Can particle physics at the LHC have a practical purpose? | 125 comments (125 topical, 0 editorial, 0 hidden)

Re: Ultra high energy collisions happen all the ti (none / 0)

This time I am seriously interested in a description (for laymen) of these collisions. What happens?

by afew (afew(a in a circle)eurotrib_dot_com) on Thu Feb 21st, 2008 at 02:22:41 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

More on the cosmic ray part or the LHC part or both?

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers

by Martin (weiser.mensch(at)googlemail.com) on Thu Feb 21st, 2008 at 02:28:30 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

I interpreted the question as asking for an explanation of how elementary particle collisions lead to cascades of other particles.

Why this is possible is simple: it is possible to convert the kinetic energy of the collision into particles by means of Einstein's E=mc^2.

How it happens is a little harder to explain for laymen, but I suppose we could get away with Feynman diagrams.

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Thu Feb 21st, 2008 at 04:04:01 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 1)

Martin:

I was replying to nicta, and to his comment about collisions at the fringes of the atmosphere. What is the interaction between rays and molecules? What are solar winds and how do they interact with molecules? Do these collisions give rise to phenomena observable at the (for want, in my case, of the correct term) "macro" level?

But a description in layman's terms of the LHC project would also be welcome.

It occurs to me you may not have ET discussion threads displayed as "nested", meaning that you can see on screen which comment is replying to which. Is that the case?

by afew (afew(a in a circle)eurotrib_dot_com) on Fri Feb 22nd, 2008 at 02:25:59 AM EST
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Re: Ultra high energy collisions happen all the ti (4.00 / 2)

Cosmic rays consist of high-energy photons (gamma rays - normally outside the solar system or even extragalactic), electrons and protons (normally from the solar wind except for the very energetic ones).

These photons, electrons and protons collide with particles in the atmosphere and produce cascades of other particles by converting the kinetic energy of the incoming ray into the products. The cascades happen because the first products are energetic enough to, themselves, cause similar collisions further down the atmosphere. The cascades are seen by flying or land-based detectors.

At the LHC you have two high-energy proton beams colliding head-on. There is a cascade but it is a lot cleaner because there is nothing else for the products to collide with, so they just fly off and decay and the collection of decay products is seen by the detector array.

The LHC is a proton-proton collider. When a cosmic-ray proton hits a hydrogen aton in a water molecule you have essentially the same proton-proton collision.

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Fri Feb 22nd, 2008 at 04:04:43 AM EST
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Re: Ultra high energy collisions happen all the ti (none / 1)

I think afew will learn everything in the film linked by Pierre, but solar wind is really not the most important already in the energy regions which would be interesting compared with LHC. Sun wind reaches less than 1 GeV.

The Supernova accelerated ones are the relevant for the comparison with LHC.

The really high one, e.g. those which are watched at Auger have most likely their origin in active galactic nuclei.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers

by Martin (weiser.mensch(at)googlemail.com) on Fri Feb 22nd, 2008 at 09:12:50 AM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

Are you familiar with the kinematic argument that the highest energy cosmic ray protons that have been observed shouldn't actually be able to reach Earth and how that provides evidence of corrections to special relativity at high energies?

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Fri Feb 22nd, 2008 at 03:00:20 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 1)

A bit, we have a group of people working at Auger who have new (and unfortunately a bit boring) results.

The highest energy cosmic rays should interact with the cosmic microwave background radiation (CMBR) if the energy of the proton is about 6×10^19 eV (in the rest frame of the CMBR) into a Delta+ resonance. This is called GZK cutoff.

The average "lifetime" of a proton for flying without interaction at this energy is about 160 Megalightyears.

A Japanese experiment (AGASA) claimed to have found some of this very high energetic particles, which led to speculations, despite the big systematic uncertainties.

Auger (south, in Argentina) recently reported (maybe preliminary) to have found as well some, but
a) there is a reduction in the current on these very high energetic particles
b) they were able to track much of the particles origin (at this high energies the intergalactic magnetic fields don't bend the trajectory too much) back to known active galactic nuclei (AGN). AGNs are likely huge black holes (as we have likely a smaller one in our galactic center). On their accretion discs many light year long streams are accelareted to produce high energy particles of all types.

The surprise about their existence though shows only a lack of our understanding of the AGNs, not of the special relativity and not super heavy BigBang relicts, which would have been the most interesting for particle physics.
Of course this is not yet completely safe, as Auger is only shortly in operation and it might be that some have different origin, but I wouldn't bet against it.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers

by Martin (weiser.mensch(at)googlemail.com) on Fri Feb 22nd, 2008 at 03:52:07 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

Thanks for the update: if there is an AGN in the right direction within a distance of the order of 160 Mlyr then there's no evidence of violations of the GZK cutoff.

Which is rather a pity, because GZK violation was one of very few experimental results not explainable by the standard model of particle physics and the standard cosmological model (another one being the rotation curves of galaxies).

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Fri Feb 22nd, 2008 at 05:00:28 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

LHC - it's like banging rocks together. Only the rocks are really, really small.

by ThatBritGuy (thatbritguy (at) googlemail.com) on Fri Feb 22nd, 2008 at 08:44:02 AM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

Oh, great, that's better, thanks. But you forgot to say how many rocks would fit into a football field?

by afew (afew(a in a circle)eurotrib_dot_com) on Fri Feb 22nd, 2008 at 09:03:31 AM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

That was interesting. I thought I'd look up 'diameter of an electron' and found sizes between 10^-13 m and 10^-18 m.

The 'classical' radius is

Although all that really tells you is that you can't say how big an electron is, because unlike a rock it doesn't have hard edges. Instead it's defined by how much it bounces when you bang it with something else.

Curiously, football pitches also seem to suffer from quantum indeterminacy - very bad news for science journalists.

The official FIFA dimensions are '90-120m' wide and '45-90m' tall. Goal dimensions are defined as 7.32m x 2.44m, which presmably requires precision laser range-finding to make sure they're accurate to the nearest mm.

So - it's unlikely we'll ever know exactly how many electrons fit into a football pitch.

A bit unfortunate, that, but there it is.

by ThatBritGuy (thatbritguy (at) googlemail.com) on Fri Feb 22nd, 2008 at 09:36:32 AM EST
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Re: Ultra high energy collisions happen all the ti (none / 1)

I have to live with uncertainty. It's killing me.

by afew (afew(a in a circle)eurotrib_dot_com) on Fri Feb 22nd, 2008 at 10:15:00 AM EST
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Re: Ultra high energy collisions happen all the ti (none / 1)

You can say that the charge of the electron is contained in a sphere with radius less than 10 ^(-18)m and is assumed to be a point in the current theory.

However at LHC protons are used not electrons.
Protons have a diameter of about 10^(-15)m. Of course not the protons, but the quarks and at LHC even mainly the gluons are colliding.

For the non-experts, the following picture is an illustration of a proton by the Oxford Hera group's website.

The dots are quarks. In a proton quarks minus antiquarks net 3 quarks. The springs represent gluons. The lower energy you count the more gluons are there. These are the "force particles" of the strong interaction, which holds as well nuclei together, and have the interesting property that they carry as well strong charge.

The best known "force particle" is the photon, which is the carrier of the electromagnetic interaction. Others are gravitons (not yet directly observed, but who doubts they exist), and the W (Z) bosons, the carrier of the weak interaction, which are the only force particles known which have mass.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers

by Martin (weiser.mensch(at)googlemail.com) on Fri Feb 22nd, 2008 at 10:17:12 AM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

The radius of a proton is a more meaningful quantity than the radius of an electron, because as far as we know the electron is pointlike (no internal degrees of freedom) but a quark is a composite particle containing three quarks and so in principle one could assign it a radius just like the hydrogen atom - a composite particle of a proton and an electron can be assigned a radius by studying the electron wavefunction for its ground state.

Then again, the "radius" of a wavefunction, be it the electron orbital in an atom or the wf of a quark in the proton, is only meaningful as an order of magnitude anyway.

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Fri Feb 22nd, 2008 at 03:22:25 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 1)

You are a theory guy, right? ;-)
10^(-18)m is experimentally tested, point is what the theory says.

But more important you have a typo
"but a quark is a composite particle containing three quarks", should be proton is a composite particle...

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers

by Martin (weiser.mensch(at)googlemail.com) on Fri Feb 22nd, 2008 at 04:02:56 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

In a previous life I was a theory guy, yes, though I took my graduate Standard Model courses from a pretty good phenomenologist.

I suppose I could find it in the particle data booklet, but can you outline the experiment that tests the electron radius?

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Fri Feb 22nd, 2008 at 04:51:20 PM EST
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Electron size (4.00 / 2)

Well, how do we know that the proton has a fm? When hitting it with a charged particle with a Compton wavelength in the order of 1 fm in a scatter experiment, we start to see a formfactor, e.g. the particle doesn't see the full charge any more.
LEP II had about 100 GeV/c^2 per electron, therefore a Compton wavelength of about 10^(-17)m, and there was not the slightest sign of substructure, we still see absolutely the full structure and one would likely expect already small signs of a formfactor with a somewhat bigger wavelength than the electron size.

I don't know what measurement exactly provides the best measurement, could be a precision measurement of the B sector or whatever, but that's it pretty much.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers

by Martin (weiser.mensch(at)googlemail.com) on Fri Feb 22nd, 2008 at 05:17:09 PM EST
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Re: Electron size (4.00 / 2)

There's more than one LEP? that's scary ;-)

Any idiot can face a crisis - it's day to day living that wears you out.

by ceebs (ceebs (at) eurotrib (dot) com) on Fri Feb 22nd, 2008 at 05:32:24 PM EST
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Re: Ultra high energy collisions happen all the ti (none / 0)

The electron is, as far as we can tell, a "point" particle, meaning that it has no internal degrees of freedom: all its degrees of freedom have to do with its relation with the surrounding space(-time).

The classical radius of the electron is the radius of a sphere such as the energy of the electric field outside it matches the observed rest mass of the electron. If you assume a "classical" electron is truly pointlike you get an infinite energy for its electric field.

The funny thing about quantum field theory is that, since the electron is pointlike (see first paragraph) you need to "renormalize" the self-interaction of the electron (i.e., the interaction of the electron with its own electric field) and renormalization methods involve a "cutoff" (effectively, a mass or a radius cutoff - see second paragraph). You then ger "running coupling coefficients" which means that the "bare mass" and the "bare charge" of the electron vary with the "cutoff".

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Fri Feb 22nd, 2008 at 03:18:33 PM EST
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