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Search for neutron-rich hypernuclei in FINUDA: preliminary results. presented by M. Palomba 1 for the FINUDA Collaboration 1 INFN and Dipartimento di Fisica, Bari, Italy DA F NE2004: Physics at meson factories Laboratori Nazionali di Frascati, June 8 th , 2004. Outline.

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search for neutron rich hypernuclei in finuda preliminary results

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
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
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
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

12 l be 6 l h 7 l h known data
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
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

finuda event reconstruction nrh

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

pid and p momentum spectra
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
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

upper limit evaluation 2
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

neutron rich study m counts

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

slide12

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

slide13

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

upper limits summary table
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
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

slide16
Search for neutron-rich hypernuclei

in FINUDA: preliminary results

p spectra from background reactions monte carlo

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

osim de dx pid
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

slide19

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

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