Physicists measure the heaviest known particle with record precision

(ORDO NEWS) — The world of particle physics has been on shaky ground lately. For years, researchers have carefully studied particles to make sure the rules we use to explain the universe are correct, but the results are alarmingly inconsistent.

In addition, physicists using the Large Hadron Collider (LHC) have measured the heaviest known elementary particle with unprecedented accuracy.

It’s a much-needed victory for the Standard Model of particle physics – the set of rules that predict the behavior of all the particles that make up our world – the new calculations come with a margin of error much smaller than previous ones, giving physicists confidence in a particle’s true mass.

But this does not mean that the case is closed – this measurement can only be the beginning of a deeper understanding of how our Universe works.

The elementary particle in question is called the top quark, and it is the most massive of all known elementary particles, contributing to a fundamental part of our understanding of the universe.

Importantly, it derives its mass from its association with the elusive Higgs boson. This partnership is the strongest bond on this scale that we know of in the Standard Model.

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It is also important what the top quark decays into. After it has been smashed in a collider, the up quark can only decay under the influence of weak forces, and it decays into a W boson (and usually a down quark).

If you are a regular reader of ScienceAlert, you may recognize the W boson as the center of recent controversy.

After years of trying to punch holes in the Standard Model, researchers have recently published compelling evidence that indicates that previous estimates of the W boson’s mass may be wrong.

If these results are confirmed, it would mean that the entire Standard Model could be wrong.

And this is where the top quark comes in – we can use its mass to make predictions about both the Higgs boson and the W boson, so getting the best possible estimate is critical.

“Remarkably, our knowledge of the very stability of our universe depends on our shared knowledge of the masses of the Higgs boson and the top quark,” the European Council for Nuclear Research (CERN), which led the study, explained in a press release.

“We know that the Universe is very close to a metastable state only because of the accuracy of the current measurements of the top quark mass. If the top quark mass were even slightly different, the Universe would be less stable in the long run, and possibly eventually disappear would be the result of a turbulent event like the Big Bang.”

While it may seem simple to be able to “weigh” these particles in the same way that we weigh ordinary objects in order to find out their mass, it is actually not so simple.

To make an elementary particle, like the top quark, physicists knock together subatomic particles known as protons in devices like the Large Hadron Collider. Each collision ejects a range of other particles, allowing researchers to study these by-products in a controlled environment.

However, it is still not easy to observe the properties of each particle. When we start talking about these incredibly small scales, we get into the quantum realm where the particles get a little fuzzy and it’s hard to accurately determine their mass.

There are several ways to get around this. One is to run the experiment several times and then statistically process the results.

Another is to use different methods. In this case, the researchers directly measured the particle and simultaneously made a measurement using other forms of data in conjunction with established theory (in this case, this was called the pole mass measurement).

The researchers say their new result is 0.12 GeV more accurate than previous calculations based on the same data, at 172.76 gigaelectronvolts (plus or minus 0.3 gigaelectronvolts). This is quite consistent with what we expect from theories based on the Standard Model, the CERN researchers say.

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The improved accuracy comes from new analysis methods that use more variables than before to better deal with uncertainties between measurements.

The latest measurement examined collision data from the LHC’s CMS (Compact Muon Solenoid) detector in 2016. CERN researchers have studied five different properties of collisions that produce a pair of top quarks. The considered properties depend on the mass of the top quark, and in previous studies only three properties of events were considered.

The team then calibrated this dataset to the utmost precision to determine what uncertainties remained—they could extract those uncertainties and better understand them when calculating the best fit for the final top quark mass.

While this result in itself is a big step forward for particle physics and a tentative victory for the Standard Model, CERN says we can expect even better accuracy when the same approach is applied to the data set collected by the CMS detector in 2017 and 2018 – not to mention future, already record-breaking studies. The LHC has just started up again after a three-year shutdown and is already breaking records.

It’s safe to say that thanks to this updated mass measurement and the technology that provided it, we’re going to gain an even deeper understanding of the smallest aspects of the universe. Watch this space.


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