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