New Physics Claim from D0!

Tevatron not dead, or so it seems. Although these days all eyes are turned to the LHC, the old Tevatron is still capable to send the HEP community into an excited state. Last Friday the D0 collaboration presented results of a measurement suggesting the standard model is not a complete description of physics in colliders. The paper is already available at this link, and it should be out on arXiv tonight.

The measurement in question concerns CP violation in B-meson systems, that is quark-antiquark bound states containing one b quark. Neutral B-mesons can oscillate into its own antiparticles and the oscillation probability can violate CP (much as it happens with kaons, although the numbers and the observables are different). There are two classes of neutral B-mesons: B_d and its antiparticle B_d where one bottom quark (antiquark) marries one down antiquark (quark), and B_s, B_s with the down quark replaced by the strange quark. Both these classes are routinely produced Tevatron's proton-antiproton collisions roughly in fifty-fifty proprtions, unlike in B-factories where mostly B_d B_d have been produced. Thus, the Tevatron provides us with complementary information about CP violation in nature.

There are many final states where one can study B-mesons (far too many, that's why B-physics gives stomach contractions). The D0 collaboration focused on the final states with 2 muons of the same sign. This final state can arise in the following situation. A collision produces a b \bar b quark pair which hadronizes to B and B mesons. Bottom quarks can decay via charged currents (with virtual W boson), and one possible decay channel is b -> c μ^- ν_μ. Thanks to this channel, the B meson sometimes (with roughly 10 percent probability) decays to a negatively charged muon, B -> μ^- X, and analogously, the B meson can decay to a positively charged antimuon. However, due to BB oscillations B-mesons can also decay to a "wrong sign" muon: B -> μ⁺ X, B -> μ^- X. Thus oscillation allow the BB pair to decay into two same sign muons a fraction of the times.

Now, in the presence of CP violation the B->B and B->B oscillation processes occur with different probabilities. Thus, even though at the Tevatron we start with the CP symmetric initial state, at the end of the day there can be slightly more -- than ++ dimuon final states. To study this effect, the D0 collaboration measured the asymmetry

A_sl^b = (N_b⁺⁺ - N_b^--)/(N_b⁺⁺ + N_b^--) .

The standard model predicts a very tiny value for this asymmetry, of order 10^-4, which is below the sensitivity of the experiment. This is cool, because simply an observation of the asymmetry provides an evidence for contributions of new physics beyond the standard model.

The measurement is not as easy as it seems because there are pesky backgrounds that have to be carefully taken into account. The dominant background comes from ubiquitous kaons or pions that can sometimes be mistaken for muons. These particles may contribute to the asymmetry because the D0 detector itself violates CP: due to budget cuts the Tevatron abandoned construction of the D0 detector made of antimatter. In particular, the kaon K⁺ happens to travel further than K^- in the detector material and may fake the positive value of asymmetry.

At the end of the day, after (hopefully) carefully subtracting the backgrounds, D0 quotes the measured asymmetry to be

A_sl^b = -0.00957 ± 0.00251(stat) ± 0.00146 (syst),

that is the number of produced muons is larger than the number of produced antimuons with the statistical significance estimated to be 3.2 sigma. The asymmetry is some 100 times larger than predicted by the standard model!

Of course, it's too early to celebrate the downfall of the standard model, as in the past the bastard have recovered from similar blows. Yet there are reasons to get excited. The most important one is that the latest D0 result goes well in hand with the anomaly in the B_s system reported by the Tevatron 2 years ago. The asymmetry measured by D0 receives contributions from both B_s and B_d mesons. The B_d mesons are much better studied because they were produced by tons in BaBar and Belle, and to everyone's disappointment they were shown to behave according to the standard model predictions. However BaBar and Belle didn't produce too many B_s mesons (their beams were tuned to the Upsilon(4s) resonance which is a tad too light to decay into B_s mesons), and so the B_s sector can still hold surprises. Two years ago CDF and D0 measured CP violation in B_s decays into J/ψ φ, and they both saw a small, 2-sigma level discrepancy from the standard model. When these 2 results are combined with all other flavor physics data it was argued that the discrepancy becomes more than 3 sigma. The latest D0 results is another strong hint that something fishy is going on in the B_s sector.

Both the old and the new anomaly prompts introducing to the fundamental lagrangian a new effective four-fermion operator, ∼ (b s)² + h.c., that contributes to the amplitude of B_s B_s oscillations. At this point we have no hints whatsoever what could be the source of this new operator, and the answer may even lie beyond the reach of the LHC. In any case, in the coming weeks theorists will derive this operator using extra dimensions, little Higgs, fat Higgs, unparticles, supersymmetry, old newspapers, golf balls, and tires. Yet the most important question is whether the asymmetry is real, and we're dying to hear from CDF and Belle... Looking forward to Session 06 of ICHEP, maybe we'll know more then :-)