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What are we are made of? Vinay B Kamble (Former Adviser, DST & Former Director, Vigyan Prasar, New Delhi) Email: [email protected] Just a few particles! Today we know that molecules are made of atoms. Atoms are made of particles called protons, neutrons, and electrons. Protons and neutrons are made of even smaller particles called quarks and gluons. How about quarks? They are regarded as fundamental particles; they do not show any substructure, and hence, they cannot be further broken up into still smaller particles. Electrons and neutrinos are regarded as fundamental particles too. Thus, electrons, neutrinos, and quarks are indivisible. Protons and neutrons, and hence the atomic nucleus consists of two kinds of quarks, up quarks and down quarks. We thus need three elementary particles for all matter to exist: electrons, up quarks and down quarks. But during the 1950s and 1960s, new particles were unexpectedly observed in both cosmic radiation and at newly constructed accelerators, the new siblings of electrons and quarks. In addition to the matter particles, there are also force particles for each of nature’s four forces - gravitation, electromagnetism, the weak force and the strong force. Gravitation and electromagnetism are the most well-known. The strong force acts upon quarks and holds protons and neutrons together in the nucleus, whereas the weak force is responsible for radioactive decay, which is necessary, for instance, for nuclear processes inside the Sun. Over the years, the physicists have worked out a mathematical model - The Standard Model - that describes the known fundamental particles that make up the matter we are familiar with (classified as fermions after Enrico Fermi who proposed them); and the particles that transmit the forces (classified as bosons after Satyendra Nath Bose who proposed them (see Figure 1). As it turns out, there are only 17 particles in the Standard Model. Of these, 6 are fermions such as quarks that make up neutrons and protons in nuclei; and 6 fermions called leptons (“light” particles), such as electrons that go around these nuclei. Quarks and leptons are the particles that make up the matter. Four particles are called gauge bosons. These are the particles that transmit forces and thus allow fermions to interact. Finally, there is the Higgs boson – the heart of the Standard Model! If it were not for this particle, the Standard Model would just fall apart. It is required not to transmit force, but to give mass to other particles. But, before we continue with the story, let us describe the Standard Model and why it is important not only to particle physicists, but also for everyone and everything we come across in nature. Towards Order: The Standard Model The Standard Model is a theory concerning the three fundamental interactions called the electromagnetic, strong and weak nuclear interactions that mediate between the subatomic particles that are the building blocks of matter. It does not include gravity. Often the terms interaction and force are used interchangeably. The Standard Model was driven forward sometimes by new experimental discoveries and sometimes by theoretical advances; and spanned many decades and many continents. The current formulation was worked out in the mid 1970s when the existence of quarks was experimentally confirmed. Since then, discoveries of the bottom quark (1977), the top quark (1995), and the tau neutrino (2000) have given further credence to the Standard Model. It turns out that all forms of matter are made from just twelve fundamental particles! These particles can be divided into two distinct groups – quarks and leptons. They are distinguished by the different ways in which they react to the fundamental forces. There are 6 quarks and 6 leptons. The six quarks are called up, down, charm, strange, bottom and top (in order of mass). The six leptons are the electron, electronneutrino, the muon, muon-neutrino, tau and tau neutrino. Though the familiar electron is a fundamental particle, proton and neutron are not. Proton is composed of two up quarks and one down quark. A neutron is composed of one up and two down quarks. The charge of up quark is +2/3, while that of down quark is -1/3. The charge of the electron, however, is -1! Quarks and leptons have half-integral spin (1/2, 3/2 etc), a property of a class of particles called fermions that follow Fermi-Dirac statistics abiding by Pauli’s exclusion principle allowing only one particle in one quantum state. Gravity and electromagnetism are familiar to us. The weak and strong forces are relatively new. Electromagnetism is responsible for current in wires, electricity in our homes, radio, television and telecommunication, including chemical binding in atoms and molecules and chemical reactions. The strong nuclear force binds atomic nuclei together and makes them stable, and it acts only through quarks. Weak interactions are most noticeable when particles undergo beta decay (radioactivity in which electrons or positrons are emitted) from nuclei, and in the production of deuterium and then helium from hydrogen that powers the sun's thermonuclear process. This is how the sun shines and we get energy. The weak force is felt by both quarks and leptons, unlike strong force which is felt only by quarks. If two leptons come within the range of weak force, it is possible for them to change into other leptons. While gravity and the electromagnetic force have infinite range, strong and weak nuclear forces have short ranges of interaction, 10-15 metres and 10-17 metres respectively. In terms of their strengths, gravity is the weakest force, and then come electromagnetic, weak and strong force. Three Generations Figure 2 shows the standard way in which the quarks and leptons are grouped into families. Each column is referred to as a generation. The up and down quarks belong to 1st generation, strange and charm to 2nd generation; and top and bottom quarks to 3rd generation. Most commonly found quarks in universe are up and down quarks, since atomic nuclei are composed of protons and neutrons. The others are more massive and rarer. Now consider the leptons. Most familiar of them is, of course, the electron. Indeed, the properties of electron are mirrored in muon and tau – but only they can decay into other particles. They have the same electrical charge and respond to fundamental forces in the same way. The electron is a stable particle. The other three leptons are called neutrinos, as they are electrically neutral. Unlike neutrons, they are fundamental particles. We say that neutrons are charge zero, while neutrinos are neutral. Neutrinos have extremely small masses. For example, electron neutrino has mass less than one thousandth of an electron! Electron and electron-neutrino belong to the 1st generation; muon and muon-neutrino belong to the 2nd generation, while tau and tau-neutrino belong to the 3rd generation along with their quark counterparts. We may note that quarks and leptons are fermions with spin ½ and follow Fermi-Dirac statistics. Fermions follow Pauli’s exclusion principle, that is, only one particle can be accommodated in a particular quantum state. Let us now consider the last column indicated by bosons, viz., photon, gluon, and weak force. These are called gauge bosons and are the force carriers that mediate the electromagnetic (γ), strong (g), and weak interactions (Z0, W+ and W-). Incidentally, bosons are particles with integral spin (0, 1, 2 etc) and follow Bose-Einstein statistics. They do not follow Pauli’s exclusion principle, and hence any number of particles can be accommodated in a particular quantum state. Interactions in physics are the ways in which particles influence other particles. At macroscopic level, electromagnetism allows particles to interact with one another via electric and magnetic fields. The Standard Model explains such forces as resulting from matter particles exchanging other mediating particles, known as force mediating particles. When a force-mediating particle is exchanged between two particles, at a macroscopic level the effect is equivalent to a force influencing both of them, and the particle is therefore said to have mediated that force. Photons mediate the electromagnetic force between electrically charged particles. The photon is massless. The Z0, W+ and W- gauge bosons have non-zero mass and yet mediate the weak interactions between all quarks and leptons. They are massive, with the Z0 being more massive than the W+ and W-. The gluons mediate the strong interaction between the quarks. Gluons are massless. The gauge bosons of the Standard Model all have spin 1, thus making them bosons. Finally, we come to the Higgs boson shown separately. Why do we show it separately? Higgs is not a gauge boson. Physicists do not need it to transmit force, but to give mass to other particles. Two of the 16 other fundamental particles, the photon and the gluon, are massless. But without the Higgs, there is no explanation of where the mass of other particles comes from. Without it, the masses of all particles would be zero, and they would travel with the speed of light, as demanded by the theory of relativity. Apparently, the discovery of the particle announced by CERN on 04 July 2012 appears to have properties of the much sought after Higgs boson. However, further investigations have boosted our confidence that this must be the Higgs particle as required by the Standard Model. Then, this was the last piece of the jigsaw puzzle of the standard model! So, how many particles? We may note that the 17 particles we have described here are the basic particles of the Standard Model. We note that every fundamental particle has an antiparticle which carries an electric charge that is opposite of the charge on the particle. Thus we have 6 quarks and 6 antiquarks; and 6 leptons and 6 antileptons. As regards the bosons - photon, gluon, Z0 and the Higgs Boson H0 are their own antiparticles, while antiparticle of and W- is W+. Together, they are responsible for some 200 composite particles (protons, neutrons, mesons etc) that we observe in various circumstances through different combinations and at different energies. If we include graviton (spin 2 and hence a boson) which is supposed to be the carrier of gravitational interaction and its own antiparticle, though not included in the list of the Standard Model, the total number of the fundamental particles thus would add up to 31. The fundamental particles described above and others that are composed by various combinations of these particles are classified into several groups depending on their properties. For example, Hadrons (Heavy particles) include mesons (kaons, pions etc) which are bosons; and baryons which include protons, neutrons and other heavy particles that are fermions. These are schematically depicted in Figure 3. Colour Charge We shall briefly discuss yet another aspect of the Standard Model, the Colour Charge on quarks. Leptons, such as the electron, possess the property of the electric charge, which comes in two types, positive and negative, and allows them to feel the electromagnetic force. Quarks have fractional electric charge and feel the electromagnetic force. Bound as baryons in a nucleus, unlike leptons, quarks also feel the strong force. This implies that quarks must have a property that enables them to feel strong force and also binds them together in threes in nucleons (protons and neutrons), or as pairs in mesons. This strong force analogy of electric charge is called the colour. Quark colours come in three shades, red (r), green (g), and blue (b). Surely, this has nothing to do with real colours! They only describe a property of the quarks that determines the strength of their interactions with each other through strong force. Any stable baryon (baryon or meson) must have an overall ‘colour charge’ that is colourless. This is similar to combinations of the red, blue and green primary colours in light that mix together to make white. Baryons such as protons must have a red, blue and green quark: u(r), u(b), and u(g). Antiquarks have the corresponding anticolour, anti-red, anti-blue, and anti-green. In mesons, the colours would cancel out in quarkantiquark pairs. For example, a π+ meson would be composed of an up quark with red colour charge, and a down antiquark with anti-red colour and is overall colourless, since the red is cancelled out by anti-red. Colour is a central concept in the theory of force that holds quarks together and acts as the source of strong force. The colour force between quarks is described by a gauge theory called quantum chromodynamics. It describes an exchange force involving particles of the colour field that carry the force from one quark to another. The carrier particles are the gluons that have zero mass and spin, and hence are bosons. The colour field is analogous to the electromagnetic field and the gluon is analogous to the photon in quantum electrodynamics (QED). Incidentally, quantum electrodynamics combines electromagnetism, relativity, and quantum mechanics. The strong force, which the nuclear physicists observe as binding the nucleus together and mediated by the exchange of mesons, actually has a more fundamental origin in the colour force that binds the quarks together! When colour force is introduced, the number of quarks and antiquarks in the Standard Model would triple (12 x 3). But the number of gluons would increase to 8 from just 1! Thus there would be 36 quarks, 12 leptons, 8 gluons, photon, Z0, W- , W+ and the Higgs Boson H0, adding up to 61 particles! With graviton it would be 62! Thus we have a large family that has proliferated from just 17 basic fundamental particles! Fields! Fields! Fields! What the Standard Model of particle physics does is that it unites the fundamental building blocks of nature and three of the four forces known to us – except gravitation. Gravitation remains outside the Standard Model. For long, it was an enigma how these forces actually work. For instance, how does the piece of metal that is attracted to the magnet know that the magnet is lying there, a bit further away? And how does the Moon feel the gravity of Earth? The explanation offered by physics is that space is filled with many invisible fields. The gravitational field, the electromagnetic field, the quark field and all the other fields fill space, or rather, the four dimensional spacetime, an abstract space where the theory plays out. The Standard Model is a quantum field theory in which fields and particles are the essential building blocks of the universe. In quantum physics, everything is seen as a collection of vibrations in quantum fields. These vibrations are carried through the field in small packages, quanta, which appear to us as particles. Two kinds of fields exist: matter fields with matter particles, and force fields with force particles - the mediators of forces. Incidentally, the Higgs particle, too, is a vibration of its field - often referred to as the Higgs field. Without this field the Standard Model would collapse like a house of cards, because quantum field theory brings infinities that have to be reined in. The Standard Model would only work if particles did not have mass. As for the electromagnetic force, with its massless photons as mediators, there was no problem. The weak force, however, is mediated by three massive particles; two electrically charged W particles and one Z particle. They did not sit well with the light-footed photon. How could the electroweak force, which unifies electromagnetic and weak forces, come about? The Standard Model was threatened. This is where Englert, Brout and Higgs entered the stage with the ingenious mechanism for particles to acquire mass that managed to rescue the Standard Model. Symmetry and the Standard Model The Higgs field is a very special kind of field. All other fields vary in strength and become zero at their lowest energy level. Not the Higgs field. Even empty space would still be filled by a field - the Higgs Field - that does not become zero. We do not notice it; just the way we do not notice air. But without it we would not exist, because particles acquire mass only in contact with the Higgs field. Particles that do not pay attention to the Higgs field do not acquire mass, those that interact weakly become light, and those that interact intensely become heavy. If the Higgs field suddenly disappeared, all matter would collapse as the suddenly massless electrons would disperse at the speed of light. So what makes the Higgs field so special? It breaks the intrinsic symmetry of the world. In nature, symmetry abounds; faces, flowers and snowflakes exhibit various kinds of geometric symmetries. We come across other kind of symmetries in physics that describe the world we live in, though on a deeper level. For example, one such, relatively simple, symmetry stipulates that it does not matter for the results if a laboratory experiment is carried out in anywhere in the world, or even at any time. Special theory of relativity deals with symmetries in space and time, and has become a model for many other theories, such as the Standard Model of particle physics. It turns out that the equations of the Standard Model are symmetric; in the same way that a ball looks the same from whatever angle you look at it, the equations of the Standard Model remain unchanged even if the perspective that defines them is changed. Principles of symmetry play an important role in describing physical phenomena. In 1918, the German mathematician Emmy Noether showed that the conservation laws of physics, such as the laws of conservation of energy, or conservation of momentum, and conservation of electrical charge, also have their origin in symmetry. Symmetry, however, dictates certain requirements to be fulfilled. A ball has to be perfectly round; the tiniest hump will break the symmetry. For equations other criteria apply. And one of the symmetries of the Standard Model prohibits particles from having mass. Now, this is apparently not the case in our world, so the particles must have acquired their mass from somewhere. This is where the Higgs mechanism plays a crucial role for symmetry to both exist and simultaneously be hidden from view. How the Higgs boson was postulated By 1930s, it became clear that the ranges of the fundamental forces would be inversely proportional to the masses of the particles that transmit force. These, incidentally, can be regarded as the quantum excitations of the all pervasive force fields. For example photon is an excitation of the electromagnetic field. Hence, barring the massless photon (which transmits the electromagnetic force), the carriers of strong and weak nuclear forces need to be massive particles. Now, symmetry of equations of electromagnetism makes photon massless making electromagnetic force have an infinite range. When this idea was extended to strong and weak interactions, the extended symmetry operation implied that the excitations of these force fields when included in the theory should also be massless! Thus the symmetry required that the matter particles in the theory – quarks and leptons – should also be massless. In 1964, Peter Higgs and other particle physicists came up with a solution. They demonstrated that by introducing a “scalar field” (whose particles have zero value for quantum spin) and incorporating the mechanism of “spontaneous symmetry breaking”(See Figure 4), the problem of massless particles could be solved in theories with gauge invariance as the underlying mathematical symmetry. In physics, gauge invariance (also called gauge symmetry) is the property of a field theory in which different configurations of the underlying fields - which are not themselves directly observable - result in identical observable quantities. A theory with such a property is called a gauge theory. A transformation from one such field configuration to another is called a gauge transformation. The Higgs mechanism when incorporated into the field equations, would allow particles to have masses. Integrating Higgs mechanism into the Standard Model allowed scientists to make predictions of various quantities, including the mass of the heaviest known particle, the top quark. Experimentalists found this particle just where equations using the Higgs mechanism predicted it should be. The Higgs mechanism works as a medium that exists everywhere in space. Particles gain mass by interacting with this medium. Peter Higgs pointed out that the mechanism required the existence of an unseen particle, which we now call the Higgs boson. The Higgs boson is the fundamental component of the Higgs medium, much as the photon is the fundamental component of light. From where does Higgs particle itself get its mass? The Higgs boson gets its mass from the self-interaction in the Higgs field, which arises through a mechanism known as “spontaneous symmetry breaking” by the Higgs field of a certain universal symmetry that prevailed at the time of Big Bang. The Higgs mechanism does not predict the mass of the Higgs boson itself but rather a range of masses. Fortunately, the Higgs boson when it decays would leave a unique particle footprint depending on its mass. So it would be possible for scientists to know what to look for and they would be able to calculate its mass from the particles they saw in the detector. As discussed earlier, the electromagnetic force is carried by photons which have no mass, whereas the weak force is carried by particles called W and Z, which do have mass. The W and Z particles were discovered in a Nobel prize winning enterprise at CERN in the 1980s, but the mechanism that gives rise to their mass was not yet experimentally identified; and that is where the Higgs particle comes in (See Figure 5). The Higgs mechanism in its basic form is the simplest theoretical model that could account for the mass difference between photons and the W and Z particles, and by extension could account for the masses of a range this mechanism is not the only possible explanation. The experimental confirmation of the Higgs boson would place the last piece of jigsaw puzzle that Standard Model is into its designated slot. This explains the euphoria when a candidate for the Standard Model Higgs boson was discovered in two independent experiments at LHC on 04 July 2012. By now we feel reasonably confident that this was the particle we were looking for! Yet, all is not well with the Standard Model The Standard Model developed in 1960s and 1970s does not include answers to many basic questions such as how to unify electroweak forces with strong or gravitational forces. It cannot even explain mysteries of the Universe, answers to which have their roots in the world of fundamental particles. At the time of the Big Bang, the Universe had no dimensions at all. How did the Universe, infinitely dense at the time of the Big Bang, evolve into a vast Universe full of stars and planets we live in today? As the early universe expanded, energy should have condensed into equal amounts of matter and antimatter, which would then annihilate each other on contact, reverting to pure energy. Thus, the universe should really be empty! But, there is more matter than antimatter and therefore we exist! Given the energy and the temperatures at which the LHC works, the experiments planned may help physicists understand why the Universe grew with just enough more matter than antimatter. How about the dark matter? What is so ‘dark’ about it? Observations of the motion of distant galaxies indicates that they are subject to more gravity than their visible matter could possibly account for, implying existence of some exotic hidden matter in the mix. Where did this dark matter come from? In fact, out of the total matter and energy content in the universe, what we observe is just about 4 per cent matter in the form of stars and galaxies. What we do not observe is the 22 per cent of dark matter and remaining 74 per cent being “dark energy”. Higgs mechanism hence may account for only the 4 per cent of the matter that we observe! How do we explain the dark matter and dark energy, then? A theory called Supersymmetry could possibly explain this. According to it, every fundamental particle had a much more massive counterpart in the early Universe. Indeed, the electron might have had a massive partner that physicists refer to as the ‘selectron’. Similarly, the muon might have had the ‘smuon’, and the quark might have had ‘squark’. Many of those supersymmetric partners would be unstable. However, one kind of particles may have been just stable enough to survive till today without interacting with any other particles. Could they might be dark matter? By smashing particles like protons at energies and temperatures that existed at the earliest moments of the Universe, the LHC could reveal the particles and forces that were responsible for everything that followed. Supersymmetry also predicts a family of Higgs bosons. Where does the Higgs boson observed at LHC fit in? Let us wait and watch. Late, but not too late! François Englert and Peter W. Higgs were jointly awarded the Nobel Prize in Physics in 2013 for the theory of how particles acquire mass. As we noted earlier, in 1964 they proposed the theory independently of each other (Englert together with his now deceased colleague Robert Brout). In 2012, their ideas were confirmed by the discovery of a so called Higgs particle at the CERN laboratory outside Geneva in Switzerland. Both François Englert and Peter Higgs were young scientists when they independently of each other put forward a theory that rescued the Standard Model from collapse. Almost half a century later, on Wednesday 4 July 2012, they were both in the audience at the European Laboratory for Particle Physics, CERN, outside Geneva, when the discovery of a Higgs particle that finally confirmed the theory was announced to the world. Robert Brout, however, did not live to be a part of the audience on that day, nor be a recipient of the 2013 Nobel Prize. References: 1. 2. 3. 4. 5. 6. Particle Physics by Christopher Bishop 2002 Pub: John Murray Concepts of Modern Physics by Arthur Beiser 2003 Pub: Tata Mc Grow- Hill Introduction to Elementary Physics by David Griffiths 1987 Pub: John Wiley CERN Website: home.web.search.ch Numerous articles in Wikipedia Nobel Prize website: www.nobelprize.org Satyendra Nath Bose Figure 1: Satyendra Nath Bose with wife Usha Bala Particles are of two types - fermions and bosons. Fermions have a spin of half integrals (1/2, 3/2 etc) while bosons have integral spin (0, 1, 2 etc). The Higgs particle is a boson. Satyendra nath Bose is best known for his work with Albert Einstein that resulted into “Bose-Einstein Statistics’ that describes the behaviour of the particles with integral spin. Two or more bosons can occupy the same place at the same time, but no two fermions can occupy the same state. This is why electrons which are fermions, have to stay away from each other and be in different orbital states as in an atom. Einstein extended Bose's ideas to material particles (or matter), and predicted what is known as the BoseEinstein condensation. Several scientists who worked on these ideas received Nobel prizes. Satyendra Nath Bose was never considered by the Nobel committee. 125.3 GeV 0 (?) 0 0 H Higgs boson* b * Higgs boson (tenta ative) – anno ounced by CE ERN on 04 July 2012, pro operties of which w need to o be further stud died to identiify if this is the th Higgs as required by the Standard d Model Figure 2: 2 The Stan ndard Mod del as we know k Figure 3: 3 Particle classificatio c on. Note tha at mesons are bosons and hadrons; and baryons are hadron ns and ferm mions. The secret of T o nature iss symmetry, but much of the textture of the world is du ue to mechanissms of symmetry breakking. There are a a varietyy of mechan nisms where ein the symm metry of nature e can be hidden or broke en. A more profound wa ay of breaking or hiding g symmetry is i the phenome enon of spon ntaneous syymmetry bre eaking. Here e, the laws of o physics are symmetric but the state e of the sy ystem is no ot. Associated with sp pontaneous symmetry breaking iss the phenome enon of symmetry restorration. If one e heats a sysstem that po ossesses a broken b symm metry it tends to be restore ed at high temperature. Thus T a ferro omagnetic material m can be b magnetizzed at low temp perature (or even at roo om temperatture) with alll the little ato omic magne ets aligned in the same dirrection. This is a state of o broken rottational symm metry. As th he temperatu ure increase es the atoms vibrate more and more. Finally whe en the tempe erature is greater than a certain critical value the e fluctuation ns win out over o the forcces that ten nd to align the t atomic magnets m and d the average magnetizatiion vanishess. Above the e critical tem mperature th he system exhibits e rotattional symmetryy. Such a transition fro om a state of o broken syymmetry to one where the symme etry is restored is a phase transition. t W believe th We hat the same e phenomen non occurs in the case of o the symmetries of the fundamental forces f of natture. Many of o these are broken at lo ow temperattures. e universe, when w the te emperature was very high, h all of these t Very earrly in the history of the symmetries of natu ure were prresumably restored. r Th he resulting g phase tra ansitions, ass the d, from sym mmetric state es to those e of broken symmetry have universe expanded and cooled er example is the electtroweak inte eraction thatt can important cosmologiical implications. Anothe etic and wea ak components on spontaneous sym mmetry brea aking. manifest itself into electromagne d understand d the phenomenon of sp pontaneous symmetry breaking thro ough the exa ample We could of a Mexxican hat. Th he hat has a rotational symmetry, s an nd it is respected as lon ng as long as a the ball sits on o top. Howe ever, the mo oment ball fa alls the symm metry is spontaneously broken. b Figure 4: 4 Spontaneous Symm metry Breakking To understand the Higgs mechanism, imagine that a room full of physicists quietly chattering is like space filled only with the Higgs field.... ... a well known scientist walks in, creating a disturbance as he moves across the room, and attracting a cluster of admirers with each step ... ... this increases his resistance to movement, in other words, he acquires mass, just like a particle moving through the Higgs field ... In this analogy, these clusters are the Higgs particles. Figure 5: How particles get mass through Higgs Mechanism (Courtesy CERN)