Is There a Fourth Quark Family?

standard model of particle physics

Strange Deviations: Particle physicists have been puzzling over discrepancies in the conversion of quarks for some time now—their oscillation rates do not align with the standard model. This could indicate the existence of additional, yet undiscovered, types of these elementary particles. Physicists have recently measured a crucial parameter for these deviations in the isotope aluminum-26 and also scrutinized quark conversion—with quite surprising results.

According to the standard model of particle physics, there are six quarks divided into three families. The first family consists of up-and-down quarks, which constitute the protons and neutrons in the atomic nucleus. The next two families include charm and strange quarks, as well as top and bottom quarks. The individual quark “flavors” differ in their charge, mass, and weak interaction.

However, there’s more to it: Similar to neutrinos, quarks can dynamically change their nature. Under the influence of the weak nuclear force, they transform into another type of quark. This occurs, for example, in radioactive beta decay when the up quark of a proton transforms into a down quark. As a result, the proton becomes a neutron, and the atomic nucleus decays into a new element. The standard model predicts the oscillations and their proportions for each type of quark, illustrated in the 3×3 Cabibbo-Kobayashi-Maskawa matrix (CKM).

Quark Transformations Do Not Fit the Theory

CKM matrix
The CKM matrix gives the probabilities for different quark transformations. According to the standard model, each row and column must add up to exactly one.

The peculiar aspect is that each row and column of the CKM matrix should ideally sum to 1, as mandated by the Standard Model. However, this does not appear to be the case. Previous analyses have revealed significant deviations from 1 in the first row, specifically in the conversion of down, strange, and bottom quarks into an up quark. “A clear shift is observed in the conversion from down to up, leading to tensions regarding CKM unitarity,” observe Peter Plattner from CERN and his colleagues.

If these discrepancies are confirmed, it could indicate gaps in the Standard Model and the existence of yet-undiscovered quark variants. Theoretically, this could even imply the presence of a fourth quark family. However, current measurements of quark oscillation lack the precision required to exclude disturbances as the cause of these discrepancies. This limitation arises because only a specific, highly rare type of beta decay, known as “superallowed” beta decay, is physically simple enough to facilitate the necessary analyses.

Measurements on the Aluminum Isotope

At this juncture, Plattner and his team’s experiments come into play, where they precisely measured the oscillation from the down to the up quark (Vud) like never before. They employed two different facilities and methods to produce and measure the required excited aluminum isotopes for this decay. In the beginning, they made nuclei of the aluminum isotope 26Al and its excited isomeric state using the ISOLDE experiment at CERN and the IGISOL beamline in Finland.

The physicists then used laser spectroscopy to find the charge radius of these excited aluminum nuclei, which is an important parameter for measuring quark conversion in beta decay. Plattner and his team explain that, until now, the nuclear charge radius of the 26 mAl isomer has relied solely on extrapolated values, not directly experimentally determined. This uncertainty has also adversely affected previous measurements of quark oscillation.

Measured Value Reduces Discrepancies – But Not Enough

Some of the measurements were carried out at the ISOLDE facility at the CERN research center.
Some of the measurements were carried out at the ISOLDE facility at the CERN research center.

Physicists have achieved, for the first time, the experimental determination of the charge radius of the excited aluminum-26 nucleus. Through measurements conducted with two different methods, the determined radius for this isotope is 3.130 femtometers. Notably, this value deviates by 4.5 standard deviations from the previously extrapolated values, indicating that the nuclear charge radius of this aluminum-26 isomer is significantly larger than previously thought.

This surprising result has implications for quark conversion and the CKM matrix. The measured conversion rate for up-and-down quarks is slightly closer to the target value due to this change. Plattner and his colleagues report that this adjustment brings the value for the first row of the CKM matrix approximately one-tenth of a standard deviation closer to unitarity. However, this alone is insufficient to resolve the discrepancies of one to two standard deviations from the theoretically specified value of 1.

Question of New Physics Still Open

“The search for new physics beyond the Standard Model is a highly precise endeavor, even in the probabilities of quark transformations,” comments theoretical physicist Andreas Juttner from CERN. “This result emphasizes the importance of thoroughly scrutinizing all relevant theoretical and experimental parameters in every conceivable way.” Whether there are particles and forces beyond the Standard Model remains open, including with regard to undiscovered quark variants.

In this regard, there is still work to be done: “There are seven more superallowed beta emitters whose nuclear charge radii have not been experimentally determined,” report Plattner and his team. They have already begun determining the value for one of these isotopes, cobalt-54, at the ISIGOL facility in Finland.

Physical Review Letters, 2023; doi: 10.1103/PhysRevLett.131.222502