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CERN | Geneva | Switzerland

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LHCb is trying to crack the Standard Model

LHCb, one of the Large Hadron Collider (LHC) experiments, was designed specifically to study charge-parity (or CP) violation. In simple words, its goal is to explain why more matter than anti-matter was produced when the Universe slowly cooled down after the Big Bang, leading to a world predominantly composed of matter. This is quite puzzling since in laboratory experiments, the measured preference for the creation of matter over antimatter is too small to explain why we only see matter around us today in the Universe. So why did the Universe evolve this way?

One of the best ways to study this phenomenon is with b quarks. Since they are heavy, they can decay (i.e break down in smaller parts) in many different ways, but are light enough for us to produce in copious amounts (unlike the heaviest quark, the top quark). In addition, theorists can make very precise predictions on their decay rates using the Standard Model, the theoretical framework we have to describe most phenomena observed to this day.  Once we have predictions on how often b quarks should decay into one or another decay mode, we can compare this with what is measured with the LHCb detector, and see if there are any deviations from the Standard Model predictions. Such deviations would indicate that this model is incomplete, as every physicist suspects, even though we have not been able to define the nature of the more complex theoretical layer that must be hidden or measure anything in contradiction with the Standard Model.

Here is how LHCb wants to do it: by studying rare decays with a precision never achieved before.

When electrons or protons collide in large accelerators, b quarks are produced, but they do not come alone. They are typically accompanied by one other quark (mostly u, d or s) to form composite particles called B mesons. Such mesons have been produced at several colliders, most abundantly in b-factories in the US and Japan, but also at the Tevatron, an accelerator similar to the LHC and located near Chicago in the US.

Physicists from b-factories studied the decays of B mesons in great detail for more than ten years, but nothing new disproving the Standard Model has been uncovered so far, even after scrutinizing the decays of more than 470 millions B pairs of mesons! All decay modes inspected behaved according to the Standard Model predictions. This means we now need to study even rarer decay modes, the ones the Standard Model predicts will occur only once in a billion times. To do so, we need to look at several billion decays to detect the slightest deviation. This is in these small details that we hope to uncover new physics going beyond the Standard Model.

Recently, the Tevatron experiments, D0 and CDF, took the lead by measuring very rare decays, namely Bs → μμ, where a Bs (a meson made of an anti-b and an s quark) decays to a pair of muons, (denoted m), a particle very similar to electrons, only heavier. CDF saw a small excess of events with respect to Standard Model expectations. And when they look at the angular distributions of Bs à J/Ψ Φ , that is when the Bs meson decays into two other mesons, J/Ψ and Φ, they can measure a parameter called Φs , which is supposed to be zero according to the Standard Model. Both D0 and CDF obtained a non-zero result, but this measurement is not quite accurate enough to really challenge the Standard Model.

And that’s where LHCb, the new kid on the b-physics block, comes into play. With the LHC delivering data at a fast and furious pace, LHCb can already surpass the precision reached at the Tevatron. Already in July, LHCb (and CMS, another LHC experiment) contradicted the CDF claim of anomalous number of Bs → μμ events. They might do it again with the release of their first measurement of Φs, which is expected to be much more precise than the Tevatron result.

Will Φs be equal to zero as predicted by the Standard Model? LHCb will announce this on Saturday at the Lepton-Photon conference in Mumbai. Could LHCb be the first experiment to crack the Standard Model? With the level of precision they are already reaching, even if it’s not now, they will be in the best position to do it in the near future.

Stay tuned. The new results will be added here on Monday.

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Addition:
At the Mumbai Lepton Photon conference on Saturday, LHCb presented their new measurement in the decay of Bs → J/ψ φ . They measure the parameter φs to be near zero, as predicted by the Standard Model. Being more precise than the CDF and D0 measurement announced earlier this year, this new measurement shows that the Standard Model holds true even when tested with this unprecedented precision.

However, there is still room for new and unexpected phenomena as the LHCb precision increases as new data are being analysed. LHCb should have about three times more data available by the end of the year, putting the Standard Model under even more rigorous tests.

LHCb result

LHCb result

The color circles show the LHCb results at different degrees of precision. The theoretical prediction is shown in black with its own uncertainty. At present, the two are in fair agreement. With more data analysed, the uncertainty in the experimental measurement will decrease, allowing for an even more stringent test of the current prediction. (The extra set of circles correspond to the other solution to the equation).

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline

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