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LHC about to restart - some frequently asked questions

  1. Apr 4, 2015 #1

    mfb

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    The LHC is about to restart after a break of two years - beams could go around as early as today (you can watch it here but that is very technical). There are some misconceptions about it and questions I got asked multiple times, so I thought I write some answers to clarify how "working for the LHC" works. This is the n+1th place where you can find those answers, but with a different focus and at a different place. Feel free to ask more questions!

    With "working for the LHC", I actually mean "working for one of the experiments at LHC" - the LHC is just the accelerator, it has four big experiments (ATLAS, CMS, ALICE, LHCb) and some smaller ones.


    "You work for a CERN experiment, but not at CERN?"
    Right. Most of the work is data analysis in some way - the detectors produce huge amounts of data (roughly 1 TB per second) that have to get filtered (there is no way to store all of it) and analyzed. Data is stored in a worldwide computing grid and can be accessed from everywhere in the world. There is no need to be at CERN all the time. Detector development for future upgrades is done all over the world as well.

    There are a few scientists that actually run the detectors from the control rooms at CERN in shifts, and a few more experts are available to come to the control rooms if necessary - but these are very small groups compared to the size of the collaborations (about one thousand for ALICE and LHCb, several thousands for ATLAS and CMS).
    There are also scientists working on the actual detector hardware, exchanging parts and so on while the LHC is shut down, but again that is a relatively small group.

    Many important meetings and workshops are at CERN, so visits there (typically for 1-2 weeks) are common. In general, meetings are very important to stay up to date - what is done where, who does what (to avoid doing the same thing multiple times, or not at all) and so on. You cannot visit CERN for every meeting (typically several per week that can be related to your work), so most of the time the meetings are done via the internet.


    "When do they take your data?"
    Many experimental sites work like that - you get some hours to weeks of time to run your experiment, and then you leave again and analyze what you got. The LHC does not work that way: it collides protons with protons (sometimes lead ions) and you cannot choose what happens in the collisions. The experiments have to record everything that could be interesting, every time the LHC is colliding protons. Ideally the LHC would run 24/7 but 30%-40% of the time is more realistic due to various technical issues. Analyses often look for rare processes or need a very high precision (you want to be better than the previous analysis), so they usually need the collision data from months to years to get enough statistics. Both ATLAS and CMS had about 2*1015 collisions so far, LHCb and ALICE have lower numbers.


    "How do you collide protons?"
    This is not as trivial as it sounds like. Protons are tiny - 10-15 m in diameter. There is no way to focus them well enough to let two specific protons collide. The LHC collects about 100 billion protons in groups called "bunches", thinner than a human hair and with a length of a few centimeters. In 2012 there were about 1400 of these bunches per direction circulating, the plan for 2015 is to have 2800. These bunches are "collided" - most protons just go through without any influence, but a few of them (~25-30 in ATLAS and CMS, ~3 in LHCb, 0 to 1 in ALICE) collide. The others go around the ring to have another chance of collision later. The bunches were 15 m apart (soon 7.5 m), giving collision processes 20 million times per second (soon 40 million).


    "Do you (personally) study the Higgs boson?"
    No. The LHC experiments study many different particles, the Higgs is certainly an important one but not the only one.


    Event displays
    When you read news articles about the LHC, they often include colorful pictures like this one. While it can be possible to make a guess what happened based on the picture, no one actually looks at them to study the collisions. There is no way to look at 1015 of these pictures, you need some way to let a computer analyze them. In addition, you are interested in numbers these pictures do not show.


    None of the particles we study hit the detectors
    This is often surprising. All the particles that fly through the detector are well-studied and the detectors are not able to improve our knowledge about them. So how does it work?
    The particles we are interested in are very short-living. They decay before they would reach even the innermost parts of the detector. Their decay products might decay again, but eventually you get particles that live long enough to fly through the detector, which records their particle type, flight direction and energy. Based on these decay products, it is possible to reconstruct what happened in the collision.

    This is not completely true, a few particles are living long enough to have a chance to reach the detectors, but that is very uncommon. And even there you are mainly interested in finding their decay products instead of the actual particles.


    Is it possible to visit the detectors?
    Yes! Unfortunately, the best time to do so was in the last two years during the long shutdown. The LHC does shorter breaks every winter and longer ones are planned for the future as well, there might be more chances.

    More LHC articles:
    Part 2: https://www.physicsforums.com/insights/lhc-part-2-commissioning-2/
    Part 3: https://www.physicsforums.com/insights/lhc-part-3-protons-large-barn/
    Part 4: https://www.physicsforums.com/insights/lhc-part-4-searching-new-particles-decays/
     
  2. jcsd
  3. Apr 4, 2015 #2
    Ohh wow, had dreamed to see LHC since many years and coming to see It on this Wednesday with a group of 40 something students as a school trip, now we cant go down there? What are we gonna do, just chill in Geneva while eating Swiss chocolate?
     
  4. Apr 4, 2015 #3
    They have some good skiing spots around there.
     
  5. Apr 4, 2015 #4

    mfb

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    Things you can visit:
    - if organized (easy): a control room (the one for ATLAS is close the the main CERN site, the main accelerator control center is in Prévessin)
    - if organized in advance: some detector hardware behind the shielding walls.
    - if organized in advance: CERN has some accelerator parts that are not in operation right now. It might be possible to visit something.
    - building 40 looks interesting and has some posters and stuff
    - the globe (the wooden sphere, a small exhibition center)
    - the area around restaurant 1 ("R1"). People are sitting around everywhere, working, talking about their work, relaxing, playing games, ... all mixed together. A very nice place I think.
     
  6. Apr 4, 2015 #5

    Vanadium 50

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    CERN is a scientific research facility that sometimes allows tours. It is not a theme park that sometimes does scientific research. The first priority of the laboratory is not to give tours. Some - not all, but some - school tours forget this, and there have been some incidents of bad behavior. Enough of these, and tour groups will be seeing less and less of CERN.

    In particular, if you visit the ATLAS control room, do not tap - or worse, bang - the glass. People are trying to work, and it's disrespectful to treat them like monkeys in a zoo.
     
  7. Apr 4, 2015 #6

    mfb

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    Personal experience? It looks very specific.


    You are allowed to hit the emergency off button outside the ATLAS control room (the one with a sign telling you you are allowed to).
     
  8. Apr 4, 2015 #7

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    Not many folks visit the CMS control room in Cessy.
     
  9. Apr 4, 2015 #8
    Does CERN plan on probing further into the electroweak symmetry breaking? What is the rough estimate of the energy required to observe other symmetry breaking processes that occured in the early universe?
     
  10. Apr 5, 2015 #9

    rollingstein

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    True. But OTOH, maybe it helps to just learn to live with the visitors' inconvenience a little. After all, funding our large experiment does depend a lot on public goodwill.

    Kids will be kids, But at least they go home & tell their parents. So next time a hundred million comes up for approval maybe there's more awareness where it is going to. It is getting increasingly difficult for large basic science experiments to get taxpayer dollars.

    Maybe I'll endure being treated like a monkey in a zoo for a few hours just to humor they audience.
     
  11. Apr 5, 2015 #10

    mfb

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    The LHC is running! Beams circulated in both directions this morning. Now we'll get some weeks of commissioning, and first collisions probably end of May.

    Sure. The Higgs couplings will be measured in much more detail, together with various other measurements.
    Maybe supersymmetry, maybe something completely new is visible. The unification of the strong and the electroweak force is out of reach by orders of magnitude.
     
  12. Apr 5, 2015 #11

    rollingstein

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    @mfb

    Great post! Can you elaborate on some of the LHC risks hyped up by the media? I assume real risk factors are different. What are they? e.g. I remember a Japanese experiment in a water tank that lost thousands of photomultipliers in a massive cascading implosion. Any different kinds of concerns with LHC? Can it blow itself up? :P
     
  13. Apr 5, 2015 #12

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    "Splashes" (non-colliding beams deliberately directed to material to provide particles to the detectors) happened this morning.
     
  14. Apr 5, 2015 #13

    mfb

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    Black holes and so on? Completely pointless

    Damage to the machine? Well, that is possible.
    The accident 2008 showed how much energy gets stored in the superconducting coils, but they got improved significantly, and they have been tested up to to the full current that will be used this year, so we know they work.
    The beam is another issue - its maximal energy is similar to the kinetic energy of a large airplane before take-off. There are two "beam dumps" designed to handle this energy, but if the beam at full energy gets lost somewhere else it could burn a hole through the machine there. The low-energetic beams used now are not an issue.
     
  15. Apr 5, 2015 #14
    It would be equally interesting if instead of shedding some more light (pardon the pun) on the Higgs mechanism, the LHC made the reason behind baryogenesis after the inflationary epoch less obscure.
     
  16. Apr 5, 2015 #15

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    A few hours every time you are on shift? (At least day shift) I think you may underestimate the fraction of time tours are going on: it's pretty much non-stop M-S during the day. I am not arguing that there shouldn't be tours - I am arguing that students should behave appropriately. If by "kids will be kids" the argument is that they shouldn't be expected to, I would turn that around and say that in that case, the spaces that they should be allowed to visit should be reviewed to consider that.
     
  17. Apr 5, 2015 #16

    mfb

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    Well, the energy is not sufficient to test that scale directly. Precision measurements of CP violation, especially at LHCb, might help to find deviations from the Standard Model.

    There are some unexplained effects, but nothing that would clearly be unaccountable for in the standard model.
     
  18. Apr 5, 2015 #17

    rollingstein

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    Fair enough. Can't argue against that.
     
  19. Apr 5, 2015 #18
    Well I don't mean to dispute the validity of the theory, but I'm curious nonetheless - what does the standard model have to say about baryogenesis? Any proposed hypothetical particle which is not observable today or some unaccounted symmetry? Has any suggestive data collected by the LHC been released by CERN?
     
  20. Apr 5, 2015 #19

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    The standard model allows a small violation of the baryon number as nonperturbative process, but the violation is too small to account for the "large" baryon asymmetry we see today.
     
  21. Apr 5, 2015 #20
    Are you talking about the Sakharov conditions? And is the figure the all too often heard "1 in billion baryons survived for each baryon anti-baryon collision"?
     
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