The magnetic field grew rapidly stronger, and began to slow your flight. After fifty metres your speed had reduced dramatically and when, after a further fifty metres, you emerged into an underground cavern, you were travelling slower than you had ever done before. Here you found a gigantic, barrel-shaped object, the subterranean monster known by its human minders as “ATLAS”. They force-fed this ravenous beast with a constant diet of protons which they accelerated to almost the speed of light in the “Large Hadron Collider”. This, the largest collaboration ever attempted between the scientists of the world, lay in a huge circular tunnel in the sediment one hundred metres deep and twenty-seven kilometres long. You caught only a glimpse of all this, Majestic Monopole, before you were swallowed by ATLAS’s body, drawn in by some of the most powerful magnets ever built by mankind.

Cooled by liquid helium to almost absolute zero temperature, ATLAS’s “superconducting” magnets were designed to bend the paths of particles created by proton collisions and so allow the detector to record and identify their constituents. At thirty-four minutes and twenty-three point five-eight-four-one-two seconds past nine on that memorable morning, the array of individual detectors within ATLAS began to record your silent passage.

Although designed to trace the paths of particles travelling outwards from the centre of the detector, the various sub-systems were nevertheless perfectly capable of recording the effects of your unexpected arrival. The outer detectors in the assemblage saw you first. As you traversed the “Muon Spectrometer”, many of the hundreds of long, thin, high-pressure gas-filled metal tubes fired a pulse of electricity from their central high-voltage wires. These signals passed into the data processing system and the first-level electronic “triggers” took only two microseconds to decide that something extraordinary was happening. These data were stored in “readout buffers” in the underground counting room of the USA15 cavern close to ATLAS’s lair, awaiting confirmation from the other sub-systems that this was a real event and not a spurious signal.

You moved downwards, Oh Monopole, bending sharply in the strong field of the “toroidal magnetic coils” as you penetrated deeper into ATLAS. Here you passed easily through the “Calorimeter’s” metal plates, although collisions with the heavy nuclei of several atoms further slowed your rate of descent and triggered more data from the “scintillating plastic and liquid argon sensors”.

Very soon you entered the “Inner Tracker” causing electrical pulses to fire within excited gas-filled straws and, deeper still, state-of-the-art semiconductors measured your position with exquisite accuracy while your path curved sharply in the enormous two Tesla magnetic field of the huge “central solenoid”. Your final energy was spent in traversing the stainless steel and copper wall of the Large Hadron Collider’s beam pipe, a narrow tube which ran through the middle of ATLAS.

At first all was quiet. There was a moment of peace, when (had you been capable of feeling emotions, Oh Marvellous but totally insensible Monopole) you would perhaps have reflected, as you drifted slowly along the beam pipe, that here at last you had found the home you had been seeking since time began. You might have held your breath, waiting eagerly to meet your relatives in this exotic world, so different from anywhere else you had ever been and grateful for having found a magnetic field strong enough to finally bring your long journey to an end.

You did not have long to wait. Within a tiny fraction of a second your tranquillity was shattered and your hopes raised even higher as a bunch of protons came hurtling along the beam pipe like a vast school of excited children going on an outing. There were almost a trillion of them, flashing past only millimetres away from where you floated gently along the pipe. As the pack passed, some of the particles actually collided with you. The collisions did no harm and their momentum counteracted the effect of the magnetic field, pushing you back along the pipe towards the centre of the detector.

The particles, you would now have realised with a sharp sense of disappointment, were not Monopoles like yourself, but merely protons, the sort of thing you had frequently seen before, although you had never previously met so many with so much energy. And, just as they had inside stars, some of the protons adhered to you, increasing the size of your outer mantle.

Within less than a nanosecond the bunch of protons had passed you by, Oh Marvellous Monopole, and gone hurtling out of sight along the beam pipe. Alone once more you began drifting in the magnetic field but you had hardly gone a centimetre before another bunch arrived and the same thing happened. More protons fused with you and pushed you back up the beam pipe.

Surrounded by your growing mantle, you felt more at home now than ever before, but you could also sense a tension growing around you. Every proton carries something called an “electric charge” which makes it try to push away from all other protons. This produced a mutual repulsion within the particles in your mantle, which was in danger of breaking up and leaving you naked once more. Luckily a new phenomenon prevented this. Sometimes a passing particle came very close without actually fusing with you. As a result of this near-miss collision, one of the protons in your mantle acquired sufficient energy to emit a small object which scientists call a “positron”. This carried away the excess electrical charge, leaving your mantle more stable. In addition, an uncharged particle called a “neutrino” was emitted at the same time. These two particles went shooting through the beam pipe wall out into the detector.

Because it had no electrical or magnetic charge, the neutrino travelled out through the whole of ATLAS without producing any effect whatsoever. But things were very different for the positron. As soon as it hit the beam pipe wall it immediately fused with one of the particles it found there, a particle which was like its twin, identical in every way except it carried the opposite kind of electrical charge. Scientists, in their wisdom, call these two sorts of electrical charge positive and negative. The positron’s charge is said to be positive while the electron’s is negative. These opposite charges strongly attracted each other and the positron fused with the electron, with dramatic results. All the matter of these two particles was transformed into a pair of “gamma rays”, similar to light but with much higher energy. They radiated outwards through ATLAS’s sub-detectors, firing events in the “Electromagnetic Calorimeter”.

The computerised “trigger system” was designed to distinguish the few extraordinary events occurring in the beam pipe from the millions of ordinary ones which were of no interest. It had already decided that your arrival was interesting and had recorded every detail of your inward path. But that had been merely one event out of many, although with higher energy than almost any other. The computers had simply assigned the whole record a “StoreGate key”, added it to the database and moved on.

Now the hard-wired Level-1 trigger system embedded within ATLAS was observing the outward flow of gamma rays emanating from the beam pipe wall with equally close attention. The electronics easily determined that positrons were flowing out of the beam pipe and passed the data to the Level-2 Trigger System.

The Level-2 processors which analysed these events were unable to find any obvious energy source for the positrons. During the many years of planning and building ATLAS nobody had ever predicted such events. Indeed, despite all their precautions to ensure the safety of the experiments at CERN, not one scientist had ever evaluated the possible dangers presented by a cosmic monopole.

The processors assumed that some undetectable form of energy must be flowing out of the beam pipe in the opposite direction which they labelled as “missing transverse energy”. They had been programmed to recognise this type of event as a distinct and important signature for interesting new physics. Something truly extraordinary was happening, the banks of computers agreed, and hard discs whirred as records were copied from the “Transient Data Store” and written to permanent storage.