Why do things have mass? If this question has ever bothered you, then you have something in common with some of the most brilliant minds ever existed on our planet. When you push an object, like a table, a car, or something else, the resistance you experience is the “Mass” of that object (the measure of its inertia). The mass of an everyday macroscopic object fundamentally arises from the mass of the constituting elementary particles and the forces holding those particles together. These particles do not have any physical structure and cannot be divided any further. In other words, they are the fundamental building blocks of everything around us. The question that had bothered scientists for decades was what gives these elementary particles their mass? In 2012, this mystery was resolved by the confirmation of another elementary particle, the Higgs boson, associated with the Higgs field responsible for providing the other elementary particles with their masses by interacting with them. Theoretically, photons do not interact with the Higgs field and don’t have mass, whereas quarks, electrons, and other particles that interact with it have mass. In other words, the more it interacts, the massive the particle becomes. To understand the complex nature of the Higgs boson and Higgs mechanism, let’s first understand how the hunt for this elusive particle started in the first place?
How was the Higgs boson Discovered?
The Standard Model
In the mid-twentieth century, the world of particle physics was uncharted territory for the scientific fraternity around the world. Physicists were trying to get their hold around several new particles that were either appearing in the mathematics of Quantum Field Theory or were being detected around the world. They lacked an overall framework to explain these fundamental constituents of matter and the forces that govern their behavior. In the 1970s, this framework became popular as the Standard Model of Particle Physics. It incorporated all that was known about subatomic particles at the time, the behavior of three fundamental forces (electromagnetic, strong nuclear, and weak nuclear), and predicted the existence of additional particles as well. This model reduced all of the known particles down to just 17 elementary ones. A mathematical theorem developed by Emmy Noether in 1918, that links symmetries with nature’s conservation laws to the subatomic world, placed these fundamental particles into lists and groups, much like the elements following hierarchy in the periodic table. These elementary particles are classified into two main categories: Fermions and Bosons.
Fermions
There are 12 fermions categorized as six quarks and six leptons that obey the statistical rule described by Enrico Fermi of Italy, Paul Dirac of England, and the exclusion principle proposed by Wolfgang Pauli of Austria. In technical terms, fermions have a half-integral spin that does not allow them to have a quantum state similar to other fermions. They are known as the basic building blocks of matter. More precisely, the first-generation (first column) of the standard model contains two quarks (up and down quark) that combine in the pair of three to form protons (uud: up-up-down) and neutrons (udd: up-down-down), in addition to two leptons (electron and its associated neutrino). Hence, everything that exists in our physical world is essentially made up of these first-generation elementary particles. The second-generation (second column) consists of four almost similar particles to that of the first generation except for being heavier than them (this trend is not applicable for the neutrinos). The second-generation quarks (charm and strange quarks) and one of the two leptons (muon) are highly unstable and decay into their lighter equivalents mentioned in the first generation. Likewise, the third-generation particles are even-heavier, including two quarks (top and bottom) and two leptons (tau and tau-neutrino) that are the additional parent copies of the second-generation particles. The neutrinos in the standard model are placed based on their association with corresponding leptons. They originate in the radioactive decay of several other particles; however, their transition is an open-end topic of research, accounted by neutrino oscillations, a quantum mechanical phenomenon that creates neutrino with a specific lepton family number.
Bosons
While fermions are the material particles, bosons are known as the force particles. Technically, bosons are the force carriers that mediate the strong, weak, and electromagnetic fundamental interactions. Unlike fermions, they have an integral spin and obey the statistical rules described by Satyendra Nath Bose of India (hence the name boson) and Albert Einstien. The four gauge or vector bosons that act as a force carrier are gluons, photons, W bosons, and Z bosons. Gluons are the particle associated with the strong nuclear forces that hold the quarks together inside protons and neutrons. Photons, the quanta of light, are the particles associated with electromagnetic interactions that are responsible for electrons to stick to the nucleus. Unlike other elementary particles of the standard model, photons and gluons do not have any rest mass. The W and Z bosons are the particles associated with the weak nuclear force that causes the radioactive decay of particles, decaying them into a more stable state. For instance, when a muon decays into an electron, it happens because of the transaction of the W boson and neutrinos. Unlike other Gauge bosons, the W boson is the only one that has a charge associated with them; also, W and Z bosons are more massive than other bosons, with Z being the heaviest one. The fifth boson and also the last known member of the standard model is the Higgs boson. It does not have any spin quantum number associated with it, and therefore, it is known as the scalar boson.
The Importance of Higgs boson
The Higgs boson was the longest missing piece of the standard model puzzle till 2012. While devising the standard model back in the 1960s, physicists encountered a strange problem. The quantum mechanical equations, which were being employed to design the model, were only consistent when the mass of the elementary particles was kept zero. It was known that these particles do have mass, and when physicists modified the equations to account for this fact, those equations became complex and widely inconsistent with other observations. Fortunately, in 1964, theorists Robert Brout, François Englert, and Peter Higgs made a proposal, now known as the Brout-Englert-Higgs Mechanism, which turned out to be the solution to this problem.
The Standard Model in One Equation
Quantum Field Theory suggests that the elementary particles are not something that has a physical structure but rather an excitation in some sort of field, permeating everywhere (even in the vacuum), associated with that particle. For instance, an electron is an excitation in the electronic field that has zero energy value everywhere else except where the electron is present. Similarly, every elementary particle is a vibration or excitation in its own field, and these vibrations interact with each other by the transfer of momentum, energy, charge, etc., among them. Higgs, along with other physicists, proposed that a similar force field came into action shortly after the Big Bang. At first, this field was zero-valued (no excitation) and as the universe started to expand, it grew spontaneously to some non-zero value and started interacting with other elementary particles. They named it the Higgs field and suggested that as the particles interact with the Higgs field, they experience some resistance offered by the field that does not allow them to travel with the speed of light. The more the particle interacts with the field, the more massive it becomes. For instance, photons traveling with the speed of light do not interact with anything (otherwise they cannot travel at 299 792 458 m/s), and therefore, they do not have any rest mass. When this idea was incorporated into the quantum mechanical formulation, the results were perfectly consistent with the formulation, without any need of making the mass zero. The only problem left was to confirm the existence of Higgs Field. This is where the Higgs boson comes in. According to QFT, if such a force field exists, there should also be an elementary particle associated with it. The existence of the Higgs boson was confirmed by the largest particle collider and biggest machinery present on earth, the Large Hadron Collider, on 4th July 2012, thus confirming the Higgs field.
The Role of LHC in Finding the Higgs Boson
The Large Hadron Collider is considered the most magnificent machine ever built by human beings. It is a particle accelerator situated at the CERN (European Council for Nuclear Research) laboratory near Geneva, Switzerland. More precisely, the device is located in a 27-km-long circular tunnel located under the French-Swiss border in which particles are accelerated in the opposite directions inside two vacuum tubes near the speed of light. These vacuum tubes are surrounded by 9593 electromagnets, whose purpose is to steer the electrically accelerated proton or heavy-ion beams. These magnets are powered via 250,000 km long superconducting cables running throughout the tunnel. The proper functioning of superconducting cables and electromagnets is ensured by distributing 120 tonnes of liquid helium throughout the system that renders the temperature of -271.3 °C (colder than outer space). This whole setup can make the proton or ion beams go approximately 11000 times per second through a single point in the 27 km circular orbit, creating 1 billion collisions per second when needed.
But why did scientists built the most expensive machinery in the world to smash these tiny particles? Well, in simple terms, to understand the fundamental nature of everything that exists around us. Just like a little kid who smashes his toys to satisfy his curiosity about what’s inside, scientists make these particles collide to understand what are they made of? When they accelerate protons inside the tubes, they add energy to the protons. In other words, the proton becomes an energy carrier to the collision point. When a collision of two such proton beams takes place along with the disintegration of protons into quarks and other particles, the sum of their energies can create mass ({E}{=}{m}{c}^{2}); in other words, it can cause some excitation in the omnipresent quantum fields, which results into particles. This idea allowed the physicists to confirm the existence of the Higgs Field by looking for Higgs boson in the collision debris. Whether the particle has been created or not is confirmed by the four detectors placed in four different positions along the circular orbit.
- ATLAS: A Toroidal LHC ApparatuS, is one of two general-purpose detectors at the Large Hadron Collider (LHC). It investigates a wide range of physics, from the search for the Higgs boson to extra dimensions and particles that could make up dark matter.
- CMS: Compact Muon Solenoid is another general-purpose particle detector with similar objectives to that of ATLAS; however, the technical specifications of both the detectors are different.
- ALICE: A Large Ion Collider Experiment is a detector dedicated to heavy-ion physics at the Large Hadron Collider (LHC). It is designed to study the physics of strongly interacting matter at extreme energy densities, where a phase of matter called quark-gluon plasma is formed.
- LHCb: The Large Hadron Collider beauty (LHCb) experiment specializes in investigating the slight differences between matter and antimatter by studying a type of particle called the “beauty quark,” or “b quark”.
Although these detectors are well-efficient in detecting electrons, muons, photons, and several other elementary particles, they cannot directly detect the presence of the Higgs boson. This is because the half-life of the Higgs boson is {10}^{-22} seconds, which is too short, and the particle decays into other particles before being identified. Physicists work around this problem by going backward from daughter particles to the parent Higgs boson. QFT suggests that the Higgs boson can decay into any other elementary particle. Physicists, initially, decided to work with photons as photons didn’t have rest mass, which could reduce several complications in the calculations. They look for those two photons in the collision debris, whose invariant masses add up to give the value that falls under the predicted mass range of the Higgs boson. The only problem with this idea is that there can be several photons in the debris, which are completely unrelatable to the Higgs boson and yet consistent with the calculations.
Figure A: Invariant Mass distribution of diphoton system with a peak (at around 125 GeV) corresponding to the Higgs Decay.
This problem is countered by introducing statistics to the process. The idea is to repeat the experiment again and again until the result can show a significant mass value for the presence of those two invariant mass photons. If there is no Higgs field present, the plot will always remain random, no matter how many times the experiment is being performed. On July 4th, 2012, the idea of the Higgs field, which was initially a bunch of equation on the paper, found its practical evidence after 40 years, when the analysis of several proton-proton collisions showed a significant deviation from the randomly distributed invariant mass at around 125 GeV (Figure A). The peak in the otherwise randomly distributed data was the confirmation that a Higgs Boson has been discovered in the detector. On 8 October 2013, the Nobel Prize in physics was awarded jointly to François Englert and Peter Higgs for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, which was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider.
Interesting Facts
- The God Particle: The name God particle was first coined by the American physicist Leon Lederman in the title of a popular book in 1993. The actual term used by Lederman was “the Goddamn Particle,” however, the publisher didn’t like it and went with “the God particle” for the title. Later on, the media found the phrase catchy in reference to the omnipresent nature of the Higgs field. In reality, the Higgs boson has nothing to do with God.
- The mass of the Higgs boson (approximately 125 GeV) also appears due to its interaction with its own field.
- When Peter Higgs first submitted the idea of the Higgs field to the scientific community in 1964, it was rejected and sent back. Yoichiro Nambu, a highly regarded physicist who had reviewed the paper, suggested Higgs add a section explaining his theory’s physical implications. Higgs added a paragraph predicting that an excitation of the field, like a wave in the ocean, would yield a new particle. He then submitted the revised paper to the competing journal Physical Review Letters, which was published on 31 August 1964.
- In 1993, the then UK science minister, William Waldegrave, held a national competition for the best explanation of the Higgs boson. The competition was won by David Miller, a physicist at the University College London. Miller explained that the particles that have mass are like some well-known politicians, the particles without mass are like anonymous people, and the Higgs Field is like a crowd of the general public. Since politicians would have to talk with the general public, they would experience some sort of restrain to their path through the public, which is the mass in particle’s case. On the contrary, an anonymous person can pass through that crowd more quickly, like a photon.
- Although the Higgs field does give mass to elementary particles like quarks and electrons, it does not provide all mass to all particles, not even particles made exclusively of quarks. For example, protons and neutrons get most of their mass from the strong nuclear force that holds their quarks together.
- Peter Higgs was not the only one credible for Higgs mechanism explanation. At least a dozen of theoretical physicists were involved in paving the way for a framework that could completely explain the complex nature of the Higgs Field. In 2010, six of those scientists were awarded the J J Sakurai prize for publishing their relevant work on the topic in 1964.
- The “boson” in the Higgs boson comes from the name of an Indian Physicist, Satyendra Nath Bose, who is best known for his collaboration with Albert Einstien in 1920 in the development of Bose-Einstien statistics. The theory explains why, unlike fermions, two bosons can exist in the same place at the same time. Certain types of particles obey Bose-Einstein statistics, which is why Paul Dirac named them bosons. Einstein used Bose-Einstein statistics to predict a new form of matter called the Bose-Einstein condensate. The first condensate was produced in 1995.
- The Higgs Boson is considered the last predicted particle by the elegant mathematics of the standard model; however, its discovery does not conclude all the loose ends of quantum mechanics. For instance, the Standard Model can not yet account for the fourth fundamental force, Gravity. The hypothetically proposed elementary particle associated with the force of gravity is the graviton. It is predicted to be massless and chargeless, just like photons.
- The discovery of the Higgs boson has marked the dawn of a new era in the timeline of human understanding of the universe and everything in it or beyond it (if it exists). Some of these fascinating research areas are Supersymmetry (or SUSY), extra space-time dimensions, anti-matter, dark matter, dark energy, etc.