Last weekend, I had the pleasure to be shift leader for ATLAS. It was a real pleasure for many reasons: being right in the middle of the action, surrounded by an international team of enthusiastic and dedicated people, and taking part in great teamwork. The shift crew (about ten people plus dozens of experts on call) must keep the detector running smoothly, tackling every problem, big or small, as fast as possible.
Data was coming in at a high rate and all sub-detectors were humming nicely. Not a glitch in hours so we were getting slightly sleepy nearing the end of the shift around 22:00. So when a colleague from the trigger system (the system that decides which events are worth keeping) called to inquire about recurrent splashes of data, I was rather puzzled.
I quickly went around, asking a few shifters to check their system. Nobody had a clue. Then I took a closer look at this plot that I had not scrutinized before since everything was so seamless.
The two lower curves in beige and green show the instantaneous luminosity measured by the two largest detectors operating on the Large Hadron Collider (LHC), CMS andATLAS. This is a measure of how many collisions are happening per second in each experiment from the two beams of protons circulating in opposite direction in the LHC tunnel. If you look closely at these curves, they both have small dips at regular intervals. Since both ATLAS and CMS were registering these dips, it had to be coming from a common source, the LHC.
So I called the LHC control room to find out what was happening. “Oh, those dips?”, casually answered the operator on shift. “That’s because the moon is nearly full and I periodically have to adjust the proton beam orbits.”
This effect has been known since the LEP days, the Large Electron Positron collider, the LHC predecessor. The LHC reuses the same circular tunnel as LEP. Twenty some years ago, it then came as a surprise that, given the 27 km circumference of the accelerator, the gravitational force exerted by the moon on one side is not the same as the one felt at the opposite side, creating a small distortion of the tunnel. Since the moon’s effect is very small, only large bodies like oceans feel its effect in the form of tides. But the LHC is such a sensitive apparatus, it can detect the minute deformations created by the small differences in the gravitational force across its diameter. The effect is of course largest when the moon is full.
What came as a surprise to me was to witness the dynamic aspect of it. As the moon was rising in the sky, the force it exerted changed ever so slightly, but even these infinitesimal changes were big enough to require a periodic correction of the orbit of the proton beams in the accelerator to adapt to a deformed tunnel.
Other surprising disturbances were also observed in the LEP days like one that appeared every day at fixed times. It took months and a train company strike to figure it out. These perturbations were created by the passage of the fast train linking Geneva to Paris, the TGV, since it releases a lot of electrical energy into the ground.
The LHC is also sensitive to the hydrostatic pressure created by the water level in nearby Lake Geneva that also deforms the tunnel shape.
So next time you want to know if the moon is full, just check the luminosity plot from the LHC to see if you can spot those small glitches caused by the operator correcting the beam orbit. (The header will have to read “Proton physics: stable beams” to see these curves).
Read more: www.quantumdiaries.org