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MEASUREMENT of e+e- HADRONIC CROSS SECTION with RADIATIVE RETURN at KLOE
Barva Maura - Università Roma Tre On behalf of the KLOE Collaboration LNF Spring School - May 2004
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Summary Motivation of shadr shadr with radiative return
Results on s(e+e-p+p-) at small angle Conclusions and outlook L.N.F. Spring School - May 2004
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Why the measurement of the cross section is important?
THE ANOMALOUS MAGNETIC MOMENT OF THE MUON Magnetic moment for pointlike fermion Dirac (Dirac prediction) because there are quantum correction m+ q g* Hadronic vacuum polarization amSM = amQED + amhad +amEW L.N.F. Spring School - May 2004
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Theorical vs Experimental value A NEW MEASUREMENT IS VERY IMPORTANT!
Status of am: Theorical vs Experimental value Last result from BNL: 01/04 E821 t-data from ALEPH, OPAL, CLEO e+e- dominated by CMD-2 |am(e+ e-) –am( data)|: 2.7 s -Deviation |am(t) –am( data)|: 1.4 s - Deviation A NEW MEASUREMENT IS VERY IMPORTANT! L.N.F. Spring School May 2004
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Hadronic contribution to am
The error on am(theo) is dominated by the error on amhad This quantity is not evaluable in pQCD, but it can be calculated by DISPERSION INTEGRAL: The factors 1/s inthe dispersion relation makes the low energy region particularly relevant. The e+e-p+p- channel accounts for ~70% of the contribution both to amhad Input: a) Hadronic electron-positron cross section data b) Hadronic t- decays, which can be used with the help of the CVC-theorem and an isospin rotation (plus isospin breaking corrections) L.N.F. Spring School - May 2004
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How to perform this measurement?
traditional way : measuring cross section by varying e beams energy energy scan BUT DAFNE is a f - factory and therefore runs at fixed c.m.s.-energy: s = mf = MeV r s’ Complementary approach: Take events with Initial State Radiation (ISR) “Radiative Return” to r-resonance: e+ e- r + g p+ p- + g L.N.F. Spring School - May 2004
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s(had) through radiative return
The s(e+e- p+p-g) is a function of the 2-Pion invariant mass s’=Mpp.To extract s(e+e- p+p-) with the ISR we Need to know the RADIATOR FUNCTION H(s) H(s) is evaluable From MC Phokhara Have to get rid of “pure” FSR: which causes events with Mg* = s to be assigned to lower s´ values Have to properly include radiative corrections (including simultaneously emission of ISR and FSR) L.N.F. Spring School - May 2004
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DAFNE: the Double Annular F-factory
for Nice Experiments Ebeams MeV 2 separate rings for e+e- to minimize beam-beam interaction two interaction region: KLOE-DEAR/FINUDA crossing angle: 12.5 mrad (px(f) MeV) Lpeak (cm-2s-1) 4.4 1030 Lbunch (cm-2s-1) Bunch current (mA) Lifetime (mins) 5.3 1032 40 120 N of bunches Design DAFNE Parameters DAFNE performance: 2000 run: 25pb-1, 7.5 x107 f decay first published results 2001 run: 190pb-1, 5.7 x108 f decay 2002 run: 300pb-1, 9.0 x108 f decay Anlysis in progress L.N.F. Spring School - May 2004
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The KLOE detector:K LOng Experiment
Electromagnetic calorimeter lead/scintillating fibers (1 mm ), 15 X0 4880 PMT’s 98% solid angle coverage Drift chamber (4 m 3.75 m, CF frame) Gas mixture: 90% He + 10% C4H10 stereo–stereo sense wires almost squared cells Beam pipe : (spherical, 10 cm , 0.5 mm thick) Instrumented permanent magnet quadrupoles (32 PMT’s) L.N.F. Spring School - May 2004
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Measurement of sppg – Analysis scheme
Pion tracks at large angles from a vertex close to IP Photons at small angles to enhance ISR: dsISR/dW 1/sin2q relative contribution of “pure” FSR below the % level over entire Mpp spectrum reduce background Lose events with Mpp< 600 MeV LARGE ANGLE ANALYSIS (see after…) L.N.F. Spring School - May 2004
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Summary of syst. errors and background
Acceptance 0.3% Trigger 0.3% FILFO 0.6% Tracking 0.3% Vertex 0.3% Likelihood 0.1% Track mass % Backg. Subsraction % Unfolding % Total exp % Luminosity 0.6% Vacuum Polarization % FSR % Radiator function % Total theory 0.9% 1.Rad. Bhabhas: Pion-Electron-Separation by means of a Particle-ID algorithm 2. f p+ p- p0 e+e- m+ m- g: Kinematic Separation: “Trackmass“ p+p-p0 m+m-g + e+e-g signal region MTRK (MeV) mp p+p-gg tail mr2 mm L.N.F. Spring School - May 2004
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Analysis: s(e+e- p+p-g)
Statistics: 141pb-1 of 2001-Data 1.5 Million Events r-w interference High Statistics! High Resolution! Mpp2 (GeV2) H(Mpp2) obtained from PHOKHARA L.N.F. Spring School - May 2004
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Analysis: Pion Form Factor and amhad
|Fp|2 CMD2 - KLOE 0.5 0.7 0.9 20 40 10 30 We have evaluated the dispersions integrals for the 2-Pion-Channel in the energy range 0.35 <M2pp<0.95 GeV2 ampp = (389.2 0.8stat 3.9syst 3.5theo) 10-10 Comparison with CMD-2 in energy range 0.37 <M2pp<0.93 GeV2 (376.5 0.8stat 5.1syst+theo) 10-10 (378.6 2.7stat 2.3syst+theo) 10-10 KLOE CMD2 The results are in good agreement! L.N.F. Spring School - May 2004
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(1) amhad prospect: Large Angle analysis
The energy region close to threshold, Mpp < 600 MeV, is excluded by our selection cuts This region contributes ~20% to amhad We use events at large photon angles to access lower M2 region. (Photon tagging will be possible in this case) Check FSR parametrization (by looking to charge asymmetry) This analysis is going on... MeV Mpp (MeV) 400 300 600 500 800 700 900 N(ppg) / MeV 10 102 103 L.N.F. Spring School - May 2004
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(2) amhad prospect: the direct measurement of
We are also studying a direct measurement of R This method is independent from the luminosity estimate We are working on obtaining particle ID combining calorimetric and kinematically variables 2002 DATA!!! ppg events mmg events Likelihood Mass Trk L.N.F. Spring School - May 2004
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Conclusion: Summary: Ongoing activities
We have presented the first precise measurement of hadronic cross section using radiative return at KLOE Our result is in agreement with CMD2, therefore confirming the existing discrepancy bewteen SM and BNL result on (g-2)m Ongoing activities Large angle photon analysis: to study the low Mpp2 region and measure the asymmetry to check the FSR results The direct measurement of R= (e+e- +-g)/(e+e- +-g) L.N.F. Spring School - May 2004
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