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CDF Nails It! April 19, 2006

Posted 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!):

Delta M for both CDF and D0

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 Unitarity Triangle

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!).



1. alaska - April 19, 2006

Great post! And good new design. but you should put links for comments at the bottom.

2. dorigo - April 19, 2006

Hi Gordon,

thank you for the praise and the link… I like your animated plot, can I steal it ? Where did you get it ?

3. Jeff Hodges - April 19, 2006

Nice post and the animated plot does rock! I didn’t realize Tommaso Dorigo had a new blog, so I’ve gone ahead and made it part of Mixed States. Thanks!

4. gordonwatts - April 19, 2006

I made the plot myself. They are actually very easy to make, it turns out. I think there is all sorts of free software out there (someone told me xfig would even do it). I use photoshop elements to do my photo cataloging and my photo touch up (ahem!), and it had a trival way to make animated gifs. Since there is interest, I’ll try to get the raw versions of the plot — outside the talk — and put together a more “clean” animated gif.

You most certianly may steal the plot, Tommaso (as can anyone else). That plot represents about 10 minutes of my work compared to decades of other people’s (ok, I did work on the DAQ system in D0 that took the data, though my hardware work on CDF is probably long gone).

Thanks for the design comments. I was looking around at other people’s blogs and then I looked at mine… It turns out I can make my own gif for the banner, so I’m hoping to do something more specific to HEP soon. If I ever find the time… I agree about the comment links at the bottom. I’ll see if I can do that (wordpress gives you only limited control over the look of your site).

5. Experimental sociology | Cosmic Variance - April 19, 2006

[…] Update: Tommaso and Gordon explain more about the physics of the result. […]

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