Precision Tests of Fundamental Interactions

Laser spectroscopy of antihydrogen: In a series of the papers published in Nature, the ALPHA collaboration reported the first-ever laser spectroscopy of antimatter atoms. Over the past several decades, laser spectroscopy of the ordinary hydrogen atom has reached exceedingly high accuracies to the level of 4x10^-15 level. Now ALPHA has achieved laser measurements in antihydrogen, with an initial precision of  2x10^-10, which was subsequently improved to 2x10^-12. The latter is the most precise direct measurement of antimatter properties, and represents a test of matter-antimatter symmetry, known as CPT symmetry, with a parts-per-trillion precision. The results attracted significant attention both from the scientific community and the general public.    

Canadian laser breakthrough towards laser-cooling of antimatter: In Nature (2018), the ALPHA collaboration reports the first-ever observation of a key atomic transition in antihydrogen, the so-called Lyman-alpha transition. In ordinary hydrogen, this is one of the most important transitions in the Universe, responsible for first light produced after the Big Bang when the electrons and the protons combined to form hydrogen atoms. However, the transition is notoriously difficult to observe in antimatter, partly because of the technical challenges with producing laser light to drive the transition. The observation of the Lyman-alpha transition in antihydrogen was made possible by the development of an innovative laser system by a team from TRIUMF and the University of British Columbia. The Lyman-alpha transition not only provides important information of the structure of the anti-atom, but also can be used to control the motion of antihydrogen. In particular, the laser cooling of antihydrogen will enable the creation of ultra-cold antihydrogen, essential for future spectroscopy and the gravity studies of antimatter.

Successful construction of the ALPHA-g antimatter gravity detector: In July 2018, the construction at TRIUMF of the time-projection chamber (TPC) for the new antimatter gravity experiment, ALPHA-g, was completed. The TPC detector has been shipped to CERN, where it is currently being tested. The detector is a key component for the ALPHA-g project, whose goal is to measure the gravitational property of antimatter by dropping antihydrogen atoms inside a detector. The TPC will measure the location of the antihydrogen annihilations, from which the effect of gravity will be inferred. According to the equivalence principle in Einstein’s theory of gravity, matter and antimatter should behave identically under the force of gravity. However, no one has ever seen how antimatter falls. The ALPHA-g experiment is set to be the first to do just that.

The most accurate beta asymmetry in nuclear or neutron decay: As reported in Physical Review Letters (2018), scientists using TRINAT measured the asymmetry in the average direction of beta particles with respect to the nuclear spin of potassium-37 (³⁷K), achieving the best fractional accuracy of any nuclear or neutron decay. Since C.S. Wu's discovery of parity violation using this observable in 1957, steadily improving experiments have shown no evidence for right-handed neutrinos. When compared to other nuclear beta decay experiments, the 37K result shows a possible 2.2 sigma discrepancy in the strength of the weak interaction in different nuclei, empirically and suggestively correlated with the density of nuclear magnetism.  

A new low-stress elastopolymer viewport seal compatible with ultra-high vacuum: As reported in Review of Scientific Instruments (2014), a TRINAT co-op student developed a technique for a vacuum viewport seal that minimizes stress-related birefringence while maintaining ultra-high vacuum. One of many systematic effects that alters atomic polarization is imperfect circularly polarized light, and a common well-known difficulty is stress-induced birefringence (the same effect that produces the appearance of colors in stressed, otherwise clear and colourless, Scotch tape. The new technique keeps the circular polarization almost perfect.

Using the first small data set, NA62 improved the exclusion limits for heavy neutral leptons coupled to electrons and muons by an order of magnitude for masses less than the kaon mass (200 - 495 MeV); this study is similar to that done by PIENU in a lower mass region. As reported in Physical Letters B (2018), so far only three neutrinos have been found, corresponding to the three electron-type particles. The tiny values of the neutrino masses have stimulated theoretical speculation that additional heavy neutral leptons may exist which, if verified, could have important consequences for the origin and composition of the universe; however, the mass range for those new particles is relatively unconstrained. NA62 could directly observe heavy neutral leptons coupled to electrons and muons such as sterile neutrinos in the 2-body decays of kaons K⁺→e⁺ν_h  and K⁺→µ⁺ν_h  (where ν_h is a massive neutrino) by observing an extra peak in the positron or muon energy spectrum. After suppressing backgrounds by several orders  of magnitude, the search came up empty, allowing new order of magnitude more sensitive limits to be obtained. NA62 expects to continue improving these results with more data.  

Improved search for heavy neutrinos in the decay π→eν: Using the complete data set, PIENU has improved the exclusion limits for heavy neutral leptons coupled to electrons by an order of magnitude for masses less than the pion mass (139.6 MeV).  As reported in Physical Review D (2018), so far only three neutrinos have been found, corresponding to the three electron-type particles. The tiny values of the neutrino masses have stimulated theoretical speculation that additional heavy neutral leptons may exist which, if verified, could have important consequences for the origin and composition of the universe. However the mass range for those new particles is relatively unconstrained. PIENU could directly observe evidence for heavy neutral leptons such as sterile neutrinos in the 2-body decays of pions π⁺→e⁺ν_h where ν_h is a massive neutrino, by observing an extra peak in the positron energy spectrum. After suppressing backgrounds by five orders of magnitude,  the search came up empty allowing new more sensitive limits to be obtained; these results give the best limits in any mass region so far studied. In future, PIENU expects to have new results on heavy neutrinos coupled to muons by studying π⁺→µ⁺ν_h.  In addition, the comparison of the measured and predicted π⁺→e⁺ν branching ratio also constrains the presence of non-SM neutrinos in the lowest energy region.

Half-life measurement provides clearer view of the weak force: Scientists using GRIFFIN achieved a half-life measurement of magnesium-22 (²²Mg) three times more precise than the previously adopted world average. As reported in Physical Review C (2017), this high-precision measurement provides a clearer view of the dynamics of the weak force. Precision measurements of the ft values for superallowed Fermi β-decay transitions between isobaric analog states provide fundamental tests of the Standard Model's description of electroweak interaction. These transitions provide a stringent test of the conserved vector-current (CVC) hypothesis, and in combination with other values, they also provide the most precise determination of V_ud, the most precisely determined element of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix. Researchers used a 4π proportional gas counter and the GRIFFIN spectrometer to make the ²²Mg half-life measurement, resolving a discrepancy between the two previously published ²²Mg half-life measurements.  

First production of ultracold neutrons at TRIUMF:  In fall 2017, the Japanese-Canadian TUCAN (TRIUMF Ultra Cold Advanced Neutron source) collaboration succeeded for the first time in producing ultracold neutrons (UCN). This was a major milestone towards the search for the elusive neutron electric dipole moment (nEDM). UCN move so slowly, about 5 meters per second compared to about 500 meters per second for air molecules, and with such low energy that they can be contained and observed. Thus, UCN are ideal for determining the nEDM, which TUCAN aims to measure with the highest-ever precision. The nEDM is predicted to be vanishingly small, but if it is measured to be larger than expected, the TUCAN results could aid in solving a key cosmic puzzle: why there is much more matter than antimatter in the universe.  

A new primary proton beamline at TRIUMF for production of spallation neutrons: BL1U is a new primary proton beamline commissioned in TRIUMF’s Meson hall in Fall 2016. The beamline ends in a target made of tungsten and provides spallation neutrons for fundamental neutron research to the Ultracold Neutron Facility. BL1U is unique in that it shares the proton beam provided by TRIUMF’s 520 MeV cyclotron with BL1A, the other primary beamline in the Meson hall. This is facilitated by a special, very fast-kicker magnet which ramps its field on-and-off during the 100 microsecond gap between two proton pulses, and thus diverts single proton pulses out of BL1A into BL1U. Since the Centre for Material and Molecular Science instruments rely on BL1A proton beam, this innovative beam sharing enables the simultaneous operation of both facilities.

Isomeric states in light francium nuclei: Francium is the heaviest alkali element. In addition to its simple atomic structure, it also possesses a fairly simple nuclear structure based on what has been observed to be an inert lead core with 5 additional protons. This combination makes francium one of the leading candidates upon which to perform high precision experiments to test both nuclear and atomic theories as well as to perform fundamental tests of the standard model. A precursory experiment to investigate the nuclear structure of very light francium isotopes confirmed for the first time that several isotopes contain long-lived isomeric states. The nuclear structure of these states has been determined via laser spectroscopy, providing invaluable input and tests of both nuclear and atomic theories.    

Seeing francium nuclei as tiny magnets: The ratio of the hyperfine splittings of s and p states is not constant across isotopes due to the isotope-dependent distribution of nuclear magnetization, a phenomenon called the hyperfine anomaly. By carrying out measurements of the hyperfine splitting of the excited electronic 7p_1/2 state at the 100-ppm level, and comparing to previously known ground state 7s splittings, the hyperfine anomaly in six isotopes of francium (Fr) was experimentally determined. As reported in Physical Review Letters (2015) the measured magnetic distributions behave regularly from ²¹³Fr through ²⁰⁷Fr, but ²⁰⁶Fr stops behaving like a spherical nucleus with valence nucleons. The results are valuable input for future calculations of both the anapole moments and the neutron radii needed for small corrections to Francium Trapping Facility measurements of atomic-parity violation for ^207−213Fr.  

While some francium atoms escape, most stay trapped: As reported in the Canadian Journal of Physics (2017), laser-trapped francium atoms were irradiated with blue laser light, causing some of them to be photoionized (lose an electron) and lost from the francium trap. The probability of photoionization was in line with the general trend exhibited by the other alkali atoms. Photoionization losses from the laser trap are one of the most serious limitations for a trap-based tests of atomic parity violation, and the results of this experiment importantly support the feasibility of such experiments.

Key step towards historic measurement of atomic parity violation in francium: Francium Trapping Facility scientists made the first excitation of the highly forbidden 7s-8s transition on which future atomic-parity violation measurements will be based. As reported in Physical Review A (2018) the researchers scrutinized the accuracy of theoretical predictions of the overlap of the valence electron wavefunction with the nucleus (field shift) and electron-electron correlations (specific mass shift) in francium was carried out, another critical test towards understanding atomic theory in francium.

Pions, muons and positrons: The M11 beam channel provides low intensity beams of pions, muons and positrons for testing and calibrating detectors for particle physics experiments world-wide.  Three notable examples from the period 2013-2018 are described here. TREK The TREK experiment is searching for new physics beyond the standard model in the rare decay modes of kaons. In November 2013 and June 2014, a group led by Mike Hasinoff (UBC) tested the scintillating fibre target to establish the bias voltage offsets for each individual MPPC detector/scintillating fibre combination. Muon and positron tracks through the detector were measured to compute individual fibre efficiencies. This target was subsequently used for data taking in Experiment 36 at J-PARC in Tokai, Japan. Previous to this, in November 2012, the TREK group tested TOF counter time resolution and e-mu discrimination using a polyethylene block placed in front of a Lead-Glass-Counter to change the shower development. ATLAS Polycrystalline chemical vapor deposition (pCVD) diamond detectors are a candidate for forward calorimetry in the high luminosity environment of the CERN LHC.  One such detector was exposed to particles delivered by M11 to quantify the variation in signal response across the surface of such a detector. A discrepancy was observed in the diamond detector’s response to beam particles at different bias polarities.  This study was documented in a Carleton MSc thesis, "Characterization of diamond sensors for use in ATLAS calorimetry upgrades", by Joshua Turner, 2012, Super-B and BELLE The Super-B drift chamber group undertook two tests of drift chamber prototypes in the M11 beam line in 2013. These tests were the first to demonstrate that the particle identification capabilities of drift chambers could be significantly improved by counting individual ionization events in the gas (“cluster counting”), rather than just measuring the total energy deposited in each cell. The results were published as NIM A735, 169-183 (2014), and formed a key component of the PhD thesis of Jean-Francois Caron (UBC, 2015). After the merging of the Super-B and BELLE-II collaborations, the Belle II Canada group used the M11 beamline to test pure and thallium CsI crystals with various readout options in the summer of 2015. The group subsequently decided not to pursue pure CsI. The thallium-doped CsI data have been used, along with data collected at the proton irradiation facility at TRIUMF, to develop a new method of distinguishing hadronic from electromagnetic energy deposits using pulse shape discrimination. This technique is being implemented for the Belle II calorimeter, and will form an important component of the PhD thesis of S. Longo (Victoria).  

Electron jumps reveal subtle changes in shape of francium nuclei: As part of the commissioning process for the Francium Trapping Facility, precise measurements were carried out on the isotopic dependence of the 7s − 7p_1/2 electronic transition in a chain of different francium isotopes. As reported in Physical Review A (2014) these data were combined with previously measured isotope shifts in the 7s - 7p_3/2 transition. Isotope shifts are a sensitive measure of changes in the nuclear charge radius, or size of the nucleus, between isotopes of the same atom. Comparison of the two data sets provides insights into the change of electron behaviour as the number of neutrons in the nucleus varies. The results provide a sensitive gauge of the ability of the atomic many-body calculation to describe the francium atom at a level necessary for the interpretation of the Facility's future atomic-parity violation measurements with francium.    

A new tool for low-energy fundamental particle transport investigations: As reported in Nuclear Instruments and Methods in Physics Research Section A (2017) researchers from TRIUMF and Technical University of Munich have developed PENTrack, a tool for simulating proton, electron and neutron paths in low-energy particle transport research. Knowledge of detailed particle behaviour has a large impact on systematic studies of experimental data, and simulations are key to accounting for such phenomena as complex apparatus geometries and electromagnetic fields. The simulation tool will in particular support simulations of trajectories and spins with ultra-cold neutrons at the new TRIUMF Ultracold Advanced Neutron source.      

Direct measurement of spin polarization of decaying atoms: In 2016, TRINAT demonstrated a direct atomic-physics probe of the spin polarization of potassium-37 (³⁷K) nuclei as they decayed. Many spin-polarized decay experiments disturb the polarization or must measure it separately. As reported in the New Journal of Physics (2016), the high accuracy of the polarization achieved (99.1% +/- 0.1%) enabled a sensitive search for wrong-handed neutrinos. The result also demonstrates the potential to measure the polarization to a level of precision which would be competitive in searches for new physics. The paper was selected as one of the journal's 2016 Highlights.  

Search for K+→ π+ν¯ν at NA62: Recently, NA62 (in talks at conferences, and soon to be published) announced its first results based on a small initial data set, with the observation of one event compatible with K+→ π+ν¯ν decay.  An upper limit on the branching ratio of <14x the SM expectation was set demonstrating that the experiment basically works a planned (after 10 years of development). Since data with about 30x the sensitivity has been already acquired, the experiment is on track to meet its goals of improving the measurement sensitivity by an order of magnitude compared the work done at BNL where K+→ π+ν¯ν was initially discovered with a few events observed at the one in 10 billion level. The precise measurement of this tiny branching ratio will severely test the predictions of the SM and reveal or limit the prospects for certain new avenues of theoretical speculation which go beyond the SM.   

Improved measurement of the π→eν branching ratio: In Physical Review Letters (2015), the PIENU collaboration announced an interim result (based on 10% of the data set) which improved knowledge of the branching ratio, and consequently of the equivalence of electron and muon couplings to the weak force, by a factor of two. Since the result agreed well with the Standard Model (SM) prediction assuming universality, it serves to deepen the lepton universality puzzle. It also further constrains hypothetical non-SM theories by increasing the mass-scale limitation on those theories. The subject is highly topical since tentative measurements in B-meson decays are indicating possible deviations from universality involving the third flavor or generation of particles. This work is the latest in a long series of TRIUMF experiments on pion decay, improving the precision by more than an order of magnitude. The final results are expected in late 2018.