Production and Searches for Cascade Baryons with CLAS

Transcription

1 Production and Searches for Cascade Baryons with CLAS Photoproduction Cross sections Ground State Ξ (1320) Excited State Ξ 0 (1530) Search for Cascade Pentaquarks Elton S. Smith CLAS Collaboration Jefferson Lab SENDAI

2 SU(3) Families of Hadrons Quarks q d s u Mesons qq K π K Baryons qqq Δ N Σ Ξ Ω Baryons built from meson-baryon, or qqqqq Θ + Φ -- Φ + 2

3 Conventional Baryons Electric Charge Q Strangeness S Strangeness S 3-Q Decuplet 3-Q Octet For exact SU(3) f the masses for the states in a multiplet would be identical. 3

4 Motivation Cascade physics under-explored Only 6 states with 3 or 4 stars in PDG Most without spin-parity assignment Narrower than S=-1 hyperons and N* Decouple from Ξπ decay (predicted by Chao, Isgur, Karl) Experimental verification needed (No BR measurement) Complimentary to N* program Isospin-violating mass differences of Ξ*: Do they agree with quark model/lattice predictions? Not feasible in N* sector M(u)-M(d)~3MeV, M(Ξ - ) M(Ξ 0 ) ~ 7MeV? 4

5 JLab accelerator CEBAF Continuous Electron Beam Energy GeV 200 µa, polarization 75% 1499 MHz operation Simultaneous delivery 3 halls A B C 5

6 CLAS Detector 6

7 Production mechanisms for photoproduction Hadronic model for Ξ production Effective lagrangian approach More appropriate at medium energy than quark fusion model Model proposed after preliminary CLAS data (2005) Related to hyperon spectroscopy (S= 1) γ p K + Κ - K + Y* Model also includes other processes (s-, u-channel ) Ξ Oh et al., PRC74(2006) H. Lee GlueX Workshop 7

8 CLAS Data: γp K + K + (X - ) Ξ Ξ Parasitic data taken during high statistics γp experiment. High energy beam and vertexing could improve precision. Guo et.al. PRC 76 (2007) dσ/dcos*θ Ξ (nb/sr) Data qualitatively consistent with t-channel hyperon production High spin hyperon resonances needed (J>=3/2) ps, pv couplings cosθ* Ξ 8

9 γp K + K + π - (X 0 ), X 0 =Λ 0, Ξ 0 Λ 0 Ξ 0 (M=1316.6± 0.6MeV) Isospin Multiplet ΔM (MeV) (PDG) π + /π D +/ D p/n Σ /Σ Ξ /Ξ ±0.24 Λ from Ξ - Λπ - decay CLAS:M(Ξ - )-M(Ξ 0 )=5.5±1.8MeV Only CERN-NA48 measurement for M(Ξ 0 ) has more than 50 events. 9

10 Total cross sections σ(nb) Ξ (1320) Ξ (1530) total cross section: E γ : GeV σ=1.13± 0.27nb E γ : GeV σ=1.77 ± 0.40nb 10

11 What can we say about higher excited states? PDG Excited cascades Mass (GeV) Width (MeV) BR to Ξπ Ξ 0 (1530) % Ξ 0 (1620) (*) ~22 Ξπ Ξ 0 (1690) (***) 1.69 <30 seen Ξ 0 (1620) plausible, but not significant Ramos, Oset, Bennhold: J p =1/2 - PRL.89:252001, (2002) K* background Non-Ξ events background Further study of excited states: 3/4-track trigger decay chain reconstruction Higher photon energies Additional detectors vertex and/or RICH 11

12 Summary Cascade physics offers rich opportunities Identify new states, determine J P of existing states Verify QM and LQCD predictions Mass splitting of Ξ doublet can be used for determining (ud) quark masses Cascade program requires Production: cross section x luminosity Detection: Large acceptance needed to establish resonances Outlook Larger statistical CLAS data sample (g12) in hand Promising program of measurements in Halls B and Hall D using the 12 GeV Upgrade 12

13 Search for exotic Ξ 5 or Φ (1860) Ξ 5 Single observation NA49 CERN SPS Phys. Rev. Lett. 92 (2004) Ξ 5 0 M=1.862± GeV Ξ(1530) Θ + Ξ 5 Ξ

14 Photoproduction Limits (Hermes) Bremsstrahlung beam from 27.6 GeV positrons Deuterium target Limits on M(Ξ - π - ) and M(Ξ - π + ) σ(ξ -- σ(ξ 5 ) x BR < 1.0 nb (2.1 nb) (90% CL) 50 ) x BR < 1.2 nb (2.5 nb) (90% CL) Ξ 5 0 Ξ

15 Photoproduction Limits (FOCUS) Bremsstrahlung beam from 300 GeV electrons BeO target σ(ξ -- 5 ) x BR/σ(Ξ * (1530) < (0.091) (95% CL) Limits on M(Ξ - π - ) and M(Ξ - π + ) Link PLB (2008) Ξ 5 -- p > 25 GeV 15

16 Quark lines for production of Ξ 5 γ Σ us K + us K + ud π n ddu Ξ 5 ssd Ξ Ξ 5 is composed of (ssddu) quarks flavor exotic 16

17 Jlab Experiment (eg3) Jlab Exp : 29 days of photon beam, 40cm long LD 2 target. E e = 5.77 GeV, I e = 30nA, radiator = 5x10-4 r.l. Photon energy range 4.5 < Eγ < 5.5 GeV. Trigger: main 3-tracks, and a prescaled 2-track trigger. Ξ 5 γ ( cτ = 4.9cm ) ( cτ = 7.9cm ) Look for enhancement in the M πππp invariant mass after Λ and Ξ mass and vertex constraints. 17

18 Sample Event in CLAS π - π - p π - π + or K + 18

19 Event Selection pπ Mass Λ (1116) σ =1.5 MeV S/B = Λ events Λπ Mass σ =2.1 MeV S/B = Ξπ events Σ - (1385) Ξ - (1321) Combine proton with one π - Pick the best Λ candidate Combine Λ with second π - Pick the best Ξ - candidate 19

20 Invariant Mass Distributions σ =2.1 MeV S/B = 0.7 Λπ π Λπ π 1700 Ξπ events Side-band subtraction Mass spectrum (Ξ - π - ) M=1862 MeV NO Narrow structure Visible 20

21 Results for Upper Limit PRELIMINARY Compare to hadronic model which include couplings to and HERMES UL Liu, Ko, PR C69 (2004) CLAS UL UL for σ σ (nb) PRELIMINARY Upper limit for γd Φ - - X Ξ π - X cross section at M~1.862 GeV is ~0.6 nb (90% CL) Reduced the upper limit on the cross section ~2nb from HERMES by factor of ~3. 21

22 Summary Cascade physics offers rich opportunities to study quark dynamics Photoproduction cross sections measured with CLAS detector Ground state Ξ 0 (1320) and Ξ *0 (1520) excited state Mass difference Ξ Ξ 5.5±1.8MeV Search for Ξ comes up empty-handed Limits set on exotic σ < 0.6 nb (90% CL) 22

23 Backup Slides 23

24 Results for Upper Limit 24

25 Estimates of cross section and rates In 20 days expect 46 events nb -1 <σ> σ ~ nb σ Γ Ξ γ K + K + π n Σ Ξ 5 Ξ Elton S. Smith SENDAI08, Sendai, Japan December W. Liu 14, and 2008 C.M. Ko, nucl-th/

26 Prospects at the 12 GeV Upgrade CLAS12 E 0 ~6.6 GeV: M(Ξ )~2.6 GeV Polarization measurement: production mechanism Large acceptance and good momentum resolution Search for cascade nuclei using nuclei target GLUEX E 0 ~8.8 GeV: M(Ξ )~3GeV Uniform acceptance essential for higher resonances Same data as the meson program (γ p reaction) Spin-Parity measurement Necessary to establish resonances Feasible at both CLAS12 and GLUEX (Double Moments) 26

27 126 GeV CEBAF add Hall D (and beam line) 27

28 Search by Hera-B collaboration Ξ(1530) HERA-B hep-ex/ Ξ π + + Ξ + π Ξ π + Ξ + π + Mass (GeV/c 2 ) HERA-B collaboration at DESY (Germany) Null result with high statistical sample They also have a null result for the Θ + 28