A #thread on #penguins and why I like them.
I say complicated because most particle decays due to the weak interaction look like this. A neutron (three quarks) decays to a proton (3 quarks), and electron and a neutrino. It's mediated by the W boson.
Some decays cannot happen this way, like the first discovered penguin decay, b→sγ: a b quark goes to a s quark and a photon. You need a W boson, but also a top quark.
It was observed in 1993 by the CLEO experiment: A clear peak is seen at the mass of the B meson (the lightest particle to contain a b quark). https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.71.674
I can make this more complicated by adding two leptons (electrons, muons, taus, you choose) to the diagram.
But this pulls in the possibility of having a Z boson instead of the photon γ. And then I can also have a box, with a W boson and a neutrino.
So: I have three diagrams (counting the photon and Z as two). All are very suppressed and the process is thus rare: 1 in a million or so.
As this is quantum mechanics they all interfere, which gives interesting patterns.
What do I mean with patterns? If you take one of such decays, B→Kπμμ, you have a B meson decaying to a kaon (with a strange quark), a light pion and two muons. You can describe that decay with 3 angles and two masses. (the angle between the planes is not indicated.)
The two masses are the masses of the Kπ and μμ systems. The first is the mass of the K* meson, because that's what we select and the other is a free variable we call q.
How often a decay picks a particular combination of angles tells me something about the underlying physics. Here's the distribution (i.e. the "how often") of the angle θℓ for a given range of q.
This is data from our 2015 paper https://arxiv.org/abs/1512.04442  . The crosses are data, the line is the fit which is the sum of the B→Kπμμ signal in blue and background in red. Their relative sizes are taken from the mass fit of the Kπμμ combination.
So if you look again at the distribution of this angle (or its cosine) you'll notice that it's asymmetric. It prefers larger values of the cosine.
That is an asymmetry, which I can plot versus the squared mass of the dimuon system q² (we like squared masses).
At 6<q²<8 the asymmetry is positive as indicated by the cross. The vertical bars indicate the uncertainty. The magenta boxes are theory predictions.
This plot shows the so-called forward-backward asymmetry. It's an asymmetry that was highlighted very long ago as a potential way of spotting supersymmetry. I had this already in my PhD thesis in 2002 (with opposite sign convention).
Why is that interesting? Because if we spot asymmetries different from those predicted by the Standard Model,it means more than these three diagrams must be at play. It could be new hypothetical particles as supersymmetry, new bosons, or leptoquarks.
The forward-backward asymmetry turns out not to be too exciting. But in 2013, and then again in 2015 we spotted an anomaly in another asymmetry. You can build it from the other two angles in the decay.
Take the decays falling into the red areas minus those in the blue. Plot that versus the dimuon mass squared to get this:
There's a discrepancy between the data and the theory. We were not alone to measure this. ATLAS, CMS and Belle also have data.
The situation is somewhat unclear, but the data seems to mostly lies above the prediction. That could indicate a new spin-1 particle. Or it could be that the theory prediction needs more work. In any case we need more data.
Adding 2016 data, we roughly double the dataset. Here are the B mesons from 2016. We add them to the Run 1 (2011-12) dataset and re-run the angular fit.
The ominous P₅' asymmetry stays away from the Standard Model prediction.
If you split that by datasets you see that 2016 data is closer to the Standard Model than the Run 1 data.
However, there is more to an angular fit than only one asymmetry. Performing a global fit to all angular distributions and looking for new vector and axial currents we get this.
The data are about 3 standard deviations away from the Standard Model. The horizontal shift indicates a vector current. The overall consistency of the various asymmetries has improved. What this means for New Physics or QCD, I leave to theorists. 🚑
Here's the preprint. https://twitter.com/LHCbPhysics/status/1237351750413225984
https://twitter.com/PKoppenburg/status/1237837410244816896?s=20
You can follow @PKoppenburg.
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