The Say of the Week June 21, 2007
Posted by dorigo in games, humor.add a comment
“I am too shy to express my sexual needs except over the phone to people I don’t know“
G.Shandling
Rumsfeld hadrons June 20, 2007
Posted by dorigo in internet, news, physics, science.12 comments
An inspiring title for a preprint, indeed. Frank Close just published on the ArXiV (get your own copy here) a short paper, Hep-ph/0706.2709, with that very title. The paper is just as inspiring, and it is worth a close look if you are interested in heavy hadron spectroscopy.
The reason of citing a war criminal in the title is made clear from the abstract:
Donald Rumsfeld, in attempt to excuse the inexcusable, once (in)famously said that “there are things that we know we know; there are things that we know we don’t know; and then there are things that we don’t know that we don’t know”. Recent discoveries about hadrons with heavy flavours fall into those categories. It is of course the third category that is the most tantalising, but lessons from the first two may help resolve the third.
I found the paper very interesting to read. It is, however, a bit too technical for me to attempt a discussion of it here. Maybe I can just quote the simplest example that Frank brings to explain what we really understand well in heavy hadrons, to stay in the spirit of the last sentence of his abstract.
The example of things we do know we know comes from the B_c meson. This peculiar particle is made by a bottom and a anti-charm quark jiggling around each other. The large mass of the two quarks makes the system non-relativistic to a good approximation (heavy things move slower!), so the mass of the object must be not too different from the mass of its constituents - to the extent that such a thing is well-defined.
Now, the mass of the B_c has been measured with precision by CDF (see plot on the left, which shows the distribution of reconstructed mass of the J/psi and pion system into which the B_c decayed), and is found to be M=6276.5+-4.8 MeV , in fair agreement with theoretical calculations based on lattice QCD (F.Allison et al., PRL 94, 172001 (2005): M=6304+-12+18-0 MeV), which are basically calculations approximating space with a grid of points and computing the interactions between quarks on this grid.
What can fortify beyond doubt our confidence that we do understand the (b anti-c) system is however not so much the good agreement with lattice QCD, but a “crackpottian coincidence”: take the mass of the fundamental mesons made of (c anti-c) [the psi] and (b anti-b) [the Upsilon], add them, divide by two, and - lo and behold - the value turns out to be 6278.6 MeV, less than one part per mille away from the B_c mass.
Duh! two c-quarks worth of dough, plus two b-quark worth of dough, divided by two, is a c-quark plus a b-quark worth of dough! Or, as Groucho Marx would put it, “A child of five could understand this. Go fetch a child of five!” (for some reason I feel very grouchian lately).
What the exercise tells us is that quarks may be mysterious things when their dynamics is involved, but as heavy static sources of color field they are much less mysterious. Close makes the point that he awaits the moment when excitations of the B_c meson will be found, because it is then that more interesting effects will start posing new puzzles.
A small Peter Woit ? June 19, 2007
Posted by dorigo in Blogroll, humor, personal, physics, science.25 comments
I am rather amused, if a bit perplexed, to have fallen in disgrace at Lubos Motl’s court, The Reference Frame. In a post titled Rumors about the God particle, which he let out yesterday, he describes me as
“a person [...] (who) likes to paint himself as a very important person and he enjoys if his readers get excited about his mystifications, speculations, and propaganda. He’s like a small Peter Woit which is no coincidence because he is a frequent visitor of the notorious crackpot’s blog.”
Hmm, but doesn’t that mean that Lubos also visits Woit’s blog ? Otherwise how can he possibly know if I leave the occasional comment there ? Or maybe does he visit Not Even Wrong just because he wants to keep an archive of friends and foes ?
In any case, it transpires Lubos Motl cataloged me as a small crackpot. I am happy to see his definition is loose enough to allow very different personalities to be encompassed by it - or, to paraphrase Groucho Marx, I am happy to belong to a club that accepts people unlike me as members.
As for Lubos, I feel he deserves credit for always speaking his mind, be it fanatism about string theory, anti-global-warming propaganda, or jealousy for not being cited as a source of information by science magazines and media. But I do not mean to be too hard to him - for some reason, I find him a rather interesting person. I’d actually like to meet him one day, and maybe I will. You coming to PASCOS , Lubos ?
Intense pleasure from a chess move June 18, 2007
Posted by dorigo in Art, chess, computers, games, internet, personal.18 comments
I know, I am growing old… Do not get me wrong though - I still believe chess comes second as a means of providing pleasure to me. But this evening it came a close second indeed.
I was playing blitz chess on the internet - 5 0 games on the internet chess club (my handle there is tonno, by the way), which means five minutes for all your moves, and no move-by-move increments. After a French advance opening rather sloppily played by both of us, we reached the following position:
I am white, and black is to move. Can you see that white is better ? It should not be difficult to acknowledge, given the dominion of the c-file, the space, and the venomous “bad” bishop on g5 - a bishop which in similar positions white would love to get rid of, but which is instrumental in creating a bind here.
In the diagrammed position the correct continuation would have been 26. … Rc7 27.h3, Rhc8 28.Qb2 and the game could have gone on for many more moves. But I counted on the enticing position of my queen on c3, which had been only apparently placed there to hope for 26….Qxa3?? 27.Nc5+ (winning the black queen), as probably black thought, but to prepare a much meaner trick.
As black moved his knight with 26…. Na7, I felt a deep, deep wave of pleasure as I slid my queen up the c-file, only to stop it one square short of capturing black’s c8 rook: my move was played instantly, as if I were in a trance. I had not calculated the details in any way: it HAD to be the correct move. In these instances, which happen extremely rarely to me, I feel like a complete idiot savant: I do not have to compute, I see the solution.
27.Qc7!!!! is one of the most brilliant, beautiful, and esthetically pleasing moves it ever happened me to play in 23 years of “serious” chess practice. The queen stops en prise of the rook, immortal and yet certain of its own sacrifice. It can’t be taken, and yet it must be.
After 27….Rxc7 28.Rxc7+ Ke8 29.Rxa7, black paused and thought for a while. It is clear that he is utterly lost, but it is not easy to resign such a position. So he played 29…., Qd7 - a counter-queen sacrifice which is obviously not giving any hope. I replied 30.Ra8+, and black resigned. There follows 30….Qc8 31.Raxc8 Kd7 32.R1c7 mate.
Putting the position into Fritz 8 only diminishes my joy very slightly - everything is correct, there is nothing to blame about white’s play. But the fact that the silicon monster sees Qc7 in a fraction of a second is kind of annoying: it is just as if beauty was of no interest to computers: that move is worth 10 pawns, but it needs no more than a very quick look to them.
I now recall the last time I had such a vision during a chess game - and I reported about that experience in my Quantum Diaries blog too.
CMS Tracker blog! June 18, 2007
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Geoff Hall has recently set up a blog on the CMS tracker. Way to go! I am very happy to see that the idea of distributing information about particle physics experiments through blogs is gaining ground.
Geoff’s blog is young but is already interesting and it provides nice pictures of the silicon tracker of CMS, a daring device made of a hundred square meters of silicon wafers. I hope it will become an established source of information about that jewel of a tracking system.
Xi_b mass comparison June 16, 2007
Posted by dorigo in news, physics, science.6 comments
The recent discovery of the new Xi_b baryon has been announced first by D0 at Fermilab on June 12th, and then in a back-to-back seminar of D0 and CDF findings, which was given yesterday at the usual place - the “wine and cheese seminar” at 1-west in Fermilab’s Wilson Hall.
The mass of the baryon is in good agreement with theoretical estimates, which - for heavy hadrons - are relatively easy to make and less error-prone, because the large mass of the heaviest quark in the bag (the bottom quark in this case, but the same goes for charm-quark hadrons to some extent) makes the system easier to model.
So let us compare the findings of D0 and CDF with theory. A picture is worth a thousand words… Here it is:
As you can see, the two measurements are in agreement with the theory bands. CDF has by far the more precise determination, thanks to its better measurement of track momenta (in particular, muons in this case).
I will have more to say soon about the details of the analysis procedure, but now I am off to a weekend in the dolomites…
Ok ok before I go: here are two plots from CDF. The first is the mass of J/psi Xi combinations - the signal which provided the mass measurement in CDF. The second is the mass of Xi_c pion combinations, one that has not yielded a mass combination yet. CDF will improve its analysis with these latter decays!
And so about cascade baryons… June 15, 2007
Posted by dorigo in news, physics, science.14 comments
So it is D0, not CDF, that is credited for discovering cascade b-baryons first in the end. A Fermilab press release explains that the result has been submitted to Physical Review Letters on June 12th.
Congratulations to D0 for pulling this off! After the discovery of Sigma_b baryons by CDF, it seems only fair to have D0 get their own share of credit for the fantastic amount of information on the Standard Model that the Tevatron is producing.
The Xi_b particle (not Chi_b, with which I sometimes confuse myself - see comment by Lubos Motl below) is a baryon composed of a (bds) quark triplet. Three down-type quarks, each of electrical charge -1/3, but of very different properties: the d-quark weighs only a few MeV, the s-quark weighs thirty times more, and the b-quark weighs still a factor of thirty more than the s-quark. An odd, massive object (almost 6 times heavier than the lightest baryons, proton and neutron), whose decay is driven by the characteristics of the b-quark, which has a long lifetime. It is through that peculiarity (among others) that Xi_b baryons, although quite rare, can be identified.
My hands are itching, but I will abstain from saying what CDF has to say about these particles… I decided to play by the rules of our experiment, one of which asks bloggers to refrain from posting new results until the authors have had a chance to announce it first.
UPDATE: today at 1PM CST, D0 and CDF will give two back-to-back seminars on the discovery of the Xi_b baryon. You can follow the presentation via live stream video from this site (click on Services Offered: Streaming Video)
UPDATE 2: seems like the correct link is this one .
UPDATE 3: now that the CDF result has been presented by its authors, I break no rules if I release information about it here - I had a post out last Sunday, but decided to take it offline once I realized that the B group conveners in CDF had asked the result to be kept confidential until a formal release. In the meantime, D0 sent their own result out directly to Physical Review Letters, beating CDF in the race in a rather odd way (important results should be coordinated between the two experiments). Anyway, it does not matter much who made the announcement first - the important thing is that these particles are seen. I will discuss the details of the two analyses soon in another post.
A second intriguing MSSM paper June 14, 2007
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Only today did I have a chance, with a week of delay, to give a close look at a fresh new paper by J.Ellis, S.Heinemeyer, K.A.Olive and G:Weiglein, ”Light Heavy MSSM Higgs Bosons at Large Tan(Beta)“.
The paper is a very interesting attempt at fitting our whole knowledge of those particle physics and astrophyisics observable quantities most connected to the choices of parameters within the Minimal SuperSymmetric extension of the Standard Model (MSSM) in a way that best marries the tentative Higgs signal which appeared last January in a beautiful analysis by CDF of MSSM higgs decays to tau lepton pairs.
The analysis, produced by CDF with the first available 1/fb worth of proton-antiproton collisions collected by the Tevatron, found a 2-sigma excess of reconstructed events in the tau-tau mass distribution, compatible with a 160 GeV MSSM higgs boson for a value of tan(beta) -a crucial parameter of the theory- close to 45. The find made ripples in the web and on the media , but as exciting as it may be, the chances that it is a real new signal are zilch if compared to the odds that it is a mere fluctuation of the data.
Despite this cool-down remark, it is a perfectly legal thing to do to ask oneself how well does a “heavy” (in the sense that we are talking about the heavier of the neutral scalars - there are three in the MSSM) higgs and a value tan(beta)=45 go along with the other constraints on MSSM parameters coming from Bs decays to muon pairs, neutralino searches and bounds from dark matter searches, and g-2 measurements.
The new paper does exactly that, and it is a pleasant read. The authors use the model called NUHM, which they have shown in a previous paper to be a very interesting choice in the range of possible MSSM scenarios. From the abstract:
“…we find that a light heavy MSSM Higgs signal in the unexcluded part of the sensitive region could indeed be accommodated in this simple model, even after taking into account other constraints from cold dark matter, electroweak precision observables and B physics observables.”
It appears especially intriguing to note, as they are quick to do, that
“In this case the NUHM suggests that supersymmetric signatures should also be detectable in the near future in some other measurements such as BR(Bs->mu mu), BR(b->sgamma) and (g-2), and Mh would have to be very close to the LEP exclusion limit. In addition, the dark matter candidate associated with this model should be on the verge of detection in direct detection experiments.”
In their conclusions they specify things quite plainly in a few points:
“1) the predicted value of BR(Bs->mu mu) in the allowed region is generally > 2×10^(-8). Thus, this channel may offer good prospects within the near future for either supporting or contradicting the NUHM interpretation[...]
2) Mh must be very close to the LEP lower limit….This part of parameter space could be probed at the Tevatron with 8/fb of integrated luminosity.
3) The predicted value of BR(b->sgamma) in the allowed region is 4.6×10^(-4), which is about one standard deviation above the current experimental value [...] Consequently, an improvement in the present theoretical uncertainty might enable a discrepancy to appear.
4) The discrepancy between experimental measurement of (g-2) and the SM calculation can easily be explained in this scenario [...]“
And finally, quite interesting to note:
“Performing a chi^2 fit for nine precision and B-physics observables along the lines of Ref.[19] yields a total value of chi^2=9-10 in the allowed part of the NUHM parameter space [...]“
I must say I also appreciate a lot the realistic toning down which they put at the end of the paper:
“Very likely the weaker CDF tan(beta) bound from the search for heavy Higgs bosons compared to its expected sensitivity [i.e. the fact CDF saw a small excess at 160 GeV - TD] is due to a statistical fluctuation that will eventually evaporate. Nevertheless, it is interesting to know whether a signal at this level could be accommodated within the MSSM.”
Needless to say, the best thing about all the above is that it is a quite concrete view of the experimental panorama, and a meaningful stab in the darkness of Supersymmetric theories. At the very least, we will be able to exclude a very plausible scenario in the near future… It is good to have at least some power to falsify new physics theories nowadays!
3/fb reached! June 13, 2007
Posted by dorigo in mathematics, news, physics, science.21 comments
The Tevatron has been painstakingly producing proton-antiproton collisions for six years now, at an accelerating pace. The goal of 3 inverse femtobarns of collisions has now been crossed, as shown in the graph below (of which you can always find an up-to-date version in http://www.fnal.gov/pub/now/tevlum.html ).
To understand what the heck is a integrated luminosity of three inverse femtobarns, just think about shooting a lot of bullets with a short gun at a dime placed ten yards away. You do not expect to hit the dime, do you ? In fact, your bullets will cover a wide area around the dime rather randomly - say an area of about a square foot wide (if you are good). In order to hit the dime you will on average need to shoot a number of bullets equal to the ratio between the area covered by the dime (about a sixth of a square inch, or one cm^2) and one square foot - or about a thousand of them.
Let us put this in a tidier form: if we shot 1000 bullets per square foot, that is exactly the same concept physicists talk about when discussing the amount of collisions they managed to make: an “integrated luminosity” L = 1000/sq ft, or about 1/cm^2 since a square foot is about 1000 cm^2. Since the dime has a cross section S of one cm^2, you expect to have made N= SL = 1 cm^2 x 1/cm^2 = 1 hit on average!
Now, physicists use the same line of reasoning to estimate the chance of producing a rare process when colliding particles. Rare processes have a very small cross section, and so to produce them we need to shoot many bullets! The dreaded “inverse femtobarn” is nothing but a measure of the number of bullets we shot per unit area. A femtobarn is the impossibly small area of 10^-39 cm^2: the area of a square whose side is a few billionths of a billionth of a meter. So three inverse femtobarns means having “illuminated” with three protons every such square of the incoming antiproton.
With three inverse femtobarns, one can produce really rare processes. The total cross section of a proton-antiproton collision is about S = 10^-25 cm^2, so with L=3/fb = 3/(10^-39 cm^2) we have actually produced N = SL = 3 x 10^14 collisions (or three hundred thousand billions)! Now, a really rare process such as Higgs boson production has a cross section of a few hundred femtobarns: with 3 inverse femtobarns of data we expect to have produced several hundreds of them!
The problem, then, is finding these few hundred Higgs bosons in the three hundred thousand billions… Hehm. It is much harder than finding the dime once it was hit by your bullet!
Not to worry. CDF and D0 are up to the task. And the Tevatron is expected to more than double the total amount of collisions it produced this far by the end of 2009… More dimes to find, more chances to get rich.
Pascos 2007 June 12, 2007
Posted by dorigo in astronomy, news, personal, physics, science, travel.4 comments
Well, it looks like I am going to follow a few talks at PASCOS 2007 (PArticles, Strings, and COSmology) this year. The conference will be held at the Imperial College in London from July 2nd to 9th. London again, after another conference in cosmology in March! And Pascos for the second time, after Boston in 1998 (the conference was held at Norteastern University back then).
The list of invited speakers is yummy. Albrecht, Bond, Buchmuller, Buescher, Copeland, Cvetic, Efstathiou, Ellis, Feng, Ferrara, Frenk, Giddins, Hawking, Horgan, Hough, Kachru, Kallosh, Kaplunovsky, Linde, Matthiae, McLerran, Nakada, Ross, Seljak, Silk, Smith, Son, ‘t Hooft, Turok, Tye, Virdee, Wark.
I did not plan to submit my candidacy for any talk to represent the CDF collaboration this summer, but the chair of our Speakers Committee had the nice idea to notify us that the conference organizers had told him they were willing to consider post-deadline submissions of abstracts from our experiment, not having received any! Since my current interest for cosmology is high, I decided why the hell not… And I did.
Here is the abstract I submitted:
“Precision Standard Model Tests at the Tevatron“
The CDF and D0 experiments have recently produced exquisite new measurements in several key quantities constraining the physics of the Standard Model, with datasets in excess of one inverse femtobarn of proton-antiproton collisions at a center-of-mass energy of 1.96 TeV. New results include a 1% determination of the top quark mass, the most precise determination of the W boson mass, the first observation of WZ production, the first evidence for associated ZZ production at a hadron collider, and much more, including improved searches for the SM Higgs boson.
The talk will discuss these results as well as the general picture of our current knowledge of the Standard Model.
Of course the abstract was accepted… CDF has been producing such a string of five-star results lately that everybody wants to hear about it. And representing CDF is really a honor I feel today as much as I did ten talks ago.
So I hope I will be able to post a few notes on the interesting talks I will hear in London. Unfortunately, this trip will only be partly funded - I need to pay for my own stay, because our CDF group has no money for me any more this year. I thus decided I will only be in London for three days. That will allow me to give my talk, hear a few more, give a look at the Courtauld, and be back before my wife gets really pissed off (I have been around a bit too much lately!).
