N. 10 - 1 giu 2011
International info a cura di Cecilia Migali
The High Energy and Particle Physics Prize of the European Physical Society has been awarded for 2011 to three theoretical physicists: Sheldon Lee Glashow (Boston University, Usa), John Iliopoulos (Ecole Normale Supérieure, Paris, France) and Luciano Maiani (University of Rome La Sapienza, Italy and President of National Research Council of Italy-Cnr) "For their crucial contribution to the theory of flavour, presently embedded in the Standard Theory of strong and electroweak interactions".
In 1970, they discovered a compelling argument for the existence of a yet undiscovered particle - the "charm" quark - to solve a number of problems that particle physicists were facing at the time. Their proposal, now called "Gim mechanism" from the initials of the three authors, was spectacularly confirmed four years later, when particles containing the charm quark were finally discovered. The award ceremony will take place at the Eps-Hep 2011 conference in Grenoble on July 25.
In 1970 it was already understood that the microscopic constituents of matter are quarks and leptons (such as the electron and neutrinos), but there was evidence for only three types of quarks. It was also known that whereas the weak interactions can change the type, or "flavour", of a quark, not all flavour-changing" processes were observed. Processes associated with "neutral urrents" were not observed, as if they were forbidden, or at least strongly inhibited, by some unknown mechanism.
The three physicists showed that the introduction of a fourth quark, the "charm" quark, that was undiscovered at the time, would generalize the unified description of the weak interactions previously introduced by Nicola Cabibbo, in a way that would permit flavour - changing neutral currents only via tiny quantum effects: to explain the bserved rarity of such transitions, the charm quark mass should be at most a couple of times the proton mass. The rest is history. Neutral weak currents without flavour-changing and charmed particles were discovered in the mid-70s. The Gim mechanism became a cornerstone of the Standard Theory of Strong and Electroweak Interactions (for the electroweak part of which Sheldon Glashow shared the 1979 Nobel Prize with Abdus Salam and Steven Weinberg), and is at work also in the modern version with three families of quarks and leptons.
At present another feature of the Standard Theory is under intense scrutiny at the Large Hadron Collider at Cern, which will soon tell whether the predicted Higgs boson indeed exists or is replaced by some new physics. It is remarkable that the presence or the absence of a builtin Gim mechanism has been one of the most severe censors of the many models of new physics proposed so far, under suggestive names such as technicolor, supersymmetry or extra dimensions. Professor Paris Sphicas, the current secretary of the Eps-Hepp Board, from the University of Athens and Cern, said "The Gim mechanism was a bold step, which required nothing less than the existence of a new particle, a fourth quark that was unknown at the time. Forty years after its inception, it remains an essential and inspiring topic for any course on particle physics." Doctor Yves Sirois, a member of the Eps-Hepp Board from the Ecole Polytechnique in Palaiseau and Cnrs Research Director, added "The invention of the Gim mechanism was a decisive breakthrough, allowing a model of leptons interacting with photons and weak bosons, to become what is known today as the 'Standard Model' of the fundamental interactions for the elementary constituents of matter, the leptons and quarks".
Original publication: S.L.Glashow, J.Iliopoulos and L.Maiani, 'Weak Interactions with Lepton-Hadron Symmetry', Phys. Rev. D2 (1970) 1285.
Fonte: Fabio Zwirner, Istituto Nazionale di Fisica Nucleare- Padova, email firstname.lastname@example.org
Per saperne di più: - eps-hep2011.eu/