Measurement of the e e- ? p p

1
Measurement of the ee- ? pp(?) cross section
below 1 GeV with the KLOE detector
DAFNE 2004 Physics at meson factories
Federico Nguyen - IEKP Uni Karlsruhe for the
KLOE Collaboration 10th June 2004
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Outline

1. measuring s(ee- ? pp- g) with ISR events
2. selection and identification of ee- ? pp- g
   events
3. background rejection and estimates
4. the cross section dsppg /dMpp2
5. extraction of s(ee- ? pp-) and FSR corrections
6. results and conclusions

Federico Nguyen
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The radiative return
standard way 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 doesnt 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
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The selection of ee-? p p- g events
2 charged tracks in 50o lt qp lt 130o with a vertex
in r lt 8 cm, z lt 7 cm
g acceptance for enhancing ISR wrt FSR events
with g emitted at small angle are selected ? only
2 ps are detected and a cut on the direction of
the missing momentum is applied qS lt 15o (? 165o
lt qg lt 180o), qS dipion system polar angle, pg
- pS - ( pppp- )

• high statistics for ISR g events
• f ? pp-p0 contamination is reduced

Federico Nguyen
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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 0

• TOF of charged clusters
• Shape and quantity of energy deposition of the
  charged cluster along the EMC

• - before any cut
• pp-p0 events
• ee-g events

the event is selected if at least one of the 2
tracks is recognized as a pion (log L gt 0) it
leads to a rejection power gt 92 for e e- ? e
e- events keeping a selection efficiency 99
for e e- ? p p- g events
-10 -7.5 -5 -2.5 0 2.5
5 7.5 10
log L
Federico Nguyen
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Background identification and rejection
3500 3000 2500 2000 1500 1000 500 0
N. of events
before Pion ID
after Pion ID
e e- g
m m- g
p p-p0
0 50 100 150 200
250 300
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
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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?

Mpp2 ? 0.82,0.87 GeV2 c2 27.9/42

• data
• sum of ??? and ee?

- index i runs over mtrk bins - P Poisson
probability distribution - minimize c2 -2 ln L
Federico Nguyen
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Estimates of the residual background (II)
N. of events / MeV
N. of events / 0.5 MeV

• data
• ???
• pp-p0

• data
• ???
• mmg

Mpp2 ? 0.32,0.37 GeV2 c2 87.0/88
Mpp2 ? 0.38,0.42 GeV2 c2 104.4/88

• data
• sum of ??? and mmg

• data
• sum of ??? and pp-p0

Federico Nguyen
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The observed spectrum and efficiencies
100 80 60 40 20 0
eselection
N. of events / (0.01 GeV2)
Mpp2 (GeV2)
Mpp2 (GeV2)
High statistics 1.55 ? 106 events out of 141
pb-1 of data taken in
2001 High resolution bin width 0.01 GeV2
Federico Nguyen
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The luminosity measurement in KLOE
Large angle Bhabha
55 lt q lt 125 acollinearity lt 9 p ? 400 MeV
BABAYAGA sbha ( 428.8 0.3stat )
nb BHAGENF sbha ( 428.5 0.3stat ) nb
of ?s
Theoretical Systematic Error 0.5 Total Error
0.6
Federico Nguyen
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The cross section ds(ee-? p p- g)
fbkg ( Npp-p0 Nmmg Neeg ) / Ntot
ds/dMpp2 (nb/GeV2)
Mpp2 (GeV2)
r-w Interference
Mpp2 (GeV2)
Federico Nguyen
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From ds(ee-? p p- g) to s(ee-? p p-)
PHOKHARA (Kühn et al.) agrees with KKMC (Jadach
et al.) at the level of the claimed accuracy
0.5
for evaluating am we must get the bare cross
section
Dahad(s) from F. Jegerlehner, July 2003
Mpp2 (GeV2)
Federico Nguyen
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FSR corrections
Fp (exclusive FSR)
Fp (inclusive FSR)
the cross section in the dispersion integral must
be inclusive in FS gs
spp(g) ?
A0 1.002 0.001
our approach spp(g) inclusive in FSR
Mpp2 (GeV2)
0.4 0.5 0.6 0.7
0.8 0.9
another approach spp(g) exclusive in FSR

1. subtract FSR (by MC) from the observed sppg
2. efficiencies are evaluated from MC with only ISR
3. add FSR (Schwinger) to the measured Fp

Federico Nguyen
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Comparison with CMD-2
50 45 40 35 30 25 20 15 10 5 0
CMD-2 KLOE
Fp 2
only statistical errors are shown
we have evaluated the dispersion integral for
the pp channel in the range 0.35 GeV2 lt Mpp2 lt
0.95 GeV2
ampp ( 388.7 ? 0.8stat ? 3.5syst ? 3.5theo ) ?
10-10
KLOE PRELIMINARY
Mpp2 (GeV2)
comparison with CMD-2 in the range 0.37 GeV2 lt
Mpp2 lt 0.93 GeV2
0.4 0.5 0.6 0.7
0.8 0.9 1
ampp (375.6 ? 0.8stat ? 4.9systtheo) ? 10-10
KLOE
1.3 Error
ampp (378.6 ? 2.7stat ? 2.3systtheo) ? 10-10
CMD-2
0.9 Error
Federico Nguyen
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How the plot from M. Davier et al. (2003)
modifies
including KLOE result
Federico Nguyen
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Conclusions and outlook

• the measurement of the cross section s(ee- ?
  pp-) 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 gt Mr2 our results agree with CMD-2,
  confirming the difference between t decays and
  ee- data estimates
• our ampp value also confirms the 2.7s deviation
  between amSM(ee-) 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