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I understand that the LHC found the Higgs boson by pumping so much energy into a tiny space (via near light speed proton-proton collisions) that a Higgs boson appeared momentarily, then instantly decayed. They detected the products of the decay, and deduced that a Higgs boson must have been there. That's fine. What I don't get is:

1) Isn't the universe full of Higgs bosons, making up the Higgs field? If so, why do we need to make one ourselves? Why can't we detect the ones that are there already, like we can other bosons such as photons?

2) When we've made our Higgs out of pure energy, why does it instantly decay into other particles?

3) Does it actually directly decay into other particles, or is it rather the case that it just turns back into pure energy, and then that energy produces other, less massive particles?

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Imagine 2 bluewhales of opposite sexes mating in the ocean and we just want to detect the ocean, somehow if the mechanism(evolution) is intense enough we can notice the splashing!😀 – user6760 Jan 18 at 10:26
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"near light speed" means nothing. 7 GeV protons move at near light speed. 7 MeV protons (one millionth the energy of protons in the LHC) move at near light speed. Physicists have been building machines to accelerate charged particles to "near light speed" since back in the day when we used to call them "atom smashers." – james large Jan 18 at 16:05
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The definition of what "near light speed" means here isn't really relevant to the question, if it bothers you then you can ignore the part in brackets. I guess i was just highlighting the fact that it's a high energy collision, but again we could say that "high energy" is meaningless as it's all relative anyway. I'd have thought, though, that a 7 Mev proton has one thousandth the energy of a 7 Gev proton, rather than a millionth? (7x10e9 vs 7x10e6) – Max Williams Jan 18 at 16:11
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@corsiKa Have you seen the prices the Higgs Boson store charges? It's cheaper to make your own, and it helps challenge their monopoly! – David Richerby Jan 18 at 23:17
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@jameslarge Not long ago I was reading a blog post that made mention of desktop particle accelerators--with a hyperlink. It lead to Amazon, specifically, CRT monitors. – Loren Pechtel Jan 19 at 5:26
up vote 87 down vote accepted

Isn't the universe full of Higgs bosons, making up the Higgs field?

No. In particle physics, it is understood that the underlying (more fundamental) object is the field, not the particles. Particles are excitations of the fields that can be measured, and always carry certain properties like charge, mass, spin etc. The field that you are most familiar with is the electromagnetic field, its excitations being the photons. In another field the excitations are electrons, in another still there are gluons etc.

And there is a Higgs field, whose excitations are the Higgs bosons. The Higgs field, in contrast to the electromagnetic field has a non-zero value even if there are no Higgs bosons there.
To have an analogy in mind, think of a room full of air. When I speak, there are sound waves moving around the air. The air is the Higgs field, the sound waves are the Higgs bosons.

Why can't we detect the ones that are there already, like we can other bosons such as photons?

Higgs bosons are very massive, as particles go, so they require a lot of energy to be created in collisions. Additionally they have a number of decays pathways, so when they are created, they decay rapidly. So, even if Higgs bosons are created all the time in the atmosphere, or in supernovae or other events, they are rare and hard to detect. That is why we set up an experiment that can reproduce millions of collisions a second so to accumulate enough data.

When we've made our Higgs out of pure energy, why does it instantly decay into other particles?

This is kind of misguided. There is no clear meaning of "pure" energy. Energy is a quantity that is assigned to various phenomena, yet is common and interchangeable between them all. We speak of kinetic energy, potential energy, mass-energy, etc. but none of these forms is "purer" in any specific sense. In the particle collisions, the kinetic and rest mass energy of the protons is concentrated in a small part of spacetime, and can be redistributed in the kinetic, potential and mass energy of other particles.

Once a particle is formed, it does not really matter what way it has been formed. Just like a radioactive nucleus has the same probability of decaying in the next $10$ minutes irrespective of how long it has survived until now, a Higgs boson will decay with a certain probability into the particles it can decay to.

Does it actually directly decay into other particles, or is it rather the case that it just turns back into pure energy, and then that energy produces other, less massive particles?

Here we end up a little in metaphysics.You will have different answers depending on the interpretation of QM you choose. All we observe is the protons that go in the collision, and the shower of particles that comes out after the collision, together with their energy. That's all. Quantum theory will give you the statistics of these observations, but not what happens between the two observations; that is (for now) metaphysics, because it is unobservable.

Strictly speaking, no Higgs boson has been observed, in the sense that no Higgs boson has collided with the detectors. We have calculated how the existence of the Higgs field will affect the measurements, we found that it would affect them in a particular way, we did the experiments and indeed found that signature. The experiments and theory match so well that it is inescapable that there is a Higgs field, even though we have not "seen" (with our eyes) any Higgs bosons.

To speak about the exact way in which one particle comes into existence and decays is a bit beyond present physics (also worth exploring in other questions).

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Actually, what is giving mass to the other particles is the Higgs field, not the particles. At our temperatures and density, there just aren't that many Higgs particles flying around in our atmosphere. – Andrea Di Biagio Jan 18 at 13:23
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Of course, in the classical case, there will is always some sound created by objects moving in a room. But in the case of quantum fields, there is a minimal energy required to produce a disturbance in the field. – Andrea Di Biagio Jan 18 at 13:34
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Ah, i think i have a basic misunderstanding then: i thought that because the Higgs field is everywhere, then the Higgs bosons would be everywhere too, in a similar way to the way there are always photons flying around through any given patch of space. – Max Williams Jan 18 at 14:01
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I'm not qualified to assess the technical correctness of this answer, but if it's correct, it's one of the most clear explanations of the Higgs boson experiments I've ever read. – JiK Jan 18 at 14:19
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Yes, that is the crux. The Higgs field is non-zero everywhere, but it is hard to excite a boson out of it, so there are not many (i.e. basically $0$) Higgs bosons flying around. The Higgs field is special in that way. Photons have no mass, so no minimum energy to be created, so there are plenty all around: the EM filed is easily excitable. An example in between is the electron field. It takes a minimum amount of energy to excite an electron out of the electron field. @MaxWilliams – Andrea Di Biagio Jan 18 at 14:22

Ad 1) Yes and no. There are certainly Higgsbosons in the Universe. Everything we can create at the LHC is created in other events too. Cosmic rays can have more energy than the LHC beam, so there will be Higgs bosons for sure, just as an example. The problem is to bring such a sophisticated detector in place, in addition you don't really have the experiment fully under control.

Your second assumption is wrong, therefore I wrote yes and no. The Higgs boson is an excitation of the Higgs field, but the field itself is the "fundamental" thing so to say. It is not "made out of Higgs bosons".

Ad 2) In particle physics there is a saying: "If it is allowed to decay, it will". If it is not forbidden, a decay will happen. As a rough rule, a particle is more likely to decay if there are more options. By allowed I mean that all conservation rules have to be obeyed.

3) What do you mean by pure energy? The energy has to be there in some way, but yes it decays directly into other particles that decay further and so on. The energy we put there in the first place is stored as kinetic and mass energy in the protons. Particles decay directly into other particles as long as all conservation laws are obeyed during the decay

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Thanks @Noldig. Re question 1: if i rephrased it to say "full of Higgs bosons, resulting from the Higgs field" instead of "full of Higgs bosons, making up the Higgs field", then the question still applies: why can't we see the "naturally occurring" Higgs bosons? – Max Williams Jan 18 at 10:51
    
@MaxWilliams Maybe it will help your thinking in terms of fields, that the gluon field also is at all the space time points BUT the probability of seeing a gluon exists only within hadrons. The expectation value of the gluon field, the photon field, the neutrino field etc ( all the elementary particle table except the Higgs ) is zero. The expectations value of the Higgs field is nonzero because of symmetry breaking and giving masses to the elementary particles including the Higgs boson. – anna v Jan 18 at 17:01
    
@annav Sincere question: does the fact "the expectation value of a field is nonzero" imply "the probability of observing its excitations is nonzero"? – Andrea Di Biagio Jan 18 at 18:27
    
@AndreaDiBiagio No, it is just what distinguishes the higgs field from all the rest. I should have clarified that the expectation values of the rest of the particle fields become non zero where a particle exists. In my experimentalist' s hand waving view the Higgs field is what in confluence with the electron field, for example, gives the mass of the electron to the excitation of the electron field, which signifies that an electron of mass m_e exists at that (x,y,z,t). – anna v Jan 18 at 19:38

Another factor here:

We build these huge atom smashers because we want a look at things that are normally sealed inside larger particles. The energy is to shatter the box they're in.

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I would consider this as at least very misleading. E.g. in a electron collider you see a lot of particles after the collision. although we consider the electron to be a fundamental particle. Also the protons used in the LHC are not made out of pions, higgs bosons, photons and all kind of particles we observe. – Noldig Jan 19 at 9:55

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