Early Tevatron design days

When I started at the lab in December 1977, work on the dipole magnets for the Tevatron was well under way in what was then called the Energy Doubler Department in the Technical Services Section.

My first project was to work on the quadrupole magnets and spools, which hadn’t really been started yet. The spool is a special unit that attaches to each quadrupole and the adjacent dipole. It contains what we used to call “the stuff that wouldn’t fit anywhere else” – correction magnets and their power leads, quench stoppers to dump the energy from all the magnets, beam position monitors, relief valves, things like that.

At the time, we were located in the Village in the old director’s complex, which now houses the daycare center. We had a large open area where the engineers, designers and drafters worked and a small conference room where we kept up-to-date models of some of the things we were working on.

For several weeks we worked feverishly on the design of the quadrupole and spool combination — we in the design room and the model makers in the model shop on their full-scale models. We would work all week, then have a meeting with the lab director, Bob Wilson. Dr. Wilson would come out to see how we were doing, but more importantly to see what our designs looked like.

It turns out he was very interested in that and very fussy that things — even those buried in the tunnel — looked just so.

After every one of those meetings we’d walk back into the design room and tell everyone to tear up what we’d been working on and start over. The same would hold for the model makers. This went on for several weeks until Dr. Wilson was happy. We began to really dread going into those meetings, but in the end they served us very well.

Tom Nicol is a mechanical engineer at Fermilab.

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Neutrinos are the most abundant elementary particles in the universe that weigh something. They have no electric charge, and they pass through almost everything since they interact only weakly with matter. There are three types of neutrinos, and they change from one type into another other as they travel, a behavior called oscillation. Physicists have been engaged for years in measuring how much and how often neutrinos oscillate between their various types.

Our understanding of the process crucially depends on the measurement of the neutrino energy. We cannot measure the energy of the neutrinos themselves; we can measure only the energies of the particles that are produced by the interaction of a neutrino with the detector, typically made of material of heavy nuclei, which enables lots of interactions and so opportunities for observing the neutrino’s behavior. The nuclear environment is a complicated one, so making measurements is a difficult task.

We need a detailed understanding of the neutrino’s interaction inside the nucleus to precisely measure its energy. In the energy range of many oscillation experiments, the dominant interaction process is one in which a neutrino completely scatters off a neutron inside the nucleus and produces a muon and proton. The produced proton can interact further with other particles inside the nucleus. Thus in the presence of other protons and neutrons, the distributions of the final-state particles may differ from the primary interaction.

One way to see how the nucleus changes the picture is to look at the angle between two planes that define the interaction: If you think of the plane that includes the neutrino direction and the proton direction, and also the plane that includes the neutrino direction and the muon direction, those two planes should be “back to back,” or different by 180 degrees. MINERvA can measure the angle between those two planes precisely. It turns out that these events are not all at 180 degrees apart — not even close.

If you are a teacher, you have to deal with this kind of situation every day. Suppose you supervise a large group of kids, and you know the individual behavior of each kid. However, when they are in a large group, each kid’s behavior is affected by others, so you adjust your prediction. The behavior of a kid in a group will be different from her behavior when she is by herself.

Think of the proton and neutrons as the kids, which are inside a bound nucleus. Your estimation of how much energy the kids have will be different once you take into account the fact that they are losing lots of energy interacting with each other, compared to when they were just sitting in separate rooms by themselves.

MINERvA recently made a new measurement of this process by including all the events with a muon plus at least one proton and no lighter particles. The probability of a neutrino interaction is measured as a function of momentum transferred to the nucleus (called “Q2”), which is calculated using a measurement of the proton’s energy.

MINERvA carried out the measurement simultaneously on carbon, iron and lead. A previous MINERvA study measured this process on plastic (hydrocarbon) alone.

If all the kids in the nucleus behaved like individuals, then the ratio of cross sections would be close to a constant that’s related to ratio of the fraction of neutrons in each target nucleus. But these ratios are not at a constant value, as you can see in the plots.

This measurement also shows that the dependence on the target element (lead or iron) is not well-described in the current nuclear models and that the theorists have some homework to do before the predictions for experiments measuring oscillations will be precise. This is the first time a direct measurement of this has ever been made. This result was submitted for publication last week.

Anushree Ghosh is a postdoctoral researcher at Federico Santa María Technical University.

Invisible, imperceptible and yet far more common than ordinary matter, dark matter makes up an astounding 85 percent of the universe’s mass. Physicists are slowly but steadily tracking down the nature of this unidentified substance. The latest result from the PICO experiment places some of the best limits yet on the properties of certain types of dark matter.

PICO searches for WIMPs (weakly interacting massive particles), a hypothesized type of dark matter particle that would interact only rarely, which makes them difficult to find.

In this extreme cosmic game of “Where’s Waldo?” the newest, most technologically complex detectors are usually considered the most promising. Many of these dark matter experiments rely on hundreds if not thousands of electrical channels and require racks of computer servers just to store the data they collect.

But PICO relies on a simple phenomenon and a fairly low-key detector: bubbles, and a bubble chamber. At its core, PICO’s apparatus is simply a glass jar filled with fluid in which bubbles can form and be monitored by a video camera.

Reinventing the bubble

PICO had its beginnings in 2005 as a collaboration between the University of Chicago and the U.S. Department of Energy’s Fermilab. (The experiment started under a different name, COUPP, and later merged with the PICASSO experiment to form PICO.) In the experiment’s early days, much of Fermilab scientists’ work was devoted simply to developing bubble chamber technology. Because while the bubble chamber was hardly new — it was invented in 1952 — the technology had also been out of use for 20 years.

Bubble chambers are designed to convert the energy deposited by a subatomic particle into a bubble that can be observed. In a liquid such as room temperature water, particle collisions do nothing noticeable. To achieve sensitivity to particles, the fluid inside bubble chambers is heated to just above its boiling point, so the slightest disruption could tip the fluid to a boiling state, creating a bubble.

“You can actually watch the chamber and see the bubble form,” said Fermilab physicist Hugh Lippincott, a collaborator on PICO. In typical particle physics experiments, information about particle interactions is given solely through computer interfaces. In PICO, the interactions are visible to the naked eye as bubbles.

“It’s great to press your face up against the glass and just … pop!” said Fermilab physicist Andrew Sonnenschein, also a collaborator on PICO.

If WIMPs exist, they should occasionally interact with fluid in PICO’s bubble chamber, creating a certain number of bubbles every year.

It was a return to old-school, low-tech particle physics when Fermilab collaborators began engineering the PICO bubble chamber, which is installed 2 kilometers underground at the Canadian laboratory SNOLAB. Bubble chambers of decades past had been used to track millions of charged particles such as protons and electrons, which would leave long, winding tracks in the fluid.

“Old bubble chambers had a great run, but it ended in the ’80s,” Sonnenschein said. “They were too slow to keep up with experiments that had much larger data rates.”

As a result, bubble chambers were phased out when modern particle colliders such as Fermilab’s Tevatron and CERN’s Large Hadron Collider took over. Using complex electronics, detectors at these colliders were able to collect millions of times more data than bubble chambers.

In fact, bubble chambers had been out of commission for so long that PICO’s founders had to go back to the drawing board, return to some of the papers of the original bubble chamber pioneers, and effectively reinvent the technology for detecting dark matter.

“After the early bubble chamber designers figured out how to make them work to track high-energy particles with trails of bubbles, the basic ingredients of the recipe didn’t change. We’re looking for low-energy particles that make only single bubbles, so many things are different,” Sonnenschein said.

The new design to allow bubble chambers to detect dark matter still preserves many of the elements from older bubble chamber detectors.

“The thing that makes PICO interesting is that we’re using a relatively simple detector design compared to the other dark matter experiments,” said Dan Baxter, a Northwestern University graduate student and Fermilab fellow who was PICO’s latest run coordinator.

Unlike traditional charged-particle-detecting bubble chambers, PICO’s bubble chamber is designed to look for elusive, neutrally charged WIMPs that might take years to make an appearance.

“It’s using it in a different way,” Lippincott said. “In the old days, you would never expect to use a bubble chamber by just letting it sit there without anything happening.”

A WIMPy bubble

The weak force that governs WIMPs lives up to its name. For comparison, it’s about 10,000 times weaker than the electromagnetic force. Particles that interact through the weak force, such as WIMPs and neutrinos, don’t interact often, making them hard to capture. But even a slow-moving WIMP can deposit enough energy to be visible in a detector.

By carefully calibrating heat and pressure in PICO’s bubble chamber fluid, scientists were able to make the detector sensitive only to the interactions from massive particles like WIMPs. PICO researchers were able to avoid much of the standard background, such as signals from electrons and gamma rays, that plague other dark matter detectors.

Mastering the technology to do this took years. Predecessors to PICO started off as little more than test tubes filled with a few teaspoons of liquid. Gradually, the vessels grew larger. Then researchers added sound monitoring to their detectors to capture the “pops” from bubbles created by WIMPs.

“We see a sound chirp,” Sonnenschein said, referring to the bubbles popping. “It turns out that if you look at the frequency content of the sound chirp and the amplitude, you can tell the difference between different kinds of particle interactions.”

If a WIMP created a bubble, PICO would be able to not only see evidence of dark matter, but hear it as well. Using this acoustic technology, researchers were able to effectively veto bubbles that could not have been created by WIMPs, allowing them to eliminate background.

As it turns out, PICO did not see any bubbles from WIMPs, so they were able to place limits on both WIMP masses and the likelihood that they will interact with matter — two factors that influence the number of bubbles WIMPs produce.

Placing limits on these factors — mass and interaction rate — can tell physicists where they should look next for dark matter.

Where no bubble has gone before

“We don’t know what dark matter is, and so there’s a lot of theories about what it could be and about how it could interact with normal matter,” Baxter said.

The variety of theories calls for a variety of different experiments. Other experiments search for different sources of dark matter, such as particles called axions or sterile neutrinos. PICO’s search for WIMPs has a specific focus on so-called spin-dependent WIMPs.

“We don’t know what the WIMPs are,” Lippincott said. “But broadly speaking their interactions with normal matter would fall into two categories: one that isn’t sensitive to the spin of the nucleus, and one that is.”

Spin, like charge, is an intrinsic quantity carried by particles and atomic nuclei. PICO looks primarily for WIMP interactions that are sensitive to the spin of the nucleus. To boost their resolution of these interactions, the researchers use a fluid with a liquid containing fluorine, which has a relatively large nuclear spin. With this method, PICO increased their ability to see spin-sensitive WIMPs by a factor of 17.

Essentially, PICO’s result is that these spin-sensitive WIMPs, if they exist, must interact extremely infrequently — otherwise PICO would have seen more bubbles.

This result, which is by far the best yet for spin-sensitive WIMPs interacting with protons, does not rule out the existence of WIMPs. There are many other places left to still look for dark matter, but thanks to PICO, fewer places for it to hide.

The PICO collaboration currently has a proposal in to the Canada Foundation for Innovation to build the next generation of PICO chamber, and physicists like Lippincott and Sonnenschein remain optimistic because of the technology’s potential to scale up.

“They’re pretty cheap once the engineering is done, mainly because they’re very simple mechanically. The fiddly bits are not very fiddly,” Lippincott said. “There’s a good chance that bubble chambers will continue to play a role in the hunt for dark matter.”

PICO comprises about 50 physicists at 20 institutions in the Canada, Europe, India, Mexico and the United States and receives support from the U.S. Department of Energy Office of Science and National Science Foundation.

The U.S. Department of Energy and CERN establish contributions for next-generation experiments and scientific infrastructure located both at CERN and in the United States

The United States Department of Energy (DOE) and the European Organization for Nuclear Research (CERN) last week signed three new agreements securing a symbiotic partnership for scientific projects based both in the United States and Europe. These new agreements, which follow from protocols signed by both agencies in 2015, outline the contributions CERN will make to the neutrino program hosted by Fermilab in the United States and the U.S. Department of Energy’s contributions to the High-Luminosity Large Hadron Collider upgrade program at CERN.

Researchers, engineers and technicians at CERN are currently designing detector technology for the U.S. neutrino research program hosted by Fermilab. Neutrinos are nearly massless, neutral particles that interact so rarely with other matter that trillions of them pass through our bodies each second without leaving a trace. These tiny particles could be key to a deeper understanding of our universe, but their unique properties make them very difficult to study. Using intense particle beams and sophisticated detectors, Fermilab currently operates three neutrino experiments (NOvA, MicroBooNE and MINERvA) and has three more in development, including the Deep Underground Neutrino Experiment (DUNE) and two short-baseline experiments on the Fermilab site, one of which will make use of the Italian ICARUS detector, currently being prepared for transport from CERN.

The Long Baseline Neutrino Facility will provide the infrastructure needed to support DUNE both on the Fermilab site in Illinois and at the Sanford Underground Research Facility in South Dakota. Together, LBNF/DUNE represent the first international megascience project to be built at a DOE national laboratory.

The first agreement, signed last week, describes CERN’s provision of the first cryostat to house the massive DUNE detectors in South Dakota, which represent a major investment by CERN to the U.S.-hosted neutrino program. This critical piece of technology ensures that the particle detectors can operate below a temperature of minus 300 degrees Fahrenheit, allowing them to record the traces of neutrinos as they pass through.

The agreement also formalizes CERN’s support for construction and testing of prototype DUNE detectors. Researchers at CERN are currently working in partnership with Fermilab and other DUNE collaborating institutions to build prototypes for the huge subterranean detectors which will eventually sit a mile underground at the Sanford Underground Research Facility in South Dakota. These detectors will capture and measure neutrinos generated by Fermilab’s neutrino beam located 800 miles away. The prototypes developed at CERN will test and refine new methods for measuring neutrinos, and engineers will later integrate this new technology into the final detector designs for DUNE.

The agreement also lays out the framework and objectives for CERN’s participation in Fermilab’s Short Baseline Neutrino Program, which is assembling a suite of three detectors to search for a hypothesized new type of neutrino. CERN has been refurbishing the ICARUS detector that originally searched for neutrinos at INFN’s Gran Sasso Laboratory in Italy and will ship it to Fermilab later this spring.

More than 1,700 scientists and engineers from DOE national laboratories and U.S. universities work on the Large Hadron Collider (LHC) experiments hosted at CERN. The LHC is the world’s most powerful particle collider, used to discover the Higgs boson in 2012 and now opening new realms of scientific discovery with higher-energy and higher-intensity beams. U.S. scientists, students, engineers and technicians contributed critical accelerator and detectors components for the original construction of the LHC and subsequent upgrades, and U.S. researchers continue to play essential roles in the international community that maintains, operates and analyzes data from the LHC experiments.

The second agreement concerns the next phase of the LHC program, which includes an upgrade of the accelerator to increase the luminosity, a measurement of particle collisions per second. Scientists and engineers at U.S. national laboratories and universities are partnering with CERN to design powerful focusing magnets that employ state-of-the-art superconducting technology. The final magnets will be constructed by both American and European industries and then installed inside the LHC tunnel. The higher collision rate enabled by these magnets will help generate the huge amount of data scientists need in order to search and discover new particles and study extremely rare processes.

American experts funded by DOE will also contribute to detector upgrades that will enable the ATLAS and CMS experiments to withstand the deluge of particles emanating from the LHC’s high-luminosity collisions. This work is detailed in the third agreement. These upgrades will make the detectors more robust and provide a high-resolution and three-dimensional picture of what is happening when rare particles metamorphose and decay. Fermilab will be a hub of upgrade activity for both the LHC accelerator and the CMS experiment upgrades, serving as the host DOE laboratory for the High-Luminosity LHC Accelerator Upgrade and the CMS Detector Upgrade projects.

When I first started out here, in 1970, I worked with the farm crew. They had us going around cleaning out a lot of the barns after the farmers who were here on the land left. Hindsight being 20/20, there was a lot of really neat stuff that we threw away: We weren’t trying to preserve history or anything like that. We just wanted the barns cleaned out. For one, we needed a place to store the hay that we were growing for the buffalo.

That was some of the hardest work I ever did. We had a barn that we had stacked to the rafters with hay. But some of the baled hay hadn’t dried enough, and it spontaneously combusted. We came in the next day, and all that was left was the barn’s foundation.

I was also one of the people who cut the grass. It was more difficult than I had realized. The lab was just starting, and the equipment they had was just awful. The grass was very tall, and we had these push mowers. You’d go two feet and they’d clog up. It was similar with raking: They gave us garden rakes instead of a grass rakes. We finally got a riding mower that summer, and we were supposed to take turns using it. There were three guys that were messing around with it — fighting over it — and at some point they all bailed while it was running, and the mower ran into a tree. It was wrecked.

That was the first time I’d seen people get fired from the lab. They asked only a few of us to come back that summer. I was asked to come back, and I guess the rest is kind of history.