Sunday, July 21, 2013

Jigsaw pieces are falling to place: Neutrino Minimal Standard Model

It seems that the jigsaw pieces are really starting to fall into place, as far as proving the Neutrino Minimal Standard Model and as far as disproving supersymmetry and string theory.

For example, some data presented at the EPS-HEP Conference in Stockholm this week, is lending more evidence towards the Neutrino Minimal Standard Model and against supersymmetry (and hence against string theory as well.)

Discovery at LHC leaves less room for new particles by Symmetry Magazine
(This article discusses how the Standard Model passed an experimental test with flying colors. For more information, you can go directly to the LHCb website. There are other results on the b→sγ  transition on their website they recently presented at the same conference, but which hasn't received as much media attention, but could potentially be extremely important in constraining modification to the Standard Model.)

T2K experiment catches neutrinos in the act by Symmetry Magazine
(This article discusses the publication of an improved data set showing tau neutrinos converting into   electron neutrinos. This data now completes the experiments required to calculate the values inside of the PMNS matrix...i.e. the neutrino mixing matrix.)

Why is neutrino mixing important?  I'll answer this questions throughout the rest of this post.

The fact that neutrinos can oscillate, i.e. change flavor and convert from tau to muon to electron neutrinos, implies that neutrinos have mass (like every other fermion). And since the spin of a massive particle can change as it travels through space-time, neutrino mixing also therefore implies that there is a right-handed sterile neutrino.  (Well, there probably are three sterile, right-handed neutrinos, just as there are three active, left-handed neutrinos.) This sterile neutrinos are candidates for dark matter because they are interact only very weakly with ordinary matter. If the dark matter in the universe is neutrinos (some combination of active and sterile neutrinos), then there is no need for making major modification to the Standard Model of physics, i.e. we don't need to rely on theories like supersymmetry that predicts that there is a partner-particle for every 'normal' particle.

The reason why I think that evidence against supersymmetry is good news (and not just something I'm neutral about) is that fact that I feel that the physics community went on a collective jump over the cliff when it came to supersymmetry and hence string theory. The problem is that these theories introduced more particles than asked for and left many questions unanswered (neutrino mass, neutrino oscillations, CP violations, and baryogenesis.) My gut feeling has always been that we are close to understanding particle physics, and that we just needed some minor corrections to the Standard Model in order to account for neutrino mass, neutrino oscillations, dark matter, dark energy, and the arrow of time. 

It's looking more and more (though, still not proven completely) that the Neutrino Minimal Standard Model can explain those experimental results that the Standard Model could not, without creating any conflicts with known experimental results. For example, according to a recent paper by Canetti, Drewes, and Shaposhnikov  "Sterile Neutrinos as the Origin of Dark and Baryonic Matter" suggest that: "three sterile neutrinos alone can simultaneously explain neutrino oscillations, the observed dark matter and the baryon asymmetry of the Universe without new physics above the Fermi scale." 

Here are important points made in their paper
(1) "Baryogenesis occurs via sterile neutrino oscillations during their thermal production."  This means that there is net generation of particles (over anti-particles) because of the weak nuclear force. They suggest that the collision of top quarks and top anti-quarks can lead to the production of a mixture of active and sterile neutrinos (not anti-neutrinos.) Of course, this requires CP violations in the neutrino sector (which is predicted to occur because of the values of the  PMNS matrix, but for which direct experimental evidence is still required.) 
(2) "In most of the parameter space, the relevant CP-violation comes mainly from the sterile sector and not from the phases in the Pontecorvo-Maki-Nakagawa-Sakata matrix."  This means that the sterile neutrinos are crucial for understanding why there are more particles than anti-particles, (and perhaps why there is an arrow of time.)  If sterile neutrinos are the main source of CP violations, then this can perhaps answer the criticism raised by main physicists that the weak nuclear force is not the source of the directionality of time because it is so weak.

If we are bathed in a sea of CP and T violating particles, then it's easy to explain why there is an arrow to time. The directionality of Beta decay as well as the probabilistic nature of Beta decay (i.e. radioactive decay) can perhaps be explained by the fact that we are bathed in a sea of CP and T violating sterile neutrinos. These sterile neutrinos may explain the cosmological evidence for dark matter particles with mass of roughly 2 keV.

If you combine these hypotheses with the hypothesis that dark energy is just the fact that the universe expands when the entropy increases, then we are close to explaining most of the problems with Standard Model. Here's a list of recent papers discussing dark energy & entropy.

Dark energy from entanglement entropy

Entropic Inflation

A thermodynamic motivation for dark energy

The connection between the two ideas (NMSM & DE=Entropy) is the hypothesis that the irreversibility of weak nuclear force (i.e. CP & T violation) is the cause of the expansion of the universe. When there are weak nuclear reactions and when there is chemical/nuclear non-equilibrium, there is entropy generation and, somehow, the universe expands. In other words, there is a natural connection between General Relativity and the Standard Model because the weak nuclear force causes the expansion of space-time. Since the overall mass/energy of the universe must remain the same, as space-time expands, it becomes less dense...which makes sense. The curvature has to decrease as the 4D sphere of space-time increases.

This would also explain why the value for the dark energy does not appear to be constant (and hence why dark energy is not Einstein's cosmological constant.) The value of dark energy was extremely large during the Big Bang (because there were so many reactions occurring via the weak nuclear force), and then the amount of expansion slowed down (after the temperature cooled below the scale of nuclear reactions.) However, the universe has been expanding this whole time, and the expansion has increased in speed recently as stars started reacting via nuclear reactions and as black holes started growing.

The point here is that the expansion of the 4D surface area of the universe is explained by an increase in entropy, and the increase in entropy is due to irreversible process, which in turn are due to reactions involving the weak nuclear force...the only force that violates C, P, T, and CP. This force is also required to explain why there are why there are more particles than anti-particles. What all this means is that the "Neutrino Minimal Standard Model" coupled with "Dark Energy = Weak Nuclear Force Irreversible Entropy Generation" can explain the following:  dark matter, why there are more particles than anti-particles,  neutrino mass, neutrino oscillations, dark matter, and dark energy.

So, we are really close to understanding most of the fundamental questions in particle physics and astrophysics. But to conclude this post, I do want to emphasize there are still remaining questions even if the hypotheses listed above are correct. Some of the questions are:
(1) Why do the particles have the mass, charge, spin that they do?  For example, why is the mass of the Higgs Boson exactly half of the mass of the sum of the three carriers of the weak nuclear force (Z, W+, W-)?  There are a number of explained constants even in the Neutrino Minimal Standard Model.
(2) Why are there only 3 families for each particle?  (such as electron, muon, and tau)
(3) Did the Big Bang occur: (a) accidentally, (b) of necessity, or (c) purposely?

As an aside, I'd like to point out that researchers modeling the universe should stop making the following assumption: "isentropic expansion of the universe."  I've seen way too many astrophysicists make the assumption that the universe is like a gas expanding isentropically in a piston with a changing volume, V. This is a really bad assumption to make for the following reasons. (1) What is the piston in this analogy? (2) The universe we know is likely just the 3D surface of a 4D sphere. The expansion of the surface of a 4D sphere is not the same as the expansion of the 3D sphere. (3) The entropy of the universe is not constant, so how could you assume isentropic expansion. (4) The expansion of the universe is most likely due to irreversibility, and hence, the assumption of isentropic expansion is to hide the actual cause of the expansion. In other words, the phrase "isentropic expansion of the universe" is just as silly as the phrase "isentropic diffusion of gas from a high concentration to low concentration."

Also, I want to link to a good overview of the History of Dark Matter by Joel Primack. Dr. Primack is an expert in this area but he is also good at communicating science to a non-technical audience.

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