Production of χc pairs with large rapidity separation in kT factorization

Abstract

We calculate the production of pairs of χ cJ mesons with all possible combinations of J ¼ 0, 1, 2. The production mechanism which we consider is the crossed-channel gluon exchange in the gluon-gluon fusion reaction, the mechanism relevant for large rapidity separations between χ c mesons. The salient features of the t and u-channel gluon exchange are the broad distributions in rapidity difference Δy between χ c mesons. The building blocks are the vertices g à g à → χ cJ for off-shell gluons. We stick to the color-singlet model and calculate the gluon fusion vertices in the limit of heavy quarks with nonrelativistic motion in the bound state. These vertices are used to construct the g à g à → χ cJ 1 χ cJ 2 amplitudes. We then calculate hadronlevel cross sections using the k T -factorization approach. Several differential distributions at the pp center of mass energy ffiffi ffi s p ¼ 8 TeV are shown. We briefly discuss the contribution to J=ψJ=ψ production at large rapidity separation. For reference we show some distributions for production of χ cJ 1 χ cJ 2 pairs in double parton scattering. The discussed single parton scattering mechanisms yield similar shapes of different distributions as the double parton scattering mechanisms.

Associated production of χc pairs with a gluon in the collinear-factorization approach

“…While the distributions for χ c0 χ c0 and χ c2 χ c2 are similar, the distributions for χ c1 χ c1 are clearly less steep. Similar observation was already made in the k T -factorization study in[20]. This is particularly spectacular for the central emission diagram (diagram (C) inFig.2) when both gluons are off-mass-shell.…”

“…We see, that the cross sections for the 2 → 2 processes are consistently lower than the ones obtained in the k T -factorization approach in Ref. [20]. TABLE…”

“…[6] gives reasonable results (see e.g. [7][8][9][10][11]). In general, the inclusive cross section for J/ψ (the same is true for other quarkonia) grows with energy.…”

“…In Ref. [20] it was found that double χ c production associated with radiative decays of both χ c quarkonia leads to distributions quite similar to those from double parton scattering. A rather sizeable cross section for χ c pair production was obtained from the k T -factorization approach.…”

“…The explicit form of the tensors T µν are found in Ref. [20]. Above a, b are the color indices of incoming gluons, N c = 3 is the number of colors, and M is the mass of the χ c meson.…”