1. Nucleon spin and structure studies with COMPASS
DAPNIA
Stephane Platchkov
CERN and CEA-Saclay
On behalf of the COMPASS Collaboration
07/09/2019
S. Platchkov, PANIC 05

2. The COMPASS collaboration
Composed of
230 physicists
28 institutions
11 countries
Data taking
2002, 2003, 2004
2005: SPS stopped
LHC
SPS
COMPASS
07/09/2019
S. Platchkov, PANIC 05

3. The nucleon spin puzzle
Quark contribution to the nucleon spin
: EMC, SMC, Hermes, SLAC :
DS  0.20 – 0.25 instead of ~0.60 (QPM)
The nucleon spin is more complicated than anticipated ?
 Study Gluon contribution to the spin
 And also many other spin observables…
07/09/2019
S. Platchkov, PANIC 05

4. Physics program of COMPASS
Experiments with muon beam
Gluon spin contribution
Quark spin contribution
Flavor decomposition of spin distribution functions
Transverse spin distribution functions
Transversity – two hadron correlations
Vector meson production
Spin transfer in L-hyperon production
Pentaquarks
Experiments with hadron beams
Pion and Kaon polarizabilities
Diffractive production of exotic states
Search for glueballs
Light meson spectroscopy
Production of double charmed baryons
Other Compass talks at Panic05
J.Nassalski – Delta-G
A.Bressan – Transversity-1h
R.Joosten – Transversity-2h
J.Friedrich – Pentaquarks, hadrons
A.Sandacz – r production, DVCS
07/09/2019
S. Platchkov, PANIC 05

5. COMPASS “first” period (2002 – 2004)
Large amount of data collected with muon beam
Longitudinally polarized target
About 78% of the total
Transversely polarized target
About 22% of the total
First data with hadron beam
“Pilot” hadron run in 2004
This talk:
Quick overview of available COMPASS data
Mainly
Data from 2004 being analyzed
07/09/2019
S. Platchkov, PANIC 05

6. The COMPASS Spectrometer
Trigger-hodoscopes
RICH
ECal & HCal
μ Filter
Drift chambers
Straws
MWPC
50 m
Silicon
Micromegas
SciFi
Gems
SM1
SM2
6LiD Target
160 GeV μ
Two stage spectrometer
Polarized beam and target
~75% ≥50%
SAT,
LAT,
PID
0.003<x<0.5, 10-3<Q2<10 GeV2
07/09/2019
S. Platchkov, PANIC 05

7. Quark spin content: inclusive DIS asymmetry
Data taken in 2002 and 2003
 Good agreement with previous experiments
 Improved statistics at low x (Factor of ~2)
07/09/2019
S. Platchkov, PANIC 05

8. Quark spin content: g1d(x) structure function
Use
Data taken in 2002 and 2003
Phys.Lett. B612(2005)154.
Also: new precise proton data from HERMES
New COMPASS QCD fit to the world data on g1(x) (Q2=3 GeV2, NLO):
DS(MS) = without COMPASS data
DS(MS) = with COMPASS data
07/09/2019
S. Platchkov, PANIC 05

9. Semi-inclusive DIS Objectives: Preliminary data 2002 and 2003
Measure semi-inclusive spin asymmetries
for p+, p-, K+ K- Ks
Determine individual flavor spin distributions (isoscalar target): h
Preliminary data
2002 and 2003
 Kinematics range down to x = 0.003, Q2>1
 High accuracy data
In progress…
07/09/2019
S. Platchkov, PANIC 05

10. Transversity – Collins and Sivers asymmetries
Full description of the nucleon
Nucleon PDFs: Unpolarized Helicity Transversity
Collins: probes transverse spin quark distribution Dtq(x)
Sivers: probes intrinsic transverse momentum
Data taken in 2002 only
Collins
PRL 94 (2005)
Sivers
See A.Bressan’talk
Session III.7
Stat accuracy of the same order as Hermes
Small values of asymmetries could be due to opposite signs of the transverse du and dd distributions.
Full data set: reduce the errors by a factor of 2-3
Also: take data with a proton target – separation of transnversity
Using all hadrons increases the stat by 1.5
 Asymmetries are compatible with 0
 Possible explanation: cancellation of DuT and DdT in isoscalar targets?
07/09/2019
S. Platchkov, PANIC 05

11. Collins asymmetries – near future
COMPASS transverse spin “polarimeters”:
Collins asymmetry
Two-hadron correlation
L-polarization
Covered by:
A. Bressan
R. Joosten
In progress…
COMPASS Deuteron
Combined data
COMPASS Proton
Expected 2006 data (30d)
 Accurate data to come
07/09/2019
S. Platchkov, PANIC 05

12. Longitudinal polarization of L andL hyperons
Physics in the current fragmentation region
Spin transfer from q to L
Test of the strange quark asymmetry of the sea
Data from 2003 only
 With 2004 data: much improved statistics
07/09/2019
S. Platchkov, PANIC 05

13. Determination of DG/G Select Photon Gluon Fusion process
Via Open Charm production
Detect D°  K-p+ and D*+D°p+
Clean channel (no c quarks)
High combinatorial background
Via High-pt hadron pairs
Good S/B ratio
Physics background (QCDC, LODIS, Resolved photons contributions
Rely on MC estimates
D*+→D0π+ (2003)
1100 events
COMPASS preliminary
07/09/2019
S. Platchkov, PANIC 05

14. Determination of DG/G – Two methods
Open Charm
High-pt pairs
Data
Calculation: Glück et al.,Phys. Rev. D63 (2001)
DG=2.5
DG=0.6
DG=0.16
DG/G(xg=0.1) = ± 0.089(stat) ± 0.057(syst)
Most precise value of DG/G today
Present accuracy: 0.73
With 2004 data  0.43
With 2006 data  0.24 (estimate)
See talk by J.Nassalski, PDF-II
07/09/2019
S. Platchkov, PANIC 05

15. Search for Φ-- Pentaquarks
NA49: Peak at 1860 ± 8 MeV
Several “evidence papers” on pentaquarks published
Motivated by NA49 findings
Ali et al. PRL 92 (2004)
Search for the process:
Check of COMPASS analysis:
Look for
COMPASS : 1700 events
NA49 : events
 COMPASS does not confirm NA49
See talk by J.Friedrich, Pentaquarks session
07/09/2019
S. Platchkov, PANIC 05

16. COMPASS upgrades for 2006 New solenoid magnet RICH upgrade
Larger acceptance
70 mrad  180 mrad
Expect +30% FOM
RICH upgrade
Central region: maPMT system
More photons
Improved S/N
Outer region: New faster electronics
Expect +60% FOM
Other upgrades:
Large Drift Chamber
Full ECAL coverage
Etc…
07/09/2019
S. Platchkov, PANIC 05

17. COMPASS Programme The near future (2006 – 2010)
Improved COMPASS Factor of Merit
New OD magnet 1.3
Upgraded RICH
Full ECAL coverage
Other…
Total FoM improvement: ≥2.3
Strategy
Strategy: muon beam in 2006,
hadron beam in 2007, …
Precise data to come!
Long term future (beyond 2010)
Plans to use DVCS to measure GPD
Benefit from the high muon energy (190 GeV)
Try to access orbital momentum L
Additional detectors being developed
Compass Factor of Merit
07/09/2019
S. Platchkov, PANIC 05

18. Summary - Results New COMPASS results Better spectrometer in 2006
Improved measurement of the g1 deuteron structure function
Most precise DG/G measurement up to now
First COMPASS results on tranversity
Accurate results on semi-inclusive DIS
No evidence for pentaquarks
And a lot of data being analyzed
Better spectrometer in 2006
Expected Factor Of Merit increase: ≥ 2.3
More precise results are coming….
07/09/2019
S. Platchkov, PANIC 05