The Particle at the End of the Universe by Sean Carroll

Author:Sean Carroll [Carroll, Sean]
Language: eng
Format: epub
Publisher: Penguin Publishing Group
Published: 2012-11-13T00:00:00+00:00

Signal and background

Particle physics, since it is powered by quantum mechanics, is a lot like coin flipping: The best we can do is predict probabilities. At the LHC, we smash protons together and predict the probability of different interactions occurring. For the particular case of the Higgs search, we consider different “channels,” each of which is specified by the particles that are captured by the detector: There’s the two-photon channel, the two-lepton channel, the four-lepton channel, the two-jets-plus-two-leptons channel, and so on. In each case, we add up the total energy of the outgoing particles, and the machinery of quantum field theory (aided by actual measurements) allows us to predict how many events we expect to see at every energy, typically forming a smooth curve.

That’s the null hypothesis—what we expect without any Higgs boson. If there is a Higgs at some specific mass, its main effect is to give a boost to the number of events we expect at the corresponding energy: A 125 GeV mass Higgs creates some extra particles with a total energy of 125 GeV, and so on. Creating a Higgs and letting it decay provides a mechanism (in addition to all the non-Higgs processes) to produce particles that typically have the same total energy as the Higgs mass, leading to a few additional events over the background. So we go “bump hunting”—is there a noticeable deviation from the smooth curve we would see if the Higgs wasn’t there?

Predicting what the expected background is supposed to be is by no means an easy task. We know the Standard Model, of course, but just because we know what the theory is doesn’t mean it’s easy to make a prediction. (The Standard Model also describes the earth’s atmosphere, but it’s not easy to predict the weather.) Powerful computer programs do their best to simulate the most likely outcomes of the proton collisions, and those results are run through a simulation of the detectors themselves. Even so, we readily admit that some rates are easier to measure than to predict. So it is often best to do a “blind” analysis—use some method to disguise the actual data of interest, by adding fake data to it or simply not looking at certain events, then making every effort to understand the boring data in other regions, and only once the best possible understanding is achieved do we “open the box” and look at the data where our particle might be lurking. A procedure like this helps to ensure that we don’t see things just because we want to see them; we only see them when they’re really there.

It wasn’t always so. In his book Nobel Dreams, journalist Gary Taubes tells the story of Carlo Rubbia’s work in the early 1980s that discovered the W and Z bosons and won him a Nobel Prize, as well as his less successful attempts to win a second Nobel by finding physics beyond the Standard Model. One of the tools that Rubbia’s team used in their

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