Particle Physics_A Very Short Introduction

by Frank Close

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  Dark matter

  Protons and the nuclei of ordinary atoms seed all of the ‘luminous matter’ that shows up in astronomical observations. However, the motions of spiral galaxies, to take one example, show that there is more gravitational force about than the observed luminous matter can account for. As much as 90% of the matter present remains undetected. It appears that the universe we see by its electromagnetic radiations is outweighed by some mysterious ‘dark matter’, which does not show up at any wavelength in our telescopes.

  If there are large ‘massive compact halo objects’ (MACHOs) which could be bodies about the size of Jupiter, and not big enough to become shining stars or black holes, they would be detectable by creating double or multiple images of the distant star or galaxy through the effect of gravitational lensing. However, searches of this kind have not found enough MACHOs to explain the vast amount of dark matter that the universe appears to contain. So astrophysicists and cosmologists have had to turn to particle physics for further ideas.

  The intriguing possibility is that this dark matter could consist of vast quantities of subatomic particles that do not interact electromagnetically (otherwise we would be able to detect their electromagnetic radiation). One obvious candidate is the neutrino, whose tiny but non-zero mass could cause large clouds of them to gravitate to one another and help seed the formation of the galaxies.

  In the early universe, these neutrinos would have been highly energetic, moving at almost the speed of light. In the jargon, such flighty entities are known as ‘hot’, and computer simulation of galaxy evolution in a ‘hot dark matter’ universe shows galaxies forming in dense clusters with large voids between them. However, this computer model of the universe does not look like what the astronomers observe in practice.

  The evolution of galaxies would have been very different if the dark matter consists of massive, slow-moving, and therefore ‘cold’, particles. A problem is that there are no such entities known in the standard model, so if this is the answer to the dark matter problem, it raises another question: who are these particles?

  This brings us to the current ideas on what lies beyond the standard model. A favoured theory postulates the existence of ‘supersymmetric’ particles, the lightest of which include forms that do not respond to the electromagnetic or strong forces, but which may be hundreds of times more massive than the proton. Collisions at the highest-energy particle accelerators, in particular the Tevatron at Fermilab and the Large Hadron Collider at CERN, may have enough energy to create them. If such a particle is found, the challenge will then be to study its properties in detail, in particular to see if it could have formed large-scale clusters of dark matter in the early universe.

  Which brings us to the question: what is supersymmetry?

  Supersymmetry

  The paradox of how ‘empty’ atoms form solid matter is solved in quantum mechanics. It is a profound property of the fact that electrons (and quarks, and protons, and neutrons) all have an intrinsic spin that is one-half of an amount known as Planck’s constant, h. Such ‘spin 1/2’ particles are generically known as fermions. Quantum mechanics implies that two fermions cannot be in the same place, with the same state of motion; in the jargon they ‘cannot occupy the same quantum state’. This causes the many electrons in complex atoms to occupy specific states, and gives rise to the chemical activity, or inertness, of the various elements. It also prevents an electron in one atom encroaching too readily on one in a neighbouring atom. This underpins many properties of bulk matter, such as solidity.

  The forces among these fermions are transmitted by photons, gluons, W and Z bosons. Note the word ‘bosons’. This is a generic term referring to particles that have a spin that is an integer multiple of Planck’s quantum. All of these force carriers are bosons, having spin of unity. In contrast to fermions, which are mutually exclusive, bosons have affinity and form collective states, such as is the case for photons in laser beams.

  We have seen that the fermions – quarks and leptons – exhibit a profound unity, and also that the force carrying bosons do too. Why is it that ‘matter particles’ are all (apparently) made of spin-1/2 fermions and the forces transmitted by spin-1 bosons? Could there be a further symmetry between the forces and the matter particles, such that the known fermions are partnered by new bosons, and the known bosons by new fermions, with novel forces transmitted by these fermions? Could this lead to a more complete unification among particles and forces? According to the theory known as supersymmetry, the answer is yes.

  In supersymmetry – or SUSY as it is known – there are families of bosons that twin the known quarks and leptons. These ‘superquarks’ are known as squarks; their superlepton counterparts are known as sleptons. If SUSY were an exact symmetry, each variety of lepton or quark would have the same mass as its slepton or squark sibling. The electron and selectron would have the same mass as one another; similarly, the up quarks and the ‘sup’ squark would weigh the same, and so on. In reality this is not how things are. The selectron, if it exists, has mass far greater than 100 GeV, which implies that it would be hundreds of thousands of times more massive than the electron. Similar remarks can be made for all of the sleptons or squarks.

  An analogous statement can be made also about the super-partners of the known bosons. In SUSY there are families of fermions that twin the known bosons. The naming pattern here is to add the appendage ‘-ino’ to denote the super-fermion partner of a standard boson. Thus there should exist the photino, gluino, zino, and wino (the ‘ino’ pronounced eeno, thus for example it is weeno and not whine-o). The hypothetical graviton, the carrier of gravity, is predicted to have a partner, the gravitino. Here again, were supersymmetry perfect, the photino, gluino, and gravitino would be massless, like their photon, gluon, and graviton siblings; the wino and zino having masses of 80 and 90 GeV like the W and Z. But as was the case above, here again the ‘inos’ have masses far greater than their conventional counterparts.

  The standard, and feeble, joke is that supersymmetry must be correct: we have found half the particles already. Put another way: we have not found clear evidence for a single squark or slepton, nor photino, gluino, wino, or zino. Searching for them is a high priority at present.

  31. SUSY particles summary: massive neutrinos and oscillations.

  With such a lack of evidence for superparticles, one might wonder why theorists believe in SUSY at all. It turns out that such a symmetry is very natural, at least mathematically, given the nature of space and time as encoded in Einstein’s theory of relativity and the nature of quantum theory. The resulting pattern of superparticles turns out to solve some technical problems in the present formulation of particle physics, stabilizing the quantum theories of the behaviour of the different forces at high energies and the responses of particles to those forces. In a nutshell, without SUSY certain attempts to construct unified theories lead to nonsensical results, such as that certain events could occur with an infinite probablility. However, quantum fluctuations, where particles and antiparticles can fleetingly emerge from the vacuum before disappearing again, can be sensitive to the SUSY particles as well as to the known menu. Without the SUSY contributions, some calculations give nonsense, such as the infinite probability we saw above; upon including the SUSY contributions, more sensible results emerge. The fact that the nonsensical results have disappeared when SUSY is at work encourages hope that SUSY is indeed involved in Nature’s scheme. Getting rid of nonsense is, of course, necessary, but we still do not know if the sensible results are identical with how Nature actually behaves. So we have at best indirect hints that SUSY is at work, albeit behind the scenes at present. The challenge is to produce SUSY particles in experiments, thereby proving the theory and enabling detailed understanding of it to emerge from the study of their properties.

  SUSY might be responsible for at least some of the dark matter that seems to dominate the material universe. From the motions of the galaxies and other measu
rements of the cosmos, it can be inferred that perhaps as much as 90% of the universe consists of massive ‘dark’ matter, dark in the sense that it does not shine, possibly because it is impervious to the electromagnetic force. In SUSY if the lightest superparticles are electrically neutral, such as the photino or gluino say, they could be metastable. As such they could form large-scale clusters under their mutual gravitational attraction, analogous to the way that the familiar stars are initially formed. However, whereas stars made of conventional particles, and experiencing all the four forces, can undergo fusion and emit light, the neutral SUSY-inos would not. If and when SUSY particles are discovered, it will be fascinating to learn if the required neutral particles are indeed the lightest and have the required properties. If this should turn out to be so, then one will have a most beautiful convergence between the field of high-energy particle physics and that of the universe at large.

  Massive neutrinos

  In the standard model, neutrinos are assumed to have no mass. This was because no one had ever been able to measure a value for any mass that they might have, the amount being so tiny that it might well have been zero. However, there is no fundamental principle of which we are aware that requires neutrinos to be massless. And indeed, we now know that neutrinos do have a mass, exceedingly small compared to even the electron mass, but non-zero nonetheless.

  There are three known varieties of neutrino, the electron-neutrino, muon-neutrino, and tau-neutrino, named for their affinity for being produced in concert with the electrically charged particle that shares their name. I will refer to these as nu-e, nu-mu, and nu-tau, respectively. The fusion reactions in the heart of the Sun emit neutrinos of the nu-e variety.

  In quantum mechanics, particles have wavelike character. As the oscillations of the electromagnetic field can take on particle characteristics – the photons – so do particles such as neutrinos have wavelike oscillations as they travel through space. In effect it is a wave of varying probability. What set out as a nu-e will vary in probability as it travels, changing from nu-e to nu-mu or nu-tau as it moves away from the source. However, for this to happen, the neutrinos must have different masses, which implies that not all of them can be massless.

  Over several decades, the intensity of nu-e arriving from the Sun was measured. Given our knowledge of the way that the Sun works, it was possible to compute the number of nu-e it produced and hence the intensity of them when they reach the Earth. However, when the measurements were made, the intensity of nu-e arriving here was found to be a factor of two to three smaller than had been expected. This was the first hint that the nu-e might have a mass and be changing into the other varieties of neutrino en route. Similar anomalies were seen in the mix of nu-e and nu-mu produced when cosmic rays hit atoms in the upper atmosphere. A series of dedicated experiments towards the end of the 20th century finally established that neutrinos indeed have mass and are oscillating from one form to another in flight.

  One, in SNO – the Sudbury (Ontario) Neutrino Observatory – was able to detect not just the nu-e variety arriving from the Sun (which showed the shortfall) but also counted the total number of all varieties (which established that the totality was as predicted). This showed that nu-e indeed had changed but did not of itself determine into which variety it preferred to go.

  So have begun ‘long baseline’ experiments. At accelerators such as CERN, Fermilab, or the KEK laboratory in Japan, controlled beams of neutrinos are created. The energy, intensity and composition (mainly nu-mu) of the neutrino beams is monitored at source; it is directed through the ground to be detected several hundred km away at a remote underground laboratory. By comparing the composition of the arriving beam with that which set out it is becoming possible to determine which flavours oscillate into what, and how quickly they do so. From this it is then possible to calculate what their relative masses are (technically, it is the differences of their masses squared that is determined this way).

  During the first decade of the 21st century we anticipate a wealth of information about the enigmatic neutrinos as a result of such experiments. Determining the pattern of their masses will provide some of the missing parameters of the Standard Model. We do not know why the values of the masses of the quarks and charged leptons are as they are. That they have those values is critical for our existence, so understanding this would be a significant breakthrough. Determining the neutrino masses could therefore provide an essential clue in unravelling this enigma.

  Neutrino masses could also have impact on cosmology. Massive neutrinos could have played a role in seeding the formation of galaxies; they could play some role in explaining the nature of the dark matter that pervades the universe and there is still the unresolved puzzle of why the weak interaction experiences a violation of parity, mirror symmetry. Neutrinos are a special entree to probing the weak interaction and so the increased study of their properties may lead to unexpected discoveries.

  Determining the values of the neutrino masses is one of the major challenges currently exercising particle physicists. This leads naturally onto an even bigger question: what is the nature of mass itself?

  Mass

  The electroweak force is the force carried by the familiar photon of electromagnetism, and by the W and Z bosons, which are responsible for the weak interactions that not only initiate solar burning but also underlie certain types of radioactivity. Yet if these effects are so closely intertwined, why do they appear so different in our daily experiences, that is, at relatively low temperatures and energies? One reason is that the particle that transmits the electromagnetic force, the photon, is massless, whereas the W and Z bosons, which are associated with the weak force, have huge masses and each ‘weighs’ as much as an atom of silver.

  The Standard Model of the fundamental particles and the forces that act among them explains mass by proposing that it is due to a new field, named the Higgs field after Peter Higgs who in 1964 was one of the first to recognize this theoretical possibility. The Higgs field also permeates all of space. Were there no Higgs field, according to the theory, the fundamental particles would have no mass. What we recognize as mass is, in part, the effect of the interaction between particles and the Higgs field. Photons do not interact with the Higgs field and so are massless; the W and Z bosons do interact and thereby acquire their large masses. The building blocks of matter, the quarks and leptons, are also presumed to gain their masses by interacting with the Higgs field.

  Just as electromagnetic fields produce the quantum bundles we call photons so should the Higgs field manifest itself in Higgs bosons. In Higgs’ original theory there was just one type of Higgs boson, but if supersymmetry is correct, there should be a family of such particles.

  32. Peter Higgs with an illustration of a part of his theory on the blackboard behind him.

  Precision measurements made at LEP and other accelerators, when combined with the mathematics of quantum theory and the Standard Model, enable theorists to determine the energies at which the Higgs boson – or whatever it is that gives rise to mass – should be revealed. These calculations imply that the origins of mass were frozen into the fabric of the universe just a millionth of a millionth of a second after the Big Bang, when the temperature had ‘cooled’ to below ten thousand million million degrees. It is possible that proton-antiproton collisions at Fermilab’s upgraded Tevatron might catch the first signs of a Higgs boson. However, to make a dedicated exploration of this energy region, where the puzzle of mass should be revealed, requires the Large Hadron Collider (LHC) at CERN that will access higher energies starting in 2007.

  Quark gluon plasma

  If our picture of the origins of matter is correct, then the quarks and gluons, which in today’s cold universe are trapped inside protons and neutrons, would in the heat of the Big Bang have been too hot to stick together. Instead, they would have existed in a dense, energetic ‘soup’ known as ‘Quark Gluon Plasma’, or QGP for short.

  These intermingled swarms of quarks and gl
uons are analogous to the state of matter known as plasma, such as is found in the heart of the Sun, which consists of independent gases of electrons and nuclei too energetic to bind together to form neutral atoms.

  Physicists are attempting to make QGP by smashing large atomic nuclei into one another at such high energies that the protons and neutrons squeeze together. The hope is that the nuclei will ‘melt’ – in other words, that the quarks and gluons will flow throughout the nucleus rather than remaining ‘frozen’ into individual neutrons and protons.

  At CERN beams of heavy nuclei have been fired at static targets of heavy elements. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in the USA, has built a dedicated machine where beams of heavy nuclei collide head on. As with simpler particles, such as electrons and protons, the great advantage of a colliding beam machine is that all the energy gained in accelerating the particles goes into the collision. In 2007, RHIC will be superseded in energy by the Large Hadron Collider at CERN, which will make lead ions collide at a total energy of 1300 TeV. At these extreme energies, akin to those that would have been the norm in the universe when it was less than a trillionth of a second old, QGP should become commonplace, so that experimenters can study its properties in detail.

  Antimatter and CP

  It seems that we inhabit a volume of matter that is at least 120 million light years in diameter. Based on subtle differences in how matter and antimatter behave at the level of the fundamental particles, (technically known as breaking of ‘CP symmetry’), most physicists favour the idea that there is some subtle asymmetry between matter and antimatter at large, and that soon after the Big Bang this tipped the balance in favour of a universe dominated by matter. The challenge now is to study these differences in detail in order to identify their origins and, perhaps, the source of the asymmetry between matter and antimatter in the cosmos.