Will-o'-the-wisp around 5 sigma: the hunting of the Higgs
Mr. Hunter, we have rules that are not open to interpretation, personal intuition, gut feelings, hairs on the back of your neck, little devils or angels sitting on your shoulder.... - Capt. Ramsey ('Crimson Tide')
Particle physicists hunting for maddeningly elusive particles sometimes must feel like Mr. Hunter in the movie "Crimson Tide". The quarries which they are trying to mine seem so ephemeral, making their presence known in events with such slim probability margins, victims of nature's capricious dance of energy and matter, that intuition must sometimes seem as important as data. The hunt for such particles signifies some of the most intense efforts in extruding reality from nature's womb that human beings have ever put in.
No other particle exemplifies this uniquely human of all endeavors than the so-called Higgs boson. The man who bears the burden of imparting it its name is now a household name himself. Yet as the history of science often demonstrates, the real story is both more interesting and more complicated. It involves intense competition involving billions of dollars and thousands of careers of a kind rarely seen in science, and stories of glories and follies befitting the great tragedies. In his book "Massive", Ian Sample does a marvelous job of bringing this history to life.
Sample excels at three things. The first is the story of the two great laboratories that have mainly been involved in the race to the finish in discovering nature's building blocks- Fermilab and CERN. CERN was started in the 60s to give a boost to European physics after World War 2. Fermilab was lovingly built by the experimental physicist Robert Wilson, a former member of the Manhattan Project who was a first-rate amateur architect and saw accelerators as aesthetic things of beauty. Secondly, Sample does a nice job of explaining the reasons that led to the construction of these machines, the most complicated that mankind has ever constructed. Only human beings would put billions of dollars and immense manpower on the line purely for the purpose of satisfying man's curiosity of plumbing the depths of nature's deepest secrets. Sample also lays out the very human and social concerns that accompany such investigations. Lastly, Sample was lucky enough to get an extended interview with Peter Higgs, a shy man who very rarely does interviews. Higgs grew up in Scotland idolizing Paul Dirac and shared Dirac's view of a unifying beauty that would connect nature's disparate facts. In the late 1960s he wrote papers describing what is now called the Higgs boson. The papers were well-accepted in the US and Higgs's name soon began to be bandied about in seminars and meetings. As described below however, Higgs was not the only one postulating the theory.
So what exactly is the Higgs boson? A complete understanding would naturally need a background in theoretical physics, but the best analogy for the layman was given by a British scientist. Imagine a room full of young women who are happily chatting. In walks a handsome young man. As long as he is not noticed he can move freely across the room, but as soon as the young women spot him they cluster around him, impeding his movement. It's as though the young man has become heavier and has acquired mass from the "field" of women surrounding him. The Higgs then is the particle that imparts specific masses to all the other myriad particles discovered so far including quarks and leptons through its own field. It should be evident why it's important. The Higgs would be the crowning achievement in the Standard Model of particle physics which encompasses all particles and forced known until now except gravity.
However, the history of the Higgs particle is complicated. Sample does a great job of explaining why the credit belongs to six different people who reached the same conclusion that Higgs did. It seems that Higgs was not the first to publish, but he was the first one to clearly state the existence of a new particle. However, the most comprehensive theory of the Higgs field and particle came out later. If Nobel Prizes are to be awarded, it's not at all clear what three people should be picked, although Higgs's name seems obvious. The sociology of scientific discovery is as important as the facts and again illustrates that science is a much more haphazard and random process than is believed.
The search for the Higgs gathered tremendous momentum in the 80s and 90s. It intensified after accelerator laboratories spectacularly discovered two particles named the W and Z bosons that are responsible for mediating the electromagnetic and weak interactions (the electroweak force). These particles were predicted by Steven Weinberg, Abdus Salam and Sheldon Glashow in the 60s, and their prediction surely ranks as one of the greatest theoretical successes in modern physics. Once the theory predicted the masses of these particles, they were up for grabs. No experimentalist worth his or her salt would fail to relish nailing a concrete theoretical prediction of fundamental importance through a decisive experiment. Sample captures the pulse-quickening inter-Atlantic races to find these particles especially between CERN and Fermilab. The importance of these particles was so obvious that Nobel Prizes came in quick succession both to the theorists and the experimentalists. However the existence of the Higgs is also essential for the successful formulation of the electroweak theory, and signatures of the Higgs are thought to be produced whenever W and Z bosons are created. It again becomes obvious why finding the Higgs is so important; its existence would validate all those successes and Nobel Prizes, whereas a failure to find it would entail a stunningly hard look at some of particle physics's most fundamental notions.
These days the Large Hadron Collider (LHC) is all over the news. Yet the most exciting part of Sample's book describes not the LHC but the Large Electron Positron collider (LEP) at CERN which was the largest particle accelerator in the world at the time. Unlike protons, electrons and positrons are fundamental particles and crashing them together produces 'cleaner' results. There were some fascinating events associated with the LEP. The behemoth's circumference was 27 kilometers and it crisscrossed the Swiss-French border, so authorities had to seek permission to build the accelerator underneath some homes. It seems that French law is special just like their cheese and language; apparently if you build a house in France, it means that you own the entire ground beneath the house, all the way to the center of the earth. Suffice it to say that some negotiation with the homeowners was necessary to secure permission for underground construction. At one point the intensity of the beams inside the mammoth machine started to wax and wane. After many days of brainstorming a scientist had a hunch; it turns out that the the gravity of the moon and the sun sets up tides inside the crust of the earth. These tides put the calibration of the machine off by a millimeter, too small to be noticed by human beings, but thunderingly large for electron beams. In another case, the daily departure of a train from a nearby station sent surges of electricity into the ground and affected the beams. It seems like when you are building an accelerator you have to guard against the workings of the entire solar system.
The story of particle physics is also fraught with tragedies. One of the biggest described in the book was the construction of the Superconducting Supercollider in Texas. The SSC was supposed to be the answer to CERN and got enthusiastic backing from Reagan and Bush Sr. Unfortunately the budget spiraled out of hand, the infighting intensified, congressmen remained unconvinced and the collider never got built in spite of spending billions and affecting thousands of careers of scientists who had relocated. The fiasco just proved that public support for even projects like the LHC is never a sure thing, and scientists don't always excel at public relations.
Then of course there are all the doomsday scenarios and concerns which were raised about the LHC, from the formation of black holes to the world ending in myriad other ways. As Sample describes, these concerns go back to an accelerator at Brookhaven National Laboratory which would impact large gold ions together at furious velocities. The would-be Nobel laureate Frank Wilczek raised the theoretical yet vanishingly small probability of forming 'strangelets', entities akin to the fictitious substance 'Ice-9' in Kurt Vonnegut's novel 'Cat's Cradle'. These strangelets would coalesce together matter around themselves and form a superstable form of dead matter that would rapidly engulf the entire planet. The concern about strangelets pales in comparison however to the possibility of 'vacuum decay', in which our universe is thought to be in a perfectly happy but metastable state like a vase on a table. All it takes is a little nudge or a massive kick from a high-energy particle collision in our case to dislodge the vase or universe from its metastable state into a stable state of minimum energy. Gratifyingly, not only would this state mean the end of life as we know it but it would also mean the impossibility of life ever arising. Yes, all these scenarios seem straight out of the drug-induced, overactive imagination of a demented mind, but at least some of them are within the realm of theoretical possibility. Unfortunately when the result is the destruction of the planet, the words "improbable" and "vanishingly small" will never do much to assuage the public's fears. It just indicates that physicists will always have to grapple with public relations issues vastly more complex than the LHC.
Finally, we get a fascinating overview of the kinds of things which scientists hope to see in the LHC. The problem is that the generation of particles like the Higgs is a very low-probability event and is usually only a side-product of some other primary event. The situation is made more complicated by the immense difficulty of observing such fleeting glimpses in a hideously complex background of noise generated by the creation of other particles. Scientists working on these projects have to keep their eyes and instruments peeled for the one in a trillion event that may bring them glory. Whenever an event is observed, the scientists have to calculate the realm of probability in which it belongs. Usually if the event is outside five standard deviations ('5 sigma') then it is extremely likely to be real and not have occurred by chance alone. Not surprisingly, the observation and communication of these events is a tortuous thing. Publicity has to be avoided before you confirm such fleeting bits of probability, but leaks inevitably offer. And the media has seldom shown any restraint in announcing such potentially momentous discoveries which would bring glory, prizes and money to their originators. Scientists working today also have to deal with the presence of blogs and other instant communication conduits. As Sample narrates, at least in one case a physicist at CERN posted preliminary LHC results on the blog Cosmic Variance, and all hell broke loose. Scientists have to tread carefully especially in this era of instant data dissemination.
All this makes the scientists engaged in such endeavors live on the edge, and to us they appear like the explorers who have their eyes peeled to the sky looking out for the stray signal that would announce the presence of extraterrestrials. The mathematics of the Higgs boson is of course much more sound than that of alien contact, but the scientists who are looking for it are hanging on to such flimsy wisps of probability and interpretation that they surely must be questioning their own sanity sometimes.
In the end, even physicists are all too human. As Capt. Ramsey says, our rules are not always subject to little devils and angels sitting on our shoulders. And yet it seems that scientists like the Higgs hunters sometimes would be tempted to trust the hairs on the back of their head, especially when those hairs stand up straight at the glimpse of a peak in the graph, that 5-sigma event which would change everything. Maybe, just maybe.