The trouble with… talking about physics March 9, 2007
Posted by dorigo in Blogroll, internet, personal, physics, politics, science.17 comments
The New Scientist article about Higgs bumps did get me into some trouble, although not of the kind I could have predicted.
While I was away on vacation a few days ago, the article I discussed earlier this week appeared, and it caused concern among some of my collaborators. A few of them thought I had disclosed restricted CDF information to the reporter, maybe to try showing I was a more important person than I am, others found the claims unsupported and damaging to the image of our collaboration.
I did find that the article contained several inaccuracies, and I was not pleased by the misquoting of my thoughts, but I was enjoying my vacation in Mexico too much to feel concerned. However, I failed to realize that the article had hurt the feelings of those who feel that CDF plots and results, while blessed and approved for public consumption, must not be discussed privately by members of the collaboration - the more so if while doing that one embarks in speculative remarks.
Of course, I do not share that view. I believe that doing outreach, explaining physics results to non-physicists, is very important, and that is my motivation to keep running this blog. I do not think I harm CDF if I explain how they measured the top production mechanism, as I did yesterday, for instance.
At any rate, I must respect my colleagues’ views. And to be honest, I was indeed partly responsible for the rumor which ended up causing interest in New Scientist and other magazines. I could have been a bit more careful: if I had been, probably the final result would have been the same for all practical purposes, but my name would not have appeared in the article, and I would have been free from criticism of any kind from my peer.
So, I decided today I would come clean with my CDF collaborators, apologizing for what I thought was my share of guilt in the appearance of the New Scientist article. By writing an apology of sorts in the CDF internal newsgroup, I wanted to show I can accept criticism, and I feel no shame in admitting a mistake, even if I think it was a quite innocent one. All for one interest: the well-being of the CDF collaboration, one of the longest experiments ever - 25 years and counting! Indeed, Long Live CDF.
Before I go to the text I wrote in the CDF forum today, let me tell you what is the real trouble I got in, which was unexpected, as I wrote above. (more…)
Set Mastrogiacomo free March 9, 2007
Posted by dorigo in Blogroll, internet, italian blogs, news, politics.2 comments
And now the Economist… Oh well. March 9, 2007
Posted by dorigo in Blogroll, internet, news, personal, physics, politics, science.4 comments
Fermilab Today links an article by the Economist today. Here is an excerpt:
In recent weeks a number of physicists have got excited about bumps in the data taken at two experiments at the Tevatron. A bump is usually a sign that something has gone awry, but if all possible sources of error have been eliminated, it can indicate the presence of a particle. That both experiments on the Tevatron ring have their new bumps at precisely the same energy is particularly intriguing.
Oh well. Bubbles have the natural tendency to inflate - before they blow. Do I feel responsible for this one ? Yes and no. I was part of the bloggers who discussed these fluctuations in the last two months, and I did make some remarks that could have been equivocated here and there; but I think I was clear in stating my belief (personal, of course) that it is all a fluctuation. And whether I was speaking for myself, well, ain’t that obvious ? Come on, this is a blog!
A disclaimer March 9, 2007
Posted by dorigo in Blogroll, internet, personal.1 comment so far
I do disclaim thee, o liability!
No, not about liability - not just that. I am spending two lines here today to tell the occasional reader of this blog that all the contents of it reflect my views, and mine only. I have affiliations to the CDF and CMS experiments, to amateur astronomers clubs, to mountaineering clubs, what-not. But whatever you find in here, is only my opinion. When I make a statement here, I am speaking for nobody but myself. Not as a member, not on behalf of.
Now did I make myself clear ?
The need for this disclaimer comes from a recent querelle, of which I will talk rather impersonally later.
Probing the top pair production mechanism March 9, 2007
Posted by dorigo in news, physics, science.4 comments
CDF just released a nice analysis which, although still seriously limited by its statistical power, starts to test in a new way our understanding of the mechanism at work when a top-antitop pair is produced by the 2-TeV collisions produced by the Tevatron’s proton and antiproton beams.
QCD, the theory of the strong force keeping together hadrons (such as protons and neutrons, but also more exotic, short-lived particles) is the basis of our description of the interactions between quarks and gluons - the constituents of hadrons.
When we collide a proton with an antiproton at high enough energy, what may happen is that a quark from a proton actually withstands a close encounter with an antiquark from the antiproton. Their annihilation creates energy, which may be released by the creation of a new quark-antiquark pair - and when we are lucky (once in ten billion collisions), a rare top-antitop pair!
Another mechanism that can create a heavy quark-antiquark pair is the collision of two gluons. These are also among the projectiles’ constituents, but they usually carry too little energy to be able to create such a powerful collision as those that may end up in the creation of top quarks.
Taking in account the distribution of probabilities to find quarks and gluons of a given energy in the colliding protons and antiprotons, QCD allows us to compute with good accuracy what fraction of the top-antitop pairs were actually created by gluon collisions: about 15%.
A brief parenthesis: at the Large Hadron Collider (LHC) in construction at the CERN laboratories in Geneva, the top quark pairs will be produced with opposite frequency by gluon (85%) and quark (15%) collisions: that is because the higher energy of the projectiles favors the gluons, and the fact that both beams of the LHC are made of protons makes it harder to find a high-energy antiquark (yes, there are antiquarks in the proton! But they sit silent and get little share of dad’s energy).
What Ricardo Eusebi, Eva Halkiadakis, Sunil Somalwar, and Jared Yamaoka measured was precisely the fraction of gluon collisions yielding top pairs. They did so by considering several measurements of the kinematics of top quarks in the final state, many of them boiling down to the same detail: the angle made by the emitted quarks from the colliding beams. Indeed, a quark-quark annihilation usually yields more central top quarks, so by measuring an angle called “theta-star” one can separate the two production mechanisms. Another detail is the spin correlation of the quarks, but I won’t discuss it here.
In the picture on the left you see what kind of separation we are talking about: indeed, there is only a little difference in the cosine of theta-star to play with, and if the statistics of the top quarks data sample used is not large enough, the measurement fails to produce a significant result. This is not the case, though: with 695 inverse picobarns of collisions, CDF puts a limit: the gluon-gluon fraction is smaller than 51%, at 95% confidence-level.
What that means is that CDF excludes that gluon-gluon collisions are the largest source of top pairs. This is of interest not only to test QCD, but also to test for potential new physics mechanisms of production of top quark pairs. If, for instance, the top pairs were produced by a high-mass resonance, the experimental result would be bound to disagree with Standard Model predictions (well, QCD).
I think this is cool, and I look forward to new precise measurements of this quantity!
As for the past, I need to mention that there has already been another, largely independent measurement of the gluon-gluon fraction in top production by CDF. That analysis looked at the number of charged tracks produced by the collisions yielding top quarks, in the knowledge that gluon collisions produce more tracks, because gluons have a higher color charge! The group, led by Pekka Sinervo from Toronto University, was able to measure a gg fraction of 0.24+-0.26, in good agreement with both theory and with the newer determination. Testing the same quantity with different methods is a very useful thing to do, because when discrepancies are found, one learns a lot… Or things agree, which is also neat.
The insolence of Iberia March 9, 2007
Posted by dorigo in personal, travel.2 comments
Well, not exactly insolence, but a Shakespearish citation….
After a 27-hour trip through Cancun, Mexico City, and Madrid, we finally touched bases tonight at home in Venice. Rather exhausting, but smooth, except for an incident in Mexico City.
We were departing from Cancun with a Mexicana flight, and then had a tight connection in Mexico city with Iberia to Madrid. In Cancun the check-in personnel had not given us the boarding cards for the Iberia tract, saying they had no computer connection with their system - weird, since they are partners as far as I know. The lady at the counter had told us not to worry though, and instructed us to go directly to the departure gate in Mexico city.
Alas, we do that: once in Mexico city, we go straight at the gate instead than exiting and checking in again. But we find nobody there for a while, and I grow worried. Iberia does not know about us, and the flight will be leaving in little more than one hour! So I go to the information desk, where they tell me I would need to exit the terminal, get back in and check in with Iberia, then go through security - all this with two small kids, lots of carry-ons, and less than one hour to the flight. No way.
Then they suggest that I go alone: even worse! Imagine if I am not granted the boarding passes: I will never go back through security to my family any more…
I get them to call Iberia check in and they find nobody on the phone. The reservation office also does not answer… And time passes.
So I go back to the gate, and I find the Iberia staff finally. He says I really have to go to the check in, he cannot issue me boarding passes! I first insist that he sends the information through, and then I gradually grow angry, the more so since he refuses to call the check-in counter to signal our situation. Trying to keep calm, I argue that if we do what he says we are going to miss the flight. He agrees, and says we have to wait for a supervisor - he cannot issue the passes, nor get them done and sent to us there.
Minutes tick… And eventually, the boarding passes arrive: after I thank the Iberia staff with a hopeless smile, we are among the last to board the plane, with seats in rows 20, 27, 33 and 35, and 7-years-old and 4-years old kids.
Well, whose is the fault ? For sure Mexicana did a dirty trick to us, not giving us the passes in the first place, and sending us to the wrong place in Mexico city. But Iberia also did not behave with us. I had to talk on the phone with the Capo de Escalo to get things rolling!
Hmmm, Iberia is not high in my list of preferred airline companies any longer… Too bad. I will stick with Lufthansa next time.
