Quarks, Collisions, and the Fate of the Universe

Kate Shaw, an assistant professor at the University of Sussex and a staff scientist at the International Center of Theoretical Physics, delivered a talk on quarks, particle collisions, and their implications for the universe's fate. Shaw works on both the ATLAS experiment at the Large Hadron Collider (LHC) and the Deep Underground Neutrino Experiment (DUNE), a next-generation neutrino experiment under construction in South Dakota, USA. Her talk focused primarily on collider physics and her nearly 20 years of experience with ATLAS.

The Large Hadron Collider: A Glimpse Inside

The LHC is a 27-kilometer-long accelerator that smashes together protons, some of the smallest objects in the universe. Protons, with a size of approximately 0.8 x 10^-15 meters, are accelerated to nearly the speed of light (300 million meters per second). These proton beams circle the LHC 11,000 times per second, colliding billions of times per second. This incredible feat is achieved using superconducting magnets, which must be kept at an extremely cold temperature of 1.9 Kelvin (-270°C), colder than outer space (which is about 3 Kelvin). This makes the LHC one of the coldest places in the universe.

The primary purpose of smashing these particles together is to break them apart, study their internal structure, and recreate conditions similar to those present at the Big Bang. When particles collide at high energies, they can annihilate and create any particle that existed at the Big Bang. This process is akin to colliding oranges and having bananas emerge, meaning new and unexpected particles can be produced. Scientists use these collisions to answer fundamental questions about the origin and composition of everything, and the ultimate fate of the universe.

The Evolution of Our Understanding of Matter

The journey to understanding matter began in the late 19th century. Scientists initially understood that molecules were made of more fundamental elements. Dmitri Mendeleev created the first periodic table, organizing elements by atomic mass and observing recurring patterns in their chemical properties, which allowed him to predict new elements. Henry Moseley later refined this, showing that the atomic number (number of protons) was the key organizing principle. The arrangement of electrons in shells around the atom explains the chemical properties of elements, with noble gases, for example, having full outer shells, making them inert.

Early models of the atom included J.J. Thomson's "plum pudding" model, which proposed a positively charged sphere with electrons embedded within it. Ernest Rutherford's gold foil experiment, however, disproved this. By firing alpha particles at a thin gold foil, Rutherford observed that while most particles passed straight through, a few were violently deflected. This led to his model of the atom, where a tiny, dense, positively charged nucleus contains most of the atom's mass, with electrons orbiting around it.

The discovery of the electron (Thomson, 1897), the nucleus (Rutherford), and later the neutron, provided a seemingly simple model of the universe: electrons (negative charge), protons (positive charge), neutrons (zero charge), and photons. However, this simplicity was short-lived.

The Particle Zoo and the Rise of Quarks

New particles began to emerge: * Positrons (1932): Antimatter partners of electrons. * Neutrinos (1934): Mysterious particles that travel through matter and are difficult to detect, but are crucial for explaining stellar processes. * Muons (1937): Discovered in cosmic rays, prompting the question, "Who ordered that?" as scientists already had a seemingly complete model. * Pions: Another type of meson, further adding to the growing list of particles.

The development of "atom smashers" in the 1950s, like the Cosmotron and bubble chambers at CERN, exacerbated this problem by producing an ever-increasing number of particles (Sigmas, Rhos, Deltas, Kaons, Lambdas). This "particle zoo" threatened to make particle physics as complex as the periodic table.

In the 1960s, Murray Gell-Mann proposed the quark model, a significant simplification. He suggested that many of these particles, such as protons, neutrons, pions, and kaons, were not fundamental but were composed of even smaller particles called quarks. The word "quark" comes from James Joyce's Finnegans Wake.

The Standard Model of Particle Physics

The Standard Model organizes fundamental particles into generations: * First Generation (lightest): Up quark, down quark, electron, electron neutrino. These make up everyday matter. * Second Generation: Charm quark, strange quark, muon, muon neutrino. * Third Generation (heaviest): Top quark, bottom quark, tau, tau neutrino.

Quarks combine to form hadrons, which are categorized into: * Baryons: Composed of three quarks (e.g., protons: up-up-down; neutrons: down-down-up). * Mesons: Composed of a quark and an antiquark (e.g., pions, kaons).

Quarks cannot exist in isolation; they are always confined within hadrons. This confinement is due to the strong force, mediated by gluons. Unlike gravity and electromagnetism, where the force decreases with distance, the strong force between quarks increases as they are pulled apart. This is analogous to a spring that gets stronger the more it's stretched. If enough energy is applied to separate quarks, new quark-antiquark pairs are created from the vacuum, rather than individual quarks being released.

Inside a proton, there's a dynamic "sea" of quarks and antiquarks constantly appearing and annihilating, in addition to the three "valence" quarks that determine the proton's overall properties.

Particle Collisions and Jets

When protons collide in the LHC, their constituent quarks and gluons interact. If a quark is ejected from a hadron, the strong force pulls it back, but instead of breaking, the energy stored in the "gluon spring" creates new particles from the vacuum. This process, called hadronization, results in a "jet" of particles spraying out from the collision point. These jets are complex and challenging for scientists to analyze, as they need to reconstruct the original quarks from the spray of hundreds of particles.

The Top Quark: A Heavyweight Enigma

The top quark, predicted in 1973, was finally discovered in 1995 at the Tevatron accelerator. It is the heaviest known fundamental particle, weighing 173 GeV (gigaelectronvolts), making it 173 times heavier than a proton. Its immense mass is particularly significant.

Unlike other quarks, the top quark has a lifetime shorter than the hadronization process. This means that when a top quark is created, it decays into other particles before it can form a jet. This unique property allows physicists to study the "bare" top quark directly by analyzing its decay products, providing invaluable insights into its fundamental nature.

The Higgs Boson and the Fate of the Universe

The Higgs boson, predicted in the 1950s, is responsible for giving other particles mass. The Higgs field, which permeates all of space, has a tiny amount of energy. Particles interact with this field, and the strength of their interaction determines their mass. For example, electrons interact weakly, gaining a small mass, while top quarks interact strongly, gaining a large mass. Photons do not interact with the Higgs field and thus remain massless, traveling at the speed of light. The discovery of the Higgs boson in 2012 confirmed the existence of this field.

The masses of the top quark and the Higgs boson are intimately linked and crucial for understanding the stability of the universe. Precision measurements of these particles allow physicists to probe the "Higgs potential," which describes the energy landscape of the Higgs field.

The concept of "spontaneous symmetry breaking" is key here. Imagine a ball balanced on a hill. It's in a symmetric state, but it will spontaneously fall into a lower energy state. Similarly, the Higgs field settled into a state where particles acquired mass, allowing for the existence of atoms, stars, and life.

However, the universe's stability depends on whether the current state of the Higgs field is its absolute lowest energy state (a "stable vacuum") or merely a temporary, "metastable" state. If it's metastable, a quantum fluctuation or a high-energy event could theoretically cause the Higgs field to "tunnel" to an even lower energy state, with potentially catastrophic consequences for the universe, as it would fundamentally alter the masses of particles.

Current measurements of the Higgs and top quark masses place the universe in a "metastable" region, suggesting that it could, in principle, transition to a different vacuum state. However, scientists believe that our current understanding of particle physics is incomplete. The existence of undiscovered high-energy particles could alter these calculations, indicating a truly stable universe.

The Future Circular Collider

To resolve this mystery and further explore the fundamental laws of physics, a new accelerator, the Future Circular Collider (FCC), is being designed. This ambitious project will be a 100-kilometer-circumference tunnel, built under the Swiss-French border, dwarfing the 27-kilometer LHC. The LHC would serve as an injector for the FCC.

The FCC aims to: * Measure the mass and self-coupling of the Higgs boson with unprecedented precision. * Precisely determine the properties of the top quark. * Investigate the strength of fundamental forces. * Search for new, undiscovered particles that could complete our understanding of the Higgs potential and confirm the universe's stability.

Beyond the Higgs, the FCC will also address other major unsolved problems in physics, such as the nature of dark energy and dark matter, the matter-antimatter asymmetry in the universe, and the lack of a quantum description of gravity. The FCC represents a monumental undertaking that promises to unlock the deepest mysteries of our universe and drive technological innovation for decades to come.