1. Progress on Pion Form Factor at KLOE (large photon polar angle)
Debora Leone
(IEKP – Universität Karlsruhe)
for the KLOE collaboration
International Workshop
e+e- collision from  to 
Novosibirsk
February 27th- March 2nd 2006
Progress on Pion Form Factor at KLOE
(large photon polar angle)

2. Signal selection PRO & CONTRA the threshold region is accessible
one photon is detected
(4-momentum constraints)
lower signal statistics
large FSR contributions
irreducible background from
f to p+p- decays
large   p+p-p0 background
contamination
PRO & CONTRA
Pion tracks: 50o < p < 130o
Photons: at least one with 50o <g< 130o
and Eg > 50 MeV
tagged measurement
500< qp,g <1300 p g
50o<qp<130o
50o<qg<130o
p+p-g MC
p+p-p0 MC
50% of final
sample statistic
Mpp2 [GeV2]

3. Reducible background rejection
three sources: Radiative Bhabhas e+e- e+e- , muon pairs m+m- m+m-  and
+-0
Particle ID
Radiative Bhabhas are separated
by means of a particle-ID
(signature of EmC-Clusters
and time of flight of particles)
p+p-g – MC
p+p-p0 – MC
m+m-g - MC
Mpp2 [GeV2]
Mtrk [MeV] mp mm
mr2
TrackMass
To reject m+m-g and (partially) p+p-p0
background a cut in the plane Mtrk vs. Mpp2
is applied. Mtrk is the kinematical variable
obtained by solving
in the assumption of  x+ x- 

4. Reducible background rejection
Two further dedicated cuts to p+p-p0 rejection
Kinematic fit
Kinematic fit in the p+p-p0 background hypothesis
Two tracks in 40 < p < 140
At least two photons in time, one of them with
Eg > 40 MeV and 40 < g < 140
4-momenta conservation
Minv(gg) = m(p0) 2
Data
p+p-g MC
 Angle 2
Angle between the missing momentum
and the detected photon momentum 10 20 30 40 50 60 70
[o]
p+p-g MC
(due to NLO events)
p+p-p0 MC p+ p- W g

5. Residual background evaluation – m+m-g
Muons sample selected asking 80 < TrackMass < 110 MeV, in the same angular region
as the p+p-g sample
Preliminary
80 <TrackMass<110MeV
MC: Phokhara 5
Absolutely
normalized
DATA MC
Mpp2 [GeV2] %
Muons
contamination
Up tp 10% difference
between data and MC
for a maximum of 10%
contamination
(excluding the threshold
region...see later)
1% error on the knowledge
of m+m-g in the final
p+p-g sample
Mpp2 [GeV2]

6. Residual background evaluation – p+p-p0
In order to select a sample of p+p-p0 from data, we have applied after the angular cuts
a rigid cut on the 2 of
the p+p-p0 kinematic fit
(2 <20)
Absolutely
normalized
Data
p+p-p0 MC
Data
p+p-p0 MC
2 <20
Mpp2 [GeV2] %
Difference data-MC
of the order of 20%,
contamination in the
final sample smaller
than 10 %
accuracy on p+p-p0
subtraction at
per mil level.
3 pions
contamination
Eg [MeV]
Data
p+p-p0 MC
 [o]
Mpp2 [GeV2]

7. their amplitudes interfere
Irreducible background
m+m-g and p+p-p0 background channels well under control… but FSR events as
e+e-  ppgFSR
f  f0 g  pp g
f  r p  pg p
all of them with p+p-g final state, indistinguishable from the signal signature f f0 g p r p g f r p g
&
&
FSR f0 rp
Three processes of the same family:
their amplitudes interfere
At low Mpp2, ISR and FSR are not the only contributions to the mass spectrum and to the charge asymmetry  model dependence for the additional contributions
More phenomenological input nedeed concerning the hadronic models.

8. Trigger efficiency
In KLOE, in order to trigger, an event has to release energy over a certain threshold in two
different regions of the calorimeter.
We have evaluated the event trigger efficiency by data combining the probability that the
single particle triggers, when the other two have already triggered the event.
e(p+)
e(p-)
e(g)
Single track efficiency:
above 96% for p(p) > 270 MeV
above 99% for the photon in almost
the whole energy range
Sub-per mil probability to have
an event with low energy photon
and low momenta pions
Trigger EVENT inefficiency < 10-3

9. dN/dMpp2 spectrum KLOE preliminary 50o<qp,g<130o, Eg>50MeV
2002 Data
L = 240 pb-1
Mpp2 [GeV2]
KLOE preliminary
50o<qp,g<130o, Eg>50MeV
Both the particles not identified as electrons
Cut on 2
Cut on TrackMass vs. Mpp2
Cut on  angle
The spectrum extends down to the 2-pions threshold

10. Efficiency of the selection
Selection efficiency e
The signal selection
efficiency is never below
80% even in the threshold
region, where the ratio
signal/background is low.
p+p-g MC
Mpp2 [GeV2]
Data
p+p-g MC
Further checks proves
that the reducible
background
contribution in the
data sample after the
selection is negligible.
Data
p+p-g MC
[o]
Mpp2 [GeV2]

11. Forward-backward asymmetry
Pion polar angle [o]
90o MC
+- system: A(ISR)  C-odd
A(FSR)  C-even
 an asymmetry is expected in the variable:
test of sQED via comparison data/MC
A(f0) C-even
f0 kk model
f0 ‘no str’ af=p
f0 ‘no str’ af=p/2
no f0
20o<qp<160o
45o<qg<135o
Issue: to distinguish the effect of the interference (described in our MC by
sQED ) and the effect of f0(980).
Czyż, Grzelińska, Kühn,
Phys.Lett.B 611(116)2006
Mpp [GeV]

12. Forward-backward asymmetry
At large photon angles, the amount of FSR is large and the interference
between the two terms gives a sizeable effect. KLOE has already published
a first measurement of the forward-backward asymmetry, and proven the
sensitivity of this quantity to the presence of scalar mesons.
Phys.Lett.B634 (06), 148
Using the f0 amplitude from
Kaon Loop model, good
agreement data-MC* both
around the f0 mass and
at low masses.
data
MC: ISR+FSR
MC: ISR+FSR+f0(KL)
* G. Pancheri, O. Shekhovtsova,
G. Venanzoni, hep-ph/
Mpp (MeV)
Mpp (MeV)
for more details
see C. Di Donato’s talk
zoom

13. Conclusion
The measurement of the hadronic cross section with tagged photons is in an
advanced status.
The threshold region requires more studies.
The analysis on the r-peak and at high Mpp2 is close to the conclusion
 important check for the already published KLOE result.
Selection cuts are fixed
Evaluation of efficiencies is almost finished
test of model scalar QED possible
study of scalar mesons
Forward-backward
asymmetry

14. negligible. Sample: N(LA)=390000 (in our acceptance region)
P > 250 MeV: P(p+) = P(p-) =  The probability to not trigger is 0.077
This probability has to be combined with the trigger probabilty of the photon i.e. > 0.99
Combining the two, the probability that the event does not trigger is 810-4
P<250 MeV: lower single track efficiency, but the dinamic makes the overall probabily
negligible. Sample: N(LA)= (in our acceptance region)
N=37
(0.09% of N(LA))
MC stand alone
N=20981
(5.4% of N(LA))
N=368
(0.09% of N(LA))
And it becomes completely negligible
if we consider both the pions at low momenta