Periodic Reporting for period 4 - NEDM (The Neutron Electric Dipole Moment: pushing the precision to understand the matter-antimatter asymmetry)
Período documentado: 2021-10-01 hasta 2022-09-30
The aim of this project is to push the precision on the electric dipole moment (EDM) of the neutron, which is searched for by applying an electric field on free neutrons and looking for a change of the spin.
So far, after more than sixty years of research, all experiments are restlessly reporting a null result for the EDM of all subatomic particles.
Detecting a nonzero EDM would be of fundamental importance, it would sign directly a violation of the time reversal symmetry for a microscopic process and reveal "new physics" not accounted for by the Standard Model of particle physics.
It is believed that new physics was at play in the early Universe to generate the asymmetry between matter and antimatter, but the detailed mechanism of how this happened is presently unknown.
One must reveal this new physics in the laboratory to progress in our understanding of the Big Bang.
A promising way to proceed is to improve the precision on the neutron EDM. The task is not easy because the sensitivity of previous experiments was already fantastic.
New experiments are being constructed by international collaborations at large nuclear facilities where free neutrons are copiously produced, sin particular at the Paul Scherrer Institute (PSI).
In order to be successful, special care is needed in the design, the construction and the exploitation of the apparatuses in order to control all undesirable effects affecting the neutron spin in the experiment.
The control of systematic effects is especially important for such precision experiments, since the effect to be detected is very, very small.
The objective of this project is to conduct a program to improve the control of systematic effects in neutron EDM experiments, with three main tasks:
1 Advance the technique of quantum magnetometry using mercury-199
2 Design, construct and operate a mapping robot for the n2EDM experiment
3 Evaluate the systematic effects in the nEDM data recorded at PSI in the period 2014-2017
So far, after more than sixty years of research, all experiments are restlessly reporting a null result for the EDM of all subatomic particles.
Detecting a nonzero EDM would be of fundamental importance, it would sign directly a violation of the time reversal symmetry for a microscopic process and reveal "new physics" not accounted for by the Standard Model of particle physics.
It is believed that new physics was at play in the early Universe to generate the asymmetry between matter and antimatter, but the detailed mechanism of how this happened is presently unknown.
One must reveal this new physics in the laboratory to progress in our understanding of the Big Bang.
A promising way to proceed is to improve the precision on the neutron EDM. The task is not easy because the sensitivity of previous experiments was already fantastic.
New experiments are being constructed by international collaborations at large nuclear facilities where free neutrons are copiously produced, sin particular at the Paul Scherrer Institute (PSI).
In order to be successful, special care is needed in the design, the construction and the exploitation of the apparatuses in order to control all undesirable effects affecting the neutron spin in the experiment.
The control of systematic effects is especially important for such precision experiments, since the effect to be detected is very, very small.
The objective of this project is to conduct a program to improve the control of systematic effects in neutron EDM experiments, with three main tasks:
1 Advance the technique of quantum magnetometry using mercury-199
2 Design, construct and operate a mapping robot for the n2EDM experiment
3 Evaluate the systematic effects in the nEDM data recorded at PSI in the period 2014-2017
1 Development of a quantum magnetometry lab.
We have developed a facility at LPSC Grenoble dedicated to research on quantum mercury magnetometry, which includes
(i) a filling station allowing to fill optical cells with a controlled mixture of gases among mercury (isotopically enriched mercury-199), helium-3, helium-4 and nitrogen.
(ii) a UV continuous laser at a wavelength of 254 nm. The frequency stabilization scheme based on saturated absorption spectroscopy that we developed shows excellent performances in terms of robustness.
(iii) a magnetometry station where an optical cell is exposed to a controlled magnetic field and probed by a laser beam with controlled power and polarization. A computer controlled DAQ system allows to program automatized series of measurement cycles to optically pump the atoms in the cell and to record the optical spin-precession signals.
The facility was operational for the first scientific exploitation in 2020.
The mercury lab was first used to investigate the "light shift" phenomenon, a systematic effect that shifts the spin precession frequency due to the probing light. This study aimed at identifying a scheme for which the systematic effect due to the real light shift is canceled. We did identify such a scheme successfully, but parts of the results were unexpected, i.e. not fully in agreement with the expectations of Cohen Tannoudji's theory. We understood the origin of the disagreement: at high laser power, the optical excitation cannot be treated as a "quantum jump" and that theory is not valid. We have confirmed this understanding with dedicated measurements and calculations. This work has been presented at nEDM2021 conference in Feb. 2021 and at a seminar at LKB in July 2022. A publication is in preparation.
The mercury lab was also used for a first experiment towards mercury-199 / helium-3 comagnetometry in summer 2021. We have prepared optical cells with a mixture of mercury-199 and a buffer gas in the mbar range, and measured mercury precession signals. We have studied the influence of magnetic field gradients and buffer gas pressure on the mercury depolarization time. From a comprehensive series of measurements we have extracted the diffusion coefficient of mercury in diverse buffer gases, including helium-3. An article presenting these results (arXiv:2209.06621) will be published in Phys. Rev. A.
2 Magnetic field uniformity in neutron EDM experiments.
We have developed and operated a mapping robot to perform the magnetic cartography of the inner part of the n2EDM instrument.
The core part of the n2EDM apparatus is a large double chamber filled with polarized ultracold neutrons, where they will be exposed to a strong electric field and a weak magnetic field. Stability and uniformity of the magnetic field is of primary importance. The chambers will be installed inside a colossal magnetically shielded room (MSR) with an inner volume of 25 m3 and a quasi-static shielding factor of 100,000. The MSR is operational since 2020 and the mapping robot has been successfully installed in 2021. The mechanics of the robot is rather challenging: a remote motorization system allows precise 3D movements of magnetometer inside a large volume of interest, the inner part being absolutely amagnetic. Since June 2021 the mapper is exploited to characterize the remanent field and the inner coil system of the MSR. This work has been presented at the nEDM2021 conference nEDM2021 and at the PSI2022 conference. With this work we have demonstrated that the generation of the inner magnetic field of n2EDM satisfies the requirements of the experiment in terms of uniformity.
3 Publication of the most precise neutron EDM measurement.
As a scientific achievement, we have produced a complete analysis of the data taken in 2015-2017 with the nEDM apparatus at PSI.
The analysis was performed on blinded data by two independent groups in the collaboration, the Grenoble team formed the "west" group. An important part consisted in the correction of the main systematic effect from the analysis of magnetic field maps.
This work resulted in the letter: Measurement of the Permanent Electric Dipole Moment of the Neutron, published in Phys. Rev. Lett. (2020).
We have developed a facility at LPSC Grenoble dedicated to research on quantum mercury magnetometry, which includes
(i) a filling station allowing to fill optical cells with a controlled mixture of gases among mercury (isotopically enriched mercury-199), helium-3, helium-4 and nitrogen.
(ii) a UV continuous laser at a wavelength of 254 nm. The frequency stabilization scheme based on saturated absorption spectroscopy that we developed shows excellent performances in terms of robustness.
(iii) a magnetometry station where an optical cell is exposed to a controlled magnetic field and probed by a laser beam with controlled power and polarization. A computer controlled DAQ system allows to program automatized series of measurement cycles to optically pump the atoms in the cell and to record the optical spin-precession signals.
The facility was operational for the first scientific exploitation in 2020.
The mercury lab was first used to investigate the "light shift" phenomenon, a systematic effect that shifts the spin precession frequency due to the probing light. This study aimed at identifying a scheme for which the systematic effect due to the real light shift is canceled. We did identify such a scheme successfully, but parts of the results were unexpected, i.e. not fully in agreement with the expectations of Cohen Tannoudji's theory. We understood the origin of the disagreement: at high laser power, the optical excitation cannot be treated as a "quantum jump" and that theory is not valid. We have confirmed this understanding with dedicated measurements and calculations. This work has been presented at nEDM2021 conference in Feb. 2021 and at a seminar at LKB in July 2022. A publication is in preparation.
The mercury lab was also used for a first experiment towards mercury-199 / helium-3 comagnetometry in summer 2021. We have prepared optical cells with a mixture of mercury-199 and a buffer gas in the mbar range, and measured mercury precession signals. We have studied the influence of magnetic field gradients and buffer gas pressure on the mercury depolarization time. From a comprehensive series of measurements we have extracted the diffusion coefficient of mercury in diverse buffer gases, including helium-3. An article presenting these results (arXiv:2209.06621) will be published in Phys. Rev. A.
2 Magnetic field uniformity in neutron EDM experiments.
We have developed and operated a mapping robot to perform the magnetic cartography of the inner part of the n2EDM instrument.
The core part of the n2EDM apparatus is a large double chamber filled with polarized ultracold neutrons, where they will be exposed to a strong electric field and a weak magnetic field. Stability and uniformity of the magnetic field is of primary importance. The chambers will be installed inside a colossal magnetically shielded room (MSR) with an inner volume of 25 m3 and a quasi-static shielding factor of 100,000. The MSR is operational since 2020 and the mapping robot has been successfully installed in 2021. The mechanics of the robot is rather challenging: a remote motorization system allows precise 3D movements of magnetometer inside a large volume of interest, the inner part being absolutely amagnetic. Since June 2021 the mapper is exploited to characterize the remanent field and the inner coil system of the MSR. This work has been presented at the nEDM2021 conference nEDM2021 and at the PSI2022 conference. With this work we have demonstrated that the generation of the inner magnetic field of n2EDM satisfies the requirements of the experiment in terms of uniformity.
3 Publication of the most precise neutron EDM measurement.
As a scientific achievement, we have produced a complete analysis of the data taken in 2015-2017 with the nEDM apparatus at PSI.
The analysis was performed on blinded data by two independent groups in the collaboration, the Grenoble team formed the "west" group. An important part consisted in the correction of the main systematic effect from the analysis of magnetic field maps.
This work resulted in the letter: Measurement of the Permanent Electric Dipole Moment of the Neutron, published in Phys. Rev. Lett. (2020).
1 Measurement of the Permanent Electric Dipole Moment of the Neutron, published in Phys. Rev. Lett. (2020).
We have contributed significantly to the new measurement of the neutron EDM (still compatible with zero!). This resutls now sets the new reference in the field.
2 Progress in the undersanding of systematic effects.
During the project we obtained significant theoretical and experimental advances in the control of systematic effects in nEDM experiments. The systematical error has been reduced by a factor of 5 as compared to the previous result using essentially the same apparatus but a more advanced understanding of the basic phenomena and dedicated methods (mapping and analysis) to address the problem.
3 The n2EDM apparatus.
We brought significant contributions to the design and construction of the new apparatus n2EDM.
The very large magnetically shielded room and its inner coil system has been characterized with a dedicated mapping robot.
This new magnetically shielded room for n2EDM really constitutes a significant advance, beyond the state of the art of weak field uniformity, because the uniformity of the holding magnetic field is now typically 50 times better when compared to the previous nEDM apparatus.
We have contributed significantly to the new measurement of the neutron EDM (still compatible with zero!). This resutls now sets the new reference in the field.
2 Progress in the undersanding of systematic effects.
During the project we obtained significant theoretical and experimental advances in the control of systematic effects in nEDM experiments. The systematical error has been reduced by a factor of 5 as compared to the previous result using essentially the same apparatus but a more advanced understanding of the basic phenomena and dedicated methods (mapping and analysis) to address the problem.
3 The n2EDM apparatus.
We brought significant contributions to the design and construction of the new apparatus n2EDM.
The very large magnetically shielded room and its inner coil system has been characterized with a dedicated mapping robot.
This new magnetically shielded room for n2EDM really constitutes a significant advance, beyond the state of the art of weak field uniformity, because the uniformity of the holding magnetic field is now typically 50 times better when compared to the previous nEDM apparatus.