Search

Recent decades have seen a resurge of interest in hadron spectroscopy, driven by new, high-luminosity experiments which have identified many new hadrons, both expected and unexpected. The large number of unexpected hadrons suggest contributions from additional quark and gluonic degrees of freedom to the hadronic spectrum beyond the basic quark model. Photoproduction has emerged as an attractive process to study hadron spectroscopy, due to the range of states accessible in new and planned experimental facilities, complimentary kinematics to other experiments where rescattering effects near thresholds are reduced, and advances in our theoretical understanding of these reactions. This white paper reviews the prospects for hadron spectroscopy from three major existing and proposed facilities: the current GlueX experiment, the planned Electron-Ion Collider, and a proposed JLab 24 GeV upgrade., 13 page, 4 figures, White paper submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021)

The last two decades have witnessed the discovery of a myriad of new and unexpected hadrons. The future holds more surprises for us, thanks to new-generation experiments. Understanding the signals and determining the properties of the states requires a parallel theoretical effort. To make full use of available and forthcoming data, a careful amplitude modeling is required, together with a sound treatment of the statistical uncertainties, and a systematic survey of the model dependencies. We review the contributions made by the Joint Physics Analysis Center to the field of hadron spectroscopy., 98 pages, 58 figures. To appear in Progress in Particle and Nuclear Physics

We perform $SU(2)$ Yang-Mills lattice simulation of the electric field distribution in the Coulomb gauge for different values of $\beta$ to further investigate the nature of the Coulomb flux tube., Comment: 12 pages, 11 figures

We discuss the derivation and properties of the general representation of partial wave amplitudes in the context of improving the models currently used in analysis of three particle Dalitz distributions., 6 pages

It appears that there are two resonances with $J^P= 1/2^-$ quantum numbers in the energy region near the $��(1405)$ hyperon. The nature of these states is a topic of current debate. To provide further insight we use Regge phenomenology to access how these two resonances fit the established hyperon spectrum. We find that only one of these resonances is compatible with a three-quark state.

We present an unitary dispersive model for the $��\to 3 ��$ decay process based upon the Khuri-Treiman equations which are solved by means of the Pasquier inversion method. The description of the hadronic final-state interactions for the $��\to 3��$ decay is essential to reproduce the available data and to understand the existing discrepancies between Dalitz plot parameters from experiment and chiral perturbation theory. Our approach incorporates substraction constants that are fixed by fitting the recent high-statistics WASA-at-COSY data for $��\to ��^+ ��^- ��^0$. Based on the parameters obtained we predict the slope parameter for the neutral channel to be $��=-0.022\pm 0.004$. Through matching to next-to-leading order chiral perturbation theory we estimate the quark mass double ratio to be $Q=21.4 \pm 0.4$., 15 pages, 6 figures

In this work, we present different representations of Khuri-Treiman equation, the advantage and disadvantage of each representations are discussed. Using a toy model for scattering amplitude, we study the sensitivity of solution of KT equation to the left hand cut of this toy model and to the different approximate methods. At last, we briefly discuss Watson's theorem when three particles in final states are involved., Comment: EPJA publication version

We assess the two-photon exchange contribution to the Lamb shift in muonic hydrogen with forward dispersion relations. The subtraction constant $\bar T(0,Q^2)$ that is necessary for a dispersive evaluation of the forward doubly-virtual Compton amplitude, through a finite energy sum rule, is related to the fixed J=0 pole generalized to the case of virtual photons. We evaluated this sum rule using excellent virtual photoabsorption data that are available. We find that the "proton polarizability correction" to the Lamb shift in muonic hydrogen is $-(40\pm5)��$eV. We conclude that nucleon structure-dependent uncertainty by itself is unlikely to resolve the large (300$��$eV) discrepancy between direct measurement of the Lamb shift in $��H$ and expectations based on conventional Hydrogen measurements., 8 pages, 4 figures, submitted to Phys. Rev. A

We revisit the photo-absorption sum rule for real Compton scattering from the proton and from nuclear targets. In analogy with the Thomas-Reiche-Kuhn sum rule appropriate at low energies, we propose a new "constituent quark model" sum rule that relates the integrated strength of hadronic resonances to the scattering amplitude on constituent quarks. We study the constituent quark model sum rule for several nuclear targets. In addition we extract the $\alpha=0$ pole contribution for both proton and nuclei. Using the modern high energy proton data we find that the $\alpha=0$ pole contribution differs significantly from the Thomson term, in contrast with the original findings by Damashek and Gilman., Comment: 8 pages, 4 figures, 2 tables; typos corrected; submitted to PRC

We consider the electromagnetic and transition pion form factors. Using dispersion relations we simultaneously describe both the hadronic, time-like region and the asymptotic region of large energy-momentum transfer. For the latter we propose a novel mechanism of Regge fermion exchange. We find that hadronic contributions dominate form factors at all currently available energies., 4 pages, 3 figures

We consider the electromagnetic and transition pion form factors. Using dispersion relations we simultaneously describe both the hadronic, time-like region and the asymptotic region of large energy-momentum transfer. For the latter we propose a novel mechanism of Regge fermion exchange., Comment: Talk at the DIS2011; 4 pages, 2 figures

We construct an analytical model for two channel, two-body scattering amplitudes, and then apply it in the description of the three-body $J/��\to K^+K^-��^0$ decay. In the construction of the partial wave amplitudes, we combine the low energy resonance region with the Regge asymptotic behavior determined from direct two-body production. We find that resonance production in the $K��$ channel in $J/��$ decays seems to differ from that observed in direct $K��$ production, while the mass distribution in the $K\bar K$ channel may be compatible.

Advances in experiment, theory and phenomenology of hadron spectroscopy are discussed. We focus on specific developments in the meson sector., Comment: 8 pages, 4 figures. Talk presented at the DPF-2011 Conference, Providence, RI, August 8-13, 2011

We solve the dispersion relation for the P-wave pi pi amplitude.We discuss the role of the left hand cut vs Castillejo-Dalitz-Dyson (CDD), pole contribution and compare the solution with a generic quark model description. We review the the generic properties of analytical partial wave scattering and production amplitudes and discuses their applicability and fits of experimental data., 10 pages, 7 figures, typos corrected, reference added

We discuss the importance of inelasticity in the P-wave pi pi amplitude on the Dalitz distribution of 3pi events in J/psi decay. The inelasticity, which becomes sizable for pi pi masses above 1.4 GeV, is attributed to KK to pi pi rescattering. We construct an analytical model for the two-channel scattering amplitude and use it to solve the dispersion relation for the isobar amplitudes that parametrize the J/psi decay. We present comparisons between theoretical predictions for the Dalitz distribution of 3pi events with available experimental data., Comment: 10 pages, 10 figures

Recently we have shown that exclusive QCD photon-induced reactions at low Mandelstam-t are best described by Regge exchanges in the entire scaling region, and not only for low values of Bjorken-x. In this paper we explore this crucial Regge behavior in Deeply Virtual Compton Scattering from the point of view of collinear factorization, with the proton tensor written in terms of Generalized Parton Distributions, and we reproduce this feature. Thus it appears that in the Bjorken limit, a large class of hard, low-t exclusive processes are more sensitive to the meson cloud of the proton than to its fundamental quark structure. These process will then be described most efficiently by process-dependent Regge Exclusive Amplitudes rather than by universal Generalized Parton Distributions. We introduce such Regge Exclusive Amplitudes for Deeply Virtual Compton Scattering., typos corrected, changes in Fig.3

We analyze the role of final state interactions in decay of the lighest exotic meson, $pi_1$ with $J^{PC}=1^{-+}. We use the relativistic Lippmann-Schwinger equation for two coupled $��b_{1}$ and $����$ channels. The first one is the predicted dominant decay mode of the $��_{1}$, whereas in the other a narrow $��_1(1600)$ exotic signal has been reported by the E852 collaboration. The FSI potential is constructed, based on the $��$ meson exchange between the two channels. We find that this process introduces corrections to the $��_{1}$ widths of the order of only a few MeV. Therefore, we conclude that a substantial $����$ mode cannot be generated through level mixing., 7 pages, 11 figures

Starting from the QCD Hamiltonian, we derive a schematic Hamiltonian for low energy quark dynamics with quarks restricted to the lowest s-level. The resulting eigenvalue problem can be solved analytically. Even though the Hamiltonian exhibits explicit chiral symmetry the severe restriction of the number of degrees of freedom breaks the pattern of chiral symmetry breaking for finite quark masses., Comment: 7 pages

We briefly review theoretical calculations for the pseudoscalar-vector meson hyperfine splitting with no open flavor and also report a many body field theoretical effort to assess the impact of chiral symmetry in the choice of effective potentials for relativistic quark models. Our calculations predict the missing eta_b meson to have mass near 9400 $MeV$. The radial excitation $\eta_c(2S)$ is in agreement with the measurements of the BELLE and most recently BABAR collaborations., Comment: European HEP conference proceedings (Aachen, July 2003). A few typographical errors corrected

Results of a new analysis of the $K^+K^-$ photoproduction at two photon energies $E_\gamma = 4{GeV}$ and $5.65{GeV}$ with a particular emphasis on the $S$-wave production are presented. We show that the proper treatment of all the helicity components of the $S$- and $P$-waves enables one to eliminate the reported discrepancies in extraction of the $S$-wave photoproduction cross section from experimental data., Comment: 25 pages, 5 figures, new predictions at 8 GeV added in Fig. 5

We discuss the possibility that the recently reported resonance in the $D_s ��^0$ spectrum can be described in terms of residual $D��$ interactions., 3 pages, 1 figure

We discuss a coupled channel analysis of the $����$ and $��'��$ systems produced in $��^-p$ interactions at 18 GeV/$c$. We show that known $Q\bar Q$ resonances, together with residual soft meson-meson rescattering, saturate the spectra including the exotic $J^{PC}=1^{-+}$ channel. A possibility of a narrow exotic resonance at a mass near 1.6 GeV/$c^2$ cannot, however, be ruled out., 4 pages, 2 figures

We analyze the process $\vec ��+p\to ��^0 ��^{+}n$ at low momentum transfer focusing on a possibility of production of an exotic $J^{PC}=1^{-+}$ meson state. In particular we discuss polarization observables and conclude that linear photon polarization is instrumental for separating of the exotic wave., 23 pages, 6 figures

The heavy quark expansion of Quantum Chromodynamics and the strong coupling flux tube picture of nonperturbative glue are employed to develop the phenomenology of hybrid meson decays. The decay mechanism explicitly couples gluonic degrees of freedom to the pair produced quarks and hence does not obey the well known, but model-dependent, selection rule which states that hybrids do not decay to pairs of L=0 mesons. However, the nonperturbative nature of gluonic excitations in the flux tube picture leads to a new selection rule: light hybrids do not decay to pairs of identical mesons. New features of the model are highlighted and partial widths are presented for several low lying hybrid states., 13 pages, 1 table, revtex

We argue that deeply virtual Compton scattering will display Regge behavior nu^alpha_R(t) at high energy at fixed-t, even at high photon virtuality, not necessarily conventional scaling. A way to see this is to track the Reggeon contributions to quark-nucleon scattering and notice that the resulting Generalized Parton Distributions would have divergent behavior at the break-points. In addition, we show that the direct two-photon to quark coupling will be accessible at large $t$ where it dominates the DVCS amplitude for large energies. This contribution, the J=0 fixed-pole, should be part of the future DVCS experimental programs at Jlab or LHeC., 5 pages, 4 figures, Proceedings of the XVII International Workshop on Deep-Inelastic Scattering and Related Subjects DIS 2009, 26-30 April 2009, Madrid

In recent years there has been a rapidly growing body of experimental evidence for existence of exotic, multiquark hadrons, i.e. mesons which contain additional quarks, beyond the usual quark-antiquark pair and baryons which consist of more than three quarks. In all cases with robust evidence they contain at least one heavy quark Q=c or b, the majority including two heavy quarks. Two key theoretical questions have been triggered by these discoveries: (a) how are quarks organized inside these multiquark states -- as compact objects with all quarks within one confinement volume, interacting via color forces, perhaps with an important role played by diquarks, or as deuteron-like hadronic molecules, bound by light-meson exchange? (b) what other multiquark states should we expect? The two questions are tightly intertwined. Each of the interpretations provides a natural explanation of parts of the data, but neither explains all of the data. It is quite possible that both kinds of structures appear in Nature. It may also be the case that certain states are superpositions of the compact and molecular configurations. This Whitepaper brings together contributions from many leading practitioners in the field, representing a wide spectrum of theoretical interpretations. We discuss the importance of future experimental and phenomenological work, which will lead to better understandingof multiquark phenomena in QCD., 47 pages, 10 figures. Corresponding authors: Marek Karliner, Elena Santopinto

The Weisberger relation, an exact statement of the parton model, elegantly relates a high-energy physics observable, the 1/x moment of parton distribution functions, to a nonperturbative low-energy observable: the dependence of the nucleon mass on the value of the quark mass or its corresponding quark condensate. We show that contemporary fits to nucleon structure functions fail to determine this 1/x moment; however, deeply virtual Compton scattering can be described in terms of a novel F_{1/x}(t) form factor which illuminates this physics. An analysis of exclusive photon-induced processes in terms of the parton-nucleon scattering amplitude with Regge behavior reveals a failure of the high Q^2 factorization of exclusive processes at low t in terms of the Generalized Parton-Distribution Functions which has been widely believed to hold in the past. We emphasize the need for more data for the DVCS process at large t in future or upgraded facilities., 11 pages, 3 figures, invited contribution to the 11th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, sept. 10th-14th 2007, Juelich, Germany

We highlight the need for the development of comprehensive amplitude analysis methods to further our understanding of hadron spectroscopy. Reaction amplitudes constrained by first principles of $S$-matrix theory and by QCD phenomenology are needed to extract robust interpretations of the data from experiments and from lattice calculations., Comment: 15 pages, 9 figures. Contribution to Snowmass 2021