Yale’s Helen Caines takes a leadership role in international experiment

A photo of a device that tracks particles produced by ion collisions.
The left half of this image shows the Solenoidal Tracker at RHIC. It is a detector that specializes in tracking the thousands of particles produced by each ion collision at RHIC. The right half of the image shows the end view of a collision of two 30-billion electron-volt gold beams in the STAR detector at RHIC. (Image courtesy of STAR)

Helen Caines has spent much of her professional life immersed in cosmic soup.

While other physicists have chased gravitational waves, cultivated qubits, and mused about dark matter, Caines has focused squarely on the thick glop of particles that transformed into nuclear matter in the first milliseconds after the Big Bang. Through studying these particles, Caines believes, humanity can come to understand the basic processes that formed the early universe at that instant.

Now Caines is a leading voice in explaining how much we’ve learned so far and what is to come. On July 1, she became co-spokesperson for the STAR experiment, an international collaboration of more than 600 physicists searching for the theorized “critical point” that transformed the universe from a soup of quarks into what we know as matter today.

We’re doing very exciting physics, things we never dreamed we’d be able to do when we started,” said Caines, an associate professor of physics and member of Yale’s Wright Lab. “STAR is a testament to how innovative a collaboration can be. We have the whole range of experience, from undergraduates to emeritus professors working with us.”

The STAR experiment is focused on the dense, hot soup of quarks and gluons — known as the quark-gluon plasma — that is believed to have existed ten millionths of a second after the Big Bang. These conditions can be recreated in the laboratory by colliding heavy ions and studying the reactions — an endeavor that still amazes Caines even after more than 20 years of research.

It’s just so intriguing that you can smash heavy ions together and actually learn something about the early universe from it,” she said. “It’s like smashing two automobiles together and then trying to determine the make and model of each one.”

STAR launched in 1991 and is based at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. The experiment began collecting data in 2000. More than 60 institutions in 13 countries are part of STAR.

Helen Caines will co-lead the STAR experiment's investigation of what happened ten millionths of a second after the Big Bang. (Photo by Michael Marsland)

Yale’s involvement in the STAR experiment runs deep. Zhangbu Xu, co-spokesperson with Caines, has a Yale Ph.D., and Yale physics professor John Harris was the founding spokesperson, serving from 1991 until 2002. Current Yale collaborators, along with Caines and Harris, are emeritus professor Jack Sandweiss; adjunct professor Thomas Ullrich; graduate students Stephen Horvat, Daniel Nemes, and David Stewart; senior research scientist Richard Majka; research scientist Nikolai Smirnov; and postdoctoral associates Saehanseul Oh and Li Yi.

Yale has been committed to heavy ion physics research since the founding by professor D. Allan Bromley of the original Wright Nuclear Structure Laboratory in 1966 and its various upgrades of its tandem van de Graaff accelerators,” Harris said. Yale became a member institution of the STAR experiment in 1996, when Harris arrived on campus.

Caines joined the experiment in 1996 as well. Her work involves measuring the high-momentum particles that are produced when ultra-relativistic heavy ions are collided. Specifically, she focuses on the particles’ movement through the surrounding soup. The work is helping scientists start to understand the properties and characteristics of a new state of matter in transition.

This is where the so-called “critical point” becomes essential to physicists. Caines has been a major proponent for a program at RHIC called Beam Energy Scan, which has successfully concluded its first phase of experiments and is in the middle of its analysis.

BES covers the full range of collision energies at RHIC with the primary goal of potentially discovering a critical point that is predicted to exist in the phase diagram of nuclear matter,” Harris said. “At this critical point nuclear matter transforms into a plasma of quarks and gluons in a first order phase transition, where nuclear particles as we know them coexist for an instant with quarks and gluons in a very hot phase, about 100,000 times hotter than our Sun.”

Caines will co-lead STAR in its continuing investigation of this nuclear phase and help lead a second phase of experiments over the next few years. She and Yale graduate student Horvat have identified an approximate region in collision energy and temperature where researchers may find the critical point — a region where the hotter phase of quarks and gluons gives way to the cooler nuclear phase.

Caines’ colleagues say she is well suited to her new role.

These large collaborations require a lot from a spokesperson,” said Sarah Demers, the Horace Taft Associate Professor of Physics at Yale and a member of the ATLAS experiment at CERN’s Large Hadron Collider in Geneva, Switzerland. “You need to be a physics detector expert, a physics analysis expert, and you need to be able to keep your colleagues inspired and behind a common plan. Helen is an excellent physicist, and she knows how to lead a team.”

Caines received her Ph.D. from the University of Birmingham, U.K., in 1996. She was appointed assistant professor at Yale in 2004 and promoted to associate professor in 2010. She is a faculty member of Yale’s Wright Lab.

Part of the satisfaction of her job, she said, is the opportunity to be surprised even after decades of research. The STAR experiment exemplifies this, she explained.

We’re at a very interesting stage,” Caines said. “We think we may find a place in nuclear matter, where things go wild.”

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Part of the In Focus Collection: While you were away: the summer’s top stories

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Jim Shelton: james.shelton@yale.edu, 203-361-8332