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Register a new user$D$-wave effects in diffractive electroproduction of heavy quarkonia from the photon-like $Vrightarrow Qbar Q$ transition
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We analyze the validity of a commonly used identification between structures of the virtual photon $gamma^*to Qbar Q$ and vector meson $Vto Qbar Q$ transitions. In the existing studies of $S$-wave vector-meson photoproduction in the literature, such an identification is typically performed in the light-front (LF) frame while the radial component of the meson wave function is rather postulated than computed from the first principles. The massive photon-like $Vto Qbar Q$ vertex, besides the $S$-wave component, also contains an extra $D$-wave admixture in the $Qbar Q$ rest frame. However, the relative weight of these contributions cannot be justified by any reasonable nonrelativistic $Qbar Q$ potential model. In this work, we investigate the relative role of the $D$-wave contribution starting from the photon-like quarkonium $Vto Qbar Q$ transition in both frames: in the $Qbar Q$ rest frame (with subsequent Melosh spin transform to the LF frame) and in the LF frame (without Melosh transform). We show that the photon-like transition imposed in the $Qbar Q$ rest frame leads to significant discrepancies with the experimental data. In the second case we find that the corresponding total $J/psi(1S)$ photoproduction cross sections are very close to those obtained with the $S$-wave only $Vto Qbar Q$ transition, both leading to a good description of the data. However, we find that the $S$-wave only transition leads to a better description of photoproduction data for excited heavy quarkonium states, which represent a more effective tool for study of $D$-wave effects. Consequently, the predictions for production of excited states based on the photon-like structure of $Vto Qbar Q$ transition should be treated with a great care due to a much stronger sensitivity of the $D$-wave contribution to the nodal structure of quarkonium wave functions.
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In this work we present for the first time the comprehensive study of the Melosh spin rotation effects in diffractive electroproduction of S-wave heavy quarkonia off a nucleon target. Such a study has been performed within the color dipole approach using, as an example and a reference point, two popular parametrizations of the dipole cross section and two potentials describing the interaction between Q and bar{Q} and entering in the Schroedinger equation based formalism for determination of the quarkonia wave functions. We find a strong onset of spin rotation effects in 1S charmonium photoproduction which is obviously neglected in present calculations of corresponding cross sections. For photoproduction of radially excited Psi(2S) these effects are even stronger leading to an increase of the photoproduction cross section by a factor of 2-3 depending on the photon energy. Even in production of radially excited Y(2S) and Y(3S) they can not be neglected and cause the 20-30% enhancement of the photoproduction cross section. Finally, we predict that the spin effects vanish gradually with photon virtuality Q^2 following universality properties in production of different heavy quarkonia as a function of Q^2 + M_V^2.
In this work, we revise the conventional description of J/Psi(1S), Y(1S), Psi(2S) and Y(2S) elastic photo- and electroproduction off a nucleon target within the color dipole picture and carefully study various sources of theoretical uncertainties in calculations of the corresponding electroproduction cross sections. For this purpose, we test the corresponding predictions using a bulk of available dipole cross section parametrisations obtained from deep inelastic scattering data at HERA. Specifically, we provide the detailed analysis of the energy and hard-scale dependencies of quarkonia yields employing the comprehensive treatment of the quarkonia wave functions in the Schroedinger equation based approach for a set of available c-bar{c} and b-bar{b} interquark interaction potentials. Besides, we quantify the effect of Melosh spin rotation, the Q^2-dependence of the diffractive slope and an uncertainty due to charm and bottom quark mass variations.
The $D$-wave admixture in quarkonium wave functions is acquired from the photonlike structure of $Vto Qbar Q$ transition in the light-front frame widely used in the literature. Such a $D$-wave ballast is not justified by any nonrelativistic model for $Q-bar Q$ interaction potential and leads to falsified predictions for the cross sections in heavy quarkonium production in ultra-peripheral nuclear collisions. We analyze this negative role of $D$-wave contribution by comparing with our previous studies based on a simple non-photon-like $S$-wave-only $Vto Qbar Q$ transition in the $Qbar Q$ rest frame.
Recent discoveries by Belle and BESIII of charged exotic quarkonium-like resonances provide fresh impetus for study of heavy exotic hadrons. In the limit N_c --> infinity, M_Q --> infinity, the (Qbar Q qbar q) tetraquarks (TQ-s) are expected to be narrow and slightly below or above the (Qbar q) and (Q qbar) two-meson threshold. The isoscalar TQ-s manifest themselves by decay to (Qbar Q) pi pi, and the ~30 MeV heavier charged isotriplet TQ-s by decays into (Qbar Q) pi. The new data strongly suggest that the real world with N_c=3, Q=c,b and q,q = u,d is qualitatively described by the above limit. We discuss the relevant theoretical estimates and suggest new signatures for TQ-s in light of the recent discoveries. We also consider baryon-like states (Q Q qbar qbar), which if found will be direct evidence not just for near-threshold binding of two heavy mesons, but for genuine tetraquarks with novel color networks. We stress the importance of experimental search for doubly-heavy baryons in this context.
The differential cross section $dsigma/dq^2$ of diffractive electroproduction of heavy quarkonia on protons is a sensitive study tool for the interaction dynamics within the dipole representation. Knowledge of the transverse momentum transfer $vec q$ provides a unique opportunity to identify the reaction plane, due to a strong correlation between the directions of $vec q$ and impact parameter $vec b$. On top of that, the elastic dipole-proton amplitude is subject to a strong correlation between $vec b$ and dipole orientation $vec r$. Most of models for $b$-dependent dipole cross section either completely miss this information, or make unjustified assumptions. We perform calculations basing on a realistic model for $vec r$-$vec b$ correlation, which significantly affect the $q$-dependence of the cross section, in particular the ratio of $psi^{,prime}(2S)$ to $J/psi$ yields. We rely on realistic potential models for the heavy quarkonium wave function, and the Lorentz-boosted Schrodinger equation. Good agreement with data on $q$-dependent diffractive electroproduction of heavy quarkonia is achieved.
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