Searching for the Higgs boson
Part II: A New Hope?1
First off: my apologies. I was just starting to generate some real interest in this blog when I got very busy with my research. I have had a few people tell me that I need to start up again, and today seems like the perfect day to kick start this blog.
Why today? It's not to celebrate American independence. No, today we may see fireworks far more impressive than anything we've seen in the past. In a previous blog I gave a quick introduction to the Higgs boson and interpreted the data that was released around that time. Today (July 4th, 2012) CERN will be announcing new information on the search for the Higgs boson.
Before I continue, let me explain a little more about the Higgs boson.
Fermions and Bosons
There are two types of subatomic particles: Fermions and Bosons. One important difference between the two are how they fill energy levels. The graph below shows two energy level graphs. Fermions are shown on the right and bosons on the left.
Let's look first at the fermions. The blue dots represent individual fermions. Each black line represents an energy level. Notice that only one fermion is allowed on each energy line. All particles will attempt to have the lowest energy possible, but for fermions this means filling up the lowest available spot. Bosons on the other hand (the red dots on the left) are allowed to group together on one line. Since they each want the lowest energy, they each group together at the lowest energy.
Fermions and Bosons
There are two types of subatomic particles: Fermions and Bosons. One important difference between the two are how they fill energy levels. The graph below shows two energy level graphs. Fermions are shown on the right and bosons on the left.
Let's look first at the fermions. The blue dots represent individual fermions. Each black line represents an energy level. Notice that only one fermion is allowed on each energy line. All particles will attempt to have the lowest energy possible, but for fermions this means filling up the lowest available spot. Bosons on the other hand (the red dots on the left) are allowed to group together on one line. Since they each want the lowest energy, they each group together at the lowest energy.
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| Bosons (left) and Fermions (right) |
For a more real life example of the differences, think about two baseball bats (which are fermions). If I swing them together, they will collide. The two fermions (the bats) will not be allowed to be in the same place at the same time. Now think of two flashlights. Photons (light particles) are bosons. If I swing two lights together they don't collide, they pass right through each other. The two bosons (light particles) are allowed to be in the same place at the same time.2,3
So what is the Higgs boson?
Below is a model of all the interactions in particle physics. You can see that photons interact with electrons (leptons) and protons (made from quarks) as well as W bosons. Gluons, which carry the strong force,4 only interact with themselves and quarks. Higgs bosons are theoretical force carrying particles similar to gluons, photons, W bosons, and Z bosons. The Higgs boson is responsible for giving mass to all things in the universe. If the Higgs does not exist, then we have no understanding of why anything in the universe would have mass.
Sigma????
If you've been following the search for the Higgs, you've probably heard physicists talk about sigma values. So what do they mean?
Particle physicists can't just put a sample under a microscope to look for the Higgs. Instead, they use colliders. Basically, they accelerate hadrons (protons, neutrons, etc.) in opposite directions, smash them together, and see what happens. See the video below for a much better explanation of what is going on.
The Higgs Boson Explained from PHD Comics on Vimeo.
Ok, so like the video showed, we're looking for a small bump in the data. The real question now is, if we see a bump how do we know it's really a bump? What if it's just a bad measurement that we made?
To answer that we need statistics. Below is a graph showing a bell curve. Most people, when they think about making a measurement, are only concerned about the average. The average is important, but another important consideration is the standard deviation.
An example: The temperature in my house
My thermostat tells me the average temperature in my house. However, my thermostat does not tell me that my living room is always hot and my bedroom is always freezing. There are deviations in the temperature in my house, and those deviations are important. One standard deviation (which is represented by sigma) means that 68.2% of my house will be within that temperature range. Two sigma means that 95.4% of my house will be within that range and so on.
So, if the average temperature in my house is 75 degrees F, and the standard deviation is 2.5 degrees, then 95.4% of my house is between 70 and 80 degrees. What if I found a picture of a thermometer that read 60 degrees F? I could be 99.993% sure (4 sigma) that the picture of that thermometer was not taken inside my house.
When particle physicists are making measurements they have a certain amount of deviation. We don't want to accidentally confuse those deviations for a Higgs (or any other particle). So physicists make absolutely sure to record and analyze those deviations. To officially declare a "discovery" of the Higgs boson, we want to see a result of 5 sigma (or 99.99994%). So far we have seen compelling evidence for the presence of the Higgs boson - about 3 sigma (99.7%). What will be announced in the next few hours???
Updates to come....
Update #1
The CMS team is speaking right now. Joe Incandella is describing their experimental set up. Long story short, when they combine the gamma gamma and ZZ* detection methods (two different ways to theoretically create a Higgs boson) they see a bump in the data at ~125 GeV with a statistical significance of 4.9 Sigma (99.99994% certainty).
"We have observed a new boson with a mass of 125.3 +/- 0.6 GeV at 4.9 sigma significance"
-Joe Incandella of the CMS team
Update #2
The ATLAS team is speaking now. Fabiola Gianotti is speaking for the team. For me, the webcast was
cutting in and out. Luckily, I found the press release from CERN, which was put online during her
presentation! Of course, the presentation is full of Comic Sans. I guess when you're that awesome you can
use a horrible font.
So the ATLAS team is also reporting the discovery of a new boson (the Higgs) with a mass of 126.5 GeV,
at a 5.0 sigma significance.
The ATLAS team is also putting a great deal of focus on the fact that not only have they seen a particle at
this mass, but that its decay is what the standard model predicts for the Higgs boson.
Update #3
So now we are seeing two groups reporting the discovery of a new boson which acts like
the standar model Higgs boson. So what now?
Where does the research go from here? I'm not the best source to answer this question, but as I see it,
now we start studying the Higgs. We know where it is, so now we can start asking questions about it.
The Standard Model requires the existence of the Higgs, but will the Higgs mechanism sync up with what
the Standard Model predicts? How does interaction with a Higgs boson impart mass?
There are still lots and lots of questions to ask and lots to learn. More data, more data, more data!
Update #4
Peter Higgs, who was one of the scientists to originally theorize the existence of the Higgs boson in 1964,
was asked to say a few words. He said, holding back tears, "To me, it's really an incredible thing that it's
happened in my lifetime".
Notes:
[1] - Star Wars geeks: I don't need to hear that A New Hope is episode IV, not Part II.
[2] - Star Wars geeks: This is why lightsabers are stupid.
[3] - I should point out that this example is not 100% legitimate. Quantum states do not in fact describe spatial coordinates at all. That being said, I think it is a helpful example.
[4] - The strong force is what holds quarks together to form protons and neutrons, the building blocks of atoms.