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Measurement of the neutral kaon mass Using Ks-> p + p - events*

Measurement of the neutral kaon mass Using Ks-> p + p - events*. M. Antonelli M. Dreucci 2 nd KLOE Physics workshop (Otranto). Introduction center-of-mass energy determination kaon mass measurement systematics conclusions. *approval for publication. Introduction.

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Measurement of the neutral kaon mass Using Ks-> p + p - events*

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  1. Measurement of the neutral kaon mass Using Ks-> p+p- events* M. Antonelli M. Dreucci 2nd KLOE Physics workshop (Otranto) • Introduction • center-of-mass energy determination • kaon mass measurement • systematics • conclusions *approval for publication

  2. Introduction Fractional accuracy on MK ~ 6 x 10-5 (PDG) not easy to achieve with invariant mass reconstruction But… if the center-of-mass energy W ~ EKL+EKS ~ 2EK is precisely determined MK2=W2/4- PK2 momentum calibration less important dMK/MK ~ b2dPK/PK ~ DM dPK/PK ~ 0.05 dPK/PK DM = W -2 MK ~ Mf - 2 MK ~ 26 MeV

  3. Introduction 2 • Center-of-mass energy: • bhabha events (e+e- invariant mass) • calibration with f mass from • line shape fit to e+e- -> f -> KLKS • 2001 energy scan • ISR effect: • KLKS invariant mass WKK(MK)  W • WKK f(W) =W • f(W) from theory (QED)

  4. Determination of W e+ e- invariant mass distribution • e+ e- invariant mass • with 60o < qmin< 90o • Fit with numerical • function • from BHABHAYAGA • Typical accuracy • ~ 2 – 50 keV • 3 keV @ 50 nb-1 q cut stability studied

  5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calibration of W: line-shape fit • 2001 energy scan: • about 500 nb-1 11 energy points (1014 – 1025) MeV • KLKS cross section from KS -> p+p- • Luminosity from VLAB

  6. Theoretical cross section r term f term w term { fixed Phase space: Propagator: Gv(s) energy dependent width

  7. Theoretical cross section: r w terms s/s(no V) V= r,w Up to 10% contributions W(MeV)

  8. Initial state radiation correction Standard method: Radiator • Radiator from Nicrosini et al. 2ndDAFNE handbook • numerical integration with VEGAS • Fadeav radiator used for comparison (no differences) • other integration codes used REMT and Capon code

  9. Initial state radiation correction • up to 30% • correction

  10. Line shape fit: results Mf= 1019.3290.011 MeV/c2 W -> W Mf(CMD-2)/ Mf Mf(CMD-2)= 1019.504 0.011  0.033 MeV/c2

  11. Measurement of WKK KLKS invariant mass WKK(MK) from KS-> p+p- events WKK =( 2MK2 + 2EKSEKL –2PKLPKS)1/2 PKS from pion momenta; EKS=(PKS2+MK2)1/2 PKL= Pf-PKS ; EKS=(PKS2+MK2)1/2 Run by run average Pf from bhabha * * About 122/10002 correction

  12. Measurement of WKK MK(MeV) Calibration constant about 2 W(MeV)

  13. Initial state radiation effect Final state KL g Undetected radiated photon (mostly collinear to beams) KS Center-of-mass energy W reduced to WKK by fK(W) ( W dependence trough s(w)) + additional “beam energy spread” • Evaluated with: • s(s) + H(s,s’) + beam energy spread convolution • full MC simulation: all above + radiation from both • beams + detector effects

  14. Initial state radiation Correction function fK(W)=W/WKK Very small below f peak ~40 KeV (~20 for MK) • full simulation • simple convolution Larger above f (radiative return events) ~ 200 KeV +/- half correction

  15. Initial state radiation MC – data comparison at 1025 MeV radiative return to f events

  16. MK single event resolution Solve: WKK(MK)fK(W)=W Event by event MC prediction: ~370 KeV PK resolution ~220 KeV beam energy spread ~100 KeV ISR spread

  17. MK run by run results vs center-of-mass energy Stable vs W but just above Mf (rad. returns) Average for W< 1025 MeV (smaller ISR corrections) MK=497.5830.005 normalized RMS=1.3

  18. Systematic uncertainties Momentum calibration: evaluated by forcing a dp/p in the analysis dMK/MK = 0.06dp/p (very close to the expected) p calibration at about 2x10-4 dMK ~ 6 keV ISR correction: full a2 correction included higher order terms < a2 constant (W) terms dMK ~ 7 keV Checks with other radiators + MK(above f) consistent with MK(belowf) + correction is small (below f) - 20 keV

  19. Systematic uncertainties W calibration error: arises from dMf= 0.011 MeV/c2 dMf(CMD-2)=0.011  0.033 MeV/c2 dMK ~ 18 keV Error treated as uncorrelateddMK(tot)= 20 keV MK=497.583 0.005 0.020

  20. Comparison with other measurements

  21. Conclusions K0 mass measurement with competitive error Statistical error not a problem: many events + amazing resolution ~400 keV Systematic error dominated by W calibration ~20 keV 3 measurements with about 30 keV errors in ~100 keV MK?

  22. f mass in PDG Dominated by a masurements by a fixed target experiment (invariant mass reconstruction)

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