Speaker
Description
The most general massless particles allowed by Poincare' invariance are “continuous spin” particles (CSPs), a term coined by Wigner. Such particles are notable for their integer-spaced infinite tower of spin polarizations, with states of different integer (or half-integer) helicities mixing under boosts, much like the spin-states of a massive particle. The mixing under boosts is controlled by a spin-scale $\rho$ with units of momentum. Normally, we assume $\rho=0$, but this misses the most general behavior compatible with Lorentz symmetry. The interactions of CSPs are known to satisfy certain simple properties, one of which is that the $\rho\rightarrow 0$ limit generically recovers familiar interactions of massless scalars, photons, or gravitons, with all polarizations of helicity $|h|\geq 3$ decoupling in this limit. Thus, one can ask if the photon of the Standard Model is a CSP with a small, but non-zero $\rho$. One concern about this possibility - originally raised by Wigner - is that the infinite tower of polarizations could pose problems for thermodynamics.
In this talk, I discuss aspects of CSP thermodynamics, and show that the structure of CSP interactions imply that it is in fact thermodynamically well behaved. In a bath of charged particles coupled to CSP photons, the primary $h=\pm 1$ helicity modes thermalize quickly, while the other modes require increasingly long time-scales to thermalize. In familiar thermodynamic systems, the CSP photon behaves like the familiar photon, but with small, time-and $\rho$-dependent corrections to its effective number of degrees of freedom. Departures from familiar thermal behavior arise at energy scales comparable to $\rho$, which could have interesting and testable experimental consequences.
