Direct Measurement of Lifetime of
Heavy Hypernuclei
Using Photon Beam with CLAS and
Fission Fragment Detector
Proposal to be submitted to PAC 31
Liguang Tang
Hampton University / JLAB
CLAS Collaboration Meeting, 11/01/2007
Physics Motivations


Pure BBBB weak interaction is essential to help to fully
understand the strong interaction limit of QCD.
It is still a poorly understood sector of Standard Model because:
The weak NN  NN interaction cannot be clearly observed
due to extremely high background from strong interaction
 The tiny PV amplitude in the NN scattering is very difficult
to observe, while PC amplitude is completely masked by
strong interaction
 The weak NN scattering is “contaminated” by the neutral
current, induced by both W and Z0 boson exchanges
NNN is unique to study BBBB weak interaction: it is weak
(S=1), induced only by the charge current (W), both PC & PV
amplitudes can be observed


NNN: Non-Mesonic Decay Mode
of -Hypernuclei

-hypernuclei decay via two distinguished modes

Mesonic decay (same as the free ):
  p - + 38 MeV (~64%) and   n 0 + 38 MeV (~36%)
(Pauli block to nucleon except extremely light hypernuclei)

Non-mesonic decay (unique):
n  nn + 176 MeV (n)
p  np + 176 MeV (p)
NN  nNN + 176 MeV (2)
N
N
N
N
N
N
N
, , , , , *
, , , , ,*
~ 420 MeV/c for NN
~ 320 MeV/c for NNN

N

of observable
study
Partial decay Three
widths:types
(Exclusive
but lighttohypernuclei)
the NM decay
 (PC)
NNN
- NNN transition
amplitudes
- Decay width Ratio: n /p
- I = 1/2 rule (3/2 contribution ?)
Decay asymmetry: (Exclusive but light hypernuclei)
- Polarization of hypernuclei
- Interference of PC and PV amplitudes in NNN
Lifetime (T): (Inclusive, light to very heavy)
- Characteristics of the A dependence
- Interaction range, BB short range correlation
- Role of I=3/2 (S.I., HMS) and 1/2 (L.I., OPE)
- Significance of NNNNN contribution?
- Unknown mechanisms, role  inside nucleus
Long Standing Puzzle: I = 1/2 Rule
in Weak S=1 Non-leptonic Decays





Standard model predicts: 2:1 (1/2) : (3/2)
Observations from mesonic decays of K and 
concluded: ~20:1 (I = 1/2 rule)
Suppression of I = 3/2 transition was explained
by the theory of color symmetry of constituent
quark, but under predicted the ratio n/p
Origin is still unknown
Weak NNN interaction is unique to study the
origin of this empirical rule
Progress and Puzzle on Partial Decay
Widths (Light Hypernuclei only)
Fundamental issues are not solved yet
Early puzzle: n/p=1.0 (Exp.) but 0.1 (Th.)
 Theories are in contradiction on the amplitude of
 Theory: An error was found in the calculation of kaon
I
= 3/2 transition
thegoes
NMup
decay
exchange
amplitude,for
ratio
Current
data on
the partial
decay
widths
offinal
light
Experiment:
Better
handle on
the low
energy
state protons,cannot
adding conclude
neutron detection,
adding twoof
hypernuclei
the requirement
nucleon
I
= 3/2 correlation
transition study,…, ratio comes down
 Agreement: R=n/p  0.5
 There are suggestions
that partial decay width
5 He)=0.450.110.03, R(12 C)  0.5
Example:
R(
4

study on H is essential for the conclusion
 Puzzle on this ratio
4 is settled!5
4


Extrapolated R( H) from He and He data
were too poor in uncertainty to make conclusion
Puzzle in Decay Asymmetry
(NNN with  Naturally Polarized)
12 C in
Ref. anddata
Model
Experimental
from two5light

He hypernuclei are
Theory
A. Ramos
et al.
contradictory
with
theory (one agree but one
OPE
-0.524
-0.397
disagree +atKleast on the sign-0.509
of )
-0.375

C. Barbero
 (I=1/2)
-0.4346
-0.4317
 Theories
cannot conclude the
role of HME -0.4185
and I
 (I=3/2)
-0.4207
= 3/2 component,
K (I=1/2)
both are-0.1676
dominated by short
-0.1757
K (I=3/2)
-0.1675
-0.1766
range interactions.
Exp.
M. Oka
H. Bhang et al.
 Fundamental
issues
T. Maruta
et al. are
0.090.08
0.070.08
not0.110.08
solved
-0.240.26
-0.200.26
3rd Observable: Lifetime (A Dependence)
- Strictly range related
Pure NM decay region
n/p  0.5
2

(A)


dr


(r)

A(r)a
COSY
result
suggested
NM precision lifetime
 for heavy
High
hypernuclei
is essential to of
help
to fully
continued
lifetime
If interaction isdecrease
short ranged, the
understand the mechanisms of the NM
lifetime
should saturate
at mediumby
and
cannot
be explained
decay
and the JLAB
experiment is
capable
A or below
to
have antheoretical
unprecedented
accuracy
current
models
Goal of the JLAB Experiment



Direct time measurement
Unprecedented precision:
Sys. error <  3 ps Stat. error <  5 ps
Challenge or confirm COSY-13 result to
 Put more stringent limits to the n/p ratio
to test future validity of the I=1/2 rule on a
higher confidence level
 Test the significance of other possible decay
mechanisms (3-body decay and rescatterings)
Beauty of the JLAB Experiment
Fission Fragment Detector
- LPMWPC
Excellent timing
resolution
Precise
Beam Structure
t
1.67 ps
 Double TOF measurements
2.0 ns
 Excellent position reconstruction
plus correction by position-time
correlation due to Hall
the correlated
B g11
mass relation for the two FF
Hall C HKS

Super Stable with
 ~ 130-200 ps
Photon Beam
t
Excellent decay T0 calibration

Tested in Hall B
Large amount of prompt fissions
(Summer, 2007)
from background  and p
productions
 Precise T0 and line shape
determination
Coincidence
time
 (sys)
<  3 ps
Schematic Layout of the Experiment
- FFD, 1.5 m upstream
- Small forward angle,  < ~14o
- Similar to CLAS angle  < ~19o
- Clean photon beam
- Large  and kinematics range
- Short flight path
0.6
HKS limit
209

Production
threshold
E (GeV)
Pb

1.5 M
E02-017 E limit
1.7
2.0
0.0
0.5
0.67
PK (GeV/c)
1.05
1.35
1.6
Offline Particle ID (CLAS Simulation)
E = 2.1 GeV/c
+/p/K+ ratio: 100:10:1
Small angle cut applied
PID is reasonably good
Background induced
fissions is essential as
tools for calibrations to
ensure high precision
Momentum Region
Successful Beam Test in Hall B
Effective target thickness: 500mg/cm2 (10x thicker)
CLAS Rates at 27 (nA)(%)
ADifferential
reconstructed
T0 w/o
beam
position
information
amplifier
can
eliminate
the
RF 18
noise
FFD performed
very
stable
and
good
FFD
Rates
Drift chamber peak current
A
characters at
maximum
beam
intensity
TOF (top two average)
9.5 kHz
TOF (bottom two average)
 = 279 ps
EC (top two average)
2-fold:
x 1.9
EC (bottom two
average)
1-fold: x 1.92
26.3 kHz
24.2 kHz
133 kHz
GainExperimental
Over E02-017Condition
(Using HKS)
Electron beam (I)
Experimental condition
Radiator (r.l.)
Beam current (nA)
Intensity (nA%)
50 nA
E02-017
Hall B
100
50
5%
Gain
0.5
250 nA%
Radiator (%)
5
5
Summed
TOF rate
Kinematics
acceptance
1
3.0 ~90 kHz 3.0
Average
Summed
kaon survival
EC rate
(%)
28
48 ~380 kHz 1.7
Integrated
over 
CLASd/d
DC peak
current
1
1.5 ~10A
Overall
FFD 1/4 rate
1.0
1.7
~600 Hz 3.83
FFD 2/4 rate
~300 Hz
Coincidence (CLAS & FFD) rate
~ 5 Hz
Yield Rate
JLAB-Hall B
390
Fission Probability
0.5
COSY
10,000
2.8
Much better ratios of
Prompt/Delayed fissions
Yield Rate
Item
Bi
Au
Kaon single
0.91/s
0.88/s
Kaon coincidence (Prompt)
40/hour
8/hour
Kaon coincidence (Delayed)
30/hour
17/hour
Kaon accidental
610-7/s
610-7/s
Beam Time Request (E = 2.1 GeV)
Item
Commissioning
Bi target
Au target
Beam hours
72
100
178
Total Yield
3000
3000
Lifetime Extraction & Precision

N(t) = Nd dt’R(t-t’)e-t/ + NpR(t) + NKIDR’(t)
-
Systematic
Statistical precision
precision depends
depends on:
on:
Four
parameters
Precise
line
shapes
Nfree
~ 3000
counts
d
Precise production & decay time zeros
If t =induced
200 ps fissions
and  = are
200essential
ps
Background
Systematic
Statistical error: <~  35 ps
Summary

High precision measurement on lifetime of heavy
hypernuclei is crucially important to help
uncovering the mastery of short range interaction
and role of I = 1/2 rule

The proposed JLAB experiment is the only one can
reach such precision

Real photon, FFD, and the Hall B CLAS is the
cleanest and best way to carry out this experiment