Advances in High Energy Physics

Volume 2019, Article ID 8091865, 9 pages

https://doi.org/10.1155/2019/8091865

## Hidden-Beauty Broad Resonance in Thermal QCD

^{1}Department of Physics, Kocaeli University, 41380 Izmit, Turkey^{2}Özyeğin University, Department of Natural and Mathematical Sciences, Çekmeköy, Istanbul, Turkey^{3}Physik Department, Technische Universität München, D-85747 Garching, Germany^{4}Faculty of Education, Kocaeli University, 41380 Izmit, Turkey

Correspondence should be addressed to J. Y. Süngü; moc.liamg@liyelaj

Received 2 December 2018; Revised 18 February 2019; Accepted 6 March 2019; Published 18 June 2019

Academic Editor: Alexey A. Petrov

Copyright © 2019 J. Y. Süngü et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.

#### Abstract

In this work, the mass and pole residue of resonance is studied by using QCD sum rules approach at finite temperature. Resonance is described by a diquark-antidiquark tetraquark current, and contributions to operator product expansion are calculated by including QCD condensates up to dimension six. Temperature dependencies of the mass and the pole residue are investigated. It is seen that near a critical temperature , the values of and decrease to 87% and to 44% of their values at vacuum.

#### 1. Introduction

Heavy quarkonia systems provide a unique laboratory to search the interplay between perturbative and nonperturbative effects of QCD. They are nonrelativistic systems in which low energy QCD can be investigated via their energy levels, widths, and transition amplitudes [1]. Among these heavy quarkonia states, vector charmonium and bottomonium sectors are experimentally studied very well, since they can be detected directly in annihilations. In the past decade, observation of a large number of bottomonium-like states in several experiments increased the interest in these structures [2–6]. However, these observed states could not be conveniently explained by the simple picture of mesons. The presumption of hadrons containing quarks more than the standard quark content ( or ) is introduced by a perceptible model for diquarks plus antidiquarks, which was developed by Jaffe in 1976 [7]. Later Maiani, Polosa, and their collaborators proposed that the X, Y, Z mesons are tetraquark systems, in which the diquark-antidiquark pairs are bound together by the QCD color forces [8]. In this color configuration, diquarks can play a fundamental role in hadron spectroscopy. Thus, probing the multiquark matter has been an intensely intriguing research topic in the past twenty years and it may provide significant clues to understand the nonperturbative behavior of QCD.

In 2007, Belle reported the first evidence of and first observation for decays near the peak of the state at [2]. Assigning these signals to , the partial widths of decays and were measured unusually larger (more than two orders of magnitude) than formerly measured decay widths of states. Following these unusually large partial width measurement, Belle measured the cross sections of and and reported that the resonance observed via these decays does not agree with conventional line shape. These observations led to the proposal of existence of new exotic hidden-beauty state analogous to broad resonance in the charmonium sector, which is a Breit-Wigner shaped resonance with mass () MeV/, and width () MeV/, and is called [5]. In literature, there are several approaches to investigate the structure of exotic resonance. In [9], is considered as a bound state with a highly large binding energy. In [10, 11], is interpreted as a tetraquark and its mass is estimated by using QCD sum rules at vacuum.

Moreover, it is likely that at very high temperatures within the first microseconds following the Big Bang, quarks and gluons existed freely in a homogenous medium called the quark-gluon plasma (QGP). One of the first quark-gluon plasma signals proposed in the literature is suppression of particles [12]. In 2011, CMS collaboration reported that charmonium states and melt or were suppressed due to interacting with the hot nuclear matter created in heavy-ion interactions [13, 14]. Following these observations in the charmonium sector, CMS collaboration also reported suppression of bottomonium states, and relative to the ground state [15, 16]. The dissociation temperatures for the states are expected to be related to their binding energies and are predicted to be and for the , and mesons, respectively, where is the critical temperature for deconfinement [16–18].

In addition to these studies, one of the first investigations on thermal properties of exotic mesons was done in [19], in which the authors investigated in medium properties of under the hypothesis that it is a state or state, but making no assumption on its structure. They estimated that the mass of the molecular state decreases with increasing temperature; the mass of charmonium or tetraquark state is almost stable.

Inspired by these findings and motivated by the aforementioned discussions, we focus on the resonance and its thermal behavior. This paper is organized as follows. In Section 2, theoretical framework of Thermal QCD sum rules (TQCDSR) and its application to are presented, and obtained analytical expressions of the mass and pole residue of are given up to dimension six operators. Numerical analysis is performed and results are obtained in Section 3. Concluding remarks are discussed in Section 4. The explicit forms of the spectral densities are written in Appendix.

#### 2. Finite Temperature Sum Rules for Tetraquark Assignment

QCD sum rules (QCDSR) approach is based on Wilson’s operator product expansion (OPE) which was adapted by Shifman, Vainshtein, and Zakharov [24] and applied with remarkable success to estimate a large variety of properties of all low-lying hadronic states [25–27]. Later, this model is extended to its thermal version that is firstly proposed by Bochkarev and Shaposnikov and led to many successful applications in QCD at [28–35]. Very recently, in [36], the authors claimed that any QCDSR study on tetraquark states should contain contributions to OPE, which are unknown. It is a very important and strong argument for studying tetraquark states within QCDSR; however, those terms and their thermal behaviors are not known, and their calculation is beyond the scope of this work. Thus we follow the traditional sum rules to investigate the thermal behavior of hadronic parameters of , which were used very successfully in predicting properties of tetraquark states as well. In this work, we proceed with traditional sum rules analysis by using a tetraquark current, following several successful applications to exotic hadrons [37–42].

In this section, the mass and pole residue of the exotic resonance are studied by interpreting it as a bound tetraquark via TQCDSR technique which starts with the two point correlation functionwhere represents the hot medium state, is the interpolating current of the state, and denotes the time ordered product. The thermal average of any operator in thermal equilibrium is given aswhere is the QCD Hamiltonian, and is inverse of the temperature, and is the temperature of the heat bath. Chosen current must contain all the information of the related meson, like quantum numbers, quark contents and so on. In the diquark-antidiquark picture, tetraquark current interpreting can be chosen as [10]where , is the charge conjugation matrix and are color indices. Shorthand notations and are also employed in (3).

In TQCDSR, the correlation function given in (1) is calculated twice, as in two different regions corresponding two perspectives, namely the physical side (or phenomenological side) and the QCD side (or OPE side). By equating these two approaches, the sum rules for the hadronic properties of the exotic state under investigation are achieved. To derive mass and pole residue via TQCDSR, the correlation function is calculated in terms of hadronic degrees of freedoms in the physical side. A complete set of intermediate physical states possessing the same quantum number as the interpolating current are inserted into (1), and integral over is handled. After these manipulations, the correlation function is obtained ashere is the temperature-dependent mass of meson. Temperature-dependent pole residue is defined in terms of matrix element aswhere is the polarization vector of the satisfyingAfter employing polarization relations, the correlation function is written in terms of Lorentz structures in the formwhere dots denote the contributions coming from the continuum and higher states. To obtain the sum rules, coefficient of any Lorentz structure can be used. In this work, coefficients of are chosen to construct the sum rules and the standard Borel transformation with respect to is applied to suppress the unwanted contributions. The final form of the physical side is obtained ashere is the Borel mass parameter. In the QCD side, is calculated in terms of quark-gluon degrees of freedom and can be separated into two parts over the Lorentz structures as where and are invariant functions connected with the scalar and vector currents, respectively. In the rest framework of (), can be expressed as a dispersion integral, where corresponding spectral density is described as The spectral density can be separated in terms of operator dimensions asIn order to obtain the expressions of these spectral density terms, the current expression given in (3) is inserted into the correlation function given in (1) and then the heavy and light quark fields are contracted, and the correlation function is written in terms of quark propagators as where are the full quark propagators and is used. The quark propagators are given in terms of the quark and gluon condensates [25]. At finite temperatures, additional operators arise due to the breaking of Lorentz invariance by the choice of thermal rest frame. Thus, the residual invariance brings additional operators to the quark propagator at finite temperature. The expected behavior of the thermal averages of these new operators is opposite of those of the Lorentz invariant old ones [43]. The heavy-quark propagator in coordinate space can be expressed asand the thermal light quark propagator is chosen aswhere is the external gluon field, with Gell-Mann matrices, symbolizes color indices, implies the strange quark mass, is the four-velocity of the heat bath, is the temperature-dependent light quark condensate, and is the fermionic part of the energy-momentum tensor. Furthermore, the gluon condensate related to the gluonic part of the energy-momentum tensor is expressed via relation [43]:where and coefficients areIn order to remove contributions originating from higher states, the standard Borel transformation with respect to is applied in the QCD side as well. By equating the coefficients of the selected structure in both physical and QCD sides, and by employing the quark hadron duality ansatz up to a temperature-dependent continuum threshold , the final sum rules for are derived asTo find the mass via TQCDSR, one should expel the hadronic coupling constant from the sum rules. It is commonly done by dividing the derivative of the sum rule given in (18) with respect to to itself. Following these steps, the temperature-dependent mass is obtained aswhere the thermal continuum threshold is related to continuum threshold at vacuum via relation[44, 45]. For compactness, the explicit forms of spectral densities are presented in Appendix.

#### 3. Phenomenological Analysis

In this section, the phenomenological analysis of sum rules obtained in (18) and (19) is presented. First, the input parameters and temperature dependance of relevant condensates are given. Following, the working regions of the obtained sum rules at vacuum are analyzed. The behavior of QCD sum rules at is used to test the reliability of our analysis.

##### 3.1. Input Parameters

During the calculations, input parameters given in Table 1 are used. In addition to these input parameters, temperature-dependent quark and gluon condensates, and the energy density expressions are necessary. The thermal quark condensate is chosen aswhere is the light quark condensate at vacuum and which is credible up to a critical temperature . The expression given in (21) is obtained in [46, 47] from the Lattice QCD results given in [48, 49]. The temperature-dependent gluon condensate is parameterized via [46, 50]where is the gluon condensate in vacuum state and . Additionally, for the gluonic and fermionic parts of the energy density, the following parametrization is used [46]which is extracted from the Lattice QCD data in [51].