Precision measurement of the total hadronic cross-section with CMD-2 at VEPP-2M e+e- collider

E. Solodov

Budker Institure of Nuclear Physics (Novosibirsk, Russia)

CMD-2 Collaboration

ICHEP – 2004Beijin, China, August 16-22, 2004

Outline

1. VEPP-2M and available data2. New results on e+e-

3. New results on e+e- 44. New results on -meson

parameters5. Some other new measurements6. Summary

R, the definitionR(s) is defined as:

R(s) is one of the most fundamental quantities in high energy physics:• its global structure reflects number of quarks and their colors; used for QCD tests and as a source of QCD parameters• plays special role in precision measurements:

R(s)(0)(ee * hadrons) (0)(ee )

had (M Z2 ) (0)M Z

2

3Re ds R(s)

s(s M Z2 ) i4m

2

had (l.o.)

m

3

2

ds K (s)s2

4m2

R(s)

had (l.o.)

<2 GeV

<2 GeV

2-5 GeV

2-5 GeV

>5 GeV

>5 GeV

Measurement of R in Novosibirsk

• VEPP-2M collider: 0.36-1.4 GeV in c.m., L31030 1/cm2s at 1 GeV• Detectors CMD-2 and SND: 60 pb-1 collected in 1993-2000• All major hadronic modes are measured:

e+e- e+e- Still a lot of data to analyzed !

Measurement of R in Novosibirsk

Total hadronic cross-section measured by CMD-2 and SND

The approach

• Luminosity L is measured using Bhabha scattering at large angles detector inefficiencies are measured• Efficiency H is calculated via Monte Carlo + corrections for imperfect detector • Radiative correction accounts for ISR effects only

All modes except 2

• Ratio N(2)/N(ee) is measured directly detector inefficiencies are cancelled out• Virtually no background• Analysis does not rely on simulation• Radiative corrections account for ISR and FSR effects

2

F2 N2

Nee

ee (1ee ) 2 (po int like)(12 )

Formfactor is measured to better precision than L

Systematic errorsRadiative corrections

0.4%

“Experimental” (event selection, separation, efficiencies,…)

0.4%

Total 0.6%

Radiative corrections

1%

Luminosity 1-2%Efficiency 1-5%Total 1.5-

7%

(ee H )NH Nbg

LH (1H )

What is really measured?Definition of (e+e-hadrons) depends on the application

• Hadron spectroscopy: vacuum polarization (VP) is the part of the cross-section (“dressed”), final state radiation (FSR) is not• “Bare” cross-section used in R: vice versa – FSR is the part of the cross-section, VP is not • Measured number of events include VP and part of FSR allowed by the event selection

CMD-2 published 2 cross-sections e+e-2:radiative correction take into account part of FSR, allowed by the event selection (thus remove FSR completely from the measured cross-section); VP is left untouched. Used to get rho-meson mass, width, …VP is removed, all FSR is added. Used for R calculation VP FSR

0 () 1 (s) 2 1

(s)

The radiative correctionsVacuum

polarizationInitial and final state radiation

• the correction factor |1-(s)|2 is the same for all final states• R(s) itself is used for (s) evaluation iterations

ISR+FSR

ISR+FSR+VP

• ee, , final states: 1 at large angle, multiple ’s along initial or final particles (0.2%)• CMD-2 results are consistent with independent calculations (BHWIDE, KKMC)• Other final states: multiple ’s along initial particles (1%)

(s) s4 2

Pee * final

(s' )ds'

s s' i

ee * final(s)

4m2

e+e- —> +- >0.6 GeV

1-2% 0.6-1% 1-5%Systematic error

4 separate scans over 5 years, >1 million events, 100k published

ee

• e// separa-tion using energy deposition• N()/N(ee) is fixed according to QED

• e// separation using particles momentum• can measure N()/N(ee) and compare to QED

<0.6 GeV

ee |F|2

-meson parameters

Data 94-95 113 k events

Data 98 ( only)520 k events

Data 94-95-98670 k events

m (MeV/c2) 775.69 0.64 0.59 776.04 0.55 775.36 0.43 (MeV) 143.89 1.33 0.80 145.64 0.94 143.53 1.08

B 1.30 0.24 0.05 1.58 0.16 1.47 0.12

(rad.) 0.232 0.064 0.003 0.170 0.037 0.192 0.029

ee (keV) 7.05 0.11 0.05 7.083 0.074 7.076 0.062

a’;’ (rad.) 0.290.07; (fixed) 0.300.06;(f) 0.340.03;-2.20 0.01

m’ (MeV/c2) 1467 (fixed) 1467 (fixed) 1317 16’ (MeV) 309 (fixed) 309 (fixed) 466 30

a”;” (rad.) - - 0.0035(f);-2.17(f)

m” (MeV/c2) - - 1733 (fixed)

” (MeV) - - 175 (fixed)

PDG

775.8 0.5

150.3 1.6

1.70 0.27

7.02 0.11

1465 25

400 60

1720 20

250 100

Syst. Errors are under studypublished

Systematic error 5%

CMD-2/SND discrepancy recently resolved

e+e- —> 4

Phys.Lett. B595(2004)101 Ready for publication

New measurements of mesone+e- e+e-

Systematic error 3%363000 events104000 events

-meson parameters

CMD-2 (2004) PDG(average)

BeeBKK(14.460.070.40)1

0-5(13.930.140.99) 10-

5

m, MeV/c2 1019.4480.0100.080 1019.4560.020

, MeV 4.250.030.03 4.260.05

BeeB3(4.620.070.12)10-

5 (4.52 0.19) 10-5

m, MeV/c2 1019.220.020.15

, MeV 4.260.080.05

e+e-

Br( ) = ( 4.9 +2.8-2.2 0.4)10-4

Br( ) = ( 2.9 +1.5-1.3 0.3)10-4

Br( ) = 1.355 0.014 0.063 %

e+e-

Br( ) = 9.46 0.17 0.57 %Br( ) = ( 4.2 +1.0

-0.9 0.4)10-4

Br( ) = (1.214+0.045-0.036 0.061) 10-3

e+e-

’” ?

e+e- e+e-

Using data collected with CMD-2 detector in 720-840 MeV c.m. energy range the branching fraction of the conversion decay was determined: Br( e+e-0)=(8.160.730.63)10-4

The upper limits on the branching fractions of the following decays have been set for the first time:

Br( e+e-0) < 2.210-5 (90% CL);Br( e+e-) < 1.510-5 (90% CL);Br( e+e-) < 2.310-5 (90% CL);

Future measurements at VEPP-2000

• Factor >10 in luminosity (round beams)• Up to 2 GeV c.m. energy• CMD-3: major upgrade of CMD-2 (new drift chamber, LXe calorimeter)

• measure 2 mode to 0.2-0.3%• measure 4 mode to 1-2%• overall improvement in R precision by factor 2-3

Under construction. Data taking is expected to start in 2006-2007.

CMD-3

VEPP2000 construction (August 2004)

SNDCMD-3

Lxe calorimeter for CMD-3

Conclusion• Measurement of R is still very active and important field

• Important for interpretation of g-2 experiment, evaluation of (MZ), tests of QCD• Energy range 0-1.4 GeV contributes 80% (value and error) to a

had - motivation to continue data analysis collected with CMD-2

• New data on , 4, precision measurements of , - have been presented

• New high luminosity machine VEPP2000 is under construction• CMD-3 and SND detectors are expected data taking in 2006• Expect to reach 0.3-2% precision in the 0-2 GeV range in few years (factor of 2 improvement)

New

Implication to a

Final state Today2004

CMD-2+ISR2005-2006

+VEPP-20002010

2 5.9 <4.0 <2.0

, 1.4 1.0 <0.7

Other, <2 GeV 2.3 1.5 1.0

>2 GeV 1.8

Total 7.2 <5.0 <3.0

BaBar

VEPP-2000

Uncertainty of the hadronic contribution to a , 10-10

Our estimation

Current/Future activities in R

VEPP-2M

VEPP-2000

CLEO

BaBar ISR

KLOE

CLEO-cBES

VEPP-4M

Conclusion

• Measurement of R is still very active and important field• Important for interpretation of g-2 experiment, evaluation of (MZ), tests of QCD

• Recent improvements: VEPP-2M, BES• Lots of data are being analyzed: VEPP-2M, KLOE, BaBar, CLEO• Many future projects: VEPP-2000, BESIII, CLEO-c• ISR experiments have demonstrated impressive potential: KLOE, BaBar.• Expect to reach 0.3-5% precision over the whole 0-10 GeV range in few years (factor of 2 improvement)