Prompt charmonia production and polarization at LHC in the NRQCD with -factorization. Part I: meson

S. P. Baranov; A. V. Lipatov; N. P. Zotov

doi:10.1140/epjc/s10052-015-3689-x

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2023 Impact factor 4.2

Particles and Fields

Open Access

Eur. Phys. J. C (2015) 75: 455
https://doi.org/10.1140/epjc/s10052-015-3689-x

Regular Article - Theoretical Physics

Prompt charmonia production and polarization at LHC in the NRQCD with -factorization. Part I: $\psi (2S)$ meson

S. P. Baranov¹, A. V. Lipatov²^,3^* and N. P. Zotov²

¹ P.N. Lebedev Physics Institute, Moscow, 119991, Russia
² Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, 119991, Russia
³ Joint Institute for Nuclear Research, Dubna, Moscow Region, 141980, Russia

^* e-mail: lipatov@theory.sinp.msu.ru

Received: 1 September 2015
Accepted: 16 September 2015
Published online: 28 September 2015

Abstract

In the framework of the $$k_T$$ -factorization approach, the production and polarization of prompt $\psi (2S)$ mesons in pp collisions at LHC energies is studied. Our consideration is based on the non-relativistic QCD formalism for bound states and off-shell amplitudes for hard partonic subprocesses. The transverse momentum dependent (unintegrated) gluon densities in a proton were derived from the Ciafaloni–Catani–Fiorani–Marchesini evolution equation or, alternatively, were chosen in accordance with the Kimber–Martin–Ryskin prescription. The non-perturbative color-octet matrix elements were first deduced from the fits to the latest CMS data on $\psi (2S)$ transverse momentum distributions and then applied to describe the ATLAS and LHCb data on $\psi (2S)$ production and polarization at $\sqrt{s} = 7$ TeV. We perform the estimation of the polarization parameters $\lambda _\theta$ , $\lambda _\phi$ , and $\lambda _{\theta \phi }$ , which determine the $\psi (2S)$ spin density matrix and demonstrate that taking into account the off-shellness of the initial gluons in the color-octet contributions leads to unpolarized $\psi (2S)$ production at high transverse momenta, in qualitative agreement with the LHC data.