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International Workshop
e+e- collision from  to 
Novosibirsk
February 27th- March 2nd 2006
Debora Leone
(IEKP – Universität Karlsruhe)
for the KLOE collaboration
Progress on Pion Form Factor at KLOE
(large photon polar angle)
D. Leone
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
PRO & CONTRA
Signal selection
Pion tracks:
qp <
Photons: at least one with 50o <qg< 130o
and Eg > 50 MeV
tagged measurement
50o <
130o
500< qp,g <1300
 the threshold region is accessible
 one photon is detected
(4-momentum constraints)
 lower signal statistics
 large FSR contributions
 irreducible background from
 to p+p- decays
 large   p+p-p0 background
contamination
50o<qp<130o
50o<qg<130o
p+p-g MC
p+p-p0 MC
50% of final
sample statistic
g
p
Mpp2 [GeV2]
D. Leone
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
Reducible background rejection
three sources: Radiative Bhabhas e+e- e+e- g, muon pairs m+m- m+m- g and
p+p-p0
Particle ID
Mtrk [MeV]
Radiative Bhabhas are separated
by means of a particle-ID
(signature of EmC-Clusters
and time of flight of particles)
mp
mm
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
2


 
M - | p + |2 + M 2trk - | p - |2 + M 2trk - | p + + p - |2  0
p+p-p0 – MC


p+p-g – MC
in the assumption of  x+ x- g
m+m-g - MC
2
2
mr2 Mpp [GeV ]
D. Leone
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
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 < qp < 140
At least two photons in time, one of them with
Eg > 40 MeV and 40 < qg < 140
4-momenta conservation
Minv(gg) = m(p0)
W Angle
p+p-g
Data
p+p-g MC
Angle between the missing momentum
and the detected photon momentum
MC
 
p γ .p miss
W  acos(  
)
| p γ | | p miss |
(due to NLO events)
p+p-p0 MC

pγ
0
10
D. Leone
20
30
40
50
60
70
W[o]
Pion Form Factor @ KLOE
2
p+
g
W
2
p-

p miss
Novosibirsk, 27.02- 02.03, 2006
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]
%
Up tp 10% difference
between data and MC
for a maximum of 10%
contamination
(excluding the threshold
region...see later)
Muons
contamination
1% error on the knowledge
of m+m-g in the final
p+p-g sample
Mpp2 [GeV2]
D. Leone
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
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
Absolutely
the p+p-p0 kinematic fit
normalized
(2 <20)
Data
MC
p+p-p0
Data
MC
p+p-p0
2 <20
Mpp2 [GeV2]
Eg [MeV]
Data
MC
p+p-p0
W [o]
D. Leone
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.
Pion Form Factor @ KLOE
%
3 pions
contamination
Mpp2 [GeV2]
Novosibirsk, 27.02- 02.03, 2006
Irreducible background
m+m-g and p+p-p0 background channels well under control… but FSR events
as
e+e-  ppgFSR
  f0 g  pp g
  r p  pg p
all of them with p+p-g final state, indistinguishable from the signal signature
p
r
p
FSR
g
g

&
f0
p
&
f0
p
p

rp
g
r
p
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.
D. Leone
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
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-
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
eg
D. Leone
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
dN/dMpp2 spectrum
 50o<qp,g<130o, Eg>50MeV
 Both the particles not
identified as electrons
 Cut on 2ppp
 Cut on TrackMass vs. Mpp2
 Cut on W angle
Mpp2 [GeV2]
2002 Data
L = 240 pb-1
Mpp2 [GeV2]
The spectrum extends down to the 2-pions threshold
D. Leone
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
Efficiency of the selection
e
Selection efficiency
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
W[o]
D. Leone
Mpp2 [GeV2]
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
Forward-backward asymmetry
90o
p+p- system: A(ISR)  C-odd
A(FSR)  C-even
 an asymmetry is expected in the variable:
θπ
θπ
-
+
test of sQED via comparison data/MC
MC
f0 kk model
f0 ‘no str’ ap
f0 ‘no str’ ap/2
no f0
A(f0) C-even
Pion polar angle [o]
20o<q
o
p<160
45o<qg<135o
Issue: to distinguish the effect of the
interference (described in our MC by
sQED ) and the effect of f0(980).
Mpp [GeV]
D. Leone
Czyż, Grzelińska, Kühn,
Phys.Lett.B 611(116)2006
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
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,
Mpp (MeV)
Mpp (MeV)
for more details
see C. Di Donato’s talk
zoom
D. Leone
G. Venanzoni, hep-ph/0506332
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
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
D. Leone
Forward-backward
asymmetry
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006
P > 250 MeV: P(p+) = P(p-) = 0.96  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)=390000 (in our acceptance region)
MC stand alone
N=37
(0.09% of N(LA))
N=20981
(5.4% of N(LA))
N=368
(0.09% of N(LA))
D. Leone
And it becomes completely negligible
if we consider both the pions at low momenta
Pion Form Factor @ KLOE
Novosibirsk, 27.02- 02.03, 2006