The “Bump”: The obvious blue line represents an “excess” of particles that deviate from the Standard Model (red line). Although it is far from conclusive, this excess could represent “new physics” — an unpredicted particle or new gauge boson carrying a new force of nature (from the publication “Invariant Mass Distribution of Jet Pairs Produced in Association
with a W boson in ppbar Collisions at sqrt(s) = 1.96 TeV“). Credit: CDF Collaboration.
If you’re a little hazy about the details of Wednesday’s buzz surrounding the potential discovery of “new physics” in Fermilab’s Tevatron particle accelerator, don’t worry, you’re not alone. This is a big week for particle physicists, and even they will be having many sleepless nights over the coming months trying to grasp what it all means.
That’s what happens when physicists come forward, with observational evidence, of what they believe represents something we’ve never seen before. Even bigger than that: something we never even expected to see.
In the quest to probe the very edge of our understanding of how the Universe works, massively powerful particle accelerators need to be built.
The more powerful the accelerator, the more energetic the collisions and the more rare the particles produced.
The Higgs boson for example, a particle predicted by the Standard Model, can only hope to be found by using the immense power of accelerators such as Europe’s Large Hadron Collider (LHC) or the U.S. Tevatron.
So what has the Tevatron discovered?
In a nutshell, after colliding protons and antiprotons in the Tevatron over and over again, a peculiar pattern started to emerge. After the proton-antiproton collisions took place, a flash of energy caused other particles to form. The vast majority of these post-collision particles were expected to appear, abiding by the theoretical framework of the Standard Model.
However, a very small number of particles were produced that appear to originate beyond the physics predicted by the Standard Model.
A rough analogy could be two freight trains colliding head-on. Out of the resulting fire and carnage, dozens of cars spontaneously form, spraying out from the wreckage.
In this fantasy collision, the trains are the protons and antiprotons, and the cars could represent post-collision particles detected in the Tevatron, most of which are common and expected to be created (Fords, Chevvys, Hondas). However, a very small number are not predicted and are considered “exotic” (a Ferrari here, a Lamborghini there). Suddenly, we’re very interested in the exotic cars.
The CDF detector at Fermilab’s Tevatron (Fermilab).
Back in the real world, the energies of particles produced after the collisions were measured by physicists from the CDF (Collider Detector at Fermilab) Collaboration and they noticed a “bump” in their data (pictured top, blue line).
Basically, this bump — or “excess” as CDF project physicist Viviana Cavaliere described in her live Fermilab announcement on Wednesday — is an anomaly that doesn’t “fit” with the Standard Model (the Standard Model particles are depicted by the red line in the picture).
Physicists tend to get excited when they see these kinds of bumps in the energy profiles of collision products. It could mean that a new type of particle has been discovered.
However, many, many collisions are needed before a “bump” like this is produced. Of the countless billions of predicted particles that are generated after particle accelerator collisions, only a very tiny proportion may be considered “exotic” (or not predicted by the Standard Model).
It’s like a camera taking a picture of a very faint, distant object. For that object to come into focus, longer exposure times are needed. In the case of the Tevatron collisions, many collisions over many months (or even years) are needed before this “bump” appears from the background.
If this bump is real, what exactly is it?
“They are in fact two ‘jets’ of particles, which when you add their momenta together give you a ‘mass peak’ [the "bump"] suggesting they might have come from a new (unknown) particle decaying,” Jon Butterworth, an LHC physicist working with the ATLAS detector and physics writer for the UK’s Guardian.co.uk, told Discovery News.
Although the excitement surrounding this “unknown” particle finding is tangible, Butterworth is keen to emphasize that this “bump” might not be real. “It could certainly still be an anomaly. It’s on the edge,” he said. “Statistically the significance isn’t great, and there are substantial systematics (such as the backgrounds) which could undermine the significance further. I think they’ve done a decent job, and it is interesting but rather far from compelling.”
It is what is known as a “three-sigma event,” and this refers to the statistical certainty of a given result. In this case, this result has a 99.7 percent chance of being correct (and a 0.3 percent chance of being wrong). “Three-sigma isn’t seen as a ‘discovery,’” Butterworth said in a previous Discovery News article. “Really, a ‘five-sigma’ is classed as a discovery. Five-sigma is the ‘Gold Standard.’”
So this Tevatron finding is an interesting result, but it needs further work to be confirmed.
If it is confirmed to be real, Butterworth said that the only possible Standard Model candidate is the Higgs boson. But that is unlikely, he said, because other particles predicted to occur from a decaying Higgs aren’t present. Therefore, it’s unlikely this is a Higgs signal.
“So if not that, it has to be something pretty weird and unexpected,” Butterworth continued. “But unless and until confirmed by more data and other experiments, my money is on combination of statistical fluctuations and systematic error in the background estimation and jet measurement, and it will (sadly) fade away.”
“Hope I’m wrong.”
Publication: “Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in ppbar Collisions at sqrt(s) = 1.96 TeV,” CDF Collaboration, 2011. arXiv:1104.0699v1 [hep-ex]
