Questions for Jeremiah Mans

Learn more about my research In November 2008, Jeremiah Mans answered visitors questions about particle physics.

Your Comments, Thoughts, Questions, Ideas

DO's picture
DO says:

Many particles have such odd names like quarks. Who names them?

posted on Fri, 11/21/2008 - 4:46pm
Jeremiah Mans's picture

Traditionally, the naming of particles has been decided by those who discover or predict them. Usually, there is a "theme" for a given type of particle. For example, there are two other particles similar to the electron which are given Greek names (derived from the names of Greek letters in fact): the muon and tau particles. On the other hand, the first quarks were discovered by American physicists with a more whimsical sense of humor -- they ended with names like "up", "down", "strange", "charm" and so on. The word "quark" itself is said to come from a line of James Joyce's book "Finnegan's Wake" and was coined by one of the theoretical physicists who proposed it : Murray Gell-Mann of Caltech.

posted on Tue, 11/25/2008 - 6:43am
Beach bum's picture
Beach bum says:

Why build CERN in the middle of one of the most densely populated areas of the world? What was the criteria for picking this location?

posted on Sat, 11/22/2008 - 8:42pm
Jeremiah Mans's picture

The region of Switzerland and France near CERN is actually a relatively unpopulated region. In fact, when CERN was founded in 1952, the area was completely farmland -- notice the cow in the original news posting on the foundation of CERN! As the first accelerators began operation, the site was an island of large buildings in a sea of pastoral farmland. Even today, the area around CERN is mostly farmland, though there are certainly larger apartment buildings in the area than in 1957.

As for the criterion, it was a political compromise between France, Italy, and Germany -- the main countries involved in CERN initially. It has been a good choice, as Geneva hosts many international agencies related to the United Nations. On a practical level, the international nature of Geneva means that there are many international schools, clubs, and even markets where one can buy British or American specialities -- those few classic foods (like baked beans or Goldfish crackers) which you can't find in the French grocery stores!

posted on Wed, 11/26/2008 - 10:00am
Joe's picture
Joe says:

What inspired you to pursue a career in particle physics?

posted on Sun, 11/23/2008 - 2:09pm
Jeremiah Mans's picture

Hi Joe,

I have always enjoyed doing a wide variety of tasks -- looking at data, thinking about it, designing experiments, developing electronics, working with computers, etc. As an experimental scientist, I have the opportunity to do different things every day.

For the choice of particle physics: I actually made this choice during graduate school. As an undergraduate, I had worked on biophysics projects for several summers. I had expected to continue this during graduate school, but I wanted to try something different between undergraduate and graduate school. I was offered the opportunity to go to CERN and work with the electron-positron collider which was operating at that time. I found experimental particle physics fit my interests and skills perfectly, and the rest is history.


Jeremiah Mans

posted on Wed, 12/10/2008 - 4:34am
Julia's picture
Julia says:

So, is the Higgs field the same thing as the quantum field?

posted on Mon, 11/24/2008 - 2:02am
Jeremiah Mans's picture

Hi Julia,

The Higgs field is only one many quantum fields found in nature. Particle physics views all particles as quantum fields : radio waves (photons), electrons, quarks (nuclei) are all understood as quantum fields.

The Higgs field is a special sort of quantum field, however. Most quantum fields have a "vacuum expectation value" of zero, meaning that in empty space one expects to find no photons, no electrons, no quarks, etc. The Higgs, however, has a non-zero "vacuum expectation value" -- this means that the Higgs field is present everywhere all the time. This is how the Higgs is able to give mass to all particles -- it's present everywhere in space where a particle might be.

J. Mans

posted on Mon, 12/01/2008 - 2:55pm
JGordon's picture
JGordon says:

So, if the Higgs boson isn't found at the LHC... what then? Do we look harder? Or does that mean that the Higgs probably doesn't exist?

And if the Higgs boson doesn't exist, how much of particle physics needs to be rethought? Would the Standard Model have to be thrown out altogether?

posted on Mon, 11/24/2008 - 11:27am
Jeremiah Mans's picture

Hi JGordan,

One of the interesting things about quantum fields is that particles can affect measurements which are "too weak" to actually create the particle. Conceptually, one can recall the Heisenberg Uncertainty Principle which allows a massive particle to appear temporarily, as long as it disappears within a time inversely proportional to its mass. This means that the Higgs can affect lower-energy measurements. In fact, this technique was used at the LEP collider to predict (roughly) the mass of the top quark before it was discovered directly at the TeVatron. If we play the same game with the Higgs, we see a prediction that the Higgs exists and has a low mass. Another way to express the result is that we already observe the effect of the Higgs on our measurements -- if the Higgs didn't exist, the measurements would be different.

Given all this, it would be a very big surprise if we don't find something which behaves like a Higgs boson. It might not be the "simple" Higgs boson of the Standard Model. In fact, most theoretical physicists expect that we will find something different than the simple Higgs. There is a big market in alternatives: supersymmetric Higgs, "technicolor", little Higgs, composite Higgs, and more. The hardest job is probably going to be to figure out what sort of Higgs it is when we find one!


Jeremiah Mans

posted on Wed, 12/10/2008 - 5:50am
Taylor and Jessica's picture
Taylor and Jessica says:

What is particle physics? I've never heard of it til now!!

posted on Wed, 11/26/2008 - 12:37pm
Jeremiah Mans's picture

Hi Taylor and Jessica,

"Particle physics" means a study of the world as defined by very small interacting parts, rather than extended solid objects, liquids, gasses, or even stranger forms of matter seen in neutron stars. In general, particle physics attempts to understand the interactions of matter at the very smallest scales -- the fundamental particles -- but includes a large number of "simple" composite objects, made out of just a few fundamental particles.

What sorts of questions do particle physicists study?

One broad area of study is how the proton and neutron are constructed out of simpler pieces. The quick answer is that each contains three quarks, but in fact the reality is quite a bit more complex -- there are many virtual ("temporary") particles which appear and disappear within the proton and these have a major impact on what happens when two protons interact.

Looking even deeper, particle physicists are trying to construct a complete view of all the components of matter and how they can interact. All the work completed to date has been synthesized into a "Standard Model" of fundamental interactions. This theory has been extraordinary successful explaining observations from radioactive decay to exploding stars, but it makes a number of assumptions which have not been tested: the most important of these is the Higgs boson which I've discussed in a few other postings.

To study the "extreme" phenomena which are the most effective way to test the assumptions of the Model and to explain mysteries such as "dark matter", we use very powerful (and large) machines to accelerate particles (protons in this case) to very high speeds and collide them -- this concentrates enough energy in one place to produce and observe rare and "extreme" phenomena clearly.

J. Mans

posted on Thu, 01/01/2009 - 5:32pm
Teague's picture
Teague says:

So do you think that the String Theory is plausible?

posted on Sun, 11/30/2008 - 5:52pm
Jeremiah Mans's picture

Hi Teague,

"String Theory" is a very broad topic. Looked at from one point of view, it's an attempt to construct a coherent model of nature which can include gravity at the very lowest levels -- right at the interaction between particles. On the smallest scales (10^-35 m), gravity interacts with spacetime in such a way to make conventional quantum mechanics uncalculable. String theory provides reasonable quantum gravity, but it is harder to provide a consistent and compelling explanation for "long distance" phenomena such as the electromagnetic force.

On the other hand, String Theory originated as a calculational tool to try to understand the fundamental structure of protons and other hadronic particles (composite particles containing quarks). It developed in the direction of a "Theory of Everything", but in fact has recently returned to its roots and has proven quite useful in understanding the results of heavy-ion collisions. In this area, at least, String Theory is "proven" as a tool.

The "heavy-ion" success does not imply anything about the relevance of String Theory to understanding physics at the smallest scales, however! Fundamentally, we need more experimental data to motivate and constrain a theory as complex and rich as String Theory -- thus the LHC!


J. Mans

posted on Thu, 01/01/2009 - 3:20pm
Anonymous's picture
Anonymous says:

Will the LHC make a black hole? Is it dangerous?

posted on Sun, 11/30/2008 - 7:06pm
Jeremiah Mans's picture

Considering only conventional physics, there would be no possibility whatsoever of observing any gravitational effects at the LHC, much less the creation of a black hole! The energies of the LHC, though high, are many orders of magnitude from the Planck scale where such effects might be observed -- it would be similar to worrying about hearing loss from a rock concert 3000 km away.

Some recent theories have postulated that gravity might change its behavior on length scales somewhat smaller than the proton, which might allow for the creation of extremely small black holes, which would 'evaporate' immediately due to Hawking radiation. We know from observation that if such black holes were created, they could not be stable and destructive. The LHC is not unique and special -- these sorts of collisions occur all the time in the atmosphere of the earth and the Sun as high energy cosmic rays collide with these bodies. The difference at the LHC is that the collision occurs at a well-defined place and time, so that we can study the results better. However, if black holes were able to be created in such collisions and cause damage, the Earth and sun would have disappeared a long time ago. There is a nice review of this discussion and further theories here:

J. Mans

posted on Thu, 01/01/2009 - 3:45pm
Anonymous's picture
Anonymous says:

whats up with the particle accelerrator breaking? is it a big deal?

posted on Wed, 12/03/2008 - 10:53am
Jeremiah Mans's picture

The LHC began operations on September 10. The process of getting such a large system working (27 kilometers of magnets, vacuum pumps, accelerating devices, beam instrumentation, etc) is quite challenging. However, the beams group at CERN was able to get the first particles to successfully circulate around the accelerator exactly on schedule, with a great deal of press coverage.

On September 10th, however, the accelerator was not fully commissioned -- it was ready to hold a beam at low energy but not to accelerate it to high energy. Very powerful magnets are required to keep the protons moving in a circle around the accelerator. In the case of the LHC, these are superconducting magnets which operate at 1.9 degrees above absolute zero. To produce the large magnetic fields required, very large electrical currents are needed : up to 14,000 amps. As of September 19th, seven of the eight "octants" into which the accelerator is divided had been tested up to 9,300 amps, the level required for initial operation. The last sector was being tested at 7,800 amps when one of the connections between the magnets failed. Given the large amount of energy in the current, the breaking of this connection created an electrical arc which destroyed the connector and punched some holes in the magnet.

After study, it appears likely that this connection was not properly constructed. The magnets which were affected by the incident have been removed from the tunnel and the first cleaned magnets have recently been returned to the tunnel. We expect to restart the accelerator next summer after all the magnets have been returned to the tunnel, reconnected, and commissioned in place.

This was a disappointing experience -- particularly since the failure occured in the final test of the final sector. It has delayed the arrival of collision data by about a year -- as experimental physicists we are using the time to complete our preparations for collisions so that we get the best possible results as soon as collisions occur.



posted on Wed, 12/10/2008 - 5:32am
curious's picture
curious says:

Please explain a little bit about the "god" particle and what it means in terms of physics. who thought of it first and why/what were the situations that led him/her think of a particle such as the "god' partice?

posted on Tue, 12/09/2008 - 9:51pm
Jeremiah Mans's picture

The term "the god particle" refers to the Higgs boson which I described above. The Higgs quantum field (of which the boson would be the fundamental excitation) is postulated to fill all of space and to provide masses to all the particles (in particular to the fundamental particles such as the electron and the quarks) through its interactions with them. The "god particle" term is almost completely unused within the physics community and generally appears in the popular media. The term came from Leon Lederman and is the title of a book he wrote. There are several explanations for the term.

One explanation refers to the difficulty which physicists have had in producing a Higgs boson and thus confirming or denying the theory. The community has been searching for more than 30 years and the Higgs has not been directly seen (though it seems to affecting measurements made by precision experiments). Dozens of doctoral theses (including my own) have been written searching for but not finding the Higgs. This has led some discouraged researchers to term it "that godd*mn particle", which shortened nicely into an exciting book title as the "god particle".

Looking at the physics of the Higgs field, however, there is one unique and arguably godlike characteristic of the Higgs field: it is omnipresent. All other quantum fields have an expectation value of zero in the vacuum -- electrons, photons, quarks, etc don't fill all of space. They must be created (generally in particle/antiparticle pairs). The Higgs field, however, has a non-zero presence in the vacuum, which allows it to interact with all other particles to provide them mass.

In the end, the most conventional explanation for the use of the term has to be that it catches the eye and attention which guarantees its continued use in the press, despite the complete disuse of the term among active physicists.

J. Mans

posted on Wed, 12/31/2008 - 12:41pm
jenn's picture
jenn says:

protons are in the center of atoms which compose all matter. how is it that only the protons are collided in the machine and not the electrons and neurons also

posted on Fri, 12/19/2008 - 1:42pm
Jeremiah Mans's picture

Hi Jenn,

That's a great question! Accelerators use electric and magnetic fields to control the particles they collide. These fields only have an effect on particles with net charge -- thus an accelerator can't collide neutral atoms (with same number of electrons as protons) or neutrons alone. However, it is possible to collide electrons with protons (as was done in the HERA collider in Germany), electrons with anti-electrons (positrons), as well as protons with protons (or anti-protons).

In fact, one part of the LHC's research will be to collide not just protons with protons, but rather lead ions -- 82 protons and 126 neutrons colliding with another 208 nucleons! This research is designed to study how nuclei are held together and how nuclear matter behaves at very high temperature. We expect the first data-taking session with lead ions in 2010.


J. Mans

posted on Tue, 12/30/2008 - 11:28am