Of Particular Significance

Excitement at Fever Pitch

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 07/02/2012

Excitement is reaching fever pitch as the presentation at CERN on the Higgs search approaches!  What’s important is that they measure By now the experiments have actually finished their analyses, and the press is reporting (harmless) rumors that both of the big Large Hadron Collider experiments, ATLAS and CMS, are seeing something.  Presumably, assuming the rumors aren’t wrong, what they are observing, using this year’s data, is signs of a Higgs-like particle with a mass of 125 GeV/c2 that are roughly similar to what they reported in December, January and March using last year’s data.  The details will be fascinating to see!

UPDATE: The CDF and DZero experiments at the now-closed Tevatron just updated their Higgs searches, and they find some amount of evidence in favor of a Higgs particle being produced through its interaction with W and Z particles and decaying to bottom quark/anti-quark pairs.  Resonaances has a summary.  This process is interesting because neither this production mechanism nor this decay mode are yet accessible to ATLAS and CMS; they don’t have enough data collected yet.  One should be a bit cautious about such difficult analyses, but taken at face value these measurements are complementary to what ATLAS and CMS can tell us on Wednesday.

What’s all this fuss about?  For the non-technical reader (and/or for your non-technical friends) I’ve written an article trying to put this historic moment in perspective, using language that is as non-technical and unfrightening as I can manage.  If you agree that this is a good article for non-experts, please forward the link to others!

For the reader who wants to follow more details: Let me remind you of the sharpest questions that we’ll be asking on Wednesday, many of which are the same or almost the same as those we asked last December. (You can review my various background articles on the Higgs if you want to understand why these are the right questions):

For both ATLAS and CMS, we will ask:

  • Is there a new particle apparent, as a bump in a plot, in the search for a Higgs decaying to two photons?
  • Is there supporting evidence of a bump, at around the same mass as for the photon search, in the search for a Higgs decaying to two lepton/anti-lepton pairs?
  • Is there supporting evidence for an excess of some sort in the searches for a Higgs decaying (a) to a tau lepton/anti-lepton pair, (b) to a bottom quark/anti-quark pair, and (c) to a lepton, anti-lepton, neutrino and anti-neutrino?

These are just questions about what the data actually shows; there are some additional sub-questions, but we’ll get back to them after the data actually appears.  Then,

  • Does evidence in favor of a new particle support the idea that this is a Higgs particle?

The easiest way to get a positive answer would be to see evidence that the particle decays to two lepton/anti-lepton pairs, or to a lepton, anti-lepton, neutrino and anti-neutrino — because a Higgs particle is a ripple in a Higgs field, and a Higgs field, by definition, gives mass to the W and Z particles, and consequently the Higgs particle must be able to decay, pretty often, to two W or two Z particles (more precisely, for a lightweight Higgs particle, to a real W or Z particle and a virtual W or Z particle).  If the answer is yes, then we ask:

  • Does evidence in favor of this particle suggest that it is a Higgs, but not a Higgs of the simplest type (i.e., that it is not a Standard Model Higgs)?

The easiest way to get a positive answer would be to see evidence that rates for the various modes of production of the Higgs, or the probabilities for various ways that the Higgs can decay, or both, are very different from those predicted for a simplest Higgs of mass of about 125 GeV/c2.  Most likely we’ll get an ambiguous answer this week, and the answer to this question may not become clear for several years — but we could get lucky even now, if the Higgs in nature is dramatically distinct from the simplest case.

Then the next questions will be:

  • Is what CMS observes largely consistent with what ATLAS observes?  In short, do the results for one experiment largely confirm the results for the other experiment?
  • Are there any notable discrepancies between the two experiments that might undermine confidence in their results?

Obviously, consistency between the two experiments will greatly increase confidence in the results!

Onward to Geneva, and IndependHiggs Day!!!

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13 Responses

  1. Can I just say what a relief to discover an individual who genuinely knows what they are talking about on the internet. You actually realize how to bring an issue to light and make it important. More and more people need to look at this and understand this side of your story. I was surprised you aren’t more popular since you certainly possess the gift.

  2. Yesterday, not only CDF/DZero but also ATLAS published an update on their 2011 (7 TeV) data:


    They are talking about 2.9sigma (like D0 and CDF) – of course, no 2012 data is included which evidently will be presented tomorrow.

  3. So now that OPERA had 5 sigma and Minos had more than 2 sigma, we could have considered the FTL neutrinos a given? … No!

    Let’s be careful about combining independent experimental results and, also, let’s make sure we are actually dealing with a 5 sigma machine.

    One question I have for the Professor is are our sensors getting lost in the ” “vacuum noise”? In other words, to find the SM Higgs boson we need to create extremely high temperatures at a very small volume. So, is this bump the SM Higgs boson or is it a transition region of the unstable vacuum and turns into and stable field? If so, are we messing around in a region we should not be? Could we be opening another Pandora’s box?

  4. Physicists Inch Closer to Proof of Elusive Particle By DENNIS OVERBYE:

    Judging by his continuous writing, Dennis must have passion for particle physics. To six physicists he mentions, who won J. J. Sakurai Prize for Theoretical Particle Physics in 2010 – Carl R. Hagen, Francois Englert, Gerald S. Guralnik, Peter W. Higgs, Robert Brout (May 3, 2011), T.W.B. Kibble, he should have also added two important names – Philip Anderson and Yoichiro Nambu . And going even further, the work that too was somewhat instrumental and helpful at the time was 1957 BCS theory, by Bardeen, Cooper, and Schrieffer.

  5. Prof. Strassler: if this is the wrong place for this question, please feel free to move it. I have always understood (in my simplistic way) that the weak force and the electromagnetic force are united by a symmetry relation, and that since the W+- and Z particles have mass but the photon doesn’t, this symmetry must be broken. I also understand that it is the Higgs field, or the Higgs “mechanism” that breaks this symmetry and gives the weak force particles their mass. That’s all fairly clear (if I have it right), but what is the basis for claiming that the Higgs field gives other particles (quarks, electrons, etc.) their mass? In my naive view of this, to say that the Higgs field gives quarks their mass assumes that there is a symmetry relating the strong force with the electroweak force, and that this symmetry is also broken by the Higgs (I don’t have a clue how the electron/muon/tau would figure in this, let alone gluons). As I understand current theory, the electroweak force is not related to the strong force except under “GUT” theories, which, to say the least, haven’t panned out.

    I really enjoy your blog, and I’d appreciate any response you could give to me.

    1. One of the problems (before the work of Higgs et al.) with massive gauge bosons was that their mass wasn’t “gauge invariant.” If you naively give a gauge field a mass, then you lose the gauge symmetry which defines its structure/interactions. So, we start by looking at the electroweak symmetry, which in technical group theory terms means a symmetry structure of “SU(2)_W x U(1)_Y”. Here W refers to weak and Y refers to hypercharge. The is point that we don’t have an independent electromagnetism or photon until we “break” this symmetry to a smaller part of itself, U(1)_EM, and group theory tells us that there are four independent fields here. Three from SU(2)_W and one from U(1)_Y. Eventually we will arrange the fields in such a way that particular combinations are the charged Ws, Z, and photon.

      The Standard Model gives a mass to the W and Z, but not the photon, by postulating a four-component Higgs field that has two properties: it carries an electroweak charge but not an electric charge, and it has a vacuum expectation value (VEV). When you deduce the consequences of the VEV, you find that the charged weak boson fields (W’s) and the combination of the electrically neutral components of the electroweak gauge boson field that does couple to weak charge (the Z) gain a mass from three of the components of the Higgs field. Since the (simplest) Higgs doesn’t have an electric charge it doesn’t interact with the other combination of the neutral electroweak fields which doesn’t couple to the weak charge. This combination we identify as the photon. Similarly, since the Higgs carries no color charge the gluons remain massless. We’re left with one more component of the Higgs field which is physical and can propagate on it’s own. This is what we call the Higgs boson when we talk about producing Higgs field quanta.

      Now, mass terms for the quarks and leptons also violate the gauge invariance of whatever gauge groups (i.e. symmetries) they’re charged under. The fermion fields are not defined by the symmetries of the Standard Model like the gauge fields are, but rather arranged to interact with the gauge fields in a systematic way. Having no gauge symmetry that defines them, we can’t use the Higgs mechanism to grant them mass. However, in the equations of the SM we can write a term involving the fermion fields and the Higgs field that is gauge invariant (so called Yukawa terms). When we go through the process of breaking the electroweak symmetry, the fermion fields also acquire a mass term involving the Higgs VEV. But since we don’t have a gauge symmetry to tell us how strongly the Higgs and fermions interact, we’re left with one free coupling parameter for each fermion field. By enforcing these mass terms to be equal to the observed mass, we set the coupling parameter between the Higgs and each fermion field (with caveats about neutrinos which aren’t important here).

      The Yukawa term, after symmetry breaking, gives not just a fermion mass but also an interaction between the physical component of the Higgs field that remains and each fermion. These come complete with that unspecified coupling parameter above, but since we then define it through the fermion mass terms, we now know the interactions between the Higgs and fermions as well as the Higgs and the weak gauge bosons.

  6. If the rumors are true, and the Higgs particle is indeed the simple one, what is the next step for theory? I assume a lot of alternative theories get ruled out, but what would should the next experiment search for in fundamental particle physics?

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