Reconstruction of a collision in the CMS detector.
Late last year, the people behind the Large Hadron Collider announced that they may have found signs of a new particle. Their evidence came from an analysis of the first high-energy data obtained after the LHC’s two universal detectors underwent an extensive upgrade. While the possible new particle didn’t produce a signal that was statistically significant, it did show up in both detectors, raising hopes that the LHC was finally moving towards a new physics.
This week, that hope was officially dashed. Physicists used a conference to release their analysis of the deluge of data that came out of this year’s run. According to their data, the area of the apparent signal is filled with nothing but statistical noise.
The search for new particles in data from the LHC starts with a calculation of the kinds of things we would expect at a given energy. The Standard Model, which describes particles and forces, can be used to make predictions about the frequency with which specific particles emerge from collisions, and into which those particles will decay. For example, the Standard Model might state that two electrons should appear in five percent of the collisions that occur at a specific energy. Searching for new particles means searching for deviations from those predictions.
In some cases, those searches are partly motivated by theoretical considerations. That’s what happened in December. A specific hypothesis about gravitons (hypothetical force carriers for gravity) suggests that they could decay through a process that produces two high-energy photons. The teams behind the ATLAS and CMS detectors searched their data for collisions that produced two photons, then compared their numbers to predictions of what they should see based on the Standard Model.
That analysis yielded an apparent excess at an energy of 750 Giga-electron Volts. While the excess was visible in the data from both detectors, it didn’t reach levels significant enough to call it a discovery. Under these circumstances, physicists can only wait for more data. Fortunately, the LHC is delivering data at a phenomenal rate.
The entire 2015 energetic run yielded just under four inverse-femtobarns (don’t ask, it’s just what physicists use) worth of data. Now, with only a portion of the data from this year’s run, they were able to add another 12.9 inverse femtobarns to the analysis. And with this new data, the bump in the earlier analysis disappeared. “No significant overrun is observed over the predictions of the Standard Model,” said a summary of the CMS detector team.
As a result of this analysis, the LHC team is able to put hard limits on the possible masses of this type of graviton (they probably don’t exist below four Tera-electronVolts). This doesn’t mean gravitons don’t exist – there are other theoretical types – just that we can’t find evidence for these specific gravitons.
The same goes for new particles in general. The LHC researchers only have a limited amount of computer time to analyze all the incoming data, so they focus on things that are likely to be of interest. Clearly, a hint of a new particle fits that description. But there are plenty of other analyzes that won’t happen until sometime after the current data collection ends in the fall.
Still, it’s hard not to be disappointed that the most promising hint of a new particle (since the Higgs) has turned out to be a bust.