Saturday 3 April 2010

Farewell to the Noughties - Theory

The LHC just started colliding protons at 7 TeV, marking the symbolic beginning of a new decade in particle physics. A good moment to complete the summary of the past one. Some time ago I made a list of most important particle-related experimental results. Time for theory. What significant developments took place in particle theory during the noughties?

Well...none, in the first approximation. The past decade has been marked by inertia and intellectual masturbation. The last truly novel ideas, like AdS/CFT, Randall-Sundrum, or ADD, were all born back in the 90s. No surprise that the list of 50 top cited articles last year contains only 3 particle theory papers written in the 00s, none of which in a prominent position.

Nevertheless, our understanding of particle theory has progressed, somewhat. Here is my subjective, biased, and utterly unfair summary of the most interesting developments.
  • Extra dimensions are strong dynamics
    Warped extra dimensions a-la Randall-Sundrum have dominated new physics model building. Perhaps the most interesting aspect of that industry is a qualitative analogy between five-dimensional warped models and purely four-dimensional strongly coupled models. This of course is not completely unexpected given the AdS/CFT conjecture. Nonetheless it is interesting that the correspondence extends to more down-to-Earth and phenomenologically relevant examples, even if in a vulgarized form. And so, 5D Higgsless theories are large N technicolor models in disguise, 5D gravity in a black hole background captures some aspects of heavy ion physics, etc. Even low-energy QCD can, to a certain extent, be modeled this way, and some quantitative predictions for the parameters of the effective chiral lagrangian can be derived.
  • Higgs can be stabilized without supersymmetry
    The bulk of particle theory is driven by the fact that the Higgs boson mass in the standard model receives large, quadratically divergent corrections at the quantum level. The common expectation, or maybe just wishful thinking, is that new symmetries and new particles appear at the TeV scale to fix that problem. The best known example - supersymmetry - is based on a boson-fermion interplay, for example, quantum corrections from the top quark are canceled by its scalar partners called stops. This is however not the only possibility, and the cancellations can occur between the same-statistics particles, for example top quark contributions can be canceled by another heavy colored fermion. This option has been known since 70s, but only during the last decade it was systematically understood and classified in the framework of little Higgs and gauge-Higgs unification theories. The end results is rather depressing though: all models we have constructed so far are just as good, or rather just as bad, as supersymmetry.
  • QCD is boring but it's here to stay
    Theorists working on QCD have always been looked down upon by smartasses building new fancy models of the universe. Yet in the past and in the coming decade hadron colliders are the sad reality, and an input from QCD theory is necessary to isolate new physics from mundane background processes. Definitely, tons of good work in that direction has been done. At the most basic level, we have seen heroic computations of higher-order corrections to SM processes like W+jets, Z+jets or ttbar+jets, and so on, without which life at the LHC would be much harder. At a more sophisticated level, new jet algorithms better suited for hadron colliders have been developed, and new ways to search for new physics using jet substructure have been proposed. One should also mention the progress in theoretical handling of QCD, for example the soft-collinear effective theory.
  • There is more to dark matter than meets the eye
    Models of dark matter are more numerous than stars in the sky, so why bother about another thousand spawned during the last decade? However, some recent proposals are important because they changed the way we search for dark matter. On one hand, models based on KK parity and T-parity prompted us to explore new collider signatures. Even more important was the impact on direct detection experiments. Not so long ago experimenters, brainwashed by MSSM preachers, searched only for spin-independent (coupled to nucleus' mass) or spin-dependent (coupled to nucleus' spin) elastic WIMP scattering. Experimental set-ups as well as data analyses were tailored for these 2 possibilities to the point that less standard dark matter signals would simply be discarded as background. This embarassing situation has been greatly improving in recent years. Thanks in part to inelastic dark matter models, or the recent offensive of light GeV scale elastic dark matter models, experimental analyses are becoming more flexible and developing alternative experimental techniques is being encouraged.
  • There are more ways to compute scattering amplitudes
    Anybody who ever computed scattering amplitudes in gauge theories can't help the feeling that there is something wrong with the standard way of doing it. In the approach via Feynman diagrams, hundreds of complicated expressions at the end of the day magically combine into something far more simple. It is becoming more and more clear that gauge theories may hide surprising mathematical structures that control scattering amplitudes. During the last decade some of these structures have been uncovered thanks to e.g. BCFW recursion relations, CSW rules, or fancy twistor space techniques. More recently, a new approach based on Grassmannians suggests that the hidden simplicity extends to higher loop levels, at least in the maximally supersymmetric case. But this last one might be more appropriate for my Farewell to the Teenies...

So much for the last decade, now dying to see the new one. Clearly, it can't get much worse :-)


Luboš Motl said...

This is a very tendentious description, Jester.

In the noughties, we've seen BMN which is topcite 1000+, a very fine realization of string theory including exciting strings within field theory.

And there has been the KKLT direction, mapping the apparent vastness of the landscape. We may dislike the conclusions but that can't change the fact that there's evidence that the multiplicity of solutions is real.

At any rate, drought is how the decades will look like after "the theory of everything" is really understood - and we may not be that infinitely far from that. As Feynman said, it's clear that the same rate of progress can't last indefinitely. This was just clear a priori. It's preposterous to be "disappointed".

Of course that people must be excited about the things that have already been found. It's just wrong to be making bets for "permanent new revolutions".

With this said, I am convinced that the theoretical physicists are not yet finished with the big things but there's no guarantee by Nature that things have to be finished by 5 or 10 years.

Anonymous said...

Lean years indeed, especially for particle phenomenology (``formal theorists'' always seem to find ways to keep busy). By now most people realize that ADD and RS are grossly overrated, but papers citing this weird stuff continue pouring in. Can't wait for the LHC to unleash some real particle physics!

Per said...

Hi there

I like your posts. Especially this one. You have a good way to summarize thing in a kinda laid back way and still keep the important stuff in focus.

Keep it burning!


Anonymous said...

Dark matter experiments have always looked for signals from low mass candidates. Inelastic models are really forced. Axions have always been searched for, as have near-strongly interacting candidates.

Jester said...

Yes they have been looking for light dark matter but the sensitivity is much worse. The main reason is that most experiment use fairly heavy targets such as germanium or xenon. This is perfect for detecting 100 GeV dark matter particles, but what if dark matter is 1-5 GeV? Only recently people started thinking about direct detection experiments based on helium...thanks in part to theorists I believe.

Anonymous said...

Sucks to be pwned by condensed matter:

Anonymous said...

To write this: "all models we have constructed so far are just as good, or rather just as bad, as supersymmetry." right after talking about Little Higgs models is definite proof you do smoke illegal substances to write your posts. Could you share with us what you take exactly? Looks like worth a try!

Anonymous said...

No need of job search committees, and all that. After the citation summary, the h index, now the blogs. Fantastic !!!

Did you read all the 100 000 papers or so written during the decade?

You forgot for the noughties Maldacena instead of RS,in particular for QCD related stuff, the latter did not bring anything (but did for the hierarchy issue). You forgot that your holographic model of hadrons fails (no partons).

Putting little Higgs at the level of SUSY is surprising too. A simple model for collective breaking, and a symmetry in nature between bosons and fermions have the same scientific implications???

But I support you in forgetting KKLT, which generated (many of low quality) papers but did not solve neither the susy breaking issues, nor anything useful.

You forgot instead, the unparticles (and their soft-wall cousins), the Horava-Lifshitz gravity which represent lead to the lowest level of theoretical physics papers ever written, where one studies phenomenology of theories before they build the theories themselves!

The poor DM experimentalists are more and more depicted by theorists as stupid and narrow minded; They believed in the stupid WIMPs, and they releasing data that did not check seriously, just to make the buzz. Where are the GZK related papers?

Ervin Goldfain said...


Unfortunately, few people realize that during the last decade or so we became aware of how critically important non-equilibrium statistical physics is in strongly coupled field theory, inflationary cosmology and condensed matter phenomena. Non-equilibrium dynamics of quantum fields is a key player in electroweak baryogenesis, chiral phase transitions and quark-gluon plasma in heavy ion collisions, dynamics of phase transition in Bose-Einstein condensates, spin glasses, stochastic models of the dark sector, non-extensive statistics and complex dynamics in high-energy particle interactions.
It is also unfortunate, in my opinion, that there are too many theorists that continue to believe
in the myth of field unification via larger symmetry groups. While it is true that EW unification occurs at high energies, they keep forgetting that the same EW sector or physics beyond SM are responsible for broken symmetries in meson physics (P and CP violations). The same goes for the anomalous magnetic moment of leptons, the excess of positrons @ PAMELA and muons @ CDF and other deviations from SM that the LHC will likely uncover.



Jester said...

Remember that this list is "...subjective, biased, and utterly unfair..." and "tendentious". Nevertheless, a few comments are in order:
i) Maldacena belongs to the nineties,
ii) KKLT, Horava-Lifshitz, or unparticles: no way. I'm surprised, though, that nobody complains about ISS...
iii) Unfortunately, I know nothing about non-equilibrium field theory to appreciate the progress there,
iv) By no means I wanted to depict DM experimentalists as narrow-minded. On the contrary, this is the field where the most amazing progress is happening right now. The (mostly past) domination of narrow theoretical ideas should be blamed on us theorists.