Precise Measurement of the Positive Muon Anomalous Magnetic Moment

Abstract

A precise measurement of the anomalous g value, aµ = (g − 2)/2, for the positive muon has been made at the Brookhaven Alternating Gradient Synchrotron. The result a µ + = 11 659 202(14)(6) × 10 −10 (1.3 ppm) is in good agreement with previous measurements and has an error one third that of the combined previous data. The current theoretical value from the standard model is aµ(SM)= 11 659 159.6(6.7) × 10 −10 (0.57 ppm) and aµ(exp)−aµ(SM) = 43(16) × 10 −10 in which aµ(exp) is the world average experimental value.PACS number: 14.60.Ef 13.40.EmPrecise measurement of the anomalous g value, a µ = (g−2)/2, of the muon provides a sensitive test of the standard model of particle physics and new information on speculative theories beyond it. Compared to the electron, the muon g value is more sensitive to standard model extensions, typically by a factor of (m µ /m e ) 2 . In this Letter we report a measurement of a µ for the positive muon from Brookhaven AGS experiment 821, based on data collected in 1999.The principle of the experiment, previous results, and many experimental details have been given in earlier publications [1,2]. Briefly, highly polarized µ + of 3.09 GeV/c from a secondary beamline are injected through a superconducting inflector [3] into a storage ring 14.2 m in diameter with an effective circular aperture 9 cm in diameter. The superferric storage ring [4] has a homogeneous magnetic field of 1.45 T, which is measured by an NMR system relative to the free proton NMR frequency [5,6]. Electrostatic quadrupoles provide vertical focusing. A pulsed magnetic kicker gives a 10 mrad deflection which places the muons onto stored orbits. The muons start in 50 ns bunches and debunch with a decay time of about 20 µs due to their 0.6% momentum spread. Positrons are detected using 24 lead/scintillating fiber electromagnetic calorimeters [7] read out by waveform digitizers. The waveform digitizer and NMR clocks were phase-locked to the Loran C frequency signal.The muon spin precesses faster than its momentum rotates by an angular frequency ω a in the magnetic field B weighted over the muon distribution in space and time. The quantity a µ iswhere ω a is unaffected by the electrostatic field for muons with γ = 29.3. Parity violation in the decay µ + → e +ν µ ν e causes positrons to be emitted with an angular and energy asymmetry. Because of the Lorentz boost, the positron emission angle with respect to the muon spin direction in the muon rest frame is strongly coupled to its energy in the laboratory frame. The number of decay positrons with energy greater than E is described byin which the time dilated lifetime γτ ≈ 64.4 µs. Some 140 g − 2 periods of 4.37 µs were observed. Most experimental aspects of the data taking in 1999 were the same as in 1998 [1]. However, some improvements were made. Care was taken in tuning the AGS ejection system to minimize background from any extraneous proton beam extracted during the muon storage time. Scintillating fiber detectors which could be moved in and out of the storage region were u...

Universal extra dimensions and the muon magnetic moment

“…In this paper, we addressed the implications of this idea of "universal extra dimensions" for the muon anomalous magnetic moment. For one or two extra dimensions, we computed the one-loop contribution of the KK modes of the standard model fields and found that it is too small to be detected by the Muon (g − 2) experiment at BNL [6]. We then analyzed higher-dimension operators in the context of the 6-dimensional standard model.…”

“…In the previous section we showed that, with one or two universal extra dimensions, the value of g µ − 2 ≡ a µ induced by loops with standard model KK fields below the effective theory cutoff M s is smaller than the final expected 1σ sensitivity of the muon g − 2 experiment at BNL [6]. However, physics above M s also contributes to a µ , and these effects could be large given that the SU(3) C × SU (2) In six dimensions, the standard model is chiral as in four dimensions and is highly constrained by anomaly cancellation and Lorentz invariance.…”

Lower limit on the neutralino mass in the general MSSM

“…For practically all the other (s)particles of the model one does now have some lower bounds on their masses from extensive phenomenological studies and searches both at colliders and in astroparticle experiments. One can for example mention the impact of both LEP and the Tevatron in constraining the model parameters through the direct searches, the importance of rare decays notably b → sγ or even low energy experiments like the recent measurement of the muon anomalous moment (g − 2) µ [4]. Cosmological considerations like the thermal relic density of the LSP also put some stringent bounds on the MSSM.…”

“…For the latter we include both the strong α s contribution as well as the Yukawa contribution for the SUSY Wilson coefficients as well as the charged Higgs and the Goldstone contribution. 4 Our standard model value (with scale parameters set at m b ), gives Br(b → sγ) = 3.72 × 10 −4 while the scale and other parameter uncertainty (α s , CKM matrix elements) are about 10%. To bound the SUSY contribution we take the world weighted average of the CLEO [65] BELLE [66] and ALEPH [67] measurements…”

LHC τ -rich tests of lepton-specific 2HDM for (g − 2) μ

“…The muon g − 2 anomaly has been a long standing puzzle since the announcement by the E821 experiment in 2001 [1,2]. During the past 15 years, development in both experimental and theoretical sides has been made to reduce the uncertainties by a factor of two or so, and thus establish a consistent 3σ discrepancy.…”