QCD sum rules approach to the X, Y and Z states

Abstract

In the past decade, due to the experimental observation of many charmonium-like states, there has been a revival of hadron spectroscopy. In particular, the experimental observation of charged charmonium-like, Z c states, and bottomonium-like, Z b states, represents a challenge since they can not be accommodated within the naive quark model. These charged states are good candidates of either tetraquark or molecular states and their observation motivated a vigorous theoretical activity. This is a rapidly evolving field with enormous amount of new experimental information. In this work, we review the current experimental progress and investigate various theoretical interpretations of these candidates of the multiquark states. The present review is written from the perspective of the QCD sum rules approach, where we present the main steps of concrete calculations and compare the results with other approaches and with experimental data.

Tetraquark-adequate formulation of QCD sum rules

“…Inspired by earlier partial results [12][13][14][15][16], we performed a thorough analysis [11] of four-quark singularities in the Mandelstam variable s due to the possible existence of tetraquark poles in Green functions. Its outcomes give reason to question the intrinsic consistency of investigating multiquark hadrons by means of traditional QCD sum rules [17,18]: the latter must be adapted to the challenge.…”

Tetraquark-adequate QCD sum rules for quark-exchange processes

“…Numerous works deal with the application of Shifman-Vainshtein-Zakharov (SVZ) sum rules [6] to tetraquark and pentaquark states [7,8] (and references therein). The method of SVZ sum rules (or QCD sum rules) makes use of dispersion representations to calculate QCD Green functions, or correlators, in two different ways: First, by applying the Wilson operator product expansion (OPE), which gives the OPE (theoretical) side of a QCD sum rule.…”

“…Previous applications of SVZ sum rules to exotic states [7,8] have followed the same route by calculating the O(1) QCD diagrams (and in some cases also radiative corrections) and the corresponding power corrections, and borrowing the same criteria for continuum subtraction as prescribed for the ordinary hadrons [6]. As a result, the tetraquark or pentaquark properties have been related to O(1) QCD diagrams.…”

“…of Eqs. (8) and (9) correspond to condensate insertions in these diagrams. The typical lowest-order diagrams contributing to the OPE side of Eq.…”

Decay properties of P-wave bottom baryons within light-cone sum rules

“…Similar methods were applied to study heavy mesons and baryons in Refs. [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60]. Very quickly, one notices that in order to make a complete QCD sum rule studies of P -wave heavy baryons within HQET,…”