Hypernuclear Spectroscopy in Hall A at JlabA.Acha, H.Breuer, C.C.Chang, E.Cisbani, F.Cusanno, C.J.DeJager, R. De Leo, R.Feuerbach, S.Frullani, F.Garibaldi*, D.Higinbotham, M.Iodice, L.Lagamba, J.LeRose, P.Markowitz, S.Marrone, R.Michaels,

Y.Qiang, B.Reitz, G.M.Urciuoli, B.Wojtsekhowski

And the Hall A Collaboration

16O(e,e’K)16NL16O(e,e’K)16NL

12C(e,e’K)12BΛ12C(e,e’K)12BΛ

High quality high intensity electron beam

Excelle

nt parti

cle Id

entifica

tionHigh Resolution Spectrometers

with small angle capability

9Be(e,e’K)9LiΛ

Hadron identification using Aerogel Threshold Cherenkov detectors

p

k All events

AERO1 n=1.015

AERO2 n=1.055

p

k

ph = 1.7 : 2.5 GeV/c

Protons = A1•A2

Pions = A1•A2

Kaons = A1•A2

2005 E-94107: Running on waterfall target

Be windows H2O “foil”

H2O “foil”

P

K

Coincidence Time selecting kaons on Aerogels and on RICH

AERO K AERO K && RICH K

Pion rejection factor ~ 1000

Rich – PID – Effect of ‘Kaon selection’:

PK

Coincidence Time selecting kaons on Aerogels and on RICH:

AERO K AERO K && RICH K

GREATLY improved AEROGEL performance!

N interaction

most of information is carried out by the spin dependent part doublet splitting determined by , s, T

(r)

Electroproduction of Hypernuclei

e

p

K+

e’

*

N … N

nucleus

Hypernucleus

p

beam

Scattered electronDetected by HRSe

Kaon detected by HRSk

N … N ( )+ −++′→+ 1 ZKeZe AA ( )+ −++′→+ 1 ZKeZe AA

High luminosity, high duty cycle, excellent beam energy spread

Better energy resolution than hadronic induced reactions, BUT smaller cross section

CsI PhotoCathodes for the RICH detector

RICH detector in the Hall A detector stack

ABSTRACT:Information about the force between nucleons and strange baryons, i.e. the N interaction, can be obtained by studying the spectroscopy of nuclei where a nucleon is replaced by a Lambda hyperon to form a bound state (see [I]). Hypernuclear spectroscopy via electromagnetic induced reaction has been measured in Hall A at Jefferson Lab on 1p-shell nuclei, 12C, 9B and 16O. The reaction constitutes an innovative experimental approach to study hypernuclei, providing an alternative to the hadronic induced reactions studied so far [II]. Jefferson Lab's electron beam features and the available experimental equipment in Hall A offered a unique opportunity to perform this experiment. Among the features needed are a narrow beam energy spread (E/E~10E-5), high resolution spectrometers (missing energy resolution less than 500 KeV FWHM) and good particle identification. [III]Modifications were made to the Hall A standard apparatus: two superconducting septum magnets were added to the spectrometer systems to allow particle detection at very forward angles [IV]. The particle identification for the experiment required separating kaons from large pion and proton backgrounds, and electrons from pion backgrounds. Particle identification (PID) of kaons, positive pions and protons in the left (hadron) High Resolution Spectrometer arm used two threshold aerogel Cerenkov detectors [V] as well as a RICH detector [VI]. In the right (electron) arm a gas Cerenkov detector was used to distinguish electrons and negative pions. A lead glass preshower and shower detector was used to enhance the separation between electrons and negative pions. A timing coincidence between double layers of scintillators in each arm was used to identify kaons coincident with the electrons and reject background events.

CONCLUSIONS:

Data have been collected on 12C, 9Be and 16O targets (water fall target [VII]). Very clean background free hypernuclear spectra of very good quality were obtained.In the case of the 12B hypernuclear spectroscopy, for the first time a measurable strength with high statistical significance has been measured for two peaks in the core excited part of the missing mass spectrum. Ongoing analysis is showing new information on the spectroscopy of the 12B hypernucleus. Data on 9Li and 16N hypernuclei are still preliminary [III]. However, the comparison of the data with different theoretical models shows that very promising physics insights can be obtained. This constitutes the first “systematic” study of 1p-shell hypernuclei with an electromagnetic probe that has begun in Hall A at Jlab. The new experimental devices have proven to be very effective. In particular septum magnets did not affect significantly the HRS’s optics [VIII] and the RICH confirmed its essential role for unambiguous kaon identification.

View of Septum Magnets:

RICH detector –C6F14/CsI proximity focusing RICH

Chϑ“MIP”

Performances: Np.e. # of detected photons (p.e.)and (angular resolution)

..

..

ep

ep

Nc

ϑϑ

=

Cherenkov angle resolutionSeparation Power

cϑϑϑ n12 =−

A tile of aerogel:

[VII] [VI]

[V]

[I]

[II]

[VIII]

[IV]

[III]