Measurement of cross section   K + K –

1. Measurement of cross section   K+ K– • M. Martemianov • Last results of  (  K+ K–) • Method of cross section measurement • Selection criteria • Radiative correction for cross section • Results 2nd KLOE Physics Workshop, Otranto, 10-12 June 2002

2.   K+ K– cross section Cross section : • trig - trigger efficiency • rec - reconstruction efficiency • geom - geometrical acceptance • L - luminosity rad - radiation coefficient, where '(S) / (S) - measured / real cross section

3. CMD-2 / SND results World data (obtained in VEPP-2M collider / Novosibirsk) 1995 CMD -2:  = 2001  65  82 nb Total L  300 nb-1 Peak value : 2317 nb New results will come soon 2001 SND :  = 1968  20  140 nb Total L  4400 nb-1 CMD-2 SND

4. Method for   K+ K– cross section • Method to calculate cross section used on K0 • Two types events are used : • NKK (and) - two of charged kaons has K0 decay and NKK (and) = 2geom (and) trig  2recBR(K0 )2NKK • NKK (OR) - one of charged kaons has K0 decay and N(or)NKK (OR)= 2NKKrec  trig BR(K0 ) [geom (or) (1–BR(K0 ))+geom (and)(1-rec)BR(K0 )] NKK (and) K+ K +  NKK (or) K+ +

5. Charged kaon selection 1. Kaon tag Rxy(PCA) < 10 cm |Z| < 20 cm 70 MeV/c < P < 130 MeV/c 2. Secondary vertex tag Number of tracks at vertex : 2 Same charge 40 cm < Rxy < 150 cm 180 MeV/c < P (CM of kaon) < 270 MeV/c |pK - psecondary | > 120 MeV/c | pK - psecondary | < 320 MeV/c MC Data P, MeV/c Momenta distribution in the rest frame of charged kaon

6. K 0 - selection / I • Charged kaon selection • Momenta in the rest frame of kaon and pion hypothesis of secondary particles (P) | P - 205 MeV/c | < 6 MeV/c • E > 25 MeV • E 1+ E 2 > 100 MeV • t1 = t  1 - R1(cl - vertex)/c • t2 = t  2 - R2(cl - vertex)/c • |t1 - t2 | < 3 ns MC Data MeV 0 - mass distribution Meff (MC) = 134.45 MeV Meff (Data) = 133.04

7. K 0 - selection / II MC Data MC Data E, MeV P, MeV/c Total enegry E+ E0 Missing momenta : PK - P0 - P

8. Angle distribution / I • In Tag selected events background is below few % level. • The K geometrical acceptance seems well reproduced by MC. • Coefficient is extracted from MC data directly for cross section measurement MC Data K Angle distribution for charged kaons

9. Angle distribution / II K2 K1 (K1 + K2)/2 NKK / R - function of acceptance (?)

10. Geometrical acceptance tot 1 tag 2 tag Acceptance Acceptance geom(and) • The number of kaon events NKKis a function of geometrical acceptances and NKK (and) , NKK (or) only • Sample of 50000 events, where no K - decays Acceptance geom(or) Acceptance vs min. number of hits / track R = 0.975

11. Radiative corrections / I Radiative correction for  - meson peak  e+ K+ (1020) K e • Corrections known with accuracy 0.5 % • Radiative corrections for charged kaons is negligible MeV - peak calculated by BW formula (red) and BW + rad. corrections (blue)

12. Radiative corrections / II 1+rad 1+rad MeV MeV Radiation coefficient at peak vs beam energy spread Radiation coefficient vs beam energy at beam energy spread 300 keV at peak 1+rad  70.9 %

13.   K+ K– cross section Results of   K+ K– Integrated luminosity : Lum = 6.331 pb-1 Nkk(or) = 44838 events Nkk(and) = 5065 events Background less than 1 % for double tag and few for single tagged Krad = 1/(1+ rad )  1.4 Nkk = 10117434 events Method takes us :  (  K+ K–) = NKK  Krad /Lum 1598 nb 1.4  2200 nb  (  K+ K–) = NKK/(Lum BR(K0))  3200 nb

14. Conclusion • Preliminary estimation of (  K+ K–) was performed at the peak of  - meson resonance at high statistics • Obtained result is close to world data • Method of calculation don’t depend from trigger and reconstruction efficiencies • Background to be evaluated • Next step: investigation of  - meson launch resonance shape