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M easurement of the η’N scattering length at LEPS2

M easurement of the η’N scattering length at LEPS2. 2014/2/20 Keigo Mizutani Kyoto Univ. η’N interaction. Few experiments have measured the η’N interaction. absorption of η ’ is relatively small. . scattering length : order of 0.1 fm . .

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M easurement of the η’N scattering length at LEPS2

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  1. Measurement of the η’Nscattering length at LEPS2 2014/2/20 Keigo Mizutani Kyoto Univ.

  2. η’N interaction • Few experiments have measured the η’N interaction. • absorption of η’ is relatively small. • scattering length : order of 0.1 fm. • [P. Moskalet al., Phys. Lett. B 482, 356 (2000)] [M. Nanovaet al., Phys. Lett. B 710, 600 (2012)]

  3. γpη’p process η’ γ Tγpη’p Vγpη’p Vγpη’p Tη’pη’p + = Re(a) = -2.7 fm, Re(re) = 0.25 fm (bound state w/ 〜 6 MeV B.E.) [S. Sakai, D. Jido, Phys. Rev. C88, 064906 (2013)] η’ η’ γ γ p p Chiral effective theory p p p p phase shift effective range scattering length Imaginary parts are not calculated.

  4. ωN scattering length ωphotoproduction near threshold at FOREST preliminary preliminary interaction is repulsive • [ref. Hashimoto’sdoctoraldissertation(2011)] We are planning η’ version of this experiment.

  5. Measurements of the scattering length σtotspectrum by CBELSA (⊿Eγ is not sufficiently small.) • The large η’N scattering length (-2.7 fm) will make threshold enhancement of total cross sections. • Near-threshold σtot measurement w/ high Eγ resolution and statistics is necessary. • [V. Credeet al., Phys. Rev. C80, 055202 (2009)]

  6. Cross section near threshold a = -2.7 + i0.0 fm re = 0.25 fm black : w/ interaction red : w/o interaction (T-matrix = 0)

  7. Dependency on imaginary part a = -2.7 - i0.5 fm re = 0.25 fm a = -2.7 + i0.5 fm re = 0.25 fm black : w/ interaction red : w/o interaction a = -2.7 – i1.0 fm re = 0.25 fm a = -2.7 + i1.0 fm re = 0.25 fm

  8. BGO experiment at LEPS2 • BGO experiment has started data-taking from Dec. 2013. • Maximum 107 cps photon beam (1.4 GeV – 3 GeV) is available. e- γ • BGO egg detector has 1320 crystals, and it covers from 24° to 144°.

  9. γ-beam production • UV laser (351 nm or 266 nm) is injected to the storage ring. • Maximum 107 cps photon beam (1.4 GeV – 3 GeV) is available. e- Tagger

  10. Experimental setup 144° 24° 7° TOF wall (〜200 ps) BGOEGG γ BGOEGG Drift Chamber BGOEGG RPC (〜 50 ps) Inner Plastic Scinti. target (LH2, 4cm) p 12 m 4 m γp η’p γ

  11. RPC ± 7° ± 4.3° proton

  12. Detection of proton detected by RPC (or TOF wall) γ p η’ p Eγ = 1.45 GeV ηπ0 π0 (br: 22 %) γγ(br: 39 %) RPC (50 ps) TOF wall (200 ps) γγ(br: 2.2 %) (br = branching ratio) Eγ = 1.46 GeV • Around Eγ = 1.45 GeV, protons are emitted at forward angle. • detection efficiency of p : 〜 100 % RPC (50 ps) TOF wall (200 ps) by RPC

  13. γγ(br: 2.2 %) Detection of η’ acceptance for 2 γ γ p p η’ ηπ0 π0 (br: 22 %) BGO egg γγ γγ(br: 39 %) acceptance for 6γ BGO egg BGO egg

  14. Yield estimation [V.Credeet al.(CBELSA), Phys. Lett. C 80, 055202 (2009)] • cross section of γppη’ : 0.3 ub (at Eγ= 1.45 GeV) • target : 4 cm length LH2 • beam 2.7 * 105 cps (1430 – 1500 MeV) • branching ratio * acceptance : 〜0.04 〜10000 events / 6months invariant mass of 2 γ invariant mass of 6 γ

  15. Photon energy resolution • High photon energy resolution is necessary. • Kinematic Fitting • 6γ (or 2γ) invariant mass = η’ mass (1 constraint) • four-momentum conservation (4 constraints) resolutions of proton Eγ = 1.45 GeV by RPC • δP/P = 0.5 % • ( δt = 100 ps, L = 12 m for 0.7 GeV/c proton) • δθ = 0.005, • δφ = 0.005/tanθ ( δx = 50 mm ) • tagger resolution : • 15 MeV • BGOEGG resolution : ⊿Eγ = 0.9 MeV black : 6γ red : 2 γ

  16. Expected spectra w/ and w/o interaction a = -2.7 + i0.0 fm r = 0.25 fm 6 months black : w/ interaction red : w/o interaction Two histograms are different ( > 4σ ).

  17. Expected spectra w/ and w/o interaction a = -2.7 + i0.0 fm r = 0.25 fm 6 months black : w/ interaction red : w/o interaction Two histograms are different ( > 3σ ).

  18. a = -2.7 ± i0.5 [fm] distinguishable ( > 4σ ) a = -2.7 + i0.5 fm r = 0.25 fm black : w/ interaction red : w/o interaction a = -2.7 - i0.5 fm r = 0.25 fm

  19. a = -2.7 ± i1.0 [fm] distinguishable ( > 3σ ) a = -2.7 + i1.0 fm r = 0.25 fm black : w/ interaction red : w/o interaction a = -2.7 - i1.0 fm r = 0.25 fm

  20. How about a = -0.1 fm ? Not distinguishable a = -0.1 + i0.0 fm re = 0.0 fm black : w/ interaction red : w/o interaction black : w/ interaction red : w/o interaction

  21. Summary • Production cross section of η’ near threshold η’N cross section may enhance due to η’N interaction. • Large scattering length (-2.7 fm) has been predicted theoretically and we can measure with over 3σconfidence level at LEPS2. The sensitivity for small scattering length will be investigated. • 104η’ (near threshold) / half year at BGO experiment at LEPS2 • ⊿Eγ 〜 1 MeV ( Kinematic Fitting ) • σtotdepends on imaginary part of a and re. • Theoretical help to evaluate imaginary part is necessary.

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  23. Effects of virtual π Nη’p η’ γ • Diagrams as above are possible, and they may distort the shape of near-threshold cross section even if η’ and N do not interact. • γpπN ( few μbarn ) , πNη’p are small? π p p N

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