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    The LHC Will Run At 7 TeV In 2011 And 2012
    By Tommaso Dorigo | January 31st 2011 11:59 AM | 9 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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    A week ago a meeting was held in Chamonix to discuss in detail the schedule for the near future of the Large Hadron Collider, and to take a decision on the schedule, in particular for 2012.

    Previously, for 2012 a long shutdown had been foreseen, to allow the retro-fitting of LHC with systems which will prevent future incidents such as the one of September 2008. The possibility that ATLAS and CMS would only get about one inverse femtobarn worth of proton-proton collisions in 2011, and then stay inactive until late 2013, was rather disappointing for the experimenters. CMS and AtLAS would then have a significant chance of missing the discovery of the Higgs boson with the maybe insuficient data, and to even have to stay put while the Tevatron, running at its best, would continue to rush toward a first evidence of the elusive boson.

    This is now no more a possibility. For two reasons.

    First came the news that the Tevatron will finally shut down at the end of 2011, after a fantastic, over 25-years-long career. This means that CDF and DZERO will not be able to see a first hint of the Higgs, even in the most favourable case. Note that despite what everybody nowadays knows already -i.e. that a light-mass Higgs boson is more challenging for the LHC than for the Tevatron-, even for the Tevatron experiments the most observable mass of the Standard Model Higgs is 165 GeV, just as for the LHC; this is due to the sufficiently clean and frequent decay of the Higgs into a pair of W bosons, which has its highest rate if the Higgs mass is just above twice the mass of the W -or 165 GeV, as stated. 

    10 inverse femtobarns per experiment just might have been enough for CDF and DZERO to see a 3-sigma evidence of a 165 GeV Higgs, but the problem is that that particular mass point has already been excluded, at 95% confidence level, by their analysis of the first 6 inverse femtobarns they put their hands on. This makes it necessary for the new data yet to be analyzed to first fill the signal void of the earlier datasets, before a significant signal can be obtained with the combined statistics. Quite unlikely.

    7 TeV, multiple inverse femtobarns

    Now, the second news -the one I report on today- is that LHC will continue to run through 2012, increasing as much as possible the instantaneous luminosity of the beams. This means that while at the end of 2011 CMS and ATLAS might still only have one inverse femtobarn of collisions to play with, by the end of 2012 it is reasonable to expect at least a factor of 5 more data. And that, eventually, will allow the CERN experimenters to find the Higgs.

    The conclusion stems from a re-analysis that the LHC experiments produced for Chamonix. Note that before the Chamonix meeting everybody was sure that the decision would be taken to run at the higher energy of 8 TeV, i.e. 4 TeV per beam. The more conservative decision taken last week may disappoint some -at 7 TeV many new-physics processes have a significantly lower cross section and are thus harder to detect- but this is a wise choice by the management, in my opinion.

    The point was made by Myers in his summary talk. The chance of a new incident similar to that of September 2008 is not zero: it grows with time, and it grows much more rapidly with the beams energy. That is because a larger beam energy requires higher currents in the dipole magnets, and the latter increases the chance of quenches -the prerequisite of a possible resistance failure like the one which triggered the 9/2008 blast.

    For 7 TeV running, chances of an incident appear to be negligible, but for 8 TeV running they are not so any more, although they still remain quite small. A new incident would not cause as much damage as the early one, but it would still force a 8-10 month shutdown, plus a significant, unacceptable loss of CERN's reputation. The risk analysis assessed the risk as ``medium'', and the return in higher physics reach was not found to be sufficient to justify such a course of action.

    What does this mean for the Higgs hunt ?

    So let us finally see what CMS predicts to do with one, two, or more inverse femtobarns of 7-TeV collisions in 2011 -and 2012, if needed. The benchmark is, of course, the Standard Model Higgs boson; however, you should bear in mind that a lot more might be enabled by such data.

    The graph below shows the number of standard deviations equivalent to the observable Higgs boson signals that may be obtained by a combination of sensitive search channels by CMS. The curves show the median sensitivity, i.e. they show the minimum significance achievable 50% of the time. Different curves refer to 7- or 8-TeV running conditions (now ruled out for 2011-2012), and for different integrated luminosities. As you clearly see, 50% chances of a 3-sigma or larger evidence for the Higgs boson exist in a wide range of masses, even with the 7 TeV running and 5 inverse femtobarns ( the full blue line -a reasonable estimate for 2012!). And bear in mind that this is CMS only: a combination with ATLAS would typically make a 3-sigma signal become a 4-sigma one, roughly. If it were real!



    The last graph shows instead the range of masses which can be excluded, at 95% confidence level, by CMS searches, again as a function of beam energy and integrated luminosity. A limit below the red line at 1.0 means that the Higgs boson is excluded... If the Higgs is not there, we will know soon! The blue line is under 1.0 for the whole mass range!


    More information on the Higgs reach by CMS can be found here.

    Και αυτό το άρθρο μπορεί να διαβαστεί στα Ελληνικά στον Ελληνικό μπλόγκ μου.

    Comments

    The press release does not rule out higher energy for 2012, only for 2011. It was suggested at Chamonix that some extra measurements made this year using thermal amplifiers could lead to a reassessment of the risk allowing a higher energy during 2012. Obviously they don't want to emphasise this possibility in the press release but it is carefully worded not to discount it.

    dorigo
    Hi Philip,

    to me the question is settled by a graph shown by Steve Myers in his conclusive talk at Chamonix last Friday. The risk analysis will depend very little on the actual value of the probability of an incident, since the risk is too big to take. What matters is that the number of quenches expected is rising a lot with the needed currents, and this drives the decision.

    Cheers,
    T.
    Bonny Bonobo alias Brat
    Tommaso, I wondered how often they give the LHC a service or whatever you call the equivalent for a particle collider? Or is it just an ongoing process? Do certain parts have to be replaced after a specific amount of time and do they do stress tests and xrays as they do for aeroplanes? Does the helium ever have to be replaced or reconditioned? I once read that the LHC had the largest amount of helium contained in one place on the planet and that it was becoming quite scarce and cannot be easily replaced, is that also true? Sorry if these questions are stupid and/or not part of your expertise, if so maybe you could provide a link?
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    They make like 5 day long cryo stop every 6 weeks to test/repair compressors pumping helium and also 2 month long technical stop in winter when all the helium is transferred into the external tanks.

    I agree that staying at 7 TeV makes sense. The physics gain from going to 8 TeV does not really justify any additional risks.

    However, I get the impression that now, after the decision to shut down the Tevatron in October this year, the LHC expectations concerning luminosity and reach in Higgs mass look more modest than some months or weeks ago.
    As I understand, the (previous) LHC expectations played a crucial role in the decision to shut down the Tevatron...

    dorigo
    No, Sven, this is not my understanding. The decision to stop the Tevatron had to do with money...
    There was no extra money for HEP to run the Tevatron in 2012-2014, simply. I do concur that the LHC experiments may have decided to ask for continued running in 2012 based on the earlier Tevatron extension chances, though.

    Best,
    T.
    LHC will not find anything, like LEP. It is just a waste of time and money...

    dorigo
    Ι might agree with you Paolo, but not finding anything will be at least as interesting as finding something new! This is true, in particular, for the Higgs boson.

    Cheers,
    T.
    "LHC will not find anything, like LEP. It is just a waste of time and money..." this is meaningless.

    The role of LHC is to uncover the nature of EW symmetry. Something will be found. Maybe you wanted to say "nothing more than a weakly coupled higgs boson".