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Confirmation for a kaon-bound state
K-pp observed in antiproton-4He annihilation at rest
G. Bendiscioli 1, T. Bressani 2, A Fontana 1, L. Lavezzi 1, A Panzarasa 1, A. Rotondi 1
1 Dipartimento di Fisica Nucleare Università di Pavia and INFN, Sezione di Pavia
2 Dipartimento di Fisica Sperimentale Università di Torino and INFN, Sezione di Torino
2
Deeply Bound Kaonic StatesTheoretical predictions assert that bound states can be formed in light systems
N(s)K
0INK
[Akaishi and Yamazaki (2002)]
Theoretical estimatesof the potentials
The attractive force is strong enough to produce a contraction of the surrounding nucleus extremely dense nuclear system
3
Experimental results
Bound State Reaction Ref.
K-pnn 4He(K-stop,p)
Suzuki et al.[2005]
≤ 21.6 MeV
K-ppn 4He(K-stop,n)
Suzuki et al.[2005]
≤ 21.6 MeV
15OK- 16O(K-,n)Kishimoto et
al.[2005]
K-pp A(K-stop,N)
Agnello et al.[2005]
MeV0.9193.4B (stat)(sys)2.03.8
MeV2.3169.3B (stat)(sys)0.83.0
MeV67Γ
MeV115B
(stat)1411(sys)
23
(stat)65(sys)
34
4
annihilation at restHep4
Annihilations where several nucleons are involved produce an environment rich of strange quarks which is suitable for the creation of a bound stateppK
The bound state is searched through the decay
nXπpπpπnXΛpKXYnKHep 0S
04
XnKπpπpπnXKπΛpπXKπYnπHep 0LS,
0LS,
-0-4
The analyzed events belong to the set with two negative and three positive tracks, connected to the same vertex:
ΛpppK ppK
The searched signal is a peak in the invariant mass distribution 2p- (since p-)
Analyzed events have been collected by the OBELIX experiment
5
Selected eventsParticle identification is performed through independent measurement of velocity (), momentum (p) and dE/dx
Only pions and protons withthese momenta are detected:p() > 80 MeV/cp(p) > 300 MeV/c
Total number of analyzed events 9829
6
Inclusive distribution
Moreover the entries in the histogram are twice the events
The inclusive distribution of the invariant mass 2p- has no hint for the bound state
7
invariant mass cut
From fit: mass = 2212.1 ± 4.9 MeV width = 27.0 ± 7.8 MeV
A peak is visible in the subset of events with IM(p-) in [1085 – 1145] MeV
8
nXπpπpπnXΛpKXYnKHep 0S
04
Angular correlations and background reduction
angle between proton and p- ()
If and p come from the bound state they are emitted back-to-back in the center of mass system of the bound state cos = -1Here the system 2p- is emitted together with other particles. Thus it has momentum different from 0.
angle angle
IM(p-) in [1085 – 1145] MeV and IM(2p-) in [2200 - 2240] MeV
-0.5
9
nXπpπpπnXΛpKXYnKHep 0S
04 angle angle
-0.65
angle between 2p- (bound state) and +- (K0)
if cos = -1Here:1. nucleons are not initially at rest2. neutral particles are present3. final particles interact
IM(p-) in [1085 - 1145] MeV andIM(2p-) in [2200 - 2240] MeV
πpπpπΛpKYKppnp 0s
0
Angular correlations and background reduction
10
angle angle
Cuts: IM() in [1085 – 1145] MeV and cos < -0.4
Cuts: IM() in [1085 – 1145] MeV and cos < -0.55
angle angle
11
statistical significance
194N
statsysb54143N
3.41551
41194
143194σN
Ss
S
130N
statsysb4482N
3.71348
32130
82130σ
NS
s
S
Background evaluation:The background contribution is calculated evaluating the difference between the results of fits with good chi2, then statistically undistinguishable, from different background models (polynomials, gaussian).
IM(p-) in [1085 – 1145] MeVIM(p-) in [1085 – 1145] MeV … and cos < -0.4
… and cos < -0.4
12
Conclusions
• A peak has been observed in antiproton – 4He annihilation at rest in correspondence to the bound state K-pp, with statistical significance higher than 3, with mass 2212.1 ± 4.9 MeV and width 27.0 ± 7.8 MeV • The binding energy is 159 ± 5 MeV
• The B.R. for the production of the peak is 2.5 x10-4 per stopped antiproton
• The discrepancy with FINUDA results concerning the same bound state K-pp ( , [Agnello et al.]) can be assigned to the fact that FINUDA findings is evaluated summing up the contributions from three different nuclear targets (6Li, 7Li, 12C); moreover effects like the mass and width modification of a resonance in different nuclear media, the different inital states of the nuclear targets, etc. should be kept into account.
MeV(stat)(sys)67Γ eMeV (stat)(sys)115B 1411
23
65
34
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