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Anna Phan | USLHC | USA

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Why B physics? Why not A Physics?

In my last post, I showed that LHCb is the best LHC detector for B physics, using the decay of the \(B_s\) meson into a \(J/\psi\) meson and \(\phi\) meson as an example. Today I’m going to try and explain why we want to study this particular decay and show you our latest result.

The reason we are interested in studying the decays of B mesons is that they may shed light on one of the major mysteries of the universe, namely the source of the observed matter-antimatter symmetry. Matter and antimatter are assumed to exist in equal amounts at the beginning of the universe, but as the universe expanded and cooled, an asymmetry developed between them, leaving a universe completely dominated by matter.

The Standard Model predicts an asymmetry between matter and antimatter, but at a level that is too small to explain the observed asymmetry in the Universe. Deviations from the predictions would indicate new physics.

As an aside, the difference between the properties of matter and antimatter is called CP violation. I bring up this factoid as it makes up part of the LHCb logo, which I thought was quite clever when I first saw it.

Anyway, one area in which the Standard Model predicts an asymmetry is the \(B_s\) meson system, that is, anti-\(B_s\) mesons are not exact mirror images[*] of \(B_s\) mesons. This difference is encapsulated in the \(B_s\) mixing[***] phase \(\phi_s\). This phase is what can be measured from the decay, \(B_s \rightarrow J/\psi + \phi\), which we just presented at the Lepton Photon conference in Mumbai.

I’ll spare you all the technical details of the analysis (the details of which should be appearing here soon) and skip to the result…

Okay, I know there’s a lot of information on this graph, so let’s go through it piece by piece. Firstly, the x-axis represents the \(B_s\) mixing phase, \(\phi_s\), while the y-axis represents the \(B_s\) decay width[****] difference \(\Gamma_s\). Both of these properties are shown as it is not possible to measure them independently. The Standard Model prediction for both of these variables is shown as the black point, while the CDF, D0 and LHCb results are shown as coloured contours, with the solid line representing the 68% confidence limit and the dashed line showing the 95% confidence limit.

The results of the measurements favour two regions, one of which is located around the Standard Model prediction, though not centered on it, indicating the possibility for new physics. The LHCb result however, is disappointingly much closer to the prediction than the CDF and D0 results.

Only more data will tell us the answer to the million dollar question: Is it Standard Model or New Physics?

[*] I’m assuming here that you all know what antimatter is. If not, a common analogy is that antimatter is the mirror image of matter. More technically, antimatter has all the same properties of matter, apart from opposite charge and parity[**]. For example, the antimatter particle of a negatively charged left-handed electron is a positively charged right-handed positron.

[**] Parity is another name for chirality, which Flip explains very well in this post.

[***] A very interesting property of neutral mesons, such as the \(B_s\), is that they can spontaneously transform themselves into their own antiparticles (and visa versa). This phenomenon, known as flavor oscillation or mixing and I’ll definitely be discussing it in a future post.

[****] It turns out that one of the possible differences between \(B_s\) mesons and anti-\(B_s\) mesons is a property called decay width.

Oh, if anybody was wondering, there is no such thing as A Physics in particle physics, which is why we don’t study it…


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  • EDBM

    One comment on the diagram: Assuming the horizontal axis is an angle, the whole diagram is not visible, especially part of the wrapped-around” region, which leads me to wonder if a better presentation might not have been to cut it at about -pi/2, where there is a minimal probability for a solution. Is that angle of any physical significance, by the way?

    Second, doesn’t the dumbbell shape of the region kind of suggest that there might be two solutions, is that a physical possibility? (Obviously, I don”t know what mixing phase is…)

    Curious layperson

  • Anna Phan

    Dear Curious layperson,

    You are right that the graph does not display the full angular range of the mixing angle. If you are interested, Pauline has a posted the LHCb only resultshere, showing the full angular range with the split in an area with no results. The horizontal axis on the graph I show is constrained by how the other experiments have chosen to present their results.

    There isn’t really a physical meaning to the mixing angle; I can’t go and get my protractor and measure it in an event. It is an abstract quantity which happens to be an angle.

    If you look carefully at the graph, you will see that the results of each of the experiments have two solutions (the CDF and LHCb ones are in the wraparound region). This is an artifact of the analysis. The set of equations which need to be solved to find the mixing angle have a two fold ambiguity, that is, they are the same for a simultaneous 180 degrees rotation of the mixing angle and if the sign of the decay width difference is flipped. We are currently working on adding an extra constraint to the system so we can remove one of the solutions.

    I hope that all made sense,

    PS. I like the pseudonym. :)