An Experimental Search for Deeply Bound Kaonic Nuclei with Stopped Kaons Takatoshi Suzuki(U-Tokyo), for KEK-E471 collaboration Outline: 1. Introduction 2.Experiment and devices 3.Low level analysis 4. 4 He(K - stopped, p) spectra and discovery of S 0 (3115) 5. 4 He(K - stopped, n) spectra and indication of S + (3140) 6.Discussion and Conclusion

1. Introduction 1-0 Does deeply bound kaonic nuclear states with narrow widths exist ? -> Yes they do! * Wycech (cf. NPA 450 (1986) 399c ) coupled channel potential and in-medium t-matrix (from Martin) * Kishimoto (PRL 83 (1999) 4701) Atomic DD potential + SU(3) Chiral Lagrangian (from Batty-Friedmann-Gal) (from Waas-Kaiser-Weise) * Akaishi-Yamazaki (PRC 65 (2002) ) low energy KN scattering +  (from Martin + Iwasaki) -> No, they don’t! They must be shallow and broad. * Many authors.

1-1. AY predictions1 - Method of Calculation 1.Bare KN coupled channel potential constructed from (1) KN scattering (I=0,1) (2)  (1405) (I=0) 2.In-medium t-matrix -> g-matrix

1-2. AY predictions2 - Method of Calculation2 3. Consideration of nuclear “contraction” In strongly bound system, nuclear size parameter  may deviate from the ordinary value. Optimization of the nuclear size parameter  by the minimization of by introducing E core (  ) representing nuclear incompressibility -> Further binding!! Indication of dense nuclear system….

1-3. AY predictions3 - Isospin dependence of the kbar-nuclear states State g I=0 g I=1 Un-contracted (MeV) E K  Contracted (MeV) E K  3 K H(T=0) 3/ K H(T=1) 1/2 5/ K He(T=1/2) First prediction of a strongly bound narrow kbar-NNN state with T=0. Shallow and broad state with T=1 Y. Akaishi and T. Yamazaki, PRC 65 (2002)

He(K - stopped, n/p) reactions - population of the kbar - NNN states 4 He(K - stopped, n) T=0,1 formation channel -> (K He) T=0, (K He) T=1,Tz=0 Appearance of a narrow peak was expected. 4 He(K - stopped, p) T=1 formation channel -> (K H) T=1,Tz=-1 No narrow peaks were expected.

1-5. Experimental principle Kaonic helium4 formation from stopped K - Kaonic nuclear formation via nuclear Auger effect Neutron(proton) emission as a spectator Strong decay into mesonic or non-mesonic final states Triple coincidence of 1. Incident kaon 2. Auger nucleon 3. Secondary charged particle Missing mass spectroscopy.

2. Experiment Stopped K - method. 1. Incident K - and its track by beamline detectors - 2. secondary charged particle and its track by TC-VDC - 3. nucleon and its stop timing(TOF) by NC - are measured in coincidence. Those 3 signals are required hardwarewise K-beam & VTC OR &NC OR semi-inclusive spectra 2-1. Experiment - an overview M. Iwasaki et al., NIM A 473 (2001) 286

2-2. kaon beam line Secondary K - beam extracted through KEK-PS K5 * 2TP(Tera Proton)/spill(1.6 sec duration with 4 sec cycle) * 6cm thick Pt production target * p K ~ 650 MeV/c  p/p ~ 4 % * Wedge-Shaped-Degrader (stopped K + yield: 1.5 times enriched)

2-3. Beam line devices 1 - a side view Beam definition -> Scinitllators K/  separation -> Lucite Cherenkov Counters … K  ratio at T0: Kaon track detection -> Beamline Drift Chamber (BDC) ~ 4.7 k/spill ~ 0.88 M/spill

2-4. Beam line devices 2 - Beamline Drift Chamber * Planer-Shaped drift chamber for incident kaon track detection just before its stop. * 16 sensitive planes (8 for x-z tracking, 8 for y-z tracking ) with 8 normal- dashed modules. * Maximum drift length 2.5mm (wire spacing 5mm) with 32 sense wires covering 16cm*16cm of x-y plane with 512 channels. * Very thin to suppress multiple scattering of kaon. * Read-out by ASD(Amplifier-Shaper-Discriminator) cards.

2-5. Beam line devices 3 - Liquid helium4 target Only a brief summary…… * g/cm 3 super fluid helium 4 * Cell structure: cylindrical tube * 15 cm thickness (2.18 g/cm 2 ) * 23.5 cm diameter * Stable operation below 2.0 K

2-6. Vertex detector system - Vertex Drift Chamber * Feed-through type drift chambers for secondary charged particle track detection. * 12 sensitive planes (6 for y-x tracking, 6 for y-z tracking ) with 4 modules each consisting of 3 planes. * Hexagonal symmetric electric field. * 20 mm spacing of sense wires. * Covers 24.1 % of solid angle seen from the target center. * Read-out by ASD(Amplifier-Shaper-Discriminator) cards.

2-7. Neutron detector system - cross sections of NC and NCV Plastic scintillator segments to detect nucleon stop timing and position. * Two arms(32(E471)+ 24(SKS)segments) * 8.3 % of total solid angle * NCV to cover NC segments * Fe plate between NC and NCV to induce  e + e -

3. Low Level Analysis Track detection of incident K just in front of its stopping. z-x and z-y track detection is separately performed by independent two  2. Incident kaons are broadly distributed in their Position and direction Track detection of incident K Position resolution 0.13mm(  ).

y-x and y-z independent tracking under cylindrical approximation of the field. Position resolution 0.3~0.4mm (  ) 3-2. Track detection of secondary charged particle

Reaction vertex is defined by the two pre-defined tracks. Vca: vector connects two closest- approaching points, which represent final state hyperon motion DCA : |Vca| 3-3. Reaction vertex and DCA

With two-dimensional correlation between T0 pulse height and Vertex z position… Stopped K selection K+K+ K-K-

With reaction vertex position….. Events originate from outer materials are omitted Target fiducial cut

Stopped K - number estimated from Stopped K + number. Estimated stopped K - number: 65 M in 9.4 days Yield estimation of stopped K - Not very accurate. Triple coincidence has been imposed hardwarewise. Fairly accurate. detected number + solid angle + computer dead time

We measure the time difference between kaon injection onto T0 and nucleon detection on NC. With  ray events…. 1. Slewing correction factors 2. z-vertex correction factor (Flight time of K from T0 to its stop:) 3. Time origin in order to adjust 1/  for  events  are determined TOF analysis on NC (1)

 spectrum for neutral particles… TOF analysis on NC (2) Variation of the center and  of 1/  distribution...

Procedure… Charged particle is Selected by NCV hit. 2. Total light output, is obtained. 3. The proton events appear on the two- dimensional correlation between  and  E NC Proton PID on NC

PID of secondary charged Particle: Stage 1 TCthin dE/dx vs TOF Stage 2 (Fig.) dE/dx vs dE/dx Proton and pion are separated PID of secondary charged particle

For the events(pion) within the polygon, momentum value can be defined Secondary pion momentum cut We divide the pion events at 125 MeV/c -255 MeV/c (hypernuclear formation) exclusively appear on  -decay side.

V N : a unit vector along with detected hyperon motion VcaV N ~ 0 -> Quasi-free hyperon production or hypernuclear formation non-pionic absorption or kaonic nuclear formation and its non-mesonic decay >>0 -> hyperon decay Study of the hyperon motion in the final state - VcaV N

4. 4 He(K - stopped, p) spectra 4-1. Semi-inclusive momentum spectrum A significant peak structure Exists just below 500 MeV/c, on a continuum due to 1. Formation of a strange tribaryon via 2. Hypernuclear formation and its non-mesonic two-body decay 3.A fake due to instrumental bias? Discovery of a strange tribaryon S 0

4-2. Classification of the momentum spectrum 1 - by secondary charged particle ID and momentum (a)pion cut (b)proton cut (c)fast(  decay) pion cut (d)slow(  decay) pion cut

4-3. Classification of the momentum spectrum 2 - VcaVp selection Under pi-cut condition- (a)VcaVp : -6 ~ 0 -> hyper nuclear two-body decay (b)VcaVp : 0~+6 -> hyper nuclear two-body decay (c)VcaVp : -60~-6 -> Formation of a strange tribaryon and its non-mesonic decay (d)VcaVp: 6~60 -> nothing to generate mono- chromatic structure…

4-4. Classification of the momentum spectrum 3 - neutron coincidence 1. Strange tribaryon or Two-nucleon absorption -> on both 2. Hypernuclear decay -> only on n-anticoincidence 3. Instrumental bias -> on both ….. Hyper nuclear decay is NOT likely.

4-5. Summary of the analysis a): event topology of hypernuclear cascade b),c): indication of analysis 1,2

He(K - stopped, p) missing mass spectrum and S 0 (3115) Event-by-event energy loss correction -> Function fitting with P3 BG + Gaussian signal ->    DOF = 8.76/12 Gaussian center: MeV/c 2 Gaussian sigma: 8.7 MeV/c 2 Statistical significance: S/  S = 2441/297= 8.2  (Unexpected) Discovery of a Starnge Tribaryon S 0 (3115) T. Suzuki et al., PLB 597 (2004) 263

4-7. Systematic error of the mass and the natural width Systematic error: Source1: The deviation of Gaussian center due to the variation of BG functional shape ->  M 1 = 0.0 ~ -1.0 MeV/c 2 Source2: Uncertainty of TOF origin,  = ~ >  M 2 = +3.8 ~ -1.0 MeV/c >  M S 0 =  M 1 +  M 2 = +3.8 ~ -2.0 MeV/c 2 Natural width: Missing mass resolution is not clearly known, and the lower limit of the fitted Gaussian sigma is below the upper limit of the estimated resolution-> Only upper limit of the width has been estimated. Source1: Statistical error of the P3-BG fitting Source2: Systematic error of Gaussian  due to the variation of BG functional shape. Source3: Missing mass resolution lower limit 6.4 MeV/c 2 By simple de-convolution of the resolution… > 21.6 MeV/c 2 (by 95 % C.L.)

4-8. Decay modes and formation ratio - 1 Decay and formation branching ratio can be determined under several moderate assumptions….. Assumption 1. Pionic decay branch can be neglected, namely, Assumption 2. The strong decay is simply generated uniformly in the 3-body phase space. Assumption 3.    atomic formation is neglected (all weak decay). Then, detection probability of each decay modes in the realistic setup can be obtained by a Monte-Carlo simulation for each cut condition concerning the secondary charged particle.

4-9. Decay modes and formation ratio - 2 By using signal numbers (N i ) included in the spectra under 1   -decay cut - equation (i=1), 2   -decay cut - equation (i=2), 3 proton cut -equation (i=3), and simulated detection efficiency,  YNN i, decay branching ratio, B YNN, are obtained by solving ---->  dominance is shown. Formation branching ratio is determined by To large formation ratio to be assigned to the hyper nuclear two-body decay.

5. 4 He(K - stopped, n) spectra 5-1 Momentum referential peak Monochromatic neutron source from   stopped     n is expected in the stopped K - reaction at 185 MeV/c (1/  ). The peak position is exactly adjusted even for the events with small pulse height, and it also ensure the proton momentum scale as well as neutron.

5-2. Semi-inclusive neutron momentum spectrum Neutron semi-inclusive momentum spectrum at 10 MeVee threshold After constant BG subtraction…. The overall shape is well reconstructed by Monte-Carlo simulation. No clear peak-like structure is found in. The intensity over 400 MeV/c is exclusively due to non-pionic absorption process….

5-3. Classification of the spectrum by pion momentum An indication of a peak structure is found only in   -decay cut spectrum -> indication of another kind of strange tribaryon S + mainly decays into  ? -> further study by VcaVn selection is required…...

5-4. Classification by VcaVn and indication of S + Once decay mode and mass value has been estimated, we can set relevant VcaVn window to enhance the signal using Monte-Carlo. Comparison between (a) -25<VcaVn<-5 (HSN) AND -5<VcaVn<-2 or -60<VcaVn<-25 (LSN) (b) 5<VcaVn<25 AND 2<VcaVn<5 or 25<VcaVn<60 We find a clear enhancement of HSN spectrum on (a) -> Indication of S +

He(K - stopped, n) missing mass spectrum and S + (3140) A enhancement of the HSN missing mass spectrum at around 3140 MeV/c 2 -> An indication of another kind of strange tribaryon S + (3140) state. * No structure at around 3140 MeV/c 2 on the proton spectrum -> T=0. A global fitting to the HSN data points of 1 exponential BG + 2 Gaussian (S + (3140) and S + (3115))    DOF = 28.52/31 Gaussian center: MeV/c 2 Gaussian sigma: 7.3 MeV/c 2 Statistical significance: S/  S=120/32= 3.7  Consistent yield with S 0 (3115) M. Iwasaki et al., nucl-ex/ v2

6. Discussion and conclusion 6-1 Comparison between kaonic nuclear calculation S 0 (3115) = K - pnn T=1,Tz=-1, S + (3140) = K - ppn T=0 (= 3 K H) kaonic nuclear state? Problem?: 1. level reversing 2. large deviation of kbar binding energy up to 100 MeV. Other possibilities? Experiment: nucl-ex/ (v2), PLB 597 (2004) 263. Non-relativistic calc.: PRC 65 (2002) , PLB 535 (2002) 70.

6-2. Experiments for kaonic nuclear study in year 2005 For B=3…. 1. KEK E549(Iwasaki) - Study of 4 He(K - stopped, p) inclusive spectrum - Extension of E471 for 4 He(K - stopped, n) 2. KEK P570(Hayano) - Kaonic 4 He 3d->2p X-ray measurement - Extension of E471 for 4 He(K - stopped, n) For other baryon numbers… 1. FINUDA at Frascati(Nagae) - Invariant mass spectroscopy for B=2,3 - Missing mass spectroscopy for heavier nuclei 2.KEK E549(Kishimoto) - Missing mass spectroscopy via 16 O(K -, p) reaction

6-3. Conclusion 1. We have discovered a new kind of strange tribaryon S 0 (3115) with total isospin T=1 in the 4 He(K - stopped, p) reaction. 2. An indication of another kind of strange tribaryon S + (3140) with total isospin T=0 has been obtained in the 4 He(K - stopped, n) spectrum.