The Standard Model More Deeply: The Nature of Neutrinos

Earlier this week I explained how neutrinos can get their mass within the Standard Model of particle physics, either by engaging with the Higgs field once, the way the other particles do, or by engaging with it twice. In the first case, the neutrinos would be “Dirac fermions”, just like electrons and quarks. In the second, they’d be “Majorana fermions”. Decades ago, in the original Standard Model, neutrinos were thought not to have any mass at all, and were “Weyl fermions.” Although I explained in my last post what these three types of fermions are, today I want go a little deeper, and provide you with a diagrammatic way of understanding the differences among them, as well as a more complete view of the workings of the “see-saw mechanism”, which may well be the cause of the neutrinos’ exceptionally small masses.

[N.B. On this website, mass means “rest mass” except when otherwise indicated.]

The Three Types of Fermions

What’s a fermion? All particles in our world are either fermions or bosons. Bosons are highly social and are happy to all do the same thing, as when huge numbers of photons are all locked in synch to make a laser. Fermions are loners; they refuse to do the same thing, and the “Pauli exclusion principle” that plays a huge role in atomic physics, creating the famous shell structure of atoms, arises from the fact that electrons are fermions. The Standard Model fermions and their masses are shown below.

Figure 1: The masses of the known elementary particles, showing how neutrino masses are much smaller and much more uncertain than those of all the other particles with mass. The horizontal grey bar shows the maximum masses from cosmic measurements; the vertical grey bars give an idea of where the masses might lie based on current knowledge, indicating the still very substantial uncertainty.

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5th Webpage on the Triplet Model is Up

Advanced particle physics today: Another page completed on the explanation of the “triplet model,”  (a classic and simple variation on the Standard Model of particle physics, in which the W boson mass can be raised slightly relative to Standard Model predictions without affecting other current experiments.) The math required is still pre-university level, though complex numbers … Read more

Fourth Step in the Triplet Model is up.

Advanced particle physics today: Today we move deeper into the reader-requested explanation of the “triplet model,”  (a classic and simple variation on the Standard Model of particle physics, in which the W boson mass can be raised slightly relative to Standard Model predictions without affecting other current experiments.) The math required is still pre-university level, though … Read more

Third step in the Triplet Model is up.

I’m continuing the reader-requested explanation of the “triplet model,” a classic and simple variation on the Standard Model of particle physics, in which the W boson mass can be raised slightly relative to Standard Model predictions without affecting other current experiments. The math required is pre-university level, mostly algebra and graphing.

Triplet Model: Second Webpage Complete

Advanced particle physics today: I’m continuing the reader-requested explanation of the “triplet model,” a classic and simple variation on the Standard Model of particle physics, in which the W boson mass can be raised slightly relative to Standard Model predictions without affecting other current experiments. The math required is pre-university level, mostly algebra and graphing. The second … Read more

Triplet Model: First Webpage Complete

Advanced particle physics today: Based on readers’ requests, I have started the process of explaining the “triplet model,” a classic variation on the Standard Model of particle physics, in which the W boson mass can be raised slightly relative to Standard Model predictions without affecting other current experiments. The math required is pre-university level, so … Read more

The Importance and Challenges of “Open Data” at the Large Hadron Collider

A little while back I wrote a short post about some research that some colleagues and I did using “open data” from the Large Hadron Collider [LHC]. We used data made public by the CMS experimental collaboration — about 1% of their current data — to search for a new particle, using a couple of twists (as proposed over 10 years ago) on a standard technique.  (CMS is one of the two general-purpose particle detectors at the LHC; the other is called ATLAS.)  We had two motivations: (1) Even if we didn’t find a new particle, we wanted to prove that our search method was effective; and (2) we wanted to stress-test the CMS Open Data framework, to assure it really does provide all the information needed for a search for something unknown.

Recently I discussed (1), and today I want to address (2): to convey why open data from the LHC is useful but controversial, and why we felt it was important, as theoretical physicists (i.e. people who perform particle physics calculations, but do not build and run the actual experiments), to do something with it that is usually the purview of experimenters.

The Importance of Archiving Data

In many subfields of physics and astronomy, data from experiments is made public as a matter of routine. Usually this occurs after an substantial delay, to allow the experimenters who collected the data to analyze it first for major discoveries. That’s as it should be: the experimenters spent years of their lives proposing, building and testing the experiment, and they deserve an uninterrupted opportunity to investigate its data. To force them to release data immediately would create a terrible disincentive for anyone to do all the hard work!

Data from particle physics colliders, however, has not historically been made public. More worrying, it has rarely been archived in a form that is easy for others to use at a later date. I’m not the right person to tell you the history of this situation, but I can give you a sense for why this still happens today.

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Breaking a Little New Ground at the Large Hadron Collider

Today, a small but intrepid band of theoretical particle physicists (professor Jesse Thaler of MIT, postdocs Yotam Soreq and Wei Xue of CERN, Harvard Ph.D. student Cari Cesarotti, and myself) put out a paper that is unconventional in two senses. First, we looked for new particles at the Large Hadron Collider in a way that … Read more

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