Scientists study tiny particles called neutrinos to learn about how our universe evolved. These particles, well-known for being tough to detect, could tell the story of how matter won out over antimatter a fraction of a second after the Big Bang and, consequently, why we’re here at all.
Getting to the bottom of that split-second history means uncovering the differences, if any, between the neutrino and its antimatter counterpart, the antineutrino.
The MINERvA neutrino experiment at Fermilab recently added some detail to the behavior profiles of neutrinos and antineutrinos: Scientists measured the likelihood that these famously fleeting particles would stop in the MINERvA detector. In particular, they looked at cases in which an antineutrino interacting in the detector produced another particle, a neutron — that familiar particle that, along with the proton, makes up an atom’s nucleus.
MINERvA’s studies of such cases benefit other neutrino experiments, which can use the results to refine their own measurements of similar interactions.
It’s typical to study the particles produced by the interaction of a neutrino (or antineutrino) to get a bead on the neutrino’s behavior. Neutrinos are effortless escape artists, and their Houdini-like nature makes it difficult to measure their energies directly. They sail unimpeded through everything — even lead. Scientists are tipped off to the rare neutrino interaction by the production of other, more easily detected particles. They measure and sum the energies of these exiting particles and thus indirectly measure the energy of the neutrino that kicked everything off.
This particular MINERvA study — antineutrino enters, neutron leaves — is a difficult case. Most postinteraction particles deposit their energies in the particle detector, leaving tracks that scientists can trace back to the original antineutrino (or neutrino, as the case may be).
But in this experiment, the neutron doesn’t. It holds on to its energy, leaving almost none in the detector. The result is a practically untraceable, unaccounted energy that can’t easily be entered in the energy books. And unfortunately, antineutrinos are good at producing energy-absconding neutrons.
Researchers make the best of missing-energy situations. They predict, based on other studies, how much energy is lost and correct for it.
To give the scientific community a data-based, predictive tool for missing-energy moments, MINERvA collected data from the worst-case situation: An antineutrino strikes a nucleus in the detector and knocks out the untraceable neutron so nearly all of the energy bestowed to the nucleus goes “poof.” (These interactions also produce positively charged particles called muons which signal the antineutrino interaction.) By studying this particular disappearing act, scientists could directly measure the effects of the missing energy.
Scientists at Fermilab use the MINERvA to make measurements of neutrino interactions that can support the work of other neutrino experiments. Photo: Reidar Hahn
Other researchers can now look for these effects, applying the lessons learned to similar cases. For example, researchers on Fermilab’s largest operating neutrino experiment, NOvA, and the Japanese T2K experiment will use this technique in their antineutrino measurements. And the Fermilab-hosted international Deep Underground Neutrino Experiment, centerpiece of a world-leading neutrino program, will also benefit from this once it begins collecting data in the 2020s.
The neutron production case is just one type of missing-energy interaction, one of many. So the model that comes out of this MINERvA study is an admittedly imperfect one. There can’t be a one-size-fits-all-missing-energy-scenarios model. But it still provides a useful tool for piecing together a neutrino’s energy — and that’s a tough task no matter what particles come out of the interaction.
“This analysis is a great testament to both the detector’s ability to measure neutrino interactions and to the collaboration’s ability to develop new strategies,” said Fermilab scientist and MINERvA co-spokesperson Deborah Harris. “When we started MINERvA, this analysis was not even a gleam in anyone’s eye.”
There’s a bonus to this recent study, too, one that bolsters an investigation conducted last year.
For the earlier investigation, MINERvA focused on neutrino (instead of antineutrino) interactions that knocked out proton-neutron pairs (instead of lone neutrons or protons). In a detector such as MINERvA, a proton’s energy is much easier to measure than a neutron’s, so the earlier study presumably yielded more precise measurements than the recent antineutrino study.
How good were these measurements? MINERvA scientists plugged the values of the earlier neutrino study into a model of this recent antineutrino study to see what would pop out. Lo and behold, the adjustment to the antineutrino model improved its ability to predict the data.
The combination of the two studies gives the neutrino physics community new information about how well models do and where they fall short. Searches for the phenomenon known as CP violation — the thing that makes matter special compared to antimatter and enabled it to conquer in the post-Big Bang battle — depend on comparing neutrino and antineutrino samples and looking for small differences. Large, unknown differences between neutrino and antineutrino reaction products would hide the presence or absence of CP signatures.
“We are no longer worried about large differences, and our neutrino program can work with small adjustments to known differences,” said University of Minnesota–Duluth physicist Rik Gran, lead author on this result.
MINERvA is homing in on models that, with each new test, better describe both neutrino and antineutrino data — and thus the story of how the universe came to be.
These results appeared on June 1, 2018, in Physical Review Letters.
The following CERN press release celebrates the groundbreaking of a major upgrade to the Large Hadron Collider, called the High-Luminosity LHC. The HL-LHC will produce five to seven times more collisions than the current LHC, greatly increasing the chances of new discoveries.
Fermilab is leading the U.S. contribution to the HL-LHC, in addition to building new components for the upgraded detector for the CMS experiment. The main innovation contributed by the United States for the HL-LHC is a novel new type of accelerator cavity that uses a breakthrough superconducting technology. Fermilab is also contributing to the design and construction of superconducting magnets that will focus the particle beam much more tightly than the magnets currently in use in the LHC. Fermilab scientists and engineers have also partnered with other CMS collaborators on new designs for tracking modules in the CMS detector, enabling it to respond more quickly to the increased number of collisions in the HL-LHC.
Major work starts to boost the luminosity of the LHC
The Large Hadron Collider (LHC) is officially entering a new stage. Today, a groundbreaking ceremony at CERN celebrates the start of the civil-engineering work for the High-Luminosity LHC (HL-LHC), a new milestone in CERN’s history. By 2026 this major upgrade will have considerably improved the performance of the LHC by increasing the number of collisions in the large experiments and thus boosting the probability of the discovery of new physics phenomena.
CERN members work on crab cavities for the HL-LHC. Photo: CERN
The LHC started colliding particles in 2010. Inside the 27-kilometer LHC ring, bunches of protons travel at almost the speed of light and collide at four interaction points. These collisions generate new particles, which are measured by detectors surrounding the interaction points. By analyzing these collisions, physicists from all over the world are deepening our understanding of the laws of nature.
While the LHC is able to produce up to 1 billion proton-proton collisions per second, the HL-LHC will increase this number, referred to by physicists as luminosity, by a factor of between five and seven, allowing about 10 times more data to be accumulated between 2026 and 2036. This means that physicists will be able to investigate rare phenomena and make more accurate measurements. For example, the LHC allowed physicists to unearth the Higgs boson in 2012, thereby making great progress in understanding how particles acquire their mass. The HL-LHC upgrade will allow the Higgs boson’s properties to be defined more accurately and to measure with increased precision how it is produced, how it decays and how it interacts with other particles. In addition, scenarios beyond the Standard Model will be investigated, including supersymmetry, theories about extra dimensions and quark substructure (compositeness).
“The High-Luminosity LHC will extend the LHC’s reach beyond its initial mission, bringing new opportunities for discovery, measuring the properties of particles such as the Higgs boson with greater precision, and exploring the fundamental constituents of the universe ever more profoundly,” said CERN Director-General Fabiola Gianotti.
The HL-LHC project started as an international endeavor involving 29 institutes from 13 countries. It began in November 2011, and two years later was identified as one of the main priorities of the European Strategy for Particle Physics, before the project was formally approved by the CERN Council in June 2016. After successful prototyping, many new hardware elements will be constructed and installed in the years to come. Overall, more than 1.2 kilometers of the current machine will need to be replaced with many new high-technology components such as magnets, collimators and radio-frequency cavities.
“Audacity underpins the history of CERN, and the High-Luminosity LHC writes a new chapter, building a bridge to the future,” said CERN Director for Accelerators and Technology Frédérick Bordry. “It will allow new research, and with its new innovative technologies, it is also a window to the accelerators of the future and to new applications for society.”
To allow all these improvements to be carried out, major civil-engineering work at two main sites is needed, in Switzerland and in France. This includes the construction of new buildings, shafts, caverns and underground galleries. Tunnels and underground halls will house new cryogenic equipment, the electrical power supply systems, and various plants for electricity, cooling and ventilation.
During the civil-engineering work, the LHC will continue to operate, with two long technical stop periods that will allow preparations and installations to be made for high luminosity alongside yearly regular maintenance activities. After completion of this major upgrade, the LHC is expected to produce data in high-luminosity mode from 2026 onward. By pushing the frontiers of accelerator and detector technology, it will also pave the way for future higher-energy accelerators.
A groundbreaking ceremony will be held tomorrow to celebrate the start of civil engineering work for a major upgrade to the Large Hadron Collider at CERN in Geneva, Switzerland. When complete, the High-Luminosity LHC (HL-LHC) will produce five to seven times more proton-proton collisions than the currently operating LHC, powering new discoveries about our universe.
For the last decade, scientists, engineers and technicians from the U.S. Department of Energy’s Fermi National Accelerator Laboratory have been working with partners around the world to conduct R&D on new accelerator components that would make operations at the HL-LHC possible. The U.S. research was conducted via the LHC Accelerator Research Program, or LARP. Now the research turns into reality, as construction of the new components begins.
The primary components contributed by the United States for the HL-LHC construction are powerful superconducting magnets and superconducting deflecting cavities, called crab cavities of a novel compact design never before used in an accelerator.
“This is a truly major milestone for the whole U.S. accelerator community,” said Fermilab scientist Giorgio Apollinari, who leads the DOE Office of Science-funded U.S. HL-LHC Accelerator Upgrade Project (AUP). “More than 10 years of research work funded by DOE under LARP have gone into developing these cutting-edge magnets and crab cavities and in demonstrating their technical feasibility for the intended application at HL-LHC. We now look forward with much anticipation to shipping the first components to CERN and seeing them operate as part of the world’s foremost particle collider.”
Fermilab is developing magnets such as this one, which is mounted on a test stand at Fermilab, for the High-Luminosity LHC. Photo: Reidar Hahn
In the LHC, superconducting quadrupole magnets focus the beams into collision at four points around the 27-kilometer ring. In the HL-LHC, these focusing magnets must be more powerful to focus the stream of particles much tighter than in the LHC. Fermilab, in collaboration with DOE’s Brookhaven and Lawrence Berkeley national laboratories, developed the basic technology for these new magnets through LARP. The final design was completed in collaboration with CERN for application in the HL-LHC upgrade.
These new magnets are made of a niobium-tin alloy that allows the magnets to reach the desired high magnetic field of 12 tesla. This powerful field is created by running a very high electric current through coils of superconducting wire, which conduct electricity without resistance when cooled to almost absolute zero. Fermilab is the lead U.S. laboratory for this project and is fabricating half of the coils and conducting the final assembly and testing of 11 full cryoassembly magnet structures before shipping them to CERN. The U.S. in total is delivering half of the quadrupole magnets for the upgrade, while CERN is completing the other half.
“These are the next generation of superconducting magnets for accelerators,” said Fermilab’s Ruben Carcagno, the deputy project manager for the HL-LHC AUP. “This is the first time that this new technology will be deployed in a working machine. So it’s a big step.”
Fermilab is developing and constructing cavities like this one for the future HL-LHC. The cavity proper is the structure situated between the four rods. Photo: Leonardo Ristori
In addition to the magnets, the United States will deliver half of the crab cavities to CERN for the HL-LHC, while CERN completes the remaining cavities. The cavities to be produced in the United States are of a radio-frequency dipole (RFD) design and are the product of more than 10 years of research through LARP by Old Dominion University and SLAC National Accelerator Laboratory, with contributions from Thomas Jefferson National Accelerator Facility and U.S. industry. Fermilab will be responsible for fabricating and testing the RFD cavities before delivering them to CERN. These novel cavities will kick or tilt the beams just before they pass through each other to maximize the beam overlap and therefore the possibility of proton collisions.
Once it’s up and running, the HL-LHC will produce up to 15 million Higgs bosons per year, compared to the 4 million produced during the LHC’s 2015-2017 run. The higher luminosity will mean big changes for the LHC experiments as well, and the ATLAS and CMS detectors are undergoing major upgrades of their own. Learn more about Fermilab’s contributions to the HL-LHC upgrades to the CMS detector.