Hidden worlds of fundamental particles
Date: Thursday, June 01, 2017 @ 11:30:23 EDT

From PhysicsToday: Spectacular bursts of particles that seem to appear out of nowhere may shed light on some of nature’s most profound mysteries. ...

Figure 1. Portals to hidden sectors. The diagram represents the energies of possible hidden-sector states in relation to those of the standard model. Colored arrows indicate possible transitions between states. At the Large Hadron Collider, hidden-sector states can be created by means of the production and decay of heavy mediators (a), exotic decays of the Higgs boson (b), or small direct couplings (c). Once produced, the states decay through the same portals. Hidden-sector states have long lifetimes because the direct couplings are small or because energy must be borrowed, courtesy of the Heisenberg uncertainty principle, to excite the intermediate mediator or Higgs boson.

Why is there something rather than nothing? It’s a question so basic and so vast in scope that it seems beyond the realm of quantitative inquiry. And yet surprisingly, it is a question that can be framed and at least partially answered in the study of particle physics.

The standard model (SM) is our current theory of matter and its interactions. Written in the language of relativistic quantum field theory, it has stood up to several decades of tests at high-energy colliders. We know, however, that it cannot be the complete description of the universe, in large part because in several important regards it fails to answer the something-versus-nothing question.

Material existence is connected to three fundamental mysteries. First, why can matter clump to form a rich array of structures without collapsing into black holes? The requisite weakness of gravity arises because the particles that make up everyday matter are incredibly light. In the SM, the mass of all fundamental particles must be less than the electroweak mass, whose value of a few hundred GeV/c2 follows from the physics of the Higgs boson. However, when the effects of gravity are taken into account, the SM electroweak scale receives quantum contributions on the order of the Planck mass, 1018 GeV/c2. That puzzling discrepancy of scale is called the hierarchy problem. In the SM, a particle’s lightness is recovered only thanks to an incredible cancellation among unrelated parameters. There exist more attractive solutions, including supersymmetry and the Higgs boson as a composite particle, that also predict high-energy signatures of new physics at the Large Hadron Collider (LHC).

Second, stuff exists only because at some point in the early universe, there was one extra particle of SM matter for every one billion matter–antimatter pairs. That tiny excess is all that remains today, and it must have been created dynamically in the primordial plasma of the Big Bang. In the SM, that process—baryogenesis—is too weak by many orders of magnitude; evidently, additional particles and interactions must have been present. Theorists have suggested many possibilities, including additional Higgs bosons and modified Higgs couplings that could be detected at the LHC.

Third, strong interlocking evidence from a multitude of astrophysical and cosmological observations suggests that dark matter makes up about 80% of the matter in the universe. We have no definitive knowledge of what it is or how it connects to the SM. The most popular theories predict a tiny interaction of dark matter with ordinary matter. Such a coupling would enable direct detection by nuclear recoils in sensitive detectors and indirect detection in cosmic-ray data that show evidence of dark-matter annihilation into SM particles.

The hierarchy problem, the riddle of matter–antimatter asymmetry, and the nature of dark matter drive much of experimental and theoretical particle physics. Many experimental searches for answers to those mysteries are proceeding vigorously, but so far with null results across the board. That doesn’t mean nothing is to be discovered. On the contrary, the null results may well point us toward the true nature of the universe. There could be hidden sectors—additional particles and forces—with only tiny couplings to the SM. Far from being inconsequential, those new sectors can address all three big mysteries. Their signatures are subtle and easily missed, but luckily, their hidden nature is also the key to their discovery. Invisible long-lived particles (LLPs) can be produced at colliders and decay into energetic SM particles after traveling an appreciable distance. Can we catch those revealing flashes?

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