Teasing strange matter from the ordinary

In a unique analysis of experimental data, nuclear physicists have made the first-ever observations of how lambda particles, so-called "strange matter," are produced by a specific process called semi-inclusive deep inelastic scattering (SIDIS). What's more, these data hint that the building blocks of protons, quarks and gluons, are capable of marching through the atomic nucleus in pairs called diquarks, at least part of the time. These results come from an experiment conducted at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility.

It's a result that has been decades in the making. The dataset was originally collected in 2004. Lamiaa El Fassi, now an associate professor of physics at Mississippi State University and principal investigator of the work, first analyzed these data during her thesis project to earn her graduate degree on a different topic.

Nearly a decade after completing her initial research with these data, El Fassi revisited the dataset and led her group through a careful analysis to yield these unprecedented measurements. The dataset comes from experiments in Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF), a DOE user facility. In the experiment, nuclear physicists tracked what happened when electrons from CEBAF scatter off the target nucleus and probe the confined quarks inside protons and neutrons. The results were recently published in Physical Review Letters.

"These studies help build a story, analogous to a motion picture, of how the struck quark turns into hadrons. In a new paper, we report first-ever observations of such a study for the lambda baryon in the forward and backward fragmentation regions," El Fassi said.

In like a lambda, out like a pion

Like the more familiar protons and neutrons, each lambda is made up of three quarks.

Unlike protons and neutrons, which only contain a mixture of up and down quarks, lambdas contain one up quark, one down quark and one strange quark. Physicists have dubbed matter that contains strange quarks "strange matter."

In this work, El Fassi and her colleagues studied how these particles of strange matter form from collisions of ordinary matter. To do so, they shot CEBAF's electron beam at different targets, including carbon, iron, and lead. When a high-energy electron from CEBAF reaches one of these targets, it breaks apart a proton or neutron inside one of the target's nuclei.

"Because the proton or neutron is totally broken apart, there is little doubt that the electron interacts with the quark inside," El Fassi said.

After the electron interacts with a quark or quarks via an exchanged virtual photon, the "struck" quark(s) begins moving as a free particle in the medium, typically joining up with other quark(s) it encounters to form a new composite particle as they propagate through the nucleus. And some of the time, this composite particle will be a lambda.

But the lambda is short-lived -- after formation, it will swiftly decay into two other particles: a pion and either a proton or neutron. To measure different properties of these briefly created lambda particles, physicists must detect its two daughter particles, as well as the beam electron that scattered off the target nucleus.

The experiment that collected this data, EG2, used the CEBAF Large Acceptance Spectrometer (CLAS) detector in Jefferson Lab's Experimental Hall B. These recently published results, "First Measurement of ? Electroproduction off Nuclei in the Current and Target Fragmentation Regions," are part of the CLAS collaboration, which involves almost 200 physicists worldwide.

SIDIS

This work is the first to measure the lambda using this process, which is known as semi-inclusive deep inelastic scattering, in the forward and backward fragmentation regions. It's more difficult to use this method to study lambda particles, because the particle decays so quickly, it can't be measured directly.

"This class of measurement has only been performed on protons before, and on lighter, more stable particles," said coauthor William Brooks, professor of physics at Federico Santa María Technical University and co-spokesperson of the EG2 experiment.

The analysis was so challenging, it took several years for El Fassi and her group to re-analyze the data and extract these results. It was her thesis advisor, Kawtar Hafidi, who encouraged her to pursue the investigation of the lambda from these datasets.

"I would like to commend Lamiaa's hard work and perseverance in dedicating years of her career working on this," said Hafidi, associate laboratory director for physical sciences and engineering at Argonne National Lab and co-spokesperson of the EG2 experiment. "Without her, this work would not have seen fruition."

"It hasn't been easy," El Fassi said. "It's a long and time-consuming process, but it was worth the effort. When you spend so many years working on something, it feels good to see it published."

El Fassi began this lambda analysis when she herself was a postdoc, a couple of years prior to becoming an assistant professor at Mississippi State University. Along the way, several of her own postdocs at Mississippi State have helped extract these results, including coauthor Taya Chetry.

"I'm very happy and motivated to see this work being published," said Chetry, who is now a postdoctoral researcher at Florida International University.

Two for one

A notable finding from this intensive analysis changes the way physicists understand how lambdas form in the wake of particle collisions.

In similar studies that have used semi-inclusive deep inelastic scattering to study other particles, the particles of interest usually form after a single quark was "struck" by the virtual photon exchanged between the electron beam and the target nucleus. But the signal left by lambda in the CLAS detector suggests a more packaged deal.

The authors' analysis showed that when forming a lambda, the virtual photonhas been absorbed part of the time by a pair of quarks, known as a diquark, instead of just one. After being "struck," this diquark went on to find a strange quark and forms a lambda.

"This quark pairing suggests a different mechanism of production and interaction than the case of the single quark interaction," Hafidi said.

A better understanding of how different particles form helps physicists in their effort to decipher the strong interaction, the fundamental force that holds these quark-containing particles together. The dynamics of this interaction are very complicated, and so is the theory used to describe it: quantum chromodynamics (QCD).

Comparing measurements to models of QCD's predictions allows physicists to test this theory. Because the diquark finding differs from the model's current predictions, it suggests something about the model is off.

"There is an unknown ingredient that we don't understand. This is extremely surprising, since the existing theory can describe essentially all other observations, but not this one," Brooks said. "That means there is something new to learn, and at the moment, we have no clue what it could be."

To find out, they'll need even more measurements.

Data for EG2 were collected with 5.014 GeV (billion electron-volt) electron beams in the CEBAF's 6 GeV era. Future experiments will use electron beams from the updated CEBAF, which now extend up to 11 GeV for Experimental Hall B, as well as an updated CLAS detector known as CLAS12, to continue studying the formation of a variety of particles, including lambdas, with higher-energy electrons.

The upcoming Electron-Ion Collider (EIC) at DOE's Brookhaven National Laboratory will also provide a new opportunity to continue studying this strange matter and quark pairing structure of the nucleon with greater precision.

"These results lay the groundwork for upcoming studies at the upcoming CLAS12 and the planned EIC experiments, where one can investigate the diquark scattering in greater detail," Chetry said.

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Physicists reveal why matter dominates our universe

Why does matter dominate our universe?

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.

Distinguished Professor Sheldon Stone says the findings are a first, although matter-antimatter asymmetry has been observed before in particles with strange quarks or beauty quarks.

He and members of the College's High-Energy Physics (HEP) research group have measured, for the first time and with 99.999-percent certainty, a difference in the way D0mesons and anti-D0 mesons transform into more stable byproducts.

Mesons are subatomic particles composed of one quark and one antiquark, bound together by strong interactions.

"There have been many attempts to measure matter-antimatter asymmetry, but, until now, no one has succeeded," says Stone, who collaborates on the Large Hadron Collider beauty (LHCb) experiment at the CERN laboratory in Geneva, Switzerland. "It's a milestone in antimatter research."

The findings may also indicate new physics beyond the Standard Model, which describes how fundamental particles interact with one another. "Till then, we need to await theoretical attempts to explain the observation in less esoteric means," he adds.

Every particle of matter has a corresponding antiparticle, identical in every way, but with an opposite charge. Precision studies of hydrogen and antihydrogen atoms, for example, reveal similarities to beyond the billionth decimal place.

When matter and antimatter particles come into contact, they annihilate each other in a burst of energy -- similar to what happened in the Big Bang, some 14 billion years ago.

"That's why there is so little naturally occurring antimatter in the Universe around us," says Stone, a Fellow of the American Physical Society, which has awarded him this year's W.K.H. Panofsky Prize in Experimental Particle Physics.

The question on Stone's mind involves the equal-but-opposite nature of matter and antimatter. "If the same amount of matter and antimatter exploded into existence at the birth of the Universe, there should have been nothing left behind but pure energy. Obviously, that didn't happen," he says in a whiff of understatement.

Thus, Stone and his LHCb colleagues have been searching for subtle differences in matter and antimatter to understand why matter is so prevalent.

The answer may lie at CERN, where scientists create antimatter by smashing protons together in the Large Hadron Collider (LHC), the world's biggest, most powerful particular accelerator. The more energy the LHC produces, the more massive are the particles -- and antiparticles -- formed during collision.

It is in the debris of these collisions that scientists such as Ivan Polyakov, a postdoc in Syracuse's HEP group, hunt for particle ingredients.

"We don't see antimatter in our world, so we have to artificially produce it," he says. "The data from these collisions enables us to map the decay and transformation of unstable particles into more stable byproducts."

HEP is renowned for its pioneering research into quarks -- elementary particles that are the building blocks of matter. There are six types, or flavors, of quarks, but scientists usually talk about them in pairs: up/down, charm/strange and top/bottom. Each pair has a corresponding mass and fractional electronic charge.

In addition to the beauty quark (the "b" in "LHCb"), HEP is interested in the charmed quark. Despite its relatively high mass, a charmed quark lives a fleeting existence before decaying into something more stable.

Recently, HEP studied two versions of the same particle. One version contained a charmed quark and an antimatter version of an up quark, called the anti-up quark. The other version had an anti-charm quark and an up quark.

Using LHC data, they identified both versions of the particle, well into the tens of millions, and counted the number of times each particle decayed into new byproducts.

"The ratio of the two possible outcomes should have been identical for both sets of particles, but we found that the ratios differed by about a tenth of a percent," Stone says. "This proves that charmed matter and antimatter particles are not totally interchangeable."

Adds Polyakov, "Particles might look the same on the outside, but they behave differently on the inside. That is the puzzle of antimatter."

The idea that matter and antimatter behaves differently is not new. Previous studies of particles with strange quarks and bottom quarks have confirmed as such.

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Physicists solve 35-year-old mystery about quarks

Quarks, the smallest particles in the universe, are far smaller and operate at much higher energy levels than the protons and neutrons in which they are found. In 1983, physicists at CERN, as part of the European Muon Collaboration (EMC), observed for the first time what would become known as "the EMC effect": In the nucleus of an iron atom containing many protons and neutrons, quarks move significantly more slowly than quarks in deuterium, which contains a single proton and neutron.

Now physicists from Tel Aviv University, the Massachusetts Institute of Technology (MIT) and the Thomas Jefferson National Accelerator Facility know why quarks, the building blocks of the universe, move more slowly inside atomic nuclei.

"Researchers have been seeking an answer to this for 35 years," says Prof. Eli Piasetzky of TAU's Raymond and Beverly Sackler School of Physics & Astronomy. Prof. Piasetzky; Meytal Duer, also of TAU's School of Physics; and Prof. Or Hen, Dr. Barak Schmookler and Dr. Axel Schmidt of MIT have now led the international CLAS Collaboration at the Thomas Jefferson National Accelerator Facility to identify an explanation for the EMC effect. Their conclusions were published on February 20 in the journal Nature.

The researchers discovered that the speed of a quark depends on the number of protons and neutrons forming short-ranged correlated pairs in an atom's nucleus. The more such pairs there are in a nucleus, the larger the number of slow-moving quarks within the atom's protons and neutrons.

Atoms with larger nuclei intrinsically have more protons and neutrons, so they are more likely to have a higher number of proton-neutron pairs. The team concluded that the larger the atom, the more pairs it is likely to contain. This results in slower-moving quarks in that particular atom.

"In short-range correlated or SRC pairs, an atom's protons and neutrons can pair up constantly, but only momentarily, before splitting apart and going their separate ways," Duer explains. "During this brief, high-energy interaction, quarks, in their respective particles, may have a larger space to play in."

The team's new explanation can help to illuminate subtle yet important differences in the behavior of quarks, the most basic building blocks of the visible world.

For the research, the scientists harnessed a Large Acceptance Spectrometer, or CLAS detector, a four-story spherical particle detector, in an experiment conducted over several months at the Continuous Electron Beam Accelerator Facility (CEBAF) at the Thomas Jefferson National Accelerator Facility. The experiment amassed billions of interactions between electrons and quarks, allowing the researchers to calculate the speed of the quark in each interaction based on the electron's energy after it scattered, and to compare the average quark speed among the various atoms.

"These high-momentum pairs are the reason for these slow-moving quarks," Prof. Hen explains. "How much a quark's speed is slowed depends on the number of SRC pairs in an atomic nucleus. Quarks in lead, for instance, were far slower than those in aluminum, which themselves were slower than iron, and so on."

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