February 20, 2004  


Those Eureka! Moments at the B Factory

By Davide Castelvecchi

Claiming new discoveries about elementary particles requires a great deal of care.

In that tiny, bizarre world, the laws of physics can never tell you exactly what will happen—but they usually predict with great precision how likely it is that things will go a certain way. Though very rare, freak accidents, such as particles appearing out of nowhere, are possible.

In a typical BABAR measurement, researchers look for very uncommon patterns of particle decay. At the forefront of research, it is the rarest events that can teach us new physics. Results come from months of painstaking data taking and analysis, rather than from a single ‘Eureka!’ moment. "It’s usually not the case that you look at one event and say ‘Wow!’" said Jeffrey Richman, BABAR’. Physics Analysis Coordinator.

Rare particle decays can be compared to ‘special’ coins that don’t land on tails or heads with equal probability. If you had a bag with a million normal coins in it, and only one special coin—one that, say, always lands on tails —how would you go about finding the special coin?

Say you pick a random coin out of the bag. If you toss it once, and it lands on tails, that doesn’t say much, does it? What if it lands on tails twice in a row? A regular coin has a 25% chance of doing that, so it’s not too unusual. Such strings of deceptively favorable events are called false positives, or, as particle physicists would say, ‘background’.

Now say you are a skeptic, and you toss your coin ten times—tails every time. A little math shows that there was less than one chance in a thousand for a regular coin to do that. Sounds more promising. But is ten flips enough? At that point, if you stop looking you are vulnerable to things you don’t anticipate as background.

To prevent themselves from jumping to conclusions too early, physicists often employ a procedure called blind analysis. You decide ahead of time how many coins you’re going to test—say, a million—and how many times you want to toss each one—say 100. Even if a coin’s first 50 tosses give all tails, you force yourself to keep on tossing until the end. After weeks and weeks of tossing coins and recording the results, you finally look at your records, and see something like this: most coins behaved roughly like normal coins, some landed more often on tails than heads, some the other way around, but just one coin landed on tails 99 times.

For a normal coin, that would happen about once in 10 billion billion billion times, so a false positive is extremely unlikely. On the other hand, you should expect even the special coin to land on heads once in a while. Because of quantum accidents, there’s no such thing as absolute certainty.

For each BABAR measurement, the moment of truth is scheduled to come at the end of the blind analysis procedure, in what people often call the ‘unblinding party.’ Unblinding parties usually involve a small group of graduate students and postdocs, staying up late looking at someone’s laptop in the Research Office Building kitchen. "If there’s a ‘Eureka!’ moment, that’s it." Richman said. "You look at your final output file and it could be: ‘Bang! There is a big signal!’


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Tuesday February 17, 2004 by Emily Ball