Evidence for Why We Exist

The DZero collaboration comprises about 500 scientists from 19 countries who designed and built the 5,500-ton DZero detector and now collect and reconstruct collision data. Credit: DZero collaboration

Scientists at the Fermi National Accelerator Laboratory may have found an answer for a basic cosmological question. If the universe is composed of matter and anti-matter, then how do we exist?

When matter and anti-matter particles collide in high-energy collisions, they turn into energy and produce new particles and antiparticles. At the Fermilab proton-antiproton collider, scientists observe hundreds of millions every day. Similar processes occurring at the beginning of the universe should have left us with a universe with equal amounts of matter and anti-matter, which would have then annihilated each other. But the world around is made of matter only and antiparticles can only be produced at colliders, in nuclear reactions or cosmic rays. “What happened to the antimatter?” is one of the central questions of 21st–century particle physics.

The Fermilab scientists discovered a significant violation of this mathematically perfect view of matter-antimatter symmetry. The violation occurred in the behavior of particles containing bottom quarks, and was beyond what is expected in the current theory, the Standard Model of particle physics.

The new result, submitted for publication in Physical Review D by the DZero collaboration, an international team of 500 physicists, indicates a one percent difference between the production of pairs of muons and pairs of antimuons in the decay of B mesons produced in high-energy collisions at Fermilab’s Tevatron particle collider. In other words, the particle collisions produced one percent more muons than anti-muons — a net gain of matter over anti-matter.

The DZero detector records particles emerging from high-energy proton-antiproton collisions produced by the Tevatron. For this measurement of CP violation, scientists analyzed 10 trillion collisions collected over the last eight years. Credit: Fermilab

The dominance of matter that we observe in the universe is possible only if there are differences in the behavior of particles and antiparticles. Although physicists have observed such differences (called “CP violation") in particle behavior for decades, these known differences are much too small to explain the observed dominance of matter over antimatter in the universe.

However, the new study found that the B mesons, which constantly shift back and forth between a state of matter and anti-matter, are slightly slower in transitioning from one phase than in the other. Namely, they shift back into "matter" faster than they shift into "antimatter." This disparity led to the one percent gain of matter seen in the experiment.

If confirmed by further observations and analysis, the effect seen by DZero physicists could represent another step towards understanding the observed matter dominance by pointing to new physics phenomena beyond what we know today.

Using unique features of their precision detector and newly developed analysis methods, the DZero scientists have shown that the probability that this measurement is consistent with any known effect is below 0.1 percent (3.2 standard deviations).

"This exciting new result provides evidence of deviations from the present theory in the decays of B mesons, in agreement with earlier hints," said Dmitri Denisov, co-spokesperson of the DZero experiment, one of two collider experiments at the Tevatron collider. Last year, physicists at both Tevatron experiments, DZero and CDF, observed such hints in studying particles made of a bottom quark and a strange quark.

To obtain the new result, the DZero physicists performed the data analysis "blind," to avoid any bias based on what they observe. Only after a long period of verification of the analysis tools, did the DZero physicists look at the full data set. Experimenters reversed the polarity of their detector’s magnetic field during data collection to cancel instrumental effects.

“Many of us felt goose bumps when we saw the result,” said Stefan Soldner-Rembold, co-spokesperson of DZero. “We knew we were seeing something beyond what we have seen before and beyond what current theories can explain.”

The Fermilab accelerator complex accelerates protons and antiprotons close to the speed of light. The Tevatron collider, four miles in circumference, produces millions of proton-antiproton collisions per second, maximizing the chance for discovery. Credit: Fermilab

The precision of the DZero measurements is still limited by the number of collisions recorded so far by the experiment. Both CDF and DZero therefore continue to collect data and refine analyses to address this and many other fundamental questions. The DZero result is based on data collected over the last eight years by the DZero experiment: over 6 inverse femtobarns in total integrated luminosity, corresponding to hundreds of trillions of collisions between protons and antiprotons in the Tevatron collider.

According to Dennis Kovar, Associate Director for High Energy Physics in DOE’s Office of Science, "The Tevatron collider is operating extremely well, providing Fermilab scientists with unprecedented levels of data from high energy collisions to probe nature’s deepest secrets.”