Exclusive production processes at hadron collider are something magical. You direct two trucks at 100 miles per hour one against the other head-on, and the two just gently push each other sideways, continuing their trip perfectly unaffected, but leave behind a new entity (a cart?) produced with the energy of the glancing collision. 

That is more or less the picture. Now think at a hadron collision at the LHC, and we can suddenly talk about subnuclear physics without changing much of it. In exclusive production processes, the two projectiles emerge unbroken from the collision, much like it happens in what are called "elastic collisions" which are governed by electromagnetic interactions.  But the difference is that there has been a significant exchange of energy, as proven by the produced particles!

In fact in exclusive production processes the involved interactions can be strong, weak, or electromagnetic; the common denominator is that the projectiles retain their identity. These are the cleanest ways to produce new states of matter in particle collisions: you get what you want, and nothing more. No strings attached, in a literal sense (strings are the common representation of colour connections between two states in quantum chromodynamics, if you have not got the pun).

One exclusive production process that has been studied recently is the one of Upsilon mesons, in collisions of lead ions and protons. Why Upsilons ? And why lead-proton collisions ? I will try to explain it below.

First of all, what is a Upsilon. Upsilon mesons are bound states of a bottom quark with its antiparticle. You can think of a Upsilon (also labeled with the corresponding greek letter Y) as a bag where the two quarks jiggle around each other. The Y, discovered in 1977 by a team led by Leon Lederman at Fermilab, is the heaviest bound state of two quarks we know of. Top quarks, unfortunately, are believed to not form bound states as they decay too quickly for quantum chromodynamical interactions to allow the formation of a composite particle one can speak of.

The Y is also very interesting to study because it can decay into a very clean signature - a pair of energetic muons, particles that even in the usually messy collisions of heavy nuclei can be detected easily. But we are not talking of messy collisions here, as indeed we are talking about the Y and nothing else. So why lead-proton collisions ?

The lead nucleus carries a strong electric field, due to the number of protons it contains. This means it is an intense source of electromagnetic interactions, which are at the source of these exclusive production processes. At this point it is useful to have a look at the Feynman diagram of the process I am describing.

How to read this graph ? Here are the rules. 1) do it left to right - the abscissa describes the time evolution of the reaction; 2) each particle gets a line, sometimes oriented with an arrow; 3) p stands for "proton"; Pb stands for lead; the letter γ indicates a photon; g are gluons; and the b are bottom quarks bound in the Y meson.

As you can see, the lead nucleus "talks" with the proton by exchanging a photon. This photon is "quasi-real", so the lead nucleus does not break apart. As for the proton, it contributes to the reaction by emitting two gluons (the springy lines at the bottom). Why two gluons and not one ? Well, if the proton emitted only one gluon, it would lose its colour-neutrality (the gluon carries away colour quantum numbers). A coloured proton would immediately break apart due to the repulsive forces that would take place between its constituents. Instead, with two gluons, the proton can emit and reabsorb the same quantum numbers and remain intact.

The process is interesting for several reasons, but I won't describe them here. Rather, let me show a couple of graphs from the article describing the study, performed by the CMS collaboration using data collected in 2013. The first one is the dimuon mass distribution, with a fit to the three resonance shapes (the Y comes in several different mass states depending on the angular momentum of the system - they are called Y(1S), Y(2S), Y(3S) etcetera). 

The second graph is a summary of the cross section for exclusive production of Y mesons as a function of the center-of-mass energy of the photon-proton system that effectively generates the Y particles. As you can see, the process has been observed by several different experiments (whose measurements are shown with different coloured markers; the CMS ones are black bullets) in different kinematical regions and with different techniques. A comparison of the data to the theoretical predictions -the coloured curves, which are admittedly a bit all over the place- can teach us a lot about the details of this complex and fascinating particle production mechanism.