1. Precision Measurement of R Value in the continuum Region
Zhen Gao①, Bingxin Zhang②
①:University of Science and Technology of China
②Institute of High Energy Physics
On behalf of the R working group

2. Outline Introduction Data sets Event selection criteria
Background analysis
Determination of hadronic events and R-value
Summary & next to do

3. Introduction(1) 𝑅= 1 𝜎 𝜇𝜇 0 ∙ 𝑁 ℎ𝑎𝑑 − 𝑁 𝑏𝑘𝑔 𝐿∙ 𝜀 ℎ𝑎𝑑 ∙ 𝜀 𝑡𝑟𝑖𝑔 ∙(1+𝛿)
At lowest order,
𝑅≡ 𝜎( 𝑒 + 𝑒 − →ℎ𝑎𝑑𝑟𝑜𝑛𝑠) 𝜎( 𝑒 + 𝑒 − → 𝜇 + 𝜇 − )
= 𝑞 𝜎( 𝑒 + 𝑒 − →𝑞 𝑞 ) 𝜎( 𝑒 + 𝑒 − → 𝜇 + 𝜇 − )
In experiment,
𝑅= 1 𝜎 𝜇𝜇 0 ∙ 𝑁 ℎ𝑎𝑑 − 𝑁 𝑏𝑘𝑔 𝐿∙ 𝜀 ℎ𝑎𝑑 ∙ 𝜀 𝑡𝑟𝑖𝑔 ∙(1+𝛿)
𝜎 𝜇𝜇 0 : born-level cross section for 𝑒 + 𝑒 − → 𝜇 + 𝜇 −
𝑁 ℎ𝑎𝑑 : number of hadronic events
𝑁 𝑏𝑘𝑔 : number of background hadronic events
𝐿: integrated luminosity
𝜀 ℎ𝑎𝑑 : detection efficiency for hadrons
𝜀 𝑡𝑟𝑖𝑔 :𝑡𝑟𝑖𝑔𝑔𝑒𝑟 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦
1+𝛿: radiative correction factor

4. Introduction(2)
One of the most fundamental quantities in particle physics that directly reflect the flavor and the color of the quarks
A necessary input for :
𝛼 𝑠 −𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝑟𝑢𝑛𝑛𝑖𝑛𝑔 𝑐𝑜𝑢𝑝𝑙𝑖𝑛𝑔 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝑎 𝜇 −anomalous magnetic moment of the muon

5. Introduction(3)
BESII measured the R-value between 2 and 5GeV , with an average precision of 6.6%.
BESIII is goal to low the precision of R both in the charmonium region( 3% ) and continuum region.

6. Potential backgrounds
QED process :
Bhabha , 𝛾 𝛾𝛾 , 𝛾 𝜇 + 𝜇 − , 𝛾 𝜏 + 𝜏 −
𝑒 + 𝑒 − → 𝑒 + 𝑒 − + hadrons
Cosmic ray
Beam-associated backgrounds :
beam-wall; beam-gas; Touschek effect

7. Data sets
Collision data at : ,2.4,2.8,3.05,3.06,3.08,3.4,3.5,3.5424,3.5538,3.5611,3.6002,3.65,3.671GeV 14 energy points
BOSS version: BOSS664p01
MC sample: (2.2324~3.671 GeV 13 energy points)
e+e-→hadrons(LUNDARLW+exclusive )
e+e-→e+e- (babayaga)
e+e-→μ+μ- (babayaga)
e+e-→τ+τ- (KKMC)
e+e-→γγ (babayaga)
e+e-→e+ e- X (two photon process)

8. MC Generator
Comparison of summed cross section for exclusive decays to the measured ones.
Black : measured cross section
Red : summed exclusive cross section

9. Event Selection(I) Veto Bhabha and 𝑒 + 𝑒 − →𝛾𝛾 (Only EMC)
Two showers with maximum deposited energy
𝜃 1 + 𝜃 2 −180 <10° & 𝐸 > 0.65∙ 𝐸 𝑏𝑒𝑎𝑚
Good hadronic tracks (track level)
|Vr|<1.0 cm, |cosθ|<0.93, no Vz cut.
Momentum < 1.1· 𝐸 𝑏𝑒𝑎𝑚
𝑑𝐸 𝑑𝑥 𝑚𝑒𝑎. − 𝑑𝐸 𝑑𝑥 𝑝𝑟𝑜𝑡𝑜𝑛 𝜎 𝑝𝑟𝑜𝑡𝑜𝑛 <10
Veto large momentum electrons, if E/p > 0.8, p<0.65· 𝐸 𝑏𝑒𝑎𝑚
Veto gamma conversion, if M( 𝑒 + 𝑒 − )<0.1 and angle( 𝑒 + 𝑒 − )<15 °

10. Event Selection(II) Hadronic event candidates
𝑁 𝐺𝑜𝑜𝑑 ≥2 ;𝑉𝑖𝑠𝑖𝑏𝑙𝑒 𝑒𝑛𝑒𝑟𝑔𝑦(𝐸𝐸𝑀𝐶) >0.4∙ 𝐸 𝑏𝑒𝑎𝑚
If 𝑁 𝐺𝑜𝑜𝑑 = 2 :
Veto: 𝜃 1 + 𝜃 2 −180 <15°& 𝜑 1 − 𝜑 2 −180 <10°
Number of isolated photon ≥ 2
If 𝑁 𝐺𝑜𝑜𝑑 ≥3 :
Veto: Angle of two largest energetic tracks,
𝜃 1𝑠𝑡 + 𝜃 2𝑛𝑑 −180 <15°& 𝜑 1𝑠𝑡 − 𝜑 2𝑛𝑑 −180 <10°

11. Veto Bhabha and 𝒆 + 𝒆 − →𝜸𝜸
@ 2.23GeV
Veto :
Signal survive ratio : > 99%
Background veto ratio : ~70%

12. Vr
@ 2.23GeV
|Vr| < 1cm for each track is applied!

13. dE/dx
@2.8 GeV
@3.4 GeV
cut： 𝑑𝐸/𝑑𝑥(𝑚𝑒𝑎𝑠.)−𝑑𝐸/𝑑𝑥(𝑝𝑟𝑜𝑡𝑜𝑛) 𝞼(𝑝𝑟𝑜𝑡𝑜𝑛) <10

14. Veto large momentum e±

15. Veto large momentum e±

16. Event selection : Veto: E/p > 0.8 && p > 0.65Ebeam
After adding this cut :

17. Veto gam-conversion @2.23 GeV @3.08 GeV e± : E/p > 0.8
Gamma conversion : if M( 𝑒 + 𝑒 − )<0.1 and angle( 𝑒 + 𝑒 − )<15 °

18. Visible energy 𝐶𝑢𝑡 : 𝑉𝑖𝑠𝑖𝑏𝑙𝑒 𝑒𝑛𝑒𝑟𝑔𝑦 >0.4∙ 𝐸 𝑏𝑒𝑎𝑚 @ 3.4GeV
Visible energy : all energy deposition in EMC both for charged tracks and neutral ones.

19. Isolated photon @ 2.23GeV @ 3.08GeV
Only for NGood = 2, the minimum isolated photon requirement is added
Isolated photon : Angletrack > 20 ° , Eγ > 0.1GeV and 0 < T < 15
Number of isolated photon >= 2
@ 2.23GeV
@ 3.08GeV

20. Data&MC comparison nGood ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06
① : ② : ③ : ④ : ⑤ : 3.06
⑥ : ⑦ : ⑧ : ⑨ : ⑩ :
⑪ : ⑫ : ⑬ : ⑭ : 3.671

21. Data&MC comparison cos𝞱 ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06
① : ② : ③ : ④ : ⑤ : 3.06
⑥ : ⑦ : ⑧ : ⑨ : ⑩ :
⑪ : ⑫ : ⑬ : ⑭ : 3.671

22. Data&MC comparison nGam ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06
① : ② : ③ : ④ : ⑤ : 3.06
⑥ : ⑦ : ⑧ : ⑨ : ⑩ :
⑪ : ⑫ : ⑬ : ⑭ : 3.671

23. Data&MC comparison Phi ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06
① : ② : ③ : ④ : ⑤ : 3.06
⑥ : ⑦ : ⑧ : ⑨ : ⑩ :
⑪ : ⑫ : ⑬ : ⑭ : 3.671

24. Data&MC comparison M(γγ) ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06
① : ② : ③ : ④ : ⑤ : 3.06
⑥ : ⑦ : ⑧ : ⑨ : ⑩ :
⑪ : ⑫ : ⑬ : ⑭ : 3.671

25. Data&MC comparison M(pipi)
① : ② : ③ : ④ : ⑤ : 3.06
⑥ : ⑦ : ⑧ : ⑨ : ⑩ :
⑪ : ⑫ : ⑬ : ⑭ : 3.671

26. Data&MC comparison M(KK) ① : 2.2324 ② : 2.4 ③ : 2.8 ④ : 3.05 ⑤ : 3.06
① : ② : ③ : ④ : ⑤ : 3.06
⑥ : ⑦ : ⑧ : ⑨ : ⑩ :
⑪ : ⑫ : ⑬ : ⑭ : 3.671

27. Fitting the average Vz @ 2.23GeV 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑉𝑧= 1 𝑁 𝐺𝑜𝑜𝑑 𝑉𝑧(𝑖) 𝑁 𝐺𝑜𝑜𝑑
Black dots : data
Green : MC shape
Red : fitting background
Blue : total fitting result
@ 2.23GeV
Signal : MC shape convoluted Gaussian
Bkg : 3rd polynominal
N(signal) : ± 312
N(bkg) : 2364 ± 95
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑉𝑧= 1 𝑁 𝐺𝑜𝑜𝑑 𝑉𝑧(𝑖) 𝑁 𝐺𝑜𝑜𝑑

28. Cross check: Fitting result @ 2.6444 GeV
Black dots : data
Green : MC shape
Red : fitting background
Yellow : Separated beam data
Blue : total fitting result
ChiSquare = 5.48
Signal : MC shape
Bkg : 2nd polynominal
N(signal) : ± 566
N(bkg) : ± 248

29. QED background estimation
@ 2.23GeV
Bhabha
digam
dimu
eeX
Total
500000
nGood == 2
145
289
527
1431
nGood >= 3
185 41
265
332
Remain
330
792
1763
Survive ratio
0.066%
0.15%
0.35%
Total background after scaled : 3.08 %
@ 3.08GeV
Bhabha
digam
dimu
eeX
Total
500000
nGood == 2
106
212
374
1551
nGood >= 3
167 28
190
549
Remain
273
240
564
2100
Survive ratio
0.054%
0.048%
0.11%
0.42%
Total background after scaled : 2.49 %

30. QED background estimation
Scaled Number!

31. Efficiency

32. Difference between two requirements
① . Veto large momentum electrons, if E/p > 0.8, p<0.65· 𝐸 𝑏𝑒𝑎𝑚
② . Veto all electrons, E/p > 0.8
Data①
Data②
MC①
MC②
Diff
2.23
87675
75014
71.58%
62.36%
-1.79%
2.4
101614
87593
73.58%
65.06%
-2.51%
3.4
32667
29030
82.67%
76.36%
-3.79%

33. Costheta distribution using cut2
Costheta of all the hadronic tracks
② . Veto all electrons, E/p > 0.8

34. Summary & next to do
Preliminary result of the R-value using 14 energy points in the continuum region;
selection criteria needs to be studied further;
More work on MC tuning needs to be done;
Systematic uncertainty.

35. Back up

36. Fitting result @2.2324, 2.4 GeV N(signal) : 90463 ± 312
N(bkg) : 2364 ± 95
N(signal) : ± 336
N(bkg) : 3348 ± 108