New measurements of neutron electric dipole moment

Transcription

1 New measurements of neutron electric dipole moment A.P.Serebrov ), E.A.Kolomenskiy, A.N.Pirozhkov, I.A.Krasnoshekova, A.V.Vasiliev, A.O.Polyushkin, M.C.Lasakov, A.K.Fomin, I.V.Shoka, O.M.Zherebtsov, P.Geltenbort +, O.Zimmer +, S.N.Ivanov +, E.B.Alexandrov *, S.P.Dmitriev *, N.A.Dovator * B.P.Konstantiov Petersburg Nuclear Physics Institute of National Research Centre Kurchatov Institute, Gatchina, Leningrad region, Russia + Institut Max von Laue Paul Langevin, BP 56, 3802 Grenoble Cedex 9, France * Ioffe Physical Technical Institute RAS, 902 St. Petersburg, Russia The results of measurements of neutron electric dipole moment obtained by PNPI EDM spectrometer at reactor ILL are presented. As a result of given measurements the limit for EDM of neutron есm has been set on the confidence level 90%. Increase of measuring accuracy by 3- times is being discussed. Existence of electric dipole moment (EDM) of an elementary particle is possible only at simultaneous violation of space parity and invariance relevant to time reversal. The difficulties of the Standard Model in explanation of CP-violation are formally surmounted by other models such as super symmetry, models with multiple Higgs-particles and left-right symmetric theories. In these models CPnonconservation arises in the first order due to weak interaction, with neutron EDM being at the level accessible to a modern experiment. Baryon asymmetry of the Universe is predicted by such models at the observable level which points at possible validity of proposed versions of the theory. Search for neutron EDM in low energy physics is an alternative to search for new particles with hadrons colliders in high energy physics. A value or a new constraint on neutron EDM magnitude is a very sensitive test for choosing the theory with CP-violation. To illustrate it in coordinates of mass of particles being sought for in super symmetric theoretical models, Fig. shows areas (for possible existence of super particles) excluded by other experiments [, 2]. The vertical axis gives the super symmetric Higgs/Higgsino mass, while the horizontal axis gives the U()Y gaugino mass parameter M. The shaded region indicates the region for these masses needed to obtain the observed baryon asymmetry. The dark grey region is excluded by LEP. The grey region is excluded by the present electron EDM limit, while the dash-doted line gives prospective future experimental sensitivity of neutron EDM. The dashed line indicates sensitivity of future direct search measurements at the LHC. The area accessible to the Large Hadrons Collider (LHC, Switzerland) is shown with a dotted line. The area accessible for EDM E mail: serebrov@pnpi.spb.ru

2 experiment in increasing the experimental accuracy by two orders of magnitude is marked with a dot-dashed line. It will be quite possible to achieve it using new UCN sources of high intensity [3]. Fig.. Illustrative sensitivity of present and future EDM measurements to supersymmetric mass parameters relevant for electroweak baryogenesis [, 2]. Thus, the neutron EDM measurement is of significance for search for phenomena outside the scope of Standard Model and its discovery is likely to become the determining factor for choosing a version of the theory adequately describing processes with violation of CPsymmetry. At present the most accurate constraints on neutron EDM value have been obtained using the magnetic resonance method with the reversible electric field and prolonged holding of a neutron in the trap of EDM spectrometer [, 5]. Sensitivity of EDM spectrometer is determined by the number of registered neutrons, by width of magnetic resonance and by strength of electric field. Value of neutron EDM is calculated by the following formula: d n h( N N ) N 2( E E ) f, () where N and N are detector counts correspondingly in parallel and antiparallel directions of an electric field relative to magnetic field; h - the Planck constant, E is strength of electric N field; is slope of resonance curve at the operation point. f 2

3 Statistical error of measured EDM is expressed by the following formula: d n res h 2N N 2( E E ) f 2( E res h 2N E ) T N res, (2) res where N is neutron count at resonance frequency accumulated during the measuring time at one of the points of electric field; T is time interval between two impulses of oscillating field; N N N N is parameter determining the scope of a resonance curve. max min max min So, an experimental sensitivity d n is determined by neutron count N, interaction time in the installation and by the value of electric field intensity E. The experiment concerned is a new stage of experiments made by PNPI on search for neutron electric dipole moment by using the two chamber EDM spectrometer. Investigations are started within the framework of making preparations for putting into operation the reactor PIK built in PNPI. The extracted reactor beam is expected to have UCN source on the basis of super fluid helium. Besides, some preparation is in progress at reactor WWR-M PNPI for creating a new UCN source with a moderator based on super fluid helium. Both sources are supposed to have UCN density by two-three orders higher than the available source at high flux reactor in the Institute Laue Langevin (ILL, Grenoble, France). At present in PNPI there is no operating UCN source, therefore EDM installation is situated in ILL. It is located in the position of PF2 MAM turbine of UCN with UCN flux density of about n/сm 3 [6]. The project is supported by an international scientific committee ILL which recommends conducting prolonged (2-3 years) measurements for collecting statistics. Now ILL reactor has stopped its operation for a year for the planned reconstruction to be carried out. Thus we represent a new result obtained by PNPI installation with results of EDM-measurements on the latest three reactor cycles. Our work makes use of PNPI differential magnetic resonance spectrometer [] (Fig.2), the specific characteristic of which is availability of the two UCN storage chambers provided with a common system of magnetic fields and with electric fields being equal in magnitude but oppositely directed. Storage chambers are located in four layer magnetic screen made out of permalloy. Control and stabilization of magnetic conditions is maintained with the help of eight quantum magnetometers at optical pumping up of cesium atoms. They are placed around UCN storage chambers in pairs: one magnetometer over another one and provide monitoring of the mean magnetic field within the resonance range. They are also responsible for formation of resonance frequency by means of dividing cesium frequency with regard to gyro magnetic factors of cesium and neutron. For an electric field to be induced one applies a high voltage 3

4 source of 200 kv with polarity reverse of output voltage without switching off loading. Either glass ceramic or quartz is used as material for side walls of UCN storage chambers. In changing polarity of electric field EDM neutron effects in different chambers will have opposite signs whereas instability of common magnetic field results in the resonance frequency shift of similar signs. Difference in results of measurements in different chambers leads to adding effects of neutron EDM, with effects produced by correlated count alterations irrelevant to EDM being considerably suppressed. Fig.2. EDM spectrometer scheme. A system of double analysis of polarization is another peculiarity enhancing the installation sensitivity. At the exit from each storage chamber of the spectrometer there are two detectors with their own analyzers of polarization and flipper. This system allows detecting both components of neutron polarization at the leading field direction. This almost twice increases the summed up neutron count during measurements and allows compensation of variation of results related to fluctuations of neutron intensity from the source. Correlation data analysis of four detectors and regularly measured parameters of leakage currents as well as the number of breakdowns and recordings of magnetometers enable to reveal possible systematic effects. Actually, with the help of each of four detectors one measures neutron electric dipole moment d, d 2, d 3, d. The signs of these four effects account the sign of the derivative at the resonance point as well as direction of the electric field with respect to the magnetic field. Using these values one can deduce new four linear-independent values as shown below.

5 EDM [( d d2) ( d3 d)] [( d d2) ( d3 d)] N [( dd2 ) ( d3 d )] Z [( d d2) ( d3 d)] The first of them determines the neutron EDM effect, the second ( ) determines the effect of electric field influence on resonance conditions, the third ( N ) determines the effect of electric field influence on detector counting rate, with each of the above mentioned effects compensating other effects. Finally, in the last combination ( Z ) all above mentioned effects must be compensated, which is regarded as a significant criterion of compensation scheme operation. The measurements have been carried out for phase shifts between radio impulse 90 and 270 it means with different sign of derivative of resonance curve. It is very useful to compensate different type of drift and possible systematic effects. But t is to be noted that for detecting influence of electric polarity alteration on the detector counting rate ( N ) measuring results with phase shifts between radio impulses 90 and 270 must be subtracted rather than added. UCN storage time in spectrometer chambers was from 70 sec to 00 sec. Fig.3 shows resonance curves of the spectrometer for phase shifts 90 and 270. The depth of resonance peak-gap was 70% at the time of 95 sec between two pulses. Fig.3. Resonance curves for detectors D and D2 and phase shifts 90 and 270 for holding time 00 s. 5

6 An example of one of measuring series is presented in Fig.. The duration of this series was equal to 5 hours, electric field was 8kV/cm and UCN holding time in the traps of the spectrometer was 00 sec. Each point represents the result of a single measurement of EDM value in accordance with formula () in switching the electric field polarity in sequences (+ +) or ( ++ ) in order to eliminate the effect of the linear drift. The result of measurements of EDM in the top storage chamber is Dtop=(2.59±3.90) 0-25 e cm and the result for the bottom chamber is Dbott=-(3.98±.22) 0-25 e cm. The total result D=-(0.70±2.7) 0-25 e cm demonstrates the compensation of fluctuations of the magnetic field and permits to assess the sensitivity of the experimental set-up, which for this series of measurements amounted to e cm/day. Fig.. An example of one of measuring series. EDMtop and EDMbott are measuring value of EDM for top and bottom chamber; EDM is measured for both chambers. However, the time of collecting statistics during the experiment, as a rule, for different reasons is three times less than the current time. At present this installation has made a series of measurements of neutron EDM with collecting statistics during 3 reactor cycles. Table presents the results of EDM neutron measurements and values, N and Z controlling possible systematic effects in units e cm. As pointed out, the above mentioned systematic effects must be compensated in neutron EDM combination. For this reason, possible systematic effects at the level of 2-3 standard deviations must be compensated, thus, we do not consider that it is necessary to introduce any estimations on systematic in neutron EDM measuring results. It should be mentioned that control of possible effects on cesium magnetometers has also been conducted. The difference in recordings of magnetometers in the upper and lower planes (correspondingly in the vicinity of the top and

7 bottom chambers) at electric field switching does not exceed 0-6 nt. This limit can be recalculated in regard to the constraint on false EDM effect at the level of е сm, i.e. by the order of magnitude lower the stated accuracy of measurements. Table : Results of measurements in units 0-26 е сm. Old [] New Total EDM 0.7± ± ± ± ± ±5.0 N -.5±. 8.62± ±3.35 Z -0.8± ±.72.05±3.05 Variations of measurement results for normalized values EDM,, N and Z are presented in Fig.5. Distribution of EDM and Z results is determined by a statistical data variation. Distribution of and N results is somewhat extended because of magnetic field instability and because of neutron intensity correspondingly. Fig.5. Distribution of measurement results for EDM values,, N and Z. For each value are shown: 2 the ratio of distribution width to its statistical width as well as at fitting by means of normal distribution. Accuracy of measurements in ILL was есm, and on account of earlier made measurements at the reactor WWR-M the measuring accuracy is есm. This measuring 7

8 accuracy is as high as the level of modern experimental limit on neutron EDM value [5]. Limit set on 26 neutron EDM in our measurements is 5.20 есm, which corresponds to the confidence level of 90%. Such a result and confirmation of the existing constraint in an independent experiment made at another installation is of crucial importance for the fundamental problem involved. We are planning to increase accuracy of our measurements at the expense of enhancing 3- times UCN intensity at the entrance of the spectrometer by transporting it to another beam of higher intensity, as well as at the expense of enhancing 3 times the transmitting capability of the spectrometer using a new scheme of spectrometer traps. Thus, we are going to increase accuracy 26 of measurements by 3- times up to 0.70 есm. Concluding, the authors would like to express their gratitude to the staff of the workshop of experimental facilities in PNPI for their assistance in reconstructing the spectrometer and also to the personnel of ILL reactor for their help in assembling the installation and making measurements. Particularly we would like to thank T. Brenner from ILL and A.I. Egorov from PNPI for their assistance. References. NSAC Long Range Plan: The Frontiers of Nuclear Science, 2007; [ 2. M.J. Ramsey-Musolf, in: Proc. of the Intensity Frontier Workshop 20; [Fundamental Physics at the Intensity Frontier, arxiv: v [hep-ex]]. 3. A.P. Serebrov, V.A. Mityuklyaev, A.A. Zakharov et al., NIM A 6, 276 (2009).. I.S. Altarev, Y.V. Borisov, N.V. Borovikova et al., Phys. of At. Nucl. 59, 52 (996). 5. C.A. Baker, D.D. Doyle, P. Geltenbort et al., PRL 97, 380 (2006). 6. A.P. Serebrov, P. Geltenbort, I.V. Shoka et al., NIM A 6, 263 (2009). 8