Syracuse University Magazine

The Particle Detectives

The Particle Detectives

In an amazing adventure of modern science, a group of Syracuse physicists is providing breakthroughs on some of the most mysterious particles of the universe

By Rob Enslin

Some 2,000 years ago, Julius Caesar built a 19-mile rampart from Lake Geneva to the Jura Mountains to fend off hordes of Gallic attackers, thus laying the groundwork for the Swiss city of Geneva. Today, the same stretch of land boasts another marvel of engineering, almost as long—17 miles in circumference—and mostly underground. Locals agree the edifice has a futuristic, if not vaguely mysterious, quality about it. But what goes on there is of cosmic proportions.

The European Organization for Nuclear Research—or CERN, an acronym derived from the first letters of its original French name—lies north of Geneva, in the shadow of the Swiss Alps. With its labyrinth of tunnels, the facility is home to the aptly named Large Hadron Collider (LHC), the world’s biggest, most powerful particle accelerator. Included is an enormous, state-of-the-art vacuum system, guided by powerful superconducting electromagnets that, in turn, are cooled by liquid helium. The accelerator hurtles beams of protons, in opposing directions, through miles of vacuum, until they collide with one another. The result is a blinding flash of energy that some say is brighter than a thousand suns. This subatomic collision course is not just the stuff of spy-techno thrillers—it is physics’ present and future.

Founded after World War II, at the dawn of international collaboration, CERN is as much an interdisciplinary research organization as it is the most sophisticated science laboratory on Earth. Thousands of scientists and engineers annually flock there, hoping to better understand the physical makeup of the universe—or, as some say, peer into the “mind of God.” Chief among them is a team of physicists from Syracuse University, led by Distinguished Professor Sheldon Stone. “This is more than just pushing the boundaries of physics,” he says during a recent meeting in his book-lined office on campus. “We want to know what really happened after the Big Bang, 13.7 billion years ago, that has allowed matter to survive and build the universe we inhabit today.”

Stone oversees the efforts of some 20 people at CERN, including four other professors, two research assistant professors, and a swath of graduate students and undergraduates from the Department of Physics in the College of Arts and Sciences. To say several of them—Stone, along with professors Tomasz Skwarnicki, Marina Artuso, and Steven Blusk—are rock stars in the high-stakes field of particle physics is to flirt with understatement. Case in point: Since spring 2014, all four have garnered international attention for their groundbreaking work in new physics—that is, physics beyond the Standard Model, a decades-old theory used to describe the fundamental forces and particles that make up the universe.


Physics professors Tomasz Skwarnicki (left), Sheldon Stone, Marina Artuso, and Steven Blusk, pictured here in a Physics Building lab, are involved in groundbreaking research at CERN.

Photo by Steve Sartori

The following is a brief look at their accomplishments:

July 2015: Tomasz Skwarnicki and Sheldon Stone—along with doctoral student Nathan Jurik G’16 and Liming Zhang, a former Syracuse research associate who is now a professor at Tsinghua University in Beijing—virtually break the Internet, with news of their discovery of two rare pentaquark states. Their finding puts to rest a 51-year-old mystery, in which American physicists Murray Gell-Mann and George Zweig independently proposed that all baryonic matter is made of either three quarks, or four quarks plus an antiquark that’s called a pentaquark. While many three-quark baryons have been found, the pentaquark sighting is a first.

July 2015: Marina Artuso addresses the European Physical Society Conference on High-Energy Physics in Vienna, showing that baryonic decays, containing beauty (b) quarks, are consistent with being “left-handed.” Her talk focuses on the decay of a particular Lambda baryon into a proton, as well as a muon and a neutrino. (A muon is charged like an electron, but with more mass. A neutrino has no charge and very little mass, and, thus, is hard to find.) Until now, this decay has never been seen.

November 2014: Steven Blusk identifies two never-before-seen baryons with three quarks and large masses, while sifting through CERN data that is several years old. Thanks to heavyweight b quarks, each particle is six times more massive than a proton.

April 2014: Stone and Zhang suggest that a particle called f0(980)—widely thought to be “exotic” and have four quarks—has only two quarks (i.e., a quark and an antiquark). Their finding stems from a theoretical model of their own design that determines the particle’s composition. This work supports their primary objective, which is to analyze the charge-parity (CP) violation in the decay of a Bs meson, in an attempt to explain the absence of matter in the universe. “Without CP violation, equal numbers of protons and antiprotons would form and then instantly annihilate each other, resulting in no net-creation of matter,” Stone says. “Right after the Big Bang, antimatter disappeared, leaving behind matter to form everything around us, from stars and galaxies to life on Earth.”

April 2014: Three weeks earlier, Skwarnicki is the lead author of a paper confirming the existence of a particle with two quarks and two antiquarks, known as a tetraquark. Dubbed Z(4430), the state was discovered by the Belle Collaboration in Japan in 2007, only to be disputed by the BABAR experiment at Stanford. Belle responds a few years later with even more rigorous analysis of the same data set. Drawing on Belle’s and BABAR’s analysis techniques, Skwarnicki and company comb over their own particle data from CERN and confirm that Z(4430) is, indeed, real.  

Blueprint for Existence
Since time immemorial, people have sought to explain the birth of the universe. “Who are we?” “Where do we come from?” These questions are at the heart of ancient creation myths and modern scientific theories, not to mention countless philosophical and theological debates. To describe the very early universe, physicists generally rely on the Big Bang Theory, which posits that part of the universe visible today was originally a few millimeters long. In fact, everything associated with physics—fundamental particles, stars, galaxies, even space and time—was packed so tightly into one spot that it built up tremendous energy and heat. Scientists put the surrounding nascent temperature at 10 billion degrees Fahrenheit.

No one really knows why, but in a billionth of a billionth of a billionth of a billionth of a second, the point suddenly expanded and began to cool, and has been doing so ever since. Thus, the Big Bang was not as much of a “bang,” as it was an unprecedented inflation, with all the galaxies functioning like points on the surface of the balloon. Remnants of those first few moments still persist in radiation left over from primordial gravitational waves.

Physicists have determined that, for one-trillionth of a second after the Big Bang, the universe was roughly the size of a baseball. Within it, particles formed, collided, and disintegrated trillions of times over, until they stabilized into elements of hydrogen, helium, or lithium. Interactions of these elements and some 90 others became the blueprint for existence.

It is the Big Bang that informs much of what goes on at CERN and, by extension, in Syracuse’s High-Energy Experimental Physics Group. Fresh from a two-year upgrade, the $7 billion LHC was built by CERN from 1998 to 2008, in collaboration with 10,000-plus scientists from more than 100 countries. Stone chuckles at the irony of using the world’s biggest machine to pinpoint the universe’s smallest fragments. “We’re recreating the first millionth of a second of creation,” says Stone, a Fellow of the American Physical Society (APS). “By flinging particles together to see what happens, we’re focusing energy on an awesome scale. It’s pretty amazing, really.”

With its ring-shaped tunnel straddling the Franco-Swiss border, the LHC circulates beams of protons in opposite directions. The faster the beams, the more energy they generate upon impact. The result? “Energy is sometimes converted into heavy particles that are not normally found in nature,” says Skwarnicki, also an APS Fellow. “By examining debris from these high-energy collisions, we learn more about the building blocks of matter and the forces controlling them.”

The LHC came into public view in 2012, when scientists proved the existence of the Higgs boson, the so-called “God Particle” that is 100 times more massive than the proton, and is able to transmit forces. (Until then, the Higgs boson was the only fundamental particle predicted by the Standard Model that had not been observed.) Skwarnicki says that, immediately after the Big Bang, the universe’s four known fundamental forces—strong, electromagnetic, weak, and gravitational—emerged, along with the Higgs boson and its associated field. “The LHC enables us to study exotic particles, such as the Higgs and tetraquarks and pentaquarks, to see how these forces work,” he says.

LHC experiments also search for new heavier particles that may explain physics beyond what is already known. One such theory, supersymmetry, stipulates that every particle in the Standard Model has a heavier twin with vastly different properties, such as different spins and electrical charges. These experiments may provide alternate explanations of such concepts by finding additional spatial dimensions.

Tetraquarks and pentaquarks are currently on the minds of physicists everywhere. Guy Wilkinson—who heads up CERN’s Large Hadron Collider beauty (LHCb) experiment, where Team Syracuse is based—considers the University an important ally. “Syracuse has been a valued participant of LHCb for many years, well before the LHC delivered its first collisions in 2009,” says Wilkinson, also a physics professor at Oxford University (UK). “Professors Artuso, Blusk, Stone, and Skwarnicki have an exceedingly strong track record in producing important, high-profile publications, of which the pentaquark discovery analysis is the most recent example. The scientific output of the experiment would have been much weaker, without their unstinting efforts.”   

Wilkinson goes on to say that Syracuse’s findings not only explain how protons and neutrons are bound together, but also how matter is constituted. “This has profound consequences, for example, on what happens to stars at the end of their lives,” he says. “As stars collapse, having burnt all their fuel, the nature of hadrons [composite particles made of quarks] also changes. Tetraquarks and pentaquarks could play an important role in this process.”

John Rennie, editorial director of McGraw-Hill Education’s AccessScience online platform and former editor-in-chief of Scientific American magazine, echoes these sentiments. “These are some of the most fascinating and important results coming out of experimental physics today,” he says, regarding tetraquarks and pentaquarks. “These particles force physicists to rethink theories about how atomic nuclei behave under the most extreme conditions, like during the super-dense earliest universe and inside neutron stars. That may not have much day-to-day significance for most of us, but it could have big implications for our understanding of how stars evolve at the end of their lives.”

What Particles Tell Us
CERN is home to many projects, but probably none more innovative than LHCb. It involves approximately 800 scientists from 16 countries, seeking to explain why the universe is made up of matter, instead of antimatter. The “b” stands for “beauty quark,” or “bottom quark,” but “b quark” is just fine, thank you very much.

While absent in today’s universe, b quarks were prevalent after the Big Bang, and have been generated by the LHC in the billions. Integral to this process is LHCb’s 5,600-ton detector, which is located 330 feet below the French village of Ferney-Voltaire, and is in the throes of a multiyear upgrade. “We use a series of subdetectors to detect particles, which are thrown forward in one direction, during a collision,” says Stone, who spends an average of three months a year at CERN, working on LHCb. “A lot of different quarks are created by the LHC, before they fall apart or decay into other forms. Our goal is to catch these b quarks, which are usually part of some baryon or meson, and analyze their decays.”

To appreciate LHCb, one must understand quarks, which make up most of the matter in the universe. Based on a nonsensical word in James Joyce’s Finnegan’s Wake (“Three quarks for Muster Mark”), quarks come in different types, or flavors: up, down, charm, strange, top, and bottom. Up and down quarks are the most common and stable in the universe. The rest are heavier and less stable, and are produced only during high-energy collisions.

That quarks are bound together by a short-range force is of vital importance. The theory describing this force is quantum chromodynamics (QCD). Rennie says Syracuse’s findings have huge repercussions for the study of QCD. “This is the model that aims to describe precisely how quarks interact inside particles, like the proton,” he adds. “QCD’s predictions are now often only roughly accurate, but physicists may learn things from the study of pentaquarks that could refine QCD’s math.”

Murray Gell-Mann, a Distinguished Fellow at the Santa Fe Institute, as well as the Robert Andrews Millikan Professor Emeritus at the California Institute of Technology, is proud to see his iconic work come full circle. “This is part of a long process of discovery of particle states,” he says, regarding pentaquarks. “Every baryon is composed mostly of three quarks. Also, part of the time, it’s three quarks and a pair. What they [seem to have found] here is a particle that is, part of the time, made of three quarks and a pair, and not just three quarks, which would be conventional.”

The recipient of the 1969 Nobel Prize in Physics, Gell-Mann is hopeful that more particles await discovery. “There is even the possibility of a particle made of gluons and no quarks,” he says, pointing out that gluons, which keep protons and neutrons intact, carry the strong force.

Doctoral student Nathan Jurik G’16, for one, is not lost on the significance of his involvement with LHCb. Of all the theories governing the four fundamental forces, QCD, he says, is probably the least understood. “We have a long way to go to understand what particles are telling us,” Jurik says. “Just because we know that five quarks can form a particle doesn’t mean we know how it happens. We’re at the stage where a lot of questions are being raised, in hopes of finding answers to some pretty fundamental questions. To do this, we need to perform more sensitive studies of pentaquarks, as well as discover new pentaquarks and tetraquarks. The whole thing is pretty surreal.”

An Unfolding Journey
Syracuse’s high-energy research group operates like a well-oiled machine. While their expertise overlaps, each member brings specific skills to the table. Stone, a driving force behind LHCb, does a lot of physics analysis, as do Artuso, Skwarnicki, and Blusk. Artuso is also in charge of the Upstream Tracker (UT) Project, which recently received a $5 million grant from the National Science Foundation (NSF) to build an inner tracking device. Professor Matthew Rudolph, who specializes in precision measurements, joined the Syracuse group in August. “I guess you can say that Syracuse has outsized its role at LHCb,” Stone says, leaning forward in his chair. “Of the 250 papers that the collaboration has published so far, our group [at Syracuse] has written about 50 of them. That’s more than 20 percent. Pretty impressive, if you figure that we account for less than 2 percent of the size of the entire collaboration.”

Under Artuso’s direction, the UT is scheduled to be installed in 2019, and, with the rest of the upgrade, will increase the amount of data that LHCb can handle by factors of five to 10. She also extracts fundamental constants—numbers and values that dictate the strengths of forces, such as gravity and the masses of elementary particles—from LHCb data. An APS Fellow, as well as an advisor to the U.S. Department of Energy and NSF, Artuso is a highly respected leader in the development, design, and construction of detectors for elementary particle physics experiments. She says the UT will help explain things that the Standard Model cannot, such as the relationship between matter and antimatter, the properties of invisible dark matter, and the values of the masses of quarks and leptons (subatomic particles not affected by strong interactions). “The new detector will substantially increase the luminosity that LHCb can handle, providing more accurate measurements of fundamental particles and enabling observations of rare processes that occur below the current sensitivity level,” says Artuso, who, several years ago, founded a group of LHCb scientists, dedicated to studying the semileptonic decay of hadrons. “Just as importantly, the NSF grant will enable students to participate in the construction and testing of the detector at Syracuse.”

Faculty cannot overstate the importance of students and postdocs to LHCb. In fact, Skwarnicki says it was former SU research associate Liming Zhang, along with Jurik, who instigated the pentaquark project and wrote the code for the analysis. “All four of us cross-checked the data results with one another, from beginning to end, and then quickly got the paper out,” recalls Zhang, who presented the group’s findings at CERN in July. “I am proud to have come up with a model that was able to be proven, mathematically, and was essential to the project.”

Blusk, who oversees UT’s test-beam studies program, considers training the next generation of particle physicists as mission-critical. “The test-beam program, in many ways, mimics a scaled-down version of the detector we plan to install, so our students get a broad view of the complexities of planning and building an experiment,” he says. “This is vital to the overall LHCb upgrade, and is essential for the collaboration to continue producing cutting-edge physics results.”

Wilkinson applauds the Syracuse physicists’ contributions to advancing fundamental science, and looks forward to their continuing efforts. “LHCb is one the great adventures of modern science,” he says.
Stone agrees: “It’s said that physics is one of humanity’s great success stories. But it’s one that is far from over.” «


Last summer at CERN, Syracuse physicists discovered a class of particles known as pentaquarks. In a pentaquark particle, five quarks could be assembled into a meson (one quark and one antiquark) and a baryon (three quarks). A pentaquark could also feature four quarks and one antiquark that are tightly bound.

Photos and art courtesy of CERN

What's the Matter

Most of the matter around us is made from protons and neutrons, both of which are composed of quarks. There are six kinds of quarks, but they are usually thought of in pairs: up/down, charm/strange, and top/bottom. Whereas protons and electrons have integer charge values of +1 and -1, respectively, quarks have fractional charges of +2/3(e.g., up, charm, and top quarks) or -1/3 (e.g., down, strange, and bottom quarks). All quarks have an intrinsic angular momentum called spin, with a value equal to 1/2.

Quarks are like social creatures, in that they exist in groups with other quarks, and are never found alone. Composite particles made up of quarks are called hadrons, which are divided into baryons (usually containing three quarks) and mesons (usually a quark and an antiquark).

According to the Standard Model, there are two fundamental classes of particles: fermions and bosons. Fermions have half integral spin, and include quarks, protons, neutrons, and electrons. Bosons are force-carrier particles with integral spin (including zero), and include photons, gluons, and the Higgs boson.


CERN’s Large Hadron Collider circulates beams of protons at close to the speed of light before they smash together, creating subatomic debris that scientists study.


The LHCb experiment’s powerful magnet helps scientists identify particles after protons are smashed together. Particles normally travel in straight lines, but the presence of a magnetic field causes the paths of charged particles to curve, with positive and negative particles moving in opposite directions. By examining the path’s curvature, scientists can calculate the momentum of a particle and thus establish its identity.

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