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EPS (July 17th-23rd 2003) in Aachen, Germany

The Hadronic Cross Section Measurement at KLOE Marco Incagli - INFN Pisa on behalf of the KLOE collaboration. EPS (July 17th-23rd 2003) in Aachen, Germany. Im[ ]  | hadrons | 2. Still measuring hadronic cross section: why?.

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EPS (July 17th-23rd 2003) in Aachen, Germany

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  1. The Hadronic Cross Section Measurement at KLOEMarco Incagli - INFN Pisaon behalf of the KLOE collaboration EPS (July 17th-23rd 2003) in Aachen, Germany

  2. Im[ ]  | hadrons |2 Still measuring hadronic cross section: why? • The hadronic cross section is a fundamental tool to evaluate the hadronic contributions to am and to a(MZ) • These quantities are not evaluable in pQCD, but one can use DATA by means of optical theorem + analyticity: • For example am can be evaluated with the dispersion integral: amhad = K(s)~ 1/s (kernel function)

  3. Contributions, as of today, to the error s2(amhad) • The factors 1/s and s(e+e- hadr) in theintegrand of the dispersion relation make the low energy region and the large resonances particularly relevant • The e+e-p+p- channel accounts for ~70% of the contribution both to amhad and to s2(amhad) pp (s<0.5GeV) pp (except r) pp (r region) w + ppp f rest (<1.8GeV) rest (1.8-5 GeV) pQCD (>5GeV) s2(amhad) (from Davier, Eidelman, Hoecker, Zhang)

  4. The role of the t • amHAD can also be evaluated starting from t data and using the (approximate) isospin invariance The recent very precise BNL determination of am and some discrepancies between the value of amHADas evaluatated with ee energy scan and t data, make a new measurement relevant e+e-p+p- tp-p0n

  5. s(had) through the radiative return at KLOE • A way to get the hadronic cross section s(e+e-p+p-) vs Q2 at a fixed energy machine: the Radiative Return (Binner, Kuehn, Melnikov, Phys.Lett. B 459 (1999) 279) Eg Radiation function H(Q2,qg) Q2

  6. Radiative ReturN: PROs and CONs • luminosity and energy scale is estabilished at s=Mf and applies to all values of Mpp2=Q2 • do not need to run the collider at different energies • requires precise understanding of radiative processes • MC used by KLOE : PHOKHARA ver.2.0 (on Tuesday 10, Jul 2003 we have received ver.3.0 which includes FSR!)

  7. DAFNE e+e- machine at Frascati (Rome) Cross sections: f3.3 mb eeg 100 mb eeg 6.2 mb (qe>20o) mmg 0.1 mb ppg 0.05 mb KLOE detector e+ e- • e+e f s ~ mf = 1019.4 MeV • beams cross at an angle of 12.5 mrad • LAB momentum pf~ 13 MeV/c

  8. qp qS 7 m qp 6 m KLOE detector and Fiducial Volume • Definition of fiducial volume: 50o<qp<130o qS<15o or qS>165o where S is the two-pion system • This cut enhances the signal wrt eeppg events in which the photon is radiated from the pion (final state radiation: FSR) The price is that the kinematic region below Q2=0.3GeV2 cannot be probed by thesesmall photon angle events

  9. Getting the ppg cross section 1 548 036 events • L=140.7pb-1 of data collected in 2001 • 1.5106 evts  ~11000 evts/pb-1 • 0.35<Q2<0.97 GeV2 (592-985 MeV) • Bin width = 0.01 GeV2 (~7 MeV) • To get the cross section must evaluate: background ; efficiencies ; luminosity 50 40 number of events (x103) 30 20 Background 10 0 Mpp2 (GeV2) Selection efficiency Luminosity

  10. Background rejection I - e/p separation • e/p separation using a likelihood method: • electron and pion likelihood definition based on TOF and cluster shape • the log of the ratio of the two likelihoods is the discriminating variable eff(p) ~ 98% eff(e) ~ 3% - signal + bkgd events • p+p-p0 events • e+e-g events log(Lpion/Lelectron)

  11. Background rejection II - closing the kinematics MTRK (MeV) • Kinematic separation between signal and background in the (Mpp2,MTRK) plane where MTRK is defined as: (pf-p+-p-)2=pg2=0 with: p=(p2+MTRK2,p) • this cut effects multiphoton processes (eeppgg) • efficiency evaluated using MC p+p-p0 p+p-gg tail signal region 140 105 m+m-g + e+e-g Mpp2 (GeV2)

  12. s s d ( ISR & FSR ) d ( ISR ) - 1 2 2 dQ dQ Efficiency of kinematic separation and FSR • The efficiency of the (Mpp2,MTRK) cut has been evaluated by MC • This efficiency evaluation does not include ppgg events with a FSR photon MTRK efficiency 90% • without TrackMass cut • with TrackMass cut Mpp2(GeV2) A preliminary run with the new PHOKHARA shows that the FSR contribution is at most 2-3% As of now, we do not apply any correction for FSR and add a contribution of 2% to the systematic error +2% -2% A.Denig, H.Czyz r peak Mpp2(GeV2)

  13. Luminosity with Large Angle Bhabhas • Luminosity measured with Large Angle Bhabhas: 55o<qe<135o • 2 independent generators used for radiative corrections: • BABAYAGA (Pavia group): seff = (428.80.3stat) nb • BHAGENF (Berends modified): seff = (428.50.3stat) nb Systematics from generator claimed to be 0.5% Experimental systematic error determined by comparing data and MC angular and momentum distributions

  14. Summary of systematics • Experimental • Acceptance 0.3% • Trigger 0.2% • Tracking 0.3% • Vertex 1.0% • Likelihood 0.1% • Track Mass 0.2% • BKG subtr. 0.5% • Unfolding 0.6% • TOTAL 1.4% (1%) • Theory • Luminosity 0.6% • Vacum Pol. 0.1% • TOTAL 0.7% • FSR (NNLO processes) 2.0% (<1%) Systematic error can be reduced to • in a short time scale

  15. 2 g p+ e+ V.P. Fp(Q2) g* p- e- g 2 p+ e+ 1 g* p- e- Observed cross section • Absolute e+e-p+p-g cross section after bkg subtraction • To get s(e+e-p+p-) we need the H(Q2) function e+e-p+p-gISR(g) ds/dQ2 (nb/GeV2) s(e+e-p+p-) ~ H(Q2) is obtained from PHOKHARA MC setting Fp(Q2)=1 and swithcing off vacuum polarization Radiation function H(Q2) Mpp2 (GeV2)

  16. Hadronic cross section • Hadronic cross section after dividing by the function H(Q2) • The cross section to be inserted in the dispersion integral is the bare cross section e+e-p+p-(g) ds/dQ2 (nb/GeV2) Dahad(s) from F. Jegerlehner d(s) (correction to s(s)) Mpp2 (GeV2) Must correct for running of a : Mpp2 (GeV2)

  17. Preliminary value for amhad • In order to see how our result compares with existing data, we have integrated the bare cross section in the same region covered by CMD2 (0.37<Q2<0.95): • damhad(0.37:0.95) = 374.1  1.1stat5.2syst 2.6theo(+ 7.5-0. FSR) • The published CMD-2 result is : • damhad(0.37:0.95) = 368.1  2.6stat 2.2syst+theo • The two numbers are compatible, given the systematic error, butFSR corrections must be included before performing a detailed point to point comparison

  18. Comparison e+e- vs t data Q2KLOE damhadCMD2* damhad 0.37:0.6 256.2  4.1 (+5.1-0FSR)249.7  2.2 0.6:0.95 117.9  2.1 (+2.3-0FSR)119.8  1.1 • The difference with CMD2 value is mostly below the r peak • It is very difficult, with our data, to explain the discrepancy between e+e- and t data in the region above the r resonance * our evaluation based on CMD2 published table Q2 (GeV2) r peak 10-15% relative difference

  19. Summary and outlook • KLOE has shown the feasibility of using initial state radiation to obtain the hadronic cross section at low energies • Measurement using small angle photon events (qg<15o) is almost finalized we have a new MC for a more precise evaluation of FSR • Preliminary result on damhad slightly higher, but compatible with, CMD2 value • Next steps: • Finalize current analysis • Study events at large photon angles (qg>40o) which allow us to cover the region (2mp)2<Mpp2<0.35 GeV2 • Usemmg events as normalization sample to reduce the systematic error

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