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of the of the KKLL K KSS regeneration cross section: regeneration cross section:

F. Ceradini, A. Ferrari, A. PasseriF. Ceradini, A. Ferrari, A. PasseriUniversita’ di Roma Tre & INFNUniversita’ di Roma Tre & INFN

2nd KLOE Physics Workshop2nd KLOE Physics WorkshopOtranto, June 10th 2002Otranto, June 10th 2002

Measurement Measurement

a status reporta status report

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eNoss = NKL BR (KS+- ) rec(r) Preg -r(rig)/L

Preg = reg nd

nd=NA i df /Ai

d regenerator thickness densityf fraction in volume of the material i

Preg

Be

He

C

2.5 10-4

0.56 10-5

3.5 10-4

all wires 3.5 10-6

(Al +W)

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The 1999 analysis has been redone on 2000 data sample (17 pb-1)

~ 4.7 pb-1 of Monte Carlo events have been used for comparison

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Monte Carlo simulationMonte Carlo simulationModel dependent description, based on the eikonal approximation:

f(k, ) = (2l + 1) fl(k) Pl(cos ) [ 1 -e i0(b)] ~ /(1- ik)

KLOE: s-wave small momenta

bf(k, ) = ik b db J0(kb sin ) [ 1 -e i0(b)]

0(b) depends on forward scattering amplitudes for n and pdistribution of nuclear matter

impact parameter

scattering length

freg = f() - f() 2

2dreg f() - f() 2d =

The angular distribution has been implemented in a simplified way:

g() = p1 + p2 exp (-2/2 p32) kaon momentum

atomic weightsp1, p2 , p3 depend on

g() is truncated at 90o

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KL - Helium KL - CarbonKL - Berillium

reg reg vs pvs p

p(MeV/c) p(MeV/c) p(MeV/c)

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KLOE 1999:KLOE 1999:

(reg)bp = 75.7 9.6 10.6 mb

(reg)dc = 51.9 6.2 5.3 mb

110Beinc

110 BeT

Lack of experimental data on KL cross sections on nuclei:Lack of experimental data on KL cross sections on nuclei:

110 Cinc

545

Theory (mb)

(CMD-2)(CMD-2)

545 (CMD-2)(CMD-2)

Experiment (mb)pK (MV/c)Material

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Event selectionEvent selection

• KL TAGGING

• cut on mKs and pKs : 4 from the peak value • use BPOS only as constraint to evaluate the –decay point event by event• cut on the – decay position: 3 around the decay vertex• select KL candidates by looking at vertices inside a cone around the estimated KL flight path

I. KL candidate selection

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The angular cut KL flight path

pS

p V

d

F(d) = a2 + (b/d)2

The angular cut is a function of the distance - secondary vertex:

F() = p1 + [ p2 exp (-/2 p32) + p4 exp (-/2 p5

2) ] For different intervals in d the angular distribution has been studied:

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p3 and p5 are fitted in each d-interval

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F(d) = a2 + (b/d)2

= 3 (39.4)2 + (1111/d)2 mrad

Angular cut:

Fit results: p3 cone: a = 17.3 0.1 mrad b = 498.9 4.5 mrad cm

p5 cone: a = 39.4 0.8 mrad b = 1111 23 mrad cm

KL decay length

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KL candidate sample

KL decay length

cm

N(KL candidates) = 1096068

sel =N(KL candidates)N(KL tag)

= 12.8%

Data/MC = 93.3%

MC (KL charged) = 40.8%

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KL invariant mass selection

KL invariant mass

MeV/c2

MeV/c2

minv in [493.7, 501.7] MeV/c

Data/MC = 86.5%After the selection: 16025 events

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p distribution

p = |pest| -|pmeas|

-6 <p < 12 MeV/c

Cut:

Data/MC = 74.2%After the selection: 6940 events

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r(cm)

(cm)

After p cut:All CP violation + regeneration candidates

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to separate the internal Be regenerator:secondary vertex in [60o,120o]

(cm)

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(angular distribution)

All CP violation + regeneration candidates

MC generator angular distrib. is truncated at 90 degrees!

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CP violation events:

cut at 75 mrad

r(cm)

Good agreement !

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Specialized Angular distributions

Pipe

DC wall

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Selection efficiencies vs rSelection efficiencies vs r

CP violatingCP violating

regeneratedregenerated

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Conclusions and outlookConclusions and outlook

The 2000 data sample analysis is going to be completed - the MC correction reg/CP will be evaluated with high statistics specialized sample - a memo will be written by the summer In the next future:

- analysis on 2001 data sample - reduce systematics due to the knowledge of the regenerator thickness: measure it