1. M. Palutan, T. Spadaro, P. Valente Laboratori Nazionali di Frascati
Status of the analysis of KS semileptonic decays Update on BR(KS p-p+)/BR(KS p0p0)
M. Palutan, T. Spadaro, P. Valente
Laboratori Nazionali di Frascati
C. Gatti
Università Statale di Pisa  Università di Roma “La Sapienza”

2. Physics motivations e+p-nHWKK0 = a + b e-p+nHWKK0 = a* - b*
Two clean predictions within the Standard Model:
Transition rates: G(KS  pen) = G(KL  pen)  DS=DQ rule
Charge asymmetries: AS = AL  CPT symmetry
e+p-nHWKK0 = a + b
e-p+nHWKK0 = a* - b*
e-p+nHWKK0 = c + d
e+p-nHWKK0 = c* - d* T
Im a = Im b = Im c = Im d = 0 CP
Im a = Re b = Im c = Re d = 0
CPT
b = d = 0
DS=DQ
c = d = 0
eS = eK + dK
eL = eK - dK
CPT violation in decay
CPT violation in mixing

3. BR and DS=DQ rule (I) e+p-nHWKK0 Re x+  Re c*/a 
Test of the DS=DQ rule. The relevant parameter is:
CPLEAR measurement (-1.86.1)10-3
KLOE statistical error on Re x+
KLOE 2000: Phys. Lett. B535 (2002)
(assuming d=0)
Re x+ can be extracted by measuring the ratio of semileptonic KS and KL decay widths summing up both charge final states
GSsemi
GLsemi
= Re x+
BR(KSpen) tL
BR(KLpen) tS
810-3
710-3
110-3

4. AS = 2Re eK + 2Re dK + 2Re b/a - 2Re d*/a
G+S,L - G-S,L
Charge asymmetry (I)
AS,L =
G+S,L + G-S,L
AS = 2Re eK + 2Re dK + 2Re b/a - 2Re d*/a
AL = 2Re eK - 2Re dK + 2Re b/a + 2Re d*/a
No measurement CP
CPT in mixing
CPT in decay
DSDQ and CPT
AS - AL0 implies CPT
(AS + AL  4Re eK)
Re dK = (2.9  2.7) CPLEAR Phys Lett B (1998)

5. G+S,L - G-S,L Charge asymmetry (II) AS,L = G+S,L + G-S,L
3 levels of accuracy can be considered:
In order to test if AS is consistent with 2Re eK , need 2 fb-1
In order to measure AS with O(30%) significance, need 20 fb-1
In order to improve limits on dK= (AS – AL)/4 , need 50 fb-1
+ precise measurement of ALe from KTeV
ALe = (3322  58stat  47syst ) KTeV Phys Rev Lett 88 (2002)
KLOE is able to improve on the measurement of ALe, using the whole 500 pb–1 data set

6. KS p+p- and KS p0p0 decays
Both the isospin (I=0 and 2) amplitudes and the pp phase-shifts can be estimated from the measured K pp branching ratios:
A(K0 p+p-) = A0 eid0 + 2A2 eid2
A(K0 p0p0) = A0 eid0 - A2 eid2/2
A(K+ p+p0) = 3/2 A2 eid2
t+ p+0 BR+- MK02
tS p+- BR+0 MK+2
1 + 1/ w2 = (1+p+-/ p00R)
R p00/p+- =
1 + w2/2 + w 2 cos(d0-d2 )
1 + 2 w2 - w 22 cos(d0-d2 )

7. KS p+p- and KS p0p0 decays
Using the PDG values for the branching ratios, one gets:
d0-d2  (56.73.8) (and w=0.045)
This value is inconsistent with the prediction from O(p2) cpT (456), the measurement from pp scattering (45.21.3 1.5), and the estimate from the KL decays
With the KLOE published measurement of
R = G(KS p+p-)/G(KS p0p0)=2.239 based on 17 pb-1
(R = PDG):
d0-d2  (483) (while w is unchanged)
In pipeline, planned for mid 2003:
Measurement of R with an error of 10-3
Measurement of both the absolute branching ratios

8. KS beam KL
A pure KS beam can be selected by identifying the KL in the opposite hemisphere  KS tag
Due to the very different lifetimes lS  0.6 cm, lL  350 cm, a very efficient tag is given by KL’s that reach the calorimeter and interact therein (Kcrash)
ecrash  30%
The Kcrash cluster is cleanly identified by its low velocity, b  0.2, as measured from the time of the e+e- interaction (t0).
This also provides an estimate of the KL direction (s  2) and momentum (s  3MeV) Pf KS

9. Ks beam
KS p0p0
Events tagged by a ‘Kcrash’ cluster
KS p+p-
Looser cut on b*, from
(0.195, ) to (0.17, 0.28)
Higher tagging efficiency, from
0.3 to 0.35 at 100 MeV
Lower systematic error in the estimate of the ratio of p+p-/p0p0 and p+p-/pen tagging efficiencies, especially for Kcrash energy cut above 200 MeV

10. Selection of KSp+p- events
Measurement method
‘Kcrash’ cluster
N a
BR(KS a) ea = 
N b
BR(KS b) eb
Events tagged by a ‘Kcrash’ cluster p p
Selection of KSp+p- events
2 tracks close to the IP, impact on the EmC
At least one track associated to an EmC “T0” cluster (0.2% loss)
Less than 3 prompt EmC clusters, not associated to any track (0.08% loss)

11. Selection of KSpen events
Measurement method
‘Kcrash’ cluster
N a
BR(KS a) ea =  KL
N b
BR(KS b) eb
Events tagged by a ‘Kcrash’ cluster e
boost p KS
Selection of KSpen events
2 tracks and 1 vertex close to the IP, to EmC n
Reject events with invariant mass Mpp close to the K0 mass
Use time information from calorimeter clusters to perform PID for charged tracks

12. p/e identification e-p+ dt(me1)dt(mp2) dt(me2)dt(mp1) e+p-
Time of flight e/p identification (Dt  2 ns) :
dt(m) = tcluster – t.o.f. calculated with mass hypothesis m
Sign of the charge is determined  semileptonic asymmetry accessible
e-p+
MC KS  p+p-
MC KS  pen
Data
pe signal
dt(me1)dt(mp2)
dt(me2)dt(mp1)
6 ns
e+p-

13. Close the kinematics, by using:
pen final selection
Close the kinematics, by using:
KLOE 2000: PLB B535 (2002)
1) e-p identification from t.o.f.
2) KS momentum from KL crash direction and  momentum:
N(pen) = 627  30
Emiss = ES - Ee - Ep
Pmiss = |PS - p1- p2|

14. Charge-selected pen spectra
Data 166 pb-1 (2001)
Emiss-Pmiss (MeV)
e+p-
e-p+
Entries 62738
Entries 48917
Form of signal distribution approximately independent of sign of charge

15. Charge-selected pen spectra
Data 166 pb-1 (2001)
e+p-
e-p+
Emiss-Pmiss (MeV)
Substantial charge dependence of the KSp+p- background spectrum far away from the signal peak

16. Event yields by sign of charge
 Data 166 pb-1 (2001)
 MC fit result
N(p-e+n) =  92
c2/n.d.f = 1.5
Fitting to data unselected by charge gives a result compatible with the sum: N(pen)=7732127
N(p+e-n) =  88
 Data 166 pb-1 (2001)
 MC fit result
c2/n.d.f = 1.1

17. Efficiency estimates (I)
epen = e(fiducial cuts) e(t.c.a) e(t0trigger) e(t.o.f.)
1) “Natural” data control sample: KLpen decays close to the IP
2) Correction obtained by weighting Monte Carlo kinematics with single-particle efficiencies extracted from data:
KSp +p- (p and m efficiencies)
fp +p- p0 (p and m efficiencies)
Radiative Bhabha (e efficiencies)
KLpen (p, m, and e efficiencies)
Pion calorimeter clusters needed for t.o.f PID,
correct parametrization of calorimeter response to low-energy (100 MeV) pions is therefore a crucial task

18. Efficiency estimates (II)
3) Data control sample of KLp+p-p0 decays to tag the KS (Rtag)
4) Data control sample of KSp0p0 and KLpen close to the IP to study vertex efficiency:
The vertex requirement led to the biggest source of systematic error in the 2000 anaysis (1% uncertainty)
Official routine has 2% inefficiency.
A new recovery algorithm has been implemented, which releases the quality cuts of VTXMIN on the c2 (it just looks for a mimimum)
New vertex efficiency practically 100%

19. Data of year 2000: branching ratio measurement
e(fiducial cuts) = (29.70.5)%
e(t.c.a.) = (92.50.6)%
e(t0trigger) = (92.20.4)%
e(t.o.f.) = (82.00.7)%
epen = (20.80.4)%
N(pen) = 627  30
N(p+p-) = 1.6106
Normalizing to the KS p+p- channel, one gets:
BR(KS pen) = (6.910.37)10-4
From the KL semileptonic decay width and assuming Re x+=0,BR(KSpen)=(6.700.07)10-4
Published year 2000 result based on 17 pb-1 is 5 times more accurate than the only existing measurement

20. Data of year 2001: efficiencies by charge
e+p-
e-p+
Average
e(fiducial cuts)
0.34070.0014
0.33750.0014
0.33910.0011
e(t.c.a.)
0.92170.0021
0.92490.0025
0.92090.0012
e(t0)
0.99730.0007
0.99790.0007
0.99760.0007
e(trigger)
0.94120.0045
0.93880.0045
0.94010.0045
e(t.o.f.)
0.73220.0076
0.75540.0081
0.74390.0079
e(pen overall)
0.22930.0016
0.23530.0017
0.23170.0014
e(pp overall)
0.62150.0010
etag(pp) / etag(pen)
0.99240.0020
0.99120.0020
0.99180.0015
Systematic and statistical uncertainties nearly equally contribute to the error on the pen total selection efficiency

21. Data of year 2001: branching ratio measurement
epen = (23.170.14)%
N(pen) = 7732  127
N(p+p-) =  106
Normalizing to the KS p+p- channel, one gets:
BR(KS pen) = (6.610.110.070.040.03)10-4
Statistical
Fit syst
Effi syst
BR(pp) error
From the KL semileptonic decay width and assuming Rex+=0, BR(KSpen)=(6.700.07)10-4
Result based on 166 pb-1 of 2001 data

22. Charge asymmetry
To get the asymmetry, one has to correct the e+p- and e-p+ event yields using the efficiencies for each sign of charge…
AS =
N+ e+-N- e-
N+ e++N- e-
Present PRELIMINARY result is based on 166 pb-1
AS = 0.012 0.017stat 0.004syst with an overall error of 2%
BR(KS p-e+n) = (3.3300.0810.0330.0180.014)10-4
BR(KS p+e-n) = (3.2490.0760.0320.0170.013)10-4

23. Conclusions and prospects on KSpen
Present preliminary result for the branching ratio has an overall error of 2%, allowing an accuracy on Re(x) of 0.5%
Present preliminary result (2001 data) for the charge asymmetry has an overall error of 2% and is compatible with 0
Work is needed to improve the MC simulation for the Emiss-Pmiss background spectrum
Full analysis of year sample will allow measurement of:
Re x with an accuracy better than
Charge asymmetry to within 1.%

24. Status of KSp+p- / KSp0p0
Selection efficiency for KSp+p- is evaluated by folding data/MC tracking, TCA, T0 efficiencies into the MC simulation
Efficiencies have been extracted for the whole 2001 sample and for a selected sample of 60 pb-1 of 2002
Ratio of tagging efficiency with the new b* window is now stable at 0.1% level throughout the samples
Work is needed to address the selection efficiency for KSp0p0:
Photon efficiencies
Calorimeter response
Run over the whole data set to check stability and to measure data/MC corrections over the entire detector
Effect of accidental clusters on the prompt counting

25. Selection of KSp0p0 events
Measurement method
‘Kcrash’ cluster
N a
BR(KS a) ea = 
N b
BR(KS b) eb g
Events tagged by a ‘Kcrash’ cluster g g
Selection of KSp0p0 events
At least 3 prompt EmC clusters not associated to any track