(AGENPARL) – WASHINGTON (D.C.), mer 25 novembre 2020
One possibility is that symmetries are still at play but in an unexpected form. In this case, the unexpected symmetry relates the standard model to an identical twin, which has its own particles and interactions that are mirror reflections of those in the standard model . In these “twin Higgs” scenarios, the only connection between the standard model and its mirror twin are the Higgs bosons of the two sectors (Fig. 4). The particles of the standard model and their mirror counterparts act in tandem to control the self-energy of the Higgs, explaining at least some (but not all) of the hierarchy between the weak scale and the Planck scale.
Twin Higgs models could offer unique experimental signatures. Although these models predict a host of new particles—an entire standard-model’s-worth near the weak scale—the mirror particles do not interact via standard model forces, so they could have evaded detection at the LHC. Even so, they may not be entirely invisible. The strong interactions of the mirror sector lead to bound states, just like in the standard model, giving rise to a zoo of mirror mesons and baryons. Some of these particles can mix with the Higgs, providing a portal through which mirror particles can be produced and eventually decay back into standard model particles. Remarkably, these processes are slow enough that mirror particles, if produced at the LHC, would travel distances ranging from centimeters to kilometers before decaying. Detecting such long-lived particles requires a dedicated approach to recording and analyzing data at the LHC, which the CMS and ATLAS collaborations are energetically pursuing.
Another possibility is that symmetries play no decisive role, but rather, the Higgs mass is determined dynamically by the evolution of other fields in the early Universe—similar in spirit to Dirac’s proposed solution for the proton mass. Specifically, this idea assumes a new field, called the relaxion field , that behaves like the hypothetical axion field, which theorists have proposed as a fix to a fine-tuning problem in nuclear physics. The amplitude of the relaxion field, which evolves along a gently sloping potential in the early Universe, helps to control the mass of the Higgs. In other words, the Higgs mass is determined by the combination of self-energy from known standard model fields and the background value of the relaxion—both of which may be very large in the early Universe. Only when the total mass of the Higgs becomes small, do features appear in the relaxion potential that cause the evolution to stop, thus fixing the Higgs mass at its observed value.
In this dynamic scenario, the relaxion is the only new particle associated with the value of the weak scale, leaving few possibilities of detectable signals at the LHC but allowing for a wealth of possible signatures in other experiments. Depending on the mass of the relaxion, such signatures might show up as new long-range forces, energy density in dark radiation, rare meson decays at beam dump experiments, or exotic Higgs decays at the LHC . Current axion searches could be sensitive to relaxions, which means any limits placed on axions, such as recent results from the CASPEr experiment , would also apply to relaxions.
Finally, it may be that the difficulty of finding a naturalness solution to the hierarchy problem is symptomatic of something larger: the omission of gravity in the standard model. Perhaps the Higgs mass fine-tuning problem will disappear in a theory that unifies quantum mechanics and gravity. We don’t yet have such a theory, but researchers are able to identify quantum field theories that don’t fit with quantum gravity expectations. These gravity-inconsistent theories are said to belong to the “swampland” (see Trend: Cosmic Predictions from the String Swampland). By studying limits of the swampland, researchers can use gravity as a guide for finding a viable theory that goes beyond the standard model. If the Higgs mass happens to be fixed by these sorts of gravity constraints, then one would expect new long-range forces and light particles that couple to the Higgs. These phenomena might be observable in a future Higgs factory (see Opinion: Exploring Futures for Particle Physics).
It’s too soon to say which path nature has chosen among the present options: conventional naturalness hiding just around the corner, other-naturalness in one form or another, or something entirely different. In the meantime, the recent proliferation of ideas surrounding the hierarchy problem has broadened the landscape of possibilities, drawing attention to a host of new experimental signatures to explore.