new result from Θ search experiment at leps · leps new ld2 and lh2 runs • data taken from oct....
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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
20
40
60
80
100
120
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
20
30
40
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
0
20
40
60
80
100
120
140
160
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
0
25
50
75
100
125
150
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
50
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
300
400
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
150
200
250
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
0
25
50
75
100
125
150
175
200
1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8
0
20
40
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
0
10
20
30
40
50
60
70
80
1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.80
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
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
40
50
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
0
20
40
60
80
100
120
140
160
180
1.4 1.5 1.6 1.7 1.8 1.9 20
50
100
150
200
250
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
25
50
75
100
125
150
175
200
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.
0
10
20
30
40
50
60
70
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
0
10
20
30
40
50
60
70
80
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
0
10
20
30
40
50
60
70
80
0.9 1 1.1 1.2 1.3 1.40
25
50
75
100
125
150
175
200
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
5
10
15
20
25
30
35
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
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
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
50
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
20
30
40
50
60
70
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
10
20
30
40
50
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.