Fermilab Today

[On Thursday] engineers at the Large Hadron Collider successfully collided several tightly packed bunches of particles at 13 trillion electronvolts. This is one of the last important steps on the way toward data collection, which is scheduled for early June.

As engineers ramp up the energy of the collider, the positions of the beams of particles change. The protons are also focused into much tighter packets, so getting two bunches to actually intersect requires very precise tuning.

"Colliding protons inside the LHC is equivalent to firing two needles 6 miles apart with such precision that they collide halfway," says Syracuse University physicist Sheldon Stone, a senior researcher on the LHCb experiment. "It takes a lot of testing to make sure the two bunches meet at the right spot and do not miss each other."

Engineers spent the last two years outfitting the LHC to collide protons at a higher energy and faster rate than ever before. Last month they successfully circulated low-energy protons around the LHC for the first time since the shutdown. Five days later, they broke their own energy record by ramping up the energy of a single proton beam to 6.5 trillion electronvolts.

High-energy test collisions allow engineers to practice steering beams in the LHC.

—Sarah Charley

From Argonne National Laboratory

Physicist Peter Winter wins Department of Energy Early Career Award

Argonne's Peter Winter has won a DOE Early Career Award for the Fermilab-based Muon g-2 experiment. Photo: Argonne National Laboratory

High-energy physicist Peter Winter of the U.S. Department of Energy's (DOE) Argonne National Laboratory has received a DOE Early Career Award, a prestigious five-year research grant totaling $2.5 million.

The grant will help to fund Winter's contributions to the Muon g-2 ("g minus 2") experiment currently being assembled at Fermi National Accelerator Laboratory in Batavia, Illinois. Winter's work centers on performing benchmarking tests and calibrations of the nuclear magnetic resonance probes that will be used to precisely measure the magnetic field near the 150-feet-around storage ring magnet. As part of this effort, Winter reused a decommissioned MRI magnet from a hospital in California.

"It is a great honor to receive this Early Career award from DOE," Winter said. "With this funding, Argonne will become a leader in the measurement of the magnetic field for the g-2 experiment."

—Jared Sagoff

Photo of the Day

Monday is Memorial Day

Julie Kurnat created this chalk drawing for Memorial Day, May 25. Photo: Julie Kurnat, TD

In the News

Extraordinary magnetic shield could reveal neutron's electric dipole moment

From Physics World, May 20, 2015

One of the "quietest" magnetic environments in the Milky Way has been unveiled at the Technical University of Munich (TUM). Built by physicists based in Germany, the US and Switzerland, the shielded chamber is claimed to be the most effective for its size, and is able to reduce magnetic fields by a factor of more than one million. It could be used to measure the charge distribution within the neutron and, ultimately, determine whether the particle has an electric dipole moment (EDM). The shield could also be used in biomedical applications such as brain scanning and treating cancer using magnetic nanoparticles.

Frontier Science Result: CMS

Two Z's or three?

A coupling is a connection in the paths of particles viewed in a diagram in which one axis represents the flow of time. A triple coupling could be one particle decaying into two or two particles colliding and forming one. In the Standard Model, W-W-photon is allowed, but Z-Z-photon is not.

Most everyday phenomena, from magnetism to the chemical processes of life, are due to exchanges of photons. The only exceptions are gravity and radioactive decay (if you consider that an "everyday phenomenon"). Despite this apparent universality, the photon is only the first in a family of intermediate particles: Z bosons are like photons except that they have mass, and W⁺ and W⁻ bosons are massive and also charged.

These four bosons, Z, W⁺, W⁻ and the photon, can also interact with each other. Since a W boson is charged, it can emit a photon and change its trajectory. Viewed in a space-time diagram (above) this is known as a coupling between the initial W, the final W and the photon. Similarly, W, W and Z can interact, but couplings between three or more neutral bosons, such as Z-photon-photon, Z-Z-photon and Z-Z-Z, are not allowed in the Standard Model and have never been observed. This does not mean they're impossible, though.

New phenomena beyond the Standard Model could make these so-called anomalous couplings possible. A recent study by CMS scientists searched for evidence of such couplings by looking for two Z bosons in the same event. Why not three? Take a look at the space-time diagram above: In a three-particle coupling, at least one was from the initial state.

However, other Standard Model processes can make pairs of Z bosons, so finding two Z's is not enough. To determine whether the Z pair is due to an anomalous coupling or one of the allowed Standard Model processes, the scientists studied the momentum distribution of one of the Z's and the relative rate of ZZ production in 7- and 8-TeV collisions. The effects of anomalous couplings would grow with collision energy.

This search revealed exactly as many pairs of Z bosons as the Standard Model predicts (within uncertainties) and no anomalous couplings. In fact, this analysis combined with a previous CMS result places the most stringent limits on anomalous couplings to date, sharpening our view of what might lie beyond the Standard Model.

—Jim Pivarski

The physicists pictured above made significant contributions to this analysis.

Pictured above are U.S. CMS postdocs and students who work at Fermilab and CERN on hardware development of tracking trigger R&D advanced telecommunications architecture test stands for the high-luminosity LHC.

In the News

Billion-dollar particle collider gets thumbs up

From Nature, May 19, 2015

A machine that would allow scientists to peer deeper than ever before into the atomic nucleus is a big step closer to being built. A high-level panel of nuclear physicists is expected to endorse the proposed Electron-Ion Collider (EIC) in a report scheduled for publication by October. It is unclear how long construction would take.

The panel is the Nuclear Science Advisory Committee, or NSAC, which produces regular ten-year plans for the US Department of Energy (DOE) and the National Science Foundation. Its latest plan is still being finalized, but NSAC's long-range planning group "strongly recommended" construction of the EIC at a meeting last month, says NSAC member Abhay Deshpande, a nuclear physicist at Stony Brook University in New York. The EIC will almost certainly be formally endorsed in the NSAC report, he says. It must then be approved by the DOE, but most projects backed by the expert panel have come to fruition, he says.

The collider would allow unprecedented insights into how protons and neutrons are built up from quarks and the particles that act between them, known as gluons.