Electric dipole moment searches using storage rings
EElectric dipole moment searches using storagerings
Frank Rathmann ∗† Institute for Nuclear Physics (IKP), Forschungszentrum Jülich GmbH, 52428 Jülich, GermanyE-mail: [email protected]
Nikolai N. Nikolaev
L.D. Landau Institute for Theoretical Physics, 142432 Chernogolovka, RussiaE-mail: [email protected]
The Standard Model (SM) of Particle Physics is not capable to account for the apparent matter-antimatter asymmetry of our Universe. Physics beyond the SM is required and is either probedby employing highest energies (e.g., at LHC), or by striving for ultimate precision and sensitivity(e.g., in the search for electric dipole moments). Permanent electric dipole moments (EDMs) ofparticles violate both time reversal ( T ) and parity ( P ) invariance, and are via the CPT -theoremalso CP -violating. Finding an EDM would be a strong indication for physics beyond the SM, andpushing upper limits further provides crucial tests for any corresponding theoretical model, e.g.,SUSY.Up to now, EDM searches focused on neutral systems (neutrons, atoms, and molecules). Stor-age rings, however, offer the possibility to measure EDMs of charged particles by observing theinfluence of the EDM on the spin motion in the ring. Direct searches of proton and deuteronEDMs, however, bear the potential to reach sensitivities beyond 10 − e cm. Since the CoolerSynchrotron COSY at the Forschungszentrum Jülich provides polarized protons and deuteronsup to momenta of 3.7 GeV/c, it constitutes an ideal testing ground and starting point for such anexperimental program.Besides the discussion of the achievements of the JEDI collaboration, and the description of aneffort to perform a first direct deuteron EDM measurement at COSY, the report highlights inaddition future technical developments that will pave the way toward EDM searches in dedicatedrings. A recent advancement that grew out of the successful work performed by JEDI is theformation of the CPEDM Collaboration ‡ , which aims at the design of an EDM prototype ring thatcould be hosted either at CERN or at COSY, will be discussed as well. ∗ Speaker. † for the JEDI collaboration http://collaborations.fz-juelich.de/ikp/jedi/ (Jülich ElectricDipole moment Investigations) ‡ Charged Particle Electric Dipole Moment Collaboration http://pbc.web.cern.ch/edm/edm-default.htm c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] A p r lectric dipole moment searches using storage ring Frank Rathmann
1. Introduction
Electric dipole moments (EDMs) are one of the keys to understand the origin of and the baryo-genesis in our Universe. In 1967 Andrei Sakharov formulated three conditions for baryogenesis [1]:1. Early in the evolution of the Universe, the baryon number conservation must be violatedsufficiently strongly.2. The C and CP invariances, and T invariance thereof, must be violated.3. At the point in time when the baryon number is generated, the evolution of the Universe mustbe out of thermal equilibrium. CP violation in kaon decays is known since 1964, it has been observed in B -decays and in charmedmeson decays, and based on the existing data can be described by the CP -violating phase in theCabibbo-Kobayashi-Maskawa matrix [2, 3]). CP and P violation entail non-vanishing P and T vio-lating EDMs of elementary particles. Although extremely successful in many aspects, the StandardCosmological Model (SCM) has one pronounced weaknesses; it fails miserably in the expectedbaryogenesis rate. The observed baryon asymmetry η of the Universe is expressed via η = ( n b − n ¯ b ) / n γ , (1.1)where n b and n ¯ b denote the number of baryons and anti-baryons, and n γ the number of relicphotons. The discrepancy between observation and expectation from the Standard CosmologicalModel (SCM) amounts to about 9 orders of magnitude (see Table1). η = ( n b − n ¯ b ) / n γ Observation (cid:0) . + . − . (cid:1) × − Best Fit Cosmological Model [4] ( . − . ) × − WMAP [5]Expectation from SCM ∼ − Bernreuther (2002) [6]
Table 1:
Observation and expectation from Standard Cosmological Model (SCM).
Simultaneously, the SM predicts exceedingly small electric dipole moments of nucleons 10 − < d n < − e cm [7], way below the current upper bound for the neutron EDM, which is d n (cid:47) . × − e cm [8], and also beyond the reach of future EDM searches [9].In the quest for physics beyond the SM one could follow either the high energy trail or lookinto new methods which offer very high precision and sensitivity. Supersymmetry is one of themost attractive extensions of the SM. The SUSY predictions span typically a range of 10 − < d n < − e · cm and precisely this range is targeted in the new generation of EDM searches [9],discussed here.There is consensus among theorists that measuring the EDM of the proton, deuteron and he-lion is as important as that of the neutron [10, 11]. The EDMs could have a non-trivial isospindependence and d d (cid:54) = d p + d n , even if the CP -violation comes from the isoscalar QCD θ -term [12].Furthermore, it has been argued some 25 years ago that T -violating nuclear forces could substan-tially enhance nuclear EDMs [13, 14]. At the moment, there are no significant directly determinedupper bounds available on d e , d p and d d . The current status of EDM searches is reflected in Table 2.1 lectric dipole moment searches using storage ring Frank Rathmann
Particle Current limit Goal d n equivalent goal Date [ref] Electron < . × − ≈ − < . × − < × − < × − ( − . ± . ) × − ≈ − − Hg < . × − − < . × − [19] 2016 [20] Xe < . × − ≈ − to 10 − ≈ − to 10 − < × − ≈ − − ≈ − ≈ × − to 5 × − Table 2:
Current limits, goals and d n equivalent goals for various particles.
2. Charged particle EDM searches using storage rings
The experimental requirements for charged particle EDM searches using storage rings are verydemanding and require the development of a new class of high-precision, primarily electric storagerings. Precise alignment, stability, field homogeneity, and shielding from perturbing magnetic fieldsplay a crucial role. Beam intensities around N = × particles per fill with a polarization of P = . E =
10 MV / m and long spin coherence times ofabout τ SCT = A y (cid:39) . f (cid:39) .
005 need to be provided. In terms of the above numbers,this would lead to statistical uncertainties of σ stat = h √ N f τ SCT
P A y E ⇒ σ stat ( ) = . × − e cm , (2.1)where for one year of data taking 10 000 cycles of 1000 s duration is assumed. The experimentalist’sgoal must be to provide systematic uncertainties σ syst to the same level. In the rest frame of the particle in a storage ring, the equation of motion for the spin vector (cid:126) S in the presence of an electric field (cid:126) E and magnetic field (cid:126) B can be written asd (cid:126) S d t = (cid:126) Ω × (cid:126) S = (cid:126) µ × (cid:126) B + (cid:126) d × (cid:126) E , (2.2)where µ denotes the magnetic moment, and d the electric dipole moment. The spin precessionfrequency of a particle on the closed orbit due to its magnetic dipole moment (MDM) relative tothe direction of flight can be expressed as (cid:126) Ω = (cid:126) Ω MDM − (cid:126) Ω cyc = − q γ m (cid:34) G γ (cid:126) B ⊥ + ( + G ) (cid:126) B (cid:107) − (cid:18) G γ − γγ − (cid:19) (cid:126) β × (cid:126) Ec (cid:35) . (2.3)2 lectric dipole moment searches using storage ring Frank Rathmann (cid:126) Ω = frozen spin , because in this case momentum and spin stay aligned. In the absenceof magnetic fields ( B ⊥ = (cid:126) B (cid:107) = (cid:126) Ω = , if (cid:18) G γ − γγ − (cid:19) = . (2.4)This can be realized only for particles with G >
0, such as proton ( G p = . G e = . magic momentum p magic G p − γ − = ⇔ G p = m p ⇒ p magic = m (cid:112) G p = .
740 MeV c − . (2.5)Storing protons in a ring with purely electrical deflection elements at magic momentum freezesthe horizontal spin precession, i.e. , the proton spins remain aligned along the direction of flight. In apurely electric machine with (cid:126) B =
0, Eq. (2.2) then implies the development of a vertical polarizationcomponent p y ( t ) . The derivative of which is proportional to the electric dipole moment. Here itshould be noted that freezing the spin precession works for any spin orientation. Obviously, thehighest sensitivities can be reached when (cid:126) d and (cid:126) E are orthogonal, hence when (cid:126) d points along themomentum.Magic machines for light ions with frozen spin can be envisioned to allow for a measurementusing different particle types. The general solution for the ratio of outward electric field E x to thevertical magnetic field B y fulfilling the magic condition, derived from the Thomas-BMT equation,can be expressed as E x B y = Gc β γ − G β γ , (2.6)(right-handed coordinate system, with z along beam direction). Equating the Lorentz force andthe relativistic centrifugal force, yields then for a specific radius the required electric and magneticfields. The required parameters for electric and magnetic field for a circular machine with radius r =
25 m are listed in Table 3.particle
G p [ MeV c − ] T [ MeV ] E x [ MV m − ] B y [ T ] proton 1 .
793 700 .
740 232 .
792 16 .
772 0 . − .
143 1000 .
000 249 . − .
032 0 . − .
184 1200 .
000 245 .
633 14 . − . Table 3:
Example for frozen spin conditions for protons, deuterons and helions with and without magneticfields for a circular machine with radius r = .
000 m using Eq. (2.6).
Measurement of the EDM of protons, deuterons and helions can be anticipated to take placein one and the same machine.
3. Progress toward storage ring EDM experiments
The COoler SYnchrotron COSY has been formerly used as spin-physics machine for hadronphysics experiment. It provides phase-space cooled internal and extracted beams of polarized3 lectric dipole moment searches using storage ring
Frank Rathmann protons and deuterons at momenta of p = . . / c. Since about 2012, COSY is heavilyused to complement the spin-physics tool box for storage ring EDM experiments, as it providesan ideal starting point for accelerator related R&D. In addition, as will be outlined below, COSYwill be used to carry out a first direct measurement of deuteron EDM. Figure 1 shows the maininstallations presently in use for this purpose at COSY. Figure 1:
Landscape of COSY with the main installations employed to perform a first direct measurementof the deuteron EDM.
The JEDI collaboration developed a new technique to determine the spintune ν s in a ma-chine [22]. The spin tune ν s is determined to about 10 − in a 2 s time interval, and in a 100 s cycleat t ≈
38 s, the relative uncertainty of the spin tune amounts to ∆ ν s / ν s ≈ − . With this, a newprecision tool for accelerator physics has become available to study systematic effects in a storagering, e.g. , the long term stability of an accelerator. One of the main obstacles for any storage ring EDM experiment is the decoherence of thein-plane polarization. Using sextupole magnets to correct higher order effects, in 2014 at COSYspin coherence times (SCT) of about τ SCT ≈
400 s could be reached [23]. Since 2016, typicalvalues routinely exceeding τ SCT =
800 s are available [24]. This pronounced progress has not beenanticipated. It should be emphasized that large spin coherence times are of particular importance,because σ stat ∝ τ SCT − (see Eq. (2.1)). In a machine with purely magnetic deflection and focusing like COSY, it is not possible tofreeze the spins. Using an RF device that operates on a harmonic of the spin-precession frequencyis the only possible approach toward an EDM measurement in COSY. In order to achieve a good4 lectric dipole moment searches using storage ring
Frank Rathmann precision for such a measurement, phase-locking is necessary, making sure that phase betweenthe spin-precession and the device RF is maintained throughout the measurement. To this end,a feedback system has been developed that stabilizes the phase of the spin precession relative tothe phase of an RF devices, providing a so-called phase-lock . The feedback system maintainsthe resonance frequency, and the phase between spin precession and device RF ( e.g. , solenoidor Wien filter). As a major achievement, an error of the phase-lock of σ φ = .
21 rad has beenachieved [25, 26].In the presence of a long spin-coherence time, phase-locking of the in-plane polarization canbe viewed as providing a co-magnetometer for the resonant buildup of a vertical polarization com-ponent using an RF Wien filter (cf. Sec. 5).
Precision experiments, such as the search for electric dipole moments of charged particles us-ing storage rings, demand for an understanding of the spin dynamics with unprecedented accuracy.As the ultimate aim is to measure the electric dipole moments with a sensitivity up to 15 ordersin magnitude better than the magnetic dipole moment of the stored particles. For this reason, thebackground to the signal of the electric dipole from rotations of the spins in spurious magneticfields of the storage ring must be understood. One of the observables, especially sensitive to theimperfection magnetic fields in the ring is the angular orientation of stable spin axis. For the firsttime, the JEDI collaboration succeeded to determine experimentally the stable spin axis. A newmethod called spin tune mapping was developed, and the angular orientation of the stable spin axisat two different locations in the COSY ring has been determined to an unprecedented accuracy ofbetter than 2 .
4. Technical challenges and developments
Charged particle EDM searches require development of a new class of high-precision ma-chines with mainly electric fields for bending and focusing. Some of the technical challengesinvolved in this will be discussed in the following sections: • Spin coherence time τ SCT ∼ • Large electric field gradients ∼
10 to 20 MV / m (see Sec. 4.2). • Beam position monitoring with precision of 10 nm (see Sec. 4.3). • Continuous polarimetry with relative errors < • Magnetic imperfections (see Sec. 4.6). • Prototype EDM storage ring (see Sec. 4.7). • Alignment of ring elements, ground motion, ring imperfections.5 lectric dipole moment searches using storage ring
Frank Rathmann (a) 64t dipole magnet. (b) Setup to be installed in the magnet chamber.
Figure 2:
The magnet can produce up to B max = . h g =
200 mm. The electrodelength is (cid:96) = h e =
90 mm, and the electrode spacing S =
20 to 120 mm. Themaximum applied voltage field U = ±
200 MV. Foreseen material is aluminum coated by TiN. • For deuteron EDM with frozen spin: precise reversal of magnetic fields for CW and CCWbeams required. E / B deflector development In the framework of the CPEDM collaboration , a prototype EDM storage ring is presentlybeing developed (see Sec. 4.7). In conjunction with this development, electrostatic deflector ele-ments are being designed that provide radial electric fields. Combined elements that generate inaddition vertical magnetic fields are being developed as well.The development takes place in two stages that are jointly organized by IKP of Forschungszen-trum Jülich and RWTH Aachen University. In stage 1, a laboratory setup, developed at RWTHAachen, employs scaled-down electrodes. The purpose of this investigation is to identify potentialmaterials, coatings and surface treatment that can be applied in order to achieve high electric fields.With a 30 kV power supply, and appropriately reduced distances of up to a few mm between theelectrodes, large electric fields of interest can be achieved. First results using polished stainlesssteel electrodes are reported in [29].Stage 2 of the deflector development program aims at tests with real-size deflector elements ofa length of about (cid:96) = Storage ring EDM experiments require very precise orbit measurements along the circumfer-ence of the ring. The JEDI collaboration has begun to develop a new type of compact beam-position Charged Particle Electric Dipole Moment Collaboration http://pbc.web.cern.ch/edm/edm-default.htm lectric dipole moment searches using storage ring Frank Rathmann (a)
Conventional split-cylinder beam-position monitor withan installation length of ≈
20 cm. (b)
Rogowski pickup based on a seg-mented toroidal coil.
Figure 3:
The main advantage of the Rogowski design is that with a toroid diameter of d t =
10 cm, and acoil diameter d c = monitor based on a segmented Rogowski coil [30].The main advantage of this design is a short installation length of ≈ dC polarimetry data base Due to the large analyzing power and differential cross section in the forward region, dC elasticscattering constitutes a well-suited polarimeter reaction for deuteron EDM measurements. In orderto provide precise input to Monte-Carlo simulations for an optimized beam polarimeter design, theanalyzing powers and the differential cross sections were measured at six different deuteron beamkinetic energies in the range of 170 MeV to 380 MeV [32, 33] Up to now, the EDM-related COSY experiments, carried out by the JEDI collaboration, em-ployed existing detector installations as polarimeters ( e.g. , EDDA [34] and WASA [35, 36]). A fewyears ago, the decision was taken to develop a high-precision beam polarimeter with an internalCarbon target based on LYSO scintillation material.This detector material, produced by Saint-Gobain Ceramics & Plastics , is a Cerium dopedLutetium based scintillation crystal, Lu . Y . SiO :Ce. Compared to NaI, LYSO provides higherdensity (7.1 vs 3 .
67 g / cm ), and a very fast decay time (45 vs 250 ns) [37]. After several commis-sioning runs with external beam, the detector system will be installed at COSY in 2019. lectric dipole moment searches using storage ring Frank Rathmann
JEDI developed a new method to investigate magnetic machine imperfections based on thehighly accurate determination of the spin-tune. This spin-tune mapping technique used the twoavailable cooler solenoids of COSY as (makeshift) spin rotators to generate artificial imperfectionfields. The measurement of the shifts of the spin tune as function of the spin kicks of the twosolenoids yields the map [27, 38].The location of the saddle point of the map determines the tilt of the stable spin axis causedby the magnetic imperfections. It is possible to control the background to the direction of thestable spin axis (cid:126) c from magnetic dipole moment rotations at a level ∆ c = . × − rad [27].The systematics-limited sensitivity for a deuteron EDM measurement at COSY amounts to σ d ≈ − e cm. In view of the various technical challenges involved in building the final all-electric ring, as e.g. , described in [39], as next step, the CPEDM collaboration decided to design and build a demon-strator ring for charged-particle EDM searches. The new CPEDM collaboration, which evolved outof the success and the achievements of JEDI, brings together scientists from CERN and the JEDIcollaboration. The project is part of the Physics Beyond Collider (PBC) process presently carriedout at CERN, and the European Strategy for Particle Physics Update. A possible host site for theprototype EDM storage ring is either COSY or CERN.The scope of the project is to provide for protons at a kinetic energy of T =
30 MeV an all-electric machine operation with simultaneous clockwise (CW) and counter-clockwise (CCW) or-biting beams of the machine. The circumference of the machine is about 100 m. At T =
45 MeVusing vertical magnetic fields superimposed on the radial electric fields in the deflector elements,frozen-spin operation for protons shall be possible. Items to be studied with the prototype ringinclude: • Storage time investigations, • CW/CCW operation. • Spin coherence time studies. • Polarimeter studies. • Studies of magnetic moment effects due to imperfect shielding and artificially introducedmagnetic fields. • A direct measurement of the EDM of the proton. • Tests of stochastic cooling.Further details about this project can be found in a contribution to these proceedings [40].8 lectric dipole moment searches using storage ring
Frank Rathmann
5. Proof of principle EDM ( precursor ) experiment using COSY
Highest EDM sensitivity shall be achieved with a new type of machine, namely with an elec-trostatic circular storage ring, where the centripetal force is produced by electric fields. This E fieldcouples to the EDM of the orbiting particles and provides the desired sensitivity ( < − e cm). It isobvious that in such an environment, magnetic fields mean evil, since the MDM ( µ N = e ¯ h / m N c ≈ − e cm) is vastly larger than the EDM we are after.The idea behind such a proof-of-principle experiment (so-called precursor experiment) is touse a novel (cid:126) E × (cid:126) B RF Wien filter [41, 42] to accumulate the EDM related spin rotation in order tomake them measurable. In a magnetic machine, the particle spins precess about the local stablespin axis. In an ideal machine, this axis corresponds to the vertical ( y ) direction of the magneticfields (cid:126) B dipole ∝ (cid:126) e y in dipole magnets. In this situation, an RF device that is operating on someharmonic of the spin-precession frequency can be used to accumulate the EDM effect as functionof time in the cycle, provided the particle ensemble is coherently precessing in the horizontal plane(see Sec. 3.2).In order for the RF system of the Wien filter to stay tuned precisely on a harmonic of the spin-precession frequency, a phase-lock between the spin-precession of the particle ensemble in thering and the RF of the Wien filter is needed, as described in Sec. 3.3. The horizontally precessingpolarization serves as a co-magnetometer for the buildup of the vertical polarization (EDM) signal.The goal of the experiment is to show that a conventional magnetic storage ring can be employedto obtain a first direct EDM measurement of the deuteron (or proton). The technical realization and a report about the commissioning of the RF Wien filter [43] atCOSY is available in these proceedings in Ref. [31]. Two additional aspects of this developmentshall be mentioned here.Mechanical tolerances and misalignments decrease the simulated field quality of the RF Wienfilter, and it is therefore important to consider them in the simulations. In particular, for the EDMmeasurement, it is important to quantify these field errors systematically. Since Monte-Carlo sim-ulations are computationally very expensive, an efficient surrogate modeling scheme based on thePolynomial Chaos Expansion method to compute the field quality in the presence of tolerances andmisalignments has been developed, which was subsequently used to perform a sensitivity analysisof the RF Wien filter at zero additional computational cost [44].We have developed an implementation of the polynomial chaos expansion as a fast solver ofthe equations of beam and spin motion of charged particles in electromagnetic fields, and it could beshown that, based on the stochastic Galerkin method , this computational framework substantiallyreduces the required number of tracking calculations compared to the widely used Monte Carlomethod [46]. The Galerkin method [45] constitutes one of the many possible finite element method formulations that can be usedfor discretization. lectric dipole moment searches using storage ring Frank Rathmann -3 -3-2-101234 10 -4 Figure 4:
Model calculation of the buildup of a vertical polarization component P y ( t ) for four differentEDMs ranging from 10 − to 10 − and the conditions as indicated with an initial polarization (cid:126) P ( t = ) =( , , ) . In order to make things visible on this timescale, a field amplification factor of f ampl = EDM induced vertical polarization oscillations in an experimental situation with an RF Wienfilter can generally be described by p y ( t ) = a sin ( Ω p y t + φ RF ) . (5.1)The associated EDM resonance strength ε EDM can be defined as the ratio of angular frequency Ω p y relative to the orbital angular frequency Ω rev in the machine, ε EDM = Ω p y Ω rev . (5.2)The term “EDM” in Eq. (5.2) applies to the case that only the EDM contributes to Ω p y . Inpractice, the resonance strength will receive contributions from other sources, such as rotations ofthe RF Wien filter and solenoidal fields in the ring that generate unwanted spin kicks.A model calculation of the polarization buildup, essentially showing only the very beginningof the polarization oscillation, is shown in Fig. 4. Actually, the term buildup is meant here as anout-of-plane rotation of the initial purely in-plane polarization due to the presence of either an EDMand/or unwanted MDM rotations due to field imperfections and a non-ideal closed orbit in the ring,because ideally, the magnitude of the polarization | (cid:126) P ( t ) | remains constant.The model calculation reflects the situation of an ideal COSY ring with stored deuterons at p d =
970 MeV / c ( G d = − . γ = . f s = f rev ( γ G ± K ) ≈ .
765 kHz for K = . (5.3)10 lectric dipole moment searches using storage ring Frank Rathmann zx y (cid:126) P ( t ) (cid:126) P xz ( t ) (cid:126) P y ( t ) α ( t ) φ RF Figure 5:
The deuteron spins are precessing in the horizontal ( xz ) plane and the RF Wien filter is running onthe corresponding frequency with a certain RF phase φ RF that is maintained using the phase-locking system,discussed in Sec. 3.3. As can be seen in Fig. 4 the oscillation due to the spin precession is vastly faster than the changed P y ( t ) / d t due to the EDM. The electric field integral assumed in the model calculation is f ampl × (cid:90) E WF · d (cid:96) ≈ , where f ampl = . (5.4)The assumed electric and magnetic fields are by a factor 1000 larger than the fields of the real WFoperated (without ferrites) at an input power of 1 kW [43]. Alternatively, ε EDM can be determined from the measured initial slopes ˙ p y ( t ) | t = of the polar-ization buildup through a variation of the RF phase φ RF using the phase-lock (see Sec. 3.3): ε EDM = ˙ p y ( t ) | t = a cos φ RF · Ω rev . (5.5)One can show that the evaluation of ε EDM from Eqs. (5.2) and (5.5) is equivalent if | (cid:126) P | = i.e. , ˙ p y ( t ) = ˙ α ( t ) . This situation is indicated in Fig. 5.The first measurements of EDM-like buildup signals by JEDI are shown in Fig. 6. Both plotsshow the rate of the out-of-plane rotation angle ˙ α ( t ) | t = as function of the Wien filter RF phase( φ RF ) for different rotations of the RF Wien filter around the beam axis ( φ WFrot ) and different spinrotations in the Snake solenoid ( χ Snakerot ). For these measurements, the B field of RF Wien filter isoriented normal to the ring plane (along (cid:126) e y ). The RF Wien filter was operated at f WF =
871 kHz.Variations of φ WFrot and χ Snakerot affect the pattern of observed initial slopes ˙ α . During the measure-ments shown in Fig. 6, the magnets of the electron cooler were switched off altogether on flattop toreduce unwanted spin precessions of the stored particles in the cooler magnets.Further details about this type of EDM measurement are discussed in the contribution to theseproceedings by Alexander Nass [31]. As next step the first EDM production run using COSY isscheduled for Nov.-Dec. 2018. 11 lectric dipole moment searches using storage ring Frank Rathmann (a) ˙ α for φ WFrot = − ◦ , 0 ◦ , + ◦ and χ Snakerot = (b) ˙ α for χ Snakerot = −
1, 0, + ◦ and φ WFrot = Figure 6:
Rate of the out-of-plane rotation angle ˙ α ( t ) | t = as function of the Wien filter RF phase φ RF fortwo situations. In panel (a), only the RF Wien filter is rotated around the beam axis, and in (b) only theSiberian snake solenoid in the opposite straight section of COSY (see Fig. 1) rotates the spins around thebeam axis.
6. Axion-EDM search using storage ring
The motivation to search for oscillating axion-EDMs using storage rings is derived from arecent paper by Graham and Rajendran [47]. An oscillating axion field couples to gluons andinduces an oscillating EDM in hadronic particles. The measurement principle relies on the factthat when the oscillating EDM resonates with the particle g − (cid:126) v × (cid:126) B ), the sensitivity is improved significantly. Limits for axion-gluon couplings tooscillating EDMs are discussed in Ref. [48].Without any additional equipment, a measurement of axion-like oscillating EDMs can be real-ized in the magnetic storage ring COSY. A proposal for a test beam time has been accepted by theCOSY Beam time Advisory Committee. A first experiment is scheduled for the first half of 2019.
7. Summary
The search for charged particle EDMs offers a new window to disentangle sources of CP violation, and to possibly explain the matter-antimatter asymmetry of the Universe. The JEDIcollaboration is making steady progress in the field of spin dynamics of relevance to future searchesof EDMs. For these investigations, COSY remains to be a unique facility.The first direct JEDI deuteron EDM measurement at COSY is well underway. A first run tookplace in Nov.-Dec. 2018, a second run shall take place at the end of 2019. The anticipated deuteronEDM sensitivity of the measurements is about 10 − to 10 − e cm.There is a strong interest of the high-energy physics community in storage ring searches for theEDM of protons and light nuclei as part of physics program of the post-LHC era. In the framework Available from http://collaborations.fz-juelich.de/ikp/jedi/public_files/proposals/Axion_Search_at_COSY.pdf lectric dipole moment searches using storage ring Frank Rathmann of the recently formed CPEDM Collaboration that evolved out of JEDI, the design of a 30 MeVall-electric EDM prototype storage ring is being prepared. Possible hosts are CERN or COSY.
Acknowledgments
This work is supported by an ERC Advanced-Grant of the European Union (proposal number694340).
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