Weak decay of hypernuclei with FINUDA. Alessandro Feliciello I.N.F.N. - Sezione di Torino

Transcription

1 Weak decay of hypernuclei with FINUDA Alessandro Feliciello I.N.F.N. - Sezione di Torino

2 2 Outlook Introduction The FINUDA experiment Weak decay of hypernuclei mesonic nucleon stimulated rare decay channels Conclusions

3 3 Physical motivations see B.F. Gibson's talk see E. Oset's talk see H. Bhang's talk see W.M. Alberico's talk see C. Bennhold's talk see H. Outa's talk see R.L. Gill's talk

4 Hypernuclear physics at DAΦNE (brilliant) source of of monochromatic, collinear, background free, tagged neutral and and charged kaons KLOE CP, CP, CPT CPT violation chiral dynamics and and more... K S K L (3%) ρπ (13%) FINUDA hypernuclear physics K + K - (9%) DEAR K-N K-N scattering

5 The FINUDA experiment at DAΦNE 5 straw tubes interaction/target region clepsydra end cap LMDC beam pipe TOFONE superconducting coil compensating magnets magnet yoke KEK data C( C( K,, π - stop ) - ) 12 C Entries /.25 MeV/c MC MC data data m Y - m A [MeV] ~ 2 MeV MeV res. res. ~ MeV MeV res. res. ~.7.7 MeV MeV res. res.

6 6 Hypernucleus decay mesonic Pauli blocked in in medium-heavy hypernuclei* Γ m = Γ π + Γ π * A 5 partial rate transition N π Γ π n + π W Γ π p + π - non-mesonic Dominant in in medium-heavy hypernuclei* Γ = Γ + Γ + Γ nm n p 3N * A 5 partial rate transition N N Γ n + n n + n W π S Γ p + p n + p N Γ 3N * + N + N N + N + N * not taken into account in the following evaluations

7 7 The FINUDA strategy Carbon targets K stop C + C + π reference mark K + 1 K 2 y 1 cm x Lithium targets K 6 5 stop Li a a good good opportunity opportunity to to study study light light hypernuclear hypernuclear systems systems + He + p + π K 6 stop Li + He + p + n + π K 6 stop Li + H + p + p + π

8 8 How to distinguish the different hypernuclei ( H, H He, 5 He) the excellent momentum resolution ( p/p.3% FWHM at at 27 MeV/c) on on the π -- from HY formation will allow to to clearly identify 5 He He more difficult will be be the recognition between He He and H however the abundant production (Coulomb assisted) allows to to search for rare decays channels He π d + d p n + H 3 H 3 will be be identified through its decay mode He

9 9 mesonic decay kinematic features: 2-body decay A A Z (Z + 1) + A Z A Z +π π monochromatic p ~ 1 MeV/c n-body decay A Z A Z + π + xn x 1 π spectrum is peaked around 1 MeV/c The The FINUDA spectrometer is is not not designed to to study mesonic decay... π 3 π - - from from HY HY formation p = MeV/c MeV/c ρ max = max 8 8 cm cm both both π π - are - are measured measured in in coincidence K K π 1 cm µ + 6 y x π - - from from HY HY decay decay p = 1 1 MeV/c MeV/c ρ max = max 3 3 cm cm

10 anyway FINUDA can can measure π - - from mesonic decay by by means of of two two µstrip arrays 1 K 2 π - - from from HY HY formation p = MeV/c MeV/c 3 hits hits on on µstrip µstrip K cm y x π - - from from HY HY decay decay p = 1 1 MeV/c MeV/c poor poor momentum resolution p/p p/p % % (FWHM) due due to to multiple scattering Energy lost [MeV] ππ - - from from mesonic mesonic decay decay K ± good pid pid by by means of of de/dx in in µstrip p from non-mesonic decay π + from K + π + π µ + from K + µ + ν π - from HY formation e ± Momentum [MeV/c] Acceptance is is 2% 2% (3 (3 hits hits on on µstrip)

11 11 p-induced decay A ( A Z 2) ( Z 1) + p + n π π - -,, p and and n are are measured measured in in coincidence coincidence µ + 6 n 5 K K π 3 n n is is detected in in thick thick scintillators p ππ - - is is tracked p p is is tracked 1 cm y x reconstruction: proton acceptance 3% energy resolution 1.3 MeV (at 8 MeV) T.o.F. neutron acceptance 71% energy resolution ~ 1 MeV detection efficiency 1% measured measured 5 ps (FWHM) Entries /.5 MeV 3 2 by by T.o.F. T.o.F. (MC (MC data) data) / 6 Constant Mean Sigma Difference between sim. and rec. n kin. energy [MeV]

12 12 p-induced decay injected* Entries /.5 MeV n p Entries /.5 MeV reconstructed Entries /.5 MeV Injected nucleon kinetic energy [MeV] 2 n 2 p *A. Ramos et al., Phys. Rev. C (1997) reconstructed n kinetic energy [MeV] Reconstructed p kinetic energy [MeV] main background: π γγ from K + π + π (21%) K + π + π π (2%) both n n and and p p spectra from HY HY decay can can be be obtained! signal background signal background.6 2 without any selection selection criteria on candidate events: n T.o.F. p-n angular correlation p-n total energy signal 8% 8% background 2.5%

13 13 n-induced decay A Z ( A 2) Z + n + n π π - - and and 1 1 (2) (2) n are are measured measured in in coincidence coincidence ππ - - is is tracked π 3 1 cm µ + 6 n K + K 1 2 y n 5 x n n is is detected in in thick thick scintillators n n is is detected in in thick thick scintillators main background: π γγ from K + π + π (21%) K + π + π π (2%) signal background.2 without any selection signal background 11 selection criteria on candidate events: n T.o.F. n-n angular correlation n-n total energy signal 93% 93% background 1.5%

14 1 What FINUDA may do with 5 pb -1-1 (2 (2 sets sets of of targets) CC observable B.R. (%) collected events statistical error (%) high resolution hyp. spectr. for C g.s. τ hyp (Γ tot ) ~ 3 Γ π- /Γ.1 [1] ~ 5 Γ p /Γ.31 [1] ~ 3 only p detect Γ p /Γ.31 [1] 93 ~ 1 both p and n detected Γ n /Γ.58 [1] 8 ~ 1 both n detected Γ n /Γ p ~ 1 [1] H. Noumi et al., Phys. Rev. C52 (1995) CB an an equivalent number of of events for B ( C C excited states at at ~ 1 1 MeV)

15 15 Hypernuclear decay observables 12 C Γ tot Γ total decay rate (lifetime) (strength of the interaction) () 1.25 ±.18 Γ Γ proton induced decay rate +.11(2) p.31 ±.7 -. Γ Γ neutron induced decay rate n Γ Γ total non-mesonic decay rate (1) nm 1.1 ±.2 Γ π Γ Γ Γ n p mesonic (π ) decay rate (pion nucleus distortion) n/p induced decay rate (isospin structure of interaction) (1) J.J. Szymansky et al. Phys. Rev. C3, (1991) 89 (2) H. Noumi et al., Phys. Rev. C52 (1995) 2936 (3) A. Montwill et al., Nucl. Phys. A23 (197) 13 () R. Grace et al., Phys. Rev. Lett. 55(1985) ±.15 ±.3.1 ±.7 ±.3 (1) ( 1 ) ± ± ( 3). 52 ±. 16 ( 3) ( 2 ) (2) (2) Few measurements Few measurements with large errors with large errors

16 He d + d decay 16 ππ - - is is tracked d d is is tracked Entries / 1 MeV/c d d is is tracked reconstruction: deuteron acceptance 32% D 1 cm π 3 K + 1 µ + 25 K 2 y D 5 π π - - and and 1 1 (2) (2) d are are measured measured in in coincidence coincidence momentum resolution 2% (at ~ 57 MeV/c) T.o.F. 5 ps (FWHM) raw raw reconstructed reconstructed spectrum spectrum d reconstructed momentum [MeV/c] "forward" and and "backward" dd "forward" dd Entries / 1 MeV/c x 7.91 / 8 Constant Mean Sigma d reconstructed momentum [MeV/c]

17 He p + 3 H decay 17 t t is is tracked p p is is tracked ππ - - is is tracked T 5 π 3 1 cm π + 6 K 2 K π + γ 17 8 p γ y x 9 π π - -,, p and and t are are measured measured in in coincidence coincidence Entries / 1 MeV/c reconstruction: proton acceptance 38% momentum resolution 1% (at ~ 51 MeV/c) T.o.F. 5 ps (FWHM) triton acceptance 15% poor momentum resolution p reconstructed momentum [MeV/c] Entries / 1 MeV/c raw raw reconstructed reconstructed spectra spectra H reconstructed momentum [MeV/c] Entries / 1 MeV/c / 6 Constant Mean 5.3 Sigma p reconstructed momentum [MeV/c]

18 18 He p + 3 H decay main background: p from hypernuclei non-mesonic decay A ( A Z 2) ( Z 1) + p + n Entries / 1 MeV/c p reconstructed momentum [MeV/c] Entries / 1 MeV/c / 6 Constant Mean 5.3 Sigma p reconstructed momentum [MeV/c]

19 19 What FINUDA may do with 5 pb -1-1 (2 (2 sets sets of of targets) 5 12 He observable B.R. (%) collected events statistical error (%) τ hyp (Γ tot ) ~ 2 Γ π- /Γ. [1] ~ 1 Γ p /Γ.21 [1] ~ 2 only p detect Γ p /Γ.21 [1] ~ 6 both p and n detected Γ n /Γ.2 [1] 82 ~ 11 both n detected Γ n /Γ p ~ 11 [1] J.J. Szymansky et al. Phys. Rev. C3, (1991) He observable Γ d /Γ both d detected Γ t /Γ both p and 3 H detected B.R. (%) collected events statistical error (%) ~ ~ 23

20 2 Hypernuclear decay observables * * from V.J. Zeps, Nucl. Phys. A639 (1998) 261c [18] G. Keyes et al., Il Nuovo Cim. 31A (1976) 1 [21] J.J. Szymanski et al., Phys. Rev. C 3 (1991) 89

21 The case of He π + decay 21 He π + + n + H 3 experimentally: Γ π + / Γ 5% π G. Keyes et al., Il Nuovo Cim. 31A (1976) 1 C. Mayeur et al., Il Nuovo Cim. (1966) 698 G. Bohm et al., Nucl. Phys. B 9 (1969) 1 no no π + + decay mode! possible mechanisms: π + n π + p π + + n + p Σ + + n Σ + π + + n Γ π Dalitz Dalitz & & von von Hippel Hippel + / Γ 1% π Gibson & Gibson & Timmermans Timmermans + p Σ + + n Σ + + N π + + n + N π + emission is a s-wave process π + energy spectrum flat FINUDA could potentially collect a large number of of events (~ (~ ))

22 The case of He π + decay 22 e + 7 K + 1 π µ K 2 y µ + + stops and and decays τ ~ µs µs 1 cm x K + 1 π + + stops and and decays τ ~ ns ns π + µ + 5.1cm 29 < p π+ < 95 MeV/c: hard to be tracked p µ+ = 3 MeV/c: impossible to be tracked We need a clever strategy to to filter the events

23 23 Conclusions FINUDA is ready to be installed errors on measurements of some observables related to hypernuclei weak decay are 1 order of magnitude lower than for the existing data some rare decay channels can be measured for the first time the success of the FINUDA hypernuclear program depends on the DAΦNE luminosity

24 2 DAΦNE luminosity & FINUDA DAΦNE proposal 5 5 pb pb day day DAΦNE luminosity [cm -2 s -1 ] FINUDA proposal 5 5 pb pb week year HYP2 5 5 pb pb months