First images of collisions at 13 TeV | Hacker News Reader

First images of collisions at 13 TeV

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160 pointsjonbaer47 comments

I followed the links to the CMS, Atlas and other detectors that the images appear to be from but couldn't find any information on what these images actually mean. By reading the layman's descriptions (which I most certainly am) I assume that the images show where along the "surface", if there is such a thing, of the detector various particles were detected and the length of the bars show either how much energy the particle(s) have or the density of particles at that point.

Is there anywhere I can go to get a more detailed description of these without delving into academic papers that I have no hope of understanding?

http://www.extremetech.com/extreme/206123-first-images-of-re...

You've got the right general idea. It's a tricky thing to simultaneously display three-dimensional tracks and the amount of energy deposited in each detector system.

I'm surprised that Atlas and CMS don't have a better viewer's guide immediately at hand, but perhaps this article will help:

http://www.symmetrymagazine.org/breaking/2010/03/18/atlas-ev...

I've worked at two accelerator facilities (RHIC and CEBAF) writing software for doing particle tracking and pseudo-data generation, and I always wished that they could divert more time into preparing articles like the one you linked. They're super helpful to laypeople, especially family and friends, but they're also great for getting students at universities interested in maybe coming to pitch in.

Sadly, the lab personnel (especially those coordinating outreach) are generally crazy overworked and don't have a lot of time, and the interns are just too inexperienced and lack the requisite domain knowledge to do it themselves.

If you ever feel like writing a paper on this I can highly suggest Ludwik Fleck as a starting point (his writing on thought collectives). At least that's where I started when I needed a concept of marketplace for ideas and why laypeople matter.

Thanks! this is exactly what I was looking for. I may be missing something but I found it odd that the CERN website didn't have any more detailed breakdowns of the experiments and setups like the article you linked.

I understood more after using some citizen science project where you classified particles by their trajectories using images like these. Basically, if you didn't figure out that part, the images are cross sections of the vacuum pipe and show the trajectories of the particles that come to exist in the collision of the accelerated particles

A detector is typically a series of concentric cylinders, with the beam pipe, where the collisions occur, running through the center. The inner layers are tracking chambers, which detect the paths of charged particles. This is what produces all the curved lines radiating from the center.

The outer layers are calorimeters, which catch particles and measure their kinetic energy. As you correctly assumed, these produce the bar plots. Often there will be one layer of calorimeters for photons and electrons, and a second for hadrons (protons, mesons, etc.)

This Wikipedia article is a good starting point: https://en.wikipedia.org/wiki/Hermetic_detector

Very helpful, thank you. Do you know if they have the ability to plot the various events of the collision by time, such that the bar plots could be shown appearing one after another as collision events are recorded?

I'm not familiar with the nitty-gritty of the LHC experiments, but in general the various detector subsystems have different response times and rate capabilities. There are certain types of detectors with explicit time granularity (e.g. time of flight) but for most detectors there would be no time structure within a single collision record ("event").

But you could work backwards to make an animation for each event. From a scientific perspective it's not that interesting though.

I'd expected it to be trivial to hunt down some animated gifs, but to no avail.

Here's a movie derived from an actual event that will give you a feel for things: http://www.atlas.ch/multimedia/proton-event.html

CMS, too: https://www.youtube.com/watch?v=poNftWVbwmM

Wow, thank you. That CMS video is awesome! This really gives me more of a foothold on what is going on.

The amount of data that comes out of the different detectors on the LHC is staggering. It takes a fair amount of post processing to get meaningful results from collisions, and there can be millions of collisions in a run. There's some more information about how they process these results on the LHC computing grid page: http://wlcg-public.web.cern.ch/.

When I did my traineeship at ATLAS DAQ around 10 years ago, one issue was how to per-process the data that could be considered invalid fast enough, to be able to throw it away before it arrived into the cluster.

Talking about ATLAS experiment: protons in LHC collide 40 million times a second, but "only" around 100 events per second are stored for later analysis. LHC grid concerns the 100; three levels of trigger (one hardware and two software levels) are used to select the useful 100 out of 40 millions, in real time.

That's insane.

(Copying from elsewhere:) I highly recommend the "Particle Adventure" for lots of info about basic particles physics that's accessible to the layman

http://www.particleadventure.org/

In particular, starting at this page should give you an idea of how many components there are and what sorts of information come out of them:

http://www.particleadventure.org/component_detector.html

Although the cartoons seem a bit silly, this is what my professor gave me as an undergrad to get oriented, and it was really helpful.

Here's the LHC dashboard for your projector: https://op-webtools.web.cern.ch/op-webtools/vistar/vistars.p...

Are there others? This is the one I use: http://meltronx.com/index.html

Wow, I haven't seen that. As you can see, my link goes straight to cern; you appear to have found someone else's meta-dashboard.

The cern one has the dropdown in the top left that takes you to a lot of other ones too.

I know HN is a 'serious' place, but I never not laugh when I see BEAM SETUP: INJECT AND DUMP. It just seems so...euphemistic

Can someone explain what that means in the LHC context?

The LHC is a big ring around which particles travel at high speed and, from time to time, collide with one another.

Given that the particles were not travelling around the ring 100 years ago, and will probably no longer be travelling around the ring 100 years from now, there needs to be a way of getting them in and out.

In (INJECT): beams of particles (travelling rapidly but in a straight line) are produced by other accelerators and then diverted into the LHC ring.

Out (DUMP): the particles are diverted out of the LHC ring, the exiting beam is spread out using electromagnets, and it is sent into a big block of graphite encased in concrete.

Thank you!

Do you happen to know what kind of time scales are involved in these operations? How long does it take to spin the beam up and down?

I don't know, I'm afraid. According to the Wikipedia page on the LHC, though, injection takes "several minutes", acceleration after that takes "20 minutes", and then the beams circulate for 5-24 hours while the detectors watch for products of collisions. (Beams plural because the LHC has two beams circulating in opposite directions.)

I would expect dumping to be very quick.

What does the raw data that generates these renders look like, or do their dashboards literally spit out images that look like that? I'm guessing their instrumentation can roughly figure out what energy/size/charge of particle is where, but the how just seems like magic to me.

I highly recommend the "Particle Adventure" for lots of info about basic particles physics that's accessible to the layman

http://www.particleadventure.org/

In particular, starting at this page should give you an idea of how many components there are and what sorts of information come out of them:

http://www.particleadventure.org/component_detector.html

+1 for Particle Adventure.

I wish more things were taught that way!

this one bothered me around the time of the higgs announcement, i did some research and found a number of exciting and upsetting results:

the exciting :

    * SCOPA3 : (i)(ii)
        cerns commitment to open access publishing 
    * public data samples (iii)
    * open data portal (iv)

the upsetting :

"LHC data are exotic, they are complicated and they are big. At peak performance, about one billion proton collisions take place every second inside the CMS detector at the LHC. CMS has collected around 64 petabytes (or over 64,000 terabytes) of analysable data from these collisions so far."

    * exotic : requires special analisys tools 
         that require vms to install smoothly (v)
    * big : by nature of personal computing 
         it would be impossible for you to get 
         all of the data even on a single event

why is this upsetting? it seems possible,probable, that the data released will only be data that correlates with 'discovery', meaning there is little that can be done to aide the effort from home, and seemingly even less of a chance of finding counter theories or errors

(i) http://press.web.cern.ch/press-releases/2012/10/scoap3-open-...

(ii) http://cms.web.cern.ch/news/lhc-data-be-made-public-open-acc...

(iii) http://cms.web.cern.ch/content/cms-public-data-samples

(iv) http://opendata.cern.ch/

(v) http://opendata.cern.ch/getting-started

Once again, Scientific Linux delivers :P

I remember the days when that distribution was put out together. :)

A description of the parts of the Large Hadron Collider, in 90's rap form, is here:

https://www.youtube.com/watch?v=j50ZssEojtM

This was worth the watch. Thanks for the link, haha!

Thanks, that was a nice one!

If one had to explain to somebody who has absolutely no clue whatsoever what the LHC does and why, one could do worse than point them to this video. :)

"Mr. Singularity, come here. I want to see you."

How long will it take the black holes to swallow the earth?

Well, there's been some alarmism about AI recently, and in that same vein I'll admit it - even with a physics degree I'm uneasy about the LHC. By definition we don't know what will happen. We are generating kinetic energies that haven't been seen since literally the dawn of time. I would just be a lot more comfortable if these experiments were being done in space, far away from life. It's just that even 99.999% certainty that nothing catastrophic will happen really isn't high enough considering the stakes.

EDIT: add 'kinetic'

Re: "We are generating kinetic energies that haven't been seen since literally the dawn of time."

Is 13 TeV really considered a lot of energy in terms of what happens daily with particles hitting our atmosphere? http://en.wikipedia.org/wiki/Cosmic_ray seems to suggest that we've observed naturally occurring 3 × 10^20 eV, which is a lot ( larger than 13 TeV (10^12).

Absolutely. Nothing in the natural universe moves that fast anymore.

Sure they do.

http://en.wikipedia.org/wiki/Ultra-high-energy_cosmic_ray

http://en.wikipedia.org/wiki/Oh-My-God_particle was 300,000,000 TeV. Not all of that's available for interaction, so:

> For the Oh-My-God particle, this gives 7.5×10^14 eV, roughly 50 times the collision energy of the Large Hadron Collider.

> The speed of the particle (0.999 999 999 999 999 999 999 9951c), if it was a proton, is so high that it would experience relativistic time dilation by a factor of about 320 billion. At that rate, the particle could have traveled for the entire duration of the universe's existence while experiencing less than sixteen days subjective time.

it wasn't clear at a glance for me, haven't seen that unit before.. but now that you mention:

    TeV == Tera electron Volt

so thanks!

*electron volt

thanks!

You have to look at center of mass energy, the energy actually available for reactions in a collision. The collisions of ultra-high-energy-cosmic-rays (UHECRs) with air occur at hundreds of TeV center of mass energy. And they may be nucleons, in which case this energy is spread over several nuclei. So assuming that Lorentz symmetry holds (probably the safest assumption in modern physics), 13 TeV is in a similar ball park as UHECR collisions.

> And they may be nucleons, in which case this energy is spread over several nuclei.

???

I remember the black hole jokes when I used to work there (> 10 years ago).

Most likely they are still being told.

On the one hand, yes. There should always be a healthy concern for safety.

But keep in mind these sorts of things happen relatively frequently - https://en.wikipedia.org/wiki/Oh-My-God_particle Furthermore, they have a really good idea of what's going to happen. With bowling, I can't tell you if i'm going to roll a strike or a split, but i'm sure i won't hit you with a bowling ball.

To put it another way, if there was some scary possible outcome, there would be evidence of that from the many particles that hit us regularly. Mini black holes sound scary, but they're really tiny and just evaporate because they're, well, mini. Nuclear reactions are well understood. If there was some other effect, we'd see it in the sun. Just think about how hard it is to get fusion power working. What are the chances of that happening accidentally?

So yes. You and I are right to be concerned, we're facing the unknown. But if you look into it, you'll see the folks doing the research, well, it's not unknown to them. They're looking for small details in well understood systems. Maybe something bad will happen, but they're not shooting from the hip.

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