The Higgs Boson: Searching for the God Particle

by Scientific American Editors

  His pronouncement caused a stir in the global physics community, and the Scottish physicist Peter Higgs, whose name had gotten attached to the hypothetical particle (Higgs had done some work in the 1960s, as had several other physicists, paving the way for the theoretical existence of the mass-imparting boson) took the challenge personally, complained about Hawking, and later lamented that to answer Hawking’s challenge would have been “like criticizing the late Princess Diana.”

  In fact, informal polls of physicists over the last decade have shown that an overwhelming majority believed that the existence of the Higgs was a foregone conclusion and that all that was needed was simply to run the LHC long enough: the Higgs would eventually show up. Hawking—known for controversial and contrarian pronouncements—was seen as simply throwing around his weight.

  But the Higgs boson never appeared. Running continually at an unprecedented energy level of seven trillion electron volts since March 31, 2010, the LHC has been amassing petabytes of data that are being analyzed by a grid of interlinked computers worldwide in search of the missing boson. And yesterday, August 22, at the Biennial International Symposium on Lepton-Photon Interactions at the Tata Institute of Fundamental Research in Mumbai, India, the bombshell was dropped: CERN scientists declared that over the entire range of energy the Collider had explored—from 145 to 466 billion electron volts—the Higgs boson is excluded as a possibility with a 95% probability.

  The search for the Higgs is a statistical hunt that involves looking at the particles that emanate from the high-energy collisions of protons inside the LHC, measuring their energies and directions of flight, as well as other parameters, and trying to assess whether it is likely that some of these particles result from the decay of a Higgs boson created by the collision. These assessments carry a probability measure, such as 95%, 99%, or—as traditionally required in particle physics for a “definitive” conclusion about the existence of a new particle: 99.99997% (this is the infamous “five-sigma” requirement).

  To be sure, the new, negative results presented in Mumbai yesterday are of a different nature. They state that, with a 95% probability, the Higgs does not exist within the range of energies the LHC has so far explored, between 145 and 466 billion electron volts. The probability of nonexistence is not overwhelming—there is still a 5% chance that the Higgs is hiding somewhere within this energy range. And, more importantly, the lower energy range from 114 to just under 145 billion electron volts, a region of energy that Fermilab has

  determined, through earlier experiments, may harbor the Higgs, has not been ruled out. But the Higgs is quickly running out of places to hide. Lower energy levels have been accessible to smaller accelerators, such as the Tevatron at Fermilab and the LEP—the LHC’s predecessor at CERN—and neither collider had found it. Perhaps the Higgs does not exist at all.

  So while CERN will continue its search for the Higgs at least until the end of this year, if no positive results about the Higgs should come out, Stephen Hawking—betting against the entire world of physics, as it were—would be able to cash in on his wager. And in that case, Congress may feel that even though its 1993 decision to cancel the American alternative to CERN—the Superconducting Super Collider—was generally met with chagrin by the American physics community, it may have been the right move one after all: to spend billions of taxpayer dollars in search of a particle that likely does not exist would have been wasteful.

  But if the Higgs doesn’t exist, where does mass in the universe come from? Theories that go beyond the “standard model” of particle physics (of which the Higgs is the keystone—the one missing piece needed to explain how the universe we know came to be) may be necessary. Steven Weinberg, who in his landmark 1967 paper on the unification of the electromagnetic and the weak interactions had made key use of the Higgs for “breaking the symmetry” and separating the electromagnetic from the weak forces, has since gone beyond the standard model in his research. Weinberg has proposed a theory called Technicolor, within which the primeval symmetry of our universe can be broken through a different mechanism than the action of the elusive Higgs. But to prove the validity of the Technicolor theory may require an energy level that would dwarf that available to the LHC—at an equally astronomical cost.

  -Originally published: Scientific American online August 23, 2011

  Waiting for the Higgs

  by Tim Folger

  Underneath a relict patch of illinois prairie, complete with a small herd of grazing buffalo, protons and antiprotons whiz along in opposite paths around a four-mile-long tunnel. And every second, hundreds of thousands of them slam together in a burst of obscure particles. It’s another day at the Tevatron, a particle accelerator embedded in the verdant grounds of the 6,800-acre Fermi National Accelerator Laboratory complex in Batavia, about 50 miles due west of Chicago. There have been many days like this one, some routine, some spectacular; of the 17 fundamental particles that physicists believe constitute all the ordinary matter and energy in the universe, three were discovered here. But there won’t be many more such days. By October 1 the power supplies for more than 1,000 liquid-heliumcooled superconducting magnets will have been turned off forever, the last feeble stream of particles absorbed by a metal target, ending the 28-year run of what was until recently the most powerful particle accelerator in the world.

  For several hundred physicists here who have spent nearly two decades searching for a hypothetical particle called the Higgs boson, the closure means ceding the hunt—and possible Nobel glory—to their archrival, the Large Hadron Collider, a newer, more powerful accelerator at CERN on the Swiss-French border. With its 17-mile circumference and higher energies, the LHC has displaced the Tevatron as the world’s premier particle physics research instrument, a position it will retain well into the next decade.

  -Originally published: Scientific American 305(4), 74-79 (October 2011)

  Waiting for the Higgs, With the Man Who Built the LHC

  by Davide Castelvecchi

  * * *

  CERN

  * * *

  They call it “the machine.”

  Thousands of physicists working at the LHC are looking for the Higgs boson and other new particles, and many of them have contributed to building the gigantic detectors that are taking most of the media limelight these days.

  But humming 100 meters under the Franco-Swiss border is the apparatus that makes it all possible. The “machine” is the collider itself: the particle accelerator that delivers swarms of protons to the detectors—funneling them through intense magnetic fields, pumping them with energy, and eventually smashing them into each other at an interaction point that is the width of a hair. Building particle accelerators is an entirely different job than building particle detectors or looking for new particles. The specialists who do it are called accelerator physicists.

  Particle physicists live in a quantum world—that of the processes that destroy particles and create new ones and that underlie the fundamental forces—and dream of discovering the new laws of nature for the 21st century.

  Accelerator physicists toil in relative obscurity, with tools such as radio-frequency waves and giant tesla coils, and mostly rely on physics that is more than a century old—classical electromagnetism, with a good dose of special theory of relativity.

  While we all wait here in Geneva for tomorrow’s update on the Higgs boson, I met with Lyn Evans, who recently retired after four decades as an accelerator physicist at CERN. During those years he took part in the inception of the LHC and, starting in 1994, he oversaw its design and construction.

  Evans picked me up at CERN’s visitors center this morning. We walked through a maze of connected hallways until we got out to his car. A quick drive took us to another building toward the outer edge of this citadel of science.

  There, we sat and chatted in his office. Like every other expert I talked to, Evans says that tomorrow’s announcement will only be a step toward the Higgs, not the final answer. “It’s obvi
ous to everybody that we don’t have enough data yet,” he says.

  But to get more data faster, the particle physicists rely on the machine—and so far, the machine has delivered. This year CERN’s accelerator physicists have been able to ramp up the intensity of the beams faster than expected, and to produce five times as many collisions, than the particle physicists were hoping to get. “I think everybody is astonished—even I, a little bit” at how the machine has performed so far.

  It was not always this way. Only three years ago, the machine lay crippled after a severe accident. It happened at Sector 34 of the LHC ring. On September 19, 2008, just over a week after the LHC first got started up, a cable connecting two of the 15-meter-long, 35-ton magnets that form the LHC melted down, producing an electrical arc. Suddenly, the liquid helium that keeps magnets at their superconducting temperature of 1.9 kelvin vaporized. Valves designed to release the resulting gas were not able to do so fast enough, and a shock wave ensued–so violent that it gravely damaged 53 magnets.

  “It was really hard to pick ourselves up from that one,” Evans says. At the time, he recalls, he was in the personnel department, and he received a call from the accelerator’s control room. He quickly went down to inspect the damage, wearing a respirator as the tunnel had filled with helium gas. Evans says it was not surprising that an electrical joint could fail. “It was the collateral damage that was unexpected.”

  The LHC cools helium to low temperatures to make the magnets superconducting, so that they can carry more current and create more powerful fields. But at 1.9 kelvin, Evans explains–the helium is colder than that at the Tevatron, the LHC’s precursor at Fermilab, near Chicago. In particular, it is below a critical temperature at which it becomes a superfluid.

  Supefluidity is an exotic state of matter that drastically lowers viscosity, and thus it enables the liquid to soak the porous material the magnet is made of, carrying any stray heat away more efficiently. (Superfluid helium also conducts heat 10,000 times better than any other materials, Evans says.)

  (As it happens, both the magnets’ superconductivity and the helium’s superfluidity are quantum effects, so it’s no longer quite true that particle accelerators are based entirely on classical physics.)

  While particle physicists gear up for big discoveries, the machine experts at CERN are already looking ahead to the upcoming upgrade. In part as a result of the Sector 34 accident, CERN has decided to do a first run at half the energy. But in 2013, the lab will completely shut down the accelerator for an entire year.

  First, the CERN team will pump the liquid helium out. Part of it will be liquefied and stored, but CERN does not have enough storage space for all of its 150 tons of it, so it will sell about half of it on the market. Then, they will circulate helium gas inside the machine to slowly bring all of its 50,000 tons up to room temperature, a process that will take weeks. “There are constraints on the rate you can do it,” Evans says: less-than-gentle temperature gradients could easily break things up.

  During the shutdown, CERN will bring the LHC up to its design specs, and then the laborious cool-down process will begin, so the accelerator can restart. Once again, it will be the machine people’s job to make all of that happen.

  -Originally published: Scientific American online December 12, 2011

  Is Supersymmetry Dead?

  by Davide Castelvecchi

  For decades now physicists have contemplated the idea of an entire shadow world of elementary particles, called supersymmetry. It would elegantly solve mysteries that the current Standard Model of particle physics leaves unexplained, such as what cosmic dark matter is. Now some are starting to wonder. The most powerful collider in history, the Large Hadron Collider (LHC), has yet to see any new phenomena that would betray an unseen level of reality. Although the search has only just begun, it has made some theorists ask what physics might be like if supersymmetry is not true after all.

  “Wherever we look, we see nothing—that is, we see no deviations from the Standard Model,” says Giacomo Polesello of Italy’s National Institute of Nuclear Physics in Pavia. Polesello is a leading member of the 3,000-strong international collaboration that built and operates ATLAS, one of two cathedral- size general-purpose detectors on the LHC ring. The other such detector, CMS, has seen nothing, either, according to an update presented at a conference in the Italian Alps in March.

  Theorists introduced supersymmetry in the 1960s to connect the two basic types of particles seen in nature, called fermions and bosons. Roughly speaking, fermions are the constituents of matter (the electron being the quintessential example), whereas bosons are the carriers of the funda mental forces (the photon in the case of electro magnetism). Supersymmetry would give every known boson a heavy “superpartner” that is a fermion and every known fermion a heavy partner that is a boson. “It is the next step up toward the ultimate view of the world, where we make everything symmetric and beautiful,” says Michael Peskin, a theorist at SLAC National Accelerator Laboratory.

  The monumental collider at CERN near Geneva should have the oomph to produce those superparticles. Currently the LHC is smashing protons with an energy of four trillion electron volts (TeV) apiece, up from 3.5 TeV last year. This energy is divided among the quarks and gluons that make up the protons, so the collision can generate new particles with the equivalent of about 1 TeV of mass. But despite the high expectations (and energies), so far nature has not cooperated. LHC physicists have been searching for signs of particles new to science and have seen none. If superparticles exist, they must be even heavier than many physicists had hoped. “To put it bluntly,” Polesello says, “the situation is that we have ruled out a number of ‘easy’ models that should have showed up right away.” His colleague Ian Hinchliff e of Lawrence Berkeley National Laboratory echoes him: “If you look at the range of masses and particles that have been excluded, it’s quite impressive.”

  Many are still hopeful. “There are still very viable ways of building supersymmetry models,” Peskin says. Expecting to see new physics after just a year of data taking was unrealistic, says Joseph Lykken, a theorist on the CMS team.

  What has theorists on edge, however, is that for super symmetry to solve the problems for which it was invented in the first place, at least a few of the superparticles should not be too heavy. To constitute dark matter, for example, they need to weigh no more than a few tenths of 1 TeV.

  Another reason most physicists want some superparticles to be light lies in the Higgs boson, another major target of the LHC. All elementary particles that have mass are supposed to get it through their interaction with this boson and, secondarily, with a halo of fleeting “virtual particles.” In most cases, the symmetries of the Standard Model ensure that these virtual particles cancel one another out, so they contribute only modestly to mass. The exception, ironically, is the Higgs itself. Calculations based on the Standard Model yield the paradoxical result that the boson’s mass should be infinite. Superpartners would solve this mystery by providing greater scope for cancellations. A Higgs mass of around 0.125 TeV, as suggested by preliminary results announced in December 2011, would be right in the range where supersymmetry predicts it should be. But in that case, the superparticles would need to have a fairly low mass.

  If that proves not to be the case, one explanation is that heretofore underappreciated symmetries of the Standard Model keep the Higgs mass finite, as Bryan Lynn of University College London suggested last year. Others say Lynn’s idea would provide at best a partial explanation, leaving a vital role for physics beyond the Standard Model—if not supersymmetry, then one of the other strategies that theorists have devised. A popular plan B is that the Higgs boson is not an elementary particle but a composite of other particles, just as protons are com posites of quarks. Unfortunately, the LHC simply does not have enough data to say much about that idea yet, says CERN’s Christophe Grojean. More exotic options, such as extra dimensions of space beyond the usual three, may forever lie beyond the LHC’s r
each. “Right now,” points out Gian Francesco Giudice, another theorist at CERN, “every single theory has its own problems.”

  As ATLAS and CMS continue to accumulate data, they will either discover superparticles or exclude wider ranges of possible masses. Although they may never be able to strictly disprove supersymmetry, if the collider fails to find it, the theory’s usefulness may fade away, and even its most hardcore supporters may lose interest. That would be a blow not just to supersymmetry but also to even more ambitious unified theories of physics that presume it, which include string theory and other approaches. LHC physicists take this uncertainty in stride and expect the collider to find some new and exciting physics—not just the physics theorists had expected. Hinchliffe says, “The most interesting thing we will see is something that nobody thought of.”

  -Originally published: Scientific American 306(5) 16-18 (May 2012)

  SECTION 4

  The Game Is Afoot

  New Particle Resembling Long-Sought Higgs Boson Uncovered at Large Hadron Collider

  By John Matson

  NEW YORK—The city that never sleeps was mostly asleep. The bars were closed. But at 4:45 A.M., inside a library on Columbia University's Manhattan campus, Michael Tuts was getting ready to pop the champagne.

  The physicist had good reason to celebrate. The massive team of scientists of which he is a part—3,000 researchers working on the ATLAS experiment at Europe's Large Hadron Collider—had just announced the discovery of a new particle. The particle looks an awful lot like the long-sought, and long-hypothetical, Higgs boson, most famous for explaining why elementary particles, such as quarks, have mass. A competing, comparably sized experiment, known as CMS, had arrived at a very similar finding at the collider facility.