Tag Archive | Higgs boson

The Higgs boson Part 3: Exotic Higgs decays

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‘.

An exotic Higgs decay producing two off-centre vertices

An exotic Higgs decay producing two off-centre vertices

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?)

Simulation Overload

5460d0cab0c78227780074a9_1

We had a great launch last week and Higgs Hunters was off to a flying start. After a day or two though, we realised that something wasn’t right. Over the weekend, you all began seeing way too many simulations. That should start to fix itself now. Here’s what happened, and how we want to move forward:

On many Zooniverse projects, there are simulated data mixed in with the real data. We do this to help calibrate the project as a whole, and, in the case of Higgs Hunters, to see what sort of events can easily be seen in this data, and what can’t. It became apparent over the weekend that the balance of real and simulated data on Higgs Hunters was wrong. Several weeks ago when the ATLAS team delivered the Higgs Hunters data to the Zooniverse, a piece of metadata was missing and the developers at Zooniverse thought that a whole batch of simulated data was actual data. The entire dataset was uploaded and the project launched as planned.

We realised a day or two after launch that we weren’t telling volunteers when they had seen a simulation. This is important to know — especially if you think you found something really cool — and so we moved quickly to get the sims flagged as such after each classification. However, as many of you noticed, this fixed made it apparent that, in fact, 8/10 images were sims! Fixing the one mistake brought the other to the surface — and after a lot of searching we figured out what had happened.

Today we have done the following:

  1. We have paused 90% of the simulated data — so you won’t be asked to classify it now.
  2. We are aggressively retiring the simulations — so they’ll be removed 4x faster than the real data.
  3. ATLAS are preparing more data for Higgs Hunters — 10 times as much!
  4. Hopefully we can upload the new data before the end of 2014, and we’ll email participants to let them know.

After all of this, we now have ~8k subjects in Higgs Hunters and ~30% are sims — that means we’ll be done in ~100k classifications. So let’s call this Round One and see where it takes us. In the meantime, we thank everyone for their efforts on Higgs Hunters, and we apologise for all the confusion and frustration that this has caused some users.

We’re really sorry, and hope that this helps to explain some of the problems that have occurred and the ways in which we’re moving forward.

The Higgs boson Part 2: Discovery of the Higgs

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.

Experimental detection of the Higgs boson

Experimental detection of the Higgs boson

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).

Illustration of first 3 sigma levels

Illustration of first 3 sigma levels

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 Higgs boson Part 1: What is the Higgs?

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.

Professor Peter Higgs Image credit: Claudia Marcelloni/ATLAS

Professor Peter Higgs
Image credit: Claudia Marcelloni/ATLAS

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)

Introduction to Particle Physics Part 4: Beyond the Standard Model

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.

Galaxy rotation curves - evidence for dark matter

Galaxy rotation curves – evidence for dark matter

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 Calabi-Yau manifold – welcome to the strange world of string theory

A Calabi-Yau manifold – welcome to the strange world of string theory

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?)

Introduction to Particle Physics Part 3: Order from chaos – the Standard Model arrives

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…

Bewilderment abounded at all the new particles

Bewilderment abounded at all the new particles

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.

The Standard Model of Particle Physics

The Standard Model of Particle Physics

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)