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The proton electric dipole moment (pEDM) experiment as a most sensitive CP-violation probe
Selcuk Haciomeroglu
Abstract
The origin of the asymmetry between the amounts of matter and antimatter in the early Universe,
leading to the domination of matter at our time, still remains a mystery. Many potential solutions
have been explored, but a mechanism based on the CP-violation (i.e. breaking a combined charge
and mirror image symmetry) seems to be most promising.
In this article we present the work of the Center for Axion and Precision Physics (CAPP) of the
Institute of Basic Science on the pEDM experiment, aiming at probing the CP-violation via EDM, a
CP-violating quantity. We explain the basics of the experimental techniques to be employed, as well
as their roles and purposes. We point out the consequences of a potential strong CP-violation
detection for the New Physics.
13.8 billion years after the big bang, we observe that the universe is dominated by matter. The
source of the asymmetry between matter and antimatter is not known yet. But the favorable process
to explain this mystery makes use of CP-violation.
There are some symmetries in the physical processes. CP-symmetry asserts that the physics of
matter at some spatial coordinate is equivalent to the physics of antimatter at the mirror symmetric
coordinate. This symmetry is observed to be violated in some experiments in the last 50 years [1,2].
Then, the standard model of particle physics was slightly modified [3] via CKM mechanism to give
consistent results with those experiments. However, the CP-violation mentioned above is too strong
to be explained by the current version of the standard model. Therefore, the experiments to probe
stronger CP-violation are candidates to lead to “new physics”.
CAPP works on proton electric dipole moment (pEDM) experiment to probe CP-violation via
EDM, a CP-violating quantity. Standard model predicts very small EDM values for fundamental
particles, based on the CKM mechanism mentioned above. However, some theories beyond the SM,
like supersymmetry, predict bigger EDM values, by introducing stronger CP-violating sources.
The pEDM experiment aims to probe proton EDM with 10-29 e.cm sensitivity, which is
approximately 100,000 times better sensitivity than the current experimental limit [4]. This
corresponds to a measurement of the first moment of the proton charge distribution along its spin
direction with 10-29 cm resolution. This is indeed a very small number. The ratio of this distance to
the size of the proton is equivalent to 1 cm with respect to the size of the solar system. (Check
whether this is correct!)
The experiment is based on storing proton longitudinally polarized beams in a ring in an all-electric
for about 15 minutes at a time. If there is an EDM along the proton spin, the radial electric field will
precess it in the vertical direction. In a successful EDM experiment the overall vertical spin
polarization should have a measurable rate of change, proportional to the EDM of the proton (See
Figure 1). The pEDM experiment will consist of approximately 104 injections each with about 103
seconds of storage.
The injection of the beams into the ring will be in two opposite directions, namely clockwise (CW)
and counter-clockwise (CCW). This is one of the several novel elements in the pEDM ring. The
counter-rotating beams will pass through each other for billion times during one storage, without
considerable interaction. This idea of counter-rotating beams increases the symmetry of the
experiment to avoid several systematic errors.
The pEDM ring will have a circumference of about 500 meters. There will be about 100 bunches in
each direction running inside the ring at 60% of the speed of light. This makes about 3 microsecond
of revolution period. The beams will be running between two sets of concentric parallel plates of
240 kV potential difference at a separation of 3 cm. This will provide 8 MV/m radial electric field
on the beams to produce a vertical spin precession due to the proton EDM. This is such a high
electric field that a specially made electric field plates need to be made.
The ring is not composed of continuous parallel plates. They will be separated (at about 50
locations) by straight drift regions (Figure 2). These drift regions are necessary for hosting
additional ring elements like quadrupoles, sextupoles, radio frequency (RF) cavities and beam
position monitors (BPM).
Quadrupoles are used for focusing the beam around the design orbit via application of force
proportional to its displacement from the design orbit. Therefore, it leads to beam oscillations both
vertically and horizontally. This oscillation makes sure that the beam does not get out of the storage
region. (Figure 3).
Sextupoles are similar to quadrupoles, but they are used for momentum corrections. Quadrupoles
have four poles, sextupoles have six.
The most fundamental feature of the pEDM experiment is that each proton must have a specific
momentum on average. That momentum is 0.7 GeV/c, and called as “magic momentum”. The
deviation from that specific momentum leads to very fast horizontal spin precession, which reduces
significantly the sensitivity of the EDM measurement. In reality only a few of the protons in the
beam will be at magic momentum. Therefore, an RF cavity will be utilized to correct this issue. It is
an element at a specific location in the ring, producing an oscillating electric field in the
longitudinal direction. It works at the revolution frequency of the particle with magic momentum,
such that if a particle lags, it gains energy in the RF cavity and vice versa (Figure 4 and 5). This
makes sure that every proton in the beam will be at magic momentum on average. So that horizontal
spin precession will be below 1 radian in 103 seconds.
Background magnetic field is the most critical source of systematic errors in an EDM experiment,
since the proton has a large magnetic moment and it may lead to a vertical spin precession.
Therefore, it should be avoided in order to have a pure EDM signal. The pEDM collaboration has
developed a solution to this problem, using extremely sensitive beam position monitors (BPMs)
based on superconducting-quantum-interference-devices (SQUID) gradiometers. The systematic
error elimination will be done in two steps. The first one is to shield the whole ring of more than
500 meters to less than 1 nanoTesla. One should bear in mind that a car passing 50 meters away
perturbs the magnetic field by 50 nanoTesla. This shows how precise the experiment will be. CAPP
is currently working on a shielding prototype (Figure 6), initially designed by a Germany-based
group lead by Prof. Peter Fierlinger (Figure 7). They have already achieved one of the best
magnetic shielding in the world [5]. A successful magnetic shielding requires a good combination
of mu-metal, degaussing strategy and geometrical optimization. Their technology is now being
transferred and optimized for the pEDM experiment.
The second step is the measurement of remaining time-stable radial magnetic field inside the shield,
and compensating for it. This will be done making use of the counter rotating beams. A radial
magnetic field splits the counter-rotating beams vertically. Then, they will induce some horizontal
magnetic field proportional to the vertical separation between them, depending on the strength of
the vertical focusing. This field is at the order of attoTesla, beyond the reach of any magnetometer
in the world. CAPP has a plan to measure this tiny field with a large signal-to-noise ratio over the
durations of the experiment. The plan is to modulate the quadrupole strength, and hence the vertical
focusing strength, at a specific frequency and use SQUID gradiometers to measure the beam
separation at the modulation frequency. Dr. Yong-Ho Lee (Figure 8) is one of the top experts of
SQUIDs in the world. CAPP is collaborating with his group at KRISS for the development and
optimization of ultra-low noise SQUID gradiometers for the pEDM experiment.
Another important item in the pEDM experiment is the measurement of the spin polarization. The
beams will be continuously extracted from the ring during the 103 s of storage. At each revolution,
some tiny fraction of the protons will hit a carbon target, which is designed to be a limiting beam
aperture at a specific location in the ring and some of the scattered protons will then hit the detector.
The position of the particles in the detector will give information about the average transverse spin
components. This way, the spin precession of the proton beam will be determined over the course of
the experiment. Measurable spin precession rate implies non-zero EDM. There are several
approaches for the detectors, including silicon, micromegas and GEM detectors. CAPP is currently
focused on GEMs under the leadership of Dr. SeongTae Park.
So far, the pEDM collaboration has addressed most of the possible systematic errors and achieved
several candidate working lattices with stable beam and spin dynamics. The items like BPM and
polarimeter are currently under development. The experiment is a world-wide effort with
considerable contribution and leadership role from Korea. It aims to measure the proton EDM with
almost 105 better sensitivity than ever achieved. This is a very important experiment, since a
successful EDM measurement will show the existence of a stronger CP-violation source than
currently available from the SM, and will open the doors of “new physics”.
References
[1] Christenson, J. et.al. (1964). Evidence for the 2π decay of the K02 meson, Phys. Rev. Lett.,
13(4), 138–140.
[2] Aubert, B., et. al. (2001). Observation of CP Violation in the B0 Meson System, Phys. Rev.
Lett., 87(9), 091801 1–8.
[3] Kobayashi, M. and Maskawa, T. (1973). CP-Violation in the renormalizable theory of weak
interaction, Prog. of Th. Phys., 49(2), 652–657.
[4] Baker, C., et. al. (2006). Improved Experimental Limit on the Electric Dipole Moment of the
Neutron, Phys. Rev. Lett., 97, 131801.
[5] Altarev, I., et. al. (2015), A large scale magnetic shield with 106 damping at milliHertz
frequencies, J. Appl. Phys. 117, 183903
Figure 1: The proton beams will be injected into the ring with their spins polarized in the same direction with
momentum (t=0). They will be stored under in an all-electric ring. In case of a measurable EDM, vertical spin
component will grow over time. The rate of this grow is proportional to the proton EDM.
Figure 2: The ring will be composed of a sequence of deflectors and drift regions. The deflectors are essentially
concentric plates with some curvature. They will be separated by 3 cm, through which the beam will pass. The
deflectors will be separated by drift regions. The drifts will host the measurement and control units like quadrupoles,
BPMs, RF cavity etc. The figure is not to scale for better visibility.
Figure 3: The cross section of an electrostatic quadrupole with
the beam (shown in red) passing through. Quadrupoles are
utilized for focusing the beam around the design orbit (origin in
the cross section). It is ideally composed of four plates of
hyperbolic shape, but simpler geometries are also possible in
practice. A quad always focuses the beam in one direction
(vertical for this case) and defocuses in the other. So, both
focusing and defocusing quadrupoles are used in a typical
accelerator in an alternate focusing configuration. The direction
of focusing is determined by the sign of the potential on the
plates.
Figure 4: An RF cavity creates an oscillating electric field inside, in the
longitudinal direction. It is utilized for synchronizing the particles to the
ideal particle. This makes sure that the momentum of every particle will
average to the “magic momentum” (See Figure 5 for additional
information).
Figure 5: The longitudinal electric field in the RF cavity oscillates in the revolution
frequency of the ideal particle. Therefore the ideal particle comes to the cavity at zero phase.
However, the slow particle reaches the cavity in a positive phase and gains energy inside the
RF cavity. Similarly, the particle with larger momentum than the magic reaches the cavity at
the negative phase. Then, the RF cavity takes out some energy from the particle. This makes
sure that every particle has a magic momentum on average.
Figure 6: A computer aided design (CAD) drawing of shielding prototype. It is
basically composed of two layers of mu-metal, separated by a few cm. The length of
the prototype is 2.25 m and the outer radius is 65 cm. The beampipe will be covered
by these shielding layers.
Figure 7: CAD drawing of shielding prototype. It is basically composed of two
layers of mu-metal, separated by a few cm. The length of the prototype is 2.25 m
and the outer radius is 65 cm. The beampipe will be covered by theseFigure
shielding
15: Dr. Yong-Ho Lee
layers.
from KRISS.
Figure 14: Prof. Dr. Peter
Fierlinger.