Search for neutron-rich hypernuclei in FINUDA: preliminary results

1. Search for neutron-rich hypernuclei in FINUDA: preliminary results presented byM. Palomba1 for the FINUDA Collaboration 1INFN and Dipartimento di Fisica, Bari, Italy DAFNE2004: Physics at meson factories Laboratori Nazionali di Frascati, June 8th, 2004

2. Outline • Introduction to neutron-rich hypernuclei: physics interest, state of the art • Neutron-rich hypernuclei production in FINUDA • Data analysis description • Preliminary results • Conclusions and prospects Search for neutron-rich hypernuclei in FINUDA: preliminary results

3. Introduction Z N = Z • The existence of hypernuclei with a large neutron excess (N/Z[HYP]~ 2 N/Z[ORDINARY NUCLEI]) has been theoretically predicted [L. Majling, NP A 585 (1995) 211c] • The L hyperon does not suffer the Pauli principle constraints   “extra-binding” energy a larger number of neutrons may be bound with respect to ordinary nuclei • Search for the not yet observed neutron-rich L hypernuclei with very large N/Z and study of theirnuclear structure; links to the astrophysics field (phenomena related to high-density nuclear matter in neutron stars) BL (MeV) xxx = unstable core N to be searched in FINUDA observed not observed From: Nuclear Wallet Cards 2001, NNDC, BNL L. Majling, NP A 585 (1995) 211c Search for neutron-rich hypernuclei in FINUDA: preliminary results

4. Neutron-rich hypernuclei production in FINUDA (K-stop, p+) reaction • Two elementary reactions channels: • Double charge exchange • 2-steps reaction K- + p  L+p0; p0 + p  n + p+ • Strangeness exchange + L-S coupling • reaction K- + p  S-+ p+ (S- + p L + n) • Global reactions: • 12C target: K- + 12C  12LBe + p+ • 6Li target: K- + 6Li  6LH + p+ • 7Li target: K- + 7Li  7LH + p+ • A p+ is produced in the final state • The p+ momentum contains the information on the L binding energy BL inside the neutron-rich hypernucleus Search for neutron-rich hypernuclei in FINUDA: preliminary results

5. 12LBe, 6LH, 7LH: known data • At present our knowledge about neutron-rich hypernuclei on the experimental side is rather poor: Search for neutron-rich hypernuclei in FINUDA: preliminary results

6. Data analysis: event selection • October 14th,2003–March 22nd,2004 full data analyzed • p+ momentum spectra observed in the ~ 250 MeV/c region (where the NRH peaks are expected) • Candidates:positive tracks coming out from K- • Selection criteria: • K- stopped in the selected target • Positive charged tracks crossing the entire spectrometer (“long” tracks) • 4-points track fit successful • p+ PID (p + d contamination is present) Search for neutron-rich hypernuclei in FINUDA: preliminary results

7. m+ m+ K+ p+ K- p p+ p FINUDA: event reconstruction (NRH) More difficult goal than the p-- case: • Lower statistics (~ 10-2 10-3) & Lower acceptance Search for neutron-rich hypernuclei in FINUDA: preliminary results

8. PID and p+ momentum spectra • p+ separation from p and d is obtained from: • The OSIM dE/dxvs. momentum p information • Proper mass intervals are defined, according to the Bethe-Bloch function • TheTOFONE-TOFINO time-of-flightinformation • Proper b zones are defined, according to the relativistic formula: b = p / (p2 +m2c2)1/2 p+ spectrum Search for neutron-rich hypernuclei in FINUDA: preliminary results

9. Upper limit evaluation for the NRH formation probability: • The FINUDA peculiar characteristics allow the m+ (produced by K+ decay on the opposite side) to be used as a “reference beam” to normalize the p+ spectrum in the region of interest: • For a given target we collect the following counts: • in the p+ spectrum: Np = N(stoppedK-)·ep·PB • in the m+ spectrum: Nm = N(stoppedK+)·em·BR(K+ m+ + nm) ·eT • ep , em=p+ ,m+detection efficiencies ; eT=time gateefficiency ( 0.510%) • PB = probability per stopped K- corresponding to the Np counts (i.e. probability for an event to fall in the region where the NRH peak is searched) • Assuming N(stoppedK-)  N(stoppedK+)and epem: • PB (Np/Nm) · BR(K+ m++ nm) ·eT Search for neutron-rich hypernuclei in FINUDA: preliminary results

10. Upper limit evaluation (2): • If there was a signal S over the background B = Np: • Total counts: T = B + S = (1 + S/B)B = (1+S/B)Np with statistical uncertainty: sT = [(1+S/B)Np]1/2 • Background counts: B = Np; uncertainty:sB = Np1/2 • No evidence of signal means that, within k s (kR), it must be: • Final expression for the neutron-rich formation probability: Search for neutron-rich hypernuclei in FINUDA: preliminary results

11. preliminary Neutron-rich study: m+ counts • Nm is evaluated as the integral in the whole region: p > 215 MeV/c (same region delimited in the reconstruction program to identify the m+; peak @ 236 MeV/c) Search for neutron-rich hypernuclei in FINUDA: preliminary results

12. Neutron-rich study: p+ counts preliminary • Np is evaluated as the integral in the regions: • p = 262.9 MeV/c  2 sp(12LBe) • p = 249.1 MeV/c  2 sp(6LH) • p = 246.4 MeV/c  2 sp(7LH) • Momenta central values correspond to BL values in the ground states • sp = actual (0.9%)or nominal (0.35%)momentum resolution Search for neutron-rich hypernuclei in FINUDA: preliminary results

13. Neutron-rich study: p+ counts • Np is evaluated as the integral in the regions: • p = 262.9 MeV/c  2 sp(12LBe) • p = 249.1 MeV/c  2 sp(6LH) • p = 246.4 MeV/c  2 sp(7LH) • Momenta central values correspond to BL values in the ground states • sp = actual (0.9%)or nominal (0.35%)momentum resolution preliminary Search for neutron-rich hypernuclei in FINUDA: preliminary results

14. Upper limits summary table: • Uncertainties dominated by the error on the m+ time gate efficiency ( 10%) KEK result for 12LBe: 6.1 · 10-5 (90% C.L., no error given) Search for neutron-rich hypernuclei in FINUDA: preliminary results

15. Conclusions and prospects Search for neutron-rich hypernuclei: • No evidence of peaks associated to the searched states yet, but: • Already possible, with the available statistics and looking at thep+inclusive spectra only (no coincidences), to evaluate an experimental upper limit for the NRH formation probability: • better than the measured one so farfor the already studied hypernuclei (12LBe) • given for the first time in the other cases(6LH,7LH) • Reconstruction program improvement continuously in progress  New analysis of available data • Coincidence techniques for the physical background rejection in study (further background reduction of a factor ~ 25) • Neutron-rich peaks ?… Search for neutron-rich hypernuclei in FINUDA: preliminary results

16. Search for neutron-rich hypernuclei in FINUDA: preliminary results

17. TOT K- + p  + + - +  n + + K- + p  - + + K- + (NN)  + + N +  n + + NEUTRON-RICH p+ spectra from background reactions (Monte Carlo) Search for neutron-rich hypernuclei in FINUDA: preliminary results

18. OSIM dE/dx PID 3 zones defined (p+, p, d) p • p+- p separation curve  Bethe-Bloch with M = (mp+mp)/2 • p - d separation curve  Bethe-Bloch with M = (mp+md)/2 d p+ Search for neutron-rich hypernuclei in FINUDA: preliminary results

19. Time Of Flight and PID • For the reconstructed events in FINUDA the following relation between the TI (TOFINO) and TE(TOFONE) times may be set: TE= TI+L/bc • This giving an expression for the particle b:b= L / c (TE- TI) • L = reconstructed track length • The track is associated to a p+ only if bis greater than a proper threshold value, depending on the reconstructed momentum p: b > bT = p / (p2 +m2 c2)1/2 Search for neutron-rich hypernuclei in FINUDA: preliminary results