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The say of the week February 11, 2007

Posted by dorigo in Blogroll, humor, internet, personal, physics, science.

The neutrino masses force an extension of the Standard Model more or less as revolutionary as a rearrangement of the furniture in the kids’ room forced by the arrival of a new baby.” 

T.D. (in a comment on a post on Not Even Wrong).

There I go… At the risk of looking self-centered, I am again using a sentence I wrote myself… Oh well, this is my blog!



1. Carl Brannen - February 12, 2007

The best introduction to the standard model and the problem of putting neutrino masses into it (that I’ve come across) is the new textbook by Burgess and Moore, The Standard Model, a Primer

The last few chapters cover the issues.

2. dorigo - February 12, 2007

Thank you for the hint, I will give it a look!

3. Carter Hall - February 12, 2007

Greetings Tommaso –

It’s true that the discovery of neutrino mass
was not the revolutionary breakthrough that supersymmetry or
extra-dimensions would be. Nevertheless, it was somewhat more interesting
than your “say” indicates. Finite neutrino mass is telling us that whatever
is responsible for the masses of the charged fermions (presumably
the Higgs mechanism or something equivalent) is most likely *not*
the entire of the story for the masses of the fundamental particles.
The is readily apparent when you plot the masses of the quarks,
charged leptons, and the neutrino mass (limits) on a log scale
as a function of generation. While the charged particles masses have
an orderly partern from 1st to 2nd to 3rd generation, the neutrino
masses are completely out of whack by being so tiny. It doesn’t
take much imagination to infer that something new is going on in
the neutrino sector.

But the plot thickens further. Because neutrinos are electrically
neutral, it’s possible that the neutrino mass term is of the
Majorana form, where the particle and antiparticle are the same
object. In fact, most theorists expect that neutrinos will turn
out to be Majorana, in part because it offers a natural way of
explaining the suppressed neutrino mass scale in terms of GUT
scale physics. If neutrinos are shown to be Majorana experimentally,
this will be a smoking gun for non-SM physics, because the Higgs
mechanism cannot generate such a mass term.

So the importance of neutrino mass is not that it solved the solar
neutrino problem (which is a very nice piece of work), but that
it raises a new question: why is neutrino mass so small and yet
finite? It’s a hierarchy problem similar to the gauge hierarchy
or cosmological constant problem.

4. dorigo - February 12, 2007

Hi Carter, what a pleasant surprise! 😉

Thank you for your insightful explanation of the importance of neutrino masses… I agree that my position is a bit extreme, but you might agree that the hints and the promises that the neutrino mass spectrum gives are still… well, hints and promises.

I see no alternative model to the SM which fits the data better, explains the spectrum (something the SM is blamed of being unable to do), and gives it a totally new flavor. Well, ok, there is no spectrum yet, but I know there will be one one day.


5. Tony Smith - February 13, 2007

In his book Journeys Beyond the Standard Model (Perseus 1999) Pierre Ramond says:
“… the standard model must be extended to accommodate massive neutrinos … the extensions do not put in question the nature of the standard model, but rather add more parameters to it …
To preserve lepton number and massive neutrinos we need to introduce new fermions to serve as the Dirac partners of the left handed neutrinos. These new fermion degrees of freedom can have any electroweak quantum numbers …
Electroweak breaking then generates a neutrino Dirac mass of the order of 245 x Y(0) GeV …
The experimental limits on neutrino masses imply that the Y(0) cuopling constants must themselves be very small, in the range of Y(0) less than or equal to ( 10^(-10) – 10^(-4) ). If one accepts such tiny couplings (after all, we already have m_e = 10^(-6) M_W …), this represents a viable extension of the standard model …”.

To me, that sounds like Dirac mass for neutrinos would indeed be much like Tommaso’s “say of the week”:
“… a rearrangement of the furniture in the kids’ room forced by the arrival of a new baby”.
To verify the “say” in that way,
the question is: Are neutrino mass states Dirac mass states?

In a Particle Data Group review NEUTRINO MASS, MIXING, AND FLAVOR CHANGE, Revised September 2005, B. Kayser (Fermilab) says:
“… In the Standard Model (SM), neutrinos are assumed to be massless. Now that we know they do have masses, it is straightforward to extend the SM to accommodate these masses in the same way that this model accommodates quark and charged lepton masses. …
if the cosmological assumptions … are correct, then 0.04 eV is less than …[ the mass of the heaviest neutrino ]… is less than (0.2 – 0.4) eV …
To accommodate the nu mass in the same manner as quark masses are accommodated, we add nu_R to the Model. Then we may construct the “Dirac mass term” …
Suppose the right-handed neutrinos required by Dirac mass terms have been added to the SM. If we insist that this extended SM conserve …[lepton number]… L, then, of course, Majorana mass terms are forbidden. …
One approach that shows great promise is the search for neutrinoless double beta decay ( 0 nu beta beta ). … This process manifestly violates L conservation, so we expect it to be suppressed. …
Are the neutrino mass eigenstates Majorana particles?
The confirmed observation of neutrinoless double beta decay would establish that the answer is “yes.”
If there are only three nu_i, knowledge that the spectrum is inverted and a definitive upper bound on … the “effective Majorana mass for neutrinoless double beta decay” … that is well below 0.01 eV would establish that it is “no” …”.

In hep-ph/0611243 ( Lecture notes at TASI2006, Boulder, CO, June 2006 ) Petr Vogel says:
“…[in]… the Heidelberg-Moscow experiment … no obvious peak at the …[ 0 nu beta beta ]… expected position can be seen …
Nevertheless, a subset of members of the Heidelberg-Moscow collaboration reanalyzed the data (and used additional information, e.g. the pulse-shape analysis and a different algorithm in the peak search) and claimed to observe a positive signal corresponding to the effective mass of … 0.39 +0.17 -0.28 eV … That report has been followed by a lively discussion …
the next generation of experiments … will, among other things, test this recent claim. …
let me briefly comment on the most advanced of the forthcoming … experiments CUORE, GERDA, EXO, and MAJORANA … These four experiments are in various stages of funding and staging. First results are
expected in about 3 years, and substantial results within 3-5 years in all of them …”.

So here too we have a drama of a collaboration with one view of experimental results and a subgroup of the collaboration holding a very different view, and the prospect that bigger/better experiments may resolve the matter within 3 to 5 years.
This is the sort of thing that, to me, is the heart of physics, and what makes physics fun.

Tony Smith

PS – Based on my own (unconventional) physics model, my bet is on no Majorana masses, and for Dirac masses of two of the three neutrino states nu_2 and nu_3, at around 0.009 eV and 0.054 eV, with nu_1 being massless.
Very roughly my physical picture is of a 4+4=8-dim Kaluza-Klein structure in which the higher (massive neutrino) generations have interactions related to all 8 dimensions while the first (massless neutrino) generation lives in 4-dim physical spacetime. It is somewhat like the overall structure of the model of Arkani-Hamed, Cheng, Dobrescu and Hall in hep-ph/0006238 (Phys. Rev. D62, 096006 (2000)), but differing in details.

6. dorigo - February 13, 2007

Hi Tony,

thank you for your insightful review of the issue, and your comment on your model. I did read your neutrino mass matrix paper a few months ago, by the way, and I found it intriguing, even in my usual confusion when trying to absorb that kind of information.

I think your comment is worth a post here, and I will paste it one level up, in the hope you do not mind… If so, let me know.


7. More on neutrino mass in the SM « A Quantum Diaries Survivor - February 13, 2007

[…] Posted by dorigo in internet, science, books, physics, Blogroll. trackback Quite unusually, my last instantiation of the periodic posting of “The Say of the Week” resulted in some constructive criticism […]

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