nucl-ex digest · Jan 30, 2025

01*

Zubarev response approach to polarization phenomena in local equilibrium

Using the expansion of Zubarev's density operator, we develop a linear response approach to study various spin physics in a locally equilibrated medium, particularly focusing on various polarization phenomena in heavy-ion collisions. Specifically, we connect familiar correlation functions and diagrammatic methods to the Zubarev formalism, enabling the use of established techniques like the Matsubara/imaginary time formalism to facilitate calculations. For a spin-1/2 particle, we re-derive its vector polarization using this Zubarev response approach, which exactly reproduces with our previous results based on Luttinger's method. For a spin-1 particle, we calculate the vector polarization and find the expected contributions from vorticity, temperature gradients, and shear, which are identical to those for spin-1/2 particles except for a factor of 4/3 as expected. For the tensor polarization and spin alignment of a spin-1 boson, we explicitly prove that the non-dissipative contribution is zero at leading order in gradients, and briefly reiterate our previous findings for the dissipative contribution with further discussions on several concerns. Additionally, we discuss several relevant subtleties and questions, including an alternative derivation for Zubarev response approach, the covariance issues of different spin density matrix definitions, a further explanation of slow and fast modes, the mode selection scheme, etc. We also discuss skeleton expansions, higher-order contributions, and non-perturbative methods, particularly their potential connection to lattice field theory. In summary, this work discusses the foundations and subtleties of Zubarev response approach, with specific examples from spin physics in heavy-ion collisions.

↳ nucl-thhep-phhep-thnucl-exPRC(2026)·12 citations

The Fermi function and the neutron's lifetime

The traditional Fermi function ansatz for nuclear beta decay describes enhanced perturbative effects in the limit of large nuclear charge

Z

and/or small electron velocity

β

. We define and compute the quantum field theory object that replaces this ansatz for neutron beta decay, where neither of these limits hold. We present a new factorization formula that applies in the limit of small electron mass, analyze the components of this formula through two loop order, and resum perturbative corrections that are enhanced by large logarithms. We apply our results to the neutron lifetime, supplying the first two-loop input to the long-distance corrections. Our result can be summarized as \begin{equation*} \tau_n \times |V_{ud}|^2\big[1+3\lambda^2\big]\big[1+\Delta_R\big] = \frac{5263.284(17)\,{\rm s}} {1 + 27.04(7)\times 10^{-3} }~, \end{equation*} with

∣ V_{u d} ∣

the up-down quark mixing parameter,

τ_{n}

the neutron's lifetime,

λ

the ratio of axial to vector charge, and

Δ_{R}

the short-distance matching correction. We find a shift in the long-distance radiative corrections compared to previous work, and discuss implications for extractions of

∣ V_{u d} ∣

and tests of the Standard Model.

↳ hep-phnucl-exnucl-thPLB(2025)·15 citations