The Large Hadron Collider: Bring it on!
- 27 January 2007
- Davide Castelvecchi
my highlights / edits
IT'S official: 2007 is the year of the LHC. In case you haven't heard, the initials stand for the Large Hadron Collider, which is nearing completion at the CERN laboratory near Geneva, Switzerland. Just a snowball's throw from Mont Blanc, it is not only the hottest thing in physics but also the largest, most elaborate scientific instrument of all time.
After 20 years of anticipation, the LHC is set to switch on this November, and soon thereafter CERN, the European Organization for Nuclear Research, will become the proud operator of the world's most powerful particle accelerator. It will smash protons together with seven times the energy and at 100 times the rate of the top collider to date, the Tevatron at Fermilab near Chicago. That will allow it to probe the interactions of particles down to the unprecedented scale of 10-17 centimetres, roughly the size of the universe a trillionth of a second after the big bang, when the known fundamental forces of nature were born. The LHC is widely expected to usher in a new era of particle physics, perhaps even pointing the way to the fabled "theory of everything".
Not surprisingly, what lies ahead is making physicists positively giddy. "We are like children waiting for Christmas," says JoAnne Hewett, a theorist at the Stanford Linear Accelerator Center in Menlo Park, California. "You can't imagine the excitement." Immense technical hurdles remain, however, not least the problem of handling the amount of raw data the LHC will produce over its projected 20-year lifetime, which could exceed that in all the words spoken in human history.
Help might be on the way in the form of a controversial method for automating the data analysis. Some researchers, including Bruce Knuteson of Fermilab and the Massachusetts Institute of Technology, say their software is the fastest way to reveal hints of new physics. Others see it as a recipe for confusion and wild goose chases. The way future experiments are done and analysed may hinge on who is right.
The past dozen years have been a dry spell for particle physics. Despite all the theorists' fancy ideas, experimenters have found no new particles since 1995 when the top quark - the last in the family of six quarks which are among the building blocks of matter - was discovered at the Tevatron.
Since then, the immense multinational effort to build the LHC has gone into high gear, and the machine will start collecting data just over a year from now. Inside its 27-kilometre underground ring it will turn nanograms of hydrogen into two merry-go-rounds of protons travelling in opposite directions. Each beam will be loaded with 0.3 gigajoules of energy, equivalent to that of a 400-tonne French TGV train running at 140 kilometres per hour. The two beams of protons will crash head-on at several points in the ring where monumental detectors are now nearing completion. The largest of these, called Atlas (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid), are where thousands of researchers will be looking for new physics.
How exactly? Inside the detectors, the quantum shrapnel produced by colliding protons will scatter like colours in a Jackson Pollock painting. Courtesy of Einstein's E = mc2, much of the protons' energy will mutate into mass, and the particles that come out as debris will often be heavier - even hundreds of times heavier - than the protons that went in. The researchers hope that among those heavy particles will be ones of a kind nobody has seen before. "The most exciting thing the LHC can potentially discover is something we cannot predict," says CERN theorist Alvaro De Rujula.
The LHC could produce particles that turn into ephemeral, mini black holes; particles that solve the mystery of dark matter; particles that spend much of their lives in other dimensions of space; or even particles that explain why the universe seems perfectly tuned for us to exist. No one will catch them directly, though. Rare particles tend to have very short lives and almost instantly decay into more ordinary stuff. So physicists must reconstruct the nature of the rare particles by looking at this debris.
At previous colliders, experimentalists usually knew how to do this. Virtually all the phenomena they observed fit the so-called "standard model" of particle physics developed in the 1970s, in which quarks, electrons, neutrinos, photons and other known particles play the fundamental roles. From its equations, physicists were able to calculate the patterns that should appear in the zillionth of a second after a collision. The main tool for this was invented by Richard Feynman in the 1950s: Feynman diagrams show how old particles can combine into new ones, annihilate into pure energy, procreate, and decay by splitting into other particles. Diagrams can include the cameo appearance of a hypothetical new particle, along with the signature combinations of outgoing stuff that betray its presence. Catch that signature in your detector and you can claim discovery of a particle even if you haven't observed it directly.
Hello Higgs
At the LHC, this time-honoured method should be useful for at least one task: finding the Higgs boson, the only particle in the standard model that has yet to be observed. The Higgs is the crucial link that explains how other particles such as quarks and electrons acquire their masses. Its discovery is expected to confirm the standard model as the ruler over all known forces except gravity. Despite the efforts of researchers at four machines, including ongoing measurements at the Tevatron, the Higgs has remained elusive.
All the predictions point to its mass being no more than 200 times the mass of a proton, which is well within the LHC's reach. If the LHC doesn't find it, that would mean that either Einstein's E = mc2 or quantum theory - the two pillars of modern physics on which the standard model is predicated - is wrong. That would lead to a far greater upheaval in physics than the discovery of any new particle, but it's very unlikely. "The Higgs has to be there," says Fermilab theorist Joe Lykken.
So far so good, but the LHC's discoveries are expected to go beyond the standard model and include a zoo of new particles. Theorists have proposed hundreds of models for what such particles would look like, but they are all speculative. The LHC could validate one of these models, or perhaps show that they are all wrong. What is challenging is that some of the most popular models predict very similar signatures, which could give researchers fits when they try to figure out which is correct.
Take supersymmetry, which unites the various forces of nature at high energies. This model says that for each standard-model particle there exists a heavier "superpartner" particle. At the LHC, such superpartners should appear in pairs of identical, electrically charged particles. Seeing such pairs, or the particles they decay into, would be a sure sign of new physics, says Gordon Kane of the University of Michigan, Ann Arbor.
Trouble is, a similar signature is also predicted by a model called universal extra dimensions, in which each particle we see has more massive counterparts shadowing it in an additional dimension of space that we cannot see. Like superpartners, these would show up by decaying into pairs of identical particles with a net electric charge. The two types of counterparts would differ in a crucial quantum property known as spin, but the LHC does not have a direct way of measuring this.
For now, though, most experimentalists are less concerned about validating theories than they are about discovering new particles, a feat which would likely earn them a Nobel prize. "People always ask me, 'If you discover a new particle, how will you distinguish supersymmetry from extra dimensions?'" says Ian Hinchliffe, who leads one of the Atlas teams. "I'll discover it first, I'll think about it on the way to Stockholm, and I'll tell you on the way back."
That's all well and good, but a legitimate issue remains that could ultimately determine the success of the LHC. Waiting for specific patterns to hit the detectors - which is what most researchers plan to do, with each group specialising in just a few possible outputs - might mean missing the most interesting new physics.
Researchers at the Tevatron were able to discover the top quark because they knew it would tend to decay into one electron or muon (a heavier electron-like particle), one neutrino and hundreds of less interesting particles - and that's precisely what they found in the detectors. This "top-down" approach has worked fine at the energies probed so far, where virtually all observations have been in agreement with the standard model. On the rare occasion that unexplained phenomena have occurred, physicists have erred on the side of caution and put them down to statistical flukes, quantum fluctuations or measurement errors. That is expected to change at the energies produced by the LHC, where brand new phenomena should be plentiful. "The most likely scenario is that we're going to have a ton of weird stuff to explain," says Nima Arkani-Hamed, a theoretical physicist at Harvard University.
"The most likely scenario is that we're going to have a ton of weird stuff to explain
That's why some researchers, including Arkani-Hamed, say a revised approach is called for. Instead of trying to check whether a particular model can fit a predetermined signature, they would examine all patterns of debris hitting the detectors that can't be explained by the standard model, and which are frequent enough to not be statistical flukes. They would then work upwards from these observations to make a guess at which model is likely to fit the data. This process, says Arkani-Hamed, needs the participation of theorists who in the past have rarely taken part in analysing data. It would rely on sophisticated software to select which patterns of debris, or "channels", to look at.
Such software has recently been developed by Knuteson, Stephen Mrenna and others at Fermilab, and by Sascha Caron and collaborators at the DESY lab in Hamburg, Germany. Knuteson started working on these methods in the late 1990s, and when he has applied them to Tevatron data the results have matched the speed and accuracy of conventional top-down methods. That much is uncontroversial. Where Knuteson and Mrenna have broken contentious new ground is in creating an algorithm aimed at a more ambitious bottom-up search for new physics (www.arxiv.org/hep-ph/0602101).
Reverse Feynman
For a given channel - an unusual combination of quarks and neutrinos, say - the computer uses quantum theory to reverse-engineer the hundreds of possible Feynman diagrams that could have produced these particles, and suggests ways to tweak the standard model to explain them - perhaps by introducing a new particle at an intermediate stage. This might be a superpartner of a regular particle, or something else entirely. "It begins to automate the building of new models," says Knuteson.
Critics say that this could lead to too many false alarms. "When you look at such immense sets of data, there are always statistical fluctuations," warns Michelangelo Mangano of CERN. "This kind of approach has made people claim false discoveries, slowing down the progress of physics." Knuteson's team insists that the software has safeguards against this, and can discern which hints of new physics are statistically most relevant. One event in an unexpected channel would be dismissed, but hundreds of them - especially if the standard model says they should be rare - would flag up something worth paying attention to.
It is too early to know who is right, and what exactly the bottom-up approach will yield. Knuteson plans to join the CMS experiment, where he says that looking at hundreds of channels simultaneously should accelerate the process of discovery. Caron is already a member of Atlas, where he plans a similarly broad search. If their approach proves successful it could eventually change the way theoretical physics is done. "We will get to the point where developing new theories, something currently in the human domain, will be done by computers," Knuteson says.
However the results from the LHC are interpreted, hopes are high that they will lead us to new and unexpected discoveries about the most basic influences in the universe. That is where the excitement really begins, and Arkani-Hamed is confident that it will be people, not machines, that make the breakthroughs. "Going from the data to a beautiful theory," he says, "is something a computer will never do."
Davide Castelvecchi is a science writer based in Washington DC
It further highlights the importance of eating a healthy diet for reducing the risk of cancer 
