An unexpected discrepancy in the mass of the W boson

In particle physics, data lasts much longer than the detectors that generate it. A decade ago, Fermilab’s Collision Detector (CDF), a 4,100-ton instrument, reached the end of its useful life and was disassembled for use in other experiments. Now, a new analysis of old CDF data has revealed a surprising discrepancy in the mass of an elementary particle, the boson. Wwhich could be an indication of still unknown particles and interactions.

the boson W it is very massive, about 80 times more than a proton. This particle is responsible for certain forms of radioactive decay, and allows neutrons to convert to protons. Its mass is limited by many other particles and parameters of the standard model (the physical theory that describes fundamental particles and their behavior), and in turn imposes restrictions on them. That is why the boson W it has become a target for researchers seeking to understand where their best theories fail and why.

Although physicists have long known the approximate mass of the boson WThey have not yet been able to define it completely. However, if we plug the available data into the Standard Model framework, theory predicts that mass should be 80,357 megaelectronvolts (MeV) plus or minus 6 MeV. (Because of the equivalence between mass and energy, the mass of particles is usually expressed in electron volts. One MeV is about twice the mass of the electron.) But in a recent analysis published last April in Sciencephysicists from the CDF collaboration have found that the mass of the boson W is 80,433.5 ± 9.4 MeV. The new measurement, which is more precise than all the previous ones combined, exceeds the prediction of the standard model by almost 77 MeV.

Although the two results differ by only one part in 1,000, the uncertainties in each are so small that this small divergence is of enormous statistical significance: it is highly unlikely to be an illusion produced by chance. It seems that the well-studied boson W it still holds many secrets about how the subatomic world works, or at least about how we investigate it. Particle physicists, who have been taken by surprise by the result, are still beginning to understand its implications.

“No one expected this discrepancy,” says Martijn Mulders, an experimental physicist at CERN who was not involved in the new research but co-authored an accompanying commentary in Science. “It’s very unexpected. You almost feel betrayed because all of a sudden they’re sawing off one of the legs that holds up the whole structure of particle physics.”

Quark Search

A rough measurement of the boson’s mass W was the one that allowed physicists to predict the mass of the quark top (top) with reasonable precision back in 1990, five years before that particle was first observed. Then, using the boson mass W and that of the top quark, the researchers made a similar prediction for the Higgs boson, which would be dramatically confirmed in 2012.

More recently, physicists making such measurements have focused not so much on refining the basics of the Standard Model as on exploring its shortcomings: it doesn’t explain, for example, gravity, dark matter, neutrino masses, and the like. series of problematic phenomena. Physicists believe that investigating the points where the Standard Model fails or deviates from observations is one of the best ways to search for “new physics,” the term they use to refer to additional and possibly more fundamental components of the universe.

Until the CDF result, some of the most promising discrepancies from the standard model were an anomaly studied in Fermilab’s Muon g-2 experiment and the results of the LHCb experiment (where “b” refers to the quark beauty) of the Large Hadron Collider (LHC) at CERN.

Small anomalies are very common, and the vast majority are nothing more than statistical fluctuations arising from the enormous number of subatomic events produced and recorded by typical particle physics experiments. In those cases, the anomaly disappears as even larger volumes of data are obtained. But the latter looks more promising, because there was already a lot of highly accurate data on the boson’s mass. W and the theoretical prediction presents a very low uncertainty.

And perhaps most importantly, the collaboration of the CDF has been extremely careful. The experiment was done with ‘blinding’ to minimize the risk of human bias, meaning that the physicists analyzing the data were blinded to the results until their work was done. When the team members knew the resulting value of the boson mass WIn November 2020, “it was a moment of stunned silence,” recalls Ashutosh Kotwal, one of the study’s authors. “Realizing what that newly revealed number meant, of course, is priceless.”

Since then, the results have passed several rounds of peer review, but that only guarantees that the physicists have done their job well, not that they have found new physics.

data collection

To measure the mass of the boson W, first you have to build a particle collider. The Tevatron, which operated from 1983 to 2011, was a 6.3-kilometer ring in which protons and antiprotons collided with energies of up to 2 teraelectron volts (TeV), about 25 times the mass of a boson. W. The CDF experiment, located along the ring, looked for signs of bosons W in these collisions from 2002 until the Tevatron stopped working.

But you can’t directly observe a boson W: Decays into other particles too fast to register on any detector. Therefore, physicists must infer their presence and properties by studying these decay products, especially electrons and muons. The CDF team found in the experiment data about four million events attributable to the decay of the boson W. By measuring the energy deposited in the CDF detector by electrons and muons from those events, physicists worked “backward” to find out the energy (or mass) of the boson. W original.

That work took a decade because of the many uncertainties in the data, Kotwal explains. The CDF team managed to determine the mass of the boson W with unprecedented precision—twice as high as the best measurement ever made by the ATLAS collaboration—by quadrupling its dataset and employing new techniques. For example, they modeled collisions between protons and antiprotons, and carried out a deeper re-examination of the operational details of the decommissioned detector, even using old cosmic ray data to characterize it down to the micron.

All of this was enough to give the researchers’ anomalous result remarkable statistical significance—nearly seven sigmas, in mathematical jargon. That means that if there were no new physics affecting the boson W, we would have to perform the experiment 800 billion times for a discrepancy equal to or greater than the observed one to arise by chance. That seems far-fetched even in the world of particle physics, where astronomical figures are the norm: the field’s “gold standard” for statistical significance is five sigma, which corresponds to an effect that appears by chance. in one of every 3.5 million experiments.

The most important thing is that this value of seven sigma No implies that the CDF team’s result has a 99.999999999 percent chance of being due to new physics, or that other measurements of mass W are wrong. What it means is, rather, that the result obtained by the collaboration of the CDF is not the result of chance. It is a call for further investigation, not a conclusion.

detective work

To determine the origin of the anomaly, it needs to be confirmed by other experiments. “It’s a spectacular result,” says Guillaume Unal, ATLAS physics coordinator who is not involved in the new study. “It is a very complex measurement, which is very challenging, and it is also very important to verify the standard model very accurately.” The ATLAS collaboration is working to improve its measurement of the boson’s mass Wand Unal assures that the data from the second round of LHC experiments, which concluded in 2018, will allow them to approach the precision of the CDF.

Meanwhile, theoretical physicists will rush to study the new result to come up with a myriad of possible explanations. Although the LHC has ruled out many versions of supersymmetry (a set of theories that posit that all elementary particles have a “supersymmetric partner” associated with them), the slight shift in the boson’s mass W it could be due to a set of relatively light supersymmetric particles.

“Of course, [las restricciones del LHC] they are becoming more stringent,” stresses Manimala Chakraborti, a theoretical physicist at the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, who is not part of the CDF collaboration. “But you can still find regions of the parameter space where supersymmetry is feasible.”

At a time when new colliders are being proposed and the LHC is preparing to start a new round of experiments after a comprehensive review, it may seem strange that an anomaly at the seven sigma level is announced from an experiment already disappeared and whose detectors have been recycled.

But the collaboration continues to meet to evaluate and refine the fruits of the experiment. “That detective work is what drives us,” Kotwal concludes. “All the clues are there… It’s like [en los relatos de] Sherlock Holmes. The person may be gone, but the traces remain.”

Daniel Garisto

Reference: “High-precision measurement of the W boson mass with the CDF II detector”, CDF Collaboration in Science, vol. 376, pp. 170-176, April 7, 2022.

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