1. Transverse Spin Effects @ COMPASS
Andrea Bressan
(University and I.N.F.N. Trieste)
on behalf of the COMPASS Collaboration
18 April 2019
Andrea Bressan University -INFN Trieste

2. THE NUCLEON STRUCTURE three distribution functions
are necessary to describe the spin structure of the nucleon at LO:
q(x) : number density or unpolarized distribution
Dq(x) = q- q: longitudinal polarization or helicity distribution
DTq(x) = q- q: transverse polarization or transversity distribution
First data from ’70 (SLAC)
Starting from the ’80 (E80, EMC)
Still unmeasured
Theoretically transverse spin effects have been neglected for long time (re-raised in the ’90), not measurable in inclusive DIS
18 April 2019
Andrea Bressan University -INFN Trieste

3. Andrea Bressan University -INFN Trieste
Transversity
In the last ten years:
Great development in the theory of transversity;
Remarkable role of ΔTq(x), notably complementary to Δq(x).
In the last couple of years:
Role of the kT structure functions clarified (Cahn and Sivers effects, …).
Tensor charge (’91 – ’92):
in analogy with:
Soffer inequality (95):
Leader sum rule (04):
Key features of transversity:
Probes relativistic nature of quarks
No gluon analog for spin-1/2 nucleon
Different Q2 evolution and sum rule than Δq(x)
Sensitive to valence quark polarization
18 April 2019
Andrea Bressan University -INFN Trieste

4. Andrea Bressan University -INFN Trieste
Measuring ΔTq(x)
Tq(x) is chiral odd: decouples from leading twist DIS because helicity of quark must flip. It doesn’t play any role in inclusive DIS;
How to access it?
18 April 2019
Andrea Bressan University -INFN Trieste

5. Measuring ΔTq(x) How to access it:
Chiral-odd: requires another chiral-odd partner
ppl+l-X
epe’hX
Direct
Measurement
Convolution with fragmentation functions
(measurements of ff ongoing at BELLE)
How to access it:
SIDIS (e.g. COMPASS and HERMES )
Hard scattering (e.g. RHIC)
Drell-Yan
Single Spin Asym (e.g. p↑p→πX)
Hard scattering (e.g. GSI)
18 April 2019
Andrea Bressan University -INFN Trieste

6. Andrea Bressan University -INFN Trieste
More than 220 physicists from 30 Institutes and 12 Countries
18 April 2019
Andrea Bressan University -INFN Trieste

7. Andrea Bressan University -INFN Trieste
LHC
COMPASS
SPS
18 April 2019
Andrea Bressan University -INFN Trieste

8. Andrea Bressan University -INFN Trieste
18 April 2019
Andrea Bressan University -INFN Trieste

9. Andrea Bressan University -INFN Trieste
Beam
18 April 2019
Andrea Bressan University -INFN Trieste

10. Andrea Bressan University -INFN Trieste
Target
superconductive
Solenoid (2.5 T)
3He – 4He Dilution
refrigerator (T~50mK)
Dipole (0.5 T)
For transversity: 1 2
reversed once a week
Polarization: %
Dilution factor: 0.38
two 60 cm long Target-Containers
with opposite polarization
During data taking for transversity
dipole field always 
Relaxing time > 2000 hrs
18 April 2019
Andrea Bressan University -INFN Trieste

11. Andrea Bressan University -INFN Trieste
COMPASS acceptance
Q21 (GeVc)2
Excellent for non-perturbative & perturbative physics
- small xBj & very small Q2  Q2 > 100 (GeV/c)2
18 April 2019
Andrea Bressan University -INFN Trieste

12. used events in the analysis
COMPASS data sample
In so far ( 3 years )
only DIS off 6LiD
20% of the time with transverse target polarization
only runs with transverse polarized target being analyzed
runs
good runs
used events in the analysis
2002
475
453
(100 SPS spills)
1.6 · 106
2003
479
429
~4•(2002)
2004
496
470
(200 SPS spills)
~2•(2003)
RICH PID
E-Calorimetry
18 April 2019
Andrea Bressan University -INFN Trieste

13. SIDIS off trans. polarized target
3 possible quark polarimeters are being explored in COMPASS:
Collins effects of (leading) h±
Part A: first analisys finalized on 2002 data
(accepted for publication on PRL)
Azimuthal dependence of the plane containing hadrons pairs
Part B: first test and preliminary results on data
Measurement of transverse polarization of spin ½ baryons
(e.g.  hyperon)
Analysis ongoing, no results yet
18 April 2019
Andrea Bressan University -INFN Trieste

14. Andrea Bressan University -INFN Trieste
SHA
COLLINS and SIVERS
18 April 2019
Andrea Bressan University -INFN Trieste

15. SIDIS off transversely polarized target
Collins effect predicts an azimuthal asymmetry in the quark fragmentation
The quark prefers to fragment in one direction;
Look at leading hadron (it comes from the struck quark);
The larger is “z” (fraction of the available energy carried by the hadron), the stronger is the signal ;
But an azimuthal asymmetry can also come from the un-polarised quarks; namely from an azimuthal modulation of quark transverse momentum for a transversely polarized nucleon (Sivers effect) q h x z y z q x y kT
18 April 2019
Andrea Bressan University -INFN Trieste

16. Collins and Sivers asym’s
S = azimuthal angle of spin vector of fragmenting quark (before scattering)
S’ = azimuthal angle of spin vector of
fragmenting quark (after scattering)
h = azimuthal angle of hadron
Breit frame:
ref. system with z axis defined by
g direction and x-z plane defined by
the scattering plane
18 April 2019
Andrea Bressan University -INFN Trieste

17. Andrea Bressan University -INFN Trieste
Collins Effect
Calculated as function of x, z and pt and for “Leading Hadrons” and for “All Hadrons”
18 April 2019
Andrea Bressan University -INFN Trieste

18. Andrea Bressan University -INFN Trieste
Sivers Effect
The quark intrinsic moment cannot be neglected  an azimuthal asymmetry not connected with PDF is introduced
But
Azimuthal angular dependence different  sin(siv )
18 April 2019
Andrea Bressan University -INFN Trieste

19. Andrea Bressan University -INFN Trieste
Event Selection
DIS cuts:
Q2>1 (GeV/c)2
0.1<y<0.9
W>5 GeV/c2
Hadrons
Track Length<10 X0
Energy Deposit in HCALs > 5(8) GeV
Leading Hadron
0.25<z<1
Pt>0.1 GeV/c
z-missing cut
All Hadrons
z>0.2
18 April 2019
Andrea Bressan University -INFN Trieste

20. Kinematical distributionsp
Q2=2.4 (GeV/c)2
Y=0.33
W=9.4 GeV/c2
pT0.51 GeV/c
z 0.45
xBj0.035
z 0.45
xBj0.035
18 April 2019
Andrea Bressan University -INFN Trieste

21. Collins and Sivers effects
Only statistical errors shown
CERN-PH-EP/
hep-ex/
PRL
Systematic errors are smaller than the quoted statistical errors.
18 April 2019
Andrea Bressan University -INFN Trieste

22. Collins and Sivers effects
Only statistical errors shown
CERN-PH-EP/
hep-ex/
PRL
Systematic errors are smaller than the quoted statistical errors.
18 April 2019
Andrea Bressan University -INFN Trieste

23. COMPASS vs. HERMES Positive hadrons
Corrected for π phase difference in the definition of ΦC between HERMES and COMPASS
Deuteron(COMPASS) vs. Proton(HERMES)
With present errors the 2 data sets are compatible
COMPASS higher energy  lower x (but with present statistics large errors at high x)
In Hermes:
Negative Collins asymmetries;
Positive Sivers asymmetries.
In COMPASS:
No sizeable effect…
Possible cancellations in iso-scalar target?
HERMES data points from:
A. Airapetian et al, Phys. Rev. Lett. 94 (2005) [DC53] (hep-ex/ )
18 April 2019
Andrea Bressan University -INFN Trieste

24. COMPASS vs. HERMES Negative hadrons
Corrected for π phase difference in the definition of ΦC between HERMES and COMPASS
Deuteron(COMPASS) vs. Proton(HERMES)
With present errors the 2 data sets are compatible
COMPASS higher energy  lower x (but with present statistics large errors at high x)
In Hermes:
Large Positive Collins asymmetries;
No Sivers effect.
In COMPASS:
No sizeable effect…
Possible cancellations in iso-scalar target?
HERMES data points from:
A. Airapetian et al, Phys. Rev. Lett. 94 (2005) [DC53] (hep-ex/ )
18 April 2019
Andrea Bressan University -INFN Trieste

25. Expected accuracy for transversity
Statistical accuracy increased in years 2003/4:
trigger system upgraded;
DAQ upgraded;
2004 longer run.
2003 data analyzed (systematics evaluation in progress)
2004 data production over (analysis in progress)
AColl statistical errrors:
Positive hadrons
(to the left):
Green;
2002 – 2004 Red.
Negative hadrons
(to the right):
18 April 2019
Andrea Bressan University -INFN Trieste

26. Andrea Bressan University -INFN Trieste
2 HADRON
ASYMMETRIES
18 April 2019
Andrea Bressan University -INFN Trieste

27. Andrea Bressan University -INFN Trieste
Two Hadron System
First Measurement following Jaffe et al. (Phys. Rev. Lett. 80, 1166 (1998) )
Using two hadrons (e.g. p+p-) the trasversity distribution can be accessed in conjunction with a interference fragmentation function 
fS’ = azimuthal angle of spin vector of fragmenting quark with:
fS’ = p - fS (spin flip)
18 April 2019
Andrea Bressan University -INFN Trieste

28. Andrea Bressan University -INFN Trieste
Definitions
18 April 2019
Andrea Bressan University -INFN Trieste

29. Interference Fragmentation Function H
One model !
Another model !
AMh
sinf
R. L. Jaffe, X. Jin and J. Tang,
Phys. Rev. Lett. 80, 1166 (1998)
H∢ (z,M2p+p-)∼sind0 sind1 sin(d0-d1)H∢(z,M2p+p-) ^
Radici, Jakob, Bianconi, PRD 65,
18 April 2019
Andrea Bressan University -INFN Trieste

30. Andrea Bressan University -INFN Trieste
Event Selection - I
DIS events and hadron identification as in the 1 hadron analysis
No p/K/p separation by using RICH information implemented yet
<N>=1.9
<z>=0.17
18 April 2019
Andrea Bressan University -INFN Trieste

31. Andrea Bressan University -INFN Trieste
Hadron Pair Selection
All combinations of positive (h1) and negative (h2) hadrons fulfilling:
z1>0.1 + z2>0.1
xF1>0.1 + xF2>0.1
zh = z1 + z2 < 0.9 (to cut exclusive h production)
1.020.22 combinations/DISevent (2.8106 events in only)
18 April 2019
Andrea Bressan University -INFN Trieste

32. Andrea Bressan University -INFN Trieste
2-Hadron Asymmetries
18 April 2019
Andrea Bressan University -INFN Trieste

33. Andrea Bressan University -INFN Trieste
ONE WORD ON
NEAR FUTURE
18 April 2019
Andrea Bressan University -INFN Trieste

34. Andrea Bressan University -INFN Trieste
2006 COMPASS proton run
Projections for 30 days of data taking with NH3 target
(theoretical predictions by A. Efremov et al.(*) superimposed):
Taking into account:
Variation of statistical errors:
taking into account the variation of:
Positive hadrons
Negative hadrons
(*) Eur.Phys.J.C32: ,2003
18 April 2019
Andrea Bressan University -INFN Trieste

35. Andrea Bressan University -INFN Trieste
Outlook
COMPASS has a multi-purpose spectrometer which will take data at least until 2010;
Collins and Sivers SSA calculated from 2002 data
(first measurements on a deuteron target) accepted for publication
(PRL);
First results of the analysis concerning two opposite charge hadrons asymmetries were shown ( ).
In both cases the asymmetries are small and compatible with zero
The total collected statistics allows to increase the presented accuracy
on SSA (2002 data only) by a factor of 3 and 2h ( ) by 1.4
Complementary data (of comparable statistics) will be collected in 2006 on a transversely polarized proton target (NH3).
18 April 2019
Andrea Bressan University -INFN Trieste

36. Andrea Bressan University -INFN Trieste
THANK YOU
18 April 2019
Andrea Bressan University -INFN Trieste

37. Andrea Bressan University -INFN Trieste
Save slides
18 April 2019
Andrea Bressan University -INFN Trieste

38. 2-Hadron Asymmetry vs xBj and zAx
sinf
H (z) ∢
Radici, Jakob, Bianconi, PRD 65,
18 April 2019
Andrea Bressan University -INFN Trieste

39. Theoretical work on the subject
“The role of Cahn and Sivers effects in DIS”
M. Anselmino et al. (hep-ph/ )
Phenomelogical model whose parameters are constrained by HERMES proton measurements;
COMPASS preliminary results for Sivers effect (shown at SPIN04) are in agreement with the model.
Interpretation of Collins effect in progress.
18 April 2019
Andrea Bressan University -INFN Trieste

40. Andrea Bressan University -INFN Trieste
Flavor decomposition
18 April 2019
Andrea Bressan University -INFN Trieste

41. Andrea Bressan University -INFN Trieste
Estimators
Weaker assumptions on acceptance effects
less sensitive to distorsions set by Cahn effects
Reconstruction time independent in the solid angle
18 April 2019
Andrea Bressan University -INFN Trieste

42. Evaluation of systematics
Stability of the asymmetries:
i) in time;
ii) in two halves of the target cells;
iii) according to the hadron momentum.
Evaluated 3 different estimators and check the compatibility with the standard one.
Look at the paper:
“First Measurement of the Transverse Spin Asymmetries of
the Deuteron in Semi-Inclusive Deep Inelastic Scattering”
(The COMPASS Collaboration)
CERN-PH-EP/
hep-ex/
(accepted by PRL)
18 April 2019
Andrea Bressan University -INFN Trieste

43. Extraction of Collins (Sivers) asym’s
Evaluating the population as a function of Φ, independently in both target cells;
Extracting 2 asymmetries and calculating the weighted mean;
j = C, S, (Φj calculated always with spin = ↑) and F is the muon flux, n the number of target particles, σ the spin averaged cross-section, and aj the product of angular acceptance and efficiency of the spectrometer;
18 April 2019
Andrea Bressan University -INFN Trieste

44. The COMPASS Experiment in 2002
beam
18 April 2019
Andrea Bressan University -INFN Trieste

45. Andrea Bressan University -INFN Trieste
18 April 2019
Andrea Bressan University -INFN Trieste

46. Andrea Bressan University -INFN Trieste
Detectors
Straw mW
Inside RICH1
Vessel
GEM
Drift
SciFi
18 April 2019
Andrea Bressan University -INFN Trieste

47. Andrea Bressan University -INFN Trieste
Spectrometer 2002  2004
Beam: 160 GeV µ+
µ/spill
(4.8s/16.2s)
Pm ~ 80%
trigger-hodoscopes
DW45
straws
SM2 dipole
Muon-filter2,MW2
RICH_1
HCAL1
Gem_11
SM1 dipole
ECAL2,HCAL2
Polarised Target
MWPC Gems Scifi
Muon-filter1,MW1
Veto
straws,MWPC,Gems,SciFi
Gems,SciFi,DCs,straws
Silicon
SciFi
Micromegas,DC,SciFi
BMS
18 April 2019
Andrea Bressan University -INFN Trieste