1. 1 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

2. 2 Physics motivation…

3. 3  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) V  N (~30 MeV) < V NN (~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  n p

4. 4 Physics issues 12 C  12  B Precision analysis of core excited states. p orbit states splitting? Comparison with the mirror hypernucleus, 12  C (KEK/SKS). 28 Si  28  Al The first precision spectroscopy beyond the p-shell. ls splitting in the p, d orbits? Other targets ( 6,7 Li, 9 Be, 10,11 B, 51 V, 89 Y). Rate study for heavier targets for next exp. p-shell spectroscopy. Target mass dependence --- quasifree K + electroproduction.

5. 5 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+K+ e - beam Target nucleus p **

6. 6 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

7. 7 Previous Experiment…

8. 8 12  B spectrum of previous exp. d  /d  nb/sr/0.3 MeV -B  (MeV) -15051015-5-10 (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 structure of core-nucleus excited states.

9. 9 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 12 C target. Required improvements for the new experiment. 1. Reduce the accidental rate in e’ spectrometer. 2. Improve the energy resolution of Kaon spectrometer.

10. 10 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 7.75 o Decrease singles rate  improve signal to accidental ratio. be able to increase beam current.

11. 11 Present experiment.

12. 12 Experimental setup K+K+ e’ e beam Enge HKS

13. 13 Experimental setup

14. 14 New spectrometer Dipole Q1 Q2 Configuration Q+Q+D Momentum range 1.0 – 1.4 GeV/c Momentum resolution 2 x 10 -4 (FWHM) Dispersion 4.7 cm/% Solid angle 16 msr Momentum acceptance 12.5 % Made in Japan 。 High resolution Kaon Spectrometer -HKS

15. 15 HKS detector – Kaon trigger 1X 1Y AC 2X WC K+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

16. 16 Tilt angle of Enge (e’ arm) Side view Accepted region. 7.75 degree

17. 17 Tilted Enge spectrometer 7.75 degree

18. 18 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.

19. 19 Data summary Target 6 Li, 7 Li, 9 Be, 10 B, 12 C, 28 Si, 51 V, 89 Y, 208 Pb, CH 2 calibration data / physics data / trigger study Electron Beam Intensity, I  26  A for 12 C 18  A for 28 Si Energy stability ~ 50 keV

20. 20 Trigger condition HKS (Kaon trigger) --- 1.2 x 10 4 Hz 1X x 1Y x 2X x AC x WC ( 1X x 2X : 1.1 x 10 6 Hz ) Rejection rate by AC / WC is 1/100 Enge (e’ trigger) --- 1.2 x 10 6 Hz (  1 x 10 8 Hz) Hodoscope 1layer x 2layer Coincidence of K and e’ --- ~500 Hz DAQ dead time ~5% *Rates are with 12 C target (100 mg / cm 2 ), 26  A Previous exp.

21. 21 Previous vs. Present experiment Old : New Beam intensity, 1.6  A : 26  A Target thickness, 22 mg/cm 2 : 102 mg/cm 2 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 msr Energy Resolution, 750 keV : 400 keV Kaon arm (  p/p), 5x10 -4 : 2x10 -4 Tilt method is quite useful!

22. 22 Data analysis…

23. 23 Detector performance HKS (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

24. 24 Water and aerogel cherenkov  p K+K+ Sum of WC npe p K+K+ Sum of A C npe  tof - K Aerogel : Reject pions Water : Reject protons Veto conditions are loose in trigger. Off-line analysis Reject pions Reject protons

25. 25 Time-of-flight Average TOF resolution : TOF = 1X – 2X, 180 ps  tof –  track p K  After cherenkov cut

26. 26 Coincidence time e’ time at Enge K + time at HKS Target Beam bunch 2ns (499MHz) Reconstruct timing at target from timing at detector position. From coincidence time, select true Coincidence events (reject accidental events)

27. 27 Previous experiment Ratio of true / accidental in coincidence time Present experiment With 1  A, CH 2 targetWith 1.5  A, CH 2 target S/N Improved!  ~300 ps

28. 28 Kaon PID coincidence time (ns)  tof –  track  p K

29. 29 Particle and Trigger Rate 11 13032 28 Si(65) 41115021 12 C(100) e + [kHz] p [kHz] π [kHz] K [Hz] Target (mg/cm 2 ) HKS single arm particle rate at 30 uA 18 30 Beam Current (uA) 1.6 1.3 Enge Single (MHz) 91015.3 28 Si 74014.8 12 C Coin (Hz)HKS single (KHz) Target Trigger rate 89 Y 13 15.4 1.8 1040

30. 30 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 CH 2 target   masses are well known. 12  B ground state binding energy was measured in the previous experiments.

31. 31 Calibration data from the p(e,e’K + )  0 reactions 12 C(e,e’K + ) quasi-free Accidental Previous experiment Present experiment S/N Improved! 210 Lambdas 2000 Lambdas  ~930 keV  

32. 32 Accidental True Coincidence time with 12 C target, 26  A Coincidence time (ns)

33. 33 Carbon ( 12  B) data ~ 500 counts (~10/hr) ~2 MeV(FWHM) (Previous exp. 165 counts with 900 keV. HallA 300 counts with 700 keV.) Very preliminary s-shell p-shell Accidental events

34. 34 Coincidence time with 28 Si target Coincidence time (ns) True/accidental ~ 2 Accidental True

35. 35 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 12 C and 18  A for 28 Si. 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.