By Dr Manish Sinha
Physicists have long attempted to understand the universe from the smallest scale to the largest, probing inward to matter’s tiniest components. Since the 1960s, the ‘Standard Model’ has been vigorously tested under the most extreme conditions ever created. Whilst some aspects of the Model have been verified, others displayed a stubborn elusiveness to scientific detection – for forty years preceding July 2012 the “Higgs Boson” was one such feature. It transpires that this attribute is fundamental to the existence of all visible matter in the universe; without it, nothing would have mass and we would not exist.
Matter is comprised of molecules, which are comprised of atoms. Atoms are further composed of a cloud of electrons orbiting approximately one-hundred-millionth of a centimetre away from a nucleus, which itself is one-hundred-thousandth the size of the electron cloud. Buried deep within these impossible-to-imagine dimensions is where scientists seek enlightenment on the structure of matter. How does matter acquire mass? Can matter not possess mass simply by existing? According to the Model, the answer is an unequivocal ‘no’, yet nobody could authenticate why. The discovery of the Higgs Boson changed that in an instant.
The Higgs Boson was postulated to exist by British theoretical physicist Peter Higgs at the University of Edinburgh in 1964. Under the Standard Model, this particle is what permits matter to acquire mass. It has been referred to as “the most sought after particle in modern physics” owing to its importance and both Higgs and Francois Englert were awarded the 2013 Nobel Prize in physics upon this discovery.
The Standard Model lies within the realm of particle physics, a discipline dedicated to reducing the structure of the universe to its most elementary constituents. Subsequent to the discovery of atoms in 1803, physicists have identified several of the particles that constitute atoms. The Standard Model is somewhat ‘two-dimensional’: it describes the smallest indivisible particles comprising matter in addition to elucidating a mechanism through which forces permit particles to interact with each other. Particles not only make up matter, but also constitute the forces transmitted between them. In other words, the transmission of forces through the universe is itself achieved through discrete particles.
The Standard Model matter comprises two particles with contrasting characteristics: quarks and leptons. The protons and neutrons positioned at the atom’s nucleus are comprised of quarks, whilst the electrons are comprised of leptons. The difference between them lies in variations to three fundamental characteristics: mass, electric charge and “spin”. The latter quantity describes how strongly an electrically charged particle responds to a magnetic field whilst moving through it. Overall, twelve varieties of quarks and leptons have been discovered. These are held together by precisely four different types of forces and the pertinence of each force between any pair of particles depends upon each particle’s weight and the distance between them. The four forces are: gravity, electromagnetic force, strong nuclear force and the weak nuclear force. Gravity is the weakest but interacts between particles irrespective of their separation. In contrast, the strong and weak nuclear forces are restricted to acting over distances smaller than the size of an atom.
The Standard Model stipulates that forces bridge the gap between particles through ‘carrier particles’. Collectively, these ‘carrier particles’ are referred to as “bosons” and each fundamental force possesses its own dedicated boson. The electromagnetic force field is carried by bosons called photons, whilst the strong and weak nuclear force fields are transmitted through the ‘W’ and ‘Z’ bosons. By hunting for particles with specific mass, electric charge and spin, scientists have successfully forecast the existence of fundamental particles long before they were verified through controlled experiments.
Physicists have two alternatives when searching for such particles: to observe them in nature, or artificially re-create conditions conducive to their existence. Since finding them in nature is extremely difficult, the only alternative is to physically recreate conditions similar to when the universe was allegedly formed (i.e. the Big Bang). Particle physicists take a large number of protons and neutrons (collectively referred to as hadrons), accelerate them to virtually the fastest speeds humanly possible, smash the hadrons together and observe the result. Due to the incredibly high energies involved – temperatures that exceed 100,000 times hotter than the centre of the sun – it is possible to fragment the hadrons into their constituent particles and measure the mass, electric charge and spin of particles within the resulting trail. Formidable challenges still remain: the collision of 350 trillion protons may result in fewer than 10 reliable candidate particles for the Higgs boson. Scientists are however confident and estimate that the chance of a Higgs Boson being incorrectly discovered is less than one in 3.5 million.
The facility responsible for this discovery was the $10 billion Large Hadron Collider, constructed by the European Organization for Nuclear Research (CERN) and designed by 10,000 scientists from 100 different countries. The facility consists of a circular tunnel 17 miles long buried 175 metres below the France-Switzerland border. The hadrons are accelerated around the facility to approximately 99.9999991% of the speed of light and one quadrillion-particle collision typically ensues. The resulting particles are visible for a microscopic instant only. Thus, the key to uncovering the inner particles released lies in examining the immediate “spray”. The data challenges of this analysis are immense. The collision data are typically produced at a rate of approximately 15 petabytes per year and analyzed by a grid-based computer network infrastructure connecting 140 computing centres in 35 countries. This annual data production is 20% greater than that required to store the DNA structure of every human being on earth.
The Model as described has withstood the test of time but is still incomplete. In its current state, it unifies three of the four fundamental forces of nature. Particle physicists have developed mechanisms – consisting of well-defined bosons that carry forces in specific ways – which unify the electromagnetic force, strong and weak nuclear forces. To date, no corresponding particle that transmits gravity has been discovered. The Standard Model still provides a very good description of the universe because gravity is by far the weakest of the four forces; its inclusion would represent a rounding-error to observations over microscopic scales but would, conversely, be crucial over large distances. Another limitation of the Standard Model is its inability to incorporate a range of new theories for more complicated structures of matter, commonly referred to as “dark matter” and “super-symmetry”.
Boson particles (photons) that carry the electromagnetic force were discovered in 1905 by Albert Einstein, whilst the W and Z bosons carrying the strong and the weak nuclear force were discovered at CERN in 1983. If the Standard Model is correct, the gravitational force between any two particles should be carried by bosons specific to gravity (e.g. gravitons). Not one single graviton has hitherto ever been discovered. Additionally, if bosons that carry forces between matter particles are as real and fundamental as matter particles themselves, how do particles acquire mass in the first instance? This is as fundamental as the question of life itself. Without a mechanism consistent with the Standard Model there would be no Sun, no planets, no life and the universe would consist solely of massless particles moving in straight lines. Clearly, this does not conform to common observation. The Standard Model predicts that a particle, of a specific mass, electric charge and spin must exist which interacts with matter in such a manner giving it the crucial property of mass. Whilst it is but one particle amongst many, its significance is undisputed.
In the Standard Model, the concept of energy equivalence is paramount to understanding how mass is created. A fundamental tenet of physics states that energy cannot be created or destroyed, but merely transferred from one form to another. Higg’s original proposition contained two elements: an elementary particle with zero energy, and a Higgs field with its own energy content spanning the universe. Through the so-called Higgs mechanism, elementary particles gain mass by interacting with the invisible field. A simple analogy compares the Higgs field to a crowd of people spread evenly throughout a room. An anonymous person entering the crowd can move through unhindered. However, an entering celebrity would attract significant attention leading to a congregation forming around the celebrity, thereby slowing them down. This deceleration manifests as an increase in metaphorical "mass." Similarly, as matter moves through the field proposed by Higgs, bosons with previously zero energy interact with, and are slowed down by, the field. The slowing down results in a corresponding increase in the mass of the particle through the mass-energy equivalence – no energy is created or destroyed, merely transferred from one form to another.
The Standard Model is neither perfect nor complete. Indeed, its limitations are exposed by its current inability to incorporate gravity with the remaining trio. It does however provide a robust Model through which an understanding of the smallest scale structure of matter can be acquired and which has, thus far, stood the test of time. The discovery of the Higgs Boson has been hailed as one of the most significant breakthroughs in modern physics. For the first time, we have a validated mechanism to explain how matter acquires mass. The implications are immense, and the billions of dollars spent unifying the efforts of tens of thousands of scientists worldwide have clearly been successful. As scientists investigate the various properties of the Higgs Boson, it is difficult to predict what discovery will be made next but it will almost certainly originate deep underground within the laboratories of CERN. One of the most sophisticated and expensive projects ever undertaken is crucial to the advancement of our understanding of the structure of matter within the universe.