It could be a real bad day to be a hadron if you hang out near Geneva today.
Q. What's a hadron, anyway?
A. It's any particle made up of quarks (e.g. three-quark baryons such as as protons and neutrons, or two-quark mesons).
Q. What would happen if you stuck your head in the LHC beam?
A. I discussed this obliquely in an article for sciencefiction.com back in 2011.
The LHC as a weapon system (abridged)
The Large Hadron Collider is not at first sight an ideal weapon system. First of all it’s physically a 17 mile (27 km) long circular tunnel, buried beneath the French-Swiss border at a depth in some place of over 570 feet (175 m). Not much use against most military targets. However, the concept of a beam weapon was explored in the SDI (‘star wars’) programme for ICBM defense, and a weaponised LHC would be a formidable anti-spacecraft system.
The LHC is a proton accelerator. It gets its protons from a tank of hydrogen gas and then cranks them up to near the speed of light. How fast are the protons finally going? Their velocity is only 3 metres per second slower than light but in relativity the factor which measures relativistic effects is the gamma factor (γ) which tends to infinity as v → c. At the LHC the proton γ is 7,460. At that speed a starship journey from earth to the nearest star, Proxima Centauri (4.2 light years away) would be just a five hour jaunt for the crew!
The LHC scientists are interested in those few protons from the contra-rotating bunches which hit each other: collisions lead to new physics. But from a weapons point of view we are more interested in the majority which miss. These are completely lethal. In dealing with them, the LHC engineers had to answer the question: what do you do with the fastest human-produced objects ever to travel on the earth?
The engineering team simulated the effects of the proton beam hitting a copper tube 10 centimetres in diameter and five metres long. In the top picture of the diagram below we see the situation half a microsecond after beam impact. The beam has already penetrated 2 metres (around 7 feet) into the tube with a core overpressure of 30 GigaPascals (300,000 atmospheres). The protons and their collision debris are literally gouging out the centre of the target.
The lower picture shows the situation at 9.5 microseconds where the beam has now penetrated five metres (more than 16 feet) into the copper and has compromised the entire tube. This is a density plot and shows the vaporised core as having less than 1% of the original copper density. Based on this simulation, the entire 89 microsecond beam would penetrate 35 metres (115 feet) of solid copper.
At CERN they take care to defocus the beam first, spreading it over the face of the beam dump. It turns out that carbon is better than copper in stopping the now-defocused beam so the actual LHC beam dump absorber is engineered as a two foot diameter, 23 foot long graphite cylinder contained in an outer steel cylinder. This is water-cooled and surrounded by about 750 tonnes of concrete and iron shielding in a dedicated enclosure. The LHC protons finally come to rest in a cascade of secondary particles, deep inside a blazingly-hot carbon tube.
In a future particle beam weapon system, it could have been a rocket hull or spacecraft: stabbed, incinerated and disabled in 100 microseconds.
OK, I'm conscious that I evaded the question. About your head. No matter, it was answered brilliantly here.