The Standard Model of particle physics takes a hit
The story so far: On April 7, researchers from the Collider Detector at Fermilab (CDF) Collaboration, in the United States, announced, through an article in Science, that they made an accurate measurement of the mass of the so-called W boson. They said that this precisely determined value did not match what was expected from estimates using the Standard Model of particle physics. This result is highly significant because it implies the incompleteness of the description of the standard model. This is a major claim, since the standard model has enjoyed extraordinary success over the past few decades. Therefore, physicists are looking for confirmation from other independent future experiments.
What is the standard model of elementary particle physics?
The standard model of elementary particles is a theoretical construct in physics that describes the particles of matter and their interaction. It is a description that views the elementary particles of the world as being connected by mathematical symmetries, just as an object and its mirror image are connected by bilateral (left-right) symmetry. They are mathematical groups generated by continuous transformations, say, from one particle to another. According to this model, there is a finite number of fundamental particles which are represented by the “eigen” states characteristic of these groups. The particles predicted by the model, like the Z boson, have been seen in experiments and the latest discovery, in 2012, is the Higgs boson which gives mass to heavy particles.
Why is the standard model considered incomplete?
The Standard Model is considered incomplete because it gives a unified picture of only three of the four fundamental forces of nature – electromagnetic, weak nuclear, strong nuclear, and gravitational interactions – it omits gravity altogether. So, in the grand scheme of unifying all forces so that a single equation would describe all interactions of matter, the Standard Model turned out to be lacking.
The other shortcoming of the Standard Model is that it does not include a description of dark matter particles. So far, these have only been detected by their gravitational pull on surrounding matter.
How are symmetries related to particles?
Standard Model symmetries are called gauge symmetries because they are generated by “gauge transformations” which are a set of continuous transformations (as rotation is a continuous transformation). Each symmetry is associated with a gauge boson.
For example, the gauge boson associated with electromagnetic interactions is the photon. The gauge bosons associated with weak interactions are the W and Z bosons. There are two bosons W — W + and W –.
Inspired by the success of quantum electrodynamics, in the 1960s Sheldon Glashow, Abdus Salam and Steven Weinberg developed the similar but more general, “electroweak” theory in which they predicted these three particles and how they mediated the weak interactions. They received the Nobel Prize for their efforts in 1979. The W boson was first seen in 1983 at CERN, located on the Franco-Swiss border. Unlike the photon, which is massless, W bosons are quite massive, so the force they carry – the weak force – is very short-ranged.
Unlike the photon, which is electrically neutral, W-plus and W-minus are both massive and charged. By exchanging such W bosons, a neutron can for example turn into a proton. This is what happens in beta decay, a radioactive interaction that occurs in the sun. Thus, the W boson facilitates the interactions that cause the sun to burn and produce energy.
What is the main result of recent experience? What is the difference they got?
The recent experiment at the CDF, which measured the mass of the W boson at 80,433.5 +/- 9.4 Mev/c 2, which is about 80 times the mass of a hydrogen nucleus, showed this to be more than expected from the Standard Model. The expected value using the standard model is 80,357 +/- 8 MeV/c 2 . This is estimated from a combination of analytical calculations and high precision experimental observations of a few parameters that go into the calculation like the mass of the W boson, the strength of the electromagnetic interaction, the Fermi constant, the mass of the Higgs boson and the mass of the top quark. Thus, the mass of the W boson itself is a prediction of the Standard Model. Therefore, any deviation in its mass signifies a lack of self-consistency in the Standard Model.
However, this is not the last word, because the mass deviation of the W boson must be checked and confirmed with the same precision by other facilities, for example the Large Hadron Collider (LHC).
Where are we now in terms of new physics?
New physics is in the air and experiments have been brewing for some years to detect new particles. The Large Hadron Collider itself has been revamped for “Run3” which will perform special experiments to research physics beyond the Standard Model. A Perspective article by Claudio Campagnari and Martijn Mulders in Science highlights several high-precision experiments that are in the pipeline such as the International Linear Collider in Japan, the Compact Linear Collider and Future Circular Collider at CERN, the Electron-Positron Circular Collider in China, etc. With its high-precision determination of the mass of the W boson, the CDF has struck at the heart of the Standard Model, so this is a significant discovery and if confirmed by the LHC and other experiments, it will open the door to ideas. and experimentation.
On April 7, researchers from the Collider Detector at Fermilab (CDF) Collaboration, in the United States, announced that they had made an accurate measurement of the mass of the W boson. They said that this precisely determined value did not correspond to estimates of the standard model of particle physics.
The recent experiment which measured the mass of the W boson at 80,433.5 +/- 9.4 Mev/c2 is greater than expected from the Standard Model. The expected value using the standard model is 80,357 +/- 8 MeV/c2. This implies the incompleteness of the description of the standard model.
This mass difference of the W boson must be verified and confirmed with the same precision by other research facilities.