# Stefan E. M üller Laboratori Nazionali di Frascati

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Stefan E. Mller Laboratori Nazionali di FrascatiEvaluation of ampp between 0.35 and 0.95 GeV2 with data from the KLOE detector2007 Europhysics Conference on High Energy PhysicsManchester, 19-25 July 2007 (for the KLOE collaboration)

ahadr can be expressed in terms of (e+e- hadrons) by the use of a dispersion integral:

Ecut is the threshold energy above which pQCD is applicable s is the c.m.-energy squared of the hadronic system K(s) is a monotonous function that goes with 1/s, enhancing low energy contributions of hadr(s)

Particle factories have the opportunity to measure the cross section s(e+ e- hadrons ) as a function of the hadronic c.m. energy M hadr by using the radiative return:

This method is a complementary approach to the standard energy scan. Requires precise calculations of the radiator H EVA + PHOKHARA MC Generator(S. Binner, J.H. Khn, K. Melnikov, Phys. Lett. B 459, 1999)(H. Czy, A. Grzeliska, J.H. Khn, G. Rodrigo, Eur. Phys. J. C 27, 2003)Neglecting FSR effects:

DEAR, e+e- - collider with =m1.0195 GeVIntegrated Luminosity Peak Luminosity Lpeak= 1.4 1032cm-2s-1Total KLOE int. Luminosity: L dt ~ 2500 pb1 (2001 - 05)2006: Energy scan with 4 points around m-peak 225pb-1 at = 1000 MeV This talk is based on 240pb-1 from 2002 data!

Driftchambersp/p = 0.4% (for 900 tracks)sxy 150 mm, sz 2 mmExcellent momentum resolution

Electromagnetic Calorimeter

Pion tracks at large angles 50o< p 165o No photon detection! High statistics for ISR photons Very small contribution from FSR Reduced background contamination

b) Photons at large angles 50o < < 130oPhoton is observed in the detector! Threshold region accessible Increased contribution from FSR Contribution from f0(980)Pion tracks at large angles 50o< p 165o No photon detection! High statistics for ISR photons Very small contribution from FSR Reduced background contamination

To further clean the samples from radiative Bhabha events, a particle ID estimator for each charged track based on Calorimeter Information and Time-of-Flight is used. Experimental challenge: Fight background from ee ee (g) ee (), separated by means of kinematical cuts in trackmass MTrk (defined by 4-momentum conservation under the hypothesis of 2 tracks with equal mass and one photon)and Missing Mass Mmiss(defined by 4-momentum conservation under the hypothesis of e+e-+-x

- 30% cosmic veto inefficiency recovered in 2002 by introducing additional software trigger levelImproved offline-event filter reduces its systematic uncertainty to
Impact of update on trigger correction on 2001 cross section:KLOE 2001 TRG updatedKLOE 2001 published KLOE 2001 TRG updatedKLOE 2001 publishedChanges published value on app by 0.4%

r-w InterferenceM2 (GeV2)Statistics: 242pb-1 of 2002-data 3.4 Million EventsqMiss1650 Preliminary!!!M2 (GeV2)

- At KLOE, Luminosity is measured using Large angle Bhabha events (55o
ISR-Process calculated at NLO-level PHOKHARA generator (Czy, Khn et.al) Precision: 0.5%Radiator-Function H(s) (ISR):Radiative Corrections:i) Bare Cross Section divide by Vacuum Polarisation from F. Jegerlehner: http://www-com.physik.hu-berlin.de/~fjeger/ii) FSR - Corrections Cross section spp must be incl. for FSR

FSR corrections have to be taken into account in the efficiency eval. (small angle acceptance, MTrk) and in the passage M2FSR contr. (sQED)Net effect of FSR is ca. 0.8%:

Systematic errors on ampp:STotal= 1.1%Preliminary!!!

Offline FilternegligibleBackground0.3%Trackmass/Miss. Mass0.2% (prelim)p/e-ID0.3%Vertex0.5%Tracking0.4%Trigger0.2%Acceptance ()negligibleMpp2 Mg* (FSR corr.)0.3% (prelim)Software Trigger 0.1 %

Luminosity 0.3%Acceptance (Miss)0.1%Radiator H0.5%Vacuum polarizationnegligible

(updated)

- 2001 published result (Phys. Lett. B606 (2005) 12):Applying update for trigger eff. and change in Bhabha-cross section used for luminosity evaluation: app(0.35-0.95GeV2) = (388.7 0.8stat4.9syst) 10-10Dispersion integral for 2p-channel in energy interval 0.35
Threshold region non-trivial due to irreducible FSR-effects, to be estimated from MC using phenomenological models (interference effects unknown)&&FSRf0rp

important cross check with small angle analysisthreshold region is accessible photon is detected (4-momentum constraints)

lower signal statistics FSR not negligible anymore large p+p-p0 background irreducible bkg. from f decays240 pb-12002 DataMpp2 [GeV2]Nr. of events / 0.01 GeV2Preliminary!!!

2002 Data L = 240 pb-1Mpp2 [GeV2]Apply dedicated selection cuts: Exploit kinematic closure of the event Cut on angle btw. ISR-photon and missing momentum

Kinematic fit in p+p-p0 hypothesis using 4-momentum and p0-mass as constraints

FSR contribution added back to cross section (estimated from PHOKHARA generator)

Error on LA dominated by syst. uncertainty on f0 contr. LA stat.+syst. error Reducible background from p+p-p0 and +-g well simulated by MCPreliminary!!! Model dependence of irreducible background from f f0g p+p-g is the dominating uncertainty. Estimated using different models for f0-decay and input from dedicated KLOE f f0g analyses (with f0 decaying to charged and neutral pions).

Range of comparisonSmall angle: app(0.50-0.85GeV2) = (255.4 0.4stat 2.5syst) 10-10Large angle: app(0.50-0.85GeV2) = (252.5 0.6stat 5.1syst) 10-10Range of comparisonLA stat.+syst. error (sLA-sSA)/sSAapp between 0.5 - 0.85 GeV2: Preliminary!!!(60% of systematical error due to f0-uncertainty)

Jegerlehner (hep-ph/0703125):

Using new KLOE result would increase difference from 3.2s to 3.4sSummary of the small angle results:Preliminary!!!

Comparison with ampp from CMD2 and SND in the range 0.630-0.958 GeV :Phys. Lett. B648 (2007) 28Preliminary!!!

We have obtained app in the range between 0.35 - 0.95 GeV2 using cross section data obtained via the radiative return with photon emission at small angles.

The preliminary result from 2002 data agrees with the updated result from the published KLOE analysis based on 2001 dataData from an independent and complementary KLOE measurement (Large angle analysis) of the 2p-cross section has been used to obtain app in the range between 0.5 - 0.85 GeV2

All three KLOE results are in good agreement KLOE results also agree with recent results on app from the CMD2 and SND experiments at VEPP-2M in Novosibirsk

Refine the small angle analysis by unfolding for detector resolution, evaluating further possible backgrounds, etc.

Continue evaluation of resonance contributions in the large angle analysis

Measure the pion form factor via bin-by-bin ratios of pions over muons (Normalization to muons instead of absolute normalization with Bhabhas)

Obtain pion form factor from data taken at= 1000 MeV (outside the f resonance) suppression of background from f-decays determination of f0-parameters

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