CsI光電面と GEMを用いたガスチェレンコフ検出器

omega meson
in nucleus,
experimental study
K. Ozawa
(Univ. of Tokyo)
Contents
• Physics motivation for w meson
• Experimental approaches
• Previous experiments
• Proposed experiment at J-PARC
• Summary
Collaboration with, or helped by
Prof. R.S. Hayano,
Prof. H. Nagahiro,
Prof. S. Hirenzaki,
K. Utsunomiya, S. Masumoto,
Y. Komatsu, Y. Watanabe
I need more helps from you!
2009/2/22
NQCD symposium, K. Ozawa
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Hadrons in QCD
• hadron can be undestood as
excitation of QCD vacuum
Precise measurements of hadron
property at nuclear medium can provide
QCD information
Mass [GeV]
 many experimental and theoritical efforts
to search for and study in-medium
modifications of hadrons
Figure by Prof. V. Metag
Modification of vector meson mass
is expected, even at nuclear
density.
I’d like to focus on vector mesons, such as w.
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Experimental approaches
– Meson spectroscopy
Nucleon Hole
p, p, g
Meson
Emitted Proton
Neutron
Decay
Target
– Direct measurements of mass spectra
2009/9/18
PUHF WS, K. Ozawa
4
Results from LEPS
Chiral ’05
N. Muramatsu
There some
hints of a
bound state.
Missing mass
resolution of
~30MeV/c2 is
expected.
Forward
measurements
are essential.
Large statistics
data and further
analysis are
waited.
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Results from CBELSA/TAPS
TAPS, w  p0g with g+A
D. Trnka et al., PRL 94 (2005) 192203
p
g
w
g
p0
g
mw 
advantage:
after background subtraction
gA  w + X
m
p0g
g
p p  p g 2
gg
m
 3.0 %
• p0g large branching ratio (8 %)
• no -contribution (  p0g : 7  10-4)
disadvantage:
• p0-rescattering
2009/2/22
NQCD
symposium, K. Ozawa
mw = m
0 (1 -  /0) for  = 0.13
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TAPS results II
Large w width in nuclei due to w-N interaction.
M. Kotulla et al, PRL 100 (2008) 192302
Essential:
Focus on Small momentum
Issue:
Yield estimation of decays
60 MeV/c2 even at stopped w.
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Proposed experiment
Two measurements at the same time.
– Meson spectroscopy
– Direct measurements of mass spectra
Nucleon Hole
p-
w
Emitted Neutron
p0 g decay
Target
Clear measurements in small momentum!
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Bound w state search
J-PARC
• Beam Energy：
• Beam Intensity:
2009/2/22
50ＧｅＶ
(30GeV for Slow Beam)
3.3x1014ppp, 15mA
(2×1014ppp, 9mA)
NQCD symposium, K. Ozawa
Hadron Hall
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Hadron hall
NP-HALL
56m(L)×60m(W)
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Reaction and Beam momentum
n
w
g
mw 
g
Generate w meson using p beam.
Emitted neutron is detected at 0.
p A  w + N+X
Decay of w meson is detected.
p0
p0g
g
gg
p p  p g 2
To generate stopped modified
w meson, beam momentum is
~ 1.8 GeV/c. (K1.8 can be used.)
As a result of KEK-E325,
9% mass decreasing (70 MeV/c2) can
be expected.
Focus on forward (~2°).
2009/2/22
If p momentum is chosen carefully,
momentum transfer will be ~ 0.
w momentum [GeV/c]
p
Stopped w meson
0.4
0.2
0
0
2
p momentum [GeV/c]
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Note: Forward measurements
• Forward proton
– Good
• High mass resolution
• High efficiency
– Bad
• No separation between proton and p beam.
Triggering generated protons is too hard.
• Forward 1~2°will be excluded.
• Forward neutron
– Good
• 0 degree measurements
– Bad
• Need long TOF for high resolution
• Low efficiency < 30%
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Experimental setup
Beam
Neutron
p-p  wn @ 1.8 GeV/c
 p0 g
 gg
Target: Carbon 6cm
Small radiation loss
Clear calculation of w
bound state
Gamma
Detector
Ca, Nb, LH2 are under
consideration.
Neutron Detector
Flight length 7m
60cm x 60 cm (~2°)
Gamma Detector
Assume T-violation’s
75% of 4p
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SKS for charge sweep
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Neutron Measurement
Timing resolution
Beam test is done at Tohoku test line
Timing resolution of 80 ps is
achieved (for charged particle).
It corresponds to mass resolution of
22 MeV/c2.
Neutron Efficiency
Iron plate (1cm t) is placed to increase neutron efficiency.
Efficiency is evaluated using a hadron transport code, FLUKA.
Neutron efficiency of 25% can be achieved.
We can not see a clear bound peak.
At this moment, there is no beam
line at J-PARC to have enough TOF
length and beam energy
2009/2/22
NQCD symposium, K. Ozawa
Bound
region
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Gamma detector
CsI EMCalorimeter
T-violation’s one is assumed.
（D.V. Dementyev et al., Nucl. Instrum. Meth. A440(2000), 151）
Obtained p meson spectra for
stopped K decays
Assumed Energy
Resolution
Muon holes should be filled by additional crystals.
Acceptance for w is evaluated as 90%.
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NQCD symposium, K. Ozawa Fast simulation is tuned
to reproduce existing data.
Decay Yield Evaluation
Based on measured crosssection of
p-p  wn for backward w
(G. Penner and U. Mosel, nucl-th/0111024,
J. Keyne et al., Phys. Rev. D 14, 28 (1976))
Production cross section
0.02 mb/sr (CM) @ s = 2.0 GeV
0.17 mb/sr (Lab) @ s = 2.0 GeV
H. Nagahiro et al calculation based
on the cross section and known
nuclear effects.
Assumed potential is consistent
with w absorption in nucleus.
Beam intensity
107 / spill, 6 sec spill length
Neutron Detector acceptance
Dq = 2°(60 cm x 60 cm @ 7m)
Gamma Detector acceptance
90% for w
Radiation loss in target
11%
Survival probability in final state interaction
60%
Beam Time
100 shifts
Branching Ratio
1.3 %
8.9 %
2009/2/22
Total
No
interact
Interact
w nuclei
Large width ~ 60 MeV/c2

w p 0g
*
total  abs ,
 1.26 %
*
total
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Results for three potentials
Generation of w
H. Nagahiro et al
Large abs.
No int.
Large abs.
Large int.
Decay of w (Invariant Mass)
2366
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2755
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938
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Final Spectrum
One can select
bound region as
Energy of w < E0,
which is measured
by the forward
neutron counter.
Bound
region
Including Background: Main background is 2p0 decays and 1g missing
Strong
kin.
effects
symposium, K.
Ozawa
Invariant MassNQCD
spectrum
for
the bound region
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“Mass” Correlation
 Invariant Mass VS Missing Energy
 Non-correlated model
Correlated model
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Correlation analysis will useful for reducing kinematical effects.
Issue
• It’s hard to find a bound state peak using
forward neutron measurements at J-PARC
due to a limited hadron hall space at this
moment.
– Note: proton measurements are also hard.
• Effects of relatively large angle to form a bound state
• Effects of beam spread and halo for a trigger.
• When we focus on “mass modification” of w
meson in nucleus, large nucleus should be
used.
– In addition, we can measure a large mass
width (absorption cross section) of w in
nucleus in small momentum range.
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Summary
• Hadrons can be understood as a excitation
of “QCD vacuum” and carried “vacuum”
information.
• Experimental efforts are underway to
investigate this physics. Some results are
already reported.
– Still, there are problems to extract physics
information.
• New experiments for obtaining further
physics information is being proposed.
– Measurement with stopped mesons
– Measurement of bound states
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