Quantum Diaries

I felt like a giddy child when I found out about these results! See below for the article I wrote for Physics World (unedited version).

Muons could explain missing antimatter in the universe

Particle physicists at Fermilab’s Tevatron accelerator in the US have found an exciting new result that could explain one of the big mysteries of cosmology – why there is matter in our universe.

Guennadi Borissov of Lancaster University in the UK led an international team of researchers in the analysis of proton-antiproton collisions at the DO experiment. In these collisions, they were searching for B_d and B_s mesons decaying to muons.

New Physics

CP violation is a fundamental difference between the behaviour of a particle and its antiparticle. Without it, matter would not have survived in the universe after the Big Bang, because matter and antimatter were created in equal amounts and should have annihilated completely.

To date, precision B-factory experiments BELLE, Japan and BABAR, US have accurately measured CP violation manifesting in the decay of kaons and B_d mesons, and the results have been consistent with predictions from the Standard Model of particle physics. However, this is not nearly enough to explain the matter-antimatter asymmetry in the universe, so scientists are thrilled to finally see signs of CP violation beyond the theory’s expectations. B-physicist Tim Gershon of Warwick University, who has worked on these experiments, explains:

“Measurements from the B factories have placed stringent limits on many of the possible deviations from the Standard Model. The B_s system has long been thought a good place to look for the extra CP violation that we know exists in nature.  Results from both CDF and D0 have hinted at new physics effects before, causing great excitement in the community.”

Now, D0 has shown that B_s decays may indeed hold the key to understanding our existence. The recent measurement suggests a comparatively large asymmetry that could overthrow Standard Model predictions and help to explain the universe’s matter dominance. This, Borissov says, is “the most important implication of our result.”

Measuring asymmetry

Neutral B mesons can oscillate between their particle and antiparticle, which means that spotting which one is which to measure any asymmetry can be tricky. One way to do this is to look for semileptonic decays, such as to muons. In this case, a W boson carries the charge of the flavour-changing bottom quark to the muon. The meson can then be identified as B0 or anti-B0 by the muon’s charge.

In this measurement, D0 looked for two muons of the same sign coming from the same B anti-B pair, meaning that one oscillated to its antiparticle before decaying. “Each one could decay into a muon, a neutrino and, say, a charm-flavour meson”, Borissov explains. Asymmetry between the B and anti-B is then measured as an overall preferred charge for the measured muon-pairs.

However, he is keen to point out that it isn’t as easy as it sounds, warning that many muons in the proton anti-proton collisions come from kaon decays. This background is serious because kaons have an artificial preference over anti-kaons for decaying in the D0 detector, so that if they were mistaken for B mesons a fake asymmetry would be seen. To get around this problem, the asymmetry in control samples of kaon decays was measured and removed.

Considering the challenges, the result is remarkable and has exciting implications. The final measured asymmetry deviates from the Standard Model prediction by 3.2 s. “This means that the probability of the result being simply a statistical fluctuation is around 1 in 1000”, says Prof Terry Wyatt of University of Manchester, former spokesperson of D0. Clearly the measurement is striking, but more work needs to be done before scientists can be certain the deviation is real.

Outlook for new physics

The largest uncertainty limiting D0s measurement is from low statistics, so they are continuing to gather data. Wyatt continues, “We hope to increase the collected data set by about a factor of 2. In addition, we can hope for improvements in the analysis techniques that could reduce the uncertainty further.” An agreement from D0’s sister experiment, CDF, would also be a promising test of the measurement.

Possibly the most excited by this result are those at the LHCb experiment at the LHC. Gershon, who is now working on the experiment, remarks, “The LHCb’s data samples will soon become the envy of the global B physics community.”  Guy Wilkinson, LHCb’s physics coordinator, explains why.

“LHCb can record up to 2000 of the interesting decays every second, and our instrumentation is optimised for reconstructing these decays. These two factors mean that for many studies LHCb expects to surpass the sensitivity of the Tevatron experiments rather early in this, the first LHC run, of 2010-11.”

Measurements of muon asymmetry are already underway to compare with D0s result. The potential changes to the Standard Model also have exciting implications for another potential CP-violation measurement, which LHCb are calling their “golden channel”. LHCb B-physicist and CERN fellow Rob Lambert gushes, “The release of that paper was like Christmas come early for me. I stayed up into the small hours getting overly excited by the great things we can do at LHCb”.

You can find the published version here.