in the LEPS2/BGOegg experiment N. Tomida (RCNP, Osaka Univ.) - PowerPoint PPT Presentation

The search for the η’ -mesic nuclei in the LEPS2/BGOegg experiment N. Tomida (RCNP, Osaka Univ.) 2018/Nov/13 QNP2018, Tsukuba

η’ -nucleus optical potential ρ(r) V 0 = Δm η’ (ρ 0 ) : η’ mass shift U(r) = ( V 0 + iW 0 ) x ρ 0 W 0 = - Γ(ρ 0 )/2 : η’ absorption • linear sigma model : V 0 = 80 MeV Theory (PRC 88 (2013) 064906) • NJL model : V 0 = 150 MeV • QMC model : V 0 = 37 MeV (PRC 74 (2006) 045203) (PLB 634 (2006) 368) Experiment • GSI • CB-ELSA no PID p 12 C->d X γ 12 C-> η’X (PRL 117(2016)202501) (PLB 727(2013)417) Depending unbound on the scale of the Comparison theoretical with cross section theoretical calculation calculation (pn-> dη’ cross section is V 0 = 39 ± 7(stat) ± 15(syst) MeV Upper limit not known)

What we do • η’ -mesic nuclei search by MM( γ ,p) @LEPS2 using BGOegg γ + 12 C -> η’ x 11 B + p • MM ( γ , forward p) -M 11B -M η’ Missing Mass 12 C forward γ p p η’ 1N absorption p p η absorption side • Tag decay product = back-to-back ηp pair from 1N absorption of bound (stopped) η’ • MM resolution : 12~30 MeV => Cannot see “peak structures” => Compare yield below threshold with the theoretical calculation • Data taken in 2015 (8.0x10 12 photons) • Blind analysis [mask : -100 < MM ( γ , forward p) -M 11B -M η’ < 100 MeV]

Contents • η’ -nucleus optical potential 12 C forward γ p • What we do p η’ 1N absorption • Experimental set up p • η, side p selection cuts p η side • Particle identification cuts • Kinematical cuts • Signal selection cuts <= from QMD signal simulation • BG reduction cut <= η angle • Expected yield • Quasi- free η’ data used for normalization of cross section • 1/3-data (signal region masked) • Summary

Experimental set up forward γ + 12 C -> η’ x 11 B + p 1.3-2.4 GeV side η’+ p -> η + p SPring-8 Tagger η LEPS2 beamline 24 ° 144 ° 7 ° g e - e - p target p 12.5m BGOegg DC (1320 BGO crystals) (Inner Plastic Scintillator) IPS RPC-TOF • η -> 2 γ (br=39%) • Forward proton • Side proton energy (50 MeV<E kin <250 MeV) 2m • η’ -> 2 γ (normalization) • ηπ 0 (BG study) 1m 3.2m

η and side proton selection cuts Particle identification cut Kinematical cut 2 γ invariant mass η Remaining BG : 1/3 data reaction emitting Few combina η torial BG η + side p + forward p => Multi π BG • γ pp-> ηp p is strongly • γ p- >πη p, πp - >π p suppressed ηp -> η p π pn->pn M γγ [MeV] dE(IPS)-E(BGOegg) • γ p- >ππ p, Energy deposit in IPS [arb.] πp -> ηp side proton 1/3 data 2 protons proton Signal selection cut (forward, side) Select signal kinematics => BG from π • η’(stop)p -> ηp primary BG reduction cut reactions is suppressed from BG data Energy deposit in BGOegg [MeV]

Signal selection cut γ C-> ηX η angle η momentum QMD (Quantum Molecular Dynamics) E γ =750 MeV • η, p interaction with nuclei data experimental data reproduced well QMD (PLB 639 (2006) 429) Signal simulation with T. Maruyama • γ + 12 C-> N*+ 10 Be + p Input : γ , forward p momentum => MM( γ ,p) Remaining momentum is shared by N*+ 10 Be system N*-> ηp => N* almost at rest => back-to-back ηp Signal selection cut η -p opening angle η kinetic E • η, p kinetic energy • ηp opening angle • ηp invariant mass ηp escape rate from nuclei • ~33% (both η,p escape) cos( ηp ) η kinetic E / remaining E

Using ratio to “remaining energy” side forward γ + 12 C -> η + p +X+ p remaining E = E γ +m 12C -m η -m side p -m 10Be -E forward p = Available energy for η, side p kinetic energy η kinetic E / remaining E η kinetic E η kinetic energy [MeV] η kinetic E/remaining E Selection 1.5σ line becomes independent on MM signal region MM [MeV] MM [MeV]

BG reduction cut η boost BG signal η γ p η’ p γ p p p • γ pp-> ηp p • γ p- >πη p, πp - >π p • from η’ at rest ηp -> η p or • γ p- >ππ p, • isotropic η angle distribution πp -> ηp • all cases : forward peak η η polar angle η polar angle QMD γ C-> π 0 η pX signal data sim. backward η selection signal selection cut η kinetic E / remaining E η kinetic E / remaining E

Yield estimation Nucl. Phys. A 435 (1985) 727 • Calculation using Green’s function method (by H. Nagahiro) • Normalized by γ p-> η’p cross section ( ⇔ GSI exp.) => Still absolute value of the cross section is not so reliable • The spectra is separated to absorption and η’ escape (quasi -free) η’N - > πN, KΛ, KΣ, ηN 1N absorption Measure • absorption η’NN -> NN 2N absorption • Normalize the cross section by η’ escape event • Obtain information of ηp branch (including η,p escape rate from nuclei) from absorption events @ 0<E ex -E 0 <60 MeV V 0 =100 MeV V 0 = 0 MeV E γ =2.5 GeV forward proton V 0 = 100, 50, 20, 0 MeV = 1 degree W 0 = 12 MeV η’ escape Calculation up to L η’ =7 absorption η’ escape (E ex -E 0 < 60 MeV) absorption

η’ escape (quasi -free) events • γ + C - > η’+ X + p • 2015 same data set 2 γ (br=2%) BGOegg MM γ ,p – M 11B - M η’ MM γ ,p – M 11B - InvM 2 γ invariant mass ~300 η’ Rising up from η’ threshold side band MM [MeV] MM [MeV] InvM [MeV] ~30 events @ 0<MM<60MeV ~300 events @ in all MM region (L η’ <~7)  Consistent with recent η’ -p coincidence measurement at ELSA (arXiv:1810.01288)

Expected yield MM<0 MeV 0<MM<60 MeV absorption η’ escape absorption V 0 (MeV) 100 50 100 50 100 50 Original estimation 266*Br 94*Br 189*Br 154*Br 118 136 Normalize by ~1/4 61*Br 19*Br 43*Br 31*Br ~30 η’ escape ex) ηN Br = 20 6 14 10 ~30 0.4(1N)*0.8( ηN ) • Normalized by η’ escape = original estimation × ~1/4 -> BG suppression is very important • Obtain information of ηN branch (including escape rate from nuclei) from absorption events @ 0<E ex -E 0 <60 MeV

1/3-data • before kinematical selection cuts • -100<MM<100 MeV mask η -p opening angle MM ( γ , forward p) -M 11B -M η’ η angle vs kinetic E cos(η) BG reduction signal cut signal region selection cut signal selection cut cos( ηp ) η kinetic E / remaining E MM [MeV] • About x20 BG events before kinematical selection cuts • Using both signal selection cuts and BG reduction cut, we can reduce BG enough to observe signals • We are now evaluating MM dependence of η angle distributions => MM dependent backward η selection cut will be determined soon

Summary • We search for η’ bound state via missing mass spectroscopy of 12 C( γ , p)X using BGOegg @ LEPS2 • We tag back-to-back ηp pair from 1N absorption of bound η’ • The yield is estimated by using Green’s function method • We normalize the cross section using η’ escape events • We obtain info of ηp branch using ηp events @ 0<MM<60 MeV • We defined signal selection cut condition using QMD signal simulation • We also define BG reduction cut (backward η selection cut) to remove remaining BG • MM dependence of BG events are being studied using 1/3-data • After fixing all cuts, we will open the box