New result from Θ+ search experiment at LEPS

T. Nakano（RCNP, Osaka University）

•No introduction on the Θ+

•New Analysis idea

•Analysis details

•Summary

Chiral05 @ RIKEN., February 16, 2005

How to search for Θ+?

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

γ Θ＋γ Θ＋

Λ(1520)p K－ p K－n n

p p

LEPS New LD2 and LH2 runs

• Data taken from Oct. 2002 to Jun. 2003.

• ~2・1012 photons on a 15cm-long LD2 target.

• LH2 data were taken in the same period with ~ 1.4・1012 photons on the target.

# of photons: LH2:LD2 ≈ 2:3

# of events from a proton: LH2:LD2≈ 2:3 (e.g. KKp from φ production)

# of events from a nucleon: LH2:LD2≈ 1:3

γ

TOF wall

Beam

Dipole magnet

Beam dump

DC2

DC3

DC1

Vertex detector (SSD)

Target

Start counter

Aerogel Cerenkov counter

LEPS detector

1m

Max: 2.4 GeV

Good acceptance in the forward angles

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

Λ(1520).

γ Λ(1520)

K+/K0

0

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140

160

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

PK GeV/c

PK obtained by missing momentum

Formation momentum

LD2

p/nΘ+n/p

Energy and momentum calibration

0

200

400

600

800

1000

1200

1400

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

100

200

300

400

500

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

MMp(γ,K－p) GeV/c2

γp→K－pKππ－

mis-ID as K－

K＋

π－ mis-ID events aresuppressed.

γ p Λ(1520) XK- p

γ p K- p X

MMp(γ,K－p) GeV/c2

LH2 LH2

0

100

200

300

400

500

600

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.80.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Event selection

MMp(γ,K－p) GeV/c2 M(K－p) GeV/c2

Λ(1520)K mass is smeared by Fermi motion. (assumed proton at rest)

LD2 data:

after selecting Λ(1520)

inelastic events

select

LD2

0

50

100

150

200

250

300

350

400

Select Λ(1520) in 1.50–1.54 GeV/c2

⇒ calculate K- p missing massof γ d K- p X reaction

0

10

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50

60

70

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

K－p missing mass in 1.50<M(K－p)<1.54 GeV/c2

MMd(γ,K－p) GeV/c2

Θ＋ signal？ γ d Λ(1520) XK- p

Good understanding of the background spectrum shape is crucial.

Major background process

• Quasi free Λ(1520) production must be the major background.• The effect can be estimated from the LH2 data

γ Λ(1520)

pn n

K+

0

50

100

150

200

250

1.46 1.48 1.5 1.52 1.54 1.56 1.58

Extracting Λ(1520) contribution

M(K－p) GeV/c2

Λ(1520) contribution can extracted by sideband subtraction method.

(1520) 1.5 0.5N WΛ = × − ×

N

W

LH2 γ p φ p reaction and non-resonant KKpproduction can contribute in the signal region. But they do not make a peak in K-p invariant mass.

contributions from φ, KKp,,,.

Energy Dependence of Λ(1520) production

0

0.005

0.01

0.015

0.02

0.025

0.03

1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4

0

0.2

0.4

0.6

0.8

Eγ GeV1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4

Eγ GeV

K+ detected

Identify Λ* in the missing mass.

K- p detected

in the forward angle.

Energy dependences are similar in the both angle regions. Energy dependence of the angular distributions must be

small.

Λ(1520) angular distribution

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180

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0 0.2 0.4 0.6 0.8 1

After sideband

subtraction. Acceptance corrected.

Cos Θ Kp in CMSCos Θ Kp in CMS

Angular distributions is flat.

0

5

10

15

20

25

30

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

LH2 distribution: Comparison with MC

MMd(γ,K－p) GeV/c2

MC:

Energy dependence is obtained by fitting to data.

Angular distribution is assumed to be flat.

LH2

•deuteron at rest was assumed in the missing mass calculaions.

•small missing mass events correspond to soft undetected kaons.

φ contribution

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25

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175

200

0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4

M(K－K+) GeV/c2

K+ momentum is estimated by missing momentum technique.

Smeared by Fermi motion.

φ

LD2 data

∀10.and.∀20.and.∀86

1

1.1

1.2

1.3

1.4

1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4

K－p invariant mass in the low energy region

INVM

(K－

K+ )

GeV

/c2

Eγ GeV

∀10.and.∀20.and.∀86

1

1.1

1.2

1.3

1.4

1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4

Eγ GeV

LD2 LH2

K－p invariant mass after φ exclusion

0

50

100

150

200

250

300

350

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

M(K－p) GeV/c2 M(K－p) GeV/c2

0

20

40

60

80

100

120

140

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

Eγ > 2.2 GeVall energy

LH2 LH2

non-resonant KKp events

non-resonant KKp events

Λ(1520)

Non-resonant KKp (phase space) contribution reproduces the spectrum shape under L(1520) well.

0

25

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75

100

125

150

175

200

1.4 1.5 1.6 1.7 1.8 1.9 2

K－p missing mass for Λ(1520) and non-resonant events in the

MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

0

100

200

300

400

500

600

700

800

1.4 1.5 1.6 1.7 1.8 1.9 2

non-resonant KKp Λ(1520)

Energy dependences are set to be the same.

No angular dependence has been introduced in the both reactions.

K－p invariant mass after φ exclusionLD2 data

M(K－p) GeV/c2 M(K－p) GeV/c2

0

100

200

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500

600

700

800

900

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.80

50

100

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200

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300

350

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

Eγ > 2.2 GeVall energy

LD2 LD2

K－p invariant mass after subtracting KKp

M(K－p) GeV/c2 M(K－p) GeV/c2

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25

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100

125

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175

200

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

0

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60

80

100

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

Eγ > 2.2 GeV

LH2LD2

Eγ > 2.2 GeV

Λ(1405)

Λ(1520)

Y*

Enhancement of Λ(1405) and Y* productions from neutron?

K－p missing mass in the sideband regions

MMd(γ,K－p) GeV/c2 MMd(γ,K－p ) GeV/c2

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80

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.80

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70

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

1.46< M(K－p) < 1.50 GeV/c2 1.54< M(K－p) < 1.58 GeV/c2

LD2 LD2

φ: Energy and angular dependences were obtained by fitting to the data.

KKp: Enegy dependence fit to the data. Angulardependence flat.

K－p missing mass for Λ(1520)

MMd(γ,K－p) GeV/c2

0

10

20

30

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60

70

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

MMd(γ,K－p) GeV/c2

0

10

20

30

40

50

60

70

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

Quasi-free Λ(1520) and non-resonant KKp

φ

Test by side-band subtractionNarrow gate (1.52<M(K－p)<1.54 GeV/c2) = L + N

Λ(1520) is enhanced.Wide gate (1.46<M(K－p)<1.58 GeV/c2) = L + 3N

Non-resonant KKp is enhanced.⇒ L = 1.5 (L+N)－ 0.5 (L+3N)

narrow gate wide gate

M(K－p) GeV/c2

L

NNN

K－p missing mass for non-resonant KKp events

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1.4 1.5 1.6 1.7 1.8 1.9 20

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1.4 1.5 1.6 1.7 1.8 1.9 2

MMd(γ,K－p) Ge V/c2MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

0

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1.4 1.5 1.6 1.7 1.8 1.9 2

1.46< M(K－p) < 1.50 GeV/c2 1.50< M(K－p) < 1.54 GeV/c2 1.54< M(K－p) < 1.58 GeV/c2

Missing mass spectrum shapes are almost identical. Sideband subtraction works well.

K－p missing mass vs. Eγ for KKpevents

MMd(γ, K－p) GeV/c2

EγG

eV

∀10.and.∀20.and.∀32.and.∀86

1.6

1.65

1.7

1.75

1.8

1.85

1.9

1.95

2

1.4 1.45 1.5 1.55 1.6 1.65 1.7

Events in low Eγconcentrated in the signal region.

Sideband subtraction will not work well in this energy region.

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1.4 1.425 1.45 1.475 1.5 1.525 1.55 1.575 1.6

K－p invariant mass in the low photon energy regions

M(K－p) GeV/c2

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30

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90

1.4 1.425 1.45 1.475 1.5 1.525 1.55 1.575 1.6

M(K－p) GeV/c2

Eγ < 1.75 GeV 1.75 < Eγ < 1.85 GeV

LD2 LD2

Contribution from Λ(1520) is small below 1.75 GeV.

S/N (Λ(1520)/Others)

Eγ GeV

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4

S/N

LD2cut

S/N is bad near threshold and in the high energies.

Side-band subtraction in KK invariant mass

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0.9 1 1.1 1.2 1.3 1.40

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0.9 1 1.1 1.2 1.3 1.4

M(K+,K-) GeV/c2 M(K+,K-) GeV/c2

after side-band subtraction

Since φ is not related with Λ(1520),φ peak disappears.

LD2

LD2

-5

0

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30

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40

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

Side-band subtraction in K－p missing mass

MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

after side-band subtraction

Confirmed that the observedexcess is related with Λ(1520).

Bump structure

0

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30

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60

70

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

LD2

LD2

Eγ > 1.75 GeV

K－p missing mass (assuming deuteron at rest) from LH2 (proton) data

MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

-5

0

5

10

15

20

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.80

10

20

30

40

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1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

No peak

sideband regionsideband subtracted spectrum

contribution from φ in the large missing mass region

LH2 LH2Eγ > 1.75 GeV

K－p missing mass in sideband regions

MMd(γ,K－p) GeV/c2

0

10

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30

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80

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

γ Λ(1520)

K+/K0

Θ+

γ p

K+/K0

Θ+

K-

No peak!

Θ+ formation cross-section by simple kaon re-scattering should be small.

LD2

A. Titov estimated it is small.

WidthThe resolution study is underway.

The resolution depends on both photon energy and momenta of charged particles.

need to know energy and angular distributions of the signal.

The estimation of the width depends on the BG shape and level.

0

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60

70

1.4 1.45 1.5 1.55 1.6 1.65 1.7

Width: Comparison with a MC spectrum

MMd(γ,K－p) GeV/c2

MC gives ~10 MeVresolution while Real data resolution is about ~ 8 MeV.

Conclusion

• Peak is seen at ~1.53 GeV/c2 in the missing mass of the (γ,Λ*) reaction from a deuteron.• The peak cannot be seen in the K-p invariant mass region outside of the Λ(1520). • If the peak is due to the Θ+, its production by re-scattering seems to be small in our kinematicregion.• Enhancement (bump structure) around 1.6 GeV is also observed in the (γ,Λ*) reaction.• Width seems to be very narrow.