CryoEDM
Encyclopedia
CryoEDM is a particle physics
experiment aiming to measure the electric dipole moment
(EDM) of the neutron
to a precision of ~10-28ecm. The name is an abbreviation of cryogenic neutron EDM
experiment. The previous name nEDM is also sometimes used, but should be avoided where there may be ambiguity. The project follows the Sussex/RAL/ILL nEDM experiment, which set the current best upper limit of 2.9×10-26ecm. To reach the improved sensitivity, cryoEDM uses a new source of ultracold neutrons
(UCN), which works by scattering cold neutrons in superfluid helium.
The experiment is located at the Institut Laue–Langevin in Grenoble
. The collaboration includes the nEDM team from Sussex University and RAL
, as well as new collaborators from Oxford, and Kure
, Japan. The collaboration is remarkably small for a modern particle physics experiment (around 30 people).
In 2008 the experiment was ranked as an alpha 5 (top priority) project by STFC
, together with the much larger CERN
experiments: ATLAS
and CMS
.
Although electrically neutral overall, the neutron is made up of charged quarks. An imbalance of charge on one side would cause a non-zero EDM. This would be a violation of parity
(P) and time reversal
(T) symmetries. A neutron EDM is believed to exist at some level to explain the matter-antimatter asymmetry of the Universe
, although to date every measurement has given a value consistent with zero.
Limits on the neutron EDM are a significant constraint on many particle physics theories. The Standard Model
of Particle Physics predicts a value 10–31 – 10–32 ecm, while supersymmetric theories predict values in the range 10–25 – 10–28ecm.
, when the applied electric field E is reversed. This is given by
where d is the EDM,
is the magnetic dipole moment, B is the magnetic field, and h is the Planck constant
, (the
depends on whether the fields are parallel or antiparallel). Clearly when the electric field is reversed, this produces a shift in the precession frequency proportional to the EDM. As the neutron magnetic dipole moment is non-zero it is necessary to shield or correct for magnetic field fluctuations to avoid a false positive signal.
The precession frequency is measured using the Ramsey separated oscillatory field magnetic resonance
method, in which a large number of spin
polarized ultra-cold neutrons are stored in an electric and magnetic field. An AC magnetic field pulse is then applied to rotate the spins by
. The signal generator used to apply the pulse is then gated off while the neutron spins precess about the magnetic field axis at the precession frequency; after a period of ~100s, another field pulse is applied to rotate the spins by 
. If the frequency of the applied signal is exactly equal to the precession frequency, the neutrons will all be synchronised with the signal generator, and they will all end up polarized in the opposite direction to how they started. If there is a difference between these two frequencies, then some neutrons will end up back in their original state. The number of neutrons in each polarization state is then counted and by plotting this number against the applied frequency, the precession frequency can be determined.
Moving from a room temperature to a cryogenic measurement, means it has been necessary to rebuild the entire apparatus. The new experiment uses superconducting
lead
magnetic shields, and a SQUID
magnetometer system.
The experiment has now finished construction, and is in its optimization and data acquisition phase. Several years of running are now foreseen to collect data to make a new EDM measurement.
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
experiment aiming to measure the electric dipole moment
Electric dipole moment
In physics, the electric dipole moment is a measure of the separation of positive and negative electrical charges in a system of charges, that is, a measure of the charge system's overall polarity with SI units of Coulomb-meter...
(EDM) of the neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
to a precision of ~10-28ecm. The name is an abbreviation of cryogenic neutron EDM
Neutron electric dipole moment
The neutron electric dipole moment is a measure for the distribution of positive and negative charge inside the neutron. A finite electric dipole moment can only exist if the centers of the negative and positive charge distribution inside the particle do not coincide. So far, no neutron EDM has...
experiment. The previous name nEDM is also sometimes used, but should be avoided where there may be ambiguity. The project follows the Sussex/RAL/ILL nEDM experiment, which set the current best upper limit of 2.9×10-26ecm. To reach the improved sensitivity, cryoEDM uses a new source of ultracold neutrons
Ultracold neutrons
Ultracold neutrons are free neutrons which can be stored in traps made from certain materials. The storage is based on the reflection of UCN by such materials under any angle of incidence.- Properties :...
(UCN), which works by scattering cold neutrons in superfluid helium.
The experiment is located at the Institut Laue–Langevin in Grenoble
Grenoble
Grenoble is a city in southeastern France, at the foot of the French Alps where the river Drac joins the Isère. Located in the Rhône-Alpes region, Grenoble is the capital of the department of Isère...
. The collaboration includes the nEDM team from Sussex University and RAL
Rutherford Appleton Laboratory
The Rutherford Appleton Laboratory is one of the national scientific research laboratories in the UK operated by the Science and Technology Facilities Council . It is located on the Harwell Science and Innovation Campus at Chilton near Didcot in Oxfordshire, United Kingdom...
, as well as new collaborators from Oxford, and Kure
Kure University
is a private university in Kure, Hiroshima, Japan. The school first opened as a junior women's collegein 1986 and became a four-year college in 1995.-External links:*...
, Japan. The collaboration is remarkably small for a modern particle physics experiment (around 30 people).
In 2008 the experiment was ranked as an alpha 5 (top priority) project by STFC
Science and Technology Facilities Council
The Science and Technology Facilities Council is a UK government body that carries out civil research in science and engineering, and funds UK research in areas including particle physics, nuclear physics, space science and astronomy .-History:It was formed in April 2007 as a merger of the Particle...
, together with the much larger CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
experiments: ATLAS
ATLAS experiment
ATLAS is one of the six particle detector experiments constructed at the Large Hadron Collider , a new particle accelerator at the European Organization for Nuclear Research in Switzerland...
and CMS
Compact Muon Solenoid
The Compact Muon Solenoid experiment is one of two large general-purpose particle physics detectors built on the proton-proton Large Hadron Collider at CERN in Switzerland and France. Approximately 3,600 people from 183 scientific institutes, representing 38 countries form the CMS collaboration...
.
The neutron electric dipole moment
For more information see Neutron electric dipole momentNeutron electric dipole moment
The neutron electric dipole moment is a measure for the distribution of positive and negative charge inside the neutron. A finite electric dipole moment can only exist if the centers of the negative and positive charge distribution inside the particle do not coincide. So far, no neutron EDM has...
Although electrically neutral overall, the neutron is made up of charged quarks. An imbalance of charge on one side would cause a non-zero EDM. This would be a violation of parity
Parity (physics)
In physics, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it is also commonly described by the simultaneous flip in the sign of all three spatial coordinates:...
(P) and time reversal
T-symmetry
T Symmetry is the symmetry of physical laws under a time reversal transformation: T: t \mapsto -t.Although in restricted contexts one may find this symmetry, the observable universe itself does not show symmetry under time reversal, primarily due to the second law of thermodynamics.Time asymmetries...
(T) symmetries. A neutron EDM is believed to exist at some level to explain the matter-antimatter asymmetry of the Universe
Baryon asymmetry
The baryon asymmetry problem in physics refers to the apparent fact that there is an imbalance in baryonic matter and antibaryonic matter in the universe. Neither the standard model of particle physics, nor the theory of general relativity provide an obvious explanation for why this should be so;...
, although to date every measurement has given a value consistent with zero.
Limits on the neutron EDM are a significant constraint on many particle physics theories. The Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of Particle Physics predicts a value 10–31 – 10–32 ecm, while supersymmetric theories predict values in the range 10–25 – 10–28ecm.
Measurement principle
Modern EDM experiments work by measuring a shift in the neutron Larmor spin precession frequencyLarmor precession
In physics, Larmor precession is the precession of the magnetic moments of electrons, atomic nuclei, and atoms about an external magnetic field...
, when the applied electric field E is reversed. This is given bywhere d is the EDM,
is the magnetic dipole moment, B is the magnetic field, and h is the Planck constantPlanck constant
The Planck constant , also called Planck's constant, is a physical constant reflecting the sizes of energy quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory, who discovered it in 1899...
, (the
depends on whether the fields are parallel or antiparallel). Clearly when the electric field is reversed, this produces a shift in the precession frequency proportional to the EDM. As the neutron magnetic dipole moment is non-zero it is necessary to shield or correct for magnetic field fluctuations to avoid a false positive signal.The precession frequency is measured using the Ramsey separated oscillatory field magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
method, in which a large number of spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
polarized ultra-cold neutrons are stored in an electric and magnetic field. An AC magnetic field pulse is then applied to rotate the spins by
. The signal generator used to apply the pulse is then gated off while the neutron spins precess about the magnetic field axis at the precession frequency; after a period of ~100s, another field pulse is applied to rotate the spins by . If the frequency of the applied signal is exactly equal to the precession frequency, the neutrons will all be synchronised with the signal generator, and they will all end up polarized in the opposite direction to how they started. If there is a difference between these two frequencies, then some neutrons will end up back in their original state. The number of neutrons in each polarization state is then counted and by plotting this number against the applied frequency, the precession frequency can be determined.The Sussex/RAL/ILL neutron EDM experiment (nEDM)
The nEDM experiment was a room temperature neutron EDM experiment which ran at ILL, using ultra-cold neutrons from the ILL reactor. Magnetic field fluctuations (a significant source of systematic error) were monitored using atomic mercury magnetometer. The results of the measurement were published in 1999 giving an upper limit on the neutron EDM of 6.3×10-26ecm.. A further analysis published in 2006 improved this to 2.9×10-26ecmCryoEDM
The cryoEDM experiment is designed to improve the sensitivity of the nEDM experiment by a factor of two down to ~10-28ecm. This will be achieved by a number of factors: the number of UCN will be increased using a new source, in which a beam of cold neutrons is downscattered inside superfluid helium; the use of liquid helium instead of vacuum will allow the applied electric field to be increased; improvements to the apparatus will increase the possible storage time and polarization product.Moving from a room temperature to a cryogenic measurement, means it has been necessary to rebuild the entire apparatus. The new experiment uses superconducting
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
magnetic shields, and a SQUID
SQUID
A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions....
magnetometer system.
The experiment has now finished construction, and is in its optimization and data acquisition phase. Several years of running are now foreseen to collect data to make a new EDM measurement.