This morning the Large Hadron Collider set yet another record as it injected 13 proton bunches in each beam to reach a luminosity of 2 x 1029 cm-2 s-1, about three times previous records. The next step will probably be to increase the intensity of the bunches from 20 billion protons in each bunch up to the nominal design limit of 120 billion.
With such high bunch intensities the beam controllers must take extra care. If the beam becomes unstable the protons can go astray and plough into the sensitive particle detectors causing damage to the instruments. If instabilities grow the beams must be dumped quickly before all control is lost. This is why the gradual increase in luminosities has been a slow process.
Since they started up the LHC at the end of last year the physicists have been dogged by a mysterious source of interference christened “The Hump”. Some unknown vibration is causing an extra vertical oscillation in the beams. The movement is very tiny, just microns in magnitude, but when its frequency hits the tune frequency of the beam it resonates and the protons start to spread out from the beam vertically, causing beam losses. So far this has been just a nuisance decreasing the lifetimes of the beams and sometimes triggering an emergency dump, but as beam intensities increase it becomes a bigger threat and could lead to damage of the accelerator.
Over the past few months the engineers have been trying to trace the source of the interference, but without success. Accelerators are very sensitive to any kind of movement. The previous accelerator at CERN, known as LEP was affected by the tidal pull of the moon and by the passing of France’s high-speed train, the TGV. Recently the Tevatron in the US detected the devastating Earthquake in Haiti thousands of miles away when it caused a small wobble in the beams.
But The Hump is different. It is a high frequency vibration which drifts in frequency between about 3.1 and 3.8 KHz, sliding up and down the spectrum repeatedly over a period of 7 to 10 minutes. It could either be a sonic vibration or a low-frequency radio wave. It is unpredictable, being sometimes there and sometimes not. The CERN engineers have looked for sources of vibration from equipment in the accelerator such as vacuum pumps or the cryogenics, but nothing matches the observed interference and now they are stumped.
At this point they must start to consider a more sinister possibility. Could The Hump be a deliberate plot to sabotage the LHC? There are plenty of people who would be motivated to attempt such an act. Since its inception the collider has been a source of controversy because a few fringe physicists have suggested that it could create black holes of other unnatural entities that could grow in size to swallow up the Earth. Scientists at CERN have dispelled such theories, pointing out that anything created will fly off at nearly the speed of light and will not be captured by the Earth. Black holes would decay just like any other particle and could not be dangerous, even in the unlikely event that they are created. It has also been pointed out that the kind of particle collisions that the LHC produces are commonly produced in the atmosphere by cosmic rays and would have caused problems long ago if there was any danger. Further studies of particle interactions around neutron stars have eliminated any possibility of catastrophic events beyond any reasonable doubt.
This has not satisfied the more hardened detractors who claim that the only people qualified to make such a risk assessment are particle physicists with a self-interest in running the experiment. Some activists have even instigated court cases to try to get the LHC closed down by legal process. With the failure of such initiatives it is certainly not beyond the realms of possibility that some fanaticists would attempt to disrupt the LHC directly.
The Hump has especially annoying characteristics. The LHC beam has a tune frequency of around 3.5 KHz which can be modified to avoid most sources of noise, but The Hump varies in frequency, crisscrossing the range in which the tune can be set. If you wanted to design a source of interference to disrupt the accelerator you could do no better than this.
If The Hump is caused by a sonic wave or radio wave it must be coming from either directly above or below the collider ring at some point because it is disturbing the beams in the vertical direction. More distant sources would cause horizontal vibrations. Natural causes from below can be ruled out because they would have been seen before. This points to a source of interference coming from above the ring, probably at ground level. It is possible that a sonar device or radio transmitter operating at the observed frequency could be deliberately targeted at the ring from a station or movable vehicle above ground.
The LHC scientists are nearly ready to step up the beam intensity so that real physics experiments capable of discovering new phenomena can begin. the saboteurs, if they are really out there, will be waiting for the right moment to throw the collider beams of course, damaging the detectors and making further experiments impossible.
Perhaps this is just another conspiracy theory and a benign source for The Hump will be found soon but after several months of searching without result no possibility can be ruled out.