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CERN | Geneva | Switzerland

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The Standard Model checked to the ninth decimal

At the European Physics Society conference in Stockholm, two experiments operating at the Large Hadron Collider (LHC) at CERN, LHCb and CMS reported on July 19 solid evidence that the Standard Model of particle physics still shows no sign of wear and tear by checking a prediction of the model to the ninth decimal place.

The Standard Model makes very accurate predictions but theorists know this theory has its limits. At higher energy, its equations start breaking down. Theorists are convinced that despite all the success of this model, it is not giving us the big picture. Hence, scientists have been trying to find a “secret passage” to the next level, a more encompassing and more robust theory.

One way to achieve this is to look for a small deviation in a measured quantity from the value predicted by the Standard Model and a good place to find such a deviation is in an extremely rare process. It is much easier to hear a faint noise in a quiet place than in the middle of traffic during rush hour.

Specifically, the scientists measured how often composite particles denoted Bs and Bd (pronounced “b sub s and b sub d)” mesons decay into a pair of muons (particles similar to electrons but about 200 times heavier). A Bs meson is a composite particle containing b and s quarks while Bd mesons are made of b and d quarks. These heavy particles are unstable and quickly break apart into lighter particles.

The Standard Model predicts that Bs mesons decay into a pair of muons about three times in a billion while for Bd mesons, it occurs thirty times less often. This gives two excellent places to look for small deviations that could reveal the existence of new phenomena not foreseen within the Standard Model.

All theories going beyond the Standard Model come with new particles that would affect how other particles decay, i.e. how they break apart. Decays are very much like making change for a big coin. Imagine a coin of one euro. It can be broken into pieces of 1, 5, 10, 20 or 50 cents Now, say a new 25-cent coin is introduced. An automatic teller would not give change for one euro in a particular way (say with coins of 50, 20, 20 and 10 cents) as often as before just because new possibilities now exist.

By measuring how often a Bs meson decays into muons, scientists were hoping to see the first deviations from the predictions of the Standard Model. On the contrary, the two experiments confirmed this prediction within experimental errors.

CMS, whose name stands for Compact Muon Spectrometer, and LHCb, an experiment designed specifically to study particles containing b quarks, are particularly suited for these types of measurements. CMS got (3.0 +1.0-0.9) x 10-9 and LHCb obtained (2.9 +1.1-1.0) x 10-9, while the Standard Model prediction stands at (3.5 ± 0.3) x 10-9. The significances of the CMS and LHCb signals correspond to 4.3σ and 4.0σ, respectively, which means, the excesses of events that are seen most likely come from signal and not from background. Two other experiments presented new results based on smaller data samples. ATLAS (using a partial data sample) and D0 (final result with their full data sample) and they obtained the same upper limit at 15 x 10-9.Bs-mumu-combo

The results obtained by LHCb and CMS, as well as their combined value, is compared to the prediction from the Standard Model shown by the vertical black line and its theoretical uncertainty (green band).

For Bd decays, 95% confidence level upper limits were set at 7.4 x 10-10 for LHCb while CMS obtained 11 x 10-10. The Standard Model predicts this to be less than 1 x 10-10.

All these values are consistent with the Standard Model predictions but they do not yet rule out new physics. After the LHC resumes operation at higher energy in 2015, the LHC experiments will continue improving their Bs measurements. In particular, they will aim to get a first measurement for Bd mesons instead of an upper limit, and then evaluate the ratio for the Bs and Bd mesons, such that some of the experimental and theoretical uncertainties will cancel out, to obtain an even more precise measurement. Since no deviations were found in the ninth decimal position, it means the experiments need to check the tenth decimal position.

More details can be found on the CMS and LHCb websites.

Pauline Gagnon

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