Archive | December 2014

Tell me the truth!

As you know, on HiggsHunters we have been giving you not just real data images from the ATLAS detector. We have also been showing simulated images of what ATLAS detector events would look like if the Standard Model were extended with various new particles and interactions that would give displaced vertices. So what are these simulations of exactly, and why are they important for the science at HiggsHunters?

The simulations are of Z plus Higgs events, where the Z decays to a pair of muons (see last post!) and the Higgs boson decays to a pair of new, long-lived, neutral particles (called “a” bosons). Such a model is predicted by various theories which are possible extensions of the SM. These “a” bosons can then themselves decay, after some lifetime, back to SM particles. The parameters of this model are:

  • the mass of the “a” boson
  • the average lifetime of the “a” boson
  • what SM particles the “a” boson decays to

Since this is unknown new physics, these parameters are not known! But we can make some educated and motivated guesses based on theories of SM extensions and exclude values that would have allowed the “a” boson to be seen at previous experiments. For our simulations on HiggsHunters, we chose the following values:

  • mass of either 8, 20, or 50 GeV. Recall the Higgs boson mass is 125 GeV, so we need the “a” boson mass to be less than half the Higgs boson mass if the Higgs boson is to decay to a pair of “a” bosons.
  • average lifetime (times the speed of light) of 1, 10, or 100 mm. We’d like to have a chance of seeing it decay displaced, and within the detector. Note that this does not mean the displaced vertex will always be 1, 10, or 100 mm away from the collision point, anymore than dogs who live an average of 11 years will all die on their 11th birthday! The probability that a particle with average lifetime, L, decays after a length of time, t, is proportional to e^(-t/L). Dogs have a more complicated distribution!)
  • “a” boson decays to either a pair of bottom quarks (for an “a” mass of 20 or 50 GeV) or tau leptons (for an “a” mass of 8 GeV). This is based on the expectation that Higgs-like bosons usually decay to pairs of heavy SM particles more often, since they couple to mass.

This gives us 3×2+3×1=9 possible parameter combinations to simulate. We present a roughly equal number of all 9 on HiggsHunters. Below is an image of an event simulated with the parameters: “a” mass = 20 GeV, “a” decay to a pair of bottom quarks, and “a” lifetime = 100 mm. Since this is a simulation, we know where the decays took place – they are drawn on the image as pink dots.

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Here are a bunch more of these (XY, XYzoom, RZzoom), for your viewing pleasure! (These have “a” mass = 20 GeV, “a” decays to bottom quarks, and “a” lifetime = 10 mm.)

But when we give them to you on the site, you don’t get to see the pink dots! Wouldn’t that make life easier for you?! Well, yes, but the point is that by testing you all with these simulations we can measure your ability (efficiency) to have found such displaced vertices if they were to exist in the real data. This is very important for us to draw scientific conclusions from the results of the HiggsHunters project! (We also can’t show the pink dots to you right after you classify it, since each image gets classified by ~20 people. We can’t reveal the truth until after the image is done being classified by everyone! We could make the truth images available on talk at some point in the far future.)

If you all manage to find evidence for some new particle giving displaced decays, we need to know how often this process is occurring in the LHC pp collisions. (Well, we would also need to celebrate with some Champagne first!) How often it happens is given by how often you see it happening divided by your efficiency to see it happen. It is this efficiency which we can measure using the simulations, for each set of model parameters.

If instead you manage to go through all the data and find no new particles, we have still learned something very interesting! (No Champagne needed though, unfortunately.) We will have learned that these new particles are not being created at a rate that is large enough so that you would have seen them. This rate (for each particular set of model parameters) is very interesting to theorists who work on possible extensions to the SM and is given by about 3 divided by your efficiency to see it happen. So here again, these efficiencies measured with the simulations are essential. (In case you’re curious, the “3” in the formula above comes from the fact that to have at least a 95% chance of having at least 1 event occur, you need to expect about 3 events to occur on average. You can always just get a little unlucky and the events don’t occur even if on average you would have expected them to! For more info, read about the Poisson distribution. I also love this Poisson calculator – try typing in x=1 and avg. rate of success=3, you should see P(X>=1)=0.95!)

That’s it! Be proud that your abilities have been calibrated, and you are now ready to tackle the larger sets of real data we plan to upload to the site soon!

What you’re seeing on HiggsHunters

Since you’ve all been diligently classifying for over a week (and doing a fantastic job!), we from the science team thought we should explain to you all a bit more about the physics you’re looking at and what these events are! How do we go from the 20 million proton bunch collisions each second happening inside the ATLAS detector in 2012 to the images on your screens?

First, remember we’re trying to find exotic decays of the Higgs boson. If you just chose random collisions, you’d have just about a 1 out of 10 billion chance of picking one with a Higgs decay in it! So we need to select events (proton bunch collisions) which are enriched in Higgs bosons. Most of the time, the Higgs is produced by itself, and then decays. Unless you know what it is decaying to and can select on that, you have no way of enriching the sample. (This works for many SM Higgs searches/measurements, e.g. where its decays to photons or Z bosons can be selected.) We chose a method that is blind to what the Higgs decays to, thus letting us potentially see any new exotic decay. About 2% the time, the Higgs boson is produced along with a Z boson, in the same event (according to the SM calculations). About 3.3% of the time, this Z boson decays to a pair of muons. We select this 0.07% of Higgs bosons using the di-muon trigger and record them. There should be about ~150 events like this in our 2012 data (with 2 muons with transverse momentum above 20 GeV/c and |eta|<2.5 and dimuon invariant mass within 20 GeV of the Z boson mass).

Unfortunately, these 150 events are accompanied by ~4 million events that just have a Z decay to two muons and no Higgs boson! To improve our chances of having a Higgs, we use another trick – require the Z to be moving away from the beamline. If there’s a Higgs and a Z, they tend to move away from each other in opposite directions. A Z by itself tends to go almost nowhere. By asking that the Z has a transverse momentum above 60 GeV/c, just 5% of lonely Z events survive, but we retain 60% of the 150 Z+Higgs events, so 90 left out of 200k Z background. (Not bad, we’ve gone from about 1 / 10 billion to about 1 / 2000!) Finally, we enhance the chance of something interesting going on by asking for missing transverse momentum  above 40 GeV/c. This means that some particles have escaped without interacting (like neutrinos, or dark matter!), or some of the particles were mismeasured. This leaves us finally with 50 Z+Higgs events if the Higgs decays to long-lived particles which themselves decay to pairs of bottom quarks or tau leptons (more on our simulations and models later!) and “just” ~50,000 Z events for you all to search through to find them (along with those simulations).

Now that the events are selected, we have to reconstruct those displaced tracks! To do this, we save all events with a Z decaying to muons in a format that has all tracker hits saved (those thousands of little dots in the tracker pictures). Standard ATLAS tracking only reconstructs tracks that start from within 10 mm of the collision point, but for this study we reconstruct track paths that start up to 10 cm from the collision point. We also run a custom vertexing algorithm which attempts to find displaced vertices (hence the colored tracks and little ovals). But we hope you will do better! Reconstructing the data with these settings is slow… it takes about 10 minutes on average per event! For 50,000 events, that’s ~1 year of CPU time. Of course, we are clever and split the work up over hundreds of machines as part of the “world wide grid”, so is just takes a couple days.

Finally, we download all the reconstructed data (about ~250 GB, from 5 MB per event) and make these wonderful pictures! This process happens on a machine in my office (see below). Running several instances in parallel, I can make about 1 image each second, which lets me finish all 50,000 overnight.

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Not everything is being shown in the pictures. We cut down the information so you can ignore the boring stuff and more easily see displaced vertices. We only draw tracks with transverse momentum above 2 GeV/c, which start at least 0.5 mm from the collision point. (Most particles come directly from the collision point, and are low momentum.) We also don’t draw tracks that start more than 20 cm from the center of the detector or the selected collision point in the direction along the beamline. To cut down on fake tracks, they must have at least 7 hits in the silicon strip tracker. Vertices shown must have at least 3 tracks. Muon tracks must have transverse momentum above 10 GeV/c, and jets of hadrons must have transverse momentum above 40 GeV/c. Other objects (photons, electrons, bottom quark jets, etc.) are drawn as long as they have transverse momentum above 5 GeV/c.

Once done, they’re zipped up and sent to the Higgs Hunters site!

That’s it! Happy hunting!

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 (-271.3C, 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 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

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!

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

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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)