1. R Measurement from BES Motivation R Scan in 2-5 GeV
Z.G. Zhao
6/27/2002, PSI
R Measurement from BES
Z.G. Zhao
(Representing BES Collaboration)
University of Michigan, Ann Arbor, MI, USA
IHEP of CAS, Beijing, China
Motivation
R Scan in 2-5 GeV
QCD Test Topics with R scan Data
Summary
9/22/2002, FNAL
Zhengguo Zhao
Z.G. Zhao

2. The Beijing Electron Positron Collider (BEPC)
A unique e+e- machine operating in Ecm~2-5 GeV since
L~51030 /cm2s at J/ peak
9/22/2002, FNAL
Zhengguo Zhao

3. BESII Detector (General Purpose Collider Detector)
Z.G. Zhao
6/27/2002, PSI
BESII Detector (General Purpose Collider Detector)
VC: xy = 100 m TOF: T = 180 ps  counter: r= 3 cm
MDC: xy = m BSC: E/E= 22 % z = 5.5 cm
dE/dx= 8.4 %  = 7.9 mr B field: T
p/p=1.8%(1+p2) z = 2.3 cm
9/22/2002, FNAL
Zhengguo Zhao
Z.G. Zhao

4. Definition of R Experimentally R: one of the most
Z.G. Zhao
6/27/2002, PSI
Definition of R
R: one of the most
fundamental quantities in
particle physics, counts
directly the number of quarks,
their flavor and colors
Experimentally
Nhad: observed hadronic events
Nbg: background events
L: integrated luminosity
had: detection efficiency for Nhad
: radiative correction
9/22/2002, FNAL
Zhengguo Zhao
Z.G. Zhao

5. R Values Below 10 GeV 15% R/R ~ 15% below 5 GeV
Z.G. Zhao
R Values Below 10 GeV
6/27/2002, PSI
15%
R/R ~ 15% below 5 GeV
Unclear & complex structure in GeV
9/22/2002, FNAL
Zhengguo Zhao
Z.G. Zhao

6. Motivations calculated measured
Reducing the uncertainty of (MZ2)  essential for precision tests of the SM
calculated measured
At MZ 0.0009
Hunting for new physics from a(g-2)/2
 Interpretation of E821 at BNL
New physics
9/22/2002, FNAL
Zhengguo Zhao

7. Relative uncertainties of three input parameters in the SM fit
The indirectly determination of mH
depends critically on the precision
of (MZ2)
9/22/2002, FNAL
Zhengguo Zhao

8. Relative Contributions to the Uncertainties of a and (MZ2)
BEPC energy region
9/22/2002, FNAL
Zhengguo Zhao

9. BES R Scan March-May, 1998: 6 energy points
at 2.6, 3.2, 3.4, 3.55, 4.6, 5.0 GeV + separated beam operation at each
Feb.- June, 1999:
85 energy points at GeV
+ 24 energy points separated beam operation
9/22/2002, FNAL
Zhengguo Zhao

10. Events Recorded by BESII
Events collected at BESII
Event rate
9/22/2002, FNAL
Zhengguo Zhao

11. Hadronic Events Selection
Select Nhad(e+e-hadrons) from all other possible contaminations. e-
9/22/2002, FNAL
Zhengguo Zhao

12. Typical Cuts Used for Selecting NhadMC
Data
9/22/2002, FNAL
Zhengguo Zhao

13. Trigger Efficiency (trg)
trg measured by comparing the responses to different trigger requirements in
- special runs taken at J/
- R at 2.6, 3.0, 3.5 GeV with larger data
sample
trg = 99.96%, 99.33%, 99.76% for Bhabha,
dimuon and hadrons with errors of 0.5%
9/22/2002, FNAL
Zhengguo Zhao

14. Background Subtraction
Background sources:
1. Cosmic ray
2. Lepton pair production subtracted by event
3. Two-photon processes selection and MC
4. Beam associated  most serious
fit the event vertex distribution with Gaussian for hadronic events, polynomial(degree 2) for BG
b) separated beam and single beam operation data
Nbg = f  Nsep
Both a) and b) are consistent
9/22/2002, FNAL
Zhengguo Zhao

15. Subtraction of the Beam Associated Background
Gaussican for Nhad
P2 for BG
9/22/2002, FNAL
Zhengguo Zhao

16. Luminosity Measurement
Bhabha events
Bhabha
events
Rely on large-angle Bhabha e+e- e+ e- ()
e+e-  cross check
9/22/2002, FNAL
Zhengguo Zhao

17. Detection Efficiency for the Hadronic Events
Rely on hadronic events generatormodel dependent
LUARLW is developed to improve JETSET for low energy region, particularly for Ecm< 3 GeV
- final states are simulated exclusively
- less parameters in the fragmentation function
- remove high energy assumption
D, D*, Ds, Ds* productions are simulated according to Eichiten Model
A generator is built into LUARLW to handle decays of resonances in the radiative return processes e+e- J/ or (2S)
9/22/2002, FNAL
Zhengguo Zhao

18. Some Hadronic Event Shapes at Ecm=2.2 GeV (LUARLW)
Tune the parameters
to reproduce 14
distributions of the
observed kinematic
variables and
hadronic event shape.
Hatched region: MC
Histogram: Data
9/22/2002, FNAL
Zhengguo Zhao

19. Some Hadronic Event Shapes at Ecm=3.55 GeV
9/22/2002, FNAL
Zhengguo Zhao

20. Initial States Radiative Correction
ISR: remove high order
effects from
Leading order [O(2), diagram (a)]
All the high order contributions
vertex
Bremsstrahlung
Born term
Vacuum
polarization
9/22/2002, FNAL
Zhengguo Zhao

21. Initial States Radiative Correction
Four schemes are compared to calculate ISR.
(1) G. Bonneau and F. Martin, NP B 27(1971)387
(2) F.A. Berends and R. Kleiss, NP B 178(1981)14
(3) E. A. Kuraev and V.S. Fadin, Sov. J. NP 41(1985)3
(4) A. Osterheld et al., SLAC-PUB-4160, (T/E)
(2) + multi-soft photon = (4)
(1) – (4) are consistent within 1% in the continuum region, 1-3% above charm threshold
9/22/2002, FNAL
Zhengguo Zhao

22. Variation of Detection Efficiency and ISR in 2-5 GeV
Syst. error
9/22/2002, FNAL
Zhengguo Zhao

23. Some Values at a Few Typical Energy Points to Determine R
Ecm(GeV)
Nhad
N+N
L(nb-1)
had(%)
1+
2.0
1155.4
19.5
47.3
49.50
1.024
3.0
2055.4
24.3
135.9
67.55
1.038
4.0
768.7
58.0
48.9
80.34
1.055
4.8
1215.3
93.6
84.4
86.79
1.113
Systematic Errors (in %) at a Few Typical Energy Points
Ecm(GeV)
Nhad
 triiger
L(nb-1)
 had
(1+)
Total
2.0
7.07
0.5
2.81
2.62
1.06
8.13
3.0
3.30
2.30
2.66
1.32
5.02
4.0
2.64
2.43
2.25
1.82
4.64
4.8
3.58
1.74
3.05
1.02
5.14
9/22/2002, FNAL
Zhengguo Zhao
largest

24. R Values in 2-5 GeV Preliminary results reported at ICHEP2000
PRL 88:101802,2002
(3770)
Preliminary results
reported at ICHEP2000
are the same as the final
published ones in 2002
9/22/2002, FNAL
Zhengguo Zhao

25. R Below 10 GeV Before BES R Scan After BES R Scan 9/22/2002, FNAL
Zhengguo Zhao

26. Impact on α(MZ) Δαhad=0.02768±0.00036 Δαhad=0.02769±0.00018
From Paolo Gambino
α(MZ) appropriate parameter for EW
Spectral function enters its calculation
similarly, but higher energy data have more weight. Results are converging
δα(MZ) is no more the main bottleneck for precision EW
Further improvements expected
Conservative estimate
(upper bound of uncertainty)
Δαhad= ±
Burkhardt & Pietrzyk 2003
With more efficient use of exp data
Δαhad= ±
Hagiwara et al 2003
Use of τ data + ~0.002
9/22/2002, FNAL
Zhengguo Zhao

27. The Impact of BES R-value on SM Fit to mH
(95% C.L.)
Without BES data
With BES data
9/22/2002, FNAL
Zhengguo Zhao

28. had(MZ2) and Its Relative Contribution in Magnitude and Uncertainty
BESIII
CLEOC
CMD,SND
KROE
CLEO
9/22/2002, FNAL
Zhengguo Zhao

29. R(pQCD) and R(BES) pQCD calculation agree amazingly well with BES data
(D. Michel et.al.) under current uncertainty
9/22/2002, FNAL
Zhengguo Zhao

30. Evaluation of s and s(MZ) from R(BES)
Exhibits QCD correction known to O(s3(s)) so far.
Precision measurement of R can determine s and thus
evaluate s(MZ).
Using R at 3.0 and 4.8 GeV, one has:
Which gives:
PDG2000:
J.H. Kuhn, M.Rreinhauser
Nucl. Phys. B619(2001)588
consistent
9/22/2002, FNAL
Zhengguo Zhao

31. Hadron Production from e+e- Annihilation
A wonderful laboratory to study E.W. and strong interaction.
Unique place to investigate pQCD, hadron production mechanism
With BES R scan data
 distribution (MLLA/LPHD)
Multiplicity: second binomial moment R2 (NLO)
e+e-(K) + X processes (polarized parton density)
MLLA: Modified Leading Log Approximation
LPHD: Local Parton Hadronic Duality
9/22/2002, FNAL
Zhengguo Zhao

32.  Distribution: =-ln(2p/s)
MLLA/LPHD calculations
Limiting spectrum when Q0=eff
BES data can be reasonably
well described by MLLA/LPHD
9/22/2002, FNAL
Zhengguo Zhao

33. *: Peak Position of  Distribution
MLLA/LPHD predict:
eff: free parameter
C=0.2915/0.3190/0.3513
For nf = 3/4/5
BES data support MLLA/LPHD prediction
9/22/2002, FNAL
Zhengguo Zhao

34. KLPHD and eff
BES
9/22/2002, FNAL
Zhengguo Zhao

35. Multiplicity
Average multiplicity
NLO
9/22/2002, FNAL
Zhengguo Zhao

36. Second Binomial Moment
NLO calculation
c=0.5603/0.3196/0.3513
for nf=3,4,5
BES data systematically
lower than what NLO
Predicted, and consistent
with the tendency of that
from high energy e+e- data
9/22/2002, FNAL
Zhengguo Zhao

37. Summary
BES measured R in 2-5 GeV with average uncertainties of 6.6% (a factor of 2-3 improvement)
R scan data at continuum is used to test QCD. However precision test of QCD need more accuracy data (~1-3% level)
BES is planning to collect hadronic event sample with high statistics at a few energy points between 2-3 GeV
Further meaningful improvement on R measurement, and understand the broad structure between GeV need BESIII at BEPCII.
9/22/2002, FNAL
Zhengguo Zhao