Fermilab Today

Donato Passarelli, Leonardo Ristori, Sergey Kazakov and Oleg Pronitchev, all of the Technical Division, stand next to the recently tested SSR1 cavity. Photo: Reidar Hahn

Eight cavities might sound like a nightmare to the average person. But when it comes to speeding up particles, it's an aspiration.

In July, a team of scientists and engineers finished designing, building and testing the first of a series of eight special cavities for the planned PIP-II project. All components of the cavity were designed at Fermilab and were built in U.S. industry. The team anticipates completing and testing all eight cavities, plus two cavities received by Indian collaborators, by summer 2016.

The cavities will fit into the first single-spoke resonator cryomodule, SSR1, to be tested with particle beam in the next few years. Altogether the PIP-II project would require 116 cavities of five different types to propel protons to 800 MeV, or 84 percent the speed of light.

"This milestone is exciting because it was really the last step in R&D for this type of cavity," said Leonardo Ristori, task manager for the spoke resonator section of PIP-II. "We spent all these years designing, building and testing prototypes. Now we feel comfortable that we can produce these single-spoke cavities."

Not to be confused with the painful tooth decay that comes from eating too much sugar, accelerator cavities are meticulously designed metal structures, pumped with radio-frequency power, that give particles a boost using rapidly oscillating electric fields.

SSR1 cavities are cylindrical, about the size and build of car tires. They are called "single-spoke" because individual cavities are divided by a hollow hourglass-shaped partition that resembles the spoke of a wheel. They are fashioned from pure niobium, a superconducting metal that, when kept under 9.3 Kelvin (or minus 443 degrees Fahrenheit), presents no electrical resistance when a voltage is applied.

The most recent cavity test simulated the configuration of the full SSR1 cryomodule using the same pieces that would be used in the PIP-II superconducting linac. In this integrated test, the team tested the performance of the power coupler and the frequency-tuning system, making sure they didn't interfere or degrade the performance of the cavity. The team was interested in measurements of how large of an accelerating electric field the cavity could support, called the gradient, and how efficiently it uses the power put into it, referred to as the quality factor.

Ristori said that the cavity, the coupler and the tuner all passed the tests, meeting and exceeding project requirements.

One of the main challenges in designing the cavity was desensitizing it to helium pressure variations and other sources of vibration. This was for the most part achieved by developing a state-of-the-art, self-compensating behavior.

"This is an encouraging result," Ristori said. "Everybody did an excellent job in each portion, and it all came together. It motivates the team to move forward and push the design to the limits for the other sections of the planned accelerator."

Once all eight are complete, the team will assemble the SSR1 cryomodule in Lab 2, where they are currently installing cleanrooms. It will be the first spoke cryomodule ever completed in the United States.

"It's our first fully equipped cavity, tested at full power for PIP-II," said Slava Yakovlev, head of the SRF Development Department. "We will use all the lessons we learned from this cavity in order to develop and build all the other cavities in the project and put them into operation."

—Ali Sundermier

In Brief

A top 10 paper claim

The paper "PYTHIA 6.4 Physics Manual," published in 2006 by scientists at Lund University and Fermilab, has made it into the top 10 most highly cited papers in the INSPIRE high-energy physics literature database.

It is the only paper published in the last 10 years that has made it into the latest top 10 list (excluding the PDG's Review of Particle Properties). Published by Torbjorn Sjostrand (Lund), Stephen Mrenna (Fermilab) and Peter Skands (now Monash University, Australia), it has amassed more than 6,500 citations. Congratulations to the authors.

Photo of the Day

Well, hello there

nature, bird, animal, pigeon, Wilson Hall, closeup

This pigeon recently flew into a Wilson Hall window. Fortunately, it wasn't hurt. In fact, it looks quite bright-eyed. Photo: Jesus Orduna, Brown University

Frontier Science Result: DZero

When barns collide

This plot shows the recent double-parton scattering result (σA σB)/ σAB from DZero, in blue, compared to previously published results, in red.

Disponible en español

Protons have parts that, as mentioned in previous columns, have the not terribly imaginative name of "partons." When a proton collides with another proton, a parton from one proton can collide directly with a parton from the other proton. Cases in which the collision is nearly head-on are interesting for discovering new forms of energy and matter; that is the basic reason for the Tevatron and LHC science programs.

Occasionally, two partons from the first proton can collide individually with two partons from the other proton. This double-parton scattering process does not involve the creation of new forms of energy or matter, but it can look that way; it can form a "background." So it is important to measure the rate of double-parton processes to separate them from new physics.

The natural thing to want to measure is the ratio of single-parton to double-parton collisions. But what exactly is it that one measures? What are the numbers that go into that ratio? This is where the barns come in.

If two objects that go whizzing by each other are very likely to collide, they are in some sense fat, wide objects; if they are likely to pass without colliding, they are narrow objects. We say that the wide object has a large cross section; the narrow object, a small cross section. In particle physics, the nucleus of a uranium atom is huge; to hit it with another particle is no harder than to "hit the broad side of a barn," as the saying goes. And so the unit for measuring cross sections, the barn, is an area corresponding to roughly the area that a uranium nucleus would cover on the top of your desk (assuming you could get away with having a uranium atom on top your desk, which is pretty unlikely).

The symbol for a cross section is σ, sigma. A single-parton collision that creates some set of particles, say A, has a cross section σ_A; a single-parton collision that creates set B has cross section σ_B and the double-parton collision, σ_AB. That interesting ratio, the ratio of single parton to double parton collisions, is (σ_Aσ_B)/σ_AB. The smaller this ratio is, the more double-parton collisions occur and the more background one has to new physics.

DZero has recently measured this ratio (called σ_eff) in the case where one parton collision created a pair of photons and another parton collision created a pair of jets (sprays of particles all moving in the same direction). Then, having measured this ratio for this kind of collision, we compared it with the same ratio in other processes. The new result shown in the figure agrees well with previous studies and gives us confidence that the double-parton scattering backgrounds to new production are understood, so that we can allow for their contributions when looking for new physics.

This is my last Frontier Science Result article for DZero. I'd like to thank my DZero colleagues and my Fermilab Today editor Leah Hesla for all their help in producing them.

—Leo Bellantoni

Dmitri Bandurin (U. Virginia), Georgy Golovanov and Alexander Verkheev (JINR, Dubna, Russia), Peter Svoisky (U. Oklahoma) and, not pictured, Philipp Gaspar (LAFEX, CBPF, Rio de Janeiro, Brazil) are the primary analysts for this measurement.

The DZero collaboration relies upon many of its collaborators to carefully review analyses for scientific quality before they are released. This analysis was guided by Editorial Board Chair Don Lincoln of Fermilab (left). From June 2014 to the present, Leo Bellantoni (right) has written the monthly Frontier Science Results for DZero with flair and imagination.

In the News

Hunting for dark energy

From Cosmos, Sept. 7, 2015

It seems an impossible task. How do you detect a force you know next to nothing about? But this is exactly what physicists wishing to understand dark energy — the elusive force that is expanding the Universe — are attempting to do. And by conducting an experiment that recreates the conditions of deep space in the lab, Paul Hamilton from the University of California in Los Angeles and colleagues just helped narrow the search.

University of Queensland cosmologist David Parkinson says the study, published in Science in August, is "fantastic, and really well executed". While the team didn't detect dark energy, the results help establish what dark energy is not.