Ratios of helicity amplitudes for exclusive $$\rho ^0$$ ρ 0 electroproduction on transversely polarized protons

Abstract

Exclusive ρ 0 -meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV 2 < Q 2 < 7.0 GeV 2 , 3.0 GeV < W < 6.3 GeV, and −t < 0.4 GeV 2 . Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing ρ 0 -meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude F 0 1 2 0 1 2 , which describes the production of a longitudinal ρ 0 -meson by a longitudinal virtual photon. The ratios of nucleon-helicity-nonflip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to F 0 1 2 0 1 2 . Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the π − ρ transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 ρ 0 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.

ρ meson unpolarized generalized parton distributions with a light-front constituent quark model

“…It is believed that the studies of GPDs are closely related to the processes of deeply virtual Compton scattering and the deeply virtual meson electroproduction [3][4][5]. By comparing with experimental measurements, one can obtain possible constrains on the GPDs of a hadron [6,7]. With the help of sum rules, the unpolarized GPDs are directly connected to the electromagnetic FFs of the system.…”

“…Thus, it is of a great interest to see what the GPDs of the ρ meson look like with the help of the LCCQM model. This study may be even useful to understand the processes involving the ρ meson lepton production such as e + N → e + ρ 0 + N [7,25,52,53] or the process of γγ * → ρρ [54] and the future Electron-Ion Collider(EIC) experiments [55,56].…”

Prospects for quarkonium studies at the high-luminosity LHC

“…Exclusive and diffractive production has been studied in lepton-hadron interactions, both at the FT experiments HERMES [231][232][233][234][235][236][237][238][239][240][241][242][243], COMPASS [244][245][246][247][248][249][250][251], and the experiments at Jefferson Lab [252][253][254][255][256][257][258][259][260][261][262][263][264], and at the collider experiments H1 (see, among others, [265][266][267][268][269][270][271][272][273][274][275][276][277][278][279]…”

Central Exclusive Production at LHCb

“…The hard scale is provided here by the large virtuality, 2 = − 2 ≫ 1 GeV 2 , of the photon exchanged between the lepton and the nucleon. There exists a multitude of such measurements at fixed-target experiments [6][7][8][9][10][11][12][13] and at lepton-proton colliders, by the H1 and ZEUS collaborations [14][15][16][17]. The former series of measurements are mainly sensitive to larger values of Bjorken-, B , with ≈ B /(2− B ), and thus to quark GPDs, while the latter probe lower values of B down to 10 −4 , where gluons dominate.…”