Further to Andy’s identification of a hadronic interaction, where an apparent OCV is produced by interactions between particles and parts of the detector, I’ve identified some candidates for hadronic interactions. This is a small subset of those candidates. What do you think? Do these tracks that originate from a piece of detector material?
How many of these have you seen during your Higgs Hunting escapades?
We’ve selected a small subset of the data images that a lot of people flagged as unusual and we’re hoping to analyse the data from these events in more detail. An interesting phenomenon, which may well be caused by an error in the system that draws particle tracks or in the detector, are the muon tracks that bend sharply. A few of these also look like cosmic rays. Feast your eyes!
This event from the Higgs Hunters project is a prime, lovely example of what we’re hunting for in that project: a well-defined four-track off-center vertex (OCV). Researcher andy.haas provided some additional insight into exactly what happened here:
Beautiful! This is clearly a “hadronic interaction.” A standard particle (like a pion) has come from the collision and hit a bit of material (probably a silicon tracking detector sensor) in the detector, producing a secondary assortment of standard charged particles, which we see as displaced tracks coming from the displaced vertex. You can tell because the vertex is found exactly at a layer of tracking detector—the third layer, in both the normal and slice views.
Join the hunt at Higgs Hunters today!
We’ve been analysing your clicks on the images to see just how good human beings are at identifying off-centre vertices, and the news so far is pretty encouraging – I’ve produced a few images that highlight the clusters of clicks on data images where you saw three or more vertices: here are a few of the more striking images. This is just a very small sample, as the algorithm identified over a thousand such clusters of clicks:
AHH0000k2d has an interesting track that appears to go backwards!
As discussed in the FAQ, the colourful tracks usually indicate OCV tracks found by a computer algorithm. In this case it’s pretty clear that humans found some tracks that the computer missed!
Again, a mixture of tracks that were found by the computer and those which required humans to find!
This is one of the many images you’ve identified with missing muons – we’ve seen a few of these which were diagnosed as cosmic rays, but this one seems to have an OCV as well.
Another notable observation from this data is that lots of people have been identifying the colourful ellipses that are sometimes produced in the detector as “weird”. They do look very unusual, but again, these are artefacts produced by the algorithm that draws the tracks, and aren’t (unfortunately) physically interesting. A reminder that things like large deposits of energy, tracks that bend suddenly in the detector, or beams of particles at wide angles are potentially candidates for “weird” observations – although, if in doubt, it’s best to flag it as weird, as we’re looking into analysing the images that lots of you have flagged up as being unusual (regardless of where you click.)
Good work, everyone – keep it up!
Since the last time this blog was updated, the LHC has been restarted and, in the last month or two, the floodgates have been reopened and data from the detector has again started to flow. In the finest traditions of Hollywood, the sequel will take all the elements of the original and turn it up to eleven – or, in this case, 13 TeV, the new collision energy for protons in the LHC. These record-breaking energies mean that the protons are travelling at a brisk 0.99999999c. At such speeds, the relativistic effect of time dilation means that – if you survived – every second you experienced would be nearly two hours for your friend remaining on earth. These effects, while fascinating, have been well-understood for many years: even the Higgs boson had been predicted in 1964. The exciting prospect with the second run of the LHC is that we are entering the realm of new and sometimes controversial physics. While the first run had a clear target, the Higgs, at a known energy, the second run aims to ramp up the collision energy and simply see what we find.
The potential candidates for discovery by this second run-through are numerous. If there is a new ‘holy grail’ of particle physics after the discovery of the Higgs, then arguably it’s dark matter – a name to make film studio executives drool, certainly. On the face of it, you might wonder how particles which are apparently so arcane and difficult-to-produce can really be relevant to our world at large. After all, none of the potential candidates for dark matter have ever been isolated or observed in the laboratory. But it is a testament to the beauty and complexity of the universe that the smallest details of the fundamental particles that make it up can have a huge impact on the grandest possible scale. Take another boson – not the Higgs, but the W boson, which mediates the weak interaction, detailed in this post.
If the mass of this particle were to be slightly greater, the rate of nuclear fusion in the sun would be quicker, it would have burned for a briefer time, and humanity may never have had time to evolve. If its mass were below a certain threshold, the Sun may never have produced enough energy to allow life to evolve. For the first particle physicists, this fortunate lifespan was observed by measuring the length of a line of bubbles in a cloud chamber. The detectors in the LHC are the spiritual heirs of these slightly less sophisticated methods, but the inferences we can make still have profound consequences.
Dark matter provides another example of how the nature of subatomic particles can influence structure on the grandest scale. The predicted gravitational interactions between stars, galaxies and other large-scale structures were inconsistent with the amount of mass that was actually observed. As well as this, the universe seems to be flat – having no overall curvature. At the same time, cosmologists can measure how its expansion is accelerating using the light from distant supernovae. For these results to be consistent, matter in the universe should have a certain average density of around 6 hydrogen atoms per cubic metre: but visible matter is only 4% of this. Most cosmologists are certain that dark matter explains these phenomena, so particle physicists are looking for a new subatomic particle that – unlike protons and electrons – doesn’t interact electromagnetically, and so doesn’t absorb or emit light. Like the Higgs boson, predictions of dark matter have been around for decades, and models that feature it have become very sophisticated, but until it is actually observed there will still be some controversy, with leading alternative models including modifications to our current theory about gravity.
One of the historical candidates for dark matter are WIMPs, or weakly interacting massive particles. The fact that we expect dark matter particles to be “invisible” and also stable makes them difficult to detect. A lot of the experiments attempting to detect WIMPs use the same method as the Kamiokande experiment in Japan, which principally looks for neutrinos. A large vat of water, usually deep underground to reduce the effects of background radiation, is filled with detectors which look for the effects of collisions between the WIMPs and the nuclei of atoms in the water. These experiments have not yet produced convincing evidence of WIMPs, and it’s also worth noting that their mass was predicted to be around 100GeV, so not beyond the range of the LHC even in its original run.
With WIMPs still not yet observed, the search has moved towards more exotic particles. The theory of supersymmetry, which was also discussed in a previous blog post, is a key theoretical development that physicists hope can be confirmed by this run.
Elementary particles are divided into fermions, with spin ½, 3/2 etc. and bosons, with spin 0, 1, 2…. Supersymmetry suggests that for every particle in the standard model there is a partner with a significantly higher mass, and in the opposite category – so for each of the quarks, there’s a “squark”, and for gluons which are the force-carriers for the strong interaction, there are “gluinos”, and so on for every elementary particle. Each boson has a superpartner that is a fermion, and vice versa. One advantage of the supersymmetry extension to the standard model is that it predicts a set of particles over a range of masses, and the lowest of these is expected to be stable; it could be a weakly interacting particle that still exists from the early universe. Again, the lightest supersymmetric particle could be produced in the LHC, but would be difficult to detect. To date, there has not been convincing evidence of superparticle partners, although the wide range of possible masses that these models allow means that we also can’t eliminate the theory based on data from the LHC. Particle physicists instead focus on narrowing down the possible ranges for the parameters which define this model.
There are other targets for the latest run of the LHC. Just recently the news has reported the discovery of the pentaquark – a new type of baryon. Previously, all detected subatomic particles that were made up of quarks either had three quarks (baryons) or a quark and an antiquark (mesons) bound together by the strong force. As the name suggests, the pentaquark consists of five – more specifically, four quarks and one antiquark, giving it a spin of 1 and a range of possible charges. The specific configuration of quarks that has been observed is two up quarks, a down quark, a charm quark and an anti-charm quark. The pentaquark was certainly an unexpected discovery by the LHC, and has not yet been confirmed entirely; scientists are likely to be cautious, because the claims that a pentaquark was discovered ten years ago were eventually refuted. Pentaquarks are not candidates for dark matter, because they will interact electromagnetically, but the fact that new and unexpected discoveries have been made so soon after the start of the new run is indicative of just how much there is left for us to find in the ever-increasing particle zoo!
In the next entry, I’ll give you a quick update on the analysis of your clicks so far. Happy hunting!
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.
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!
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.
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!