LEPs - past, present and future - Regional Voices presentation
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![Page 1: 25 May 2005SPring-8/LEPS facility @STORI'051 GeV photon beam experiments at the SPring-8 laser-backscattering facility Introduction LEPS (Laser-Electron.](https://reader036.fdocuments.in/reader036/viewer/2022062516/56649e1c5503460f94b0a5bd/html5/thumbnails/1.jpg)
25 May 2005 SPring-8/LEPS facility @STORI'05 1
GeV photon beam experiments at the SPring-8
laser-backscattering facility
• Introduction
• LEPS (Laser-Electron Photon at SPring-8) facility
• Some experimental results using the forward spectrometer
- photon beam asymmetry for the K+ photoproduction
- meson photoproduction
- search for pentaquark +
• Present status and future prospect
M. Yosoi, RCNP Kyoto for the LEPS collaboration
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25 May 2005 SPring-8/LEPS facility @STORI'05 2
Characteristics of BCS photons(BCS:Backward Compton Scattering)
• rather flat energy distribution with small spreading
(Unlike the Bremsstrahlung, where low energy photons are
dominated, ~1/E)
• high linear- or circular-polarization
• photon energy can be tagged by recoil electron
Ee=8 GeV=351 nm
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25 May 2005 SPring-8/LEPS facility @STORI'05 3
Synchrotron Radiation Rings with laser-backscattering facilities
LEGS@NSLS/BNL2.8 GeV
GRAAL@ESRF 6 GeV LEPS@SPring-8 8GeV ~100mA
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25 May 2005 SPring-8/LEPS facility @STORI'05 4
With LEPS, what can be aimed at ?
Threshold region of (ss) meson and hyperon resonances
Key words :1. Forward angle measurement including 0 deg.2. Polarization observables3. Strangeness
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25 May 2005 SPring-8/LEPS facility @STORI'05 5
LEPS facility
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25 May 2005 SPring-8/LEPS facility @STORI'05 6
Beam line map of Spring-8
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25 May 2005 SPring-8/LEPS facility @STORI'05 7
Schematic view of the LEPS facility
b) Laser hutch
a) SPring-8 SR ring
c) Experimental hutch
Inverse Compton -ray
Laser light
8 GeV electron Recoil electron
Tagging counter
Collision
Energy spectrum of
BCS photons
Bremsstrahlung
Backward-Compton scattering
36m70m
SSD + Sc phodoscope ScFi + Sc phodoscope
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25 May 2005 SPring-8/LEPS facility @STORI'05 8
LEPS forward spectrometer
2m
• Target LH2, LD2, etc.• AC index = 1.03 to reject e+e- pairs• SSD 120m pitch• DCs ~ 200 m• Magnet 135 x 55 cm2, ( 35o x 15o) B = 0.7T ( ~106 /sec)
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25 May 2005 SPring-8/LEPS facility @STORI'05 9
Particle identification
•TOF : RF signal - TOF wall, t = 120 ps
•Momentum : SSD, DCs, Tracking
p ~ 6 MeV/c for 1 GeV/c K
Reconstructed mass spectra
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25 May 2005 SPring-8/LEPS facility @STORI'05 10
Other detectors :1. EM calorimeter for 02, 2
• Main detector
Lead scintillating fiber
252 modules
• Covered solid angle
2.08 (str)
: 30o ~ 100o
: 0o ~ 360o
• Length of each module
22cm ( 13.7 X0 )
• Angular interval (segment)
10 degree2
2
%)2.04.4(%1.02.5
EE
E
• Energy resolution
( 6.8% for 1 GeV photon)
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25 May 2005 SPring-8/LEPS facility @STORI'05 11
Other detectors :2. TPC for Hyperon Resonances
B
3/2-
1/2-
)
)30 MeV
(1405) : 3-quark state orKN bound state ?
KN
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25 May 2005 SPring-8/LEPS facility @STORI'05 12
Photon beam asymmetry for the p(,K+) and p(,K+)0 reactions
R.G.T. Zegers et al. PRL29,092001
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25 May 2005 SPring-8/LEPS facility @STORI'05 13
Missing resonances N* and *
• Knowledge of N* and D* is essential to understand the internal structure of baryons.
• Many nucleon resonannces predicted by quark models are still missing.
• So far, N channel K or K channel
K+
Y()p
N*, *
W~ 1.9 GeV
without D13
with D13
SAPHIR data
D13(1895) resonance ?
Recent CLAS data show more than one resonance !
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25 May 2005 SPring-8/LEPS facility @STORI'05 14
Description of K photoproduction
• Choice of included resonances• Coupling constant• Hadronic form factors• Treatment of background terms
Need more study to fix parameters.
• Additional observables are useful for further studies.• There are the data of cross sections and recoil polarizations from SAPHIR and CLAS collaborations. • Photon beam asymmetry is one of the good candidates.
Ambiguitiestree-level effective-Lagrangian approach
- should be cautious to define conclusions with cross sections only
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25 May 2005 SPring-8/LEPS facility @STORI'05 15
Missing mass spectrum and photon beam asymmetry
For all events
Nv(h) : normalized yield of K+ photoproduction for vertical (horizontal) pol.: K+ azimuthal angleP : Polarization of photon
)2cos( PNN
NN
hv
hv
and 0 events 2 cuts : contamination < 2%
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25 May 2005 SPring-8/LEPS facility @STORI'05 16
Experimental Results and comparison with model calculations
Mart & BennholdPRC61, 012201(with D13 )
Janssen et al.PRC65, 015201, PRC66,035202
Currently no models reproduce our data consistently. Strong conclusions for D13 etc., are very premature.
Data : positive sign become large as E
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25 May 2005 SPring-8/LEPS facility @STORI'05 17
meson photoproduction on protons near threshold
cf.) photoproduction from Li, C, Al, and Cu has also been measured study modification of in the nuclear medium (T. Ishikawa et al. PLB608, 215)
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25 May 2005 SPring-8/LEPS facility @STORI'05 18
Vector meson photoproduction
p
p
uudPomeron
(~ss)
p uud???
Pomeron exchange
Meson exchange
Glueball exchange ??
hep-ph/0302195,0211430
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25 May 2005 SPring-8/LEPS facility @STORI'05 19
KK invariant mass distributions
Missing mass (, Kp)X (GeV)
Selection of KKp final state:
3 cut in missing mass
Missing mass resolution=10 MeV
Invariant mass resolution=2-3 MeV
KK invariant mass (GeV)
Missing mass (, KK)X KK invariant mass (GeV)
selection cut:
|MKK-1.019|< 10 MeV
KK
Kp
p
K
BG
from hypero
ns
BG
from K
Kp
BG subtraction by usingweighted MC which fits to the real data.
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25 May 2005 SPring-8/LEPS facility @STORI'05 20
Differential cross sections
Solid : fit with E independent slopeDashed : fit with E dependent slope
min||exp||min
ttbttdt
d
dt
d
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25 May 2005 SPring-8/LEPS facility @STORI'05 21
Differential cross section at t=-|t|min
Curve : Pomeron + Pseudo scalar exchange model (A. Titov et. al, PRC 67, 065205)
A peaking structure is seen in d/dt near E=2 GeV, which has not been explained by model calculation.
Smaller t-slope near threshold.
min||exp||min
ttbttdt
d
dt
d
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25 May 2005 SPring-8/LEPS facility @STORI'05 22
Decay polarization observables with linearly polarized photon
Decay Plane // natural parity exchange (-1)J (Pomeron, 0+ glueball, Scalar mesons)
Photon Polarization
K
K
K
Decay Plane unnatural parity exchange -(-1)J
(Pseudoscalar mesons ,)
Relative contributions from natural, unnatural parity exchanges (U/UN)
Decay angular distribution of meson
→KK
K
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25 May 2005 SPring-8/LEPS facility @STORI'05 23
Decay angular distribution
• W ~ sin2 : Helicity-conserving process is dominant.• Natural parity exchange is dominant. (no energy dependence)
Forward angles; -0.2 < t+|t|min < 0 GeV2
around the peak region of the cross section
above the peak region
~2 GeV peak : mainly natural parity exchangeNeed more data above 2.4 GeV
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25 May 2005 SPring-8/LEPS facility @STORI'05 24
Search for pentaquark +
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25 May 2005 SPring-8/LEPS facility @STORI'05 25
What are pentaquarks ?
M1890-180*Y] MeV
D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305
(Chiral Soliton Model)
• Exotic: S = +1• Low mass: 1530 MeV• Narrow width: ~ 15 MeV• J=1/2+
• Minimum quark content : 5 quarks• Quantum numbers of “Exotic” pentaquarks : not 3-quark
qqqqq
Theoretical Prediction of
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25 May 2005 SPring-8/LEPS facility @STORI'05 26
First evidence of from LEPS
n K KK n
M = 1.540.01 GeV< 25 MeVGaussian significance 4.6
Target: neutron in Carbon nucleus
Background level is estimated by a fit in a mass region above 1.59 GeV.
Assumption:• Background is from non-resonant KK production off the neutron/nucleus• … is nearly identical to non-resonant KK production off the proton
+
T. Nakano et al., PRL91, 012002
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25 May 2005 SPring-8/LEPS facility @STORI'05 27
Positive results
Final state:
Ks + p
K+ + n
(Ks + p )
A few % difference from zero, but ~20% difference from the KN threshold.
Statistics is not enough.
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25 May 2005 SPring-8/LEPS facility @STORI'05 28
Negative results• Many negative results have been reported with good st
atistics from high energy facilities.
(HERA-B, HyperCP, CDF, BaBar, LEP, Belle, …)
• New CLAS data for pKs+ has shown no peak ! (April APS meeting)
Cross section
p Ks)
< 1-4 nb
Inconsistent with
SAPHIR data
g11@CLAS
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25 May 2005 SPring-8/LEPS facility @STORI'05 29
LEPS new search for
1. n KKKn in deuteron
MM (GeV) MM (GeV)
preliminarypreliminary
(1520)
pKKp nKKn
LEPS new LH2/LD2 data (Oct. 2002 – Jun, 2003) Fermi motion is corrected to get the missing mass spectra exclusion cut is essential Background is estimated by mixed events
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25 May 2005 SPring-8/LEPS facility @STORI'05 30
LEPS new search for
2. d (1520)Kp
is identified by Kp missing mass from deuteron. No Fermi correction is needed.
p
n
K
p
p
n
K -
p
(1520)
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25 May 2005 SPring-8/LEPS facility @STORI'05 31
Major background process
• Quasi free (1520) production must be the major background.
• The effect can be estimated from the LH2 data
p
n
(1520)
K+
n
LH2
LH2 distribution: comparison with MC
MMd(γ,K - p) GeV/c2
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25 May 2005 SPring-8/LEPS facility @STORI'05 32
Event selection
MMp(,K - p) GeV/c2 M(K - p) GeV/c2
(1520)
Select (1520) in 1.50–1.54 GeV/c2
⇒ calculate K p missing mass
K mass is smeared by Fermi motion. (assumed proton at rest)
LD2 data:
after selecting (1520)
inelastic events
select
LD2
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25 May 2005 SPring-8/LEPS facility @STORI'05 33
Kp missing mass for (1520) production from deuteron
MMd(,K - p) GeV/c2 MMd( ,K - p) GeV/c2
Quasi-free and non-resonant KKp
preliminary
preliminary
1.53 GeV, width <10 MeV
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25 May 2005 SPring-8/LEPS facility @STORI'05 34
Extracting (1520) contribution
M(Kp) GeV/c2
(1520) contribution can extracted by sideband subtraction method.
(1520) 1.5 0.5N W
N
W
p p reaction and non-resonant KKp production can contribute in the signal region. But they do not make a peak in Kp invariant mass.
LH2
contributions from , KKp,,,.
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25 May 2005 SPring-8/LEPS facility @STORI'05 35
Side-band subtraction in Kp missing mass
MMd(,K - p) GeV/c2 MMd(,K - p) GeV/c2 after side-band subtraction
Confirmed that the observedexcess is related with (1520).
Bump structureLD2
LD2
E > 1.75 GeV
preliminary
preliminary
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25 May 2005 SPring-8/LEPS facility @STORI'05 36
Summary• LEPS@Spring-8 has been in operation since 2000 for the stu
dy of the nuclear and particle physics.• From the first LH2 run with the forward spectrometer - photon beam asymmetry of K+ photoproduction polarization observables are strong tools to pin down the missing baryon resonances. - photoproduction near the threshold found peaking structure around E=2 GeV, new mechanism ? more study (LD2, E upgrade) - + was first discovered in n(,K-) mode for n in the carbon of the trigger counter• New LH2/LD2 exp. Re-observed + peak in two ways n(,K-) mode (n in d), and d(,(1520)) mode
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25 May 2005 SPring-8/LEPS facility @STORI'05 37
Present status and prospect• photoproduction from nuclear target (Li,C,Al,Cu) data taking completed in 2001, already published
• p(,Ks) experiment with gas cherenkov counter data taking completed in 2002• Experiment using the EM calorimeter with nuclear target (C
H2,C,etc) data taking completed in 2003• Hyperon resonance study with TPC and spectrometer for n
uclear target (CH2,C,Cu) data taking completed in 2004• Energy upgraded : 2.4 GeV 2.9 GeV now going on for the nuclear target• LH2/LD2 + TPC(new) + 2.9 GeV will start after this summer
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25 May 2005 SPring-8/LEPS facility @STORI'05 38
LEPS collaborationD.S. Ahna, J.K. Ahnb, H. Akimunec, Y. Asanod, W.C. Change, S. Datef, H. Ejiria,f, T. Emorij,
H. Fujimurah,j, M. Fujiwaraa,b, K. Hicksi, K. Horiea, T. Hottaa, K. Imaij, T. Ishikawak, T. Iwatal,Y. Katoa,H. Kawaim, Z.Y. Kimh, K. Kinoa, H. Kohria, N. Kumagaif, S. Makinon, T. Matsumuraa,
N. Matsuokaa, T. Mibea, K. Miwaj, M. Miyabej, Y. Miyachio, M. Moritaa, N. Muramatsud,T. Nakanoa, Y. Nakatsugawaj, M. Niiyamaj, M. Nomachip, Y. Ohashif, T. Oobam, H. Ookumaf,
D. S. Oshueve, C. Rangacharyuluq, A. Sakaguchip, T. Sasakij, T. Sawadaa, P. M. Shagina, Y. Shiinom, H. Shimizuk, Y. Sugayap, M. Sumihamaa, T. Tsunemia, H. Toyokawaf, A. Wakaio, C.W. Wan
ge, S.C. Wange, K. Yoneharac, T. Yoritaf, M. Yosoij and R.G.T. Zegersa, a Research Center for Nuclear Physics (RCNP), Ibaraki, Osaka 567-0047, Japan
b Department of Physics, Pusan National University, Pusan 609-735, Koreac Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
d Japan Atomic Energy Research Institute, Mikazuki, Hyogo 679-5148, Japane Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
f Japan Synchrotron Radiation Research Institute, Mikazuki, Hyogo 679-5198, Japanh School of physics, Seoul National University, Seoul, 151-747 Koreai Department of Physics, Ohio University, Athens, Ohio 45701, USA
j Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japank Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826, Japan
l Department of Physics, Yamagata University, Yamagata, Yamagata 990-8560, Japanm Department of Physics, Chiba University, Chiba, Chiba 263-8522, Japan
n Wakayama Medical College, Wakayama, Wakayama 641-0012, Japano Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan
p Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japanq Department of Physics, University of Saskatchewan, Saskatoon, S7N 5E2, Canada
r Department of Applied Physics, Miyazaki University, Miyazaki 889-2192, Japan
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Backup slides
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LH2 TargetCerenkov
Detector
SSD
Drift
Chamber
Trigger
Counter
Dipole Magnet
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Recent experimental setupwith TPC
A TPC(Time Projection Chamber)is newly installed with a superconducting solenoidalMagnet (~ 2 T)
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A possible reaction mechanism• + can be produced by re-scattering of K+.• K momentum spectrum is soft for forward going (1520).
p/n
n/p
(1520)
K+/K0
Formation momentum
LD2
PK GeV/c
MM
d(
,K p
) G
eV/c
2
LH2
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Kp missing mass in sideband regions
MMd(,K - p) GeV/c2
(1520)
K+/K0
p
K+/K0
K
No peak!
+ formation cross-section by simple kaon re-scattering should be small.
LD2
A. Titov estimated it is small.