1. MEASUREMENT of e+e- HADRONIC CROSS SECTION with RADIATIVE RETURN at KLOE
Barva Maura - Università Roma Tre
On behalf of the KLOE Collaboration
LNF Spring School - May 2004

2. Summary Motivation of shadr shadr with radiative return
Results on s(e+e-p+p-) at small angle
Conclusions and outlook
L.N.F. Spring School - May 2004

3. Why the measurement of the cross section is important?
THE ANOMALOUS MAGNETIC MOMENT OF THE MUON
Magnetic moment
for pointlike fermion
Dirac
(Dirac prediction)
because there are
quantum correction m+ q g*
Hadronic
vacuum
polarization
amSM = amQED + amhad +amEW
L.N.F. Spring School - May 2004

4. Theorical vs Experimental value A NEW MEASUREMENT IS VERY IMPORTANT!
Status of am:
Theorical vs Experimental value
Last result from BNL: 01/04
E821
t-data from
ALEPH, OPAL,
CLEO
e+e- dominated
by CMD-2
|am(e+ e-) –am( data)|: 2.7 s -Deviation
|am(t) –am( data)|: 1.4 s - Deviation
A NEW MEASUREMENT IS VERY IMPORTANT!
L.N.F. Spring School May 2004

5. Hadronic contribution to am
The error on am(theo) is dominated by the error on amhad
This quantity is not evaluable in pQCD, but it can be calculated by DISPERSION INTEGRAL:
The factors 1/s inthe dispersion relation makes the low energy region particularly relevant.
The e+e-p+p- channel accounts for ~70% of the contribution both to amhad
Input:
a) Hadronic electron-positron
cross section data
b) Hadronic t- decays, which can be used
with the help of the CVC-theorem
and an isospin rotation (plus isospin
breaking corrections)
L.N.F. Spring School - May 2004

6. How to perform this measurement?
traditional way : measuring cross section by varying e beams energy energy scan
BUT
DAFNE is a f - factory and therefore runs at fixed c.m.s.-energy: s = mf = MeV r s’
Complementary approach:
Take events with
Initial State Radiation (ISR)
“Radiative Return” to r-resonance:
e+ e- r + g  p+ p- + g
L.N.F. Spring School - May 2004

7. s(had) through radiative return
The s(e+e-  p+p-g) is a function of the 2-Pion invariant mass s’=Mpp.To extract s(e+e-  p+p-) with the ISR we
Need to know the RADIATOR FUNCTION H(s)
H(s) is evaluable
From MC Phokhara
Have to get rid of “pure” FSR:
which causes events with Mg* = s to be assigned to lower s´ values
Have to properly include radiative corrections (including simultaneously emission of ISR and FSR)
L.N.F. Spring School - May 2004

8. DAFNE: the Double Annular F-factory
for Nice Experiments
Ebeams MeV
2 separate rings for e+e- to minimize
beam-beam interaction
two interaction region: KLOE-DEAR/FINUDA
crossing angle: 12.5 mrad (px(f) MeV)
Lpeak (cm-2s-1)
4.4 1030
Lbunch (cm-2s-1)
Bunch current (mA)
Lifetime (mins)
5.3 1032 40
120
N of bunches
Design
DAFNE
Parameters
DAFNE performance:
2000 run: 25pb-1, 7.5 x107 f decay first published results
2001 run: 190pb-1, 5.7 x108 f decay
2002 run: 300pb-1, 9.0 x108 f decay
Anlysis in progress
L.N.F. Spring School - May 2004

9. The KLOE detector:K LOng Experiment
Electromagnetic calorimeter
 lead/scintillating
fibers (1 mm ), 15 X0
 4880 PMT’s
 98% solid angle coverage
Drift chamber (4 m   3.75 m, CF frame)
 Gas mixture:
90% He + 10% C4H10
 stereo–stereo
sense wires
 almost squared cells
Beam pipe :
(spherical, 10 cm , 0.5 mm thick)
Instrumented permanent
magnet quadrupoles
(32 PMT’s)
L.N.F. Spring School - May 2004

10. Measurement of sppg – Analysis scheme
Pion tracks at large angles
from a vertex close to IP
Photons at small angles
to enhance ISR:
dsISR/dW  1/sin2q
relative contribution of “pure” FSR below the % level over entire Mpp spectrum
reduce background
Lose events with Mpp< 600 MeV
LARGE ANGLE ANALYSIS
(see after…)
L.N.F. Spring School - May 2004

11. Summary of syst. errors and background
Acceptance 0.3%
Trigger 0.3%
FILFO 0.6%
Tracking 0.3%
Vertex 0.3%
Likelihood 0.1%
Track mass %
Backg.
Subsraction %
Unfolding %
Total exp %
Luminosity 0.6%
Vacuum
Polarization %
FSR %
Radiator
function %
Total theory 0.9%
1.Rad. Bhabhas:
Pion-Electron-Separation by means
of a Particle-ID algorithm
2. f  p+ p- p0
e+e- m+ m- g:
Kinematic Separation: “Trackmass“
p+p-p0
m+m-g + e+e-g
signal
region
MTRK (MeV) mp
p+p-gg tail
mr2 mm
L.N.F. Spring School - May 2004

12. Analysis: s(e+e-  p+p-g)
Statistics: 141pb-1 of 2001-Data 1.5 Million Events
r-w interference
High Statistics!
High Resolution!
Mpp2 (GeV2)
H(Mpp2)
obtained from
PHOKHARA
L.N.F. Spring School - May 2004

13. Analysis: Pion Form Factor and amhad
|Fp|2
 CMD2
- KLOE
0.5
0.7
0.9 20 40 10 30
We have evaluated the dispersions integrals for the 2-Pion-Channel in the energy range 0.35 <M2pp<0.95 GeV2
ampp = (389.2  0.8stat  3.9syst  3.5theo) 10-10
Comparison with CMD-2 in energy range 0.37 <M2pp<0.93 GeV2
(376.5  0.8stat  5.1syst+theo) 10-10
(378.6  2.7stat  2.3syst+theo) 10-10
KLOE
CMD2
The results are in good agreement!
L.N.F. Spring School - May 2004

14. (1) amhad prospect: Large Angle analysis
The energy region close to threshold, Mpp < 600 MeV, is excluded by our selection cuts
This region contributes ~20% to amhad
We use events at large photon angles to access lower
M2 region. (Photon tagging
will be possible in this case)
Check FSR parametrization (by looking to charge asymmetry)
This analysis is going on...
MeV
Mpp (MeV)
400
300
600
500
800
700
900
N(ppg) / MeV 10
102
103
L.N.F. Spring School - May 2004

15. (2) amhad prospect: the direct measurement of
We are also studying a direct measurement of R
This method is independent from the luminosity estimate
We are working on
obtaining particle ID combining calorimetric and kinematically variables
2002 DATA!!!
ppg events
mmg events
Likelihood
Mass Trk
L.N.F. Spring School - May 2004

16. Conclusion: Summary: Ongoing activities
We have presented the first precise measurement of hadronic cross section using radiative return at KLOE
Our result is in agreement with CMD2, therefore confirming the existing discrepancy bewteen SM and BNL result on (g-2)m
Ongoing activities
Large angle photon analysis: to study the low Mpp2 region and measure the asymmetry to check the FSR results
The direct measurement of R= (e+e-  +-g)/(e+e-  +-g)
L.N.F. Spring School - May 2004