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 was recently 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.