1. Azimuthal asymmetries of di-hadron in e+e- annihilation at BESIII
Guan Yinghui(管颖慧)1, Li Haibo(李海波)1, Lu Xiao-Rui(吕晓睿)2
1. IHEP 2. UCAS

2. Outline
Introduction
Methods to probe Collins effect on experiments(e+e- annihilation)
Implement the azimuthal asymmetries analysis at BESIII
Useful information about TMD evolution
Difficulties:
absence of high thrust two-jet events;
backgrounds
MC Test
Summary and discussion
PRD

3. Introduction(1)—Single spin asymmetries
PRD 78,
Introduction(1)—Single spin asymmetries
H: describes the relation between the
transverse spin of the fragmenting quark
and the azimuthal distribution of
final-state hadrons around the
quark momentum
z: fractional energy of hadron
Ph: transverse momentum of hadron
Single Spin Asymmetries

4. Introduction(2) --two methods to probe DCA
PRD 78,
Introduction(2) --two methods to probe DCA
Double Collins Asymmetries(DCA)
Method 12
Method 0

5. Introduction(2)– two methods to probe DCA
Define the ratio: R
Method0,
Method12
The distribution expected with azimuthal asymmetries
Backgrounds need to be understood well   2

6. Introduction(3)– Double Ratio
Unlike:
Like:
Charged:
Define Double ratio
To cancel asymmetries produced by detector acceptance and radiative effects

7. Introduction(4)--event shape and thrust axis
backgrounds
Thrust value is used to suppress backgrounds at Belle/BaBar
The thrust axis is used as a approximation of the q-qbar axis
The thrust axis is the key issue:
to separate h1 and h2
is the reference axis in Method12

8. Data sample Sample: Event Selection MC@4.2GeV
event generator: KKMC/Pythia
Event Selection
Good charged tracks: Ncharged>=3
Identify p
Evisible>2.5GeV
Thrust>0.8 (All good charged and neutral tracks are used to reconstruct the thrust axis)
z>0.2

9. Thrust value—comparison with BABAR
BESIII
@4.2GeV
BABAR
@10.6GeV
arXiv:
Select high thrust two-jet events

10. Thrust axis--comparison with BABAR
BESIII
@4.2GeV
BABAR
@10.6GeV
The thrust axis is not a good approximation of the q-qbar axis
at low energy

11. MC Test --Method12 qqbar sample Black line: MC sample
MC Truth
Using Truth axis
Using Thrust axis
Black line: MC sample
Red line: weighted MC sample
with asymmetry=10%

12. MC Test--Method0 qqbar sample Black line: MC sample
MC Truth
Using Truth axis
Using Thrust axis
Black line: MC sample
Red line: weighted MC sample
with asymmetry=10%

13. MC Test –Double Ratio To cancel detector effects MC Truth MC Truth
accepted by detector
Double Ratio

14. MC Test — Double Ratio (Method0)
MC Truth
Double Ratio
Miss-combinations(h1,h1) may contribute

15. MC Test– Method0’ High z means large angle between h1 and h2
They probably came from different quark
cut: z1>0.5 && z2>0.5

16. MC Test –Double Ratio(Method0’)
require z1>0.5&&z2>0.5
Without assigning them to hemisphere

17. 谢谢！ Summary/Discussion Lesson from Belle and BABAR At BESIII Ideas: …
Suppress backgrounds
Double ratio to cancel instrumental effects
At BESIII
Difficulties: isotropic event topology; backgrounds
Ideas:
Using Method0
Selecting sufficiently high z 
Other energy point (~3.6GeV)
Others:
Does factorization hold at BEPCII energy region？ …
谢谢！

18. BACKUP

19. Multiplicity

20. Thrust data vs MC

21. Fractional energy z
High z

22. How many hadron-pairs we have?
Nominal cuts
Further cuts with z1>0.5&&z2>0.5
~10k p-pair survive

23. Results from BABAR
the asymmetry appears to grow with z

24. Z vs

25. Thrust vs

26. Thrust axis at Belle

27. MC Test-- Double Ratio(Method12)
Using Truth axis
Using Thrust axis

28. Method0’
cut: z1>0.2 && z2>0.2
cut: z1>0.5 && z2>0.5

29. Double Ratio - -Method0  Method0’
cut: z1>0.2 && z2>0.2
belong1*belong2<0
cut: z1>0.5 && z2>0.5

30. Double ratio- - backgrounds(M’)
hadrons DD
ISR
ditau
gXYZ
DDpi

31. MC Test—background level
cut: z1>0.2 && z2>0.2
cut: z1>0.5 && z2>0.5

32. Other variables could be used to suppress backgrounds
Also related to event shape
Not good 4GeV
R2=H2/H0

33. R value