abstract

In order to search the pentaquark state \({\mit \Theta }^{+}\), the \(\gamma n \to K^+K^-n\) reaction has been measured at the Laser Electron Photon facility at SPring-8 (LEPS), where the photon beam is produced by the backward Compton scattering of laser photons off the 8-GeV electrons circulating in the storage ring. The present status and prospects of the experimental study of the \({\mit \Theta }^{+}\) at the LEPS are reported.

abstract

Experimental evidences for the existence of exotic 5-quark baryons have been reported recently by several groups. These data may open new perspectives in hadron spectroscopy as well as in QCD. However, the statistical significance of these results is quite low, and the pentaquark signal has not been seen in various other experiments. The new dedicated experiments currently underway at CLAS will hopefully give a clearer answer on the existence and, possibly, on the properties of pentaquarks. This paper summarizes the results obtained so far in the search for 5-quark states and describes the characteristics and goals of the new experiments running at CLAS.

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Using the TOF detector at the COSY storage ring the hadronic reaction \(pp \rightarrow {\mit \Sigma }^+ K^0 p\) was measured exclusively at a beam momentum of 2.95 GeV/\(c\). A narrow peak was observed in the invariant mass spectrum of the \(K^0 p\) subsystem at \(1530 \pm 5\) MeV/\(c^2\) with a significance of 4–6 standard deviations, depending on background assumptions. The upper limit of 18 \(\pm 4\) MeV/\(c^2\) (FWHM) for its width is given by the experimental resolution. Since a resonance in this subsystem must have strangeness \(S = + 1\) we claim it to be the \({\mit \Theta }^+\) state for which very recently evidence was found in various experiments. To come to a final decision on the existence of the \({\mit \Theta }^+\) very recently COSY-TOF performed a measurement which will give results with strongly improved statistical accuracy.

abstract

The current status of pentaquark searches at the ZEUS experiment at HERA is summarised.

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An experimental search for quasi-real photoproduction of possible \({\mit \Theta } ^{+} \) and \({\mit \Theta } ^{++}\) resonances was performed using the decay modes \(pK_{\rm S}\) and \(pK^{+}\) with the HERMES experiment at DESY. We report evidence for the existence of a \({\mit \Theta } ^{+} \) peak with a statistical significance of \(4 \sigma \). The absence of a peak in the \(pK^{+}\) spectrum in the vicinity of the \({\mit \Theta } ^{+} \) mass yields an estimate of the upper limit for the quasi-real photoproduction cross-section of the \({\mit \Theta } ^{++}\) resonance. In addition, a search for exotic baryon resonances with \(S=-2\), \(Q=-2\) or 0 has been performed in quasi-real photoproduction on a deuterium target by isolating the decay channel \({\mit \Xi }^- \pi ^{-} \to {\mit \Lambda }\, \pi ^- \pi ^{-} \to p \pi ^- \pi ^- \pi ^{-}\) and \({\mit \Xi }^- \pi ^{+} \to {\mit \Lambda }\, \pi ^- \pi ^{+} \to p \pi ^- \pi ^- \pi ^{+}\). No evidence for the previously reported \({\mit \Xi }^{--}(1860)\) and \( {\mit \Xi }^{0}(1860) \) resonances is found in the \({\mit \Xi }^- \pi ^{-}\) and \({\mit \Xi }^- \pi ^{+}\) invariant mass spectra, respectively. Upper limits for their photoproduction cross sections of 2.1 nb and 2.5 nb are found at the 90% confidence level. The photoproduction cross section for the \({\mit \Xi }^{0}(1530)\) resonance, which has the same decay mode as the hypothetical \({\mit \Xi }^{0}(1860)\) peak, is found to be between 9 and 24 nb.

abstract

A large data set of proton-nucleus collisions at \(\sqrt {s} =41.6\) GeV has been searched for \({\mit \Theta }^+\) and \({\mit \Xi }(1862)\) pentaquark candidates. In \(2 \times 10^8\) inelastic events we find no evidence for narrow signals (\(\sigma \approx 5\) MeV/\(c^2\) ) in the \({\mit \Theta }^+ \rightarrow p K^0_{\rm s}\) and \({\mit \Xi }(1862) \rightarrow {\mit \Xi } \pi \) channels. Upper limits on production cross sections at mid-rapidity and on ratios of production cross sections to those of well established resonances are presented.

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We report the results of the \({\mit \Xi }^{-}\pi ^{\pm }\) and \(\overline {{\mit \Xi }}^{+}\pi ^{\pm }\) mass distribution studies performed on the COMPASS data taken in 2002 and 2003. The studies were triggered by the NA49 experiment observation of the exotic \(\phi ^{--}\) state at the \({\mit \Xi }^{-}\pi ^{-}\) invariant mass of 1860 MeV and width \(\lt \)20 MeV. COMPASS data analysis does not confirm the existence of this state.

abstract

The results of resonance searches in \({\mit \Xi }^{-} \pi ^{-}\), \({\mit \Xi }^{-} \pi ^{+}\), \(\overline {{\mit \Xi }}^{+}\pi ^{-}\) and \(\overline {{\mit \Xi }}^{+}\pi ^{+}\) invariant mass spectra in proton-proton collisions at \(\sqrt {s}=\)17.2 GeV are presented. A narrow state was observed in \({\mit \Xi }^{-} \pi ^{-}\) spectra with mass of 1.862 \(\pm \) 0.002 GeV/\(c^{2}\) and width below the detector resolution of about 0.018 GeV/\(c^{2}\). This state is identified as a candidate for the hypothesized exotic \({\mit \Xi }_{5}^{--}\) baryon with \(S = -2\), \(I = {3}/{2}\) and a quark content of (\(dsds\bar {u}\)). \({\mit \Xi }^{-}\pi ^{+}\) and the corresponding antiparticle spectra show an indication of enhancements at the same mass. Very preliminary results of resonance search in \(p K_{\rm S}^{0}\) channel show a resonant structure at 1.526 GeV/\(c^{2}\).

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Given the existing empirical evidence for an exotic \({\mit \Theta }^+\) baryon, we analyze possible properties of its SU\((3)_F\)-partners, paying special attention to the nonstrange member of the antidecuplet \(N^\ast \). A modified \(\pi N\) partial-wave analysis results in two candidate masses, 1680 MeV and 1730 MeV. In both cases, the \(N^\ast \) should be rather narrow and highly inelastic. Our results suggest several directions for experimental studies that may clarify properties of the antidecuplet baryons, and structure of their mixing with other baryons. Recent experimental evidence from the GRAAL and STAR Collaborations could be interpreted as observations of a candidate for the \({\mit \Theta }^+\) nonstrange partner. We also briefly discuss recent negative results regarding the \({\mit \Theta }\)-baryon and the possibility of higher exotic multiplets.

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We briefly review the formulation of chiral quark soliton model and explain the difference and similarities with the Skyrme model. Next, we apply the model to calculate non-exotic and exotic mass spectra. We concentrate on large \(N_{c}\) counting both for mass splittings and decay widths. It is shown that pure large \(N_{c}\) arguments do not explain the small width of exotic pentaquark states.

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The present work is a summary of the status of lattice pentaquark calculations. After a pedagogic introduction to the basics of lattice hadron spectroscopy we give a critical comparison of results presently available in the literature. Special emphasis is put on presenting some of the possible pitfalls of these calculations. In particular we discuss at length the choice of the hadronic operators and the separation of genuine five-quark states from meson–baryon scattering states.

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We report on the observation of a narrow resonance in \(D^{*-}p\) and \(D^{*+} \bar p\) invariant mass combinations in deep-inelastic \(ep\) scattering at centre-of-mass energies of 300 and 320 GeV at HERA. The mass of the resonance is measured to be \(3099 \pm 3 ({\rm stat.}) \pm 5 ({\rm syst.}) \, \textrm {MeV}\), the Gaussian width of \(12 \pm 3 ({\rm stat.}) \, \textrm {MeV}\) is compatible with the experimental resolution. The state can be interpreted as an anti-charmed baryon with minimal constituent quark composition \(uudd\bar c\), together with the charge conjugate.

abstract

The chiral doublers scenario is discussed from the basic point of view of the heavy–light quark symmetry. Simple estimation of the masses of charm and beauty mesons and baryons are presented. The case of heavy–light pentaquark is also reviewed.

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The Belle detector at the KEKB asymmetric \(e^+e^-\) collider is not only an excellent experimental environment for the measurements connected to the neutral and charged \(B\) mesons, but also a rich source of different hadronic states containing charm quarks. We present discoveries of new and property measurements of several recently observed such hadron states. Some of them pose questions regarding their nature and represent a challenge for the description in terms of QCD. Specifically we address the properties of \(D_{sJ}\) mesons, newly observed \(X(3872)\) and \(Y(3940)\) states, analysis of the hadrons produced in association with a \(c\overline {c}\) system and searches for possible pentaquark states.

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Recent results from on hadronic spectroscopy are presented, based on data collected by the BaBar experiment between 1999 and 2004. The properties of the recently discovered \(D^*_{sJ}(2317)^+\) and \(D_{sJ}(2460)^+\) states are studied: resonance parameters and ratios of decay rates are measured from continuum \(e^+e^-\) production, and production rates are measured from \(B\) decays. A search for the \(D^*_{sJ}(2632)^+\) state whose observation has been recently reported by the SELEX Collaboration, and a search for a charged partner of the charmonium-like \(X(3872)\) state, are performed, yielding negative results. Finally, extensive searches for several pentaquark candidates, both fully inclusive and in \(B\) decays, result in no positive evidence.

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Selected CLEO results on hadron spectroscopy are reviewed.

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I present results of a heavy hadron chiral perturbation theory analysis of the decays and masses of the recently discovered excited charm mesons. The present data on the electromagnetic branching ratios are consistent with heavy quark symmetry predictions and disfavor a molecular interpretation of these states. I also discuss model independent predictions for the strong decays of pentaquarks in the \(\mathbf {\overline {10}}\) representation of SU(3) which can be used to constrain the angular momentum and parity quantum numbers of these states.

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The reaction \(pp \rightarrow pK^+ Y\) has been studied with the ANKE spectrometer at COSY-Jülich in order to investigate heavy hyperon production. The missing mass spectra MM\((pK^+)\) have been analyzed and compared with Monte Carlo simulations. Indications for a hyperon resonance \(Y^{0*}(1480)\) have been found.

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Preliminary results from exclusive production of \(\rho ^{0}\) mesons at the COMPASS experiment at CERN, using a 160 GeV polarized muon beam and a large polarized nuclear target, are presented. The studied reaction is exclusive incoherent \(\rho ^{0}\) production, \(\mu N \rightarrow \mu N \rho ^{0}\), with \(N\) being the nucleon in any of the nuclei of the target. The measurements were done in a wide range of \(Q^{2}\): \(0.01 \lt Q^{2} \lt 10\) GeV\(^{2}\), at \(\langle W \rangle \approx 10\) GeV. The results for \(r^{04}_{00}\), \(r^{1}_{1\,-1}\), \(r^{04}_{1\,-1}\) and \({\rm Im}\,r^{3}_{1\,-1}\) \(\rho ^{0}\) spin density matrix elements and the ratio \(R = \sigma _{\rm L} / \sigma _{\rm T}\), based on 700 000 events selected from the year 2002 data, are shown and commented.

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Radiating a photon from the initial state provides a useful tool for studying a range of low energy physics using a high-energy \(e^+ e^-\) accelerator. Accurate results require careful calculation of the first order virtual photon corrections. We compare exact results for initial state radiative corrections, finding agreement to an order of \(10^{-5}\) or better as a fraction of the Born cross-section for most of the range of photon energies, at CMS energies relevant in both high-energy collision and radiative return experiments.

abstract

Predictions for the radiative return with the muon pair and pion pair final state from KKMC and PHOKHARA Monte Carlo programs are compared and discussed. The case of muon pairs is well understood, especially of the initial state radiation (ISR), where three different second order calculations agree very well. The case of the final state radiation (FSR) requires more tests. Matrix element in KKMC of the EEX type with the incomplete second order NLL corrections is not good enough for the radiative return at \(Q^2\lt 1\) GeV with the precision requirement better than 1%. A method of extending the superior CEEX-type matrix element in KKMC to the pion pair final state is described.