1. SPIN @ Prague 2007 Collins and Sivers asymmetries
Girisan Venugopal
University of Bonn
(on behalf of the COMPASS collaboration)
Collins and Sivers asymmetries
on the deutron target

2. COMPASS experiment @ CERN
LHC
SPS N
fixed target experiment at the CERN SPS
240 physicists from 28 Institutes, 11 Countries
11/12/2018
Girisan Venugopal University of Bonn

3. Girisan Venugopal University of Bonn
COMPASS spectrometer
high energy beam
large angular acceptance
broad kinematical range
longitudinally polarised muon beam
longitudinally or transversely polarised target
MuonWall
SM2
E/HCAL
E/HCAL
Straws DC
MWPC
SciFi
Silicon
Micromegas
GEMs
SM1
Polarised
Target
Muon Wall
RICH
m beam
11/12/2018
Girisan Venugopal University of Bonn

4. The COMPASS polarized target
11/12/2018
Girisan Venugopal University of Bonn

5. Girisan Venugopal University of Bonn
Data taking
transversely polarized deuteron target (6LiD) ~ 20% of the running time
2002
11 days
2003
9 days
2004
14 days
trigger (large x, Q2)
DAQ, on line filter
Reconstructed DIS events
(106)
11/12/2018
Girisan Venugopal University of Bonn

6. Girisan Venugopal University of Bonn
Event selection
DIS cuts:
Q2 > 1
0.1 < y < 0.9
W > 5 GeV/c
Hadron selection:
z > 0.2
pt > 0.1 GeV/c
Q2 > 1
Statistics :
8.5 * 106 positive hadrons
7.0 * 106 negative hadrons
11/12/2018
Girisan Venugopal University of Bonn

7. Hadron identification
Cherenkov thresholds: π ~ 3 GeV/c
K ~ 9 GeV/c
p ~ 17 GeV/c
5 m
6 m
3 m
C4F10
photon
detectors
2 σ π/K separation at 43 GeV/c
Statistics
positive
negative 
3.4 * 106
2.8 * 106 K
0.7 * 106
0.4 * 106
11/12/2018
Girisan Venugopal University of Bonn

8. Transverse Spin Physics
At leading order, the inner structure of the nucleon can be described with three Parton
Distribution Function (PDF)
q(x) Momentum distribution
Well known – Unpolarized DIS
Δq(x) Helicity distribution
known – Polarized DIS
ΔTq(x) Transversity distribution
Largely unknown
11/12/2018
Girisan Venugopal University of Bonn

9. Measuring Transversity
Inclusive DIS
11/12/2018
Girisan Venugopal University of Bonn

10. Measuring Transversity
Drell-Yan process
SIDIS FF
DTq(x)
Measured in COMPASS
11/12/2018
Girisan Venugopal University of Bonn

11. Measuring Transversity
Properties of Transversity distribution fuction
is chiral-odd: decouples from inclusive DIS
Observable effects are given only by the product of Tq (x) with an other chiral-odd function
Can be measured in SIDIS on a transversely polarised target
via “quark polarimetry”
“Collins” asymmetry
“Collins” Fragmentation Function
two-hadron asymmetry ( presented by Frank Massmann)
“Interference” Fragmentation Function
Λ polarisation
Fragmentation Function of q↑Λ
11/12/2018
Girisan Venugopal University of Bonn

12. Collins effect in SIDIS
The “quark polarimetry” relies on the Collins effect
The fragmentation function of a transversely polarised quark has a spin dependent part
“Collins” FF:
11/12/2018
Girisan Venugopal University of Bonn

13. Collins effect in SIDIS
Fragmentation of transversely polarized quarks
to unpolarized hadrons
The “Collins angle” is
C = h + S - p
h,,s azimuthal angles of the hadron,
transverse spin of the initial quark
Transversity distribution
function
Collins fragmentation
Function (is being
measured in Belle Expt)
11/12/2018
Girisan Venugopal University of Bonn

14. Girisan Venugopal University of Bonn
Collins asymmetries π K
only statistical errors shown (~1%), systematic errors considerably smaller
small asymmetries compatible with 0 for both +π, K and -π, K (ΔTu, ΔTd cancel in deuterium )
11/12/2018
Girisan Venugopal University of Bonn

15. Collins: Interpretation
Global fit performed by M. Anselmino et.al
2006: final results from data NPB765 (2007)31
Talk given by A. Prokudin (International Workshop on Structure and Spectroscopy, Freiburg)
11/12/2018
Girisan Venugopal University of Bonn

16. Girisan Venugopal University of Bonn
Sivers effect
The intrinsic transverse momentum of unpolarized quarks in a transversely
polarized nucleon can introduce a different azimuthal asymmetry
It is related to the parton orbital angular momentum in a transversely polarized nucleon
The “Sivers angle” is
S = h - S
h,,s azimuthal angles of the hadron,
transverse spin of the initial quark
Sivers
Function
11/12/2018
Girisan Venugopal University of Bonn

17. Girisan Venugopal University of Bonn
Sivers asymmetries π K
only statistical errors shown (~1%), systematic errors considerably smaller
small asymmetries compatible with 0 for both +π, K and -π, K
11/12/2018
Girisan Venugopal University of Bonn

18. Sivers: Interpretation
Global fit performed by M. Anselmino et.al
2006: final results from data NPB765 (2007)31
Talk given by A. Prokudin (International Workshop on Structure and Spectroscopy, Freiburg)
11/12/2018
Girisan Venugopal University of Bonn

19. Sivers: Interpretation
The measured asymmetry on deuteron compatible with zero has been interpreted as
Evidence for the Absence of Gluon Orbital Angular Momentum in the Nucleon
S.J. Brodsky and S. Gardner, PLB643 (2006) 22
11/12/2018
Girisan Venugopal University of Bonn

20. Girisan Venugopal University of Bonn
Summary & Outlook
The measured asymmetries on a Deuteron target are very small
and compatible with zero
Theoretical work aimed at a first extraction of transversity and
Sivers distribution function is ongoing by different groups
COMPASS measures this year with proton target
11/12/2018
Girisan Venugopal University of Bonn

21. Girisan Venugopal University of Bonn
Thank you all !!!
11/12/2018
Girisan Venugopal University of Bonn

22. Collins : Interpretation
A.V. Efremov, K. Goeke and P. Schweitzer, PRD73 (2006) (ΔTq from Chiral Quark Soliton Model )
COMPASS
11/12/2018
Girisan Venugopal University of Bonn

23. Sivers: Interpretation
M. Anselmino et al., PRD72 (2005)
13 parameter fit to HERMES and COMPASS 2002 data to extract the Sivers functions
comparison with COMPASS data ok
COMPASS
11/12/2018
Girisan Venugopal University of Bonn