Of Particular Significance

At CERN, waiting. Here’s what to watch for on Tuesday.

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 12/12/2011

This week I’m at CERN, home of the Large Hadron Collider [LHC] (and have been assigned the office of none other than John Ellis, who perhaps surprisingly is out of town.) Tomorrow, some new results on the Higgs particle search at ATLAS and CMS, the two general-purpose experiments at the LHC, will be presented.  In today’s post, I’ll first summarize the  situation, and then I’ll explain what I’ll be paying closest attention to in tomorrow’s presentation.

First, a summary of basics, with links.

Now, about the presentations on Tuesday.  Despite various rumors, I do not expect (and most experts I talk to do not expect, and CERN itself warns us not to expect) any evidence presented tomorrow to be convincing yet; here’s why.  But we might (or might not) see some evidence that is moderately compelling.  How should we evaluate it?  What should we be watching for?

[My reasoning here may be opaque to you unless you’ve read through the Standard Model Higgs trilogy (at least the first half of part 3, and also the older article it links to) or you’re already somewhat knowledgeable about the subject.]

First, how compelling are the hints of the Higgs particle shown by ATLAS and CMS, treating the experiments separately?

  • More precisely, how much of the signal for each experiment comes from the clean searches for Higgs –> two-photons and Higgs –> ZZ –> two lepton-antilepton pairs, which have low systematic and theoretical errors and give a sharp measurement of the Higgs mass, and how much comes from the messy WW –> lepton/anti-neutrino anti-lepton/neutrino signal, which has much higher systematic and theoretical errors and gives a vague estimate of the Higgs mass?
  • In particular, if you were to drop the Higgs –> WW measurement entirely, are the signals of the Higgs in two-photons and in two lepton-antilepton pairs convincing by themselves? (This tests whether the result’s significance relies most heavily on a tricky and controversial measurement or on the simpler non-controversial ones.)
  • If Higgs –> two lepton-antilepton pairs plays an important role, this will be because of one or two collisions that suggest the same Higgs mass as a signal in Higgs –> two photons. How much less convincing would the result become if you were to remove one of those collisions? (This checks sensitivity of the result to an individual chance event.)
  • How much of the region near to the signal has actually been excluded? Should we believe evidence in favor of, say, a 125 GeV Higgs if a Higgs at 128-135 or 118-122 has not yet been excluded? (This helps clarify whether the search is really close to completion.)
  • If the WW measurement plays a large role in the evidence, how confident do the theory experts on Higgs –> WW, and on the various backgrounds to this search, seem to be with the theoretical errors stated by the experiments? Do the experiments have detailed evidence that they understand their systematic errors on this measurement? How well under control is their ability to measure the “missing momentum” signal of the neutrinos with sufficient accuracy and precision? (Until we have convincing answers to these questions, I personally think we should be very cautious trusting the WW contribution to the significance of the result.)

Next, we should view the experiments together.  Are they really seeing evidence for the same thing?

  • Are the ATLAS and CMS signals for a Higgs particle really sitting at the same Higgs mass? or do the preferred masses differ by more than 2%, which would be inconsistent with a Higgs particle’s signal?
  • Does the evidence really become much more compelling when you combine the evidence from the two experiments (which we’ll have to do by eye, since the statistically valid, officially authorized combination of the two will probably not be carried out for quite some time)?

I’ll try to get most or all these questions answered for you, to the best of my ability, by tomorrow evening or Wednesday morning. Whether I’ll be able to get you any information during and/or right after the presentation tomorrow may depend on how much of a zoo it is at CERN tomorrow, and how well the wireless network can handle the inevitable massive flood of data going in and out of CERN!

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16 Responses

  1. I have been reading since you came and gave a talk at McGill. As a condensed matter physicist, I don’t always find the time to follow all this stuff and I find your blog very convenient.

    In short, thanks for that and keep up the good work.


  2. I’ll be watching closely. You said elsewhere on your blog that science is the world’s best spectator sport. Well, you’ve achieved that, as I await the announcement with anticipation akin to the Super Bowl (more, actually, as I sadly am a Bengal fan.)

  3. Its really good to see all the caviats above. I think the whole section is better than some other articles jumping to conclusions. The CERN teams will of course play it both ways. On the one hand they will emphasize caution, on the other hand there will be talk of possibly the most important discovery in physics for the last 50 years. Experimentally a signal in the gam/gam channel is not good enough. The WW is about as sensitive in that region (slightly more by my calcs) and we should see an accompanying ‘hump’ in the WW with a best fit to 125GeV (just as we ‘saw’ at 140 GeV 4 months ago, when I was emphasizing the absence of an accompanying, but less significant, ‘hump’ in the gam/gam channel).

    Experimentally, I am sceptical of the Higgs being found in the last bastion left to it, where (by definition) the experiment is least sensitive. Theoretically, I find the whole idea of a single particle giving mass to everything (via an ad hoc potential) a bit to simplistic – a kind of a nirvana. Many are often quoted as saying they put forward the Higgs scenario only as a simple of how mass can be generated preserving renormalizability, and manifest symmetries. Reality is more complex.

  4. Take some pictures of the office. Can you actually move inside the office without tripping into any stack of preprints? Or I am thinking another one’s office?

  5. As a physicist, you are obviously doing this to satisfy yourself. But with your blog, you have helped everyone who cares for the ultimate truth. There is no “word” in any language can express our (at least, mine) appreciation for your kindness and a job well done. Thanks 14 trillion.

  6. Hi Matt,
    I think the necessary calculation has not been done to determine whether atoms would exist if the Standard Model existed in its current form except the Higgs didn’t get a VeV to break EW symmetry. Certainly, QCD would break electroweak symmetry at the confinement scale and give masses to the W,Z. In the case where the fermions remain massless, the world would look quite different (see Quigg and Shrock arXiv:0901.3958 [hep-ph]): nuclei may or may not exist in abundance, depending on the proton-neutron mass difference (not calculated), but with a massless electron, the Bohr radius would be huge, so the notion of atoms wouldn’t make much sense. But, Quigg and Shrock assumed there was nothing around to give masses to fermions. If the Higgs sector is still there, with all the Yukawa interactions, now fermions can get masses. I could imagine that hypothetical universe to be completely different from, but just as complicated as, our own real universe.

    1. George (George Fleming, research scientist at Yale and an expert in quantum field theory) — thanks for the message.

      Strictly speaking you are right, the situation is a bit ambiguous as I have stated it. This deserves a longer discussion than we can have here, but one thing is clear: if you leave the Higgs field in the theory, but with a value of zero and with a mass at the TeV scale (which is an arbitrary choice), the electron would indeed get a mass through quantum effects, but it would be extremely small, far too small to prevent protons from decaying to neutrons in that universe. So I think there could certainly not be anything like hydrogen.

      I am not sure which nuclei would be stable; to figure that out would require some very hard work to calculate the nuclear physics of this hypothetical world. It probably is beyond our technical abilities. And very few of these nuclei would form in the early universe.

      If instead we took the Higgs field out of the theory altogether, then the electron would be strictly massless and there could be no atoms of any size.

      Let’s discuss this again once tomorrow’s insanity is over.

      But I think we can all agree, without any subtleties, that a world in which the Higgs field had a value of zero would be completely unrecognizable, and that the Higgs field plays an absolutely essential role in making our world the way it is.

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