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Flip Tanedo | USLHC | USA

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More Post-Higgs silliness

I recently got to eavesdrop on a delightful and silly e-mail exchange between US LHC’s very own Burton and Aidan, both ATLAS physicists, after I pointed out that Wikipedia now mentions the ATLAS Higgs talk as a “notable use” of the infamous font Comic Sans. The quotes below are lifted directly from their e-mail exchange (with their permission), as illustrated by yours truly.

For more substantial physics discussion, check out Aidan and Seth’s Higgs postgame video and Anna’s ongoing posts from ICHEP.

Update [7/08]: the “4.9 sigma” comment below is a mistake, the actual “global significance” includes the ‘look elsewhere effect’ and is lower than this.

 

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9 Responses to “More Post-Higgs silliness”

  1. djb says:

    seriously comic sans? sounds like a science fair project…

  2. Ted says:

    Hi Flip,

    A few questions from an interested non-scientists.

    1. Could the Higgs have been discovered in the Tevatron at Fermilab, given that the Higgs is less massive than the top quark?

    2. If the Higgs could have been detected at the Tevatron, then why wasn’t it found before the top quark? Is this an issue with to total amount of data collected before the Tevatron was shut down?

    3. I saw a YouTube video with physicist Bryan Cox on BBC explaining how space is “crammed with Higgs particles” and how other particles have mass because they are “bumping into” all of the Higgs particles. (I watched it twice because this sounded so wrong. Here it is: http://www.youtube.com/watch?v=yKz07k04D70&feature=youtube_gdata_player) Just how common is the Higgs in nature? Even if a Higgs boson is made during a high-energy collision in the universe outside the LHC, won’t it decay almost instantaneously, like those observed inside the LHC detectors? How does the Higgs give mass to anything if there are no (or relatively few) Higgs found in nature?

    Sorry — I thought I had short questions and then they kept building.

    I appreciate your excellent posts. If my questions are too cumbersome, perhaps you could refer me to source material.

    Thanks,

    Ted

    • Flip Tanedo says:

      Hello Ted, my apologies for the late reply. These are great questions. Unfortunately I don’t have the technical information to answer your questions completely—I’m sure some of the other bloggers do (e.g. Aidan, who explained a lot of this to me)—but I can at least address some of the subtleties that make these great questions.

      Could the Higgs have been discovered at Fermilab?

      Probably, but it would have required a lot more data. You’re right that the Higgs is lighter than the top quark, so naively we can say “if we can produce top quarks, then, why can’t we produce Higgses?”. But there are some subtleties with this statement: the actual relevant collision energy at a modern proton–(anti)-proton collider is the energy of the quarks/gluons that interact, which is a fraction of the nominal energy of the machine.

      Top quarks are produced at a much higher rate than Higgses. Pairs of tops are produced readily by, say, gluons of sufficient energy. Higgs bosons are produced at a much lower rate. This has to do with how the Higgs is produced and to what it decays to. There are a few effects worth noting:

      * At high energies, protons are basically bunches of gluons that collide. (This is a kind of quantum screening effect analogous to what I’ve written about the electron charge being screened at short distances.)

      * In its last few years, the Tevatron had focused on b-physics so it was really optimized for tagging b quarks. This made them more sensitive to different types of Higgs decay than the LHC. The LHC also has the disadvantage of having more QCD background, making it harder to tag the h-> bb decay.

      * The production mechanisms were also a little different between the Tevatron and LHC: at the Tevatron you could produce Higgses through interactions of the proton/antiproton valence quarks. At the LHC Higgs production occurs primarily through gluons.

      * But the main point addressing your question is that the top quark and the Higgs are produced differently and decay differently. This means that the efficiency for tagging top quark events is different from the efficiency for tagging Higgs events. So while it’s “easy” to acquire enough statistics to discover the top quark at the Tevatron, practically we had to wait for the LHC to find the Higgs.

      The proposed Tevatron extended run plan (~3 years, I think) would have covered the entire ‘reasonable’ SM Higgs mass range, including 125 GeV, at 5 sigma but by then it was clear that the LHC would beat them to it. (This, I believe, answers your second question.)

      Another great question would be whether the Higgs could have been discovered at LEP, which was an electron/positron collider that went up to 115 GeV. If people had known for sure that the Higgs was at 125, I wonder if they could have run the machine at higher energies to discover it. (LEP era ended when they started building the LHC in the same tunnel.)

      Regarding your last question… you’re right, it does sound wrong! But it’s partially a matter of semantics. In a sense the statement is correct, that Standard Model fermions get their masses from ‘bumping into’ the Higgs. (And gauge bosons get their mass from doing this and additionally “eating” the Higgs.) But what we mean by “Higgs” is different. The thing which fermions bump off of is the Higgs field, which is a classical idea, just like the electric field of a charged object. The particles are ‘bouncing’ off of the Higgs field in the same way that an electron ‘bounces’ off of the photon field (electromagnetic field) of a bar magnet.

      This ‘field’ is DIFFERENT from the Higgs particle. The Higgs particle is a quantum excitation of the field. This is in the same way that the photon is the quantum excitation of the electromagnetic field. These Higgs particles indeed decay, but the Higgs field is some background value that exists everywhere. Subtly different ideas! I suggest checking out the Higgs videos on Minute Physics (on YouTube) for a pithy and graphic explanation.

      Great questions,
      F

  3. Matt M says:

    Hi Flip,

    I have had a question in my head regarding the Higgs particle/field for a while, and seeing as you always manage to give pretty straight forward and understandable (to a layman!) answers, I was wondering if you’d be able to answer it if possible?

    I understand that particles generated in colliders such as the LHC tend to decay in to less massive ones, as tends to be the universe’s want to move things in to the least energetic states it can i.e. the Higgs Boson can decay in to two Z bosons, which can each then decay in to pairs of electrons or muons.

    My question is why aren’t the Higgs Bosons that are giving me and everything around me mass this very instant not decaying constantly in to other, less massive particles? What is providing them with the energy to remain stable and massive that isn’t doing so with particles created in the LHC? What is the difference between these particles (if any?).

    I hope that makes sense, many thanks for your consistently interesting blogs and posts.

    Thanks

    Matt

    • Flip Tanedo says:

      Hi Matt, my apologies for the late reply.

      See the response to Ted’s comment above. This is a great question—the answer is that there’s a difference between the Higgs field and the Higgs particle.

      The Higgs field is the thing which gives mass. It’s easiest to think of it as a classical object, like the electric field or the gravitational field. This is the thing which elementary particles “bounce off of” to get mass.

      You might argue: this is weird! The gravitational and electric fields cause forces. I don’t feel any Higgs force. Recall from high school or college physics, though, that the forces are caused by differences (gradients) in the fields, i.e. how the fields change in space. The Higgs field is absolutely constant everywhere, so it doesn’t generate any long range forces. (In the quantum picture, this has to do with the Higgs being massive.)

      The Higgs particle is the excitation (the quantum) of the Higgs field, and this is a particle in its own right, that can also ‘bounce off of the Higgs field’. Sometimes it’s easiest to think of the Higgs particle and the Higgs field as two different things, though really the former is the quantum excitation of the latter.

      Cheers,
      F

  4. maximal says:

    Hi Flip:

    I read the comments… both their questions and comments and your straight answers are really interesting and mind provoking.

    Scientifically I do no believe in the higgs field, similar to those of electric or magnetic fields, because it has not been proved experimentally up till this minute. The higgs boson which is claimed as excitation of the higgs field must imply that the higgs field is in constant excitation state since the BIG BANG and still unstoppable, and this is not a proven scientific fact. The massive boson that has been discovered recently is still not certified by scientist as being “the higgs” boson itself from the excited mother “higgs field”. So please let me understand what we really have.

    Thank you.

    • Flip Tanedo says:

      Hi maximal, the Higgs field and the Higgs particle are two different things. Thing which was recently discovered was the Higgs particle.

      Ok, now some semantics: the Higgs particle is an excitation of the Higgs field, so yes, the particle implies the existence of the field. But the particular aspect of the Higgs field which is special is that it obtains a ‘vacuum expectation value’ (vev, see some of my Higgs posts on this blog). The Higgs vev is something which we know must exist from the consistency of a quantum theory of massive gauge bosons; at least within the framework of quantum field theory.

      You can interpret the vev in different ways, though one has to be careful that these interpretations are always consistent with the mathematical framework of the theory. Your statement about the HIggs being in a ‘constant excitation state’ is one interpretation of the vev, though the term ‘excitation’ is a bit arguable: the ground state of the Higgs has a vev, this is the field configuration with the lowest energy. The Higgs particle—the thing we discovered—is an excitation on top of this vev.

      I do not understand what you mean by “unstoppable.”

      But you’re right: we’ve found a particle that seems to be produced and to decay in the way that we expect a Higgs particle to do so. It’s in a mass range which is reasonable for the Standard Model.

      If it’s not the Standard Model Higgs, it is most likely a variant of the Higgs rather than being a random new particle that has nothing to do with electroweak symmetry breaking.

      There’s one “oddball” alternate possibility that’s not completely unmotivated: a “dilaton” (a.k.a. “radion”) which is a particle that shows up in certain theories of strong coupling that happens to behave similarly to the Higgs.

      The question now is to figure out if the interactions of the newly discovered particle indeed match what is expected from a Standard Model Higgs boson.

      -F

  5. Stephen Brooks says:

    About the 5 sigma discovery standard: I think there was a proper 5 sigma (accounting for look-elsewhere) but only if you take both experiments into account.

    If you look at those p-value plots of ATLAS or CMS individually, the peak may be at 4.9/5 sigma but this gets deflated to 4ish if you account for the “look elsewhere” effect – those graphs were sampling scores of distinct points. However, the probability of the 2nd experiment having a signal _where the first one’s peak was_ does not need to be deflated by look elsewhere because then we’re only sampling at one point. So overall the two experiments saw two signals but because they’re coincident only one needs to be deflated by look elsewhere. So that’s your 4.9 sigma combined with a 3-4ish value, which under most assumptions would push it above 5 globally.

    • Flip Tanedo says:

      Hi Stephen, yes, I agree that it’s probably true that when the experiments are combined you get over 5 sigma—but that combination hasn’t been done yet, at least not officially.

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