Friday, September 29, 2006

The Omega-Minus gets a Spin (part 1)

Last Friday, I could not come to any decision which of the many Perimeter photos I could write something sensible about, and browsed through the latest issues of the PRL instead. There, I stumbled across a paper whose title really amazed me: Measurement of the Spin of the Omega-Minus Hyperon, by the BaBar collaboration at SLAC (B. Aubert et al.), Phys. Rev. Lett. 97, 112001 (2006), and hep-ex/0606039.

My first thought was: How comes that such a paper is published as a PRL? I mean, every child knows that the Omega-Minus has spin 3/2 - just read the textbooks! After all, the Omega-Minus is that famous closing particle of the baryon decuplet!

But then, when reading the paper, I was surprised: Although this hyperon was discovered more than 40 years ago, there has not been a really conclusive measurement of its spin so far! And since the story of the Omega-Minus, its discovery, and its spin is, I think, a quite remarkable one, with connections to lots of interesting physics and some twists maybe not known to every child, I had the idea I should post something about it. I started reading more about the history of the Omega-Minus, came across many interesting details which I thought I could mention, and so, eventually, this has become a somewhat longer post... in fact, so long, that I decided to split it in two. So here is something about

  1. The baryon zoo of the 1960's and flavour SU(3)

  2. The prediction of the Omega-Minus by Ne'eman and Gell-Mann

What will follow next will cover

  1. From the Eightfold Way to the quark model and static SU(6)

  2. Quark colour

  3. The spin of the Omega-Minus

The baryon zoo of the early 1960's and flavour SU(3)

After the discovery of the neutron, it became clear that atomic nuclei are built up of two types of particles, protons and neutrons, bound together by the so-called strong force. The number of protons and neutrons does not change in strong interactions, and the corresponding conserved charge was called the baryon number, B. Moreover, proton and neutron are so similar under the strong interaction that they were considered as two different projections of one particle, the nucleon, in an abstract space called isospin space. As a spin-1/2 particle can come in two projections of its spin on an axis, s3=+1/2 and s3-1/2, the nucleon was considered a isospin T=1/2 particle, with the proton having isospin projection T3=+1/2, and the neutron having T3=-1/2. The formalism of isospin is, indeed, completely identical to the formalism of spin.

During the 1950s, more particles with the same baryon number B=1 as the nucleon were discovered: the Λ, named for the V-shaped tracks in a cloud chamber when it decays into a proton and a negative pion, the Σ's, and the "Cascades", Ξ, which got their name because of their cascading decay pattern Ξ → Σ + ... → nucleons + ... The concept of isospin could be applied also to these new baryons: the Λ is a singlet with T=0, the cascades Ξ- and Ξ0 are a doublet, as the neutron and the proton, and the Σ's form a triplet with T=1 - in fact, Murray Gell-Mann predicted the neutral Σ0 based on the assumption of the triplet, once the Σ+ and Σ- were known.

In order to classify these baryons, a new quantum number introduced by Gell-Mann, and called "strangeness", S, was useful. Strangeness (or "hypercharge" Y, which is related to strangeness by Y = B + S) can change in weak decays - similar to the decay of the neutron into proton - but not in strong interactions. Then, the Λ and the Σ's have strangeness S = -1, or hypercharge" Y = 0, while the Ξ's have strangeness S = -2, or hypercharge Y = -1. When trying to cast the description of baryons by isospin and strangeness in a unified, symmetric framework, Gell-Mann discovered that the eight baryons can be identified with the eight-dimensional, adjoint representation of the Lie group SU(3). This group, the group of special unitary transformations of a complex three-dimensional vector space, is an extension of the isospin group SU(2) to include strangeness as one further degree of freedom. It has eight generators, instead of the three generators of SU(2), which span the adjoint representation. Gell-Mann called the classification of baryons as an octet of SU(3) the "eightfold way". The very same classification scheme was discovered, independently of Gell-Mann, by the Israeli physicist Yuval Ne'eman. Ne'eman, who was then an army officer on leave to do his Ph.D. with Abdus Salam in London, died earlier this year. The SU(3) scheme discovered by Gell-Mann and Ne'eman is known today as flavour-SU(3).

The "eightfold way": the baryon octet, corresponding to the weight diagram of the eight-dimensional, adjoint representation of the group SU(3). S is strangeness, Y = B + S = 1 + S is hypercharge, and T3 is the isospin projection. Representations of SU(3) can be labelled by two numbers, p and q, which also determine the shape of the multiplet. There are two states in the centre of the multiplet, corresponding to the Λ and the Σ0. All baryons in the octet have spin 1/2.

The prediction of the Ω- by Ne'eman and Gell-Mann

Tables of elementary particles in the early 1960s were quite crowded, and the eightfold way only a first step on the road to a systematic understanding. There were man more baryonic particles known besides the octet baryons. Most of these are very short lived. Typically, they show up in scattering experiments of pions or kaons, the strange mesons, on octet baryons. There, they are visible as bumps in the scattering cross section as a function of energy. They are called resonances for this reason. The first such particles, the Δ resonances, had been discovered in 1952 by Fermi's team using pion-proton scattering experiments. Δ resonances have spin 3/2, and come in four different electric charges (-, 0, +, and ++) at the same mass of 1232 MeV, so they must belong to a state with isospin T=3/2.

In July 1962, when elementary particle physicists from all over the world met at the 11th International Conference on High-Energy Physics at CERN, there was news about resonances in scattering experiments on strange baryons. Two years before, the Σ* resonances (with isospin 1, then still called Y*) had been discovered when scattering negative kaons on protons (Margaret Alston et al., PRL 5 (1960) 520-524), which had spin 3/2 as the Δ's (Robert P. Ely et al., PRL 7 (1961) 461-464). At CERN, the detection and properties of Ξ* resonances was reported. These resonances with strangeness S=-2 seemed to form an isospin doublet, thus to have isospin 1/2 (G. M. Pjerrou et al., PRL 9 (1962) 114-117), and there were strong hints that their spin was 3/2, as for the Δ's and the Σ*s. Was there a way to make sense of these resonances, or to fit them in a classification scheme?

Two physicists (Gell-Mann on the right?) discussing the table of known hadronic particles and resonances at the CERN conference in July 1962. The second column indicates strangeness, the third column isospin. Further columns give mass and width, and the last one, with the many question marks, spin and parity. (Does anyone know what SNOW stands for?) The then recently discovered Σ* resonance is noted as Y1* in the third to last row, and the brand-new Ξ* shows up in the last row. (Credits: CERN, via Rochester Roundabout: The Story of High Energy Physics, by J. C. Polkinghorne)

Both Ne'eman and Gell-Mann attended the CERN conference. Ne'eman had submitted an abstract about his work on the SU(3) classification scheme of baryons, but he wasn't given a slot to talk about it. It seems that SU(3) wasn't taken that serious yet. But this didn't stop him from thinking hard about possible ways to integrate the new resonances into his scheme. Several multiplets could, in principle, accommodate for the new resonances: a decuplet, corresponding to (p=3, q=0), a 15-plet, with (p=2, q=1), and a 27-plet, with (p=2, q=2), whose weight diagram would have the same sixfold symmetry as the octet diagram. Ne'eman had no clue about which one to chose, when he met a husband-wive pair of experimentalists originating from Israel on the bus trip from the conference hotel to CERN, Sulamith and Gerson Goldhaber. (Gerson is now involved in the supernova Ia measurements of the Perlmutter group that established cosmic acceleration - that's amazing!). They started talking about physics, the Goldhabers asked him about SU(3), and they told him that they had, without success, tried to repeat the Alston et al. scattering experiments, but using positive kaons on neutrons instead of negative kaons on protons. That was just the piece of information that was missing! Both the 15-plet and the 27-plet, if they were the correct multiplets to classify the baryon resonances, would have required resonances of positive kaons on neutrons! Thus, they were excluded by the negative results of the Goldhaber experiment!

The 27-plet of SU(3), with (p=2, q=2), could, in principle, accommodate all the baryonic resonances known in 1962. But then, there should also be resonances with strangeness S=1, which should show up in scatterings of positive kaons on neutrons. The experiments of the Goldhabers and their group excluded the existence of such resonances. This negative result, later called the Goldhaber Gap, eliminated the 27-plet, and the 15-plet, from the possible multiplets to classify the resonances, leaving only the decuplet. Amusingly, the Goldhaber Gap corresponds exactly to the position of the elusive pentaquark Θ+, which made such a fuss in the last three years, but seems not to exist, after all.

Now, Ne'eman had everything he needed to know to come to a conclusion: The baryon resonances fitted neatly in the decuplet (p=3, q=0) of SU(3). Moreover, most excitingly, there was exactly one resonance still missing in this multiplet, the resonance with strangeness S=-3. This resonance should exist, if the decuplet scheme was right, and he could even say something about its mass, using a formula of Gell-Mann and Okubo. This formula predicted a linear splitting of the masses of the decuplet resonances with strangeness, and, indeed, the mass difference between the the Δ's and the Σ*s was about 150 MeV, as was the mass difference between the Σ*s and the Ξ*s. Thus, Ne'eman was confident that there should be a isosinglet resonance with S=-3, negative electric charge, spin 3/2, and a mass of about 1680 MeV, and he decided to make this point in the discussion following a review talk on the new baryonic resonances.

Sulamith Goldhaber, of the Goldhaber gap, with Yuval Ne'eman, visiting the Goldhabers at Berkeley (Credits: Lawrence Berkeley Lab Magnet, Vol. 8, No. 4, April 1964, p. 2)

He didn't have luck. Gell-Mann had also heard the rumours about the negative results of the Goldhaber experiment, and, consequently, he had come to very the same conclusions as had Ne'eman. So, following the presentation on Strong interactions of strange particles by G. A. Snow, both Ne'eman and Gell-Mann raised their hands to ask for permission to speak. The chairman called Gell-Mann, who was the more eminent physicist of both, and Gell-Mann announced that "[...] we should look for the last particle called, say, &Omega-, with S=-3, I=0. [Here, I is isospin.] At 1685 MeV it would be metastable and should decay by weak interaction [...]". This was the public prediction of the closing resonance of the baryon decuplet, fittingly named after the last letter of the Greek alphabet, and subsequently published in the proceedings of the conference. It seems that Gell-Mann got to know Ne'eman in person for the first time just on the way back to his chair in the auditorium, when he read Ne'emans name on the name tag. And it seems that Ne'eman was not too bitter of being scooped in the last second - at least, they started a long-lasting collaboration, and published together the Eightfold Way.

The SU(3) decuplet of the baryon resonances. The resonances known in 1962 are shown in black. There was one particle missing, at the lower tip of the triangle. This particle was called &Omega- by Gell-Mann, who predicted its strangeness, spin, isospin, and mass. Ne'eman had come to the very same prediction at the same time.

In the break following the talk of Snow, Gell-Mann and Ne'eman discussed with two experimentalists from Brookhaven National Lab, Nicholas Samios, who later became director of BNL, and Jack Leitner. They thought about possible ways to detect the &Omega- in experiment. Indeed, a search program was set up at BNL, which was successful two years later: the &Omega- was found on a bubble chamber picture (V. E. Barnes et al., PRL 12 (1964) 204), and its properties were exactly as predicted by Gell-Mann and Ne'eman.

At that point, there was no more doubt that the SU(3) classification scheme of particles had some truth about it.

The discovery of the &Omega- in a bubble chamber picture. The &Omega- leaves the short, thick track in the lower left corner. (Credits: BNL and V. E. Barnes et al., PRL 12 (1964) 204).

The actual detection of the &Omega- was a big success of the SU(3) classification scheme, but it was not the end of the story. Surprisingly, the spin of the &Omega- could not been measured so easily - even the 2006 particle data book entry on the &Omega- still states on the baryon summary page: JP is not yet measured; 3/2+ is the quark model prediction.

So, stay tuned, and read next time about:

  1. From the Eightfold Way to the quark model and static SU(6)

  2. Quark colour

  3. The spin of the Omega-Minus

and why, maybe, SLAC could have pushed the headline;

Quark model prediction finally proven after 40 years!

The story leading to the prediction of the &Omega- at the 1962 CERN conference is related in several sources. I have used (thanks, in part, goes to Google Search Inside):

  1. The Second Creation: Makers of the Revolution in Twentieth-Century Physics, by Robert P. Crease, Charles C. Mann

  2. The Particle Century, edited by G Fraser, with contributions by Ne'eman and Samios

  3. Strange Beauty: Murray Gell-Mann and the Revolution in Twentieth-Century Physics by George Johnson

  4. Rochester Roundabout: The Story of High Energy Physics by J. C. Polkinghorne

  5. The Periodic Table and Genetic Code of the Hadrons by Yuval Ne'eman

  6. From SU(3) to Gravity: Festschrift in Honor of Yuval Ne'eman edited by Errol Gotsman and Gerald Tauber, with a contribution of Gerson Goldhaber about the encounter in the bus on pages 103-105.

  7. The Eightfold Way, by Murray Gell-Mann, Yuval Ne'eman. The new edition contains the historically oriented survey Hadron Symmetry, Classication and Compositeness by Yuval Ne'eman, first published in Symmetries in Physics (1600-1980): Proceedings of the 1st International Meeting on the History of Scientific Ideas, held at Sant Feliu de Guíxols, Catalonia, Spain, September 20-26, 1983. edited by M. G. Doncel, A. Hermann, L. Michel and A. Pais (Barcelona, 1987).

Tuesday, September 26, 2006


News on my move: no news. My furniture is still subject to maximal position uncertainty, and nobody is responsible for anything. Meanwhile, I came to the conclusion that the word couch is derived from the word ouch. I felt very grown up buying a couch, really, it is the first time in my now 30 years that I own such an object of settlement! But I wasn't really prepared to sleeping on it for some weeks. I am definitely too old for that.

But here is the true progress report: effective noon today I have high speed internet in my apartment. If nothing else, I am at least connected to the rest of the world.

Seriously, in the old days before internet, I would sometimes pick up the phone receiver and listen to the dial tone when I felt lonely. It was like an open channel to the rest of the world. Nowadays its a high-speed channel, and it doesn't only come with a doooooot, but with emails, photos and videos. Actually, the only thing that's still missing is the transmission of solid objects. Imagine that: instead of getting spam emails, your monitor would throw sample pills in your face when your filter sucks, and Mohammed Send-me-bucks, the son of Emir Nowhereland would drop in and ask for your help.

Besides this I tried to use my stove on the weekend, but had to find out that the oven didn't work. So I called my landlord yesterday, and she had someone come and look at the thing. To my eyes the stove looks like manufactured in the 19st century, but it's actually not even 10 years old as I was told.

Anyway, the guy came in total silence, replaced some fuses, pushed and shoved the stove around a bit. Then he carefully knocked at the clock and the timer which are in the panel. He frowned at me in a really scary way and said: Did-you-push-these-buttons? The one with the timer? Well, when I noticed the stove didn't work I pushed about every button I could find, not that there were so many, and yes, I also turned on the timer (which almost caused me a heart attack when it went off some hours later). So I said yes. Never-push-these-buttons, the guy said, still frowning. I asked: what are they good for? NOTHING! I looked confused. TROUBLE, he said. And then I learned that turning the time makes everything go out of sync.

That's my current problem with time, everything is out of sync.

But the oven works now. I promised I won't try to set the clock.

And here is a picture of my ouch:

Besides this, I encourage you to speculate on the to-be-announced 2006 Nobelprize over at Peter Woit's blog.

Friday, September 22, 2006

Micro Black Holes

Black holes are fascinating! They merge together completely different fields of physics: From General Relativity over thermodynamics and quantum field theory, they do now also reach into the regime of particle and collider physics.

As I discussed in the earlier post about extra dimension, in the presence of additional large compactified dimensions, it would be possible to produce tiny black holes at future colliders. In this case, we would be able to experimentally test Planck scale physics and the onset of quantum gravity with the Large Hadron Collider (LHC), which is scheduled to start next summer.

The formation of black holes is a fairly robust prediction and one of the most general expectations that one can have, even though the details are still subject to research.

For me, it is quite amazing to see how this field has evolved during the last decade. Starting from a smiled upon speculation, it has by now become a widely accepted scenario for physics beyond the standard model, which is included in simulations of LHC events.

1. Micro Black Holes in Large Extra Dimensions

In the standard 3+1 dimensional space-time, the production of black holes requires a concentration of energy-density which can not be reached in the laboratory. But in a higher dimensional space-time, gravity becomes stronger at small distances and therefore the event horizon is located at a larger radius. This radius can be so large that we could bring particles closer together than their horizon. A black hole could be created.

The presence of extra dimensions results in a modification of the predictions of the standard model, which become important from a certain energy scale 'the new fundamental scale', and which might be accessible at the LHC. Due to the Heisenberg uncertainty, it requires a large energy to get particles into a small volume. Only energies close by the new fundamental scale would be sufficient to produce a black hole out of this same energy.

For collider physics one is therefore interested in the case where the black hole has a mass close to the new fundamental scale. This corresponds to a horizon radius close to the inverse of the new fundamental scale, which is much much smaller than the radius of the extra dimensions. To a good approximation, this tiny black hole just does not notice that the extra dimensions are compactified, and one can neglect the boundary condition. (The higher dimensional Schwarzschild-metric for this case has been derived by Myers and Perry in '86)

On the other hand, for astrophysical objects we expect to find back the usual 3-dimensional description. In this case, the horizon radius is much larger than the radius of the extra dimensions and the influence of the extra dimensions is negligible.

Those two case are depicted in the figure below. We will be interested in the case depicted on the right side. R is the radius of the extra dimensions (all of them have the same radius) and RH is the horizon radius of the black hole.

2. Production of Black Holes

Let us consider two elementary particles, approaching each other with a very high kinetic energy in the center-of-mass system close to the new fundamental scale. At those high energies, the particles can come very close to each other since their high energy allows a tightly packed wave package despite the uncertainty relation. If the impact parameter is small enough, which will happen to a certain fraction of the particles, we have the two particles plus their large kinetic energy in a very small region of space time. If the region is smaller than the Schwarzschild radius connected with the energy of the partons, the system will collapse and form a black hole.

The production of a black hole in a high energy collision is probably the most inelastic process one might think of. Since the black hole is not an ordinary particle of the standard model, and its correct quantum theoretical treatment is unknown, it is commonly treated as a metastable state, which is produced and decays according to the semi-classical formalism of black hole physics.

To compute the production details, the cross-section of the black holes can be approximated by the classical geometric cross-section Pi R2. A common approach to improve the naive picture of colliding point particles is to treat the creation of the horizon as a collision of two shock fronts in an Aichelburg-Sexl geometry describing the fast moving particles.

Looking at the figure on the left, we also see that, due to conservation laws, the angular momentum of the formed object only vanishes in completely central collisions with zero impact parameter. In the general case, we will have an angular momentum, and the black hole might also carry an electric charge.

Another assumption which goes into the production details is the existence of a threshold for the black hole formation. From general relativistic arguments, two point like particles in a head on collision with zero impact parameter (the b in the figure above) will always form a black hole, no matter how large or small their energy. At small energies however, we expect this to be impossible due to the smearing of the wave functions by the uncertainty relation. This then results in a necessary minimal energy to allow for the required close approach. This threshold is of order of the new fundamental scale, though the exact value is unknown since quantum gravity effects should play an important role for the wave functions of the colliding particles.

Using the geometrical cross section formula, it is now possible to compute the differential and total cross sections for black hole production. This also allows us to estimate the total number of black holes, that would be created at the LHC per year. Inserting the expected technical details for the collider, one finds a number of approximately 109 created black holes per year! This means, about one black hole per second.

3. Evaporation of Black Holes

It was shown by Hawking in '75 that a black hole emits particles with a temperature that is inverse to its mass. This means, the smaller the black hole, the hotter it will be. Since we are talking about really tiny black holes, they are very hot. The typical temperature of the micro black holes is about 200 GeV or 1016 Kelvin!

The evaporation rate (massloss per time) of the higher dimensional black hole can be computed using the thermodynamics of black holes. Once produced, the black holes will undergo an evaporation process whose thermal properties carry information about the number and the radius of the extra dimension. An analysis of the evaporation will therefore offer the possibility to extract knowledge about the topology of our space time and the underlying theory.

The evaporation process can be categorized in three characteristic stages:

1. Balding phase: In this phase the black hole radiates away the multipole moments it has inherited from the initial configuration, and settles down in a hairless state. During this stage, a certain fraction of the initial mass will be lost in gravitational radiation.

2. Evaporation phase: The evaporation phase starts with a spin down phase in which the Hawking radiation carries away the angular momentum, after which it proceeds with emission of thermally distributed quanta until the black hole reaches Planck mass. The radiation spectrum contains all Standard Model particles, which are emitted on our brane, as well as gravitons, which are also emitted into the extra dimensions. It is expected that most of the initial energy is emitted in during this phase in Standard Model particles.

3. Planck phase: Once the black hole has reached a mass close to the Planck mass, it falls into the regime of quantum gravity and predictions become increasingly difficult. It is generally assumed that the black hole will either completely decay in some last few Standard Model particles or a stable remnant will be left, which carries away the remaining energy.

To perform a realistic simulation of the evaporation process, one has to take into account the various particles of the standard model with the corresponding degrees of freedom and spin statistics. In the extra dimensional scenario, standard model particles are bound too our submanifold whereas the gravitons are allowed to enter all dimensions. For a precise calculation one also has to take into account that the presence of the gravitational field will modify the radiation properties for higher angular momenta through backscattering at the potential well.

These energy dependent greybody factors can be calculated by analyzing the wave equation in the higher dimensional spacetime and the arising absorption coefficients. A very thorough description of these evaporation characteristics has been given by Kanti in 2004 which confirms the expectation that the bulk/brane evaporation rate is of comparable magnitude but the brane modes dominate.

4. Observables of Black Holes

One of the primary observables in high energetic particle collisions is the transverse momentum of the outgoing particles, pT (pee-tee), the component of the momentum transverse to the direction of the beam. Two colliding partons with high energy can produce a pair of outgoing particles, moving in opposite directions with high pT but carrying a color charge, as depicted in the figure to the right.

Due to the quark confinement, the color has to be neutralized. This results in a shower of several bound states, the hadrons, which includes mesons (consisting of a quark and an antiquark, like the pions) as well as baryons (consisting of three quarks, like the neutron or the proton). The number of these produced hadrons and their energy depends on the energy of the initial partons. This process will cause a detector signal with a large number of hadrons inside a small opening angle. Such an event is called a jet.

Typically these jets come in pairs of opposite direction. A smaller number of them can also be observed with three or more outgoing showers. This observable will be strongly influenced by the production of black holes.

To understand the signatures that are caused by the black holes we have to examine their evaporation properties. As we have seen before, the smaller the black hole, the larger is its temperature and so, the radiation of the discussed tiny black holes is the dominant signature caused by their presence. The high temperature results in a very short lifetime such that the black hole will decay close by the collision region and can be interpreted as a metastable intermediate state.

Due to the high energy captured in the black hole, the decay of such an object is a very spectacular event with a distinct signature. The number of decay products, the multiplicity, is high compared to standard model processes and the thermal properties of the black hole will yield a high sphericity of the event. Furthermore, crossing the threshold for black hole production causes a sharp cut-off for high energetic jets as those jets now end up as black holes instead, and are re-distributed into thermal particles of lower energies. Thus, black holes will give a clear signal. A schematic picture of this process is shown on the left.

It is apparent that the consequences of black hole production are quite disastrous for the future of collider physics! Once the collision energy crosses the threshold for black hole production, no further information about the structure of matter at small scales can be extracted. As it was put by Giddings and Thomas, this would be ''the end of short distance physics''.

By now, several experimental groups include black holes into their search for physics beyond the standard model. Ideally, the energy distribution of the decay products allows a determination of the temperature (by fitting the energy spectrum to the predicted shape) as well as of the total mass of the object (by summing up all energies). This then allows to reconstruct the fundamental scale, and the number of extra dimensions.

The quality of the determination depends on the uncertainties in the theoretical prediction as well as on the experimental limits e.g. background from standard model processes. Besides the formfactors of black hole production and the greybody factors of the evaporation, the largest theoretical uncertainties turnout to be the final decay and the time variation of the temperature. In case the black hole decays very fast, it can be questioned whether it has time to readjust its temperature at all or whether it essentially decays completely with its initial temperature. Also, the determination of the properties depends on the number of emitted particles. The less particles, the more difficult the analysis.

However, in my opinion the most crucial uncertainty are the latest stages of the evaporation. For hadron colliders like the LHC, the last stages with black hole masses close by the production threshold will dominate the signature, since most of the black holes are actually produced out of parton collisions with a total center-of-mass energy close by even this threshold. In hadronic collisions there are thus very little black holes which actually capture the total available energy of 14 TeV, since the proton's energy gets distributed on its constituents. Such a problem would not be present for a lepton collider.

See also:

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First Day of Fall

See here for: First Day of Fall 2007

From Wikipedia:

Autumn (also known as fall in North American English) is one of the four temperate seasons, the transition from summer into winter. In the temperate zones, autumn is the season during which most crops are harvested, and deciduous trees lose their leaves. It is also the season where days rapidly get shorter and cooler, the nights rapidly get longer, and of gradually increasing precipitation in some parts of the world.

Fall is an alternative English word for the season of Autumn. In use now only in North American English, the word traces its origins to old Germanic languages. The exact derivation is unclear, the Old English fiæll or feallan and the Old Norse fall all being possible candidates. However, these words all have the meaning to fall from a height and are clearly derived either from a common root or from each other.

[Photo by Ron Day]


        I had gotten tired, or maybe just old,
        Had nowhere to go to and noone to hold,
        For one carless moment, I loosened my grip,
        I made a wrong step and time started to slip,
        One careless moment, just one that was all,
        Time slipped away and spring turned into fall.

        I tried to stay focused and not to look down,
        I could not stop thinking that I should have known,
        That things far below me would drop out of sight,
        And I could not tell what was wrong and what right,
        One careless moment, I lost my connection,
        Time slipped and left me without a direction.

        ...more poems

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Thursday, September 21, 2006

USA in transit

When looking for flights to see Bee in Canada, I was delighted to learn that Waterloo has, indeed, an International Airport, and that the connection by Northwest/KLM from Frankfurt, either directly or via Amsterdam, is even quite inexpensive. OK, direct means in any case that you have to change planes in Detroit, MI. I do not know whether you are planning to travel to Canada (or to some other country) via the US, but I found the current procedures for transit quite remarkable.

The first, positive thing is that - besides all things fluid or gely outside your body - you are allowed to bring carry-on luggage on board as always. I had the impression that there is quite an incertitude about this among travellers (I was not sure myself before whether I could bring my ibook into the cabin), since I have never before seen the overhead compartments as empty as this time: most overhead compartments on the Amsterdam-Detroit flight were not even half-filled.

But then, the lady at the KLM desk at Frankfurt had told me that my suitcase would be checked through straight to Waterloo. This was not true. At Detroit, all passengers had to collect their luggage at the baggage claim, and go through immigration and customs, including leaving fingerprints and being photographed. Of course, I also had to fill a visa waiver from. I did not really expect to be subject to this whole procedure, since I had a ticket to leave the US two hours later. So, the green from was stapled in my passport, and keeping in mind that you never ever should leave the states with this piece of paper still in your hands to avoid any trouble when you ever should have to go back there, I insisted that the lady at the boarding for the turboprop to Waterloo took it out.

That would not have been necessary, as I learned on my way back. This time, I even had already a boarding pass for the flight to Frankfurt, but immigration and customs were unavoidable. I had to fill, again, a visa waiver form, which I could get neither in Waterloo, nor on the plane to Detroit. Fortunately, there were forms available at the immigration post, and a German speaking Northwest employee was very helpful and even borrowed me her pen. And no Jumbo from Tokyo or so had just arrived, so the huge immigration hall was essentially empty, and the immigration officers quite relaxed and friendly. And, after fingerprinting and photographing, I was told that I can, indeed, keep my visa waiver form in the passport when leaving the US for Canada for a period of less then 30 days. On the other hand, filling that form is no big deal compared to the whole immigration/customs/security recheck procedures...

But this was not the end of the measures necessary for a simple transit: I just had decided, one hour before boarding, to spend the $9.95 for a non-resident access to the Detroit Airport Wireless to transfer my million of Perimeter photos to the Frankfurt server, when a TSA agent informed all of us waiting passengers that right now, fingerprinting and photographing was also required when leaving the US. This is comparably easy, since it is done by machines which scan the passport, give exact instructions what to do with your fingers and where to look for the photo, and finally print out a paper slip with a picture-like pixel code, but you have to walk back the mile or so from the gate to the centre of the terminal, where the machines are located. I was a little upset, since I would have had plenty of time before if anyone had told it me, but now I had to interrupt the file transmission and hurry to these machines, since I did not know how long all this would take...

I the end, all went fine, I got the plane in time, and even all my picture files arrived in Frankfurt 8 hours before me.

However, I really wonder if it is necessary that my fingerprints and my portrait are taken twice within two hours, just because I happen to change planes in Detroit. And, for my next visit in Waterloo, I will probably be looking for affordable flights via Toronto...


Blind loops

Okay folks, this is just an extended complaint about the so-called customer service, and I am writing merely for therapeutic reasons, coz otherwise I'll probably go out on the street and bite some innocent pedestrians.

Yesterday I tried to get an internet connection into my new apartment. Since I already have two cellphones, and make most of my calls via skype anyhow, I don't really need another phone line in the apartment. It wasn't difficult to find out that Rogers is apparently the Canadian company who does that stuff. So I went into the next store I could find.

There, I was told a long list of special offers and rate plans for internet over cable, but the guy knew about no technical details except those which end in dollar signs, and they also don't sell wireless routers. But okay, I could just drop into the next Walmart and get one. After the guy began to repeat his list of offers for the third time, I was getting tired and said fine, I'll take that plan, where do I sign. First problem was that last week I had to treat in my pretty California driver's licence for a temporary Ontario one, which doesn't have a photo, and therefore isn't a photo ID. But I convinced them to accept my International Student ID, which I am actually not sure why I have a valid one, since I haven't been a student for 5 years or so.

What then happened was that some 15 year old employee, an original with dental braces, named Tericita of Tiffany or something tried to enter my data in an online form. Which didn't work, and which I could have done by myself in my office btw. She tried that repeatedly, but it still didn't work. Then she called the customer-service hotline, where a tape was apologizing that all representatives are busy, but would you please hold. Tifanny-or-Tericita turned on the speaker, and began surfing the web while I was listening to the tape telling her to hold and hold and hold. After 1/2 hour or so, I asked her how long she thought that would take, but just got a shoulder shrug. After 1 hour, the representative was actually available on the phone but said essentially that his system had crashed down and he couldn't do anything about nothing.

So, after one hour looking at Tiffany browsing the web, I was asked to come back tomorrow.

I went to get some coffee and asked the next person in line which internet service he uses. Bell, he said. So, I went into the next Bell store and asked them to please, please set up internet service in my apartment.

Since I was pretty pissed off, I told the guy who had a funny beard my experience with the Roger's people. This complete incompetence of employees is something that upsets me frequently. I mean, the only thing they do is look up their own website and call customer service. That I can do on my own. Why don't they just put a computer there and a courtesy phone? You know what? That's exactly what you find in the Bell's store. A courtesy phone to call customer-service. But the guy with the funny beard was all sympathetic, and, yes Rogers sucks, good you came to see us.

Funny beard was actually quite nice, but told me they don't do cable stuff, just internet via phone, which is actually much better because blahblahblah. After some back and forth he told me it would be possible to set up what he called a 'blind loop', a phone line from which I couldn't call but get an high-speed internet. Bell actually does wireless networking with their own modems and has a provider called Bell Sympatico.

Again the guy with the funny beard couldn't answer any of my questions regarding security of the wireless or even basic things like the range of the sender. But after he found my Dr. on the credit card, he told me he'd grown the beard because he has this funny rash which wouldn't go away, and if I could give him some advice. I said I couldn't and asked him what internet service he uses at home. Rogers, he said.

Since I was already tired, I just said finefinefine, and could we please set that up now, I need a drink. Then we played the same game again. Funny beard tried to enter data into a form. Which didn't work. Then he went to the courtesy phone and called customer service. And here's the new part of the story: gave me the receiver to talk to the customer service representative. After 1/2 hour I found out that they don't have any wireless offers they can make me on the phone, and I should speak to the store guy, or call another number. The other number turned out to be a tape saying 'this service it not available', even after verifying that the number was correct. Funny beard shrugged his shoulders. I left without having any internet.

This morning I called the Bell's hotline again where they were at least able to tell me that in my apartment there's no high speed internet available anyhow, only dial up.

Then I found a flyer in the drawer of my kitchen with a business card of the local cable guy. I called him, said I need a wireless in my apartment, if possible yesterday. He said yes, no problem, he'll send someone over in the next days.

So, I have actually hope that I will be online again sometime in this century...

Tuesday, September 19, 2006

The Glasshouse Effect

The reactions I have encountered regarding the Perimeter Institute usually come in a three step scenario:

    b) ... but its in WATERLOO... (alias: the-end-of-the-world), funny faces come with this point, and
    c) But they have this cool building!

On the websites of the Perimeter Institute you find the following amusing sentence summarizing the alleged end-of-the-world problem:

"Perimeter's founders recognized from the outset that in order to establish a Waterloo-based Institute of the highest international standing it would be necessary to create a landmark building, both functionally and aesthetically, to attract and retain the very best researchers the world has to offer."

Now, after the first three weeks, I can tell you

b) Waterloo is better than you'd think. E.g. it is indeed possible to buy eatable cheese and chocolate. Besides this, one gets dinner after 8 pm, and so far nobody has asked me whether we do have fridges in Germany. I'll comment on the Canadian liquor disaster another time.
c) The building looks interesting, yes. But you feel like sitting in a glasshouse. A glasshouse in a zoo, with visitors waving at you and taking pictures. No kidding, it belongs to the sightseeing schedule in the region. I can hear the tourists with their digital cameras telling their kids: Look! A theoretical physicist! He's eating the chalk. Wait, it is a she!

I am looking forward for someone to throw peanuts at me.

Yesterday, Stefan took one million photos of the building. The glasshouse effect is most impressively shown on this one:

    My office is on the uppermost floor the third from the right. I share this room with Stefan Hofmann, who currently is on a trip to Germany. The small windows are where you find the printers.

    To get a three-dimensional impression look at these great 360 degrees images:

    See also: You're BEAUTIFUL...

    And a word in total privacy: Happy birthday mum! Tried to call, but I always get the fax mashine, what's wrong there?

    Update Jan 10th 2007: The new websites have more of the panoramic photos!

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    Wednesday, September 13, 2006

    The End of Clothes

    After three weeks living out of a suitcase I have reached the end of clothes. My furniture's position is still uncertain and I still don't have a car. But I am making progress: I have a bank account, social and health insurance, and I managed to convince my new roommate to clean out my desk so I can mess it up with my own stuff.

    Buying a car turned out to be a real challenge. It seems I can't buy a car without having an insurance, and I can't get an insurance without a local drivers licence. Please don't ask me why, I have given up to expect the world to make sense.

    So, yesterday I went to change my California driver's licence to an Ontario licence. Since I almost failed the written test in California, I actually read the driver's handbook to see how many laws I have broken in the last years. This was one of the most entertaining reads I have had lately. The book contains an enormous amount of valuable information. For example, while driving you should not shave or use a laptop.

    Also, the handbook has an extra section how to deal with emergency situations where you learn "If your brakes fail and you manage to stop the car do not drive away." The same section also clarifies what to do in case you have to stop on a freeway. Since I had to do that repeatedly in Arizona when the water began to boil at an outside temperature of 112 F, I am relieved that I eventually know the correct procedure: After stopping the car, I was supposed to knot a white cloth to my antenna, and as the book says explicitly "Do not open the hood".

    You see, I was really prepared for the test. The only problem was that they weren't able to access my Arizona record. As I had to learn, Ontario has graded driver's licences. And without the Arizona record I have driving experience that qualifies me as a beginner, despite the fact that I have had my licence since 12 years or so. Which is of course great for the insurance.

    But even better, I called the Motor Vehicles Department in Arizona, and they aren't able to send me my own record because I can't recall the drivers licence number. Now what I have to do (!!) is to find an US-notary who confirms that my signature is my signature. Then I can fill out form 46-4416 and request to have my record sent.

    Bottomline is that I spend every day running around from one office to the next, filling out forms, treating in forms for other forms, arguing with employees, spelling my name over and over again, sending and receiving faxes, exchanging numbers for other numbers, and cards for other cards.

    August/September seems to be the postdoc's depression time. Its like all my friends are constantly in a bad mood in these weeks, either busy with preparing applications for the coming year, or with organizing their move and settling somewhere else. I had a friend who used to call postdocs 'the homeless people', and I think there is some truth in it.

    However, it seems that Americans are much more used to moving around than Europeans. And I admit there is some fun in moving. At least I have something entertaining to tell you :-)

    "We spend our time searching for security and hate it when we get it."

    But being European I have to say I am looking forward to staying in one place for some years. I actually think about buying a couch and flatware. Seriously!

    Saturday, September 09, 2006


    I am in Canada. My furniture is somewhere. On a ship as I have been told by the moving company. I asked them how that ship is supposed to go from Santa Barbara to Toronto? They told that me its an international move. And international moves always go by ship. I suggested they take out a map of North America and think about it. After spending about one hour on hold, and being forwarded to several people, they decided they would indeed put my stuff on the road.

    When the moving truck came last week, I asked the muscle guys where the boxes go next. "To LA", they said. "There we put them on the ship." So, I am waiting for the ship.

    Meanwhile the Dept. in Santa Barbara decided to ping back my emails. Which was a major disaster because my new email-account at Perimeter had a forward to Santa Barbara, which I could not remove. The result was that a couple of emails entered loops back and forth from Waterloo to Santa Barbara, spitting out about 10 error messages every minute. After banging my head against the next wall for some while, I ran to see the system administration. The result is that now I can neither receive nor send any emails. With none of my accounts. And how great is that? I AM ON WITHDRAWAL! CAN'T CHECK MY EMAIL.

    Besides this, last week there was a very interesting workshop here on Natural Ultraviolet Cutoffs In Expanding Space-Times:

    The question of how to describe a natural ultraviolet cutoff in an expanding space-time is of significance in several respects. First, it concerns the fate of general covariance in the presence of a natural UV cutoff. Second, it concerns the continued generating of degrees of freedom through expansion, which carries with it the possibility of an associated generating of vacuum energy. Finally, through inflation, a natural ultraviolet cutoff may have left observable imprints in the CMB.

    The question whether it might be possible to extract signatures of quantum gravitational effect from Cosmological data is currently one of the most exciting topics there is. All of the talks from the workshop have been recorded, and I will let you know when they become available online. Unfortunately, I missed a big part myself, since I was busy with signing about 1 million forms, and running around trying to get my life organized.

    On Friday, I also gave a talk on the workshop about my Minimal Length model, which made me realize all the things that I intended to work out but didn't... I really hope that I will have some time in the next months to think straight. Something that I wasn't able to do for quite some while.

    Wednesday, September 06, 2006

    Pencils, Black Holes, and the Klein Paradox

    Yesterday morning, when scanning the news at spiegel online, a headline in the science section made me curious: Bleistift statt schwarzer Löcher, pencils instead of black holes. That short piece turned out to be a quite sensible description of a recent experiment on the Klein paradox in single layers of graphite. There was even a link to the original paper on the archiv: cond-mat/0604323. I had heard before of the funny features of electrons in graphene, as these single atomic layers of graphite are called, and I followed up the story. While I am not an expert on these things, I find them quite remarkable and interesting.

    In 1929, Oskar Klein, of Klein-Gordon and Kaluza-Klein fame, applied the Dirac equation to the typical textbook problem of an electron hitting a potential barrier. While in nonrelativistic quantum mechanics, the electron can tunnel into the barrier, albeit with an exponential damping, in the relativistic problem, something strange happens if the the barrier is on the order of the electron mass, V ∼ mc². Then, as Klein found out, the barrier is nearly transparent for the electron, and even perfectly transparent in the limit of infinite barrier height.

    Oskar Klein (

    This very odd situation, called the Klein paradox, is nowadays usually explained by the effect of pair creation: The barrier, which is repulsive for electrons, is attractive for positron. Thus, there are positron states inside the barrier with the same energy level as the incoming electron state. This means that electron-positron pairs are created, which are responsible for the transparency of the barrier.

    A steep and high potential barrier implies a very strong electric field. The pair creation at the barrier thus corresponds to the pair creation in strong fields. Experimental evidence for this effect - the so-called charged vacuum - was long sought-after in heavy ion collision, but so far without success. The problem is that electric fields strong enough for the spontaneous creation of electron-positron pairs occur only in the vicinity of superheavy nuclei, with Z ∼ 170. Such nuclei do not exist in nature - they have to be created, albeit for a very short while, in heavy ion collisions.

    The problem with the experimental verification of spontanous pair creation in high-energy physics is, obviously, the electron mass, which necessitates very strong fields. Things would be much easier if one would have massless charged Dirac particles at hand. Enters the graphene:

    Carbon atoms in graphite from very neat layers with a hexagonal, honeycomb structure. This layered arrangement of the atoms explains nicely the properties of graphite, such as its suppleness, which is why it is used in pencils. There are now even pictures of the honeycomb structure, thanks to atomic force microscopy:

    Graphite layer (

    Graphite is a quite good electric conductor. If one prepares single layers of graphite, or graphene, the conductance electrons are constrained to this layer. Now, in this two-dimensional system, the peculiar hexagonal structure leads to a linear relation between momentum and energy for the excitation of conductance electrons. Thus, these electronic excitations behave as massless Dirac fermions, instead of massive electrons! This remarkable feature has been exploited in several recent experiments - and one of these experiments is the experimental study of the Klein paradox referred to in the spiegel piece.

    The barrier in the experiment is created by some semiconductor material inserted into the graphene layer. Applying different electrostatic potentials to the semiconductor, the barrier height for the massless quasi-electrons can be tuned. Now, potential differences of some 100 meV instead of some 0.5 MeV do the job for reaching the regime of the pair creation and the Klein paradox. In the experiment, reflexion and transmission coefficients are measured, and they correspond neatly to Klein's calculations!

    This is definitely one more example where some of the standard textbook situations of quantum mechanics is, actually, realised in a beautiful experiment.

    Much more information about the experiment, and the special features of graphene, can be found on the News and Publications web page of the Mesoscopic Physics Group at the University of Manchester who actually started the experimental exploration of graphene, and did the Klein paradox experiment. For the Klein paradox as such, I am studying now a paper from the arxiv, quant-ph/9905076.

    But what about the Black Holes? The spiegel piece probably took it form a news item at Science: Black Hole in a Pencil.

    I guess, Bee is much more qualified to comment on that, once she will find a little time to breathe. The point is, I suppose, is that charged small black holes would naturally provide strong enough electric fields for pair creation, and thus for testing situations as in the Klein paradox in experiment. If only charged black holes could be produced more easily than nuclei with Z = 170...

    Best, stefan

    Sunday, September 03, 2006

    Good Bye Santa Barbara

      "There's nothing left to try
      There's no place left to hide
      There's no greater power
      Than the power of good-bye

      Your heart is not open, so I must go
      The spell has been broken...I loved you so
      You were my lesson I had to learn
      I was your fortress

      There's nothing left to lose
      There's no more heart to bruise
      There's no greater power
      Than the power of good-bye"

    Madonna, The Power of Good Bye

    Saturday, September 02, 2006

    Smart Crash

    The ESA satellite Smart-1 will end his life this evening, September 2nd at 10:43 p.m. PDT (5:43 GMT). It is scheduled to crash into the the Lake of Excellence (Lacus Excellentiae) in the Moon's southern hemisphere, with an impact speed of about 7,200 km/h (4,500 mph). Thereby it is still contributing to science by blowing up stuff from the moon's surface.

    For observation details look up

    You find more information at NASA, BBC, Universetoday, Sciencedaily, Spacedaily, and while we're at it, make sure you get your Milkywaydaily.

    What a way to end a life! For once, George W should be glad to hear that the Lake of Excellence is not located in the land of the free and the home of the brave.
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