1. Measurement of the Hadronic Cross Section with KLOE
Graziano Venanzoni
INFN – LNF Frascati
Seminar at BNL, 7 July 2004

2. Outline Motivation of s(e+e- hadrons) at low energy
Status of amhad and comparison of e+e-hadrons vs t decays
The KLOE experiment at the DAFNE f-factory
Results on s(e+e-  p+p-)
Conclusions & outlook
G. Venanzoni
– 7 July 2004

3. Motivation I will concentrate on these two items
A precise measurement of low energy hadronic cross sections enters in many important tests of the Standard Model:
Anomalous magnetic moment of the muon am= (gm-2)/2
Running fine structure constant at Z0 mass aQED(MZ)
Test of CVC (hadronic t decays vs e+e-hadrons)
I will concentrate on these
two items
G. Venanzoni
– 7 July 2004

4. Hadronic contribution to am
am is one of the most precise measurement in high energy physics (5 •10-7 relative accuracy!)
Contribution to am
amexp = (  6)10-10
G.W. Bennett at. Al. 2004
EXP
amth = amQED + amEW + amhad + amNO SM (?) EW
amhad = amhad,LO+NLO + amhad,LBL
LBL
Had (LO+NLO)
QED
10-10
G. Venanzoni
– 7 July 2004

5. Contribution to the uncertainty on am
The largest contribution comes
from amhad
Contribution to dam
EXP EW
LBL
had (LO+NLO)
K(S) 1/s; it emphasizes the
role of low energies; particular
important is the reaction e+e-  p+p-
(below 1 GeV)
QED
10-10
G. Venanzoni
– 7 July 2004

6. Hadronic contributions to amhad
12 - 
(+)
(+J/, ) 4
3 (+,) 2
> 4 (+KK)
< 1.8 GeV
Calculations based on
Davier, Eidelman, Höcker, Zhang
ahad
2p contrib. ahad
8% [2mp GeV]
54% [ GeV]
10% [Rest<1.8 GeV] r
<1.8 GeV
2[ahad]
2p contrib. dahad
8% [2mp GeV]
34% [ GeV]
31% [Rest< 1.8 GeV]
> 1GeV r
<1.8 GeV 1%
e+e- data only!
G. Venanzoni
– 7 July 2004

7. Estimate of amhad 1-1.5% 0.6% 1-5% L= 317.3 nb-1
So far, estimates of amhad from:
Hadronic electron positron collider data (mostly from CMD2, SND)
Measurement of s(e+e-  p+p-) with CMD-2 :0.6% syst. error
1-1.5%
0.6%
1-5%
|Fp|2
L= nb-1
 events in  meson region
CMD2
Energy scan!
1 GeV
G. Venanzoni
– 7 July 2004

8. Estimate of amhad (II)
2) hadronic t- decays, which can be used with the help of the
CVC-theorem and an isospin rotation (plus isospin breaking corrections)
hadrons   W  e+ e–
CVC: I =1 & V
W: I =1 & V,A
: I =0,1 & V
However, amth(e+e-) – amth(t)  (15±8)10-10
G. Venanzoni
– 7 July 2004

9. am SM prediction vs experiment
New cross section data have recently
lowered theory error:
a) CMD-2 (Novosibirsk/VEPP-2M) p+ p-
channel with 0.6% precision < 1 GeV
b) t-Data from ALEPH /OPAL/CLEO
THEORY
’20/‘03
Reanalysis of e+e-
e+ e- - Data: 2.7 s - Deviation
t – Data: s - Deviation
Experiment BNL-E821
Values for m+(2002) and m-(2004)
in agreement with each other.
Precision: 0.5ppm
Experiment
’20/‘04
G. Venanzoni
– 7 July 2004

10. Comparison of e+e- vs. t for 2p channel
Deviation btw. CMD-2 and t-Data in a limited Energy Range above the r-Peak!
p p channel
t-Data are
systematically
higher  10% !
Possible Explanation Fred Jegerlehner: m(r0)  m(r) ???
 correct t-Data (but how much?)
G. Venanzoni
– 7 July 2004

11. Conclusions on am and e+e- vs t data:
Many reasons to have a new measurement of s(e+e-  p+p-)!
KLOE has already finished the analysis on this channel (using the radiative return), which I’m going to present you with today
G. Venanzoni
– 7 July 2004

12. Radiative Return e+ e-  g + Hadrons, 4mp2£ sp = ( mf2 –2mf·Eg) £ mf2
The standard method of measuring (e+e-hadrons) is an energy scan by varying the beam energy within the desired range. Since at DANE the collision energy is fixed, one uses the radiative return which exploits the process
e+ e-  g + Hadrons,
where the photon is emitted in the initial state (ISR).
[Binner, Kühn, Melnikov]
sp , the invariant mass of the hadronic system, varies continuously between:
4mp2£ sp = ( mf2 –2mf·Eg) £ mf2
In this approach the overall normalisation enters only once!
However careful evaluation of radiative corrections: PHOKHARA
Generator (J. Kühn, H. Czyż, G. Rodrigo)
G. Venanzoni
– 7 July 2004

13. DAFNE: a f-factory at INFN-National
Laboratories in Frascati (near Rome)
Main Rings
Linac
Damping
ring
Beam test
facility
KLOE hall
Synchrotron
light
G. Venanzoni
– 7 July 2004

14. DAFNE : e+ e - high luminosity collider
(Ldesign= 5x1032cm-2s-1) set at the F peak:
s = MF = 1.02 GeV
s(F)  3.3 mb
a f-factory is mostly a kaon factory
kaons are produced back to back in the CoM with |p|~110 MeV
e+e- in two separate rings with crossing angle 25mrad at IP (small F momentum pF13MeV)
DEAR
Decay
BR(%)
f  K+ K-
49.1
f  KS KL
33.8
f  r p / p+p-p0
15.6
f  h g
1.26
G. Venanzoni
– 7 July 2004

15. DAFNE performances DAFNE Parameters  L(pb-1) 450 pb-1 from 2001-2002
Design
2002
(KLOE)
N bunches
51+51
Lifetime (mins)
120 40
Bunch current(mA) 20
Lbunch (cm-2s-1)
4.4 · 1030
1.5 · 1030
Lpeak (cm-2s-1)
5.3 · 1032
0.8 · 1032
Standard analysis sample:
450 pb-1 from
Beam trajectory length ~ 98 m
Beam crossing frequency MHz
Bunch spacing : 2.7 ns
Bunch I.P.:
sx sy sz
2mm 20 mm 3 cm
Lpeak = 2 · 1032 cm-2s-1
L int / day = 10 pb-1
L int / year = 2 fb-1
(2109KSKL)
2004 goal
G. Venanzoni
– 7 July 2004

16. Physics reach vs Integrated luminosity
1999
2000
Today
2001
5 pb-1
50 pb-1
500 pb-1
5 fb-1
KS physics
BR(KS  p+p-)/BR(KS  p0p0)
BR(KS pen)
f radiative decays
f  f0g, a0g
f  hg, hg
KS semileptonic asymmetry,
KS  3p0, KL  2p, KL gg,
K  mn,pp,ppp,pen,ppen,
s(e+e-  hadrons) to 1%
Interferometry
e/e to O(10-4) via double ratio
rare KS,L decays, CPT tests
G. Venanzoni
– 7 July 2004

17. The KLOE detector Electromagnetic Calorimeter (EMC)
Fine sampling Pb (0.5 mm thick) / Scifi (1 mm ø)
Hermetical coverage
High efficiency for low energy photons
E/E = 5%/E(GeV)
t = 50ps/E(GeV)
Central drift chamber (DCH)
Large detection volume
Uniform tracking and vertexing in all volume
Helium based gas mixture
v = 1 mm pt /pt= 0.5%
r, =200 m z = 2 mm
Quadrupoles’ calorimeter (QCAL)
Pb/Sci tile calorimeter covering quads inside KLOE
G. Venanzoni
– 7 July 2004

18. The KLOE detector DC Lead Scifi ECAL Quadrupoles End-cap ECAL
SC coil B=6kG
Lead Scifi ECAL DC
Spherical Beam Pipe e- e+
Quadrupoles
End-cap
ECAL
Spherical Beam Pipe
G. Venanzoni
– 7 July 2004

19. Kaon physics at KLOE: current status of the analyses
KS  p0p0p0
Preliminary results
KS  p+p-(g)
KS  p0p0
Phys. Lett. B (2002)
Update with ’01-’02 data
KS pen
Phys. Lett. B (2002)
Preliminary update with ’01-’02 data
K0 mass
KLOE Note 181 (
KL  gg/KL  3p0
Phys. Lett. B (2003)
KL  pen
KL  pmn
KL  p+ p- p0
In progress
Vus
CP violation & interference
K+  p+p0p0
Preliminary results (hep-ex/ )
G. Venanzoni
– 7 July 2004

20. Hadronic physics at KLOE: current status of the analyses
f meson parameters
Preliminary results
f  p+p-p0
Phys. Lett. B (2003)
h  p+p-p0
h  3g
hep-ex/ , submitted to Phys. Lett.
f  hg
Phys. Lett. B (2002)
Update with ’01-’02 data in progress
f  f0g, a0g
Phys. Lett. B (2002), B (2002)
s(e+e-  p+p-g)
This talk !
G. Venanzoni
– 7 July 2004

21. sp is the invariant mass
Measurement of s(e+e-+) from
e+e-+-g events
Selection of +-g events
Measurement of ds(e+e-+-g)/dsp
Extraction of s(e+e-+) p+ e+
sp is the invariant mass
squared of the 2 pion system
~Fp(s) e- p-
G. Venanzoni
– 7 July 2004

22. Selection of e+e-p+p-g: 1) acceptance cuts
Side view e+ e-
Front view p+ p- g
Pion tracks at large angles
50o < qp < 130o
NO PHOTON TAGGING
Photons at small angles
qg < 15o and qg > 165o
are shadowed by
quadrupoles near the I.P.
• High statistics for ISR events
Reduced background contamination
Low relative contribution of FSR
G. Venanzoni
– 7 July 2004

23. Selection of e+e-p+p-g: 2) identification of pion tracks (PID method)
To reduce Bhabha contamination, a -e separation is performed using a particle ID estimator based on:
mTRK
  
e e 
before cutting on
particle ID estimator
after cutting on
m+ m- g
p+ p-p0 mp
TOF of charged clusters in EMC
Shape and energy deposition of the cluster
The event is selected if at least one of the charged tracks is identified to be a pion.
mTRK is the mass of the charged particle, under the hypothesis that the final state consists of two particles with the same mass and one photon.
G. Venanzoni
– 7 July 2004

24. Selection of e+e-p+p-g: 3) rejection of
m+m-g, p+p-p0 events
rp preselection
already applied
mTRK (MeV)
p+ p- p0
p+p-p0
Kinematical selection in the plane (sp,mTRK)
p+p-gg tail
p+p-g events
Mtrk~mp correponds to ppg events
Mtrk>mp corresponds to the multi-photon emission (ppgg): most of
these events are retained by this cut
p+ p- g
m+m-g
m+ m- g
sp (GeV2)
G. Venanzoni
– 7 July 2004

25. +-g observed spectrum
Ni/0.01GeV2
141.4 pb-1 of 2001 data were selected by the mentioned cuts
After selection: events
(11000 evts/pb-1)
statistical error/bin < 1%
Our cuts select events with sp >550 MeV
We will measure the cross section near
threshold looking at the photon at large angle
(analysis is in progress, see later)
sp(GeV2)
G. Venanzoni
– 7 July 2004
Acceptance: qpp<15o (qpp >165o), 50o<qp<130o, ES>10 MeV

26. Measurement of ds(e+e-+g)/dsp
An absolute measurement!
Residual Background
Signal
Acceptance
Luminosity
Selection efficiency
Unfolding of the experimental resolution is
omitted
G. Venanzoni
– 7 July 2004

27. Estimate of residual background
N. of events / 0.5 MeV
data

mmg
Residual contamination from +-0, , and Bhabha‘s is evaluated by fitting the shape of signal and background in the trackmass distribution for different slices of sp
The estimated number of background events is then subtracted from the spectrum.
Mpp2  [0.32,0.37] GeV c2 = 87.0/88
data
sum of  and mmg
G. Venanzoni
– 7 July 2004

28. Estimates of the residual background (II)
N. of events / MeV
data

p+p-p0
Observed
spectrum
Total background
estimate
Mpp2  [0.38,0.42] GeV c2 = 104.4/88
data
sum of  and p+p-p0
The systematic error on the
background subtraction
is <0.3% above 0.55 GeV2
G. Venanzoni
– 7 July 2004
Federico Nguyen

29. Evaluation of efficiencies
Trigger
including Cosmic-ray Veto Eff.
Total Efficiency
Reconstr. Filter
Tracking / Vertex
60%
-e separation
Trackmass
blue = estimated from data and/or
indep. control samples p+p-p0, p+p-
Kinematics simulated by MC
red = estimated from MC and
compared with data
sp(GeV2)
G. Venanzoni
– 7 July 2004

30. Unfolding MC • data The detector matrix “almost” diagonal
80% in the
central bins
sp true
sp obs
The detector matrix “almost” diagonal
We unfold the spectrum using
GURU (A. Höcker et.al./ALEPH), a package
based on SVD decomposition
Issues: - Reliability of MC simulation 
- Correct choice of the regularization
parameter MC
• data
Systematics:
Bin-by-Bin changes for different values of the
regularization parameter large (up to 3%)
 uncorrelated systematic error!
Due to nature of the dispersion integral the effect on am is almost negligible
G. Venanzoni
– 7 July 2004

31. Luminosity measurement
* Data
Babayaga (MC)
(Pavia theory group)
Bhagenf (MC)
(Berends adapted
by E. Drago and
G. Venanzoni)
track momentum (MeV)
Large angle Bhabha:
55° < q < 125°
acollinearity < 9°
p  400 MeV
BABAYAGA: sbha = ( ± 0.3stat ) nb
BHAGENF: sbha = ( ± 0.3stat ) nb e- e+ g
of s
Theoretical Systematic Error = 0.5%
Total Error = 0.6%
G. Venanzoni
– 7 July 2004
Federico Nguyen

32. The cross section ds(e+e-+g)/dsp
Efficiencies:
- Trigger & Cosmic veto
Tracking, Vertex
p/e- separation
Reconstruction filter
Trackmass-cut
Unfolding resolution
Acceptance
ds/dMpp2 (nb/GeV2)
sp (GeV2)
r-w Interference
Errors:
0.9%
Background:
e+ e- e+ e- g
e+ e-  m+ m- g
f  p+ p- po
0.3%
Luminosity:
Bhabhas at large angles
seff = 430 nb,
0.3%exp
0.5%theo
G. Venanzoni
– 7 July 2004

33. |Fp(s)|2 2) Extraction of s(e+e- p+ p- ) Neglecting FSR emission:
sp is invariant mass
of the 2p
Neglecting FSR emission: e+ e- p+ p-
~Fp(s) ~1
H(s)= =
|Fp(s)|2
dsppg(sp)=
s=sp
s is invariant mass
of the intermediate
photon s
H(s)
From which we obtain
G. Venanzoni
– 7 July 2004

34. Fp(s) s s= mf2 mf2 Which Kind of FSR?
The cross section e+e- p+p- has to be inclusive with respect to NLO FSR events:
G. Venanzoni
– 7 July 2004

35. LO FSR Corrections p+ e+ ~Fp(mf2) e- s=mf2 p-
LO FSR is a background since the Pion form factor
is evaluated at the same fixed scale s=mf 2 e+ e- p+ p-
~Fp(mf2)
s=mf2
Must to be subtracted as much as possible!
sp(GeV2)
LO - FSR %
This contribution becomes < 1% due to our acceptance cuts
G. Venanzoni
– 7 July 2004

36. NLO FSR Correction: simultaneous emission of ISR and FSR
10%
FSR (nlo) /ISR 8% 6% 4%
Before trackmass cut 2%
This contribution is as large as 5% in low M2pp bins
FSR contribution + cut efficiency are evaluated by MC (PHOKHARA Generator) 0%
-2%
After trackmass cut
-4%
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
M2pp
G. Venanzoni
– 7 July 2004

37. Extraction of s(e+e- p+ p- ) in presence of simultaneous emission of ISR and FSR
In presence of NLO FSR, our procedure must be modified since
ssp p+ e+ sp
dsppg(s)=
~Fp(s) e-
ssp p-
|Fp(s)|2 = p+ e+ sp
H(s)= ~1 e-
s=sp p-
H(s)
Acceptance corrections
G. Venanzoni
– 7 July 2004

38. which is inclusive in FSR (hard, soft & virtual)
In this way we get:
which is inclusive in FSR (hard, soft & virtual) p+ e+ e- s p-
Remember that for am calculation we need
spp inclusive w.r.t. FSR and corrected for VP (bare cross section)
G. Venanzoni
– 7 July 2004

39. FSR Correction:How to cross-check the above procedure?
We have checked our procedure with a different method:
N(e+e-  p+ p- gISR gFSR)
Subtract FSR (nlo) contribution to the observed ppg spectrum
subtract FSR contribution
Apply the analysis corrections, as in the case of pure ISR
s(e+e-  p+ p- gISR)
s(e+e-  p+ p )
Schwinger term
0.8% for s<1GeV2
gFSR
% Schwinger ‘90
Divide for radiator H
Add (pointlike) FSR correction
(1+dFSR)
How much do they agree?
G. Venanzoni
– 7 July 2004

40. Excellent agreement (D = 0.002  0.001)
FSR corrections: comparison of the two methods 2
Excellent agreement (D =  0.001)
1.05
Pion Formfactor (KLOE)
Rel. difference inclusive -
1.04
exclusive approach
1.03
1.02
1.01 1. 1
0.99
0.98
0.97
D = 0.2% ± 0.1%
0.96
0.95
0.4
0.5
0.6
0.7
0.8
0.9
FSR systematics=0.3%, two contributions:
0.2% = difference incl.-excl. correction
we put an upper limit of 20% for the
model of scalar QED: 20% * 1% = 0.2%
G. Venanzoni
– 7 July 2004

41. Charge asymmetry: test of FSR model
At large Photon angles the amount of FSR is large!
test the model of scalar QED (i.e. poinlike pions), which usually is
taken for the radiation of photons from pions in MC generators
 Measure Charge Asymmetry
and compare data with MC
 Charge asymmetry is due to
different C-Parity of ISR- and
FSR-amplitudes
 preliminary results in a
complementary ppg – analysis
at large photon angles show a
good agreement of data with
the model of scalar QED 50 70 90
110
130
-20
-10 20 10
Asymmetry [%]
Polar Angle [°]
Data MC
K L O E
P R E L I M I N A R Y
G. Venanzoni
– 7 July 2004

42. CMD-2 error: exp.+syst.+stat.
Cross Section s(e+e-  p+p-)
TOTAL ERROR %
To be compared the 0.9%
CMD-2 error: exp.+syst.+stat.
G. Venanzoni
– 7 July 2004

43. 2p contribution to amhadr
At large values of sp (>mr2) we are consistent with CMD and therefore
we confirm the deviation from t-data! .
Pion Formfactor
CMD-2
KLOE
0.4
0.5
0.6
0.7
0.8
0.9
sp [GeV2] 45 40 35 30 25 20 15 10 5
2p contribution to amhadr
We have evaluated the Dispersions Integral for the 2-Pion-Channel
in the Energy Range 0.35 <sp<0.95 GeV2
ampp = (388.7  0.8stat  3.5syst  3.5theo) 10-10
Comparison with CMD-2 in the Energy Range 0.37 <sp<0.93 GeV2
(375.6  0.8stat  4.9syst+theo) 10-10
(378.6  2.7stat  2.3syst+theo) 10-10
KLOE
CMD2
1.3% Error
0.9% Error
G. Venanzoni
– 7 July 2004

44. Preliminary Conclusion Including KLOE result 1
KLOE has confirmed a 3s deviation btw. theory and experiment for (g-2)m!
Including
KLOE result
Preliminary
G. Venanzoni
– 7 July 2004

45. Conclusions s(e+e- p+p-)
KLOE has measured spp with 1.3% precision.
The analysis is finished and we are going to publish it.
KLOE confirms a 3s - deviation between the experimental
and the theoretical value for the am, as well as
10% discrepancy with t-data above the r-peak
We expect to reduce the systematic error well below 1%,
by repeating the analysis with 2002 data. Improvements
from theory are also expected in the next future (now our
theoretical error on luminosity is 0.5%)
Analysis with the photon at large angle (to study the region
near the threshold) is in progress. See next slide
G. Venanzoni
– 7 July 2004

46. Measurement of s(e+e- p+p-) down to the threshold
Measure s(pp) in the region close to threshold, Mpp < 600 MeV,
responsible for 20% of ampp
This region excluded by angular selection in small angle photon approach
Complementary Analysis at large photon angles
Mpp (MeV)
400
300
600
500
800
700
900
1000
2000
3000
4000
N(ppg) / MeV
huge rp-Background
large FSR-component
Photon-Tagging possible
investigate the possibility
of an R-measurement, i.e.
normalization to muons
2002 data
ISR+FSR MC
G. Venanzoni
– 7 July 2004