The light scalar mesons, discovered over forty years ago, became a challenge for the naive quark-antiquark model from the outset. At present the nontrivial nature of these states is no longer denied practically anybody. Two-photon physics has made a substantial contribution to understanding the nature of the light scalar mesons. Recently, it entered a new stage of high statistics measurements. We review the results concerning two-photon production mechanisms of the light scalars, based on the analysis of current experimental data.
The crucial points of physics of light scalar mesons are discussed. The special attention is given to the chiral shielding of lightest scalar mesons and the production of the well established light scalar mesons in the ϕradiative decays. Arguments in favor of the four-quark model of the a0(980) and f0(980) mesons are given.
The following topics are considered. 1. Confinement, chiral dynamics and light scalar mesons. 2. $ϕ$-meson radiative decays about nature of light scalar resonances. Arguments in favor of the four-quark model of the $a_0(980)$ and $f_0(980)$ mesons are given.
The structure of the scalar mesons has been a subject of debate for many decades. In this work we look for $\bar{q}q$ states among the physical resonances using an extended Linear Sigma Model that contains scalar, pseudoscalar, vector, and axial-vector mesons both in the non-strange and strange sectors. We perform global fits of meson masses, decay widths and amplitudes in order to ascertain whether the scalar $\bar{q}q$ states are below or above 1 GeV. We find the scalar states above 1 GeV to be preferred as $\bar{q}q$ states.
For the first time, as far as we know, in field theory are found explicit forms of propagators satisfying the Källen -- Lehmann representation. To be exact, it is shown that scalar meson propagators, taking into account a virtual intermediate state contribution to the resonance self-energy, satisfy the Källen -- Lehmann representation in the wide domain of coupling constants of the light scalar mesons with the two-particle states. It is proposed to use these propagators in routine fitting data about light scalar mesons to reveal physics underlying the light scalar mesons.
Decays of hidden-charm scalar tetra-quark mesons as the partners of D_{s0}(2317) which has successfully been assigned to the iso-triplet scalar tetra-quark [c,n][sbar,nbar]_{I=1} meson are studied. Because OZI-rule allowed strong decays are kinematically limited, their radiative decays are expected to be important.
Despite the apparent simplicity of meson spectroscopy, light scalar mesons cannot be accommodated in the usual $q\bar q$ structure. We study the description of the scalar mesons below 2 GeV in terms of the mixing of a chiral nonet of tetraquarks with conventional $q\bar q$ states. A strong diquark-antidiquark component is found for several states. The consideration of a glueball as dictated by quenched lattice QCD drives a coherent picture of the isoscalar mesons.
We study the influence of the meson cloud of the nucleon on predictions of the Monte Carlo Glauber model for the charged particle multiplicity density at mid-rapidity in $AA$ collisions. We find that for central $AA$ collisions the meson cloud can increase the multiplicity density by $\sim 16-18$\%. The meson-baryon Fock component reduces the required fraction of the binary collisions by a factor of $\sim 2$ for Au+Au collisions at $\sqrt{s}=0.2$ TeV and $\sim 1.5$ for Pb+Pb collisions at $\sqrt{s}=2.76$ TeV.
The photonuclear production of vector mesons in ultraperipheral heavy ion collisions is investigated within the QCD color dipole picture, with particular emphasis on the saturation model. The integrated cross section and the rapidity distribution for the A + A --> V + A + A (V = rho, omega, phi, J/Psi) process are computed and theoretical estimates for scattering on both light and heavy nuclei are given for energies of RHIC and LHC. A comparison with the recent STAR data on coherent production of rho mesons is also presented. Furthermore, we calculate the photoproduction of vector mesons in pr…
We study the semileptonic decays of $D_{s}^{+}$, $D^{+}$, and $D^{0}$ mesons into the light scalar mesons [$f_{0} (500)$, $K_{0}^{\ast} (800)$, $f_{0} (980)$, and $a_{0}(980)$] and the light vector mesons [$ρ(770)$, $ω(782)$, $K^{\ast} (892)$, and $ϕ(1020)$]. With the help of a chiral unitarity approach in coupled channels, we compute the branching fractions for scalar meson processes of the semileptonic $D$ decays in a simple way. Using current known values of the branching fractions, we make predictions for the branching fractions of the semileptonic decay modes with other scalar and vector …
We discuss coherent and incoherent photoproduction of $J/Ψ$ vector mesons in high energy heavy ion collisions. In a dipole picture for the photon both can be naturally related to the dipole cross section that is also probed in inclusive DIS. We compare results of a particular calculation to ALICE data.
We review our recent results on exclusive production of $μ^+ μ^-$, heavy quark-antiquark, and meson-antimeson pairs in ultraperipheral, ultrarelativistic heavy ion collisions.
We study the effects of light scalar mesons on the skyrmion properties by constructing and examining a mesonic model including pion, rho meson, and omega meson fields as well as two-quark and four-quark scalar meson fields. In our model, the physical scalar mesons are defined as mixing states of the two- and four-quark fields. We first omit the four-quark scalar meson field from the model and find that when there is no direct coupling between the two-quark scalar meson and the vector mesons, the soliton mass is smaller and the soliton size is larger for lighter scalar mesons; when direct coupl…
We present the calculations of collisional and radiative energy loss of $B$ and $D$ mesons in the medium produced in PbPb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. The nuclear modification factor $R_{AA}$ of $B$ and $D$ mesons including shadowing and energy loss are calculated and compared with the measured data. While the $D$ meson $R_{AA}$ can be described in terms of the radiative energy loss alone, both the collisional as well as radiative energy loss are required to explain the $B$ meson measurements.
We calculate the centrality dependence of inclusive cross section of large-pT charmed-meson ($D^{0}$, $D^{*}$, $D^{*+}$, and $D_{s}^{+}$) from heavy quark fragmentation by the hard photoproduction processes for the nucleus-nucleus collisions in ultrarelativistic heavy ion collisions. The numerical results indicate that the modification of the hard photoproduction processes cannot be negligible for the charmed-meson production in Au-Au collisions at Relativistic Heavy Ion Collider (RHIC) and Pb-Pb collisions at Large Hadron Collider (LHC).
In this work, I studied the spectrum of scalar mesons and axial-vector mesons via Dyson-Schwinger equation and Bethe-Salpeter equation approach in the rainbow-ladder approximation. An interaction model with a repulsive term added to the one used for the pseudoscalar and vector mesons is proposed. My results are consistent with the experiment data and other model results, which shows that this interaction model is effective for all the heavy, heavy-light, and light scalar and axial-vector mesons.
We argue that the NICA fixed target experiment will be able to provide very important new experimental data on dilepton and $ϕ$ meson production in the basically undiscovered energy domain between the SIS and SPS energies. Experimental information about elementary cross sections in this energy region is an essential ingredient of models of nuclear collisions in the same energy range.
We discuss the possible production of the $σ$ meson and detection of its two photon decay mode in peripheral heavy ion collisions at RHIC. Assuming the mass and total decay width values reported recently by the E791 Collaboration, we verify that the $σ$ meson can be observed in the subprocess $γγ\to γγ$ if its partial decay width into photons is of the order of a few keV.
We make predictions for cross sections of $ρ$ and $ϕ$ vector meson photoproduction in ultraperipheral Xe-Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV. Analyzing the momentum transfer distribution of $ρ$ mesons in this process, we explore the feasibility of extracting the nuclear density of $^{129}$Xe, which is needed in searches for dark matter with Xenon-based detectors.
For some experimental guidelines of the next millenium, I review the determinations of the masses, decays and mixings of the gluonia, scalar and hybrid mesons from QCD spectral sum rules and low-energy theorems, and compare them with the lattice.
We estimate the rates for radiative transitions of the lightest scalar mesons f_0(980) and a_0(980) to the vector mesons rho and omega. We argue that measurements of the radiative decays of those scalar mesons can provide important new information on their structure.
We study the effect of the nucleon meson cloud on centrality dependence of the jet nuclear modification factor $R_{pA}$. We find that the meson-baryon Fock components may lead to a noticeable deviation of $R_{pA}$ from unity. Our results for $R_{pA}$ show the same tendency as that observed by ATLAS in $p+Pb$ collisions at $\sqrt{s}=5.02$ TeV. The meson cloud suppresses the central jet events and enhances the peripheral jet events. But quantitatively the effect is somewhat smaller than in the data.
We study whether light scalar meson nonet can survive in large values of Nc through observing how scattering amplitudes behave as Nc increases from 3 within a unitarized chiral approach. We obtain the result that vector mesons such as rho and K* survive as narrow width resonances, but all of the scalar meson nonet below 1 GeV fade out as Nc exceeds a rather small number about 6.
We study the hadronic effects on the X(3872) meson abundance in heavy ion collisions. We evaluate the absorption cross sections of the X(3872) meson by pions and rho mesons in the hadronic stage of heavy ion collisions, and further investigate the variation in the X(3872) meson abundance during the expansion of the hadronic matter for its two possible quantum number states; $J^P=1^+$ and $2^-$. We show that the absorption cross sections and the time evolution of the X(3872) meson abundance are strongly dependent on the structure and quantum number of the X(3872) meson. We thus suggest that stu…
In this letter we study the photoproduction of scalar mesons in the intermediate energies considering distinct mixing scenarios in the description of meson physical states. The differential and integrated total cross section are computed for the cases of the mesons a_0(980), f_0(1500) and f_0(1710) focusing the GlueX energy regime with photon energy equal to 9 GeV. Our results indicate that light-quark scalar meson photoproduction is well suited for studying hybrid mesons structure.
We study the production of a $ψ(2S)$ meson in heavy ion collisions. We evaluate Wigner functions for the $ψ(2S)$ meson using both Gaussian and Coulomb wave functions, and investigate the wave function dependence in the $ψ(2S)$ meson production by recombination of charm and anti-charm quarks. The enhanced transverse momentum distribution of $ψ(2S)$ mesons compared to that of $J/ψ$ mesons, originated from wave function distributions of the $ψ(2S)$ and $J/ψ$ meson in momentum space, provides a plausible explanation for the recent measurement of the nuclear modification factor ratio between the $ψ…
We evaluate the decay branching ratios of $χ_{c1}\to PS$, in a quark model parametrization scheme, where $P$ and $S$ stand for pseudoscalar and scalar meson, respectively. An interesting feature of this decay process is that the $c\bar{c}$ annihilate via the pQCD hair-pin diagram is supposed to be dominant. Hence, this decay process should be sensitive to the quark components of the final-state light mesons, and would provide a great opportunity for testing the mixing relations among the scalar mesons, i.e. $f_0(1370)$, $f_0(1500)$ and $f_0(1710)$, by tagging the final state pseudoscalar meson…
We study the $K^*$ meson reduction in heavy ion collisions by focusing on the hadronic effects on the $K^*$ meson abundance. We evaluate the absorption cross sections of the $K^*$ and $K$ meson by light mesons in the hadronic matter, and further investigate the variation in the meson abundances for both particles during the hadronic stage of heavy ion collisions. We show how the interplay between the interaction of the $K^*$ meson and kaon with light mesons in the hadronic medium determines the final yield difference of the statistical hadronization model to the experimental measurements. For …
Relativistic heavy ions are copious sources of virtual photons. The large photon flux gives rise to a substantial photonuclear interaction probability at impact parameters where no hadronic interactions can occur. Multiple photonuclear interactions in a single collision are possible. In this letter, we use mutual Coulomb excitation of both nuclei as a tag for moderate impact parameter collisions. We calculate the cross section for coherent vector meson production accompanied by mutual excitation, and show that the median impact parameter is much smaller than for untagged production. The vector…
We study the production and evolution of charm and bottom quarks in hot partonic medium produced in heavy ion collisions. The heavy quarks loose energy in the medium which is reflected in the transverse momentum spectra of heavy mesons. The collisional energy loss of heavy quarks has been calculated using QCD calculations. The radiative energy loss is obtained using two models namely reaction operator formalism and generalized dead cone approach. The nuclear modification factors, $R_{AA}$ as a function of transverse momentum by including shadowing and energy loss are calculated for $D^{0}$ and…
We study the two point-function for the scalar mesons $σ, κ, f_0(980)$ and $a_0(980)$ as diquak-antidiquark states. We also study the decays of these mesons into $ππ$, $Kπ$ and $K\bar{K}$. We found that the couplings are consistent with existing experimental data, pointing in favor of the four-quark structure for the light scalar mesons.
I review a proposed pattern of the light scalar mesons with q\bar{q} mesons and glueball above 1 GeV and tetraquark mesoniums below 1 GeV. Several challenges and caveats of calculating these light scalar mesons with dynamical fermions are discussed.
Following the re-establishment of the σ(600) and the κ(900), the light scalar mesons a_0(980) and f_0(980) together with the σ(600) and the κ(900) are considered as the chiral scalar partner of pseudoscalar nonet in SU(3) chiral symmetry, and the high mass scalar mesons a_0(1450), K^*_0(1430), f_0(1370) and f_0(1710) turned out to be considered as the L=1 q\bar{q} scalar mesons. We assume that the high mass of the L=1 q\bar{q} scalar mesons is caused by the mixing with the light scalar mesons. For the structure of the light scalar mesons, we adopted the qq\bar{q}\bar{q} model in order to expla…
We investigate the effect of the electromagnetic field generated in relativistic heavy-ion collisions on the dissociation of $Υ$ mesons. The electromagnetic field is calculated using a simple model which characterizes the emerging quark-gluon plasma (QGP) by its conductivity only. A numerical estimate of the field strength experienced by $Υ$ mesons embedded in the expanding QGP and its consequences on the $Υ$ dissociation is made. The electromagnetic field effects prove to be negligible compared to the established strong-interaction suppression mechanisms.
We study, within the Monte-Carlo Glauber model, centrality dependence of the midrapidity charged multiplicity density $dN_{ch}/dη$ and of the anisotropy coefficients $\varepsilon_{2,3}$ in Pb+Pb collisions at $\sqrt{s}=5.02$ TeV and in Xe+Xe collisions at $\sqrt{s}=5.44$ TeV. Calculations are performed for versions with and without nucleon meson cloud. The fraction of the binary collisions, $α$, has been fitted to the data on $dN_{ch}/dη$ in Pb+Pb collisions. We obtain $α\approx 0.09(0.13)$ with (without) meson cloud. The effect of meson cloud on the $dN_{ch}/dη$ is relatively small. For Xe+Xe…
We explore the system size dependence of heavy-quark-QGP interaction by studying the heavy flavor meson suppression and elliptic flow in Pb-Pb, Xe-Xe, Ar-Ar and O-O collisions at the LHC. The space-time evolution of the QGP is simulated using a (3+1)-dimensional viscous hydrodynamic model, while the heavy-quark-QGP interaction is described by an improved Langevin approach that includes both collisional and radiative energy loss inside a thermal medium. Within this framework, we provides a reasonable description of the $D$ meson suppression and flow coefficients in Pb-Pb collisions, as well as …
We get the distributions of shower partons initiated by heavy quarks $c$ and $b$ by studying the fragmentation functions in the framework of the recombination model. The transverse momentum spectra of heavy flavored mesons are predicted with these distributions. We find that the contribution from the recombination of thermal-shower partons is an important part in the total spectrum for the mesons. We predict the heavy flavored meson productions for different centralities with the heavy quark fugacities fitted by the experimental data of $J/ψ$ transverse momentum spectra in Au+Au collisions.
The double meson production in ultraperipheral heavy ions collisions is addressed, focusing on the particular case of rho-J/Psi from two-photon reactions. The cross section at photon level is obtained using distinct parameterizations for the gluon distribution on the light meson. The resulting estimates for the nuclear case are presented and discussed. As a by product, we estimate the double rho production cross section using the Pomeron-exchange factorization relations.
We present a formalism for studying the exclusive production or decay of mesons with any value of the internal orbital angular momentum L. As an application, we discuss the production of meson pairs (involving tensor and pseudotensor mesons) in photon-photon collisions.
We consider chiral perturbation theory with an explicit broad $σ$-meson and study its contribution to the scalar form factors of the pion and the nucleon. Our goal is to learn more about resonance saturation in the scalar sector.
The scalar mesons $σ(600)$, $κ(800)$, $f_0(980)$ and $a_0(980)$ together with the pseudo Goldstone bosons $π$, $K$, and $η$ may be considered as the Higgs sector of strong interaction. After a long time of uncertainty about the internal structure of the scalar mesons there now seems to be consistency which is in line with the major parts of experimental observations. Great progress has been made by introducing the unified model of Close and Törnqvist. This model states that mesons below 1 GeV may be understood as $q^2\bar{q}^2$ in S-wave with some $q\bar{q}$ in P-wave in the center, further ou…
Evaluating the ratio of the exclusive vector meson photoproduction to inclusive hadron/jet production cross section in ultraperipheral heavy ion collisions
Universidade Federal de Santa Catarina（圣卡塔琳娜联邦大学）；Federal University of Rio Grande do Sul (UFRGS)（南里奥格兰德联邦大学）
We discuss in detail the extraction of hadronic mixing strengths from lattice studies. We apply this to the mixing of a scalar glueball and a scalar meson in the quenched approximation. We also measure correlations appropriate for flavour-singlet scalar mesons using dynamical quark configurations from UKQCD. This enables us to compare the results from the quenched study of the mixing with the direct determination of the mixed spectrum. Improved methods of evaluating the disconnected quark diagrams are also presented.
Charmed scalar mesons are studied. By assigning the newly observed charmed strange meson with a narrow width to one of the iso-triplet four-quark mesons, decays of the multiplet members are investigated and, as the consequence, existence of additional narrow scalar mesons is predicted. Decays of the ordinary charmed scalar mesons are also studied. In addition, it is demonstrated that four-quark mesons can play an important role in hadronic weak decays of charm mesons.
We analyze the mass spectroscopy of low and high mass scalar mesons and get the result that the coupling strengths of the mixing between low and high mass scalar mesons are very strong and the strengths of mixing for $I=1, 1/2$ scalar mesons and those of I=0 scalar mesons are almost same. Next, we analyze the decay widths and decay ratios of these mesons and get the results that the coupling constants $A'$ for $I=1, 1/2$ which represents the coupling of high mass scalar meson $N'$ -> two pseudoscalar mesons $PP$ are almost same as the coupling $A'$ for the I=0. On the other hand, the coupling …
The production of $f_{1}$ ($J^{PC} = 1^{++}$) mesons in proton-proton collisions via pomeron-pomeron fusion is discussed. Two ways to construct the pomeron-pomeron-$f_{1}$ coupling are presented. Comparisons with data from the WA102 experiment are made and predictions for RHIC and LHC experiments are given.
Combining the recent lattice calculation of $a_0(1450)$ and $σ(600)$ mesons with the overlap fermion in the chiral regime with the pion mass less than $300 {\rm MeV}$, the quenched lattice calculation of the scalar glueball, and the phenomenological study of the mixing of isoscalar scalar mesons $f_0(1710)$, $f_0(1500)$, $f_0(1370)$ through their decays, a simple pattern for the light scalar mesons begins to emerge. Below 1 GeV, the scalar mesons form a nonet of tetraquark mesoniums. Above 1 GeV, the nonent $q\bar{q}$ mesons are made of an octet with largely unbroken SU(3) symmetry and a fairl…
We propose that there coexist two scalar mesons of different structures (a conventional meson and a four-quark meson) in the recently observed broad bumps just below the large peak of the tensor meson in the D-pi mass distribution. We base this proposal on the interpretation of the charm-strange scalar meson of mass 2317 MeV as a four-quark meson. The strange counterparts of these scalar mesons are also studied.
In the covariant level-classification scheme of hadrons proposed recently, the existence of "chiral particles", scalar and axial-vector mesons as the partners of the ground-state pseudo-scalar and vector mesons, respectively, were predicted in the D meson and B meson systems, realizing a linear representation of chiral symmetry. In this work we reanalyze the B0 pi mass spectra through Z0 decays to show a possibility for the scalar B_0chi.
Aspects of Scalar Meson Dominance in semi- and non-leptonic weak pseudoscalar meson decays are shortly discussed on the basis of the Quark-Level Linear Sigma Model.
Interactions in coupled channels pipi, KKbar and an effective 2pi2pi in scalar-isoscalar wave have been analysed. Influence of interchannel couplings on analytical structure of multichannel interaction amplitudes has been studied. Interplay of S-matrix zeroes and poles and their relation with parameters of scalar resonances has been investigated.
We calculate $p_T$-dependences of diffractive vector meson photo- and electroproduction in electron-proton collisions with and without proton dissociation at small momenta transfers. The calculated slopes are in good agreement with the data.
We study the photon-triggered light and heavy meson production in both p+p and A+A collisions. We find that a parton energy loss approach that successfully describes inclusive hadron attenuation in nucleus-nucleus reactions at RHIC can simultaneously describe well the experimentally determined photon-triggered light hadron fragmentation functions. Using the same framework, we generalize our formalism to study photon-triggered heavy meson production. We find that the nuclear modification of photon-tagged heavy meson fragmentation functions in A+A collision is very different from that of the pho…
We discuss the salient features of exclusive vector meson production in heavy ion collisions at LHC energies. Special attention is paid to the space-time picture of the process. We account for both coherent and incoherent contributions. The explicit quantitative predictions are given for the $ρ$-meson differential cross section in lead-lead collisions in different kinematical configurations relevant for the LHCb and ALICE experiments.
Heavy quarks are a powerful probe for investigating the properties of the Quark-Gluon Plasma created in heavy-ion collisions, since they are produced in initial hard scattering processes and experience all the stages of the medium evolution. ALICE has measured the production of $\mathrm{D}^0$, $\mathrm{D}^+$, $\mathrm{D}^{*+}$, and $\mathrm{D}_{\mathrm{s}}^+$ mesons at central rapidity in their hadronic decay channels in various collision systems and energies. We present recent results for D-meson production measured by the ALICE Collaboration in pp collisions at $\sqrt{s}=7$ and 2.76 TeV, Pb-…
We investigate the finite-temperature spectral functions of heavy quarkonia by using the soft-wall AdS/QCD model. We discuss the scalar, the pseudo-scalar, the vector, and the axial-vector mesons and compare their qualitative features of the melting temperature and growing width. We find that the axial-vector meson melts earlier than the vector meson, while there appears only a slight difference between the scalar and pseudo-scalar mesons which also melt earlier than the vector meson.
We study the production of spin-one mesons in high-energy heavy-ion collisions with peripheral kinematics in the framework of QED. The cross sections of the production of a single vector meson and of two different ones are presented. The explicit dependence on the virtuality of the intermediate vector meson is obtained within a quark model. The effect of reggeization of the intermediate vector meson state in the case of the production of two vector mesons is taken into account.
Rapidity distributions of vector mesons are computed in dipole model proton-lead ultraperipheral collisions(UPCs) at the CERN Larger Hadron Collider(LHC). The dipole model framework is implemented in the calculations of cross sections in the photon-hadron interaction. The bCGC model and Boosted Gaussian wave functions are employed in the scattering amplitude. We obtain predictions of rapidity distributions of $J/ψ$ meson proton-lead ultraperipheral collisions. The predictions give a good descriptions to the experimental data of ALICE. The rapidity distributions of $ϕ$, $ω$ and $ψ(2s)$ mesons i…
