Central exclusive production of polarised charmonia Roman Pasechnik a , Antoni Szczurek b,c , Oleg Teryaev d a Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden b Institute of Nuclear Physics PAN, PL-31-342 Cracow, Poland c University of Rzesz´ ow, PL-35-959 Rzesz´ ow, Poland d Bogoliubov Laboratory of Theoretical Physics, JINR, Dubna 141980, Russia Abstract We consider exclusive double diffractive production of polarised axial-vector χ c (1 + ) and tensor χ c (2 + ) charmonia in proton-(anti)proton collisions at Tevatron energy. The corresponding amplitudes for these processes are derived within the k t -factorisation approach. Contributions from different polarisation states of axial-vector and tensor charmonia are quantified. Corresponding experimental consequences are discussed. Keywords: central exclusive production, k t -factorisation, spin effects, charmonium 1. Introduction Recently, the central exclusive charmonia production has attracted a lot of attention from both experimental [1] and theoretical [2] sides. Such a process provides the unique opportunity to test the QCD diffractive Khoze- Martin-Ryskin (KMR) mechanism [3] based on k t - factorisation incorporating nonperturbative small- x/k t gluon dynamics described by the unintegrated gluon distribution functions (UGDFs) against accessible data. The final hadronic central system is, however, rather complicated and composed of three spin states χ c (0 + , 1 + , 2 + ), and there is no yet any reliable way to measure them separately. Such a measurement would significantly reduce an overall theoretical uncertainty, as different χ c spin contributions come from different phase space regions [4, 5, 6]. Indeed, in the asymp- totical forward limit χ c (1 + , 2 + ) are strongly suppressed with respect to χ c (0 + ) due to the J z = 0 selections rule [7]. However, corrections to this asymptotics may be important in the total cross section leading to a notice- able contribution of the χ c (1 + , 2 + ) states to the observ- able signal from radiative χ c decays [2, 5, 6]. Generally, separate measurements of χ c states in diffractive pro- duction would impose more strict bounds on the produc- tion mechanism under consideration and extract some new information on the underlying QCD dynamics. Email addresses: roman.pasechnik@fysast.uu.se (Roman Pasechnik), antoni.szczurek@ifj.edu.pl (Antoni Szczurek), teryaev@theor.jinr.ru (Oleg Teryaev) The goal of the present paper is to analyze polari- sation effects in the central exclusive charmonia pro- duction. Such effects can be identified potentially by measuring the angular distribution of J/ψ mesons from radiative decays of χ c ( J + ) giving more detailed infor- mation on partial spin contributions. Moreover, certain combinations of polarisation observables can be less sensitive to unknown nonperturbative effects leading to unique opportunities for model-independent analysis of diffractive processes. 2. Central exclusive production of polarised axial- vector and tensor charmonia According to the Khoze-Martin-Ryskin approach (KMR) [3], we write the amplitude of the exclusive dou- ble diffractive color singlet production pp → ppχ cJ as M g ∗ g ∗ J,λ = s 2 π 2 δ c1c2 N 2 c − 1 ℑ  d 2 q 0,t V c1c2 J,λ f off g,1 f off g,2 q 2 0,t q 2 1,t q 2 2,t . (1) where f off g,i = f off g,i ( x i , x ′ i , q 2 0,t , q 2 i,t , t i ) are the off-diagonal unintegrated gluon distributions for “active” gluons with momenta q i = x i p i + q i,t and color indices c i , and the screening soft gluon with small fraction x ′ ≪ x i , q 0 ≃ q 0,t , J and λ are the spin and helicity of a pro- duced meson with momentum P = q 1 + q 2 and mass M in the center-of-mass frame of colliding protons and z axis directed along the meson momentum P, respec- tively. Hard subprocess parts V c1c2 J,λ (g ∗ g ∗ → χ c ) were Nuclear Physics B (Proc. Suppl.) 214 (2011) 110–113 0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. www.elsevier.com/locate/npbps doi:10.1016/j.nuclphysbps.2011.03.068