Quantum Diaries

I’ve been reading a lot of the comments to the blogs and realized that it’s a little unclear what physicists mean when we say we’ll find the Higgs. All the bloggers have posted pretty event display plots which are a visual representation of what we get in the detector. You can see lines of tracks in the inner detector, energy deposition in the calorimeters and tracks in the muon system. You’ll also see particle tracks bend in the magnetic field. This is beautiful and shows exactly what the detector “sees” when an event occurs. This, however, is not what physicists will use to discover the Higgs.

So what do you do exactly?

It’s never a matter of finding “one” Higgs and declaring victory. Based on different theoretical models we’ll be looking for different signatures. Each of these signatures will have certain background events. Background events are events that  have the same or similar signature as the signal (or in this case, the Higgs).

So here’s an example:
One signature we can look for is the Higgs decaying to two Z bosons. Z’s have a nice signature because they can decay into 2 electrons each. (for example… they also can decay into muons, quarks… etc, but this is just one example).

So now we have something to look for: 4 electrons which “come from” 2 Z’s. But we can’t just look for 4 electrons because there are other processes that also decay into 4 electrons. For example we can create 2 Z bosons without coming from a Higgs. This is called diboson production. This would be a background to the Higgs->ZZ signal.

So now what do we do? We have to somehow distinguish the difference between the signal (H->ZZ->4e) from the background (ZZ->4e) because in the data there is a probability that all of them will be produced. Well there are lots of ways we try to do this. One is that there would be a peak at the mass of the Higgs if we reconstruct and combine all the decay particles. In order to get this peak we’d need lots of signal events because it’s about getting enough statistics to distinguish them from the ZZ->4e background. We’d need a signal that’s ~5 sigma above the background to claim discovery. What that means is that it has a <0.01% chance of being a fluctuation in the background.

Although this is just one example of how we’d have a discovery, it’s similar for all discoveries. Some particles are easier to discover because they have a very high signal to background ratio, so it will take fewer statistics to get to ~5 sigma. The Higgs is difficult though, for a multitude of reasons. What this means in the end is that it will take more time to get the statistics to distinguish it from the background.

Then once we’ve been able to find an event that we believe is a Higgs candidate we’ll reconstruct it with the event display software that’s been seen so often on the blogs. I hope this sheds a little light on how discoveries will be made at the LHC. Although I’ve never gone through the process myself, there are lots of physicists (and LHC blogers) that have done this before (like during the top quark and W/Z discoveries). So please feel free to ask questions if you have them.

-Regina

This entry was posted on Friday, December 11th, 2009 at 10:57 am and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.