$ mesons in the entire p$_\mathrm{T}$ range.
Fraction of J/psi mesons coming from the decay of b hadrons, i.e. nonprompt J/psi meson fraction, as a function of dimuon pT for pp and PbPb collisions, for all centralities.
Fraction of J/psi mesons coming from the decay of b hadrons, i.e. nonprompt J/psi meson fraction, as a function of dimuon rapidity for pp and PbPb collisions, for all centralities.
Differential cross section of prompt J/psi mesons as a function of dimuon pT in pp and PbPb collisions. The PbPb cross sections are normalised by TAA for direct comparison. Global uncertainties arise from the integrated luminosity uncertainty in pp collisions, and the number of minimum bias events and TAA uncertainties for PbPb collisions.
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