In Support of a Proton EDM Experiment at Fermilab
Steve Vigdor, Feb. 1, 2014
It is natural and inevitable that a long-range planning process such as P5’s ends up focusing most
attention on the several major projects that compete for the lion’s share of (hopefully) available funds.
In the case of HEP, these major projects, even when divided into stages, have costs near or above a
billion dollars per stage. History has taught us that maintaining the necessary funding levels for such
large projects over long terms, in the face of frequent reviews, and bureaucratic and political hurdles, is
challenging. It is thus also vital for the future health of the field in the U.S. to reserve sufficient flexibility
in plans and budgets to encourage and fund nimble, innovative projects at an order of magnitude
smaller scale when they address issues of central intellectual import for the field.
I write to support and to provide some historical perspective on one such project: a proton electric
dipole moment search proposed for Fermilab. I am not a collaborator on the pEDM project, but I did
devote considerable effort during my years as Associate Lab Director for Nuclear and Particle Physics at
Brookhaven to steering the proponents in the general direction of the real axis. After a lengthy
gestation period in the wake of the Brookhaven muon g-2 experiment, the collaboration that formed
around pursuing charged particle EDM’s in storage rings originally proposed a search for a deuteron
EDM, utilizing a polarized deuteron beam from the AGS, to the RHIC/AGS Program Advisory Committee
in May 2008. The PAC endorsed the scientific merit strongly, but was concerned about collaboration
strength and the absence of a clear approach to achieving the goal precision in stages.
Over the intervening years, the collaboration made good progress in simplifying the concept of the
experiment and establishing R&D goals and milestones. An important step was to focus first on
measurements for the proton, which permitted use of the “magic momentum” (corresponding to a
modest kinetic energy of 200 MeV) at which an in-plane polarization vector would remain stationary
with respect to the orbiting beam direction in a purely electrostatic storage ring. This eliminated many
of the complications associated with the combined electric and magnetic fields that would be needed
for a deuteron experiment. A non-zero intrinsic proton EDM would then be manifested by a gradual
buildup in vertical polarization of an initially longitudinally polarized stored beam.
The use of counter-rotating stored beams, and of oppositely polarized beam bunches, allows extensive
cancellation of many otherwise troublesome systematic errors. Clever techniques were developed for
using rf modulation to monitor the relative separation of the counter-rotating beam paths to the
exquisite precision needed to monitor potential contaminating radial magnetic fields and to reach
systematic sensitivity at the 10-29 ecm level. Excellent progress was made in experiment runs at the
COSY facility in Jülich to demonstrate that polarimetry could indeed provide the needed sensitivity to
tiny vertical polarization components. A ring lattice was designed that holds the promise to combine
long spin coherence times with sufficient available straight section lengths to add initially unforeseen
diagnostic equipment that will undoubtedly become important when one gets beyond the preconceptual level in experiment design. Reasonable cost estimations for the experiment, given its preconceptual (i.e., pre-CD1) stage, were carried out by the proponents with essential input from the
Collider-Accelerator Department at BNL. In very round numbers, an eventual construction project for
the experiment would cost on the order of $100M.
Along the way, I organized at BNL two technical reviews of the evolving experiment design, involving
panelists with expertise in EDM measurements, in precision polarization physics and in storage ring
accelerator science. A December 2009 review came to the following general conclusions:
“The Committee judges the proton EDM experiment to be intriguing, important and very
challenging, and the overall design to be informed by many clever ideas, especially the use of
the magic momentum and simultaneously counter-rotating beams. A proton EDM
measurement to the goal sensitivity of 10-29 ecm would represent a profound contribution to
the search for non-Standard Model CP violation in nature. Furthermore, the unique systematics
of a storage ring EDM search allow it to provide crucial independent confirmation of an EDM
signal from another, more conventional, EDM experiment. While the Committee expresses a
number of significant concerns about technical performance requirements needed for the
experiment and about some of the proposed solutions, it has not identified anything considered
to be a technical showstopper at this point. The Committee is thus enthusiastic about seeing
this project move forward, and encourages the collaboration to continue to identify key
systematic issues and crucial R&D.”
A different review team in March 2011 “strongly endorses the program and finds excellent potential for
the group to contribute on a significant and competitive level to the worldwide efforts.”
I should not need to convince P5 of the scientific importance of improving limits on fundamental
particle EDM’s: they address squarely one of the grand challenge intellectual goals of particle physics,
understanding the origin of the CP violation needed to establish the baryon asymmetry of the universe.
A storage-ring EDM experiment is novel, but it is neither cheap nor without very significant technical
challenges. So why pursue it? Neutron EDM experiments have a 60-year head start on the learning
curve needed to approach sensitivities even an order of magnitude less ambitious than that of the
pEDM collaboration. What does a proton experiment add? I would offer three answers to this
question:
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The availability of intense proton beams makes attainment of statistical sensitivity less daunting
than for neutrons, so that one can plausibly contemplate attaining an order of magnitude better
sensitivity than the best goals of next-generation neutron experiments.
A proton EDM measurement complements neutron EDM experiments. If a non-zero signal were
to be seen in a neutron experiment, one would want to check it with protons. If the signal were
real, the proton experiment offers the promise to determine the isospin-dependence of the
violation source. (Subsequent deuteron EDM measurements in storage rings could then further
delineate the source of the violation.) If no signal is seen for the neutron, it is desirable to make
measurements with even better sensitivity, but completely different systematic error sensitivity,
with protons.
Developing a storage ring EDM experiment with the stability, spin coherence time, monitoring
sensitivity and understanding of beam and spin dynamics needed to reach the pEDM goal will
likely push the state of the art in accelerator science and technology at the precision frontier as
much, though in different ways, as developments at the energy or intensity frontiers.
There are, of course, also exciting developments in new techniques for measuring electron EDM’s, and
those experiments are likely to come in at yet an additional order of magnitude lower cost. But neutron
and proton EDM’s bring sensitivity to different sources of CP violation, such as QCD, than electrons.
Given the positive arguments, why has the pEDM concept languished as long as it has? Even the
contemporary dovetail joint boundary between nuclear physics and high energy physics funding at DOE
– a boundary that has ONP supporting the nation’s only operating collider facility and OHEP providing
the support for U.S. contributions to a reactor neutrino experiment in China – leaves cracks large
enough for a project of this magnitude to fall through. I did not succeed in attracting ONP funding even
for R&D toward the experiment at Brookhaven, its most natural home given the depth of expertise at
the laboratory in the production, acceleration and monitoring of polarized beams. This is not surprising,
given ONP’s reluctance to devote an even larger fraction of its budget to BNL, combined with their
growing nervousness over difficulties encountered in a similarly challenging new neutron EDM
experiment of comparable scale (an R&D program inadvisably funded as a DOE project, and bending
severely under the weight of all the usual project management scrutiny). At the same time, OHEP
(which historically supported the major BNL proponents of the EDM experiment) insisted that EDM
research was in the ONP bailiwick, and would not be supported at all by OHEP. Somewhat to my
surprise, this attitude persisted even after the intellectual prominence given to EDM experiments at the
first Intensity Frontier Workshop, when an opportunistic research office might have decided to put
down a stake in previously unclaimed portions of that territory.
Finally, I urged the pEDM proponents to consider forming a collaboration with Fermilab scientists and
proposing the experiment to run at Fermilab, where a well-managed experiment could provide a worthy
component of Fermilab’s future scientific program and could hopefully attract greater HEP interest.
That collaboration has now formed and led to significant strengthening of all aspects of the proposal,
including project management capability. An Expression of Interest has been warmly received and
encouraged by the Fermilab PAC. In parallel, the possibility of adding significant funding for R&D and for
an eventual project from South Korea has provided additional impetus to move forward.
I am in a small minority that believes that missed opportunities at the interfaces between nuclear and
high energy physics funding in the U.S. cause more damage to both fields than the perceived dangers of
diluting priorities in each field if the two offices were joined. In any case, it is beyond the scope of the
P5 charge to address questions of where these boundaries should lie. But P5 could do some good by
noting explicitly that the pEDM proposal represents a creative opportunity to take a fresh and
technologically exciting approach to address a half-century old grand challenge for particle physics, and
that it is worthy of R&D support to help demonstrate its technical feasibility, even in tight-budget
scenarios.