q + search at leps

+ Search at LEPS

First evidence from LEPS nK+K-n Phys.Rev.Lett. 91 (2003) 012002

hep-ex/0301020

+Low statistics: but

Tight cut: 85% of events are rejected by the exclusion cut.

Unknown background: BG shape is not well understood. Events from a LH2 target were used to estimate it. Possible kinematical reflections.

Correction: Fermi motion correction is necessary.

4.6S

B 3.2

S

S B

Search for + in nK+K-n

MM (GeV) MM (GeV)

•A proton is a spectator (undetected).

• Fermi motion is corrected to get the missing mass spectra.

•Tight exclusion cut is essential.

•Background is estimated by mixed events.

preliminary

preliminary

L(1520)

pK+K-p nK+K-n

+ search in d (1520) KN reaction

Θ ＋ is identified by K － p missing mass from deuteron. ⇒ No Fermi correction is needed.

K- n and pn final state interactions are suppressed. If ss(I=0) component of a is dominant in the reaction, the

final state KN has I=0. (Lipkin)

γ

p

n

Θ ＋

K －

p

(1520)

A possible reaction mechanism• + can be produced by re-scattering of K+.• K momentum spectrum is soft for forward going (1520).

γ

p/n

n/p

(1520)

K+/K0

missing momentum

Formation momentum

LD2

Pmiss GeV/c

LEPS acceptance has very little overlap with CLAS acceptance.

Event selection

MMp(γ,K － p) GeV/c2 M(K － p) GeV/c2

Λ(1520)

Select Λ(1520) in 1.50–1.54 GeV/c2

　　　　　 ⇒ calculate K- p missing mass of d K- p X reaction

K mass is smeared by Fermi motion. (assumed proton at rest)

LD2 data:

after selecting (1520)

inelastic events

select

LD2

γp→K － pKπ

Background processes• Quasi-free (1520) production must be the major

background.• The effect can be estimated from the LH2 data. γ

p

n

(1520)

K+

n

• The other background processes which do not have a strong pK- invariant mass dependence can be removed by sideband subtraction.

Sideband subtraction to remove non-resonant background

S = - 0.4

1.50 < M(K － p) < 1.54 1.45 < M(K － p) < 1.50 or 1.54 < M(K － p) < 1.59

LD2 LH2

M(K － p) GeV/c2M(K － p) GeV/c2

E > 1.75 GeV

Remove fluctuation by smearing E

MMd(γ,K － p) GeV/c2 MMd(γ,K － p) GeV/c2

•Fluctuations in the sideband spectra are removed by smearing E with 10 MeV smearing (nearly equal to the resolution).

•E smeared spectrum gives 2/n.d.f ~ 1 when compared with the original spectrum.

• contribution in the signal region is slightly larger than that in the the sideband region. The underestimtion is corrected by using the MC simulation.

correction for contribution

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Comparison of Esmeared spectra

M(K － p) GeV/c2

MMd(γ,K － p) GeV/c2


correction for contribution

•Validity of the sideband method with E smearing was checked by using two independent regions of the sideband.

•Channel-to-channel comparison gives

mean=-0.04 and RMS=2.0.

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Estimate * contribution from LH2 data


•Estimate quasi-free* contribution using LH2 data.

•Missing mass is calculated by assuming deuteron mass in the initial state.

•MC study shows the Fermi motion effect is small.

•Non-resonant and contributions are subtracted by sideband subtraction method.

•Small fluctuations in the large missing mass region (MM>1.55 GeV) could not be completely removed.


MC

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K － p missing mass spectrum


sideband

*

sum

5S

S B

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Excesses are seen at 1.53 GeV and at 1.6 GeV above the background level.

1.53-GeV peak:

pre

lim

inar

y


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Normalization of * is obtained by fit in the region of MMd < 1.52 GeV.

(in the 5 bin = 25 MeV)

pre

lim

inar

y

Variation of width for the signal region

M(K － p) GeV/c2 M(K － p) GeV/c2

MMd(γ,K － p) GeV/c2 MMd(γ,K － p) GeV/c2

* : +13%

EX: +22%

* : -17%

EX: -12%

(BG: -50%)

pre

lim

inar

y

pre

lim

inar

y

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Background estimation

• Sideband method overestimate BG level because of * leakage into the sideband region.• However, a slight change of the BG level does not change the fitting result much.• 10% reduction of BG level requires 15% increase of * contribution. It results in a 5% smaller BG level in the + region.MMd(γ,K － p) GeV/c2


• If we fit the full missing mass region (instead of MMd<1.52 GeV), the* contribution increases by 33%. •The background level in the + region becomes 6.5% more, and the significance drops to 3.8.•The 2 of the fit is bad: 2/ndf=2.8 in the full region and 2/ndf=2.4 in the region MMd<1.52 GeV.

pre

lim

inar

yp

reli

min

ary

x 0.9

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Photon energy dependence

1.53 GeV peak:•No change in the peak position. not likely due to kinematical reflections.

1.60 GeV bump:•Only seen in the low energy region. threshold effects?•Not seen in LH2 data•Associated with (1520).•Different reaction mechanism from that of the 1.53 peak.


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E< 2.1 GeV

E> 2.1 GeV

K － p missing mass in sideband regions


γ (1520)

K+/K0

γ p

K+/K0

K-

+ formation cross-section by simple kaon re-scattering is small.

A theoretical estimation by Titov is small (nucl-th/0506072) .

1.45<MpK<1.50

or

1.55<MpK<1.59

1.44<MpK<1.49

or

1.56<MpK<1.60

10 MeV away from the* region

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Summary of LEPS new result• We searched for + in the in the d (1520) KN reaction •A ~ 5 Peak is seen at ~1.53 GeV/c2 in the missing mass of the (,*).•The peak can be explained by the narrow S=+1 resonance + , and we think it is likely.• The peak cannot be seen in the K-p invariant mass region outside of the (1520). • If the peak is due to the +, its production by re-scattering seems to be small in our kinematic region.• Enhancement (bump structure) around 1.6 GeV is also observed in the (,*) reaction.• Width of the 1.53 peak is consistent with the detector resolution.

q + search at leps

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