January 19, 2010

A backdoor to new physics [, Electric Dipole Moment] The Neutron Optics and Physics group at KEK (NOP) is preparing to begin the research and development of an experiment to measure the neutron electric dipole moment. The result may answer one of the most fundamental questions about the universe. To make this measurement, the group proposes an innovative

The NOP group members gather in front of their beamline at J-PARC: (Back left) two physicists from KEK, Dr. Kenji Mishima and Dr. Tamaki Yoshioka, and two PhD students from the University of Tokyo, Hidehiko Otono (front left) and Hideyuki Oide.

Current cosmology tells us that our such as spin. In 2001, both the Belle indirect CP violation search, physicists use a experiment at KEK and the BaBar experiment more fundamental symmetry in the laws of universe started out from ‘nothingness’. When at SLAC succeeded in providing experimental physics, CPT symmetry. The CPT symmetry our universe first burst into existence, there evidence for mechanism of CP symmetry says that the laws of physics should be were equal numbers of particles and violation in B meson decay. unaltered when charge conjugation, parity, and antiparticles. Yet today, our universe is almost time reversal are applied simultaneously. entirely composed of matter, with just a very The outstanding problem with the CP violation little bit of antimatter. is that size of this known violation is too small This means that one can look for violation of to explain the large matter/antimatter time reversal symmetry to deduce violation of Physicists have proposed several mechanisms asymmetry in our visible universe. So particle CP symmetry. If time reversal T symmetry is to explain how this matter-antimatter accelerators around the world are working to broken, then unbroken CP operation cannot asymmetry came about. Recently, they found increase the energy of the collisions to look for restore the CPT symmetry. Because the CPT physical evidences for one such mechanism: additional, hidden CP violations. Interesting symmetry must be preserved at all time, T violation of charge conjugation-parity (CP) candidates for additional CP violation includes symmetry violation automatically implies CP symmetry. CP symmetry says that the laws of those in new exotic physics such as in theories symmetry violation. This method—looking for physics must stay the same when a particle is of supersymmetry (SUSY). CP violation by means of time reversal violation swapped with its antiparticle (charge —is called indirect CP violation search. conjugation) and then takes mirror image However, colliding particles at high energy isn’t (parity operation) of its physical properties the only way to search for CP violations. In the

[1] Neutron electric dipole moment The Neutron Optics and Physics (NOP) group at the Japan Proton Research Complex (J- PARC) is a collaboration of seven universities and research institutions in Japan including KEK. The mission of the group is to perform The leader of research on a number of topics in fundamental Neutron Optics physics using neutron beams. One of the main and Physics objectives of NOP is to search for CP violation indirectly by measuring the size of the electric (NOP) group at dipole moment of . KEK Prof. Hirohiko Shimizu An electric dipole moment is a slight explains the asymmetry in the distribution of electric theory behind charges inside neutrons. Since neutrons are the neutron electrically neutral particles, they have an equal electric dipole amount of positive and negative charge inside. moment However, the positive and negative charges experiment. may not be evenly distributed. For example, there may be more positive charges on one side, and more negative charges on the other. This asymmetry is called an electric dipole the order of 10-31 e centimeters,” says Shimizu. storage cell. However, some supersymmetric models predict a moment of 10-27 ecentimeters, meaning that As the neutrons travel to the storage cell, the just one order of magnitude improvement in the neutron pulse smears out as some particles measurement will prove or disprove some travel faster while some travel slower. To reduce models. the velocity differences, physicists use the spin property of neutrons. When a varying external Since neutrons have a small magnet-like magnetic field of a particular type is applied to property called spin, they precess in the the neutrons, the spin of the neutrons flips, presence of a magnetic field. If physicists apply releasing energy from the neutrons, and an alternating electric field in addition to a slowing the faster neutrons. This makes the static magnetic field, the interaction of the neutron bunch much denser. Additionally, the electric field and the electric dipole moment slowest neutrons are cut out of the bunch by a causes the neutrons to precess at a faster rate. shutter before entering the storage cell. This “Spin causes neutrons to precess a few times combination of magnetic field and shutter is per second, while the electric dipole moment called a neutron rebuncher. The team found increases that by something like one that repeating this process ten times would precession in 100 years,” says Shimizu. “To multiply the number of neutrons in the cell by A schematic picture of the ultracold measure this tiny difference, we need to roughly three times. with a converter, in produce and store as many neutrons as which cold neutrons are slowed to possible.” become . Neutron storage The gist of effective neutron storage is to Ultracold neutron source produce ultracold neutrons and let them moment, and means that neutrons can be The NOP’s proposed affected just a little by the presence of a neutron electric dipole powerful electric field. The world’s most moment beamline advanced neutron experiment at Institut Laue- will produce high Langevin (ILL) has shown that the value cannot intensity pulsed be larger than 10-26 e centimeters. beam of cold neutrons from “The neutron electric dipole moment is the protons provided by most promising probe for new physics. With a linac at J-PARC. this tool, theoretical predictions of models The resulting cold beyond the Standard Model are experimentally neutrons are then testable,” explains the NOP group leader Prof. slowed, transforming Hirohiko Shimizu of KEK. them into ultracold neutrons by a The uncertainty principle of quantum converter. Ultracold mechanics states that the elementary particles neutrons are very which make up a neutron—an up quark and slow neutrons, two down quarks—can spontaneously moving at the speed transform into much heavier particles for a brief of just a few meters periods of time. Those heavier particles may be per second. The either known particles from the Standard team estimates that Model, or exotic new particles from theories the system produces beyond the Standard Model. Either way, these several thousands short-lived heavy particles can occasionally neutrons per cubic (From left) Dr. Takashi Ino, Dr. Kaoru Taketani, Dr. Tamaki interact with external particles like photons. centimeter per pulse. Yoshioka, and Dr. Kenji Mishima from KEK gather around The ultracold their neutron guides with supermirrors on the inner surfaces neutrons are then “In order to prove the existence of new particle, to guide slow neutrons. They are the four members who guided to the we need to be able to measure a moment on constructed BL05.

[2] convert as many cold neutrons as possible to ultracold neutrons. Cold neutrons move at a speed of 1,000 meters per second. The current supermirror technology only allows reflection of neutrons moving slower than 42 meters per second. Any neutrons moving faster than that can escape the container. In practice, this means that significant portions of neutrons are lost before they can be used in the experiment.

A schematic “We know that there are a sufficient number of view of the neutrons which travel at under 200 meters per ultracold second being produced at BL05,” says neutron Shimizu. “We need a supermirror that can reflect back those neutrons.” source at J- PARC. This He says a supermirror that could reflect source is neutrons with a speed under 100 meters per used for the second would be sufficient. Simple kinematics neutron says that if the mirror is in motion at 100 electric dipole meters per second away from the neutron moment source, those neutrons moving at 200 meters experiment. per second would come to complete halt when they hit the mirror. Neutrons moving less than 200 meters per second would be slowed. The team therefore has designed a clever bounce around inside of a small volume. The turbine of supermirrors that rotates at a few challenge is to make the cloud of neutrons as Accelerator scientists generally use magnets Mach speed in vacuum to move the mirrors dense as possible. to skew and focus beams of charged particles. away at 100 meters per second. They call this For electrically neutral neutrons, however, that turbine a Doppler shifter. “The neutrons in the storage cell are moving is not possible. To prevent dispersion of around randomly and behave like a gas, so neutron beams during transport to an Supermirror when compressed, the temperature rises, experimental station, the team introduced a The big challenge is to create a supermirror making them move faster,” explains Shimizu. neutron guide with specially designed mirrors that can reflect back neutrons moving at 100 “Faster neutrons will simply penetrate through on the inner surfaces to reflect neutrons away meters per second. The current supermirror the container.” from the guide walls and contain them inside consists of ten thousand layers of 5- the guide. This was At ILL, about 10 ultracold neutrons per cubic one of Shimizu’s centimeter enter the storage container. This previous means that, ideally, their 20-litter volume can contributions to hold a total of 200,000 ultracold neutrons. An the neutron alternative to increasing density is to make the science. volume larger. However, neutron precession can be disturbed by very small magnetic Each mirror is itself fields. Using a larger volume is not practical a work of art. since a larger cell requires a larger magnetic Because neutrons shield. are electrically neutral, they easily For this reason, neutron laboratories from penetrate ordinary around the world are trying to increase storage substances, density, rather than increase storage volume. undisturbed by the Both the Paul Scherrer Institut (PSI) in electrons and Switzerland and the National Laboratory for protons in Particle and Nuclear Physics (TRIUMF) in everyday matter. Canada aims to hold neutrons of the order of To reflect them A supermirror consists of layers of thin films to create several thousands per cubic centimeters. back, the guide effective potentials with finely spaced nuclei so as to prevent That's a two-digit improvement. “Our group at uses special neutrons from passing through. KEK is aiming for at least two-digit mirrors with fine improvement, and possibly more. This would granularity film increase the sensitivity of the device by a coating for the inner nanometer thick films. The new mirror will factor of at least ten,” says Shimizu. surface. Even though neutrons are undisturbed require between 100 thousand to a million by electric charges in atoms, they still layers of 2-nanometer thick films. Neutron guide experience nuclear potential due to nucleus. The potential is very small at 10-4 eV, but The NOP team has just launched into the The enabling technology is called ion beam tightly spaced nuclei can form a potential to research and development of the neutron spattering. Nearly a decade ago, the neutron reflect back slow-moving neutrons. This electric dipole moment experiment using a team from Kyoto University and the Institute of specially fabricated mirror is called a neutron beamline called BL05. BL05 produces Physical and Chemical Research Laboratory supermirror. cold neutrons from protons provided by a 3 (RIKEN) first used ion beam spattering to form GeV proton synchrotron at J-PARC, and sends films for the first . Now, the them to three beamline branches. The R&D is Doppler shifter technology has become standard in neutron conducted at an experimental station installed For their R&D work, the team proposes to take experiments worldwide. on one of the branches. a step further and use this supermirror to

[3] Over the past few years, “By determining the lifetime of the neutron, we Europe has caught up with can answer some very fundamental questions the technologies required in physics. For example, we will improve our for supermirror production. understanding of fundamental properties of the However, Japan still holds Standard Model, and of the composition of the strongest technological elements during the nucleosynthesis phase of base in these technologies, the early universe,” says Hidetoshi Otono, a and is best positioned to PhD student at the University of Tokyo. advance the frontier further. Shimizu says that “We will hopefully put a period to the long his team’s goal is to create debate over the lifetime of neutrons sometime a mirror which can reflect this year,” says Dr. Tamaki Yoshioka of KEK. He neutrons with velocities up and Dr. Kenji Mishima of KEK have been to 119 meters per second. working on the proposal for the neutron electric dipole moment experiment. Neutron decay The neutron lifetime is very difficult to measure. Depending measurement on the experimental methods, past experiments have At another BL05 reported different values for the lifetime. experimental station, the NOP group at KEK is now starting the first phase of the project: neutron decay time. About 15 minutes after its birth, a neutron decays into a proton, an A neutron electron, and an decays into antineutrino. Many a proton, an laboratories report that the electron, lifetime to be around 886 seconds. One laboratory in and an Russia, Petersburg Nuclear antineutrino Physics Institute (PNPI), via a W reports it to be at 879 boson. seconds, one percent less than the world’s average. PNPI, however, has the What’s impressive is that just a handful of best experimental physicists completed the designs and actual sensitivity. The team’s aim construction of the beamline in less than three is to determine the precise years. “We are conducting processes that Sohei Imajo is a PhD student from Kyoto University lifetime of a neutron. would have normally taken more than ten working on Doppler shifter. years,” says Shimizu. "In the 25 years since According to Shimizu, the KEK produced the world's first neutron beam research and development from an accelerator, there has been little for the neutron electric neutron science at KEK. We are now trying to dipole moment experiment make up for those 25 years. will benefit from the physics of the neutron The team has successfully applied their decay experiment. An innovative ideas about neutron optics using observable called spin- supermirrors. In the very low energy, clean electron asymmetry in background, high quality neutron beamline at neutron decay can explore J-PARC, their physics endeavor now starts with some of the same physics unprecedented experimental sensitivity. as the neutron electric Mishima who has worked on the project since dipole moment. Although its inception three years ago says, “Finally, we the measurement will not are at this moment of beginning that we have be as precise as that for long waited for. We are looking forward to the electric dipole moment, presenting the world with new conclusive At J-PARC, the 3 GeV synchrotron proton ring provides the team thinks that the results.” proton beams to the neutron experimental hall at the result will help refine the Materials and Life Science Facility. The proposed electric dipole moment experiment dipole moment experimental station will be built at the setup. end of the linac.

HIGH ENERGY ACCELERATOR Related Link: Paper: Neutron Optics and Physics Design of neutron beamline for RESEARCH ORGANIZATION (KEK) fundamental physics at J-PARC BL05 (Nuclear Instruments and Address : 1-1 Oho, Tsukuba, Ibaraki Methods in Physics Research 305-0801 JAPAN Section A, Volume 600, Issue 1, p. Home : http://www.kek.jp/intra-e/feature/ 342-345.) Mail : [email protected]

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