Origin of matter

Updating Magic Universe

Why Mr Sakharov’s Coat Fits Better Now

You may suspect that some big stride has occurred when 500 physicists from 86 institutions begin to report the results of 8 years of effort by referring to a paper written by a celebrated predecessor more than 40 years ago. The old fellow was Andrei Sakharov, writiing about antimatter back in January 1967. The report in May 2010 tells of an experiment at Fermilab near Chicago that helps to explain why our Universe contains anything substantial at all, in the way of atoms, stars or people. That’s to say, how it came about that matter survived the Big Bang.

Andrei Dmitrievich Sakharov, H-bomb inventor and rebellious peacemaker. His chief legacy to "pure" physics was his prompting about the antimatter problem, which has guided scientists ever since. Source: the A.D. Sakharov Museum

As Magic Universe recalls in the story about Antimatter:

On a reprint of his four-page paper in the Soviet Journal of Experimental and Theoretical Physics Letters, Sakharov jotted in his own hand a jingle in Russian, summarizing its content. In translation it reads:

From the Okubo Effect/ At high temperature/ A coat is cut for the Universe/ To fit its skewed shape

To grasp what he was driving at, and to find out, for example, who Okubo was and how temperature comes into it, you have to backtrack a little through 20th-Century physics. The easier part of the explanation poses the cosmic conundrum that Sakharov tackled. What happened to all the antimatter? …

[later] For creating matter, the only way known to physicists involves concentrating twice the required energy and making an exactly equal amount of antimatter at the same time. And there’s the problem. If the Universe made equal amounts of matter and antimatter, it should all have disappeared again, in mutual annihilation, leaving the cosmos completely devoid of matter.

Well, the Universe is pretty empty. Just look at the night sky. That means you can narrow the problem down, as Sakharov did. For every billion particles of antimatter created you need only 1,000,000,001 particles of matter to explain what remains. To put that another way, in supplying the mass of the Earth, the Universe initially made the equivalent of two billion and one Earths and threw two billion away in mutual annihilation. The traces of the vanished surplus are all around us in the form of invisible radiation.

Yet even so small a discrepancy in the production of matter and antimatter was sufficient for Sakharov to call the Universe skewed. And in his 1967 paper he seized on recent discoveries about particles to say how it could have come about. Here the physics becomes more taxing to the human imagination, because Mother Nature is quite coy when she breaks her own rules.

Previous cases of rule-breaking cited in the Magic Universe story simply weren’t sufficient to fix the matter-survival problem. They were (1957) “parity violation” concerning a bias in the direction of emission of positrons (anti-electrons) from radioactive atoms, and (1964) long-lived neutral K particles sometimes breaking up into two pions when the theory said three. The latter result vindicated the reasoning of Susumu Okubo of Rochester, in 1958, so he was the physicist celebrated in Sakharov’s jingle. But as I comment in Magic Universe: the K particles … were like naughty toddlers, nowhere near strong enough for the heist that stocked the Universe with matter.

Now the Fermilab result tells us that, during the decay of beefier shortlived sub-atomic particles, Mother Nature slightly prefers ordinary muons (heavy electrons, negatively charged) to anti-muons (positively charged), by roughly 101 to 100.

The 5500-ton DZero detector counts positively and negatively charged muons emerging from proton-antiproton collisions produced by the Tevatron particle collider. A magnetic field makes muons with opposite charges follow paths that curve in opposite directions, so that they can be counted. Credit: Fermilab.

The sub-atomic particles in question are B mesons made of a pair of heavy particles called bottom quarks. Magic Universe tells how, from the 1990s onwards, great “B factories” were set up at the world’s particle accelerators in the hope of finding misbehaviour among the B mesons. That’s what the the Fermilab team seems to have accomplished at last.

One per cent of favouritism towards the ordinary muons may not sound much. But it’s fifty times larger than is permitted by the Standard Model, which has dominated particle physics (rather disappointingly) since the 1970s. Quite apart from the relevance of the Fermilab result to tailoring Sakarov’s coat, the result reassures physicists that there really are important discoveries about the workings of the microcosmic world waiting to be made.

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

A lesser update for the same Antimatter story in Magic Universe concerns a space project.

Also being prepared for the International Space Station was the largest particle detector of sub-atomic particles ever operated beyond the Earth’s atmosphere, in a collaboration involving Europe, the USA, China, Taiwan and Russia. The Alpha Magnetic Spectrometer was designed to look for antimatter. One possible source would be the break-up of exotic particles in the mysterious Dark Matter that fills the Universe.

The crazy-seeming aspect of this experiment was that the Alpha Magnetic Spectrometer was also to look for anti-helium nuclei. This took scientists right back to re-examining the axiom of the Sakharov scenario, that all antimatter was wiped out very early in the history of the Universe. If, on the contrary, significant amounts have survived, anti-helium would be a signature. Anti-protons from anti-hydrogen won’t do, because they are too easily made in present-day collisions of ordinary cosmic-ray particles.

The news is that the Alpha Magnetic Spectrometer will fly on the very last Space Shuttle mission before the fleet is mothballed, no earlier than mid-November 2010.

References

A.D. Sakharov, Pisma Zh. Eksp. Teor. Fiz. 5, 32, 1967, translated as Sov. Phys. JETP Lett. 5, 24,1967, and available at http://www.jetpletters.ac.ru/ps/1643/article_25089.pdf

N. Calder, Magic Universe, “Antimatter: does the coat that Sakharov made really explain its absence?”, pp. 15-23, Oxford UP, 2003

The new DZero paper will be published in Physical Review D, and is available at http://arxiv.org/abs/1005.2757

The quote from Stefan Soldner-Rembold is in a Fermilab press release http://www.fnal.gov/pub/presspass/press_releases/CP-violation-20100518.html