The W boson opens a new crack in the theory that explains nature

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Next July will be the 10th anniversary of one of the most important discoveries in physics, the discovery of the Higgs boson.

The elusive particle whose existence Peter Higgs predicted four decades earlier was the only one left to prove experimentally to complete the so-called Standard Model of Physics, the theory that describes nature at its most fundamental level by explaining the composition of matter.

But scientists have not stopped experimenting at large accelerators to try to discover new particles and challenge the Standard Model.

And they are getting surprises even with particles that were thought to be well known, like the one a team is publishing this Thursday in the journal

The protagonist of this research is the W boson, an elementary particle that had been studied in depth in particle colliders such as the LEP (already dismantled) or the LHC at CERN, the European Laboratory for Particle Physics.

It is one of the heaviest known particles in the universe and has a mass that is about 80 times that of a proton.

Now, by subjecting it to measurements twice as precise as those previously made, the international team that makes up the CDF collaboration (acronym for the Collider Detector at the Fermilab laboratory in Chicago) has seen that it appears to be heavier than established by the Standard Model .

A total of 400 scientists from 23 countries -including Spanish physicists from the CIEMAT and the Physics Institute of Cantabria (IFCA, CSIC-UC)- have participated in the data collection and analysis that have required two decades of work.

If their surprising results are confirmed, the repercussions in the world of physics could be significant and varied.

It could suppose the verification that the Standard Model has to be improved or expanded, as had already been suggested by the results with experiments with other particles, new phenomena different from those indicated by the model or a sample of physics that exists beyond that. Standard model.

"That the mass of the W boson turns out to be really inconsistent with the indirect estimates within the Standard Model

. But the Current measurements, including the recently published one, have systematic uncertainties that are difficult to interpret," Juan Alcaraz, Research Professor at the CIEMAT-Particle Physics Unit and member of the CMS collaboration, told this newspaper, without being linked to this work.

"This is a new measurement using the data from the CDF experiment and the result is so precise, twice the best measurement done before, that it does not agree with what is obtained with the calculation from the Standard Model, which is also a very precise value. ", explains Óscar González, member of the CDF and CMS collaborations and senior scientist at CIEMAT.

The value itself is not much higher than what was obtained in some of the previous measurements, but their precision did not stand out so much their difference with the theoretical calculation", he details through an email.

From his point of view, we are facing "a very important result, since the calculation of the mass of the W is done within that model and with enough precision. That there is a discrepancy with the experimental measurements implies that something in the calculation does not correspond precisely with the reality of the W boson".

Nobel in 1984

The existence of the W boson was theorized in the 1960s and confirmed in 1983 -Carlo Rubbia, former director of CERN, shared the Nobel Prize in Physics the following year with Simon van der Meer for their contributions to discovering the W and Z boson.

The W boson

(it is named for

,

in English)

in the universe (the other three fundamental forces are electromagnetic, the gravitational and the strong nuclear, causing for example nuclear energy).

The weak nuclear force is responsible, for example, for the processes that make the Sun shine or for radioactivity, as the IFCA has explained.

"It can be considered one of the most studied particles in the field. What stands out about the new measurement of its mass is its precision compared to other previous measurements," says Oscar González.

Scientists have used data collected during high-energy particle collisions produced by the Tevatron collider at Fermilab Laboratory in Chicago from 1985 to 2011.

Alberto Ruiz, an IFCA researcher, admits that the results have been "a surprise: "Until now all the predictions and measurements given by the Standard Model were compatible, however we know that the model only explains 5% of the universe, and that it does not It may be an ultimate theory," he says. "We knew that the model could fail and this,

he stated in a press release. press.

"The surprisingly high value for the mass of the W boson provided by the CDF collaboration directly challenges a fundamental element at the heart of the Standard Model, where both theoretical predictions and realized observations were thought to be firmly established and well understood," write the authors. Physicists Claudio Campagnari, from the University of California, and Martijn Mulders, from CERN, in an article also published in

in which they comment on the discovery.

However, these two physicists recall that "additional experiments will be necessary to provide additional confirmation" and paraphrasing Carl Sagan, they maintain that "extraordinary claims require extraordinary evidence".

If other known particles were subjected to the same measurements, could they also turn out differently than previously thought?

"The theoretical calculation to obtain the mass of the particles depends on the values ​​that are had for others, which means that there is a hierarchy between them and also differences in precision that can be obtained in the calculations," explains González.

"In the specific case of the W boson, the advantage is that it can be calculated very precisely with the theory, which is part of the key to being able to see differences with the experimental measurement. In the case of other particles, it cannot be done with such precision, or even, its experimental value is so precise that in fact they are used as 'input' parameters to make the theoretical calculations, as it happens with the Z boson, companion of the W".

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