Three new physics experiments could revamp the standard model
Physicists build giant machines to study tiny particles
Diana Parno’s head swam when she first stepped inside the enormous, metallic vessel of the experiment KATRIN. Within the house-sized, oblong structure, everything was symmetrical, clean and blindingly shiny, says Parno, a physicist at Carnegie Mellon University in Pittsburgh. “It was incredibly disorienting.”
Now, electrons — thankfully immune to bouts of dizziness — traverse the inside of this zeppelin-shaped monstrosity located in Karlsruhe, Germany. Building the experiment took years and tens of millions of dollars. Why create such an extreme apparatus? It’s all part of a bid to measure the mass of itty-bitty subatomic particles known as neutrinos.
KATRIN, which is short for Karlsruhe Tritium Neutrino Experiment, started test runs in May. The experiment is part of a multipronged approach to the study of particle physics, one of dozens of detectors built in an assortment of odd-looking shapes and sizes. Their mission: dive deep into the standard model, particle physicists’ theory of the subatomic building blocks of matter — and maybe overthrow it.
Developed in the 1960s and ’70s, the standard model has some sizable holes: It can’t explain dark matter — an ethereal substance so far detected only by its gravitational effects — or dark energy, a mysterious oomph that causes the cosmos to expand at an increasing rate. The theory also can’t explain why the universe is made mostly of matter, while antimatter is rare (SN: 9/2/17, p. 15). So physicists are on a quest to revamp particle physics by probing the standard model’s weak points.
Major facilities like the Large Hadron Collider — the gargantuan accelerator located at CERN near Geneva — haven’t yet found where the standard model goes wrong (SN: 10/1/16, p. 12). Instead, particle physics experiments have confirmed standard model predictions again and again. “In some sense we are victims of our own success,” says Juan Rojo, a theoretical physicist at Vrije Universiteit Amsterdam. “We don’t have hints about what is the next step.”
New experiments like KATRIN might be able to ferret out answers. Also joining the ranks are Muon g-2 (pronounced “gee minus two”) at Fermilab in Batavia, Ill., and Belle II in Tsukuba, Japan. A behind-the-scenes look at these experiments reveals the sweat, joy and sacrifice that goes into each of these difficult enterprises. These efforts involve hundreds of researchers, sport price tags in the tens of millions of dollars and require major technological undertakings: intricate electronics, powerful magnets and ultraclean conditions. Researchers have built complex apparatuses with their own hands, lugged tons of equipment across continents and cleaned the insides of detectors until they gleam.
Here’s a glimpse at three of the latest standard model challengers.
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KEK High Energy Accelerator Research Organization
Approximate cost: $50 million
How it works
Electrons and their antimatter partners, positrons, take laps around a 3-kilometer long, ring-shaped accelerator and collide at the center of the Belle II detector, producing a class of particles called B mesons. These particles contain a bottom quark, an exotic particle not found in run-of-the-mill matter. Scientists sift through the data produced when B mesons decay inside the 8-meter-tall detector to learn about the particles’ weird ways.
1. An accelerator sends electrons from one end and positrons from the other into Belle II.
2. Tracking detectors follow particles’ paths after collision, pinpointing B mesons.
3. Quartz sensors distinguish between similar types of particles.
4. A calorimeter measures energies of particles.
5. Outer layers spot particles that get past inner sections.
OK, but why?
Certain B mesons seem to prefer to decay into electrons, rather than their heavier cousins, muons (SN: 5/13/17, p. 16). That goes against the standard model, which says electrons and muons should appear in equal amounts. If this unexpected behavior holds up to scrutiny, something big must be wrong with the theory. B mesons also partake in a process called CP violation, in which antimatter and matter don’t behave like perfect mirror images.
Studying CP violation might help scientists understand why the universe is composed of matter and not antimatter. In the Big Bang, matter and antimatter were produced in equal measure and should have annihilated into nothingness, but somehow matter gained an upper hand. It’s “the most fundamental question human beings can ask … ‘Why are we here?’ ” says physics graduate student Robert Seddon.
Like an onion
Each layer of the detector has a different purpose. The innermost layers spot the tracks that particles take through the detector. Farther out, sensors tell one particle from another and measure particles’ energies. The outermost section spots muons and other particles that can travel that far. When the accelerator is running, it creates a high-radiation environment in the lab that Seddon, of McGill University in Montreal, calls “completely off-limits. You go in there, you die.”
Parsing the particles
Pristine, lab-grown quartz makes up the sensors that discern between different types of particles. Creating the sensors required gluing together bars of quartz over a meter long, precisely aligning them to within about 10 microns — close in size to a human red blood cell. Scratching or smudging the quartz damages it, so handling the bars took a soft touch. Physicists who had arrived recently from overseas were banned from the work, says physicist Saurabh Sandilya of the University of Cincinnati; there’s no room for jet lag–induced clumsiness.
Let’s get it started
On April 26, the first electrons and positrons collided in the new detector. Running the experiment was thrilling but tense. “Somebody brought me some whiskey because I was really scared,” says physicist Tom Browder of the University of Hawaii at Manoa. He worried there might be a failure of a system called the trigger, which identifies interesting collisions from the deluge of boring events that the detector sees. After several hours, when the first events began rolling in around 1 a.m., the team finally took a breath.
Like a baby, a new particle detector can interfere with its creators’ sleep. The detectors run all night; if a malfunction occurs, experts might get an after-hours phone call. Physics graduate student Laura Zani of the University of Pisa in Italy certainly did. But the newborn detector, a piece of which she helped build, also inspired pride. When Zani saw the first particle tracks appear on computer screens, she thought, “We did it.”
Karlsruhe Institute of Technology, Germany
Approximate cost: $70 million
How it works
Physicists aim to measure the mass of neutrinos, wily subatomic particles that are nearly impossible to detect. At one end of the 70-meter-long KATRIN, radioactive decays of tritium produce electrons and the antimatter twins of neutrinos. Those antineutrinos escape while the electrons cruise through KATRIN’s blimp-shaped tank and are detected at the other end (SN Online: 10/18/16). The tank, a spectrometer, divvies up the particles according to their energies. Some energy from each tritium decay goes to generating the antineutrino’s mass. That limits how much energy the electron gets. So measuring the electrons’ energies can reveal the mass of neutrinos. KATRIN should officially start taking data next spring.
1. Tritium decays, releasing electrons and antineutrinos, which escape.
2. Electrons travel along beamline to spectrometer.
3. The spectrometer sorts electrons by their energies.4. A magnetic field (dotted lines) shepherds high-energy electrons to a detector at the other end.
5. An electric field turns low-energy electrons back.
6. Magnets focus electrons onto the detector.
OK, but why?
A neutrino’s mass is a tiny fraction of an electron’s. “Why is it so light?” Parno asks. “That’s mysterious.” The standard model initially predicted that neutrinos have no mass at all. But measurements indicate that the particles must have mass, though how much is still a question. Neutrinos barely interact with matter and are incredibly numerous: Billions of neutrinos sail through your thumbnail each second. These particles are so quirky that scientists want to know more.
It all starts with tritium. This radioactive version of hydrogen, pumped through the experiment in a gaseous form, emits 100 billion antineutrino and electron pairs each second. In the tritium lab, special rules are in place because of the radioactivity — scientists enter via an air lock and must wash their hands when they leave. The place has a spaceship vibe, says Larisa Thorne, a physics graduate student at Carnegie Mellon University. “I did feel quite like I was on Star Trek.”
End of the line
At the opposite end from the tritium lab, powerful magnets focus high-energy electrons on a detector, which counts the electrons that arrive. Credit cards must be stashed in a locker or they’ll be wiped by the magnetic field.
The big bake
The entire spectrometer is kept under ultrahigh vacuum, eliminating molecules of air or other substances that could interfere with the electrons’ journeys. It’s the largest ultrahigh vacuum vessel ever created. To get that extreme vacuum, the researchers temporarily heat the whole shebang to more than 200° Celsius, baking off water and other contaminants on the vessel’s surface. Metal expands when heated, so the spectrometer bulges by about 12 centimeters during the process. “It’s pretty strange to think that this large tank, which is filled with nothing … actually expands,” Thorne says.
COMING HOME KATRIN’s spectrometer was built off-site and had to be carefully transported to the lab in Germany, just squeezing between nearby houses.
Fermilab, Batavia, Ill.
Approximate cost: $46 million
How it works
Muons, heavier relatives of electrons, behave like tiny magnets with a north and south pole. Muon g-2, which started up in February, studies the properties of those minimagnets. Researchers beam thousands of muons into a doughnut-shaped electromagnet about as wide as the width of a basketball court. As muons circulate inside the electromagnet, their poles pivot like wobbling tops. Muons are unstable, so as they circulate, they decay into lighter particles known as positrons. The angles at which those positrons fly off can reveal the rate of the muons’ magnetic gyrations and, therefore, the strength of the muons’ magnets. The researchers will compare the measurement to predictions based on the standard model.
2. Muons circle in the same direction repeatedly.
3. Muons decay into positrons, which are picked up by detectors that measure energy and particle tracks.
OK, but why?
Transient particles blip in and out of existence everywhere in space. Those particles tweak the rate at which the muons gyrate. If undetected particles are out there, Muon g-2’s measurement might not square with predictions. A similar experiment performed at Brookhaven National Laboratory in Upton, N.Y., in the 1990s hinted at a mismatch (SN: 2/17/01, p. 102). Muon g-2 will make a more precise measurement to follow up on that lead.
Muon g-2’s magnetic field is about 30,000 times as strong as Earth’s magnetic field. Such strength is useful only if the magnetic field is ultrauniform. So physicists strategically placed thousands of tiny metal shims — many just a fraction of the thickness of notebook paper — to adjust the magnetic field. Hours of “shimming” left physicists’ hands “covered in dirt and oil and grease,” says physics graduate student Rachel Osofsky of the University of Washington in Seattle. The dirty job was worth it: The magnetic field is now uniform to within 0.0015 percent.
The electromagnet, a hand-me-down from the Brookhaven Lab, had to be shipped from Upton to Fermilab in Illinois. But how to transport an enormous, fragile doughnut?
In 2013, the magnet took a boat trip down the East Coast and cruised up the Mississippi and other rivers to Lemont, Ill. A truck carried the cargo the rest of the way, going about 8 kilometers per hour on closed-off highways in the middle of the night. The magnet barely squeaked through the tight passages of electronic tolling arches. No word on whether the magnet had to pay.
Building a particle detector takes lots of painstaking work, much of it done by graduate students. For Muon g-2, building the tracking detectors, which observe the trajectories of the emitted positrons, required threading wires 25 microns thick through 100-micron-wide holes. Imagine trying to stick a piece of spaghetti through a straw, both small enough for a Lego figurine to use. “That was like a year of our lives just getting wires down tiny holes,” says Saskia Charity, a physics graduate student at the University of Liverpool in England.
This article appears in the September 29, 2018, issue of Science News with the headline, “Massive machines: An inside look at three big efforts to study tiny particles.”