Narrow Results

The CLAS Collaboration is operating the CLAS detector at theThomas Jefferson National Laboratory (JLab) in USA. The unique combination of the detector large acceptance and high intensity of the continuous electron beam of CEBAF has opened the way to a comprehensive study of the hadrons structure in a kinematic domain between nuclear and particle physics. Meson spectroscopy plays a central role in the physics program of the Collaboration. Many exclusive channels have been studied with virtual and real photon beams in a wide kinematic domain providing key information about the hadron structure as well as the reactions dynamic. In this contribution, the rich physics program covered by present and future experiments will be reviewed.

Diffractive dissociation reactions at COMPASS provide clean access to mesons with masses below ≈ 2.5 GeV/c². This paper presents a partial wave analysis of about 420 000 π^-Pb → π^-π^-π⁺Pb events at 190 GeV/c beam momentum and with four-momentum transfer t' ∈ [0.1,1] GeV²/c². The well-known a₁(1260), a₂(1320) and π₂(1670) mesons are resolved with high quality. Also the less established states π(1800) and a₄(2040) are seen. In addition, a resonance in the spin-exotic J^PC = 1^-+ wave is observed. A mass-dependent fit results in a mass and width of and , respectively, which is consistent with the disputed hybrid candidate π₁(1600).

The present status and future physics program of Central Production using the STAR detector at RHIC will be described. The program focuses on particle production resulting from the Double Pomeron Exchange (DPE) process. Forward protons from the DPE interaction are detected in the Roman Pot system installed at 55.5 m and 58.5 m on both sides of the STAR interaction point. The recoil system of charged particles from the DPE process is measured in the STAR Time Projection Chamber (TPC). The first data were taken in 2009 during the RHIC-Run 9 using polarized proton-proton collisions at . The preliminary spectra of two pion and four pion invariant mass reconstructed by STAR TPC in central region of pseudo-rapidity |η| < 1, are presented. Plans to take data with the current system at and plans to upgrade the forward proton tagging system, so that it can reach higher masses and obtain large data samples in searching for glueballs that could be produced in the DPE process, are also discussed.

We present results from a Partial-Wave Analysis (PWA) of diffractive dissociation of 190 GeV/c π^- into π^-π⁺π^- final states on nuclear targets. A PWA of the data sample taken during a COMPASS pilot run in 2004 on a Pb target showed a significant spin-exotic J^PC = 1^-+ resonance consistent with the controversial π₁(1600), which is considered to be a candidate for a non- mesonic state. In 2008 COMPASS collected a large diffractive π^-π⁺π^- data sample using a hydrogen target. A first comparison with the 2004 data shows a strong target dependence of the production strength of states with spin projections M = 0 and 1.

The mass spectra of light-heavy tetraquarks $cq\bar{c}\bar{q}$ ($q=u,d$) are computed in a nonrelativistic diquark model with one-gluon exchange plus confining potential. In the diquark model, a $cq\bar{c}\bar{q}$ state is regarded to be made of a light-heavy diquark ($qc$) and an antidiquark $\bar{q}\bar{c}$ in antitriplet and triplet color configuration, respectively. The masses of charm mesons were calculated in order to fit the model parameters used to create the masses of tetraquarks and therefore enhance the model’s reliability. The masses of $(cq\bar{c}\bar{q})$ tetra-quark states are determined to be in the range of 3.8–4.7GeV, which is consistent with the experimentally reported charmonium-like states. In particular, the $Z_c(3900)$, $Z_c(4430)$ and $\psi (4660)$ tetraquarks, which have been seen experimentally, may all be described by our model.

A simple, causal and analytic model of final state rescattering is used to describe all available data on the exotic resonances $Z_c(3900)$ and $Z_c(4025)$. The model provides a compelling and accurate representation of experiment with no need for poles in the scattering matrix.

A few of the newly discovered XYZ states might be bound states of a heavy meson and antimeson. These molecular states, as they are also known, respect heavy quark symmetries at the potential level and as a consequence they might appear in multiplets. Here we exploit their symmetry to predict new states from already known ones. For instance, we expect the existence of a bottom counterpart of the X(3872) at 10580 MeV. Conversely, the assumption that the Z_b(10610) and are molecular implies a pair of analogous resonances with hidden charm located at 3870 and 4010 MeV. They might be identified with the Z_c(3900) and Z_c(4025), supporting the idea that they are and bound states.

Using high precision numerical method based on Gaussian expansion method for few-body systems, we studied the spectra of possible exotic meson states composed of (Q = c, b, and q = u, d, s) in non-relativistic constituent quark model. Some newly released experimental data by the BES, Belle, BaBar Collaboration can be interpreted in our calculation. Several possible exotic states are also predicted.

The QCD analysis of the recent experimental data (L3@LEP) of the hard exclusive ρρ production in two photon collisions shows that these data can be understood as a signal for the existence of an exotic isotensor resonance with a mass around 1.5 GeV. We also argue that hard exclusive reactions are a powerful tool for an experimental study of exotic hybrid mesons with J^PC = 1^-+.

In 2003 the X(3872) particle was discovered by the Belle collaboration. Despite results collected since then, the nature of the state still remains unclear. In this contribution we report on new results on properties of the X(3872) state using data collected with CDF II detector at the Fermilab Tevatron. The dipion mass spectrum and angular distributions are used to determine the J^PC quantum numbers of the state.

A surprising number of new mesons with masses above the threshold have been discovered at the B factories. Some of them are ordinary charmonium states, but others are definitely exotic mesons. The current theoretical status of the new mesons is summarized.

The interaction between pseudoscalar and/or vector mesons can be studied using hidden gauge Lagrangians. In this framework, the interaction between charmed mesons has been studied. Furthermore, doubly charmed states are also predicted. These new states are near the D^*D^* and thresholds, and have spin-parity J^P = 1⁺. We evaluate the decay widths of these states, named as R_cc(3970) and S_cc(3970) (with strangeness), and obtain 44 MeV for the nonstrangeness, and 24 MeV for the doubly charm-strange state. Essentially, the decay modes are DD_(s)π and DD_(s)γ, being the D_π and D_γ emitted by one of the D^* meson which forms the molecule.