Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Surprising high-energetic event at the LHC

  1. Sep 5, 2015 #1

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Probably just a weird fluctuation, but still something to watch.
    Event Display of a Candidate Electron-Positron Pair with an Invariant Mass of 2.9 TeV

    Edit: Follow-up after data-taking in 2015 ended: no further events with that high energy, now one event is reasonable. Was just a weird fluctuation to have this event so early on.
    CMS result with the full 2015 dataset
    ATLAS result with the full 2015 dataset

    The CMS detector found an event with a very high-energetic electron and a very high-energetic positron. If they are from the decay of a particle, this particle would have a mass of around 2.9 TeV (the so-called "invariant mass" of the electron/positron pair). The most likely candidate would be the hypothetical Z', a heavier version of the known Z boson.
    The event is significantly more high-energic than everything seen in run 1 (2010-2012). Sure, the proton energy increased, but the analyzed data from this year represents just 0.3% of the collisions from run 1.

    How surprising is it? There are well-known processes that can produce those pairs, but they rarely do that at very high energy. One could calculate the probability to find an event with an invariant mass of at least 2.9 TeV, but that is unfair - the choice of 2.9 comes from the one observed event. CMS calculated the expected number of events above 1.0, 2.0 and 2.5 TeV for the size of their analyzed dataset:
    Above 1.0 TeV: 0.21
    Above 2.0 TeV: 0.007
    Above 2.5 TeV: 0.002

    Those numbers don't have proper systematic uncertainties yet, so they might change a bit in the future, but it is clear that an event with 2.9 TeV was not expected that early in run 2.

    What comes next? CMS studied just 1/3 of their dataset, and the full ATLAS dataset is even a bit larger. I'm quite sure both collaborations look at that now. If they find a second event, things get really interesting. If they find nothing, it is probably just a statistical fluctuation (with then ~0.04 expected events above 2 TeV it is not that surprising any more. Keep in mind that there are many possible particle combinations where one can look for events).

    If it is a Z', it should also decay to muons with the same probability. Looking for high-energetic events with a muon and an antimuon will be interesting as well.
     
    Last edited: Feb 12, 2016
  2. jcsd
  3. Sep 5, 2015 #2
    How does it work? They can only analyze the other data they already have, or wait for another event like that to happen, or they can fine tune the collisions energy to 2.9 TeV to increase the probability of it?
    Thanks.

    --
    lightarrow
     
  4. Sep 5, 2015 #3

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Processing and analyzing the collected data takes some time, so there are collisions recorded that still need analysis.
    Besides that, more collisions will come soon.

    There is no way to improve the collisions (a higher energy would help, but preparing the machine for that takes months so it won't happen this year) - all you can do is try to get as many collisions as possible for better statistics.
     
  5. Sep 5, 2015 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    They wait. Reducing the energy will not help. The rate will go way down.

    The odds of this being anything are small. Every couple of days the experiments get an oddball event that is extreme in some way or another. More importantly, 65 pb-1 at 13 TeV corresponds to 1.3 fb-1 at 8 TeV (at 2.9 TeV). The experiments have analyzed the equivalent of 15x as much data already and not seen anything.
     
  6. Sep 5, 2015 #5

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    On the other hand, the Z' exclusion limit did not reach 3 TeV with run 1 data. Sure, you probably don't expect 1 event every 65/pb, but even combined with the whole run 1 data the observed event rate is above the expectation (but the significance goes away).
     
  7. Sep 5, 2015 #6

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    The Z-prime exclusion limit is for a SSM Z-prime benchmark model. We know that's not the case (precision electroweak tells us) so it's just a benchmark: the exact number for the mass doesn't tell us much. I can always come up with a number where the limit is less (or for that matter, more) stringent.

    The best apples-to-apples comparison is the partonic luminosity.
     
  8. Sep 6, 2015 #7
    I would much appreciate it if someone would define for me "pb-1". I can guess that pb might mean "part per billion", and that "-1" might mean inverse, but "inverse parts per billion" makes no sense to me, and in the conexs of the discussion about Z', I cannot imagine what "pb-1" means.

    Ah, I found the following in a list of possible abbreviations for pb: petabyte. Is this what is intended? If so, then it might be useful to mention what the petabyte accumulation rate is for the LHC test runs.
     
    Last edited: Sep 6, 2015
  9. Sep 6, 2015 #8
    It means (pico barn)-1. So "65 pb-1" means 65 events for every pico barn of cross section. Multiplying that value for the cross section of a specific reaction you find the total number of events for that reaction.
    Or at least this is how I've understood it :smile:

    --
    lightarrow
     
  10. Sep 6, 2015 #9
    Hi lightarrow:
    Thaks very much for the definition.

    Regards,
    Buzz
     
  11. Sep 6, 2015 #10

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    I wrote about cross-sections and luminosity in this insights article.
    Okay.
    Well, we still have more events than expected, but it is probably just a weird fluctuation, as I wrote in post 1.
     
  12. Feb 12, 2016 #11
    Hi @mfb:

    Although the paper had a September 2015 date, the CERN cite gives August 30 as the date of the "discovery". How do you interpret that there has been no further reports about this since then?

    Regards,
    Buzz
     
  13. Feb 12, 2016 #12

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

  14. Feb 12, 2016 #13
    Hi mfb:

    I interpret your post that the original finding was a false positive. Is that right?

    Regards,
    Buzz
     
  15. Feb 12, 2016 #14

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Nothing false about it. It's a real event. We expect to see one event or so.
     
  16. Feb 12, 2016 #15
    Hi Vanadium:

    Thnaks for your prompt answer.

    My mistake. I thought that to get an acceptable confidence level, a second detection was required.

    Regards,
    Buzz
     
  17. Feb 12, 2016 #16

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Confidence level of what?
    You can always construct confidence levels. If no event is observed, they typically include zero of whatever parameter would lead to more events, so they are called "upper limits". If one event is observed, the confidence interval depends on your expected background and on what you want to measure.

    Some event had to be the most high-energetic in 2015, just by the ordering of energies. This event is at 2.9 TeV. So what? It is not surprising to have an event at 2.9 TeV in the full 2015 datasets.
     
  18. Feb 12, 2016 #17
    Hi mfb:∈

    I mean a calculated value (confidence level) X∈[0,1] WRT which a measured value M with an error measurement range +/-Y is such that the probability that a random measurement R is in the range M+/-Y is < 1-X.

    Regards,
    Buzz
     
  19. Feb 12, 2016 #18

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    I know what confidence levels are, but what is the measured value you want to consider?
     
  20. Feb 12, 2016 #19
    Hi mfb:

    WRT quote: the measured value is 2.9 TEV as the energy at which data from the experimental run indicates that a "candidate" particle was created from the annihilation energy of the electron/positron pair.

    BTW, as I think I understand it, the analysis of the data was not specific about the specific nature of the particle. Is this correct?

    Regards,
    Buzz
     
  21. Feb 12, 2016 #20

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    The 2.9 TeV are the measured value of a single event. That is not a physical parameter you could constrain. You can ask what the true invariant mass of the pair was, then you get a very narrow window around the measured value, but I don't think that is what you mean.

    The LHC is a proton-proton collider. The dominant production process is quark+antiquark -> photon/Z -> electron+positron. No electron/positron annihilation.
    I don't understand that question.
    Which particle? And no, that is not the main part that I don't understand.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Surprising high-energetic event at the LHC
  1. The LHC (Replies: 18)

  2. LHC, what then? (Replies: 7)

Loading...