Field of Science

Refining the mass of W

In our standard model of elementary particles we have four fundamental interactions: gravity, electromagnetism, strong nuclear force and weak interaction. In particular the last force is responsible for the radioactive decay and for the hydrogen fusion in stars. The bosons of the interaction (the particle exchanged between two fermions) are $W^\pm$ and $Z$ bosons. An example of weak interaction is $\pi^+$ decay:
The weak bosons are predicted in 1968 by Glashow, Weinberg and Salam(1) and discovered at CERN in 1983 in a series of experiments conducted by Carlo Rubbia and Simon van der Meer(2). Now, from one of the last analysis from Tevatron, we have the last measure of W bosons. Indeed CDF's researchers propose the following preliminary value for $W$: \[M_W = (80.387 \pm 0.019) GeV\] and combining it with previous measures, the new preliminary world average is
I must remember that $(80.390 \pm 0.016) GeV$ will became the new $W$ mass only after the publication of the CDF's preprint (pdf) in a peer review journal and after the publication of the further calculation on the Particle Data Group. Indeed Wired (and en.wiki, following Wired), following Tommaso Dorigo, who simply described the experimental process that carries to the measure and to the new proposal, has just setted the new mass, forgetting the reviewing scientific process. So, until then, the average mass is $(80.399 \pm 0.023) GeV$(3).
(1) The three theorists won Nobel Prize in 1979
for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current
I would emphasazie three key concepts in the theory: symmetry, gauge theory and renormalization.
Symmetry, I hope, is clear: a transformation that not change, for example, a physical system. But in physics we use a lot of gauge theories: in this case our systems are invariant (not change) under local (and not global) symmetry transformations (and the set of these transformations form a continuous group). So it's very important to clarify the difference between global and local: for global transformation, we mean a transformation that acts on the whole space; for local transformation, we mean a transformation that acts, unchanging the system, in the neighbourhoods of the point that I study (in few words, a local transformation acts in a little parte of the space).
And finally the renormalization is a trick used to forgot the divergence in theory (I hope to write for a future post a more detailed explenation of renormalization)!
In every case, you can read the theoretical background in the three Nobel lessons, in particular Glashow's one, that is simple but not trivial:
Glashow, S. (1980). Towards a unified theory: Threads in a tapestry Reviews of Modern Physics, 52 (3), 539-543 DOI: 10.1103/RevModPhys.52.539 (pdf)
Salam, A. (1980). Gauge unification of fundamental forces Reviews of Modern Physics, 52 (3), 525-538 DOI: 10.1103/RevModPhys.52.525 (pdf)
Weinberg, S. (1980). Conceptual foundations of the unified theory of weak and electromagnetic interactions Reviews of Modern Physics, 52 (3), 515-523 DOI: 10.1103/RevModPhys.52.515 (pdf)
I would notice that in 1973, at CERN, there were the first observation of the neutral current, and this is one of the most intriguing problems solved in the model.
(2) The two experimenatlists won Nobel Prize only one year later. The discovery was resumed in the Nobel lessons:
Rubbia, C. (1985). Experimental observation of the intermediate vector bosons W+, W-, and Z0 Reviews of Modern Physics, 57 (3), 699-722 DOI: 10.1103/RevModPhys.57.699 (pdf)
van der Meer, S. (1985). Stochastic cooling and the accumulation of antiprotons Reviews of Modern Physics, 57 (3), 689-697 DOI: 10.1103/RevModPhys.57.689 (pdf)
(3) Nakamura, K., & , . (2010). Review of Particle Physics Journal of Physics G: Nuclear and Particle Physics, 37 (7A) DOI: 10.1088/0954-3899/37/7A/075021 (W data)
Update: I forgot to write somthing about the Particle Data Group. It is an international collaborations of particle physicists that compile the results about particles and fundamental forces after the pubblication of the experimental data on peer review journals. We consider the official publications of the PDG (Review of Particle Physics and Particle Data Booklet) like the Bible of particles and the official source for constants and mass that we must use in papers. The data are updated annually on web and biennially on the Review of Particle Physics.

No comments:

Post a Comment

Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="http://www.fieldofscience.com/">FoS</a> = FoS