LHCb is trying to crack the Standard Model

LHCb will reveal new results tomorrow that will shed more light on the possible CP-violation measurement reported recently by the Tevatron experiments, different from Standard Model predictions. Quantum Diaries blogger for CERN, Pauline Gagnon, explains how.


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 antimatter 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 we do not measure a preference for the creation of matter over antimatter. Hence the CP-conservation law in physics that states that Nature should not have a preference for matter over antimatter. 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 into 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 located near Chicago in the US.

Physicists from b-factories have 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 of 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. It 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 → μ μ, where a Bs (a meson made of an anti-b and an s quark) decays to a pair of muons, (denoted μ), 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 → 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 is already able to surpass the precision reached at the Tevatron. Already in July, LHCb (and CMS, another LHC experiment) contradicted the CDF claim of an anomalous number of Bs → μ μ 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.

Pauline Gagnon for CERN's Quantum Diaries Blog.