1. Matteo Negrini Frascati, Jan 19, 2006
Measurement of e+e- to multihadron cross sections using Initial State Radiation in Babar
Matteo Negrini
Frascati, Jan 19, 2006

2. Outline The Babar experiment Initial state radiation (ISR) at Babar
Multihadron cross sections
e+e-  p+p-p0
e+e-  2(p+p-), p+p-K+K-, 2(K+K-)
e+e-  3(p+p-), 2(p+p-p0), 2(p+p-)K+K- pp
Disclaimer: extensive study of J/y decays in the same channels is not presented here

3. The PEP-II B-factory 9.0 GeV e- 3.1 GeV e+ ECM=10.58 GeV
Peak Luminosity:
1034 cm-2 s-1

4. The Babar Detector SVT: 97% efficiency 20mm z resolution SVT+DCH:
s(pt)/pt=0.13%·pt+0.45%
EMC:
s(E)/E=2.32%·E-1/4+1.85% 1m

5. Particle Identification

6. ISR at Y(4S) energy s of the machine fixed at 10.58 GeV
gISR
hadrons
s of the machine fixed at GeV
Radiation of a hard photon  s’=0-5 GeV
The gISR is detected in the analysis (acceptance ~10-15%)
Eg ~ 3-5 GeV  small fake beam gas background
The hadronic state is collimated on the recoil
Final state radiation contribution small and well separated
s’ determined from the final state
energy resolution depends on tracking
entire energy scan of the cross sections in a single “shot”
m+m- used for luminosity normalization

7. ISR luminosity
ISR cross section for the production of the final state f:
x=2Eg/s
s’=s(1-x)
Probability of ISR photon emission:

8. Cross section calculation
dNfg = number of ISR f events
dNmmg = number of ISR mm events
dFSR = final state radiation correction (negligible for most f)

9. e+e-  p+p-p0 89 fb-1 PRD 70, 072004 (2004) Selection:
particles inside fiducial volume
Eg>3 GeV for at least one g
kinematic fit applied
fit c2 used for event selection
Backgrounds:
p+p-p0p0, p+p-, K+K-p0, t+t-, ...
Bkg suppression cuts (improve S/B)
no track identified as K
Ep0>0.4 GeV
kinematical fit to 4p (to reject p+p-2p0)
Mpg>1.5 GeV/c2 (to reject tt)
c2 distribution in the range 1.05<M3p<3 GeV/c2
Prel. selection (data/MC)
p+p-p0p0
other bkg processes
Selected
Bkg/efficiency study
89 fb-1
PRD 70, (2004)

10. e+e-  p+p-p0 Inconsistent SND with DM2 Babar DM2 w’’ M=1350±28 GeV/c2
G=450±100 GeV/c2 w’ w
M=1660±10 GeV/c2
G=230±36 GeV/c2 f

11. Systematic uncertainties
e = efficiency
eMC = eff. measured on MC
d = correction

12. e+e-  4h p+p-p+p- K+K-p+p- K+K-K+K- 89 fb-1 PRD 71, 052001 (2005)
Selection:
particles inside fiducial volume
Eg>3 GeV for at least one g
4 charged tracks
kinematical fits applied
4p always applied
2p2K and 4K applied if K are present
fit c2 used for event selection
Background:
ISR t+t- and multihadron
Separation of different channels based
on the c2 for the different kinematical fit
4p candidates
4p MC
Non ISR bkg (JETSET)
89 fb-1
PRD 71, (2005)

13. e+e-  p+p-p+p- Signal Signal region (MC)
ISR background (control region)
Control region (MC)
Non ISR background (JETSET)
Selection efficiency vs mass (MC)
10%
<1%
3-5%
Syst. error due to bkg subtraction

14. e+e-  p+p-p+p- Comparison with previous experiments
Measured 4p cross section
Only statistical error shown

15. e+e-  p+p-p+p-

16. e+e-  p+p-K+K- c2(2p2K)<20 c2(4p)>30 c2(4K)>20
0.5% 4p contamination
negligible 4K contamination
15% systematic uncertainty
Dominated by:
10% uncertainty in the acceptance (MC)
5% difference data/MC in K ID
Only statistical error shown

17. e+e-  p+p-K+K- 2p2K channel dominated by intermediate K*0Kp
After removing events in K*0 bands f
K*0(892) r
“Other” Kp combination

18. e+e-  K+K-K+K- c2(4K)<20 high purity sample
25% systematic uncertainty
Contributions:
absence of detailed model for acceptance
uncertainty in bkg subtraction
difference data/MC in K ID
Only statistical error shown

19. e+e-  6h 3(p+p-) 2(p+p-)p0p0 K+K-2(p+p-) 232 fb-1 Selection:
particles inside fiducial volume
Eg>3 GeV for at least one g
6 tracks or 4 tracks and 4g (Eg>20 MeV)
kinematical fits applied
fit c2 used for event selection
Background:
ISR t+t- and multihadron
232 fb-1

20. e+e-  3(p+p-) Signal ISR background (control region)
Signal region (MC)
Non ISR background (JETSET)
Control region (MC)
<3%
3-5%
Selection efficiency vs mass (MC)
Syst. error due to bkg subtraction

21. e+e-  3(p+p-)
Intermediate state r0(770)2(p+p-) assumed in MC production
All pions
point = data
histogram = MC

22. e+e-  3(p+p-)
r0(770)2(p+p-) model gives a very good description of the data
No other significant structures observed (but full partial wave analysis not done)
A 6p phase space simulation does not produce deviations in the angular distribution
The acceptance varies by less than 3%, which is taken as a systematic uncertainty MC
(J/y not included)
Data

23. e+e-  3(p+p-) Structure at 1.9 GeV also seen by FOCUS FOCUS collab.
PLB 514, 240 (2001)
Only statistical error shown

24. e+e-  3(p+p-)

25. e+e-  2(p+p- p0) Additional bkg reduction cuts: require Eg>50 MeV
K veto on tracks
Structure at 1.9 GeV confirmed
in 2(p+p-p0)
Only statistical error shown

26. e+e-  2(p+p- p0)

27. e+e-  2(p+p- p0) Data MC Presence of resonant structures w r+ J/y w h
p+p-p0 mass
“other” p+p-p0 mass
Evidence of wh production

28. e+e-  2(p+p- p0) Structure in wh channel 2(p+p-p0) wp+p-p0 f(1680) ?
Fit to resonance:
m=1.645±0.008 GeV/c2
G=0.114±0.014 GeV/c2
peak cross section=3.08±0.33 nb
2(p+p-p0)
wp+p-p0
f(1680) ?
w(1650) ? wh

29. Combining the 6p channels
Fit to Breit-Wigner structure by several states decaying to the same mode
Continuum
Resonance
m = 1.88±0.03 GeV/c2
G = 0.13±0.03 GeV
f = 21±40˚
m = 1.86±0.02 GeV/c2
G = 0.16±0.02 GeV
f = -3±15˚

30. e+e-  2(p+p-)K+K- At least one particle identified as K
Kinematical fit
c2(6p)>20
15% systematic uncertainty
Dominated by:
10% uncertainty in the acceptance (MC)
5% difference data/MC in K ID
Only statistical error shown

31. e+e-  pp 232 fb-1 hep-ex/0512023 Study of:
cross section (continuous spectrum from threshold, s=2mp, in a single measurement)
form factor in the time-like region
C = Coulomb correction factor
Allows s finite at threshold
Experimental challenging because of larger 2-body backgrounds: mm, pp, KK
Good PID and kinematical fit required
232 fb-1
hep-ex/

32. e+e-  pp 30% signal loss Selection: particles inside fiducial volume
ISR bkg suppression factors:
15·103 for pp
500·103 for mm
2·103 for KK
Selection:
particles inside fiducial volume
Eg>3 GeV for at least one g
2 charged tracks, both with proton ID
kinematical fit to C+C-g applied
C = e, m, p, K, p
25% signal loss
ISR bkg suppression
factors:
50 for pp, mm
30 for KK p K p K

33. e+e-  pp 1. ISR backgrounds Background Data ~4000 events
N. of bkg events
pp 5.9±2.5
KK 2.5±1.0
ee 2.5±1.0
mm <11 K p m

34. e+e-  pp 2. pp + photons background Expected mpp distrib.
from ppp0 events
2. pp + photons background
Dominant background contribution from ppp0
Kinematical fit to ppgg performed

35. e+e-  pp Detection efficiency 10% systematic error
From the difference between pure electric
(GM=0) and pure magnetic (GE=0) values

36. e+e-  pp
Cross section (systematic error included)

37. e+e-  pp |GE|/|GM| measurement from p angular distribution
Two samples of ppg events generated: one with GE=0 the other with GM=0
Angular distributions HM(cosq*,s) and HE(cosq*,s) obtained from the simulation
Fit to the distribution:
Free parameters:
A (normalization)
|GE/GM|

38. e+e-  pp Angular distributions for different s regions
|GE/GM| measurement from
angular distributions
(stat. and syst. errors included)
GM=0
GE=0

39. e+e-  pp
Definition of an “effective form factor” under the assumption |GE|=|GM|
Fit to asymptotic behavior
At threshold

40. Summary and perspectives
Extensive ISR analysis program in Babar
Full spectrum from threshold to ~4.5 GeV
All main hadronic channels under study
pp, KK, LL
p+p-2p0, p+p-3p0
K+K-p0, K+K-2p0, fp0
p+p-p0h, fh
...

41. BACKUP

42. Systematcs overlook Luminosity from mm: 3%
Background subtraction: ~1-10%
larger where the cross section is small
Acceptance from simulation: 1-3%
~10% in some cases (model dependence)
Data/MC differences in tracking/PID: ~3-5%