The Muon g-2 experiment at Fermilab is a major upgrade of the previous version of the Muon g-2 experiment, which ended data taking at Brookhaven National Laboratory in 2001. The measurement of the anomalous magnetic moment of the muon provoked world-wide interest because of the tantalizing evidence of deviation from the standard model. This experiment reuses Fermilab's accelerator system and the storage ring from the original BNL experiment. It is expected that this g-2 measurement will have a four-fold improvement in the final uncertainties compared to the last result. This includes both an improvement in collected statistics and in the systematic uncertainties of the measurement.

[Photo of Dr. Breese Quinn]
Bresse Quinn
[Photo of Dr. Jenny Holzbauer]
Jenny Holzbauer

Motivation and Physics

    The anomalous magnetic moment of the muon is particularly interesting because it is a sensitive probe for new physics at relatively high scales. It might be noted that one can also measure the electron magnetic moment, which has been done to higher precision than the muon magnetic moment. However, the electron is less sensitive to new physics because heavy virtual particles contributions scale according to the lepton mass squared. Therefore the muon anomalous moment is about 43,000 times more sensitive to these particles, which could include those from new physics, such as SUSY. This type of indirect probe of new physics is useful in general because it may cover energy scales or particular kinematic regions with problematic backgrounds that are difficult to reach with direct searches.

    In addition to work to produce new results from experiment, theorists are working to improve the standard model values for the muon anomalous moment. The biggest challenge is the hadronic contribution, which dominates the uncertainty on the theory value. Even without these improvements, if the central muon g-2 value remains the same, the expected significance of a new result would be around 5 sigma, while with reduced uncertainties on the expected value from theory, the significance could be close to 9 sigma. This potential for a discovery-level deviation from theory indicating the presence of new physics, to confirm or deny the evidence level deviation from the previous result, is a strong motivator for doing this experiment and making the uncertainties as low as possible.

    The Muon g-2 experiment performs the measurement of the anomalous magnetic moment using a beam of polarized muons contained in a storage ring. The muons' magnetic moment is related to their spin, which precesses as the muons travel through the ring's magnetic field. Decay electrons, which are preferentially emitted in the direction of the electron spin, are observed at detectors located on the inside of the ring. Therefore, the oscillating rate of electron detection measures the spin precession and magnetic moment. The muons themselves are produced when a beam of protons from the Fermilab accelerator complex hits a target and produces pions, which then decay to muons and neutrinos. Energy selected muons are then transported to the storage ring. It is important to precisely understand the properties of the muon beam and the storage ring's fields so any unexpected precession of the muons can be limited and/or accounted for.

    There are various beam line instruments to measure the muon beam properties from pro- duction at the target, through the transport lines, around the delivery ring, and finally into the Muon g-2 storage ring. There are toroids, ion chambers, wall current monitors, SWICs, SEMs, and beam loss monitors to measure the muon beam profile, position, intensity, and losses. Finally, there is a threshold Cherenkov counter to measure the beam's species composition. The Cherenkov will be used during commissioning to verify the particle production rates immediately after the target, and the muon beam purity immediately prior to injection into the storage ring.

Cherenkov Counter Particle Identification System

The University of Mississippi's High Energy Physics group's at the Muon g-2 exeriment includes work on the Cherenkov counter. This subdetector of Muon g-2 will perform an integral role in Particle identification (PID) of the particles in the beam. During commisioning, the Cherenkov will be used to verify the particle production rates immediately after the target, and the muon beam purity immediately prior to injection into the storage ring. Dr. Breese Quinn has been in charge of the Cherenkov detector's commisioning.

Cherenkov Counter Commissioning

    Dr. Quinn acquired a Cherenkov counter that was previously used at BNL. He made several modifications to the device for use at Fermilab. The chamber gas C4F8O was selected, which allows full operation below 1 atmosphere pressure. This greatly simplified safety certification and eliminated the need for overpressured secondary windows to keep the inner windows convex. Wet pumps were replaced with dry pumps to simplify operation, and a new low-gain PMT was tested to eliminate the need for neutral density filters. New electronics were fabricated, and a simple DAQ system was constructed.

    The completely refurbished device was bench tested, and then installed in the M3 beam-line tunnel for beam tests in winter 2014. With the C4F8O gas, the counter will start producing Cherenkov radiation from different mass particles at different pressure threshold points. When exposed to beam just downstream of the production target, the pressure scan was performed to measure the relative contributions of electrons, muons, and pions to the beam composition.

Electric Field Correction

    One source of g-2 measurement corrections is related to the vertical focusing of the beam. Focusing is done using an electric quadrupole field and under certain conditions, namely at a muon momentum of 3.09 GeV/c, the electric field does not contribute to the spin equation, which is the rate the spin turns relative to the momentum. The machine is operated so particles have this momenta. However, this is not strictly true in real life, as not all muons have this momenta and the condition that the velocity be transverse to the magnetic field may not be precisely true in the case of all particles. There is a correction applied due to the radial electric field related to this uncertainty about the particle momenta, which is what we will study

    A postdoc for the University of Mississippi group, Dr. Holzbauer, has already begun work to become familiar with the existing simulation infrastructure for the group, with the goal of writing code necessary to estimate the electric field correction. Here we refer to the radial electric field correction, which was (for a particular portion of the data) 0.47±0.05 ppm in the BNL experiment. This is present because there are muons which do not have momentum of exactly 3.09 GeV/c, the value at which contributions of the electric field to the spin precession are zero. Because of this, the major value we will be determining is the average of the square of the muon's equilibrium radius of curvature relative to the central orbit. This is done via fast-rotation analysis.