The year of the Higgs
Things are looking good at Cern. The Large Hadron Collider (LHC) is back in business and for the first time, there is a real sense that major discoveries are within reach.
It seems as good a time as any to zip through some of the basics surrounding the hunt for the Higgs boson. To cut to the chase (and barring any mishaps) the LHC is likely to get at least a sniff of the Higgs particle later this year, with probable confirmation coming next year. But for the next 10 months this is still a two horse race. The world’s second most powerful collider, the Tevatron at Fermilab in Illinois, may glimpse the missing boson before it closes down for good at the end of the year.
I didn’t write about the US Department of Energy’s decision to switch off the Tevatron this year, instead of running on for three more as the lab had hoped. I spoke to people on the committee involved and there was certainly disappointment in some quarters. The announcement was disheartening for a country of America’s means, but the bigger issue now has to be whether the US has a good enough strategy in place to inspire its next generation of high-energy physicists. That is another story.
I’ll cover some simple stuff in the paragraphs below, including how a collider makes particles; how the Higgs boson might be made; what it might look like, and how do you claim an official discovery. Then I’ll lay out the status of the search to date and some future prospects.
In the olden days, accelerators sent particles slamming into targets in the hope of smashing atoms into pieces and revealing their constituents. Hence the nickname “atom smasher”. Colliders like the LHC do things differently. They slam particles together in head-on collisions to produce intense and concentrated flashes of energy. The energy released in each collision is available to make particles, in line with Einstein’s infamous equation of mass and energy. Some of the energy of the collision effectively condenses into matter.
The LHC was designed to accelerate protons (which are simply hydrogen atoms with their electrons ripped off) and collide them at energies of 14 teraelectronvolts (TeV). One electron volt is the kinetic energy an electron gains when it is accelerated by a potential difference of one volt. Physicists use prefixes to refer to large energies, such as kilo (1000), Mega (1 million), giga (1 billion) and Tera (1 trillion). The mass of a Z boson is around 91GeV/c2, so a collision needs to liberate at least this much energy to make one.
Although the LHC was built to crash protons together at 14TeV, there is far less energy available in each collision to make new particles. The machine is running at half energy at least until the end of 2011, meaning the total energy is only 7TeV. But that is not the only issue. More important is that inside the LHC, collisions are not between protons themselves, but their constituents. Each proton is made up of three quarks and three gluons and the collisions are between these. So when two protons crash into each other, it is actually the gluons and quarks that hit each other. This matters because the energy is shared out among the constituents. So if two protons collide at 7TeV, the maximum amount of energy released when the bits inside hit one another is only around 1TeV.
So how might the Higgs boson be made in a collider? This depends on many things. In the LHC and the Tevatron, two colliding gluons can produce a Higgs particle. Once made, the Higgs boson doesn’t just sit there waiting to be discovered. It is highly unstable and decays immediately into more familiar particles. Which ones depend on how much mass – and so how much energy - the Higgs particle has. In the LHC, one option is for the Higgs to decay into two high-energy photons. I’ll be blogging about the fuss over a potential signal in this decay mode pretty soon.
I’ve had conversations with non-scientists (let’s call them the Guardian newsdesk) about the LHC during which it’s become clear they don’t grasp why results can be a long time in coming from a particle collider. The confusion is understandable. Plenty of people are used to school experiments where an instrument is switched on, you take a measurement and get the answer. Life is not so easy with colliders. Thanks to the uncertainties of quantum mechanics, you never know whether or not a particle will be created in a collision. The best you can do is work out the probability, analyse countless collisions, and look for an excess in the number of particles the Higgs, in this case, decays into.
You can look at this in another way. Suppose the LHC was colliding wallets instead of protons. Most of the time, the debris from the collisions would include some loose change, a few notes and some bankcards. The problem is that even when a Higgs is made, it immediately decays into familiar stuff – more loose change, notes and bankcards in this analogy. So what you look for is the right amount of extra debris that you’d expect were the Higgs particle popping into existence and immediately decaying.
Physicists rank the strengths of these excesses in standard deviations, denoted by the greek letter sigma. To claim a cast-iron discovery takes a five sigma signal, which means the surfeit of decay particles you measured has less than a one in a few million chance of being a statistical fluke. Officially, “evidence” means at least a three sigma signal.
So where is the Higgs search at right now? In 2000, Cern’s Large Electron Positron (LEP) Collider ruled out a Higgs below 114.4GeV, but saw what might have been signs of the Higgs at a slightly greater mass shortly before it closed down. Since then, the Tevatron has ruled out a range of masses, from 157GeV to 173GeV. Indirect measurements of the Higgs mass pretty much rule out the particle (that is, with 95% confidence) above 185 GeV.
In January, Rolf Dieter Heuer, Cern’s Director General, gave a talk to personnel and among other things, discussed the prospects for a Higgs discovery. You can see the whole presentation here. Slide 17 is key. Here, the DG says the Tevatron can pretty much rule out the Higgs (if it doesn’t exist) between a mass range of 114 to 185GeV by the end of the year. In that period, the collider could see three sigma evidence for a Higgs particle with a mass of 115 GeV or somewhere in the range from 150 to 180GeV.
But what about the LHC? A more detailed prediction is given on slide 17, but one fairly optimistic scenario (one that sees the machine collect 2.5 inverse femtobarns of data at eight teraelectronvolts) suggests the LHC could exclude the Higgs if it doesn’t exist iver essentially the entire mass range; find three sigma evidence for a Higgs weighing 123 to 530 GeV, and an all important five sigma discovery if the Higgs lies at 138GeV to 220GeV. The Higgs particle may well weigh less than 123GeV and not reveal itself at the LHC until 2012.
So what next? Neither Cern nor Fermilab will sit on a major discovery should one appear in their data. Large collaborations leak like sieves and any news will likely emerge as rumours on the physics blogs before a research paper is posted on the arxiv and any press conference called. Barring any surprises, expect the next news to be unveiled in the summer, at the European Physical Society High Energy Physics meeting in Grenoble at the end of July.