1. Federico Nguyen - IEKP Uni Karlsruhe for the KLOE Collaboration
DAΦNE 2004: Physics at meson factories
Measurement of the e+e- → π+π–(γ) cross section below 1 GeV with the KLOE detector
Federico Nguyen - IEKP Uni Karlsruhe
for the KLOE Collaboration
10th June 2004

2. Outline: measuring s(e+e-  p+p- g) with ISR events
selection and identification of e+e-  p+p- g events
background rejection and estimates
the cross section dsppg /dMpp2
extraction of s(e+e-  p+p-) and FSR corrections
results and conclusions
Federico Nguyen

3. The radiative return standard way: Advantages: Additional issues:
measuring se+ e-  hadrons(s) by varying e beam energies
alternative approach:
given a fixed s1/2, by studying Initial State Radiation events
Advantages:
uncertainties related to beam energy and
luminosity are the same for the full spectrum
it doesn’t require a dedicated set-up (measurement
performed in parallel with typical KLOE programs)
H = radiation function
qmin = emitted g min. ang.
Additional issues:
it requires a precise knowledge of ISR rad. corr.
FSR effects need to be under control
for H we used the MC code Phokhara
with the full NLO ISR corrections
[H.Czyz et al., Eur Phys J C24 (2002) 71]
Federico Nguyen

4. The selection of e+e- p+ p- g events
2 charged tracks in 50o < qp < 130o with a vertex in r < 8 cm, |z| < 7 cm
g acceptance: for enhancing ISR wrt FSR events with g emitted at small angle are selected  only 2 p’s are detected and a cut on the direction of the missing momentum is applied: qS < 15o ( 165o < qg < 180o), qS = dipion system polar angle, pg = - pS = - ( pp++pp- )
high statistics for ISR g events
f  p+p-p0 contamination is reduced
Federico Nguyen

5. Pion identification
/e separation is performed using a Likelihood function based on:
0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
TOF of charged clusters
Shape and quantity of energy deposition of the “charged” cluster along the EMC
- before any cut
p+p-p0 events
e+e-g events
the event is selected if at least
one of the 2 tracks
is recognized as a
pion (log L > 0)
it leads to a rejection power > 92% for e+ e-  e+ e- events
keeping a selection
efficiency ~ 99% for
e+ e-  p+ p- g events
log L
Federico Nguyen

6. Background identification and rejection
p+p-p0
m+m-g + e+e-g
signal
region
mtrk (MeV) mp
p+p-gg tail mm
Mr2
Mpp2 (GeV2)
3500
3000
2500
2000
1500
1000
500
N. of events
before Pion ID
after Pion ID
e+ e- g
m+ m- g
p+ p-p0
mtrk (MeV)
the effect is seen in “track mass” distr., mtrk, defined by 4-mom. conserv. under the hypothesis of 2 equal mass tracks and a g
Federico Nguyen

7. Estimates of the residual background (I)
i) evaluate mtrk histograms in Mpp2 slices for
MC signal and background samples
ii) fit data mtrk histograms with MC ones to
get the relative weights of background
N. of events / MeV
data

ee
L = Pi P(di | wS Si+wB Bi )
only free parameters
data
MC signal, MC background
Mpp2  [0.82,0.87] GeV c2 = 27.9/42
- index i runs over mtrk bins
- P = Poisson probability distribution
- minimize c2 = -2 ln L
data
sum of  and ee
Federico Nguyen

8. Estimates of the residual background (II)
N. of events / 0.5 MeV
N. of events / MeV
data

p+p-p0
data

mmg
Mpp2  [0.32,0.37] GeV c2 = 87.0/88
Mpp2  [0.38,0.42] GeV c2 = 104.4/88
data
sum of  and mmg
data
sum of  and p+p-p0
Federico Nguyen

9. The observed spectrum and efficiencies
100 80 60 40 20
eselection [%]
N. of events / (0.01 GeV2)
Mpp2 (GeV2)
Mpp2 (GeV2)
High statistics:  106 events out of 141 pb-1
of data taken in 2001
High resolution: bin width = 0.01 GeV2
Tracking +
Vertex
Acceptance
Trigger
Reconstr. filter
Pion ID
Trackmass
Federico Nguyen

10. The luminosity measurement in KLOE
Large angle Bhabha:
* Data
Babayaga (MC)
(Pavia theory group)
Bhagenf (MC)
(Berends adapted
by E. Drago and
G. Venanzoni)
track momentum (MeV)
55° < q < 125°
acollinearity < 9°
p  400 MeV
BABAYAGA: sbha = ( ± 0.3stat ) nb
BHAGENF: sbha = ( ± 0.3stat ) nb e- e+ g
of s
Theoretical Systematic Error = 0.5%
Total Error = 0.6%
Federico Nguyen

11. The cross section ds(e+e- p+ p- g)
fbkg = ( Np+p-p0 + Nmmg + Neeg ) / Ntot
ds/dMpp2 (nb/GeV2)
Mpp2 (GeV2)
r-w Interference
Mpp2 (GeV2)
Federico Nguyen

12. From ds(e+e- p+ p- g) to s(e+e- p+ p-)
PHOKHARA (Kühn et al.) agrees with
KKMC (Jadach et al.) at the level
of the claimed accuracy: %
for evaluating am we must get the
bare cross section:
Dahad(s) from F. Jegerlehner, July 2003
Mpp2 (GeV2)
Federico Nguyen

13. it is coherently included
FSR corrections
Fp (exclusive FSR)
Fp (inclusive FSR)
the cross section in the dispersion
integral must be
inclusive in FS g’s
spp(g) 
A0 = ± 0.001
our approach: spp(g) inclusive in FSR
Mpp2 (GeV2)
it is suppressed
by acceptance cuts
another approach: spp(g) exclusive in FSR
subtract FSR (by MC) from the observed sppg
efficiencies are evaluated from MC with only ISR
add FSR (Schwinger) to the measured Fp
it is coherently included
in the MC simulation of the signal [Czyz et al., Eur. Phys. J C27 (2003) 563]
Federico Nguyen

14. Comparison with CMD-2 KLOE50 45 40 35 30 25 20 15 10 5
|Fp | 2
CMD-2
KLOE
Comparison with CMD-2
only statistical
errors are shown
we have evaluated the
dispersion integral for the pp channel
in the range 0.35 GeV2 < Mpp2 < 0.95 GeV2
ampp = (  0.8stat  3.5syst  3.5theo )  10-10
KLOE PRELIMINARY
Mpp2 (GeV2)
comparison with CMD-2 in the range 0.37 GeV2 < Mpp2 < 0.93 GeV2
KLOE
ampp = (375.6  0.8stat  4.9syst+theo)  10-10
1.3% Error
CMD-2
ampp = (378.6  2.7stat  2.3syst+theo)  10-10
0.9% Error
Federico Nguyen

15. How the plot from M. Davier et al. (2003) modifies…
…including
KLOE result
Federico Nguyen

16. Conclusions and outlook
the measurement of the cross section s(e+e-  p+p-) via ISR events using a sample of 141 pb-1 of data taken in 2001 has been presented
high statistics and high mass resolution have allowed to release results competitive with CMD-2
for Mpp2 > Mr2 our results agree with CMD-2, confirming the difference between t decays and e+e- data estimates
our ampp value also confirms the 2.7s deviation between amSM(e+e-) and the recent amexp value
we started the large g angle analysis, allowing to probe the threshold region, and also Particle ID studies for measuring R
Federico Nguyen