5 Key Findings from the Large Hadron Collider (To Date)

Sometimes, the smallest details can lead to the biggest breakthroughs. By the early 1900s, physicists believed they had the universe figured out, thanks to Newton's laws of gravity and Maxwell's electromagnetic theories. However, the mystery of radioactivity remained unsolved, sparking a revolution in science that uncovered the profound complexity hidden within the tiniest particles.

The fields of particle physics and quantum mechanics, which explore the smallest building blocks of matter, introduced two additional fundamental forces and a variety of exotic particles. After the 1970s, the focus shifted to testing and refining the standard model. Decades of experiments with particle accelerators filled in many gaps, but questions persisted: Why do certain particles have mass while others do not? Is it possible to unify the four fundamental forces or reconcile general relativity with quantum mechanics?

Could one of these unresolved questions lead to another scientific revolution? To find out, scientists built the most powerful particle collider ever conceived: a 16.8-mile (27-kilometer) ring of superconducting magnets, operating at temperatures colder than space, designed to collide particles at nearly the speed of light in an ultrahigh vacuum. On September 10, 2008, the $10 billion Large Hadron Collider (LHC), a global collaboration involving hundreds of scientists and engineers, became part of CERN's accelerator complex and quickly set new records for particle collisions.

Let's revisit the key findings, beginning with the most renowned discovery to date.

5: The Higgs Boson

In the macroscopic world, we take it for granted that all particles possess mass, no matter how minuscule. However, in the microscopic realm, electroweak theory, which unifies the electromagnetic and weak forces, suggests that certain particles known as mediators should be massless. This presents a challenge, as some of these particles do, in fact, have mass.

Mediators act as carriers of forces: Photons are responsible for electromagnetism, while W and Z bosons transmit the weak force. Interestingly, photons are massless, whereas W and Z bosons are quite heavy, each weighing roughly 100 times the mass of a proton [source: CERN].

In 1964, physicist Peter Higgs from the University of Edinburgh, along with François Englert and Robert Brout from the Free University of Brussels, independently proposed a groundbreaking idea: a unique field that imparts mass to particles based on their interaction with it. If this Higgs field existed, it would necessitate a corresponding mediator particle, the Higgs boson. Detecting such a particle, however, would require a facility as advanced as the LHC.

In 2013, scientists confirmed the discovery of a Higgs boson with a mass of approximately 126 giga-electron volts (GeV), equivalent to the mass of about 126 protons (thanks to mass-energy equivalence, electron volts can be used as a unit of mass) [sources: Das]. This discovery didn't conclude research; instead, it opened new avenues for exploring the universe's stability, the imbalance between matter and antimatter, and the nature and prevalence of dark matter [sources: Siegfried].

4: Tetraquarks

In 1964, two researchers grappling with the complexities of hadrons -- subatomic particles influenced by the strong force -- independently theorized that these particles were composed of a fundamental particle with three variants. George Zweig referred to them as aces, while Murray Gell-Mann named them quarks and categorized their types, or flavors, as "up," "down," and "strange." Later, physicists identified three additional quark flavors: "charm," "top," and "bottom."

For decades, physicists classified hadrons into two groups based on their quark composition: baryons (such as protons and neutrons) consisted of three quarks, while mesons (like pions and kaons) were made up of quark-antiquark pairs [sources: CERN; ODS]. However, this raised the question: were these the only possible quark combinations?

In 2003, Japanese researchers discovered an unusual particle, X(3872), which seemed to consist of a charm quark, an anticharm quark, and at least two additional quarks. During their investigation, they also identified Z(4430), a particle that appeared to be composed of four quarks. The LHC has since provided evidence for several such particles, challenging or significantly altering the traditional model of quark arrangements. These Z particles are short-lived but may have existed briefly after the Big Bang [sources: O'Luanaigh; Diep; Grant].

3: Missing Supersymmetry

The supersymmetry theory, often abbreviated as SUSY, was proposed to address several unresolved issues in the standard model, such as the origin of particle mass, the potential unification of electromagnetism and the strong and weak nuclear forces, and the composition of dark matter. It also suggested a fascinating relationship between quarks and leptons (the building blocks of matter) and the bosons that mediate their interactions. Like baryons, leptons (such as electrons) belong to a class of subatomic particles called fermions, which have quantum properties opposite to those of bosons. According to SUSY, every fermion has a corresponding boson, and vice versa, with the ability to transform into one another [sources: CERN; Siegried].

If SUSY holds true, it would imply that fermions and bosons, the two fundamental particle types, are interconnected aspects of the same reality. This theory would eliminate certain infinite values in mathematical equations by allowing corresponding particles to cancel each other out. Additionally, it could incorporate gravity—a significant gap in the standard model—by suggesting that fermion-boson and boson-fermion transformations might involve gravitons, the hypothetical carriers of gravitational force.

Physicists anticipated that the LHC would either validate SUSY or uncover new challenges, guiding researchers toward unexplored theoretical and experimental domains. So far, neither outcome has materialized, but supersymmetry remains a viable possibility. SUSY encompasses numerous versions, each tied to specific assumptions; the LHC has only ruled out some of the more elegant and probable variants.

2: Coordinated Motion

During calibration of LHC instruments, scientists deviated from the standard proton-proton collisions and instead collided protons with lead nuclei. They observed an unexpected phenomenon: the typically random trajectories of subatomic debris were replaced by a coordinated pattern, suggesting a previously unseen level of order in particle interactions.

One proposed explanation for this phenomenon suggests that the collision produced a rare state of matter known as quark-gluon plasma (QGP), which behaved like a liquid and generated synchronized particles as it cooled. Both Brookhaven National Laboratories and the LHC have previously created QGP—the densest form of matter outside a black hole—by colliding heavy ions such as lead and gold. If QGP can indeed form from proton-lead collisions, it could reshape our understanding of the conditions immediately following the Big Bang, when QGP briefly dominated. However, there's a catch: the collision likely lacked the energy required to produce this hypothesized quark soup [sources: CERN; Grant; Roland and Nguyen; Than].

While most physicists support this idea despite its challenges, others propose an alternative explanation involving a theoretical field generated by gluons, the particles responsible for the strong force and binding quarks and antiquarks into protons and neutrons. This hypothesis suggests that gluons traveling at near-light speeds create fields that enable them to interact. If validated, this model could offer profound insights into the structure and interactions of protons [sources: Grant].

1: Signs of New Physics After All ... or Not

Despite seeming counterintuitive, many physicists hoped the LHC would reveal flaws in the standard model. After all, the framework has its issues, and a groundbreaking discovery could validate supersymmetry or open new research directions. However, as previously noted, the LHC has consistently reinforced the standard model while challenging exotic physics. While not all data has been analyzed, and the LHC has yet to reach its maximum energy of 14 tera-electron volts (TeV), the prospects for undermining the standard model appear slim.

Alternatively, a 2013 study on B-meson decay might hold clues. The research revealed B-mesons decaying into a K-meson (also known as a kaon) and two muons (particles akin to electrons), which would typically be unremarkable. However, the decay pattern observed deviated from the standard model's predictions. While the findings currently lack the significance to warrant celebration, they are promising enough to fuel optimism. Further analysis of additional data could elevate these results from uncertainty to confirmation. If validated, this unusual decay pattern might provide the first evidence of the new physics many researchers are seeking [sources: Johnston; O'Neill].