1. BESIII Physics Programs
This is not a BESIII logo!
BEST
B  (looks like DD for D or charm physics)
E  (looks like cc for charmonium(like) physics)
S  (for light hadron Spectroscopy [+exotics])
T  (for tau physics, looks like a Roman number “III”)

2. Yuan Changzheng (苑 长 征) IHEP, Beijing 2013年7月
 Physics in a Nutshell
The t is a clean laboratory to test the Standard Model
Yuan Changzheng (苑 长 征)
IHEP, Beijing
2013年7月

3. Outline The discovery of  The properties Lepton Universality
Mass, lifetime, and Michel parameters
Production and decays
Lepton Universality
Asymptotic behavior of QCD
Strange quark mass
a (反常磁矩）[I will skip this, in backup]
Search for rare decays and new physics
 physics at BESIII
Future?

4. Tsai(蔡永赐) and Perl Everything was in Yung-Su Tsai’s paper
Discovered by Martin Perl
One of the best known particles
Refs.
M. L. Perl, Phys. Perspect. 6 (2004) 401; “The discovery of the tau lepton and the changes in elementary-particle physics in forty years”
Proceedings of the tau workshops (90,92,…,04,06,08,10,12)

5. PRD4, 2821 (1971) Weak decays; well understood
All the properties are predicted for mass 0.6, 0.8, 0.938, 1.2, 1.8, 3.0 and 6.0 GeV. (m=1.777 GeV)
PDG:
Total rate=1/290.6E-15=344E10s

6. PRL35, 1489 (1975)
64 e- events
Mass = GeV
1995 Nobel Prize

7.  mass
BES threshold scan method, used by KEDR recently. New development for BESIII!
PDG: ±0.16 MeV

8.  lifetime Mainly from LEP experiments and recently from BaBar!
PDG: 290.6±1.0 fs

9. Michel Parameters

10. Production in e+e- experiment
Most important production mechanism
Pair production in vector decays [(2S), (1,2,3S), Z0]
Pure leptonic decays of particles (D, Ds, B, W±)
Other mechanisms (H, …)

11. Tau Decay
Universal W Couplings

12.  decays Pure leptonic Semi-leptonic Rare and forbidden
Cabibbo favoured
Cabibbo suppressed
Rare and forbidden
Lepton Flavor Violation
Lepton Number V
Baryon Number V

13. Why  BR and SF Lepton Universality Asymptotic behavior of QCD
Strange quark mass & Vus
Test CVC
a and running 

14. Magnetic Anomaly    QED Prediction: QED
Computed up to 5th order [Kinoshita et al.] 
Schwinger 1948  
QED
Hadronic
Weak
SUSY...
... or other new physics ?

15. Magnetic Anomaly
Contributions to the Standard Model (SM) Prediction:
Source
(a)
Reference
QED
~ 0.3  10–10
[Schwinger ’48 & others]
Hadrons
~ (15  4)  10–10
[Eidelman-Jegerlehner ’95 & others]
Z, W exchange
~ 0.4  10–10
[Czarnecki et al. ‘95 & others]
Dominant uncertainty from lowest order hadronic piece. Cannot be calculated from QCD (“first principles”) – but: we can use experiment (!)
The Situation 1995
had  
had  

16. e+e- data Pi form factors from all the available experiments data:
DM1, TOF, OLYA, CMD, DM2, CMD2, SND (KLOE excluded due to systematic bias)
Latest BaBar results not in this plot.
Fit with ++’+’’ describes data pretty well (Gounaris and Sakurai’s parametrization).

17. The Role of  Data through CVC – SU(2)
W: I =1 & V,A 
CVC: I =1 & V
: I =0,1 & V e+  
hadrons W
e –
hadrons
Hadronic physics factorizes in Spectral Functions :
fundamental ingredient relating long distance (resonances) to short distance description (QCD)
Isospin symmetry connects I=1 e+e– cross section to vector spectral functions:
branching fractions mass spectrum kinematic factor (PS)

20. Three energies for  physics
Threshold: GeV
 almost static; 非共线与非共面性
Eff ~ 10-20%
high precision m measurement, maybe BR?
B factory: GeV
Cross section: 0.8nb
Background: large; eff~10%
However, statistics is huge
Low BR channels; modes with K; searches; single/double tag
Z-pole: 91 GeV
Cross section: 1.5nb; Lorentz boost large
Low multiplicity, Z background low
Back-to-back; eff~80%, mainly acceptance
Global analysis of all channels, limited by statistics

21. Global analysis
Very clean  events can be selected with very simple selection criteria.
Low track multiplicity
Back-to-back
Low invariant mass
Total energy not very small
Subtract background
Classify all the events

22. Classification Divide into 14 classes according to
PTID [ cannot separate /K! ]
n(TRK)
n(0)

23. Statistical error from global analysis
ni follows multinomial distr.
2ni=Npi(1-pi)
N=ni
pi=ni/N
No error from N
Non-global case, Poisson distr.
2ni=ni
+ additional error from N
300k ’s

24. Non- background at 1.2% level
Non- background measured from data directly
Bhabha
Dimu
Two-photon processes
Low multiplicity qq events

25. The EM calorimeter Experiments ALEPH DELPHI L3 OPAL Type
Lead sheet + wire chamber
[radius~200 cm]
Lead-gas High-Density Proj. Chamber
BGO crystal
[radius=52 cm]
Lead glass
Read out
45 layers grouped in 3 stacks readout in longitudinal
40 layers grouped in 9 readout in longitudinal
11000 crystals
No longitudinal readout
11704 lead glass blocks
Radiative length
22X0
18X0
21X0
25X0
Granularity
12x12 mrad2
17x2 mrad2
27x27 mrad2
40x40 mrad2
Angular resolution
/E mrad
1 mrad in 
2 mrad in 
~ 3 mrad
~ 11 mrad
Energy resolution
/E
0.31/E0.440.027
5% at 100 MeV, 1% > 2 GeV
/E
E threshold
>350 MeV
> 500 MeV
Analysis technique
cuts NN
 Paper
Phys. Rep.
EPJC
Unpublished

26. Real/fake photons Lots of low energy photons Lots of fake photons
ALEPH Data
MC truth

27. Real/fake photons Use 6 variables to discriminate photons Real photons

28. Resolved & merged 0
Minimum opening angles between two photons from a 0 decay
=(2m/p)=14 mrad for E=20 GeV
Depends strongly on the granularity
Transverse & longitudinal information of the cluster helps resolve merged 0

29. Efficiency matrix Global efficiency is large (~80%)
Inefficiency mainly due to geometric coverage
Cross contamination due to missing /0

30. Systematic errors Dominant error is /0 reconstruction
Could be improved with better EM calorimeter
Better fake photon simulation will also help

31. Results on BRs Statistical error > systematic error
Statistical precision can improve with more data
Systematic errors are measured with data, also limited by statistics, can be improved with more data

32. Tau Branching Fractions (combined and corrected)
ALEPH, Phys. Rep. 421, 191 (2005)
Spectral functions can be obtained by unfolding the mass spectra!

33. Lessons Need to do global analysis For charged tracks For /0
Good momentum measurement
Good /K separation (PID for tracks up to 40 GeV?)
Good vertex if wants to measure lifetime
For /0
Good geometric coverage
Very fine granularity with longitudinal readout
Good energy resolution and angular resolution
Very low photon energy threshold, better < 200 MeV

34. Lepton Universality 轻子普适性 (assumption of SM)

36. LEPTON UNIVERSALITY

37. CHARGED CURRENT UNIVERSALITY

38. Spectral function Strong coupling constant & Asymptotic Freedom

39. Only lepton massive enough
Hadronic Tau Decay
Only lepton massive enough
to decay into hadrons
probes the
hadronic V-A current
e+ e- H0 probes the hadronic electromagnetic current
Isospin :

40. Tau Hadronic Spectral Functions (SFs)
Hadronic physics factorizes in (vector and axial-vector) Spectral Functions :
branching fractions mass spectrum kinematic factor (PS)
Fundamental ingredient relating long distance hadrons to short distance quarks (QCD)
Isospin symmetry connects I =1 e+e– cross section to vector spectral functions:

41. Tau Branching Fractions measured by ALEPH
 (7525)% used (CLEO)
ALEPH, Phys. Rep. 421 (2005)

42. BELLE
CLEO

43. Inclusive V+A and VA Spectral Functions
Results from ALEPH and OPAL  and their comparison
Of purely nonperturbative origin
ALEPH, Phys. Rep. 421 (2005)
OPAL, EPJ, C7, 571 (1999)

44. Braaten-Narison-Pich
Im (s)
Re (s)
OPE
(fitted from data)

45. The most precise test of Asymptotic Freedom
(ALEPH 2005)
The most precise test of Asymptotic Freedom

46. Running within the Tau Spectrum
The spectral functions allow to measure R(s0 < m2); the previous fit allows us to compare the measurement to the theoretical expectation assuming RGE running
running strong coupling: evidence for quark confinement
assuming quark-hadron duality to hold, this is a direct evidence for running strong coupling
also: test of stability of OPE prediction, and hence of trustworthiness of s(m2) fit result

47. Strange Spectral Function strange quark mass and Vus

48. Strange Spectral Function: SU(3) Breaking Vus and QCD uncertainties
(k,l)
ALEPH
OPAL
(0,0)
0.39  0.14
0.26  0.12
(1,0)
0.38  0.08
0.28  0.09
(2,0)
0.37  0.05
0.30  0.07
(3,0)
0.40  0.04
0.33  0.05
(4,0)
0.34  0.04
determination
Vus and QCD uncertainties

49. Strong sensitivity to Vus
Taking as input (from non t sources) :
(k=0,l=0)
(k0,l=0)
Simultaneous ms & Vus fit possible with better data
The t could give the most precise Vus determination

50. New phenomena & new physics

51. Lepton Flavour Violation
LEPTON MIXING
Lepton Flavour Violation
Mixing Structure U  V :
Open Questions: n Masses (Dirac, Majorana). Leptonic
Leptogenesis (Baryon Asymmetry) CP

52. LEPTON FLAVOUR VIOLATION
90 % CL Upper Limits on Br(l -  X -) [BABAR / BELLE]
Decay
U.L.
m- e-g
1.2  10-11
m- e-e+e-
1.0  10-12
m- e-gg
7.2  10-11
t- e-g
1.1  10-7
t- e-e+e-
2.0  10-7
t- e-e+m-
1.9  10-7
t- m-g
6.8  10-8
t- e-m+m-
t- m-e+m-
1.3  10-7
t- e-e-m+
t- m-m+m-
t- e-p0
t- m-p0
4.1  10-7
t- e-h’
10  10-7
t- m-h’
4.7  10-7
t- e-h
2.3  10-7
t- m-h
1.5  10-7
t- e-K*
3.0  10-7
t- e-KS
5.6  10-8
t- m-KS
4.9  10-8
t- m-r0
t- e-K+K-
1.4 · 10-7
t- e-K+p-
1.6 · 10-7
t- e-p+K-
3.2 · 10-7
t- m-K+K-
2.5 · 10-7
t- m-K+p-
t- m-p+K-
2.6 · 10-7
t- e-p+p-
1.2 · 10-7
t- m-p+p-
2.9 · 10-7
t- L p-
0.7  10-7
t- e+K-K-
1.5 · 10-7
t- e+K-p-
1.8 · 10-7
t- e+p-p-
2.0 · 10-7
t- m+K-K-
4.4 · 10-7
t- m+K-p-
2.2 · 10-7
t- m+p-p-
0.7 · 10-7

53. And there are more …
Such as lepton number violation,
Baryon number violation …
(un)fortunately no signal observed so far …

54. Tau physics at BESIII MC simulation in progress

55.  mass measurement BESI threshold scan Redone by KEDR
BESIII simulation indicates –
Taking data at one point is enough!
Precision of 0.1 MeV is possible
Use Compton backscattering to measure beam energy in high precision

56.  mass
PRD 53 (1993)20
12 points, Lum.: 5 pb1
M =  0.18  MeV
M / M = 1.7 10 – 4
 0.21  0.17
Ecm (GeV)
BESI results: stat. err. (0.18  0.21 )
is compatible with syst. (0.25  0.17)
PDG10: ±0.16 MeV

57. e-tagged final state One point With lum. Ltot Ltot (pb–1) Sm (MeV) 9
0.2488 18
0.1692 27
0.1402 36
0.1213 45
0.1065 54
0.0978 63
0.0904 72
0.0842
100
0.0678
1000
0.0214
10000
0.0068
Ecm= MeV
Smτ=0.1MeV
Ltot=54 pb–1

58. BES:PRD53(1996)20
Fix all other fit parameters except for M
KEDR:hep-ex/
eliminated

59. Compton backscattering technique,
BEPCII Storage Ring SR RF RF
Compton backscattering technique,
accuracy
up to
5  10– 5
Total systematic uncertainty on beam energy measurement can reach 90keV IP

60. Sketch of energy measurement system
5  10 – 5 e+ e–
North
Income laser
Backscattering laser

61. Relative error: Meas.: 4.610–5 Design: 510–5
137Cs: E= keV, E=4.54x10-6
60Co: E= keV, E=3.01x10-6
232Th: E= keV, E=4.98x10-6
Pu-C: E= keV, E=6.53x10-6
Relative error:
Meas.: 4.610–5
Design: 510–5

62. T ’ Cross Section Scan PDG2010: 3686.09 ± 0.04 MeV m=1750 keV
Accuracy: 2x10-5
Beam spread: 1.650.04 MeV
No efficiency correction
Cross section in arbitrary unit
Published in NIMA 659, 21 (2011)

63. T
 Mass measurement in 2012
Data at 4 energy points were taken, ~5 pb-1 at the  threshold
Expect stat. precision is 0.3 MeV, systematic error <0.1 MeV
More data expected in 2012 to reduce stat. precision to 0.1 MeV

64.  branching fraction at threshold
Static , mono-chromo , K,  system – is it easy to tag?
MC simulation underway

65. -pair at rest (1) ± →±+ (2) ± →K±+
Ecm=3.554 GeV (  pair threshold)
p/p=0.32%p0.37%
-pair at rest
(1) ± →±+
(2) ± →K±+
Achim Stahl ,Inter. J. of Modern Phys. A Vol.21,No.27(2006)5667
MC simulation
pπ = GeV
pK = GeV
pπ/mτ = 0.497
pK /mτ = 0.461
mτ= GeV
mπ= GeV
mK = GeV

66. W/o energy spread
Ecm = GeV
Energy Spread=1.3 MeV
W/ energy spread
To be addressed (via MC study):
Backgrounds
Tau
Non-tau
No part ID?
Fit momentum spectrum?
Precision versus luminosity
Other decay modes? SF?

67. Future experiments ILC can run at Z-pole
3 proposals in parallel in China
Circular Higgs factory (e+e-/) [L=1034/cm2/s, ￥30B]
Super Z factory [L=1035/cm2/s, ￥10B]
Super -charm factory [L=1035/cm2/s, ￥3B]
All are suitable for  physics study
Precision  measurement
CPV in  decays
Lepton number violation
Bright future ahead

68. Summary 结束！ The t is a clean laboratory to test the SM
Lepton Universality
QCD coupling constant and asymptotic freedom
Vacuum polarization from hadron loop
(g-2)
Fine structure constant  at MZ
Search for physics beyond standard model
Future facilities: TcF (and more), B-factories
Work to be done
Improve precisions for strange decay modes
Pin down systematic errors of BR & SF
Search for CPV
结束！

70.  Anomalous Magnetic Moment tau data versus e+e- data

71. Most recent progress Recent data Recent analyses
--0 from Belle PRD78, (2008)
e+e-+- from
KLOE PLB670, 285 (2009)
BaBar PRL103, (2009)
Recent analyses
Tau based: EPJC66, 127 (2009)
by M. Davier, A. Hoecker, G. Lopez Castro, B. Malaescu, X. H. Mo, G. Toledo Sanchez, P. Wang, C. Z. Yuan, and Z. Zhang
e+e- based: EPJC66, 1 (2009)
By M. Davier, A. Hoecker, B. Malaescu, C. Z. Yuan, and Z. Zhang

72. Hot topic in HEP 被选为 EPJC 封面

73. ”Dispersion relation“
The Muonic (g –2)
Contributions to the Standard Model (SM) Prediction:
Source
(a)
Reference
QED
~ 0.3  10–10
[Schwinger ’48 & others]
Hadrons
~ (15  4)  10–10
[Eidelman-Jegerlehner ’95 & others]
Z, W exchange
~ 0.4  10–10
[Czarnecki et al. ‘95 & others]
Dominant uncertainty from lowest order hadronic piece. Cannot be calculated from QCD (“first principles”) – but: we can use experiment (!)
The Situation 1995
had 
”Dispersion relation“ 
had  
...

74. Magnetic Anomaly    QED Prediction: QED
Computed up to 4th order [Kinoshita et al.] (5th order estimated) 
Schwinger 1948  
QED
Hadronic
Weak
SUSY...
... or other new physics ?

75. Contributions to the dispersion integral2
3 (+,) 4
> 4 (+KK)
(+J/, )
(+)
12 - 
< 1.8 GeV
ahad,LO 2
 2[ahad,LO] 2

76. Today‘s e+e-  data comparison

77. “Sum of 2958 diagrams should give a good
estimate of 10th-order muon g-2”
T. Kinoshita & M. Nio, PRD 73 (2006)
“Can we calculate the remaining 6122 diagrams?
Yes, we can do it in a few years”
Set V diagrams

78. Evaluating the Dispersion Integral
use data
Agreement bet-ween Data (BES) and pQCD (within correlated systematic errors)
use QCD
Better agreement between exclusive and inclusive (2) data than in analyses
use QCD

79. Contributions to ahad [in 10 –10] from the different energy domains
Modes
Energy [GeV]
e+e – 
Low s expansion
2m – 0.5
55.6 ± 0.8 ± 0.1rad
56.0 ± 1.6 ± 0.3SU(2)
 + – (+SND+CMD2)
0.5 – 1.8
449.0 ± 3.0 ± 0.9rad
464.0 ± 3.0 ± 2.3SU(2)
 + – 20
2m – 1.8
16.8 ± 1.3 ± 0.2rad
21.4 ± 1.3 ± 0.6SU(2)
2 + 2 – (+BaBar)
13.1 ± 0.4 ± 0.0rad
12.3 ± 1.0 ± 0.4SU(2)
 (782)
0.3 – 0.81
38.0 ± 1.0 ± 0.3rad –
 (1020)
1.0 – 1.055
35.7 ± 0.8 ± 0.2rad
Other excl. (+BaBar)
24.3 ± 1.3 ± 0.2rad
J /,  (2S)
3.08 – 3.11
7.4 ± 0.4 ± 0.0rad
R [QCD]
1.8 – 3.7
33.9 ± 0.5theo
R [data]
3.7 – 5.0
7.2 ± 0.3 ± 0.0rad
5.0 – 
9.9 ± 0.2theo
Sum (w/o KLOE)
2m – 
690.8 ± 3.9 ± 1.9rad ± 0.7QCD
710.1 ± 5.0 ± 0.7rad ± 2.8SU(2)

80. Borrowed from Michel Davier