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CERN | Geneva | Switzerland

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Wrapping it up on the Higgs boson

As the Large Hadron Collider (LHC) is preparing to shut down for the end of the year holidays, the LHC experiments presented on Thursday morning a summary of the last three years of operation. For CMS and ATLAS, the highlight was of course the discovery of what looks more and more like the Higgs boson.

The certainty for the presence of a new boson has been reinforced. As Sara Bolognesi, speaking on behalf of the CMS collaboration, put it: “The signal is so strong, the probability of having it wrong is as low as the chance of flipping a coin 40 times and getting 40 heads in a row”. Marumi Kado, representing ATLAS, showed that even when using a single decay channel, the signal is strong enough to claim a discovery. Hence, the focus is now on finding the exact properties of this new boson to reveal its identity.

ATLAS showed their first results on the spin and parity of the new boson. The parity seems positive, as expected for the Standard Model Higgs boson, reaching the same observation as CMS. But the jury is still out on the value of its spin although the results are more compatible with 0, the value expected by the Standard Model, but a value of 2 is still possible. A clearer answer might come once the 23 inverse femtobarns of data delivered this year by the LHC will have been processed and combined for the two experiments.

What’s new on the more-and-more-Higgs-like new boson? CMS showed the first results on a Higgs boson decaying into a Z boson and a photon. This decay channel should be very small unless there are contributions from processes predicted by theories going beyond the Standard Model, and these could be huge. Nothing is seen so far but this is a promising avenue.

A few facts are nevertheless puzzling. For example, ATLAS measures two different masses when the Higgs decays to two photons as opposed to four leptons, the two decay channels giving the best precision on the mass measurement.

Each one of these decay channels represents one way the Higgs boson can break apart. It is very much like making change for one dollar. No matter if you give the change with coins of ten, twenty or fifty cents, the total sum should always add up to one dollar. As it stands, it is as if ATLAS obtains $1.05 and $0.95 when adding up all the coins, despite having checked each channel with extreme scrutiny for a possible mistake.

This is most likely due to a statistical fluctuation since the data gives only one mass value in the global combination but it might take more data than is at hand to resolve this apparent discrepancy. CMS obtains similar masses in both channels but the results need to be updated with more data for the two-photon channel.

Another slightly intriguing fact: both experiments measure more Higgs boson decays into two photons than what is predicted by the Standard Model. I summarized the situation in the table below.

The error margins are still fairly large which means more data will be needed to sort it all out. The LHC will undergo a major upgrade starting in March 2013, to restart at higher luminosity and higher energy beginning of 2015.  It takes a lot of patience to do high energy physics!

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.




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20 Responses to “Wrapping it up on the Higgs boson”

  1. Jim Goodman says:

    Anyone think the difference in mass might be due to neutrinos?

    • CERN says:

      Hello Jim,

      the apparent mass difference is 3 GeV (this is roughly 3 times the mass of a proton). You are saying that the Higgs might have decayed into 4 leptons + neutrinos, the neutrinos carrying the excess mass in energy form. This would be highly unusual. According to the theory, the Higgs should decay into two Z bosons, then each Z would decay in turn into two leptons (electrons or muons). There should not be any neutrinos there if all goes according to the Standard Model. Certainly not for the Z decays, which we know very well. We normally detect the presence of neutrinos by checking that the energy is balanced in all directions. This is something we always check since it tells us that all has been taken into account. It has been checked for these events and nothing unusual was noticed.

      Most likely, this is one of those nasty statistical fluctuations that plague physicists all the time. But you are right that this is one good possibility to check (but it was not the case)

      Cheers, Pauline

  2. Jim Goodman says:

    Thank you very much for the prompt and definitive answer. I was also wondering how could a Top Quark (171Gev) be produced by a Higgs (125Gev)?

    • CERN says:

      Hello Jim,

      well, as you pretty much figured out, this is not possible. A Higgs boson is not heavy enough to break apart and produce two particles heavier than itself. It would be like trying to buy two $174 toys with only $125 in your pocket. Stealing is not permitted for particles either, so this would not work.

      Happy Christmas shopping, Pauline.

  3. Jim Goodman says:

    Thank you for the prompt reply. I also noticed that Z’s are 93 Gev and this is a well known channel so how does a Higgs (125 Gev)split into two particles whose sum is 186 Gev? Sorry to keep on this track.

    I think I need to shop for Christmas presents to change my single track thinking. Thanks for reminding me I need to get my wife another present.

    Maybe the low level of Higgs signature is due to the necessity of two Higgs smashing into each other to be detected?

    Happy Christmas,

    • CERN says:

      Hello again,

      indeed, it is tricky. A particle can be off-shell, which means that it is highly virtual. A particle has a mass corresponding to its most likely value but it can take other values. Its mass generally follows a sort of bell curve distribution called a Breit-Wigner. A Z boson most often times comes out with a mass of 91 GeV. But once in a while, it can have more or less mass. So when a Higgs decays into a pair of Z bosons, one is real, with a mass of 91 GeV, the other one is virtual, with a mass as low as 25-40 GeV. Because it is difficult to produce a Z boson so far from its mass, it makes it more difficult to produce such a pair. This is why a Higgs decay into 2 bosons is so rare.

      I hope this helps. Pauline

  4. Jim Goodman says:

    Thank you for the information, Pauline. I now see how scarce detection of the Higgs can be.

  5. Jim Goodman says:

    Thank you again for your valuable time. Are there many masses above 4Gev produced in LHC that are not well known names? Can the LHC produce a 1760 Gev particle? I think it would be stable.

    Again thanks for your patience with me.


    • CERN says:

      Hello Jim,

      no other peak has been found anywhere. There have been all sorts of attempts to find one but nothing came up. So this is still the only unnamed peak around.

      Since the LHC operates at 8 TeV, in principle, it could produce particles with several TeV each. So 1760 GeV, or 1.76 TeV, is possible. But of course, the heavier the particle, the harder it is to produce it. Usually, it is always possible to produce a pair of particle whose mass is below the detector center-of-mass energy.

      I m heading off on vacation now so will be quiet for a few weeks. Enjoy the end of the year celebrations, Pauline

  6. Valentina says:

    What do these new discoveries mean for theories like String Theory? What information about the Higgs boson might confirm or contradict this theory?

    Thank you in advance!

  7. pauline gagnon says:

    Hello Valentina,

    the Higgs boson discovery as I understand it has no impact on String theory at this point to refute or trash the theory. The Higgs boson was predicted completely outside of string theory. To me it appears to be independent of it. But like most mortals, I do not know enough about this theory to be certain. I will ask a string theorist after the vacation to see what he or she might have to say about this.


    • Valentina says:

      Thank you for your reply!

      My question I suppose is: is the existence of the Higgs Boson, with the features found so far, compatible with String Theory? And from a String Theory perspective, what features in the Higgs Boson would be expected for ST models?

      I really appreciate your time and reply :)


  8. Jim Goodman says:

    There was a dip on the mass graphs in addition
    To the 125Gev dip that disappeared when the
    Graphs were combined. Was that a new mass
    Or the top quark?

    Thank you for your patience and valuable time.

    Jim Goodman

    • CERN says:

      Dear Jim,

      unfortunately, I have seen so many plots of this type for the Higgs boson search that I don’t know exactly which one you are talking about. But most likely, little dips that disappear when more data is combined are generally due to small statistical fluctuations. They are completely random and do not pertain to any particular physical effect, be it the top quark or any other real particle. It has to do with fluctuations associated with small numbers.

      Think of a simple example: say there is a bag with marbles and 10% of the marbles are red, the other ones are yellow. If I ask you to take 10 marbles, how many red ones do you expect to get? One? None? Two Three? All these answers are possible. But if you pick randomly 100 marbles, the number of red one will be getting closer to 10%, and even more so if you pick 1000 marbles. As the numbers get bigger, the statistical fluctuations get smaller. This is probably what happen with the secondary peak you saw. Most likely it had no physical basis, just a random variation.

      I hope this helps. Pauline

  9. Jim Goodman says:

    Sorry to be so dense but why does not the Top show up?

    I appreciate your patience and valuable time.

    Jim Goodman

    • CERN says:

      Hello Jim,

      don’t worry, there is no dense questions, only dumb answers.

      I am not sure I get your question right. Are you puzzled because we see the Higgs boson there but not the top quark? If this is what you meant, the reason is that both particles have different mass and different decay modes. So we are not looking at the same place. The top mass is around 174 GeV where the Higgs boson mass appears to be somewhere around 126 GeV. So we are not looking at the same place at all. But more importantly, the top quark decays in very different ways than the Higgs so we would not be looking at the same signature. For top quarks, we would look at objects called jets created by quarks, essentially a collection of tracks originating from the same point. For Higgs boson, we look at two photons or 4 leptons (that is, electrons or muons), things like that. Both particles decay in very different ways so the trace left by their remnants are very different.

      Let me know if I understood your question right. If not, please rephrase and I will try answering better.


  10. Jim Goodman says:

    You got the question right. If we are not looking for the decay mode we won’t
    See the particle. With all the work done I feel that all modes of decay and all possible
    Masses have been found. For example a jet of neutrinos only. Please let me
    Know if I get why no more particles are possible. Thank you for your patience
    And valuable time.
    Jim Goodman

    • CERN says:

      Indeed, to see something, we need to design a a way to zoom on a set of specific characteristics. We are doing several things right now:
      1. studying with greater accuracy everything we already know. We want to make sure everything is not only well understood but also that it corresponds **exactly** to what the current theoretical model (the Standard Model) predicts. Any deviation would mean something else is there, something new not accounted for by the current theory. So we check things up to 10, sometimes 11 decimals… When we say “precise measurements”, we mean it.

      2. Look in all sorts of directions and with different ways for unusual and new phenomena, things predicted by models going beyond the Standard Model. Theorists keep imagining what else could be there to explain the few things the Standard Model leaves unexplained. For this, we imagine special ways to catch hypothetical particles. Every time experimentalists like myself say: we have looked for this hypothetical particle and do not see anything with a mass below 1 TeV or 2 TeV, we are telling theorists what possibilities are still there and what models do not correspond to reality. This we hope will eventually set them on the only possible answer, the right one! All modes have not been tried yet since we have not found the right one that would explain the few questions left unanswered by the Standard Model. We will keep trying until we find it.

      As for neutrino jets, usually, neutrinos are produced like single particles and not regrouped in jets. Neutrinos do not interact with our detectors so we find them by seeing that something is missing since all energy must be balanced. A jet or group of neutrinos would be detected this way. Many searches are done for neutrinos and some of them are looking for more than just one neutrinos. I am not aware of any search for several neutrinos all being created at once though.

      Thanks for your interest in all this, I hope this helps. Pauline

  11. Jim Goodman says:

    Very helpful. Thank you for the information. My pet theory is masses decaying into 0.938Mev*n^2 from a particle with mass l*0.938*n^2.. Thank you for your patience.
    Jim Goodman

  12. Jim Goodman says:

    Fermi IL looked for other masses and one graph seemed to show particles at about the predicted mass using a charge of +1 and -1. Thank you for your consideration.

    Jim Goodman

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