A1256655917TVOZw

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International Workshop ee- 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)
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Signal selection
Pion tracks 50o lt ?p lt 130o Photons at least
one with 50o lt?glt 130o and Eg gt
50 MeV tagged measurement
pp-g MC pp-p0 MC 50 of final sample statistic
50oltqplt130o 50oltqglt130o
Mpp2 GeV2
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Reducible background rejection three sources
Radiative Bhabhas ee-? ee- ?, muon pairs mm-?
mm- ? and ????-?0
Particle ID
Radiative Bhabhas are separated by means of a
particle-ID (signature of EmC-Clusters and time
of flight of particles)
TrackMass

• To reject mm-g and (partially) pp-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- ?

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Reducible background rejection Two further
dedicated cuts to pp-p0 rejection
Kinematic fit
Kinematic fit in the pp-p0 background
hypothesis Two tracks in 40? lt ?p lt 140? At least
two photons in time, one of them with Eg gt 40
MeV and 40? lt ?g lt 140? 4-momenta conservation
Minv(gg) m(p0)
?2
Data pp-g MC
? Angle
?2
Angle between the missing momentum and the
detected photon momentum
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Residual background evaluation mm-g
Muons sample selected asking 80 lt TrackMass lt 110
MeV, in the same angular region
as the pp-g sample
Preliminary
80 ltTrackMasslt110MeV
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 mm-g in the
final pp-g sample

Mpp2 GeV2
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Residual background evaluation pp-p0
In order to select a sample of pp-p0 from data,
we have applied after the angular cuts a rigid
cut on the ?2 of the pp-p0 kinematic fit
(?2 lt20)
Absolutely normalized
Data pp-p0 MC
Data pp-p0 MC
?2 lt20
Mpp2 GeV2

Difference data-MC of the order of
20, contamination in the final sample smaller
than 10 accuracy on pp-p0 subtraction at
per mil level.
3 pions contamination
Eg MeV
Data pp-p0 MC
Mpp2 GeV2
? o
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Irreducible background

• mm-g and pp-p0 background channels well
  under control but FSR events as
• ee- ? ppgFSR
• f ? f0 g ? pp g
• f ? r p ? pg p
• all of them with pp-g final state,
  indistinguishable from the signal signature

FSR
rp
f0
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.
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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.
Single track efficiency above 96 for p(p?) gt
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 lt 10-3
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dN/dMpp2 spectrum

• 50oltqp,glt130o, Eggt50MeV
• 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
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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.
pp-g MC
Mpp2 GeV2



Data pp-g MC
Further checks proves that the
reducible background contribution in the data
sample after the selection is negligible.
Data pp-g MC
?o
Mpp2 GeV2
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Forward-backward asymmetry
??- system A(ISR) ? C-odd
A(FSR) ? C-even ? an asymmetry is expected
in the variable
test of sQED via comparison data/MC
f0 kk model f0 no str afp f0 no
str afp/2 no f0
A(f0)? C-even

20oltqplt160o 45oltqglt135o
Issue to distinguish the effect of the
interference (described in our MC by sQED ) and
the effect of f0(980).
Czyz, Grzelinska, Kühn, Phys.Lett.B 611(116)2006
Mpp GeV
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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 ISRFSR
• MC ISRFSRf0(KL)

G. Pancheri, O. Shekhovtsova, G.
Venanzoni, hep-ph/0506332
Mpp (MeV)
Mpp (MeV)
for more details see C. Di Donatos talk
zoom
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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
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P gt 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. gt 0.99 Combining
the two, the probability that the event does not
trigger is 8?10-4 Plt250 MeV lower single track
efficiency, but the dinamic makes the overall
probabily negligible. Sample N(LA)390000
(in our acceptance region)
N37 (0.09 of N(LA))
MC stand alone
N20981 (5.4 of N(LA))
N368 (0.09 of N(LA))
And it becomes completely negligible if we
consider both the pions at low momenta