A search is performed for the central exclusive production of pairs of charmonia produced in proton-proton collisions. Using data corresponding to an integrated luminosity of $3{rm fb}^{-1}$ collected at centre-of-mass energies of 7 and 8 TeV, $J/psi
J/psi$ and $J/psipsi(2S)$ pairs are observed, which have been produced in the absence of any other activity inside the LHCb acceptance that is sensitive to charged particles in the pseudorapidity ranges $(-3.5,-1.5)$ and $(1.5,5.0)$. Searches are also performed for pairs of P-wave charmonia and limits are set on their production. The cross-sections for these processes, where the dimeson system has a rapidity between 2.0 and 4.5, are measured to be $$ begin{array}{rl} sigma^{J/psi J/psi} &= 58pm10{(rm stat)} pm 6{(rm syst)} {rm pb} , sigma^{J/psipsi(2S)} &= 63 ^{+27}_{-18}{(rm stat)}pm 10{(rm syst)} {rm pb} , sigma^{psi(2S)psi(2S)} &< 237 {rm pb}, sigma^{chi_{c0}chi_{c0}} &< 69 {rm nb}, sigma^{chi_{c1}chi_{c1}} &< 45 {rm pb}, sigma^{chi_{c2}chi_{c2}} &< 141 {rm pb}, end{array} $$ where the upper limits are set at the 90% confidence level. The measured $J/psi J/psi$ and $J/psipsi(2S)$ cross-sections are consistent with theoretical expectations.
The differential cross-section as a function of rapidity has been measured for the exclusive production of $J/psi$ and $psi(2S)$ mesons in proton-proton collisions at $sqrt{s}=7$ TeV, using data collected by the LHCb experiment, corresponding to an i
ntegrated luminosity of 930 pb$^{-1}$. The cross-sections times branching fractions to two muons having pseudorapidities between 2.0 and 4.5 are measured to be $$begin{array}{rl} sigma_{pprightarrow J/psirightarrow{mu^+}{mu^-}}(2.0<eta_{mu^pm }<4.5)=&291pm 7pm19 {rm pb}, sigma_{pprightarrowpsi(2S)rightarrow{mu^+}{mu^-}}(2.0<eta_{mu^pm}<4.5)=&6.5pm 0.9pm 0.4 {rm pb},end{array}$$ where the first uncertainty is statistical and the second is systematic. The measurements agree with next-to-leading order QCD predictions as well as with models that include saturation effects.
Exclusive $J/psi$ and $psi(2S)$ vector meson production has been observed in the dimuon channel using the LHCb detector. The cross-section times branching fractions to two muons with pseudorapidities between 2.0 and 4.5 are measured to be $ sigma_{pp
rightarrow J/psi (rightarrow mu^{+} mu^{-})} (2.0 <eta_{mu^{pm}}< 4.5) = 307 pm 21 pm 36 text{pb}, sigma_{pprightarrow psi(2S) (rightarrow mu^{+} mu^{-})} (2.0 <eta_{mu^{pm}}< 4.5) = 7.8 pm 1.3 pm 1.0 text{pb}, $ where the first uncertainties are statistical and the second are systematic. The measurements are found to be in good agreement with results from previous experiments and theoretical predictions. The $J/psi$ photoproduction cross-section has been measured as a function of the photon-proton centre-of-mass energy. The results are consistent with measurements obtained at HERA and confirm a similar power law behaviour for the photoproduction cross-section.
