Transcript Spin Physics Results from COMPASS

Spin Physics at COMPASS
Christian Schill
Universität Freiburg
on behalf of the COMPASS collaboration
Selection of Results
Polarized DIS and polarized distribution functions
Direct measurements of gluon polarization Dg/g
Transversity measurements
Rutherford Centennial Conference on Nuclear Physics 2011
Manchester, August 8, 2011.
1
COMPASS polarized target system
3He
– 4He dilution
refrigerator (T~50mK)
solenoid
2.5T
dipole magnet 0.5T
acceptance
± 180 mrad
6LiD
or NH3
50/90% polarization
40/16% dilution factor
Longitudinally or transversely
polarized
μ
160 GeV long.
polarized muon beam
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Longitudinal spin asymmetries in DIS
• Inclusive scattering
• Semi-inclusive scattering
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Proton A1 asymmetries
incl. & semi-incl. asymmetries
for identified π’s and K’s
incl.
π+
π–
K+
K−
PLB 693 (2010) 227
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Deuteron A1 asymmetries
Inclusive & semi-inclusive asymmetries
for identified π’s and K’s
incl.
π+
π–
K+
K−
PLB 680 (2009) 217–224
5
Proton & deuteron g1(x,Q2)


unique kinematical domain
important for global QCD analyses
new
p
From first moment of g1d:
∆Σ=a0=0.33±0.03±0.05 (evol. Q2=∞)
d
CLAS
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Flavour distributions
LO analysis of 5p+5d asymmetries, DSS FF
Line: NLO DSSV not including these data
PLB693 (2010) 227
PRD80 (2009) 034030
5-flavour fit, assuming Δs = Δs
D. de Florian, R. Sassot, M. Stratmann, and W. Vogelsang
Phys.Rev.Lett.101:072001,2008
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Flavour asymmetry?

Rather small effect, Δu > Δd
PLB693 (2010) 227
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Flavour distributions: Ds = Ds ?
LO analysis of 5p+5d asymmetries, DSS FF
PLB693 (2010) 227
6 flavours: u, d, s, u, d, s
No significant difference! Go on assuming Ds = Ds
Varenna, July 2011
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Gluon polarization Dg
1 1
 D  Dg  LZ
2 2
quarks
gluons
small ~0.15
orbital
unknown
poorly known
How to measure Dg?
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Photon–gluon fusion (PGF)
Gluon polarization is measurable in PGF
A ||  R pgf aˆpgf
Dg
g
• Measure A||
N
• calculate R pgf , aˆpgf and
background by Monte Carlo
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Hadron production

Analysis of hadron-pair asymmetries:

open charm: single D meson
cleanest process wrt physics background
AROMA
D0 → K π
D0 → K π π0

high-pT hadron pairs with Q2 > 1 GeV2 LEPTO

high-pT hadron pairs with Q2 < 1 GeV2 PYTHIA
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Results for Δg


All results compatible with zero!
Confirmed by RHIC results in pp
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Transverse Spin Effects
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Transversity PDFs

Transversity DTq(x)


Transverse quark spin distribution in transversely polarized
nucleon
Measurement



Semi-inclusive DIS
transversely polarized target
Collins azimuthal asymmetry
q(x)
Dq(x)
DTq(x)
AColl 
2
0
h
h
e
D
q
(
x
)
D
D
(
z
,
p
q q T
T q
T)
2
h
h
e
q
(
x
)
D
(
z
,
p
q q
q
T)
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Collins asymmetry
π
x
z
PT
positive
negative
K
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Collins Asymmetry: Global Fit


Fit to COMPASS d, HERMES, BELLE (FF, e+e-)
Compared to COMPASS proton data
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Proton Sivers Asymmetry
•
non-zero asymmetry for pos. hadrons on the proton
h+
h−
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Summary

COMPASS is a major player in the nucleon spin physics

PRESENT



Many important results with µ+ beam
Lots of data are being analyzed, additional results to come
FUTURE ( >2012): COMPASS phase II
1. GPD measurements (µ+ and µ- beams)
2. DRELL-YAN measurements
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Proposal for 2012 and beyond

Study of the Generalized Parton Distributions (GPDs)

Natural extension of:
Form factors
(spatial distributions)
PDFs
(momentum distributions)
Access through: - Deeply Virtual Compton Scattering (DVCS)
- Hard Exclusive Meson Production (HEMP): r, w, f, p, h
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Measurements of TMDs using Drell-Yan
p  p  ll  X
Single Target Spin Asymmetry AUT
Drell-Yan (pp ) gives direct
access to chiral odd PDFs like
the Sivers function
COMPASS Projected
Models for Sivers function
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F2(x,Q2)
g1(x,Q2)
p
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Proton & Deuteron g1(x) world data
Proton data - world
Subm. Phys. Lett. B (hep-ex/1001.4654)
COMPASS data: 2007
From first moment
of g1d:
Deuteron data - world
Phys. Lett. B 647 (2007) 8
COMPASS data: 2002 – 2006
∆Σ=a0= 0.33±0.03±0.05 (evol. to Q2=∞)
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Test of the Bjorken sum rule

NS
1
1 g A NS 2
  
C1 (Q )
6 gV
p
1
n
1
3 Par. NLO Fit
NS
g1
 p
g1d (x ,Q 2 ) 
2
(x , Q )  2  g 1 (x , Q ) 

1  1.5wD 

2
|gA/gV| = 1.2694 ± 0.0028 from neutron beta decay (PDG)
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Results for Δg – next to leading order
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Collins Asymmetries
proton
new
h+
h−
deuteron
h+
h−
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Transversity ∆Tq(x) from hadron pairs

Alternative: couple ∆Tq(x) to chiral odd 2-hadron interference FF
cross-section
asymmetry:
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Two-hadron asymmetry


large asymmetries
interference FF and transversity sizable
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Comparison to a recent Fit

Recent fit (dominated by HERMES, COMPASS p not yet in)
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GPDs at COMPASS – kinematical coverage

Unique features

Both µ+ and µ- polarized beams
ZEUS
H1
L=1032 with a 2.5m LH2 target
Prediction for 1 year of data taking
Complementary to HERA
and HERMES/JLAB
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Generalized Parton Distributions and DVCS

Cross section
known
ds Deep VCS + Bethe-Heitler2
DVCS
DVCS
  e a BH Re(T DVCS )  P Im(T DVCS )
ds  ds BH  ds unpol
 P ds pol

Experimental apparatus must detect



Incident and scattered lepton
Outgoing real photon
Recoiling soft proton
g
g*
x+x
x-x
GPDs
P1
t
P2
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