The search for the η -mesic nuclei in the LEPS2/BGOegg experiment

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

2 Theory η -nucleus otical otential U(r) = ( V 0 + iw 0 ) x NJL model : V 0 = 150 MeV (PRC 74 (2006) ) Exeriment CB-ELSA γ 12 C->η X unbound Comarison with theoretical calculation ρ(r) ρ 0 V 0 = 39±7(stat)±15(syst) MeV V 0 = Δm η (ρ 0 ) : η mass shift W 0 = -Γ(ρ 0 )/2 : η absortion linear sigma model : V 0 = 80 MeV (PRC 88 (2013) ) QMC model : V 0 = 37 MeV (PLB 634 (2006) 368) GSI 12 C->d X no PID (PLB 727(2013)417) (PRL 117(2016)202501) Uer limit Deending on the scale of the theoretical cross section calculation (n->dη cross section is not known)

3 γ η What we do η -mesic nuclei search by using BGOegg γ + 12 C -> η x 11 B + 12 C 1N absortion η side Missing Mass forward Tag decay roduct = back-to-back η air from 1N absortion of bound (stoed) η Data taken in 2015 (8.0x10 12 hotons) Blind analysis [mask : -100 < MM (γ, forward ) -M 11B -M η < 100 MeV] MM (γ, forward ) -M 11B -M η absortion MM resolution : 12~30 MeV => Cannot see eak structures => Comare yield below threshold with the theoretical calculation

4 η -nucleus otical otential What we do Contents Exerimental set u 1N absortion η, side selection cuts Particle identification cuts η side Kinematical cuts Signal selection cuts <= from QMD signal simulation BG reduction cut <= η angle Exected yield Quasi-free η data used for normalization of cross section 1/3-data (signal region masked) Summary γ 12 C η forward

5 SPring-8 Tagger e - BGOegg IPS GeV e - (1320 BGO crystals) (Inner Plastic Scintillator) 1m Exerimental set u γ + 12 C -> η x 11 B + η + -> η + LEPS2 beamline g 144 target forward η 24 η -> 2γ (br=39%) Side roton (50 MeV<E kin <250 MeV) η -> 2γ (normalization) ηπ 0 (BG study) DC side m RPC-TOF 3.2m Forward roton energy 2m

6 Energy deosit in IPS [arb.] η and side roton selection cuts Particle identification cut 2γ invariant mass 1/3 data η de(ips)-e(bgoegg) roton 1/3 data π Energy deosit in BGOegg [MeV] η Few combina torial BG => Multi π BG is strongly suressed M γγ [MeV] side roton 2 rotons (forward, side) => BG from rimary reactions is suressed Kinematical cut Remaining BG : reaction emitting η + side + forward γ->η γ->πη, π->π η->η πn->n γ->ππ, π->η Signal selection cut Select signal kinematics η (sto)->η BG reduction cut from BG data

7 η, interaction with nuclei Signal selection cut η, kinetic energy η oening angle η invariant mass Signal selection cut QMD (Quantum Molecular Dynamics) exerimental data reroduced well (PLB 639 (2006) 429) Signal simulation γ+ 12 C-> N*+ 10 Be + N*-> η with T. Maruyama γc->ηx data QMD η kinetic E η angle η momentum Eγ=750 MeV Inut : γ, forward momentum => MM(γ,) Remaining momentum is shared by N*+ 10 Be system => N* almost at rest => back-to-back η η- oening angle η escae rate from nuclei ~33% (both η, escae) η kinetic E / remaining E cos(η)

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

9 η olar angle η olar angle BG η γ γ->η γ->πη, π->π γ->ππ, π->η BG reduction cut η->η all cases : forward eak η γc->π 0 ηx data boost or signal γ from η at rest isotroic η angle distribution QMD signal sim. η η backward η selection signal selection cut η kinetic E / remaining E η kinetic E / remaining E

10 Yield estimation Calculation using Green s function method (by H. Nagahiro) Normalized by γ->η cross section ( GSI ex.) => Still absolute value of the cross section is not so reliable The sectra is searated to absortion and η escae (quasi-free) absortion 1N absortion η N -> πn, KΛ, KΣ, ηn Measure 2N absortion η NN -> NN Normalize the cross section by η escae event Obtain information of η branch (including η, escae rate from nuclei) from absortion 0<E ex -E 0 <60 MeV V 0 =100 MeV V 0 = 0 MeV Nucl. Phys. A 435 (1985) 727 absortion η escae E γ =2.5 GeV forward roton = 1 degree η escae absortion V 0 = 100, 50, 20, 0 MeV W 0 = 12 MeV Calculation u to L η =7 (E ex -E 0 < 60 MeV)

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

12 Exected yield MM<0 MeV 0<MM<60 MeV absortion absortion η escae V 0 (MeV) Original estimation 266*Br 94*Br 189*Br 154*Br Normalize by η escae ex) ηn Br = 0.4(1N)*0.8(ηN) 61*Br 19*Br 43*Br 31*Br ~ ~30 Normalized by η escae = original estimation ~1/4 -> BG suression is very imortant Obtain information of ηn branch (including escae rate from nuclei) from absortion 0<E ex -E 0 <60 MeV ~1/4

13 cos(η) 1/3-data before kinematical selection cuts -100<MM<100 MeV mask MM (γ, forward ) -M 11B -M η η- oening angle η angle vs kinetic E signal region MM [MeV] signal selection cut cos(η) BG reduction cut signal selection cut η kinetic E / remaining E 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 deendence of η angle distributions => MM deendent backward η selection cut will be determined soon

14 Summary We search for η bound state via missing mass sectroscoy of 12 C(γ, )X using LEPS2 We tag back-to-back η air from 1N absortion of bound η The yield is estimated by using Green s function method We normalize the cross section using η escae events We obtain info of η branch using η 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 deendence of BG events are being studied using 1/3-data After fixing all cuts, we will oen the box