Just a note August 30, 2007
Posted by dorigo in personal, travel.29 comments
This is just to say that although I received many comments to the recent posts, and I want to answer to several of them in a meaningful way, I am about to travel back to Venice by car. I will resume blogging tomorrow with a post on the nice seminar given by T.D.Lee on the fiftieth anniversary of his conjecture.
And Lubos Motl also is discussing the issue. For once we mostly agree, although I will have something to say about that too… Tomorrow.
UPDATE: I did my homework: you can find answers to your comments in the two previous posts. I would also like to mention here that Louise Riofrio has also discussed the matter today, wisely giving the emphasis of her post to the rather more interesting topic of the physics of black holes. And there are a couple of comments there which I found interesting to read, maybe you will too. Finally, let me mention that Louise defends me by mentioning my recent post discussing the case of Pegah Emambakhsh, which I wrote because I only found it a moral obligation to diffuse outrage about the story in the blogosphere. And that, to my eyes, is worth a thousand outraged posts about sexism.
Am I a sexist ? August 30, 2007
Posted by dorigo in Blogroll, humor, language, personal, physics, politics, science, social life.90 comments
It seems that my description of the speaker and the audience in a CERN seminar yesterday raised some eyebrows, and the comments column of that post got filled with a discussion that has little to do with the physics. I prefer to answer some of the comments in an independent post, i.e. here. For other columns discussing the issue, see what Kea (an often discriminated woman physicist) or Clifford (if you are sexophobic) have to say.
I am not an extraterrestrial, and no human being is happy to be criticized, nor is any blogger happy to hear from a reader “I won’t be reading much of this blog in the future, I don’t think“. I will not try to feign unconcern. But by far the strongest feeling today, as I checked my blog comments - mostly concentrated in the column under yesterday’s post on Lisa Randall’s seminar , and the incoming links, was amusement.
And as usual when I find myself amused by the criticism I receive, my first reaction is to beg for more. Hannibal Lektar, interviewed in his cell in “Silence of the Lambs“, replies to the shock of the investigator at the display of the monster’s personality by mentioning how he once ate a man’s liver with a bottle of Chianti, and showing how the thought of it makes him drool. Likewise, my impulse would be to drop a casual, really sexist remark which would only drive a larger wedge between me and my detractors, like “My next grad student in CMS is quite skilled, and she is also quite sexy - I’m working to exploit the latter feature”. (She will laugh at you all when I show this to her).
Instead, let me disciplined here. I will try to answer to the criticism by taking it seriously, but bear with me: this will not be totally devoid of the occasional sarcasm which is part of my writing style. You had a taste of it just above…
1) First of all, a call to keep a perspective. Maybe you never cared to read the fine print, but the subtitle of this blog is “private thoughts of a physicist and chessplayer“. That is right, private. And indeed, in my blog you will be just as likely to find a discussion of physics issues as a chess game or a report of an observing session under the stars, or a picture of my kids. Did you get the message ? If I venture in a description of a person I meet - be it a woman, a man, whatever - it is because this is my diary, and I want to keep a record of my ideas, my feelings, my thoughts.
Moreover, I write posts here as I would write a book, or a novel, and not an essay on particle physics: I like to describe the characters, and I am better at describing women than men. You, dear reader, are the ultimate judge of the quality of the output, and you are perfectly entitled to decide it is not of your liking. What you cannot do, I think, is criticize the contents because they are at odds with the way you would have written things, or because they do not fit with your personal idea of what is kosher content in the internet. I of course love to be read, but maybe this blog is just not for you. And that stil does not entitle you to criticize it as sexist: these are personal thoughts.
Maybe you can criticize me as a sex maniac, but that would be a little bit over the edge for having written “left the arms exposed“, don’t you think ?
2) Related to the above, is a general lack of perspective in some of the comments I got to read today. The internet is full of child porn, home-made bomb manuals, neonazist sites, climate change skeptics. And the world, too, is full of maniacs who bomb countries for their personal gain, serial rapists, killers, readers of New Scientist.
You feel I have not done a good service to the cause of reducing sexism in academia with my post of yesterday ? Maybe you have a point, but is it such a big deal ? Did it really require your royal, thought over intervention ? This blog is visited less than a thousand times a day (maybe by the same people over and over). By attacking a paragraph which even carried a initial disclaimer (”if I am allowed a slip…“), and which was clearly only meant at giving the atmosphere of the seminar (as a few readers of both sexes seem to have understood without guidance), you show concern for this blog content and that is fine, but you also show the kind of random, compulsive 360-degree action that is typical of fundamentalists. One cannot argue with fundamentalists, so what am I doing here ?
3) Perhaps what I am doing here is trying to explain that I, too, hate the situation a pretty girl faces when she starts an academic career. But the whole world works the same way. By applying restraint, censorship, and dogged control inside your institution, in the behavior of your colleagues, in what you read in blogs, you may be successful in creating an apparently sexism-free environment. But the people the girl will meet in his academic life are human beings, with their own pulsions, their fallacies, their sexism - even if covert. You may walk in your neat corridor devoid of pics of busty girls, but you will not have purged the mind of Dr. Brown three doors down - he has those pics hung on his garage walls.
I guess the point I am making is: bombing Iraq is not the way to eradicate fundamentalism and arab terrorism. Quite the opposite.
4) Finally, I deeply regret the level of paranoia we have reached with the whole issue in academia. The world is changing, thank god. In some ways it is even getting better.
Let’s take JoAnne Hewett’s comment as an example. She starts off by writing “You started off on a sexual tone“. Excuse me, what is a sexual tone, describing a dress ? Describing jewelry ? Saying somebody looks nice and fit ? I have to exercise restraint to avoid saying what I think here. I know a sex maniac could smell sex in that sentence, and in a way, that is exactly what annoys me in the criticism: the people who criticize are those most obsessed with sex, not the other way round! Get a life, folks!
But then things get worse: “And then brought in a Southpark reference to penises ? Got sex on your mind while writing this post, do you ?“… What can I say ? She really does not understand, but she is excused because she probably does not usually read this blog, which is full of the same stuff. One example (the first that springs to my mind because of the similarity of the topic, but there are dozens) is from a post of mid-May this year, when I talk about back-of-the envelope calculations as opposed to theoretical calculations with too many digits of accuracy: “we need it about as much as a man needs a two-feet-long penis - great for bragging, but ineffective and redundant“.
JoAnne concludes “why make the point that she was not intimidated by the questions…? Think the girl can’t take tough questions, do you ?“. Good lord. Is that sexist ? JoAnne, that is insulting, and I think you meant to insult me. You managed to do it. I might excuse you if you wrote the comment out of an impulse, but that requires your apologies. Otherwise, please walk away, you have crossed the line.
Lisa Randall: Black holes out of reach of LHC August 29, 2007
Posted by dorigo in news, personal, physics, science.103 comments
This week I am spending a few days at CERN to get back on track with CMS activities after my long vacations. I was planning to follow the talks of the Physics Days this afternoon, but as I learned that Lisa Randall was about to give a seminar on “Black Holes and Quantum Gravity at the LHC“ I could not resist and changed my plans to accommodate it.
If you allow a slip to inappropriate comments, Lisa Randall is notoriously not only an esteemed and well-known theorist, but also a quite attractive woman - a powerful mix, capable of turning to jello the knees of most men. Today she wore a nice black and white dress which left her shoulders and arms exposed, a necklace of mother-of-pearl, and a wide-band silver bracelet with colored stones. Her hair was collected in a pony tail. She looked nice and fit - for some reason it made me think she probably works out on a regular basis.Lisa talked in front of a large audience - I could count about 150 heads - which packed the auditorium. She was introduced by the convener by what must have been the shortest introduction in the history of theoretical seminars ever, something like:
”Hre’s Lisa, she’ll tlk ’bout bloles quagity”.
I’m serious. Hilarious, but an odd note if combined with the fact that, despite my attempt at an initial applause, the audience stood quite still. She balked not, and started to talk with no further ado.
So let me do the same and try to put together some notes from her seminar. As always when I discuss things above my head, be compassionate with my ignorance, and grateful for my effort. In other words, don’t shoot the pianist.
The talk focused on models with higher dimensions of quantum gravity in the context of a low quantum gravity scale. How low ? Well, as low as one can hope for - about 1 TeV or so. Naturally, at the LHC one would expect quite dramatic signatures. Should LHC be looking at black hole production or elsewhere ? It turns out that the best way is to look for compositeness scales. But let’s not jump to the conclusions yet.
The questions experimentalists have to ask themselves at the start of a project like the LHC, which will explore unknown new energy scale and domains, are like “Are we optimizing existing searches for the signatures we might have access to ?”, “Are we sure we are not missing possible searches ?”, “Will this get me a salary increase ?”. The latter is my own contribution to the list. One interesting question, connected to the scope of Lisa’s talk, is: “If there is new physics, but it lies at a higher energy scale than the one directly accessible by the machine, how do we maximize our chances to see it ?”
Historically, the reason that black holes appear so promising as compared with other possible signatures is the predicted huge cross section for their production if there is a low quantum gravity scale. Lisa ventured to compute that if quantum gravity turns on at a scale of a TeV, one gets 100 pb cross sections at the LHC for producing black holes, naively. There is no suppression from gauge couplings, so it is indeed a large signal. Also, the signature is spectacular, since these objects are predicted to decay into large multiplicity final state, with highly spherical distributions. Very distinctive, unmistakable new physics.
But the problem is that the idyllic picture is not very realistic. The onset of a non-perturbative regime where black holes are produced and decay with those signatures is much above the QG scale, and this appears to be above the reach of even our brand new pupil, the LHC. The poor sucker has not emitted the first burp yet, and it is already criticized for being a midget. In any case, at threshold one would not see the striking signatures, but maybe something can be saved. How much could we learn from that ?
Randall was very clear in stating that LHC is unlikely to make classical BH states decaying with Hawking radiation. She appeared to be interested in assessing the damage: and the answer is that, if you have a low quantum gravity scale and you cross it, you will have a change in the two-particle final states. Things are not calculable, but there appear to still be distincitve experimental signatures that are capable of distinguishing among different models.
Lisa did not discuss much what brought the optimism down in the last few years. She just stated that you have to go well above M, the energy scale of quantum gravity, to be sure to hit the striking signatures publicized in the past. The parton distribution functions of the proton drop rapidly with the fraction of parton momentum, and since we are by necessity near threshold, the value of the latter is very important in determining what the rate of the new process is going to be. To make matters worse, M is convention-dependent. Factors of 
Lisa discussed some of the models and the resulting conventions and equations for the schwarzschild radius, the energy scale, and the other main characteristics. I prefer to avoid entering these details in this short writeup, because I am sure I would drop some dimensional factor here or there. Not that many of you would notice, but I’m a perfectionist 
One point which looked important is that in the models considered, the black hole lifetime is bigger than the inverse of the energy scale of quantum gravity. This drives some of the phenomenology of the black hole decay. Another point is that every degree of freedom should carry an insignificant amount of energy with respect to the total; and since we are never going to get far above threshold at the LHC, we will have to be careful to call what we produce a real classical black hole. These things have low entropy close to threshold, and the multiplicity of the decay will be affected.
A critical factor in the computation of the number of particles emitted in the black hole decay is the assumption of the dimensionality of the space: particles emitted in the bulk have more directions in which to oscillate. Furthermore, since the threshold for producing black holes is not M, but a higher energy, even if we did see a black hole, we would not be able to extract M from the total cross section, because of inelasticity effects: not all the energy of the colliding partons goes in the creation of the black hole, due to initial state radiation.
The difficult question to answer is in fact, what fraction of the energy gets trapped inside the horizon? It is of course important since the PDF fall rapidly with energy. What is clear is that the inelasticity effectively increases the threshold. The reduction in cross section due to this effect is enormous, and it is the lack of considering it which has brought some overoptimistic predictions in the past.
So, the upshot is that BH production threshold is higher than originally thought. It means a lower production cross section, a lower reach in black hole mass, and it translates into lower entropy reach as well. The conclusion of Lisa Randall is that we will not produce classical thermal black holes at the LHC. What will we still be able to produce, then ? And what kind of multiplicities should we expect ?
Lisa discussed the calculation of the multiplicity of final state particles. She said that the calculation is totally unreliable. But one thing stays clear: low multiplicity final states will dominate even if we call it black holes. So we have to face the facts, and study 2-body final states: jets and leptons. Can they be distinguished from backgrounds by rate, kinematics, bra size? Yes. For jets, transversality is the key. QCD is dominated by t-channel exchange, i.e., forward scattering. Black hole events are isotropic. So this is really becoming like any other compositeness search: massive states produced at low rapidity.
The most concrete proposal of the talk came now. At the LHC, one should measure the differential cross section for dijet production by determining the angular dependence through 
While describing a scenario where the LHC will have to walk the walk of unclear kinematical analyses rather than being hit in the face by those firework-like signatures that experimentalists have started to dream more and more frequently as of late, Randall was careful to insist that the LHC is indeed a powerful machine, although she fell short of declaring it will make everything clear about quantum gravity. It reminds me of an episode of South Park where chinese conspirators keep american people happy and oblivious by telling them they have larger penises.
After discussing the signature of black holes, Randall really took a walk on the wild side, by delving with the possible signatures of the same kind from alternative models of quantum gravity, such as a weakly coupled string theory. There one apparently expects a resonance behavior, followed by a dramatic drop in transverse cross section, which can be used to distinguish the stringy behavior from the simple production of a new Z’ boson, blah blah.
In addition to the resonance, you would also see a drop in the quantity 
Lisa said she would love to see these studies done by Atlas and CMS: energy-dependent angle studies in dijet production. I bet she will.
After Lisa concluded her talk there were several questions. Some of them were quite critical of the reliance that had been put on the model discussed. I found even some point of acrimony in a couple of questions, but Lisa was not intimidated. She explained she had stated quite clearly the limitations of the results she had been discussing. I was slightly surprised to find a non totally friendly atmosphere. Evidently, times are hard for theorists these days.
In the end, I myself asked a question. I knew from previous blogging on the issue that when one reaches a quantum gravity regime, the QCD cross section of dijet production has to go down, but Lisa had not discussed this feature. She explained that before one reaches the regime when QCD 2-particle cross section gets reduced, the cross section has to go up, in any case. So the dijet cross section reduction that Sabine has first studied happens at a regime that LHC will fail to cover.
MSSM Higgs at 160 GeV: one more piece of non-evidence August 27, 2007
Posted by dorigo in news, physics, science.12 comments
2007 has been a rather bumpy year for blogging about Higgs searches, and in particular those for a MSSM Higgs decay to third-generation fermions. It all started when John Conway discussed at Cosmic Variance his brand new result on the decay to tau lepton pairs, which showed an intriguing 
Just as the buzz level on the internet was getting back down to a physiological level, rumors started again about a possible large signal found by D0 in a search for MSSM H->bb decays in events with three or four b-quark jets. Since that is another good sample where to search for that particle, people were again very interested in the issue, and a new wave of interest arose everywhere, and got picked up by slate and other high-traffic web sites, and still more newspapers.
The rumor was not based on a public result, so there was not much to do but sit and wait for the moment when D0 would be ready to discuss their achievements. But D0 did not produce anything, only making the curiosity grow.
I am too lazy to replicate all the links here, so if you want to dig in the issue please find all the required addresses in this more comprehensive summary .
Anyway, that is the past. The news now are that, for the Lepton-Photon 2007 conference, CDF has produced a public result on the same search. Since CDF and D0 have similarly sized datasets, and similar sensitivities, it is quite interesting to see what they find! So let us look at the new result, whose main authors are Thomas Wright and Dante Amidei - two knowledgeable and skilled physicists who usually (and this is no exception) produce very accurate results.
The search is based on a trigger collecting multi-jet events enriched with b-quarks. The trigger is made possible in CDF by the Silicon Vertex Tracker, a wonderfully complex set of custom-designed hardware boards organized in a highly parallel architecture. The parallelism allows speed of execution: in less than 20 microseconds, the hardware collects information on hits in the silicon layers, compares hit patterns to those stored in associative memory boards, and performs a linearized fit, achieving a measurement of charged track transverse momentum, azimuthal angle, and -crucially- impact parameter with respect to the beam position with a precision quite similar to the best achievable offline with a glass of brandy in one’s hands. A sketch of the operation flow of SVT is shown below, where you also get to see the actual boards performing the operation.
The multijet trigger collects events with at least three jets, two of 
The sample of data collected by the SVT multijet trigger is enriched in b-quark jets, but a more stringent selection is needed. Such is provided by the SecVtX algorithm, which explicitly searches inside jet cones for sets of charged tracks which can be fit to a common origin displaced from the interaction point: a secondary vertex. CDF selects events with all three jets tagged by SecVtX, to select a sample enriched with the process 
Tom and Dan found that to search for MSSM production of Higgs and one b-jet, the ”smoking gun” distribution was the invariant mass of the two highest-
Monte Carlo generators do not predict with the necessary accuracy the correct mixture of processes with b-jets and c-jets in multijet events. So one has to rely on the data to model them. Events with only two b-tagged jets provide a good handle, but the key to understanding how the different backgrounds contribute to the 3-tag sample is given by a variable called 
where 
So here is the result of the fit: The dijet mass distribution with the best fit overlaid is shown in the plot on the left, for a hypothesized MSSM Higgs of 150 GeV. The black points are CDF data with three b-tags, the four main backgrounds are shown in light blue (bbb= three b-jets), dark blue (bb+light-quark jet), violet (bcb= events where a charm jet produced along with two b-jets is one of the two leading jets), and yellow (bqb=events where a mistagged light-quark jet is one of the leading jets). The best fit allows some small fraction of signal (in red), but it is also compatible with the no signal hypothesis, such that a limit can be set to the cross section of 
The limit above, shown with a black line, is compared -as has become customary lately- with expectations from pseudo-experiments (dashed black line, and red 1-sigma and purple 2-sigma bands). You can see that between 140 and 160 GeV CDF obtained a worse limit than the one they expected to set, mainly because of the small fluctuation which is shown in the dijet mass fit above. In all cases, the fluctuation is at most a 1.5-sigma excess, which again, is one more piece of non-evidence for MSSM Higgs at the Tevatron. Nonetheless, let me say that this analysis is a very beautiful attempt at something which I know out of experience to be a quite difficult task: understanding the composition of multijet datasets in terms of their flavor composition, and modeling correctly the different nuisances of the mass distribution of the leading jet pairs, is no small feat.
For the more technically inclined, I should also mention that the plot above was derived by assuming no width effects in the Higgs mass templates. That is to say, the increased bbH coupling due to a large value of tan(beta) will increase the width of the breit-wigner shape. The cross section also gets modified by the change, and so one has better compute a limit taking those effects into account. Of course, both including and neglecting the effect is useful. You can find the width-included limit in the public note describing the result.
A win against IM Vladimir Eljanov August 24, 2007
Posted by dorigo in chess, computers, games, internet, personal.9 comments
It is always nice to win a game against a titled chessplayer, even if it is only a blitz game (5 minutes per player) and it is played online in the Internet Chess Club, thus preventing you from receiving a fair handshake at the end.
It happens rarely to me, but mostly because getting to play with international masters or grandmasters is tough even on the ICC, where many titled players are logged at any given time of the day. And then again, of course 90% of the times they dispose of me as quickly as you can dump the garbage.
To get the honor of playing against an IM or a GM you have to first boost your rating to a level which allows the automatic pairing system to give you that chance. It happened tonight, when I was lazily playing 5-minute chess while my wife was telling a story to the kids to get them to sleep. My opponent, patola(IM) on ICC, does not have a top notch rating for blitz games online, but has a FIDE rating of almost 2400, which is rather average for International Masters. He is the father of Pavel Eljanov apparently - and Pavel is a strong grandmaster.
Anyway, here is the game. Apparently, I managed to surprise him in the opening with a risky and bold pawn sacrifice… And then things snowballed for him, until he managed to mend his position to a level which still allowed him to hope. But with a few precise moves, I came out on top.
patola(IM)-tonno, ICC 5 0 - 24/8/2007
1.c4 c6 2.d4 d5 3.Nf3 Nf6 4.Qc2 g6 5.cxd5 Bf5!?
(see diagram 1 below)
A risky move, and not a totally correct one, but quite viable in blitz games, where the surprise factor is very important. My opponent indeed started to think… He knew h could take the pawn, but he would thus give black some play… On the other hand, if he did not take the pawn, he would have to lose a tempo with the queen… So he went for it. 6.Qb3 Nxd5 7.Qxb7 Nd7 8.Qb3 here 8.a3 would have brought white a small but steady advantage after 8… Rb8 9.Qa6 (but worse is 9.Qxa7 e5! with some play against the white queen, exemplifid by 10.dxe5? Ra8 11.Qd4 Nb4!) Rb8 9.Qd1 Bxb1!
(see diagram 2 below)
Here is the surprise! White loses the exchange. 9.Qa4 would have amounted to the same thing (9…. Bxb1 10.Rxb1 Nc3) and even the stubborn 9.Qc4 would have left black on top after 9…. Nb4! 10.Na3 Be6 11.Qc3 Nxa2 12.Qc2 Nxc1 13.Rxc1 Qa5, where black has gotten even with pawns and retains the bishop pair, the initiative, and a better overall structure. 10.Rxb1 Nc3 11.bxc3 Rxb1 12.e3 Qa5 13.Qc2 Ra1 14.Bc4 Nb6 15.Bb3 Nd5 16.Bxd5 cxd5
(see diagram 3 below)
17.a4? But here white had a chance to obtain some dynamical compensation by means of 17.Qb2! Rxa2 18.Qb8+. I do not really know what would have happened in that case, but for sure I was glad to see him playing the pawn instead. After this missed chance, the game goes downhill for Eljanov. Probably he played 17.a4 to divert my queen from the attack of c3, but it is an inaccurate move. 17…. Qxa4 18.Qb2! Still good now, but it does not win a tempo by attacking the rook any longer, so that black has time to create an escape for the king!(see diagram 4 below)
18…. f6! the last required accuracy. Now black is really safe. Other means of defending against the impending sortie of the white queen would have been much more troublesome. f6 creates the f7 square for the king, and avoids any knight jumps to e5 and g5. 19.0-0 Kf7! 20.Qb7 Qa5 21.e4 dxe4 22.Qxe4 Qxc3 23.Bd2 Rxf1+ 24.Kxf1 Qc4+ 25.Kg1 e6 26.h4
(see diagram 5 below)
A desperate attempt, but black’s position is easy to play and I by now had even more time on the clocks… 26…. Be7 27.h5 Qd5 28.Qf4 Bd6 29.Ne5+ Bxe5 30.dxe5 Qxe5 and in this hopeless position white allowed his time to end. A nice game, although admittedly not too well played by either of us. I am curious to know whether 5. …Bf5!? is a novelty or if it has been played before. My guess is that it must have been played, if only by another fool. The database of my fritz 8 is not working for some reason. Carl, can you search the position for me ? :)
Higgs limits at the Tevatron: <1.4 x SM !! August 23, 2007
Posted by dorigo in news, physics, science.32 comments
With pleasure I visited the public web page of the CDF experiment today, finding a fresh new combination of most of the searches for Standard Model Higgs bosons ongoing at the CDF and D0 experiments. And it is an exciting piece of news!
As I discussed recently here, CDF has produced a combination of its results. D0 did the same, and then, with a technically very complicated but conceptually very simple operation, the two results have been combined.
So let us have a look at the combination, and then discuss it briefly. It is shown in the plot below:
As usual, the y axis shows the 95% CL limit in units of Standard Model cross section for Higgs production, for each value of the Higgs mass (the x axis). A limit at a value y=2.5 for a value x=145 GeV means that the Tevatron experiments exclude, with 95% confidence, that the Higgs boson has a production cross section at the Tevatron larger than 2.5 times the SM prediction IF its mass is 145 GeV.
The yellow band (yellow has become a classic for LEP exclusion regions) “cuts away” the part of the plane where the Higgs cannot be, having been excluded by the LEP II experiments.
Then there are the results. Let us forget about individual expected limits by CDF and D0 (red and blue dashed lines) and concentrate instead on the black ones. The dashed black line shows the 95% CL limit that the Tevatron experiments were supposed to be able to set, given the datasets analyzed in each analysis and every small feature of each. You get that black dashed line only after combining the results of combinations of pseudoexperiments, taking care of nuisance parameters, integrated luminosities, cross correlations, correlated efficiencies, statistical effects of any kind, and other devilish details. But that is only an expected limit: it tells you what is the real power of the experiments.
The real limit is the one shown with the continuous black line. And here we get to see that, had the Lepton-Photon conference (for which this plot was finalized) been held in october or november this year, the Tevatron could have had a chance to start crying “No higgs at 160 GeV” on newspapers throughout the world. In fact, the integrated luminosity of the analyses most contributing at 160 GeV (H->WW decay searches) were of about 1.9/fb in each experiment, but both CDF and D0 have about 60% more data already in their pockets, and only lacked the time to process them, validate them, and include them in the analyses.
The Tevatron has been a bit lucky there though: as you can see in the plot, the limit set at 160 GeV is about twice as good as the one they expected to set. Lucky ? A debatable point indeed. However, the first point to bring home is definitely that the CDF and D0 experiments are starting to bite in the Higgs hunt business.
Then, you get to see something strange. What is going on at 120 GeV ? The limit there is twice as bad as pseudoexperiments predict!
That fact only highlights the amount of variability of the results that the Tevatron can display, with respect to their “expected limits”. Indeed, the thin dashed black line (and the other dashed lines too) is a misrepresentation of reality: instead of a thin line you should be looking at a wide band - a 1-sigma contour of expectations. The lack of the band is certainly only due to lack of time and hurry in producing the plot in time for summer 2007 conferences, and I think this plot will get more detail in a month or two. In any case, in the absence of the band, I am unable to speculate on how likely it was to get such a lousy limit at 120 GeV if the Higgs was not there.
Indeed, a unwitty soul could be looking at the discrepancy between dashed and continuous black lines above the mark at 120 GeV and claim that what the Tevatron experiments are observing at 120 GeV is exactly what they would be seeing if there was a Standard Model Higgs boson produced with a larger-than expected cross section sitting right there. Because the lousier limit there means that the experiments saw more candidate events at 120 GeV than they expected to find from background sources…. An excess ? No way! A fluctuation, and be sure, a mild one.
When the +-1-sigma bands of expected results will be released, we will be able to quantify just how unlucky - or unlikely - the result is at 120 GeV, if the Higgs boson is not there. In the meantime, the second point to bring home from the plot is that that 95% CL line at 120 GeV is still at 8.5 times the expected Higgs cross section.
That underlines how hard the Higgs boson search is at low mass - something the LHC experiments have already started to dread. The ATLAS and CMS experiments, I wish to remind you, will be in great embarassment if the Higgs mass happens to be just a few GeV higher than what LEP II excluded. And that not because they would be taking data next year only by virtue of having shut down the LEP II experiment only months short of discovering the Higgs boson: but because the Higgs at 120 GeV is tough, very tough, and at the LHC life is even tougher than at the Tevatron. Proof be the wagons of money the experiments have invested in exquisite electromagnetic calorimeters, to detect a blip in the diphoton invariant mass from the H->gamma gamma decay!
UPDATE: in a comment below, an anonymous visitor asks why the actual limit at 120 GeV is so much worse than the expected one, given that the combination by CDF (which I posted a few days ago) is not terribly above their own expectation. I mentioned that the combination of limits is a tricky business, and that one could not really guess until one had all the data available. So here is the D0 combination by itself, the missing piece:
One clearly sees that D0, too, got a larger exclusion at 120-140 GeV than they expected. If you combine this with the CDF limit, I have no reason to doubt that the combination will be worse than expectations in that range.
But let us look at numbers a bit more carefully: at 120 GeV, D0 expected to set a limit at 7xSM and got 10xSM, and CDF did exactly the same: 7xSM expected, 10xSM obtained. At 140 GeV, though, D0 expected 7.5xSM and got 10xSM, CDF expected to set limits at 7.5xSM and actually got 5.8xSM (or so I read off the plot - I am too lazy to fish out the numbers from internal CDF documentation, and I would still be unable to diffuse it here!).
What we get is that a combination of those numbers yields, according to the CDF-D0 group who did the exercise, at 120 GeV a 5xSM expected limit and x8 obtained, while at 140 GeV a expected 4xSM and an obtained 3xSM. Do these numbers make sense ? I think that simple statistical arguments justify those figures, and I thus have to dismiss the rather annoying hypothesis put forward by the commenter that ”for the _expected_ overall curve, WHlvbb accidentally used its 68% CL expected, rather than 95%, in its contribution to overall, resulting in a lower than intended overall expected curve“.
UPDATE II: as I posted the above update, a suspicion arose in my mind. Indeed, the numbers for 120 GeV make more sense to me than those at 140 GeV, when a 10xSM combined with a 5.8xSM yield a 3xSM - suspicious. So I take back what I wrote above for the case of the point at 140 GeV. Rather, let us give a closer look at the “point” at 140 GeV in the combination plot, which I paste here magnified for your convenience:
One observes something weird: while there appear to be kinks in the red and blue dashed lines at 140 GeV (the red one is evident, the blue one less so, but still is there), one sees no kink for the black dashed line at the same abscissa.
A suspicion might arise that the black dash was only computed at 130 and 160 GeV, and a straight line drawn between those points. That would explain the slightly better-than-expected combined result at 140!
I will investigate.
UPDATE III (yes, they keep coming): I just revised the title to “1.4xSM”, since that is the actual value at 160 GeV.
UPDATE IV: I checked the issue with the Higgs conveners in CDF and basically, what I can do here is mention a few facts:
- the combination is preliminary
- it is based on preliminary results from the individual analyses
- a better combination, with 1- and 2-sigma bands on the expected limit, and with more data points will be produced soon.
All the above is to say that there is no real mystery, and that one should be looking at the combined plot with a grain of salt. Let us wait for a better version of the plot, which is red hot from the press ….
The say of the week August 23, 2007
Posted by dorigo in games, humor, travel.add a comment
“What ? No wooden legs ?”
(as a wise man said upon being shown all the crutches at Lourdes, supposedly thrown away by cripples that had been miraculously healed)
Courtesy Bad (an anonymous poster here)
Save Pegah Emambakhsh August 23, 2007
Posted by dorigo in news, politics.33 comments
An iranian citizen refuged in London, Pegah Emambakhsh, will be deported back to Tehran on flight BA6655, departing from Heathrow tonight at 21.55. Pegah is a lesbian and the iranian government sentenced to death by lapidation her lover two years ago, when Pegah flied the country. She is destined to the same fate if the english government does not stop her return to Iran.
Death by lapidation is way more cruel and inhumane than other ways to kill a human being. The agony can last for tens of minutes as smaller stones are thrown first, to be followed by others that break the bones, until deathly blows are inflicted on the head. If you are a death penalty aficionado, you still should find it repulsive, leaving alone the outrage of killing a person whose only guilt is her homosexual love.
Pegah is detained in Yarlswood (Sheffield), but if her departure is not delayed or canceled, she is probably already on her way to Heathrow. Hopes to prevent her flight to a horrendous death for a sexual preference which is not a crime in more civilized countries than Iran are getting thinner by the minute.
Thousands of emails have flooded the english government from around the world, asking to stop the deportation. Members of the European Parliament had also started a mobilitation a few days ago, but in response the english visa office seems to have actually anticipated the departure to tonight, to prevent the protest from being successful.
UPDATE: the extradition has been delayed only a few hours ago to the 28th of August. There are still four more days, therofore, for a civil action to stop the lapidation of Pegah. If you are in speaking terms with any member of the british government, please get your ass off the chair and do what you should have already.
The Tevatron Hirise as you have never seen it before August 23, 2007
Posted by dorigo in Art, physics.7 comments
In a talk recently given by prof. Robin Erbacher at Lepton-Photon 2007 I found today a picture which I am glad to steal. It shows the Tevatron Hirise (also known as “Wilson Hall” to commemorate Robert Rathbun Wilson), the building symbolizing the Fermi National Accelerator Laboratory, in a way I had never seen it before, which summarizes the monumental achievements in top quark physics produced by the lab:
My compliments to Robin (or whoever did this), I really appreciated the composition.
Homeopathy losing ground in Italy August 22, 2007
Posted by dorigo in news, physics, science.25 comments
For the series “let’s cheer up if we can”, a small reason to be optimistic about the future of mankind -or at least, of my sorry country- came today, when I read on the italian newspaper “La Repubblica” that my fellow citizens seem to be slowly withdrawing from curing their diseases with homeopathic drugs and other alternative treatments.
Homeopathy is founded on nasty pseudo-science, and is a big business - as anything pharmaceutical companies handle. The idea is that diseases can be cured by assuming microscopic amounts of substances which in larger amounts cause the same symptoms of the disease. The preparation of the drug is usually performed by dissolving ridiculously small amounts of the active substance in water, and then shaking it such that water will “retain a memory” of the molecular structure it was in contact with. The whole concept is at odds with hundreds of years of studies of chemistry, physics, and medicine: which only shows how, if you take a lie and shout it loud enough and long enough, you will get somebody to believe it. The situation is made worse by the human mechanisms of attaching hope to whatever treatment they are subjected to: the very well known “placebo effect”.
According to a research performed by the italian ISTAT (national institute of statistics) quoted in the article, the estimated 8.2 million users of homeopathic drugs in 2000 (a huge number, reached at the peak of a boom in Italy) have gone down to 7 million in five years. A drop of 15% which does not mean collapse but still smells of crisis. Of those seven million users, the large majority say they integrate homeopathic and other “alternative” treatments with more conventional drugs - maybe a sign that, when a bacterial infection is in progress in your kid’s lungs, you are more likely to come to your senses.
More depressing is to know, however, that there is a sick correlation between the instruction level and the use of alternative treatments to standard drugs: twice as many people who completed high-school studies or got a bachelor have used alternative medicine than people who abandoned school earlier. Among the categories that make most use of homeopathy and other non conventional treatments, businessmen and white collars are above 20%, while blue-collar workers are at 12.5%. The picture of the homeopathic drug user is clear: the peak usage is in the category of women between 33 and 44 years of age, with a high level of instruction, and living in northern Italy.
I read the 15% decrease in the use of homeopathic treatment with some caution, anyway. A factor which has to be taken into account with care is the strong decrease in buying power between 2000 and 2005, brought about by the introduction of the new unified european currency in 2002. Homeopathic treatments are usually more expensive in my country than regular drugs, although many of them can be obtained through prescription; they also last longer. So it is unclear by how much the decrease of users is really due as I hope to an increased understanding of the total ineffectiveness of these substances. For sure, in 2004 the italian Superior Health Council has done its part, by inviting pediatricans to be careful when prescribing homeopathic treatment to children.
I am curious to know if any of my readers who consider themselves true scientists one way or another (a nice way to say I accept PhDs and also crackpots with a ToE in their pocket to speak up, or even internationally acclaimed string theorists) have used with confidence homeopathy to cure themselves or their relatives in the past. In fact, while I am with Feynman when he says “I believe that a scientist looking at non scientific problems is just as dumb as the next guy”, I hope the same can’t be said by “a scientist looking at scientific problems just outside his narrow field of research”!
