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

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

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

ωN scattering length ω photoproduction near threshold at FOREST [ref. Hashimoto’s doctoral dissertation (2011)] We are planning η’ version of this experiment. interaction is repulsive preliminary

Measurements of the scattering length The large η’N scattering length (-2.7 fm) will make threshold enhancement of total cross sections. Near-threshold σ tot measurement w/ high E γ resolution and statistics is necessary. [V. Crede et al., Phys. Rev. C 80, (2009)] σ tot spectrum by CBELSA ( ⊿ E γ is not sufficiently small.)

Cross section near threshold black : w/ interaction red : w/o interaction (T-matrix = 0) a = i0.0 fm r e = 0.25 fm

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

BGO experiment at LEPS2 BGO experiment has started data-taking from Dec Maximum 10 7 cps photon beam (1.4 GeV – 3 GeV) is available. BGO egg detector has 1320 crystals, and it covers from 24° to 144°. e-e- γ

γ-beam production UV laser (351 nm or 266 nm) is injected to the storage ring. Maximum 10 7 cps photon beam (1.4 GeV – 3 GeV) is available. Tagger e-e-

Experimental setup BGOEGG target (LH 2, 4cm) γ 24° 144 ° Inner Plastic Scinti. 4 m 12 m 7° TOF wall ( 〜 200 ps) γ p BGOEGG Drift Chamber RPC ( 〜 50 ps) γp  η’p

RPC proton ± 7° ± 4.3°

Detection of proton γ p η’ p detected by RPC (or TOF wall) η π 0 π 0 (br: 22 %) γ γ (br: 39 %) γ γ (br: 2.2 %) RPC (50 ps) TOF wall (200 ps) RPC (50 ps) TOF wall (200 ps) Eγ = 1.45 GeV Eγ = 1.46 GeV (br = branching ratio) Around Eγ = 1.45 GeV, protons are emitted at forward angle. detection efficiency of p : 〜 100 % by RPC

Detection of η’ γ p p η’ η π 0 π 0 (br: 22 %) γ γ (br: 39 %) γ γ (br: 2.2 %) BGO egg γ BGO egg acceptance for 2 γ acceptance for 6 γ

Yield estimation cross section of γp  pη’ : 0.3 ub (at E γ = 1.45 GeV) target : 4 cm length LH 2 beam 2.7 * 10 5 cps (1430 – 1500 MeV) branching ratio * acceptance : 〜 0.04 〜 events / 6months invariant mass of 2 γ invariant mass of 6 γ [V.Crede et al.(CBELSA), Phys. Lett. C 80, (2009)]

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) 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 : E γ = 1.45 GeV ⊿ E γ = 0.9 MeV by RPC resolutions of proton black : 6γ red : 2 γ

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

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

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

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

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

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 3σ confidence level at LEPS2. The sensitivity for small scattering length will be investigated η’ (near threshold) / half year at BGO experiment at LEPS2 ⊿ E γ 〜 1 MeV ( Kinematic Fitting ) σ tot depends on imaginary part of a and r e. – Theoretical help to evaluate imaginary part is necessary.

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Effects of virtual π N  η’p 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? γ p N π η’ p