1. Study of e+e collisions with a hard initial state photon at BaBar
E. Solodov (BINP, Novosibirsk)
for the BaBar collaboration

2. Initial state radiation (ISR) method
High PEP-II luminosity at s = GeV precise measurement of the e+e- cross section 0 at low c.m. energies with BaBar.
Improved hadron spectroscopy
Input to (gm-2) and aem calculations.
Few previous data in the GeV range.
Comprehensive program at BaBar.
Today: preliminary results for +0, 2+2, K+K+, 2K+2K from 89.3 fb-1

3. ee  0 Event selection: Kinematic fit:   J/
Excited  states ?
Event selection:
Isolated ISR photon with ECM > 3 GeV
At least 2 good photons with E > 0.1 GeV
Two good, non-K tracks from IP
Events/0.01 GeV/c2
Kinematic fit: 
Energy and momentum balance enforced
Mass of two soft photons constrained to 0
2 < 40 for fit in  0g hypothesis selects signal events
Reject events with extra photons if 2 < 40 for   0 0 g hypothesis
J/

4. Background for 3pg Most dangerous backgrounds:
e+ e-  K+ K- 0 , e+ e-  qq  + - 0 0
Other backgrounds:
e+e-  2, 4, 5, …, +-, 0
Two methods of background subtraction:
background mass distribution measured in data, subtracted bin-by-bin from signal mass distribution
taken from simulation, corrected to real experimental distribution MC
Data
Total background level:
( )% in ,  regions
( )% at higher masses
accuracy in background level
~25% up to 2 GeV

5. Detection efficiency for 3
The detection efficiency (m):
determined from a Monte Carlo simulation that includes additional corrections extracted from special control event-samples
quite uniform in 0.5<m<3 GeV/c2
systematic error currently ~4% - will be improved with more data

6. Fit of the 0 mass spectrum
3(m) - Born cross section of e+e-  + - 0
is a coherent sum of 4 resonances: , , ,
R(m)~1 - radiative correction function from calculation
dL/dm - ISR luminosity taken from total integrated luminosity and photon radiator function W(s,x); (checked with mmg events)
f(m,m) - detector resolution taken from simulation
(with floating extra Gaussian smearing)
Fix ,  widths to PDG values
Fix - relative phase to experimental value (1637)
Fix - relative phase to 180,  -  to 0

7. Fit results:  -  region
2/d.f.=146/148
consistent with known properties
of these resonances
(,  widths fixed to PDG values) f
The resolution is about 6, 7, 9 MeV/c2 at , , J/ masses.
Fitted resolution smearing is
~1 MeV/c2
BaBar Preliminary PDG
B(ee)B(3)=(6.700.06 0.27) (6.350.11)10-5
B(fee)B(f3) =(4.300.08 0.21) (4.590.14)10-5

8. Fit results: higher mass region

Good fit obtained for the range up to 1.8 GeV/c2.
Extending the fit to masses above GeV/c2 may require a more complicated fitting function taking into account non-resonant 3 production.
Mass and width parameters are dependent upon our assumed phases - interference effect is strong
BaBar Preliminary PDG
B(ee)B(3)=(0.820.050.06) M()= 13502020 MeV/c
()= 4507070 MeV
B(ee)B(3)=(1.30.10.1) M()= 1660102 MeV/c  30
()= 2303020 MeV  35

9. e+ e-  +  0 cross section
DM2
BaBar Preliminary
SND
coverage of wide region in this experiment -
no point-to-point normalization problems
consistent with SND data E C.M. < 1.4 GeV
inconsistent with DM2 results
overall normalization error ~5% up to 2.5 GeV

10. J/  3 decay The J/ meson is narrow - clean signal
After sideband subtraction -
NJ/ = 92034
Detection efficiency - =(9.20.6)%
The result: (J/ ee)B(J/ 3)=
0.1220.0050.08 keV
We previously measured (mmg)
(J/ ee)=5.610.20 keV
Events/0.004 GeV/c2
sideband
sideband
Phys. Rev. D69, (2004)
B(J/ 3) = (2.180.19)% BaBar Preliminary
(1.500.20)% PDG
(2.100.12)% BES 2003

11. e e  22 , KK , 2K2K
Event selection:
Isolated ISR photon with ECM > 3 GeV
At least four good tracks from IP
Kinematic fit:
Energy and momentum balance enforced
Energy and angles of hard ISR photon are not used - 1C fit
Fit in 3 hypotheses:
4 for all events
2K2 if 1 or 2 identified kaons
4K if 2, 3 or 4 identified kaons
Background subtraction:
Other ISR processes (5, …) – using difference in 2 distributions
e+e-  qq – using JETSET simulation

12. ee  2 2 cross section
BaBar
Preliminary
Systematic errors:
12% for m4 < 1 GeV,
5% for 1 < m4 < 3 GeV,
16% for higher masses)
best measurement above 1.4 GeV
Coverage of wide region in one experiment
No point-to-point normalization problems
Intermediate states:
a1(1260) - dominant, structure which may be f0(1370) final state is seen.
For detailed study, a simultaneous analysis of 2+2- and +-20 final states is required.

13. e e  2 2 cross section
Good agreement with direct e e measurements
Most precise result above 1.4 GeV

14.  substructures
BaBar preliminary
MC generator:
H.Czyz and J.H.Kuehn,
Eur.Phys.J C18(2000)
Includes a1(1260)p and f0(1370)r
Does not include J/
a1(1260)
J/
f0(1370)
r(770)

15. e e  K K   f, KK contribute strongly
Systematic error – 15% (model dependence, kaon identification)
Much more precise than previous measurement
J/
Substantial resonance sub-structures observed:
K*(890)K dominant
f, KK contribute strongly
K*2(1430)K seen.

16. KK substructures
BaBar preliminary
K*(890)Kp dominated
No studies in previous
e+e- experiments!
K*Kpg MC generators
are not available yet
K* regions excluded r f
No signal from ff0(980) yet
Connection to f f0(980)g ?
ff0(980) ?
Events from f band

17. e e  2K 2K BaBar preliminary First measurement
Overall normalization systematic error – 25% (model dependence, kaon identification)
J/
No clear mass structure in the two- or three-body subsystems
No ’ s !

18. J/ and (2S) decays BaBar preliminary PDG
2+2-
K+K-+-
2K+2K-
+-
BaBar preliminary PDG
B(J/ +-+-)=(3.610.26 0.26) (4.01.0) 10-3
B(J/ K+K-+-) =(6.090.50 0.53) (7.22.3) 10-3
B(J/ K+K-K+K-) = (6.71.1 1.0) (9.23.3) 10-4
B((2S)  J/ +-)=0.3610.015  0.011

19. Summary from all  data 12.35  0.96exp  0.40SU(2)
Using ISR method the cross sections of e+e  +0, 2+2, K+K+, 2K+2K reactions have been measured from threshold to 4.5 GeV.
These are the most precise measurements to date for c.m. energies greater than 1.4 GeV.
Example: contribution to ahad (1010) from 2+ 2 (0.56 – 1.8 GeV)
from all e+ e exp  0.87exp  0.23rad
from all  data  0.96exp  0.40SU(2)
from BaBar  0.64exp  0.13rad
Several B(J/ -> X) measurements better than current world average
Detailed papers to be submitted to PRD
More modes to come; aim for systematic errors 1% (in +)
Davier-Eidelman-
Hoecker-Zhang 2003
696.37.2