Rapidity Gaps between Jets

Of all the subatomic objects explored with the HERA electron proton collider, the pomeron is perhaps the most enigmatic. Even its status as a real 'object' is controversial. It carries the quantum numbers of the vacuum: that is to say it's the closest thing to nothing you can get. And yet, at very high energies, it is the dominant mechanism by which all particles which feel the strong force scatter off one another. The problem in understanding the pomeron is that, within quantum chromodynamics, our theory of the strong  force, there is no fundamental object which carries the quantum numbers of the vacuum. There are only quarks and gluons, which carry the strong force charge, known as colour.

If colour charge seems unfamiliar, that's because we never see it directly in everyday life. It remains hidden within the protons and neutrons which make up the nuclei of atoms. This is perhaps fortunate, since it is about ten thousand billion billion billion billion times stronger than gravity. The mechanism by which the strong force remains hidden is known as confinement. Protons and neutrons are made up of three quarks, each of which must carry one of the three different colour charges, known as red, green and blue, each bound together by gluons. When put together, these three charges mix to give a colour neutral object, in the same way that mixing the three primary colours of paint gives white (this is the origin of the name 'quantum CHROMOdynamics'). All particles made up of quarks and gluons that we see in nature (known as hadrons) must be colour neutral.

At HERA, when protons and electrons collide at very high energies, the proton is smashed apart. Quarks and gluons are literally knocked out of the proton. Since we never see a single quark or gluon in nature (they are coloured) some mechanism must be at work which transforms them into colourless hadrons. The picture is simple. When a quark leaves the proton, it remains connected to the other quarks and gluons by a colour string: think of it like an elastic band stretched between the outgoing quark and the rest of the proton. Just like an elastic band, as the colour string gets tighter it will eventually break, and when it does, more quarks and gluons are formed, which in turn continue to rush away from each other, connected by still more colour strings. By this process, the outgoing quark has its energy transformed into many more quarks and gluons, which eventually get together to form lots of colourless hadrons, which we see as a shower of particles in our detector. This process, known as hadronisation, is not well understood, but the basic rule is simple. If two quarks (or gluons) are colour connected (that is a 'colour elastic band' is stretched between them), then a shower of new hadrons will be produced along the line of the elastic band. This will show up as a large deposit of energy on our detector.

What has this got to do with the enigmatic pomeron? Remember that the pomeron is colourless; it carries the quantum numbers of the vacuum. So if a pomeron is exchanged between two quarks for example, there will be no colour string between them, and therefore no shower of hadrons. We should see a region of our detector which is literally empty. This would be the unmistakable signature of the pomeron. The plot below shows this signature. The black points show the number of collisions with a particular amount of energy between two quark or gluon 'jets', which are the showers of hadrons formed by the breaking colour strings. The solid and dotted lines are the results of simulations in which there are no pomerons. Both the simulations agree fairly well with the measured points, except in the far left of the plot. This is where there is very little energy between the outgoing quark or gluon jets: the smoking gun revealing the presence of the pomeron.

So we see the pomeron. It's obvious in the plot below. But what is it? The answer is, we still don't know for sure. It may be a colourless combination of gluons, an object known as a glueball: like a proton, but with no quarks. Or it may just be a phenomenon linked to the hadronisation process itself. Precision measurements like this one are the only way to find out.
The pomeron appears