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A direct search for K s 3 0 decay at KLOEPowerPoint Presentation

A direct search for K s 3 0 decay at KLOE

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### A direct search for Ks 3 0 decay at KLOE

for the KLOE collaboration.

7-11 June 2004

INFN, LNF

KS3p0

- This decay has never been observed but is allowed by SM
- [CP violating; CP(KS)=+1, CP(3p0)=-1].

- From theoretical point of view:

All KLOE statistics 0.9 signal events produced (e=100%)

Current published limit: BR(KS3p0) < 1.4x10-5 (SND, PL B459 1999)

- The observation of this decay would confirm CP violation in KS
- system.

- Uncertainty on KS3p0 amplitude limits precision on CPT test
- (Bell-Steinberger relation).

KS3p0

KLCRASH

b0.22

- KS tagging: from KL interactions on EMC

KLCRASH(e~30%)

This algorithm searches KL which interact directly on EMC.

KLCRASH calorimetric clusters are selected using energy and b*.

- High bkg rejection needed:
- major expected background: KS2p0 (BR31%) + 2 fake photons either from
- shower fragments or accidental overlap with clusters from machine
- background.

-- KS3p0 (MC)

-- MC BKG

DATA

- A kinematic fit is applied on the
- Ks side requiring the conservation of
- 4-momentum (NDOF=11).
- c2FIT

DATA=450pb-1 (2001+2002); MC =540pb-1 (all available statistics)

- Preselected signal sample (KLCRASH and 6 photons): 39728 events
- Normalization Sample (KLCRASH
- and 4 photons): 23.5x106 events

c2FIT< 30 is not enough

(2/3 of bkg rejected)!

Other discriminating variable have to be used:

(c22p, c23p)

The c22p

KLCRASH + 4 photons

DATA

-- MC

The c22p is built selecting 4 out of 6 clusters

which satisfies better the kinematics of KS 2p0

- The parameters used are:
- mass distribution
- opening angle between pions in
- KS C.M. frame
- 4-momentum conservation

The calibration is done using KS2p0sample.

The c23p

KS3p0

MC

DATA

MCbg

The c23p is

based only on

the 3 “best reconstructed” pion masses

c23p

DATA-MC comparison

- A small fraction of the background (3%) is constituted by fake KLCRASH
- (KSp+p-) + KL3p0. Our MC well reproduces such fakes.

Analysis summary

- To increase bkg rejection we use
- these analysis cuts:
- Track veto:
- to reject events with tracks
- coming from IP.
- DECUT:
- After finding the four photons satisfying the kinematics
- of KS2p0, the residual energy of the KS is measured:
- E(KS) = EKs- Eg
- 2 bodies decay E(KS)~ 0 MeV
- 3 bodies decayE(KS)~ Mp

Rejecting events with:

DE(KS)<10 MeV

Candidates

- We have optimized our analysis cuts looking at MC
- c22p
- c23p

- c2FIT
- DECUT

c22p

DATA

MC

c23p

- e = (22.6±0.8)%
- We count 4 candidates with
- 3.2±1.4±0.2
- expected by MC
- N3p ≤ [email protected]% CL
- Folding the proper bkg
- uncertainty.

- Using this upper limit we can calculate some parameters
- directly related to CP and CPT test.

Using PDG values and our limit:

- We derive from our analysis:

This improves of a factor ~70 the previous limit.

- Our limitcan be compared with NA48
- - interference term between KSKL in the
- same final state (proportional to Re(h000)
- and Im(h000)).

KLOE versusNA48

(hep-ph/0310152)

SND

90% CL

- Assuming CPT invariance and ignoring
- transition into I=3 final state:
- Re(h000)=Re(e).

- We can calculate the contribution of
- 3p0 decay at Im(d) (Im(d)000) using
- unitarity relation (eS,L=e±d):

- Since these are 2 opposite values,
- we obtain the limits:

Im(d)000

becoming negligible on the

determination of Im(d)

f000(rad)

This value is based only on unitarity and increases the precision on Im(d) of 2.5

- The most conservative approach is to assign a phase toa000
- which gives the minimum and maximum values of Im(d)000:

- A direct search for KS3p0 decay has been performed
- using the whole statistics collected at KLOE during 2001-
- 2002 data taking.
- We set an upper limit on the branching ratio to
- 2.1x10-7 @ 90% C.L.
- This reduce of a factor ~70 the previous limit.
- This limit makes completely negligible the contribution of
- this decay on Im(d) reducing of a factor~2.5 the
- uncertainty on Im(d)

The c23p

DATA

MCbg

Due to DAFNE bkg which satisfy the trigger.

Reduced by our Splash Filter:

rmean > 70 cm

Emean > 70 MeV

“SPLASH” events

Good agreement DATA-MC in all the regions

DATA

MCbg

DATA

MCbg

KS3p0

MC

c2FIT < 30

Thec23pis based only on the 3 reconstructed pion masses

DATA-MC comparison

DATA

?

FAKEKCRA

MCbg

In the data, a new category of BKG events (not simulated by the standard Kcrash MC) appears. The simulation takes into consideration only KL decaying after a cilinder bigger than DCH and smears the KL MC direction with the Kcrash resolution observed in data.

The Kcrash fake background

- A small fraction of the background (3%) is constituted by
- a fake Kcrash and a KL 3p0 decay.
- The fake KLCRASH is generated mostly by KSp+p- event,
- where the pions, spiralizing around the pipe, interact on
- the quadrupoles creating late clusters in the event.
- We have add the simulation of these events in the MC and
- calibrated their quantity.

- Outside the
- KLCRASHb*
- windows DATA are
- dominated by fakes
- In the central band
- there is also a small
- percentage of
- KLCRASH.

DATA

-- MC Fake

DATA

-- MC Fake

EKLCRA

EKLCRA

EKLCRA

b*

Comparing events with tracks from IP.

b*

MC fakes only (i.e. no Kcrash)

DATA dominated by fakes

The agreement between DATA and MC is better than 10% in all

the regions.

c22p

Region

Nev (Data)

Nev (MC)

SBOX

337 18

309.5 16.3

UP

Cup

UP

493 22

479.8 21.2

DOWN

382 19

426.5 14.4

CSbox

5502 74

5595.1 51.0

CSox

Cup

11249 106

11744.1 74.0

SBOX

Cdown

24396 156

23844.9 103.8

DOWN

Cdown

c23p

DATA-MC comparison

Monte Carlo calibration

-- DATA

-- MC

-- KCRAMC

-- FAKEKCRA

Normalized with the full sample Ng=6

(A)

(B)

Mean

KCRAMC

0.446

0.447

0.447

FAKEKCRA

1.002

0.895

0.949

c23p < 3.5

FIT data using shapes from MC:

ID(DATA) = a1 ID(MC)+ a2 ID(FAKES)

Two different regions used:

c23p < 3.5 (A)

c23p < 8.8 (B)

After removing SBOX

DATA-MC comparison (I)

c23p all

c23p > 3.5

DATA

-- MC

DATA

-- MC

c23p < 3.5

c23p< 8.8

DATA

-- MC

DATA

-- MC

Normalization with the mean values of the coefficients

DATA-MC comparison (II)

c22p all

c22p > 40

DATA

-- MC

DATA

-- MC

14<c23p<40

c23p< 14

DATA

-- MC

DATA

-- MC

MC KS 3p0, BR=1.4x10-5

Normalization with the mean values of the coefficients

The track veto

MC SignalDATAMC bkg

c22p all

c22p > 40

14<c23p<40

c23p< 14

All events with tracks from IP are rejected rPCA < 4 cm ; |zPCA| < 10 cm

MC signal, BR(KS3p0)=1.4x10-5

The topological DECUT

-- MCBG no 2fitcut

MCBG 2fit < 30

MCBG 2fit + TRKveto

MCSIG

DECUT < 10 MeV

After finding the four photons satisfying the kinematics of

KS2p0, the residual energy of the KS is measured:

E(Ks) = Eks- Eg

- Summarizing:
- c2FIT<30
- TRKveto
- Sbox

- DATA/MC evaluated in Signal and control boxes varying the cut on the kin. Fit:
- c2FIT = 500, 400, 300, 200, 100
- No dependence observed as a function of this
cut

Syst.error on BKG

Syst1 = 9% (Calibration)

Syst2 = 10% (Data/MC)

Systot = 14 %

Analysis efficiency and bkg systematics

Sample of 10 Kevents

KS 3p0

e3p = ( 21.30.7stat 0.3syst )

Optimization of UL cut on the kin. Fit:

- We have optimized the UL by MC looking at 4 different
- parameters:
- c2FIT
- c22p (both sides)

- c23p
- DECUT

- The best choice of the cuts is the following (CUTMAX):
- c2FIT < 31
- 14 < c22p < 60

- c23p < 3.7
- DECUT > 37 MeV

- Optimization is done by minimizing the expected 90% CL
- upper limit (ref. J.F.Grivaz, F. Le Diberder LAL 92-37)
- All is done varying cuts on MC only.

NA48 interference function cut on the kin. Fit:

K0 K0 Dilution factors

KL decay

KS decay

Interference KL-KS

Related parameters cut on the kin. Fit:

- The more conservative approach is to give ata000a phase which
- maximizeIm(D)000:

- Because there are 2 opposite values, we obtain a range for
- |Im(D)|[email protected] 90% C.L.:

Im(D)000 KLOE

Im(D)000 CPLEAR

Im(D)pln

This value is obtained using only unitarity, and increases the precision on CPT test.

Im(D)00

PDG 03

Im(D)+-

x10-5

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