When does fermentation occur?

In stars, fusion reactions occur when protons collide. Do those same fusion reactions occur when two protons collide at the LHC supercollider at CERN?  If not, why not?

  • Are the same things happening in stars that happen when two protons collide at the LHC supercollider at CERN? If not, why doesn't a fusion reaction at the LHC? If the reactions at CERN are different than in stars does that mean the models for fusion reactions in stars are incorrect?

  • Answer:

    Fusion reactions at the LHC will be extremely rare and will probably never ever happen (although it is not impossible).  The only fusion reaction that occurs in the Sun that could possibly be duplicated at the LHC is: [math]P + P \rightarrow D + e^+ + \bar{\nu}[/math]​ (proton plus proton becomes deuterium nucleus (bound proton and neutron) plus a positron and an anti-neutrino) This reaction will be extremely rare compared to all the other possible reactions that we observe at the LHC. In fact, I would guess that this particular reaction has never happened at the LHC. To see why, consider that the LHC typically collides two protons together such that each proton have a kinetic energy of 7 TeV (this kinetic energy is 7,460 times the rest mass of the proton!). On the other hand, the binding energy of the deuterium nucleus is only 0.42 MeV (see http://www.wolframalpha.com/input/?i=2+*+proton+mass+-+deuterium+mass). Now this binding energy compared to the kinetic energy of the colliding protons is only 1 part in 16 million. So for the fusion reaction shown above to occur at the LHC the kinetic energy of the incoming protons must be carried away by the deuterium nucleus and by the positron and anti-neutrino.  This is extremely unlikely to occur - it is difficult to have the relatively fragile deuterium nucleus plus the positron and anti-neutrino share the 14 TeV of kinetic energy.  It is much more likely that many additional particle pairs will be created and share this tremendous kinetic energy. Consider that a proton has 3 valence quarks (two up quarks and a down quark). So you are really taking 3 quarks with a huge amount of kinetic energy and colliding it with 3 other quarks coming from the opposite direction with a huge amount of kinetic energy and somehow one of these up quarks has to change into a down quark with the emission of a W+ boson that decays into a positron and an anti-neutrino. Then somehow these 6 quarks have to bind together even though the binding energy is only 0.42 Mev and the 6 quarks have to go off in the same direction (by becoming the Deurteron) sharing the tremendous kinetic energy of the collision with the positron and anti-neutrino.  Doesn't sound very likely, does it? In addition, the up quark converts to a down quark via the weak interactions: [math]q_u \rightarrow q_d + W^+ \rightarrow q_d + e^+ + \bar{\nu}[/math]​   and the probability that a weak interaction (via the weak nuclear force) will occur is very small compared to the very high probability that quarks with interact via the color force (the strong nuclear force). In contrast, at the center of the Sun there is a high density, high temperature completely ionized plasma. The density at the center of the Sun is estimated to be 150 times the density of water (about 15 times the density of lead) and the temperature is estimated to be 15 million degrees Kelvin. That temperature corresponds to an average kinetic energy of 1.4 KeV per proton. This is one 50 billionth of the kinetic energy of the LHC protons. This kinetic energy is so low that no other particles can be created since there is not enough energy in the collision to create, for example, even one electron-positron pair (this requires a minimum of 1 MeV of energy) At the kinetic energy of 1.4 KeV, there is a probability for the protons to overcome the coulomb barrier caused by the electrostatic repulsion of two like charges by quantum tunneling through the barrier. The probability of tunneling is very small, but the density and temperature is so high such that many many collisions take place per second so that a significant number of tunneling events do take place.  Thus whenever one of the protons does manage to tunnel through the coulomb barrier, the two protons will get close enough together to permit the weak interaction to convert one proton into a neutron to produce the Deuterium nucleus - and there is not a significant amount of excess kinetic energy that needs to be handled by the Deuteron and the positron and anti-neutrino as in the case of the LHC! That brings up another reason why fusion will not occur at the LHC: "luminosity".  The LHC has high luminosity for particle physics ("luminosity" measures roughly how many collisions are possible per second - for the LHC there are 115 billion protons in each bunch and bunches are colliding every 25 nanoseconds). However at the center of the Sun there are about [math]10^{25}[/math]​  (10 trillion trillion) protons per cubic centimeter that are colliding at much higher rate than LHC's once per 25 nanoseconds, so the Sun's luminosity is huge compared to the LHC. That is why fusion reaction take place on the Sun but there is only a very small chance that a fusion reaction would ever take place at the energies and the conditions at the LHC.  On the other hand, the reactions that take place at the LHC has nothing to do with what happens in the Sun since the kinetic energy available in the Sun is completely insignificant compared to the LHC kinetic energy.

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's answer is correct.  The basic issue is that fusion is like a little delicate reaction, like trying to put together a Lego set.   To get the right little house built, things have to be just right.  In particle physics, we throw big bags of Legos together with such ferocity that the Lego bricks evaporate. You need to collide protons with about 1 MeV of energy, and and instead we are colliding them with 10,000,000 times that much energy.

Jay Wacker

I believe that the energy levels and conditions are very different. The accelerators are at such high energy that other particles are formed. The stars do not operate at such temperatures or energy levels.

Daniel Spector

CERN protons of course collide far too fast to fuse. Generally they are blown apart as thoroughly as two teacups colliding at a thousand miles an hour.  They are blasted into jets of quarks that become mesons and many other things. Conditions in the Sun or a bomb are FAR more gentle, and keep the particles from disintegrating. Proton-proton (p-p) fusion even at the right temperature (hot enough but not too hot to destroy the particles) is very difficult, because protons don't "stick" without beta decay to make deuterium (D), and that decay is slow, due to weak-force mediation. So the protons come apart almost every time. That is why mean life span of a proton at the core of the Sun is a billion years (even at those fantastic pressures and densities), while any D produced lasts just 4 seconds before becoming He-3, and He-3 lasts 400 years before going to He-4 (where the process ends, at least in our Sun, and at least at present). I doubt p-p fusion has ever been seen experimentally, anywhere! (if you know of the experiment, give me the cite). That's why bombs and fusion experiments use D and tritium isotopes, not light hydrogen-1.  A hydrogen bomb (thermonuke) or fusion reactor that used H-1 (protium) would be (in Larry Niven's immortal words) halfway to science-fiction. The energy per volume from this reaction, even at the center of the Sun, is less way less than your own (volumetric) metabolic rate. It's that hard to do. Stars-only are capable of it, and it takes a star larger or denser even than ours, to do it "well" (fast). Of course the slowness is lucky for us. == Addendum: I just checked and artificial proton-proton fusion has never been seen. The cross section can't be measured and must be calculated from first principles. Nor would you get enough extra energy to measure if you were dumb enough to make a thermonuke bomb using H-1. Only a star can do it. That seems unlikely to change without new physical laws. Our distant descendants won't be getting energy from H-1 either unless they use stars. Which means most high energy resources in the universe (where you don't have to wait for slow sunlight production) are actually dependent on deuterium in space and in planets! D from the Big Bang (its only source) is the ultimate "fossil fuel". Only 25  ppm but there's a lot of H outside stars out there.

Steve Harris

In fact deutrons are produced at LHC but rather in lead-lead collisions, in which some of the final state nucleons can stick together via a mechanism called coalescence. Even antideuterons are produced and observed, which gives an opportunity to test equality of different properties of matter and antimatter. Well, even antihelium was detected by ALICE but at extremely low statistics.

Adam Jacholkowski

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