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Search for neutron-rich hypernuclei in FINUDA: preliminary results

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### 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

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

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

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

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

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

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

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

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 ( 0.510%)
- 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 epem:
- PB (Np/Nm) · BR(K+ m++ nm) ·eT

Search for neutron-rich hypernuclei

in FINUDA: preliminary results

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 (kR), it must be:

- Final expression for the neutron-rich formation probability:

Search for neutron-rich hypernuclei

in FINUDA: preliminary results

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

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

- 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

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

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 ~ 25)
- Neutron-rich peaks ?…

Search for neutron-rich hypernuclei

in FINUDA: preliminary results

Search for neutron-rich hypernuclei

in FINUDA: preliminary results

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

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

- 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

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