Shuyang Cao (曹书阳)

2024

1. arxiv

   Field mixing in a thermal medium: A quantum master equation approach

   Shuyang Cao

   Aug 2024

   Abs arXiv Bib

   We studied the nonequilibrium dynamics of the indirect mixing of two (pseudo-)scalar fields induced by their couplings to common decay channels in a medium. The effective non-Markovian quantum master equation (QME) for the two fields’ reduced density matrix is derived to leading order in the couplings of the two fields with the medium, but to all orders of the couplings among degrees of freedom in the medium. The self-energy and noise-kernel in the QME satisfy a fluctuation-dissipation relation. The solutions show that an initial expectation value (condensate) of one field induces a condensate of the other field through the indirect mixing and that the populations and coherence of the two fields thermalize and approach to non-vanishing values asymptotically. The nearly-degenerate field masses and coupling strengths resonantly enhance the quantum beats and asymptotic coherence, and induce a prominent dynamics of the vacuum after the switch-on of the couplings. We argue that a time-dependent definitions of particles due to the changing vacuum must be introduced so as to obtain results consistent with the calculations of equilibrium states in the asymptotic limit. A coupling strength hierarchy breaks down the resonant enhancement in the nearly-degenerate case but leads to different power countings of the coupling strengths in the magnitudes of the observables and time-scales in the evolution, suggesting the possibility of detecting extremely long-lived particles using prepared short-lived particles within a practical experimental period.

   @article{cao2024fieldmixingthermalmedium,
     title = {Field mixing in a thermal medium: A quantum master equation approach},
     author = {Cao, Shuyang},
     month = aug,
     year = {2024},
     eprint = {2408.08460},
     archiveprefix = {arXiv},
     primaryclass = {quant-ph},
     url = {https://arxiv.org/abs/2408.08460}
   }

2023

1. PhysRevD

   Chern Simons condensate from misaligned axions

   Shuyang Cao, and Daniel Boyanovsky

   Phys. Rev. D, Apr 2023

   Abs arXiv Bib HTML

   We obtain the non-equilibrium condensate of the Chern Simons density induced by a misaligned homogeneous coherent axion field in linear response. The Chern-Simons dynamical susceptibility is simply related to the axion self-energy, a result that is valid to leading order in the axion coupling but to all orders in the couplings of the gauge fields to other fields within or beyond the standard model except the axion. The induced Chern-Simons density requires renormalization which is achieved by vacuum subtraction. For ultralight axions of mass m_a coupled to electromagnetic fields with coupling g, the renormalized high temperature contribution post-recombination is ⟨\vecE⋅\vecB⟩(t) = -\fracg\,\pi^2 T^415 \,\overlinea(t)+ \fracg  m^2_a T16\,\pi\,\dot\overlinea(t) with \overlinea(t) the dynamical homogeneous axion condensate. We conjecture that emergent axion-like quasiparticle excitations in condensed matter systems may be harnessed to probe cosmological axions and the Chern-Simons condensate. Furthermore, it is argued that a misaligned axion can also induce a non-abelian Chern-Simons condensate of similar qualitative form, and can also “seed” chiral symmetry breaking and induce a neutral pion condensate after the QCD phase transition.

   @article{PhysRevD.107.083531,
     title = {Chern Simons condensate from misaligned axions},
     author = {Cao, Shuyang and Boyanovsky, Daniel},
     journal = {Phys. Rev. D},
     volume = {107},
     issue = {8},
     pages = {083531},
     numpages = {14},
     year = {2023},
     month = apr,
     publisher = {American Physical Society},
     doi = {10.1103/PhysRevD.107.083531},
     url = {https://link.aps.org/doi/10.1103/PhysRevD.107.083531},
     archiveprefix = {arXiv},
     eprint = {2303.04197}
   }

2022

1. PhysRevD

   Brownian axionlike particles

   Shuyang Cao, and Daniel Boyanovsky

   Phys. Rev. D, Dec 2022

   Abs arXiv Bib HTML

   We study the non-equilibrium dynamics of a pseudoscalar axion-like particle (ALP) weakly coupled to degrees of freedom in thermal equilibrium by obtaining its reduced density matrix. Its time evolution is determined by the in-in effective action which we obtain to leading order in the (ALP) coupling but to \emphall orders in the couplings of the bath to other fields within or beyond the standard model. The effective equation of motion for the (ALP) is a Langevin equation with noise and friction kernels obeying the fluctuation dissipation relation. A “misaligned” initial condition yields damped coherent oscillations, however, the (ALP) population increases towards thermalization with the bath. As a result, the energy density features a mixture of a cold component from misalignment and a hot component from thermalization with proportions that vary in time (cold) e^-Γt+(hot)\,(1-e^-Γt), providing a scenario wherein the “warmth” of the dark matter evolves in time from colder to hotter. As a specific example we consider the (ALP)-photon coupling g a \vecE⋅\vecB to lowest order, valid from recombination onwards. For T ≫m_a the long-wavelength relaxation rate is substantially enhanced \Gamma_T = \fracg^2 m^2_a T16\pi . The ultraviolet divergences of the (ALP) self-energy require higher order derivative terms in the effective action. We find that at high temperature, the finite temperature effective mass of the (ALP) is m^2_a(T) = m^2_a(0)\Big[ 1-(T/T_c)^4\Big], with T_c ∝\sqrtm_a(0)/g, \emphsuggesting the possibility of an inverted phase transition, which when combined with higher derivatives may possibly indicate exotic new phases. We discuss possible cosmological consequences on structure formation, the effective number of relativistic species and birefringence of the cosmic microwave background.

   @article{PhysRevD.106.123503,
     title = {Brownian axionlike particles},
     author = {Cao, Shuyang and Boyanovsky, Daniel},
     journal = {Phys. Rev. D},
     volume = {106},
     issue = {12},
     pages = {123503},
     numpages = {24},
     year = {2022},
     month = dec,
     publisher = {American Physical Society},
     doi = {10.1103/PhysRevD.106.123503},
     url = {https://link.aps.org/doi/10.1103/PhysRevD.106.123503},
   }