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Observation of an Exotic Baryon with S
1 in Photoproduction from the Proton V. Kubarovsky,1,3 L. Guo,2 D. P. Weygand,3 P. Stoler,1 M. Battaglieri,18 R. DeVita,18 G. Adams,1 Ji Li,1 M. Nozar,3 C. Salgado,26 P. Ambrozewicz,13 E. Anciant,5 M. Anghinolfi,18 B. Asavapibhop,24 G. Audit,5 T. Auger,5 H. Avakian,3 H. Bagdasaryan,28 J. P. Ball, 4 S. Barrow,14 K. Beard,21 M. Bektasoglu,27 M. Bellis,1 N. Benmouna,15 B. L. Berman,15 N. Bianchi,17 A. S. Biselli,7 S. Boiarinov,20 S. Bouchigny,19 R. Bradford,7 D. Branford,12 W. J. Briscoe,15 W. K. Brooks,3 V. D. Burkert,3 C. Butuceanu,37 J. R. Calarco,25 D. S. Carman,7 B. Carnahan,8 C. Cetina,15 S. Chen,14 L. Ciciani,28 P. L. Cole,33 J. Connelly,15 D. Cords,3,* P. Corvisiero,18 D. Crabb,36 H. Crannell,8 J. P. Cummings,1 E. De Sanctis,17 P.V. Degtyarenko,3 H. Denizli,29 L. Dennis,14 K.V. Dharmawardane,28 C. Djalali,32 G. E. Dodge,28 D. Doughty,9 P. Dragovitsch,14 M. Dugger, 4 S. Dytman,29 O. P. Dzyubak,32 H. Egiyan,3 K. S. Egiyan,38 L. Elouadrhiri,9 A. Empl,1 P. Eugenio,14 L. Farhi,5 R. Fatemi,36 R. J. Feuerbach,7 J. Ficenec,35 T. A. Forest,28 V. Frolov,1 H. Funsten,37 S. J. Gaff,11 M. Garc¸on,5 G. Gavalian,25 G. P. Gilfoyle,31 K. L. Giovanetti,21 P. Girard,32 R. Gothe,32 C. I. O. Gordon,16 K. Griffioen,37 M. Guidal,19 M. Guillo,32 V. Gyurjyan,3 C. Hadjidakis,19 R. S. Hakobyan,8 D. Hancock,37 J. Hardie,9 D. Heddle,9 P. Heimberg,15 F.W. Hersman,25 K. Hicks,27 M. Holtrop,25 J. Hu,1 Y. Ilieva,15 M. M. Ito,3 D. Jenkins,35 K. Joo,10 H. G. Juengst,15 J. H. Kelley,11 M. Khandaker,26 K. Y. Kim,29 K. Kim,22 W. Kim,22 F. J. Klein,3 A.V. Klimenko,28 M. Klusman,1 M. Kossov,20 L. H. Kramer,13 S. E. Kuhn,28 J. Kuhn,7 J. Lachniet,7 J. M. Laget,5 J. Langheinrich,32 D. Lawrence,24 A. Longhi,8 K. Lukashin,3 R.W. Major,31,* J. J. Manak,3 C. Marchand,5 S. McAleer,14 J.W. C. McNabb,7 B. A. Mecking,3 S. Mehrabyan,29 J. J. Melone,16 M. D. Mestayer,3 C. A. Meyer,7 K. Mikhailov,20 R. Minehart,36 M. Mirazita,17 R. Miskimen,24 V. Mokeev,39 L. Morand,5 S. A. Morrow,5 M. U. Mozer,27 V. Muccifora,17 J. Mueller,29 G. S. Mutchler,30 J. Napolitano,1 R. Nasseripour,13 S. O. Nelson,11 S. Niccolai,15 G. Niculescu,27 I. Niculescu, 21 B. B. Niczyporuk,3 R. A. Niyazov,28 J. T. O’Brien,8 G.V. O’Rielly,15 A. K. Opper,27 M. Osipenko,18 K. Park,22 E. Pasyuk, 4 G. Peterson,24 S. A. Philips,15 N. Pivnyuk,20 D. Pocanic,36 O. Pogorelko,20 E. Polli,17 S. Pozdniakov,20 B. M. Preedom,32 J.W. Price,6 Y. Prok,36 D. Protopopescu,16 L. M. Qin,28 B. A. Raue,13 G. Riccardi,14 M. Ripani,18 B. G. Ritchie, 4 F. Ronchetti,17 P. Rossi,17 D. Rowntree,23 P. D. Rubin,31 F. Sabatie´ ,5 K. Sabourov,11 J. P. Santoro,35 V. Sapunenko,18 M. Sargsyan,13 R. A. Schumacher,7 V. S. Serov,20 A. Shafi,15 Y. G. Sharabian,38 J. Shaw,24 S. Simionatto,15 A.V. Skabelin,23 E. S. Smith,3 T. Smith,25 L. C. Smith,36 D. I. Sober,8 M. Spraker,11 A. Stavinsky,20 S. Stepanyan,38 I. I. Strakovsky,15 S. Strauch,15 M. Taiuti,18 S. Taylor,30 D. J. Tedeschi,32 U. Thoma,3 R. Thompson,29 L. Todor,7 C. Tur,32 M. Ungaro,1 M. F. Vineyard,34 A.V. Vlassov,20 K. Wang,36 L. B. Weinstein,28 A. Weisberg,27 C. S. Whisnant,32 E. Wolin,3 M. H. Wood,32 A. Yegneswaran,3 and J. Yun28 (CLAS Collaboration) 1

Rensselaer Polytechnic Institute, Troy, New York 12180-3590, USA 2 Vanderbilt University, Nashville, Tennessee 37235, USA 3 Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA 4 Arizona State University, Tempe, Arizona 85287-1504, USA 5 CEA-Saclay, Service de Physique Nucle´aire, F91191 Gif-sur-Yvette, Cedex, France 6 University of California at Los Angeles, Los Angeles, California 90095-1547, USA 7 Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA 8 Catholic University of America, Washington, D.C. 20064, USA 9 Christopher Newport University, Newport News, Virginia 23606, USA 10 University of Connecticut, Storrs, Connecticut 06269, USA 11 Duke University, Durham, North Carolina 27708-0305, USA 12 Edinburgh University, Edinburgh EH9 3JZ, United Kingdom 13 Florida International University, Miami, Florida 33199, USA 14 Florida State University, Tallahassee, Florida 32306, USA 15 The George Washington University, Washington, D.C. 20052, USA 16 University of Glasgow, Glasgow G12 8QQ, United Kingdom 17 INFN, Laboratori Nazionali di Frascati, Frascati, Italy 18 INFN, Sezione di Genova, 16146 Genova, Italy 19 Institut de Physique Nucleaire ORSAY, Orsay, France 20 Institute of Theoretical and Experimental Physics, Moscow, 117259, Russia 21 James Madison University, Harrisonburg, Virginia 22807, USA 22 Kyungpook National University, Daegu 702-701, South Korea

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Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA 24 University of Massachusetts, Amherst, Massachusetts 01003, USA 25 University of New Hampshire, Durham, New Hampshire 03824-3568, USA 26 Norfolk State University, Norfolk, Virginia 23504, USA 27 Ohio University, Athens, Ohio 45701, USA 28 Old Dominion University, Norfolk, Virginia 23529, USA 29 University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA 30 Rice University, Houston, Texas 77005-1892, USA 31 University of Richmond, Richmond, Virginia 23173, USA 32 University of South Carolina, Columbia, South Carolina 29208, USA 33 University of Texas at El Paso, El Paso, Texas 79968, USA 34 Union College, Schenectady, New York 12308, USA 35 Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0435, USA 36 University of Virginia, Charlottesville, Virginia 22901, USA 37 College of William and Mary, Williamsburg, Virginia 23187-8795, USA 38 Yerevan Physics Institute, 375036 Yerevan, Armenia 39 General Nuclear Physics Institute, Moscow State University, 119899 Moscow, Russia (Received 21 November 2003; published 21 January 2004; publisher error corrected 22 January 2004) The reaction p !  K  K  n was studied at Jefferson Laboratory using a tagged photon beam with an energy range of 3–5.47 GeV. A narrow baryon state with strangeness S  1 and mass M  1555  10 MeV=c2 was observed in the nK  invariant mass spectrum. The peak’s width is consistent with the CLAS resolution (FWHM  26 MeV=c2 ), and its statistical significance is 7:8  1:0. A baryon with positive strangeness has exotic structure and cannot be described in the framework of the naive constituent quark model. The mass of the observed state is consistent with the mass predicted by the chiral soliton model for the  baryon. In addition, the pK invariant mass distribution was analyzed in the reaction p ! K  K  p with high statistics in search of doubly charged exotic baryon states. No resonance structures were found in this spectrum. DOI: 10.1103/PhysRevLett.92.032001

The constituent quark model describes light mesons as bound states of a quark and an antiquark (qq ), and baryons as bound states of three quarks (qqq), where q is u, d, or s. It is a remarkable feature of meson and baryon spectroscopy that practically all well-established particles can be categorized using this naive model. At the same time, quantum chromodynamics (QCD) predicts the existence of so-called exotic mesons and baryons with more complicated internal structures. Exotic mesons may be classified such as glueballs (ggg), hybrids (qq g), and four-quark (qq qq ) states, and exotic baryons as qqqg or qqqqq  states. A baryon with strangeness quantum number S  1 is an excellent example of a particle whose exotic structure is manifest. Diakonov, Petrov, and Polyakov [1], in the framework of the chiral soliton model, have predicted a spin 1=2, isospin 0, and strangeness S  1 exotic baryon  with mass M  1:53 GeV=c2 . Also, the pentaquark state was considered in the quark models [2] and lattice QCD [3]. The possible quark structure of  is uudds. Experimental evidence for a narrow S  1 baryon state has been reported in the interactions of photons and kaons with nuclear targets: n ! K  K  n (on a 12 C target) [4], K  Xe collisions in the pK0 decay mode [5], and the exclusive photodeuteron interaction in the nK  decay mode [6]. In all these reactions the primary interaction is with a neutron in the initial state. The CLAS 032001-2

PACS numbers: 13.60.Rj, 14.20.Jn, 14.80.–j

Collaboration has reported the observation of the same state in the photoproduction reaction from the proton target [7]. The SAPHIR Collaboration [8] has also reported the observation of this state in the reaction p ! nK  Ks0 . A narrow peak with mass around M  1:54 GeV=c2 and width less than 25 MeV=c2 was observed and interpreted as the production and decay of the exotic  baryon. More recently, the ITEP Collaboration observed a narrow baryon resonance in the invariant mass of the pKS0 system formed in neutrino and antineutrino collisions with nuclei [9]. This Letter reports a more comprehensive study than in [7] of the  production on a proton target which includes data from three distinct runs under different experimental conditions in CLAS [10]. Two reactions, p !  K  K  n and p ! K  K  p, have been analyzed. Of the three runs, run (a) and run (b) had identical geometrical acceptance and trigger requirement with run (c) having slightly different running conditions. Runs (a), (b), and (c) had a tagged photon beam in the energy range of 3.2 –3.95, 3–5.25, and 4.8–5.47 GeV, respectively [11]. CLAS is a sixfold segmented toroidal magnetic spectrometer (details described in [10]). Run (c) triggered on the events with at least two out of six CLAS sectors having charged tracks, while runs (a) and (b) triggered on events with hits in opposite sectors. Runs (a) and (b) had the hydrogen target in the standard position, but in run (c) the target was moved upstream by 032001-2

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1 m to improve the CLAS acceptance in the forward direction, especially for the negative charged particles. The estimated integrated luminosity for the combined data of runs (a) and (b) is 2 pb1 , and run (c) is 2:7 pb1 . The combined analysis of these three runs offers access to a wider range of acceptance and energies. Events having a  , K  , and K  in the final state, identified by time of flight, were selected for the analysis of the reaction p !  K  K  n. The missing mass distributions for the reaction p !  K  K  X are shown in Fig. 1. A neutron peak is clearly seen in each of these distributions. The fitted masses of the neutron peaks are consistent with each other within 3 MeV=c2 . The mass resolution for runs (a) and (b) is about twice as good as in run (c) due to the fact that run (c) had higher energy and the magnetic field was reduced by a factor of 2. Events within 2 of the neutron peak were retained for each run individually, resulting in a total of 14 000 events. There are about 200 mesons in the selected sample, which were removed by eliminating events with the K  K  effective mass less than 1:06 GeV=c2 . The final nK  invariant mass spectrum calculated from missing mass in the reaction p !  K  X, combining data from all three data runs, is shown in Fig. 2. No obvious structure is seen in this spectrum. We explored various possible t-channel processes to understand the potential production mechanisms for the  as well as the background, examples of which are illustrated in Fig. 3. A peak appears most clearly when requiring cos  > 0:8, where   is the center-of-mass angle between the  and the photon beam. This requirement approximately corresponds to t < 0:28 GeV=c2 where t  k  p2 , k is the photon 4-momentum, and p 032001-3

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MM(π K K ) (GeV/c )

FIG. 1 (color online). The missing mass distributions in the reaction p !  K  K  X for the three different runs [runs (a), (b), and (c)] and the combined data spectrum [(d)]. The mass resolutions are 10, 11, and 19 MeV=c2 for runs (a), (b), and (c), respectively.

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FIG. 2. The nK  invariant mass spectrum in the reaction p !  K  K  n. The neutron was measured from the missing 4-momentum.

is the pion 4-momentum. This would correspond to an enhancement of the t-channel process as shown in Fig. 3(a). The spectrum is shown in the inset in Fig. 4. As a systematic check, we varied the pion angular cut from 0.7 to 0.9 and found that in all cases the peak was clearly visible. The background reaction p !  K  K  n is dominated by meson resonance production decaying to K  K  with the excitation of baryon resonances decaying to n , or meson resonance production decaying to K  K   , both with small momentum transfer to the

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FIG. 4 (color online). The nK  invariant mass spectrum in the reaction p !  K  K n with the cut cos  > 0:8 and cos K < 0:6.   and  K are the angles between the  and K  mesons and photon beam in the center-of-mass system. The background function we used in the fit was obtained from the simulation. The inset shows the nK  invariant mass spectrum with only the cos  > 0:8 cut.

meson system. These processes have the K  moving forward in the center-of-mass system [Figs. 3(b) –3(d) ]. To suppress such backgrounds, a cut was applied to eliminate events having a positive kaon going forward with cos K > 0:6, where  K is the center-of-mass angle between the K  and the photon beam. The remaining data sample is virtually free of the contaminating events that have baryons decaying to n in this final state since the  from such an event will most likely not move very forward in the center-of-mass system. The  peak was clearly observed in each of the three data sets; the resulting nK  mass spectra are combined and shown in Fig. 4. An investigation has been conducted to test whether a narrow peak in the nK  invariant mass spectrum can be artificially manufactured. First, we checked the sidebands around the neutron in Fig. 1; the resulting nK  effective mass distribution is structureless. We also considered the effect of the kinematic requirements that we applied by performing a Monte Carlo simulation based on nK  K   4-body phase space, nK  K 0 3-body phase space, and t-channel meson production. The meson events in the latter process are generated using K  K   3-body phase space and the shape of the K  K   invariant mass distribution from the data. We found no structure generated using the same cuts on the simulated events as applied on the data. Furthermore, one may consider whether some particular combination of meson waves can be reflected as a narrow peak in the nK  invariant mass distribution. We performed a full partial wave analysis of the K  K   meson system on run (c) data and utilized the prediction of this analysis to further probe the possibility of meson reflection into the nK  032001-4

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invariant mass spectrum. Again, we found that with a set of meson partial waves that well describes the entire data set we did not generate a narrow  peak when the same angular cuts as above are applied. The final nK  effective mass distribution (Fig. 4) was fitted by the sum of a Gaussian function and a background function obtained from the simulation. The fit parameters are N  41  10, M  1555  1 MeV=c2 , and   26  7 MeV=c2 (FWHM), where the errors are statistical. The systematic mass scale uncertainty is estimated to be 10 MeV=c2 . This uncertainty is larger than our previously reported uncertainty [6] because of the different energy range and running conditions and is mainly due to the momentum calibration of the CLAS detector and the photon beam energy calibration. The statistical significance for the fit in Fig. 4
over a 40 MeV=c2 mass p





window is calculated as NP = NB , where NB is the number of counts in the background fit under the peak and NP is the number of counts in the peak. We estimate the significance to be 7:8  1:0. The uncertainty of 1:0 is due to the different background functions that we tried. When a simple polynomial background is used, the statistical significance is higher. In the present analysis we used the background function obtained from the simulation as discussed above. The fact that the angular cuts we applied enhanced the  signal suggests the possible production of an N = that decays to  and K  . If the  is an isosinglet, the intermediate state can be only an N . The nK  K  invariant mass is shown in Fig. 5 for the events with nK  effective mass between 1.54 and 1:58 GeV=c2 . The apparent excess of events near 2:4 GeV=c2 is suggestive of an intermediate baryon state. A possible production mechanism that could contribute to the  production is shown in Fig. 3(a). Similar processes were also theoretically considered in Ref. [12]. The simulation we described previously also demonstrated that the angular cuts we applied could not generate a narrow peak in the nK  K  invariant mass spectrum from any of the three data runs. In addition, a search for a manifestly exotic baryon (Q  2, S  1) was performed in the reaction p ! K  X ; X ! pK  . There were 225 000 events with a proton and K  in the final state, which were selected for the analysis of this reaction. The K  was identified by the missing mass technique. After the removal of  ! K  K  and 1520 ! K  p, we observe no resonant structures in the pK  invariant mass distribution for the remaining 130 000 events. The pK invariant mass spectra for different cos K were analyzed as well, where  K is the angle between the K  and incident photon in the center-of-mass system. There are no resonance structures evident in any of these distributions. A more detailed analysis will be presented in a future paper. In summary, the reaction p !  K  K  n was studied at Jefferson Laboratory with photon energies from 3 to 5.47 GeV using the CLAS detector. A narrow 032001-4

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We acknowledge the outstanding efforts of the staff of the Accelerator and the Physics Divisions at Jefferson Laboratory that made this experiment possible. This work was supported in part by the U.S. Department of Energy, by the National Science Foundation, by the Istituto Nazionale di Fisica Nucleare, by the French Centre National de la Recherche Scientifique, by the French Commissariat a` l’Energie Atomique, by an Emmy Noether grant from the Deutsche Forschungs gemeinschaft, and by the Korean Science and Engineering Foundation. The Southeastern Universities Research Association (SURA) operates the Thomas Jefferson National Accelerator Facility for the United States Department of Energy under Contract No. DE-AC0584ER40150.

M(nK K ) (GeV/c )

FIG. 5 (color online). The nK  K  invariant mass spectrum calculated from the missing mass off the  in the reaction p !  K  K  n with the cuts cos  > 0:8 and cos K < 0:6.   and  K are the angles between the  or K  mesons and photon beam in the center-of-mass system. These events have MK  n between 1.54 and 1:58 GeV=c2 . The shape of the background curve was obtained from the simulation as discussed in the text. The inset shows the nK  K  invariant mass spectrum for all other events in Fig. 4.

baryon state with positive strangeness S  1, mass M  1555  10 MeV=c2 , and width  < 26 MeV=c2 (FWHM) was observed. The width is close to the experimental mass resolution of the CLAS detector. The peak’s statistical significance is 7:8  1:0. In addition, the pK  invariant mass distribution was analyzed in the reaction p ! K  K  p with high statistics. No resonance structures were found in this spectrum.

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*Deceased. [1] D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359, 305 (1997). [2] M. Karliner and H. J. Lipkin, Phys. Lett. B 575, 249 (2003); R. L. Jaffe and F. Wilczek, Phys. Rev. Lett. 91, 232003 (2003). [3] F. Csikor et al., J. High Energy Phys. 0311 (2003) 070. [4] T. Nakano et al., Phys. Rev. Lett. 91, 012002 (2003). [5] V. Barmin et al., Phys. At. Nucl. 66, 1715 (2003). [6] S. Stepanyan et al., Phys. Rev. Lett. 91, 252001 (2003). [7] V. Kubarovsky and S. Stepanyan, hep-ex/0307088. [8] J. Barth et al., Phys. Lett. B 572, 127 (2003). [9] A. E. Asratyan, A. G. Dolgolenko, and M. A. Kubantsev, hep-ex/0309042. [10] B. A. Mecking et al., Nucl. Instrum. Methods Phys. Res., Sect. A 503, 513 (2003). [11] D. Sober et al., Nucl. Instrum. Methods Phys. Res., Sect. A 440, 263 (2000). [12] P. Page, hep-ph/0310200.

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