• have a much bigger charge density as the protons (and especially lead nuclei),
  • aren't compound particles as the protons (and especially lead nuclei)
  • are able to get a much bigger energy with the same fields as the protons (and especially the lead nuclei).

Why seems common for the big colliders after the LEP to use protons (and much bigger nuclei)?

Reacting comments: Yes, to get good experimental data about quark matter they need a lot of hot quarks (= collided big nuclei). But to create new particles, the energy per degree of freedom needs to be maximized, and this maximum is at the single particle with the highest charge density, and this is the electron.

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    $\begingroup$ I feel that the fact that protons and lead nuclei are compound particles is probably an important part of the reason why it's interesting to bang them together as hard as possible ;) $\endgroup$ – Danu May 13 '14 at 9:07
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    $\begingroup$ @Danu Yes, to find good experimental data about quark matter they need a lot of hot quarks. But to create new particles they needed to have as much energy per grad of freedom as possible, which can't be done with so highly compound particles as a proton. $\endgroup$ – peterh - Reinstate Monica May 13 '14 at 9:09
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    $\begingroup$ Peter, energy has not been the limiting factor for discovery for approximately a generation; it is all about luminosity and the ability to generate and take data fast. Hadrons for discovery, leptons for precision. $\endgroup$ – dmckee --- ex-moderator kitten May 13 '14 at 15:43
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    $\begingroup$ Because they'd have to rename it and scrap all the t-shirts, obviously. $\endgroup$ – KutuluMike May 13 '14 at 17:59
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    $\begingroup$ It's tough to smash something into itself $\endgroup$ – James May 13 '14 at 20:00

Whenever you accelerate a charged particle it emits EM radiation known as Bremsstrahlung, and obviously charged particles moving in a circle are accelerating (towards the centre). This means that any circular collider emits a continual stream of Bremsstrahlung radiation. To counteract the energy lost to Bremsstrahlung you have to put energy in, and that costs money and annoys the local power companies.

For a given beam energy the Bremsstrahlung losses increase with decreasing particle mass, so it costs a lot more to run an electron collider than to run a proton collider of the same energy and beam current. The LEP collider, with a maximum energy of about 200GeV consumed around 70MW when running, while the LHC with a vastly higher beam energy only consumes around 120MW. These figures are a bit misleading since they include the costs of cooling, etc, and not just running the beam. According to this article the power required for maintaining the beam at the LHC is only around 20MW. I haven't been able to find the corresponding information for LEP.

All the proposed future electron/positron colliders are linear. This avoids the Bremsstrahlung losses when you bend the particle beam.

  • $\begingroup$ You are right, but still the disadvantages of a lepton collider are counterbalanced by the much cleaner events you get. So it seems to me that the two approaches have been competing, and I wouldn't be so decisive in declaring circular electron/positron collider not viable. There have been some serious proposals: arxiv.org/abs/1305.6498 $\endgroup$ – zakk May 13 '14 at 11:17
  • $\begingroup$ @zakk & JohnRennie: I created a new question about the recycling of the bremsstrahlung power here: physics.stackexchange.com/questions/112502/… , maybe it deserves a fix or an answer. $\endgroup$ – peterh - Reinstate Monica May 13 '14 at 12:18
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    $\begingroup$ It is not true that all future e-e+ colliders will for sure be linear. While there are huge losses due to Bremsstrahlung radiation, it is still feasible with a large enough ring. A new proposal that has got a lot of traction is TLEP, tlep.web.cern.ch $\endgroup$ – JeffDror May 13 '14 at 12:38
  • $\begingroup$ Have you tried to google the power consumption of the proposed future linear electron/positron colliders (CLIC or ILC)? Reducing everything to the energy costs and to the annoyance of the local power companies is simply missing the point. $\endgroup$ – DarioP May 13 '14 at 14:25
  • $\begingroup$ @DarioP: I don't know what the energy use of the ILC will be, but to be honest without precise figures, not just the overall power use stats Google found me, you can't make a fair comparison. BTW I think your answer is excellent :-) $\endgroup$ – John Rennie May 13 '14 at 14:36

The energy lost by a particle doing one turn in a circular machine is

$$U_0\propto E^4R^{−1}m^{-4}$$

where $E$ is the beam energy, $R$ is the bending radius, $m$ is the mass of the particle that you want to accelerate.

It comes out that for the mass of heavy particles such as muons, protons and heavy ions, the field strength of the bending magnets is still the limiting factor, but light particles such as electrons and positrons are simply radiating too much energy.

The problem is not simply wasting energy, but how to push that energy back to the beam. The acceleration is normally done in straight sections of the ring by using a radio-frequency electric field. If the field is not strong enough (in relation to the available space) to compensate the energy lost in the bending sections, the machine will never work even with an endless amount of energy available from the grid.

So you are forced to increase the radius to reduce the energy loss and have more space to compensate it, but looking at the relation above, you see that you cannot go too far. For instance TLEP plans to have a radius tree times larger than LEP, but its energy won't double the LEP one and it might be built only in view of a much more energetic proton machine coming in future in the same tunnel.

The other way is to build straight colliders such as CLIC or ILC projects, which however do not come without technical difficulties.

  • $\begingroup$ In addition to technical difficulties (Would that be mostly beam cooling? I am a beam user, not a beam maker...), linear machines suffer from the simple cost implications of only getting one pass through each klystron. $\endgroup$ – dmckee --- ex-moderator kitten May 13 '14 at 15:50
  • $\begingroup$ @dmckee the cooling is done in dedicated damping rings before injecting in the linac. The difficulties are mostly in alignment. Wikipedia has a good page explaining how the Compact LInear Collider solves the single-pass-through-each-klystron problem. $\endgroup$ – DarioP May 13 '14 at 16:05
  • $\begingroup$ Cute. I believe that regeneration technique was first used on a large scale at the JLAB FEL which I have seen a number of talks on. $\endgroup$ – dmckee --- ex-moderator kitten May 13 '14 at 16:08
  • $\begingroup$ @dmckee yep, the idea of moving energy between two beams has been around for a while. However nobody in the accelerator community thinks about CLIC as a novel JLAB FEL, there is almost no common problem and technology transfer between the two machines. $\endgroup$ – DarioP May 13 '14 at 18:36
  • $\begingroup$ @DarioP - is it possible for you to provide a source for the Power $\propto E^4 R^{-1} m^{-4}$ relation? Thanks in advance :) $\endgroup$ – 299792458 Jun 27 '14 at 9:12

Apart from the reason mentioned in previous answers (Bremsstrahlung) there is one more thing why proton collider is used: it can scan wide range of collision energies.

Because protons are compound particles, their collisions are in fact collisions of the quarks or gluons. These constituents have random energies and thus each collision typically has a different energy. This is handy when you look for a particle with unknown mass (like the Higgs boson). So you smash protons at 1.4 TeV, but in fact the actual collisions are in wide range of energies from MeV to TeV. This way some of the collisions will be at the level required to create the unknown particle. And you collect a lot of events to capture significant statistics to detect the particle.

On the other hand: lepton colliders collide leptons always with the working energy. You cannot explore energies outside the design range of the lepton collider. Thus lepton accelerator is handy when you want to focus on research of a single particle with well known mass. You tune the energy to the mass of the particle and get the data much faster. But you have to know the particle mass upfront, which was not the case for the Higgs.

  • $\begingroup$ Just for the interested, there exists a proposal to revive in the same tunnel as LEP, LEP2 and LHC, LEP3, espace.cern.ch/LEP3/SitePages/Home.aspx . With LEP2 we just missed the Higgs. It is a proposal to just study everything about the Higgs boson. $\endgroup$ – anna v Jun 14 '14 at 7:04

Proton proton colliders are much better for discovery than electron positron colliders. The reason is that the mass of a new particle is unknown and the likelyhood of production peeks around this center of mass energy in the various hypothesized production channels. Roughly speaking the quarks in a proton get fractional shares of the total collision energy, so a proton antiproton collider is better at "scanning" a mass range for a new particle at constant beam energy. Whereas the beam energy of an electron positron collider would have to be carefully tuned.

Furthermore in the case of the Higgs boson, the channels that were best suited for discovery involved gluons, which are produced in quark collisions. For example one of the collisions leading to the discovery is depicted by the following diagrams, where $H^0$ denotes the Higgs boson, $t,b$ are the top and bottom quark, which both couple to the Higgs field and $g$ denotes gluons, which carry the strong force and therefore also couple to $t$ and $b$. In the first diagram the signal were two photons $\gamma$ with an combined energy of roughly 125 GeV in the second case two lepton anti-lepton pairs.:
Feynman diagrams showing the cleanest channels associated with the low-mass (~125 GeV), Higgs boson candidate observed by ATLAS and CMS at the LHC. The dominant production mechanism at this mass involves two gluons from each proton fusing to a Top-quark Loop, which couples strongly to the Higgs field to produce a Higgs boson. enter image description here

To study the properties of a newly discovered particle it is indeed better to use a electron collider, which then is tuned to the peak of this particles production probability. For example the LEP collider ran at the Z-peak in order to do electro-weak precision measurements. Edit: mpv beat me to this answer.


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