As you can see elsewhere on these pages, I am intrigued by the possibility that the particles and symmetry of the Standard Model may be “emergent” – arising from an underlying theory without these fields or symmetries. While I would also like to find a viable theory with this property, it is also important to think of ways to test this possibility. As explained in a talk that I gave at the Meeting on CPT and Lorentz Symmetry, this can happen if we observe a violation of gauge invariance or of general covariance.
Mohamed Anber, Ufuk Aydemir and I are involved in a set of projects that develops this idea. The paper describing this program has been posted on the arXiv. Space limitations prevent full comments and also full referencing, but hopefully we can provide these in our future papers.
This idea is pretty unconventional in particle physics. However, that is an attraction. Thinking about this new topic leads to many interesting ideas and who knows where it will lead!

This week Thibault Damour and I put out two papers on potential violations of the equivalence principle due to light scalars. These are:

a short paper with the main results designed to be easier to read, and

a longer paper with more detail and discussion of several theoretical issues

I think that these will be useful for the field. Maybe the most unusual aspect is that they took 7.5 years to complete. On my sabbatical to the IHES, Thibault was discussing the EP and noted that the main unknown was in the effects of nuclear binding. This lead to my work on the quark mass dependence of nuclear binding, and our other collaborative work on limits on the variation of quark masses. So it has been a fruitful effort, but it sure did take longer than it needed to.

Mohamed Anber, Ufuk Aydemir and I have recently posted a paper on a new line of research. We are interested in the possibility that the gauge theories in nature (and gravity) are emergent phenomenon. I have some comments elsewhere on this site about this idea. However, the key new feature is that if symmetries are emergent, they are not absolutely true at all scales and there can be some small breaking of the symmetry. Normally one looks for new physics by considering possible interactions that conserve the gauge symmetries of the standard model. The basic argument for this is in a classic paper by Buchmuller and Wyler.
However, if the symmetries are emergent this is not sufficient and one must also consider terms whih break the invariance.

We argue that gravity is a good place to look for such breaking, because gravity is already extremely weak.
In our paper , we test out a sample interaction that breaks general covariance involving two derivatives of the metric. Indeed we find a very strong constraint, with a dimensionless number of order 10^-20.

I feel that this approach can provide a unique method to test the idea of emergence. There are many other things that we can attempt and hopefully these will be carried out in the future.

In February I gave a talk with the title “When effective field theory fail”
as the opening talk to a conference on effective field theory in Valencia in February. The paper has now been posted and will be published in the proceedings of the conference.

The title was somewhat provocative, and in some ways the whole talk was also. One of the things that interests me is the limits of effective field theory. The talk describes four cases in which I think that the effective theory fails in ways that are not expected by conventional practice. The most detailed is a study of kaon loops in chiral perturbation theory, which I have come to realize are not reliable. The Regge physics of my paper with Daniel Wyler is a second example. I have long had some concerns about the extremely long distance behavior of gravity – these are phrased more clearly in this paper. Finally, there is a paper (likely two) with Mohamed Anber and Ufuk Aydemir that is foreshadowed in the talk – hopefully this paper should appear soon.

One of the puzzzles that interests me about high energy perturbative processes is the role of Regge physics. Regge particles are somewhat out of fashion, being a major topic of the 1960s – long ago. However, from that time and from continuing work by Lipatov Fadin and their school, it is clear that Regge physics is real and that it occurs in QCD. These Regge studies exists as distinct subfield of QCD.

This is a problem for SCET. So far, Regge physics has not been found in SCET. This is a clear conflict, since SCET is supposed to describe the high energy limit of QCD and Regge physics should be part of this. This is of personal interest since one of my old papers using Regge physics on final state interactions is in apparent conflict with classic perturbative work by BBNS on factorization. In fact there is not a direct conflict, because Regge physics can convert perturbative logs into powers s^alpha. (To explain this fully needs a longer conversation.) But the conclusions that the community draws draw from the two works are different and conflicting.

Daniel Wyler and I have explored a part of this problem – the kinematics. This was done in the spring of 2008 but we finally got a written version out in our new paper. This leads to an plausible understanding of why Regge physics has been missed in SCET. There needs to an infinite number of scales involved, as well as one step that involves a Glauber or Coulombic gluon.

This summary of the kinematics is only part of the answer. Daniel and I would like to also understand how to match the Regge manifestation onto perturbative QCD. We have not yet done this, and this is a key task that needs to be done in SCET. I would argue that SCET is incomplete, and many of its predictions are uncertain, until the power corrections due to Regge physics are incorporated into the theory.

I recently attended a workshop in Madrid with the title “Light quark masses and hadron physics and the sub-title “(from quarks to life)”. The sub-title hinted at a good portion of the agenda of the workshop, which was the impact of quark masses on anthropic reasoning. This comes about because if the light quark masses were slightly different, nuclei and atoms would not exist. This is a topic that I have been interested in, both early on with Dave Seckel and Steve Barr and recently with Thibault Damour. I talked on my work on heavy nuclei and the resulting anthropic constraints. You can find my talk and the others on the conference web page. This is getting to be almost a respectable subject.

While teaching last year, I realized that there was a way to make the Standard Model with only vectorial currents, rather than the usual left-handed ones. The usual chiral structure then emerges after the Higgs separates the mass eigenstates into those with left-handed and right handed interactions. In some ways it is a beautiful variant, because the gauge sector does not have this crazy mix of left and right handed currents used in the usual standard model. I was astounded that this version was not known back in the early days of weak gauge theories, where there was a popular but less economical variant with separate left and right gauge bosons. After a good deal of literature searching, it turns out that it was uncovered by Montvey in the lattice community, but had not been treated much phenomenologically. With Mohamed Anber, and Ufuk Aydemir and Preema Pais, we have recently revisited the model. It has troubles with the electroweak precision parameters, but there is still a region of parameter space that is still viable. There is some possibility that the mirror particles involved can be viable dark matter candidates. The LHC should do a good job of probing this remaining parameters space.

Today, Preema Pais and I posted on the arXive our work on the idea of gauge federation. The point here is that there may be extra SU(N) gauge groups that exist independently of the SU(3) x SU(2) x U(1) groups of the Standard Model, and are not unified into a single GUT group at high energy. The couplings of these groups would run mostly independently of each other, aside from modifications if the fermions of the higher gauge groups also influence the lower N groups. We motivate this possibility from the idea of emergent gauge theories. There are many successful examples of gauge federation in which the couplings come together to a common strength at high energy. This provides an alternative paradigm to the idea of gauge unification.

Andi Ross, Koushik Dutta, Max Tegmark and I recently put our paper on the likely values of the Higs vev on the web – here is a link. This touches on the issue that maybe the constraints on the existence of atoms may be related to the unnaturally small value of the Higgs vacuum expectation value (vev). This idea was first suggested in my paper with Steve Barr, Dave Seckel and V Agrawal. We had been inspired by Weinberg’s anthropic discussion of the cosmological constant and realized that the constraint that atoms exist could explain the key puzzle – called the fine tuning problem – of why the Higgs mass scale was so low. In this work we considered the variation of the vev with all other parameters of the Standard Model held fixed.
In the present work with Andi, Koushik and Max we address what would happen if the other parameters of the Standard Model were also variable. We argue that the most relevant ones are the Yukawa couplings that are related to quark and lepton masses. For these, we use the experimentally observed mass distribution, found in previous work..
Since the Yukawa couplings vary, there is not just an allowed window for the vev, but rather a liklihood distribution. It is this that we describe in the paper. The result works in the sense that there is a modest spread about the most likely value, and the observed value is pretty typical of the allowed values. While this observation certainly does not show that the atomic constraints are responsible for the low value of the vev, nevertheless the results support the possibility that atomic constraints are at play in this hep puzzle.