Transcript Physics with the HKS

Hypernuclear spectroscopy up to
medium mass region through
the (e,e’K+) reaction in JLab
Mizuki Sumihama
For HKS collaboration
Department of Physics
Tohoku university
2006 HNP
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Physics motivation…
2
•
LN interaction
L hypernuclei
L
• Unified view of baryon-baryon interaction by
including new degree of freedom, strangeness.
• Central and spin-dependent LN interaction.
much smaller than NN interaction.
ex) VLN (~30 MeV) < VNN (~50 MeV)
Narrow widths of excited states
High precision spectroscopy is necessary
• Unique structure of
hadronic many-body system
•
•
•
Deeply bound states, no Pauli blocking.
Core excited states.
Glue role of a L hyperon in nucleus.
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p
n
Physics issues
12C
12LB
Precision analysis of core excited states.
p orbit states splitting?
Comparison with the mirror hypernucleus,
12 C (KEK/SKS).
L
28Si
 28LAl
The first precision spectroscopy beyond the p-shell.
ls splitting in the p, d orbits?
Other targets (6,7Li, 9Be, 10,11B, 51V, 89Y).
Rate study for heavier targets for next exp.
p-shell spectroscopy.
Target mass dependence --- quasifree K+
electroproduction.
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Basic characteristics
+
of (e,e’K ) spectroscopy
Hadron (K or p) beam –BNL/AGS, KEK/SKS..:
• Large cross section,
• Energy resolution ~ 1.45 MeV,
limited by energy resolution of beam.
Electron beam :
• Small cross section,
recovered by high intensity
continuous e beam in JLab.
p
• 400 keV (FWHM)
g*
- beam
e
energy resolution.
K+
e’
Target nucleus
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The
+
(e,e’K )
reaction
Proton converted to L
 Charge symmetry
Neutron rich L hypernuclei
Large momentum transfer
 Similarly to (p+,K+) reaction
Spin-flip amplitude
 Unnatural parity hypernuclear states
400 keV resolution
 High quality primary beam
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Previous Experiment…
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12
LB
spectrum of previous exp.
ds/dW nb/sr/0.3 MeV
Ground state doublet
(1-,2-)
90
80
Binding energy
BL = 11.4±0.5 MeV
(2+,3+)
Emulsion data
BL = 11.37 MeV
1 month
(1-,0-)
Cross section
140±17(stat) ±18(sys) nb/sr
Motoba’s calculation
138 nb/sr
70
(2-,1-)
60
50
40
-15
-10
-5
0
-BL(MeV)
5
10
15
More statistics and better
resolution are required
to see more precise structure
of core-nucleus excited states.
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First experiment
of X(e,e’K+)LX
Existing Kaon spectrometer in JLab/HallC
the energy resolution - 750keV
0 degree tagging geometry.
large backgrounds of electrons/positrons
from pair creation.
only 1.6 mA beam current with 12C target.
Required improvements for the new experiment.
1. Reduce the accidental rate in e’ spectrometer.
2. Improve the energy resolution of Kaon
spectrometer.
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Improvement
in present experiment
New Kaon spectrometer –HKS
200 keV  400 keV (old)
in total 400 keV  750 keV (old).
Tilt e’ spectrometer to avoid 0 degree.
Tilted angle 7.75o
Decrease singles rate 
improve signal to accidental ratio.
be able to increase beam current.
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Present experiment.
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Experimental setup
e’
Enge
HKS
K+
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Experimental setup
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New spectrometer
High resolution Kaon Spectrometer -HKS
Configuration
Momentum range
Momentum resolution
Dispersion
Solid angle
Momentum acceptance
Dipole
Q2
Q+Q+D
1.0 – 1.4 GeV/c
2 x 10-4 (FWHM)
4.7 cm/%
16 msr
12.5 %
Q1
Made in Japan。
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HKS detector –Kaon trigger
Aerogel cherenkov (n=1.05)
 pion rejection
1X 1Y
AC
2X WC
K+
DC1
Drift chamber
(uu’xx’vv’ wire)
 x, x’, y, y’
Water cherenkov
(n=1.33)
 proton rejection
Plastic scintillator
 time-of-flight
DC2
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Tilt angle of Enge (e’ arm)
Accepted region.
7.75 degree
Side view
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Tilted Enge spectrometer
7.75 degree
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Enge detector
2 layers of hodoscope
detect charged particle (e’)
make trigger, timing at focal plane.
Drift chamber
10 planes, xx’,uu’,xx’,vv’,xx’
measure positions/angles, x,x’,y,y’
at focal plane.
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Data summary
Target
6Li, 7Li, 9Be, 10B, 12C, 28Si, 51V,
89Y, 208Pb, CH
2
calibration data / physics data / trigger study
Electron Beam
Intensity, I ~ 26 mA for 12C
18 mA for 28Si
Energy stability ~ 50 keV
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Trigger condition
HKS (Kaon trigger) --- 1.2 x 104 Hz
1X x 1Y x 2X x AC x WC
Previous exp.
( 1X x 2X : 1.1 x 106 Hz )
Rejection rate by AC / WC is 1/100
Enge (e’ trigger) --- 1.2 x 106 Hz ( 1 x 108 Hz)
Hodoscope 1layer x 2layer
Coincidence of K and e’ --- ~500 Hz
DAQ dead time ~5%
*Rates are with 12C target (100 mg / cm2), 26 mA
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Previous vs. Present
experiment
Old
Beam intensity,
1.6 mA
Target thickness,
22 mg/cm2
Luminosity,
1
Singles rate of e’ arm, >100 MHz
: New
: 26 mA
: 102 mg/cm2
: ~75
: 1.2 MHz
Tilt method is quite useful!
~10-2
(Coincidence trigger 500 Hz with 5% dead time)
Kaon acceptance,
6 msr
: 16 msr
Energy Resolution,
750 keV : 400 keV
Kaon arm (Dp/p), 5x10-4 : 2x10-4
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Data analysis…
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Detector performance
HKS (K+ detection)
Enge (e’ detection)
Drift chambers
Drift chamber
Position resolution
Position resolution
s~220 mm
s = 300~370 mm
Detection efficiency
Detection efficiency,
~98%
~99%
TOF counters
s~180 ps
Hodoscope
Aerogel cherenkov (veto p)
s~150 ps
index = 1.05
efficiency > 98%
Water cherenkov (veto p)
index = 1.33
efficiency > 98%
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Water and aerogel cherenkov
Veto conditions are loose in trigger.
btof-bK
btof-bK
Reject pions
K+
p
K+
p
p
Reject protons
Sum of AC npe
Aerogel : Reject pions
Sum of WC npe
Water : Reject protons
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Time-of-flight
Average TOF resolution : TOF = 1X – 2X, 180 ps
p
K
p
btof – btrack
p
K
p
After cherenkov cut
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Coincidence time
•Reconstruct timing at target
from timing at detector position.
•From coincidence time, select true
Coincidence events (reject accidental events)
K+ time at HKS
e’ time at Enge
Target
Beam bunch 2ns (499MHz)
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Ratio of true / accidental
in coincidence time
Previous experiment
Present experiment
s ~300 ps
With 1 mA, CH2 target
With 1.5 mA, CH2 target
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btof – btrack
Kaon PID
p
K
p
coincidence time (ns)
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Particle and Trigger Rate
HKS single arm particle rate at 30 uA
Target
p
K
π
e+
(mg/cm2)
[kHz]
[Hz]
[kHz]
[kHz]
12C(100)
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150
11
4
28Si(65)
32
130
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Trigger rate
Target
Beam
Current (uA)
HKS single
(KHz)
Enge Single
(MHz)
Coin (Hz)
12C
30
14.8
1.3
740
28Si
18
15.3
1.6
910
1.8
1040
89Y
13
15.4
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Calibration data
for spectrometer optics
Need new optics parameters for both arms.
Enge is tilted. HKS is new.
Angle calibration.
Data with sieve slits were taken.
Momentum calibration.
p(e,e’K+)L/S0 reactions wirh CH2 target
L, S0 masses are well known.
12 B ground state
L
binding energy was measured in the
previous experiments.
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Calibration data from
the p(e,e’K+)L/S0 reactions
Previous experiment
Present experiment
s~930 keV
12C(e,e’K+)
quasi-free
L
S0
Accidental
210 Lambdas
2000 Lambdas
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Coincidence time
with 12C target, 26 mA
True
Accidental
Coincidence time (ns)
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Carbon
12
(
LB)
data
Accidental events
~ 500 counts (~10/hr) ~2 MeV(FWHM)
(Previous exp. 165 counts with 900 keV.
HallA
300 counts with 700 keV.)
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Coincidence time with 28Si target
True/accidental ~ 2
True
Accidental
Coincidence time (ns)
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Summary
Experiment was carried out in JLab/HallC
by using ‘tilted ENGE’ and ‘new spectrometer HKS’.
Comparing with the previous experiment,
the accidental rate decreases dramatically.
We took data with 26 mA for 12C and
18 mA for 28Si.
Physics run with Si target. About 214 hrs.
L/S0 peaks and 12LB ground state are observed.
Optics study is in underway.
The data will provide medium-heavier hypernuclear
spectra with good statistics and good resolution ever
achieved.
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