Tommaso Dorigo

Spinning neutrinos

Peter's last post on Maurice Goldhaber stimulates me to write about one of the most brilliant experiment of the twentieth century, one he performed in 1957 with Grodzins and Sunjar, and which has become a cornerstone of the physics of weak interactions and of particle physics in general.

The reader be warned: although I kept this at a level which should be accessible to anybody who knows what conservation of momentum is, explaining why the Goldhaber experiment is so phenomenal requires delving into details and many will get bored before reaching the end...

Anyway here we go. During the fifties, physicists were busy trying to understand weak interactions of elementary particles. At the time, the first accelerators were starting to produce evidence for the existence of many new unexpected particles. One used to smash protons against targets, and study the bodies produced downstream. Many new "resonances" were thus found as peaks in the distribution of the invariant mass of the final state particles: the mass is a well defined attribute of a particle, and it can be reconstructed by measuring energy and momentum of all the bodies created in the particle's disintegration.

Physicists were hoping that by studying the properties of these new found bodies they could understand the details of the weak interaction, the one which governed the decay of the neutron and of many other "strange" particles - strange because they were produced copiously by the strong interaction, but were slow (due to the weakness of the interaction responsible) to disintegrate. Enrico Fermi, with brilliant intuition, had already laid down in the thirties the correct formalism for making theoretical calculations of the decay rate of these particles, but had ignored the fact that the weak interaction violates a symmetry of nature called Parity, a fact postulated by Lee and Yang in 1956 and quickly demonstrated afterwards by Wu and her collaborators at the National Bureau of Standards, where she observed a clear anisotropy in the direction of emission of decay electrons from radioactive Cobalt 60 isotopes, kept at low temperature and with their spins aligned along the direction of a strong magnetic field.

If weak interactions violated the parity symmetry, Fermi's "V" formalism had to be modified. Two options were borne by experimental data: the interaction was either "V-A" or "S-T", these letters identifying the symmetry properties of the interaction potential. If the interaction was V-A, then beta decay (the mechanism through which radioactive atoms decay, by transforming one neutron into a proton, an electron, and a neutrino) had to produce (anti)neutrinos which spinned clockwise; if the interaction was S-T, the antineutrinos would all spin counterclockwise.

Neutrinos are the most elusive particles we know. They are enormously copious in the universe, since the fusion mechanisms in the star cores emit gazillions of them. But they almost never interact with matter when they traverse it: their cross section, or probability of yielding an interaction, is so tiny that most of the time they cross the entire earth unscathed. So if they do not interact, they cannot be seen! We in fact detect particles by observing their interaction with matter. How then, thought Goldhaber and company, can we be so clever as to measure their spin, which is an elusive property to begin with ?

Enter Europium 152, a radioactive isotope with quite peculiar characteristics. Fast to decay, it has zero total angular momentum (J=0: no net spin), and transmutes into excited Samarium by capturing one of its own inner electrons, thereby emitting a neutrino in one direction, and a Samarium nucleus in the other -the two bodies balancing the total impulse, thanks to conservation of momentum.

Excited Samarium has a total spin J=1 (in units of Planck's constant), and is also quite peculiar: it decays into its fundamental state - it de-excitates - by emitting a 960 keV photon so quickly (in 0.07 picoseconds) that its direction of motion has no time to be lost by thermic effects, even in a solid sample of Europium. The photon, a particle with one unity of spin, has the spin either aligned or anti-aligned with its own direction of motion, and, due to conservation of angular momentum, aligned with the original spin of excited Samarium. If the photon has been emitted along the direction of motion of excited Samarium, its spin will be aligned with the original spin of the neutrino emitted in the original decay of Europium, and it will have the same sign! This means that if we measure the photon spin properties, we infer the neutrino's spin properties. Bingo number 1!

But how can we select those photons emitted exactly in the direction of excited Samarium ? Bingo number 2 is to realize that the photons do not possess all the energy of the de-excitation of Samarium: when the photon is emitted, the Samarium atom recoils, keeping to itself some of the energy released in the form of kinetic energy. So these photons, when they encounter another atom of Samarium, will not be able to pump it back to the excited state, UNLESS... Unless the photon was emitted by Samarium traveling in the same direction, thereby being "kicked forwards", doppler-shifted to higher energy. So it will be only those photons that are able to do resonant scattering with a Samarium nucleus. Bingo number 3 is the fact that the momentum acquired by the Samarium nucleus in the decay of Europium is roughly equal to the amount necessary for the photon to gain just the needed kick forward!

To summarize: if you want to measure the neutrino's spinning property (AKA its polarization) you just need to measure the polarization of those photons which do resonant absorption by Samarium atoms (those that were kicked forward).

Ok, but how did they do that ? That is easy! Photons with energy of about a MeV have different interaction properties in a sample of iron which is magnetized parallel or antiparallel to the photons direction. That is due to the electrons in the iron being more likely to absorb photons of a given spin if their spin is antiparallel to it.

The end result ? Take a sample of Europium 152, place it before a block of magnetized iron. Place a sample of Samarium behind it, and an instrument to detect the secondary radiation from de-excitation of the Samarium undergoing resonant scattering (a photomultiplier will do). Count the ticks of the tube, turn the iron by 180 degrees, count again. Easy as pie!!

 

The picture above shows a sketch of the experimental apparatus. A small source of Europium is placed in the niche at the left, immersed in a block of magnetized iron. a big lead shielding prevents photons from reaching th photomultiplier directly from the Europium source. The rectangles labeled "Sm2O3" identify a ring of Samarium oxide, which is used as the scatterer, where photons can do resonance absorption. The photomultiplier tube is connected to a sodium iodide scintillating crystal, which records the photon signal.

Here is the original paper (click on the gif thumbnails to enlarge):

Page 1

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Through inventiveness and cleverness, Goldhaber and company designed and carried out one of the most extraordinary experiments in modern Physics. Reducing the measurement of one of the most elusive properties of subnuclear matter to a mere count of ticks of a photomultiplier tube is an enormous achievement! The neutrino is spinning counterclockwise, antineutrinos are clockwise. The interaction is V-A: a fundamentum of Particle Physics.   

Filippo!

In the afternoon, after a nap (the jet-lag!), I went to pick up Filippo at school. I brought his bycicle, so he could play with his friends in Campo del Ghetto, an very nice open space within the ghetto of Venice, close to his school (which is a 100 yards walk from my doorsteps). The picture shows him cycling around; I chose this one although you can't see much of him because I wanted to show the houses of the ghetto, a very peculiar place in Venice. On the left there is a wall with a monument to the Holocaust victims.

The picture above instead is from our Christmas vacation. Filippo was very proud of our snowman!

Finally, a few more pics of Filippo that I like:

...No, I'm not forcing chess on him! It's the "Polgar method": you play chess and pretend it's a grown-up thing, they'll get interested!

Ilaria!

Back home, I was cheered by Ilaria, which flew into my arms as I opened the door. How do I long these moments of joy after nine days away!

Ilaria had a flu until yesterday, and she still looks a bit tired. But I spent a joyful time with her, cooked her lunch (Mariarosa had some errands to do), and played a bit with her. Here she is, eating her lunch.

Maybe I should put here a better picture from our Christmas vacation on the Dolomites (a part of the italian Alps):

Here's another good picture:

And finally, with Mariarosa:

Fly like a vip

The flight to Munich yesterday was not very crowded. I had reserved seat ***, which is the one I prefer (there are only two seats like that in the economy cabin of the A340's). I won't go into the detail of explaining which seat it is, and why it is way better than all others, because I fear you, dear reader, might one day be on my same flight, and reserve it before I do!

Anyway, even in the rather spartan context of an economy ticket in a 9-hour flight, I got my satisfactions. The reason is that well-earned gold card Lufthansa donates to their most faithful customers. It makes you feel like you're a vip, and today I got one more bit of reason to feel that way.

Of course, with the card you can avoid long lines at check in, and form a discreet queue-of-one in front of the first class booth. That I very well know. Of course, you get the seat next to you blocked if there is a chance, which means a lot in terms of cabin space. Of course, you can wait for boarding in the lounge, and get munchies for free. Of course, you board with first and business class ticket holders, which means no trouble finding overhead bin space and grabbing a copy of Newsweek.

Today, though, I was sitting in my seat just before take-off, and a hostess appeared, calling me by my last name, and welcoming me on board, asking if there was anything she could do for me. I was slightly embarassed, because I felt all other passengers around me looking at me slightly puzzled... But it is nice to have a glass of wine from the business class selection, so the net result is positive... Lufthansa is really paying attention to their Senators these days!

Seen from a stranger

America is a great country, I must admit, but every so often I run into something that makes me smile as nonsensical.
It of course happens to any stranger in any country,
but the fact that I am well acquainted to the American way of life makes me a privileged observer. I do see the rationale behind seemingly paradoxical concoctions, but I am still tickled by their hilarious side.

Today I was driving to the airport (finally I'm heading back home!), and I decided to listen to some silly commercials rather than turn my ears off.
The pattern is often the following: a funny dialogue, which highlights why you cannot live with whatever they're advertising, then some information about how to find out more about the product or buy it, and finally the clinch: a series of sentences pre-cooked by some lawyers studio, whispered at blitzing speed by a male voice.

I can feel the pain of the creators of the ad as they gloomily append to their inventive dialogue those three seconds (it really is not more than that, but they squeeze in about 10 words a second) of warnings and punctualizations. I'm sorry for them, but the last bit is often the funniest to me! Companies feel compelled to add those sentences, because if they don't they fear they will get sued by some low-IQ
listener who will no doubt claim that the ad misled her or him in some very destructive way.
"The ad did not say I should not use that bleach to wash Kitty, and now she's lost all her fur!"

I've once read that Americans hate lawyers, but they love movies featuring them. I think that's true. "What's 15 lawyers chained together on the bottom of the Atlantic Ocean ? A good start". This I also heard in an american movie...

A challenge!

While replying to a recent post by David Waller, I got inspired to do the following, as a test for the grander project I mentioned (no it is not point one in my list there, it is point three, you perverts :-).

I will take on anybody who wishes to challenge me at a chess game on the Internet Chess Club. Whoever is interested should just reply to this post and we'll set out a meeting time on ICC (my handle there is tonno). I will then post here the most interesting games, with my comments.

Ok, I'm waiting. Can you humiliate the scientist ??

Beauty in a plot

In the last few days, I finally managed to extract a solid, as-predicted, meaningful Z signal from the data collected by the ZBB trigger. It took us more than a year to decide that we cannot model well enough jets of low energy to be able to use them when we reconstruct the jet-jet mass, which is the variable where a Z boson signal shows up as a bump in an otherwise smooth distribution. So, since the Z does not produce jets of low energy, I decided in the end to just cut these events away, increasing the signal to noise ratio, and forgetting about the idea of keeping more background at low jet-jet mass to model its shape better (to understand what the hell I'm talking about, have a look at the plot from Run I here).

The Z signal is buried in a much more numerous background, so it is a bit like looking for the needle in the haystack. In a dataset of 21 million events, only about 20,000 of them are present (one in a thousand), so a careful selection is needed. But even after that, the signal looks ridiculously small... That is as good as one can do, and it also is why the analysis is interesting to me: it is challenging! (I remember when I first discussed the idea of extracting it from our Run I CDF data: some of my colleagues looked at me in disbelief, not taking me seriously. But then I did do it, and the looks changed).

So, is there beauty in a plot ? I think so. The plot is the final result of many analyses. If crafted skillfully, it speaks volumes by itself. It has to be pleasing to the eye, not overcrowded, esthetically composed. The data it shows has to look well modeled by whatever mean one uses to describe it - a fit function overimposed to it, for instance, has to pass through most of the data points - physicists are born with a eyes-interfaced fitting algorithm in a part of their brain, and they will be able to tell if a fit has a bad chisquared (the measure how how well the fit models the data) by just looking at it.

In the case of the plot that shows the Z boson peak, beauty is there for two other reasons. The first is that this is the mass of two b-quark-originated jets, and b quarks are also known as "beauty quarks" (others call them bottom quarks, but the usage is deprecated for victorianesque reasons by the scientific community). The second is that, when you find a small signal in a large background stack, you feel as if you were able to salvage these beautiful events from oblivion. That reminds me of another piece of beauty, which I used in the frontispiece of my PhD thesis:

"Full many a gem of purest ray serene

The dark unfathomed caves of ocean bear,

Full many a flower is born to blush unseen,

And waste its sweetness on the desert air."

(Thomas Gray, "Elegy written in a Country Churchyard", 1751, vv 53-56).

So, having found the gems, I have to do a plot today. I'm struggling with root, a program physicists use to do their final data analysis and to produce graphs and plots. I unfortunately cannot show here the results of my efforts, since they are not blessed (see here for what is a blessing of results in CDF) yet! However, here is how my computer screen looks today:

So, since I am not telling you anything about the plot on the screen, I feel I am not breaking our publishing rules... But if you look closely, you will see a teeny-tiny green histogram - that is the Z signal!

A great evening with my friends

Yesterday evening I drove to Morris, where Gina lives. Gina is a very good friend of Mariarosa and I. She has four lovely kids, and whenever I have a chance I spend an evening with her and her friends, particularly Nikki, Jennifer, and Eddie. They are such fun to be with!

We saw a very funny movie together, "Meet the Fockers". I must admit that when they told me the title of the movie we were going to go see I had to ask, half-mockingly, whether it was a restricted movie... Misunderstood the family's name :) We did joke about that, but I was not completely sure about the right spelling until we lined up for tickets, when finally I came out with "Oh, meet the fOckers, now I get it!", which provoked a burst of hilarity in the people around us.

The movie was indeed very funny. It was great seeing De Niro and Hoffman together in a movie (anybody remembers other movies thay did together ? I don't), and the funniest scenes involved a minuscule dog which tried to hump anything it could. At one point, the dog grabbed a baby's Einstein puppet and started to copulate with it, missionary-style... So much for paying due homage to the  world year of physics! (BTW I do think the director thoughtfully decided that Einstein's puppet had to pose in a layback posture while being humped, rather than be taken from behind. I believe he may have thought that suggesting a real act of sodomy would be too outrageous, although I think it's just a detail).

Later, we had a few beers at a local place called the Lagoon. Here are a few pictures of us:

Left: Jennifer and Eddie. Nikki's visible between the two columns.

Right: here I am with Jennifer.

And here are the three of them: Nikki, Jennifer, and Gina, dancing together (left).

Last but not least, Gina and me (right):

Parelia!

Today I finally met Caolionn, after 5 years! (see here for an earlier post about her). She does look great. We had a coffee at a Starbucks in Geneva, a town near Fermilab. Caolionn is such fun! I had a memorable time with her, talking about the past and the present with an endless stream of jokes.

Driving to Geneva I had to stop my car to take a picture. These days the temperature is well below freezing around here, and these conditions favor the appearance of parelia (aka sun-dogs). I in fact saw them, and started screaming "look! Sun dogs there". And Caolionn did not understand what the fuss was about (she later explained my bad accent had suggested her I was talking about sand dogs, which she could not figure out why could be so exciting). I went as far as explaining the phenomenon to her, mocking her about the fact that a physicist should know what they are!

So here is one of the two parelia, barely visible just below the street lamp (it's the bit of rainbowish glow in the sky). I'm surprised I was able to pick it up, given the basic digital camera I took the picture with.  Parelia are a rather rare phenomenon. They are formed when a particular kind of crystalline ice (I think hexagonal crystals) is borne in the atmosphere, and the sun's light is reflected preferentially at a particular angle (each color at a slightly different angle, thus the rainbow effect) by those which lie at the same angle above the horizon as the sun.

A painful team loss and a nice personal success

So, yesterday's match against the St. Charles Chess Club ended in personal success, but my team lost the match 2.5/3.5. How disappointing!

My game (here I am, with the black pieces, immersed in deep thought at move 7) was very uncharacteristic of my style. I usually am a positional player, and I prefer to defend stubbornly than to seek activity at the price of a small material deficit. This time, I sacrificed a whole knight at move four (!!) for only positional compensation, in the form of the disorganization of the white army, a little initiative, some development lead, and the white king stuck in the centre, having lost the right to castle. The sacrifice is objectively dubious, and it leads to a probably lost position for black (my side), but the game is much harder to play for the defender, and indeed, after having almost solved his problems, and starting to reorganize, my opponent (who was short of time, having to play the last 20 moves in 9 minutes) committed an imprecision, then a mistake, and soon the game was over.

Gustavo (a physicist who works in the D0 experiment, and our very strong first board) played a beautiful game, but was only able to draw against his worthy opponent. Here Gervasio (on the right, with the white pieces) is formulating his middlegame plan. In the background, Lenny (also with the white pieces) is starting a long think.

After the end of the first four games, which included a brilliant win by Arkadij on board 6 and a unlucky loss by Carl on board 5, we were up 2.5/1.5. The two last games seemed to be both liable to warrant a draw... In the end, though, both Lenny (our third board, and our Captain) and Irwin (fourth board) lost their difficult endings.

Lenny (left, with the white pieces)seemed on top for a while, but as the time control approached his position started to deteriorate, and as he played his 45th move he had just two seconds left (you lose if you overstep the time limit, which was set at 1h30m for 45 moves and then 1h every other 30 moves).

Irwin (right, with the black pieces), on the other hand, was a pawn down in a rook ending. Rook endings are often drawn, but unfortunately not the one he ended in, and despite his ingenuity and his opponent's lack of understanding of the basic winning manouver, he lost the game, and with it we lost the match. We were unlucky this time! The SCCC is indeed a strong team, but in better days we can do more than lose by the smallest margin!

If you got to this point, you are really interested in chess, and you deserve to read about my game in more detail. Here it is, with very scarce comments.

Joe Splinter - Tommaso Dorigo, Chicago Industrial Chess League 2004/05, Far west division, round 6, board 2:

1.e4, Nf6; 2.d3, e5; 3.f4, Bc5?!N; [a novelty, of dubious value, played to avoid the setup chosen by white to counter my Alechin defence] 4.fxe5, Nxe4; [forced: after 3....Bc5 there really is no turning back, since 4...Ng8 would mean suicide after 5.d4] 5.dxe4, Qh4+; 6.Kd2, Nc6; 7.Nf3, Qf4+; 8.Ke1, Qxe4+; 9.Be2, Nxe5; 10.Nxe5, Qxe5; [10...., Qxg2, although alluring, fails to 11. Rf1, Qxh2; 12.Bf4!. Now the smoke has settled: B is a N down for two pawns, but W is seriously disorganized and his king is stuck in the center. The position is better for W, but not terribly so] 11.Nc3, Bb4; 12. Qd3, 0-0; 13.Qe3, Qf5; 14.Bd3, Qg4; [now 15.Qe4?? would fail to 15....Qxe4! 16.Bxe4, Bxc3; 17.Bxc3, Re8; gaining back the piece with a won game] 15.Qe2, Qh4+?!; [played to weaken the kingside before going to d4 anyway, this move causes problems to B later] 16.g3, Qd4; 17.Bd2, d5; [to prevent 18.Qe4, since now the trick does not work: 18....Qxe4 19.Nxe4!] 18.Qe3, Qg4; [the d5 pawn can be sacrificed to keep the initiative, but black's position is deteriorating thanks to white's precise play] 19. Nxd5, Bd6; 20.Kf2! b6?! 21.Qe4, f5!; [here B is paying for his "finesse" 15.... Qh4, since on 21....Qh5 22.Qh4 would keep threatening mate on h7 and force the Q exchange] 22. Qxg4, fxg4+; 23.Kg2, Bb7; 24.c4, c6; 25.Nc3, Bc5; [black is lost with correct play, but getting to this point has cost white a lot of thinking and he now has only nine minutes for the remaining 20 moves before the time control... B just needs to complicate the game, and W will soon blunder somewhere!] Diagram:

26.Rhf1?? Rfd8!; [and here it is, punctual as a swiss watch: under pressure, W errs. Necessary was 26.Be1, but B would still have pressure; 26.Ne4??, Rad8; also does not solve W's problems. A fun variation is instead 26.Rhd1?!, Rf2+; 27.Kh1, Rd8; 28.Bc1? [necessary is 28.Bg5], Rxd3!! 29.Rxd3, Rf1+; 30.Kg2, Rg1 checkmate!] 27.Bxh7+, Kxh7; 28.Bf4, Rd4!; 29.b3, Re8; 30.Rae1, Rxe1; 31.Rxe1, Bb4; 32.Rc1, b5!; 33.a3, Bxa3; 34.Ra1, b4; 35.Ne2, Rd3; 36.Be5??, Re3; and W immersed in a long think and allowed his flag to fall. 0-1.