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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.