Precision Nuclear Probes for New Physics
• Physics 15, 108
Tests of standard models of particle physics using nuclear isotopes are becoming increasingly precise but they have a way to go before they can confirm the existence of new particles.
Diana Prado Lopes Aude Craik/Harvard University
Traditionally, to find new fundamental particles, physicists have used pulverizers, smashed high-energy particles, and surveyed the wreckage. But in recent years, the search for low-energy nuclear has emerged as a more refined path to particle discovery. Two new studies hint at their potential [1, 2].
The standard model of particle physics describes the basic field of the Universe and its associated particles. But that doesn’t say anything about phenomena like gravity, dark matter, and dark energy—disappearances that cause particle physicists to speculate about yet-to-be-seen particles.
Isotopes are ideal laboratories for the precision nuclear search for new particles because they have a constant number of protons and electrons. Adding a neutron to make a new isotope causes only a small, predictable change in the energy level of its electronic orbital—the region where the electron is located—and in the g factor—a measure of how strongly an electron bound to an isotope reacts to a magnetic field. Hypothetical particles paired with neutrons and electrons can make deviations from the predicted values of these parameters.
Team Sailer, nuclear physicists at the Max Planck Institute for Nuclear Physics (MPIK), Germany, and colleagues have now developed a method to compare g factor of two ions; in this case, the two isotopes of neon [1]. A different team led by physicist Vladan Vuletić of the Massachusetts Institute of Technology (MIT) have enhanced their pre-existing study of the electron energy levels of ytterbium. [2]. None of the studies found evidence for the new particle, although the MIT group observed unexplained aberrations in the isotope’s energy levels.
In 1963, Oxford University physicist William King realized that plotting the energy levels of the heavier isotopes yielded straight lines. More recently, physicists have proposed that the nonlinearities, or kinks, in the so-called King plots could show clues to physics beyond the standard model. [3]. The MIT group began looking for this tangle in the isotopes of the ytterbium ion. Two years ago, the group measured the energy level shifts for the two electron configurations in D declared and found evidence for considerable obstinacy, signaling the discrepancy between their values and the expected straight King plot. But a group at Aarhus University in Denmark working with calcium isotopes saw no such deviation (see Synopsis: Hints on the Dark Boson). The measurement of calcium is more precise, but the ytterbium has a better sensitivity to the new particles.
Now the MIT group has confirmed the presence of nonlinearity in ytterbium. By measuring the energy level shift for F state electron configurations, they increase their sensitivity to nonlinearities 20-fold. Using density-functional theory, MIT team members Paul-Gerhard Reinhard at the University of Erlangen in Germany and Witold Nazarewicz, at Michigan State University calculated nonlinear causes. They found that most of the deviations from the King’s plot resulted from the effects of the nuclear structure. However, nuclear theory cannot explain everything, suggesting a second, unknown source for the deviation.
The MPIK-led method took a different route to discover new physics, which Sailer found simpler and more direct. For their method, it is not necessary to find nonlinearities in the plot. Instead, they simply trap two ions with different isotopic numbers and directly compare g factor, which they obtained by repeatedly comparing the spins of the isotopes. By observing the correlation between spins, they were able to see the small frequency difference caused by the extra neutrons. By analogy, Sailer says it’s like listening to two instruments with mismatched frequencies and trying to find the beat.
Initially, Sailer and his team worked on calcium isotopes. But after half a year of hardship (Sailer says restrictions during much of the pandemic restricted access to the source of the needed ion) the team turned to neon isotopes, which are much easier to produce. With a precision comparable to that of the MIT group, the MPIK group saw no deviations in the measurements g factor.
Even though MPIK g-difference factor measurements have high precision, this method is currently limited by uncertainties in the measurement of third-party properties such as the charge radius of different neon isotopes. And, like measurements of isotope energy levels, gThe-factor method is also susceptible to nuclear effects which are not new physics. “I spent a lot of time worrying and trying to rule out the possible effects of these other unwanted effects,” said Vincent Debierre of MPIK, a co-author of the study. “This is known to physics but in practice, it is often difficult to calculate [its] proper contribution.”
Where these two results leave the possibility of a hypothetical new particle is unclear. The nonlinear sources of the two MIT groups have not been corroborated, and there are strong limitations on each new particle from both previous calcium measurements and from previous electrons. g– measurement factor. But those concerns haven’t bothered other researchers in the field.
“From a theorist’s point of view, it’s very motivating and rewarding to see that [isotope measurements] of interest to experimentalists,” said Elina Fuchs, a theoretical physicist at CERN, Switzerland, who was not involved in either study. He estimates that about ten groups are currently working on the search for similar isotopes, up from two or three in 2020—a number that could grow with g-factor technique. “This is an impressive precision measurement and an interesting alternative method,” said Vuletić. “This new work is a great addition to a series of low-energy precision experiments looking for new physics beyond standard models.”
Fuchs is also happy with the data coming out of the other groups. In May, a group at Kyoto University in Japan also claimed to see nonlinearity in their King of ytterbium plot. [4]. Both Sailer and Vuletić believe that they can improve the precision of their respective methods by an order of magnitude over the next few years. To reduce uncertainty, one possible future method involves a combination of: g-factor and energy level measurement. “In principle, you can be very sensitive to new physics and you can clean up all the uncertainty,” says Debierre.
–Dan Garisto
Dan Garisto is a freelance science writer based in New York.
Reference
- T. Sailor et al.“Measurement of bound electrons g-difference factor in coupled ions,” Natural 606 (2022).
- J. Hur et al.“Proof of Nonlinear Plots of King Two Sources in Spectroscopic Search for New Bosons,” physique. Rev. Lett. 128 (2022).
- JC Frowning et al.“Investigating New Long Distance Interactions with Isotope Shift Spectroscopy,” physique. Rev. Lett. 120 (2018).
- K. Ono et al.“Generalized Nonlinear Observation of King Plots in the Search for New Bosons,” physique. Rev. X 12 (2022).
Subject area
#Precision #Nuclear #Probes #Physics
Comments
Post a Comment