Mankind is always curious to know how everything started. One of the branches of science dedicated to answering this question is particle physics — the study of really tiny pieces of things i.e. particles. These particles are the really small pieces that build up the world around us and are best described in the language of mathematics. Particle physics is the purest disclosure of human curiosity about the world in which we live. It deals with the study of elementary fractions of matter and the interaction between them.

Human beings are doing a boundless struggle to discover the basic rules guiding the universe. Everything in the universe, from stars and planets to us, is made of the same basic building blocks: quarks. One mysterious and essential property of these particles is mass, as some particles like protons and neutrons have mass, and others like photons do not.

Standard Model of Elementary Particles. Image courtesy: Google Images

Over the past few years, scientists have developed an accurate model to sketch out the basic elements of matter and the forces that act on them. This model is known as the Standard Model. It conveys two basic ideas: first, that all fundamental particles are made up of matter; and second that all the particles interact with each other. In this model, there are 17 elementary particles divided into two groups: fermions and bosons. These particles are distinguished on the basis of their spin.  Since the early 20th century physicists have known that there is a natural unit of spin called h-bar (ħ). Bosons are particles having integral spin i.e. they have the spin value of 0, 1, 2, 3, … Fermions, on the other hand, are particles having half-integral spin such as ½, 3/2 and 5/2, etc.

In our understanding of the working of the universe, the concept of symmetry plays an important role. In 1915, the German mathematician Emmy Noether proved the relation between symmetry and the mechanics of the universe with her remarkable theorem, according to which whenever there is a symmetry of nature, there is a corresponding conservation law. It can be described as a transformation that leaves the system completely unchanged after the transformation has been performed. According to the Standard Model, the fundamental forces (electromagnetic, strong, weak, and gravitational) arise from symmetry and are transmitted by particles called gauge bosons. The Higgs mechanism was considered by Peter Higgs and a group of other physicists in 1964, who were trying to explain why particles have mass. Moreover, this mechanism signified the existence of Higgs boson.

Peter Higgs. Image credits: physicsworld.com

In keeping with the Standard Model, a field termed as the “Higgs field” is needed to exist throughout space, and should break the symmetry laws. For decades, the existence of the Higgs field could not be determined because, at that time, the technology required for its detection was not advanced enough. The simplest way for finding the presence of this field was to find a particle that would give rise to that field. The detection of Higgs bosons was difficult due to the high energy required to produce them. It took much time to develop advanced equipment capable of detecting the Higgs boson.

Particles entering into the Higgs field  Picture credits: symmetry magazine

Mass is a kind of energy and energy can be converted from one form to another as long as its total amount remains constant. For instance, an electron-positron pair is generated when two photons collide. Particles of mass zero that interact with quarks are termed as gluons. Like photons, the particles of light, gluons are also chargeless and massless; however, they carry strong nuclear forces while photons carry electromagnetic forces. Moreover, gluons carry color charge — like photons couple only to charged particles, gluons couple only to colored particles. When two massless gluons come together, they make a Higgs boson. The Higgs boson is more than a hundred times heavier than a proton, that’s why it is so hard to create.

Event in CMS experiment shows a decay into two photons Image courtesy: Google images

Finding Higgs Boson is no small feat. It needs to be produced because big questions require big answers and very large scientific facilities and setups. Among the very largest scientific facilities ever built is the Large Hadron Collider (LHC). The LHC is a particle accelerator at CERN (a European organization for nuclear research) in Geneva, Switzerland where it brings protons (particles made up of quarks) into collision . To transform pure energy into matter, the equivalence principle between energy and mass comes into play. There are four main detectors at the LHC — ATLAS, CMS, ALICE and LHCb — each with unique features enabling them to study millions of particle collisions per second. On 4th July, 2012, the ATLAS and CMS detectors at CERN observed a new particle in the mass region of 125 GeV. This particle was the same as the Higgs boson, as predicted by the Standard model. In simple terms, the Higgs boson can be defined as a vibration in the Higgs field, just as a photon of light is a vibration in the electromagnetic field.

ATLAS and CMS detectors setup. Image credits: cds.cern.ch

On 8th October 2013, Peter Higgs and François’s Englert were awarded the Nobel Prize for the theoretical discovery of a mechanism to understand the origin of mass. By the confirmation of the existence of Higgs bosons, we came across the mechanism of how particles gain mass. The discovery and confirmation of the Higgs boson is a start of a new era in particle physics particularly. Particle physics arises directly from our restless desire to understand our world. It is not the particles that motivate us; it is our passion to figure out what we don’t understand.

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