Scientific Method / Science & Exploration

Updates from the LHC and Tevatron keep the door open for the Higgs

A recent physics meeting saw the latest updates from the detectors at the LHC …

The CMS detector, with some puny humans shown for scale.

We're only about a week away from restarting the LHC, but the physics community is still analyzing data generated by earlier runs of that collider and the Tevatron. Their latest results were presented at a physics meeting in Italy this week. The biggest news is probably the fact that the full analysis of Tevatron data turns out to be consistent with the presence of the Higgs where early indications from the LHC say it's likely to be. The LHC data, however, shows a slightly reduced signal, and the two detectors are still placing it at slightly different masses.

The data is easiest to understand via pictures, and the meeting has conveniently posted slides from each of the presentations. The Tevatron's data is a nice summary, since it shows all the areas where previous experiments have excluded the presence of the Higgs in color. These leave a narrow window open in the area of 120GeV for the Higgs to hide.

Combined, the two detectors at the Tevatron now see a potential Higgs signal within the narrow region that hasn't been excluded by the detectors at the LHC.

The solid black curve shows the presence of any signal for the Higgs, while the dashed line shows the background of noise predicted by the standard model (green and yellow areas are one and two standard deviations from background). As this data shows, there's a possible Higgs signal over a broad area of masses in the Tevatron data. Some of these have been excluded by the LHC's detectors, but there's a clear signal within the region that's still available near 120GeV. That signal is about 2.2 sigma, however, which is not much better than the LHC detectors had this summer. It is a lot better than the Tevatron had done previously, as shown here:

With more data (top), a clearer signal has appeared in the data from the Tevatron's CDF detector.

Back when the CDF detector had only analyzed 8.2 inverse femtobarns, it didn't see much of a signal at all in the area. Now that it has added additional data, the strength of the signal has gone up. It's been further enhanced by combining the data from CDF with Fermi's other detector, Dzero.

Meanwhile, the LHC detectors have continued processing their data, with the clearest results coming from ATLAS. The ATLAS team has now added additional analyses, based on different ways that a Higgs boson could decay (each decay pathway is called a "channel). With the new work, the peak that had been reported last fall is still present, just beyond 125GeV, although the signal has gone down slightly.

The ATLAS detector still sees a clear peak centered just above 125GeV.

The other detector, CMS, also has additional data. Unfortunately, the data presentation differs from the format used by everyone else. Still, it should be clear that there's a potential signal just below 125GeV.

The CMS detector also sees a signal in this area, although it peaks just below 125GeV.

There's a small problem with that: ATLAS places the peak signal just above 125GeV. Adding the extra channels hasn't actually brought them any closer into alignment.

What to make of all of this? The signals remain weak (well below the standard of discovery for particle physics), and there's that annoying discrepancy between the signals seen in the two LHC detectors. But on the plus side, we have four different detectors, using a number of different potential mechanisms of Higgs decay, all more or less pointing in the same general direction.

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7 Reader Comments

  1. Thanks for the great write up! Excited to see what we hear this summer.
    63 posts | registered
  2. Any idea what the "ATLAS Exclusion" area beside the "LEP Exclusion" is supposed to indicate? Is that an overlapping zone peeking out from behind LEP? Or is that cross-hatching supposed to indicate something else entirely?

    And when are these guys going to use better colours? I think my eyes are bleeding.
    1630 posts | registered
  3. This just in: we haven't found anything but we might in the future! More at 11.

    Don't get me wrong, I liked your write-up, and the analysis is complicated so I don't expect miracles. I just WANT miracles, because it would be faster. What can I say? I'm impatient, because I want the future to hurry up and happen ;)
    50 posts | registered
  4. I understood very little of this, but I just was glad to have something to read that wasn't about the new ipad.
    53 posts | registered
  5. TK, keep in mind that they are looking for something that makes finding a needle in a haystack easy by comparison (Heck even the mythbusters can do that). To make a dreaded car analogy, it's like trying to calculate the gas mileage of a car by examining the wreckage of a 120mph head on collision, and all they get is a couple of pictures taken after it's happened.
    1926 posts | registered
  6. As I read this, I was playing Large Hadron Collider Rap on YouTube. I honestly think that this is the only song that should be played during serious scientific discussions.

    Props to Alpinekat.
    242 posts | registered
  7. while you guys wait... something much bigger than this is being constructed in India. The magnet of that machine alone weighs 50000 tonnes.. and it is going to be used for studying atmospheric neutrinos. ... -in-india/
    4 posts | registered

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