Measurement of the DAFNE Luminosity with the KLOE Experiment EURIDICE Midterm Collaboration Meeting Measurement of the DAFNE Luminosity with the KLOE Experiment Federico Nguyen Università Roma TRE – INFN Roma III February 10th 2005

Outline Motivations: the error on spp Selection criteria of large angle Bhabha events Comparisons among MC codes Main systematics of the experimental analysis Conclusions: how can we improve?

DAFNE and KLOE Superconducting Coil B=0.52 T DAFNE, e+ e- collider at s ~ 1.019 GeV ~ MF Electromagnetic Calorimeter Pb / Scintillating Fibres Endcap + Barrel = 98% (4p) Drift Chamber 4 m , 3.3 m length 90% He, 10% i-C4H10

Resolutions in KLOE srf = 150 mm , sz = 2 mm svtx ~ 3 mm EMC features: e+ e-  e+ e- g st = 56ps /E(GeV)  133ps EMC features: Drift Chamber features: sE/E = 5.4%/E(GeV) sp/p = 0.4% (q > 45°) srf = 150 mm , sz = 2 mm svtx ~ 3 mm

Selection of large angle Bhabha a = angle btw the 2 most energetic clusters nr. of events the luminosity is given by Bhabha events divided for a s evaluated folding theory (QED rad. corrs.) with the detector simulation e+ e-  e+ e- 2 clusters with: 1) 300 MeV < E < 800 MeV 2) 55o < q1,2 < 125o 3) z = |q1 + q2 - 180o| < 9o e+ e-  g g 2 tracks with: 1) r < 7.5 cm, |z| < 15 cm 2) p > 400 MeV 3) opposite curvature

C. M. Carloni Calame et al., Nucl. Phys., B584, 459, 2000 The MC code BABAYAGA C. M. Carloni Calame et al., Nucl. Phys., B584, 459, 2000 BABAYAGA: 1) each e± can emit up to 4 photons 2) multiphoton emission parameterized by structure functions D(x,Q2), folding the Born cross section s0 3) evolution equations are solved by the numerical method Parton Shower 4) the accuracy is evaluated comparing with the exact O(a) evaluation, the authors quote 0.5% within our cuts 5) Pavia group has worked at an improved version

B contains the infrared divergence due to virtual corrections The MC code BHWIDE BHWIDE: 1) up to 100 g’s per event 2) complete O(a) corrections 3) exact phase space integration 4) collinear and infrared logarithms accounted for to all orders 5) claimed precision is < 1% according to the LEP2 requirements B contains the infrared divergence due to virtual corrections S (k,mg) contains the infrared divergence due to bremsstrahlung corrections W. Placzek et al., hep-ph/9903381, 1999 S. Jadach et al., Phys. Lett., B390, 298, 1997

v =1-Mee2/s, no significant difference has been found in the shapes BHWIDE vs. BABAYAGA we compared the 2 MC codes applying our kinematic cuts E > 400 MeV 55o < q < 125o E > 400 MeV 55o < q < 125o z = |q1 + q2 - 180o| < 9o we tested the differential cross sections in acollinearity, z, and lost energy, v =1-Mee2/s, no significant difference has been found in the shapes

Comparisons of cross sections comparing the MC codes in treating virtual (f exchange and vacuum polarization are “switched off”), soft and hard g corrections, the cross sections for our kinematic cuts are: BHAGENF (460.8  0.1stat) nb BABAYAGA (459.4  0.1stat) nb BHWIDE (456.2  0.1stat) nb MCGPJ (455.3  0.1stat) nb 0.7 % 0.1 % 0.3 % E. Drago and G. Venanzoni, Report INFN-AE-97-48, 1997 F. Berends and R. Kleiss, Nucl. Phys., B228, 537, 1983 Monte Carlo Generator with Photon Jets: 1) use of structure functions 2) complete O(a) corrections 3) authors quote O(0.2%) accuracy BHAGENF features: 1) complete O(a) corrections 2) corrections due to e+ e- → e+ e- g 3) authors quote O(0.5%) accuracy A. Arbuzov et al., Report Budker INP-2004-70, 2004 A. Arbuzov et al., JHEP 9710:001, 1997

Momentum and z comparisons we checked that the MC code folded with the detector simulation reproduces the kinematic variables, especially at the borders p > 400 MeV - MC data dn/dp [(0.5 MeV)-1] z < 9 o + Data Babayaga (MC) Bhagenf (MC) perfect agreement in the acollinearity distributions very good agreement near p ~ 400 MeV

Polar angle systematics global agreement is very good but the cut occurs in a steep region of the distributions  estimate of border mismatches after normalizing MC to make it coincide with data in the region 65o < q < 115o, we estimate as a systematic error: - MC data ~ 0.2%

Background estimate (I) mtrk: 4-mom. conserv. under the hypothesis of 2 equal mass tracks and a g A0 A1 events/3 MeV DATA MC other than Bhabha, there are events with mtrk ~ 137 MeV, e+e-  p+ p- around mtrk ~ [100,170] MeV the exponential is subtracted from data

Background estimate (II) a second method consists in discr. e/p, at least one track identified as p an average of background content of 0.55% and a systematic error of 0.10% are estimated p+p-g pions deposit ~ 40 MeV in each plane E (last plane) (MeV) E (first plane) (MeV) electrons deposit mostly in the first plane and negligible amount in the rest E (last plane) (MeV) e+e-g E (first plane) (MeV)

Dependence on sqrt(s) DL/L = -Ds/s since the cross section is evaluated at the nominal value of s1/2 = 1019.5 MeV, we corrected for DAFNE variations of s1/2 in time DL/L = -Ds/s is parameterized as a function of s1/2, from Monte Carlo:

List of systematics 0.5 % (theory)  0.3 % (experiment) = BABAYAGA: seff = ( 431.0 ± 0.3stat ) nb BHAGENF: seff = ( 430.7 ± 0.3stat ) nb both groups of authors claim 0.5% 1+d(due to s1/2) putting all together: 0.5 % (theory)  0.3 % (experiment) = 0.6 % (total error) summing in quadrature: 0.3 %

Conclusions and perspectives the measurement of the luminosity is performed using large angle Bhabha events, detected in KLOE in 2001 the major source of uncertainty is from theory we are going to use the improved version of BABAYAGA, and also BHWIDE and MCGPJ to pin down the 0.5% factor what about implementing recent 2 loop evaluations? from the experimental side we are going to analyse the 2002 data, and we are going to look at the process e+e- → gg, as an independent luminometer reaction, which is the precision of radiative corrections in this channel? A. A. Penin, hep-ph/0501120 R. Bonciani et al., hep-ph/0411321 M. Czakon et al., hep-ph/0412164

a fraction of 0.41% large angle Bhabha events are lost because of this Loss of cosmic events genuine cosmic event Bhabha event rejected because taken as cosmic at the trigger level, events are rejected because they have characteristics similar to cosmic rays (amount of energy released at the outer planes of the EMC) a fraction of 0.41% large angle Bhabha events are lost because of this