IN DEP TH
PARTICLE PHYSICS
Excitement, anxiety greet LHC restart Some worry the massive accelerator could produce nothing besides the Higgs boson By Adrian Cho
PHOTO: MAXIMILIEN BRICE/© 2014 CERN
L
ater this month, scientists at the European particle physics lab, CERN, near Geneva, Switzerland, will reawaken their slumbering giant, the Large Hadron Collider (LHC), after 2 years of repairs. They are hoping that the past is not prologue. In July 2012, physicists at CERN scored the field’s crowning achievement by discovering the Higgs boson, the particle key to explaining how other fundamental particles get their mass and the last missing piece in a 40-year-old theory called the standard model. But in its 3-year first run, the LHC also produced nothing that would point to a deeper theory. For decades, in fact, atom smashers haven’t produced anything the standard model can’t explain. So some physicists worry that the LHC won’t find anything besides the Higgs—for many, the nightmare scenario. “The likelihood for this to happen is, honestly, not very small, because we haven’t seen anything new so far,” says Maurizio Pierini, a physicist at CERN who works on CMS, one of four massive particle detectors fed by the LHC. But Michael Peskin, a theorist at SLAC National Accelerator Laboratory in Menlo Park, California, says it’s too early to fret. The LHC should have three runs over the next 15 years, he notes. “I think most people in the community were disappointed that we did not see new particles,” he says. “Before we worry about this
too much, let’s see what the next run of the LHC has to say.” The LHC was designed to smash countercirculating beams of protons at energies of 14 trillion electron volts (TeV), enough to produce particles dozens of times as massive as the Higgs or the top quark, the heaviest known particles. So far it has run at 8 TeV or less and has produced less than 1% of the expected data. That energy limit was imposed after the LHC broke down catastrophically in September 2008, just 9 days after first smashing protons, when a soldered electrical connection between two of its 1695 superconducting magnets melted. The LHC didn’t take data until March 2010. Technicians have now reworked the electrical connections—10,170 of them—so that they can safely carry the 13,000 amps of current required when the LHC runs all out. Researchers also aim to crank up the rate at which the LHC smashes protons, from 360 million to 700 million collisions per second. In that torrent, physicists hope to find something beyond the standard model, which they say must be incomplete. For example, it doesn’t account for the force of gravity. It also doesn’t explain why nature uses the so-called Higgs mechanism to give other particles their mass. Fundamental particles gain mass by interacting with a field of Higgs bosons lurking “virtually” in the vacuum. But the standard model assumes the field exists without explaining its origins.
SCIENCE sciencemag.org
Physicists’ list of things to look for first resembles the one they had in 2010, but the emphases have evolved. Most prominently, the quest to find the Higgs boson has grown into a program to study it. “Obviously, the number one thing that we are going to do is to measure the Higgs’s properties as well as we can,” says Marcela Carena, a theorist at the University of Chicago and Fermi National Accelerator Laboratory in Batavia, Illinois. “Any deviations [from the standard model predictions] will point to new physics.” For example, the standard model predicts the probabilities with which the Higgs decays into combinations of other known particles, such as a massive bottom quark and its antimatter partner. Having studied just a few thousand Higgses, physicists have only begun to measure those probabilities. Researchers will also search for multiple Higgs bosons, which many theories that attempt to expand the standard model predict. The prime candidates for new particles remain those predicted by a concept called supersymmetry, which posits that every standard model particle has a heavier partner with a different amount of spin. Supersymmetry would solve a conceptual puzzle. If new, more-massive particles exist, then as they lurk in the vacuum they should interact with the Higgs boson to cause its mass to skyrocket. That doesn’t happen, so the new particles must come with just the right 13 MARCH 2015 • VOL 347 ISSUE 6227
Published by AAAS
1183
Downloaded from www.sciencemag.org on March 13, 2015
Technicians have redone 10,170 splices in the superconducting wire in the LHC’s magnets.
NEWS | I N D E P T H
1184
ATMOSPHERIC SCIENCE
California fogs are thinning Warming linked to urbanization prevents low clouds from forming, Los Angeles area study shows By Cally Carswell
F
or years, sun worshippers have flocked to southern California’s coast, only to be disappointed each spring when the weather turns drab and foggy, a phenomenon locals call “May Gray” and “June Gloom.” But the low-hanging marine clouds responsible for that gloom have declined dramatically over the past 60 years, a new study concludes. It fingers the growth of cities and their heat-retaining concrete as a prime cause. The research, recently published online in Geophysical Research Letters, offers insight into how increasing urbanization may erode coastal fog banks in the future, with potentially serious consequences for people and ecosystems. It’s “the first definitive look at how fog might change for a specific coastal region,” says Travis O’Brien, a research scientist at Lawrence Berkeley National Laboratory in California, who was not involved in the research. To understand how California’s coastal fog might be changing, some 6 years ago lead author Park Williams, now a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, began assembling data from 24 southern California airports, which keep hourly records of fog conditions. He hoped to identify key climate variables influencing fog, such as sea surface temperatures and wind patterns. He revisited the data for years, but was unable to
spot any notable trends. Last year, however, Rachel Schwartz, a Ph.D. candidate at the Scripps Institution of Oceanography in San Diego, California, sent Williams a study she was finishing. It showed that low clouds had declined slightly over the past 60 years from coastal California to Alaska. But fog had nearly disappeared at the Los Angeles and Long Beach airports over a similar period, earlier studies by another researcher concluded. Those contrasting results gave Williams an idea: Perhaps the low clouds had not disappeared, exactly, but gained height. He realized that he had lumped together two potentially distinct data sets on stratus clouds, which form between the cool sea surface and a warm, stable inversion. The clouds can form close to the ground, as fog, which tends to burn off by afternoon; they can also sit higher in the sky and linger all day. Reanalyzing his data, Williams saw—at last—some intriguing trends. The lowest tier of clouds, it turned out, was heavily influenced by land use—and changing more dramatically than the higher clouds. At some airports, fog was becoming a threatened species; at one strip in Ontario, California, the average height of the summer clouds lifted by 170 meters between 1950 and 2014. That amounted to an 87% decline in the lowest clouds; summer fog that once regularly cloaked nearby mountains in moisture now visits them rarely. Overall, the Los Angeles region showed a 64% reduction in
Foggy days are an increasingly rare sight in Los Angeles.
PHOTO: © SUPERSTOCK/CORBIS
combination of properties to keep the Higgs light. In supersymmetry that would happen “naturally,” as the effects of partnered particles on the Higgs mass would cancel because of the difference in their spins. Unfortunately for physicists, those superpartners didn’t show up in the LHC’s first run. So now researchers are focusing mainly on finding the “stop squark,” the superpartner of the top quark, the standard model particle that most influences the Higgs’s mass. Even those searches are looking a bit iffy, as the failure to see the stop so far shows that it must weigh more than 0.7 TeV, a value that strains the theory. Still, Sanjay Padhi of the University of California, San Diego, says he’s optimistic. “I will start to worry if the stop is not found up to 1.5 TeV,” says Padhi, who works on CMS, one of the detectors that discovered the Higgs. Interest is also rising in trying to blast out particles of dark matter—the mysterious stuff whose gravity binds the galaxies. Cosmological measurements show that it makes up 85% of all matter in the universe. “We know there is something beyond the Higgs,” says Stephanie Majewski, a physicist at the University of Oregon in Eugene who works on the ATLAS particle detector, which also discovered the Higgs. “We know that there is dark matter, and we ought to be able to produce it with the LHC.” The dark matter particles themselves would not be directly detectable. To deduce their presence, physicists will look for lopsided “mono-X” events: proton-proton collisions that send a standard model particle or a jet of them flying in one direction and nothing detectable going the opposite way—an absence that would reveal a hidden particle. But John Ellis, a theorist at King’s College London, cautions that dark matter particles could have different, more complicated signatures. If nothing new shows up in the next few years, people may drift away from the LHC, Pierini says. But Ellis says physicists have a good decade before they should worry. CERN plans to upgrade the LHC’s particle detectors in 2018 and the accelerator itself around 2022, to boost its intensity. A bigger concern, Pierini says, is that experimenters might inadvertently overlook a new effect. To keep the data rate manageable, physicists rely on computerized “triggers” to sift out the few encounters between countercirculating bunches of protons that contain interesting proton-proton collisions. The triggers toss data from all but a few of every 100,000 bunch crossings. “It’s really terrifying to think that there is something there but that we didn’t discover it because we just didn’t look for it,” Pierini says. “That’s what keeps me up at night.” ■
sciencemag.org SCIENCE
13 MARCH 2015 • VOL 347 ISSUE 6227
Published by AAAS
PARTICLE PHYSICS
Excitement, anxiety greet LHC restart Some worry the massive accelerator could produce nothing besides the Higgs boson By Adrian Cho
PHOTO: MAXIMILIEN BRICE/© 2014 CERN
L
ater this month, scientists at the European particle physics lab, CERN, near Geneva, Switzerland, will reawaken their slumbering giant, the Large Hadron Collider (LHC), after 2 years of repairs. They are hoping that the past is not prologue. In July 2012, physicists at CERN scored the field’s crowning achievement by discovering the Higgs boson, the particle key to explaining how other fundamental particles get their mass and the last missing piece in a 40-year-old theory called the standard model. But in its 3-year first run, the LHC also produced nothing that would point to a deeper theory. For decades, in fact, atom smashers haven’t produced anything the standard model can’t explain. So some physicists worry that the LHC won’t find anything besides the Higgs—for many, the nightmare scenario. “The likelihood for this to happen is, honestly, not very small, because we haven’t seen anything new so far,” says Maurizio Pierini, a physicist at CERN who works on CMS, one of four massive particle detectors fed by the LHC. But Michael Peskin, a theorist at SLAC National Accelerator Laboratory in Menlo Park, California, says it’s too early to fret. The LHC should have three runs over the next 15 years, he notes. “I think most people in the community were disappointed that we did not see new particles,” he says. “Before we worry about this
too much, let’s see what the next run of the LHC has to say.” The LHC was designed to smash countercirculating beams of protons at energies of 14 trillion electron volts (TeV), enough to produce particles dozens of times as massive as the Higgs or the top quark, the heaviest known particles. So far it has run at 8 TeV or less and has produced less than 1% of the expected data. That energy limit was imposed after the LHC broke down catastrophically in September 2008, just 9 days after first smashing protons, when a soldered electrical connection between two of its 1695 superconducting magnets melted. The LHC didn’t take data until March 2010. Technicians have now reworked the electrical connections—10,170 of them—so that they can safely carry the 13,000 amps of current required when the LHC runs all out. Researchers also aim to crank up the rate at which the LHC smashes protons, from 360 million to 700 million collisions per second. In that torrent, physicists hope to find something beyond the standard model, which they say must be incomplete. For example, it doesn’t account for the force of gravity. It also doesn’t explain why nature uses the so-called Higgs mechanism to give other particles their mass. Fundamental particles gain mass by interacting with a field of Higgs bosons lurking “virtually” in the vacuum. But the standard model assumes the field exists without explaining its origins.
SCIENCE sciencemag.org
Physicists’ list of things to look for first resembles the one they had in 2010, but the emphases have evolved. Most prominently, the quest to find the Higgs boson has grown into a program to study it. “Obviously, the number one thing that we are going to do is to measure the Higgs’s properties as well as we can,” says Marcela Carena, a theorist at the University of Chicago and Fermi National Accelerator Laboratory in Batavia, Illinois. “Any deviations [from the standard model predictions] will point to new physics.” For example, the standard model predicts the probabilities with which the Higgs decays into combinations of other known particles, such as a massive bottom quark and its antimatter partner. Having studied just a few thousand Higgses, physicists have only begun to measure those probabilities. Researchers will also search for multiple Higgs bosons, which many theories that attempt to expand the standard model predict. The prime candidates for new particles remain those predicted by a concept called supersymmetry, which posits that every standard model particle has a heavier partner with a different amount of spin. Supersymmetry would solve a conceptual puzzle. If new, more-massive particles exist, then as they lurk in the vacuum they should interact with the Higgs boson to cause its mass to skyrocket. That doesn’t happen, so the new particles must come with just the right 13 MARCH 2015 • VOL 347 ISSUE 6227
Published by AAAS
1183
Downloaded from www.sciencemag.org on March 13, 2015
Technicians have redone 10,170 splices in the superconducting wire in the LHC’s magnets.
NEWS | I N D E P T H
1184
ATMOSPHERIC SCIENCE
California fogs are thinning Warming linked to urbanization prevents low clouds from forming, Los Angeles area study shows By Cally Carswell
F
or years, sun worshippers have flocked to southern California’s coast, only to be disappointed each spring when the weather turns drab and foggy, a phenomenon locals call “May Gray” and “June Gloom.” But the low-hanging marine clouds responsible for that gloom have declined dramatically over the past 60 years, a new study concludes. It fingers the growth of cities and their heat-retaining concrete as a prime cause. The research, recently published online in Geophysical Research Letters, offers insight into how increasing urbanization may erode coastal fog banks in the future, with potentially serious consequences for people and ecosystems. It’s “the first definitive look at how fog might change for a specific coastal region,” says Travis O’Brien, a research scientist at Lawrence Berkeley National Laboratory in California, who was not involved in the research. To understand how California’s coastal fog might be changing, some 6 years ago lead author Park Williams, now a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, began assembling data from 24 southern California airports, which keep hourly records of fog conditions. He hoped to identify key climate variables influencing fog, such as sea surface temperatures and wind patterns. He revisited the data for years, but was unable to
spot any notable trends. Last year, however, Rachel Schwartz, a Ph.D. candidate at the Scripps Institution of Oceanography in San Diego, California, sent Williams a study she was finishing. It showed that low clouds had declined slightly over the past 60 years from coastal California to Alaska. But fog had nearly disappeared at the Los Angeles and Long Beach airports over a similar period, earlier studies by another researcher concluded. Those contrasting results gave Williams an idea: Perhaps the low clouds had not disappeared, exactly, but gained height. He realized that he had lumped together two potentially distinct data sets on stratus clouds, which form between the cool sea surface and a warm, stable inversion. The clouds can form close to the ground, as fog, which tends to burn off by afternoon; they can also sit higher in the sky and linger all day. Reanalyzing his data, Williams saw—at last—some intriguing trends. The lowest tier of clouds, it turned out, was heavily influenced by land use—and changing more dramatically than the higher clouds. At some airports, fog was becoming a threatened species; at one strip in Ontario, California, the average height of the summer clouds lifted by 170 meters between 1950 and 2014. That amounted to an 87% decline in the lowest clouds; summer fog that once regularly cloaked nearby mountains in moisture now visits them rarely. Overall, the Los Angeles region showed a 64% reduction in
Foggy days are an increasingly rare sight in Los Angeles.
PHOTO: © SUPERSTOCK/CORBIS
combination of properties to keep the Higgs light. In supersymmetry that would happen “naturally,” as the effects of partnered particles on the Higgs mass would cancel because of the difference in their spins. Unfortunately for physicists, those superpartners didn’t show up in the LHC’s first run. So now researchers are focusing mainly on finding the “stop squark,” the superpartner of the top quark, the standard model particle that most influences the Higgs’s mass. Even those searches are looking a bit iffy, as the failure to see the stop so far shows that it must weigh more than 0.7 TeV, a value that strains the theory. Still, Sanjay Padhi of the University of California, San Diego, says he’s optimistic. “I will start to worry if the stop is not found up to 1.5 TeV,” says Padhi, who works on CMS, one of the detectors that discovered the Higgs. Interest is also rising in trying to blast out particles of dark matter—the mysterious stuff whose gravity binds the galaxies. Cosmological measurements show that it makes up 85% of all matter in the universe. “We know there is something beyond the Higgs,” says Stephanie Majewski, a physicist at the University of Oregon in Eugene who works on the ATLAS particle detector, which also discovered the Higgs. “We know that there is dark matter, and we ought to be able to produce it with the LHC.” The dark matter particles themselves would not be directly detectable. To deduce their presence, physicists will look for lopsided “mono-X” events: proton-proton collisions that send a standard model particle or a jet of them flying in one direction and nothing detectable going the opposite way—an absence that would reveal a hidden particle. But John Ellis, a theorist at King’s College London, cautions that dark matter particles could have different, more complicated signatures. If nothing new shows up in the next few years, people may drift away from the LHC, Pierini says. But Ellis says physicists have a good decade before they should worry. CERN plans to upgrade the LHC’s particle detectors in 2018 and the accelerator itself around 2022, to boost its intensity. A bigger concern, Pierini says, is that experimenters might inadvertently overlook a new effect. To keep the data rate manageable, physicists rely on computerized “triggers” to sift out the few encounters between countercirculating bunches of protons that contain interesting proton-proton collisions. The triggers toss data from all but a few of every 100,000 bunch crossings. “It’s really terrifying to think that there is something there but that we didn’t discover it because we just didn’t look for it,” Pierini says. “That’s what keeps me up at night.” ■
sciencemag.org SCIENCE
13 MARCH 2015 • VOL 347 ISSUE 6227
Published by AAAS
