Bulletin of the American Physical Society
2005 2nd Joint Meeting of the Nuclear Physics Divisions of the APS and The Physical Society of Japan
Sunday–Thursday, September 18–22, 2005; Maui, Hawaii
Session DE: Mini-symposium on Pentaquarks I |
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Sponsoring Units: DNP JPS Chair: Makoto Oka, Tokyo Institute of Technology Room: Ritz-Carlton Hotel Amphitheater |
Tuesday, September 20, 2005 7:00PM - 7:30PM |
DE.00001: Pentaquarks: do they exist or not? Invited Speaker: Pentaquarks, if they exist as narrow resonances, would provide a new testing ground for non-perturbative QCD. After an initial round of positive evidence for a strangeness S=+1 resonance at about 1535 MeV, called the $\Theta ^{+}$, there is a growing body of null results by high-statistics experiments. In addition, constraints from older KN scattering data suggest that the width of the $\Theta ^{+}$ resonance must be uncomfortably narrow ($<$1 MeV) if it has spin J=1/2. This has led some people to pronounce the $\Theta ^{+}$ to be ``dead''. However, the body of evidence is not yet complete. Several new experiments specifically designed to search for the $\Theta ^{+}$ in different reaction channels have yet to announce their results. Furthermore, it is possible that the $\Theta ^{+}$ could exist with spin J=3/2, as suggested by recent lattice gauge calculations, which allows for a more reasonable resonance width. In any case, convincing evidence for the $\Theta ^{+}$ is still lacking, and we should be skeptical about its existence at the present time. [Preview Abstract] |
Tuesday, September 20, 2005 7:30PM - 7:45PM |
DE.00002: $\Theta^{+}$ Search with $\gamma n\rightarrow pK^{0}_{s}K^{-}$ at Jefferson Lab Nathan Baltzell, David Tedeschi Two photoproduction experients in search of the $\Theta^{+}$ pentaquark have been conducted by the CLAS Collaboration in the last year at Jefferson Lab. Although the one on hydrogen has already reported finding zero evidence, theoretical estimates suggest that photoproduction on the neutron is an order of magnitude more likely. The other experiment by CLAS, with a deuterium target, has analyzed a variety of final states produced off the neutron. This talk will present an analysis in search of the possible reaction $\gamma n\rightarrow\Theta^{+}K^{-}$, with the pentaquark's decay to $pK^{0}_{s}$. Produced with a tagged photon beam of endpoint energy 3.6 GeV, the $pK^{0}K^{-}(p)$ final state is completely reconstructed by detecting the $\pi^{+}\pi^{-}$ decay of the neutral kaon and using missing mass for the proton. With well defined strangeness, no charged meson background, and a fully reconstructed final state, this channel is an important place to look for the $\Theta^{+}$. [Preview Abstract] |
Tuesday, September 20, 2005 7:45PM - 8:00PM |
DE.00003: Search for $\Theta^{+}$ via $\pi^-p \to K^{-}X$ reaction near production threshold Koji Miwa We have searched for $\Theta^{+}$ via $\pi^{-}p \to K^{-}X$ reaction using 1.87 and 1.92GeV/c $\pi^{-}$ beam at the K2 beamline of KEK-PS 12GeV Proton Synchrotron. We irradiated 2.9$\times 10^{9}$ and 3.0$\times 10^{9}$ $\pi^{-}$ beam to the SCIFI and the polyethylene targets respectively at beam momentum 1.87GeV/c. At 1.92GeV/$c$, 7.4$\times 10^{9}$ $\pi^{-}$ beam was irradiated to only polyethylene target. In the missing mass spectrum at beam momentum 1.92GeV/$c$, a bump was found at 1530MeV/$c^{2}$ which is consistent with the mass reported by several experiments. The statistical significance of this bump, however, was only 2.5$-$2.7$\sigma$(preliminary). Therefore we derived the upper limit of $\Theta^{+}$ production cross section via $\pi^{-}p \to K^{-}\Theta^{+}$ reaction and obtained 4.1$\mu$b(preliminary) at 90\% confidence level assuming that $\Theta^{+}$ was produced isotropically in the center of mass system. [Preview Abstract] |
Tuesday, September 20, 2005 8:00PM - 8:15PM |
DE.00004: Search for $\Theta^{+}$ via $p(K^{+},\pi^{+})$ reaction at KEK-PS-E559 Toshiyuki Takahashi, Seishi Dairaku, Ken'ichi Imai, Kouji Miwa, Tomofumi Nagae, Daisuke Nakajima, Sin'ya Sawada, Masashi Hayata The E559 aims to investgate the $\Theta^{+}$ via the $p(K^{+},\pi^{+})$ reaction using a liquid H$_2$ target and a high resolution spectrometer, SKS, at the KEK-PS K6 beamline. $\Theta^{+}$ would be identified in the missing mass spectrum with the resolution better than 2 MeV. In order to detect particles decayed from $\Theta^{+}$ and impove the S/N ratio, a large acceptance range counter system is installed in front of the SKS. The commissioning and data-taking just started this May. In this talk, analysis status as well as experimetal purposes and methods will be presented. [Preview Abstract] |
Tuesday, September 20, 2005 8:15PM - 8:30PM |
DE.00005: Photoproduction of Pentaquark on Deuteron Target Takayuki Nakamura, Makoto Oka In 2002, discovery of a five quark state ($\Theta^+$) was reported by LEPS (Laser Electron Photon at SPring-8) Collabolation in Japan. At present, a lot of detailed researches are being done from both sides of theory and experiment all over the world. We study production of $\Theta^+$ with $J=1/2,3/2$ in $\gamma+D\to\Lambda+\Theta^+$ reaction. This production process is the lowest energy reaction among two-body to two-body processes. We calculate the differential cross section and the total cross section of the $\gamma+D\to\Lambda+\Theta^+$ near the threshold energy. We assume that $\Theta^+$ has the mass $m_\Theta =1540\mathrm {MeV}$ and the width $\Gamma =10\mathrm{MeV}$. The cases of both negative and positive parities are considered, because the parity has not been determined from experiments. The cross sections are estimated in three cases, (1) for unpolarized photon and deuteron, (2) polarized photon and deuteron and (3) polarized photon and deuteron with the helicity of $\Lambda$ particle specified. [Preview Abstract] |
Tuesday, September 20, 2005 8:30PM - 8:45PM |
DE.00006: Phenomenology of spin 3/2 baryons with pentaquarks Tetsuo Hyodo, Atsushi Hosaka We examine several assignments of spin and parity for the pentaquark $\Theta^+$ state ($J^P=1/2^{\pm}, 3/2^{\pm}$) in connection with known baryon resonances. Assuming that the $\Theta^+$ belongs to an antidecuplet representation which mixes with an octet, we calculate the mass spectra of the flavor partners of the $\Theta^+$ based on the SU(3) symmetry. The decay widths of the $\Theta^+$ and nucleon partners are analyzed for the consistency check of the mixing angle obtained from the masses. It is found that a suitable choice of the mixing angle successfully reproduces the observed masses of $\Theta(1540)$ and $\Xi_{3/2}(1860)$, when their spin and parity are assigned to be $J^P=3/2^-$, together with other $J^P=3/2^-$ resonances. The decay widths of $\Theta\to KN$, $N(1520)\to \pi N$, and $N(1700)\to \pi N$ are also reproduced simultaneously. [Preview Abstract] |
Tuesday, September 20, 2005 8:45PM - 9:00PM |
DE.00007: Suppression of $\Theta ^{+}$ (J$^{P}$ = 3/2) photoproduction from the proton Seung-Il Nam, Atsushi Hosaka, Hyun-Chul Kim We investigate the photoproduction of $\Theta ^{+}$ from the proton and neutron, $\gamma $N $\to $Kbar$\Theta ^{+}$. Assuming that spin and parity of $\Theta ^{+}$ are J$^{P}$ = 3/2, it is shown that the production from the proton is strongly suppressed as compared with that from the neutron. This could provide a possible explanation for the null result of the recent CLAS experiment in finding $\Theta ^{+}$ via the reaction $\gamma $p $\to $Kbar$^{0}\Theta ^{+}$. [Preview Abstract] |
Tuesday, September 20, 2005 9:00PM - 9:15PM |
DE.00008: Anisotropic lattice QCD studies of spin 3/2 penta-quark Noriyoshi Ishii, Takumi Doi, Yukio Nemoto, Makoto Oka, Hideo Suganuma Anisotropic lattice QCD results for the penta-quark(5Q) $\Theta^+$ in $J^{P}=3/2^{\pm}$ channel are presented for a high-precision mass measurement using a large number of gauge configurations as $N_{\rm conf}=1000$. The standard Wilson gauge action at $\beta=5.75$ with the renormalized anisotropy as $a_{\rm s}/a_{\rm t} = 4$, and $O(a)$ improved Wilson (clover) quark action with $\kappa=0.1210(0.0010)0.1240$ are employed on a $12^3 \times 96$ lattice. Several Rarita-Schwinger interpolating fields with isospin $I=0$ are examined such as (a) the NK$^*$-type, (b) the (color-)fused NK$^*$-type, (c) a diquark-type. The chiral extrapolation leads to only massive states as $m_{5Q} > 2$ GeV. The analysis with the hybrid BC(HBC) is performed to investigate whether these states are compact 5Q resonances or not. No localized 5Q resonance states are found. [Preview Abstract] |
Tuesday, September 20, 2005 9:15PM - 9:30PM |
DE.00009: QCD sum rule for spin-$3/2$ pentaquarks Jun Sugiyama, Takumi Doi, Makoto Oka Most QCD-based-approaches, i.e, sum rules and lattice simulations, for pentaquark baryons have been done under the assumption that the pentaquarks have spin-$1/2$. But, the quark model calculations indicate possibility of spin- $3/2$. Because the $\Theta^+$ with $J^{\pi}=3/2^-$ decays into $D$- wave $NK$ states, this scenario may explain the narrow decay width of $\Theta^+$. Thus, we study the spin-$3/2$ pentaquarks using QCD sum rule technique. The spin-$3/2$ field is treated as a Rarita-Schwinger field. We consider two kinds of the diquark-type interpolating field operators and analyze which one is preferable. We perform parity projection and explore the existence of the pentaquark with $J^{\pi}=3/2^+$ and $J^{\pi}=3/2^-$. We find that $\Theta^+$ both of $3/2^+$ and $3/2^-$ are possible to exist, the $3/2^-$ state comes lower in energy than $3/2^+$ by about $60\mathrm{MeV}$ and their masses are around $1.5\mathrm{GeV}$, but they depend on the threshold parameters. We will report the results of the other pentaquark baryon. [Preview Abstract] |
Tuesday, September 20, 2005 9:30PM - 9:45PM |
DE.00010: Can the chiral quark soliton model describe the exotic baryon state? Yoichi Ohnishi, Masashi Wakamatsu We study the internal quark structure of the exotic baryon state $\Theta^+$ on the basis of an effective quark model, i.e. the SU(3) chiral quark soliton model, which has an intimate connection with the SU(3) Skyrme model. The model contains only one parameter, the dynamical quark mass $M$ which plays the role of the coupling constant between the quark fields and the chiral fields, i.e. the (composite) pions. We evaluate the distribution functions of the $\bar{s}$-quark as well as the $s$-quark in the $\Theta^+$ as a function of $M$. It turned out that the distribution of $\bar{s}$-quark has no valence-like structure and it is peaked around $x=0$. On the other hand, the $s$-quark distribution is found to violate the positivity bound in an intorelable way for the physically reasonable value of $M$ around $350 \,\mbox{MeV}$. The positivity of the $s$-quark distribution in the $\Theta^+$ is restored only for unrealistically large values of $M$ around $750 \,\mbox{MeV}$, close to the strong coupling limit. This analysis throws a little doubts on the validity of the chiral soliton pictures of the exotic Penta-quark baryons in contrast to the ordinary nonexotic baryons like the octet and decuplet baryons. [Preview Abstract] |
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