It’s an exciting time for your humble LHC blogger. She may just have a thesis topic… So what does that mean? (I often times wonder that myself).
With the recent success and in anticipation of high energy collisions (and therefore data), it’s time to figure out what can be found and what can’t given the projected amount of data. (We’re going to be running at ~7 TeV for the first part of the year, then ~10 TeV the latter half). Now lots of people are doing cross sections measurements – which is a different beast than searches (see below). Cross section measurements take a particle that we know – Zs and Ws for example – and check to see if we measure what we predict. This is very important to do and I’m over simplifying but that’s the basic idea. Despite it’s importance, I personally feel like if I’m working on the highest energy accelerator in the world, I’d at least like to try to do a particle search.
The Cross Section Beast
This isn’t a completely trivial question because ever since the Tevatron turned on, theorists have been making predictions as to what was out of reach to our current experiments. So what makes for a good early search? Lots of things, I’ll list some here:
- Of interest…
Maybe this goes without saying, but I’m going to go ahead and say it anyway. A search has to be well defined and predicted. One doesn’t just look for the Higgs, or SUSY or Z’, they look for specific decay products that could come from the predicted particle and can not be explained by sometime else. Although we’re going to be 3.5x and 5x higher energy than the Tevatron, there has been years of data collected at the Fermilab experiments. Now there are some particles that are simply outside of their reach. For example just due to conservation of energy, nothing can be created >2TeV, but due to statistics (need that high fluctuation over the background again…) some limits are in the 100-200 GeV range. Increasing the energy will allow us, even with less data to raise limits.
- High Signal:Background ratio
Since there is going to be a smaller data set (only 1 year of running), we simply won’t have enough statistics to say with confidence that we discovered certain particles. We need to say that the signal is an actual signal – not just a fluctuation of the background. I elaborate this in my Higgs post. This also means that there would be a distinct signature for example: something that would decay to 2 very high energy electrons and 2 very high energy jets. It could be di-boson production or W/Z+jets, but the electrons would come from a W/Z which was very far off mass shell – which is not impossible, but maybe not as probable.
- Missing Energy (MET to be more specific)
This is a bit contentious, and maybe more a personal taste than anything. We won’t have a completely calibrated detector initially. The detector is calibrated by taking standard particles (Ws and Zs for example) and reconstructing them. We then convert the electrical signal out of the machine to energy and momentum. To do this, the more Z and W events the better – which like everything takes time. So the energy of the signal can be off. This isn’t a bad thing, but the way we calculate missing energy (say in the form of neutrinos) is by balancing the energy in the detector. For example if there is 40 GeV deposited in 1 part of the xy plane, then there has to be another 40 GeV in another part of the xy plane to balance it out. If we don’t really know if it’s 40 GeV or 45 GeV, then it’s hard to calculate missing energy. (I should also point out, it’s transverse energy, not just energy – which I can elaborate on if anyone is interested).
So these requirements gives us a whole range of particles to search for. I’m involved in a physics group called exotics. Exotics are a generic term for anything beyond the standard model and isn’t the Higgs or SUSY. This isn’t to say that Higgs/SUSY searches aren’t beyond the standard model… I guess they get their own groups since so many people are interested in them. It makes the exotics working group more intimate :-). My interests (and potential thesis) are in particles that would unite quarks and leptons (like how a W unites the family of quarks and the family of leptons). These generically are called leptoquarks.
So what’s wrong with the Higgs? It like the captain of the high school football team and head cheerleader all rolled into one particle to the high energy physics community. I don’t know… I’m just not that into it.
-Regina
