paper appearing today<\/a> in the journal Nature, an international team including MIT physicists reports a new, ultraprecise measurement of the mass of the W boson.<\/p>\nThe W boson is one of two elementary particles that embody the weak force, which is one of the four fundamental forces of nature. The weak force enables certain particles to change identities, such as from protons to neutrons and vice versa. This morphing is what drives radioactive decay, as well as nuclear fusion, which powers the sun.<\/p>\n
Now, scientists have determined the mass of the W boson by analyzing more than 1 billion proton-colliding events produced by the Large Hadron Collider (LHC) at CERN (the European Organization for Nuclear Research) in Switzerland. The LHC accelerates protons toward each other at close to the speed of light. When they collide, two protons can produce a W boson, among a shower of other particles.<\/p>\n
Catching a W boson is nearly impossible, as it decays almost immediately into two types of particles, one of which, a neutrino, is so elusive that it cannot be detected. Scientists are left to measure the other particle, known as a muon, and model how it might add up to the total mass of its parent, the W boson. In the new study, scientists used the Compact Muon Solenoid (CMS) experiment, a particle detector at the LHC that precisely tracks muons and other particles produced in the aftermath of proton collisions.<\/p>\n
From billions of proton-proton collisions, the team identified 100 million events that produced a W boson decaying to a muon and a neutrino. For each of these events, they carried out detailed analyses to narrow in on a precise mass measurement. In the end, they determined that the W boson has a mass of\u00a080360.2 \u00b1 9.9 megaelectron volts\u00a0(MeV). This new mass is in line\u00a0with predictions of the Standard Model, which is physicists\u2019 best rulebook for describing the fundamental particles and forces of nature.<\/p>\n
The precision of the new measurement is on par with a previous measurement made in 2022 by the Collider Detector at Fermilab (CDF). That measurement took physicists by surprise, as it was significantly heavier than what the Standard Model predicted, and therefore raised the possibility of \u201cnew physics,\u201d such as particles and forces that have yet to be discovered.<\/p>\n
Because the new CMS measurement is just as precise as the CDF result and agrees with the Standard Model along with a number of other experiments, it is more likely that physicists are on solid ground in terms of how they understand the W boson.<\/p>\n
\u201cIt\u2019s just a huge relief, to be honest,\u201d says Kenneth Long, a lead author of the study, who is a senior postdoc in MIT\u2019s Laboratory for Nuclear Science. \u201cThis new measurement is a strong confirmation that we can trust the Standard Model.\u201d<\/p>\n
The study is authored by more than 3,000 members of CERN\u2019s CMS Collaboration. The core group who worked on the new measurement includes about 30 scientists from 10 institutions, led by a team at MIT that includes Long; Tianyu Justin Yang PhD \u201924; David Walter and Jan Eysermans, who are both MIT postdocs in physics; Guillelmo Gomez-Ceballos, a principal research scientist in the Particle Physics Collaboration; Josh Bendavid, a former research scientist; and Christoph Paus, a professor of physics at MIT and principal investigator with the Particle Physics Collaboration.<\/p>\n
Piecing together<\/p>\n
The W boson was first discovered in 1983 and is predicted to be the fourth heaviest among all the fundamental particles. Multiple experiments have aimed to narrow in on the particle\u2019s mass, with varying degrees of precision. For the most part, these experiments have produced measurements that agree with the Standard Model\u2019s predictions. The 2022 measurement by Fermilab\u2019s CDF experiment is the one significant outlier. It also happens to be the most precise experiment to date.<\/p>\n
\u201cIf you take the CDF measurement at face value, you would say there must be physics beyond the Standard Model,\u201d says co-author Christoph Paus. \u201cAnd of course that was the big mystery.\u201d<\/p>\n
Paus and his colleagues sought to either support or refute the CDF\u2019s findings by making an independent measurement, with an experiment that matches CDF\u2019s precision. Their new W boson mass measurement is a product of 10 years\u2019 worth of work, both to analyze actual particle collision events and to simulate all the scenarios that could produce those events.<\/p>\n
For their new study, the physicists analyzed proton collision events that were produced at the LHC in 2016. When it is running, the particle collider generates proton collisions at a furious rate of about one every 25 nanoseconds. The team analyzed a portion of the LHC\u2019s 2016 dataset that encompasses billions of proton-proton collisions. Among these, they identified about 100 million events that produced a very short-lived W boson.<\/p>\n
\u201cA particle like the W boson exists for a teeny tiny moment \u2014 something like 10-24 seconds \u2014 before decaying to two particles, one of which is a neutrino that can\u2019t be measured directly,\u201d Long explains. \u201cThat\u2019s the tricky part: You have to measure the other particle \u2014 a muon \u2014 really well, and be able to piece things together with only one piece of the puzzle.\u201d<\/p>\n
Gathering momentum<\/p>\n
When a muon is produced from the decay of a W boson, it carries half of the W boson\u2019s mass, which is converted into momentum that carries the muon away from the original collision. Due to the strong magnetic field inside the CMS detector, the electrically charged muon follows a path whose curvature is a function of its momentum. Scientists\u2019 challenge is to track the muon\u2019s path and every interaction it may have with other particles and its surroundings, in order to estimate its initial momentum.<\/p>\n
The muon\u2019s momentum is also influenced by the momentum of the W boson before it decays. Decoding the impact of the W boson\u2019s motion from the effects of its mass presented a major challenge. To infer the W boson mass, the team first carried out simulations of every scenario they could think of that a muon might experience after a proton-proton collision in the chaotic environment of the particle collider. In all, the team produced 4 billion such simulated events described by state-of-the-art theoretical calculations. The simulations encoded diverse hypotheses about how the muon momentum is affected by the physical features of the CMS detector, as well as uncertainties in the predictions that govern W boson production in LHC collisions.<\/p>\n
The researchers compared their simulations with data from the 2016 LHC run. For every proton-proton collision event that occurs in the collider, scientists can use the CMS detector at CERN\u2019s LHC to precisely measure the energy and momentum of resulting particles such as muons. The team analyzed CMS measurements of muons that were produced from over 100 million W boson events. They then overlaid this data onto their simulations of the muon momentum, which they then converted to a new mass for the W boson.<\/p>\n
That mass \u2014\u00a080360.2 \u00b1 9.9 megaelectron volts \u2014 is significantly lighter than the CDF experiment\u2019s measurement. What\u2019s more, the new estimate is within the range of what the Standard Model predicts for the W boson\u2019s mass, bolstering physicists\u2019 confidence in the Standard Model and its descriptions of the major particles and forces of nature.<\/p>\n
\u201cWith the combination of our really precise result and other experiments that line up with the Standard Model\u2019s predictions, I think that most people would place their bets on the Standard Model,\u201d Long says. \u201cThough I do think people should continue doing this measurement. We are not done.\u201d<\/p>\n
\u201cWe want to add more data, make our analysis techniques more precise, and basically squeeze the lemon a little harder. There is always some juice left,\u201d Paus adds. \u201cWith a better look, then we can say for certain whether we truly understand this one fundamental building block.\u201d<\/p>\n
This work was supported, in part, by multiple funding agencies, including the U.S. Department of Energy, and the SubMIT computing facility, sponsored by the MIT Department of Physics.\u00a0<\/p>\n","protected":false},"excerpt":{"rendered":"When fundamental particles are heavier or lighter than expected, physicists\u2019 understanding of the universe can tip into the…\n","protected":false},"author":2,"featured_media":574351,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[49],"tags":[252655,188318,252656,3723,89432,8126,199,79,6766,145949],"class_list":{"0":"post-574350","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-physics","8":"tag-christoph-paus","9":"tag-cms-collaboration","10":"tag-kenneth-long","11":"tag-large-hadron-collider","12":"tag-mit-physics","13":"tag-particle-physics","14":"tag-physics","15":"tag-science","16":"tag-standard-model-of-physics","17":"tag-w-boson"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/574350","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/comments?post=574350"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/574350\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media\/574351"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media?parent=574350"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/categories?post=574350"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/tags?post=574350"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}