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CERN | Geneva | Switzerland

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Charm and beauty: LHCb has it all!

This week, the LHCb experiment reported an anomaly they have just observed in the decays of charmed mesons. Could this be the tip of the iceberg for new physics?

As I reported before, the main goal of the LHCb experiment is to use heavy quarks (called beauty and charm quarks) to make precise measurements in the hope of detecting small deviations from the Standard Model, the theoretical framework that has been guiding particle physicists for a few decades. But it has a few known shortcomings that make us think new physics should be discovered soon.

This is a great model that allows theorists to make very accurate predictions. So far, every single one of them has proven to be true but if we were to find a flaw, it would be like discovering the secret passage guiding us further into our investigation of how matter works.

The results presented this week by the LHCb experiment hint in this direction, although as usual, further checks are needed. The scientists involved were looking at decays of charmed mesons denoted D0, particles made of one charm quark c (with electric charge +2/3) and one antiquark u (-2/3).  The D0 is therefore electrically neutral and can decay into a pair of kaons, K+K (mesons containing an s quark) or pions π+π(mesons made of light quarks, u or d).

But one could also make a charmed antimeson with one antiquark c (-2/3) and one quark u (+2/3). This is the meson antimatter. These antimesons also decay into pairs of kaons or pions, K+K and π+π.

How can one know if he or she is dealing with a charmed meson or antimeson since both decay to the same final products? One way is to “tag” the charmed mesons when they are created. For this measurement, the LHCb team selected excited charmed mesons D*+ and D*, which decayed to a positively charged pion and a D0 charmed meson, or into a negative pion when charmed antimesons are created. The pion charge gives it away.

What LHCb measured is the difference between how often charmed mesons and how often charmed antimesons decay into K+K. The same measurement was repeated with D0 Þ π+π as final decay products. The idea is to see if there is a difference in the behavior of matter (the mesons) and antimatter (the antimesons), what we call charge-parity (or CP) violation.

Then they looked at the difference of the difference between the K+K and π+π channels. The advantage is that many potential experimental biases cancel out, while a true signal would remain, as the CP violation need not be the same in the two channels.

Our Standard Model of particle physics predicts that this difference should be very small, of the order of 0.01% to 0.1% (the theoretical uncertainly is fairly large here). LHCb measures −0.82 ± 0.21 (statistical uncertainty) ± 0.11 (systematic uncertainty)%, a 3.5 standard deviation away from zero. In other words, if every source of experimental uncertainty has been properly accounted for, there is only a 0.05% chance this is due to chance. Other experiments had detected a hint of this effect before, but with much less precision, so this in itself is an accomplishment.

Does this mean the Standard Model has been proven wrong? Not yet. We need to see if this effect will remains once the group has a chance to analyze all of the 2011 data, which should be completed by March next year. Only 60% were used for this analysis.

As Mat Charles, one of the four physicists directly involved in this analysis told me: “This could be the hint that something interesting is going on. Very much worth pursuing”. They plan to add a different channel, tagging the D0 mesons and antimesons using B mesons, those containing the beauty quark, instead of D*, just to be sure this is true. Let’s hope they’ll be lucky. Such a discovery would be a great step forward.

Pauline Gagnon

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