The Higgs Boson
Discussing the Significance of the Higgs Boson and the CERN LHC
The world has just bid farewell to Peter Higgs at the age of 94. This moment marks the end of an era in theoretical physics. Few names resonate in this field as profoundly as that of his. Peter Higgs’ groundbreaking work paved the way for one of the most significant scientific discoveries of our time - the Higgs Boson.
Keeping true to Higgs’ own personal preference in this regard, I shall not sensationalise or oversimplify his discovery by referring to it as the ‘God Particle’. Instead, we will focus on discussing the very essence of Higgs' thesis and the machinery of the Large Hadron Collider (LHC) at CERN that turned theory into reality.
What was Higgs’ Thesis?
How do particles acquire mass? This was a major question in theoretical physics until the arrival of Peter Higgs. The answer, as proposed by Higgs and independently by several other physicists in the 1960s, revolves around the concept of spontaneous symmetry breaking in quantum fields.
The Standard Model of particle physics, our best theory describing the fundamental forces and particles, relies heavily on the concept of symmetry. Yet, for the universe to function as we observe, some of these symmetries must be broken. The mechanism that facilitates this symmetry breaking, now famously known as the Higgs mechanism, provides a framework through which particles gain mass.
To understand this, we will have to discuss quantum field theory. Every particle is associated with a field that permeates all of space. The Higgs field is such a field, unique in that it has a non-zero value even in its lowest energy state, or vacuum state. As particles interact with this field, they acquire mass. The interaction strength or coupling with the Higgs field determines the mass of a particle. This can be expressed through the following equation, where represents the Higgs field, and denotes the mass of a fundamental particle
Here, represents the coupling constant, a parameter that varies between different types of particles.
The existence of the Higgs field, while solving the mass puzzle, predicted a corresponding quantum, i.e., the Higgs Boson, as a manifestation of field fluctuations.
This approach to mass and symmetry breaking was a game changer. It suggested that the properties of the vacuum itself, through the Higgs field, could impart mass to other particles.
The Large Hadron Collider
The Large Hadron Collider (LHC) is the world's largest and most powerful particle accelerator. It was designed to recreate conditions a fraction of a second after the Big Bang, in order to observe particles like the Higgs Boson.
The collider's construction was an engineering feat in itself, consisting of a 27-kilometre ring of superconducting magnets, cooled to temperatures colder than outer space, to guide and accelerate protons to near the speed of light.
Protons are accelerated in opposite directions in the LHC's dual ring structure, gaining energy with each pass until they collide at points where massive detectors are positioned. These detectors, including ATLAS and CMS, are designed to observe the aftermath of proton-proton collisions, searching for new particles among the outcomes.
ATLAS and CMS
The ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid) experiments were specifically designed to detect the presence of the Higgs Boson, among other research goals. These detectors work by tracking the particles produced in collisions, with each component designed to measure different types of particles and energies. The complexity of these experiments necessitate high precision in both the equipment and the analysis of the data they produce.
The Discovery
Finding the Higgs Boson in the humongous amount of data generated by the LHC collisions was like finding a needle in a cosmic haystack. Physicists and engineers employed sophisticated statistical methods to identify the signature of the Higgs Boson. One key concept in this search was the idea of "decay channels," i.e., the specific patterns of particle decay that indicate the presence of a Higgs Boson. The decay process of the Higgs Boson into other particles, such as photons or bosons, provides indirect evidence of its existence.
The statistical significance of the observed data was measured in terms of "sigma" levels. The discovery of the Higgs Boson was confirmed with a sigma level of 5, indicating a less than 1 in 3.5 million chance that the observed pattern was due to random chance, rather than the presence of the Higgs Boson.
This monumental task of data analysis and interpretation culminated on July 4, 2012, when teams from both ATLAS and CMS experiments announced the discovery of a new particle consistent with the Higgs Boson. This announcement was nothing short of a moon landing moment for science, showcasing what can be achieved through global collaboration and ingenuity.
Decay Channels and Sigma Levels
2012 was a monumental year for CERN and the field of particle physics, culminating in the awarding of the Nobel Prize in Physics to Peter Higgs and François Englert in 2013.
Before moving onto the recent discoveries being made at CERN by the LHC, I would like to share some short notes on Decay Channels and Sigma Levels. Although we did briefly discuss these concepts in the previous section, I find it pertinent to share some more information about them in order to underscore their significance.
Decay Channels
The Higgs Boson, by nature, is incredibly short-lived. Once produced in a high-energy collision within the LHC, it decays almost instantaneously into other particles. These decay channels are what experiments like ATLAS and CMS aim to detect. The primary decay channels through which the Higgs Boson was observed include its decay into two photons
The decay into two photons is particularly significant due to its clear signature in the detectors. The equation governing this decay process can be represented as
While the decay into four leptons, through an intermediate pair of Z bosons, offers a distinct and measurable final state, represented as
These channels provided the clearest evidence of the Higgs Boson's existence, with the data from these decay processes meticulously analysed to ensure the observed particles were indeed the products of a Higgs Boson decay.
Sigma Levels
In particle physics, the certainty of a discovery is often measured in "sigma" levels. This is a statistical measure indicating how unlikely it is that an observed effect is due to chance. The gold standard for a discovery in particle physics is 5 sigma, which corresponds to a probability of about 1 in 3.5 million that the result is a fluke.
The announcement of the Higgs Boson's discovery was made when both the ATLAS and CMS experiments independently reached and surpassed this 5 sigma threshold, providing compelling evidence for the Higgs Boson's existence. This rigorous statistical analysis was crucial in confirming that the signals detected were not merely anomalies but indicative of the Higgs Boson.
Recent Discoveries after The Higgs Boson
The LHC experiments, especially the LHCb (Large Hadron Collider beauty) experiment, have been at the forefront of discovering new particles.
The LHCb collaboration has observed three never-before-seen particles. A new kind of "pentaquark" and the first-ever pair of "tetraquarks". These discoveries are crucial as they provide insights into how quarks bind together to form composite particles, expanding our knowledge beyond the conventional hadrons made up of two or three quarks.
Over the past decade, more than 59 new hadrons have been identified. This has helped shed light on the complex interactions governed by quantum chromodynamics (QCD), the theory describing the strong force that holds quarks together inside hadrons. These discoveries include both excited states of known baryons and mesons.
This "particle zoo 2.0" not only tests the limits of the quark model but also enhances our understanding of the strong interaction, crucial for accurately modelling collisions at the LHC and potentially hinting at new physics phenomena.
These experiments have also provided invaluable insights into the interactions of the Higgs boson with other particles, confirming that W and Z bosons, as well as the heaviest fermions like the top quark, bottom quark, and tau lepton, obtain their mass through interactions with the Higgs field (CERN).
The Future of Particle Physics
The identification of the Higgs Boson was a milestone marking the completion of the Standard Model of particle physics. But as is the case with every discovery, it also presents new puzzles.
One of the most pressing questions is the nature of dark matter, which makes up about 27% of the universe but doesn't interact with the electromagnetic force, making it invisible and detectable only through its gravitational effects. The Standard Model, even with the Higgs Boson, does not account for dark matter.
Another significant area of research is the imbalance between matter and antimatter in the universe. Theories and experiments, including those at the LHC, are actively investigating why the universe is dominated by matter, despite the expectation that the Big Bang should have produced equal amounts of matter and antimatter.
The discovery of the Higgs Boson has also spurred the development of new particle accelerators and experiments designed to explore these unanswered questions. Future projects, such as the High-Luminosity LHC (HL-LHC) and proposed colliders like the Future Circular Collider (FCC) and the International Linear Collider (ILC), aim to provide higher energy levels and collision rates. These enhancements are crucial for probing deeper into the Standard Model and beyond, searching for evidence of supersymmetry, extra dimensions, or other phenomena that could revolutionise our understanding of the universe.
This was a fun read. Particle physics always amazed me. Knowing it takes 5 sigma to ascertain the presence of a particle is insane. I don’t think I can spell my own name to that level of certainty 🫠
Nicely done, and RIP Peter Higgs! What a cool pioneer, although he would certainly imply otherwise.
I wrote this piece shortly after the discovery took place, and you might really enjoy it: https://discover.hubpages.com/education/higgsboson (I'm a better writer now, but I was pretty plugged in to the physics)