Guest Post: Matt Strassler on Hunting for the Higgs

Perhaps you’ve heard of the Higgs boson. Perhaps you’ve heard the phrase “desperately seeking” in this context. We need it, but so far we can’t find it. This all might change soon — there are seminars scheduled at CERN by both of the big LHC collaborations, to update us on their progress in looking for the Higgs, and there are rumors they might even bring us good news. You know what they say about rumors: sometimes they’re true, and sometimes they’re false.

So we’re very happy to welcome a guest post by Matt Strassler, who is an expert particle theorist, to help explain what’s at stake and where the search for the Higgs might lead. Matt has made numerous important contributions, from phenomenology to string theory, and has recently launched the website Of Particular Significance, aimed at making modern particle physics accessible to a wide audience. Go there for a treasure trove of explanatory articles, growing at an impressive pace.

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After this year’s very successful run of the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, a sense of great excitement is beginning to pervade the high-energy particle physics community. The search for the Higgs particle… or particles… or whatever appears in its place… has entered a crucial stage.

We’re now deep into Phase 1 of this search, in which the LHC experiments ATLAS and CMS are looking for the simplest possible Higgs particle. This unadorned version of the Higgs particle is usually called the Standard Model Higgs, or “SM Higgs” for short. The end of Phase 1 looks to be at most a year away, and possibly much sooner. Within that time, either the SM Higgs will show up, or it will be ruled out once and for all, forcing an experimental search for more exotic types of Higgs particles. Either way, it’s a turning point in the history of our efforts to understand nature’s elementary laws.

This moment has been a long time coming. I’ve been working as a scientist for over twenty years, and for a third decade before that I was reading layperson’s articles about particle physics, and attending public lectures by my predecessors. Even then, the Higgs particle was a profound mystery. Within the Standard Model (the equations that used at the LHC to describe all the particles and forces of nature we know about so far, along with the SM Higgs field and particle) it stood out as a bit different, a bit ad hoc, something not quite like the others. It has always been widely suspected that the full story might be more complicated. Already in the 1970s and 1980s there were speculative variants of the Standard Model’s equations containing several types of Higgs particles, and other versions with a more complicated Higgs field and no Higgs particle — with a key role of the Higgs particle being played by other new particles and forces.

But everyone also knew this: you could not simply take the equations of the Standard Model, strip the Higgs particle out, and put nothing back in its place. The resulting equations would not form a complete theory; they would be self-inconsistent. Though still effective in many contexts, they would become useless for predicting certain high-energy processes, including ones that the LHC, a few years from now, will directly study. So it was widely known, over thirty years ago, that something like a Higgs particle had to be in those equations to make them sensible. In fact, the condition is even stronger than that. The equations of the Standard Model will require significant and historic modifications unless the Standard Model Higgs particle is found with a mass below about 800 GeV/c². (For scale, the mass of a hydrogen atom is about 1 GeV/c².)

It’s this last point that explains why the current moment is such a critical one. Sure, previous experiments have looked for the Higgs particle too. And they were able to sweep some areas clean; we know, from these experiments, that the mass of the SM Higgs cannot lie below 115 GeV/c². But the LHC is special; it is the first accelerator capable of finding the SM Higgs over the entire allowed mass range still remaining, from 115 up to and beyond 800 GeV/c².

Phase 1, the search for the SM Higgs, is the easy part of the quest for the Higgs particle (or particles or whatever). What makes it easy? The Standard Model equations are so detailed and well-specified that everything about the SM Higgs particle is already known, except for one thing: its mass. More precisely, if you told me the SM Higgs particle’s mass, I could tell you how it is produced at the LHC and at what rate, and what it decays to, and how often it decays to one set of particles rather than another. This makes life relatively simple for the experimenters at ATLAS and CMS, because all they have to do is this: pick a mass in the allowed range, ask theorists to calculate the properties of an SM Higgs particle of that mass, figure out the best way to seek it in their data, and look at the data: is there evidence for or against its presence? They must then repeat this across the entire range of possible masses systematically, until they’ve covered all the allowed territory. (Actually they do all of these searches simultaneously, not sequentially.) When their coverage is complete — or when they find something — Phase 1 is over.

Current status of the search for the Standard Model Higgs particle, showing that the LHC's November results, using the summer's data, leave a gap from 115 to 141 GeV/c², as well as a region above 470 GeV/c² that is disfavored by other considerations.

It is useful to think of Phase 1 as three subprojects, going on all at the same time but proceeding at different rates, involving the search strategies for a lightweight, middleweight and heavyweight SM Higgs. We’re almost done with the middleweight case, the easy one, in which one looks mainly for a Higgs decaying (i.e., disintegrating) to two W particles or two Z particles. The entire range from 141 to 470 GeV/c² is now excluded (according to the combination of the summer’s data from ATLAS and CMS that was announced a few weeks ago). The lightweight range, down to 115 GeV/c², is dominated by a search for a Higgs particle decaying to two photons. To observe this decay, a very rare process, requires a lot of data, so exploring this range fully will take another six months to a year. But we should already learn more about the lightweight Higgs on December 13th, when CERN will be providing an update on the Higgs search.

A lightweight Standard Model Higgs particle must be found through the very rare process in which it disintegrates ("decays") into two particles of light (photons). Discovery of a medium- or heavyweight Standard Model Higgs particle can be made using its common decays into two W particles or two Z particles (each of which immediately decays into two other particles).

The heavyweight range — above 450 GeV/c² or so — is a little more subtle. Many theorists argue this window is already closed, by indirect experimental evidence. There are processes, carefully measured over the past 20 years, that are indirectly sensitive to the mass of the SM Higgs, and that strongly suggest it should be on the lighter side… below something like 300-400 GeV/c², though reasonable people might disagree on where exactly to set this bound. But even if you didn’t buy this powerful argument, it wouldn’t trouble the experiments. Depending upon exactly how much data the LHC takes in 2012, we should see most of the heavyweight range explored experimentally by late next year. The experimental results, combined with the theoretical arguments, should allow Phase 1 to conclude, to the satisfaction of almost all experts, once the 2012 data is fully analyzed.

So what are the possible outcomes of Phase 1?

1) The SM Higgs particle, already known with substantial confidence not to be in the middleweight range, might turn up in the lightweight or heavyweight range.
2) The SM Higgs particle might be entirely excluded, from 115 up to 800 GeV/c² or so. (Remember, though, that this would not mean there is no Higgs particle of any type — it would mean only that the simplest type is not found in nature.)
3) A Higgs-like particle that is clearly not a Standard Model Higgs particle (because it has the wrong production rates, or the wrong decay rates, given its mass) might be found instead.
3a) Some other great discovery at the LHC might move the SM Higgs search off the front pages for a while.

What would be the pros and cons of these different scenarios?

1) If the SM Higgs is found, that will be a historic discovery by the LHC, provisionally confirming the Standard Model’s simplest Higgs. That said, in some ways it will be a bit disappointing, since the Standard Model leaves many important questions in particle physics unanswered, and only by finding flaws in its equations do we have much hope of answering those questions.

2) If the SM Higgs is excluded, that will be an even more historic discovery, implying that the Standard Model’s equations are not the complete story at the LHC. For most particle physicists, this will be a much more exciting outcome! There will be a great opportunity for the LHC to teach us something profound about nature that we may not currently even suspect, although we’ll be on tenderhooks, potentially for quite a while, wondering whether the LHC’s data will provide clear guidance as to how to modify the Standard Model, or only give us some suggestive hints.

3) If a new non-SM Higgs particle is found, that will be the best possible outcome! Not only will we see the Standard Model’s equations fail, we’ll have a direct clue, in the form of the new particle, as to how to begin modifying them. In this case the LHC will immediately help us to start writing the new chapter in particle physics textbooks.

3a) And if something else unexpected is found in LHC data in the meantime, no one will complain! This of course would also mean the failure of the Standard Model’s equations, and new clues into nature’s mysteries.

You may have noticed that on this list there’s really no bad outcome. That’s right: as long as there are no technical problems at the LHC that limit the amount of data it collects in 2012, we are in a no-lose situation in the short term. This does not happen very often! And this is why there’s so much excitement right now in the field. We’re not wondering if we’ll get some historic information over the next year or so, or whether it will change the field of particle physics. We’re just wondering what it will be. (Needless to say, we’re all wishing the accelerator physicists and engineers over at the LHC, who keep the machine running efficiently, the very, very best — and while we’re at it, let’s hear a big round of applause for them right now, for what they’ve achieved in 2011!)

A highly simplified illustration of how Phase 2 of the Higgs search will proceed, depending on the outcome of Phase 1.

No matter what, there will be a very important Phase 2 to the Higgs search, which will extend for perhaps ten years beyond Phase 1. If Phase 1 finds something that looks like an SM Higgs particle, Phase 2 will be all about checking its details with high precision. The new particle may look at first as though it is just what the simplest version of the Higgs story would predict, but if even the slightest detail is out of place, it would show the Higgs is not so simple after all, which would be an exciting turn of events. If instead Phase 1 rules out the SM Higgs, then a great host of new search strategies will be brought to bear, and the experimentalists will (figuratively) fan out like a massive search party, looking for all varieties of exotic types of Higgs particles. Also — remembering that there may be no Higgs particle, but if so, the Standard Model’s equations can’t be entirely right — they’ll step up their efforts to look for the many other types of particles and forces that we might need to add to the Standard Model to make its equations sensible again, absent any type of Higgs particle in nature. And if a non-SM Higgs particle is found in Phase 1, Phase 2 will involve all of these strategies at once, producing the new Higgs particle in large quantities and studying it in detail, while looking for more clues as to what else is missing from the Standard Model’s equations.

It has been decades since a moment in particle physics looked as bright as do the next couple of years (healthy LHC operations permitting.) We’ve seen turning points in other fields: the human genome project was guaranteed to revolutionize genetics and genomics, and the study of the cosmic microwave background radiation was guaranteed to change our understanding of the early universe, as long as the experimental methods worked. These were great historic achievements that gave new life to those subjects; but we do not yet know whether the same is in store for particle physics. Will the flash of new understanding provided by Phase 1 quickly fade, or will it brighten into a dawn of a new age? Will the Standard Model’s equations work perfectly at the LHC, giving us a sense of satisfaction but no clues for future understanding? Will the equations fail, but in an obscure fashion, leaving us uncertain as to how to fix them? Or will their failure be clear and instructive, as has been the case for so many sets of equations before them, allowing the LHC, along perhaps with other experiments, to provide us with the insights we need to proceed to a more profound understanding of nature?

There is only one way to find out: run the experiment, and let nature speak.

So keep your eyes on Phase 1 of the Higgs search as it progresses toward a conclusion over the coming weeks and months. If the LHC works as hoped, the year ahead will be a memorable one.