From Don Lincoln at FNAL: “The triumphant Standard Model”

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June 17, 2016

FNAL Don Lincoln
Don Lincoln

In high-end research, there are a couple of deeply compelling types of data analyses that scientists do. There are those that break the existing scientific understanding and rewrite the textbooks. Those are exciting. But there are also those in which a highly successful theory is tested in a regime never before explored. There can also be two types of outcome. If the theory fails to explain the data, we have a discovery of the type I mentioned first. But it is also possible that the theory explains the data perfectly well. If so, that means that you’ve proven that the existing theory is even more successful than was originally known. That’s a different kind of success. It means that predictions made in one realm taught scientists enough to understand far more.

In the LHC, pairs of protons are collided together with the unprecedented energy of 13 trillion electronvolts of energy.

CERN/LHC Map
CERN LHC Grand Tunnel
CERN LHC particles
LHC at CERN

Before 2015, when the data in this analysis was recorded, the highest energy ever studied by humanity was only 8 trillion electronvolts. So, already we know that the new data is 63 percent higher in terms of energy reach as compared to the old data. To get a visceral sense of what that means, imagine that your bank told you that they made a mistake and that for every dollar you thought you had in your account, you actually had $1.63. I’m guessing you’d start planning for an awesome vacation or perhaps an earlier retirement.

When the protons collide, most commonly, a quark or gluon from each proton hits a quark or gluon from the other proton and knocks them out of the collision area into the detector. As the quarks and gluons leave the collision area, they convert into sprays of particles that travel in roughly the same direction. These are called jets. Physicists study the location and energy of the jets in the detector and compare them to the predicted distribution.

CMS scientists studied the production patterns of jets at a collision energy of 13 trillion electronvolts and found that they agreed with the predictions of the Standard Model with the same level of precision seen at lower energy measurements.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth

This result comes with a small sadness because this means that new physics hasn’t been discovered. On the other hand, it is a resounding endorsement of the theory of quantum chromodynamics, or QCD, which is the portion of the Standard Model that deals explicitly with quark and gluon scattering. QCD, first worked out nearly half a century ago, continues its decades-long track record of success.

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Scientists are constantly exploring the universe, seeing what happens when existing theories are tested in new realms. In today’s analysis, scientists put the leading theory of quark scattering to the test, studying what happens when it is compared to data taken at energies over 60 percent higher than ever before achieved.

See the full article here .

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Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
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