C. Pai

Improved measurement of the positive muon anomalous magnetic moment

A new measurement of the positive muons anomalous magnetic moment has been made at the Brookhaven Alternating Gradient Synchrotron using the direct injection of polarized muons into the superferric storage ring. The angular frequency difference omega (a) between the angular spin precession frequency omega (s) and the angular orbital frequency omega (c) is measured as well as the free proton MMR frequency omega (p). These determine R = omega (a)/omega (p) = 3.707 201(19) x 10(-3). With mu (mu)/mu (p) = 3.183 345 39(10) this gives a(mu+) = 11 659 191(59) x 10-(10) (+/-5 ppm), in good agreement with the previous CERN and BNL measurements for mu (+) and mu (-), and with the standard model prediction.

The Brookhaven muon storage ring magnet

Abstract The muon g-2 experiment at Brookhaven National Laboratory has the goal of determining the muon anomalous g-value a μ (=(g−2)/2) to the very high precision of 0.35 parts per million and thus requires a storage ring magnet with great stability and homogeniety. A superferric storage ring with a radius of 7.11 m and a magnetic field of 1.45 T has been constructed in which the field quality is largely determined by the iron, and the excitation is provided by superconducting coils operating at a current of 5200 A. The storage ring has been constructed with maximum attention to azimuthal symmetry and to tight mechanical tolerances and with many features to allow obtaining a homogenous magnetic field. The fabrication of the storage ring, its cryogenics and quench protection systems, and its initial testing and operation are described.

The superconducting inflector for the BNL g-2 experiment

The muon g-2 experiment at Brookhaven National Laboratory (BNL) has the goal of determining the muon anomalous magnetic moment, a(mu) (= (g-2)/2), to the very high precision of 0.35 parts per million and thus requires a storage ring magnet with great stability and homogeneity. A super-ferric storage ring has been constructed in which the field is to be known to 0.1 ppm. In addition, a new type of air core superconducting inflector has been developed and constructed, which successfully serves as the injection magnet. The injection magnet cancels the storage ring field, 1.5 T, seen by the entering muon beam very close to the storage ring aperture. At the same time, it gives negligible influence to the knowledge of the uniform main magnetic field in the muon storage region located at just 23 rum away from the beam channel. This was accomplished using a new double cosine theta design for the magnetic field which traps most of the return field, and then surrounding the magnet with a special superconducting sheet which traps the remaining return field. The magnet is operated using a warm-to-cold cryogenic cycle which avoids affecting the precision field of the storage ring. This article describes the design, research development, fabrication process, and final performance of this new type of superconducting magnet

The muon anomalous magnetic moment

The muon g-2 experiment at the Brookhaven National Laboratory is described, including its motivation, goal and present status. The latest result based on 1998 data is aμ+=g−2/2=11 659 191(59)×10−10 (5 ppm), where the error is primarily statistical. This value agrees with the present theoretical value. Data obtained thus far and now being analyzed should have a statistical error of about 0.5 ppm.

An Ultra-Precise Storage Ring for the Muon g — 2 Measurement

An ultra precise 3 GeV/c storage ring with a 14.5 kG super-ferric magnet is under construction at the Brookhaven AGS for the measurement of the muon anomalous magnetic moment to 0.35 PPM accuracy. This requires a magnetic field which is constant to ≈ 1 PPM and is known sufficiently well that the magnetic field integral averaged over the muon orbits can be calculated to 0.1 PPM. First the magnetic field will be statically shimmed by various techniques. Pole face winding will be used for final small static and dynamic corrections. Very elaborate NMR field monitoring techniques are required. A “movable trolley” located inside the vacuum chamber and the electrostatic focusing quadrupoles will measure the field throughout the muon storage volume. The trolley “siding” is 180˚ from the injection point where no electric quadrupoles are located. Injection can be interrupted so the trolley can circle the ring. Also ≈ 200 NMR probes located outside the vacuum chamber monitor the field during physics running and control the pole face windings. The very large (≈ 15 m diameter) superconducting coils (SC) are designed. Test winding will soon commence. Orders for the magnet steel can now be placed. R and D on various pulsed and SC dc injection methods is ongoing.

Muon g-2 experiment at Brookhaven National Laboratory

By the end of an excellent data taking in 1999, we collected ≈ 1 billion decay positrons with energy greater than 2 GeV and 30 μs after injection. The analysis of the 1999 data set were performed in parallel by various teams in the collaboration and each team provides a different approach to the analysis. The projected errors are expected to be of order 1.3 ppm statistical and below 0.5 ppm systematic. The data obtained in the 2000 run contains ≈ 4 times more decay positrons compared to 1999.

First results from the new muon (g-2) experiment

A new and improved experiment for measuring the muon magnetic anomaly at the Brookhaven National Laboratory (BNL) was successfully started and has yielded first results. New major components of the experiment include a superferric storage ring, a superconducting inflector, electrostatic quadrupoles, lead-scintillating fiber electron calorimeters and a high precision NMR magnetic field measurement and control system. The first measurement of the ratio R of the spin precession frequency of the positive muon relative to that of a free proton gives R=(3.707 219±0.000 048)×10−3. It is similar in accuracy and in good agreement with previous CERN measurements for μ+ and μ−. A muon kicker has been installed to boost the number of stored particles in the storage ring magnet and was successfully commissioned recently. The data acquired so far is expected to lower significantly the uncertainty in R. First extensive data-taking will start soon.

Status of the BNL muon (g−2) experiment

The muon (g−2) experiment at Brookhaven has just completed a 3-month run for checkout and initial data-taking. In the first two months beam was taken in a parasitic mode where one out of ten AGS pulses was delivered for commissioning of the beam line, quadrupoles, detectors, and data acquisition system. This was followed by four weeks of dedicated data collection. The main components of the experiment, which include the pion/muon beam line, the superconducting inflector, the superferric storage ring with its pulsed electric quadrupoles and magnetic field measurement system, and the detector system based on lead-scintillating fiber electron calorimeters, have been satisfactorily commissioned. The muon (g−2) precession frequency is clearly seen as a large signal. It is estimaed that over 25×106 decay positrons with energies greater than 1.5 GeV have been detected.

Beam vacuum chambers for Brookhaven's muon storage ring

An experiment is being built at Brookhaven to measure the g-2 value of the muons to an accuracy of 0.35 ppm. The muon storage ring of this experiment is designed to produce a dipole field with homogeneity to 1 ppm using a continuous superconducting magnet. The beam vacuum system in the storage ring will operate at 10/sup -7/ Torr and consists of twelve sector chambers. The chambers are constructed of aluminum and are approximately 3.5 m in length with a rectangular cross-section of 16.5 cm high by 45 cm at the widest point. The design features, fabrication techniques and cleaning methods for these chambers are described. Monte Carlo simulation of the pressure distribution and finite element analysis of the chamber deflection are summarized with good correlation shown to measured values obtained during tests of the prototype chamber.