Tommaso Dorigo

Color flow

In my post Hadronization two days ago I tried to describe what happens when two quarks are created at high energy in the decay of a Z boson. The quarks escape the decay point with light speed, pulling a color string between them. Once energetically favourable, the color string breaks, creating a new quark-antiquark pair. The process continues until quarks and antiquarks recombine in colorless combinations - hadrons.

In the post, I included a picture that described the process and tried to bring home a few points. However, I was not completely happy with the technical correctness of that picture. So here I go again - I will try to make a more precise one.

This time, I will not use smileys for quarks. Instead, I will picture their travel in time and space with colored lines. Time increases going down (see the arrow), space is exemplified by the horizontal coordinate. Here is the picture:

 

The pair of red quarks emitted by the disintegrating Z boson at the top vertex of the figure above travel apart (one towards the left and one towards the right), until the quark on the right emits a red/antiyellow gluon, and becomes yellow. The gluon (the double line with red and yellow color) shortly thereafter splits into a quark-antiquark pair, a red quark and a yellow antiquark.

After a while, the original yellow quark emits a yellow-antiblue gluon, and it becomes blue. Later, also the original red antiquark emits a gluon - a blue-antired one, becoming blue.

In the end, all colors recombine, and quark-antiquark pairs are created which are colorless: hadrons! This fragmentation process has involved the emission of three gluons, the materialization of three quark-antiquark pairs, and the final production of four hadrons traveling away from the Z decay point.

One last bit to note: in the figure above, the energy stored in a string when two partons fly apart can be thought to be proportional to the area of the polygons. That is part of the reason why I like this one better than the one with smileys.

Addendum 2/28: if you think the four hadrons created at the bottom are respectively blue, red, yellow, and blue, you're missing the whole picture! Hadrons are colorless objects... They got those colors because, well, I haven't figured out how to paint with a anti-brush yet... Red+antired= no color, etcetera!

Grasping big numbers

When I try to describe something about my job to Filippo I always end up having to face the problem of communicating the meaning of huge numbers to him. For a 5 year-old kid, large numbers are really hard to grasp: already a million is beyond his reach if you do not give him the right hints.

So I am trying to put together a few examples, which may turn out useful in discussions not only to Filippo, but actually any time I explain something about elementary particles or astronomy to people not in that business. Here are a few leads:

• The surface of Earth, a concept of extension which in the jet era we became able to grasp, is about 512 million square kilometers, that is 5x10^14 square meters. Therefore, if we place a small stone on top of each square centimeter on the Earth's surface, we are going to need 5x10^18 of them. We are still five orders of magnitude below Avogadro's number (the number of molecules in a mole of a substance)!
• How many little stones we will need to pave the surface of a sphere where the Earth is orbiting the Sun ? 3.14x(1.5x10^13cm)^2=6.5x10^26 stones, one for each square centimeter. And if we were to put a proton per each square millimeter of the same surface, how many would we need ? 10^2 times as much, or 6.5x10^28 protons: this many protons are roughly the content of an average-sized person.
• What is the age of the Universe, in seconds ? About 15 billion years, which correspond to 300x100000x15x10^9=4.5x10^18 seconds. Therefore, going at the rithm of one stone per second, one could pave the entire Earth during that time! But how much could we manage to pave in just a lifetime ? About 2 billion stones, which is no more than about 20 soccer fields.

Larger numbers still are probably meaningless even to the most hard-boiled mathematical minds. Can anybody suggest something that could equate to 10^39 ? That number is of some relevance for some theories of the cosmos - more on that later!

Hadronization

A couple of days ago I tried to describe a few cool things about quarks and gluons:

1. They cannot live as free particles, since the strong force confines them within the proton or the neutron - or other more exotic particles.
2. They possess a charge called color, and that's why they oblige to the strong force. On the other hand, all stable particles composed of quarks - hadrons, that's how we call them - do not feel the strong force, because they are colorless, neutral.

Now let's discuss what happens when a Z boson decays. Z bosons are the carriers of the Weak Interactions, together with W bosons (bosons are force carriers, in general: other examples are the gluon - strong force! - and the photon - electromagnetic force!). Z bosons are heavy things: more or less like silver atoms. They possess a large mass (91 GeV), and when they disintegrate, the mass becomes energy!

So what comes out of a Z decay ? Most of the times, a quark-antiquark pair. And since quarks are light compared to the Z boson (the top quark is the only exception, since it is actually TWICE as massive - more than a whole gold atom!), the two quarks end up happily endowed with an energy corresponding to half the mass of the Z boson, 45 GeV each: conservation of Energy! That means that they are kicked away so hard they travel practically at light speed.

The picture is as follows: now a Z boson is sitting there, and now it's there no more. At its place, a quark-antiquark pair materialized. The quark is shot one way, and the antiquark is shot exactly in the opposite direction - conservation of Momentum!

As the two bodies fly apart, they pull a string of color between them. Remember, the color force prevents a quark from parting from its peer. This string becomes more and more energetic as the quarks separate, decelerating. Once it has stored enough energy to materialize a quark-antiquark pair, it will do so, and the string will become two strings, each with a quark on one end and an antiquark on the other. A picture will clarify this a bit:

Although not completely correct, the picture above describes intuitively what happens. The sun-like symbol at the top is the Z boson disintegrating; the arrow shows the direction of time. On the following line, we see a red quark-antiquark pair kicked towards the left and the right, respectively. As they part, they pull a color string between them, which eventually breaks, materializing a second quark-antiquark pair, of different colors. These also travel away from the Z decay point in different directions, being pulled by the original red quarks. Eventually, the secondary strings also break, creating new pairs, and these may finally combine to form four colorless mesons - quark-antiquark combinations with zero net color.

It remains to note that in the case of Z decay this process typically yields not just four, but a couple dozen particles (baryons - three quarks combinations, not pictured above- and mesons), traveling in two collimated streams opposite to each other: these are what we call jets.

Incremental steps

On a recent post were I showed the blessed results of my analysis of Z->bb decays, I received a very interesting question:

[...] Why you and fellow particle physicists don't (at least publicly) get discouraged that experiments like this don't yield physics information that wasn't already known? [...] Didn't you and your particle physics colleagues--among the brightest, hardest-working, most mathematically talented scientists--go into physics to discover more new things than this? From my (uninformed!) point of view, it seems that particle physicists are celebrating very incremental steps like these, rather than worrying that the return of truly significant, worldview-altering scientific knowledge appears to be very low at this point. [...] - Ben

Ben, you are right when you say that we study Particle Physics to hit on big new things, which can bring sudden advances to our field and to science in general. However, there are then several constraints:

• First of all, one is not always free to pick the kind of research one thinks is the most interesting, the most useful, or the most likely to produce a Nobel prize. To first order, after a PhD all but the very brightest (who are able to pick exactly what they want) are typically just happy to be able to stay in the business, and will conform to the research program of the University or Institute that hires them.
• After a few years, one gets to a tenured position and can then have more freedom to choose... But will one then be still able to have the energy and the stimulus to do groundbreaking research ? Not easy!
• The other constraints come from one's background. It is hard to leave one's specific expertise (which is a valuable asset) to move to another more promising research.
• Finally, the most promising things are the ones where there is more competition, and one cannot always afford to have to fight.

Most of the big advancements in Physics, and I would say in Science in general, are indeed pictured as quantum leaps, but are rather the result of the collection of a mass of independent, smallish pieces of information. Then there is a stroke of genius, sure. But if you look closely, the genius was first of all a very well informed person! And without all the work of obscure colleagues, even a Planck would not have figured out how to fit a power spectrum the right way. In order to have the need of hypothesizing a quantization of energy Planck needed a very precise measurement of the emission spectrum of a blackbody to have to fit: and the higher precision of that measurement was - one could argue - a small incremental step from less precise measurements available at that time, to use your words.

Another instance of that mechanism is the discovery of Parity violation. Lee and Yang produced a earthquake when they speculated that Weak Interaction did not conserve the Parity symmetry, in a paper in 1956. But that paper was just a small bit of genius sprayed on top of a very detailed, precise and methodical review of all data available to that point: small experiments, tiny bits of information, sometimes imprecise, sometimes incomplete. Incremental steps!

So, am I worried by doing my tiny little bit of Science instead than concentrating on the big issues ? No. I am a soldier, not a general. Maybe one day I will be a general, but I have the feeling I will enjoy Particle Physics less then!

Colored partons

Ok, this post is not going to get renamed. I want to tell you about colored partons today. Fasten your seat belts, so you don't run away before I say you may!

Partons (quarks and gluons) are the constituents of hadronic matter - the protons and neutrons we all know and love. We could live happily ignoring them, but once you are in the business of smashing protons, well, that's what you get: quarks and gluons.

Quarks and gluons have exceedingly interesting properties. First of all, they are impossible to isolate. Suppose you pick a quark inside a proton and try to pull it out of it. If you try doing so, you extend a string of gluons that connects the one you picked to the rest of the proton. Strange, because the quark appeared to come with no strings attached!

The more you pull, the more you sweat. Energy gets stored in the string of gluons, until POP! The string breaks, but what you are left with in your hand is not just the quark you wanted to pull out. The broken string's energy has materialized a quark-antiquark pair out of the blue. Remember E=mc^2 ? Well, that's exactly true in this case: the energy of the string has converted to the mass of the two brand new quarks! (Quarks are massive, while gluons have no mass).

A quark has disappeared inside the proton, which appears unbothered by the whole thing, and the antiquark has stuck to the quark you wanted alone, creating a meson (a fancy particle, which has a very short lifetime, made up by a quark-antiquark pair orbiting one around the other). Isn't that weird ??

The other property I like a lot about quarks is that they are colored. Yes, they come in three colors: red, blue, and yellow (or pick another combination if you like). No, that's not your everyday life color. But still something which has color-like properties. See, you can make stable particles with quarks, only if you combine them in combinations with no net color, because stable particles have no color. If they had, they would break apart instantaneously, due to the strong force -QCD- whose charge is the color itself: it would be like putting together two electrons and pretend that they stick together.

How do you make uncolored objects with the three basic colors, red, blue and yellow ? Well, you know this since the school days when you were shown a colored disk getting white as it spinned! One part of each will do the trick. Likewise, three quarks of different colors will make up a uncolored baryon - a neutron or a proton, that is. But with quarks, you get a second chance! You can create a stable meson by putting together a quark (say a red one) and an antiquark (anti-red!). The two colors annihilate like the Cointreau and the Martini dry in a glass of Gordon's Gin, and you get a colorless cocktail - pardon particle.

So, what does color do ? Color is the charge of the quarks. By being colored, they stick together so strongly that protons and neutrons lose a little bit of their individuality when you pack them together in the nucleus of an atom. Ever asked yourself why your necklace does not vaporize in a cloud of Hydrogen ? Yeah: gold atoms have nuclei which contain 79 protons. That's an electric charge of +79 in a tight space! No way you could keep them together without the force of the color field between the quarks of protons and neutrons. The strong force is stronger than the electromagnetic force, and by far so!

I think that's enough for today. Tomorrow I will tell you what is color radiation (yes, just like you can do an antenna with electrons running up and down, so quarks and gluons broadcast radiation!), and just why it is necessary to understand it before you try to isolate a Z->bb signal in hadron collider data!

Collaboration feedback

After blessing a PR plot of the Z->bb signal I've been after during the last couple of years, I am starting to receive all sorts of questions from my fellow collaborators about the details of the analysis.

I am quite amused, because it should rather work the other way around: one first gets to answer lots and lots of questions (some interesting, some trivial, some nasty ones too), and only then does one receive the blessing of his results.

However, I do understand why that is so... We are all too busy and some of the new stuff our experiment produces gets overlooked - until it makes to the public web pages. Going to all the Physics meetings (a safe way not to miss anything) is out of the question: there are too many!

I was about to talk about the color flux in heavy flavor production at a hadron collider here - but as always my fingers are faster than my brain. So I will rename this post Colored partons -> Collaboration feedback, and will talk about partons in the next one!

Halal or Haram ?

I paid a visit to my friend  Tim Krabbé's highly entertaining web site today. Tim Krabbé is a renowned novelist (many moviegoers will remember the movie The Vanishing (1993), which is based on his novel Het gouden ei) as well as a strong chess master and an expert of chess studies and trivia. I found the following piece on his chess diary, and I decided to cut and paste it here. All what follows is intellectual work of Tim.

275. 13 February 2005: Relief

The Iraq elections have been won by the Shiite alliance, backed by the Grand Ayatollah Ali al-Sistani, although only with 48 % and not with an absolute majority. On his website, al-Sistani answers many questions about what in his view is permitted or halal (cricket, anal sex, oral sex (provided that no liquid gets into the mouth)) and what is not permitted, or haram. Among the latter, as has already been widely noted in the chess world, is chess.
    Answering two questions about this subject, al-Sistani is clear:

No: 1
Question: Is playing a chess allowed?
Answer: It is absolutely unlawful.
No: 2
Question: Chess is Halal or Haram?
Answer: Chess is absolutely forbidden.

It should be remarked that the French part of the site is clearer still. There, the question is:

I would like to ask you on the subject of chess. Is it permissible to play it to augment to the intelligence, knowing that I do not bet on it and that I have no bad intentions playing it?

And the answer:

It is not permissible to play chess.

However, irked that chess was once again equated with playing chess, and noticing it was possible to ask further questions, I sent in this question:

It is absolutely forbidden to play chess. But is it permissible to compose and solve chess problems? This is more like inventing riddles and creating poetry.

In a few days, I had my answer.

Who is destroying research in Italy ?

The italian government has been cutting the funding of Universities (which are state owned) and scientific research with a blind ax ever since the 2001 elections which gave it the power to do so. Their idea is that anything - anything - is justified if they manage to decrease the fiscal drag by half a percent to the wealthiest 1% of italian taxpayers.

The situation is so bad, that it brought together in an identity of view all University presidents (an unprecedented feat), who have been warning the Ministry of Education with a single voice that the situation is untenable and that a serious deterioration in the quality of teaching and research alike is already happening due to the financiary cuts - they were on the verge of not even being able to guarantee the salary to Professors last year. The budget for research in Italy has dropped to 0.9% of GNP, third-to-last in Europe, better only than Greece and Portugal (Sweden invests 3.5%, for a comparison with a more intelligently managed country).

But our beloved Minister Moratti (who incidentally, before being designated Minister of Education and University, was a successful businesswoman with zero experience of the academic world, let alone the school system) has more in store: she decided that a reform of University is needed, and she is trying to cancel the very institution of the tenured research scientist, substituting it with untenured positions that guarantee a life of uncertainty to thousands of scientists and researchers.

I, for one, would have stayed in the US when in 2000 -then a post-doc with Harvard University- I decided to go back to Italy to pursue an academic career there, if the research scientist positions had already been declassed to untenured, fixed assignment temporary jobs.

Fortunately, the academic world is not watching it happen without a fight. Spontaneous protests are happening everywhere and at all levels, and today I read with pleasure in Marcoscan's blog a quote from Prof. Margherita Hack, a respected and internationally renowned Astrophysicist from University of Trieste:

...dovremmo renderci tutti conto dell'enorme danno che si reca al paese tagliando i finanziamenti alle università e agli enti di ricerca, demoralizzando i ricercatori a tutti i livelli, e in particolare i giovani futuri ricercatori che vedono frustrato il loro entusiasmo per quello sviluppo della conoscenza che è fondamentale per il progresso del nostro paese, ma anche per tutta l'umanità.

(...we should all become aware of the enormous damage that our country is receiving by the cutting of funds to Universities and research institutes, which demoralizes researchers at all levels, and in particular young future researchers who witness the mortification of their enthusiasm for that development of knowledge which is a fundamentum to the progress of our country, as well as that of humanity as a whole.) (free translation by TD).

Free Giuliana Sgrena

Giuliana Sgrena, an italian journalist and correspondant for Il Manifesto, has been kidnapped in Iraq on February 4th.

Giuliana is a peace activist as well as a reporter. She is in Iraq to report on the conditions of the Iraqi people, on the effects of the war, and on the political landscape in Iraq.

All italian reporters are now leaving Iraq, since the situation has gotten even worse than it was in the past months, and foreign journalists are now considered explicitly as potential targets by the insurgents. So much for bringing freedom and order to Iraq!

This post is just to express my hope that Giuliana is set free as soon as possible. Italy already lost another excellent journalist, Enzo Baldoni, who was killed last August after being kidnapped by arab terrorists in Iraq. Ciao, Enzo.

The say of the week

"It is not who votes that counts, but who counts the votes"
Josif Stalin
(Any reference to the 2000 US election is totally voluntary)

...And ok, for this time once, in the interest of par condicio, another one from this side of the iron curtain:

Lady: "Winston, you're drunk!"
Winston: "Yes, lady, and you are ugly. But tomorrow I will be sober!"
(W.Churchill)