Hyperon polarization in a quarkquark scattering model

1
Hyperon polarization in a quark-quark scattering
model

• Eduard De La Cruz Burelo
• Homer A. Neal

2
Outline

• Hyperon polarization remains a major open
  question even at energies of hundreds of GeV
• Extraordinary behavior of hyperon polarization
  has no simple explanation
• Polarization in pp? pp and pp? H X could both
  be the result of simple quark-quark scattering?
• A proposed model based on quark-quark scattering.
• The qualitative explanation of the hyperon
  polarization for a variety of processes

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Hyperon polarization

• An unresolved question
• What causes the large polarization in inclusive
  hyperon production in p-p collisions?
• Vanishingly small polarizations are predicted in
  QCD.
• No model is now able to explain all features of
  Hyperon polarization at the same time.

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Hyperon polarization characteristics
(Diquark)J state
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?0 Lowest mass hyperon more intriguing

• Very little dependence on the initial proton
  energy and in the target material.
• Grows negative linearly with pT until a plateau
  is reached around 1 GeV/c.
• For constant xF (pL/pmax) value, the plateau
  extends from 1 GeV/c to the highest measured pT.

6
Intriguing questions
Why do the ?0 and ? polarizations look similar?
Why is the S polarization positive?
Why is the O polarization almost zero?
Why is there a plateau in the ?0 polarization?
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Features we would like to explain

• The shape and magnitude of the ?0 polarization
• The different signs in the ?0 and S polarization
• The similarity of the polarization for the ?0 and
  ?
• The small value of the O polarization

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q-q scattering relating pp ? pp to pp ? ? X
process

• Is there any connection between the observed
  polarization in these two processes?
• They could come from the same source quark-quark
  scattering.

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pp?pp in a quark-quark scattering model

• Neal et al. proposed a q-q scattering model to
  explain the polarization in elastic p-p
  scattering
• First region is due to q-q scattering
• Second region is due to 2q-2q or q-2q
• Subsequent data is consistent with 3q-3q
  scattering.
• Can we apply the same idea to pp? ? X process?

H.A. Neal et al. Phys. Lett. B 51 (1974) 79
) H.A. Neal et al. Phys. Lett. B 439 (1998) 407
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pp? ? X in a quark-quark scattering model

• From a preexisting pair in the incoming
  proton, the s quark scatters, get highly
  polarized, and end up in the hyperon.
• Hyperon polarization PHPq(x,pT). Pq(xF,pT)
  quark polarization effect due to a q-q scattering
  process.

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?0 polarization from polarization in pp?pp (1)

• Model PHPq(x,pT)
• In a multiple quark scattering P mqPq(pT)

Assuming ms2mu
) J. Szwed, Phys. Lett. B 105 (1981) 403.
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?0 polarization from polarization in pp?pp (2)

• What pT(?0) vs. pT(p) should we compare?
• For a elastic p-p scattering in a q-q region
  (Neal et al.)
• For pp ? ? X in q-q region
• We should compare at

Assuming s-quark account for half of the ?0 mass.
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Comparison of proton and ?0 polarization
Proton polarization multiplied by 2
Proton pT multiplied by 2/3
If we let proton data to tell us what values
prefer to match ?0 data, selected values are
around to 2 and 2/3.
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What about the plateau?
Polarization
Polarization

• The plateau in the ?0 polarization can be
  understood as the saturation of the s quark
  polarization (P100).

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Explaining the other features

• Why the similar polarization for the ?0 and ??

If the polarization of both hyperons is due to
one s quark becoming highly polarized from the
scattering process, then we expect the
polarization of both hyperons to be similar.
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Why the different sign in the ?0 and S
polarization?

• Beam quarks does not contributed to the ?0
  polarization
• s quark is responsible for the large polarization
  of the ?0.
• s quark polarization is negative.

• s quark polarization is negative which force the
  polarization of the u and d quarks to be
  positive, so overall polarization is positive.
• If we assume the s quark being responsible of the
  polarization, it can be showed that the sign of
  the S polarization is opposite to that of the ?
  polarization.

d
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Why O polarization is 0?
?0 polarization compared to the O polarization.
Statistics too low to make strong conclusions.

• One of 3 s quarks get polarized in the
  scattering, setting a maximum of 33 to the
  polarization of the O.
• Should be similar to the ?0 polarization in the
  one quark scattering region.

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Future tests/Implications

• ?b polarization is planned to be measured in LHC
  (ATLAS LHCb)
• Polarization could be huge if the q-q
  contribution to production is important and the
  Szwed mass scaling holds.
• Simple relation should exist between ? and ?b
  polarization is our q-q idea is correct.

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Conclusions

• A model based in a simple q-q scattering process
  is proposed
• Polarization due the scattering process Pq(pT) is
  the same for all quark species.
• s quark scatters from the incoming proton is the
  responsible for the large hyperon polarization.
• ? and proton polarization seems to have a common
  source.
• The model is able to qualitatively address all of
  the main observed features of hyperon
  polarization.