Axial-vector and pseudoscalar tetraquarks [ ud ][ cs ]

Spectroscopic parameters and widths of the fully open-ﬂavor axial-vector and pseudoscalar tetraquarks X AV and X PS with content [ ud ][ cs ] are calculated by means of the QCD sum rule methods. Masses and current couplings of X AV and X PS are found using two-point sum rule computations performed by taking into account various vacuum condensates up to dimension 10. The full width of the axial-vector state X AV is evaluated by including into analysis S -wave decay modes X AV → D ∗ (2010) − K + , D ∗ (2007) 0 K 0 , D − K ∗ (892) + , and D 0 K ∗ (892) 0 . In the case of X PS , we consider S -wave decay X PS → D ∗ 0 (2300) 0 K 0 , and P -wave processes X PS → D − K ∗ (892) + and X PS → D 0 K ∗ (892) 0 . To determine partial widths of these decay modes, we employ the QCD light-cone sum rule method and soft-meson approximation, which are necessary to estimate strong couplings at tetraquark-meson-meson vertices X AV D − D ∗ (2010) − K + , etc. Our predictions for the mass m AV = (2800 ± 75) MeV and width Γ AV = (58 ± 10) MeV of the tetraquark X AV , as well as results m PS = (3000 ± 60) MeV and Γ PS = (65 ± 12) MeV for the same parameters of X PS may be useful in future experimental studies of multiquark hadrons.


I. INTRODUCTION
Recent LHCb information on new structures X 0 (2900) and X 1 (2900) observed in the invariant D − K + mass distribution of the process B + → D + D − K + [1,2], enhanced activity of researches to investigate fully openflavor exotic mesons.In fact, by taking into account dominant decays of the resonance-like peaks X 0(1) (2900) → D − K + and assuming that they are four-quark systems, one sees that X 0(1) (2900) are built of quarks c, s, u, and d.The LHCb collaboration measured masses and widths of the structures X 0(1) (2900), and fixed their spin-parities.It was found that X 0 (2900) and X 1 (2900) bear the quantum numbers J P = 0 + and J P = 1 − , respectively.Let us note that, alternatively, structures X 0(1) (2900) may appear as triangle singularities in some rescattering diagrams: such interpretation was not excluded by LHCb as well.
In the four-quark picture, widely accepted to explain the LHCb data, X 0(1) (2900) may be considered in the framework of both the molecule and diquark-antidiquark (tetraquark) models.Thus, in publications [3,4] the resonance X 0 (2900) was analyzed as the scalar diquarkantidiquark state [ud][cs], whereas in Refs.[5] and [6] it was treated as a molecule D * − K * + or D * 0 K * 0 .The situation is almost the same for the vector resonance X 1 (2900): it was studied in the context of the tetraquark and molecule models, for instance, in Refs.[5,7,8].
There are numerous articles devoted to investigations of X 0(1) (2900) using different methods and schemes of high energy physics: relatively complete list of such papers can be found in Refs.[6,7,9].
An interesting conjecture about nature of X 0 (2900) was made in Ref. [10], where it was interpreted as a ra-dial excitation X ′ 0 of the scalar tetraquark X 0 = [ud][cs].We addressed this problem in our work [9], and calculated masses and widths of the ground-state 1S and radially excited 2S tetraquarks X (′) 0 using the QCD sum rule method.We modeled X (′) 0 as particles composed of the axial-vector diquark [ud] and axial-vector antidiquark [cs].We also constructed the tetraquarks X (′) S by utilizing a scalar diquark and antidiquark, and found their parameters.It was demonstrated that, the groundstate particles X 0 and X S are lighter than the resonance X 0 (2900), whereas radially excited tetraquarks X ′ 0 and X ′ S with the masses around ≈ 3320 MeV are heavier it.It other words, none of these four-quark states can be identified with the resonance X 0 (2900).Therefore, it is reasonable to treat X 0 and X S as new hypothetic exotic mesons to be searched for in experiments.
Fully open-flavor tetraquarks, to be fair, were already objects of theoretical studies, which intensified after information on the resonance X(5568) presumably composed of b, s, u, and d quarks [11].Though existence of X(5568) was not confirmed by other experimental groups, its charmed partners b → c are still under detailed analysis.In fact, the spectroscopic parameters and full width of scalar tetraquark X c = [su][cd] were calculated in Ref. [12].Masses of exotic mesons with the same content, but quantum numbers J P = 0 + and J P = 1 + were estimated in Ref. [13].
In various combinations c, s, u, and d quarks form different classes of four-quark mesons, features of which deserve investigations.Interesting class of fully open-flavor particles is collection of states Z ++ = [cu][sd], which carries two units of electric charge.The scalar, pseudoscalar, axial-vector and vector members of this group were studied in our articles Refs.[14] and [15], respectively.It was pointed out that scalar and vector tetraquarks Z ++ S and Z ++ V may be observed in the D + K + mass distribution of the decay B + → D − D + K + [15].
Tetraquarks with a content [ud][cs] establish new class of open-flavor particles.Observation of the resonances X 0(1) (2900) by the LHCb collaboration, available experimental data, and possible interpretation of X 1 (2900) as a vector state X V = [ud][cs] make these particles objects of special interest.In the present article, we continue our studies started in Refs.[7,9] by calculating spectroscopic parameters and full widths of axial-vector and pseudoscalar four-quark states X AV and X PS with the same [ud][cs] content.
Masses and current couplings of X AV and X PS are evaluated in the context of the two-point sum rule method [16,17].In calculations, we take into account various vacuum condensates up to dimension 10.The full width of the axial-vector state X AV is found by including into analysis its S-wave decay modes and D 0 K * (892) 0 .To estimate width of the pseudoscalar tetraquark X PS , we consider kinematically allowed Swave channel X PS → D * 0 (2300) 0 K 0 , and P -wave decay modes Partial widths of these processes are governed by strong couplings at relevant vertices, for example, at X AV D * (2010) − K + for the first process.To calculate required couplings, we use the QCD light-cone sum rule (LCSR) method [18] and soft-meson approximation [19,20].The latter is necessary to treat tetraquarkmeson-meson vertices, which due to unequal number of quark fields in tetraquark and meson interpolating currents differ from standard three-meson vertices [21].
This paper is organized in the following manner: In Section II, we calculate the masses and current couplings of the tetraquarks X AV and X PS .In Section III, we determine strong couplings g i , i = 1, 2, 3, 4 corresponding to vertices , and X AV D 0 K * (892) 0 .In this section, we compute partial widths of corresponding processes, and estimate full width of X AV .In Section IV, we consider the decays X PS → D * 0 (2300) 0 K 0 , D − K * (892) + , and D 0 K * (892) 0 , and find strong couplings G j , j = 1, 2, 3 at relevant vertices.Using G j , we calculate partial width of these decays and evaluate full width of X PS by saturating it with these channels.Section V contains our conclusions.

II. MASSES AND CURRENT COUPLINGS OF THE TETRAQUARKS XAV AND XPS
In this section, we compute spectroscopic parameters of the states X AV and X PS by means of the two-point sum rule method.It is an effective nonperturbative approach elaborated to evaluate parameters of ordinary mesons and baryons.The QCD sum rules express various phys-ical quantities in terms of universal vacuum condensates which do not depend on a problem under consideration.At the same time, they contain auxiliary parameters s 0 and M 2 specific for each computation.The first of them is the continuum subtraction parameter s 0 that separates contribution of a ground-state particle in the phenomenological side of a sum rule from effects of higher resonances and continuum states.The Borel parameter M 2 is required to suppress these unwanted continuum effects.By introducing M 2 and s 0 into analysis and employing an assumption about quark-hadron duality one connects phenomenological and QCD sides of sum rules and gets a sum equality.The latter can be used to express physical observables in terms of different vacuum condensates.The parameters M 2 and s 0 generate theoretical uncertainties in results, which nevertheless can be estimated and kept under control.
In what follows, we calculate the mass m and current coupling f of the axial-vector meson X AV (we employ also m AV and f AV ), and provide only final results for X PS .The starting point in computation of the spectroscopic parameters of the tetraquark X AV is the correlation function where, T is the time-ordered product of two currents, and J µ (x) is the interpolating current for the axial-vector state X AV .We model the tetraquark X AV as a compound formed by the scalar diquark u T Cγ 5 d and axial-vector antidiquark cγ µ Cs T , which are antitriplet and triplet states of the color group SU c (3), respectively.Therefore, corresponding interpolating current is given by the formula and belongs to [3 c ] ud ⊗ [3 c ] cs representation of the color group.In expression above, ǫ ǫ = ǫ abc ǫ ade , where a, b, c, d and e are color indices.In Eq. ( 2) c(x), s(x), u(x) and d(x) denote quark fields, and C is the charge conjugation matrix.
The phenomenological side of the sum rule Π Phys µν (p) is derived from Eq. ( 1) by inserting a complete set of intermediate states with quark contents and spin-parity of the tetraquark X AV , and carrying out integration over x.The momentum and polarization vector of X AV are denoted by p and ε, respectively.It should be noted that in Π Phys µν (p) the ground-state term is written down explicitly, whereas contributions of higher resonances and continuum states are shown by ellipses.
In Eq. (3), we have assumed that the phenomenological side of the sum rule Π Phys µν (p) can be approximated by a single pole term.But in the case of multiquark systems this approximation has to be used with some caution, because the physical side receives contribution also from two-hadron reducible terms.Indeed, a relevant interpolating current couples not only to a multiquark hadron, but also to a two-hadron continuum.This problem was raised in Refs.[22,23] when considering pentaquarks, and revisited recently in the case of tetraquarks [24], where it is argued that the contributions at the orders O(1) and O(α s ) in the operator product expansion (OPE) are canceled out exactly by the meson-meson scattering states at the hadronic side and the tetraquark molecular states start to receive contributions at the order O(α 2 s ).Then the reducible contributions should be subtracted from the sum rule, which can be done by means of two methods.One of them is direct subtraction of two-hadron terms from Π Phys µν (p) by calculating current-two-hadron coupling constant using an independent QCD sum rule.This strategy was realized, for example, in Ref. [25] to investigate anti-charmed pentaquark state.Existence of a two-hadron continuum below a multiquark system means that such particle is unstable and decays to these conventional hadrons.In other words, a two-hadron continuum generates the finite width Γ(p 2 ) of a multiquark system.Relevant effects can be taken into account by modifying the quark propagator in Eq. ( 3) This second method was used to study the tetraquarks [26].Rather detailed investigations demonstrated that effects of the modification Eq. ( 4) can be taken into account by absorbing two-meson contributions into a current-tetraquark coupling constant and keeping stable tetraquark's mass [27,28].Uncertainties generated by changing of a coupling are numerically smaller than theoretical errors of sum rule analysis itself.In fact, twomeson effects lead to additional ≈ 7% uncertainty in the current coupling f T for doubly charmed pseudoscalar tetraquark ccss with the mass m T = 4390 MeV and full width Γ T ≈ 300 MeV [27].In the case of the resonance Z − c (4100) these uncertainties do not exceed ≈ 5% of the coupling f Zc [28].Therefore, one can neglect twomeson reducible terms and use in Π Phys µν (p) single-pole zero-width approximation, as it has been done in Eq. (3).
To simplify the correlation function Π Phys µν (p) and express it in terms of the tetraquark's mass and current coupling, we use the matrix element and recast Π Phys µν (p) into the following form The function Π Phys µν (p) has two Lorentz structures determined by g µν and p µ p ν .One of them can be chosen to continue sum rule analysis.We work with the structure proportional to g µν and corresponding invariant amplitude Π Phys (p 2 ).Advantage of this structure is that it is formed due to contributions of only spin-1 particles, and is free of any contaminations.
The QCD side of the sum rules Π OPE µν (p) should be computed in the operator product expansion with some accuracy.To this end, we substitute into Π µν (p) explicit expression of the current J µ (x), contract relevant quark fields, and replace contractions by appropriate propagators.These operations lead to the expression where and Here, S c (x) and S u(s,d) (x) are the heavy c-and light u(s, d)-quark propagators, respectively: Their explicit expressions are presented in Appendix (see, also Ref. [29]).
The function Π OPE µν (p) is a sum of components proportional to g µν and p µ p ν .We choose the invariant amplitude Π OPE (p 2 ) corresponding to structure ∼ g µν , and use it to derive sum rules for m and f , which read and In expressions above, Π(M 2 , s 0 ) is the Borel transformed and subtracted invariant amplitude Π OPE (p 2 ), and Computing the function Π(M 2 , s 0 ) and fixing of regions for parameters M 2 and s 0 are next problems in our study of m and f .Calculations prove that Π(M 2 , s 0 ) has the form (11) where M = m c + m s .In the present paper, we neglect the masses of u and d quarks, as well as set m 2 s = 0 saving, at the same time, terms ∼ m s .The spectral density ρ OPE (s) is found as an imaginary part of the function Π OPE (p 2 ).Borel transformation some of terms are computed directly from expression of Π OPE µν (p): They form the second component Π(M 2 ) in Eq. (11).Our analysis includes contributions of different quark, gluon and mixed vacuum condensates up to dimension 10.Full analytical expressions of ρ OPE (s) and Π(M 2 ) are written down in Appendix.
The vacuum condensates, that enter to sum rules Eqs. ( 9) and (10), are universal quantities obtained from analysis of various hadronic processes [16,17,30,31].Below, we list their values used in our numerical computations qq = −(0.24± 0.01 As is seen, the vacuum condensate of strange quarks differs from 0|qq|0 [30].The mixed condensates qg s σGq and sg s σGs are expressed using the corresponding quark condensates and parameter m 2 0 , numerical value of which was extracted from analysis of baryonic resonances [31].For the gluon condensate g 3 G 3 , we employ the estimate given in Ref. [32].This list also contains the masses of c and s quarks from Ref. [33] in the MS-scheme.▲ ▲ Another problem is a choice of working windows for parameters M 2 and s 0 .They are fixed in such a way that to meet constraints imposed on Π(M 2 , s 0 ) by a pole contribution (PC) and convergence of the operator product expansion.These constraints can be quantified by means of the following expressions and where In what follows, we require fulfilment of the restrictions The PC and R(M 2 ) are employed to fix the higher and lower limits of the Borel parameter M 2 , respectively.These two values determine boundaries of the region where M 2 can be varied.Calculations show that intervals are appropriate regions for the parameters M 2 and s 0 , and comply with limits on PC and convergence of OPE.Thus, at M 2 max = 3.2 GeV 2 on average in s 0 the pole contribution is 0.51, whereas at M 2 min = 2.5 GeV 2 it becomes equal to 0.73.To visualize dynamics of the pole contribution when varying the Borel parameter, we plot PC as a function of M 2 at different s 0 in Fig. 1.One can see, that except for a small region M 2 ≥ 3.1 GeV 2 at s 0 = 9.5 GeV 2 the pole contribution exceeds 0.5.On average in s 0 , the condition PC ≥ 0.5 is fulfilled in the whole working region Eq. ( 16).To be convinced in convergence of OPE, we calculate R(M 2 min ) at the minimum point M 2 min = 2.5 GeV 2 , and get R(2.5 GeV 2 ) ≈ 0.027 in accordance with constraint from Eq. (15).Results of more detailed analysis are depicted in Fig. 2. In this figure, we show the perturbative and nonperturbative components of the correlation function Π(M 2 , s 0 ): A prevalence of the perturbative contri- bution to Π(M 2 , s 0 ) over nonperturbative one is evident.Without regard for some higher dimensional terms, the nonperturbative contributions reduce by increasing the dimensions of the corresponding operators.
The region for s 0 has to meet constraints coming from dominance of PC and convergence of OPE.Selfconsistency of performed analysis can be checked by comparing the parameter √ s 0 and the X AV tetraquark's mass extracted from the sum rule: Evidently, an inequality m < √ s 0 should be satisfied.Additionally, √ s 0 bears information on a mass m * of the first radial excitation of the tetraquark X AV , therefore the restriction m * ≥ √ s 0 provides low limit for m * .
We extract the mass m and coupling f by computing them at different M 2 and s 0 , and finding their mean values averaged over the regions Eq. ( 16).Our predictions for m and f read The results in Eq. ( 17) effectively correspond to sum rules' predictions at approximately middle point of the regions in Eq. ( 16), i.e., to predictions at the point M 2 = 2.8 GeV 2 and s 0 = 10 GeV 2 , where the pole contribution is PC ≈ 0.62.This fact guarantees a dominance of the pole contribution in extracted parameters m and f .
FIG. 4: Dependence of the mass mPS on the Borel parameter M 2 (left), and on the continuum threshold parameter s0 (right).
In Fig. 3, we depict m as functions of M 2 and s 0 , in which is seen its dependence on the Borel and contin-uum subtraction parameters.Strictly speaking, physical quantities should not depend on M 2 , but computations demonstrate that such effects, nevertheless, exist.Therefore, in a chosen region for M 2 this dependence should be minimal.Due to a functional form of the sum rule for the mass Eq. ( 9) given as the ratio of correlation functions, variation of m in the region for M 2 is mild.There is also dependence on the parameter s 0 which contains information about the lower limit for the mass of the excited tetraquark.
The pseudoscalar tetraquark X PS and its parameters have been explored by the manner described just above.Here, we model X PS as a tetraquark built of the pseudoscalar diquark u T Cd and scalar antidiquark cγ 5 Cs T .The relevant interpolating current J PS (x) is determined by the expression and belongs to the antitriplet-triplet representation of the color group SU c (3).The physical side of the sum rule in this case has relatively simple form where m PS and f PS are the mass and current coupling of the tetraquark X PS , respectively.To derive Π Phys (p), we have used the matrix element of the pseudoscalar particle The function Π Phys (p) has trivial Lorentz structure proportional to I, therefore the invariant amplitude Π Phys (p 2 ) is equal to r.h.s. of Eq. (19).
The QCD side of new sum rules is given by the formula The spectroscopic parameters of X PS can be obtained from Eqs. ( 9) and (10) The Borel and continuum subtraction parameters M 2 and s 0 used to extract m PS and f PS are given by Eq. ( 23) In these regions the PC changes inside limits 0.74 ≥ PC ≥ 0.50.
Dependence of m PS on the parameters M 2 and s 0 is shown in Fig. 4. In the left panel one can see a relatively stable nature of m PS under variation of M 2 .

III. DECAYS OF THE AXIAL-VECTOR TETRAQUARK XAV
The mass and spin-parity of the tetraquark X AV allow us to classify its decay channels.We restrict ourselves by analysis of S-wave decay channels of X AV which are X AV → D * (2010 and D 0 K * (892) 0 .The full width of the axial-vector state X AV is estimated by including into analysis namely these channels.
We are going to provide rather detailed information about computation of a partial width of the decay X AV → D * (2010) − K + , and outline important steps in analyses of other processes.A quantity to be extracted from a sum rule is the strong coupling g 1 of particles at the vertex X AV D * (2010) − K + .This coupling is defined in terms of the on-mass-shell matrix element where the mesons K + and D * (2010) − are denoted as K and D * , respectively.Here, p ′ , p and q are four-momenta of the tetraquark X AV , and mesons D * and K, and ε ′ ν and ε * µ are the polarization vectors of the particles X AV and D * .
In the framework of the LCSR method the coupling g 1 can be obtained from the correlation function with J ν (x) being the current for the tetraquark X AV from Eq. ( 2).The interpolating current for the meson D * (2010) − is abbreviated in Eq. ( 26) as J D * µ (x), and defined by the expression where j is the color index.
The main contribution to the correlation function Π µν (p, q) comes from a term with poles at p 2 and p ′2 = (p + q) 2 .This term is given by the formula where m D * and f D * are the mass and decay constant of the meson D * (2010) − .To derive Eq. ( 28), we have used Eq. ( 25) and the following matrix elements The term written down explicitly in Eq. ( 28) corresponds to contribution of ground-state particles in X AV and D * channels: effects of higher resonances and continuum states in these channels are shown by dots.The function Π Phys µν (p, q) constitutes the phenomenological side of a sum rule for the coupling g 1 .It contains two terms determined by structures g µν and p µ p ′ ν .In our studies, we use the term ∼ g µν and corresponding invariant amplitude Π Phys (p 2 , p ′2 ) which is a function of two variables p 2 and p ′2 .
The correlation function Π µν (p, q) calculated in terms of quark-gluon degrees of freedom forms the QCD side of the sum rules and is equal to where α and β are the spinor indices.
As is seen, besides c and d quark propagators the function Π OPE µν (p, q) contains also local matrix elements of the K + meson, which carry spinor and color indices.We can rewrite K|u b α s e β |0 in convenient forms by expanding us over the full set of Dirac matrices Γ J and projecting them onto the colorless states Operators uΓ J s sandwiched between the K meson and vacuum generate local matrix elements of the K meson, which are known and can be implemented into Π OPE µν (p, q).As usual, Π OPE µν (p, q)-type correlators depend on nonlocal matrix elements of a final meson (for example, K meson), which are convertible to its distribution amplitudes (DAs).This is correct while one treats strong vertices of three conventional mesons in the context of the LCSR method.In the case of tetraquark-meson-meson vertices relevant correlation functions instead of DAs of a final meson contain its local matrix elements.These matrix elements are determined at the space-time point x = 0 and are overall normalization factors.Within the LCSR method similar behavior of correlation functions was seen in a limit q → 0 of three-meson vertices, which is known as a soft-meson approximation [19].This approximation requires adoption of additional technical tools to deal with new problems appeared in a phenomenological side of corresponding sum rules [19,20].It turns out that the soft limit and related technical methods can be adapted to investigate also tetraquark-meson-meson vertices [21].It is worth to emphasize that the soft limit should be implemented in a hard part of the correlation function Π µν (p), but in matrix elements one takes into account the terms with q 2 = m 2 K .The term proportional to g µν in Π Phys µν (p, q) in the limit q → 0 with some accuracy can be transformed into the expression where m 2 = (m 2 + m 2 D * )/2.The invariant amplitude Π Phys (p 2 ) depends on the variable p 2 , and has a double pole at p 2 = m 2 .The Borel transformation of Π Phys (p 2 ) is given by the formula The ellipses in Eq. ( 34) stand not only for terms suppressed after this operation, but also for contributions which remain unsuppressed even after Borel transformation.Therefore, before performing usual subtraction procedure, one should remove these contributions from BΠ Phys (p 2 ).To this end, we have to apply the operator to both sides of a sum rule equality [19,20], and subtract conventional terms in a usual way.
Then the sum rule for the strong coupling g 1 reads where Π OPE (M 2 , s 0 ) is the Borel transformed and subtracted invariant amplitude Π OPE (p 2 ) that corresponds to the structure g µν in Π OPE µν (p, q).To finish calculation of the strong coupling g 1 , we need to specify local matrix elements of the K meson which contribute to the function Π OPE µν (p, q).Details of calculations necessary to find Π OPE µν (p, q) in the soft limit were presented in Refs.[21], therefore we skip further features of relevant analysis and provide only final expressions.First of all, our computations demonstrate that in the soft-meson approximation the correlator Π OPE µν (p, q = 0) receives contribution from the matrix element with m K and f K being the mass and decay constant of the K + meson.The Borel transformed and subtracted correlation function Π OPE (M 2 , s 0 ) is calculated by taking into account condensates up to dimension 9 and given below where the formula Partial width of the process X AV → D * (2010) − K + can be obtained by employing the following expression The sum rule for the strong coupling g 1 contains different vacuum condensates, numerical values of which have been collected in Eq. (12).Apart from that, the equation (36) depends on the spectroscopic parameters of particles involved into decay process.The mass and current coupling of the tetraquark X AV have been calculated in the current work.The masses and decay constants of the mesons D * (2010) − and K + are collected in Table I.This table contains also spectroscopic parameters of other mesons which appear at final stages of different decay channels.For masses all of the mesons and decay 1.67 ± 0.35 23.6 ± 7.6 3 XPS → D 0 K * (892) 0 1.70 ± 0.38 24.5 ± 7.9 TABLE II: Decay channels of the tetraquarks XAV and XPS, strong couplings gi, Gj and partial widths Γ i AV , Γ j PS .The star-marked coupling G1 has a dimension GeV −1 .constants of K, K * and D mesons, we use information from Ref. [33].The decay constant f D * of the vector mesons D * (2010) − and D * (2007) 0 , and decay constant f D * 0 of the scalar meson D * 0 (2300) 0 are borrowed from Ref. [34].
The Borel and continuum subtraction parameters M 2 and s 0 required for calculation of the coupling g 1 are chosen in accordance with Eq. ( 16).Numerical computations yield Then it is not difficult to find The decay of the tetraquark X AV to a meson pair D * (2007) 0 K 0 is another process with K meson in the final state.This process is a "neutral" version of the first channel differences being encoded in masses m 1 and m 2 of mesons D * (2007) 0 and K 0 , respectively.Treatment of this decay mode does not differ from analysis described above.Therefore, we provide final results for the coupling g 2 and partial width of the process The remaining two decay channels X AV → D − K * (892) + and X AV → D 0 K * (892) 0 have been explored by a similar manner.Let us consider, for instance, the process X AV → D − K * (892) + .The strong coupling g 3 that corresponds to the vertex X AV D − K * (892) + is defined by the matrix element where ǫ * µ is polarization vector of the meson K * (892) + .The correlation function that allows us to extract g 3 is with J D (x) being the interpolating current for the pseu- The main term that contributes to this correlation function and determines phenomenological side of a sum rule for g 3 has the following form where m 3 and m 4 are masses of D − and K * (892) + , respectively.Here, f D is the decay constant of the meson D − .To find Π Phys ν (p, q), we use the matrix elements of the tetraquark X AV and vertex, as well as new matrix element The same function Π ν (p, q) calculated in term of quark-gluon degrees of freedom gives QCD side of the sum rule Our analysis demonstrates that in the soft-meson approximation a contribution to Π OPE ν (p, q) comes from the local matrix element of the meson K * (892 with f K * being its decay constant. The sum rule for g 3 can be determined using structures proportional to ǫ * ν in Π Phys ν (p, q) and Π OPE ν (p, q), and corresponding invariant amplitudes Π Phys (p 2 ) and Π OPE (p 2 ).The amplitude Π OPE (p 2 ) is calculated by including effects of condensates up to dimension 9.After Borel transformation and subtraction it takes the form The function F non−pert.(M 2 ) in Eq. ( 53) is given by the expression (54) After manipulations described above in detail, for g 3 we get the sum rule 3 )/2.The sum rule prediction for g 3 reads The partial width of the process X AV → D − K * (892) + can be computed using Eq. ( 40 II.This information allows us to estimate full width of the tetraquark X AV which characterizes it as a resonance with a relatively narrow width. In this section, we investigate decays of the pseudoscalar tetraquark X PS with the mass m PS and current coupling f PS which have been extracted from two-point sum rules in Section II.We consider the S-wave process X PS → D * 0 (2300) 0 K 0 , as well as P -wave decay modes X PS → D − K * (892) + and X PS → D 0 K * (892) 0 of this four-quark state.We begin our analysis from the decay X PS → D * 0 (2300) 0 K 0 .The coupling G 1 required to calculate partial width of this process can be defined using the on-mass-shell matrix element The correlation function, which should be considered to determine strong coupling G 1 of particles at the vertex X PS D * 0 (2300) 0 K 0 , is given by the formula where D * 0 stands for meson D * 0 (2300) 0 .Here, J PS (x) and J D * 0 (x) are interpolating currents for the tetraquark X PS [see, Eq. ( 18)], and for the scalar meson D * 0 (2300) 0 .The latter is defined by the expression A contribution to the correlation function Π Phys (p, q) with poles at p 2 and p ′2 = (p + q) 2 comes from the term Here, m 7 and f D * 0 are the mass and decay constant of the meson D * 0 (2300) 0 , respectively.In order to find Eq.(62), we employ Eqs. ( 59) and ( 20), as well as the matrix element The QCD side of the sum rule Π OPE (p, q) has the following form The functions Π Phys (p, q) and Π OPE (p, q) have trivial Lorentz structures ∼ I, therefore both of them contain only one invariant amplitude.The amplitude Π OPE (p 2 ) is calculated with dimension-9 accuracy and given by the following expression where the function F non−pert.(M 2 ) is determined by the formula and µ K 0 = m 2 2 f K /m s .In the soft-meson approximation the coupling G 1 can be found by means of the sum rule where m 2 = (m 2 PS + m 2 7 )/2.The width of the decay X PS → D * 0 (2300) 0 K 0 is found by utilizing the formula in which λ = λ(m PS , m 7 , m 2 ).Our computations for the coupling G 1 and partial width of the process yield and The coupling G 2 that describes strong interaction of particles at the vertex X PS D − K * (892) + and determines partial width of the channel X PS → D − K * (892) + is defined by the matrix element The sum rule for G 2 is obtained from the correlation function In terms of involved particles' physical parameters this function has the form To extract this expression, we use the matrix elements from Eqs. (50) and (52), as well as one defined by Eq. (71).The correlation function Π ′ (p, q) calculated using quark-gluon degrees of freedom fixes the QCD side of the sum rule and is equal to The sum rule for the strong coupling G 2 can be obtained by employing standard manipulations.The width of the process where λ ′ = λ(m PS , m 3 , m 4 ).
For the coupling G 2 numerical computations give and the partial width of the decay under analysis equals to The decay channel X PS → D 0 K * (892) 0 of the tetraquark X PS which is last process considered in the present paper, can be treated in a similar way.Therefore, we refrain from further details and provide all relevant information in Table II.
The full width of X PS Γ PS = (65 ± 12) MeV, (78) does not differ considerably from the width of the axialvector tetraquark X AV .

V. CONCLUSIONS
In the current article, we have investigated the axialvector and pseudoscalar tetraquarks X AV and X PS from a family of exotic mesons [ud][cs] containing four different quark flavors.We have calculated their masses, and also estimated full widths of these states using different decay channels.
Interest to fully open-flavor structures renewed recently due to discovery of resonances X 0(1) (2900) made by the LHCb collaboration.One of these states X 1 (2900) was studied in Ref. [7] as a vector tetraquark X V = [ud][cs] .The mass and width of X V are close to physical parameters of the resonance X 1 (2900) measured by LHCb, which allowed us to interpret X V as a candidate to the vector resonance X 1 (2900).
The masses and widths of the ground-state scalar tetraquarks X 0 and X S , and their first radial excitations were calculated in Ref. [9].The scalar particles X 0 and X S were modeled using axial-vector and scalar diquarkantidiquark pairs, respectively.
Information gained in our studies about tetraquarks [ud][cs] with spin-parities J P = 0 + , 0 − ,1 + and 1 − is shown in Fig. 5.As is seen, the pseudoscalar X PS state is heaviest particle in family of tetraquarks [ud][cs], whereas light particles are scalar ones with different internal organizations.The vector and axial-vector states have comparable masses and widths.
It is interesting to consider hadronic processes, where exotic mesons X AV and X PS may be observed.We FIG.5: Mass and width of tetraquarks [ud][cs] with different spin-parities.The lower and upper red (solid) and blue (dashed) lines are 1S and 2S scalar states, respectively.The red lines correspond to the scalar tetraquark X0, whereas blue lines show parameters of XS.The mass and width of the vector particle XV were determined in Ref. [7].The axial-vector and pseudoscalar states have been explored in the current article.Theoretical uncertainties of extracted observables are not shown.
have noted in Section I that resonances X 0 (2900) and X 1 (2900) were discovered in the invariant D − K + mass distribution of the process B + → D + D − K + .For the scalar tetraquark X 0 the decay X 0 → D − K + is S-wave process, whereas X 1 → D − K + is P -wave channel for the vector particle X 1 (2900).The invariant mass distribution of D − K * (892) + mesons in exclusive decays of B + meson may be explored to observe tetraquarks X AV and X PS .In fact, decays to D − K * (892) + mesons are S-wave and P -wave channels for the tetraquarks X AV and X PS , respectively.Because branching ratios of these channels amount to 0.30 and 0.36, respectively, they may be employed to fix structures X AV and X PS .These and relevant problems require further theoretical and experimental studies.(A.12) Components of the function Π(M 2 ) are:

M 2 (PCFIG. 1 :
FIG.1:The pole contribution PC as a function of the Borel parameter M 2 at different s0.The horizontal black line limits a region PC = 0.5.The red triangle marks the point, where the mass m of XAV has effectively been computed.

FIG. 2 :
FIG.2: Different contributions to Π(M 2 , s0) as functions of M 2 .Dimension-9 and 10 terms are very small and not shown in the plot.All curves have been calculated at s0 = 10 GeV 2 .

FIG. 3 :
FIG. 3: Mass m of the tetraquark XAV as a function of the Borel parameter M 2 (left), and the continuum threshold parameter s0 (right).

TABLE I :
In expression above, the nonperturbative function F non−pert.(M 2 ) is determined by Masses and decay constants of the mesons D, D * , D * 0 , K, and K * , which have been used in numerical computations.