Fermilab Today Friday, July 19, 2013

ORKA experiment aims to observe ultra-rare decays

Summer workers help prepare the CDF central detector for the possible future
installation of ORKA. Photo: Reidar Hahn

CDF may have stopped collecting data in late 2011, but its detector hall is currently being repurposed to possibly house an experiment designed to observe one of the rarest events in particle physics.

The proposed experiment, called ORKA, would observe the decays of charged kaons, looking for a specific decay chain into a charged pion and two neutrinos.

The "OR" in the experiment's name is a nod to the rarity of the event. It comes from oro, the Spanish word for "gold," a metal as valuable to the world as this particular charged-kaon decay is to particle physics.

In fact, when an experiment at Brookhaven first observed the decay, it became among the rarest ever captured. Brookhaven observed seven events in five years; ORKA's team is aiming for 1,000 in the same time period.

"There's a long history at this lab of studying rare kaon decays because, from those decays, we may have indications of new physics," said scientist Jonathan Lewis, a member of the ORKA team. "This is a technique that's been done many times before, but we're advancing technology so we can do better."

Inside a charged kaon is a strange quark; a charged pion has a down quark. This means that a decay from a positively charged kaon to a positively charged pion and two neutrinos is a flavor change—from strange to non-strange—but the charge remains the same, a decay known as a flavor-changing neutral current. In the Standard Model, this particular decay occurs about seven times per 100 billion events. But new physics, such as supersymmetry, could increase this rate to 20, or even 50, per 100 billion decays.

"There are a handful of processes in the Standard Model where the noise from the Standard Model is way, way down, and these flavor-changing neutral currents are quite powerful in that way," said Bob Tschirhart, Fermilab scientist and spokesperson for ORKA. "That allows new physics from supersymmetry, for example, to come up above the noise so that we can see it."

But before ORKA can begin its work, it has to be installed first—and that means disassembling CDF. The 2,500-ton CDF central detector wasrecently mounted on eight rollers and, using hydraulics, moved out of the beamline to be worked on. The plug has been removed, allowing the removal of the CDF tracking detectors from inside the solenoid and creating the area where the ORKA detector could be inserted. Once the detector is built, scientists will send a beam of protons into a platinum target about the size of a pen, producing particle showers, including charged kaons, for the team to study.

The ORKA team is currently seeking approval from the DOE for the experiment, which partly involves keeping costs as low as possible. This is where reworking CDF comes in. In order to ensure the right decay is measured, the ORKA detector needs to tag all particles produced in the charged-kaon decay. The CDF structure is well-suited to support this, making it an ideal home for ORKA. And many CDF parts that can't be used for ORKA are being sent to experiments as close by as Fermilab and as far away as Japan or are being recycled as scrap. Further, the future Illinois Accelerator Research Center, whose construction is expected to be completed in 2014, will use the CDF assembly building for lab space. Getting an early start on the CDF detector disassembly was necessary so as not to interfere with IARC operations.

"DOE has been very supportive, very encouraging," Lewis said. "That's why we're doing this."

If all goes well, ORKA could be up and running by the end of the decade, following three years of construction.

"The thing that really gets us excited about ORKA is that we know it will work, since ORKA is an evolution of a tried and true technique," Tschirhart said. "We don't know whether we'll discover new physics. That's up to nature. But we know that what we build will work."

Laura Dattaro

Fermi National Accelerator - Office of Science / U.S. Department of Energy | Managed by Fermi Research Alliance, LLC.
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