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Archive for July, 2010

Stable proton beams colliding at some of the highest luminosities reached by the LHC so far!

Right now the luminosity is at 1030/cm2s.  Up until now, the luminosity collected these past few months has been at a luminosity around, at most, 1029/cm2s.

This is significant because this basically means that in about a day we can collect the same amount of data that we have collected over the past few months.  That’s why it’s so useful to do studies on increasing the luminosity rather than continuing to run at lower luminosities.

There’s a short term downside though: doing studies to increase luminosity makes it hard to get clean, stable beams for data taking.  It’s kind of like deciding whether you should buy a computer now, or wait a few months until prices come down and hardware is better.

The LHC has a balanced program of stable running and also doing studies to increase luminosity.  From the looks of it, it’s going well.

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SOP

Wednesday, July 14th, 2010

摘自《杜拉拉升职记》第1部,41节,“SOP的多种功能”,献给大亚湾实验现场工作的同事们:

何好德有一次和拉拉说:我们把公司的SOP(标准操作流程)全方位地健全起来。大家想办任何事情,哪个级别有权利做决定,可以办还是不可以办,该怎么办,由谁来办,多长时间内得办好,在SOP里全都规定好。还有,别忘记在SOP中规定特批的程序,对未尽事宜的审批办法设定好解决途径,因为总会有特例存在。

何好德进一步说:“这样,任何人之间都不用发生争论乃至对立,做决定的人也有依据,凡事都以SOP为行事标准,我批准什么,是依据SOP,不批准什么,也是依据SOP—大公司嘛,就应该尽量避免太多个人化的决定,让制度来管理公司才是正道。”

拉拉觉得何好德说的很对,她推荐了一位特别适合协调主管SOP的同事给何好德,何好德点头认可,柯比得和罗杰也没有话说,于是管理层首先批准了一个SOP—关于如何规范SOP的SOP,拉拉奉何好德之命,和财务部负责SOP管理的同事一起,在全国各办事处宣讲这个SOP的内容。

DB 中国(注:虚构的美国世界500强公司)上上下下掀起一股SOP的热潮,美国公司的SOP是当今世界上最专业而严谨的SOP,且五花八门丰富多彩。结果发展成,基本上,一个人在DB想走路,先抬左脚还是右脚,每次抬多高,每步花多长时间,都可以在SOP里查到依据。

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OK, I’ll admit it — instead of writing blog posts or reviewing results that are headed for ICHEP or doing something else productive, I find myself all too easily distracted by information on the current status of the LHC. As the gallant accelerator physicists work to push the machine to higher beam intensities and collision rates, I’m eager to learn about each little bit of progress. It definitely has some meaning to me — the more collisions the LHC produces, the more the experiments can record, and the greater the chance that we will see any particular physics process take place. Especially as we get close to the big ICHEP conference, we are all curious about how much data we might record before then, because that will determine what measurements might possibly be ready. (Of course it’s also determined by how quickly we can push the data through data analyses, how well we can understand detector performance and so forth; let’s not put all of the burden on the LHC.)

It’s not like I can do anything to make the luminosity go up, but I feel better (or at least distracted) by knowing what’s going on at this minute. This is akin to scoreboard watching in baseball, where the outfielders in one game might have their eye on the scoreboard above them to see how the competition is doing. (In fact, back at the Cornell Electron Storage Ring, the display that showed the luminosity numbers for the past 24 hours was called the “scoreboard”, so the analogy fits.)

So, if you want to play along at home, here are a few Web pages you can keep an eye on. Some of these have been mentioned in previous posts on this blog, but it’s been a little while and I’ll give a few more details.

To know what’s happening right now, check out LHC Page 1, which gives the current machine status and the (very) short-term running plan. Here you’ll typically see plots of the amount of beam current and the beam energy in the LHC, and, during periods of collisions for “physics” (i.e. data-taking by the experiment as opposed to studies of collisions done to optimize machine performance) there will be plots of the observed instantaneous luminosity reported by each of the four experiments. (Instantaneous luminosity is a measure of collision rate; its units of inverse centimeter squared per second deserve explanation in a second post.) The experiment reports can also be seen on the LHC Operation page. At other times, it will show the status of preparing to go to collisions, such as “ramp” (increasing beam energy to 3.5 TeV) or “squeeze” (focusing the beams to increase the collision rate). There are also helpful short messages about what’s going on, such as “this fill for physics” or the somewhat unnerving “experts have been called.”

The medium term run plan can be seen on the LHC Coordination screen. Here you can see the goals for the coming week, what administrative limits are currently in place to protect the machine, and the planned activities for the next few shifts.

While the collision rate is interesting, what really counts is the “integrated luminosity”, or the total number of collisions that have taken place. Up-to-date charts can be found here; the data go back to March 30, the start of 3.5 TeV operations. You can see here that the integrated luminosity has been increasing exponentially in time (when the LHC is not in studies periods or technical stops). If the collision rate were the same all the time, the integral would only increase linearly, so this demonstrates just how quickly the LHC physicists are figuring out how to make the machine go.

That’s what I’ve been keeping an eye on. OK, all of you stop looking at Facebook, and distract yourselves with the LHC instead!

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There is a new measurement of the size of the proton and it turns out that protons are smaller than we thought they were.

At some point in your education you probably got introduced to the Bohr model of the atom.  The nucleus is made up of protons and neutrons, and electrons orbit around the nucleus.  In the Bohr model, electrons orbit the nucleus in circular orbits like the Earth orbits the Sun, but these orbits are only allowed to have some radii (which correspond to an integer number of de Broglie wave lengths).  Electrons can transition between these levels and when they do, they either absorb a photon (in the case of an electron being excited from, say, the ground state to an excited state) or emit a photon (in the case of an electron going from an excited state to a lower state.)  This is shown below:

The Bohr model isn’t exactly right – but it’s close enough to get some feel for what’s going on.  In a more precise quantum mechanical picture, the electron isn’t actually orbiting the nucleus – it’s smeared out in what we call a wave function.  The square of the wave function tells us how likely we are to find the electron in a given place.  The ground state orbital (the shape of the wave function of the electron in the atom) is spherical.  The lowest excited state has four different possible orbitals, one spherical (S) and three which are shaped like a dumbbell (P), a sort of 3D figure-8.

What you probably learned in school was that these S and P orbitals have exactly the same energy – and they almost do.  In a simple model, the nucleus is just a point particle – meaning it exists just at a single point, with no size in any dimension.  But protons aren’t point particles – they’re just very small.  In the S orbitals, the electron spends most of its time near the nucleus, but in the P orbitals, the electron spends less of its time near the nucleus.  This difference in how much time the electron spends near the nucleus leads to a very small shift in the energy of the orbitals, called the Lamb shift.  The Lamb shift is measured by measuring the photon emitted when an electron goes from the P to the S orbital in the second shell.  It depends on the mass of the electron and the size of the proton.  (Here’s the explanation of the Lamb shift on the experiment’s web site.)

In this new measurement, they looked at hydrogen with a muon (the heavier cousin of the electron) instead of an electron.  Because the muon is about two hundred times heavier than the electron, it spends more time near the nucleus than the electron, meaning it’s more sensitive to the Lamb shift than the electron.  Previously, the best measurement of the diameter of the proton was 0.877±0.007 femtometers (m) and this measurement measured it to be 0.8418±0.0007 fm.  A femtometer is 10-15 meters.  If you were a proton (you’re somewhere between 1-2m tall), this would mean traveling one millimeter would be like traveling from the Earth to the Sun (1011 m).  This measurement would be like finding out that you’re 5’5″ instead of 5’8″ by looking at how long it takes for you to walk between Milwaukee, WI and Chicago, IL (150 km) and Milwaukee, WI and Madison, WI (141 km)*.

The fact that this measurement is so far off from our expectations indicates one of the following:

  • The precise calculation we’re comparing to is flawed.The proton is actually a really complicated object – perhaps we forgot an important component.
  • The measurement has some flaw we haven’t figured out yet.  Maybe there was some systematic shift that wasn’t taken into account.
  • Our theory is flawed.  This could indicate some physics beyond the Standard Model – exactly what we’re looking for at the LHC.

We have to seriously consider the first two options, but the third would obviously be very exciting.

So why I am writing about this here?  First, it illustrates that there are other ways of studying fundamental particle physics than by slamming things together.  Second, it’s an interesting result that may hint at exciting new physics we’re hoping to see at the LHC.  Third, it’s a great segue into my next post…

*Yes, this analogy breaks down at some point.  Don’t take it too far.

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Track Jets for ICHEP

Friday, July 9th, 2010

Along with many other particle physicists, I’ve been working hard lately to prepare results for the 35th International Conference on High Energy Physics, which will be held in Paris starting on July 22nd. That means I am again going through the complexities of reviewing my work with my (3000+) collaborators, to make sure that the work I’ve done is something we all have confidence in. After all, everyone’s name will be on it!

So far, things are going well, and it looks like (cross your fingers) the analysis will be out and ready. I just designed a poster this week, and some of the plots might also appear in one of the ATLAS talks given by one of my colleagues. The approval process has also been a great opportunity to get feedback — some of which has been included in the current analysis, and some of which will be added as we update and improve the results for a complete paper.

Momentum of raw track jets from the 900 GeV runI’ll be able to show you the latest results once the ICHEP conference starts, but for now I at least have some plots of track jets from last year’s 900 GeV run, one of which is shown here at right. You can click the image for more and bigger pictures, but I don’t promise that the text will be too comprehensible! I’ve written a more understandable explanation of the track jet analysis here. The plot on this page shows the momentum of track jets we found in the 900 GeV data (black points) and compares it to the momentum of simulated track jets (yellow graph). You can tell a few things from it: first, there are a lot more low-momentum jets than high-momentum jets, which is exactly what we expect; second, that the data and the simulation agree pretty well; third, that they don’t agree perfectly.

There are two basic reasons why data and simulation might not agree: one is that we aren’t simulating our detector accurately enough, and the other is that we’re not simulating the underlying physics that comes out of the collision well enough. Working out which is which, and coming up with our best guess as to what really happened when the protons collided, is the hard work of data analysis. How did it go? To find out, come see my poster at the ICHEP conference, or just stay tuned here!

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ESHEP 2010“Ah…. Tu vas voir, c’est une expérience superbe” m’ont dit tant de personnes quand je leur ai dit que j’allais participer à l’école européenne de physique des hautes énergies. Je peux maintenant le confirmer c’est avant tout une aventure humaine inoubliable, c’est incroyable de voir a quel point on peux se sentir proche des autres étudiants en à peine 2 (trop courtes) semaines.
L’emploi du temps était chargé, une journée typique comportait une matinée de cours (de 9h à 12h30) une pause déjeuner suivie d’une période de temps libre, puis reprise a 16h pour enfin terminer par une séance de discussion en petits groupes. Ce système est tout particulièrement abouti puisque les différents intervenants venaient discuter avec nous le soir, ce qui permettait de lever les incompréhension et de discuter plus en détails les aspects qui nous intéressaient.
Nous avons eu droit a un éventail de cours pour tous les gouts, de la physique des neutrino jusqu’aux modèles d’inflation en cosmologie avec une mention toute particulière au cour d’Antonio Pich sur le modèle standard qui a fait l’unanimité.

Le directeur général du CERN

Le directeur général du CERN

Nous avons également eu droit aux discours des directeurs du JINR et du CERN (j’ai même pu demandé à ce dernier l’effet que la découverte du Higgs au Tevatron aurait sur l’image du LHC 🙂 ).

Les activités hors cours n’étaient pas en reste puisque nous pouvions sur place aller à la piscine, au sauna, pécher ou nager dans le lac, faire du vélo ou encore jouer au “bowling” finlandais.
En dehors de l’institut nous avons pu visiter le village de Fiskars, un vilage vitrine des artistes travaillant le bois. Ainsi que le phare de Bentskär qui fu pour ma part l’occasion de voyager par bateau en pleine mer pour la première fois. J’ai eu la bonne surprise de découvrir que je n’étais pas malade en mer, ce qui n’était malheureusement pas le cas de nombreux étudiants lors de ce voyage mouvementé! Enfin le dernier mercredi, nous avons passé une après midi à Helsinki, la capitale de la Finlande, chacun était libre d’explorer à sa guise avant un diner dans un restaurant le soir.

Le lac près de l'institut

Le lac près de l'institut

Le bilan de cette école est d’un point de vue personnel une franche réussite, cela m’a permis de faire le point sur mes connaissances en physique (ce qui, je m’en suis rendu compte, n’était pas du luxe!) et de rencontrer des personnes formidables du monde entier. C’est dans un mélange de nostalgie et de motivation retrouvée que je retourne maintenant à mon travail de thèse.

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Marine Life in Hamburg

Wednesday, July 7th, 2010

Since late Sunday night, I’m at DESY in Hamburg for a series of meetings, first on the CALICE Analog HCAL, then on plans for a new EU Project, and finally for the ILD workshop. Lots of things to discuss, ranging from test beam plans, financial matters, detector engineering to physics simulations and general physics issues at linear colliders. But topics connected to marine life are coming up surprisingly frequently. The most pressing one: Could Paul be right? I surely hope not, but we’ll know in a few hours. Meanwhile, will they be serving octopus at our meeting dinner tonight? I’ll go find out right now.

Baby seal on the beach in Hamburg right at the Strandperle bar. Searching for its mother, but maybe also a secret interest in calorimetry...

Baby seal on the beach in Hamburg right at the Strandperle bar. Searching for its mother, but maybe there is also a secret interest in calorimetry...

On Monday, we had our traditional CALICE outing, due to unstable weather we first went to a Bistro, and only later to the beach for beers. Same place as last year, but with a significantly reduced crowd, and no intentions of detector building. There we were greeted by a very unusual sight: A baby seal was lying on the beach, right at the bar. Apparently the poor guy had lost its mother, swam up all the way to Hamburg, and then crawled exhausted onto the beach. The baby was rescued and brought to a seal nursing station, and I hope it is on the way to growing big and strong to move back to the ocean in a while.

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Independence day

Sunday, July 4th, 2010

Sorry for the hiatus, blog enthusiasts! I’m taking some time to catch-up while watching the fireworks out of my window on July 4th. Tis the season for summer conferences. The one in particular that I’m involved in is ICHEP (International Conference on High Energy Physics) is coming up at the end of July, which means all the papers have to be approved by ATLAS by the end of June (which just so happened to be last week – hooray independence  ;)). This year the conference is in Paris and there we’ll show the first physics results from the LHC. ATLAS alone has over 40 papers submitted. In particular I’ve been looking at material mapping using photons.

So what is material mapping?… sounds like something cartographers do.
We have lots of computer simulations of the ATLAS detector. To double check to make sure we’ve taken into account every cable we look to see that the particles interact the way we expect them to. Photons, for example, when they to through material convert predictably into an electron/positron pair. When I say predictably, I mean based on the amount of material they go through. We find these electron/positron pairs because they have a displaced vertex (an electron/positron which are close to each other and when you draw a line back from their tracks the origin isn’t the main interaction point). The more material in the way, the faster they convert. So we make sure our Monte Carlo simulations predict where most of the conversions occur to make sure we understand the detector.

I’ll try to provide a link to show a picture of the material map once they have been approved for public viewing.

Until then, Happy July 4th!

Fireworks

Regina

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For those of you who have been following our foray into the particle content of the Standard Model, this is where thing become exciting. We now introduce the W boson and present a nearly-complete picture of what we know about leptons.

We’re picking up right where we left off, so if you need a refresher, please refer to previous installments where we introduce Feynman rules and several particles: Part 1, Part 2, Part 3, Part 4, Part 5

The W is actually two particles: one with positive charge and one with negative charge. This is similar to every electron having a positron anti-partner. Here’s the Particle Zoo’s depiction of the W boson:

Together with the Z boson, the Ws mediate the weak [nuclear] force. You might remember this force from chemistry: it is responsible for the radioactive decay of heavy nuclei into lighter nuclei. We’ll draw the Feynman diagram for β-decay below. First we need Feynman rules.


Feynman Rules for the W: Interactions with leptons

Here are the Feynman rules for how the W interacts with the leptons. Recall that there are three charged leptons (electron, muon, tau) and three neutrinos (one for each charged lepton).

In addition, there are also the same rules with the arrows pointing in opposite directions, for a total of 18 vertices. Note that we’ve written plus-or-minus for the W, but we always use the W with the correct charge to satisfy charge conservation.

Quick exercise: remind yourself why the rules above are different from those with arrows pointing in the opposite direction. Hint: think of these as simple Feynman diagrams that we read from left to right. Think about particles and anti-particles.

In words: the W connects any charged lepton to any neutrino. As shorthand, we can write these rules as:

Here we’ve written a curly-L to mean “[charged] lepton” and a νi to mean a neutrino of the ith type, where i can be electron/muon/tau.

Exercise: What are the symmetries of the theory? In other words, what are the conserved quantities? Compare this to our previous theory of leptons without the W.

Answer: Electric charge is conserved, as we should expect. However, we no longer individually conserve the number of electrons. Similarly, we no longer conserve the number of muons, taus, electron-neutrinos, etc. However, the total lepton number is still conserved: the number of leptons (electrons, muons, neutrinos, etc.) minus the number of anti-leptons stays the same before and after any interaction.

Really neat fact #1: The W can mix up electron-like things (electrons and electron-neutrinos) with not-electron-like things (e.g. muons, tau-neutrinos). The W is special in the Standard Model because it can mix different kinds of particles. The “electron-ness” or “muon-neutrino-ness” (and so forth) of a particle is often called its flavor. We say that the W mediates flavor-changing processes. Flavor physics (of quarks) is the focus of the LHCb experiment at CERN.

Exercise: Draw a few diagrams that violate electron number. [If it’s not clear, convince yourself that you cannot have such effects without a W in your theory.]

Answer: here’s one example: a muon decaying into an electron and a neutrino-antineutrino pair. (Bonus question: what is the charge of the W?)

Remark (update 7 July): In the comments below Mori and Stephen point out that in the ‘vanilla’ Standard Model, leptons don’t have flavor-changing couplings to the W as I’ve drawn above. This is technically true, at least before one includes the phenomena of neutrino-oscillations (only definitively confirmed in 1998). In the presentation here I am assuming that such interactions take place, which is a small modification from the “most minimal” Standard Model. Such effects must take place due to the neutrino oscillation phenomena. We will discuss this in a future post on neutrino-less double beta decay.

Feynman Rules for the W: Interactions with other force particles

There are additional Feynman rules. In fact, you should have already guessed one them: because the W is electrically charged, it interacts with the photon! Thus we have the additional Feynman rule:

[Update, Aug 9: note that for these vertices I’ve used the convention that all of the bosons are in-coming. Thus these are not Feynman Diagrams representing physical processes, they’re just vertices which we can convert into diagrams or pieces or diagrams. For example, the above vertex has an incoming photon, incoming W+, and an incoming W-. If we wanted the diagram for a W+ emitting a photon (W+ -> W+ photon), then we would swap the incoming W- for an outgoing W+ (they’re sort of antiparticles).]

This turns out to only be the tip of the iceberg. We can replace the photon with a Z (as one would expect since the Z is a heavy cousin of the photon) to get another three-force-particle vertex:

Finally, we can even construct four force-particle vertices. Note that each of these satisfies charge conservation!

These four-force-particle vertices are usually smaller than any of the previous vertices, so we won’t spend too much time thinking about them.

Really neat fact #2: We see that the W introduces a whole new kind of Feynman rule: force particles interacting with other force particles without any matter particles! (In fancy words: gauge bosons interacting with other gauge bosons without any fermions.)

Remarks

  1. The most interesting feature of the W is that it can change fermion flavors, i.e. it can not only connect a lepton and a neutrino, but it can connect a lepton of one type with a neutrino of a different type. One very strong experimental constraint on flavor physics comes from the decay μ→eϒ (muon decaying to electron and photon). As an exercise, draw a Feynman diagram contributing to this process. (Hint: you’ll need to have a W boson and you’ll end up with a closed loop.)
  2. It is worth noting, however, that these flavor-changing effects tend to be smaller than flavor-conserving effects. In other words, a W is more likely to decay into an electron and an electron-neutrino rather than an electron and a tau-neutrino. We’ll discuss how much smaller these effects are later.
  3. W bosons are rather heavy—around 80 GeV, slightly lighter than the Z but still much heavier than any of the leptons. Thus, as we learned from the Z, it decays before it can be directly observed in a detector.
  4. The W was discovered at the UA1 and UA2 experiments at CERN in the 80s. Their discovery was a real experimental triumph: as you now know from the Feynman rules above, the W decays into a lepton and a neutrino—the latter of which cannot be directly detected! This prevents experimentalists from observing a nice resonance as they did for the Z boson a few months later. They used a slightly modified technique based on a quantity called “transverse mass” to search for a smeared-out resonance using only the information about the observed lepton. Generalizations of this technique are still being developed today to search for supersymmetry! (For experts: see this recent review article on LHC kinematics.)
  5. The W boson only talks to left-handed particles. This is a remarkable fact that turns out to be related to the difference between matter and antimatter. For a proper introduction, check out this slightly-more-detailed post.

Exercise: Now that we’ve developed quite a bit of formalism with Feynman rules, try drawing diagrams corresponding to W boson production at a lepton collider. Assume the initial particles are an electron and positron. Draw a few diagrams that produce W bosons. “Finish” each diagram by allowing any heavy bosons (Z, W) to decay into leptons.

What is the simplest diagram that includes a W boson? Is the final state observable in a detector? (Remember: neutrinos aren’t directly observable.) What general properties do you notice in diagrams that both (1) include a W boson and (2) have a detectable final state (at least one charged lepton)?

Can you draw diagrams where the W boson is produced in pairs? Can you draw diagrams where the W boson is produced by itself?

Hints: You should have at least one diagram where the W is the only intermediate particle. You should also play with diagrams with both the fermion-fermion-boson vertices and the three-boson vertices. You may also use the four-boson vertices, but note that these are smaller effects.

Remark: Try this exercise, you’ll really start to get a handle for drawing diagrams for more complicated processes. Plus, this is precisely the thought process when physicists think about how to detect new particles. As an additional remark, this is not quite how the W was discovered—CERN used proton-antiproton collisons, which we’ll get to when we discuss quantum chromodynamics.

Relating this to chemistry

Before closing our introduction to the W boson, let’s remark on its role in chemistry and simultaneously give a preview for the weak interactions of quarks. You’ll recall that in chemistry one could have β decay:

neutron → proton + electron + anti-neutrino

This converts one atom into an isotope of another atom. Let’s see how this works at the level of subatomic particles.

Protons and neutrons are made out of up and down type quarks. Up quarks (u) have electric charge +2/3 and down quarks (d) have electric charge -1/3. As we will see when we properly introduce the quarks, up and down quarks have the same relationship as electron-neutrinos and electrons. Thus we can expect a coupling between the up, down, and W boson.

A neutron is composed of two down quarks and an up quark (ddu) while a proton is composed of two up quarks and a down quark (uud). [Check that the electric charges add up to what you expect!] The diagram that converts a neutron to a proton is then:

Update: As reader Cris pointed out to me in an e-mail, the W should have negative charge and should decay into an electron and anti-neutrino!

Because the W boson is much heavier than the up and down quarks—in fact, it’s much heavier than the entire proton—it is necessarily a virtual particle that can only exist for a short time. One can imagine that the system has to ‘borrow’ energy to create the W so that the Heisenberg uncertainty principle tells us that it has to give back the energy very quickly. Thus the W can’t travel very far before decaying and we say that it is a “short range force.” Thus sometimes the weak force is called the weak nuclear force. Compare this to photons, which have no mass and hence are a “long range force.”

[We now know, however, that it is not intrinsically a nuclear force (in our theory above we never mentioned quarks or nuclei), and further its ‘weakness’ is related to the mass of the W making it a short-range force.]

Cheers!
Flip (USLHC)

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Back in Amsterdam

Thursday, July 1st, 2010

Prinsengracht This week, I am back in Amsterdam, attending the Amsterdam Summer Workshop. The attendance is of a very high caliber, and as for the lectures, there is one highlight chasing the next. Apart from that, it’s great to see my old colleagues again.
On Monday, my former boss Robbert Dijkgraaf, the most dynamic and surely most charming ever president of the Royal Netherlands Academy of Arts and Sciences, handed the Lorentz Medal to Edward Witten. He in turn regaled the workshop participants two days later with insights about the path integral of quantum mechanics.

We’re unusually lucky with the weather these days. A Dutch newspaper already ran an article on the effects of the “tropical heat” (read temperatures above 25 degrees for several days in a row) on humans and animals earlier this week.
I use the long evenings to cycle around town (how else can you get around here than by bike), revisit the green parrots of Vondelpark (definitely not an indigenous species, but somehow they found their ecological niche there), sample the highlights of Dutch cuisine such as bitterballen, kroketten and pancakes, and envy the people who go for joyrides in the canals in their own boats.

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