Taking physics one-step beyond the Standard Model

New research has revealed previously undiscovered effects in atoms — a potential stepping stone to a new era of physics.

Cross-section of the ATLAS detector at the LHC displaying one of the numerous particle collision events recorded during the search for the Higgs boson. — Atlas Collaboration/CERN

Since the discovery of the Higgs Boson — the particle that gives other particles mass — at the Large Hadron Collider in 2012 completed the Standard Model of particle physics, researchers have been on the hunt for physics that lies beyond this model. That is because, whilst the Standard Model offers the best explanation of the fundamental particles and the four fundamental forces that govern them it still can’t explain everything. Mysteries such as dark energy and dark matter do not fall under the watch of the Standard Model, meaning there must be particles and phenomena, as yet, undiscovered.

In a paper published in the journal Physical Review A researchers from Peter the Great St.Petersburg Polytechnic University (SPbPU) in collaboration with colleagues from the Physikalisch Technische Bundesanstalt (PTB) and a number of German scientific organizations discuss their findings of previously unexplored effects in atoms.

Just as the Large Hadron Collider was intrinsic to the discovery of the Higgs Boson and thus the completion of the Standard Model, it is expected to play an important role in the discovery physics beyond that model when it resumes operations in 2021. But, the researchers took a different approach in order to begin the journey to new physics employing methods of atomic spectroscopy. Studies that take this approach require far fewer resources than accelerating and colliding particles does. This different approach also has another significant advantage, high-energy physics experiments — such as the collisions of particles carried out at the LHC — aren’t as accurate as experiments in atomic physics.

The excitation of electrons by photons. (PubMed)

The team’s experiments involved measuring the electronic transitions in different isotopes of argon. Electronic transitions are the ‘leaps’ that electrons buzzing around an atomic nucleus when they absorb or emit a photon. An electron that emits a photon ‘steps down’ towards a ground state closer to the nucleus, whilst an electron that gains a photon takes a ‘step up.’ Each step requires specific energy, meaning that taking a step up or down requires interaction with a photon of a particular energy, wavelength and frequency. Much like when climbing a ladder you can rest your foot in between rungs, electrons cannot sit between states.

The team studied several states of argon ions with four, five, and six electrons — its optimal electronic configurations. These configurations can be reliably calculated, and are also are easily accessible for experiments. Scientists analyzed their results using the King plot, a widely used method for systematic studies of the isotope shifts of two atomic transitions in a chain of isotopes. This King plot should be linear to a very high degree of accuracy with possible nonlinear effects were considered much too small to be of any practical interest.

Scientists analyzed their results using the King plot. (Peter the Great St.Petersburg Polytechnic University)

When the team calculated the new effects they discovered that nonlinearities in the King plot are stronger by four-orders of magnitude than current theory suggests. Until this point, the limitations of the instruments used in experiments such as the one conducted by the team prevented these new effects from being detected. The researchers were able to avoid these limitations thanks to a new generation of spectroscopic experiments which increase accuracy significantly, thus making these effects observable.

Vladimir Yerokhin, the chief researcher at the Center for Advanced Studies of SPbPU, explains how this relates to physics beyond the Standard Model: “If the King plot turns out to be slightly curved, this may be a manifestation of new particles beyond the Standard Model of physics.

“It is necessary to continue studying these effects in other atoms with a larger number of electrons in order to reduce the influence of calculation errors.”

In the future experiments like those conducted by the team will utilise equipment which traps ions in the magnetic and electric fields allowing them to investigate using the quantum logic methods.

“If the experiment is successful, we can obtain restrictions on parameters of the proposed new particle beyond the Standard Model,” concludes Yerokhin. “In addition, such experiments will help to determine whether the fundamental constants change over time, which is of great importance for our understanding of the development of the Universe.”