Hidden valleys in our own backyard

High-energy collisions may reveal new lightweight particles if they are decay products of a heavy one.

To search for new phenomena, physicists need to go where no one has gone before. High-energy collisions have the potential to create particles too heavy to have been made in previous experiments at lower energy since the masses of the new particles come from the energy of collisions. High beam intensities shine more light on rare processes because a larger number of collisions means more chances to see an improbable reaction or decay. Sometimes, though, you need a little of both.

Suppose that there’s not just one new particle waiting to be discovered, but a whole system of new particles. Some of these may be hidden by their high masses while others may be hidden by very low reaction rates with ordinary matter. This possibility is sometimes called a hidden valley. If we can reach high enough in energy to create one of these new massive particles, it can decay into all the rest, revealing the whole landscape.

To test such a scenario, CMS scientists looked for new particles in their own backyard. That is, they scanned the debris of high-energy collisions for signs of low-mass particles to see if any new ones had slipped in among the familiar faces of the 1960s particle zoo. This is different from most searches, which look for high-mass signatures directly.

To be concrete, these scientists posed the question: what if the Higgs boson is not a single particle, but a family of interrelated particles? In addition to directly observing a new massive boson, like the one discovered last July, they should see the lightweight cousins of the Higgs from Higgs-to-Higgs decays. Or alternatively, what if dark matter has its own version of a photon, oxymoronically known as dark light? These dark photons would be produced along with invisible dark matter, but they would appear as visible low-mass particles.

In a careful analysis, no new lightweight particles were found. But when searching for the unexpected, it’s important to keep the whole landscape in view, including the part we thought we knew.

—Jim Pivarski

The physicists shown above developed and performed this search for new low-mass particles.
The efforts of the Fermilab LPC User Support Group are crucial and enable many CMS physics analyses.