In this paper we discuss theoretical motivations and the status of experimental searches to find time-reversal symmetry-violating electric dipole moments (EDM). Emphasis is given to a next generation search for the EDM of the

neutron, which is currently being set up at the FRM II neutron source in Garching, with an ultimate sensitivity goal of 5 × 10−28 cm (3σ). The layout of the apparatus

allows for the detailed investigation of systematic effects by combining various means of magnetic field control and polarized UCN optics. All major components of the

installations are portable and can be installed at the strongest available UCN beam

Altarev, I.

Beck, D. H.

Chesnevskaya, S.

Chupp, T.

Feldmeier, W.

Fierlinger, P.

Frei, A.

Gutsmiedl, E.

Kuchler, F.

Link, P.

Lins, T.

Marino, M.

McAndrew, J.

Paul, S.

Petzoldt, G.

Pichlmaier, A.

Stoepler, R.

Stuiber, S.

Taubenheim, B.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2012-11271-0Colloquia: PAVI11IL NUOVO CIMENTO Vol. 35 C, N. 4 Luglio-Agosto 2012A next generation measurement of the electric dipole momentof the neutron at the FRM III. Altarev(1), D. H. Beck(2), S. Chesnevskaya(1), T. Chupp(3),W. Feldmeier(1), P. Fierlinger(1), A. Frei(4), E. Gutsmiedl(1), F. Kuchler(1),P. Link(4), T. Lins(1), M. Marino(1), J. McAndrew(1), S. Paul(1),G. Petzoldt(1), A. Pichlmaier(4), R. Stoepler(1), S. Stuiber(1)and B. Taubenheim(1)(1) Physikdepartment Technische Universita¨t Mu¨nchen and Excellence-Cluster “Universe”Germany(2) University of Illinois at Urbana-Champaign, Department of PhysicsUrbana, IL 61801, USA(3) University of Michigan, Randall Laboratory - Ann Arbor, MI 48109, USA(4) Technische Universita¨t Mu¨nchen, Forschungsneutronenquelle Heinz Maier-LeibnitzGermanyricevuto il 14 Ottobre 2011; approvato il 5 Maggio 2012pubblicato online il 26 Giugno 2012Summary. — In this paper we discuss theoretical motivations and the statusof experimental searches to find time-reversal symmetry-violating electric dipolemoments (EDM). Emphasis is given to a next generation search for the EDM of theneutron, which is currently being set up at the FRM II neutron source in Garching,with an ultimate sensitivity goal of 5× 10−28 cm (3σ). The layout of the apparatusallows for the detailed investigation of systematic effects by combining various meansof magnetic field control and polarized UCN optics. All major components of theinstallations are portable and can be installed at the strongest available UCN beam.PACS 21.10.Ky – Electromagnetic moments.PACS 24.80.+y – Nuclear tests of fundamental interactions and symmetries.PACS 11.30.Cp – Lorentz and Poincare´ invariance.PACS 11.30.Er – Charge conjugation, parity, time reversal, and other discretesymmetries.1. – IntroductionA non-zero value of an electric dipole moment (EDM) [1] of a fundamental systemwould represent a manifestation of yet unknown time reversal symmetry (T) violation.Assuming the conservation of the combined operation of CPT, T violation in a funda-mental system also violates CP [2, 3]. Only few experiments could provide data on thisc© Societa` Italiana di Fisica 122A NEXT GENERATION MEASUREMENT OF THE ELECTRIC DIPOLE MOMENT ETC. 123Table I. – Predicted EDM values and measured upper limits for different systems.System Predicted EDM (ecm) Measured (ecm)Neutron ∼ 10−32±1(SM) dn < 2.9 · 10−26(90%) [13]∼ 10−26–10−28 (SUSY)Electron ∼< 10−38(SM) de < 1.05 · 10−27(90%) [14]Nuclear EDM: 199Hg ∼ 10−33–10−34(SM) dHg < 3.1 · 10−29(95%) [15]phenomenon which can be observed in Weak Decays involving quarks [3,4], as describedby the Cabbibo-Kobayashi-Maskawa (CKM) matrix [5]. However, there are many openquestions remaining. In this context, the measurement of an EDM is considered one ofthe most important experiments in this field [6]. Electric dipole moments of baryonicsystems can be also used to search for the CP violating product of the gluon operatorsG and G˜ in the QCD Lagrangian. This term is weighted with the parameter θ, whichis already strongly restricted by measurements to θ < 10−10 [7] and is considered un-naturally small and called the strong CP problem. Measured CP violation also does notaccommodate for the observed ratio of baryons to photons, related to the ratio of baryonsto anti-baryons in Baryogenesis models [8], by ∼ 9 orders of magnitude. It was pointedout by Sakharov [9] that, assuming the conservation of CPT, the explanation requiresnext to baryon number violation and departure from thermal equilibrium also additionalsources of CP violation. A prominent example for an EDM search is the neutron, butalso nuclei or electrons can have permanent electric dipoles [10]. The size of EDMs ispredicted by the electroweak SM based on contributions of three loop level and is thusvery small, e.g. for neutron this is on the order of dn ∼ 10−32 ecm [11]. Supersymmetry(SUSY) and most other possible extensions of the SM give rise to larger values of EDMs,as they arise already at one loop level (e.g., ref. [12]). Neutrons are favored systems dueto their comparably simple composition and due to the possibility to actually performextremely precise measurements. Table I shows the predicted and measured values foran EDM of selected systems.The Hamiltonian of such a system is(1) h¯ω = −S|S|(μB − d E),with μ the magnetic moment and d the electric dipole moment. The EDM is extractedby sequentially measuring the Larmor precession ω. A phase Δω between measurementswith parallel and anti-parallel orientations of B and E corresponds to the EDM:(2) d =h¯Δω4E.Most neutron EDM experiments deploy trapped ultra-cold neutrons (UCN) and Ram-sey’s method of separated oscillating fields, effectively forming an interferometer in time:polarized UCN start out aligned parallel to a constant magnetic field B0. A stable clockis operated outside the apparatus. An oscillating field B1 aligned normal to B0 referencedto the external clock rotates the spin into a plane normal to B0. While the oscillatorkeeps running, the spin precesses freely for a time T with an additional E-field alignedeither parallel or anti-parallel to B0. After a second coherent π/2-pulse, the polarizationis rotated back from the precession plane into the direction of B0. The phase caused by124 I. ALTAREV, D. H. BECK, S. CHESNEVSKAYA, ETC.the electric field is then analyzed. In many cases a field of B0 ∼ 1μT (ωL ∼ 30Hz) ischosen. The statistical accuracy of an EDM measurement is then(3) σd =h¯2αET√N√M,where N is number of detected neutrons, α is a “quality” parameter for the measurement,including the polarization loss during the precession time T . A factor of 1/√M is addedto the sensitivity formula with M the number of measurements. Currently, several col-laborations worldwide try to measure EDMs with next generation approaches to achievean improvement of the current limits by almost 2 orders of magnitude to the 10−28 ecmregime, e.g. the Sussex-RAL-Oxford-Kure collaboration and the US community at theSpallation Neutron Source, who are building cryogenic experiments based on super-fluid4He for production and storage of UCN with very innovative approaches. Other groupsat ILL and PSI use trapped UCN in vacuum at room temperature, their gain in sensitiv-ity relies on the stepwise improvement of existing apparatus and technology. Althoughall these measurements are much less accurate than measurements of atomic systems, itis worthwhile to perform these measurements due to their strong impact on theory.2. – The experimentAt the new source of UCN at the FRM-2 in Garching a next generation approach tomeasure the EDM of the neutron is currently being set up. It extends proven and alreadyknown technologies with the goal to achieve a statistical limit of dn < 5 · 10−28 ecm at3σ and a corresponding control of systematic effects of σd,syst < 2 · 10−28 ecm (1σ). Themain improvements are i) a strong source of UCN, ii) sufficient control of magnetic andelectric fields and iii) improved possibilities to test for systematic effects. The source ofUCN is placed in a tangential beam tube inside the reactor in a thermal neutron fluxof 1 · 1014 s−1. Solid deuterium is used as a super-thermal converter for the productionof UCN. Properties of the source and its operation could be tested previously [16-18].Operation of the source at the reactor is expected in 2013. The UCN are extracted fromthe source and guided in vacuum to the experiments. A beam line made from speciallyprepared replica foil tubes with a relative transmission of > 0.99 per meter guides theUCN to the nEDM beam position, which is placed outside the reactor building in a newexperiment hall at 27m distance from the solid deuterium source. Taking into accountproduction, volumes and losses of all components and the EDM chambers, the projectedpolarized UCN density is > 3000 cm−3 in the EDM experiment.Chosen from a finite number of possible realizations of a next generation measurementthat have been discussed in the community over the last decades, this approach is basedon UCN stored in two vertically aligned cylindrical vessels at room temperature anda vertical magnetic field B0. In between the cells a high voltage electrode is placed toenable measurements with an electric field parallel and anti-parallel to B0 simultaneously.Using the statistical sensitivity formula (eq. (3)) with practically feasible numbers forα, a precession time of T = 250 s, an electric field E = 18 kV/cm and the neutronnumber based on chamber dimensions of 12 cm height and 48 cm diameter, the statisticalsensitivity goal can be achieved in 200 days. In addition, a co-magnetometer based onpolarized 199Hg vapor with a laser based optical system is placed in these cells [19], alsoallowing for light-shift free operation. External magnetometers are placed on top andbottom of the chambers inside tubes that are accessible from outside during operation ofthe experiment. A cut through the apparatus is shown in fig. 1. Buffer gases can be addedA NEXT GENERATION MEASUREMENT OF THE ELECTRIC DIPOLE MOMENT ETC. 125Fig. 1. – EDM measurement chamber stack with HV electrode, magnetometer cells on top andbottom and access tubes for additional magnetometry.to all magnetometers to investigate various systematic effects and to eventually increasethe HV behavior. Within close distance tangentially to the magnetometer cells, tubes areplaced that reach through the vacuum chamber and can be accessed from outside duringthe measurements. These are used to online measure the field distribution with opticalmagnetometers and the fluctuations of gradients e.g. with SQUIDs. By performingtwo Ramsey experiments simultaneously in two chambers with inverted E-fields in thesame magnetic field, drifts of the homogeneous B0-field are canceled, only fluctuations inthe vertical gradient affect the measurement. Gradients are extracted from four stacked199Hg cells, similar to ref. [20] with an accuracy of < 10 fT. The measurement containseffects that shift the frequencies of both species in different ways and thus mimic a falseEDM signal as well as effects that increase the error bar on the measured result withoutapplying a shift in frequency. The analysis of the measurement is conceptually basedon the Sussex-RAL-ILL experiment with a double magnetometer [13]. The velocities ofUCN and 199Hg differ significantly, thus the average center of gravity is Δh ∼ 2.5mmlower for UCN. Therefore, a vertical gradient changes the ratio R of the spin precessionfrequencies of both species in a chamber.2.1. Systematic effects. – Generally, any systematic effects can be classified as i) di-rectly magnetic field related, mainly drifts and fluctuations of gradients, as well as leakagecurrents; ii) particle motion related effects, like geometric phases of the Hg atoms andthe neutrons in e.g. B and E inhomogeneities and iii) mechanical effects, like vibrations,alignment of electrodes or laser beams, symmetry of the chamber and shutters. Magneticfields and gradient drifts between the chamber positions can be controlled already with199Hg magnetometry to a level of 10 fT, which represents about the minimum accuracyfor a maximum allowed false EDM of dfalse ∼ 1 · 10−28 ecm in case of merely statisticalmagnetic field fluctuations. The homogeneity of the residual fields and the generatedfields is practically limited to < 0.3 nT/m gradients over the volume of the chambers andmagnetometers. A combination with the motional magnetic field of a particle moving inan electric field gives rise to a term(4) δω ∼ ∂B∂zr(E × v),linear with E, the so-called geometric phase effect. Due to the larger velocities of theHg atoms, this effect amounts to dfalse ∼ 4 · 10−27 ecm for a current generation Hgmagnetometer and dfalse ∼ 2·10−28 ecm for the neutrons. In addition to the requirements126 I. ALTAREV, D. H. BECK, S. CHESNEVSKAYA, ETC.Fig. 2. – Cut through the EDM installation with retracted innermost shields.on the overall B0 field, this also severely constrains magnetic contaminations to pT withinfew cm close to the neutron chambers and e.g. light shifts from the readout light of the199Hg magnetometers. As deduced from simulations, also e.g. the alignment of E andB0 must be controlled to 10−4, vibrations also are critical and are controlled via laserinterferometry. Fluctuations of the high voltage also cause non-negligible currents thatare avoided with a fully non-metallic layout, which also helps to avoid the introductionof RF noise from outside through the HV conductor. Whereas the velocity of Hg atomsis fixed in such a scheme, the UCN velocities can be changed without changing anyparameters inside the spectrometer. This is done deploying a superconducting solenoidalong the neutron guide that supplies UCN to the EDM spectrometer. In the centerof the solenoid, a resonant spin flipper is operated. The field strength can be adjustedwithin a range of values while keeping the spin flipper resonant. This splits the initialUCN spectrum from the solid deuterium based source with a minimum kinetic energyof 108 neV into a fast and a slow polarized spectrum with tunable energy (see, e.g.ref. [21]). While the slow spin component is stored in the EDM chambers, the acceleratedpolarization will not be trapped due to the high kinetic energy.2.2. Magnetic environment . – The EDM spectrometer is placed in a non-magneticbuilding outside the reactor. This yields several advantages for the performance of theexperiment: the floor is magnetically and vibrationally decoupled and built with lowmagnetism materials on an area of 6 × 9m2, covered by a temperature and humiditycontrolled clean-room environment. An active 24 coil ambient field compensation coilsystem with 180 magnetic field probes is used to compensate for static and dynamicambient fields and second order gradients. Passive shielding made from Mu-metal andaluminum shells in a nested cuboid arrangement then reduce electromagnetic distur-bances at different ranges of frequencies. The passive shield is split in an outer shieldedroom on a vibration controlled table with a large access door and a 1 nT residual fieldinside. Already in this environment, the EDM spectrometer can be assembled and testedwith easy access. On a rail system, an inner magnetic shield section can slide into theroom. This inner shield section is again built up from cuboid Mu-metal parts, withdetachable end-caps. A cylindrical Mu-metal shield with ∼ 1.5m diameter forms theinnermost layer inside this cuboid assembly. This cylinder is optimized to interact withthe coils required for the B0 field, as well as for demagnetization. An overview pictureof the installation is shown in fig. 2. The shielded room with all integrated systems isA NEXT GENERATION MEASUREMENT OF THE ELECTRIC DIPOLE MOMENT ETC. 127portable, it can be lifted by crane and transported by truck in one piece. In particular,it rests on a table that can be adjusted in height to modify the energy distribution ofUCN inside the EDM chambers.2.3. Schedule. – The construction work for the beam position, installation of cleanrooms, compensation system and outer magnetic shielding is ongoing. Subsequently, theinstallation of magnetometry systems and the inner magnetic environment is scheduledin 2012, after finalizing ongoing tests of a small scale prototype.∗ ∗ ∗This work is supported by the Deutsche Forschungsgemeinschaft (SPP 1491) and theDFG cluster of excellence Origin and Structure of the Universe.REFERENCES[1] Purcell E. M. and Ramsey N. F., Phys. Rev., 78 (1950) 807.[2] Barr S. M., Int. J. Mod. Phys. A, 8 (1993) 209.[3] Christenson J. H., Cronin J. W., Fitch V. L. and Turlay R., Phys. Rev. Lett., 13(1964) 138.[4] Aubert B. et al., Phys. Rev. Lett., 86 (2001) 2515.[5] Kobayashi M. and Maskawa K., Prog. Theor. Phys., 49 (1973) 652.[6] Barbieri R., Nucl. Phys. B, 449 (1995) 437.[7] Bigi I. I. and Sanda A. I., CP Violation (Cambridge University Press, UK) 2000.[8] Riotto A. and Trodden M., Annu. Rev. Nucl. 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A next generation measurement of the electric dipole moment

of the neutron at the FRM II

http://eprints.bice.rm.cnr.it/17897/1/ncc10310.pdf

A next generation measurement of the electric dipole moment of the neutron at the FRM II

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