Bump Hunting (Part 1)

By John Conway | January 26, 2007 1:00 am

I’ve been looking for the Higgs boson for almost 20 years.

So there I was, on a Saturday morning in December, at CERN as it so happened, when I saw the graph we’d been working towards all year. At first I thought it was some mistake – the hair literally rose up on the back of my neck, and I said: “Holy crap! What’s that?”

Where do I start? For a long time now our field of particle physics has been totally obsessed with finding this beast we call the Higgs. We have a very successful mathematical model we call the Standard Model which has accounted all too well for hundreds of different experiments and observations of the fundamental particles in nature and how they interact.

My favorite analogy is that a hundred years ago we had the periodic table of the elements, which organized them all into neat rows and columns according to their chemical properties, from the halogens to the noble gases. But a hundred years ago, no one had a clue as to why this was so. It took another thirty years of experimenting and theorizing to figure it out. That led to quantum mechanics, the solution to the hydrogen atom, and then the understanding of more complex atoms and molecules. Then it all broke open: nuclear energy, silicon electronics, computers, cell phones…not to mention NSA wire tapping and YouTube. But I digress.

Now we have a neat little periodic table of the smallest of the small, the fundamental particles we call quarks and leptons:

fundamental fermions

It took the last 40 years since quarks were first imagined to get to the point where we are now. This neat three-generation structure, though, is absolutely begging for answers to these questions: Why do they have such different masses? Why just three generations? Why the weird fractional charges? (Or is the charge of the electron three more fundamental units of charge?)

The Higgs boson is a particle which is essentially a by-product of the Standard Model, a sort of physical manifestation of a hypothetical “Higgs field” which permeates all space-time and with which all particles have some level of interaction. The more interaction with the Higgs field, the more massive a particle is. We call it a “boson” as opposed to a “fermion” (all the quarks and leptons are fermions) because it is thought to have no spin, no intrinsic angular momentum.

The Standard Model gave us no guidance, though, 30 years ago as to what the mass of the Higgs boson might be, except that it’s probably a good deal less than 1000 times more massive than the proton. So, 20 years ago, we had an accelerator at CERN in Switzerland called LEP that was the first one that we thought might be able to produce the Higgs. I was on one of the four big experiments there, called ALEPH (a mythical monster with eyes in all directions) and we started from the smallest masses we could, and worked our way up from there. We kind of thought we had found it once, but it was a sort of experimental mirage.

I moved back to the US in 1993 and took up the hunt at the then-new Tevatron at Fermilab on the CDF experiment. The LEP accelerator collided electrons with their antimatter opposites, positrons, at energies about 100 times that of the proton rest mass. The Tevatron collides protons and antiprotons at energies 20 times greater than that! Unfortunately, though, as we know nowprotons and antiprotons are bags of smaller particles called quarks, along with the “gluons” that hold them together. (Good name, huh?) So it’s actually the quarks and gluons that collide, and so you don’t get all the energy into the interesting part of the collision. In the end, the energy is closer to that of LEP, but occasionally greater. Sometimes a lot greater.

To make Higgs bosons, in the case we are interested in, it’s actually the gluons that collide. In fact the case we are interested in is not the Standard Model but an extension called supersymmetry. If we discover supersymmetry, we’ll have job security for a lifetime because for every particle we know about, if supersymmetry is true there is a supersymmetric partner for it. In the case of the Higgs boson, there is not one but four to discover!

Even better, if supersymmetry (SUSY for short) is there, then it’s possible that Higgs bosons will be produced at a greatly enhanced rate at the Tevatron. That’s what I have been working on for a lot of years now in the CDF experiment.

If you produce a Higgs boson, then what? It decays, nearly instantly, into the heaviest quark-antiquark or lepton-antilepton pair it can. For the Higgs masses we seek, this would be either a b and an anti-b quark, or a tau and an antitau lepton.

The tau lepton is without question my favorite. It’s really just a heavy version of an electron, but its mass, about twice that of a proton, means that it decays in many interesting ways. “Interesting” in the sense of the curse “may you live in interesting times” or “the IRS found your tax return interesting”. Finding tau decays amidst the debris of your typical Tevatron proton-antiproton collision is one of the toughest experimental challenges there is. It’s sort of like “Where’s Waldo?”because quite often the ubiquitous quarks and gluons that come screaming out of the collision look just like tau decays.

I won’t go into how we separate out the taus here – after 15 years of work we’re starting to get good at it at the Tevatron. Good enough that only a small fraction of the time do we mistake quarks or gluons for taus, and we have ways to measure that.

The sort of work we do at Fermilab and CERN takes large teams – there are hundreds of physicists on the Tevatron experiments and thousands on the new CERN experiments. Clearly you can’t work with hundreds of people at once, so we organize ourselves into much smaller working groups which meet often to discuss the progress. There is a whole pyramid structure of bigger and bigger meetings at which results are presented, criticized, and refined until, according to the rules of the collaboration, a result is “blessed” for release to the public.

I had been invited in October to present results of our search and other peoples’ searches for the SUSY Higgs at the annual Aspen Particle Physics meeting in January. This is the kick-off international conference of each new year, and I was quite lucky to have been invited. Our little team of CDF physicists saw this as a chance to show our fresh stuff, and so we worked like crazy all fall to get the result completed. There were calibrations to make, cross checks to perform, and internal notes to write. My part was the final extraction of the result, which we full well expected to be null – no sign of the Higgs. These sorts of searches are like that: it’s like playing the long odds at roulette, with a small chance of winning but a big payoff if you do.

I’ve been working with these guys, Amit Lath and Anton Anastassov, since 2001 on this project. They were my colleagues when I was at Rutgers: Amit is a professor there and we hired Anton as a postdoc in 2000. They are two of the sharpest guys I know, and like me are infatuated with the tau and its possibilities. The team also included two more junior talented guys: Dongwook Jang, formerly a grad student at Rutgers and now at Notre Dame as a postdoc, and Cristobal Cuenca, my student from Valencia. Anton has done by far the most work on the project and he is the one that keeps the engine tuned up and humming. Continuing the analogy, if the project were a car, I am the one to take it down a straight road very very fast, and Amit figures out where we are and how far we’ve gone. (Okay, enough of that.)

As honest scientists, we wait until we have performed all the checks and controls and have convinced our colleagues that we are ready before “opening the box” and seeing what is there or not. In an earlier version of the result back in 2005, we had seen nothing unusual. The collisions we select are usually mostly from the production of Z bosons, the particle which, together with the W bosons, is responsible for the weak nuclear force. It’s sort of a standard candle for us, and seeing the right number of them tells us we’ve done our job correctly.

In fact, the mass of the Z is about 100 times that of the proton, and what we were looking for is probably about 20-50% heavier than that. So in addition to the big “bump” in our graph from Z events, we’d also see a second, much smaller bump at higher masses, an excess over all the ordinary background signalling production of something new: SUSY Higgs bosons.

So, that Saturday, I was ready to go – we’d gotten the green light to open the box, and I was ready to extract the final result.

I opened the box, made the plot, and there it was. Holy crap!

To be continued…

mvis graph

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