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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. Physics motivation…. p. L. n. L hypernuclei. L N interaction - PowerPoint PPT Presentation
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Hypernuclear spectroscopy up to medium mass region through the (e,e’K+) reaction in JLab
Mizuki SumihamaFor HKS collaboration
Department of PhysicsTohoku university
2006 HNP
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hypernuclei• N interaction
• Unified view of baryon-baryon interaction by including new degree of freedom, strangeness. • Central and spin-dependent N interaction. much smaller than NN interaction. ex) VN (~30 MeV) < VNN (~50 MeV)
• Unique structure of hadronic many-body system
• Deeply bound states, no Pauli blocking.• Core excited states. • Glue role of a hyperon in nucleus.
High precision spectroscopy is necessary
Narrow widths of excited states
np
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Physics issues12C 12
B Precision analysis of core excited states. p orbit states splitting? Comparison with the mirror hypernucleus,
12C (KEK/SKS).
28Si 28Al
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 ) 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. • 400 keV (FWHM) energy resolution.
e’
K+
e- beam
Target nucleus
p*
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The (e,e’K+) reaction
Proton converted to Charge symmetry Neutron rich hypernuclei
Large momentum transfer Similarly to (+,K+) reaction
Spin-flip amplitude Unnatural parity hypernuclear states
400 keV resolution High quality primary beam
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12B spectrum of previous
exp.d/
d
nb
/sr/
0.3
MeV
-B(MeV)
-15 0 5 10 15-5-10
(2+,3+)(1-,2-)
(1-,0-)
(2-,1-)
40
50
60
70
80
90
Ground state doublet
B = 11.4±0.5 MeV
Cross section 140±17(stat) ±18(sys) nb/sr
Motoba’s calculation 138 nb/sr
Binding energy
Emulsion data B = 11.37 MeV 1 month
More statistics and better resolution are required to see more precise structureof core-nucleus excited states.
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First experiment of X(e,e’K+)X
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 A 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 experimentNew 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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New spectrometer
Dipole Q1Q2
Configuration Q+Q+D Momentum range 1.0 – 1.4 GeV/cMomentum resolution 2 x 10-4
(FWHM)Dispersion 4.7 cm/%Solid angle 16 msrMomentum acceptance 12.5 %
Made in Japan。
High resolution Kaon Spectrometer -HKS
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HKS detector –Kaon trigger
1X 1Y AC 2X WC
K+
DC1 DC2
Dipole
Drift chamber(uu’xx’vv’ wire) x, x’, y, y’
Plastic scintillator time-of-flight
Aerogel cherenkov (n=1.05) pion rejection
Water cherenkov (n=1.33) proton rejection
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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, CH2
calibration data / physics data / trigger study
Electron Beam Intensity, I 26 A for 12C 18 A 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 ( 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 A
Previous exp.
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Previous vs. Present
experiment Old : New
Beam intensity, 1.6 A : 26 ATarget thickness, 22 mg/cm2 : 102 mg/cm2
Luminosity, 1 : ~75 Singles rate of e’ arm, >100 MHz : 1.2 MHz
~10-2
(Coincidence trigger 500 Hz with 5% dead time)Kaon acceptance, 6 msr : 16 msrEnergy Resolution, 750 keV : 400 keV
Kaon arm (p/p), 5x10-4 : 2x10-4
Tilt method is quite useful!
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Detector performanceHKS (K+ detection)
Drift chambers Position resolution ~220 m Detection efficiency ~98%
TOF counters ~180 ps
Aerogel cherenkov (veto ) index = 1.05 efficiency > 98%
Water cherenkov (veto p) index = 1.33 efficiency > 98%
Enge (e’ detection)Drift chamber
Position resolution = 300~370 m Detection efficiency, ~99%
Hodoscope ~150 ps
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Water and aerogel cherenkov
p
K+
Sum of WC npe
p
K+
Sum of AC npe
to
f -K
to
f -K
Aerogel : Reject pions Water : Reject protons
Veto conditions are loose in trigger.
Off-line analysis
Reject pions
Reject protons
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Time-of-flightAverage TOF resolution : TOF = 1X – 2X, 180 ps
tof – track
p K
p K After cherenkov cut
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Coincidence time
e’ time at Enge K+ time at HKS
TargetBeam bunch 2ns (499MHz)
•Reconstruct timing at target from timing at detector position.•From coincidence time, select true Coincidence events (reject accidental events)
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Previous experiment
Ratio of true / accidental in coincidence time
Present experiment
With 1 A, CH2 target With 1.5 A, CH2 target
S/N Im
proved!
~300 ps
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Particle and Trigger Rate
11111303228Si(65)
4111502112C(100)
e+
[kHz]
p
[kHz]
π
[kHz]
K
[Hz]
Target
(mg/cm2)
HKS single arm particle rate at 30 uA
18
30
Beam Current (uA)
1.6
1.3
Enge Single (MHz)
91015.328Si
74014.812C
Coin (Hz)HKS single(KHz)
Target
Trigger rate
89Y 13 15.4 1.8 1040
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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+) reactions wirh CH2 target masses are well known. 12
B ground state binding energy was measured in the previous experiments.
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Calibration data from the p(e,e’K+)0 reactions
12C(e,e’K+) quasi-free
Accidental
Previous experiment Present experiment
S/N Im
proved!
210 Lambdas 2000 Lambdas
~930 keV
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Carbon (12B) data
~ 500 counts (~10/hr) ~2 MeV(FWHM)(Previous exp. 165 counts with 900 keV. HallA 300 counts with 700 keV.)
Very
prelim
inar
y s-shell p
-shell
Accidental events
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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 A for 12C and 18 A for 28Si.
Physics run with Si target. About 214 hrs.0 peaks and 12
B 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.