Chapter 1: Unanswered Questions in Particle Physics

As we step into 2024, our knowledge of the fundamental particles within the Standard Model remains limited. However, there are still eight significant questions that linger in the realm of particle physics.

There are numerous facets of reality that we once believed we understood, only to later discover that our prior grasp was simplistic and incomplete. Initially, we assumed that nature operated solely through classical and deterministic principles. Yet, the wave behavior of light, combined with the full realization of quantum physics, reveals a more intricate depiction of reality. Previously, we thought atoms were the building blocks of everything; now we recognize that atoms are merely one way in which more fundamental particles interact. With the establishment of the Standard Model of elementary particles and the quantum field theories that govern them, it seems naive to think we have exhausted our understanding of the entities that make up our Universe.

As researchers continue to seek new fundamental particles in an effort to address some of the pressing mysteries of our time—such as dark matter, the matter-antimatter imbalance, dark energy, the hierarchy problem, and the origins of the Universe—it's important to note that even aside from these grand puzzles, physics still has a wealth of unexplored territory. Indeed, many unanswered questions about the known particles in our universe persist, and here are eight of the most significant inquiries, along with a brief overview of our chances of finding answers.

The first video, "How we explore unanswered questions in physics," by James Beacham, delves into the methodologies and implications of exploring the fundamental questions in physics.

1. Does the proton decay?

According to the Standard Model, the proton is stable and will not decay. While there are numerous theoretical pathways for proton decay that respect energy, momentum, and electric charge, they all infringe upon a critical quantum number: the baryon number. Protons are the lightest stable baryons, composed of three quarks, and are unique in that no decay pathways have been identified. Through extensive experimental setups that involve vast numbers of protons, researchers have determined that the proton's lifetime must be at least ~10³⁴ years, which is about a septillion times the age of the Universe. However, since some mechanism must have generated a matter-antimatter asymmetry in the Universe, interactions that violate baryon number conservation must exist. By extending the proton’s lifetime to beyond ~10³⁸ years, we could dismiss many grand unified theories; conversely, if proton decay is observed, it might shed light on the baryogenesis mystery.

The second video, "The search for new particles and forces," by Michael Peskin, discusses the ongoing quest for new particles and the implications of such discoveries.

2. Can neutrinoless double beta decay happen?

Most atomic nuclei comprise both protons and neutrons. While some are permanently stable, others can decay through several mechanisms, including alpha decay, beta decay, gamma decay, and electron capture. Double beta decay is one of these processes, wherein two neutrons decay simultaneously into two protons and two electrons. If neutrinos are Majorana particles—meaning they are their own antiparticles—then neutrinoless double beta decay could occur, resulting in the emission of only two protons and two electrons. This phenomenon has yet to be observed, but ongoing experiments are actively searching for it. Discovering this decay would indicate new physics related to neutrinos that extend beyond the Standard Model.

3. Do glueballs exist?

In the context of particles bound by the strong nuclear force, we currently recognize only those composed of quarks or antiquarks. Theoretical predictions suggest that it’s possible to have bound states of gluons alone—hypothetical entities known as glueballs. Although these have been postulated for around 50 years, none have been conclusively identified. Recent observations of the X(2370) particle, which matches the expected quantum numbers and mass of the lightest glueball state, present an exciting candidate for glueball existence. If glueballs do not exist, it suggests a fundamental flaw in quantum chromodynamics, providing a critical test case for the Standard Model.

4. Do any forces unify beyond the electroweak scale?

Currently, we understand four fundamental forces in nature: gravity, the strong nuclear force, electromagnetism, and the weak interaction. The electroweak theory proposes that at energies around ~100 GeV, the electromagnetic and weak forces unify. This raises the possibility that at even higher energy levels, the strong force and perhaps gravity could unify as well. Grand unified theories introduce various phenomena and particles, and detecting any of these could offer groundbreaking insights into what lies beyond the Standard Model.

5. How does the electroweak symmetry break?

Symmetry breaking results in the emergence of several outcomes. When electroweak symmetry is restored, it resembles a ball at the top of a hill. As symmetry breaks, the ball rolls down the hill into a valley. This phase transition, known as a second-order phase transition, has significant implications, such as the potential existence of additional Higgs-like particles. Understanding this process could provide insights into the matter-antimatter asymmetry observed in our Universe.

6. How heavy is each species of neutrino?

Historically, neutrinos were believed to be massless; however, we now know they possess mass and can oscillate between different flavors. While experiments have established mass-squared differences between neutrino species, we still lack precise measurements of their individual masses. Investigating the masses of neutrinos through experiments like DUNE aims to provide critical insights into this question.

7. Can any Standard Model particles be "cracked open"?

While we have identified all Standard Model particles, including quarks, leptons, and bosons, it remains to be seen whether any of these are fundamentally indivisible. It is plausible that particles like the Higgs or quarks could be composite, made of smaller constituents. However, testing for this compositeness is limited by the energy scales of current experiments, such as those conducted at the LHC.

8. Are inertial and gravitational mass always equal?

When a massive particle is placed near another mass, it will exhibit gravitational attraction. The relationship between gravitational mass and inertial mass is crucial, as they both determine how particles respond to forces. Experiments have shown that these two forms of mass are identical to a high degree of precision, yet further tests are necessary to confirm this equivalence across various conditions and for exotic particles.

In conclusion, while we have a foundational understanding of the particles that constitute our Universe, there remain critical questions that warrant further exploration. Our assumptions about particle stability, the nature of neutrinos, and the very framework of the Standard Model must be rigorously tested before we can claim to have a comprehensive understanding of particle physics. It would be imprudent to presume we have all the answers without the requisite experimental evidence.

Starts With A Bang is authored by Ethan Siegel, Ph.D., who has written several books, including Beyond The Galaxy and Treknology. New titles, such as the Encyclopaedia Cosmologica, are forthcoming!