1. New (e,e ’ K+) hypernuclear spectroscopy with a high-resolution kaon spectrometer Osamu Hashimoto Department of Physics, Tohoku University December 4-7 EMI2001 at RCNP, Osaka Significance of the (e,e’K+) hypernuclear spectroscopy Lessons from the previous E89-009 (e,e’K+) experiment Optimization of the experimental condition New high-resolution kaon spectrometer for (e,e’K+) spectroscopy

2. Current issues of hypernuclear physics A new degrees of freedom – can examine deeply bound states – baryon structure in nuclear medium New nuclear structure with strangeness – nucleus with a new quantum number – electromagnetic properties Hyperon-nucleon, hyperon-hyperon interaction – Hyperon scattering and hypernuclear structure – S=-2 system and beyond Weak interaction in nuclear medium – decay widths, polarization high quality(high resolution & high statistics) spectroscopy plays a significant role Reaction spectroscopy : From MeV to sub-MeV resolution  -ray spectroscopy

3. １２ Ｃ（  ＋ ，Ｋ ＋ ） 12  C spectra by the SKS spectrometer at KEK 12 GeV PS E336 2 MeV(FWHM) 1.45 MeV(FWHM) 

4. Hypernuclear mass spectra of 89  Y, 139  La and 208  Pb by the (  +,K + ) reaction KEK SKS E140a

5. A  Hyperon in nuclear medium

6. The (e,e ’ K + ) reaction for hypernuclear spectroscopy Proton to  – Neutron rich  hypernculei Large angular momentum transfer Spin-flip amplitude Higher energy resolution   Hyperon production reactions for spectroscopy  Z = 0  Z = -1 comment neutron to  proton to  (  +,K + ) (  -,K 0 ) stretched, high-spin large momentum transfer In-flight (K -,  - ) in-flight (K -,  0 ) substitutional stopped (K -,  - ) stopped (K -,  0 ) large momentum transfer (e,e'K 0 ) (e,e'K + ) spin-flip (  K 0 ) ( ,K + ) & large momentum transfer

7. Comparison of the (K -,  - ),(  +,K + ), (e,e ’ K + ) reactions q~100MeV/c   l=0  substitutional states  S=0  J=0 + q~300MeV/c   l=1,2  stretched states  S=0  J=1 -,2 + q~300MeV/c   l=1,2  stretched states  S=0,1  J=2 -,3 + 12 60 Relative Strength 1-1- 1-1- 1 - +2 - 1-1- 2-2- 3+3+ 3 + +2 + 2+2+ 12 C(e,e’K + ) 12 B  12 C(  +,K + ) 12 C  12 C(  -,  - ) 12 C  2+2+ 0+0+ Ex(MeV) FWHM 2MeV FWHM 0.6MeV 0 6 12 6 0

8. Physics goals of Jlab E01-011 Hypernuclear structure up to medium-heavy mass region 28  Si(e,e’K + ) 28  Al reaction and beyond  N interaction in the p-shell region 12 C(e,e’K + ) 12  B reaction Mirror symmetric  hypernuclei 12  C vs. 12  B Explore hadronic many-body systems with strangeness through the reaction spectroscopy by the (e,e’K + ) reaction High-resolution and high hypernuclear yield rates are keys of the experiment High-resolution 3-400 keV High yield rates > a few 100/day for 12  B ground state (comparable to the (  +,K + ) spectroscopy) Immediate goals New experiment designed based on the E89-009 experience

9. The E89-009 experiment Beam Dump Target Electron beam Focal Plane ( SSD + Hodoscope ) Splitter Magnet K + K + Q D _ D 0 1m Q D _ D Side View Top View Target SOS Spectrometer Resolution 5 x 10 -4 Solid angle 4 msr(with splitter) ENGE Spectrometer Resolution 2x10 -4 (0.66  A@ 1.645 GeV/c) -4 u The first successful (e,e’K + ) hypernuclear spectroscopy u 0 degree tagging method employed u Low luminosity

10. K + arm At very forward angle (2 degrees) Maximum hypernuclear production cross section e’ arm At exactly zero degree Advantage : Maximum virtual photon flux Disadvantage : Huge backgrounds from Bremsstrahlung e-e- e’ K+K+ p K =1.2 GeV/c p e =0.3GeV/c E e =1.8 GeV E89-009 kinematics Beam Target nucleus E  =1.5 GeV

11. E89-009 results Clean observation of 12  B ground state by the (e,e’K + ) reaction About 600 keV(FWHM) resolution 0 degree tagging method employed 12 C(e,e’K + ) 12  B pp p(e,e’K + )  p(e,e’K + )   12 C(e,e’K + ) quasi-free Accidental ss

12. What limited the E89-009 hypernuclear physics experiment ? Energy resolution – Momentum resolution of the kaon spectrometer limited hypernuclear mass resolution Hypernuclear yield rates – Kaon spectrometer solid angle limited detection efficiency – High count rates at the focal plane of the Enge spectrometer set the limit of maximum beam intensity – High accidental background rate A high-resolution large-solid-angle kaon spectrometer designed “Tilt method” proposed

13. E01-011 design principle Optimize the experimental kinematics : Similar to E89-009 Avoid 0 degree Brems associated electrons in ENGE Avoid 0 degree positrons in the kaon arm Maximize acceptance of the kaon spectrometer Higher energy resolution(down to a few 100 keV) Tilt method High resolution kaon spectrometer Matching the momentum acceptances etc.

14. HKS overview Side View （ ） Resolution 2 x 10 Solid angle 30 msr BEAM Q1 Q2 TOF CHAMBER K New QQD Spectrometer 0 1m D -4 1.4 GeV/c1.2 GeV/c 1.0 GeV/c + Beam Dump Target Electron Beam Focal Plane ( SSD + Hodoscope ) Splitter K + K + Q D _ D 0 1m Q D _ D Side View Top View Target SOS Spectrometer Resolution 5 x 10 -4 Solid angle 6 msr(with splitter) ENGE Spectrometer Resolution 2x10 -4 (1.645 GeV/c) -

15. Kaon spectrometer Configuration QQD and horizontal bend Central momentum 1.2 GeV/c Momentum acceptance  12.5 % Momentum resolution(  p/p) 2 x 10 -4 (FWHM) (Beam spot size 0.1mm assumed) Solid angle 18 msr with the splitter (30 msr without the splitter) Kaon detection angle Horizontal: 7 degrees Enge split-pole spectrometer Central momentum 0.3 GeV/c Momentum acceptance  30 % Momentum resolution(  p/p) 5 x 10 -4 (FWHM) Electron detection angle Horizontal: 0 degrees Vertical : 4.5 degrees Basic parameters of the E01-011 experiment Beam condition Beam energy 1.8 GeV, Beam momentum stability < 1 x 10 -4 Beam current 30  A General configuration Splitter+Kaon spectrometer+Enge spectrometer

16. Virtual Photon energy E   1.5 GeV Elementary cross section: constant from 1.1 to 1.5 GeV Higher E  : Smaller momentum transfer  Lower hypernuclear cross section Beam energyE e ~ 1.8 GeV Higher E e : Opens other kaon production channels e’ momentum E e’ ~E e -E   GeV Lower p e’ : Better resolution Kaon momentum p K E  determines p K = 1.2 GeV/c ± 12.5 % Lower p K : Better resolution, particle ID Smaller K survival factor Reaction Threshold(MeV)  p  K   K   K   K * (892)  2 1 1.21.41.61.8 σ total (  b) 1.0 2.0 E γ (GeV) p( ,K + )  Total cross section Phys. Lett. B 445, 20 (1998) M. Q. Tran et al. General experimental condition

17. E e’ = 0.285 GeV/c,  e’ = 0,  K = 0,  K = 30 msr Hypernuclear yield vs. electron energy Kaon momentum ( MeV/c ) Survival rate of kaons 5m 6m 7m 10m 9m 8m Flight path 5-10m K+ Angular distribution Decay in flight

18. Correlation of kaon and electron momenta, and hypernuclear mass

19. What is “ Tilt method ” ? Avoid Brems electron tail – The tail extends due to multiple scattering in the target Avoid Moeller scattering electrons – Peak around 2.5 degrees for the beam energy around 1.8 GeV and momentum acceptance of 300 MeV+- 30% Allows us to run at 250 time higher luminocity compared to E89-009. 1.8 GeV electron beam on a 100mg/cm 2 12 C target Moeller ring Brems electron Scattered electrons in the Enge acceptance

20. Optimization of the tilt angle Very forward peaked Long tail at the larger angle Brems electron angular distribution is more forward peaked(dominated by multiple scattering in the target) Virtual photon flux

21. The “ Tilt method ” requires Optimization of “tilt” geometry Full tracking of scattered electrons at the focal plane of the Enge spectrometer – New tracking chamber Careful study of optics and momentum reconstruction method – Simulation and calibration methods Handling of higher singles rates – Pions, protons Higher rejection efficiency against pions and protons – Cerenkov counters – Better time resolution For the kaon armFor the scattered electron arm

22. Optics of Electron Arm (Splitter + Split-Pole) Tilt 4.5 o with respect to a virtual source point. Optimize the figure of merit for S/A (  e ~ 3 o ) Maximize the average virtual photon acceptance (~1.5%) (HNSS ~ 35%) Minimize the scattered electron rate (~3MHz) (HNSS ~ 200MHz) Clean separation/blocking of the bremsstrahlung electrons Improve momentum acceptance (~180 MeV/c) (HNSS ~ 120 MeV/c) Full measurement of X, X’, Y and Y’ is needed (HNSS – X only)

23. Splitter + Enge energy resolution  5x10 - 4 (FWHM)

24. The HKS spectrometer system under construction Design specification of HKS ConfigurationQ + Q + D Maximum momentum1.2GeV/c Dispersion4.7 cm/% Momentum resolution 210 -4 (FWHM) Solid angle 30 msr w/o splitter 18 msr w splitter Flight path length10 m Angular acceptance 170 mrad vertical 180 mrad horizontal Momentum acceptance±12.5 % Maximum dipole field1.5 T Conductornormal

25. Transport Study (R 12 =R 34 =0) (R 12 =R 44 =0) Horizontal Focusing & Vertical Focusing Horizontal Focusing & Vertical Parallel

26. Chamber position Two DCs  50cm from 1 st order focal plane (235cm downstream from D exit) Momentum resolution  = 270  m  p/p 2  10 -4 FWHM (SOS DC  =160~180  m) Momentum resolution of HKS

27. Solid angle of HKS QQD configuration flexible to adjust vertical and horizontal focusing Horizontally thin Q1, Q2 design allows to minimize distance from the target without bumping to Enge magnet

28. Expected performance Resolution – 300 – 400 keV(FWHM) depending on target mass Yield – More than factor of 50 gain over E89-009 – Comparable to the present (  +,K + ) spectroscopy Accidental background – 8 x 10 -4 /sec vs. 1.3 x 10 -2 /(100nb/sr)/sec per 100 keV bin ( an order of magnitude better than E89-009) – Can be further improved with the lower beam intensity

29. Singles rates Target HKSENGE e + (Hz)  + (kHz) K + (Hz) p (kHz) e - (MHz)  - (kHz) 12 C -8003402802.62.8 28 Si -8002902405.12.8 51 V -7702602306.93.0 I e = 30  A, 100 mg/cm 2 High rejection efficiencies of Cerenkov counters against pions and protons required Scattered electron rate is considerably lower than E89-009 Pion and proton rates of the kaon arm are high

30. A comparison of the (  +,K + ) reaction and the (e,e ’ K + ) reactions for the hypernuclear physics (  +,K + ) (e,e’K + ) (e,e’K + )/(  +,K + ) SKS E89-009 RATIO E01-011/E89-009 Cross sections ( 12  C gr or 12  B gr ) 10 0.05 5x10 -3 (  b/sr) ( ,K + ) Target thickness 1 0.01 10 -2 ~ 4.5 (g/cm 2 ) Beam intensity 10 6 10 9-10 10 3-4 ~ 45 (particle/sec) (virtual photon) K + momentum 0.72 1.2 (GeV/c) K + solid angle ~ 60 % ~ 10 % 0.18 ~ 3 coverage (%) (100 msr) (4 msr) K + survival rate(%) ~ 0.4 ~ 0.4 1 ~ 0.8 (Flight path) (5 m) (8 m) Overall ~1 x 10 - 1~-2 ~ 100 SKS at KEK vs HNSS at Jlab

31. Yield comparison of E01-011 and E89-009 Item E01- 011 E89-009 Gain factor Virtual photon flux per electron(x10 -4 ) 0.3540.0875 Target thickness(mg/cm 2 )100224.5 Scattered electron momentum acceptance(MeV/c) 1501201.2 Kaon survival rate0.350.40.88 Solid angle of K arm1844.5 Beam current(  A) 300.6645 Estimated yield ( 12  B gr :counts/h)76 0.9 (measured) 85

32. Expected hypernuclear production rates in the (e,e ’ K + ) reaction Target Beam Intensity (  A) Counts per 100nb/sr /hour Q-free K+ in HKS(Hz ) 12 C3072510 28 Si3031432 51 V3016342 Target Hypernucleu s  orbital Cross section (nb/sr) 12 C 12  B s1/2112 p3/279 p1/245 28 Si 28  Al s1/256 p3/295 p1/257 d5/2131 d3/2111 51 V 51  Ti s1/218 p3/241 p1/226 d5/252 d3/248 1s1/216 f7/232 f5/238 Calculated hypernuclear cross sections (Target thickness 100 mg/cm 2 with HKS) Hypernuclear production rates Motoba

33. Expected energy resolution ItemContribution to the resolution(keV, FWHM) TargetCSiVY HKS momentum(2x10 -4 )216 Beam momentum(<1x10 - 4 ) < 180 Enge momentum(5x10 -4 )150 K + angle(  mr  152643620 Target thickness (100mg/cm 2 ) < 180< 171< 148< 138 Overall< 400< 370< 350 2001.12.05.

34. Expected hypernuclear spectra

35. An installation plan of the new spectrometer system in Hall C

36. Summary New (e,e’K + ) hypernuclear spectroscopy, E01-011, was proposed based on the experience of the pioneering E89- 009 experiment at Jlab. With the new “Tilt method” and with a new high-resolution kaon spectrometer(HKS), twice better resolution, more than an order of magnitude higher yield rates and an order of magnitude better signal to accidental ratios will be realized. Design of the high-resolution kaon spectrometer (HKS) has been completed and construction of the spectrometer magnets and detector systems is under way supported by Monkasho. The spectrometer system will be ready for installation by the beginning of 2003, although it is subject to Jlab scheduling.

37. HKS magnet detail design Dipole Vacuum extension Enge magnet Q1Q2 Splitter Photon line Beam

38. Expected accidental coincidence rate Electron arm singles rate 2.6 MHz Kaon singles rate 340 Hz Coincidence time window 2 nsec ( Offline analysis ) N acc = 1.8 /sec At the beam current of 30  A, we expect

39. 3H3H 4  H, 4  He 6  He, 6  Li 7  He, 7  Li, 7  Be 9  Be 10  Be, 10  B 13  C  hyperon Nucleon Alpha 荷電対称ハイパー核とクラスター構造

40. Spin dependence of the (e,e’K+) reaction Spin independent termf 0 f 0 spin nonflip Spin dependent term g g o spin nonflip g 1 g -1 spin flip Natural parity states Unnatural parity states

41. Elementary electroproduction of a hyperon Virtual photon flux Virtual photon polarization Longitudinal polarization E  = E e - E e’ Virtual photon becomes almost real at very forward electron scattering angle

42. Cross Section View of the HKS Photon line Electron line

43. Basic experimental conditions of E89- 009 Electron detection including 0 degrees – greater photoproduction cross section of a  hyperon – E  > 1.2 GeV – E  = E e - E e’, E e = 1.645 GeV and E e’ ~0.285 GeV K + detection at forward angle including 0 degrees – P K + ~ 1.1 GeV/c – flight path as short as possible ~ 8 m A splitter magnet for e’ and K + separation at very forward angles Beam intensity < 1  A Target thickness ~ 10 mg/cm 2

44.  e’ (Degrees)  e’ vs  e’ (Degrees)  e’ is aimed about 3 o. Virtual process is used in the optical simulation. The long tails are from the features of tilted Split-pole and momentum dependent

45. Accepted e’ from virtual process Collimator Bremsstrahlung/Multiple scattered e’ - Bremsstrahlung separation - Max. VPF - Min. electron rate  P e’ (%) - Mom. acceptance ~ 180 MeV/c - Average VPF acceptance: ~1.5%

46. Requirement Proton rejection efficiency 4  10 -4 of Tohoku Univ. Florida Int. Univ. two identical walls with  =0.02 Water Cherenkov n=1.33 with and w/o WLS Lucite Cherenkov n=1.49 with WLS w/o WLS (Total reflection type) Prototypes were built Waiting for R&D beamtime HKS Water (or Lucite) Cherenkov counter

47. Focal Plane Correlations Reconstruction from (X f, X f ’, Y f and Y f ’ ) to (X t ’, X t ’, Y t, Y t ’ and  X f vs  Y f vs  X f ’ vs  Y f ’ vs 