1. 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

2. 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?

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

4. 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

5. 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) MeV < E < 800 MeV
2) o < q1,2 < 125o
3) z = |q1 + q o| < 9o
e+ e-  g g
2 tracks with:
1) r < 7.5 cm, |z| < 15 cm
2) p > 400 MeV
3) opposite curvature

6. 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

7. 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/ , 1999
S. Jadach et al., Phys. Lett., B390, 298, 1997

8. 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 + q o| < 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

9. 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
A. Arbuzov et al., JHEP 9710:001, 1997

10. 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

11. 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%

12. 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

13. 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)

14. Dependence on sqrt(s) DL/L = -Ds/s since the cross section is
evaluated at the nominal value of s1/2 = 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:

15. List of systematics 0.5 % (theory)  0.3 % (experiment) =
BABAYAGA: seff = ( ± 0.3stat ) nb
BHAGENF: seff = ( ± 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: %

16. 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/
R. Bonciani et al., hep-ph/
M. Czakon et al., hep-ph/

17. 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