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    LHC @ 8 TeV: First Week Of 2012 Run
    By Tommaso Dorigo | April 10th 2012 04:00 AM | 14 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    Last week the Large Hadron Collider has started producing collisions at the record high 8-TeV centre-of-mass energy to the ATLAS and CMS detectors.

    In the course of the first week of run almost 200 inverse picobarns have been delivered to CMS, which is absolutely satisfactory.

    The integrated luminosity versus time is shown below.



    And here is the peak instantaneous luminosity reached during these first few days of running:


    (NB: I believe the above figure lacks a "s^-1" units).

    Of course at this rate one could not expect to reach the 15 inverse femtobarns of data that has been set as a (conservative, in my view) objective for 2012, but synchrotrons are a bit like good wine: they improve with time, only much faster. Take last year as an example (right, with integrated luminosity on a logarithmic y scale): during the first week of run, just a few inverse picobarns had been collected. So we may look at this first week of LHC data taking as a quite promising start.

    By the way, I just finished to write a 33-page proceedings article summarizing (hmm, that's probably an euphemism) the CMS results obtained by analyzing 2011 data. I am going to submit a copy to the arxiv and will later link it (maybe in a separate piece). For now, I can offer the following excerpt, from the Introduction:

    In this paper we offer a review of some of the most interesting searches performed with 2011 data by the CMS experiment. As the reader will see, many of the results configure themselves as open questions: is the Bs meson decaying to muon pairs at the rate predicted by the standard model, or at a higher rate compatible with the exchange of virtual massive SUSY mediators ? Does the Higgs boson exist ? Is the Higgs boson mass compatible with the indications of the existing global fits to electroweak observables? Can gluinos with mass below 1 TeV be excluded for most of the SUSY parameter space, throwing a monkey wrench into the inner workings of the most studied extensions of the standard model ?
    As you see, it is my opinion that 2012 is going to be a crucial year for the LHC, and for particle physics!

    Comments

    Indeed, this is to be an exciting year! For some reason, I had 10 inverse femtobarns in my head as the goal for 2012, so it's fantastic to hear that it's actually 15, and you suspect we may get even more.

    Hi Tommaso, i have three questions about the LHC. First, what is the maximum inverse femtobarns that the LHC can reach? Could the LHC engineers modify this big collider, after it will reach its maximum capacity, in order to improve its capabilities (that is, what is the 'future' of the LHC?)? If the Higgs is found, what will be the next things that the LHC will look for? Thank you.

    Hi Tommaso, i have three questions about the LHC. First, what is the maximum inverse femtobarns that the LHC can reach? Could the engineers modify the big collider, after it will reach its maximum, in order to improve its capabilities (that is, what is the 'future' of the LHC?)? If the Higgs is found, what will be the next things that the LHC will look for (that is, will the higgs drive the future LHC searches?)? Thank you.

    dorigo
    Hi Luca,
    there is no maximum, the more we run the more luminosity we collect. Unless you are talking about instantaneous luminosity, which is not measured in femtobarns, but in cm-2 s-1 -or more typically, in nb-1 s-1. That will go up to 10^35 following further upgrades of the machine.
    I think we will look for anything at high-energy, as well as rare decays not yet seen, so there is no clear answer to your last question.

    Cheers,
    T.
    Is there a public web page where we can follow LHC total integrated luminosity throughout the 2012 run ?

    Is it accurate to say:
    the 200/pb was mostly accumulated on 2 days (9 and 10 April)
    so
    the rate seems to be about 100/pb per day
    and
    if the schedule gives about 8 weeks running to the June break
    and about 13 more weeks running to the October break
    then we might expect:
    8x7x100 = 5600/pb = 5.6/fb by the July conference
    and
    (8+13)x7x100 = 14700/pb = 14.7/fb by the October break ?

    If the Higgs search still looks inconclusive,
    could the run be extended by another 5 weeks or so
    (by cancelling the ion run)
    to get another 5x7x100 = 3500/pb = 3.5/fb ?

    Tony

    If the Higgs search still looks inconclusive after 5/fb (2011) plus 15/fb (2012) will another 3.5/fb really mean anything?

    dorigo
    The first week of running in 2011 gave a 200th of the first week of running in 2012. Should we predict 1000/fb ? No. But neither should we take the 100/pb/day figure to extrapolate !

    As for 3.5/fb plus or minus: they may mean a lot if we are lucky... Give or take two or three H->ZZ events in the right place and the picture may considerably change.

    Cheers,
    T.
    There's a nice presentation on the strategy for 2012 published in February:

    http://indico.cern.ch/getFile.py/access?contribId=2&resId=1&materialId=s...

    Slide 31 gives a very optimistic projection of the progress of integrated luminosity over the year. So in one and a half weeks time, it was projected that a little less than 1/fb will be collected, but also bear in mind that the current schedule was delayed by one week compared to the previous so maybe we're looking at 0.5/fb projected.

    The thing to look for over the coming weeks is the initial instantaneous luminosity which is currently at 2.8/nb/s with just 624 bunches. Will they manage to get it to the projected max of 6.8/nb/s with 1380 bunches beating last years 3.6/nb/s?

    As to "... 3.5/fb plus or minus ... may mean a lot if we are lucky...
    Give or take two or three H->ZZ events in the right place
    and the picture may considerably change ...".

    For example, with the 5/fb of 2011 CMS saw,
    for Higgs -> ZZ from 114 GeV to 158 GeV in 2 GeV bins
    (slide 6 of Adi Bornheim Moriond QCD 2012) this many events/bin:

    1+0+2+1+0+0+2+0+1+1+0+0+1+0+1+1+0+0+1+0+0+0+1

    for a total number of 13 events.

    None of the 2 GeV bins had more than 2 events
    (the 2-event bins were at 118 GeV and 126 GeV,
    but adjoining shoulder bins of the 126 GeV events had 0 events)
    so
    it seems fair to say that the 5/fb CMS Higgs -> ZZ results are inconclusive.

    However, due to low background (roughly 0.4 events per 2 GeV bin),
    the Higgs -> ZZ channel is really a Golden Channel whose results may well be determinative of where the Higgs really is and how it works.

    Based on the 5/fb from 2011, maybe we can hope that
    by July with a total of 10/fb we might have bins with 4 events or so
    and
    by October with a total of 15/fb we might have bins with 6 events or so.

    If we hit October with some bin at 6 events, that might tell us something clear depending on how all the events are distributed,
    but
    it could be that we may need an extra 3.5/fb of lucky data to get 2 or 3 events in an interesting bin that could tell us a lot about Higgs.

    Tony

    "Is there a public web page where we can follow LHC total integrated luminosity throughout the 2012 run ?"

    https://twiki.cern.ch/twiki/bin/view/AtlasPublic/LuminosityPublicResults

    had results for 2010 and 2011. Hopefully the 2012 luminosity will be there as well.

    dorigo
    CMS is tidier, there is already stuff online for 2012 here:

    https://twiki.cern.ch/twiki/bin/view/CMSPublic/LumiPublicResults

    Cheers,
    T.
    Tommaso and JollyJoker, thanks for the links to LHC luminosity.

    What follows is a very rough back-of-the-envelope effort
    so please feel free to criticize it (and improve it or just ignore it) .
    It is my effort to try to guess what 2012 may hold with respect to Higgs,
    based on the channel Higgs -> ZZ being the Golden Channel
    with low background and clear events:

    To see what might happen at CMS HIggs -> ZZ with another 5/fb,
    try to combine CMS for 5/fb 2011 with ATLAS events for 5/fb of 2011
    for Higgs -> ZZ from 114 GeV to 158 GeV
    (slide 8 of Ralf Bernhard Moriond QCD 2012)
    ATLAS sees only 6 events there (about half of the 13 seen by CMS)
    and uses a 5 GeV bin size (different from 2 GeV used by CMS)
    so it is not entirely straightforward to combine ATLAS with CMS,
    but
    you can do it roughly by seeing that the ATLAS events are:
    3 around 124 GeV
    1 around 148 GeV
    2 around 158 GeV

    and adding those to the corresponding 2 GeV bins for CMS 114-158 GeV
    (slide 6 of Adi Bornheim Moriond QCD 2012) this many events/bin:

    1+0+2+1+0+0+2+0+1+1+0+0+1+0+1+1+0+0+1+0+0+0+1 CMS

    0+0+0+0+0+3+0+0+0+0+0+0+0+0+0+0+0+1+0+0+0+0+2 ATLAS

    1+0+2+1+0+3+2+0+1+1+0+0+1+0+1+1+0+1+1+0+0+0+3 TOTAL

    In that crude combination the most interesting area
    is at 124 and 126 GeV for which 3+2 = 5 events are seen,
    but ATLAS sees them at 124 GeV while CMS sees them at 126 GeV
    so
    the combined 5/fb + 5/fb (total expected for CMS alone by July 2012)
    still does not give a really clear conclusive result
    especially
    when you note that the 10/fb combination has 3 events at 158 GeV
    where (based on other channels) you do not expect to see Higgs.

    In light of that, I do not expect to get clear results in July,
    and it may even be that things will not be clear by October,
    so
    it may be useful to get another 3.5/fb in late 2012 and hope that it is lucky.

    Since Higgs -> ZZ is the Golden Channel with clearest events and lowest background,
    if the 2012 results are unlucky and unclear at the end of the year,
    is there any chance of postponing the long repair/upgrade
    and continuing to run into 2013 until Higgs -> ZZ gives clear results ?

    Tony

    Hi Tommaso,

    besides improved luminosity does 8 TEV also mean that you can now produce particles with a greater mass?
    If so, what is the maximum mass possible now and what was it before?

    Cheers,
    Martin

    dorigo
    Yes, albeit only marginally. But let's be careful: while one could say that the increase is only of 8/7, it would be a wrong assessment, since what matters is the possibility to see a signal, and that ultimately depends on having a non-null probability of observing, say, three or five events of the same kind. So it depends also on the integrated luminosity.

    Taking 5/fb as the baseline, to uniform 2012 to 2011, this boils down to asking yourself the question: given 7 or 8-TeV cm energy, and given 5/fb, what is the range of masses of a quark-antiquark-produced resonance (a good test case for this issue) which grant a 50% chance of yielding 3 events ?

    This cannot be computed easily, even if you assumed you know what fraction of those resonances you will detect, and if you assume zero background. The problem is that to compute the rate of events you need the cross section, and the cross section as a function of particle mass depends on the model you choose.

    I cannot provide a sample calculation here, but the increase in production rate at high mass is considerable, and so the probability of a discovery. If I find a reference I will post it here later.

    Best,
    T.