Jan. 18, 2008Hall C Meeting

L. Yuan/Hampton U.

Outline HKS experimental goals HKS experimental setup Issues on spectrometer system calibration Calibration strategy and procedures Calibration consistency check -- accidental background comparison between different optics

Current spectra: 12B,28

Al, 7He

Summary and work to do

HKS Experimental Goals

JLab HKS experiment: Hypernuclear spectroscopy from lower p-shell to medium-heavy systems with ~400 keV resolution by electroproduction

– Electroproduction: AZ + e A(Z-1) + e’+ K+

– A few hundred keV binding energy resolution achievable by utilizing high precision electron beam

Focus:

– High spin, unnatural parity states complementary to hadronic reactions; Neutron rich systems -- Targets:6Li, 7Li, 9Be, 10Be, 12C

– High resolution spectroscopy beyond p-shell, possibly resolve spin-doublet splitting --Target: 28Si

HKS Experimental Setup

Experimental setup: Increasing yield and reduce accidental background

Accept very forward angle e’: using Splitter magnet on target

Vertically tilt electron spectrometer

Beam energy 1.8 GeV

Issues on Spectrometer System Calibration

Spectrometer system calibration: On-target splitter field – no single arm elastic scattering, delta scan data available High accidental background in calibration data

BeamTarget

Hadron arm E-arm

Sieve Slit

Standard double arm spectrometerHKS spectrometer system

Beam

Target

Splitter

HKSEnge

Sieve Slit

To beam dump

Spectrometer System Calibration Strategy

Using known masses of, from CH2 target and identified hypernuclear bound states

Angle calibration uses both the constraints of missing mass and Sieve Slit data

Kinematics scan: important to determine absolute missing mass, energy resolution

Nonlinear Least Square fitting with event weight function p--reduce effect of background events in calibration

Iteration

M)|f(X=pmΔwχ fpiii

i22

222SSSSMMMMtot χw+χw=χ

Kinematics Scan for Beam Energy, the HKS and Enge Central Momentum Nominal values: E_beam=1.853 GeV, Pe0=0. 3417GeV, PK

0=1.2 GeV Scan beam energy, HKS and Enge central P ±100 keV Define

00EKbkin ΔPΔPΔE=V 0.0451.531.125 00 =ΔP,=ΔP,=ΔE EKb

222 )m(m+)m(m= PDGΣ

fΣ

PDGΛ

fΛpos

)σ+σ+σ+σ+σ+(σ+σ+σ=χ LHePSLAlGSLAlceLBPSLBGSLBΣΛwid2

72

282

282

1122

122

12222 9

Relative Weight Scan and Momentum Calibration

Relative weights of different states wi determined by scan to find minimized 2 wid:

Region I (exfp ≥ -12 cm)

Region II (exfp < -12 cm))σ+σ+σ+σ+σ+(σ+σ+σ=χ LHePSLAlGSLAlceLBPSLBGSLBΣΛwid

27

228

228

2112

212

212

222 0.70.50.01

)σ+σ+σ+σ+σ+(σ+σ+σ=χ LHePSLAlGSLAlceLBPSLBGSLBΣΛwid2

72

282

282

1122

122

12222 0.70.5

Nonlinear Least Square Fitting Nonlinear Least Square fitting with event

weight-- a weight factor is calculated for each calibration event according to function:

Noise cut-off parameter : Annealing temperature parameter

When ∞, function pi becomes constant 1

12B gs event selection gate

12B gs weight function

)exp()exp()exp(

2

2

iiii

iii M

Mp

Excitation Energy (MeV)

Accidental Background Comparison Between Different Optics

Two optics: 05/18/2007 first optics after divide region and current optics Procedure:

– Select events from 4 accidental coincidence windows.

– In missing mass spectrum of the old optics, cut events into 1 MeV bins

– For events in each bin of the old optics, calculate missing mass with the new optics, fit its centre and width.

– The width and sigma from the old and new optics are compared bin by bin

Background comparison: old optics

Old optics: 05/18/07: Background: new optics Background: old optics

Mean value difference for each bin

Sigma of bins in new optics Sigma difference for each bin

Background: new optics Background: old optics

Mean value difference for each bin

Sigma of bins in new optics

Mean value difference for each bin

Sigma of bins in new optics

Mean value difference for each bin

Cou

nts

(0.3

MeV

/bin

)

0AccidentalsEvents from C

p(e,e’Kp(e,e’K++))&&00 used for kinematics and optics calibration used for kinematics and optics calibration

HKS-JLABHKS-JLABCH2 targetCH2 target~ 70 hours~ 70 hours

Preliminary

Preliminary

s

p

C.E. #1

C.E. #2

width: 380 keV FWHM

1212C(e,e’KC(e,e’K++))1212B used for kinematics and optics calibrationB used for kinematics and optics calibration

PreliminaryPreliminary

AccidentalsAccidentals

JLAB – HKSJLAB – HKS~ 120 hrs w/ 30~ 120 hrs w/ 30A A

77Li(e,e’KLi(e,e’K++))77He – First Observation of ½He – First Observation of ½++ G.S. of G.S. of 77

HeHeC

ount

s (0

.2 M

eV/b

in)

B- Binding Energy (MeV)

S (1/2+)

AccidentalsAccidentals

7He: added to a

neutron halo state 6He“glue role” of :

6He 7He

0++n+n ++n+n

½+

-0.69<r-n>=4.6

-6.12<rcore-n>=3.55 fm

PreliminaryPreliminary

* Hiyama 1997

PreliminaryPreliminaryS

p

d ?C.E. ?

2828Si(e,e’KSi(e,e’K++))2828Al – First Spectroscopy of Al – First Spectroscopy of 2828

AlAlC

ount

s (0

.15

MeV

/bin

)

B- Binding Energy (MeV)

JLAB – HKS JLAB – HKS ~ 140 hrs w/ 13~ 140 hrs w/ 13AA

AccidentalsAccidentals

Major shell structures seen Indication of core excited states

* Motoba 2003

Missing Mass Spectra of Li9L, He6L, Be10L

He

H6He 10

e 9Li

GS?

Binding Energy (300 keV/bin) Binding Energy (250 keV/bin)Binding Energy (250 keV/bin)

Simulated 10Be Missing Mass Spectrum

Generated simulated Be10L missing mass spectrum according to the statistics and S/A in the real spectrum and the resolution of the B12L GS: 0.161 keV.

The peak positions and strength from theoretical calculation (T. Motoba, Prog. Theo. Phys.)

10e Data

10e Theoretical 10

e Simulation

Summary

A special designed calibration procedure for HKS spectrometer system has been carried out

The preliminary spectrum has a resolution ~380 keV (FWHM) for 12B gs

The gs and major shell structures of 28Al and 7

He have been observed

Work to do:– Missing mass linearity check / Estimate systematic error due to the calibration process --

by calibrating simulated events with same procedure– Cross section calculation – virtual photon flux?

Reconstructed correlations