Answers to two of the most stubborn questions in science may come down to one point – the point where one particle smashes into another.
The first question comes from physics: What is the strong force? Particles called gluons exert this force inside subatomic particles, keeping protons and neutrons from flying apart in a shower of quarks.
Cosmology gives rise to the second question: What happened in the first microseconds after the Big Bang? A portion of the energy released in that cosmos-spawning event was transformed into the matter that makes up the known universe. But how did that matter form?
Last fall, Dragos Velicanu revealed clues to these mysteries. His presentation won the Klaus Kinder-Geiger Award for best talk at the Hot Quarks meeting, a biennial workshop for young scientists who study the physics of ultrarelativistic (near light speed) collisions between atomic nuclei.
Velicanu, a Department of Energy Computational Science Graduate Fellowship recipient and doctoral candidate in high-energy physics at the Massachusetts Institute of Technology, reported findings from a series of collisions that lasted 4 hours, 20 minutes in September 2012. The experiment was conducted by the CMS Collaboration, an international group of more than 3,000 scientists, engineers and students using the Compact Muon Solenoid detector at the Large Hadron Collider (LHC), the giant particle collider at CERN, the European physics laboratory. By pitting protons against lead nuclei, the collaboration found a new wrinkle in an unexplained correlation between charged particles that spew out of such collisions.
To analyze the results, Velicanu worked with Wei Li, a former MIT postdoctoral fellow who now is an assistant professor at Rice University. “Together Wei Li and I did the whole analysis completely independently to cross-check our results and help us find and fix bugs,” Velicanu says. “I also helped design, test and monitor some of the triggers used in this and upcoming analyses. Triggers are the algorithms and hardware/software we use to collect our data.”
Cosmologists and physicists agree that answers to their two fundamental questions must lie in quark-gluon plasma, or QGP, a species of matter a particle collider can create. QGP also existed during the universe’s first microseconds. In a QGP, quarks are freed from their partners but, still in the grip of the strong force, move around within a fireball no bigger than the original particles. The QGP winks out of existence in billionths of a second as the quarks recombine, forming everyday protons and neutrons.
However, as these pedestrian particles form, other particles escape from the fireball. By detecting these, scientists intend to find answers to both questions.
The CMS lies 100 meters underground and measures 21 meters long and 15 meters in diameter. With its array of instruments, it can detect not just the muons that inspired its name but also many other particle types. When CMS researchers sent protons into head-on collisions with lead nuclei, the instrument fielded particles from about 2 million collisions, detecting countless charged particles – mostly pions, the lightest union of quark and antiquark.
Velicanu and Li selected only collisions that shot at least 110 charged particles out to the sides – a sign of a direct hit between proton and nucleus – giving them a comparatively small sample to analyze. They calculated the angle between every conceivable pair of charged particles. Then they searched for correlations among the angles.
“We primarily use distributed computing systems,” Velicanu says. “To run thousands of jobs in parallel, I use either Condor at the MIT cluster, which can run a given program with unique parameters on thousands of computers simultaneously, or CRAB, which I can submit to the entire LHC computing grid that’s spread out around the world, and simultaneously run a program on data held in different continents.”
A few of the correlations they found offered no surprises. The first of them emerged at angles at or near zero degrees. This often-seen correlation arises when jets form, tossing out particles one after another like baseballs from a pitching machine. The angles between these paths are naturally close to zero degrees.
The second correlation shows up at 180 degrees. This correlation also is familiar because some particles shoot out in exactly opposite directions from jets that are locked together back to back.
Other correlations result from directional flows of particles that form as the fireball expands. Velicanu and Li used statistical methods to correct for these errant correlations.
The still-unexplained correlation was left standing. Viewed from the side of the collider, the two particles in each of these pairs appear to shoot in all directions, with the angle between any pair ranging from about 120 degrees to 240 degrees. But viewed looking down the collider’s barrel, the particles leave as pairs following almost exactly the same path – that is, the angle between the two paths is close to zero degrees.
This final correlation is not altogether new. It cropped up in data from Brookhaven National Laboratory’s Relativistic Heavy Ion Collider when gold nuclei were pitted against each other, and again at the LHC in lead-on-lead collisions. Random event generators predicted these correlations based on the almond-shaped fireballs such collisions produce. The asymmetrical expansion expected from such fireballs leads to the unusual correlation.
The correlation took on an air of mystery when it showed up in the data from proton-proton collisions, which do not form asymmetrical fireballs and, in fact, may not form QGP at all. The short run of proton-lead collisions in September yielded the strongest such correlation so far.
Scientists have been speculating about what might cause this correlation. Are newly formed particles exhibiting quantum entanglement, in which both bear a memory of their common-parent proton? Or is the correlation a sign of a never-before-seen type of matter?
The September experiment was a prologue to a longer run in January of this year, which generated billions of useful collisions. Velicanu and Li are analyzing the results. “The reason this is interesting is that we can finally put ‘the feet to the fire’ of some of the popular theories that arose to explain this data,” Velicanu says. In particular, the color glass condensate theory holds that the correlation denotes a novel state of matter and predicts data for collisions that give off three or four times more particles than the meager average of 110 per collision seen in September. “The new data can tell us how well this theory survives the data.”