Charmonia in an unquenched quark model
Abstract
In this work, we study the charmonium spectrum within an unquenched quark model including coupled-channel effects. In couple-channel calculations, we include all of the opened charmed meson channels with the once-subtracted method, meanwhile adopt a suppressed factor to soften the hard vertices given by the model in the high momentum region. We obtain a good description of both the masses and widths for the well-established states in the charmonium spectrum. Furthermore, we give predictions for the higher -, - and -wave charmonium states up to mass region of GeV. The magnitude of mass shifts due to the coupled-channel effects is estimated to be about MeV. Although many decay channels are opened for the higher charmonium states, they are relatively narrow states. Their widths scatter in the range of MeV. Many charmonium-like states, such as , , , , , , , , and /, can be accommodated by the charmonium spectrum when the unquenched coupled-channel effects are carefully considered.
I Introduction
Since the was first observed at Belle in 2003 Belle:2003nnu , many charmonium-like states have been observed in the past two decades at experiments such as BABAR, Belle, LHCb, and BESIII, etc.. Sixteen states (known as ), , , , , , , , , , , , , , and , are listed in the Review of Particle Physics 2023 (RPP 2023) by the Particle Data Group (PDG) ParticleDataGroup:2022pth . Recently, the center-of-mass energies of BESIII experiments have been extended to 4.95 GeV, which bring new opportunities for searching for the higher charmonium states. Lately, in the higher mass region three new vector states BESIII:2022joj ; BESIII:2023cmv , BESIII:2022kcv and BESIII:2023wsc were observed at BESIII. Since these charmonium-like states contain a pair, they may be candidates of charmonium states. However, as a assignment the observed properties, such as mass, for some of them are out of the conventional quark model expectations. They may be also candidates of exotic states, such as multi-quark state, hadronic molecule, or hybrid. About the nature of the charmonium-like states, there are many debates in the literature. To know about the experimental and theoretical status of the charmonium-like states, one can refer to the review works Dong:2021bvy ; Ali:2017jda ; Esposito:2016noz ; Olsen:2017bmm ; Lebed:2016hpi ; Liu:2019zoy ; Chen:2016qju ; Brambilla:2019esw ; Chen:2022asf ; Guo:2017jvc .
As we know that the exotic states, such as tetraquark states, with normal quantum numbers can hardly be distinguished from the normal ones, in order to search for exotic states, one need have a good knowledge of the normal hadron spectrum. In the charmonium spectrum, the low-lying states can be very well described by various quark models with linear confinement potentials, such as the famous nonrelativistic Cornell model Eichten:1974af ; Eichten:1978tg , relativized Godfrey-Isgur model Godfrey:1985xj ; Barnes:2005pb , relativistic quark model Ebert:2011jc , and so on. However, for the description of higher charmonium states, the conventional quark model will be questionable. The predictions often deviate from the observations. These discrepancies may be caused by the quantum fluctuation, i.e., the creation and annihilation of pairs in vacuum, which will bring an important effect. That is the so-called “coupled-channel effect” through virtual charm meson loops. This effect would become essential for some higher excited states.
The unquenched coupled-channel effects for charmonium states were evaluated within various coupled-channel models based on different strategies in the literature, such as Refs. Ferretti:2021xjl ; Ferretti:2020civ ; Duan:2021alw ; Chen:2023wfp ; Heikkila:1983wd ; Pennington:2007xr ; Eichten:1978tg ; Ortega:2019tby ; Fu:2018yxq ; Ferretti:2018tco ; Li:2009ad ; Barnes:2007xu ; Kalashnikova:2005ui ; Anwar:2021dmg ; Kanwal:2022ani ; Zhou:2013ada ; Ferretti:2013faa ; Bruschini:2021cty ; Lu:2017yhl . It is found that the masses of the higher excited states are often lowered by the coupled-channel effects. Thus, some screened potential models, which roughly includes such unquenched effects, were adopted to deal with the mass spectrum in the literature Dong:1996ci ; Ding:1993uy ; Li:2009zu ; Li:2009ad ; Deng:2016stx ; Wang:2019mhs . The screened potential model results show that the mass suppression tends to be strengthened from lower levels to higher ones, compared to the quenched potential model. Both the screened potential model and coupled-channel model seem to have the similar global features in describing the charmonium spectrum Li:2009ad . In theory, to carry out unquenched calculations of charmonium spectrum, the best way is Lattice QCD. There are some attempts in the literature Wilson:2023anv ; Wilson:2023hzu ; Dudek:2007wv ; HadronSpectrum:2012gic ; DeTar:2011nn ; Bali:2011rd ; Lang:2015sba .
For the aspect of phenomenology, one should face several challenges Ni:HL to seriously carry out a study of the charmonium spectrum within the unquenched framework. i) The first challenge is how to select the coupled channels. There are many coupled channels for a charmonium state. However, only the low-lying channels and are included in most of the calculations. Can the contributions of the other higher channels be neglected for the high charmonium states? ii) The second challenge is how to evaluate the coupled channel effects in the high momentum region. Where the nonphysical contributions may be involved due to the hard vertices given by the phenomenological models, such as the model Micu:1968mk ; LeYaouanc:1972vsx ; LeYaouanc:1973ldf . There may be too large mass correction of the coupled channel effects with a bare vertex. iii) The third challenge is how to obtain a unified description of both the masses and widths for the whole charmonium spectrum within the coupled-channel framework. Usually, most of the works only care about mass corrections due to the coupled-channel effects, a comprehensive description of the strong decays together with the mass spectrum is very scarce.
In this work, we study both the masses and widths for the high charmonium spectrum within a unified framework, where the unquenched coupled-channel effects are included by combining the semirelativistic linear potential model with the model Micu:1968mk ; LeYaouanc:1972vsx ; LeYaouanc:1973ldf . Two strategies are adopted to overcome the challenges existing in the unquenched framework as mentioned above. (i) First, in the coupled-channel calculations, we include all possible Okubo-Zweig-Iizuka (OZI) allowed charmed meson channels with mass thresholds below the bare states together with the nearby virtual channels. The contributions from the other virtual channels (whose mass thresholds are significantly above the bare states) are subtracted from the dispersion relation by redefining the bare mass with the once-subtracted method suggested in Ref. Pennington:2007xr . In other words, the mass shift of a bare state is entirely given by the opened and nearby virtual channels. (ii) Second, a factor is adopted to suppress the unphysical contributions of the coupled-channel effects in the high momentum region as done in the literature Morel:2002vk ; Silvestre-Brac:1991qqx ; Zhong:2022cjx ; Ortega:2016mms ; Ortega:2016pgg ; Yang:2022vdb ; Yang:2021tvc . As we know the vertices of the model are only effective in the non-perturbative region, which reflect the ability of creation in the vacuum. This ability will be suppressed in the high momentum region due to the weak interactions between the valence quarks. Thus, one need introduce a suppressed factor when extending the vertices of model to the high momentum region.
Our main purposes of this work are as follows. (i) To obtain a more comprehensive understanding of the charmonium spectrum within the unquenched quark model. Here, we focus not only the explanation of the masses but also the decay widths. (ii) To systematically explore the magnitude of mass shifts of the higher charmonium states up to the mass region of GeV due to the coupled-channel effects. By this study, we expect to clarify whether the mass shifts have an obvious increasing trend from lower levels to higher ones as that predicted with screened potentials. (iii) To know whether the charmonium-like states could be accommodated by the charmonium spectrum when including the unquenched coupled-channel effects. It is also crucial for seeking out the genuine exotic states from the charmonium-like states. (iv) To predict the masses and decay properties of the missing higher charmonium states within a unquenched quark model. We expect our predictions can provide useful references for the future experimental observations.
The paper is organized as follows. In Sec. II, we give an introduction of the framework of the unquenched quark model including coupled-channel effects. In Sec. III, the masses and strong decay properties of the -, - and -wave charmonium states up to mass region of GeV are given. Then, we further give some discussions based on the results. Finally, a summary is given in Sec. IV.
II Framework
II.1 Quark potential model
The bare states are described within a quenched semirelativistic quark potential model. In the model, the effective Hamiltonian is described by
(1) |
where the first two terms stand for the kinetic energies for the quark and antiquark, respectively. Their masses are labeled with and , respectively. While their three momenta are labeled with and , respectively. is the distance between the quark and antiquark. is the well-known Cornell potential Eichten:1978tg
(2) |
which includes the color Coulomb interaction and linear confinement, and zero point energy . The parameters and denote the strength of the confinement and strong coupling of the one-gluon-exchange potential, respectively. The spin-dependent potential, , is adopted the widely used form Godfrey:1985xj ; Barnes:2005pb
(3) |
In the above equation, the first and second terms are the spin-spin contact hyperfine potential and tensor potential, respectively. The third term is the spin-orbit interaction, which can be further decomposed into symmetric and antisymmetric terms Godfrey:2004ya ; Li:2019tbn , i.e.,
(4) |
with
(5) |
(6) |
In these equations, is the relative orbital angular momentum of the system; and are the spins of the quark and antiquark , respectively, and . It should be mentioned that, for a state, there are no contributions from the the antisymmetric term due to the equal masses for the quark and antiquark. The parameter set in the above potentials is determined by fitting the mass spectrum. To solve the radial Schrodinger equation, the Gaussian expansion method Hiyama:2003cu is adopted in this work.
II.2 Coupled-channel effects
There are creation and annihilation of pairs in vacuum for a physical hadron state. Thus, a bare meson state described within the quenched quark model can further couple to two-hadron continuum via hadronic loops, as shown in Fig. 1. Including such unquenched coupled-channel effects, in the simple coupled-channel model Kalashnikova:2005ui , the physical state is described by
(7) |
where is the final two-hadron relative momentum in the initial hadron static system, and denote the probability amplitudes of the bare state and continuum, respectively.
The effective Hamiltonian of the physical state is given by
(8) |
where is the Hamiltonian for describing the bare state , which has been given by Eq. (1). is a Hamiltonian for describing the continuum state . Neglecting the interactions between the and hadrons, the eigenenergy of is given by
(9) |
While is an effective Hamiltonian for describing the coupling of the bare state with the continuum. In the present work, this coupling is adopted from the widely used model Micu:1968mk ; LeYaouanc:1972vsx ; LeYaouanc:1973ldf , which is expressed as
(10) | |||||
where is a dimensionless constant that denotes the strength of the quark-antiquark pair creation with momentum and from vacuum; and are the creation operators for the quark and antiquark, respectively; the subscripts, and , are the SU(3)-color indices of the created quark and antiquark; and correspond to flavor and color singlets, respectively; is a spin triplet state; is the -th solid harmonic polynomial.
The Schrödinger equation including coupled-channel effects can be expressed as
(11) | ||||
where is the mass of the physical state . It can be determined by the coupled-channel equation
(12) |
is the bare mass of obtained from the potential model. While is the mass shift due to the coupled-channel effects, which is given by
(13) | |||||
In principle, all hadronic loops should contribute a mass shift to the bare state . However, it is unfeasible to calculate the self-energy function including an unlimited number of loops. In our calculations, we included all OZI-allowed two-body hadronic channels with mass thresholds below the bare states. Additionally, we account for channels with thresholds slightly (about MeV) higher than the bare mass. The contributions from the other far away virtual channels (whose mass thresholds are significant above the bare states) are subtracted from the dispersion relation by redefining the bare mass with the once-subtracted method suggested in Ref. Pennington:2007xr . In this approach, the mass shift can be evaluated with
(14) | ||||
where represents the subtracted zero-point for the states. It is chosen as MeV, which is the mass of . This method has been successfully applied to study the coupled-channel effects on heavy-light and charmonium states in the literature Ni:HL ; Zhou:2011sp ; Duan:2021alw .
The vertices given by the model are too hard at high momenta. To suppress the unphysical contributions to the mass shift from the higher momentum region, as done in the literature Silvestre-Brac:1991qqx ; Zhong:2022cjx ; Ortega:2016mms ; Ortega:2016pgg ; Yang:2022vdb ; Yang:2021tvc , a suppressed factor is introduced in the strong transition amplitudes, i.e.,
(15) |
where, is a cut-off parameter. In this study, we adopt MeV to consist with our recent study of the heavy-light meson spectrum including unquenched coupled-channel effects Ni:HL .
By combining Eq. (12) and Eq. (14), one can determine the physical mass together with the mass shift . If the mass of the initial state is above the mass threshold of final hadron states and , a strong decay process will happen. The partial decay width for the opened channel is given by
(16) |
which is equal to two times of the imaginary part of the self-energy of the loop as shown in Fig. 1.
Observed State | ParticleDataGroup:2022pth | |||
---|---|---|---|---|
II.3 Model parameters
To consist with our previous work for the study of the charmed and charmed-strange mesons in the potential model Ni:2021pce , the constituent masses for charmed, strange, and up/down quarks are taken as GeV, GeV, and GeV, respectively. The slope parameter for the linear confining potential is taken as . The other three parameters are taken as , GeV, and MeV, which are determined by fitting the masses of the low-lying well established , , and states. To overcome the singular behavior of in the spin-dependent potentials, following the method of our previous works li:2021hss ; Deng:2016ktl ; Deng:2016stx ; Li:2019tbn ; Li:2020xzs ; Ni:2021pce , we introduce a cutoff distance in the calculations, i.e., let within a small range . The cutoff distance is taken as fm, which is determined by fitting the mass of , since its mass is sensitive to the cutoff distance .
When calculating the strong transition amplitudes, one also need the masses and wave functions of the initial and final hadron states, and the strength parameter for the quark pair creation of the model. The masses of the well-established hadrons are taken from PDG ParticleDataGroup:2022pth . For the unestablished charmonium states, their masses are taken from our predictions in the present work; while for the unestablished - and -meson states, their masses are taken from our previous predictions in Ref. Ni:2021pce . The wave functions of the charmonium states and -meson states are adopted the numerical forms predicted in the present work and our previous work Ni:2021pce , respectively. The strength parameter for the quark pair creation is taken as , which is determined by fitting the widths of the well-established charmonium states , , and with , as shown in Table 1.
Exp ParticleDataGroup:2022pth | LC Li:2009zu | EFG Ebert:2011jc | BGS Barnes:2005pb | WCLM Wang:2019mhs | DLGZ Deng:2016stx | GI Godfrey:1985xj | |||||
✗ | |||||||||||
✗ | |||||||||||
✗ | |||||||||||
Belle:2014wyt | |||||||||||
✗ | |||||||||||
✗ | |||||||||||
✗ | |||||||||||
✗ | |||||||||||
Belle:2009rkh | |||||||||||
LHCb:2021uow | |||||||||||
BESIII:2022joj | |||||||||||
BESIII:2023wqy ; BESIII:2022kcv | |||||||||||
Channel | |||||
---|---|---|---|---|---|
✗ | ✗ | ||||
✗ | ✗ | ||||
✗ | ✗ | ||||
✗ | ✗ | ✗ | ✗ | ||
Total | |||||
Channel | |||||
✗ | ✗ | ||||
✗ | ✗ | ||||
✗ | ✗ | ||||
✗ | ✗ | ||||
✗ | ✗ | ||||
✗ | ✗ | ||||
✗ | ✗ | ✗ | |||
✗ | ✗ | ||||
✗ | ✗ | ✗ | |||
✗ | ✗ | ||||
✗ | ✗ | ||||
✗ | ✗ | ||||
Total | |||||
III Result and Discussion
The masses for the -, - and -wave charmonium states up to mass region of GeV obtained within the unquenched quark models are listed in Table 2. For a comparison, the results of the quenched scenario, the experimental data, and some predictions from other works are also presented in the same table. For clarity, our theoretical mass spectra compared with the data are plot Fig. 2 as well. Furthermore, the mass shift and partial strong decay width contributed by each channels for the charmonium states are given in Tables 3-15.
III.1 -wave states
The mass spectrum up to -wave states is given in Table 2 and also shown in Fig. 2. The strong decay properties for the higher -wave states are given in Tables 3-6. The masses for the low-lying -wave states , , and can be well described within the quark model. For the high-lying -wave states, from Table 2 one can see that the unquenched coupled-channel effects of intermediate hadron loops have sizeable corrections to the masses of the bare states, the mass shifts are predicted to be in the range of MeV.
III.1.1
The well established vector state , as the assignment of , from Table 3, one can see that both its mass and width are consistent with the predictions. The predicted partial width ratio between and channels,
(17) |
is close to the lower limit of the data measured by the BABAR collaboration BaBar:2009elc . However, the predicted ratio
(18) |
is inconsistent with the measured value by BABAR. It should be mentioned that the as the dominant channel of is supported by the measurements of Belle Belle:2006hvs .
III.1.2
Considering as the assignment, from Table 3, one can see that both the mass and width are consistent with the quark model expectations. The predicted partial width ratios,
(19) |
are consistent with the data and measured by the BABAR collaboration BaBar:2009elc , respectively, within uncertainties. Recently, the was observed in the final state by the BESIII collaboration BESIII:2023wsc . As the assignment, we find that the branching fraction of into is predicted to be (see Table 3), which is similar to that into . Thus, should be observed in the final states as well.
It should be mentioned that the quark model classification for still bears some controversies. In the literature Li:2009zu ; Wang:2023zxj ; Wang:2022jxj , is suggested to be a state or a - mixing state based on the screening potential model. The screening effect is considered to be partly equivalent to the coupled-channel effect. The magnitude of mass shifts due to the coupled-channel effects estimated within the screening potential model reaches up to a fairly large value MeV, which is about a factor of larger than our estimations with a more comprehensive consideration of the coupled-channel effects (see Table 3). In Ref. Gui:2018rvv , the strong decays of as the assignment based on the screening potential model have been studied by our group, it is found that the decay width MeV is notably smaller than the experimental value MeV. Thus, as the assignment should be excluded according to our study within the unquenched quark model.
III.1.3 and
The center-of-mass energies of BESIII experiments have been extended to 4.95 GeV, which provide good opportunities for establishing high vector charmonium states and . Our predictions about their masses and decay properties have been listed in Tables 5 and 6.
The may favor the assignment listed in RPP ParticleDataGroup:2022pth . The predicted mass and width of , MeV and MeV, are in good agreement with the data MeV and MeV measured at Belle Belle:2014wyt . Recently, a new vector resonance was observed in at BESIII BESIII:2023wqy ; BESIII:2022kcv , the measured mass and width are close to those of . The and may correspond to the same state. Further measurements of the main decay channels listed in Table 5, such as and , may be useful to establish the state.
Finally, it is should be mentioned that is also suggested to be assigned as the state in Refs. Ding:2007rg ; Gui:2018rvv ; Segovia:2008zz ; Zhao:2023hxc based on some quenched quark model studies.
III.2 -wave states
The mass spectrum up to -wave states is given in Table 2 and also shown in Fig. 2. The strong decay properties for the -wave states are given in Tables 7-10. All of the four low-lying -wave states, and , have been well established. However, the situation become complicated and confusing when toward establishing the higher -wave states. The unquenched coupled-channel effects play crucial roles for understanding the nature of states. The coupled-channel effects on the -wave states are small. For the - and -wave states, the unquenched coupled-channel effects systematically lower the mass spectrum, the mass shifts are predicted to be in the range of MeV.
Physical Mass | core | |
---|---|---|
MeV | ||
MeV |
III.2.1 -wave states
For the -wave states, there are several candidates, , , , , and , from experiments ParticleDataGroup:2022pth . However, about their assignments one will face several problems: (i) Can be assigned to indeed? (ii) Is the mass gap between and small or large? (iii) Is the width of broad or narrow? The unquenched quark model including couple-channel effects may shed light on these puzzles.
First, let’s focus on the state. Within the unquenched quark model, the mass of is predicted to be MeV. The coupled-channel effects due to the -loop have a significant correction to bare mass of . By solving the coupled-channel equation, it is interesting to find that there is a positive mass shift MeV. The positive mass shift was also found by the Lanzhou Group Duan:2020tsx . The should be a narrow state with a width of MeV, which is nearly saturated by the channel. Our prediction is consistent with that in Refs. Duan:2020tsx ; Liu:2009fe . Recently, in the final state the LHCb collaboration observed a new charmonium resonance with mass MeV and width MeV LHCb:2020pxc . Comparing the observations with our predictions, we find that perfectly favors the assignment. The resonance may correspond to observed in the final state by Belle Belle:2004lle ; Belle:2009and and BABAR BaBar:2012nxg ; BaBar:2007vxr .
In Ref. Guo:2012tv , the authors analyzed the Belle and BABAR data of . From the data, a broad resonance with mass MeV and width MeV was extracted. They claimed that this broad resonance should correspond to , rather than the narrow resonance. Soon after the work was published, the Belle collaboration reported the observation of a state with a mass of MeV and a width of MeV in the process Belle:2017egg . However, based our present study, the resonance disfavors the assignment. The broad width is out of theoretical expectation, although the mass seems to be consistent with the unquenched quark model predictions. Our conclusion is consistent with that in Refs. Duan:2020tsx ; Gui:2018rvv . It should be pointed out that the is not seen in a recent observation of at LHCb LHCb:2020pxc , where such a state might be expected to play a significant role. The broad resonance may be contributed by several states or nonresonant backgrounds.
Then, we focus on the state. This state has a strong -wave coupling to the channel. When solving the coupled-channel Eq. (12), it is seen that there are three solutions with masses MeV, 3888 MeV and 3990 MeV, respectively, as shown in the upper panel of Fig. 3. The first solution with a mass of MeV lies just below the threshold. The second solution with a mass of MeV is just above the threshold. While the third solution with a mass of 3990 MeV is heavier than the bare state. To uncover the nature of the solutions obtained from the coupled-channel model, we further analyze the spectral density function,
(20) |
as adopted in the literature Baru:2003qq ; Kalashnikova:2005ui ; Kalashnikova:2009gt ; Wang:2023snv . The line shape of the spectral density function is shown in the lower panel of Fig. 3. It is found that the first solution with a mass of MeV corresponds to a very narrow state with a width of about several MeV shown in the spectral density function. The third solution with a mass of MeV corresponds to a broader resonance with a width of MeV. However, the second solution with a mass of MeV does not exhibit any resonance structures in the spectral density function, thus, this solution is unphysical. Similar line shape of the spectral density function was also found in the previous studies Kalashnikova:2005ui ; Giacosa:2019zxw . Two similar solutions are also found by the other coupled-channel analysis of the Zhou:2017dwj ; Ortega:2009hj ; Kalashnikova:2005ui ; Giacosa:2019zxw . Considering the integral intervals as shown in the lower panel of Fig. 3, from the spectral density function, one can estimate the core components for the resonance structures. Our results are given in Table 4. The narrow state with a mass of MeV is dominated by the component, while the core component is only . The broad resonance with a mass of MeV is a dominant state, the core component is estimated to be . The narrow state favors the famous resonance. The nature of originating from the is also suggested in the literature Zhou:2017dwj ; Zhou:2017txt ; Duan:2020tsx ; Pennington:2007xr ; Li:2009ad ; Meng:2007cx ; Ferretti:2014xqa ; Ferretti:2013faa ; Meng:2014ota ; Coito:2012vf ; Wang:2023ovj . To confirm the nature of , it is crucial to look for the broad state with a mass of about MeV in the final state.
Finally, we focus on the state. The unquenched coupled-channel effects play a significant role as well. The and -loops lower the bare mass of the , 4002 MeV, to the physical point MeV. The mass shift reaches up to a fairly large value MeV. Considering the resonance as , the measured mass is well described within the unquenched framework, which can be clearly seen from Fig. 2. Within the unquenched picture, the mass gap between and is predicted to be MeV, which is much smaller than MeV predicted in the quenched quark model (see Table 2). Our predictions are consistent with that in Ref. Duan:2020tsx . Taking as the state, one find the decay width MeV predicted in theory is also compatible with the data MeV ParticleDataGroup:2022pth . The state dominantly decays into both and channels, the partial width ratio between them is predicted to be
(21) |
The decay mode has been observed by the Belle and BABAR experiments ParticleDataGroup:2022pth .
In summary, the unquenched coupled-channel effects are crucial for uncovering the puzzles in the states. Two resonance structures are found when considering coupled-channel effects for the bare state. The may correspond to the low-mass resonance dominated by the component. The and resonances can be well explained with the assignments and , respectively, when the unquenched coupled-channel effects are properly included. The broad structure cannot be explained as , which may be contributed by several states or nonresonant backgrounds.
III.2.2 -wave states
For the -wave states, from Table 2 one can see that the mass corrections due to the coupled-channel effects are not significant. The magnitude of the mass shifts is estimated to be within 30 MeV. Except the has a relatively low mass MeV, the masses for the other three states and are predicted to be around GeV. Our results are consistent with the those predicted with linear potentials Barnes:2005pb ; Deng:2016stx ; Ebert:2011jc , however, are about MeV larger than those predicted with screened potentials Wang:2019mhs ; Li:2009zu .
The strong decay properties of the -wave states are also studied, the results have been given in Table 7. The and are predicted to be narrow states with a comparable width of MeV. While the and are predicted to be moderate width states with a comparable width of MeV. The decay properties are roughly consistent with the predictions with a linear potential model in previous work of our group Gui:2018rvv . The may have good potentials to be observed in the and channels, the branching fractions for these two channels are predicted to be and , respectively. The may have a potential to be observed in the and channels, the predicted branching fractions may reach up to . The may have a good potential to be observed in the and channels, the branching fractions may reach up to . The mainly decays into the , , , and channels with a comparable branching fraction .
The (known as ) observed in the channel in the decay at CDF CDF:2011pep and LHCb LHCb:2016axx ; LHCb:2016nsl ; LHCb:2021uow may be a good candidate of . The averaged mass and width of , MeV and MeV ParticleDataGroup:2022pth , are consistent with the our predictions, MeV and MeV. The is also suggested to be assigned as in Refs. Lu:2016cwr ; Duan:2021alw ; Gui:2018rvv ; Wang:2022dfd ; Ferretti:2020civ . The , as the assignment, should be observed in its main decay channels and via the decays.
The structure observed in the process at Belle Belle:2009rkh may be a good candidate of . The measured mass and width are MeV and MeV, respectively. Assigning to , the predicted mass, MeV is in good agreement with the data, while the predicted width MeV is slightly broader than the data. In Refs. Gui:2018rvv ; Liu:2009fe ; Wang:2022dfd , such a possible assignment was also considered. To confirm this assignment future experimental search for its decays into and are strongly recommended.
Finally, it should be mentioned that recently the LHCb collaboration carried out observations of the decay LHCb:2022dvn ; LHCb:2022aki , there seems to be a bump structure around GeV in the invariant-mass spectrum, which may be contributed by the state. With more statistics, this state is most likely to be established in the channel by using the decay in forthcoming experiments.
As a whole, it’s time to establish the -wave states, which have a relatively narrow width of MeV. The and are good candidates of the and , respectively. Some weak signals of may have been found in the recent LHCb experiments. The may be good channel to establish the -wave states.
III.2.3 -wave states
For the -wave states, from Table 2 one can see that the mass corrections due to the coupled-channel effects are significant. The magnitude of the mass shifts is estimated to be MeV. Except the has a relatively low mass of GeV, the masses for the other three states and are predicted to be around GeV. There are some studies of the higher -wave states within some quenched Gui:2018rvv ; Sultan:2014oua ; Cao:2012du ; Ebert:2011jc ; Soni:2017wvy ; Chaturvedi:2019usm ; Mansour:2021rru ; Fang:2022bft and unquenched Duan:2021alw ; Ferretti:2021xjl quark models. Our results are close to the predictions with a linear potential in Refs. Gui:2018rvv ; Sultan:2014oua ; Cao:2012du , however, is about MeV smaller than the predictions in Refs. Ebert:2011jc ; Soni:2017wvy ; Chaturvedi:2019usm , and about MeV higher than the predictions with screened potentials Wang:2019mhs and other modified confinement potentials Mansour:2021rru ; Fang:2022bft .
The strong decay properties of the -wave states are also studied, the results have been given in Table 8. The and are predicted to be moderate width states with a comparable width of MeV. While the and have a slightly broader width of MeV. The decay properties are roughly consistent with the predictions with a linear potential model in the previous work of our group Gui:2018rvv . The -wave states mainly decay into the -wave and/or -wave -meson excitations by emitting a or meson. The rates decaying into the OZI-allowed , channels are often small.
The charmonium-like resonance (known as ) listed in RPP is a good candidate of . This resonance is found by LHCb in the final state via the decay LHCb:2016axx ; LHCb:2021uow ; LHCb:2016nsl . The newly measured mass and width of are MeV and MeV, respectively LHCb:2021uow , which are consistent with theoretical predictions MeV and MeV. If corresponds to indeed, the decay rate into channel is sizeable, the branching fraction is estimated to be . Thus, the resonance should be established in the final state by using the decay. Recently, this process has been observed by LHCb LHCb:2022dvn ; LHCb:2022aki . There seems to be a vague bump structure around GeV in the invariant mass spectrum. With more statistics, the is most likely to be established in .
III.2.4 -wave states
For the higher -wave states, from Table 2 one can see that the mass corrections due to the coupled-channel effects are significant. The magnitude of the mass shifts is estimated to be MeV. Except the has a relatively low mass MeV, the masses for the other three states and are predicted to be around GeV. There are some studies of the higher -wave states within some quenched Gui:2018rvv ; Sultan:2014oua ; Soni:2017wvy ; Mansour:2021rru and unquenched Duan:2021alw ; Ferretti:2021xjl quark models. Strong model dependencies exist in the predictions. Our results are close to those predicted with a linear potential in Refs. Ferretti:2021xjl ; Gui:2018rvv ; Sultan:2014oua , however, is about MeV smaller than those predicted in Refs. Ebert:2011jc ; Soni:2017wvy , and about MeV higher than the those predicted with a unquenched quark model Duan:2021alw and other modified confinement potentials Mansour:2021rru .
The strong decay properties of the -wave states are also studied, the results have been given in Tables 9 and 10. It is found that the has a width of MeV, while the other three states and have a slightly broader width of MeV. The strong decay properties are roughly consistent with those predicted with a linear potential model in the previous work of our group Gui:2018rvv . The -wave states mainly decay into the /-wave and/or -wave -meson excitations by emitting a light or meson. The rates decaying into the OZI-allowed , channels are negligibly small.
The charmonium-like resonance (known as ) listed in RPP may be a good candidate of . The was first observed by LHCb in the invariant mass spectrum via the decays in 2016 LHCb:2016nsl ; LHCb:2016axx , and confirmed in the same process with more statistics in 2021 LHCb:2021uow . The lately measured mass and width are MeV and MeV, respectively LHCb:2021uow . By using the decays, the LHCb collaboration also observed a similar structure in the channel with a mass of MeV and MeV, respectively LHCb:2020coc . The measured mass and width of are consistent with our predictions, MeV and MeV, with the assignment. The coupled-channel effects are crucial for understanding the mass of . These unquenched effects can lower the bare mass of with a value of MeV. If corresponds to indeed, it is most likely to be observed in the channel via the process. The branching fraction for may reach up to .
III.3 -wave states
The mass spectrum up to -wave states is given in Table 2 and also shown in Fig. 2. The strong decay properties for the -wave states are given in Tables 11-15. The mass shifts due to the unquenched coupled-channel effects on the are predicted to be in the range of MeV. The high -wave states have a relatively narrow width of MeV, although many decay channels are fully opened.
III.3.1 -wave states
For the low-lying states, except the spin singlet , all the spin triplets, , , and , have been well established. Assigning the , and as the spin triplets of the -wave states, from Fig. 2 it is seen that their masses are consistent with the quark model expectations.
The decays of both and are governed by the channel, their decay widths are consistent with the quark model expectations as well (see Table 11). For , the OZI-allowed two-body strong decays are kinematic forbidden, thus, its decays is dominated by the electromagnetic transitions. The measured electromagnetic decay properties are also consistent with the theoretical predictions Deng:2016stx ; BESIII:2021qmo .
How to find the missing spin singlet is still a challenge in experiments. It may be produced via the decay as suggested in Refs. Eichten:2002qv ; Xu:2016kbn ; Fan:2009cj , and established by the the two-photon cascade decay process Deng:2016stx .
III.3.2 -wave states
In the -wave states, only the with has been established. The resonance is usually assigned as the state. With this assignment, the measured width MeV are consistent with the theoretical prediction MeV. However, the mass MeV extracted from experiments is about MeV higher than most of the predictions in theory (See Table 2). This state dominantly decays into channel with a branching fraction of , while it also has sizeable decay rates into and channels with a comparable branching fraction of (see Table 11). The branching fraction ratios predicted in the present work and other works Gui:2018rvv ; Eichten:2005ga ; Segovia:2013kg are very different from the old observations at BABAR BaBar:2009elc . In these -wave decay channels there exist obvious interfering effects between and Gui:2018rvv . A coherent partial wave analysis combining all these exclusive channels is suggested to be carried out for extracting the resonance parameters and branching fractions for these two states.
The other three states , and have a similar mass of GeV. The corrections to the bare masses due to the coupled-channel effects are not significant, the magnitude of the mass shifts is within MeV. Our predicted masses for these -wave states are in agreement with most of the predictions in the literature, such as Refs. Ebert:2011jc ; Godfrey:1985xj ; Deng:2016stx ; Barnes:2005pb . The theoretical strong decay properties have been given in Table 11. It is found that , as the narrowest state in the -wave states, has a width of MeV, while mainly decays into the and channels with branching fractions and , respectively. Both the states, and , have a comparable width of MeV. The () mainly decays into the , , and channels with branching fractions (), () and (), respectively.
Some signals of may have been observed in experiments. In 2021, the LHCb collaboration observed a new resonance with a significance of in the final state by using an amplitude analysis of the decays LHCb:2021uow . The assignment is favored over other assignments with a significance of more than . The observed mass and width are MeV and MeV, respectively, which are consistent with the state observed in with by Belle Belle:2007woe . Considering as the state , both the observed mass and width are consistent with the theoretical predictions, MeV and MeV. To confirm the nature of , the other two dominant decay modes, and , are expected to be searched for in future experiments.
III.3.3 -wave states
The -wave states, , , and , may highly degenerate with each other in a very narrow mass range of MeV. From Table 2, one can see that the coupled-channel effects have a significant correction to the bare masses of the -wave states with and . The magnitude of the mass shifts due to these unquenched effects can reach up to about MeV. The masses of the -wave states predicted in the present work are consistent with the those predicted with linear potentials Deng:2016stx ; Sultan:2014oua ; Kher:2018wtv and a Martin-like potential Shah:2012js , however, are about MeV larger than those predicted with screened potentials Deng:2016stx ; Wang:2019mhs ; Li:2009zu , while about MeV smaller than those predicted with other potential models Ebert:2011jc ; Soni:2017wvy .
The strong decay properties of the -wave states are given in Table 12. From the table, one can see that the widths for the -wave states are not broad. In these states, the is the broadest state with a width of MeV. This state dominantly decays into , , , channels with branching fractions , , , and , respectively. The has a narrow width of MeV, and mainly decays into , , channels with branching fractions , , and , respectively. The has a moderate width of MeV, and mainly decays into and channels with a comparable branching fraction of . The is the narrowest state with a width of MeV. This state has large decay rates into the , , and channels with branching fractions of , , and , respectively.
Recently, the BESIII collaboration observed a vector resonance in the line shape of the cross sections of the process BESIII:2022joj . The mass and width are measured to be MeV and MeV, respectively. This state is confirmed in by BESIII BESIII:2023cmv . The observed mass, width and quantum numbers of can be well understood in theory with the assignment. If corresponds to indeed, it is most likely to be observed in the dominant decay channels, such as , , , in future experiments. The decay mode of observed by BESIII BESIII:2023cmv may be mainly contributed by .
III.3.4 -wave states
The -wave states are predicted to lie in the mass range of MeV (see Table 2). The mass corrections due to the unquenched coupled-channel effects are not large, the values are about MeV. The masses of the -wave states predicted in the present work are consistent with the those predicted with linear potentials Deng:2016stx ; Sultan:2014oua , however, are about MeV larger than those predicted with a screened potential Wang:2019mhs and a Martin-like potential Shah:2012js , while about MeV smaller than those predicted with other potential models Chaturvedi:2019usm ; Ebert:2011jc ; Soni:2017wvy .
The strong decay properties of the -wave states are given in Table 13. In these states, the and have a relatively broader width of MeV, while the two states and have a moderate width of MeV. The decay rates of the -wave states into the OZI-allowed and channels are tiny. The branching fractions for these channels are predicted to be less than . The state has large decay rates into the , , and channels with branching fractions , , and , respectively. The state has large decay rates into the , , and channels with branching fractions , , and , respectively. The state has large decay rates into the , , , and channels with branching fractions , , , and , respectively. The state has large decay rates into the , , and channels with branching fractions , , and , respectively.
Some signals of the state may have been seen in experiments. Recently, a new vector resonance was observed in at BESIII BESIII:2023wqy ; BESIII:2022kcv , the measured mass and width are GeV and MeV, respectively. The predicted mass and width of , MeV and MeV are consistent with those of . Lately, in the Born cross sections of , a structure () with a mass of GeV and a width of MeV was observed by BESIII BESIII:2023wsc . This might be the same state observed in . It should be mentioned that the may highly overlap with in the mass range of GeV. Only a small mass gap ( MeV) between them is predicted in theory. Both and have a comparable decay rate, , into the channel. Thus, the structure observed at around GeV may be contributed by and/or . Further observations with more data samples may be useful to uncover the nature of the structure.
III.3.5 -wave states
The -wave states are predicted to lie in the mass range of MeV (see Table 2). The mass corrections due to the unquenched coupled-channel effects are about MeV. The masses of the -wave states predicted in the present work are consistent with the those predicted within a linear potential model Sultan:2014oua .
The strong decay properties of the -wave states are given in Tables 14 and 15. It is found that these high -wave states have a relatively narrow width of MeV. The decay rates of the -wave states into the OZI-allowed and channels are tiny. The state has relatively large decay rates into the and channels with branching fractions and , respectively. The state has relatively large decay rates into the and channels with branching fractions and , respectively. The state has relatively large decay rates into the and channels with branching fractions and , respectively. The state has relatively large decay rates into the and channels with branching fractions and , respectively.
IV Summary
In this work, the mass spectrum and strong decay properties of the -, -, and -wave charmonium states up to mass region of GeV are systematically studied within the unquenched quark model including coupled-channel effects from all of the OZI-allowed opened charmed meson channels. We can obtain a good description of both the masses and widths for the well-established states in the charmonium spectrum. Although many decay channels are fully opened for the higher charmonium states, they are relatively narrow states. Their widths scatter in the range of MeV. We expect our study can provide a useful reference for establishing an abundant charmonium spectrum. Some key results from this study are emphasized as follows.
-
•
The magnitude of mass shifts of the bare states due to the coupled-channel effects are estimated to be within MeV. The mass shifts do not show an obvious increasing trend from lower levels to higher ones as that predicted in the screened potential models.
-
•
Two resonance structures are found when considering coupled-channel effects for the bare state. The favors the low-mass resonance dominated by the component.
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•
The resonance can be well understood with the dressed states in the coupled-channel model.
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•
The vector resonances and favor the and assignments, respectively.
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•
The newly observed vector states and at BESIII may favor the and assignments, respectively. However, as the assignment cannot be excluded.
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•
The and resonances observed by LHCb favor the and assignments, respectively.
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•
The resonance observed at CDF and LHCb and observed at Belle may be good candidates of and , respectively.
-
•
The resonance observed at LHCb and Belle favor the assignment
-
•
The , , , , and resonances listed in RPP cannot be accommodated by the charmonium spectrum.
Channel | [4708] | [4662] | [4983] | [4902] | Continue | [4708] | [4662] | [4983] | [4902] | ||
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Acknowledgements
We thank useful discussions from Long-Cheng Gui, Qi-Fang Lü, Xiang Liu, Zhi-Yong Zhou, Ying Chen, and Qiang Zhao. This work is supported by the National Natural Science Foundation of China under Grants Nos. 12235018, 12175065, 12205216.
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