Challenging the Standard Model? The latest W boson mass measurements are 7 standard deviations higher than the theory

In the early morning of April 8, Beijing time, the cover article of Science reported the latest measurements of the mass of the W boson released by the Tevatron Proton-Antiproton Collider CDF Experimental Group released by the Fermi National Accelerator Laboratory in the United States. The article said that with unprecedented experimental accuracy, the W boson mass is 80433.5 ±9.4 MeV/c^2, which is 7 standard deviations higher than the theoretical predicted value of 80357 ±6 MeV/c^2. So, does this result challenge the existing Standard Model of particle physics, heralding the emergence of new physics?

Compile the | Liu Hang

Review | Ren Yu and Fei Jin

At 2 o'clock Beijing time on April 8, 2022, the International Cooperation Group of the Us CDF (Collider Detector at Fermilab) released the most accurate result of the W boson mass measurement to date, which is 7 standard deviations higher than the theoretical prediction of the Standard Model of Particle Physics. The results of the experiment were published as a cover article in the April 7 issue of the journal Science.

The Standard Model of Particle Physics is the most successful elementary particle theory to date, describing all known elementary particles that make up matter and the three fundamental interactions between them: strong, weak, and electromagnetic forces. The strong force binds the quark and the nucleon together to form the nucleus, and the electromagnetic force combines the nucleus and the electron to form the atom and the molecule, so what is the role of the weak force? In fact, weak forces are crucial, especially for massive objects like the Sun. The weakly forced carriers are the W and Z bosons, and unlike the electrically neutral Z boson, the W boson carries an electric charge. This means that protons can be transformed into neutrons by emitting positively charged W+ bosons (W+ is the antiparticle of W); four hydrogen nuclei (i.e., protons) can be squeezed together to form helium nuclei. In this hydrogen fusion, a large amount of energy can be released, which will sustain the combustion of the sun's interior.

The W boson was discovered in the 1980s in the superproton-antiproton synchrotron at CERN (CERN – French: Conseil Européenn pour la Recherche Nucléaire). Its mass has been measured on the Large Electron Positron Collider (LEP, LEPII, also at CERN), the Fermilab's Tevatron Proton-Antiproton Collider, and the Large Hadron Collider (LHC) detector ATLAS. Like other elementary particles in the Standard Model of particle physics, the mass of the W boson derives from the Brout-Englert-Higgs mechanism, which is a key parameter of the Standard Model of particle physics. It was a rough measurement of the mass of the W boson that led physicists to predict the mass of the top quark with reasonable accuracy in 1990. Then, using the W boson mass and the top quark mass, the researchers made similar predictions about the quality of the Higgs boson, which was experimentally confirmed at CERN in 2012.

Figure 1. Fermilab's Collider Detector (CDF), which was reset and installed on the Tevatron particle accelerator. The CDF conducted experiments on precise measurements of the mass of the W boson, which is a key parameter of the Standard Model of Particle Physics. 丨 Image source: Alamy Stock Photo

In particle physics, data tends to last longer than the detectors that generated it. Ten years ago, Fermilab's 4,100-ton CDF detector reached its expiration date and was shut down, and its parts were disassembled for other experiments. Now, a new analysis of old CDF data has uncovered surprising differences in W boson quality. Due to the difficulty of detecting the decay mode of the W boson in experiments, its mass measurement accuracy has been stuck in the order of dozens of MeV/c^2, and the accuracy of the best single experiment is also about twenty MeV/c^2. This contrasts greatly with the mass measurement accuracy (2 MeV/c^2) of the Z boson, the sister particle of the W boson. Experimental particle physicists have made long-term efforts to improve the accuracy of measurements.

This time, the CDF Experimental Cooperation Group used all the data collected during its run II to make the most accurate measurement and assessment of the quality of the W boson to date, and the error (both statistical and systematic errors) was reduced to single digits for the first time - 9 MeV/c^2 (Figure 2). The accuracy of this result reached 0.01%, which exceeded the accuracy of any previous related experiment and the weighted comprehensive accuracy of all previous related experimental results, thus setting a new milestone in testing the Standard Model with precise measurements of the mass of the W boson. With such measurement accuracy, the W boson mass obtained by the CDF experimental cooperation group is 7 standard deviations (including experimental and theoretical errors) higher than the theoretical predictions of the Standard Model of Particle Physics.

Physicists have long known that the approximate mass of the W boson is about 80 times the mass of the proton — 80,000 MeV/c^2. The Standard Model of Particle Physics expects the W boson mass to be 80357+/-6 MeV/c^2. The latest results from the CDF collaboration group show that its mass measurement is 80433+/-9 MeV/c2, and the new measurement is more accurate than all previous measurements, nearly 77 MeV/c^2 higher than the standard model's predictions. Although the difference in these numbers is only about one-thousandth, the uncertainty of each number is so small that even this small difference has enormous statistical significance — and it is unlikely that this was created purely by chance.

Figure 2. The measurement results and accuracy of the mass of the W boson are shown in different experimental groups. The gray vertical bars represent the theoretical values of the Standard Model of Particle Physics and their margins of error (vertical bar width), while the red balls and red lines represent the measured averages and margins of error for each experimental group. It is clear that the margin of error of the previous relevant measurements is larger than that of the CDF II. 丨 Image source: Reference 1

But the excitement of the particle physics community is neutralized by a cautious attitude. Although the results of FermiLab CDF II are by far the most accurate measurement of the mass of the W boson, it is inconsistent with the measurements of two other independent, near-conforming Standard Model of Particle Physics experiments (see and compare the results of the three experiments in Figure 2: D0II, ATLAS, CDF II).

"It's not the difference we're expecting," said Martijn Mulders, an experimental physicist at CERN who was not involved in the new study at Fermilab but co-authored a related review in the journal Science. "It was very unexpected... Because suddenly they sawed off one of the pillars that underpinned the entire framework of particle physics. ”

"It's going to annoy some people," says Professor Ben Allanach, a theoretical physicist at the University of Cambridge.

"We need to know exactly what's going on. I wonder if we have two other experiments that are consistent with the Standard Model and are very inconsistent with the results of this experiment. ”

Mining data

To measure the mass of the W boson, there must be a particle collider. Running from 1983 to 2011, the Tevatron is a 6.3-kilometer-long ring device in which protons strike antiprotons with up to 2 TeV of energy, which is about 25 times the mass of the W boson. The CDF detector on the loop began operating from 2002 until Tevatron shut down, looking for signs of the W boson in these collisions.

The W boson cannot be observed directly, and it decays into other particles too fast for any detector to capture it directly. Physicists infer its existence and properties by studying its decay products, mainly electrons and μ. After careful calculations, the CDF collaborating team found that about 4 million events in the experimental data could be attributed to the decay of the W boson. By measuring the energies of electrons and μ in these events, physicists inversely deduce how much energy the W boson originally had, i.e. its mass.

Ashutosh Kotwal, the study's corresponding author and a spokesman for the CDF, said the work took a decade to complete because of the many uncertainties in the data. To achieve its unprecedented level of accuracy — twice as accurate as the best single-experiment measurement of W boson mass previously conducted by ATLAS — the CDF team increased their dataset by a factor of four. These include modeling proton and antiproton collisions and conducting new, more thorough examinations of the operation of decommissioned detectors — even using previous cosmic ray data to display to the order of microns to rule out the impact on experimental data.

Experimental credibility

Subtle anomalies abound, but the vast majority are just statistical fluctuations caused by a large number of events generated and recorded by experiments. In this case, these random anomalies disappear as more data is collected. However, this anomaly in the latest W boson mass value does not appear to be a statistical fluctuation, because there is a large amount of high-quality data on its measurements, and the uncertainty of theoretical predictions about this mass is very low. The CDF Experimental Collaboration Group is very cautious. To minimize human bias, the experiment was "double-blind," meaning that physicists analyzing the data knew nothing about the results until their work was complete. Ashutosh Kotwal said that when the CDF revealed to team members the measurements of the mass of the W boson in November 2020, "it was an exciting moment." "We understand the significance of this number." The results were then reviewed several rounds. However, popular science writer Daniel Garisto believes that this only means that physicists have obtained precise results from experimental measurements, not necessarily that they have discovered new physics.

New physics

Recently, physicists have shifted their focus on refining the details of the Standard Model of particle physics and more on its possible failures—for example, by not incorporating gravity, dark matter, neutrino masses, or other confusing phenomena (including dark matter, dark energy, and so on). Physicists believe that finding where the Standard Model of particle physics does not match or deviate from experimental observations is an effective way to find "new physics." Before the CDF yielded its results, the most promising candidates for a breakthrough in the Standard Model included deviations found in the results of the Fermilab μ-g-2 experiment and the results of the CERN LHCb (Large Hadron Collider) experiment.

There is no doubt that this anomalous result obtained by the CDF is worthy of attention, and it elevates the mass measurement of the W boson to the height of statistical significance: in statistical terms, close to 7 standard deviations. The 7 standard deviations here mean that if no new physics affects the mass of the W boson, then an error as large as the observed error will still occur in the 800 billion times the experiment is run, and the probability is very small. Even in the field of experimental particle physics, which is accustomed to astronomical numbers, this seems a bit too abrupt: the statistically significant "gold standard" threshold in this field is only 5 standard deviations, which equates to an occasional given effect every 3.5 million runs. In fact, the precise measurement results of the 7 standard deviations with different theoretical values mean that the results obtained by the CDF cooperation team are not accidental, and need to be independently verified and verified and further studied.

To determine the source of abnormal W mass values, confirmation from other experiments is also needed. "This is a very striking result," said Guillaume Unal, physics coordinator at ATLAS who was not involved in the new study, "and it's a very complex and challenging measurement method that really tests the Standard Model of Particle Physics with good accuracy." ATLAS is currently working to improve the measurement of W mass, and Yuner said that using data from the LHC's second run (which ended in 2018) could bring them closer to the W quality measurement accuracy of the CDF.

At the same time, theorists will seize on this new result and come up with a myriad of possible explanations. Although the Large Hadron Collider has ruled out many supersymmetric (SUSY) theories. "Of course, the limitations of the Large Hadron Collider are becoming more and more stringent," says Manimala Chakraborti, a theoretical physicist at the Astronomy Center of the Polish Academy of Sciences, "but you can still find the parametric space allowed by SUSY."

An example of an extension of the non-supersymmetric Standard Model is the modification of the Higgs term — adding an additional scalar field to the Higgs term that does not have the canonical interactions in the Standard Model. The model predicts mass shifts of up to 100 MeV/c2, depending on the mass of the additional scalar particles and their interaction with the Higgs boson.

Examples such as " dark photons " , the restoration of conservation of cosmology in weak interactions , the possible composite properties of the Higgs boson , and the model-independent correction of the Higgs boson interaction are also theoretically possible explanations.

Note: At 5:00 a.m. Beijing time on April 9, the US CDF International Cooperation Group will hold a press conference on the results of the W boson quality measurement experiment, and those interested can pay attention to:

https://fnal.zoom.us/j/93590155647?pwd=RmNQK0R0bVZPWERUMVBOS3VlUEIxZz09

Resources

1. https://www.science.org/doi/10.1126/science.abk1781

2. https://news.fnal.gov/2022/04/cdf-collaboration-at-fermilab-announces-most-precise-ever-measurement-of-w-boson-mass/

3.https://www.science.org/doi/10.1126/science.abm0101

4.https://www.science.org/content/article/mass-rare-particle-may-conflict-standard-model-signaling-new-physics

5.https://www.theguardian.com/science/life-and-physics/2018/feb/20/how-much-mass-does-the-w-boson-have

6.https://www.science.org/content/article/mass-rare-particle-may-conflict-standard-model-signaling-new-physics

7.https://www.sciencenews.org/article/w-boson-particle-mass-standard-model-physics

8.https://bigthink.com/starts-with-a-bang/hole-in-the-standard-model/

9. https://www.bbc.com/news/science-environment-60993523

10. https://www.scientificamerican.com/article/elementary-particles-unexpected-heft-stuns-physicists/

Live Preview: W Quality Workshop

In order to promote academic exchanges between Chinese high-energy physics peers and international counterparts, the Center for High Energy Physics of Tsinghua University, the Department of Physics of Tsinghua University, and the School of Physics and Science and Technology of Nanjing Normal University will jointly host the "W Quality Seminar" on April 14. The head of the physical analysis group of the CDF cooperation group, Professor Ashutosh V. Kotwal of Duke University, Professor Han Tao of the University of Pittsburgh, and relevant theoretical and experimentalists will participate in the seminar to fully discuss the results.

This seminar will take a combination of online and offline methods. The two offline venues were set up in zheng yutong lecture hall of tsinghua physics department and conference room 437 of xingjian building of the southern academy of physics, professor Wang Qing, director of the center of tsinghua university high-energy physics research center, and professor Xiao Zhenjun of Nanjing normal university presided over the theoretical and discussion part of the seminar; Yi Kai, a member of the CDF cooperation group, professor of Nanjing Normal University and visiting professor of Tsinghua University, will preside over the experimental part of the seminar in Zheng Yutong lecture hall. The online zoom conference room will be broadcast live by "Return to Park" on the WeChat video account.

Meeting time: April 14, 2022, 8:00-12:00 Beijing time

Special mention

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