S Evidence for = +1 Pentaquark Baryon in Photoproduction from the Nucleon Quarks are ultimate building blocks of subatomic particles although an isolated quark has never been observed. According to the quantum chromodynamics (QCD), each quark can have three colors; red, blue, or green. Only the colorless combination (superposition) of quarks can travel freely as a particle. An example of such a combination is a proton, which is made of two ‘u’ quarks and one ‘d’ quark. The relatives of a proton which are made of three quarks are called baryons. A particle consisting of one quark ( q ) and one anti - quark ( q ) is called a meson. N ote that an anti - quark can have the anti - colors; magenta (‘anti - red’), violet (‘anti - green’), or yellow (‘anti - blue’) so that a qq pair can be colorless. M esons and hadrons together form a particle type called hadrons. There were no clear experimental evidences for the existence of a hadron with a quark configuration rather than three quarks (three anti - quarks) or a qq pair, although QCD does not forbid the existence of other combinations such as qq qq or qqqqqq . The absence of the hadron state with more than three quarks was one of the big mysteries in particle physics for decades. R ecently, we, the LEPS collaboration at beamline BL33LEP , have found evidence for a pentaquark state ( qqqqq ) with the strange quark number S = +1 ( F ig. 1 ). A baryon state with S = +1 cannot be made of three quarks since they must contain one anti - strange ( s ) quark. To make the baryon number equal to 1 , the minimal quark configuration should be qqqqs , where q stands for a ‘u’ quark or a ‘d’ quark. Fig. 1. Production of the pentaquark particle studied by the LEPS collaboration. I n 1997 , Diakonov, P etrov and P olyakov studied anti - decuplet baryons using the chiral soliton model [1] . The mass splittings of the established octet and decuplet were reproduced with an accuracy of 1% in this model, and the lightest member of the anti - decuplet with S = +1 , which we now call Θ + , was predicted to have a mass of 1530 M e V and a total width of less than 15 M e V . The S = +1 baryon in this mass region has not been searched in the KN scattering experiments in the past because the momenta of kaons are too high, as pointed out in R efs. [1 , 2] . This fact together with the very narrow predicted width motivated us to search for evidence of the Θ + at the LEPS facility. TOF wall Drift chambers Start counter Liquid hydrogen target Cherenkov counter Silicon vertex detectors The laser - electron photon ( LEP ) beam is generated by backward - Compton scattering of laser photons with the 8 G e V electrons [3] . The maximum energy of the beam is currently 2 . 4 G e V , which is well above the threshold for ss production. F igure 2 shows a schematic drawing of the LEPS detector. F or the determination of the momentum of a charged particle, tracking counters were placed before and after a 0 . 7 T magnet. A time - of - flight (TO F ) scintillator array was positioned 3 m behind the dipole magnet to measure the velocity of a charged particle. The mass of a c h a r g e d p a r t i c l e w a s r e c o n s t r u c t e d f r o m t h e momentum and velocity information. Fig. 2. The LEPS detector. It analyzes the momentum and velocity of charged particles in forward angles. 130 References [1] D. Diakonov et al. : Z . Phys . A 359 ( 1 997) 305 . [ 2 ] M . P o ly akov et al . : Eur . Phys . J . A 9 (2000) 11 5 . [ 3 ] T . N akano et al . : [ LEPS C o ll a b o r a t ion] , Nucl . Phys . A 684 (200 1 ) 7 1 c . [ 4 ] T . N akano , D. S . Ah n , J . K . Ah n , H . A ki mu n e, Y . As ano , W . C . Ch an g, S . Da té, H . Ej i r i , H . Fuj i mur a , M . Fuj i w a r a , K . H i c k s, T . H o tt a , K . Im ai , T . Ish ika w a , T . Iw a t a , H . K a w ai , Z . Y . K i m, K . K ino , H . K o hr i , N . Kum a g ai , S . M akino , T . M a tsumur a , N . M a tsu oka , T . M i be, K . M i w a , M . M i y a be, Y . M i y a ch i , M . M o r i t a , N . Mur a m a tsu, M . N ii y a m a , M . N o m a ch i , Y . Oh a sh i , T . O o b a , H . Oh k um a , D. S . Oshue v , C . R an g a ch a ryulu, A . S aka guch i , T . S a s aki , P . M . Sh a g in , Y . Sh iino , H . Sh i m i zu, Y . Sug a y a , M . Sum i h a m a , H . T o y oka w a , A . W akai , C . W . W an g, S . C . W an g, K . Y on eh a r a , T . Y o r i t a , M . Y o sh i mur a , M . Y o s oi an d R . G . T . Zegers: [ LEPS C o ll a b o r a t ion] , Phys . Re v. Lett . 91 (2003) 0 1 2002 . [ 5 ] V . V . B a rm in et al . : [D IANA C o ll a b o r a t ion] , Phys . At o m . Nucl . 66 (2003) 1 7 1 5 . [ 6 ] S . Step an y an et al . : [ CLAS C o ll a b o r a t ion] , Re v. Lett . 91 (2003) 25200 1. [ 7 ] J . B a rth et al . : [ SAPHIR C o ll a b o r a t ion] , a rX iv :hep- ex/0307083 . T aka sh i N akano Rese a rch Ce n ter f o r Nucle a r Phys i cs E-m ai l: nakano @rc n p .o s aka -u .a c . jp Fig. 3. K + missing mass spectrum for the signal sample (solid histogram) and for events from the LH 2 (dotted histogram) [4]. 0 5 10 15 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8 MM γ K – (GeV/c 2 ) c Events / (0.02 GeV/c 2 ) A 0 . 5-cm-th i c k pl a st i c sc in t i ll a t o r (SC) l o c a ted 9 . 5 cm d o w n stre a m fr o m the 5-cm-th i c k l i qu i d-hydr o ge n (LH ) t a rget e n sured th a t a t le a st on e ch a rged p a rt i cle i s pr o duced in the LH 2 t a rget . F o r the Θ + se a rch a t LEPS, the e v e n ts fr o m the SC were used t o study the e v e n ts ge n er a ted fr o m n eutr on s in the c a rb on n ucle i a t the SC, by se a rch in g f o r b a ry on res onan ces w i th the str an ge n ess qu an tum n umber S = + 1 in the K – m i ss in g m a ss spectrum in the γ n → K + K – re a ct ion [ 4 ]. S in ce the LH 2 2 t a rget c on t ain ed no n eutr on , e v e n ts fr o m the LH 2 t a rget were used t o est i m a te the b a c k gr o u n d spectrum . F i gure 3 sh o ws the K – m i ss in g m a ss d i str i but ion o f the s i g na l s a mple . A pr o m in e n t pe ak a t 1. 54 GeV ( 1 540 MeV) i s f o u n d . The br oa d b a c k gr o u n d ce n tered a r o u n d 1. 6 GeV i s m o st l ik ely due t o non -res onan t K + K – pr o duct ion an d the b a c k gr o u n d sh a pe in the reg ion a b ov e the pe ak h a s bee n f i tted by a d i str i but ion o f e v e n ts fr o m the LH 2 . The upper l i m i t f o r the w i dth w a s determ in ed t o be less th an 25 MeV . The o bser v ed na rr o w pe ak str on gly in d i c a tes the ex i ste n ce o f an S = + 1 res onan ce wh i ch m a y be a ttr i buted t o the ex o t i c 5-qu a r k b a ry on pr o p o sed a s the Θ + . S oon a fter a result on the Θ + w a s anno u n ced by t h e L E P S c o l l a b o r a t i o n , i t w a s c o n f i r m e d b y exper i me n ts a t ITEP [ 5 ] , Jeffers on L a b [ 6 ] , an d ELSA [ 7 ]. All exper i me n t a l results in d i c a te th a t the w i dth o f the S = + 1 b a ry on , wh i ch i s no w c a lled Θ + , i s v ery na rr o w . The b o th me a sured m a ss an d wh i ch a re in g oo d a greeme n t w i th a pred i ct ion by Diaknov et al . I n te n s iv e the o ret i c a l inv est i g a t ion o f the Θ + + i s in pr o gress . Further exper i me n t a l eff o rts t o determ in e the sp in an d p a r i ty o f the Θ a re cruc ia l . 131
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