CPH Theory

Antimatter asymmetry: new results bring solution closer

Fermilab physicists’ fingers crossed

Physicists are inching closer to explaining why we – and anything else made of matter – exist, with new results inching closer to an explanation of the universe’s matter-antimatter asymmetry.

If matter and antimatter were created in equal quantities in the Big Bang, some kind of asymmetry is needed to explain how enough matter survived to create the universe we see. The difference in matter-antimatter properties – charge-parity violation – is very easy to predict, since we exist, but it’s proven infuriatingly difficult to characterize.

The new results, announced February 29 at a meeting in La Thuile in Italy, represent a refinement of results first reported last year by CERN’s LHCb team. The CERN result found a difference of 0.8 percent in decay rates for D0 mesons and their corresponding antiparticles – but, at 3 sigma, with a high chance that the observation was a statistical fluke.

According to Science Now, Fermilab’s CDF team says its follow-up experiments (paper below) come up with a slightly lower CP violation of 0.6 percent. The result on its own has only 2.7 sigma statistical significance, but when combined with the LHCb results, they say CP violation can now be pinned down to a 3.8 sigma. At around 1:10,000 chance that this is a statistical fluke, it is than the 5 sigma (one in a million chance of being wrong) required to announce a discovery, but it’s still a strong indication that the experiments are heading in the right direction.

The new experiment finds CP-violating asymmetries in “charmless B⁰, B_s⁰ and Λ_b⁰ decays into pairs of charged hadrons reconstructed in CDF data.”

Their data also reports direct CP violation in bottom strange mesons and bottom baryons, along with evidence for CP violation in other decays.

If the measurements are right, the next challenge will be fitting the results into standard model physics, which predicts a much lower level of CP violation. ®

Measurements of Direct CP Violating Asymmetries in Charmless Decays of Strange Bottom Mesons and Bottom Baryons

(Submitted on 29 Mar 2011)

We report measurements of direct CP-violating asymmetries in charmless decays of neutral bottom hadrons to pairs of charged hadrons with the upgraded Collider Detector at the Fermilab Tevatron. Using a data sample corresponding to 1 fb-1 of integrated luminosity, we obtain the first measurements of direct CP violation in bottom strange mesons, A_CP(BsKpi) = +0.39 +- 0.15 stat +- 0.08 syst, and bottom baryons, A_CP(Lb->ppi) = +0.03 +- 0.17 stat +- 0.05 syst and A_CP(Lb->pK) = +0.37 +- 0.17 +- 0.03 syst. In addition, we measure CP violation in Bd-->Kpi decays with 3.5sigma significance, A_CP(B->Kpi) = -0.086 +- 0.023 stat +- 0.009 syst, in agreement with the current world average. Measurements of branching fractions of Bs-->K+K- and B0-->pi+pi- decays are also updated.

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