arXiv subjects

B. Lee Roberts

Publications and source records attributed to B. Lee Roberts.

At least 19 recordsLinked to original sources

arXiv2025

Status of the Proton EDM Experiment (pEDM)

The Proton EDM Experiment (pEDM) is the first direct search for the proton electric dipole moment (EDM) with the aim of being the first experiment to probe the Standard Model (SM) prediction of any particle EDM. Phase-I of pEDM will achieve $10^{-29} e\cdot$cm, improving current indirect limits by four orders of magnitude. This will establish a new standard of precision in nucleon EDM searches and offer a unique sensitivity to better understand the Strong CP problem. The experiment is ideally positioned to explore physics beyond the Standard Model (BSM), with sensitivity to axionic dark matter via the signal of an oscillating proton EDM and across a wide mass range of BSM models from $\mathcal{O}(1\text{GeV})$ to $\mathcal{O}(10^3\text{TeV})$. Utilizing the frozen-spin technique in a highly symmetric storage ring that leverages existing infrastructure at Brookhaven National Laboratory (BNL), pEDM builds upon the technological foundation and experimental expertise of the highly successful Muon $g$$-$$2$ Experiments. With significant R\&D and prototyping already underway, pEDM is preparing a conceptual design report (CDR) to offer a cost-effective, high-impact path to discovering new sources of CP violation and advancing our understanding of fundamental physics. It will play a vital role in complementing the physics goals of the next-generation collider while simultaneously contributing to sustaining particle physics research and training early-career researchers during gaps between major collider operations.

hep-ex↗

arXiv2022

The storage ring proton EDM experiment

We describe a proposal to search for an intrinsic electric dipole moment (EDM) of the proton with a sensitivity of \targetsens, based on the vertical rotation of the polarization of a stored proton beam. The New Physics reach is of order $10^~3$TeV mass scale. Observation of the proton EDM provides the best probe of CP-violation in the Higgs sector, at a level of sensitivity that may be inaccessible to electron-EDM experiments. The improvement in the sensitivity to $θ_{QCD}$, a parameter crucial in axion and axion dark matter physics, is about three orders of magnitude.

hep-ph↗

arXiv2020

Reduction of coherent betatron oscillations in a muon g-2 storage ring experiment using RF fields

This work demonstrates that two systematic errors, coherent betatron oscillations (CBO) and muon losses can be reduced through application of radio frequency (RF) electric fields, which ultimately increases the sensitivity of the muon $g-2$ experiments. As the ensemble of polarized muons goes around a weak focusing storage ring, their spin precesses, and when they decay through the weak interaction, $μ^+ \rightarrow e^+ ν_e \bar{ν_μ}$, the decay positrons are detected by electromagnetic calorimeters. In addition to the expected exponential decay in the positron time spectrum, the weak decay asymmetry causes a modulation in the number of positrons in a selected energy range at the difference frequency between the spin and cyclotron frequencies, $ω_\text{a}$. This frequency is directly proportional to the magnetic anomaly $a_μ=(g-2)/2$, where $g$ is the g-factor of the muon, which is slightly greater than 2. The detector acceptance depends on the radial position of the muon decay, so the CBO of the muon bunch following injection into the storage ring modulate the measured muon signal with the frequency $ω_\text{CBO}$. In addition, the muon populations at the edge of the beam hit the walls of the vacuum chamber before decaying, which also affects the signal. Thus, reduction of CBO and unwanted muon loss increases the $a_μ$ measurement sensitivity. Numerical and experimental studies with RF electric fields yield more than a magnitude reduction of the CBO, with muon losses comparable to the conventional method.

physics.acc-ph↗

arXiv2018

The History of the Muon (g-2) Experiments

I discuss the history of the muon $(g-2)$ measurements, beginning with the Columbia-Nevis measurement that observed parity violation in muon decay, and also measured the muon $g$-factor for the first time, finding $g_μ=2$. The theoretical (Standard Model) value contains contributions from quantum electrodynamics, the strong interaction through hadronic vacuum polarization and hadronic light-by-light loops, as well as the electroweak contributions from the $W$, $Z$ and Higgs bosons. The subsequent experiments, first at Nevis and then with increasing precision at CERN, measured the muon anomaly $a_μ= (g_μ-2)/2$ down to a precision of 7.3 parts per million (ppm) The Brookhaven National Laboratory experiment E821 increased the precision to 0.54 ppm, and observed for the first time the electroweak contributions. Interestingly, the value of $a_μ$ measured at Brookhaven appears to be larger than the Standard Model value by greater than three standard deviations. A new experiment, Fermilab E989, aims to improve on the precision by a factor of four, to clarify whether this result is a harbinger of new physics entering through loops, or from some experimental, statistical or systematic issue.

hep-ex↗

arXiv2018

The Muon $(g-2)$ Spin Equations, the Magic $γ$, What's small and what's not

We review the spin equations for the muon in the 1.45~T muon storage ring at Brookhaven National Laboratory, which has subsequently been relocated to Fermilab. Muons are stored in a uniform 1.45~T magnetic field, and vertical focusing is provided by four sets of electrostatic quadrupoles placed symmetrically around the storage ring. The storage ring is operated at the "magic $γ= 29.3$" so that the effect of the motional magnetic field cancels for muons at the magic momentum. We point out the relative sizes of the various terms in the spin equations, and show that for experiments that use the magic $γ$ and electric quadrupole focusing to store the muon beam, any proposed effect that multiplies either the motional magnetic field $\vec β\times \vec E$ or the muon pitching motion $\vec β\cdot \vec B$ term, will be smaller by three or more orders of magnitude, relative to the spin precession from the storage ring magnetic field. We use a recently proposed General Relativity correction as an example, to demonstrate the smallness of any such contribution, and point out that the revised preprint from these authors still contains a conceptual error, that significantly overestimates the magnitude of their proposed correction. We have prepared this document in the hope that future authors will find it useful, should they wish to propose corrections from some additional term added to the Thomas equation, Eq. 13, below. Our goal is to clarify how the experiment is done, and how the small corrections due to the presence of the radial electric field and the vertical pitching motion of themuons (betatron motion) in the storage ring are taken into account.

hep-ph↗

arXiv2013

The Muon (g-2) Theory Value: Present and Future

This White Paper briefly reviews the present status of the muon (g-2) Standard-Model prediction. This value results in a 3 - 4 standard-deviation difference with the experimental result from Brookhaven E821. The present experimental uncertainty is $\pm 63 \times 10^{-11}$ (0.54~ppm), and the Standard-Model uncertainty is $\simeq \pm 49 \times 10^{-11}$. Fermilab experiment E989 has the goal to reduce the experimental error to $\pm 16 \times 10^{-11}$. Improvements in the Standard-Model value, which should be achieved between now and when the first results from Fermilab E989 could be available, should lead to a Standard-Model uncertainty of $\sim \,\pm 35 \times 10^{-11}$. These improvements would halve the uncertainty on the difference between experiment and theory, and should clarify whether the current difference points toward New Physics, or to a statistical fluctuation. At present, the (g-2) result is arguably the most compelling indicator of physics beyond the Standard Model and, at the very least, it represents a major constraint for speculative new theories such as supersymmetry, dark gauge bosons or extra dimensions.

hep-ph↗

arXiv2011

Searching for physics beyond the Standard Model through the dipole interaction

The magnetic dipole interaction played a central role in the development of QED, and continued in that role for the Standard Model. The muon anomalous magnetic moment has served as a benchmark for models of new physics, and the present experimental value is larger than the standard-model value by more than three standard deviations. The electric dipole moment (EDM) violates parity ({$P$}) and time-reversal ({$T$}) symmetries, and in the context of the $CPT$ theorem, the combination of charge conjugation and parity ($CP$). Since a new source of {$ CP$} violation outside of that observed in the $K$ and $B$ meson systems is needed to help explain the baryon asymmetry of the universe, searches for EDMs are being carried out worldwide on a number of systems. The standard-model value of the EDM is immeasurably small, so any evidence for an EDM would signify the observation of new physics. Unique opportunities exist for EDM searches using polarized proton, deuteron or muon beams in storage rings. This talk will provide an overview of the theory of dipole moments, and the relevant experiments. The connection to the transition dipole moment that could produce lepton flavor violating interactions such as $μ^+ \rightarrow e^+ γ$ is also mentioned.

hep-ex↗

arXiv2010

Status of the Fermilab Muon (g-2) Experiment

The New Muon $(g-2)$ Collaboration at Fermilab has proposed to measure the anomalous magnetic moment of the muon, $a_μ$, a factor of four better than was done in E821 at the Brookhaven AGS, which obtained $a_μ= [116 592 089 (63)] \times 10^{-11}$ $\pm 0.54$ ppm. The last digit of $a_μ$ is changed from the published value owing to a new value of the ratio of the muon-to-proton magnetic moment that has become available. At present there appears to be a difference between the Standard-Model value and the measured value, at the $\simeq 3$ standard deviation level when electron-positron annihilation data are used to determine the lowest-order hadronic piece of the Standard Model contribution. The improved experiment, along with further advances in the determination of the hadronic contribution, should clarify this difference. Because of its ability to constrain the interpretation of discoveries made at the LHC, the improved measurement will be of significant value, whatever discoveries may come from the LHC.

hep-ex↗

arXiv2009

Precision Searches for Physics Beyond the Standard Model

The "precision" frontier, which is closely related to the "intensity" frontier, provides a complementary path to the discovery of physics beyond the Standard Model. Several examples of discoveries that would change our view of the physical world are: Charged lepton flavor violation, e.g. muon electron conversion; the discovery of a permanent electric dipole moment of the electron, neutron, muon or a nucleus. In this paper I focus mostly on phenomena mediated by a dipole interaction, including the anomalous magnetic moment of the muon.

hep-ex↗

arXiv2007

The Physics Case for the New Muon (g-2) Experiment

This White Paper briefly reviews the present status of the muon (g-2) experiment and the physics motivation for a new effort. The present comparison between experiment and theory indicates a tantalizing $3.4 σ$ deviation. An improvement in precision on this comparison by a factor of 2--with the central value remaining unchanged--will exceed the ``discovery'' threshold, with a sensitivity above $6 σ$. The 2.5-fold reduction improvement goal of the new Brookhaven E969 experiment, along with continued steady reduction of the standard model theory uncertainty, will achieve this more definitive test. Already, the (g-2) result is arguably the most compelling indicator of physics beyond the standard model and, at the very least, it represents a major constraint for speculative new theories such as supersymmetry or extra dimensions. In this report, we summarize the present experimental status and provide an up-to-date accounting of the standard model theory, including the expectations for improvement in the hadronic contributions, which dominate the overall uncertainty. Our primary focus is on the physics case that motivates improved experimental and theoretical efforts. Accordingly, we give examples of specific new-physics implications in the context of direct searches at the LHC as well as general arguments about the role of an improved (g-2) measurement. A brief summary of the plans for an upgraded effort complete the report.

hep-ph↗

arXiv2007

Muon g-2: Review of Theory and Experiment

A review of the experimental and theoretical determinations of the anomalous magnetic moment of the muon is given. The anomaly is defined by a=(g-2)/2, where the Landé g-factor is the proportionality constant that relates the spin to the magnetic moment. For the muon, as well as for the electron and tauon, the anomaly a differs slightly from zero (of order 10^{-3}) because of radiative corrections. In the Standard Model, contributions to the anomaly come from virtual `loops' containing photons and the known massive particles. The relative contribution from heavy particles scales as the square of the lepton mass over the heavy mass, leading to small differences in the anomaly for e, μ, and τ. If there are heavy new particles outside the Standard Model which couple to photons and/or leptons, the relative effect on the muon anomaly will be \sim (m_μ/ m_e)^2 \approx 43\times 10^3 larger compared with the electron anomaly. Because both the theoretical and experimental values of the muon anomaly are determined to high precision, it is an excellent place to search for the effects of new physics, or to constrain speculative extensions to the Standard Model. Details of the current theoretical evaluation, and of the series of experiments that culminates with E821 at the Brookhaven National Laboratory are given. At present the theoretical and the experimental values are known with a similar relative precision of 0.5 ppm. There is, however, a 3.4 standard deviation difference between the two, strongly suggesting the need for continued experimental and theoretical study

hep-ph↗

arXiv2007

Muon Physics: A Pillar of the Standard Model

Since its discovery in the 1930s, the muon has played an important role in our quest to understand the sub-atomic theory of matter. The muon was the first second-generation standard-model particle to be discovered, and its decay has provided information on the (Vector -Axial Vector) structure of the weak interaction, the strength of the weak interaction, G_F, and the conservation of lepton number (flavor) in muon decay. The muon's anomalous magnetic moment has played an important role in restricting theories of physics beyond the standard standard model, where at present there is a 3.4 standard-deviation difference between the experiment and standard-model theory. Its capture on the atomic nucleus has provided valuable information on the modification of the weak current by the strong interaction which is complementary to that obtained from nuclear beta decay.

hep-ex↗

arXiv2005

Intense Muon Physics Working Group Summary

The intense muon beams which will be available at a neutrino factory provide a unique opportunity for searching for physics beyond the standard model, both in lepton flavor violation and in the search for a permanent electric dipole moment for the muon. Other experiments which can use intense muon beams will also be possible.

hep-ex↗

arXiv2005

Muon (g-2): Past, Present and Future

The muon (g-2) experiment E821 at the Brookhaven National Laboratory has achieved a relative precision of $\pm 0.5$ parts per million. A new experiment, E969, with scientific approval but not yet funded, aims to improve this to $\pm 0.2$ ppm. The technique and results from E821 will be described, and the proposed improvements for E969 will be discussed.

hep-ex↗

arXiv2005

Results and Future Prospects for Muon (g-2)

Spin physics had its beginnings in the famous experiments of Stern and Gerlach, which eventually resulted in the postulation of spin by Goudsmit and Uhlenbeck. The Stern-Gerlach experiment told us that the $g$-value of the electron was 2, but we now know that because of radiative corrections, the $g$-value of the leptons is slightly greater than 2, the lowest-order contribution being $α/π$, where $α$ is the fine-structure constant. Measurements of the magnetic dipole moments of the electron and muon have played a major role in our understanding of QED and of the standard model. In this talk I discuss the progress on measurements and theory of the magnetic dipole moment of the muon.

hep-ex↗

arXiv2004

Future Muon Dipole Moment Measurements

From the famous experiments of Stern and Gerlach to the present, measurements of magnetic dipole moments, and searches for electric dipole moments of ``elementary'' particles have played a major role in our understanding of sub-atomic physics. In this talk I discuss the progress on measurements and theory of the magnetic dipole moment of the muon. I also discuss a new proposal to search for a permanent electric dipole moment (EDM) of the muon and put it into the more general context of other EDM searches. These experiments, along with searches for the lepton flavor violating decays $μ\to e γ$ and $μ^- + A \to e^- + A$, provide a path to the high-energy frontier through precision measurements.

hep-ex↗

arXiv2003

Lepton Dipole Moments

From the famous experiments of Stern and Gerlach to the present, measurements of magnetic dipole moments, and searches for electric dipole moments of ``elementary'' particles have played a major role in our understanding of sub-atomic physics. In this talk I discuss the progress on measurements and theory of the magnetic dipole moments of the electron and muon. I also discuss a new proposal to search for a permanent electric dipole moment (EDM) of the muon and put it into the more general context of other EDM searches.

hep-ex↗

arXiv2002

Measurement of the Muon (g-2)-Value

The muon (g-2) experiment is described, and the recent results are presented. These results represent the final measurement for the positive muon.

hep-ex↗