Why is the universe made of matter? Not up to Standard

The Economist, Mar 1st 2012, 11:46 by J.P.   http://www.economist.com/blogs/babbage/2012/03/why-universe-made-matter

THAT the universe is made of matter is obvious enough. What bothers fundamental physicists, though, is why that is. The best current theory of particle physics, called the Standard Model, suggests that basically equal amounts of matter and antimatter should have been produced in the Big Bang. Famously, however, matter and antimatter annihilate on contact and disappear in a puff of pure energy, so a primordial equity between the two would have led to a lifeless, photon-filled universe. One possible reason for the apparent imbalance is a phenomenon called charge conjugation/parity violation (or CP violation for short).

If charge conjugation and parity were conserved, nature would treat particles and their antiversions, which carry the opposite electric charge and opposite value of a property called spin, alike. The Standard Model allows for a whiff of CP violation. But that is nowhere near enough of it to explain matter's cosmic dominance.

Last November an experiment called LHCb, which feeds off the world's biggest particle accelerator, the Large Hadron Collider (LHC) near Geneva, saw hints of excess CP violation beyond theorists' predictions. Now an experiment in America, known as CDF,has spotted something similar. It announced its results at a meeting in La Thuile, in Italy.

Like LHCb, CDF was looking at a subatomic species called mesons. These come in a variety of types, or flavours, each containing a quark and an antiquark. One flavour, known as D0-mesons, is composed of a charm quark and an up antiquark. Its antimatter twin, known as D0-bar, consists of a charm antiquark and an up quark. (A charm quark is a heavier cousin of the up quark which, together with down quarks, makes up the ordinary protons and neutrons found in atomic nuclei; lest things get too straightforward, the down quark is not simply an up antiquark.)

D0s and D0-bars are unstable and decay into other short-lived particles called pions and kaons. According to the Standard Model, D0-mesons and D0-bars should produce these in basically the same proportions. CDF has confirmed that they do not. 

The experiment has not been taking new data since its source of particles, an accelerator called the Tevatron at Fermilab, a big laboratory near Chicago, was shut down last September. But its boffins are busy crunching billions of as-yet-unanalysed collisions and have now looked at all those remaining which involve D0-mesons. They put the odds of the D0/D-bar discrepancy coming about by pure chance, something physicists have to reckon with in the probability-ruled world of quantum physics, at about one in 200. That is less than LHCb's result of one in 2,000, and well below the exacting standard of one in 3.5m required to claim discovery in particle physics. But it lends credence to the idea that CP violation is indeed lurking there.

It might yet emerge, however, that the findings do not in fact contradict the Standard Model. That is because, for complicated theoretical reasons that have to do with how the strong nuclear force works, calculating the expected number of different D-meson decays is notoriously tricky. Since until LHCb dropped its bombshell no one expected to see much in the way of CP violation in charm quarks, theorists have only recently got cracking on the difficult sums. They need more time to reach a firm conclusion.

Experimentalists aren't sitting idly by. Researchers at LHCb and others are looking for hints of CP violation which are unequivocally at odds with the Standard Model. They know that theory cannot be the last word in particle physics. Otherwise no one would be here to ponder the matter/antimatter conundrum.