[General] HA: nature of light particles & theories

[Burinskii A.Ya.]

Dear John,



Thank you very much for very good explanations and reference to good review.



I wish also to you and all colleagues Merry Christmas and Happy New Year,



Alex

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От: Dr Grahame Blackwell [grahame at starweave.com]
Отправлено: 14 декабря 2016 г. 12:48
Кому: Nature of Light and Particles - General Discussion
Копия: Stephen Leary; Vera Biryukova; Darren Eggenschwiler; Nick Bailey; Anthony Booth; Pete Delaney; Innes Morrison; Alexander Afriat; Phil Butler; Michael Wright; Ariane Mandray; Solomon Freer; Manohar .; Mike Mobley; Niels Gresnigt; Mark, Martin van der; AmancioHasty
Тема: Re: [General] nature of light particles & theories

Hi John,

Many thanks indeed for this very thorough round-up of the 'evidence' on quarks.
Very much appreciated.

Wishing all colleagues a great Christmas and an excellent New Year.
Grahame
----- Original Message -----
From: John Williamson<mailto:John.Williamson at glasgow.ac.uk>
To: Nature of Light and Particles - General Discussion<mailto:general at lists.natureoflightandparticles.org>
Cc: Stephen Leary<mailto:sleary at vavi.co.uk> ; Darren Eggenschwiler<mailto:darren at makemeafilm.com> ; Nick Bailey<mailto:nick at bailey-family.org.uk> ; Anthony Booth<mailto:abooth at ieee.org> ; Pete Delaney<mailto:piet.delaney.2 at gmail.com> ; Innes Morrison<mailto:innes.morrison at cocoon.life> ; Alexander Afriat<mailto:afriat at gmail.com> ; Phil Butler<mailto:phil.butler at canterbury.ac.nz> ; Michael Wright<mailto:mpbw1879 at yahoo.co.uk> ; Ariane Mandray<mailto:ariane.mandray at wanadoo.fr> ; Solomon Freer<mailto:slf at unsw.edu.au> ; Manohar .<mailto:manohar_berlin at hotmail.com> ; Vera Biryukova<mailto:biriukovavera at gmail.com> ; Mike Mobley<mailto:Mike.Mobley at gcu.edu> ; Niels Gresnigt<mailto:Niels.Gresnigt at xjtlu.edu.cn> ; Mark,Martin van der<mailto:martin.van.der.mark at philips.com> ; AmancioHasty<mailto:ahasty at gmail.com>
Sent: Sunday, December 11, 2016 4:13 AM
Subject: Re: [General] nature of light particles & theories

Hi everyone,

Have been meaning to explain a bit more about the proton internal structure for some time in answer to an earlier question from John D about the evidence for quarks inside the proton. I did reference the literature, but this is hard to understand if you are not in the field and the field anyway tries to hide the pure truth with a lot of dense and well-established undergrowth.  I had not got round to this earlier due to two things: pressure of other work and the fact that I forgot to note the source for a useful chapter I found on the internet. Just tracked it down and it is at:

https://www.physics.umd.edu/courses/Phys741/xji/chapter4.pdf

Did not want to send you my copy of it without crediting the source.

Anyway,the main thing I wanted to do was cut the through some of the jargon and help explain what the proton structure functions (in fig 4.6 in the above) mean. This is the essence of what is known experimentally about the internal structure of the proton – and contains the main evidence for the quark-parton model. The quark-parton model is the association of hard bits in the proton, the partons, with the pattern of existing particles explained by Gell-Mann’s quark model. This also helps to explain some things about Richard’s question in the recent email – hence the choice to spend time on this in the early hours of this morning.

Now I’m not going to explain this in detail – the chapter referenced above does a better job of this – but I want to cut the experiment a bit free from the embedded story of the QCD quark-gluon etc etc model (and it is just a model remember) and explain what the EXPERIMENT tells you.

The experiment gives the structure functions in terms of two variables Qsquared and x. Briefly, Qsquared is the measured 4-momentum transfer squared of the interaction in GeV squared. How hard you hit it (squared). To give you an idea of the scale of the hit – 100GeV squared is roughly ten times the mass-energy of the proton itself. And so the data extends out to about a 100 protons worth of “hit”. That is hard!

Now x is a more interesting variable. It is the measured fraction of the proton’s 4-momentum carried by whatever you hit. Thinking of the proton in its rest-frame – this is just its rest mass. So x tells you how much of the proton mass was carried by whatever you hit. x is 1 and you got the whole proton. This is what you would always measure if you hit a simple object like the electron. The electron is a single object and it carries all of its mass localized to the electron. This is how you know. The proton is not like that. At the quark-parton models simplest, with no forces and no confinement one thinks of it as three quarks. If each of these carried a third of the proton mass one would have data at only x = 0.33. Note that there is not even any structure there.

What one actually sees is completely different to this, or to any three-hard-bits-in-a-bag model. In the vast majority of collisions the effective “mass” of whatever you hit was very very low. Look at the scale for F2. It goes over 12 orders of magnitude. One is hundreds of millions times more likely to hit a “quark-parton” with a practically zero x of 0.000063 than one with a (simple model) x of 0.3 ish. Now precisely zero x would be hitting a rest-massless (photon-like) object, one third x would be simple rest-massive quarks in a massless bag with binding energy (gluons if you like) of the same order as the mass. A sixth x would be 3 equal mass quarks with some confinement at the same kind of energy as the quark mass-energy. You get nothing like this. What you get is gloop. There is almost no discernable structure at all.

So why do people think there are hard bits in the proton. The evidence for this comes from scaling – a flat distribution with Q squared then. This IS evidenced by the curves in the middle of the figure. At x = 0.08 it is pretty flat. Think about it. If the proton contained hard billiard-ball like bits, how likely you were to hit them with another flung billiard ball does not depend on how hard you fling it, but on the “impact parameter”. This is what is characteristic of single-hard-object scattering.

Note that this simple scaling does not apply at low x, where the data shows that it becomes rapidly more likely to find a photon-like object as one hits it harder, and at high x where it becomes rapidly less likely to hit a high-mass constituent. Explain that in a model of a bag of bits. You should resolve the hard bits better, instead it seems they break. Not very hard then. Ok, you are walloping them with a 4-momentum squared many times their mass squared, but one is doing this at lower x as well. The other thing is that, if you integrate over all the bits you hit in deep inelastic proton scattering, you only get about half the proton mass. The rest is something else, something unhittable with charges and photons. This is the meaning of equation 4.77. This is interpreted as arising from the binding. Could well be, but whatever they are binding is mostly, experimentally, a whole pile of really low mass bits (if bits indeed) – more and more of it as one looks harder and harder. Remember, to make up the proton mass there must be (at least) hundreds of millions of them. Hundreds of millions is not 3. One talks about “valence quarks and sea quarks, but this is mostly bullshit. One sees what one sees, not what one would like to see. Also the number in eq. 4.77 is so near 50 percent I favour something much more radical and far simpler. That will eventually become another paper. Quarks, why there are three and what they really are is what comes next.

If you want to see how bad it gets for the standard model (and why I left particle physics) the bullshit about the standard model picture gets (much!) worse in the next section about the “proton spin crisis” so read on if you dare …

I’m not quite up to speed with who is or is not on the general maiing list, so some of you may get this twice – apologies!

Thats it for now.

Cheers, John.
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