Tricriticality in 4D U(1) lattice gauge theory

The 4D compact U(1) gauge theory has a well-established phase transition between a confining and a Coulomb phase. In this paper, we revisit this model using state-of-the-art Monte Carlo simulations on anisotropic lattices. We map out the coupling-temperature phase diagram, and determine the location of the tricritical point, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>T</a:mi><a:mo>/</a:mo><a:msub><a:mi>K</a:mi><a:mn>0</a:mn></a:msub><a:mo>≃</a:mo><a:mn>0.19</a:mn></a:math>, below which the first-order transition is observed. We find the critical exponents of the high-temperature second-order transition to be compatible with those of the 3-dimensional <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>O</c:mi><c:mo stretchy="false">(</c:mo><c:mn>2</c:mn><c:mo stretchy="false">)</c:mo></c:math> model. Our results at higher temperatures can be compared with literature results and are consistent with them. Surprisingly, below <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>T</g:mi><g:mo>/</g:mo><g:msub><g:mi>K</g:mi><g:mn>0</g:mn></g:msub><g:mo>≃</g:mo><g:mn>0.05</g:mn></g:math> we find strong indications of a second tricritical point where the first-order transition becomes continuous. These results suggest an unexpected second-order phase transition extending down to zero temperature, contrary to the prevailing consensus. If confirmed, these findings reopen the question of the detailed characterization of the transition including a suitable field theory description.