Study of e+e collisions with a hard initial state photon at
BaBar
Michel Davier (LAL-Orsay)
for the BaBar collaboration
TM
2
Initial state radiation (ISR) method
• High PEP-II luminosity at s = 10.58 GeV precise measurement of
the e+e- cross section 0 at low c.m. energies with BaBar.• Improved hadron spectroscopy• Input to (g-2) and em calculations.• Few previous data in the 1.4-3.0 GeV range.• Comprehensive program at BaBar.• Today: results for +0, preliminary results for 2+2, K+K+,
2K+2K from 89.3 fb-1
ISR
d (s, x)
dxd(cos )W (s, x,) 0(s (1 x)),
W (s, x,) x
2 2x x2
sin2 x2
2
, x
2Es
3
ee 0
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
Kinematic fit:
• Energy and momentum balance enforced
• Mass of two soft photons constrained to 0
• 2 < 40 for fit in 0 hypothesis selects
signal events
• Reject events with extra photons if 2 < 40 for
0 0 hypothesis
J/
Excited states ?
Eve
nts
/0.0
1 G
eV/c
2
4
Background for 3
Most dangerous backgrounds: e+ e- K+ K- , e+ e- qq
Other backgrounds: e+e- 2, 4, 5, …, +-, 0
Two methods of background subtraction:
1. background mass distribution measured in
data, subtracted bin-by-bin from signal
mass distribution
2. taken from simulation, corrected to real
experimental distribution
Total background level:
• (0.5 - 1.5)% in , regions
• (15 - 50)% at higher masses
• accuracy in background level
~25% up to 2 GeV
Data
MC
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 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 (1637)
• Fix - relative phase to 180, - to 0
dN
dm
th
3 (m)dL
dm(m)R(m),
dNdm
exp
dNd m
th
f (m, m )
7
Fit results: - region
2/d.f.=146/148
consistent with known propertiesof these resonances (, widths fixed to PDG values)
The resolution is about 6, 7, 9 MeV/c2 at , , J/ masses.
Fitted resolution smearing is ~1 MeV/c2
BaBar PDG
B(ee)B(3)=(6.700.06 0.27)10-5 (6.350.10)10-5
B(ee)B() =(4.300.08 0.21)10-5 (4.520.19)10-5
8
Fit results: higher mass region
BaBar PDG
B(ee)B(3)=(0.820.050.06)10-6 M()= 13502020 MeV/c2 1400 - 1450
()= 4507070 MeV 180 - 250
B(ee)B(3)=(1.30.10.1)10-6 M()= 1660102 MeV/c2 1670 30
()= 2303020 MeV 315 35
• Good fit obtained for the range up to 1.8 GeV/c2.
• Extending the fit to masses above 1.8 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
9
e+ e- + 0 cross section
DM2
BaBar
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
3 (m)dN dm
(m)R(m)dL dm
10
J/ 3 decay
• The J/ meson is narrow - clean
signal
• After sideband subtraction -
NJ/ = 92034
• Detection efficiency - =(9.20.6)%
• The result: (J/ ee)B(J/ 3)=
0.1220.0050.08 keV
• We previously measured ()
(J/ ee)= (5.610.20) keV
Eve
nts
/0.0
04 G
eV
/c2
B(J/ 3) = (2.180.19)% BaBar (1.500.20)% PDG (2.100.12)% BES 2003
Phys. Rev. D69,011103 (2004)
sideband sideband
11
e e 22 , KK , 2K2K
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
• Other ISR processes (5, …) – using difference in 2 distributions
• e+e- qq – using JETSET simulation
Background subtraction:
12
ee 2 2 cross section
Systematic errors:
• 12% for m4 < 1 GeV,
• 5% for 1 < m4 < 3 GeV,
• 16% for higher masses)
• best measurement above 1.4 GeV
Intermediate states:
• a1(1260) - dominant, structure which
may be f0(1370) final state is seen.
• For detailed study, a simultaneous
analysis of and final
states is required.
BaBar Preliminary
Coverage of wide region in one experiment No point-to-point normalization problems
13
e e 2 2 cross section
Good agreement with direct e e measurements
Most precise result above 1.4 GeV
14
substructuresBaBar preliminary
MC generator:
• H.Czyz and J.H.Kuehn,
Eur.Phys.J C18(2000)497-509
• Includes a1(1260) and f0(1370)
• Does not include J/a1(1260)
(770)f0(1370)
J/
15
e e K K
Systematic error – 15% (model dependence, kaon identification)
Substantial resonance sub-structures observed: • K*(890)Kdominant•KK contribute strongly
• K*2(1430)K seen.
Much more precise than previous measurement
J/
16
KK substructures
No studies in previous e+e- experiments!
K*K MC generators are not available yet
K* regions excluded
K*(890)K dominated
f0(980) ?
Events from band
No signal from f0(980) yet Connection to f0(980)?
BaBar preliminary
17
e e 2K 2K
First measurement
Overall normalization systematic error – 25% (model dependence, kaon identification)
No clear mass structure in the two- or three-body subsystems
No ’ s !
BaBar preliminary
J/
18
J/ and (2S) decays
BaBar preliminary PDGB(J/ =(3.610.26 0.26)10-3 (4.01.0) 10-3 B(J/ K+K- =(6.090.50 0.53)10-3 (7.22.3) 10-
3 B(J/ K+K-K+K-) = (6.71.1 1.0)10-4 (9.23.3) 10-4
B((2S) J/ +-)=0.3610.015 0.028 0.3170.011
+- J/
+-
2+2- K+K-+- 2K+2K-
19
Summary• Using ISR method the cross sections of e+e , , 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.
• Examples: contributions to ahad (1010) from 2+ 2 (0.56 – 1.8 GeV)
from all e+ e exp. 14.21 0.87exp 0.23rad from all data 12.35 0.96exp 0.40SU(2)
from BaBar 12.95 0.64exp 0.13rad
from + 0
from all e+ e exp. 2.45 0.26exp 0.03rad
from BaBar 3.31 0.13exp 0.03rad
• Several B(J/ -> X) measurements better than current world average
• More modes to come; aim for systematic errors 1% (in +)
Davier-Eidelman-Hoecker-Zhang 2003
total 696.37.2
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