By John Timmer, Ars Technica

Two of the general purpose detectors at the LHC, ATLAS and CMS, tend to keep a high profile, as they’re designed to be able to spot anything that comes out of the collisions — the Higgs, dark matter, or something even more exotic. LHCb is quite a bit more specialized, as it is designed specifically to track those collisions that include a particle that contains a bottom quark (generically, these particles are called B mesons). In doing so, it’s meant to provide the most precise test of a number of predictions made by the Standard Model; should the test show it fails, they could provide indications of supersymmetry or a mechanism that explains why our Universe is filled with matter and not antimatter.

As with the other two detectors, the people behind LHCb have put together their preliminary data for the summer physics meetings, and so far, it all looks very good; the detector has already provided the most precise test of some features of the Standard Model. And, so far, it has emerged unscathed, which may mean bad things for supersymmetry and send theoreticians back to the drawing board on our matter/antimatter asymmetry.

Bottom quarks are a heavier counterpart to the down quarks that make up most of the matter we normally experience. They exist for only a short time before they decay, but they last long enough to form short-lived particles with other quarks. The decay of the B meson, driven by the decay of the bottom quark, can occur through a number of different intermediates; the precise frequency of each decay pathway and the particles that accompany it are predicted by the Standard Model. By looking at enough of these events, it should be possible to tell when one of these predictions is off.

Being a bit off could have some serious implications. The decay of B mesons is predicted to favor the production of matter over antimatter, but not enough to possibly explain the huge preponderance of matter seen in the present Universe. A greater deviation than that predicted could help balance the books on matter. In addition, some specific types of decays are sensitive to the presence of particles we’ve not yet detected, such as the extra family of particles predicted by supersymmetry, a candidate to replace the Standard Model.

The lightest supersymmetric particle has some interesting properties, in that it’s stable and won’t interact with regular matter; its mass is also sufficient to allow it to account for the dark matter observed by astronomers and cosmologists. Ruling out supersymmetric particles would cause the physics community to do a serious rethinking. So far, however, results from CMS and ATLAS haven’t been promising, and LHCb’s data continue that trend. In a slide from a talk by Gerhard Raven, representing LHCb, several slides were labeled as displaying data that provides a large sensitivity to new physics such as supersymmetry. In each of these slides, the data matched the Standard Model predictions, as shown here.

Another slide showed a type of B meson decay that could go through supersymmetric intermediates, like the chargino, a charged Higgs, or a neutralino. No sign of these exotic particles was present. As a result, some are suggesting that the results might be the last nail in the coffin for the simplest forms of supersymmetry.

The same thing applies to a hunt for greater matter/antimatter asymmetries. Earlier results from the Tevatron had suggested a few types of B meson decays might hint at a difference from the Standard Model, but the LHCb data, which already has a greater statistical certainty, indicates that the difference from the Standard Model is insignificant. As one of the slides summed it up, “No signs of New Physics just yet.”

The same slide, however, cautions that “We’ve only just gotten started: plenty left on the shopping list!” Over time, thanks to the high luminosity of the LHC, rare decay pathways should be observed, and even the more common ones will be measured with increased precision. Even if things don’t look good for the simplest forms of supersymmetry, LHCb might spot some flaw in the standard model that will help the theorists come up with something better.

Images: Fermilab

Source: Ars Technica

See Also: