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
Tags: ATLAS, discoveries, first results
Somewhat related, this.
Testing the big bangs theory: http://www.cartoonstock.com/newscartoons/cartoonists/mba/lowres/mban614l.jpg
Hi Regina,
I am not a particle particle physicist or even a physicist but I enjoy reading this blog for all I can learn about the LHC and developments in physics.
I have a simple (potentially silly) question: Does the Higgs boson exist in nature today (outside of the LHC) to generate the Higgs field that gives mass to particular particles? If not, then how does the Higgs field exist in the absence of the Higgs boson to give the particles around mass?
I can’t find a resource that answers such a basic question. Thanks for considering it.
Ted, Esq.
Hi Ted,
Glad you’re enjoying the blog! This is an excellent question. And to answer it properly, I’ll have to give you two answers
First as of today, there isn’t any evidence that the Higgs boson exists. There are lots of theories support their existence, however we haven’t found them yet. As you know, we’re looking for them at the LHC and the Tevatron. but I’ll believe that your question is assuming that we find them at the LHC (essentially “make” them via collisions) does they exist in nature as well. (If I’m misunderstanding this please let me know!)
So this is the second part of the answer, as physicists we can only observe things that occur in nature. If we do find the Higgs at a collider, it has to exist in nature too. We use the collider to try to create them in the lab because events that occur under laboratory conditions are easier to understand. Everything that we produce at the LHC exists in nature. We just use the LHC to make them so that they’re easier to observe.
I hope this answers your question. Let me know if you have more.
Regina
Regina,
Thank you for your response. Yes, my question was premised on the presumption that the Higgs did or does exist.
I think my confusion stemmed from reports in the popular press that the LHC and other colliders attempt to replicate the conditions of the first few instances at the creation of the universe–as collider energies increase, the closer the conditions can be to Time Zero. (Although I do understand that more massive particles require higher energies.) I presume from your response that the Higgs (if it exists) must be found throughout nature today, generating the Higgs field that results in mass. So, a Higgs boson (if it exists) is necessary to give mass to the elementary particles that make up me right now. Similarly, I think you are NOT saying that the Higgs is an exotic particle that was formed during the early universe, decayed quickly (long ago), and no longer populates the universe unless created during high-energy collisions (whether in a man-made collider or elsewhere). Correct?
Thanks for addressing my question and your patience.
Ted
Hello;
You talk about being into ‘exotics’ and I’m wondering what all that can cover? What if the standard model theory is indeed flawed or wrong? As to say, could it be possible a ‘mini black hole’ is responsible for holding the atomic nucleus together? Could the strong nuclear force and what we call gravity be the same force created by a black hole inside the atom?
Thanks,
Randy
Hi Ted,
I’m happy to answer your question. You are correct. The Higgs, (if we find if of course) would exist in nature today. It’s not something that became “extinct” and we’d have to bring back… so to speak. It’s also important to say that everything that existed at the start of the universe exists too. Things like quarks, leptons, as Carl Sagan put it “Cosmic Soup”. If anything, things are more complex now than they were then. It took thousands of years for those quarks to become Hydrogen and Helium. At least this is the general believe of the start of the universe.
Let me know if you have more questions. I’ll try my best to answer them,
Regina
Hi Randy,
Exotics is kind of a generic term that covers anything that is not directly explained by the standard model. (Higgs and SUSY are also “exotic” in this sense, but they’re more fairly popular theories so they have special working groups).
There is the definite possibility that the Standard Model is incomplete. I’d like to take a second to distinguish the important difference between wrong and incomplete. So far the Standard Model correctly describes nature as we have observed (for over 30 years!). This won’t change. However, we may discover something that is not included in the Standard Model or a process that is different than what we thought. This may break part of the SM, however for all the physics we’ve observed so far, we can and will always be able to use the SM to describe it. -Just like how we still use Newtonian Mechanics even though General Relativity is more complete-. We just may have more theories later on that can describe nature better.
As for your last question about a mini black hole holding together a nucleus… I don’t know if there are any theories about it. Flip (another LHC blogger) is a theorist so he may be a better person to ask about it. Sorry for passing the buck, but I’d hate to tell you something that was incorrect.
Thanks for your question!
Regina