Transcript Document
Study of excited baryons at BESII HongXun Yang Representing BES Collaboration IHEP [email protected] January 25-26, 2007, Outline • Introduction • FDC system for PWA • N* in the decay of J / p n • X(2075) and Nx in the decay of J / pK • Summary The Beijing Electron Positron Collider L ~ ~51030 /cm2s at J/ peak Ecm~2-5 GeV BESII Detector VC: xy = 100 m TOF: MDC: xy = 220 m BSC: dE/dx= 8.5 % p/p=1.78%(1+p2) T = 180 ps E/E= 22 % = 7.9 mr z = 3.1 cm counter: r= 3 cm z = 5.5 cm B field: 0.4 T Study of Excited Baryon States Motivation • Probe the internal structure of light quark baryons • Search for “missing” baryons predicted by quark model • Obtain a better understanding of the strong interaction force in the non-perturbative regime • Examples for FDC application J/ decays relatively large branching ratios (PDG2004) branching ratios(10-3) processes J / pp 0 J / pp J / p n J / pp J / pp ' J / pp 1.10.1 6.00.5 2.00.1 2.10.2 0.90.4 1.30.3 N* decays N * N N* N N * N N * N N * ' N N * N Pure isospin 1/2 Feynman diagram of the production of pN *, *, *, * For J / NN and J / NN , N and N systems are limited to be pure isospin 1/2. FDC system Physics model • Feynman rules • Counterms • physical parameters Physical process • Generate Feynman Diagram • Manipulate Amplitude and generate fortran source • Manipulate Kinematics and generate fortran source • Compile files and link sources Application of FDC • Loglikelyhood N L n 1 wdata w MC w | ci Ai |2 i • MC integration w MC 1 N N MC w k 1 k • Minimize lnL to get the parameters of each partial wave J/ψ pπ n from BES II data Events selection 2 good charged tracks Q1+Q2 = 0 |cos| < 0.8 PID: TOF and dE/dx Mp > 1.15 GeV 0.88<Mmiss<1.0 GeV Background < 8% N* in N*(1440) N*(1520) N*(1535) J/ψ pπ n N*(1650) N*(1675) N*(1680) ? L=0 limits it to be 3/2+ or 1/2+ N* in J/ψ pπ n Dalitz Plot: Acceptance and other reasons make the plot asymmetry. PWA with FDC • Try to determine the JPC of the resonance around 2.0 GeV/c2 • Following resonances are considered N(939), P11(1440), D13(1520), S11(1535), S11(1650), D15(1675), F15(1680), X • For the background: phase space and sideband Results of PWA (I) data fit Results of PWA (II) Results of PWA (III) • L = 0 is preferred due to the suppression of the centrifugal barrier factor for L 1 • For L = 0, JP is limited to be ½+ and 3/2+ • S decrease more than 400 if either ½+ or 3/2+ is included in the PWA fit • S decrease more than 60 if one of them are added while another has been included conclusion • The peak around 2.0 GeV/c2 cannot be reproduced by reflections of well-established N* resonances • Jp=1/2+,3/2+ is preferred by PWA J / pK c.c. Events selection • 4 good charged tracks • PID: kaon and at least 1 proton ID • 2 (4C)<20 • Bg rate:1~2% J / pK c.c. N* & * in (I) *(1520) *(1690) M pK N*(1535)? N*(1650)? M K • Two clear peaks at 1520, 1690 MeV/c2 in pK mass • N* in K mass X(2075) in J / pK c.c. N*(1535)? N*(1650)? X(2075) *(1690) *(1520) Phase Space • Bands for excited baryon states in Dalitz plot Possible N* and Λ* in PWA • mass(JP) N*: 1535 (1/2-), 1650(1/2-), 1710(1/2+), 1720(3/2+), 1900(3/2+ or 3/2-), 2050(1/2+ or 3/2+) Λ*: 1405(1/2-), 1520(3/2-), 1600(1/2+), 1670(1/2-), 1690(3/2-), 1800(1/2-), 1810(1/2+), 1890(3/2+) Mathematical fit • W/o any constrains, PWA with N* and Λ* can fit data: (S= -997) • However, it needs many unexpected big BRs and many large destructive interferences to cancel these big BRs. Big BRs Fraction of Ndata N*(1900) 3/2+ N*(2050) 3/2+ Λ*(1890) 3/2+ Λ*(1810) 1/2+ Λ*(1800) 1/2- 108% 33% 21% 9% 34% Nevent 5900 1800 1100 500 1900 (1/2- is P-wave decay, which should be suppressed ) Estimated Nevent in Ndata Nevent/2 N*(1900) 3/2+ N*(2050) 3/2+ Λ*(1890) 3/2+ Λ*(1810) 1/2+ Λ*(1800) 1/2- (each decay mode, not include c.c.) ~ 300 -400 ~ 100 -150 ~ 100 -150 ~ 150 -200 ~ 20 - 30 (1/2- is P-wave decay, which should be strongly suppressed near threshold) PWA with tight constrains and w/o X(2075) • Constrain the Nevent of near threshold states ~ 100-200 • The PWA fit cannot reproduce the enhancement near pΛ threshold (S=-900). PWA with tight constrains and with X(2075) • Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold. • Significance of X(2075) >> 5 sigma. (S=-952) PWA with looser constrains and w/o X(2075) Constrain the Nevent of near threshold states ~ 500-600 (already too big at least for Λ*(1810), Λ*(1890) as limited in J / c.c. ) The PWA fit can hardly reproduce the enhancement near pΛ threshold (S=-940). PWA with looser constrains and with X(2075) • Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold. • Significance of X(2075) > 5 sigma. (S=-962) conclusion of pΛ enhancement • Reproducing the pΛ threshold enhancement with pure N* and Λ* interferences needs many unexpected big BRs and large destructive interferences. • PWA fits with pure N* and Λ* and with constrains can hardly reproduce the enhancement. • PWA fit with X(2075) can easily reproduce the enhancement (independent of constrains) with high significance. A strong enhancement near the threshold of K c.c. mass spectrum of NX* BES II J / pK M K Λ (GeV/c 2 ) PS, eff. corrected (Arbitrary normalization) MKΛ MK M We perform PWA studies on the KΛ mass threshold structure: The most important we want to study is its production BR PWA is performed to J / pK possible N* and *states listed in PDG are fitted N(1720), N(1900), (1520), (1690), … many different combinations are tried different JP of Nx is tried also tried N(1535) to fit Nx An example of PWA fit • Mass and Width scan • Total fit (S=-952) • Nevent: Fraction Nevent NX 14.7% 799 N(1720) 17.1% 929 N(1900) 13.2% 717 (1520) 4.8% 261 (1570) 21.8% 1184 (1690) 14.4% 782 (1890) 13.8% 750 X(2075) 11.3% 614 Mass scan(GeV/c2) Width scan(GeV/c2) • NXN(1535) • Total fit (S=-932) • Nevent: Fraction Nevent N(1535) 26.0%1413 N(1720) 9.7% 527 N(1900) 11.4% 619 (1520) 4.8% 261 (1570) 22.2% 1026 (1690) 3.6% 739 (1890) 18.3% 994 X(2075) 11.2% 608 Events/10MeV Crosses: data Hist.: PWA fit projection Dalitz plot (PWA) Dalitz plot (data) JP check with various combinations • • • • • • • • JP A B C D E F G ½½+ 3/2- 3/2+ non -940 -848 -848 -930 -813 -845 -783 -806 -833 -752 -952 -841 -844 -916 -768 -880 -768 -752 -822 -650 -957 -889 -893 -944 -875 -970 -920 -925 -963 -919 -954 -925 -919 -944 -909 Fit results Cases Mass(GeV) Width(MeV) Fraction(%) Nevent Log Likelyhood a 1.52 ~ 1.62 110 22.3 1210 -940 b 1.56 80 ~110 44.4 2412 -845 c 1.62 70 14.7 799 -952 d 1.6 ~1.64 70 17.1 929 -880 e 1.57 90 20.6 1119 -957 f 1.62 70 ~ 90 19.9 1081 -970 g 1.58 80 15.6 845 -954 1.50 ~1.65 70 ~110 >14.7 >800 • A strong enhancement is observed near the mass threshold of MK at BES II. • Preliminary PWA with various combinations of possible N* and Λ* in the fits —— The structure Nx*has: Mass 1500~1650MeV Width 70~110MeV JP favors 1/2 consistent with N*(1535) The most important is: It has large BR(J/ψ pNX*) BR(NX* KΛ) 2 X 10-4 , suggesting NX*has strong coupling to KΛ. indicating it could be a KΛ molecular state (5 - quark system). Summary • FDC applied in the analysis of J / pK c.c. and J / p n c.c. • Clear signals of excited baryons observed • Seemed to be “missing” states observed in J / p n c.c. • Possible multi-quark states X(2075) and Nx in J / pK c.c. • PWA on Nx, N* and * in J / pK c.c. is still going on… Thank you!