Longitudinal polarization of ? and?? in DIS at COMPASS

1
Longitudinal polarization of ? and?? in DIS at
COMPASS

• M.G.Sapozhnikov
• Joint Institute for Nuclear Research, Dubna
• On behalf of the COMPASS Collaboration

2
Physical motivation

• Longitudinal polarization of ? and ?? in DIS is
  sensitive to
• s(x), ?s(x)
• polarization of strange quarks ?s
• ? spin structure

3
? production in DIS, quark fragmentation

Quark fragmentation
? spin structure
Spin transfer from polarized muon
Spin transfer from polarized quark
4
Spin transfer from polarized quark to ?
Polarized quarks, indeed, transfer the
polarization to ?.
5
COMPASS Detector
SAS
Muon filter 2
MWPCs
ECal2 HCal2
LAS
SM2
Muon filter 1
ECal1 HCal1
GEM MWPCs SciFi
RICH
Drift. Ch.
SM1
Silicon SciFi
GEM MWPCs
Scintillating fibers
GEM Straws
Beam
Micromegas
Polarized target
160 GeV µ beam 2.8 . 108 µ/spill (4.8 s)
Pb -0.76?0.04 - 2003 Pb -0.80?0.04 - 2004
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Polarized 6LiD target
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Production of ?(??)

No PID used
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Event selection
?
V1
p
V2
?
?

q
?-

• Primary vertex in the target
• Secondary vertex 5 cm downstream the
  target
• pT gt 23 MeV/c
• ? lt 0.01 rad
• Q2 gt1 (GeV/c)2
• 0.2ltylt0.9

9
Invariant mass distribution ? / ??
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Comparison with other experiments
N(?) N(??)
E665 750 650
NOMAD 8 087 649
HERMES, 1996-2000 7 300 1 687
RHIC 30 000 24 000
COMPASS, 2003,2004 70 000 42 000
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• ?
• ltxBjgt 0.05
• ltxFgt 0.23
• ltygt 0.46
• ltQ2gt3.31 GeV2

12

• ??
• ltxBjgt 0.050
• ltxFgt 0.22
• ltygt 0.48
• ltQ2gt3.27 GeV2

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Longitudinal polarization PL

• ?(-)0.642 0.013 - ?(??) decay parameter
• P - polarization vector
• k - unit vector along the decay proton momentum
• L-axis - along the momentum of virtual
• photon

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• Determination of the angular distribution
• sidebands subtraction ss

15

• Angular distributions of ? and ??
• 2004 run
• P(K0)0.0110.005

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• ? - between virtual photon and p in ? c.m.s.
• Kaon background is important at large cos?
• cut
• -1 lt cos? lt0.6

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Angular acceptance
unpolarized distribution by HERMES
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Longitudinal spin transfer DLL
DLL - polarization of the struck quark along
the axis L is transferred to the ? along the
secondary axis L In our case LL

PL DLL Pb D(y)
Pb beam polarization, D(y) depolarization
factor
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Comparison of ? and ?? x
DLL(?)? DLL(??)

• DLL(?) -0.012 0.047 0.024
• DLL(??) 0.249 0.056 0.049

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Comparison of ? and ?? xF
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Comparison with other experiments ?
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Comparison with other experiments ??
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PL dependence on the target polarization
?P P--P -0.03?0.04, ?
-0.01?0.05, ??
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Polarization of ? from quark fragmentation
Spin transfer from polarized muon
Spin transfer from polarized quark
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Polarization of ? PT 0
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DLL(??) gt DLL(?)
Even if s(x)?s(x), DLL(??) gt DLL(?) due to
difference of the denominator Du,d(?) gt
Du,d(??)

• Independent quark fragmentation
• DLL(??) gt DLL(?)
• Sensitivity to s(x)
• Are these conclusions still valid in a more
  advanced model?

27
Theory predictions for ? and??

• J.Ellis et al., Eur.Phys.J. C52 (2007) 603

28
Theory predictions for ? and??

• J.Ellis et al., Eur.Phys.J. C52 (2007) 603

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Sensitivity to the strange distribution s(x)
CTEQ5L
GRV98
DLL(s)0, BJ model
DLL(s)0, SU(6) model
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CTEQ5L

• Influence to different PDFs
• Q2 4 GeV2

GRV98LO
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Sensitivity to the strange distribution ?s(x)
CTEQ5L
GRV98
DLL(?s)0, BJ model
DLL(?s)0, SU(6) model
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Conclusions

• Production ? and ?? is mainly due to
  fragmentation of target remnant and u,d-quarks
• Spin transfer to ? and ?? is mainly due to
  interaction with strange quark and antiquark

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Conclusions-2

• Data on the spin transfer to ? and ?? could be
  used for the independent determination of s(x)
  and ?s(x)
• Determination of the spin transfer dependence on
  the target polarization allows to fixed ?s(x) and
  ??s(x)

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Conclusions-3

• Preliminary results on ? and ?? spin transfer in
  DIS are obtained on statistics 70000 ? and 42000
  ??
• SX(?)? SX(??)
• SX(?) 0
• SX(??) may be as large as 0.4-0.5
• Comparison with theory
• Spin transfer to ?? is sensitive to ?s(x)
• data need more s(x) than the GRV98

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Backup slides
36
Impact of LEP data
D. de Florian, M.Stratmann, W.Vogelsang, PRD
57(1998)5811
1- SU(6) 2 BJ 3 - ?u? ?d? ?s?
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Theory predictions for ?/??

• Liang Zuo-tang et al. Phys.Rev.D72 (2005) 033006

-0.6 0 0.6 xF
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Transverse polarization PY