NYTimes topic : A Giant Takes on Physics’ Biggest Questions part 2

 

The culprit is quantum weirdness, one principle of which is that anything that is not forbidden will happen. That means the Higgs calculation must include the effects of its interactions with all other known particles, including so-called virtual particles that can wink in and out of existence, which shift its mass off the scale.

As a result, if the Standard Model is valid for all energies, said Joe Lykken, a Fermilab theorist, “then you are in deep doodoo trying to explain why the Higgs mass isn’t a quadrillion times bigger than it needs to be.”

Another way to put it is to ask why gravity is so much weaker than the other forces — the theory wants them all to be equal.

Theorists can rig their calculations to have the numbers come out right, but it feels like cheating. “What we have to do to equations is crazy,” Dr. Arkani-Hamed said.

One solution that has been proposed is a new principle of nature called supersymmetry that, if true, would be a bonanza for the Cern collider.

It posits a relation between the particles of matter like electrons and quarks and particles that transmit forces like photons and the W boson. For each particle in one category, there is an as-yet-undiscovered superpartner in the other category.

“Supersymmetry doubles the world,” Dr. Arkani-Hamed said.

These superpartners cancel out all the quantum effects that make the Higgs mass skyrocket. “Supersymmetry is the only known way to manage this,” Dr. Lykken said.

Because Higgs bosons are expected to be produced very rarely, it could take at least a year or more for physicists to confirm their discovery at the collider. But some supersymmetric particles, if they exist, should be produced abundantly and could thus pop out of the data much sooner. “Suppose a gluino exists at 300 billion electron volts,” Dr. Arkani-Hamed said, referring to a putative superpartner. “We could know the first day if they exist.”

For several years, supersymmetry has been a sort of best bet to be the next step beyond the Standard Model, which is undefeated in experiments but has enormous gaps. The Standard Model does not include gravity or explain why, for example, the universe is matter instead of antimatter or even why particles have the masses they do.

In the end, Michelangelo Mangano, a theorist at Cern, said, “The standard model prediction can’t be the end of the story.”

Supersymmetry also fixes a glitch in the age-old dream of explaining all the forces of nature as manifestations of one primordial force. It predicts that at a high enough energy, all the forces — electromagnetic, strong and weak — have identical strengths.

“If supersymmetry is right, unification works,” Dr. Ellis said.

But there is no direct evidence for any of the thousands of versions of supersymmetry that have been proposed. Indeed, many theorists are troubled that its effects have not already shown up in precision measurements at accelerators.

“It doesn’t smell good,” Dr. Arkani-Hamed said. Physicists say the best indirect evidence for supersymmetry comes from the skies, where the galaxies have been found to be swaddled by clouds of invisible dark matter, presumably unknown particles left over from the Big Bang. “Dark matter is a very physical argument.” Dr. Ellis said. “If you take astrophysics seriously, there has to be some unseen stuff out there.”

On the menu of discoveries, there is always None of the Above. As Dr. Gianotti put it: “Nature has chosen another solution. This will be great.”

There are indeed other potential solutions that go by the name of Technicolor or the Little Higgs. But what if the collider sees nothing?

That, Dr. Ellis said, would be interesting for the theorists, who would have to retool and try to think even deeper thoughts about quantum mechanics and relativity, but bad for the experimentalists. Without any results, they would be unlikely to obtain financing for the next big machine planned, the $7 billion International Linear Collider.

A worse nightmare, several theorists said, would be seeing just the Higgs, but nothing else. That would leave them where they are, stuck in the Standard Model, with no answer to their embarrassing fine-tuning problem, no dark matter and no clue to a better theory.

To add to the confusion, according to the Standard Model, the Higgs can have only a limited range of masses without severe damage to the universe. If it is too light, the universe will decay. If it is too heavy, the universe would have blown up already. According to Dr. Ellis, there is a magic value between 160 billion and 180 billion electron volts that would ensure a stable universe and require no new physics at all.

But that would leave theorists with nothing more to do and a world in which basic questions would remain forever unanswered.

Dr. Ellis said, “ I can’t believe God would push the button on a theory like that.”

But, he conceded, “For the I.L.C., a boring Higgs is better than nothing.”

Sunken Cathedrals

There was more than birds singing and trees blooming outside the main Cern cafeteria in March to suggest that springtime for physics was approaching.

Some 300 feet beneath the warming grass, the magnets that are the guts of the collider, thick as tree trunks, long as boxcars, weighing in at 35 tons apiece, were strung together like an endless train stretching away into the dim lamplight and around a gentle curve.

A technician on his way to a far sector of the collider ring bicycled past.

“When you fold in the technology combined with the scale,” said Peter Limon, a Fermilab physicist on duty here, “I don’t think anything on Earth or in space that we know about beats it.”

Running through the core of this train, surrounded by magnets and cold, were two vacuum pipes, one for protons going clockwise, the other counterclockwise. Traveling in tight bunches along the twin beams, the protons will cross each other at four points around the ring, 30 million times a second. During each of these violent crossings, physicists expect that about 20 protons, or the parts thereof — quarks or gluons — will actually collide and spit fire. It is in vast caverns at those intersection points that the knee-padded and hardhatted physicists are assembling their detector, or “sunken cathedrals” in the words of a Cern theorist, Alvaro de Rujula, to capture the holy fire.Two of the detectors are specialized. One, called Alice and led by Jurgen Schukraft of Cern, is designed to study a sort of primordial fluid, called a quark-gluon plasma, that is created when the collider smashes together lead nuclei.

The other, LHCb, is led by Tatsuya Nakada of Cern and the Swiss Federal Institute of Technology in Lausanne. It is designed to hunt for subtle differences in matter and antimatter that could help explain how the universe, which was presumably born with equal amounts of both, came to be dominated by matter.

The other two, the aforementioned Atlas and Compact Muon Solenoid, or C.M.S. for short, are the designated rival workhorses of the collider, designed expressly to capture and measure every last spray of particle and spark of energy from the proton collisions.

The rivals represent complementary strategies for hunting the Higgs particle, which is expected to disintegrate into a spray of lesser particles. Exactly which particles depends on how massive the Higgs really is.

One telltale signature of the Higgs and other subatomic cataclysms is a negatively charged particle known as a muon, a sort of heavy electron that comes flying out at nearly the speed of light. Physicists measure muon momentum by seeing how much their paths bend in a magnetic field.

It is the need to have magnets strong enough and large enough to produce measurable bending, physicists say, that determines the gigantic size of the detectors.

The Compact Muon Solenoid, built by Dr. Virdee’s group, weighs 12,000 tons, the heaviest instrument ever made. It takes its name from a massive superconducting electromagnet that produces a powerful field running along the path of the protons.

Conversely, the magnetic field on Atlas wraps like tape around the proton beam. The Atlas collaboration has been led from its start by Peter Jenni of Cern. At150 feet long and 80 feet high, Atlas is bigger than its rival, but it is much lighter, about 7,000 pounds, about as much as the Eiffel Tower. The physicists like to joke that if you threw it in the ocean in a plastic bag it would float.

The two detectors have much in common, including “onion layers” of instruments to measure different particles and the ability to cope with harsh radiation and vast amounts of data. Dr. Virdee compared the central C.M.S. detector, made of strips of silicon that record the passage of charged particles, to a 60-megapixel digital camera taking 40 million pictures a second. “We have to time everything to the nanosecond,” he said

To manage this onslaught the teams’ computers have to perform triage, and winnow those events to a couple hundred per second. That is dangerous, Dr. Gianotti said, “because we are looking for something rare.” The Higgs occurs once in every trillion events, she said.

Contending Armies

The competition between Atlas and the C.M.S. is in keeping with a long tradition of having rival teams and rival detectors at big experiments to keep each other honest and to cover all the bets. As Dr. Mangano put it, “If you screw it up, others are here to crucify you.”

At the Fermilab Tevatron, the teams, several hundred strong, are called CDF and D0. In the glory years 20 years ago at Cern, they were called UA1 and UA2. Over the years, as the machines have grown, so have the groups that built them, from teams to armies, 1,800 people from 34 countries for Atlas and 2,520 from 37 countries for the C.M.S. The other two experiments — Alice with 1000 scientists, and LHCb with 663 — are only slightly smaller.

Robert Cousins of U.C.L.A. and C.M.S. joked that he was old enough so that after 25 years in the business “half my friends are on Atlas, the others on C.M.S.” Dr. Jenni said all 1,800 Atlas scientists would have their names on the first papers out of the collider, adding: “The people who work in the pit make as important a physics contribution as those who end up in front of the computers. This is a big step in energy. It’s new territory, and that’s in the end why everyone is excited.”

At the end of the day, Dr. Mangano said, unless there is a major problem both machines will perform. “It will come down to sociology,” he said. “How quickly can they analyze the date? How do you manipulate and analyze the data? The process of understanding is long.”

There could be new phenomena, he added, new particles that theorists have not thought of.

Dr. Mangano pointed out that it had been a long time since high-energy physicists had made a fundamental discovery. And back then, when Dr. Rubbia was doing his Nobel work, there were well-defined theories of what would be found. Now, everything will be new.

“There are many students who have never seen data,” Dr. Mangano said. “I don’t know how much longer we can keep going like that.”

What comes out of the Large Hadron Collider, he said, “will determine the future of the field.”

Dr. Arkani-Hamed said the tension was keeping him awake at night. “Nobody knows how this is going to go,” he said. “That’s what makes it so cool. The experiment itself is so spectacular.”

Sipping an espresso in his office, Dr. Mangano refused to consider the possibility of failure. “It’s like saying suppose you drive into a tree on the way home,” he said. “Let’s hope we get home safely and we see something.”