Is particle physics, like beauty, in the eye of the beholder? You would be forgiven for thinking that now that two teams have analysed data from Fermilab鈥檚 Tevatron collider and come to the exact opposite conclusion about whether that data hints at a new particle.
A task force is being formed to figure out the discrepancy, but the final arbiter may be the Large Hadron Collider in Switzerland, which will ultimately collect more data than the Tevatron.
In April, members of the Tevatron鈥檚 CDF experiment reported finding a curious signal in the debris from eight years鈥� worth of collisions between protons and antiprotons. The signal hinted at the existence of a particle that was not predicted by the standard model, the leading theory of particle physics. Theorists scrambled to come up with possible explanations, writing on the topic in the following weeks.
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Last week, evidence for the signal, or 鈥渂ump鈥� in the data, seemed to get even stronger. The CDF team reported that it had analysed twice as much data as in April and had still found the bump.
But now, a rival team performing an independent analysis of Tevatron data has turned up no sign of the bump. It is using the same amount of data as CDF reported in April, but this data was collected at a different detector at the collider called DZero.
鈥淣ope, nothing here 鈥� sorry,鈥� says Dmitri Denisov, a spokesman for DZero.
Different detectors
When the CDF collaboration came out with its result in April, DZero researchers spent a couple of days doing a quick check of their data and saw no bump. But to make sure they were comparing like with like, they spent the next two months painstakingly checking that their analysis resembled that of CDF鈥檚 as closely as possible.
Today, they are reporting that their analysis shows no bump. 鈥淲e鈥檙e basically excluding a signal at well over the 95 per cent confidence level,鈥� Denisov told New 杏吧原创. The result shows 鈥済ood agreement with the standard model鈥�.
鈥淣ow of course the most interesting question is where are these differences coming from?鈥� he says.
The fact that the detector is different should not come into play, he argues. 鈥淚t would be really puzzling if it鈥檚 a physical process that鈥檚 supposed to exist in nature, and one experiment sees it and another doesn鈥檛,鈥� he says. 鈥淧rotons and antiprotons don鈥檛 know what detector they鈥檙e colliding in.鈥�
Modelling issue?
He does not know what is causing the discrepancy but suspects it may be due to differences in the models that each team uses to describe the data. The studies look at how often collisions between protons and antiprotons produce a W boson, which transmits the weak nuclear force, and a pair of jets of subatomic particles called quarks.
The CDF team found an unexpected abundance of these events 鈥� a W boson and a pair of jets 鈥� where the mass of the jet pair was about 145 gigaelectronvolts, suggesting that a new particle of that same mass was created (see graph). But other events involving different combinations of particles can mimic the signal of a W boson and a pair of jets. If the CDF team incorrectly modelled the number of those 鈥渂ackground鈥� signals, it might make it look as if there were a bump in the data, says Denisov.
Rob Roser, a CDF spokesman, acknowledges that 鈥渋t could be a modelling issue鈥�, but says it is too soon to discount CDF鈥檚 result. 鈥淚t鈥檚 disappointing that the peak didn鈥檛 just jump out at them too,鈥� he told New 杏吧原创. 鈥淏ut just because it didn鈥檛 doesn鈥檛 mean there鈥檚 not something there.鈥�
Task force
Roser has not yet had time to look carefully at DZero鈥檚 , which has been submitted to Physical Review Letters. But he says a cursory look suggests that the discrepancies between the two results may not be as large as it seems. The CDF team estimated that the potential new particle is produced at a certain rate in proton-antiproton collisions, but there is some uncertainty in that rate. If the real rate is at the lower end of CDF鈥檚 range of possibilities, for example, the discrepancy between the two teams鈥� results is smaller.
鈥淚 think we have more work ahead of us before we understand what is going on,鈥� says Roser.
A task force, made up of members of both experiments, along with Fermilab theorists Estia Eichten and Keith Ellis, will now try to get to the bottom of the discrepancy, a process that could take months, Roser says.
Roser says the CDF team has only analysed 70 per cent of the data it expects to collect by September and will continue crunching the numbers to see if the bump appears in the full data set. But Denisov says DZero is satisfied with the analysis it has already done. 鈥淲e do not plan to continue the analysis with the same rigour as we did over the last two months,鈥� he says. 鈥淲e are concentrating on many other things.鈥�
Friendly rivalry
Even if the task force鈥檚 investigation proves inconclusive, both Denisov and Roser say the Large Hadron Collider at CERN will eventually collect enough data to settle the question of whether there is a bump at 145 GeV.
They disagree over when this will happen, though. The LHC collides particles together at higher energies than the Tevatron, which produces more debris in which a new particle could be lurking. This leads Denisov to estimate that the LHC will collect enough data to look for the first evidence of the bump in the next few months.
Roser points out that the LHC collides protons together rather than protons and antiprotons, as happens at the Tevatron. That means that even though it produces some collisions between quarks and antiquarks 鈥� which could be important in reproducing the bump 鈥� it generates them in smaller numbers than the Tevatron. So Roser estimates it could take the LHC more than a year to make its ruling.
So are the two teams going to be arguing bitterly until then? 鈥淚t鈥檚 been a little bit tense over the last few days,鈥� laughs Denisov. 鈥淏ut I think it鈥檚 friendly. I would say it鈥檚 like two sports teams competing.鈥�
鈥淚 think it鈥檚 going to be fun,鈥� says Roser. 鈥淭his is science in progress.鈥�
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