CDF Nails It! April 19, 2006Posted by gordonwatts in physics.
Last week CDF released their Bs results. Tommaso has a fantastic post detailing what it means and what is behind it. At the bottom of his post is the plot. I reproduce it here, a long with D0's, so you can compare (sorry if you have to widen your screen to see it!):
Along the x axis is the difference in mass between a Bs meson (a B meson containing a b and a s quark) and its anti-matter partner, the anti-Bs meson. The y-axis is the amplitude, but for sake of brevity, I'll call it the probability (I'm being very sloppy here!). Tommaso goes into a good deal of detail in his post, so I won't repeat it here. You can see how CDF's data resolves the high-probability peak much (at 17.25 ps-1) much better than D0 does (at 19 ps-1). That is for all the reasons we previously talked about both on this blog and on Tommaso's blog.
What is the point? Why did we measure this number? And why is this number worthy of more press than many of the other numbers that both D0 and CDF regularly measure? This is one of the major unmeasured parameters in our understanding of the Weak force. There are a number of parameters that we must measure to characterize the weak force. Indeed, we can over-measure it. Think of a box you are sure is square. You can measure one length and you'll know everything you need to know about the box. But what if someone comes along (a theorist, say…) and asks "How do we really know it is a square? I know we've always thought of it as a square, but what if it is a rectangle when we look at it from behind?" So you, the experimentalist will measure it from behind and, lo, it is still a square. You keep doing this and soon you'll have many measurements and all the measurements must fit together to confirm your model of the box as a square block. If any don't, then you don't have a square box… and life gets interesting. We are in a similar position. We are measuring and over measuring and then cross checking to make sure all our measurements are consistent. Of course, things are a bit more complex than just a square.
Representing this data is painful. Humans are visual, but the data is multi-dimensional. So we tend to take slices. In this case we take the slice called the unitarity triangle. The bands of color represent measurements and their errors. The wider the band, the larger the error of the measurement. That small little kidney-bean shaped red circle in the upper left represents what we don't know — and it leaves plenty of room for various crazy non-standard models of reality: one of the Tevatron's primary mission is to shrink that red area to be as small as possible.
The animation is the same unitarity triangle, representing results before D0's and CDF's, then after D0's, and finally after CDF's. As you can tell, from the point of view of effect on the unitarity triangle and our overall understanding of physics, the most important thing was the establishment of the upper bound in the mass difference. Both experiments will continue to measure this throughout Run II hoping to shrink that orange band further and further!
BTW, it was nice to see a write up in the New York Times today:
Have trouble making up your mind?
Physicists at the Fermi National Accelerator Laboratory reported what would seem to set a new standard for vacillation last week: a subatomic particle that reverses identity three trillion times a second, switching into its upside-down mirror-image evil-twin antimatter opposite and then back again.
"This finding is only the beginning of many more exciting scientific discoveries," said I. Joseph Kroll, a physicist at the University of Pennsylvania.
They even got the "CDF confirms the D0 results" right — that was nice to see (being a D0 patriot, and all). I only wish they'd gotten a quote from one of the people that put in so many months/years of work from D0 instead of just getting quotes from the CDFers (who are just as deserving — they have put in just as much work and effort!).