1. 15 cm Plots of missing mass spectrum and 90% interval for width of 0.5 and 10 MeV. Color lines show upper limit, lower limit and sensitivity. Search for Pentaquarks in Jefferson Lab Hall A Y. Qiang 1, J.-O. Hansen 2, P. Reimer 3, B. Wojtsekhowski 2 For the E04-012 and Hall A Collaborations 1 Massachusettes Institute of Technology, Cambridge, MA 2 Thomas Jefferson National Accelerator Facility, Newport news, VA 3 Argonne National Laboratory, Argonne, IL In 2002, people in Spring-8 experiment at LEPS reported the first observation of an exotic pentaquark state  + in  n scattering. The key properties of pentaquarks in the chiral soliton model are:  Narrow widths,  < 15 MeV/c 2 ;  Low mass, just above kinematics threshold;  Some of them are exotic. Typical Baryon:   Exotic Pentaquark:     signal from LEPS Left HRS:  Aerogel 1 & 2 Cherenkov Counters;  Ring Imaging CHerenkov, RICH;  Lead Glass Shower Counters. Right HRS:  CO 2 Gas Cherenkov Counter. RICH detector has a very good Cherenkov angle resolution  ~ 6 mrad, which reduced pion contamination to less than 5 %. After PID selection, coincidence TOF Spectrum showed very clean kaon and pion peaks. Longitudinal target vertex coincidence was used to reduce accidental background. The calibration from n,  and  productions showed excellent missing mass resolution:  = 1.5 MeV/c 2, which gave us a very high sensitivity to narrow structures. Visual inspection shows no significant narrow structures in any of the three channels, so a Feldman-Cousins M.C. analysis was performed to extract the 90% confidence interval with assumed width  = 0.5~10.0 MeV/c 2. This approach will automatically distinguish confidence interval type between band and pure upper limit. In most places, only upper limits, blue, were obtained. Lower limits, red, occur occasionally, however they all lie below the green sensitivity curves (90% probability of background fluctuations). In conclusion, no evidence of narrow pentaquarks was observed, and all signals seen are consistent with background.  (MeV/c 2 ) 0.52.05.010.0Mass Range (MeV/c 2 )   (nb/sr) 10.0 (2.3%)11.0 (2.6%)13.0 (3.1%)17.5 (4.2%)1550 - 1810   (nb/sr) 4.5 (1.1%)5.5 (1.3%)6.0 (1.4%)10.5 (1.5%)1610 - 1880   (nb/sr) 3.0 (0.7%)3.5 (0.8%) 4.0 (1.0%)1480 - 1590 Final spectrum of  0 channel, curves show the fit of  (1520). Number of events from different kinematics settings as a function of missing mass. The major advantages of using Hall A HRS are:  High energy resolution;  High luminosity electron beam. Freon Radiator (n = 1.28) CsI Pad Photocathode MWPC   What are Pentaquarks ? Particle Identification Combining Spectra Coincidence System Jlab Experiment E04-012 High Resolution Experimental Results Table of differential cross section upper limits as a function of width as well as ratio compared to  (1520) photoproduction cross section: 417 nb/sr. Typical Baryons have only 3 valence quarks. In 1997, in the framework of a chiral soliton model, D. Diakonov et al. predicted an SU(3) F antidecuplet of pentaquarks and the members of this group have five quark component. Chiral Soliton Model Iso-spin Partners Non-zero Iso-spin of  + was used to explain its unusual narrow width by violating strong decay (Capstick 2003). Why JLab Hall A ? The partners of   :  0, N 0 and  ++ were searched in the missing mass spectra of the following three channels. The high resolution of the HRS pair however comes at the cost of a very limited acceptance. To expand the range of our missing mass search, we have to combine spectra from different kinematics. During the combination, we transformed the counts into differential virtual photoproduction cross-section in CMS. Here we measured: (PDG: 15.6 ± 1.0 MeV/c 2 )