Measurement of charged particle EDMs at FZJ
Introduction and background
Current research of the Institute for Nuclear Physics (IKP, Institut für Kernphysik) of Forschungszentrum Jülich (FZJ) focuses on two major items:
- preparations for the design and construction of the High Energy Storage Ring (HESR) and contributions to its internal detector system PANDA (see 3.2.3 ???), and
- hadron physics at the Cooler Storage Ring COSY-Jülich, exploiting the internal detector systems ANKE, PAX, and WASA.
Recent achievements at COSY include single polarized (ANKE, TOF, WASA) and double polarized (ANKE) scattering and meson production experiments. Although the hadron physics program at COSY will soon be finalized and the detector facilities ANKE, WASA and TOF decommissioned, we intend to continue the PAX activities by a test measurement with protons at COSY, examining longitudinal spin-filtering. Currently a significant and increasing fraction of the beam time available at COSY is used for tests of detector and accelerator components, mostly connected with the future FAIR facility.
As a new project IKP is pursuing tests and feasibility studies at COSY, aiming to search for electric dipole moments (EDM) of charged particles (proton, deuteron and 3He) in storage rings with maximum sensitivity. Major steps in R&D preparations have been achieved recently: thick carbon targets that sample the edge of the COSY beam have been shown to provide the high sampling efficiency and polarization sensitivity to allow searches with proton/deuteron beams down to the 10-29 e.cm level, which is a thousand times better than current neutron limits. Once the system is calibrated, systematic geometric and rate-dependent errors in the polarization measuring system can be corrected in real time to levels below 10-5. Tests also show that sextupole fields in the ring may be tuned to extend the spin coherence time to values greater than 200 s so that EDM signals may build up to a more easily observed level. This result was made possible by the commissioning of an event time-marking system that unfolds the 120-kHz ring-plane precession and makes possible measurements of the spin precession rate with a sensitivity better than 10-7.
Since EDM searches have been an exceptional physics case and COSY-Jülich is the only hadron storage ring with polarized beams worldwide, the EDM project can in the foreseeable future only be pursued there. The JEDI collaboration (Jülich Electric Dipole moment Investigations) has been formed in 2012 as part of the FAME (Forces and Matter Experiments) section of the Jülich-Aachen Research Alliance (JARA) to advance the project.
Electric Dipole Moments - Motivation and theoretical relevance
Electric dipole moments (EDM) break parity (P), time-reversal (T) symmetry, and charge-parity (CP) symmetry. Any measurement of a non-zero EDM will constitute a significant step into new territory, because the established Kobayashi-Maskawa mechanism of CP violation predicts EDMs orders of magnitude below from current experimental limits. The Standard Model Lagrangian contains another source of CP violation, the QCD vacuum angle (Θ-term), whose value is strongly bounded by neutron EDM experiments. The extreme smallness of |Θ| is a long-standing puzzle of the Standard Model (SM). Additionally, the universal matter/antimatter asymmetry implies that there should be CP-violation from physics beyond the Standard Model (BSM), which indeed appears in almost all SM extensions, but has not been observed yet. EDMs are excellent probes for these new CP-violating sources. An important open question remains: If an EDM is measured, is it caused by strong CP violation or from physics beyond the SM?
Experimental data on the EDMs of light nuclei can provide an answer to this question. First of all, it is clear that a single EDM measurement can be fitted by any source and, at least two experiments are needed to say something more about the origin of the CP violation. In order to determine which systems are most promising, in recent years several calculations, using modern effective-field-theory techniques, have been performed for EDMs of the nucleon and several light nuclei.
It has been shown that the QCD Θ-term could be identified with good accuracy, once measurements of the EDMs of the neutron, proton, and deuteron have been performed. For this source, the EDMs of these systems are all expected to be of the same order of magnitude, but the precise quantitative relation between the EDMs is a clear prediction of the Θ-term. In this way, the existence of strong CP violation can be convincingly determined, potentially solving a puzzle which has been around for almost fifty years.
Second, the size of the deuteron EDM, with respect to the neutron and proton EDM, is a good probe for BSM physics. As mentioned, the Θ-term predicts a deuteron EDM of similar size as the nucleon EDM, while certain BSM sources predict the deuteron EDM to be significantly larger, up to an order of magnitude. Such a signal, obtained in the envisioned storage ring experiment, would be a strong sign for physics beyond the SM. Currently, calculations are under way in which EDMs of light nuclei are calculated within specific well-motivated beyond-the-SM models. The goal is to demonstrate that measurements of the EDMs of the proton, deuteron, and perhaps 3He can disentangle these models.
Such calculations are complicated because the models are formulated at very high energy and then need to be evolved down to scales where experiments take place. Using a cascade of effective field theories such a calculation becomes possible and the EDMs can be expressed in terms of the parameters appearing in the high-energy models. Preliminary results confirm that different classes of models predict different hierarchies of EDMs and thus can be disentangled once several light-nuclear EDM experiments are performed successfully. Especially the deuteron EDM has large discriminating power, because of the unique spin-isospin properties of the deuteron.
Jülich Electric Dipole moment Investigations (JEDI)
Up to now experiments concentrated on neutral systems (neutron, atoms, molecules) | for charged hadrons, no direct measurements exist. This is due to the fact that charged particles are accelerated in electric fields and thus cannot be kept in small volumes like traps. Storage rings have to be used to perform these kinds of experiments. However, charged systems (proton, deuteron and 3He) are not only complimentary and most sensitive, but they are required to disentangle the different possible EDM source(s).
The principle of such measurements is rather simple: If an electric dipole moment exists, the spin vector will experience a torque in an external electric field resulting in a change of the original spin direction which can be determined with the help of a polarimeter. Alternatively, one can search for a tiny change of the spin precession frequency due to an EDM. In view of the necessary requirements, the cooler storage ring COSY at the Forschungszentrum Jülich with its capability to provide polarized protons and deuterons up to momenta of 3.7 GeV/c is an ideal starting point for a research and development program and a charged-particle EDM project. For an ultimate precision measurement, a new class of dedicated storage rings is required, which do not yet exist. Direct searches for proton and deuteron EDMs bear the potential to reach sensitivities of 10-29 e.cm per year of running.
Although the principle of the measurement is simple, the smallness of the expected effect makes this a very challenging experiment. The newly founded JEDI collaboration, together with the US-based storage ring EDM (srEDM) collaboration, is pursuing research and development to cover all scientific and technological challenges and to come up with concepts for precision EDM storage rings.
In a staged approach, see figure below, JEDI proposes to perform the first direct measurements of proton and deuteron EDMs at COSY using resonant techniques which involve static and radio frequency (RF) Wien-filters. A dedicated precision storage ring will be necessary to achieve the proposed sensitivities for charged particles.
The staged approach for measuring charged particle EDMs
as foreseen by the JEDI collaboration.
Concepts for EDM storage rings The spin precession is governed by the famous Thomas Bargmann-Michel-Telegdi (Thomas BMT) equation. The main challenge is that in general the spin precession due to the magnetic dipole moment (MDM) is many orders of magnitude larger than the spin precession expected from an EDM. The aim is thus to find electro-magnetic field configurations where the contribution due to the MDM vanishes, i.e., the spin vector does not precess and always points along the momentum vector in the absence of an EDM. This technique is called "frozen spin". For protons with their positive anomalous magnetic moment, this can be achieved with purely electric fields for a beam momentum of p = 700.74 MeV/c. For particles with negative anomalous magnetic moment (like deuterons and 3He) a combination of electric and magnetic fields has to be used. In either case a non-vanishing EDM results in a build-up of a vertical polarization component for a beam that was initially polarized in the horizontal plane. A purely electric ring for protons is proposed by the srEDM collaboration at Brookhaven National Laboratory (BNL, USA). A radial electric field of about 17 MV/m between field plates approximately 2 cm apart results in a ring with a bending radius of about 30 m.
The JEDI collaboration pursues a combined electric-magnetic machine with radial electric and vertical magnetic fields. By suitable combinations of the E- and B-fields, a ring with a radius between 10 and 30 m could be used for protons, deuterons and 3He nuclei. For both options the use of clock-wise and counter clock-wise beams is mandatory for the following reason: The main systematic error will come from an unwanted spin precession due to the MDM in radial magnetic fields which will be indistinguishable from the EDM signal. A radial magnetic field, however, causes forces in different directions for the beams in opposite directions and thus it can be controlled to a very high accuracy.
First direct EDM measurements at COSY Design and construction of a new dedicated storage ring constitutes a long term project (> 10 years). Intermediate steps to prove the principle and to provide a first direct measurement of EDMs are important, if not necessary. As mentioned above, the existing conventional storage ring COSY provides an ideal vantage point. COSY is a world-wide unique facility, which provides all basic ingredients necessary for EDM searches: (i) polarized proton and deuteron beams, (ii) spin manipulators, and (iii) polarimeters. In addition, a huge experience for beam and target polarization experiments has been accumulated over the years.
Thus, JEDI is planning to perform measurements of proton and deuteron EDMs at COSY. With additions or modifications to the existing machine, so-called precursor experiments can be performed, not aiming at the highest sensitivity, but providing a proof-of-principle and a demonstration of the potential of the storage-ring method.
Since COSY is a purely magnetic ring the spin precesses in the accelerator plane due to the MDM. The EDM leads to tiny oscillations of the vertical polarization around zero, but not to a polarization build-up. For the build-up of a vertical polarization, additional elements have to be installed into the ring. Here the JEDI collaboration pursues several options using either static or RF Wien-filters operating at the resonance frequency fWF = (k + γG) × frev, (k is an integer, γ the Lorentz factor, G the anomalous magnetic moment, and frev the revolution frequency). The estimated statistical sensitivity of this method is 10-24, and such measurements would be the first direct ones for charged hadron (p, d) EDMs. Imperfections in the alignment and field quality of the Wien-filter, and also in the alignment of the magnetic elements in COSY play an important role for the systematics involved. We have started to study these effects with simulations and R&D work described in the next subsection.
R&D work at COSY COSY is an indisputable asset to the worldwide storage ring EDM community where new ideas can be put to test using reliable, stored beams of polarized protons and deuterons. In anticipation of a deuteron storage ring, tests starting in 2008 have shown that thick carbon targets can provide continuous ring polarimeter operation with high efficiency (up to 1% of beam used for measurement) and large spin sensitivity (over 50% effective analysing power). Calibration of a polarimeter can include its sensitivity to geometric and rate effects, thus permitting the correction of such errors in real time and enabling the observation of polarization changes as small as one part per million. A reconfiguration of the EDDA detector to operate as such an EDM polarimeter has, along with time-marking events, made possible new studies that demonstrate the use of sextupole correction fields to lengthen the normally unstable horizontal polarisation lifetime to hundreds of seconds needed for an EDM experiment. Electron cooling and an RF-solenoid located in the COSY ring have been essential to the success of this effort. The technical development of high-field E/B deflectors (static and RF) and further improvements and extensions for high-precision spin dynamics simulations are covered by the program "Matter and Technology" in the topic "Accelerator Research and Development" (ARD). Here it is only mentioned that the collaboration has asked for and will obtain electrostatic de ectors from Tevatron (Fermilab) for research and development.
Simulation tools Full spin-tracking simulations of the entire experiment are absolutely crucial to explore the feasibility of the planned storage ring EDM experiments and to investigate systematic limitations. For a detailed study of particle and spin dynamics during the storage and build-up of the EDM signal, one needs to track a large sample of particles for billions of turns. Existing spin tracking codes like COSY-INFINITY have to be extended to properly simulate spin motion in the presence of an electric dipole moment. The appropriate EDM kick and electromagnetic field elements (static and RF) have to be implemented and benchmarked.
Furthermore, energy conservation must be enforced by adapting and verifying a symplectic description of fringe fields, field errors, and misalignments of magnets. In order to provide the required CPU time for the simulations of spin motion with a time scale larger than tens of seconds, spin tracking programs have been migrated to powerful Jülich supercomputers within the HPC (High performance computing) section of the Jülich Aachen Research Alliance (JARA). Benchmarking experiments at COSY will be performed to check and to further improve the simulation tools. In a next step, the analysis of systematic spin rotations will be carried out. Spin tracking for a first measurement of a charged particle EDM in a storage ring can be performed to investigate the sensitivity of the proposed method. Finally, the layout of a dedicated storage ring has to be optimized by a full simulation of spin motion.
Investigations of the spin coherence time (SCT) and systematic effects Major steps towards a first direct EDM measurement contain preparatory measurements at COSY, focusing in particular on investigations of the spin coherence time (SCT) and on systematic effects.
Studies currently in progress are exploring the sensitivity limits set by small oscillations of the particles in the beam for the use of RF electromagnetic fields in the first search for a charged particle EDM. An important component will be the use of beam cooling and higher-order field corrections to increase the sensitivity using RF spin flippers.
It is conceivable that substantial improvements of COSY are required in order to reach the desired EDM sensitivities. False spin rotations as a function of closed-orbit excitation, quadrupole alignment and ring imperfections will be studied. The aim of this part is to reduce systematic errors and the results will provide the basis to specify the required COSY modifications: orbit correction steerer system and beam position monitors, power supply stability, magnet alignment and ring impedances. An improved closed-orbit control system for orbit correction in the micrometer range is necessary, which requires increasing the stability of correction dipole power supplies by at least one order of magnitude. The number of correction dipoles and beam position monitors has to be increased significantly, since the orbit has to be controlled along the entire path length of the beam in the COSY ring. The BPM accuracy, presently limited by electronic offset and amplifier linearity, has to be substantially improved as well. Systematic errors of the orbit measurement (e.g., temperature drift, beam current dependence) have to be studied in detail. The alignment of COSY magnets has to be verified.
Polarimeter development The polarimetry is done by scattering the stored particles off a target. To determine the transverse spin direction of the projectile the analysing power of the scattering process has to be known. The analysing power depends on the scattering process or reaction as well as its kinematics, and the polarimeter must be designed to select that subset of events which offers the highest statistical leverage for detecting polarization. Favourable target choices include protons and deuterons as well as carbon nuclei. Several detector technologies will be tested to determine their suitability for storage ring application.
Summary The outcome of the investigations, tests, and experiments of the JEDI collaboration at COSY during PoF 3 will be to establish the proof-of-principle of the storage-ring chargedparticle EDM search with a first direct EDM measurement for protons and deuterons of the order of 10-24 e cm. In addition a major deliverable will be the concept for a dedicated precision EDM storage ring in form of a conceptual design report (CDR) or possibly even a technical design report (TDR).