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Methods to measure quark polarizations at the LHC

Yevgeny Kats
While it's easy for the LHC detectors to reconstruct the momentum of an energetic quark by measuring the jet it produces, there is no straightforward way to determine the quark's polarization. We have pointed out that it is actually possible. For the bottom and charm quarks, which are heavy relative to the QCD scale, the polarization is expected to be largely preserved when they hadronize into the Λb and Λc baryons, respectively. With collaborators from the CMS experiment, we analyzed how such measurements can be done using various decays of these baryons.

The most interesting application would be characterization of new physics processes producing bottom or charm quarks. While new physics is yet to be discovered, we motivated a set of Standard Model analyses for ATLAS, CMS, LHCb, BaBar and Belle that would help calibrate the polarization measurements.

Further reading:
Heavy baryons as polarimeters at colliders, M. Galanti, A. Giammanco, Y. Grossman, Y. Kats, E. Stamou, J. Zupan, JHEP 1511, 067 (2015) [pdf]
Measuring c-quark polarization in W+c samples at ATLAS and CMS, Y. Kats, JHEP 1611, 011 (2016) [pdf]

An example top-antitop event that can be used for measuring the polarization of c quarks produced in W boson decays.

For the strange quark, the heavy-quark approximation cannot be used. However, it is known from experiments at LEP that Λ baryons in fact preserve much of the strange-quark polarization. We argued that this allows measuring the polarization of strange quarks at the LHC. Furthermore, there are reasons to believe that up and down quarks hadronizing into a Λ may also be transferring some of their polarization. This may open the way for measuring their polarization as well. We motivated studies in top-quark samples in ATLAS and CMS that would provide additional information about the polarization transfer from the strange, up and down quarks to the Λ.

Further reading: Measuring polarization of light quarks at ATLAS and CMS, Y. Kats, Phys. Rev. D 92, 071503(R) (2015) [pdf]

Several additional interesting projects on these topics are underway or planned for the near future.

For more highlights see: