A physicist’s journey to the ‘critical point’ and the ‘strong force’

Yale physicist Helen Caines has arrived at a key juncture in her long campaign to understand the “critical point” and the “strong force” of nuclear matter.

In the subatomic realm, the universe’s tiniest particles, called quarks, combine with other particles called gluons to form protons and neutrons — the nucleus of atoms.

But at one time — just a fraction of a second after the Big Bang, in fact — quarks and gluons existed in another phase of matter. It was a hot, dense, gumbo-like state known as quark-gluon plasma (QGP). The “critical point” is a unique feature along the theoretical boundary separating these two phases.

Then you have the “strong force,” one of the most powerful and least understood forces of nature. The strong force holds protons together at the heart of every atom, even though protons’ positive electrical charge should repel them from each other.

“Everything around us in the universe today is confined into protons and neutrons,” said Caines, a professor of physics in Yale’s Faculty of Arts and Sciences and a member of the Yale Wright Laboratory. “How that step happened has intrigued me for a long time.”

Caines has studied QGP-related phenomena for decades, primarily as a main contributor to the STAR collaboration at Brookhaven National Laboratory on Long Island. STAR (it stands for Solenoidal Tracker at RHIC) recreates the conditions of the QGP in a controlled environment and studies how particles interact there.

While STAR has produced a wealth of data since 2000 — including helping to confirm the existence of QGP — the next few years are likely to be particularly important in helping to understand these phenomena. The STAR detector recently completed a major upgrade and will collect data about particle collisions for three more years; a new device called sPHENIX, also located at Brookhaven, will collect even more data about particle collisions; in Switzerland, at the CERN facility, another longtime experiment called ALICE is also recreating the QGP state of matter and collecting data.

In an interview with Yale News, Caines talked about her long association with STAR and what scientists hope to learn about the critical point.

How close are we to finding the ‘critical point’?

Helen Caines: We recently recorded collisions over a very broad range of energies at STAR [part of a multi-year program called the Beam Energy Scan]. If the medium we created in these collisions is at the temperature and chemical potential close to the critical point, we should be able to find it.

We have the most precise data set we’re ever going to get, and we have prepared all the analysis tools that we think we will need to reveal the critical point if it is there. So now it’s a question of finishing those analyses, which should happen in the next year or two. We expect to have key results out by the end of that time. If it’s there and measurable, we should see it.

With all the current focus on QGP, what is it like to be in the middle of so much research activity?

Caines: I think it is great that there is a lot of momentum right now surrounding our work. We’ve gone through the stage of discovering the quark-gluon plasma, the stage of figuring out how to do the precise measurements we would need in order to search for the critical point, and the stage of setting up the right detectors that will allow us to understand these tiny, little signals.

Now we have several experiments out there, and over the next three or four years we’re going to collect a benchmark set of data. That’s the era we’re heading into and there’s so much excitement.

What upgrades did your team make to the STAR device?

Caines: We had three major upgrades for the Beam Energy Scan. We upgraded our time projection chamber, which is our main tracking detector for the whole experiment, so it now measures a much larger fraction of all the particles that are created in collisions. We also put in a second “time of flight” detector, to better determine the types of particles being created. Lastly, we put in an event-plane detector to better determine the centrality, or violence, of the collisions.

In addition, last year we installed a suite of detectors to measure particle production at angles very close to the beam line, a region that has not previously been well explored at the Relativistic Heavy Ion Collider [RHIC] at Brookhaven.

We’re running now with that whole new suite of detectors.

What would you most like to know about the critical point?

Caines: There are two things at the top of the list.

One is to find the critical point, to have it stand as a textbook measurement in the same way that we have standard measurement for the ice and steam phases of water. The other is to understand the extreme heat and other conditions necessary to create the quark-gluon plasma state. Teasing out these initial conditions is another question we’re poised to answer.

What could we do with that knowledge?

Caines: It’s part of us being able to understand, really, what the “strong force” is.

We know the strong force is there. It’s one of four fundamental forces, along with gravity, electromagnetism, and the weak nuclear force. But the strong force is the strongest, by many orders of magnitude, and it is what holds together the quarks and gluons that make up protons and neutrons.

Yet we don’t fully understand that mechanism. We have a very basic, lack of understanding about one of the fundamental forces. That’s why we have such a large community of scientists from around the globe coming together to work on this.

As this research proceeds to its own critical point, have you reflected at all on the eventual completion of the STAR experiment a few years from now?

Caines: It’s bittersweet. It’s sad that it will eventually shut down, but it will give way to the next big particle accelerator that’s coming online, the Electron Ion Collider [EIC]. EIC will continue our studies into this area of what is the strong force, what really makes up a proton, and what is the three-dimensional process involved.

This field isn’t going away, we’re just looking at it from different directions.