Higgs boson and Standard Model

December 17, 2011

“Researchers announced that two independent experiments at the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland, have turned up signs of the Higgs boson. It can be one of the top scientific achievements of the past 50 years”, the media reported. The experts say: “The most prosaic goal of the LHC is to find the Higgs boson. Of course, the physicists are hoping to make discoveries with the LHC that gets beyond the Standard Model, including an understanding of Dark Matter. Higgs boson is part of The Standard Model of particle physics, that is a mathematical model. The Standard Model lays out the basics of how elementary particles and forces interact in the universe. The Higgs boson is the particle that gives all matter its mass. The Higgs boson is the only elementary particle in the Standard Model that has not yet been observed in the experiments. In the LHC, protons can have an energy of 3 to 7 TeV (tera-electron volts; TeV = 1012 eV), colliding head-on at about 6 to 14 TeV, which should be ample to create a Higgs boson. In modern physics, mass of subatomic particles is described by the mass-energy unit GeV, Giga electron volts (E = mc2). The amount of energy an electron gains moving through a potential of one volt in a vacuum is one electron-volt, 1eV. Some suggest that any mechanism capable of generating the masses of elementary particles must become visible at energies above 1.4 TeV; therefore, the LHC could provide experimental evidence of the existence or non-existence of the Higgs boson. In December 2011, the two main experiments at the LHC (ATLAS and CMS) reported that the data collected up till then hints that the Higgs may exist with a mass around 125 GeV (or 125 times proton mass). However, the evidence is not yet
. Data collection and analysis in search of Higgs are intensifying since 2010 when the LHC began operating at 3.5 TeV. On 13 December 2011, experimental results were announced from the ATLAS and CMS experiments, indicating that if the Higgs boson exists, its mass is limited to the range 116 – 130 GeV (ATLAS) or 115 -127 GeV (CMS), with other masses excluded at 95% confidence level. The statistical significance of the observations is not large enough to draw conclusions, but the fact that the two independent experiments show excesses at around the same mass has led to considerable excitement in the particle physics community. It is expected that the LHC will provide sufficient data to either exclude or confirm the existence of the Standard Model Higgs boson by the end of 2012“. Many Iranians and non-Iranians know nothing about the Standard Model, because they don’t learn anything about it in high school or even in university (except the physic students), maybe because it’s based on advanced mathematics. Brian Greene, one of the great alive physicists and one of the founders of String Theory has a popular book, “The Elegant Universe”, for all people who love science. “The Elegant Universe” is about String theory, the fundamental particles, the Standard Model, the fundamental questions, etc. Greene writes: “The ancient Greeks surmised that the stuff of the universe was made up of tiny ‘uncuttable’ ingredients that they called atoms. In the 19th century scientists showed that many familiar substances had a smallest recognizable constituent; following in the tradition laid down by the Greeks, they called them atoms. But history has shown it to be a misnomer, since atoms surely are ‘cuttable’. By the early 1930s, the collective works of Thomson, Rutherford, Bohr, and Chadwick had established the solar system-like atomic model with which most of us are familiar. Far from being the most elementary material constituent, atoms consist of a nucleus, containing protons and neutrons, that is surrounded by a swarm of orbiting electrons. [In 1968], the Stanford Linear Accelerator Center found that protons and neutrons are not fundamental, either. Instead they showed that each consists of three smaller particles, called quarks. The experimenters confirmed that quarks themselves come in two varieties, which were named, a bit less creatively, up and down. A proton consists of 2 up-quarks and 1 down-quark; a neutron consists of 2 down-quarks and 1 up-quark. Everything you see in the world appears to be made from combinations of electrons, up-quarks, and down-quarks. No experimental evidence indicates that the quark is built up from something smaller. But a great deal of evidence indicates that the universe itself has additional particulate ingredients. In the mid-1950s, Reines and Cowan found conclusive experimental evidence for a fourth kind of fundamental particle called a neutrino – a particle whose existence was predicted in the early 1930s by Wolfgang Pauli. Neutrinos proved very difficult to find because they are ghostly particles that only rarely interact with other matter: an average-energy neutrino can easily pass right through many trillion miles of lead without the slightest effect on its motion. This should give you significant relief, because right now as you read this, billions of neutrinos ejected into space by the sun are passing through your body and the earth as well, as part of their lonely journey through the cosmos. In the late 1930s, another particle called a muon -identical to an electron except that a muon is about 200 times heavier -was discovered by physicists studying cosmic rays (showers of particles that bombard earth). Using ever more powerful technology, physicists have continued to slam bits of matter together with ever increasing energy, momentarily recreating conditions unseen since the big bang. In the debris they have searched for new fundamental ingredients to add to the growing list of particles. Here is what they have found: 4 more quarks – charm, strange, bottom, and top-[There are six flavors of quark; the last was observed at Fermilab in 1995]and another even heavier cousin of the electron, called a tau, as well as two other particles with properties similar to the neutrino (called the muon-neutrino and tau-neutrino to distinguish them from the original neutrino, now called the electron-neutrino) [These six particles are called ‘Leptons’] “.


Brian Greene added: “These particles are produced through high-energy collisions and exist only ephemerally; they are not constituents of anything we typically encounter. But even this is not quite the end of the story. Each of these particles has an antiparticle partner -a particle of identical mass but opposite in certain other respects such as its electric charge. When in contact, matter and antimatter can annihilate one another to produce pure energy – that’s why there is extremely little naturally occurring antimatter in the world around us. Physicists have recognized a pattern among these particles. The matter particles neatly fall into three groups, which are often called families. Each family contains two of the quarks, an electron or one of its cousins, and one of the neutrino species. The corresponding particle types across the three families have identical properties except for their mass, which grows larger in each successive family. The upshot is that physicists have now probed the structure of matter to scales of about a billionth of a billionth of a meter and shown that everything encountered to date – whether it occurs naturally or is produced artificially with giant atom-smashers – consists of some combination of particles from these three families and their antimatter partners. But why are there so many fundamental particles, especially when it seems that the great majority of things in the world around us need only electrons, up-quarks, and down-quarks? Why are there three families? Why not one family or four families or any other number? Why do the particles have a seemingly random spread of masses -why, for instance, does the tau weigh about 3,520 times as much as an electron? Why does the top quark weigh about 40,200 times as much an up-quark? These are such strange, seemingly random numbers. Did they occur by chance, by some divine choice, or is there a comprehensible scientific explanation for these fundamental features of our universe?” These simple questions are the fundamental questions, and it can very important and meaningful. In fact, many simple or obvious questions, are the fundamental questions or the most important questions. Anyway, the other experts say: “The Standard Model (formulated in 1970s) describes the universe in terms of Matter (fermions) and Force (bosons). The Standard Model is a mathematical theory that describes approximately 200 particles and their interactions using 17 fundamental particles, all of which are fermions or bosons: 6 quarks (fermions), 6 leptons (fermions), 4 force-carrying particles (bosons), and the Higgs boson. There are 4 known forces, each mediated by a fundamental particle (force-carrier particle): gravity (carrier: graviton), electromagnetic force (carrier: photon), Strong nuclear force (carrier: gluons), and Weak nuclear force (carrier: W and Z bosons). The graviton is not included in Standard Model (SM), and attempts to include gravity in the SM have failed. In fact, the SM falls short of being a complete theory of fundamental particles because it does not incorporate the physics of general relativity, such as gravitation, dark matter, and dark energy. The string theory, the M-theory, and other theories are more complete than the SM. The fundamental matter particles in the SM are quarks and leptons. Protons and neutrons are composites of quarks, whereas an electron is a lepton. A proton is 1,800 times more massive than an electron. Quarks are never found alone, whereas leptons never form composites. Properties of fundamental particles include spin, electrical charge, color charge, and mass. Composites made of quarks or anti-quarks are called hadrons. But ‘Dark Matter’ is not made of hadrons, and is speculated to be composed of neutrino-like neutralinos (leptons). The strong nuclear force acts on hadrons, but does not act on leptons. The weak nuclear force acts on both hadrons & leptons. Hadrons are “glued” together by gluons. The larger the number of gluons exchanged among quarks, the stronger the binding force. There are 6 flavors of quarks and 6 flavors of leptons, both grouped in 3 families. Above a certain critical temperature all particles are massless except the Higgs boson, that has been proposed to cause an interaction that causes particles to have mass. The standard model does suggest a mechanism by which particles acquire mass -the Higgs mechanism, named after the British physicist Peter Higgs. In 1960s, Peter Higgs and others proposed a mathematical theory to explain how particles obtain mass. The Higgs mechanism proposes that a so-called Higgs energy field exists everywhere in the universe. As particles zoom around in this field, they interact with Higgs bosons, and it gives them mass. Of course, the Standard Model cannot explain why a particle has a certain mass. For example, both the photon and the W particle are force carrier particles, but why is the photon massless and the W particle massive? In empty space, the Higgs field has an amplitude different from zero; i.e. a non-zero vacuum expectation value. The existence of this non-zero vacuum expectation plays a fundamental role. The Higgs boson has no spin and is its own antiparticle. According to Big Bang theory, the existing universe emerged from an explosion (in a vacuum) that occurred 13.7 billion years ago. The four forces were unified until 10−43 seconds after the Big Bang, after which first gravity and then strong nuclear force separated from the other two forces. At 10−12 seconds after the Big Bang electromagnetism separated from the weak nuclear force, and the universe was filled with a hot quark-gluon plasma that included leptons and antiparticles. At 10−6 seconds hadrons began to form. Most hadrons and anti-hadrons were eliminated by annihilation, leaving a small residue of hadrons by one second post-Big Bang. Between one and three seconds after Big Bang the universe was dominated by leptons/ anti-leptons until annihilation of these particles left only a small residue of leptons. The universe was dominated by photons created by all of the matter/ antimatter annihilations, and the predominance of matter over antimatter had been established. Between 3 and 20 minutes after the Big Bang protons and neutrons began to combine to form atomic nuclei. A plasma of electrons & nuclei existed for 300,000 years until the temperature dropped to 5,000ºC when hydrogen & helium atoms formed. Gravitational evidence suggests that dark matter is the dominant form of mass in the Universe. Dark matter reputedly caused hydrogen to coalesce into stars, and is a binding force in galaxies. Dark matter does not interact with the electromagnetic force, thus making it transparent and hard to detect, despite the fact that dark matter must permeate the galaxy. Unlike visible matter, dark matter is nonbaryonic – its composition is outside of the (unextended) Standard Model. Neutrinos may be a low-mass example of dark matter. The Standard Model is a good theory. But it does not explain everything. String Theory, M-Theory, and other theories explain more facts.”


Brian Greene adds: “Things become more complicated when we consider the forces of nature. One of these is the gravitational force. The other three are the electromagnetic force, the weak force, and the strong force. Gravity is the most familiar of the forces, being responsible for keeping us in orbit around the sun as well as for keeping our feet firmly planted on earth. The electromagnetic force is the next most familiar of the four. It is the force driving all of the conveniences of modern life – lights, TVs, telephones – and underlies the awesome might of lightning storms and the gentle touch of a human hand. The strong and the weak forces are the nuclear forces. The strong force is responsible for keeping quarks ‘glued’ together inside of protons and neutrons and keeping protons and neutrons tightly crammed together inside atomic nuclei. The weak force is best known as the force responsible for the radioactive decay of substances such as uranium [it causes transformation of protons to neutrons and vice-versa, along with other radioactive phenomena]. During the past century, physicists have found two features common to all these forces. First, at a microscopic level all the forces have an associated particle that you can think of as being the smallest packet or bundle of the force. If you fire a laser beam -an “electromagnetic ray gun” -you are firing a stream of photons. Similarly, the smallest constituents of weak and strong force fields are particles called weak gauge bosons and gluons. By 1984 experimenters had definitively established the existence and the detailed properties of these three kinds of force particles. Physicists believe that the gravitational force also has an associated particle – the graviton– but its existence has yet to be confirmed experimentally. The second common feature of the forces is that just as mass determines how gravity affects a particle, and electric charge determines how the electromagnetic force affects it, particles are endowed with certain amounts of “strong charge” and “weak charge” that determine how they are affected by the strong and weak forces. But as with particle masses, beyond the fact that experimental physicists have carefully measured these properties, no one has any explanation of why our universe is composed of these particular particles, with these particular masses and force charges. Notwithstanding their common features, an examination of the fundamental forces themselves serves only to compound the questions. Why, for instance, are there 4 fundamental forces? Why not 5 or 3 or perhaps only one? Why do the forces have such different properties? Why are the strong and weak forces confined to operate on microscopic scales while gravity and the electromagnetic force have an unlimited range of influence? And why is there such an enormous spread in the intrinsic strength of these forces? To appreciate this last question, imagine holding an electron in your left hand and another electron in your right hand and bringing these two identical electrically charged particles close together. Their mutual gravitational attraction will favor their getting closer while their electromagnetic repulsion will try to drive them apart. Which is stronger? There is no contest: The electromagnetic repulsion is about a million billion billion billion billion (10 to the 42th) times stronger! If your right bicep represents the strength of the gravitational force, then your left bicep would have to extend beyond the edge of the known universe to represent the strength of the electromagnetic force. The only reason the electromagnetic force does not completely overwhelm gravity in the world around us is that most things are composed of an equal amount of positive and negative electric charges whose forces cancel each other out. On the other hand, since gravity is always attractive, there are no analogous cancellations -more stuff means greater gravitational force. But fundamentally speaking, gravity is an extremely feeble force. Experiments also have shown that the strong force is about one hundred times as strong as the electromagnetic force and about one hundred thousand times as strong as the weak force. But where is the rationale – the raison d’etre – for our universe having these features? This is not a question borne of idle philosophizing about why certain details happen to be one way instead of another; the universe would be a vastly different place if the properties of the matter and force particles were even moderately changed. For example, were the mass of the electron a few times greater than it is, electrons and protons would tend to combine to form neutrons, gobbling up the nuclei of hydrogen and, again, disrupting the production of more complex elements. Stars rely upon fusion between stable nuclei and would not form with such alterations to fundamental physics. We could go on, but the idea is clear: the universe is the way it is because the matter and the force particles have the properties they do. But is there a scientific explanation for why they have these properties? String theory offers a powerful conceptual paradigm in which, for the first time, a framework for answering these questions has emerged. Let’s first get the basic idea. The above-mentioned particle are the “letters” of all matter. They appear to have no further internal substructure. But String theory proclaims otherwise. According to string theory, if we could examine these particles with even greater precision, we would find that each is not point-like, but instead consists of a tiny one-dimensional loop. Like an infinitely thin rubber band, each particle contains a vibrating, oscillating, dancing filament that physicists have named a string. This simple replacement of point-particle material constituents with strings resolves the incompatibility between quantum mechanics and general relativity. But it is only part of the reason string theory has generated such excitement. In Einstein’s day, the strong and the weak forces had not yet been discovered, but he found the existence of even two distinct forces -gravity and electromagnetism- deeply troubling. Einstein did not accept that nature is founded on such an extravagant design. This launched his thirty-year voyage in search of the so-called unified field theory that he hoped would show that these two forces are really manifestations of one grand underlying principle. This quixotic quest isolated Einstein from the mainstream of physics, which, understandably, was far more excited about delving into the newly emerging framework of quantum mechanics. He wrote to a friend in the early 1940s, ‘I have become a lonely old chap who is mainly known because he doesn’t wear socks and who is exhibited as a curiosity on special occasions.’ Einstein was simply ahead of his time. More than half a century later, his dream of a unified theory has become the Holy Grail of modern physics. And a sizeable part of the physics and mathematics community is becoming increasingly convinced that string theory may provide the answer. From one principle -that everything at its most microscopic level consists of combinations of vibrating strands -string theory provides a single explanatory framework capable of encompassing all forces and all matter. String theory proclaims, for instance, that the observed particle properties are a reflection of the various ways in which a string can vibrate. Just as the strings on a violin or on a piano have resonant frequencies at which they prefer to vibrate – patterns that our ears sense as various musical notes and their higher harmonics -the same holds true for the loops of string theory. But rather than producing musical notes, each of the preferred patterns of vibration of a string in string theory appears as a particle whose mass and force charges are determined by the string’s oscillatory pattern. The electron is a string vibrating one way, the up-quark is a string vibrating another way, and so on. Far from being a collection of chaotic experimental facts, particle properties in string theory are the manifestation of one and the same physical feature: the resonant patterns of vibration -the music, so to speak- of fundamental loops of string. The same idea applies to the forces.”

For more information:

[1] ParticleAdventure.org , a good website about the Standard Model and, the Fundamental particles.

[2] “The Elegant Universe”, by Brian Greene. A great popular book for all people, about the String Theory, Standard Model, fundamental particles, and modern Physics.

[3] A very brief but good illustration about Standard Model.