A Quantum Diaries Survivor

The CDF and DZERO collaborations at the Fermilab Tevatron collider just saw published on Physical Review D (PRD 86, 092003) their final combination of their most recent and precise measurements of the mass of the top quark. This result, which is probably going to be the definitive one by the Tevatron experiments, reaches a precision in the top mass which is comparable to the precision of the scale you have in your bathroom, despite the fact that measuring your body weight is a quite simpler matter than determining the mass of an elementary particle, let alone one which exists for less than a trillionth of a trillionth of a second.A digression

A journalist I am following on twitter just posted the question in the title of this post. I felt bound to try and give an answer with as simple concepts as I found meaningful. So, what makes a unstable particle unstable ?One answer is this: a particle is unstable if there is a way, not forbidden by any physical law, to convert its rest-mass into other forms of energy. One may understand this by thinking of entropy: any system left free to evolve will do so in the direction of maximum entropy. So since a single particle state is a very low-entropy system, while the decay products of its disintegration have a multitude of possible configurations and a higher entropy, the system will naturally evolve in that direction.

Researchers and academics below the full-professor level in Italy are currently busy with an idle exercise - putting together their applications to a selection for would-be assistant and full professors. By the way, this is happening to me as well, so you may understand why this blog has received little attention from me this past week: the occupation is extremely time-consuming.Becoming university professors in Italy in the last few decades has been a rather complicated business, where the merit of candidates was often overshadowed by favouritisms and private interests.

While New Yorkers (and others on the east US coast) prepare to deal with hurricane Sandy and the possible water surge resulting from the very strong winds and low pressure of the system, a similar situation is expected in Venice on Wednesday; unlike New Yorkers, Venetians are rather accustomed to the phenomenon - familiarly called "acqua alta" (high water) by residents. However, when the level of water is exceptionally high, normal protection systems to shops, offices, and ground floors of homes prove insufficient. That is probably going to be the case in the evening of October 31st, when the tide is expected to reach to the level of +1.40 meters above average sea level, due to a combination of factors -low pressure, full moon, winds.

As I reported in a post a few days ago, the Italian sentencing of seven scientists to 6-years imprisonment for their misassessment of the risks of the population of L'Aquila, soon thereafter struck by a powerful earthquake which killed 309 and injured 2000, raised interest and disconcertment worldwide and spurred a debate which is not likely to end soon.Who is guilty ?

Italy is a beautiful, crazy country. Take today's verdict, which condemns seven scientists (Franco Barberi, Enzo Boschi, Mauro Dolce, Bernardo De Bernardinis, Giulio Selvaggi, Claudio Eva, and Gianmichele Calvi) to six years of prison, plus a huge fine, for allegedly reassuring the population about the unlikelihood of an earthquake on the eve of the devastating shock of April 6th, 2009, which caused 309 deaths and destroyed most of the mid-size town of L'Aquila, in central Italy.

I had a dream. So what, we all do. Well, this was particular, because I remember all of it well, and because it involved a very interesting situation. I was at Fermilab, in an office on a top-level floor of a tall building, when a powerful earthquake hit.

See, this is what I really like of a blog: when readers contribute significantly!

Yesterday I spent a very interesting day at Comunicare Fisica 2012, a conference held in TORINO which brought together researchers, high-school teachers, journalists and other professionals working in the field of the popularization of science. The session in the afternoon dealt with the use of the web 2.0, and of course among the topics discussed in the talks was the use of blogs.

Two papers describing results of searches for high-mass resonances decaying into jet pairs have appeared on the arxiv this week. They are authored by the CMS and ATLAS collaborations, and they both report lower limits on the mass of hypothetical particles predicted by several new physics signatures. Both collaborations base their results on the analysis of their full 2011 datasets.

Five years ago I was fascinated by an analogy used by my friend Michelangelo Mangano to explain the problem of naturalness, a crucial issue in fundamental physics, and maybe the biggest single indicium we have that new physics beyond the standard model of particle physics should exist, and be not too far away from our current experimental reach.

In particle physics searches (and elsewhere) the word "significance" is associated with the quantitative measure of how discrepant is one observation with a so-called "null hypothesis". That is, one searches for a new effect in some dataset, and defines what one expects to see in the absence of anything discrepant from theoretical predictions: that is the null hypothesis. A new particle in the data will usually manifest itself as an excess of events, and this will cause the data to deviate from expectations.