ATLAS: How do particle detectors work?
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 at the LHC – an incredible feat of engineering!
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.
Diagram of ATLAS
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!
Simulation Overload
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:
- We have paused 90% of the simulated data — so you won’t be asked to classify it now.
- We are aggressively retiring the simulations — so they’ll be removed 4x faster than the real data.
- ATLAS are preparing more data for Higgs Hunters — 10 times as much!
- 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
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
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)
Higgs Hunters 
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