BU physicists sift through the shrapnel of proton collisions made by the biggest machine on Earth, searching for new physics
Bump Hunters
BU physicists sift through the shrapnel of proton collisions made by the biggest machine on Earth, searching for new physics
By Elizabeth Dougherty
Tulika Bose stands guard over the printer. She’s carried her laptop down the hall and submitted her print job on arrival to be certain that she will intercept her papers before anyone else has a chance to see them. Her documents contain secrets.
Bose is a physicist working at the
Large Hadron Collider (LHC) in Switzerland. Her work has nothing to do with weapons or national defense or space exploration or any of the usual top-secret stuff physicists work on. What her files contain are ideas about what we—everything under the sun and beyond it—are made of. Her documents could contain the secrets of the universe.
When Protons Collide: A proton collision is like a car accident—except when it isn’t. Physicist Kevin Black explains why. (Watch out for the kitchen sink!)
Video by Joe Chan
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Standard Model, have been found through experiments like those done at the LHC.
A theoretical physicist since the 1960s and a Nobel Laureate, Glashow came to BU in 2000 because of the physics department’s emphasis on experiments like those at the LHC.
Photo by Gina Manning
BU also happens to have on its faculty
Sheldon Glashow, a BU College of Arts & Sciences physics professor, who won the Nobel Prize in Physics in 1979 for his work developing the foundations of the Standard Model. Theorists like Glashow and
Kenneth Lane, also a BU professor of physics, and experimental physicists like Bose have become masters of the Standard Model and are spending their careers trying to figure out one thing: Is there more to the universe than what we know right now? The answer, almost surely, is yes. When physicists look for new particles, they are really looking for “bumps” in the data produced by their experiments. A bump indicates that something appeared with energy or mass that was different than expected. “We call it ‘bump hunting,’” says
Steve Ahlen, BU professor of physics, who represents yet another flavor of physicist. A hardware physicist, Ahlen has built muon detector chambers with his own hands and led their construction in Boston. Those built in Boston were auspiciously placed at the LHC and captured a good portion of the particles that allowed researchers to confirm the existence of the Higgs boson in 2012. The hunt is on, at a time like no other in physics. The quest reached a pinnacle with the Higgs boson, but finding it wasn’t the end. It was just the beginning. “Before the Higgs was discovered, people were absolutely convinced that it was there. The challenge was finding it,” says Glashow. “The trouble now is that we theorists don’t know what else is out there. We are no longer so confident that we know what to look for. But we hope that something interesting will show up.”
On Colliders and Detectors
The Large Hadron Collider is currently the world’s highest energy particle collider. It is also the largest machine ever built. The circular tunnel around which proton beams fly is 17 miles wide and buried 574 feet underground in a rural area on the border of France and Switzerland at the European Organization for Nuclear Research (CERN). The collider’s job is to smash particles together at high speeds and record the spray of shrapnel that results, so that physicists can look for signs of new particles. If they find something that looks interesting, they piece the data back together again, retracing the paths of all the particles in the spray back to the collision that caused them.
Proton beams fly around a racetrack 17 miles wide. It’s buried 574 feet underground in a rural area on the border of France and Switzerland.
Photo courtesy of CERN
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Kevin Black, an associate professor of physics at BU, and also Bose’s husband. Bose and her students work on an experiment called CMS at LHC. Black works on a different experiment there called ATLAS. The CMS and ATLAS experiments are, at their core, two different pieces of hardware that detect particles. They sit at opposite sides of the beam ring surrounding two separate beam intersections, where they capture all of the shrapnel from the proton-proton collisions that occur at their centers.
A view into the center of the ATLAS detector gives a sense of its scale. The eight tubes contain superconducting magnets that bend the shrapnel of collisions so that the energy of each particle can be measured.
Photo courtesy of CERN
ATLAS is approximately 46 meters (150 feet) long, and 25 meters (82 feet) in diameter. It is a nesting doll of sorts, containing layers of magnets, particle trackers, and calorimeters that measure the energy of the particles. Muon detectors wrap and cap the machine. Ahlen’s team tested ATLAS’s muon chambers by using them to detect muons produced when cosmic rays strike the atmosphere.
Photo courtesy of CERN
Construction of ATLAS began in 2004 and ended in 2008. Assembling ATLAS required the precise movement of giant components into exacting locations.
This cross-section of ATLAS shows its layers. The location of every one of the millions of electronic detector channels packed into these layers is known to within a fraction of a width of a human hair.
Photo courtesy of CERN
Workers install the proton beam pipe at the center of CMS in preparation for its very first experiments in 2009.
Photo courtesy of CERN
A schematic of the CMS detector. Particle detectors and trackers envelop the central pipeline where proton beams collide.
Photo courtesy of CERN
CMS data for a proton collision in 2011. Particles fly in all directions at varying levels of energy. This collision is consistent with the Standard Model and also showed signs of the Higgs boson.
Photo courtesy of CERN
The CMS detector is smaller and more dense than ATLAS, but still quite large at about 21.6 meters (69 feet) long and 15 meters (50 feet) in diameter.
Photo courtesy of CERN
The ATLAS detector, which Ahlen helped build, is a 75-foot-high, 140-foot-long machine in which five layers of detection hardware measure the momentum and mass of particles produced when protons smash together. CMS, for Compact Muon Solenoid, is considerably smaller and more dense. It captures the same types of particles as ATLAS, just in a different way.
A hands-on physicist, Ahlen likes to build things. He is currently building dark energy detectors and climbing mountains to install them on telescopes.
Photo by Gina Manning
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“I think of it as if a murder has happened, and you have all these clues,”
says Bose. “The detective comes in and uses all these clues to reconstruct the scene and figure out who committed the crime.”
Photo by Darrin Vanselow
The time-stamped data flows from each detector down an electrical pipe to join with others in a raging river of data. Carefully coded computer algorithms determine which events to keep and which to throw away. Bose, who is the
“trigger” in charge of this gatekeeping for CMS, saves only about a thousand events per second. A lot, but still just a tiny fraction of the data produced.
Secret Keepers
CMS and ATLAS produce and save independent sets of data and have independent teams analyzing it. Bose, for example, is one of nearly 4,000 people working on the CMS experiment, while Black is part of a team of 3,000 people working on ATLAS. The teams sift through their data in secret, without sharing early results. At a time when data sharing in science is all the rage, secrecy seems to go against the grain, but it is a necessity in physics. In the past, there have been cases where rumors about the early sightings of new particles lead to false discoveries. In one case, physicists started looking for signs of a new particle people were buzzing about. “Any run that had a little bump in the rumored location, they kept,” says Black. “Anything that didn’t, they found a reason to throw out the data. Inadvertently, they self-selected the particle into existence.” In the end, an unbiased look at the data proved that the particle did not exist.
For Black (PhD ’05), who is currently stationed at the LHC in Switzerland, patience is a virtue. The phenomena he studies occur so rarely that even with millions of collisions per second, he might not see them.
Photo by Darrin Vanselow
年代
How small is small?
Find out
27360 meters
17 miles
The Large Hadron Collider In the largest machine on earth, proton beams travel through a tunnel forming a ring nearly 9 kilometers (5.5 miles) wide—27 kilometers (17 miles) in circumference—buried 100 meters below ground. An average person would take nearly an hour to jog the diameter of the ring.
44 meters
144 feet
The ATLAS Particle Detector One of the largest and most complex particle detectors on Earth, the ATLAS detector is 46 meters long—about the same distance as a 50-yard dash.
0.1 nanometer
0.0000000039 inches
Atoms were once thought to be indivisible, but physicists now know they are mostly empty space. Their mass is concentrated in the nucleus, which is made of protons and neutrons. If an atom were expanded to the size of the ATLAS detector, its nucleus would be about a centimeter wide and a single proton about half a millimeter.
0.8768 femtometers
0.0000000000000345 inches
Protons are part of a set of particles called hadrons. Though miniscule, hadrons are packed with even smaller particles, such as quarks, antiquarks and gluons, that whiz around inside the proton at speeds near the speed of light. Smashing hadrons together releases these high-energy particles in a spectacular explosion.
1 Attometers
0.0000000000000000393 inches
Quarks are elementary particles: they are not divisible into something smaller. Physicists consider quarks and other elementary particles to be “point” particles, meaning they have a size of zero except for during interactions with other particles. Quarks come in several flavors: up, down, top, bottom, charm, and strange.
From Old Physics, New
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“Quantum mechanics ate the physics of the 18th and 19th centuries alive”
Kenneth Lane
Photo by Gina Manning
The most recent piece of experimental data confirming the Standard Model was the discovery of the Higgs boson in 2012. For Lane, the Higgs was a bit of a disappointment. “It’s kind of a simple-minded solution to a big problem,” he says. “Some people still feel burned by this discovery. Me, for example.” But ultimately, Lane is interested in figuring out what the most basic, fundamental particles are and how they interact with one another. “Right now, we have a story for that, but there’s always been a story for that,” he says. As long as people have been curious about the world they live in, they’ve been coming up with theories, testing them, and making them better. “This,” says Lane, “is an enterprise that need have no end.”
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There are 2 comments on Bump Hunters
Also worth mentioning that electronics to read out large portions of both the ATLAS and CMS detectors was built right here on the Charles River campus by the Electronics Design Facility, which develops custom instrumentation for BU researchers in all scientific disciplines.
Would a large aray of coils around the explosion chamber measure magnesium /energy of particles? Stray magnesium could be unknown particles. Just a stray thought I had. Maybe that’s how you measure already
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Boston University moderates comments to facilitate an informed, substantive, civil conversation. Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected. Moderators are staffed during regular business hours (EST) and can only accept comments written in English. Statistics or facts must include a citation or a link to the citation.