The Large Hadron Collider (LHC) is a truly sophisticated machine; weighing in a 37,000 tonnes and residing in a 27km long tunnel it is the most sophisticated machine ever built. With over 9,500 magnets super-cooled to a temperature of 1.9K (-271.3◦C, colder than space!), the LHC can accelerate two beams of protons to 99.999999% the speed of light, before smashing them together inside four giant particle detectors. In doing so, it can recreate the conditions that were present less than a billionth of a second after the Big Bang.
ATLAS is one of those particle detectors, and was one of two experiments to discover the Higgs boson in 2012. It is over 46m long, 25m in diameter, weighs over 7,000 tonnes and contains around 3,000km of cable. Over 3,000 physicists from over 175 different institutions in 38 different countries work together on the project. Following the Higgs discovery, ATLAS is working on learning more about the properties of this new particle, as well as looking to discover new exciting physics (such as extra dimensions and dark matter candidates).
When two beams of particles collide at extremely high energies, new particles of matter can be created from that energy. This is possible because of Einstein’s famous equation, E=mc^2, which tells us that mass(m) and energy(E) are actually the same thing. These events take place inside machines called particle detectors, sending showers of particles in all different directions.
As charged particles move outwards, they allow electrons to move in atoms of silicon or strip electrons from gas atoms (the gas used is Xenon). By detecting these signals in the particle tracker and joining them up, a picture of the path the charged particle took can be built up. These paths are curved because the detector is immersed in a strong magnetic field, how curved they are tells us the momentum (related to their speed) of the particles. Outside the tracker are the calorimeters, which are designed to absorb the charged and neutral particles so we can measure their energy. With this information, we can work backwards and find out the properties of the particles produced in the collision.
If we see an off-centre vertex, it tells us that an uncharged particle (which is invisible to the tracker) has decayed into charged particles, which could be a sign of an exotic Higgs decay!
The Standard Model predicts that the Higgs boson is unstable and falls apart (decays) only 10-22 s (0.0000000000000000000001 seconds) after it is created. Theoretically, it can decay to a quark-anti quark pair (except for top, anti-top), a lepton anti-lepton pair or to two bosons (like photons or gluons).
However, there is also the possibility that it could decay to new particles that have not been observed until now, so-called long-lived scalar particles (scalar means they are spinless just like the Higgs boson). Some theories beyond the Standard Model (e.g. http://arxiv.org/abs/1312.4992) predict that these particles would be unique in that only the Higgs boson would decay to them, meaning it would have been impossible to study them until now – these are one example of what we call ‘exotic particles‘.
The diagram above shows a Higgs boson decaying into two exotic particles, which then themselves decay into many more particles – this is how an off-centre vertex could be produced. In fact, looking for off-centre vertices was proposed way back in 2006 as a way of detecting the Higgs boson, before it had been discovered (http://arxiv.org/abs/hep-ph/0605193)!
Since this process is not predicted by the Standard Model, if we see these in our detector then we will finally be able to realise the dream of obtaining direct evidence for a new theory of nature – opening our eyes to an entirely new realm of physics.
If these exotic decays are real, then they have already happened at the LHC and are sitting in the data we have right now, just waiting to be found by a keen pair of eyes. By helping with this project, you can take us one step closer to a deeper understanding of reality – to answering some of the greatest unsolved mysteries in the universe.
(Next time: ATLAS: How do particle detectors work?)
On July 4th 2012, the ATLAS and CMS collaborations at the Large Hadron Collider (LHC) at CERN, Geneva, independently announced the discovery of a new particle with a mass over 130 times greater than a proton. This was in the range that had been predicted for the elusive Higgs boson, and it had been verified to extremely high certainty.
But what do we mean by ‘high certainty’? In Particle Physics everyone agrees on how sure you need to be to declare something as a discovery, and it’s quite stringent. Firstly, we assume that there is no new particle and calculate the probability that the signal we measure is consistent with there not being a particle there. If that probability is less than 0.32 than we say we are 1 sigma certain, 0.046 = 2 sigma (this is usually good enough for most science), 0.0027 = 3 sigma… We could stop at 3 sigma, as 0.27% chance of being wrong seems awfully small, but there have been a number of possible particle discoveries at 3 sigma level that later turned out just to be statistical anomalies. So just to be absolutely sure, the usual threshold to claim a discovery is 5 sigma (0.0000003 chance of observed signal being consistent with no new particle hypothesis).
Right, so we’re now pretty sure there is a new particle (that is a boson). But is it the Higgs boson? Luckily, by March 2013, evidence surfaced that the new particle is what we call ‘spinless’ (meaning it looks the same from every direction) and that its interactions match the theoretical predictions of the famed Higgs boson. Further testing will take place from 2015 onwards, as the LHC ramps up towards its maximum energy of 14 TeV for the first time (double the energy it was running at in 2011 – more on particle energies in a future blog post).
(Next time: Exotic Higgs decays)
The Standard Model is remarkably successful at explaining all the variety of particles we see in nature in terms of 16 fundamental particles. But there was one mystery yet remaining: why do some of those particles have mass but not others?
It seemed odd that photons and gluons (the force carriers of the electromagnetic and strong forces) were completely massless, and yet the W and Z particles (the force carriers of the weak force) weighed more than an entire atom of iron! It was clear that there had to be some kind of new mechanism that could give mass to some particles and none to others.
The solution came in 1964, when six physicists in three different groups (including Peter Higgs and François Englert, the 2012 noble prize winners) postulated that a field filling the entire universe, the Higgs field, is what gives fundamental particles their mass. The smoking gun signature of the existence of such a field would be a seventeenth fundamental particle: the Higgs boson.
So how does it work? Imagine a field (the kind with grass and sheep) coated in snow. A skier passes by, gliding easily over the snow. Then a woman wearing snowshoes shuffles by, being slowed down by the snow. Next, a man in heavy boots struggles onwards, at each step being slowed by the snow. Finally, imagine a bird flying overhead, completely unaffected by the snow. In this analogy, the field of snow is the Higgs field and each character is a different fundamental particle. The bird is a massless particle like a photon, passing by without interacting with the field. The skier is a really light particle such as an electron, with very little mass at all. The woman in the snowshoes is a slightly heavier particle, such as a quark, and finally the man with the heavy boots is a truly massive particle like the W and Z particles, slowed by the Higgs field at every opportunity. So we see that the mass of a particle depends on how strongly it interacts with the Higgs field.
(Next time: Discovery of the Higgs)
We know the Standard Model can’t be everything, as it only explains 3 of the fundamental forces of nature (the missing one being gravity). When we look out into the universe, we see that galaxies are spinning at the wrong speeds, that things that should be unstable are stable and that galactic clusters are bending light more than they should. This is strong evidence for a new form of matter that doesn’t interact with light – so called dark matter.
And if that isn’t crazy enough, the universe seems to be getting bigger at a faster rate (imagine throwing a ball in the air and it shooting upwards with ever increasing speed, totally counter intuitive). This is caused by something we call dark energy, which is a mysterious substance now believed to make up the vast majority of the energy content of our universe. As to what it actually is, we still don’t know.
So after all that hard work, it seems that the Standard Model only explains about 5% of the universe, oh well, it was nice trying. BUT WAIT! If there’s one thing Physicists are good at, it’s creating seemingly crazy ideas that just might turn out to be right…
One potential explanation of dark matter is that there is a whole new collection of super-heavy particles that we haven’t seen yet – the so called theory of supersymmetry. If we find evidence of such particles in particle colliders, it could even help us pave the way towards understanding if string theory is correct or not!
A recurring theme throughout our story was how things that once seemed disconnected are actually part of a deeper underlying truth, so it’s natural to wonder if one day we could unify all the fundamental forces into a single explanation – a theory of everything. The Large Hadron Collider may finally be the window to open our eyes to such new and exciting possibilities.
One thing is clear though: we stand on the precipice of perhaps the greatest change in our understanding of the universe – and the Higgs boson might just be the key to unlocking it all.
(Next time: What is the Higgs boson?)
Having thought our knowledge of the makeup of the universe complete, physics turned its attention elsewhere. Or at least, until something was found in the 1930s that shocked everyone: a new type of particle streaming in from space – the muon (created when cosmic rays hit our atmosphere). In the years that followed, even more strange particles were discovered – over 80 by the 1960s! It was even said that the Nobel Prize should go to the person who didn’t discover a new particle that year! And to top it off, antimatter was discovered in 1931 (which can annihilate normal matter particles in a burst of energy) and mysterious ghost particles called neutrinos (that can travel through a light year of lead) were seen in 1956. It was like the periodic table all over again, some called it the ‘particle zoo’, but there were tantalising signs of a new pattern in amongst the madness…
This all changed in 1964, when Murray Gell-Mann and George Zweig proposed that most of the particles observed could be built out of just three fundamental building blocks called quarks. Think of it like having 3 different types of Lego bricks – the particle zoo is then all the combinations you can build with those bricks. The three quarks were called: up, down and strange. At first, it wasn’t clear if they were just a nice way to understand the patterns, or if they were real physical objects, but over the following 10 years their existence was confirmed.
In the mid-1970s, a new theory was formulated to explain the fundamental building blocks of nature and the forces that affect them – the Standard Model of Particle Physics. Particles made of quarks are called hadrons (the most famous is the proton, made of two up quarks and one down quark). In addition to the quarks, there are particles called leptons (the electron is a familiar example), which differ from quarks in that they aren’t affected by the strong force. The Standard Model also explained that the fundamental forces are caused by the exchange of tiny particles we call bosons. Electromagnetism is caused by photons; the strong force is caused by gluons (which ‘glue’ the nucleus together), and the weak force is caused by the W and Z particles.
Over the following decades, 3 more quarks were discovered, the last being the top quark in 1995. The quarks and leptons seem to fit neatly into 3 ‘generations’ (the columns above), but we still do not know why. Could this structure be suggestive of a deeper theory, just like with the periodic table?
With the discovery of the Higgs boson (which explains why the W and Z have mass whilst photons and gluons do not) in 2012, the Standard Model is finally complete. But the story of Particle Physics is far from complete; we have learned by now that it’s dangerous to think we know everything…
(Next time: Beyond the Standard Model)
How many forces can you think of? At school you might have heard about pushing, pulling, lift, drag, friction and perhaps the more exotic electric and magnetic forces. But what if I told you that all of these were the same? In fact, everything we see and experience in our day to day lives, except for gravity, can be attributed to a single force of nature – electromagnetism.
In the early 1800s, a number of ground-breaking experiments carried out by people such as Michael Faraday discovered that electricity could make magnetism, and – incredibly important for our modern world – magnetism could be used to make electricity. In 1861, a brilliant Physicist called James Clerk Maxwell unified the two forces together, and in doing so explained that light itself is nothing more than a dancing intertwined mixture of changing electric and magnetic fields. When we push on a wall, the electrons in our hands repel the electrons in the wall – creating the force we feel. All of Chemistry, Biology, even life itself, were understood to be due to a single force of nature. This was one of the greatest triumphs in Physics; the first theory of almost everything.
But all was not well with just electromagnetism and gravity alone, can you guess why? If we examine the nucleus of any atom, then we see a mixture of positively charged and neutral particles. But wait, if positive charges repel then why don’t they fly away from each other? This thought baffled physicists for decades, leading to the proposal of an entirely new force of nature: the strong force. When we say strong, we mean really strong: the force between just two protons is enough to hold the entire weight of a small child!
Excellent, reality is complete once again! Oh wait, there’s the minor problem that in the 1930s neutrons were seen turning into protons during radioactive decay… No worries, Enrico Fermi came along and proposed a new force of nature, the weak force, to explain the observation. Whilst it may sound feeble, it’s actually this force that allows the Sun to change hydrogen into helium (helium has two neutrons whilst the hydrogen nucleus is just made of a proton, so something must turn some protons into neutrons). Importantly, the weak force makes this process go slow enough for life on our planet Earth to be possible – if it was much stronger then the Sun would have long since used all its fuel.
Physics had reached a turning point; everything in the universe was understood to be made of 3 sub-atomic particles (protons, neutrons and electrons), interacting with each other by the 4 fundamental forces of nature. But things are not so simple, as the universe had other plans in store for us…
(Next time: Order from chaos – the Standard Model arrives)
(This is the first in a 4-part introductory series on Particle Physics, a new part will be released daily after November 26th)
Our view of the composition of the world has changed remarkably over the centuries. By the middle of the 1800s, 60 different substances called elements had been discovered, out of which the entire world was thought to be made. Each element was believed to be completely different from all the rest, though no one knew why at the time. However, in the 1860s a number of brilliant scientists began to spot patterns that allowed the elements to be grouped together into a single structure – the periodic table of the elements. For the first time, we had an ordered structure hinting that there could be a common explanation behind these building blocks.
In 1897, a huge breakthrough was made when JJ Thompson discovered a brand new fundamental particle – what we now call the electron. This was followed in 1911 with Ernest Rutherford’s ‘planetary model’ of the atom, with a dense positively charged nucleus (made of particles called protons) surrounded by orbiting negatively charged electrons. There was a missing ingredient though, as some atoms of the same element were observed to be heavier than others. The final piece of the puzzle came in 1932, when J. Chadwick discovered a new particle with no charge – the neutron. And with that discovery, we finally had an explanation for all the elements in the periodic table: each element has a different number of protons, and the reason they behave differently is that they have different numbers of electrons. Elements were no longer the basic building blocks; the number of fundamental particles had been reduced to just 3!
But even before it was complete, the simple planetary model was already being replaced by a better theory of nature: the Quantum Theory. This solved many of the great problems facing Physics in the 1920s, but it faced much opposition due to its counter-intuitive nature. Quantum Mechanics allows particles to be in many places at once, objects to pass through solid walls and things to pop in and out of existence from empty space! It tells us that almost all of the matter making up our world is actually empty space – a deep insight into how reality can surprise us. But the surprises didn’t stop there, oh no, this was just the beginning…
(Next time: what holds the world together?)