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 E (e,e’K+) experiment Optimization of the experimental condition New high-resolution kaon spectrometer for (e,e’K+) spectroscopy

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

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

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

A  Hyperon in nuclear medium

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

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 -, Relative Strength C(e,e’K + ) 12 B  12 C(  +,K + ) 12 C  12 C(  -,  - ) 12 C  Ex(MeV) FWHM 2MeV FWHM 0.6MeV

Physics goals of Jlab E 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 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 E experience

The E 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 Solid angle 4 msr(with splitter) ENGE Spectrometer Resolution 2x10 -4 (0.66  GeV/c) -4 u The first successful (e,e’K + ) hypernuclear spectroscopy u 0 degree tagging method employed u Low luminosity

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 E kinematics Beam Target nucleus E  =1.5 GeV

E 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

What limited the E 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

E design principle Optimize the experimental kinematics : Similar to E 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.

HKS overview Side View （ ） Resolution 2 x 10 Solid angle 30 msr BEAM Q1 Q2 TOF CHAMBER K New QQD Spectrometer 0 1m D 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 Solid angle 6 msr(with splitter) ENGE Spectrometer Resolution 2x10 -4 (1.645 GeV/c) -

Kaon spectrometer Configuration QQD and horizontal bend Central momentum 1.2 GeV/c Momentum acceptance  12.5 % Momentum resolution(  p/p) 2 x (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 (FWHM) Electron detection angle Horizontal: 0 degrees Vertical : 4.5 degrees Basic parameters of the E experiment Beam condition Beam energy 1.8 GeV, Beam momentum stability < 1 x Beam current 30  A General configuration Splitter+Kaon spectrometer+Enge spectrometer

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)  σ total (  b) E γ (GeV) p( ,K + )  Total cross section Phys. Lett. B 445, 20 (1998) M. Q. Tran et al. General experimental condition

E e’ = 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

Correlation of kaon and electron momenta, and hypernuclear mass

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 E GeV electron beam on a 100mg/cm 2 12 C target Moeller ring Brems electron Scattered electrons in the Enge acceptance

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

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

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)

Splitter + Enge energy resolution  5x (FWHM)

The HKS spectrometer system under construction Design specification of HKS ConfigurationQ + Q + D Maximum momentum1.2GeV/c Dispersion4.7 cm/% Momentum resolution (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

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

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

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

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

Singles rates Target HKSENGE e + (Hz)  + (kHz) K + (Hz) p (kHz) e - (MHz)  - (kHz) 12 C Si V 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 E Pion and proton rates of the kaon arm are high

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 E RATIO E01-011/E Cross sections ( 12  C gr or 12  B gr ) x10 -3 (  b/sr) ( ,K + ) Target thickness ~ 4.5 (g/cm 2 ) Beam intensity ~ 45 (particle/sec) (virtual photon) K + momentum (GeV/c) K + solid angle ~ 60 % ~ 10 % 0.18 ~ 3 coverage (%) (100 msr) (4 msr) K + survival rate(%) ~ 0.4 ~ ~ 0.8 (Flight path) (5 m) (8 m) Overall ~1 x ~-2 ~ 100 SKS at KEK vs HNSS at Jlab

Yield comparison of E and E Item E E Gain factor Virtual photon flux per electron(x10 -4 ) Target thickness(mg/cm 2 ) Scattered electron momentum acceptance(MeV/c) Kaon survival rate Solid angle of K arm Beam current(  A) Estimated yield ( 12  B gr :counts/h) (measured) 85

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 C Si V Target Hypernucleu s  orbital Cross section (nb/sr) 12 C 12  B s1/2112 p3/279 p1/ Si 28  Al s1/256 p3/295 p1/257 d5/2131 d3/ 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

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

Expected hypernuclear spectra

An installation plan of the new spectrometer system in Hall C

Summary New (e,e’K + ) hypernuclear spectroscopy, E01-011, was proposed based on the experience of the pioneering E 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.

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

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

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

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

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

Cross Section View of the HKS Photon line Electron line

Basic experimental conditions of E Electron detection including 0 degrees – greater photoproduction cross section of a  hyperon – E  > 1.2 GeV – E  = E e - E e’, E e = 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

 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

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%

Requirement Proton rejection efficiency 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

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 