JPS meeting @Matsuyama, 29 Mar. 2006
液体重水素標的を用いた+バリオ
ン光生成の研究
村松 憲仁
大阪大学 核物理研究センター
For the LEPS Collaboration
阪大RCNP 村松憲仁,中野貴志,A,D.S.Ahn,郡英輝, 藤原守,
堀田智明, 堀江圭都, 與曽井優
甲南大
秋宗秀俊
釜山大
J.K.Ahn
東北大核理研 石川貴嗣, 清水肇
京大理
今井憲一, 新山雅之, 藤村寿子, 宮部学
JASRI
大橋裕二, 伊達伸, 依田哲彦
Academia Sinica D.S.Oshuev, W.C.Chang
東大CNS 木野幸一
阪大理
阪口篤志, 菅谷頼仁
千葉大
椎野祐樹
東北大理 住浜水季
NSCLMSU R.G.T.Zegers
宮崎大
戸井裕也, 松田達郎
Ohio Univ. K.Hicks, 三部勉
名古屋大 福井崇時
防衛大
松村徹
Univ. of Saskatchewan C.Rangacharyulu
他SPring-8/LEPS Collaboration
Contents
Introduction (+ Status)
 d+(1520)
- BG estimation
- Signal-like behavior
- Cross section measurement
 Summary & Prospects

Pentaquark Status @ EINN 2005
Group
+
X5
0c
Signal
Backgr.
Significance
s/ b+s
publ.
Comments
---------------------------------------------------------------------SPring8
19
17
4.6s
3.2s
SPring8
90
260
4.8s
G11 CLAS-p
SAPHIR
55
56
4.8s
5.2s
DIANA
29
44
4.4s
3.4s
? BELLE
CLAS(d)
43
54
5.2s
4.4s
G10 CLAS-d
CLAS(p)
41
35
7.8s
4.7s

18
9
6.7s
3.5s
HERMES
51
150
4.3-6.2s
3.6s
? BABAR
ZEUS
230
1080
4.6s
6.4s
COSY
57
95
4-6s
4.7s
SVD
41
87
5.6s
3.6s
SVD-2
370
2000
7.5s
Improved analysis
NA49
H1
38
50.6
43
51.7
SPring8
STAR
80
2,250
200
150,000
4.2s
5-6s
4.2s
5.0s
4.8s
3.5-5s
HERA-B, CDF, COMPASS
ZEUS, FOCUS, BABAR
*(K+n)
++ candidate
From Burkert @ EINN05
CLAS upper limits
• p+K0
s<2 nb vs. SAPHIR 300 nb / 50 nb
• d+K-p
s<450 pb
But acceptance coverage is very different
from LEPS (forward region).
↓
+ is not killed.
LEPS LD2 runs
• Collected Data (LH2 and LD2 runs)
Dec.2000 – June 2001 LH2 50 mm ~5×1012 photons
published data
May 2002 – Apr 2003 LH2 150 mm ~1.4×1012 photons
Oct. 2002 – June 2003 LD2 150 mm ~2×1012 photons
• #neutrons × #photons in K+K- detection mode
LD2 runs = 5mm-thick STC in short LH2 runs × ~5
• K-p detection mode w/o Fermi correction :γd→+K-p
Laser Electron Photon (LEP) Beam
-8 GeV electrons in SPring-8 + 351nm Ar laser (3.5eV)
⇒ 1.5-2.4 GeV photons (Backward Compton Scattering)
-Photon Flux ~106 cps, Photon Energy Resolution ~10 MeV
-Charged particle spectrometer with forward acceptance
-PID from momentum and time-of-flight measurements
SVTX DC1
PWO measurement
TOF
AC(n=1.03)
tagged

Target
Dipole Magnet
Start Counter 0.7 Tesla DC2
DC3
K-p detection mode
• + is identified by pK- missing mass from
deuteron. ⇒ No Fermi correction is needed.
Inclusive (n / p reaction + rescattering, or other mechanism)
+
γ
p
n
K-
(1520)
p
Event selections in K-p mode
K+ mass : 0.40 – 0.62 GeV/c2
π- mis-ID
as K-
γp→K-pKπ
MMp(γ,K-p) GeV/c2
Λ(1520) : 1.50 – 1.54 GeV/c2
Non-resonant
KKp + p + …
M(K-p) GeV/c2
Events with Λ(1520) production were selected.
E > 1.75 GeV was also applied.
K-p missing mass in 1.50<M(pK-)<1.54 GeV/c2
preliminary
5 MeV bins
Θ+ signal?
~1.53 GeV/c2
MMd(γ,K-p) GeV/c2
preliminary
3 MeV bins
MMd(γ,K-p) GeV/c2
Fluctuation or not ?
• Important to understand BG shape reliably
Quasi-Free BG = K(1520) + p + Non-resonant KKp + …
• MC-based & real data-based BG estimations
Non-resonant
KKp
(1520)

Fermi-corrected
MMp(,p)
M(pK-) GeV/c2 [LH2]
(1520)
M(pK-) GeV/c2 [LD2]
M(KK) GeV/c2 [LH2]

M(KK) GeV/c2 [LD2]
MC-based BG estimation by using LH2 data
- BGs were simulated by including Fermi motion. (MC ~ 20 x real data)
- Kinematics at CMS were adjusted to real LH2 data.
 ‘Filters’ in ECMS, CMS(pK-), CMS(proton), PCMS(proton), PCMS(K-)
- Non-resonant KKp ⇒ K+* ⇒ p were adjusted step by step.
K*
KKp
p
M(pK-) GeV/c2
M(KK) GeV/c2
2 test of MC to LH2 data
in MMd(,pK-) distribution
1.50<M(pK-)<1.54 GeV/c2 (Signal region)
(1)
(2)
(3)
MMd(,pK-) GeV/c2
(1+2+3) 1.400 – 1.700 GeV
2 = 34.154 ndf = 30
prob. = 0.275
(1) 1.400 – 1.500 GeV
2 = 15.253 ndf = 10
prob. = 0.123
(2) 1.500 – 1.600 GeV
2 = 5.895 ndf = 10
prob. = 0.824
(3) 1.600 – 1.700 GeV
2 = 13.006 ndf = 10
prob. = 0.223
M(pK-) distribution in LD2
Fermi motion is turned on in MC.
Preliminary
Extra events, which are
not seen in LH2 data
↓
Kinematical filters were
made from LD2 data
outside the signal region.
KKp
K*
p
M(pK-) GeV/c2
MMd(,pK-) in (1520) region [LD2 data]
1.50<M(pK-)<1.54 GeV/c2
+
Preliminary
Preliminary
1.6 GeV bump
MMd(,pK-) GeV/c2
MMd(,pK-) GeV/c2
Conservative statistical significance ~ 4s
Gaussian fit (temporary) ⇒ mass ~1.53 GeV/c2, width ~10 MeV
MMd(,pK-) below/above (1520) region [LD2]
M(pK-)<1.50 GeV/c2
Preliminary
MMd(,pK-) GeV/c2
Preliminary
small excess
M(pK-)>1.54 GeV/c2
MMd(,pK-) GeV/c2
Real data-based BG estimation
(Sideband subtraction method)
*
LH2
LD2
*
Non-resonant BGs + p
M(K-p) GeV/c2
M(K-p) GeV/c2
- Non-resonant BGs +  p : Deduced by
0.4 x [1.45<M(K-p)<1.50 or 1.54<M(K-p)<1.59 GeV/c2]
- K(1520) : LH2 data after sideband subtraction
Linearity was checked by comparing
two independent sideband regions.
K-p missing mass spectrum
* fitted in MM<1.52 GeV/c2
preliminary
1.6 GeV bump
preliminary
Counts/5 MeV
Counts/5 MeV
+
K(1520) fit to all MMd(,pK-) region
- BG level : 6.5% more.
- 2/ndf=2.8
*
from sidebands
MMd(γ,K-p) GeV/c2
MMd(γ,K-p) GeV/c2
Comparisons of the two methods
Two methods in BG estimation (Complementary)
Sideband method
Any BG involved realistically
Affected by LH2 statistics
* may be slightly under-estimated
Filtering method
Not affected by statistics
Possibility of Model variations
MMd(pK-) in different M(pK-) gates
around (1520) mass
10 MeV/c2
20 MeV/c2
(Standard)
The peak structure
looks associated with
(1520) production.
MMd(γ,K-p) GeV/c2
50 MeV/c2
MMd(γ,K-p) GeV/c2
MMd(γ,K-p) GeV/c2
100 MeV/c2
MMd(γ,K-p) GeV/c2
S/N ratio gets lower
by widening M(pK-)
gate, but the peak
height looks constant.
Photon energy dependence
Counts/5 MeV
E > 2.1 GeV
E < 2.1 GeV
MMd(,pK-) GeV/c2
+ is seen in both lower and higher energy regions.
1.6 GeV bump & higher M(pK) tail in LD2
ECMS<2.18
MMd(,pK-) GeV/c2
2.18<ECMS
MMd(,pK-) GeV/c2
ECMS<2.10
M(pK-) GeV/c2
2.18<ECMS
<2.26
M(pK-) GeV/c2
2.10<ECMS
<2.18
M(pK-) GeV/c2
2.26<ECMS
M(pK-) GeV/c2
1.6 GeV bump: n contribution? Hyperon-spectator nucleon interaction?
pion association?
Higher M(pK) tail : n contribution? Proton-neutron interaction?
Summary
Confirmation of + by using LD2 data
with K-p mode in MMd(,pK-) spectrum
- Two methods in BG shape estimation (MC-based &
sideband method) are complementary.
- 1.53 GeV/c2 peak (~4σ,preliminary) + 1.6 GeV/c2 bump
associated with (1520) production
- Signal-like behavior
[different M(pK-) gates, E dependence]
Prospects
• Differential cross section is being measured.
Luminosity(LD2) ~0.6 pb-1.
• Planning to take another data sets with LD2
target and forward spectrometer this year.
Tagger update is necessary. Photon beam
intensity will be twice by injecting two lasers.
• Time Projection Chamber is being prepared to
increase acceptance coverage. CLAS region
can be covered.
• Started to discussing about constructing new
beamline at SPring-8. Upgrades of beam
intensity and energy are expected. 4π detector
with good resolutions are under considerations.
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