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 = 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: preliminary results for +0, 2+2, K+K+, 2K+2K from

89.3 fb-1

ISR

€

dσ (s, x)

dxd(cosθ )=W (s, x,θ ) ⋅σ 0(s (1− x)),

W (s, x,θ ) =α

π x

2 − 2x + x2

sin2 θ−

x2

2

⎛

⎝ ⎜

⎞

⎠ ⎟, x =

2Eγ

s

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

=σ 3π (m)dL

dm⋅ε(m) ⋅R(m),

dN

dm

⎛

⎝ ⎜

⎞

⎠ ⎟exp

=dN

d ′ 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 Preliminary PDG

B(ee)B(3)=(6.700.06 0.27)10-5 (6.350.11)10-5

B(ee)B() =(4.300.08 0.21)10-5 (4.590.14)10-5

8

Fit results: higher mass region

BaBar Preliminary 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 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

€

3π (m) =dN dm( )

ε (m) ⋅R(m) ⋅dL dm

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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 Preliminary (1.500.20)% PDG (2.100.12)% BES 2003

Phys. Rev. D69,011103 (2004)

sideband sideband

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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

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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)K dominant• 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/

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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-

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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.

• Example: contribution 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

• 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

696.37.2