the higgs boson: A 5-page summary of theory and discovery
The Higgs Boson is a crucial piece in the universe, changing our very lives, but its workings are so miniscule that it is difficult for even the most versed scientist to comprehend. This boson’s existence was confirmed at the Large Hadron Collider (LHC) at CERN on July 4th, 2012. The boson confirms the existence of a Higgs Field, the final piece in the Standard Model of subatomic particle physics. Before we get into the complex Higgs, it is important to know the background theory of the Standard Model. After this, we will examine the Higgs’ part in the Standard Model and how CERN was able to confirm the discovery. Lastly, we will look at the implications of this discovery for the world, and the universe as a whole.
Theory - The Standard Model
We’ve all learned about electrons and protons in high shool chemistry, but scientists have gone one step deeper in the search for the groundwork of the universe. The Standard Model is currently accepted as the way that the world works on the subatomic level. It is split up into two parts; massive particles and massless particles. Particles with mass are called fermions, while massless particles are called bosons.
Within the fermions, there are quarks and leptons. Quarks make up protons and come in three kinds of pairs; up and down, charm and strange, and top and bottom. Up, charm, and top quarks have charges of +⅔, while down, strange, and bottom quarks have charges of -⅓.
Quarks can only be found in groups, called hadrons. There are two types of hadrons: mesons and baryons. Mesons are made of a quark-antiquark pair, while baryons are made of three quarks. Protons are baryons made of two up quarks and a down quark, while neutrons have two down quarks and an up quark. All of the matter in the nucleus of any atom found in the universe, therefore, is made up of quarks.
There are three different kinds of leptons: the electron, the tau, and the muon. Each of these come with their own neutrino. Neutrinos have no charge and are much smaller than their respective lepton. The electron is the lightest and most stable of the three, and the only one found in nature, at least on earth. The tau and the muon decay very quickly, so only electrons are stably present in our world. They are much less massive than quarks and orbit on the outside of the atom.
Bosons are theoretically massless particles as predicted by the Standard Model. It is good to think of them as force carriers. Bosons can be thought of as vibrations that come out of certain fields, like the photon is a vibration in the electromagnetic field. The bosons in the Standard Model (Higgs aside) are gauge bosons, meaning that they carry forces to undertake fundamental interactions.
The photon is a massless boson that is a vibration in the electromagnetic field. Photons move at the speed of light and create all of the electromagnetic radiation that we see and use on a daily basis. From light that we see to microwaves to x-rays, the energy is carried by photons.
Gluons are also massless bosons and carry the strong interaction. They are said to have “color charge” and come in red, blue, and green. Of course they are not actually colorful, but this is a playful measurement of certain characteristics that they possess. This strong force is noticeable in the ways that quarks act in the nucleus. On the atomic scale, this force is the strongest, binding quarks together to form hadrons and binding hadrons together in the nuclei of atoms.
W and Z bosons complete the gauge boson section of the Standard Model. These bosons, though, are massive. They carry the weak interaction, which is responsible for nuclear fusion, radioactive decay, and other nuclear reaction. Technically, they shouldn’t have mass by the Standard Model, but they do, so scientists knew there had to be another piece. This piece, as we now know, is the Higgs Boson.
Theory - The Higgs Boson
Interestingly enough, the Higgs Boson was not the crucial part of the discovery in itself. The importance lies in the Higgs Field, which was confirmed by the existence of a Higgs-like boson.
The Higgs Field is a scalar field, meaning that it is nonzero in empty space. This makes it unique; all other fields in the Standard Model zero out in empty space. For example, gravity gets weaker as you move farther away from an object, but the Higgs is always at the same level, at every point in space. This may not seem significant, but it affects all of the things that happen in the universe. The Higgs Field, as it is understood today, acts differently depending on the fundamental particles involved.
Confused? Don’t worry. This is quite difficult to grasp, but the field acts (and gives mass) to some particles and not others. Photons (light) pass through the field unaffected; this is why they travel the speed of light and other things do not. On the other hand, W and Z bosons are affected by the field, acquiring large masses as a result. The Higgs also gives mass to quarks and leptons, so the Higgs is literally the reason behind the mass of everything in the universe.
The Higgs field can be likened to a crowd, as shown by Sean Carroll in his book The Particle at the End of the Universe. Imagine you somehow end up in a celebrity party with Angelina Jolie. You can walk across the room uninhibited, as nobody really feels the need to talk to you. Everybody wants to talk to Angelina, on the other hand. so she would have a harder time making her way through the crowd. The Higgs Field works similarly, if you were a photon and Angelina were a W or Z boson.
This analogy is far from perfect, but it serves the general purpose. Particles buzzing throughout the Higgs Field don't actually stop or get slowed down by the field once they do start moving. Basically, the field makes things behave the way they do by acting in a different way on different particles.
What does the Higgs do to the Standard Model? Everything, basically. Mathematically, the Standard Model doesn't work without the Higgs. Theoretically, it is very well-aligned and symmetrical without the Higgs, but the universe doesn't work this way. This leads us into the primary function of the Higgs Field; a symmetry breaker.
The subatomic particle physics definition of symmetry is similar to the definitions we learn in elementary school. If two sides are symmetric, they are the same when folded over a line. If two particles are symmetric, they cannot be differentiated from each other. Electrons are all symmetric to each other, but not to tau leptons. By this logic, electrons and quarks are even more separated and not symmetrical.
Without the Higgs, though, electrons would be symmetric not only to tau leptons, but also to quarks. These indistinguishable particles would just zip around the universe at the speed of light, making the universe much less interesting.
The Structure of the LHC
The LHC is a state-of-the-art circular pipe 17 miles long. It uses large magnets to direct the beams of protons around the pipe and collide them. Other magnets concentrate the beams to make collisions more likely, which is not an easy task. CERN’s website states that “the particles are so tiny that the task of making them collide is akin to firing two needles 10 kilometres apart with such precision that they meet halfway.”
In different areas of the pipe, there are detectors. The two largest ones are called A Toroidal LHC Apparatus (ATLAS) and Compact Muon Selenoid (CMS). ATLAS, the bigger of the two, has a few different detectors to monitor what comes out of the collisions: an electromagnetic calorimeter, a hadron calorimeter, and a muon spectrometer. These monitors are necessary because each Standard Model particle moves out of reactions in a different way.
The innermost layer is called a tracker. This lightweight apparatus is able to track the paths of charged particles very accurately. By knowing the paths, it is possible to find their energy when they exit the reaction. Made entirely of silicon, the tracker can measure with accuracy up to 10 µm (micrometers). One micrometer is a millionth of a meter, so this tracker is incredibly accurate.
The next detector, an electromagnetic calorimeter (energy detector), lines the pipe. It measures the products of reactions, specifically particles that have charge, like photons and electrons. These can result from Higgs Bosons decaying, but so many other reactions produce photon blasts that this channel is not the most useful.
Hadrons, on the other hand, pass through the electromagnetic calorimeter because of their interaction with the strong nuclear force. Only particles that interact with the electromagnetic field are picked up by the electromagnetic calorimeter, so hadrons are unaffected. Just outside the first calorimeter is another to track these hadrons. This one is made of steel and absorbs the energy, much like the electromagnetic calorimeter. This calorimeter is much larger, covering large sections of the pipe and requiring huge amounts of construction materials to complete.
Next comes the muon spectrometer, designed to measure the energy of muons, which escape from both of the calorimeters. This spectrometer is even larger than the hadron calorimeter and it encompasses about 12,000 square feet. The reason for this tremendous size is the accuracy needed. If muons were not measured at the LHC, it would be impossible to accurately measure the total energy output of most hadron collisions.
Similarly to ATLAS, the CMS has an electromagnetic calorimeter, a hadron calorimeter, and a muon detector. The muon detector is particularly important, as the Higgs often decays into four muons. By creating two practically identical research facilities in different sections of the ring, scientists at CERN were able to conduct experiments separately and compare results, helping to confirm that the discovery wasn't simply due to chance.
How the Higgs Boson was Discovered
The Higgs Boson was discovered at CERN in Switzerland on July 4th, 2013 at the Large Hadron Collider (LHC). The LHC replaced the Large Electron-Positron Collider (LEP) to become the largest particle accelerator ever built and, unlike the LEP, it collides protons instead of leptons. It is basically an upgrade over the previous model, making it capable of reaching energies the LEP never could.
The LHC works by colliding protons at very high speeds and observing what comes out using state-of-the-art technology. When I first imagined the LHC, I saw two tiny protons whizzing around the ring and then bashing into each other, forming new particles. However, we don’t have nearly the precision to do this kind of thing, and trillions of protons are smashed into each other on a daily basis. Discovering the Higgs Boson was a daunting task, not aided by the fact that we can’t even see the protons we are colliding with our current technology.
Only certain particles of the Standard Model can survive for extended periods of time in Earth’s conditions. The other ones decay very quickly, in a matter of nanoseconds. The Higgs Boson will decay in 1.6×10−22 seconds. This is in the zeptosecond range, impossible to detect directly with what we have today. CERN predicts that they created over 100,000 Higgs Bosons before they actually “discovered” one. The way the Higgs was discovered is similar, in some ways, to a detective story. We used the unique clues, the fingerprint and the hair it left behind, to confirm its existence indirectly.
The way the Higgs was discovered was using decay channels. Decaying is where a certain unstable particle changes into other, more stable particles. A decay channel is measuring all of the decay of a certain type, for example all of the possible reactions that result in photon emissions. Since the Higgs Field is nonzero and at a constant everywhere in the universe, scientists expected to see telltale “bumps” on certain decay channels as a result Higgs Bosons decaying into certain particles.
The Higgs can decay into quite a few combinations, but the best channels are into particles that don’t occur too often. Rare is a good thing in this case because so many collisions take place at the LHC that the common channels become useless. For example, the Higgs can decay into two photons, one of the preferred channels. This meant that, when scientists turned up the energy of the LHC to the point where the Higgs Boson was eventually confirmed, bumps appeared to confirm the presence of an unknown factor (the Higgs) at that energy.
The Implications of the Higgs Boson
Now, the question is "who cares"? Why do we need to discover the Higgs Field, which we can barely comprehend, to see how the universe works on a scale we'll never be able to see? My favorite answer: because it's cool. Everybody has an innate curiosity for the world, and this final piece of the Standard Model jigsaw inspires more fascination than any political news could, at least in the scholar's mind.
Another more concrete reason for doing this kind of research is getting thousands of brilliant minds together in one place. With that much brainpower moving in a single direction, side effects can be massive in new development. For example, according to Sean Carroll, the people at CERN inadvertently created the beginnings of the internet in order to quickly share data worldwide. When you get down to it, though, there isn't much "practical" reason to continue expensive studies like these other than to gain a greater understanding of the universe. Luckily, CERN is already looking to amp up the LHC to test further theories beyond the Standard Model. Evidence from the LHC hints that the Standard Model might behave differently at higher energies, with other, higher Higgs energies possible. Another route scientists hope to explore is investigating the mysterious "dark matter" that inhabits vast portions of our universe.
I'd like to end with one of my favorite parts of any good scientific discovery: how it could end the universe as we know it. The Higgs Boson rests in a (fairly) stable state that can be likened to the bottom of a valley. It prefers to sit at the lowest point, the most stable state. However, there may be deeper "valleys" in our Higgs-scape, and the field could take a quantum leap to a deeper valley. This intensified Higgs state would change the way everything in the universe functions and could even expand atoms to many times their normal size, basically exploding the whole universe. Of course, this is incredibly unlikely to happen and we'll probably all be dead anyway before it does, but it's worth a mention.
The Higgs Boson's discovery has completed the Standard Model, but scientists are already looking further. Check out CERN's website (home.web.cern.ch) for more information, or read Sean Carroll's book The Particle at the End of the Universe.
Theory - The Standard Model
We’ve all learned about electrons and protons in high shool chemistry, but scientists have gone one step deeper in the search for the groundwork of the universe. The Standard Model is currently accepted as the way that the world works on the subatomic level. It is split up into two parts; massive particles and massless particles. Particles with mass are called fermions, while massless particles are called bosons.
Within the fermions, there are quarks and leptons. Quarks make up protons and come in three kinds of pairs; up and down, charm and strange, and top and bottom. Up, charm, and top quarks have charges of +⅔, while down, strange, and bottom quarks have charges of -⅓.
Quarks can only be found in groups, called hadrons. There are two types of hadrons: mesons and baryons. Mesons are made of a quark-antiquark pair, while baryons are made of three quarks. Protons are baryons made of two up quarks and a down quark, while neutrons have two down quarks and an up quark. All of the matter in the nucleus of any atom found in the universe, therefore, is made up of quarks.
There are three different kinds of leptons: the electron, the tau, and the muon. Each of these come with their own neutrino. Neutrinos have no charge and are much smaller than their respective lepton. The electron is the lightest and most stable of the three, and the only one found in nature, at least on earth. The tau and the muon decay very quickly, so only electrons are stably present in our world. They are much less massive than quarks and orbit on the outside of the atom.
Bosons are theoretically massless particles as predicted by the Standard Model. It is good to think of them as force carriers. Bosons can be thought of as vibrations that come out of certain fields, like the photon is a vibration in the electromagnetic field. The bosons in the Standard Model (Higgs aside) are gauge bosons, meaning that they carry forces to undertake fundamental interactions.
The photon is a massless boson that is a vibration in the electromagnetic field. Photons move at the speed of light and create all of the electromagnetic radiation that we see and use on a daily basis. From light that we see to microwaves to x-rays, the energy is carried by photons.
Gluons are also massless bosons and carry the strong interaction. They are said to have “color charge” and come in red, blue, and green. Of course they are not actually colorful, but this is a playful measurement of certain characteristics that they possess. This strong force is noticeable in the ways that quarks act in the nucleus. On the atomic scale, this force is the strongest, binding quarks together to form hadrons and binding hadrons together in the nuclei of atoms.
W and Z bosons complete the gauge boson section of the Standard Model. These bosons, though, are massive. They carry the weak interaction, which is responsible for nuclear fusion, radioactive decay, and other nuclear reaction. Technically, they shouldn’t have mass by the Standard Model, but they do, so scientists knew there had to be another piece. This piece, as we now know, is the Higgs Boson.
Theory - The Higgs Boson
Interestingly enough, the Higgs Boson was not the crucial part of the discovery in itself. The importance lies in the Higgs Field, which was confirmed by the existence of a Higgs-like boson.
The Higgs Field is a scalar field, meaning that it is nonzero in empty space. This makes it unique; all other fields in the Standard Model zero out in empty space. For example, gravity gets weaker as you move farther away from an object, but the Higgs is always at the same level, at every point in space. This may not seem significant, but it affects all of the things that happen in the universe. The Higgs Field, as it is understood today, acts differently depending on the fundamental particles involved.
Confused? Don’t worry. This is quite difficult to grasp, but the field acts (and gives mass) to some particles and not others. Photons (light) pass through the field unaffected; this is why they travel the speed of light and other things do not. On the other hand, W and Z bosons are affected by the field, acquiring large masses as a result. The Higgs also gives mass to quarks and leptons, so the Higgs is literally the reason behind the mass of everything in the universe.
The Higgs field can be likened to a crowd, as shown by Sean Carroll in his book The Particle at the End of the Universe. Imagine you somehow end up in a celebrity party with Angelina Jolie. You can walk across the room uninhibited, as nobody really feels the need to talk to you. Everybody wants to talk to Angelina, on the other hand. so she would have a harder time making her way through the crowd. The Higgs Field works similarly, if you were a photon and Angelina were a W or Z boson.
This analogy is far from perfect, but it serves the general purpose. Particles buzzing throughout the Higgs Field don't actually stop or get slowed down by the field once they do start moving. Basically, the field makes things behave the way they do by acting in a different way on different particles.
What does the Higgs do to the Standard Model? Everything, basically. Mathematically, the Standard Model doesn't work without the Higgs. Theoretically, it is very well-aligned and symmetrical without the Higgs, but the universe doesn't work this way. This leads us into the primary function of the Higgs Field; a symmetry breaker.
The subatomic particle physics definition of symmetry is similar to the definitions we learn in elementary school. If two sides are symmetric, they are the same when folded over a line. If two particles are symmetric, they cannot be differentiated from each other. Electrons are all symmetric to each other, but not to tau leptons. By this logic, electrons and quarks are even more separated and not symmetrical.
Without the Higgs, though, electrons would be symmetric not only to tau leptons, but also to quarks. These indistinguishable particles would just zip around the universe at the speed of light, making the universe much less interesting.
The Structure of the LHC
The LHC is a state-of-the-art circular pipe 17 miles long. It uses large magnets to direct the beams of protons around the pipe and collide them. Other magnets concentrate the beams to make collisions more likely, which is not an easy task. CERN’s website states that “the particles are so tiny that the task of making them collide is akin to firing two needles 10 kilometres apart with such precision that they meet halfway.”
In different areas of the pipe, there are detectors. The two largest ones are called A Toroidal LHC Apparatus (ATLAS) and Compact Muon Selenoid (CMS). ATLAS, the bigger of the two, has a few different detectors to monitor what comes out of the collisions: an electromagnetic calorimeter, a hadron calorimeter, and a muon spectrometer. These monitors are necessary because each Standard Model particle moves out of reactions in a different way.
The innermost layer is called a tracker. This lightweight apparatus is able to track the paths of charged particles very accurately. By knowing the paths, it is possible to find their energy when they exit the reaction. Made entirely of silicon, the tracker can measure with accuracy up to 10 µm (micrometers). One micrometer is a millionth of a meter, so this tracker is incredibly accurate.
The next detector, an electromagnetic calorimeter (energy detector), lines the pipe. It measures the products of reactions, specifically particles that have charge, like photons and electrons. These can result from Higgs Bosons decaying, but so many other reactions produce photon blasts that this channel is not the most useful.
Hadrons, on the other hand, pass through the electromagnetic calorimeter because of their interaction with the strong nuclear force. Only particles that interact with the electromagnetic field are picked up by the electromagnetic calorimeter, so hadrons are unaffected. Just outside the first calorimeter is another to track these hadrons. This one is made of steel and absorbs the energy, much like the electromagnetic calorimeter. This calorimeter is much larger, covering large sections of the pipe and requiring huge amounts of construction materials to complete.
Next comes the muon spectrometer, designed to measure the energy of muons, which escape from both of the calorimeters. This spectrometer is even larger than the hadron calorimeter and it encompasses about 12,000 square feet. The reason for this tremendous size is the accuracy needed. If muons were not measured at the LHC, it would be impossible to accurately measure the total energy output of most hadron collisions.
Similarly to ATLAS, the CMS has an electromagnetic calorimeter, a hadron calorimeter, and a muon detector. The muon detector is particularly important, as the Higgs often decays into four muons. By creating two practically identical research facilities in different sections of the ring, scientists at CERN were able to conduct experiments separately and compare results, helping to confirm that the discovery wasn't simply due to chance.
How the Higgs Boson was Discovered
The Higgs Boson was discovered at CERN in Switzerland on July 4th, 2013 at the Large Hadron Collider (LHC). The LHC replaced the Large Electron-Positron Collider (LEP) to become the largest particle accelerator ever built and, unlike the LEP, it collides protons instead of leptons. It is basically an upgrade over the previous model, making it capable of reaching energies the LEP never could.
The LHC works by colliding protons at very high speeds and observing what comes out using state-of-the-art technology. When I first imagined the LHC, I saw two tiny protons whizzing around the ring and then bashing into each other, forming new particles. However, we don’t have nearly the precision to do this kind of thing, and trillions of protons are smashed into each other on a daily basis. Discovering the Higgs Boson was a daunting task, not aided by the fact that we can’t even see the protons we are colliding with our current technology.
Only certain particles of the Standard Model can survive for extended periods of time in Earth’s conditions. The other ones decay very quickly, in a matter of nanoseconds. The Higgs Boson will decay in 1.6×10−22 seconds. This is in the zeptosecond range, impossible to detect directly with what we have today. CERN predicts that they created over 100,000 Higgs Bosons before they actually “discovered” one. The way the Higgs was discovered is similar, in some ways, to a detective story. We used the unique clues, the fingerprint and the hair it left behind, to confirm its existence indirectly.
The way the Higgs was discovered was using decay channels. Decaying is where a certain unstable particle changes into other, more stable particles. A decay channel is measuring all of the decay of a certain type, for example all of the possible reactions that result in photon emissions. Since the Higgs Field is nonzero and at a constant everywhere in the universe, scientists expected to see telltale “bumps” on certain decay channels as a result Higgs Bosons decaying into certain particles.
The Higgs can decay into quite a few combinations, but the best channels are into particles that don’t occur too often. Rare is a good thing in this case because so many collisions take place at the LHC that the common channels become useless. For example, the Higgs can decay into two photons, one of the preferred channels. This meant that, when scientists turned up the energy of the LHC to the point where the Higgs Boson was eventually confirmed, bumps appeared to confirm the presence of an unknown factor (the Higgs) at that energy.
The Implications of the Higgs Boson
Now, the question is "who cares"? Why do we need to discover the Higgs Field, which we can barely comprehend, to see how the universe works on a scale we'll never be able to see? My favorite answer: because it's cool. Everybody has an innate curiosity for the world, and this final piece of the Standard Model jigsaw inspires more fascination than any political news could, at least in the scholar's mind.
Another more concrete reason for doing this kind of research is getting thousands of brilliant minds together in one place. With that much brainpower moving in a single direction, side effects can be massive in new development. For example, according to Sean Carroll, the people at CERN inadvertently created the beginnings of the internet in order to quickly share data worldwide. When you get down to it, though, there isn't much "practical" reason to continue expensive studies like these other than to gain a greater understanding of the universe. Luckily, CERN is already looking to amp up the LHC to test further theories beyond the Standard Model. Evidence from the LHC hints that the Standard Model might behave differently at higher energies, with other, higher Higgs energies possible. Another route scientists hope to explore is investigating the mysterious "dark matter" that inhabits vast portions of our universe.
I'd like to end with one of my favorite parts of any good scientific discovery: how it could end the universe as we know it. The Higgs Boson rests in a (fairly) stable state that can be likened to the bottom of a valley. It prefers to sit at the lowest point, the most stable state. However, there may be deeper "valleys" in our Higgs-scape, and the field could take a quantum leap to a deeper valley. This intensified Higgs state would change the way everything in the universe functions and could even expand atoms to many times their normal size, basically exploding the whole universe. Of course, this is incredibly unlikely to happen and we'll probably all be dead anyway before it does, but it's worth a mention.
The Higgs Boson's discovery has completed the Standard Model, but scientists are already looking further. Check out CERN's website (home.web.cern.ch) for more information, or read Sean Carroll's book The Particle at the End of the Universe.