I’m going to do something different today and discuss a result from another experiment… I saw the result this morning and thought the topic would be an interesting one for a blog post.

So what will I be talking about? Time reversal violation!

You might be wondering why I would consider this an interesting topic, we all experience time reversal violation in our lives, everyday events definitely are not symmetric in time – we can always tell when a video is being played backwards. This isn’t the case in the world of particle physics however, where most interactions are symmetric under time reversal.

So why do we expect time reversal violation in particle interactions? It’s related to the underlying structure of the Standard Model, which relies on interactions being CPT symmetric.

What is CPT you ask? It’s the combination of three other more fundamental symmetries, Charge conjugation, Parity and Time reversal. I’ve described C and P previously and also presented results of CP violation in the B and D meson systems.

Now if we expect the Standard Model to be CPT symmetric and we’ve observed CP violation, it follows that we should also observe T violation.

And this is exactly the result that the BaBar collaboration released this morning, where they report “the first direct observation of T violation in the B meson system.”

I’m not going to go through the details of the analysis, it’s quite clever and complicated, instead here is a set of plots from the paper:





The points are the data, the blue line is the model without T violation and the red line is the model with T violation. As you can see, the model with T violation matches the data much better than the model without.

And voila, here you have it! Observation of T violation in the B meson system…

Tags: BaBar