Why Only Protons were selected for collision in LHC project?

[iluvphy]

Hi all,

I would like to ask why only Protons were selected for collision in LHC project? Why not other fundamental particles?

When I was watching the programme on big bang by Michio Kaku and Lawrence Krauss, they were saying at the earliest instant of big bang, there were four forces namely, gravity, weak and strong nuclear forces and electromagnetic forces tied together but "some how" they split apart, which resulted in the subsequent creation of matter. So, my question here is, how we are sure that proton-proton collisions at the speed of light would definitely simulate the most likely conditions of early instance of universe? I mean there could be some other particles or combination of other particles or combination of various forces and particles and other permutations and combinations.

Is the proton-proton collision to result in the simulation of early instance of big bang based on some strong scientific evidence?

Please note, I just loved physics from childhood but unfortunately had to jump into some other field. I always loved physics and Sir Isaac Newton is my idol, but I don't have necessary education in physics to understand the very many concepts. So, please don't bash me, if this is a stupid question all together.

Thanks.

 

[mfb]

There are not so many particles you can use:

- Electrons and positrons have a small mass, their energy is limited due to synchrotron radiation. LEP, the previous particle accelerator in the tunnel, accelerated electrons and positrons, with a much lower energy.
- Neutrons and antineutrons have no electric charge, you cannot accelerate and control them in a reasonable way
- Muons and antimuons decay within a few microseconds. There are concepts of a muon-antimuon collider, but that will need a lot of new technologies.
- protons are easy to get (basically, you just need hydrogen), easy to accelerate, easy to get in a circle, and easy to control
- antiprotons have to be produced, and you don't get so many of them. They would give slightly "better" collisions, but the raw number of collisions is more important at the LHC
- (heavy) ions behave similar to protons, and they are used at the LHC, too.
- all other particles are way too short-living to be interesting

So, my question here is, how we are sure that proton-proton collisions at the speed of light would definitely simulate the most likely conditions of early instance of universe?

If the energy is sufficient, you can produce all particles present shortly after the big bang, and study them. Heavy-ion collisions give a really hot "soup" of particles, which looks close to the conditions of the early universe.

 

[iluvphy]

Thank you mfb for your explanation.

Also, is there any chance that there might have been some other fundamental particle/ or some other forces which were responsible for the early instant of big bang and might have vanished after that instant or might have changed their properties and characteristics? Basically, is it 100% guaranteed that the current LHC experiments would simulate the exact conditions during the first instant of big bang?

 

[mfb]

Nothing is guaranteed. A particle/effect/whatever which could exist at that time and cannot exist today looks unlikely, however - it would require that the laws of physics changed, and there is no indication of any changes.
It is possible, and many physicists expect it, that there are unknown particles with a higher mass than the known particles. We would need even more energy to see them - to go back in time to an even hotter universe.

 

[iluvphy]

Thanks mfb for your quick responses.

Was just watching the big bang programme and some doubts crept in mind. Hope, some exciting and conclusive results come soon from this project, which may give us more knowledge.

 

[Bill_K]

- protons are easy to get (basically, you just need hydrogen)

From a CERN brochure:

Although proton beams at the LHC are very intense, only 2 nanograms of hydrogen are accelerated each day. Therefore, it would take the LHC about 1 million years to accelerate 1 gram of hydrogen.

 

[Office_Shredder]

Bill, we can only make about 2 nanograms of antimatter per year I think. At any rate
http://en.wikipedia.org/wiki/Antimatter#Artificial_production

According to CERN, it has cost a few hundred million Swiss Francs to produce about 1 billionth of a gram (the amount used so far for particle/antiparticle collisions).[41]

So that's like half a billion dollars to make one day's worth of acceleration mass.

 

[Bill_K]

Again a quote, from Fermilab's Symmetry Magazine:

Fermilab’s first antiprotons were produced in the Tevatron’s first collider run in 1988. At that time, it took more than an hour to make 10¹⁰ antiprotons. Now, the Antiproton Source can make 30x10¹⁰antiprotons an hour.

Producing a beam of antiprotons is much more difficult, the main advantage being simpler magnet design. Not much difference in the resulting collisions, except one can look for asymmetries.

 

[mfb]

Thanks mfb for your quick responses.

Was just watching the big bang programme and some doubts crept in mind. Hope, some exciting and conclusive results come soon from this project, which may give us more knowledge.

I think the existing results are quite exciting already. Sure, no unexpected new particle, but it is not clear if those exist at all.

To put the Fermilab numbers into perspective, the LHC used about 2*10¹⁴ protons per direction, with planned ~3-4*10¹⁴ after the current upgrade.
In addition, those protons can be focused to beams with a diameter of some micrometers , this is really hard to achieve with antiprotons.