First observation of N decay of S= system

T. Watanabe (Gifu University)

and KEK-E373 Collaborators

Brief status report of E373 Analysis of N decay event

– Event topology– Analysis

Discussion Summary

KEK-E373 experiment - capture events• 570 (90) events [preliminary]

estimated by the number of events of single- hypernuclei

in relation to that obtained by the previous exp. (KEK-E176).

– Double- hypernucleus (cand.) : 6– Twin single- hypernucleus : 2

– # stop is about 60 % of expected value

• Come from emulsion scanning efficiency

• 2nd analysis is underway

atom

28MeV

Hor

Λ

ΛΛ

ΛΛor H

p ⇒

Λ

Λ Λ

A

(S=-2)

5

6ΛΛHe

4He

t

Ξ -

p

π -

ΛHe

10μm500

10

5

Event topology

A

B0 5 10

(m)

10

5

0

#1

#3

#1

5.3±0.5m preliminary

#2

56.6±1.2mpreliminaryDecay angle #2#3:153.48±0.43o

#3

15539 m

Escaping from emulsion

#2

Particle identification of track #3

A

B0 5 10

(m)

10

5

0

#1

#2 #3

At point B• if mesonic decay of H (#2)

2 charged particles should be seen at Bonly 1 particle, denied

if non mesonic decay of H(#2)#3 is proton: Tp > 200 MeVQ-value of H ~170 MeVTp is too large, denied

Track#3 is from weak decay of hyperon. emul. = 1.60 MeV.g-1.cm2 (by SRIM2000) 　　 T = 66.0 ± 2.0 MeV (by dE/dx fitting, =5.1) To confirm this method,apply it to of the NAGARA event Tp = 26.1 ± 0.5 MeV (by dE/dx fitting, 2=3.4) Tp = 27.3 ± 0.5 MeV (by range, 2=4.0) consistent

Mass reconstruction of #2

A

B0 5 10

(m)

10

5

0

#1

#2 #3

= 153.48±0.43o

p (#2)

: KE=66.0±2.0 (MeV)

n: ???? (MeV)

KE() = 64 MeV

KE() = 68 MeV

M() = 1197MeV

M() = 1189MeV

T(#2): 22.6~29.6 MeV for

14.7~20.6 MeV for

Event reconstruction at point A(1)

A

B0 5 10

(m)

10

5

0

#1

#2 #3

Possibility of K absorption fromK- + 12C K+ + K- + 12

B• pK

-(beam)= 1.67 GeV/c (momentum bite 3.6%)

• pK+ = 0.987 GeV/c (max.=20 MeV/c)

• K- = 14.3°• TK- = 0.321 GeVRequired beam momentum is too high, denied

K

Event reconstruction at point A(2)

Track is truly - from (K-,K+) reaction?(miss connection SciFi emulsion)• candidates are rescanned 5 times• 11 candidates are picked up• No event of stop is found except this event

KE() : 22.6~29.6MeV KE() + KE(p) : 51.8~67.8 MeV

from (K-,K+) is absorbed at point A

+(p) X + Q1, X + p(included in #1) + Q2

X : or H

So far, the mass of X is not known. more analysis is still needed.

A

B0 5 10

(m)

10

5

0

#1

#2 #3

discussion• The first observation of an event

+ (p) (X) + p. capture as rest : 570 (±90) events

– several tens S=-2 nuclei ( estimated from KEK-E176)

Invariant mass of produced by (K-,K+) reaction in Scifi target.A peak is seen at the threshold.

?? H-dibaryon resonance ??(KEK-E522)

Theoretical approach

Possibilities of 1) mixing H 2) H-nucleus

• Decay rate ( p) for 6He [unit ]: ~10-3

(NOT H-dibaryon)References:1) K.Itonaga, T.Ueda and T.Motoba; NP. A691(2001) 1972) A.Parreno, A.Ramos and C.Bennhold; PRC65(2002) 0152053) K.Sasaki, T.Inoue and M.Oka; NP. A726 (2003) 349[4) H.Nemura, S.Shinmura, Y.Akaishi and K.S.Myint PRL94(2005) 202502]

• Decay rate(H p ) : several×10 %

Summary• An event showing weak interaction of double strangeness wa

s observed for the first time.• We can probably get information for the mixing.• We attempt to do particle identification of absorbed particle fr

om track profile in the emulsion– track volume– multiple scattering angle

• Near future– 2nd scan of KEK E373

( #stop: 560 (±90) 103 (designed value) )– J-PARC E07 ( #stop : 104 )