Strings ante portas

Tomorrow Strings 2008 kick off here at CERN. The place and the timing of the conference were carefully planned to coincide with the first LHC results that would hint toward string theory. The plan didn't quite work out due to the LHC schedule slip, which means we have to wait one more year for the ultimate confirmation of string theory. The expectation makes it all even more exciting.

The warm-up for Strings 2008 was the Summer Institute on String Phenomenology that has been taking place at CERN TH during the last month. Although I'm usually attracted by wordplay, puns and oxymorons, this time I didn't manage to attend many talks. That's partly due to my summer travelling, and partly due to my summer paresse. Among the few talks I've seen I best remember the one by Cumrun Vafa who talked about F-theory phenomenology. He presented a simple and elegant construction that connects F-theory to reality (that is to the MSSM). Although I wasn't able to grasp all the details, the picture below gives a rough idea:
For detailed experimental predictions, the experimentalists from ATLAS and CMS are encouraged to consult this paper. Although there are no specific predictions for the superparticle spectrum, your perspective may be changed by the fact that the Higgs particle you will discover is in reality a matter curve on Riemann surfaces located at the intersection between the
GUT model seven-brane and additional seven-branes in the full compactification where the U(1) hypercharge flux is non-vanishing. For the neutrino physicists, the important piece of information is that the neutrinos are Dirac or Majorana and their masses are roughly of the order of what is observed. I heard some skeptics saying that back in the old days phenomenology meant a different thing, but such grumbling should not be taken seriously.

Highlights from the conference along with more nasty comments are soon to appear on this blog. Live webcast here or via technically more advanced Lubos' blog.

Un

Since Howard Georgi taught us how to turn anything into unthing, unparticles have quickly spread to all areas of particle physics. This amazing expansion have been so far consistent with the unthropic principle which says that, in the universe that supports intelligent life, unparticles cannot have any useful application. Nothing is sacred these days, and even the Higgs was recently downgraded to the Unhiggs. Yet in spite of my trademark sarcasm, it seems to me that the Unhiggs may violate the unthropic principle and that the whole idea may turn out to be of some use.

In the good old Standard Model, the Higgs particle fulfills multiple tasks. First of all, it condensates to give mass to the W and Z bosons. But this is just a beginning. The presence of the Higgs particle renders the Standard Model well-behaved - *unitary* - at high energies. A massive W boson has three polarization states - two transverse (to the direction of motion), and one longitudinal. It turns out that the scattering amplitude for the longitudinally polarized W's grows with energy, and at some point it would violate general unitarity bounds of quantum field theory. But the diagrams with the Higgs exchange cancel the dangerous terms in the amplitude and the theory recovers the consistent high-energy behavior. In the particle physics jargon, the Higgs unitarizes WW scattering. Finally, the Higgs contributes to electroweak precision observables. The success of the Standard Model in fitting the LEP and Tevatron data relies to a large extent on assuming loop contributions of a fairly light (less than 200 GeV) Higgs particle.

Since the Higgs does his job so well, living without the Higgs particle is difficult. The Higgsless models make a try. They invoke new heavy spin 1 particles to unitarize WW scattering, but they do much worse in electroweak precision tests. It seems that the Unhiggs is a new possibility. Given the dearth of calculable ideas for electroweak symmetry breaking, any new direction is worth looking at.

In a recent paper, David Stancato and John Terning attempted to get W and Z boson masses from a scalar unparticle Higgs condensation. The first step is to construct a gauge invariant action for the Unhiggs. This is actually quite tricky. Usually, making the action gauge invariant amounts to replacing normal derivatives $\pa$ with covariant derivatives $D = \pa - i A$. But the unparticle nature of the Unhiggs implies that the kinetic term is some non-polynomial function $F(\partial^2)$, rather than the simple $\partial^2$, so that the usual procedure does not apply. Yet there is a trick that makes use of non-local objects called the Wilson lines. The final outcome is a complicated, non-local action: whereas in the normal gauge theory there are vertices with only 3 or 4 gauge bosons legs, in the Unhiggs set-up there exist vertices with an arbitrary number of gauge boson legs.

The paper shows that, even though the interactions are weird, the Unhiggs unitarizes WW scattering. It is not clear yet how well it fares with the electroweak precision tests.

The most serious problem with this approach is that it's unclear what would happen if such unthing is discovered at the LHC. If the Higgs particle is discovered, the Nobel Prize will sure go Peter Higgs who predicted a particle, but he obviously cannot be honored for an unparticle. The three possible scenarios are
1) Unpeter Unhiggs gets a Nobel Prize.
2) Peter Higgs gets the Ignoble Prize
3) There is a fatal error and the universe disappears.

We'll see. Or not.

The paper is here, but be warned that it's unpretty technical.

Burning Wimps

Malcolm Fairbairn had an interesting seminar last Wednesday here at CERN. It was about hypothetical stars whose dominant source of energy is dark matter annihilation rather than nuclear fusion. Such an object is called a dark star or, to avoid confusion with the movie, a WIMP-burner.

Dark matter is 5 times more abundant than the baryonic matter but it does not form compact objects because, interacting so feebly, it cannot efficiently dissipate energy. It can however fall onto dense baryonic objects like stars or even planets. If the dark matter particle is a WIMP, it should scatter on the nucleons that make a star and, from time to time, get captured by the gravitational field of the star. The enslaved particles form a spherical dark core of the star. Once the density in the dark core becomes large enough, the dark matter particles can efficiently annihilate with each other. It is expected that for large bodies like stars the capture rate and the annihilation rate eventually come into equilibrium. There are many unknowns in this game: the local dark matter density and velocity distribution, the mass of the dark matter particle, the scattering cross-section of dark matter on nucleons. Actually, the spin-independent cross section is constrained by XENON and CDMS to be less than $10^{-7}$ pb for a weak scale WIMP, but the spin-dependent cross-section (which is relevant for typical stars, as they are made mostly of hydrogen) can be as large as $10^{-1}$ pb. Interesting effects can be obtained provided the cross-section is close to the upper limit.

Searches for dark matter in the Earth core are sponsored by the Templeton Foundation, since a positive signal would allow us to identify the centre of the Earth as the location of Hell. There is more chance, however, to find it in the Sun. In the optimistic scenario -- a SUSY WIMP with $\sigma_{SI} = 10^{-2}$ pb and $M_{DM} = 100$ GeV -- the Sun may hold as much as $10^{13}$ tons of dark matter. The annihilation of dark matter in the core of the Sun could produce highly energetic neutrinos that are being currently searched for by terrestrial observatories, like for example the ICECUBE detector at the South Pole. Yet the Sun is hardly a dark star since, with the assumed parameters, the annihilation accounts for only $10^{-10}$ of its luminosity. In order to seriously affect the stellar dynamics one needs more infalling dark matter.

The galactic centre is expected to host much more dark matter who is attracted there by the supermassive black hole and other fireworks. The actual dark matter density at the centre is another unknown, but according to some models it can well be $10^{10}$ times larger than in the Solar System. This could be enough to make a dark star. One difficulty is that the stars move much faster close to the galactic centre which makes the capture process less efficient. However, stars with sufficiently elliptical orbits slow down from time to time and should be able to capture enough dark matter.

How would a dark star look like? As far as I understood, at first sight it's not obviously different from an ordinary star, but there are important differences in the stellar dynamics. Somewhat counterintuitively, energy release due to the annihilation makes the core temperature smaller.
As a consequence, the nuclear reactions become slower and may even shut off completely. Thus, dark stars burn more slowly so that they may live longer than ordinary stars of similar size. Therefore, one way hunt dark stars is to identify abnormally long living stars.

Astronomical observation in the galactic centre are challenging because the place is terribly polluted by dust. In the recent years, however, astronomers were able to peer behind the dust using the radio-wave and infrared observatories. Several stars orbiting around the central black hole were spotted. A few of them seem to be too massive to live long enough to wander by mistake from other places, while it's believed that new stars cannot form close to the centre because of large tidal forces due to the black hole. This is known as the youth paradox. Malcolm suggests that WIMP burning is a viable explanation of the paradox.

See also Malcolm's recent paper. The slides are here (warning: 20Mb and in fact it's all dark).

New Old Dama

A week ago Graciela Gelmini gave a seminar about DAMA here at CERN. DAMA is trying to detect dark matter by observing the nuclear recoil when a dark matter particle interacts hits the ordinary matter. Unlike all other similar experiments, DAMA has been claiming that they actually see the signal. Recently, the DAMA-LIBRA collaboration (it was originally called DAMA-VIRGO, but it was renamed) presented new results whose statistical significance is over 8 sigma. DAMA's claim is however largely ignored in the community. The main reason is that, due to the specifics of the experimental set-up, the interpretation of the DAMA results is controversial. The other experiments use at least two independent probes to discriminate the signal from the background, for example, XENON measures the scintillation and ionization signals, while CDMS focuses on ionization and phonons. As DAMA is sensitive only to the ionization signal, they have no handle to reject the hits due to ordinary particles like electrons or neutrons. Instead, DAMA's claim is based on the observation a periodic annual dependence of the number of hits in their detector. The annual modulation can be attributed to dark matter because the flux of dark matter particles pervading our galaxy varies annualy due to the motion of the Earth around the Sun. The problem is that there are more things in heaven and earth that display an annual variation: four seasons, temperature, hormone level, etc., and it is possible that one of those is responsible for the modulation of the background. Although DAMA insists that they carefully checked and excluded all possible sources of the background, dark matter doesn't seem to be the most likely explanation. Moreover, other dark matter experiments have claimed to exclude the region of parameter space favored by DAMA. Last not least, DAMA has gained a bad reputation, because of their agressive presentation strategy.

There seems to be a trend, however, to take DAMA a bit more seriously, and recently I've seen quite a few new preprints on arXiv that had DAMA in the title. The important question is if the DAMA signal can be reconciled with the null results from other experiments. According to Graciela, this is not very difficult. The point is that DAMA has a lower threshold than most other experiments. Therefore a fairly low mass dark matter particle with a fairly large cross section could show up in DAMA but escape detection in other, in principle more sensitive experiments. Graciela had a paper a few years ago pointing out that a 5-9 GeV WIMP with a $10^{-4}$ pb cross-section can explain the DAMA signal. No dirty tricks are needed -- the conventional Maxwellian velocity distribution of dark matter and the spin-independent cross section will do the job. The newest experimental results still leave that window open. The current experimental constraints are more extensively discussed in the new paper by Petriello and Zurek, from which I stole the plot.

Numerology suggests that a 5 GeV WIMP might not be a bad idea after all: the ratio of baryon density to dark matter density is $\Omega_{DM}/\Omega_{B} \sim 5$, which might be the consequence of $m_{DM} \sim 5 m_p \sim 5 $ GeV. Models that predict such a relation do exist, see for example this old idea of David Kaplan. There is another dark matter experiment in Gran Sasso called CRESST that has a lower threshold than DAMA, so that it should be able to close the low-mass window once their statistics is improved. Yet I don't expect a conclusive resolution of the DAMA puzzle soon: DAMA was being excluded many times before, but it always managed to find a new window to slip back in ;-)

Sagrada Familia et al.

I have not reported much from CERN in May because I was away most of the time. Last week I was enjoying my holidays in Barcelona. One of the numerous tourist attractions there is a cool science museum with a planetarium, physics toys, dinosaur skeletons, gaudy fish and octopussies. What kids seemed to like the most, however, was a temporary exhibition of particle physicists in their natural habitat who were attracted to that place by the conference Planck'08. On the photo you can see the exhibits observed by an intrigued American tourist. The physicists are kept behind a provisional fence so that tourists do not feed them (scientists in general require a carefully selected diet of coffee and cookies).

Planck is an annual European meeting centered around model building beyond the Standard Model. It usually gives a fair overview of what's new in the field. A short glance at the program reveals that there is not much. I could often hear this unpleasant sound of an empty barrel being scraped. Nevertheless, there were a few noteworthy talks too. Amusingly, most of the theoretical developments these days happens in the queer corner of particle physics. For more than 30 years particle physics was focused on very well motivated extensions of the Standard Model, which didn't get us very far. The recent trend, prompted by Howard Georgi's joke, is to play with perfectly unmotivated models instead. The hope is that shooting at random will prove, if not more successful, at least more fun. Thus, there's much a do about unparticles and its close relatives - the hidden valley models. A new member of the family proposed by Markus Luty is called quirks. This is an extension of the Standard Model with a hidden sector being a QCD-like confining theory. Unlike the ordinary QCD where some light quarks are much lighter than the QCD scale ~1 GeV, that hidden sector has a small (much less than TeV) confinement scale but heavy (more than TeV) quarks. The resulting phenomenology is weird. In QCD, two quarks flying apart form a string in between them whose energy density soon becomes large enough to pull new quarks out of the vacuum. For quirks this is not the case, so that a string between two quirks does not break and may stretch to macroscopic sizes. Markus is currently investigating the collider signals of his quirky scenario.

Besides that, I would mention Gia Dvali who keeps pushing his huge-number-of-degrees-of-freedom solution to the hierarchy problem, Riccardo Rattazzi who made some progress in understanding of the conformal field theories of the Luty-Okui type, and John Terning who explores the AdS/CFT approach to unparticles and hidden valley. Yet the overall impression is that theorists are regrouping their forces while waiting for the LHC. The real game will begin in a year or so.

Pauli's Other Exclusion Principle

As I am currently stretched between continents, I ponder over the differences between the US and Europe. Apart from the taste of food and the size of humans, there seems to be a fundamental difference at the level of particle physics. Let's have a closer look at the time and place of discoveries of elementary particles:

• Tau neutrino, 2000, Fermilab, United States
• Top quark, 1995, Fermilab, United States
• W and Z bosons, 1983, CERN, Switzerland
• Gluon, 1979, DESY, Germany
• Bottom quark, 1977, Fermilab, United States
• Tau, 1975, SLAC, United States
• Charm quark, 1974, SLAC/Brookhaven, United States
• Up, down, and strange quarks, 1968, SLAC, United States
• Muon neutrino, 1962, Brookhaven, United States
• Electron neutrino, 1956, Los Alamos, United States
• Muon, 1936, Caltech, United States
• Photon, 1905, Patent Office in Bern, Switzerland
• Electron...let's skip that one for simplicity...

This can be summarized as Pauli's other exclusion principle:

Fermions are discovered in the US, whereas bosons are discovered in Europe.

Pauli's other exclusion principle has been confirmed in numerous experiments, sometimes un a quite spectacular way. Around 1974, the most powerful accelerator in the world was the ISR at CERN who produced charmonium in commercial quantities. However, since a discovery of the J/Psi particle at CERN would contradict Pauli's other exclusion principle, the experiment set its cuts so as to reject events with pT < 3.2 GeV (the mass of the J/Psi particle is 3.1 GeV). Later on, some significant efforts were invested to miss the Upsilon particle. The cancellation of the SSC can also be explained by Pauli's other exclusion principle: the SSC would be able to discover the Higgs boson.

Since, there is no more doubt that Pauli's other exclusion principle is a fundamental law of nature, it can be used to formulate predictions. One obvious implication is that Higgs searches at the Tevatron are pointless, and resources should be reallocated to searches for new heavy quarks. The Higgs boson, if it exists, will be discovered at the LHC. The new physics at the LHC, however, is severely constrained. Typical solutions to the hierarchy problem predict new fermions at the TeV scale. The little Higgs or extra dimensional scenarios predict a vector-like partner of the top quark at accessible energies, while supersymmetry predicts fermionic partners of the gauge bosons. On the other hand, the Standard Model with a light Higgs is perfectly consistent with Pauli's other exclusion principle...

Thanks to Jose Ramon for pointing out the facts to me.

Crackpot for Dummies

These days it is hard to sound absurd in particle physics. When none of them along the line know what any of it is worth, you are in constant danger of being taken seriously. But if you're really determined to convince everybody that your idea is nonsense, the short tutorial below might prove handy. It is not enough to just come up with a pile of nonsense - presentation is the key. The tutorial is illustrated with examples from the seminar a week ago by John Moffat who has achieved a certain perfection in the art.

1. First of all, of course, you need a great nonsensical idea. That's the easiest part. Take any long-standing problem and write down a bold solution. For example, find a mechanism of breaking the electroweak symmetry without introducing any new degrees of freedom and without violating unitarity.
2. Your solution should rely on technical terms that have no meaning to anyone else but you.
Symmetry breaking fermion loop measure sounds just perfect. Never try to explain the meaning of that - you can always refer to your 11 previous publications in case somebody asks.
3. On the other hand, you should elaborate on trivial points. For example, you can explain at length why in a theory without a Higgs particle there is no quadratic divergences to the Higgs mass.
4. It is useful to formualate your predictions using as many digits as possible. For example, you could give the value of the non-local electroweak energy scale to be 541.189 GeV.
5. Make clear that your idea explains any experimental discrepancy that is actually on the market.
6. To make connection with the rest of theoretical physics you should often mention string theory, supersymmetry, little higgs and everything else, noting each time that your idea is none of the above.
7. To whatever question from the audience you should reply by repeating the last sentence you just said.


But seriously...fringe or non-mainstream ideas are important, even when they're weird, and even when they're not quite right at the end of the day. That's provided you make an attempt to explain your point and face criticism. Otherwise, all you get is a ridicule. At least here at CERN, where jester is at loose.