Transcript Differential cross section for gp

Single p0 photoproduction
at SPring-8/LEPS
Mizuki Sumihama
Osaka university, RCNP
JPS meeting March 2007
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Introduction
• Pion photoproduction is well studied experimentally
and theoretically as a spectroscopy of N* and D*
resonances. The proprieties of many resonances are
determined at W < 1.7 GeV. However above the
resonance region, the production mechanism is not well
studied partly due to lack of data.
• We measured single p0 photoproduction in 1.9-2.3 GeV
in total energy at backward angles where less
experimental data.
• The production mechanism will be investigated in
a transition range from nucleon-meson degrees of
freedom to quark-gluon degrees of freedom.
• High mass resonances, u-channel contribution, quark
counting rule…
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Diagram in tree level
 W < 1.7 GeV
Models are well established.
Data are well explained by
four-star N* and D*.
 W ~1.9 GeV
There are one or two star
N* and D* but doubtful.
 W > 1.9 GeV
at very backward angles
non-resonant u-channel
is expected to be dominant.
 Much higher, W > 3 GeV
in 60s, 70s. Regge baryon
pole explains data well.
Regge ~s2a(u)-2 at u~small,
g
t-channel
(forward)
g
s-channel
p0
p0
p,r,w
p
p
N*, D*
p
p
1.9 – 2.3 GeV
+Backward - medium
g
p
follow quark counting rule.(JLab)
u-channel
p
p0
g
p
p
s-channel
p
p0
p
Born term
List of four-star nucleon resonances
P33(1232),P11(1440), D13(1520),S11(1535), S31(1620), S11(1650), D15(1675),
F15(1680), D33(1700), P13(1720), F35(1905),P31(1910) and F37(1950) in MAID2005.
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Measurement of g + p  p + p0
Linearly polarized photons
g
Liquid hydrogen
p
p
Detect protons
By spectrometer
p0
in missing mass
•Detect protons by LEPS spectrometer at forward angles
•Identify p0 in missing mass spectrum.
•Measure differential cross sections and photon beam
asymmetries at backward angles -1 < cosqcm < -0.6.
•Polarization degree is ~95% at the maximum Eg, 2.4 GeV.
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LEPS spectrometer
– forward acceptance
Aerogel
Cherenkov
(n=1.03)
+-~10o in y
+-~20o in x
TOF
wall
Dipole Magnet
(0.7 T)
Start counter
Liquid Hydrogen
Target (50 mm thick)
Linearly polarized
g
Silicon Vertex
Detector
MWDC 3
MWDC 1
MWDC 2
1m
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Particle identification
by time-of-flight and momentum measurements
Proton selection with 4s.
Kaon/pion contamination is negligible.
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Missing mass spectrum
gppX
1/10 total statistics
LH2 target
~18000 counts
Z-vertex distribution
Measurement region:
1.5 < Eg < 2.4 GeV
1.9 < W < 2.3 GeV
-1 < cosqcm < -0.6
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Background reactions
1.
2.
3.
4.
gp  p p p
gp  p p p p
gp  ph
gp  pg
•2,3 reactions are above
the 2s cut point.
•4 is much less than pion
production by ~2-3
orders.
No-scale!
(demonstration)
Data
p0
h
w/r
2s cut
(~ 0.13)
MC
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Missing mass square
Background subtraction
Single pion and double pion productions generated by
MC simulation are fitted to data by a template fit.
Systematic uncertainty was estimated by doing a fits
by Gaussians for p0 peaks with 2s and 1.5s cuts.
Example of results of template fits
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Determine differential cross section
•Obtain yield by subtracting two-pion photoproduction.
•Detector acceptance is obtained Monte Carlo
simulation based on GEANT3.
•Photon normalization is obtained by tagging counter.
Systematic error
1.
2.
3.
4.
Target protons 1.0% (fluctuation of temp. and pressure)
Photon normalization 1.2%, Photon transmission 3%
AC over veto 1.6%
Background subtraction, < 5%
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Differential cross section, Eg
LEPS data
Existing data.
SAID -partial-wave analysis
(fit of existing data)
Eur.Phys.J.A26(2005)399,
PRL,94(2005)012503,
PLB,48(1974)463,
NPB60(1973)267…
MAID2005 - isobar model
PRC66,055213(2002)
nucl-th/0603012
•Good agreement with existing data (SAID).
•Not follow ~s2a(u)-2(Regge), ~s7(counting rule) at Eg > 1.8 GeV.
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Differential cross section cosqcm
LEPS data
Existing data.
SAID
MAID2005
•Discrepancy with MAID becomes large at higher energy.
•Change angular distribution drastically at Eg~1.8GeV,
•Backward peaking, small bump structure 1.85<Eg<1.95GeV.
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Photon beam asymmetry S
LEPS data
Existing data.
PLB544(2002)113
NPB104(1976)253…
PgScos(2f) =
SAID
MAID2005
nNv - Nh
nNv + Nh
Positive sign:
s < s
Negative sign:
s > s
2.1 GeV < W < 2.3 GeV
•Data show a good agreement with SAID/MAID.
•Strong angular dependence above 1.8GeV.
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Summary
• Differential cross sections at very backward angles
have been measured and new data at Eg > 1.8 GeV.
Photon beam asymmetries have been measured at the
first time at backward angles Eg > 1.5 GeV.
• Angular dependence of both observables changes at
Eg = 1.8 GeV. A strong angular dependence is seen at
Eg > 1.8 GeV. Energy dependence cannot be explain by
nucleon Regge pole nor scaling.
• At low energy region, the data is explained with models
including well-known-nucleon resonances, but the strong
angular dependence at high energy cannot be explained.
• Can the data be explained by Born term only? Does uchannel contribution dominate? Quark-hadron duality?
High spin states? Challenge to theorists.
• Measurement at side angles and higher photon energy,
Homework to experimentalist.
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LEPS collaboration
D.S. Ahn, J.K. Ahn, H. Akimune, Y. Asano, W.C. Chang, S. Date, H. Ejiri, H. Fujimura, M.
Fujiwara, K. Hicks, K. Horie, T. Hotta, K. Imai, T. Ishikawa, T. Iwata, Y.Kato, H. Kawai, Z.Y.
Kim, K. Kino, H. Kohri, N. Kumagai, Y.Maeda, S. Makino, T. Matsumura, N. Matsuoka, T.
Mibe, M. Miyabe, Y. Miyachi, M. Morita, N. Muramatsu, T. Nakano, Y. Nakatsugawa, M.
Niiyama, M. Nomachi, Y. Ohashi, T. Ooba, H. Ookuma, D. S. Oshuev, C. Rangacharyulu, A.
Sakaguchi, T. Sasaki, T. Sawada, P. M. Shagin, Y. Shiino, H. Shimizu, S. Shimizu, Y. Sugaya,
M. Sumihama H. Toyokawa, A. Wakai, C.W. Wang, S.C. Wang, K. Yonehara, T. Yorita, M.
Yosoi and R.G.T. Zegers
a Research Center for Nuclear Physics (RCNP), Ibaraki, Osaka 567-0047, Japan
b Department of Physics, Pusan National University, Pusan 609-735, Korea
c Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
d Japan Atomic Energy Research Institute, Mikazuki, Hyogo 679-5148, Japan
e Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
f Japan Synchrotron Radiation Research Institute, Mikazuki, Hyogo 679-5198, Japan
h School of physics, Seoul National University, Seoul, 151-747 Korea
i Department of Physics, Ohio University, Athens, Ohio 45701, USA
j Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan
k Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826, Japan
l Department of Physics, Yamagata University, Yamagata, Yamagata 990-8560, Japan
m Department of Physics, Chiba University, Chiba, Chiba 263-8522, Japan
n Wakayama Medical College, Wakayama, Wakayama 641-0012, Japan
o Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan
p Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
q 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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