Recently, 389 scientists from the International Cooperation Group of the Collider Detector (CDF) of the Fermi National Accelerator Laboratory in the United States jointly completed the most accurate measurement of the mass of the W boson, which is 7 standard deviations higher than expected in the Standard Model of Particle Physics, or points to a new field of physics.
Image from Science
The results of these studies were published on the cover of Science. If this result is finally confirmed, it will herald the need to revise or expand the theory of the Standard Model of Particle Physics, one of the most fundamental theories of physics.
"Particle physics is the study of the most basic matter of our world, and how it is made up." Yi Kai, a professor at Nanjing Normal University and a visiting professor at Tsinghua University, told the surging news (www.thepaper.cn) reporter.
Yi Kai is currently a member of the CDF, CMS (Compact Muzi Solenoid Experiment on the Large Hadron Collider at CERN), a member of the Belle II International Cooperation Group in Japan, and the head of the CMS group of Tsinghua-Nanshi Division, and has long been engaged in particle physics, hadron physics and the search for physical phenomena beyond the Standard Model.
Standard Model of Particle Physics, image from Quanta Magazine
"This 'disk' is the Standard Model, and every letter on it represents a particle, and these particles are elementary particles, that is, particles that have no structure and cannot be separated inside." Yi Kai said that the outermost circle of the "disk" is six quarks above and six leptons below, including the well-known electrons, μ mesons, τ mesons and their corresponding neutrinos. "Quarks and leptons make up our physical world, for example, common protons and neutrons are made up of three quarks of two different types. Among them, the interaction between neutrinos and other substances is very small and can penetrate substances. ”
The four elementary particles in the middle circle are photons, gluons, W bosons, and Z bosons. "They are particles that transmit interactions. Among them, photons are particles that transmit electromagnetic interactions, which are massless. Gluons are responsible for transporting strong interactions. The particles that transmit weak interactions are the W boson and the Z boson, which are very heavy. Yi Kai said.
At the center of the disk is the Higgs boson, also known as the "God particle," which was discovered simultaneously in 2012 in both the CMS and ATLAS experiments on the CERN Large Hadron Collider. The Higgs boson gives mass to other elementary particles. How these particles interact with each other explains the composition of the world and forms a system, which is the 'Standard Model'. Yi Kai said that the mass of the W boson can be deduced from this system.
For years, scientists have been testing the Standard Model correctly in a variety of ways, trying to find new breakthroughs. As an important parameter of the Standard Model, the quality of the W boson has always been one of the important means to test the Standard Model and detect phenomena beyond the Standard Model.
Due to the difficulty of experimental reconstruction of the decay mode of the W boson, its measurement accuracy (error) has been in the order of dozens of MeV (megaelectron volts), and the previous best single experimental accuracy was also around twenty MeV. This is in sharp contrast to the mass accuracy of the Z boson, the "sister" particle of the W boson, 2MeV. Experimental particle physicists have been working tirelessly for a long time. MeV is a mass unit used in particle physics, taking protons as an example, protons are about 938 MeV.
However, many previous measurements of the mass of the W boson have basically conformed to the Standard Model theory. "This may be because the previous measurement accuracy was not enough, this time the CDF cooperation group measured the quality of the W boson with a high accuracy of 0.01%." Yi Kai said.
Collider Detector (CDF) at the Fermi National Accelerator Laboratory, image from the Fermi National Accelerator Laboratory
It is understood that the Tevatron of Fermilab in the United States was once the world's largest collider, in the Tevatron, protons and antiprotons are accelerated to 1000 times their stationary mass, and then collide, resulting in a large number of W bosons. THE CDF is a general-purpose particle detector on Tevatron, where particle physics experimenters measure the mass of W bosons by studying information about charged leptons and other products produced by the decay of W bosons detected by the CDF.
After a decade of meticulous calibration and analysis of the collected data, the CDF team used all the data collected during the operation of the CDF Phase II to make the most accurate measurement of the quality of the W boson in the world, reducing the error (including statistical error and systematic error) to single digits for the first time, that is, 9MeV. In the Standard Model of Particle Physics, the expected W boson mass should be 80357+/-6 MeV. The measurement results show that the mass measurement is 80434+/-9 MeV, which exceeds the expectations of the Standard Model.
Range of experimental measurements and theoretical predictions of W boson mass, picture from Science paper
The above-mentioned study achieved a detection accuracy of 0.01%, surpassing the accuracy of all previous individual experiments and the weighted comprehensive accuracy of all previous experimental results, and the test of the Standard Model reached a new milestone. At this accuracy, the mass of the W boson measured by the CDF collaboration group is seven standard deviations (including experimental and theoretical errors) higher than the calculated values of the Standard Model, challenging the current Standard Model. This means that the probability that the CDF experiments will observe such experimental results is only about 10 to the minus twelfth power if the Standard Model's predictions are correct.
"If this result is correct, it indicates that our Standard Model is not perfect." "There are two possibilities for this, one possibility is that we need to correct the calculations in the Standard Model and improve the system, as Yi Kai explains. Another possibility is that the Standard Model needs to be extended, for example, if it also has undiscovered elementary particles or undiscovered fundamental forces. And these all point to new physical phenomena. ”
Guillaume Unal, a physicist from CERN's Large Hadron Collider ATLAS experimental project, said he "would like to see another independent measurement to confirm the results of the CDF collaboration." ”
In this regard, Nanjing Normal University, Tsinghua University, institute of high-energy physics of the Chinese Academy of Sciences, and Peking University jointly initiated and hosted the "W Quality Seminar" on April 14. In addition to the CDF experiments, several experiments on CERN's Large Hadron Collider (LHC) and the D0 experiment at Fermilab in the United States have done mass measurements of the W boson and other properties studies. Participants from each experiment discussed the current status and future of W boson research.
The online and offline participants of the "W Quality Seminar" took a group photo in the lecture hall of the Department of Physics of Tsinghua University
Currently, members of the collaboration between ATLAS and LHCb (The Large Hadron Collider Upper Bottom Quark Detector Experiment) are improving the analysis of the mass of the W boson. Another experimental CMS at CERN can also measure the mass size of W particles.
"The CDF results are currently only the results of one experiment, and other experiments are needed to verify. In addition, it is inconsistent with the measurement results of the world's second most accurate measurement, the ATLAS experiment. Yi Kai said that the CMS cooperation group now measures charged particles all using silicon detectors, and the resolution will be better than that of the CDF cooperation group in the past.
Talking about the experience in the CDF cooperation group, Yi Kai said that from the construction of the detector and the regulation of electronic equipment to the acquisition, calibration, reconstruction, and physical analysis of the control room data to obtain the final results, this process is long and arduous, requiring the cooperation of the cooperation group. For example, Yi Kai has twice obtained data as a silicon detector duty expert, once in 2001, which lasted more than three months, and the other from 2009 to 2010, which lasted more than a year. "The CDF cooperation group had more than 700 members at its peak, and the list was fixed at about 400." After Tevatron ceased operations in 2011, the team led by Ashutosh Kotwal in the CDF collaboration group conducted more detailed calibration and in-depth analysis of the data, and after rigorous internal review by the partner group, it took more than a decade to finally publish the paper.
In the future, measurements from more international collaboration groups will validate the results of the CDF collaboration group and will further test the current Standard Model.