Achim Denig Radiative Return @ DAFNE MENU 2004

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Measurement of the Hadronic Cross Section via
Radiative Return at DAFNE
MENU 2004 September 3rd Beijing, IHEP
Achim Denig for the KLOE Collaboration
Universität Karlsruhe Institut für Experimentelle
Kernphysik
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hep-ex/0407048, sent to Phys. Lett. B

• Outline
• Muon Anomaly
• Radiative Return at KLOE
• Results Outlook

Further KLOE-Results St. Müller (Session B2)
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Muon - Anomaly

• Motivation Determination of Hadronic Vacuum
  Polarization High Precision Test of the
  Standard Model
• Anomalous magnetic moment of the muon am
  (g-2)m
• Running fine structure constant at Z0-mass
  aQED (MZ)

Dirac-Theory (g - 2 ) 0 Quantum
corrections (g - 2 ) ? 0 due to corrections
of - electromagnetic interaction - weak
interaction - strong interaction (and
maybe NEW PHYSICS ???)
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Hadronic Cross Section
Hadronic contribution to am can be estimated by
means of a dispersion integral
- K(s) analytic kernel-function, almost flat in
s - above 25 GeV, use pQCD
Input Dispersion Integral a) hadronic
electron-positron cross section data, e.g. ee-?
p p- b) hadronic t- decays (CVC-theorem,
taking into account isospin breaking
corrections), e.g. t-? p0 p-nt
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Muon-Anomaly Theory vs. Experiment
Theoretical values taken from Davier, Eidelman,
Höcker, Zhang hep-ex/0308213
Status up to July 04

• Dispersion integral for hadronic
• contribution to am evaluated for
• e e--Data ONLY
• CMD-2 (VEPP-2M) measured p p-
• channel with 0.6 precision lt 1 GeV
• 2.7 s Deviation
• t-Data ONLY (if available)
• ALEPH /OPAL/CLEO
• 1.4 s Deviation

ee- data and t data differ for pp- channel in
a specific energy window above 0.6 GeV2 (above
the r-peak)!
THEORY 20/03
Experiment BNL-E821 Values for m(2002) and
m-(2004) in agreement with each other. Precision
0.5ppm
Experiment 20/04
am - 11 659 000 10-10
am - 11 659 000 10-10
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Radiative Return

• Standard method for cross section measurement
  is the energy scan, i.e. the
• systematic variation of the c.m.s.-energy of
  the accelerator
• DAFNE is a f - factory and therefore designed
  for a fixed c.m.s.-energy
• ?s mf 1 019 MeV a variation of the energy
  is not foreseen in near future

Complementary approach Take events with Initial
State Radiation (ISR)

Radiative Return to r(w)-resonance e e-?
r(w) g ? p p- g
g
r0
Cross section as a function of the 2-Pion
invariant mass spMpp2
sp
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Analysis pp-g

• Final state ee- ? p p- g relatively easy
  signature, however cross
• section measurement on percent level is a
  challenging task (normali-
• zation, efficiencies, background)
• KLOE Detector designed for CP violation, we
  are having a high
• resolution tracking chamber ideal for the
  measurement of Mpp !

Analysis- Items
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Selection pp-g
Drift Chamber
Pion tracks at large angles 50 lt qp lt 130
EM Calorimeter

Photons at small angles qg lt 15 and qg gt
165 are shadowed by quadrupoles near the I.P.
500lt ? lt 1300
?
NO PHOTON TAGGING
? lt 150
? gt 1650
?

• High statistics for ISR events
• Reduced background contamination
• Low relative contribution of FSR

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Background pp-g
1) Pion-Electron-Separation Rad. Bhabhas
ee-? ee- g are separated by means of a
Likelihood- Method (Signature of
EmC-Clusters and TOF of particle tracks)
MTRK (MeV)
pp-p0
pp-gg tail
signal region
mp
mm-g ee-g
mm
mr2
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Analysis s( ee- ? pp-g)

• Efficiencies
• - Trigger Cosmic veto
• Tracking, Vertex
• p- e- separation
• Reconstruction filter
• Trackmass-cut
• Unfolding resolution
• Acceptance

Errors
0.9

• Background
• e e-? e e- g
• e e- ? m m- g
• f ? p p- po

0.3
Luminosity Bhabhas at large angles gt 55, seff
430 nb,
0.3exp 0.5theo
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Extraction s( ee- ? pp-)
Radiator-Function (ISR)

• ISR-Process calculated at NLO-level
• Generator PHOKHARA (Kühn et.al)
• Comparison with KKMC (Jadach et.al.)
• Precision
  0.5

Radiative Corrections
i) Bare Cross Section divide by Vacuum
Polarisation ii) FSR - Corrections Cross
section spp must be incl. for FSR
Vacuum Polarization
Cross Section
Radiative Return requires ISR photon ? be
inclusive for ISR-FSR-events e e- ? p p-
gISR(gFSR)
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Cross Section s( ee- ? pp-)
Aim Cross Section ee-? pp-
TOTAL ERROR 1.3
To be compared with 0.9 CMD-2 Error
exp.syst.stat.
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2p Contribution to amhadr

• Dispersions Integral for the 2-Pion-Channel
• in Energy Range 0.35 ltMpp2lt0.95 GeV2

ampp (388.7 ? 0.8stat ? 3.5syst ? 3.5theo) 10-10
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Comparison with t Data
Rel. difference of ee--data with respect to
t-data (avg.)
At large values of sp (gtmr) KLOE CMD-2
deviate from t-Data gt 10 ? unsufficient
understanding of isospin breaking corrections in
t-Data ?!
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Conclusions

• Status of the anomalous magnetic moment of the
  muon today (August 04)

M. Davier, A. Höcker _at_ ICHEP04

• Including
• New KLOE data
• New 4th order QED contribution
• (Kinoshita) hep-ph/0402206
• New Light-by-light contribution
• (Melnikov, Vainshtein)
• hep-ph/0312226
• t data excluded!

Including KLOE

• Theory (SM) - Experiment
• amexp - amtheo ( 25.2 9.2 ) 10-10
• 2.7 standard deviations

• KLOE has proven feasibility of the Radiative
  Return for high-precision hadronic
• cross section measurements

hep-ex/0407048, sent to Phys. Lett. B
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Outlook KLOE

• Measure s(pp) in the region close to threshold,
  Mpp lt 600 MeV,
• responsible for ?20 of ampp
• This region excluded by angular selection in
  small angle photon approach
• Complementary analysis at large photon angles

• At large photon angles the amount of FSR is
  large!
• test model of scalar QED
• (i.e. poinlike pions)
• ? Measure Charge Asymmetry
• and compare data with MC
• Charge asymmetry is due to different
• C-Parity of ISR- and FSR-amplitudes

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Outlook Muon Anomaly
Theory

• BaBar fills gap between DAFNE, VEPP-2M ( lt 1.4
  GeV ) and BES ( gt 2 GeV)
• and will measure all relevant exclusive
  channels (see e.g. hep-ex/0408078)
• KLOE/CMD-2 threshold region lt 0.4 GeV2
• Future measurements at BES, VEPP-2000, CLEO,
  BELLE, DAFNE-2?
• Real Theory Rad. corrections,
  Light-by-light-scattering, Isospin breaking ...

Experiment

• New beam time for BNL-E821 (still limited by
  statistics)!?
• Ambitious new project for 5th generation (g-2)m
  experiment at J-PARC

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