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A fair account of the matter for once March 18, 2007

Posted by dorigo in language, news, physics, science.

I read a honest account of the status of things with the Higgs search on an italian newspaper today, “Il Sole – 24 Ore“. Here is a quick-and-dirty translation (my excuses to the author for lost or distorted meaning), for the benefit of those of you who have no access to italian press or no understanding of the language:

In search of the God Particle

The Higgs boson – the particle hypothesized by Peter Higgs in 1964 and considered crucial for the consistency of the extremely successful theory of fundamental interactions known as the “standard model” – is still at large, and chances are that it will stay there for a few more years.

A signal of decay of the Higgs particle, whose phenomenology is perfectly known within the standard model but whose mass is still unknown, has been sought with no success between 1996 and 2000 in electron-positron collisions provided by the LEP collider at CERN. The negative result of the four LEP experiments allowed to understand that if the Higgs exists, its mass is with all probability above 114 GeV – equivalent to the mass of 120 protons.

Meanwhile, the global analysis of several measurable quantities of the standard model made more and more strict the margin of agreement between all other experimental results and the absence of the Higgs boson. If the standard model is correct, that particle cannot be much heavier than the inferior limit found by LEP.

With the de-commissioning of LEP in 2000 the spotlights have passed to the Tevatron accelerator at Fermilab, where the proton-antiproton collisions grant some sensitivity to heavier Higgs particles. The CDF and D0 experiments that analyze the Tevatron collisions will collect data at least until 2009, and it is possible that they manage by then to obtain a first evidence of the existence of the elusive boson. If that is not the case, the definitive crowning of the standard model will become a task for the new super-powerful proton accelerator about to be completed at CERN, the Large Hadron Collider (LHC), and of the CMS and ATLAS experiments that will study its collisions from 2008 onwards.

And what if the standard model were not correct ? The question is a legitimate one, since there is consensus within the experts in maintaining that the theory, which has by now lived through forty years of more and more precise experimental verification, is at most an “effective” theory, that is one valid at low energy – just like Newtonian mechanics is a particular case of Einstein’s relativistic mechanics, which is valid for speed small with respect to that of light.

On what there may be beyond the standard model opinions are not univocal, however. Many hold that the most enticing and theoretically motivated extension is supersymmetry, a construction implying the existence, for every existing elementary particle, of a “super-partner” with similar characteristics but a different value of the intrinsic angular momentum – “spin”, a quantum number which distinguishes the matter constituents, quarks and leptons, from the carriers of the four fundamental forces, like the photon and the vector bosons.

If supersymmetry were the correct theory, there should exist more Higgs bosons: not one, but at least five distinct particles! And in that case, the Tevatron would have more chances to observe a first evidence, since one would then expect a larger production rate for these “exotic” Higgses.

In the meantime, the CDF and D0 experiments patiently continue to collect data, and with six-month cadence they publish newpreliminary analyses, after scrutinizing with infinite attention every detail of the obtained results. The last CDF result in the search for supersymmetric Higgs particles shows a mild excess of events, which could represent a first signal of decay of a Higgs with mass equal to about 160 GeV in pairs of tau leptons. It could, if it was not due – something infinitely more probable – to a normal statistical fluctuation. The competitor experiment, D0, is showing as a matter of fact a deficit of events for the same mass value, making it even more clear that it is much, much too early to place the champagne in the refrigerator.


1. Kea - March 18, 2007

That’s an impressive piece of English writing, Tommaso. Better than I could have done. If I had any way of getting hold of some champagne short of stealing it, I’d be putting it in the fridge…

2. Tony Smith - March 19, 2007

Kea suggested champagne to celebrate the LHC results (hopefully coming soon), finding or not finding the Higgs.
Considering the experimental and data analysis wizardry involved, how about celebrating with Strega ?

Tony Smith

3. dorigo - March 19, 2007

Hi Kea,

well, now I feel flattered! Writing good English is one of my goals in life. Thank you!
Anyway, is there any particular reason for celebrating soon ? Getting close to making QCD calculable ? Let us know!


4. dorigo - March 19, 2007

Dunno, Tony, never tried that liquor. I think I’m gonna get high when we finally see the Higgs, no matter what’s the flavor of the closest alcoholic liquid I can grab.


5. island - March 20, 2007

This article gives the same accounting of the situation that I got from reading your very informative blog.

They were overly optimistic tho… 😉

If that is not the case, the definitive crowning of the standard model will become a task for the new super-powerful proton accelerator about to be completed at CERN, the Large Hadron Collider (LHC)…

6. nc - March 20, 2007

Can I just clarify what the Higgs is (ignoring the complexity of needing 5 or more Higgs bosons if supersymmetry of some type is correct)? I know it is supposed to be a spin 0 boson that gives mass to everything. But Einstein’s equivalence principle between inertial and gravitational mass therefore implies that the Higgs boson interacts with whatever exchange radiation there is that causes gravity (spin-2 gravitons?).

If that’s the case, then the simple physical picture is that you have Higgs bosons there in vacuum, exchanging gravitons with other Higgs bosons. Because, by pair-production, photons can be converted into massive fermions, there must be a Higgs field (like a Dirac sea) everywhere in space which can allow such particles to pop into existence when fermions with mass are created from photons.

However, the Dirac sea doesn’t say the vacuum is full of pair production everywhere causing virtual particles to pop into existence. These loops of creation and annihilation can only occur in the intense fields near a charge. Pair production from gamma rays occurs where the gamma rays enter a strong field in a high Z-number nucleus.

The IR cutoff used in renormalization indicates that no virtual particle loops are created below collision energy of 0.511 MeV/particle, i.e., a distance of about 1 fm from Coulomb scattering electrons, corresponding to an electric field strength of about 10^18 v/m or so. If virtual particles were everywhere, the no real charges would exist because there would be no limit to the polarization of the vacuum (which would polarize just enough to entirely cancel out all real charges).

Does apply to the Higgs field? If the Higgs mass is say 150 GeV or whatever, then obviously they are not being created when an electron+positron pair are created from a gamma ray. It takes only 1 MeV to do that, not 300 MeV or whatever would be required to form a pair of Higgs bosons?

Or is the case really that Higgs bosons are virtual particles created from the vacuum at the energy of collisions corresponding to their mass? Assuming a Higgs mass of 150 GeV and a using Coulomb scatter to relate distance of closest approach to collision energy, then Higgs bosons would be spontaneously created at a distance on the order of 10^{-21} m from the core of an electron.

If this is the case, then the vacuum isn’t full of interacting Higgs bosons like the usual picture of an “aether” miring particles and giving them mass. Instead, the actual mechanism is that Higgs particles appear in some kind of annihilation-creation loops at a distance of 10^{-21} m and closer to a fermion, and the Higgs bosons themselves are “mired” by the exchange of radiation (gravitons) with the Higgs bosons at similar distances from other particles.

This is clearly the correct picture of what is going on, if the equivalence principle is correct and if mass (provided by the Higgs boson) is the charge in quantum gravity.

Professor Alfonso Rueda of California State University and Bernard Haisch have been arguing that radiation pressure from the quantum vacuum produces inertial mass (Physical Review A, v49, p678) and gravitational mass (because the presence of a mass warps the spacetime around it, encountering more photons on the side away from another nearby mass than on the side facing it, so the masses get pushed together – they have a paper proving this effect in principle in Annalen der Physik, v14, p479).



If Rueda and Haisch are correct in their “vacuum zero-point photon radiation pressure causes inertia and gravity” argument, then the problem is that they are using photon exchange for both electromagnetism and gravity, which is a real muddle because those forces are different in strength by a factor of 10^40 or so for unit charges. So either they’re totally wrong, or oversimplifying.

Suppose they’re not wrong and are oversimplyfying by using rejecting the Higgs boson while using the same gauge boson for electromagnetism and gravity.

Suppose there is still a Higgs boson in their theory, and there are different kinds of gauge bosons for gravity and electromagnetism.

Then, the gravity-causing exchange radiation is mediated between the Higgs bosons in the strong vacuum field near particles. The electromagnetism causing exchange radiation is mediated between the fermions etc. in the cores of the particles.

My next question is how the Higgs bosons explain electro-weak symmetry breaking? My understanding is that above electroweak expectation energy, there is a electroweak symmetry with SU(2)xU(1) producing four zero mass gauge bosons: W+, W-, Z, photon. Below that energy, the W+, W- and Z acquire great mass from the Higgs field.

Because they acquire such mass at low energies (unlike the photon), they have a short range, which makes the weak force short ranged unlike electromagnetism, breaking the electroweak symmetry.

The conventional idea is that very high energy W and Z bosons aren’t coupled to the Higg’s field? The Higgs field still exists at extremely high energy, but W and Z bosons are unable to couple effectively to it?

Normally in a “miring” medium, drag effects and miring increase with your kinetic energy, since the resistance force is proportional to the square of velocity, and drag effects become small at low speeds.

So the Higgs miring effect is the opposite of a fluid like the air or water? It retards particles of low energy and low speed, but doesn’t mire particles of high energy and high speed?

I’m wondering what the real mechanism is for why Z and W have mass at low speeds but not high speeds? It is also the opposite of special relativity, where mass increases with velocity.

Have I got this all wrong? Maybe the Higgs field disappears above the electroweak symmetry breaking energy, and that explains why the masses of Z and W bosons disappear?

7. dorigo - March 20, 2007

Well Island, writing it will be a task is not being too optimistic IMHO… It is just stating a fact. Whether the LHC finds the Higgs or not, is then another matter.


8. island - March 20, 2007

LHC finds the Higgs or not, is then another matter.

Yeah, I guess that it was the use of the term, “definitive crowning” that led me to believe that they were assuming that the LHC would not fail.

I’ll just blame it on the translator… 😉

9. dorigo - March 20, 2007

Wow NC, that is a long comment!

Part of what you say is correct, and part of it I’m dubious about. I need some time to give you a meaningful answer… Will get back to you.


10. island - March 21, 2007

However, the Dirac sea doesn’t say the vacuum is full of pair production everywhere causing virtual particles to pop into existence.

As with electric charge, the normal distribution of negative energy does not contribute to pair creation. Only departures from the normal distribution will isolate enough vacuum energy to produce virtual particle pairs. I think that it will turn out that it requires about 10^40 times the volume equal to the Compton wavelength of the particle cubed.

… and that presumably fixes your problem… and others.

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