Closing in on the Higgs Boson

By John Conway | March 13, 2009 9:00 pm

Lots of particle physics news from the Tevatron the past two weeks, including:

At some level the first two have a bearing on the last one, which has started to get some attention in the media, including a nice interview with Prof. Heidi Schellman from Northwestern today on Science Friday, and a piece in Scientific American to which I contributed quotes.

The observation of the production of single top quarks (rather than the easier-to-see production of top-antitop quark pairs) has been a goal of the Tevatron experiments for years. The success of this analysis demonstrates that extremely complex dissections of the data like this can be undertaken, and reveal faint signals like that of single top. The search for the Higgs boson is more difficult still.

But is the race for the Higgs boson heating up? Is there a race at all? Can the Tevatron see it before the LHC, given that the LHC has been delayed a year due to the quench incident last September?

It all hinges on the mass of the Higgs boson. In different mass ranges it decays to different final states, changing the experimental approach and changing the sensitivity of the Tevatron and LHC experiments. The new measurement of the W mass by D0 adds a bit more knowledge about what the Higgs boson mass might be, since it depends on the mass of the W boson (the carrier of the weak force, which governs nuclear beta decay) and the mass of the top quark directly.


The plot at left demonstrates where we are right now; click on it for an annotated version. I call it the “billion dollar plot” but it probably cost a lot more than that to produce, because it shows the results of decades of experimentation at the Tevatron at Fermilab, LEP at CERN, the SLC at SLAC, and other measurements. As you can see (I hope) we are in a very interesting situation: the world’s data seem to indicate that the best Higgs boson mass is deep into the territory already excluded by LEP 2 in 2000! They set a 95% confidence bound on the Higgs mass at about 114 GeV. Their limit does not extend very far beyond that mass at all; it was limited by the energy of the LEP accelerator. The mass range above 114 GeV is open experimentally, but if you take the W mass/top mass constraint seriously (in the context of the Standard Model you must) then it would certainly appear very likely that the Higgs must lie in the range 114-185 GeV, with a strong preference for the lower end.

The new Tevatron result takes a new bite out of the upper end of the range, excluding from 160-170 GeV the “sweet spot” where the Higgs can decay to two W bosons. This is in some sense “first blood” for the Tevatron: at last the two experiments can exclude a Standard Model Higgs boson somewhere it hasn’t already been excluded!

But, to my mind, the interesting end of the range is at the low end. The data favor it, and theory favors it in the sense that if nature is more complicated, and supersymmetry is manifest, then one would expect that the light Higgs boson in supersymmetry exists in the range 120-130 GeV or so. In this picture there would be heavier Higgs bosons lying in wait for either the LHC or the Tevatron, though the LHC has the edge here with higher energy.

Assuming that the improvements in the analyses continue to outpace the data, as shown in the plot below, it is not impossible that the Tevatron could start to extend the region excluded by LEP, by this summer. But discovery? A three sigma result is possible with a good deal more data, but a five-sigma discovery looks very hard, and always has.


For a gold-plated, five-sigma-significance discovery, my money is on the LHC, I have to say. But the LHC will initially see the Higgs boson decaying to two photons, and we really need to see it decaying to two quarks or two leptons to really know its nature. I think the Tevatron could do that before the LHC, measuring the decay of the Higgs boson to two b quarks, and that alone is reason enough to keep the machine running until it does, to my mind, provided there are sufficient personnel to run the detectors and analyze the data. That decision, though, is above my pay grade…

The LHC will likely be able to see the Higgs boson decaying to two tau leptons before the Tevatron can see it decaying to two b quarks. Is that a race? I view the two observations as complementary, and they both add to the scientific picture. Without the Tevatron, it will just take longer.

One last word…the Higgs boson is damned hard to see, and when the LHC turns on, a ton of other new physics may pour out first. It is a very interesting year, that’s for sure!

  • JoAnne

    Thanks for this post. Viva la Tevatron!

  • Excited State

    Can someone explain what is meant by a “single” top quark? It is my understanding that quarks never appear alone. Does this just mean they found a top that wasn’t with an anti-top?

  • Adam A

    @Excited State:

    When people talk about single top quarks, they mean, “not pair produced”. The type we normally see is a top quark produced in conjunction with an anti-quark. With single quarks, this doesn’t happen. On the idea that quarks never appear alone, that’s basically correct, though top quarks decay far too quickly for that to matter. They decay almost immediately to a W and b quark, and the b quark hadronizes (pulls some quark buddies out of the vaccum to form composite particles).

  • Adam A

    Also, I suspect there will be at least some personnel and money to run the Tevatron for a little longer. I think the delay of the LHC has changed the calculations a bit. Plus, I think there’s a lot of arguments that the Tevatron has a lot of cool results building. We’re starting to reach that data level and processing sophistication that we can see more subtle effects. And there’s certainly an argument to be made that even after the LHC turns on, it’s going to take them a while to do the delicate sorts of analyses that might be necessary – plus a through understanding of how to measure missing energy, photons and taus could take quite a while. That took a long time at the Tevatron, and the knowledge isn’t entirely transferable to the new detectors and unfamiliar beam environments.

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  • Sili

    Could you possibly post an annotated version the billionMilliard Dollar Plot? I’m too dense to read it.

    Is it conceivable that the public could help with analysis of these data in the same way that they (/we) are now doing with the GalaxyZoo?

  • Odani of the Wabe

    Damn, will somebody just publish my paper? I FOUND the Higgs boson! It was under the center cushion of my couch! I’ve got it here in an otherwise empty Schilling coarse-ground pepper bottle; I’ve also got pictures, a signed confession which implicates other so-called particles, and certified statements by Jim Cramer and Bernie Madoff that all this is true. What more do you scientists want? I’ve even advertised on!

  • John

    Sili, I have updated the plot with an annotated version. Thanks for the suggestion!

  • Lab Lemming

    If there is no Higgs, can Tevatron exclude it from all reasonable mass values before the LHC, or can it not reach some of them?

  • Sili

    Thank you. That helps enormously (i.e. I think I get it). I’d completely missed the faint dark band the first time around, too.

  • Ilya

    Could somebody explain what is meant by a five sigma discovery? OK, every student knows eg if data has to fit a theoretical straight line then if line passes through 68% (0ne sigma) of points, it’s a good fit. But what is meant by a five sigma?

  • Adam A

    @Ilya: If you have a theoretical distribution and a measured distribution, and the measured distribution deviates from the theoretical distribution by more than 5 times the calculated uncertainty, that’s a five sigma result. That basically means that there is only a 0.2% chance that such a deviation occurred due to statistical fluctuations.

    The statistics can get a bit more complicated than straight forward error bars, but that last line is basic idea.

  • Brian

    Thank you for bringing the various threads – LEP, MSSM, SM, Tevatron results and expected capacity – together and explaining their interrelationships.

  • graviton383

    I hear that if the Tevatron runs in FY’11 & no Higgs signals are seen th entire Standard Model allowed range will be excluded.

  • Stephen Serjeant

    I wonder what people would consider to be the bigger earthquake – non-detection of the Higgs boson or non-detection of gravitational waves?

  • Low Math, Meekly Interacting

    Thanks for the annotated plot!

  • Tracy Thomas

    Thanks for the nice summary – this is great for those of us sitting on the sidelines watching what’s going on!

  • FB

    I think Higgs boson cannot exist. Because its existence would contradict Relativity (bending of space-time due to mass). Just a thought. :-)

  • NewEnlandBob

    Stephen Serjeant, I think that non-detection of either the Higgs boson or gravitational waves just means that either the collisions are not energetic enough (Higgs) or that the equipment is not sensitive enough.

    FB, the existence of the Higgs boson doe NOT contradict general relativity in any way. On the contrary, it might show how mass is generated therefore causing the ability of mass to distort spacetime.

  • Paul

    NewEnlandBob, I agree that the Higgs doesn’t contradict general relativity. However, it’s mass-energy that causes spacetime to curve, not just rest energy. So I don’t see how discovering how mass (ie. rest energy) is generated would have any bearing on how spacetime distortion works.

  • Stephen Serjeant

    I just think it could be fun to whimsically pit one big science project against another in this way. There are lots of projects to choose from on the astronomy side (Square Kilometre Array, Planck, LIGO/LISA, Euclid/JDEM, CODEX), using lots of different techniques (CMB polarisation, cosmic shear, baryon wiggles – what a wonderful term “baryon wiggles” is by the way! Almost as good as “violent relaxation”.) I think any one of these could provide surprising fundamental physics insights. But I think non-detection of gravitational waves in LIGO/LISA would be a particularly big surprise to many – and who knows where that would leave the Inflation tests in the CMB using primordial gravitational waves, aka tensor modes.

    NewEnglandBob: in fact, gravitational waves are already providing astrophysically interesting limits. With solar neutrinos, there came a point when the astronomers started insisting that the fundamental physics needed looking at more carefully, while the fundamental physicists maintained that the astrophysics needed more scrutiny. Perhaps something similar could happen in attempts to detect gravitational waves directly? But I’ve less idea of how things would play out if the Higgs eludes detection.

    I’m loath to guess where the surprises might come though. I was pretty certain there’d be no cosmological constant, despite a certain 1992 Annual Reviews article, because it would mean the early Universe was fine-tuned. Luckily I didn’t put money on it…

  • Cormac O’ Raifeartaigh

    Superb summary, thanks. Will show it to my students in clas tomorrow. Best, Cormac

  • Phillip Helbig

    “I wonder what people would consider to be the bigger earthquake – non-detection of the Higgs boson or non-detection of gravitational waves?”

    It depends on the cause of the non-detection. If, as I assume you mean, there is not a
    detection where theory clearly indicates there should be, then of course the non-detection
    of gravitational waves would be the MUCH bigger earthquake. We have ABSOLUTELY NO
    REASON to believe the GR is not perfectly correct in the non-quantum regime. Thus, not
    finding gravitational waves would be a BIG surprise. The Higgs, on the other hand,
    is on much shakier ground. There isn’t even a clear prediction of the mass, and we are
    all sure that if it is not found where expected, then the next theory—which will sound
    just as plausible—will easily account for this.

  • Winter Solstice Man

    I cannot help but imagine what the teaming masses would make of this news, expect to complain why those science geeks are focusing on some little things that don’t matter and why aren’t the spending their time and money on fixing the economy so we can have even bigger TVs to watch American Idol and the Bachelor with?!

  • Stephen Serjeant

    Phillip: maybe a fairer comparison would be Higgs non-detection vs. Inflation tests via scalar/tensor CMB modes. I suspect you’re right about the likelihood of directly detecting gravitational waves, though there may be MOND fans or Pioneer anomaly pundits (I’m neither) who might take issue with your GR position.

    By the way I like the anti-spam mangling of your email address on your web page. I once asked someone to quote my email address with the word “ecoli” inserted, with a note “please remove the anti-spam germ for a faster response”. This elicited the comment “astrobiology gone mad”. Hmph!

  • Count Iblis

    Waiting for Dogot

    Everyone is eagerly awaiting the discovery of the Kibble boson (known colloquially as “The Dog Particle”), which was missed at the Fermi National Labrador. There have been speculations that it is too light to observe with available accelerators because it is really a gallstone boson, but such claims are unfunded.


    Why do they have to delay the LHC restart up so much, anyway?

  • isospin

    I am not quite understand the news about ‘single top quarks’. When you said “not pair produced”, how it preserves the color confinement?

  • isospin

    # QUASAR Says:
    March 17th, 2009 at 11:32 am

    Why do they have to delay the LHC restart up so much, anyway?

    They have to cool the magnet system again to 1.9K, which will take several months. By the way, it seems they also want to check whether there are anything else wrong.

  • Cormac O Raifeartaigh

    isospin: I dislike the expression too. ‘Single top quark’ production means production of top quark with a quark which is not anti-top -a reaction that is expected fro SM, but seen for the first time now

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