1. Study of the reaction e+e-  p+p- at KLOE
Debora Leone
IEKP – Universität Karlsruhe
DPG Tagung
München,
Study of the reaction e+e-  p+p- at KLOE
Introduction
Result on the small photon angle analysis (Phys. Lett. B606, 12 (2005))
Large photon angle analysis (in progress)
Conclusion

2. Hadronic contribution to (g-2)m
Magnetic momentum:
with m + q g * g*
B field
amSM = amQED + amhad + amEW
Due to quantum corrections:
amhad includes contributions not evaluable in pQCD,
but it can be provided by s(e+ e-  hadrons) by means
of dispersion relation:
K(s) is a steady function that goes with 1/s, enhancing low energy contributions
of shadr(s)
In energy range <1 GeV, e+e-  p+p- contributes to more than 60% to amhad

3. s(e+e- p+p-) with Radiative Return
DAFNE is designed for a fixed center-of-mass energy: s = mf = 1.02 GeV
We can measure s(e+e-p+p-) as a function of the hadronic c.m. energy M2hadr
(S.Binner, J.H. Kühn, K. Melnikov, Phys.Lett. B459,1999)
Precise knowledge of ISR – process: Radiator function
MC generator: Phokhara
(H. Czyz, A. Grzelinska, J.H. Kühn,
G. Rodrigo, hep-ph/ )
“Radiative Return” to (w) resonance e+e-  (w)   +-
Mhadr
ds(e+ e-  hadrons + g )
dM2hadrons
=s(e+ e-  hadrons) H(M2hadr )
This method is a complementary approach to the energy scan.

4. Small photon polar angle analysis
SELECTION
Pion tracks: 50o< p <130o
Photons: g <15o or g >165o
No photon tagging:
high statistics for ISR
low relative FSR contribution
suppressed   p+p-p0 background
threshold region not covered
no kinematic closure of event
PRO & CONTRA
Mpp2 (GeV2)
MC: FSR /(ISR + FSR)
(%)
KLOE result (140 pb-1 of 2001)
Phys. Lett. B606 (2005) 12

5. KLOE result compared with CMD-2
1.3% Error
0.9% Error
CMD-2
KLOE
0.4
0.5
0.6
0.7
0.8
0.9 45 40 35 30 25 20 15 10 5
s (GeV2)
|Fp(s)|2
Mpp2 (GeV2)
s(e+e-p+p-) result
statistical error: negligible
experimental systematic uncertainty: 0.9%
theoretical systematic uncertainty: 0.9%
Total systematic error: 1.3%
Pion Form Factor comparison
with result from CMD-2 experiment:
KLOE and CMD-2 in fair agreement
Dispersion integral evaluation
KLOE: (375.6  0.8stat  4.9syst+theo) 10-10
CMD-2: (378.6  2.7stat  2.3syst+theo) 10-10
KLOE and CMD-2 in good agreement
365
370
375
385
380
ampp·1010

6. Theory (SM) - Experiment
Muon anomaly comparison Theory - Experiment
KLOE and CMD-2 results used to evaluate the hadronic contribution and therefore
the muon anomaly
am ∙ 10-10
140
150
160
170
180
190
200
210
DEHZ’03 [e+e- based]
DEHZ’03 [t based]
DEHZ‘03
Experiment E821
Standard model prediction:
KLOE measurement
Phys. Lett. B606 (2005) 12
New 4th order QED calculation
(Kinoshita, Nio)
Phys. Rev. D70 (2004)
New ‘Light-by-light’ calculation
(Melnikov, Vainshtein)
Phys. Rev. D70 (2004)
CMD-2 and KLOE averaged in hadronic contribution
DEHZ’04 [e+e-]
New
Theory (SM) - Experiment
amexp - amtheo = ( 25.2 ± 9.2 ) ·10-10
2.7 standard deviations
difference

7. Large photon polar angle analysis - signal selection
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-g 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
Mpp2 [GeV2]

8. Reducible background rejection
three sources: Radiative Bhabhas e+e- e+e- , muon pairs e+e- m+m-  and
+-0
Particle ID
Radiative Bhabhas are separated
by means of a particle-ID
(likelihood function):
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
under the assumption of  x+ x- 

9. Reducible background rejection
Two further dedicated cuts to p+p-p0 rejection
Kinematic fit
Kinematic fit under the p+p-p0 background hypothesis
Two tracks in 40 < p < 140
At least two photons, 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
p+p-p0 MC p+ p- W g

10. 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 needed concerning the hadronic models.

11. 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

12. 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 structure’ af=p
f0 ‘no structure’ 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]

13. 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)
Mpp (MeV)
Mpp (MeV)
* G. Pancheri, O. Shekhovtsova,
G. Venanzoni, hep-ph/
zoom

14. Conclusion
KLOE has measured for the first time the hadronic cross section with radiative
return method, proving the validity of it.
The measurement of the hadronic cross section in a different angular region and
with tagged photon is in an advanced state.
The threshold region requires more studies, due to the irreducible background
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

15. Backup slide

16. The KLOE detector at the DAFNE F-factory
12582 sense wires
52140 wires in total
Track momentum resolution
sp/p  0.4% (q > 45°)
Vertex resolution
sxy  150 mm, sz  2 mm
Drift chamber
Electromagnetic
calorimeter
Energy resolution
sE/E = 5.7%/E(GeV)
Time resolution
sT = 54 ps/E(GeV)
 50 ps
Pb/Scint fibres 4880 PM
Magnetic Field of 0.52 T