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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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Physics motivation…

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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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Previous Experiment…

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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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Present experiment.

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Experimental setup

K+

e’e beam

Enge

HKS

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Experimental setup

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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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Tilt angle of Enge (e’ arm)

Side view

Accepted region.

7.75 degree

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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, 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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Data analysis…

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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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Kaon PID

coincidence time (ns)

tof –

tr

ack

p

K

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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

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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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Accidental

True

Coincidence time with 12C target, 26 A

Coincidence time (ns)

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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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Coincidence time with 28Si target

Coincidence time (ns)

True/accidental ~ 2

Accidental

True

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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.