On Monday, April 4th, the CDF collaboration from Fermilab released a new paper, which they have just now updated with stronger evidence, announcing they are seeing “something”. Read “something” as in unexpected, unusual, beyond anything known. Exactly what every experimental physicist hopes for! This news made headlines and spread through the High Energy Physics community faster than gossip. This is very exciting, and if it turns out to be real, it’d be the biggest discovery in many decades!
CDF found an excess of events when looking for some rare di-boson pairs, namely a W boson being produced in association with either another W boson or a Z boson. This second boson is seen through its debris, two jets of particles emerging when it disintegrates. Such di-boson events are rare but are predicted by the Standard Model, the theory that pretty much describes everything we have observed in this field so far. These rare but standard events are found amid so-called “background events”, more abundant, run-of-the-mill type of events that mimic the more unusual ones we are trying to study. It’s like looking at a pile of sand: in our case, we know roughly 95% of it is just sand, while 5% comes from some rare metal sprinkled on top of it. What CDF observes is that in addition to the rare metal, they might also see some unexpected gold dust. But as every one knows, all that glitters is not gold… The whole question is then: Is this real or just some fluke?
The key word here is that “roughly” 95% is background. To estimate the exact amount of rare events, you must know quite precisely not only how much background there is underneath, but also the specific characteristics of these events. Any small fluctuations in the background prediction and you would artificially create signs from a new particle. But there is another way to create an excess: even when we know precisely the amount of background to expect, this number is not fixed, but just what one should see on average. This can vary within known limits, but occasionally more than expected. That’s what we call a “statistical fluctuation”. But everybody in our field knows that, including the hundreds of experimentalists from CDF. So if they put this out in the open, they have very good reasons to believe it could be true. By going public about this, they are in fact inviting all the other experiments to search their own data.
Look at the CDF figure shown above where we can see the distribution of the combined mass of the two jets in these events. Going back to our analogy, the background is shown here on the top left plot in green, white, blue and grey, and represents the bulk of regular sand, all processes that are well known. The di-boson events shown in red would be the rare metal CDF was looking for. There, the two jets come from either a W or a Z boson. That’s why we can see a broad peak around 80 and 91 GeV/c2, the respective masses of W and Z bosons. Of course, due to some inaccuracies in the mass reconstruction, they don’t all come out at these exact values but instead form a broad peak in that area. That’s all typical.
What’s interesting here is what happens when you subtract all the background in the left top plot to obtain the right top plot. In principle, you should just be left with the events highlighted by the red line on the top right plot, but as you can see, there seems to be another peak at a higher mass, hinting at some unknown and completely unexpected new particle. This is even made more visible if you suppose those two jets are the debris from some particle with a mass of about 140 GeV/c2 which is shown by the blue line on the bottom plots, especially on the right after background subtraction. The vertical black lines indicate the level of uncertainty on each point after the subtraction. This means that even taking into account the known inaccuracies in the measurement, the second, blue peak has less than 1% chance of being due to a statistical fluctuation.
Coming back here at the LHC, you can be sure that when CDF released its paper in April, we all frantically scrutinized our own data, looking for this potential new particle. As it turned out, the ATLAS collaboration had already looked for di-boson events and even had very similar plots. So within a day, a small group of people gathered to share what they already had. A few more days, and they could reproduce exactly what the CDF Collaboration had done.
The same happened at the CMS collaboration. Again, lots of febrile activity, short-notice meetings, frantic exchanges of e-mails with various plots attached.
But so far, nothing! That does not mean it is not there. It can mean two things: either we are not sensitive enough yet or simply that this is not a real effect. We need more data to be able to make a definitive statement. And this new data is likely to be ready in time for the upcoming Summer conferences.
As can be seen from the figure released by ATLAS using the same selection criteria as CDF, we did not see a bump anywhere! The dots, the actual data collected in 2010, and the sum of all contributions from known sources represented by the black curve, are all within the experimental uncertainty shown by the hashed area. It will get more interesting once we add all of 2011 data since we already have ten times more data undergoing various checks and calibration.
The other Tevatron experiment, D0 is best positioned right now for an independent crosscheck and should release their findings very soon. If this new particle exists, they will see it and confirm it in the process. But until another experiment can provide this independent crosscheck, we will all be left pondering. D0 at Fermilab and CMS and ATLAS here at CERN need to either confirm or dismiss this claim, Until then, there is no way to tell if this is real or just one of those darned statistical fluctuations we are constantly battling… So stay tuned for the summer conferences where we are sure to hear more about this.
Pauline Gagnon
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