1. The KL reconstruction for the Belle experiment at KEK B-factory
Dec 3,
Mitsuhiro YAMAGA
Osaka Univ.

2. Introduction Primary goal of the Belle experiment : B0J/y+KL decay :
Study of the origin of CP-violation in the B decays.
B0J/y+KL decay :
One of the most important mode for the measurement of the CP-violation parameter, sin2f1.
Good confirmation of the Standard Model together with the B0J/y+KS decay.
KL information for the event full reconstruction :
Missing-energy, event vetoing.

3. KL/m detector (KLM)
Sandwich of iron plate and RPC (Resistive Plate Chamber).
Hadron shower detector.
15(14) layers.
Energy not
measurable.
Position only.

4. RPC module structure Endcap Barrel
Double-side strip-readout for X-Y position determination in each RPC layer module.
Endcap
Barrel

5. ECL (Electromagnetic calorimeter)
8376 CsI(Tl) crystals in front of the solenoid magnet.
5.5 x 5.5 x 30 cm tower-shape.
~1 nuclear interaction length.

6. KL reconstruction
KL candidates : Neutral hadron showers in KLM and/or ECL.
Classification of KL candidates :
KLM candidates : KLM hits associated.
KLM+ECL
KLM-only
ECL candidates : No KLM hits.
Main background : g , p

7. KLM candidates (1)
1. Clustering the KLM hits Charged-track vetoing.
KLM cluster
direction
less than 15 degree
 charged cluster
Charged
track
direction
Combine nearby hits.
Extrapolate the charged track.

8. KLM candidates (2) Determination of the KL candidate direction.
(KLM-only) (KLM+ECL)
# of hit layer >= 1
# of hit layer >= 2
neutral cluster
Less than 15 degree
 ECL cluster direction

9. Performance of KLM candidate
Single-KL Monte Carlo simulation
Detection efficiency
Strongly depends on the momentum.
No incident-angle dependence.
Angular resolution
~3 deg for KLM
~1.5 deg for KLM+ECL
(FWHM)
Satisfy our requirement.
PKL dependency
q dependency
pKL(GeV/c)
(cosq)
KLM+ECL
KLM only
f(meas-gen)(degree)
f(meas-gen)(degree)
(figures by S.Kumar)

10. ECL candidates (1) Likelihood analysis using 5 discriminants.  signal
distance to the
closest track
cluster energy
E9/E25 
(cm)
(GeV)
signal
background
cluster width
cluster mass
(cm)
(GeV/c2)
(figures by I. Higuchi)

11. ECL candidates (2) S/B FOM (=S/ S+B ) Likelihood signal
Likelihood requirement
Likelihood requirement
signal
Background (normalized)
(figures by I. Higuchi)

12. B0J/y+KL reconstruction
pB* : B0 momentum in the U4S rest frame
KLM candidate
ECL candidate
(GeV/c)
(GeV/c)
2332 candidates in 140fb-1
CP fitting result :
sin2f1= 0.80 0.13(stat.)
(by I. Higuchi)

13. Summary B0J/y+KL : KL in the event full reconstruction :
Important tool for the confirmation of the SM.
KL in the event full reconstruction :
Important for the missing energy or event vetoing.
KLM : Hadron shower detector for KL.
KL reconstruction algorithm :
KLM and/or ECL : Simple cut criteria + likelihood method.
Achieved to measure the B0J/y+KL decay, and obtained the CP violation parameter,
sin2f1= 0.80  0.13(stat.)