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The chirality-flipping operators of light fermions are currently poorly
constrained by experimental analyses due to the lack of interference with
Standard Model (SM) amplitudes in traditional observables. In this work, we
propose to investigate the semi-leptonic scalar/tensor four-fermion operators
of electron and quarks through the transverse double spin asymmetry (DSA) at
Electron-Ion Collider, where both the electron and proton beams could be highly
transversely polarized. Due to the chirality-flipping nature of these
operators, we demonstrate that their interference with the SM results in an
unsuppressed contribution to the DSA, and could lead to non-trivial azimuthal
$\cos2\phi$ and $\sin2\phi$ distributions that are linearly dependent on their
Wilson coefficients. This new method has the potential to significantly improve
the current constraints on these scalar/tensor four-fermion operators without
relying on theoretical assumptions about other types of new physics effects,
particularly for the tensor type operator of the $u$-quark. Additionally, our
findings indicate that both the real and imaginary parts of these operators can
be simultaneously constrained and offer a new opportunity for probing potential
$CP$-violation effects. However, it is important to note that these results
would be sensitive to the quark transversity distributions, which are currently
poorly constrained by the experimental data, but could be significantly
improved at the upcoming Electron-Ion Collider. Therefore, our work opens up a
new avenue to utilize this new spin asymmetry for exploring the new physics
effects from the scalar/tensor four-fermion operators.
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