REDTOP – a potential voyage to new physics horizons

A newly proposed venture by Fermilab scientists seeks to peek beyond the borders of the known realm of physics through particles called etas.

It’s called REDTOP, and while it may sound like the name of a brightly colored, fast-spinning toy, the physics it’s after has nothing to do with rotational motion. REDTOP scientists want to use eta mesons to study unexplored processes hidden in the vast ocean of subatomic physics, which could lead to the discovery of new physics.

Ways to decay

The eta advantage has to do with a behavior scientists call decay — and it’s right in the experiment’s name: Rare Eta Decays with a TPC for Optical Photons. (TPC stands for “time projection chamber,” a type of particle detector.)

Most subatomic particles undergo decay: They live for a fleeting moment and then decay into — become replaced by — particles of smaller mass. Various physics laws dictate how and how often the mother particle decays into particular daughter particles.

Scientists have measured hundreds of particle decay patterns, each with a particular likelihood of occurring. A negatively charged pion, for example, decays in about 99.99 percent of all cases into a muon and an antineutrino. In the remaining fraction, the pion decays into one of nine other possibilities.

The proposed REDTOP experiment to study eta particles would use the detector shown here, in gray.

The advantage of zero

Every subatomic particle has a collection of “quantum numbers,” numerical assignments for each of a particle’s various properties. Take the properties of electric charge and spin. An electron’s electric charge quantum number is -1; its spin quantum number is ½. For the Higgs, electric charge and spin are both 0.

According to physics laws, quantum numbers tend to remain unchanged throughout the decay process. If the pre-decay mother particle has a charge of 1, then the total charge of its post-decay daughter particles also adds to 1. In physicist parlance, its charge is conserved. If a quantum number changes in the decay process, the corresponding property is not conserved.

And this is where REDTOP scientists hope to find new physics: in quantum numbers that change in a never-seen-before way. Those rarely witnessed, nonconserved behaviors might point to reveal hidden truths about our universe.

Etas belong to a peculiar category of particles called pseudo-Nambu-Goldstone bosons, meaning all their quantum numbers are equal to zero. That’s what makes eta decays so interesting to scientists: Of all the possible decay paths a particle can take, an eta can take only those whose descendants’ quantum numbers add to zero, substantially limiting the number of ways it can decay.

Take the strange-sounding property called strangeness. Since an eta has a strangeness of 0 – which means it has no strangeness at all — it decays by the rarely occurring paths that don’t involve strangeness. Particle decays that do have strangeness, as in the case of kaons, would be strongly influenced by that property, taking the far more common strangeness-dominated paths and overshadowing other rarer decays.

Because all quantum numbers of an eta are zero, no single property eclipses another, making etas very “pure” states and allowing the rare decays of the subatomic realm to melt out of the woodwork. That makes etas the perfect laboratory: They’re rare-decay factories.

The small mass of the eta is a bonus. Because it’s lightweight, it tends to decay into a small number of particles, producing only three or four daughter particles. Scientists can decipher these simple decays more easily than large particle collision events, which can include decays into more than 50 particles.

The REDTOP window to new physics

So far scientists have listed 14 of these rare — and, being rare, little-understood — eta decays as possible candidates for new physics. The unknown details, once filled in, could lead to new discoveries. REDTOP scientists speculate that the decays may relate to dark matter, to new particles or even to a new “milliweak force,” a proposed force that is 1,000 times weaker than the known weak force.

“This is only the tip of the iceberg. We expect that the portfolio of eta decays we want to study with REDTOP will continue to grow,” said Corrado Gatto, spokesperson for the REDTOP collaboration and a scientist at Northern Illinois University and the Italian institution INFN. “We keep working on new simulations to really understand all our possibilities with REDTOP.”

REDTOP wants to produce a high-intensity sample of more than one trillion eta mesons in one year. That’s 10,000 times more than the current eta production worldwide.

But eta mesons come with a catch: They are hard to produce and even harder to study.

To make eta particles, REDTOP scientists propose using a proton beam that crashes into a specifically designed target to create an enormous shower of particles, many of which will be etas. The problem is that the collision would create 200 times more particles of other types than etas.

“If you want to measure etas with conventional detector technology, the background will swamp the detector, and it will be constantly lit like a Christmas tree,” Gatto said.

REDTOP scientists are proposing to develop three novel detector technologies, which are some of the project’s most challenging aspects. They could result in new technologies for the next generation of high-intensity experiments. Furthermore, Fermilab scientists working on the accelerator design for REDTOP will provide the laboratory with a new facility: a continuous proton beam with an energy selectable by the experimenters.

“REDTOP will be a challenge, because we want to push for new physics and new technologies at the same time,” said Fermilab scientist Anna Mazzacane, who is working on REDTOP physics and detector simulations. “But we also have a lot to gain: REDTOP is not an experiment of just one measurement. It has a lot of potential for great discoveries.”

The REDTOP collaboration aims to present the experiment to the Fermilab Physics Advisory Committee in January 2018. The Fermilab PAC is the body that reviews proposals for new experiments at the laboratory.