In this field a key tool is given by meson spectroscopy. Meson, being made by a quark and an anti- quark, is the simplest quark bound system and therefore the ideal benchmark to study the interaction between quarks and understand what the role of gluons is. Historically, the study of meson properties led to some of the most relevant discoveries in particle physics. Some examples are the discovery of the pion by Powell, Occhialini and Lattes in 1947  (see Fig. 1, left), the discovery of strange particles by Rochester and Butler in the same year  (see Fig. 1, right), the interpretation of the φ → KK decay by Zweig and others in 1963 , the discovery of the J/φ in 1974 . These findings were crucial to conjecture the existence of quarks of different flavors and put the grounds for the development of QCD.
Radiative decays of J/ mesons have historically been considered a good place to search for glueballs because the cc pair must decay via annihilation so the intermediate state must have gluons in it. BESIII has been operating at the Beijing Electron-Positron Collider (BEPCII) since 2008 at center-of-mass energies between 2 and 4.4 GeV. It has collected 1.2 billion J/ and 0.5 billion to date, but their completed analyses are based on a sample of 200 million J/. The BESIII detector consists of a helium- gas-based drift chamber, a plastic scintillator time-of-flight system and a CsI (Tl) electromagnetic calorimeter, all enclosed in a superconducting solenoidal magnet that provides a 1 T magnetic field . The solid angle for charged particle and photon acceptance is 93% of 4. A recent review of light meson spectroscopy with BESIII can be found in Ref. .
As already mentioned, several reactions and beams have been used so far for meson spectroscopy searches: just to remind the most important, high-energy meson (mainly pion) and proton beams, exploiting the peripheral and central production mechanisms, antinucleon annihilation at rest and in ﬂight, which convey the formation of a gluon-rich environment suitable for glueball production, and e + e − annihilation. The latter reaction has been stud- ied extensively since the LEP era, and is a suitable envi- ronment where also γγ collisions can be measured, which provide quite useful information as they are a natural anti- glueball ﬁlter. Starting in the Nineties the e + e − annihila- tion was fruitfully exploited for meson spectroscopy stud- ies at ﬂavor factories, with successful experiments like BaBar at PEPII and Belle at KEK, that operated at the Υ (nS ) masses, BES (and its present upgrade) at BEPC, operating in the charmonium mass region, and KLOE at DA Φ NE, which operated at the φ energy. The e + e − an- nihilation di ﬀ ers from hadronic reactions for the fact that only 1 −− systems can be formed, so these reactions pro- vide naturally a powerful quantum number selection.
Quantum Chromodynamics is one of the fundamental theories in modern high energy physics. Light meson spectroscopy plays a crucial role in examining and understanding the QCD theory in non- pertubative energy region. Decays of the J /ψ meson, being the lowest lying 1 −− c¯ c states, provide an ideal laboratory for light meson spectroscopy.
hadronic molecules, respectively; heavy-quark spin symmetry predicts that the binding ener- gies are independent of the heavy meson spin up to an uncertainty of about 10%. Moreover, there are two resonances in the (S , I) = (0, 1/2) channel with the lighter one located more than 100 MeV below its strange partner. Given the above discussion, it is important to test the picture outlined above as much as possible. In the following, we demonstrate that our resolution to these puzzles is backed by precise experimental data.
pled to both πη and K K ¯ – the first strongly-coupled meson-meson scattering system extracted in a lattice QCD calculation. The corresponding pole appears on a single unphysical Riemann sheet, un- like a canonical two-channel resonance where poles would be expected on two unphysical sheets, and this may be a sign that the state binds through the long-range interaction between a pair of mesons. Ref.  also presents results from including the πη 0 channel and considers D-wave scattering where
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Abstract. The goal of the COMPASS experiment at CERN is to study the structure and dynamics of hadrons. The two-stage spectrometer used by the experiment has large acceptance and covers a wide kinematic range for charged as well as neutral particles and can therefore measure a wide range of reactions. The spectroscopy of light mesons is performed with negative (mostly π − ) and positive (p, π + ) hadron beams with a momentum of 190 GeV/c. The light-meson spectrum is measured in diﬀerent ﬁnal states produced in diﬀractive dissociation reactions with squared four-momentum transfer t to the target between 0.1 and 1.0 (GeV/c) 2 . The ﬂagship channel is the π − π − π + ﬁnal state, for which
Our aim here is to extract an isoscalar meson mass spectrum and study the hidden-flavor composition of the states using lattice QCD computations. This is a challeng- ing undertaking for several reasons. It requires lattice gauge configurations with dynamical light and strange quarks in order that the flavor mixing appear in a unitary manner, while evaluation of the disconnected correlator contributions that distinguish isoscalars from isovectors is computationally expensive and the signals obtained typi- cally diminish into noise at small Euclidean times. These problems have limited calculations to a few J PC with typically low statistical precision [3–5]. Glueball studies, which produce exceptionally clean spectra in the quarkless Yang-Mills case , become challenging in QCD through strong coupling to the quark sector .
On the other hand a number of new states have recently been found at the B -factories (and confirmed by other e + e − machines and at the Fermilab Tevatron) whose nature is not yet understood. These states, which are produced through many different mechanisms (such as B -meson decay, Initial State Radiation (ISR), photon-photon fusion, double-charmonium), are usually associated with charmonium because they decay predominantly into charmonium states (such as the J / or the (2 S )) and DD , but their interpretation is far from obvious. In fact, some of them might be some previously unobserved form of hadronic matter, such as molecules, multiquark states or even charmonium hybrids. It is therefore understandable that these discoveries have stimulated a lot of theoretical and experimental activity and that they have played a significant role in determining a true renaissance of hadron physics. More than twenty such states have been discovered over the past 11 years. In the following I will briefly discuss the X (3872), the Y (4260) and the Z ± (3900).
Abstract. Within the broad physics programme of the COMPASS collabora- tion at CERN SPS, soft reactions of high-energy hadron beams on diﬀerent nuclear targets are investigated. Aiming at a better understanding of the strong interaction, novel results range from tests of chiral symmetry breaking to prop- erties of diﬀractively produced meson resonances studied in their multi-particle decays, such as the a 1 (1420) with unusual properties. The talk will highlight the
Figure 1. From ref.  (see figure 6). Lattice QCD calculation of isoscalar(black,green) and isovector(grey) meson spectrum using m π ∼ 400MeV. The black and green boxes represent the light quark (` = u and d) and the grey boxes represent the strange quark (s) content of the isoscalar states. Pink boxes are glueball states arising from quarkless Yang-Mills theory. The lightest hybrid meson states are indicated by the blue frames with the exotics grouped on the right hand side of the plot. Mixing angles between the light quark (`) and strange quark(s) parts are also shown in degrees.
strong overlap with operators proportional to the field strength tensor we interpret these states as the lightest supermultiplet of hybrid mesons and highlight them in red in Fig. 3. The pattern of states observed in the supermultiplet is the one expected if a quark-antiquark pair in an S-wave configuration is coupled to a 1 +− quasi-gluon, and its appearance at an energy scale ∼ 1200 MeV above the lightest quark-antiquark state is in agreement with what was observed in both the light meson sector  and the charmonium sector .
We present results of an exploratory study of singlet scalar states in unquenched QCD using both glueball and meson operators. Results for non-singlet non-strange scalar mesons are also presented. We use Asqtad improved staggered fermions and gauge configurations generated by the MILC collaboration at lattice spacings of .12 and .09 fm. In this formulation, the glueball mass is not significantly different from the quenched value at finite lattice spacing. Significant taste violations are present in the scalar sector. At light quark masses, decay channels complicate the mass determinations. There is some evidence that the non-strange singlet meson lies below the non-singlet meson.
The calculation of the positive-parity charmed meson spectrum on lattice is di ffi cult as their masses are close to the strongly-coupled S -wave D (∗) K thresholds, and thus it is not easy to get the correct masses without taking the D (∗) K into account. The inclusion of the latter, how- ever, requires the calculation of quark-antiquark annihilation-type Wick contractions, which remained a challanging problem for a long time.
The suitability of the COMPASS setup for meson spec- troscopy was studied in a short pilot run in 2004, where a 190 GeV /c π − beam was shot onto a fixed lead target. Based on the experience from this run the spectrometer was upgraded in order to address the challenges of the hadron spectroscopy physics program. A Recoil Proton De- tector (RPD) was installed around a 40 cm long liquid hy- drogen target. The RPD measures the time-of-flight of the recoil protons using two barrels of scintillator slats and provides information for the trigger decision. The read- out electronics of the electromagnetic calorimeters was up- graded and the central part of the second calorimeter was equipped with 800 radiation-hard Shashlik blocks. In ad- dition the tracking close to the beam axis and the vertex definition were improved by adding high-resolution Pixel- GEM detectors in the spectrometer and cryogenic silicon microstrip detectors in the target region, respectively. At the same time the material budget in the beam region was reduced. Also the particle identification capabilities were enhanced by utilizing the RICH detector in the first spec- trometer stage and two ChErenkov Di ff erential counters with Achromatic Ring focus (CEDAR) upstream of the tar- get which are able to identify the incoming beam particles. With this enhanced setup COMPASS took di ff ractive and central production data in 2008 and 2009 using 190 GeV /c negative and positive hadron beams on liquid hydrogen, nickel, tungsten, and lead targets. The goal of collecting about ten times the available world statistics for both production reactions has been achieved.
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In-medium properties of ω, φ and η ′ mesons have been studied at several accelerator laboratories. Measured transparency ratios indicate an in-medium broadening to 130-150, 30-50 and 15-25 MeV respectively, extrapolated to normal nuclear matter density. Inelastic in-medium meson-nucleon cross sections of ≈ 60, 15-25, and 3-10 mb, respectively, have been extracted. The analysis of meson line shapes has not provided evidence for an in-medium mass shift. Several π − − 121 Sn bound states have been identified. Encouraged by the relatively narrow η ′ in-medium width of ≈ 20 MeV, several exper- iments are planned to search also for η ′ -mesic states. These measurements appear to be a promising tool for systematic studies of the meson-nucleus interactions and in-medium effects.
The light vector mesons ρ, ω and Φ are particularly suited for the mass distribution measurements since their life- times of 1.3 fm / c, 23 fm / c and 46 fm / c, respectively, are so short that they decay within the nuclear medium with some probability after production in a nuclear reaction. Never- theless, momentum cuts have to be applied for the longer lived ω and Φ mesons to achieve decay lengths compara- ble to nuclear dimensions. Obviously the line shape anal- ysis is not applicable for η meson in the medium since its decay length is much larger than nuclear dimensions. In case of mesons with long lifetime decaying outside of the nucleus this method is not applicable since the information gained is the vacuum spectral function. The experimental results on the line shape analysis from the B4 project will be presented and discussed in section 5.2.1.
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Abstract. In this work pion and kaon condensation in the framework of chiral per- turbation theory is studied. I consider a system at vanishing temperature with nonzero isospin chemical potential and strangeness chemical potential; meson masses and mixing in the normal phase, the pion condensation phase and the kaon condensation phase are described. There are diﬀerences with previous works, but the results presented here are supported by both theory group analysis and by direct calculations. Some pion decay channels in the normal and the pion condensation phases are studied, ﬁnding a nonmono- tonic behavior of the Γ decay as a function of μ I .
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theoretical point of view the possible diﬀerence between transverse and longitudinal cross sections is the result of diﬀerent distribution of quarks in vector mesons depending on their polarizations. This eﬀect takes place both for valence quarks distribution  and the distribution of constituent quarks [9, 10]. The distributions of constituent quarks in ρ 0 mesons for transverse and longitudinal polarizations as a function of transverse distance r between two quarks and z -the share of the vector meson light-cone momentum carried by quark are shown in the ﬁgure 4. One can see that quarks distribution in vector meson is very di ﬀ erent for di ﬀ erent polarizations [9, 10].
The problem is a reliable and precise evaluation of the non-perturbative strong interaction eﬀects. Besides the dispersion relation (DR) approach applicable where the relevant experimental cross sec- tions are available one needs low energy eﬀective hadronic modeling like vector meson dominance (VMD), scalar QED (sQED), extended Nambu-Jona-Lasinio (ENJL) or hidden local symmetry (HLS) or similar Resonance Lagrangian Approach models, which attempt to extend chiral perturbation the- ory (CHPT) by including vector mesons (VMD) in accord with the chiral structure of QCD. Lattice QCD ab initio calculations come closer in precision and already have provided important constraints and information (see e.g. ).