BaBar was an experiment studying 10 GeV electron-positron collisions. The collider is long gone, but interesting results keep appearing from time to time. Obviously, this is not a place to discover new heavy particles. However, due to the large luminosity and clean experimental environment, BaBar is well equipped to look for light and very weakly coupled particles that can easily escape detection in bigger but dirtier machines like the LHC. Today's weekend plot is the new BaBar limits on dark photons:
conceived long ago, but in the previous decade it has gained wider popularity as the leading explanation of the PAMELA anomaly. Now, as PAMELA is getting older, she is no longer considered a convincing evidence of new physics. But the dark photon model remains an important benchmark - a sort of spherical cow model for light hidden sectors. Indeed, in the simplest realization, the model is fully described by just two parameters: mA' and ε, which makes it easy to present and compare results of different searches.
In electron-positron collisions one can produce a dark photon in association with an ordinary photon, in analogy to the familiar process of e+e- annihilation into 2 photons. The dark photon then decays to a pair of electrons or muons (or heavier charged particles, if they are kinematically available). Thus, the signature is a spike in the e+e- or μ+μ- invariant mass spectrum of γl+l- events. BaBar performed this search to obtain world's best limits on dark photons in the mass range 30 MeV - 10 GeV, with the upper limit on ε in the 0.001 ballpark. This does not have direct consequences for the explanation of the PAMELA anomaly, as the model works with a smaller ε too. On the other hand, the new results close in on the parameter space where the minimal dark photon model can explain the muon magnetic moment anomaly (although one should be aware that one can reduce the tension with a trivial modification of the model, by allowing the dark photon to decay into the hidden sector).
So, no luck so far, we need to search further. What one should retain is that finding new heavy particles and finding new light weakly interacting particles seems equally probable at this point :)
Sunday, 15 June 2014
Monday, 2 June 2014
...though it's not BICEP2 this time :) This is a long overdue update on the forward-backward asymmetry of the top quark production.
The final straw is two recent updates from Tevatron's D0 experiment. Earlier this year, D0 published the measurement of the forward-backward asymmetry of the direction of the leptons
from top quark decays. The top quark sometimes decays leptonically, to a b-quark, a neutrino, and a charged lepton (e+, μ+). In this case, the momentum of the lepton is to some extent correlated with that of the parent top, thus the top quark asymmetry may come together with the lepton asymmetry (although some new physics models affect the top and lepton asymmetry in a completely different way). The previous D0 measurement showed a large, more than 3 sigma, excess in that observable. The new refined analysis using the full dataset reaches a different conclusion: the asymmetry is Al=(4.2 ± 2.4)%, in a good agreement with the Standard Model. As can be seen in the picture, none of the CDF and D0 measurement of the lepton asymmetry in several final states shows any anomaly at this point. Then came the D0 update of the regular ttbar forward-backward asymmetry in the semi-leptonic channel. Same story here: the number went down from 20% down to Att=(10.6 ± 3.0)%, compared to the Standard Model prediction of 9%. CDF got a slightly larger number here, Att=(16.4 ± 4.5)%, but taken together the results are not significantly above the Standard Model prediction of Att=9%.
So, all the current data on the top quark, both from the LHC and from the Tevatron, are perfectly consistent with the Standard Model predictions. There may be new physics somewhere at the weak scale, but we're not gonna pin it down by measuring the top asymmetry. This one is a dead parrot:
Graphics borrowed from this talk.