Evidence for a narrow |S|=1 baryon state at a mass of 1528 MeV in quasi-real photoproduction

• DOI: 10.1016/j.physletb.2004.01.079
• Publication Date: 2004-02-27T10:22:03Z
• Authors: A. Airapetian; N. Akopov; Z. Akopov; M. Amarian; V.V. Ammosov; A. Andrus; E.C. Aschenauer; W. Augustyniak; R. Avakian; A. Avetissian; E. Avetissian; P. Bailey; D. Balin; V. Baturin; M. Beckmann; S. Belostotski; S. Bernreuther; N. Bianchi; H.P. Blok; H. Böttcher; A. Borissov; A. Borysenko; M. Bouwhuis; J. Brack; A. Brüll; V. Bryzgalov; G.P. Capitani; T. Chen; X. Chen; H.C. Chiang; G. Ciullo; M. Contalbrigo; P.F. Dalpiaz; W. Deconinck; R. De Leo; L. De Nardo; E. De Sanctis; E. Devitsin; P. Di Nezza; M. Düren; M. Ehrenfried; A. Elalaoui-Moulay; G. Elbakian; F. Ellinghaus; U. Elschenbroich; J. Ely; R. Fabbri; A. Fantoni; A. Fechtchenko; L. Felawka; B. Fox; S. Frullani; G. Gapienko; V. Gapienko; F. Garibaldi; K. Garrow; E. Garutti; D. Gaskell; G. Gavrilov; V. Gharibyan; G. Graw; O. Grebeniouk; L.G. Greeniaus; I.M. Gregor; K. Hafidi; M. Hartig; D. Hasch; D. Heesbeen; M. Henoch; R. Hertenberger; W.H.A. Hesselink; A. Hillenbrand; M. Hoek; Y. Holler; B. Hommez; G. Iarygin; A. Ivanilov; A. Izotov; H.E. Jackson; A. Jgoun; R. Kaiser; E. Kinney; A. Kisselev; M. Kopytin; V. Korotkov; V. Kozlov; B. Krauss; V.G. Krivokhijine; L. Lagamba; L. Lapikás; A. Laziev; P. Lenisa; P. Liebing; L.A. Linden-Levy; K. Lipka; W. Lorenzon; H. Lu; J. Lu; S. Lu; X. Lu; B.-Q. Ma; B. Maiheu; N.C.R. Makins; Y. Mao; B. Marianski; H. Marukyan; F. Masoli; V. Mexner; N. Meyners; O. Mikloukho; C.A. Miller; Y. Miyachi; V. Muccifora; A. Nagaitsev; E. Nappi; Y. Naryshkin; A. Nass; M. Negodaev; W.-D. Nowak; K. Oganessyan; H. Ohsuga; N. Pickert; S. Potashov; D.H. Potterveld; M. Raithel; D. Reggiani; P.E. Reimer; A. Reischl; A.R. Reolon; C. Riedl; K. Rith; G. Rosner; A. Rostomyan; L. Rubacek; J. Rubin; D. Ryckbosch; Y. Salomatin; I. Sanjiev; I. Savin; A. Schäfer; C. Schill; G. Schnell; K.P. Schüler; J. Seele; R. Seidl; B. Seitz; R. Shanidze; C. Shearer; T.-A. Shibata; V. Shutov; M.C. Simani; K. Sinram; M. Stancari; M. Statera; E. Steffens; J.J.M. Steijger; H. Stenzel; J. Stewart; F. Stinzing; U. Stösslein; P. Tait; H. Tanaka; S. Taroian; B. Tchuiko; A. Terkulov; A. Tkabladze; A. Trzcinski; M. Tytgat; A. Vandenbroucke; P. van der Nat; G. van der Steenh...; M.C. Vetterli; V. Vikhrov; M.G. Vincter; C. Vogel; M. Vogt; J. Volmer; C. Weiskopf; J. Wendland; J. Wilbert; G. Ybeles Smit; Y. Ye; Z. Ye; S. Yen; W. Yu; B. Zihlmann; H. Zohrabian; P. Zupranski
• Keywords: Isovector; Pentaquark; Invariant mass

Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel p K^0_S --> p pi^+ pi^-. A peak is observed in the p K^0_S invariant mass spectrum at 1528 +/- 2.6 (stat) +/-2.1 (syst) MeV. Depending on the background model,the naive statistical significance of the peak is 4--6 standard deviations and its width may be somewhat larger than the experimental resolution of sigma=4.3 -- 6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Theta^{+}(uuddbar(s)) pentaquark baryon. No signal for an hypothetical Theta^{++} baryon was observed in the pK^+ invariant mass distribution. The absence of such a signal indicates that an isotensor Theta is excluded and an isovector Theta is unlikely.<br/>8 pages, 4 figures<br/>