A quest for the Kpp bound state via 3. He(K, n) reaction, J-PARC E15 experiment

Transcription

1 A quest for the Kpp bound state via 3 He(K, n) reaction, J-PARC E15 experiment for E15 collaboration M. Iwasaki RIKEN Cluster for Pioneering Research 1 Meson Science Lab.

2 J-PARC E15 Strong KN attraction! Λ(1405) = K - p bound state? Many theoretical supports for the existence! Key questions : excellent introduction by Prof. Dote etc. - Can kaon (boson) be a member of nuclei? - Kaon properties change in nuclear media? - Size of kaon bound state? Could be a good probe for cold & dense QCD, to study the relation of hadron mass and χ-symmetry <qq> as QCD-Higgs condensation

3 Semi-inclusive forward n q = pn - pk q: virtual kaon momentum K - 3He n NC 1 GeV/c q CDS Does quasi-free kaon (= virtual kaon ) stick to two spectator q~ MeV/c

4 Semi-inclusive forward n accidental M(π Σp) M(Kpp) QF-K KN KN Free Kaon QF-K not clear signal observed, where we expected large yield ~ 1 mb/st below the threshold M(Kpp) but we larn KN interaction is: absorptive & attractive! QF-K is dominant KN KN

5 Exclusive: Λ p n K - p 3He simplest final state 3 baryon w/ strangeness n mis. 1 GeV/c p Λ q assuming KN KN CDS π - q: virtual kaon momentum - K + 3 He ( Λ + p ) + n PID Λpn final state w/ 4-momentum conservation

6 Λpn event selection event topology from ppπ - events with single cut b) lnl < 30 pn window p+n p+n missing n... missing K - lnl: product of 6 probability densities lnl has DCA=0? K - p vertex K - Λ vertex p π - vertex for Λ K - p vertex = K - Λ vertex M inv.pπ - = m Λ? M miss.ppπ - = m n? lnl [arb. ] n missing M [GeV/c 2 ] miss.pp n missing ( p+n ) 1 n missing ( p+n) ( p+n) DCA: distance of closest approach of two trajectories c)

7 data - E15 2nd q p [GeV/c] M inv. p [GeV/c2 ]

8 data Kpp QF - KA Sekihara Oset Ramos - E15 2nd q p [GeV/c] M inv. p [GeV/c2 ] Sekihara Oset Ramos

9 We introduce three model functions to fit Kpp, QFKA, - broad(bg) detector efficiency Λpn 3-body phase space physics process

10 M(Kpp) K - pp Sum of all q p [GeV/c] M inv. p [GeV/c2 ]

11 400 M(Kpp) KN KN, KNN Λp 300 data QF KA q p [GeV/c] forward K (or direct K) QF- KA backward K M inv. p [GeV/c2 ] 0.1 moving Gaussian with larger yields@ small & large q

12 data M(Kpp) K - pp Kpp Bound State q p [GeV/c] M inv. p [GeV/c2 ] MKpp w/ form factor

13 400 M(Kpp) broad BG 300 data BG q p [GeV/c] M inv. p [GeV/c2 ] cannot accommodate large lower q

14 M(Kpp) K - pp Sum of all q p [GeV/c] M inv. p [GeV/c2 ]

15 data q p [GeV/c] M inv. p [GeV/c2 ] 0.1

16 M(Kpp) K - pp focusing on Kpp dominant region 350 < q < 650 MeV/c q p [GeV/c] M inv. p [GeV/c2 ] QF-K escaping form region of higher q

17 M inv.λp q-selected n mis. + Λ p d /dm inv. p [nb/(mev/c 2 )] M(Kpp) 350 < q p < 650 MeV/c acceptance corrected Minv. p [GeV/c2 ]

18 conclusion A: Definitive peak observed B Kpp ~ 50 MeV, Γ Kpp ~ MeV, Q Kpp ~ 400 MeV/c arxiv: Three physical processes in Λpn final state Kpp, QFKA, - broad(bg) ( Kpp is consistent with S-wave)

19 Where is Λ*? We started study assuming Λ(1405) = K - p bound state p 3He n mis. K - if K - pp 1 GeV/c CDS p Λ π is formed Λpn final state arxiv: p 3He n mis. K - K - p must 1 GeV/c CDS n Σ Λ* π π be formed Λ*pn final state all preliminary

20 K - 3 He à E15 (π ± π n p) + n mis. ± p 3He n mis. 4-hold coin. w/ n-missing K - 1 GeV/c n Σ Λ* π CDS π Experimental challenge: neutron detection with plastic counter (t=3cm) n detection efficiency on CDH ~ 3% solid angle of CDH ~ 60% t = 3cm 10cm 79cm CDH x 55 more difficult than Λpn (ppπ - ) + n mis. 3-hold coin. 19

21 Neutron ID with CDH π + π - p events (3 tracks) in CDS with 4 CDH hits are selected a CDH hit with CDC-veto (outer-layer) is applied to identify the neutral hit de vs. 1/β < 1/β (p n <1GeV/c) 5MeVee < de neutron ID de-cut dependence preliminary n Neutron clearly identified by CDH

22 remove unfavored events ππnpn πσpn IM(nπ + ) vs MM(π + π - pn) Σ identification K 0 àπ + π - removed Σ + π - Σ + p n mis. IM(nπ - ) vs MM(π + π - pn) n miss preliminary π + Σ - p n mis. Λàpπ - M(Λ) removed n miss Σ - 21

23 Λ * (πσ)pn final state Event Selection Missing n: 0.85 < MM(π + π - pn) <1.03 GeV/c 2 Σ mass: 1.18 < IM(nπ - ) < 1.20 GeV/c 2 for Σ < IM(nπ + ) < 1.21 GeV/c 2 for Σ + counts [per 15 MeV/c 2 ] q Kn [GeV/c] M( ) M(K - p) M inv. [GeV/c 2 ] Λ * mass preliminary M inv. [GeV/c 2 ]

24 Λ * (πσ)pn final state Event Selection Missing n: 0.85 < MM(π + π - pn) <1.03 GeV/c 2 Σ mass: 1.18 < IM(nπ - ) < 1.20 GeV/c 2 for Σ < IM(nπ + ) < 1.21 GeV/c 2 for Σ + K - p smaller q in contrast to Λ(1520) counts [per 15 MeV/c 2 ] q Kn [GeV/c] M( ) M(K - p) M inv. [GeV/c 2 ] (1405) K - p (1520) M inv. [GeV/c 2 ] Λ * mass preliminary K - p = Λ(1405) / Λ(1520) = 3-quark baryon?? 3/2 -

25 Y* Cross Sec`on (q Kn <0.65 GeV/c) Λ(1405) ~ µb Flatté param.: m R ~ 1418 MeV/c g πσ ~ 1.9E-1 g KN ~ 1.7E-2 ~ µb [evaluated from Σ + (1385)!π + Λ measurement] d /dm inv.( [ b/(mev/c 2 )] (1385) M( ) M(K - p) M inv. [GeV/c 2 ] (1405) q Kn < 0.65 GeV/c (1520) acceptance corrected πσpn phases-space preliminary M inv. [GeV/c 2 ] Λ(1405) strength sharply drops at M(Kp)

26 M inv.λ*p - vs - M inv.λ* (Λ* πσ) M inv.λ*p d /dm inv. [ b/(mev/c 2 )] M inv. [GeV/c 2 ] M( p) M(K - pp) preliminary p kinematical limit M inv.( p [GeV/c2 ] M inv.λ*p M inv. all M inv. M(K - p) Λ(1520) (1405) 1.6 M inv.λ* = K - p M(K - p) M( ) d /dm inv.( [ b/(mev/c 2 )]

27 M inv.λ*p - vs - M inv.λ* (Λ* πσ) M inv.λ*p d /dm inv. [ b/(mev/c 2 )] M inv. [GeV/c 2 ] M( p) M(K - pp) preliminary p kinematical limit M inv.( p [GeV/c2 ] M inv.λ*p M inv. all M inv. M(K - p) Λ(1520) (1405) 1.6 M inv.λ* = K - p M(K - p) M( ) d /dm inv.( [ b/(mev/c 2 )]

28 Λ* + p = K - p + p 900 d /dm [nb/(mev/c 2 )] M(πΣp) > M(Λ*p) M(K - pp) kinematically, K - need to choose a proton to stick K He n+x X = p + "K - p" preliminary p + ( M inv. M(K - p) ) Minv. p [GeV/c2 ]

29 Λ* + p = K - p + p 900 d /dm [nb/(mev/c 2 )] M(πΣp) > M(Λ*p) M(K - pp) kinematically, K - need to choose a proton to stick K He n+x X = p + "K - p" preliminary p + ( M inv. M(K - p) ) Λ* is efficiently formed by kicking out another spectator proton Minv. p [GeV/c2 ]

30 K - pp Λp - vs - K - p + p πσ + p 900 d /dm [nb/(mev/c 2 )] K - can grab two protons same time M(πΣp) M(K - pp) X = "K - pp" kinematically, K - need to choose a proton to stick K He n+x X = p + "K - p" preliminary p + ( M inv. M(K - p) ) Minv. p / p [GeV/c2 ]

31 K - pp - vs - QF- KA 900 d /dm [nb/(mev/c 2 )] K - can grab two protons same time Kpp bound kinematically, K - need to choose a proton to stick K He n+x X = p + "K - p" part of QFKA? - preliminary p + ( M inv. M(K - p) ) Λp weakly by FSI Minv. p / p [GeV/c2 ]

32 conclusion B: Kp (=Λ*) formation observed with sharp drop at M(Kp) reaction summary: n + Λ* K - p + p n + Λ + p weakly by FSI n + (π + Σ) 0 + p K He n + K - pp n + Λ + p M(Kpp) virtual K - energy controls the reaction branch Kpp seems more like Kpp rather than Λ*p consistent picture w/ K - p & K - pp

33 yet another conclusion We wish to open a new era on K meson nuclear bound state Help us if you can!

34 The E15 Collaborations

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