A Quantum Diaries Survivor

I received a question which I think should be in the “FAQ” of CERN - I think the answer belongs to an independent post, for the benefit of those who never bothered to think the matter over. 

So I am curious. Why does the tevatron collide protons-antiprotons, but the LHC goes for proton-proton? This has been puzzling me for some time, and even google has no anwsers  

Running protons against antiprotons has the advantage of allowing one to use the same magnets to bend both beams, circulating in opposite directions, in the same way. The Lorentz force experienced by a moving charge in a magnetic field changes sign once for the opposite direction and once for the opposite charge, and the net effect is null, so that both protons and antiproton can travel in the same beam pipe - thus saving magnets, vacuum structures, and a lot of infrastructure.

So, what is the drawback ? It is that producing antiprotons is a maddeningly hard task. You produce antiprotons by smashing protons against a target, and sifting through the emerging bodies downstream, with magnet optics that select them and patiently direct and store them in an accumulation ring.

The art of producing antiproton beams has been perfected at the Tevatron in the last twenty years, but it is just impossible to reach the intensity required to achieve the rate of collisions that CMS and Atlas at LHC need in order to investigate very rare processes. Producing a proton beam is easy: you take hydrogen, strip electrons off, and there you go…

Finally, one must mention that colliding protons versus protons is not exactly the same thing as colliding protons versus antiprotons, as far as the physics output is concerned. Strong interactions do not care whether the projectiles are particle or antiparticle: what they care about is the color charge of quarks and gluons, which is the same in hadrons and anti-hadrons. But electroweak interactions do, because they are sensitive to the flavor of the quarks (electroweak processes do not “see” the gluons, by the way). So the relative rate, and kinematics, of electroweak processes is different at the two accelerators. In any case, these are details, and one can discover pretty much the same things one way or the other.

To summarize: deciding on protons versus antiprotons at the Tevatron at the end of the seventies was -as far as I understand it - a matter of cost versus effectiveness when a 900 GeV machine was designed. The Main Ring, a fixed-target (only protons circulating only in one verse) 400 GeV synchrotron which had helped discover the Upsilon mesons and the bottom quark at the end of the seventies, could be used to run hadron-hadron _collisions_, provided one injected antiprotons in one way and protons in the other. This yielded the possibility to try and discover the top quark without a complete redesign of the machine (although, well, major upgrades were needed anyways).

On the other hand the LHC, being a totally new machine, was instead built with two separate beampipes, and separate magnets in the bending tracts, thereby allowing proton-proton operation, with intensities that will exceed by two orders of magnitude those of the Tevatron - and allow to study 100-times rarer processes.