Earlier this week Discovery News reported that Fermilab's CDF collaboration had spotted a tiny "bump" in the data from Fermilab's CDF collaboration that seemed to indicate the presence of a new particle -- and instead of disappearing with subsequent analysis, the signal grew stronger, just shy of the threshold required to announce a bona fide discovery.
ANALYSIS: Fermilab's 'Bump Hunters' See Hints of New Particle
Now, the hotly anticipated results from its sister detector, DZero, have been announced. Alas, the news isn't what the CDF scientists were hoping for: DZero sees no evidence of the signal in their own data analysis of more than 200 trillion particle collisions.
That sound you hear is the collective sigh as particle physicists around the world swallow their disappointment and resign themselves to months of sifting through a comparative analysis of the two data sets, trying to figure out why the two experiments don't agree.
"The result is not good for the CDF," DZero spokesman Dmitri Denisov told Nature. "We are not confirming the signal. We just see nothing."
Signature Searching
The CDF data in question come from routine background analysis of the production of gauge boson pairs (force-carrying particles), part of the ongoing search for the Higgs -- although the suspected new particle is most definitely not the Higgs (regardless of what other news outlets might tell you), and more likely to be a new kind of boson pair not predicted by the Standard Model.
SEE ALSO: What Is The Higgs Boson?
It's important to run these background analyses, because the better physicists understand those backgrounds, the more precisely they can pick out potential Higgs signatures from the sea of data. And sometimes those background analyses can yield surprising results -- or, as in this case, contradictory ones.
Check out the relevant graph below. That big peak shown in red is exactly what physicists expect to see, in keeping with the predictions of the Standard Model. The CDF version of this graph showed an unexpected second peak that they took to be evidence of a possible new particle not predicted by theoretical models. You can see the outline below of where it should be -- and it's pretty clear that the data points from DZero don't match at all in that portion of the spectrum.
Two of a Kind
It's extremely rare for the two sister collaborations to disagree in their findings, although it's not unprecedented. According to Fermilab Today, over the last decade CDF and DZero have agreed on their results more than 99% of the time, spanning over 500 different ultra-precise subatomic measurements. They are specifically designed to to check each other's results.
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Both DZero and CDF are part of Fermilab's Tevatron collider, but the experiments are located at different points on the accelerator ring, and track different collisions, although the same physical processes occur at both locations.
The detectors are somewhat different in design, and in the selection criteria used to analyze events. So there will be differences in how they respond to a specific particle, like an electron or muon. But both collaborations look at the same types of collisions and measure the same quantities, and both are looking for the elusive Higgs boson.
"They gather their data and do their analyses completely independently, and make slightly different choices about how to select which data to record and analyze," Ann Nelson, a particle physics theorist at the University of Washington, told Discovery News. That said, "The data are from the same underlying microphysics and from collisions of the same beams at the same energy, so the conclusions drawn should be the same if both [teams] are doing things correctly."
It is certainly possible for CDF to see something that DZero has missed, according to Gordon Watts, a physicist with DZero. "When we first heard about [the CDF result], one worry was that our different approach had led us to miss something that CDF was seeing," he told Discovery News. "This turned out not to be the case, but sometimes a different way of looking at the data can lead to a discovery your competitors failed to make."
ANALYSIS: What Has the Tevatron Really Discovered?
Where's the Error?
So why aren't DZero and CDF agreeing in this case? The most obvious explanation is that the CDF physicists made a subtle error in their modeling of the background events. Nelson emphasizes that scientists on both teams are being as meticulous as possible in their analyses. "However, the 'bump' [in question] is a small excess on top of a large background, so a slight mistake in the background measurement or modeling could be to blame."
Watts agrees with Nelson that the discrepancy is likely to be found in the background modeling. "Since we aren't looking for a particular signal, it all comes down to how you model the detector and the backgrounds and so forth," he says, acknowledging that it's possible the error lies not with CDF but with DZero. "In DZero, perhaps we grossly over-estimated our errors and that has hidden the signal. However, there are other physics results that we have published that make that all but impossible."
Really, the only certainty at this point is that one of the two collaborations will eventually prove to be wrong. "Here are two results, both carefully vetted by teams of more than 500 physicists, and there is an obvious disagreement," says Watts. "In six months to a year, either CDF or DZero will be wearing some egg on their face."
Fermilab will set up a task force to perform a comparative analysis of the results from these two experiments in agonizing detail, plot by plot, in hopes of understanding where the discrepancies lie. Fermilab Director Pier Oddone announced that this task force will include scientists from both CDF and DZero, as well as two Fermilab theorists. While the Tevatron will shut down operations in September, there is more than sufficient data to resolve the issue, he said.
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As for the LHC...
Meanwhile, physicists at the Large Hadron Collider in Switzerland are analyzing their own data for signs of this mysterious potential particle. Thus far, there is no signal, although Guido Tonelli, a physicist with CERN's Compact Muon Solenoid (CMS) collaboration, told Nature that there is still too little data available for analysis to make a definitive conclusion. Early results from the LHC will be reported at the end of July during a high energy physics conference in Grenoble, France.
"Trust me, almost everyone, at some level, hopes that this is real," says Watts. "But we have to let the data speak, and so far, it is far from conclusive."
And what if the LHC sees the same signal as CDF? "We'll have a huge party!" Watts jokes. "Seriously, if they do see it, then all the papers proposing different models will be scoured for their distinguishing features, and all of us experimenters will run off to try to compare them with data. Very little sleep will be had. It will be a hell of a lot of fun."
Tags: Laboratories, Particle Physics, Particles
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