#standard model
Hints of an unexpected new particle could be confirmed within days.
If the observations are confirmed, it will be revolutionary. This could mean nothing less than the fall of the Standard Model of particle physics (SM), which has passed every experimental test thrown at it since it was first put together over four decades ago.
I updated my Standard Model “snowflake plot” tonight while waiting for a headache to go away. I also added the graviton to this version because I’m biased. :P
A long time ago I decided: if I have to include an SM slide in many talks from now on, it’ll at least be on my own terms!
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The Weak Interaction
Amongst the four fundamental forces in nature, the weak interaction (aka the weak force or weak nuclear force) is the most commonly unheard of.
The weak force is mainly responsible for radioactive decay and fusion in stars. In our current understanding of the Standard Model, the weak interaction itself is caused by the emission or absorption of W and Z bosons. For example, this is most commonly seen in beta decay.
The exchange of W and Z bosons not only cause the transmutation of quarks (i.e. quark flavor changing) inside hadrons, but also (by definition) the hadrons themselves. For example, the process of beta decay allows for a neutron to transform into a proton. Given that a neutron is made up of two down quarks and one up quark, a down quark will need to emit a W¯ boson in order to transform into an up quark, thus allowing for the formation of a proton (which consists of two up quarks and one down quark). At the end of the process, the W¯ boson will then further decay into an electron and antineutrino.
It is called the weak force because its field strength is several orders of magnitude less than that of both electromagnetism and the strong interaction (however, gravity is the weakest of the four forces). The weak interactions are extremely short ranged (≈ 2 x 10-3fm) because the intermediate vector bosons (W and Z) are very massive (with even higher masses than neutrons).
Unifying Fundamental Forces
Electromagnetism and the weak force are now considered to be two aspects of a unified electroweak interaction. This is the first step toward the unification the four fundamental forces.
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Dark Matter 4: What is Dark Matter made of? - The search for WIMPS
In my last post, I talked about how we can see the effects of dark matter not just in galaxies, but in the wider Universe as well. Gravity pulls the galaxies into a ‘cosmic web’ pattern - called galactic filaments.
Now we are sure that there is more mass in the universe than we can see, we can start to think about what this extra mass actually is. Obviously, it must be either very difficult or impossible to see with light, and it must be very heavy. This shows that it is not baryonic or 'normal’ matter - stuff made out of protons and neutrons that we can see all around us in everyday life. Normal matter can interact with light, and this is why we can see it - it reflects and diffracts light into our eyes. If a dark matter particle exists, then it shouldn’t interact with light at all, and this is why can not zoom in with a telescope and actually see it.
It makes sense then to assume that dark matter is not 'normal matter’ and must be made out of something more exotic. We know that normal matter is made of atoms which can be split up into particles, so can we get 'dark atoms’ made up of 'dark particles’?
The first image I have included shows a few types of particle that dark matter could be made out of. Out of all of these, the most likely theory seems to be that dark matter is made out of a new type of particle called a Weakly Interacting Massive Particle (WIMP). These basically do what they say on the tin - they don’t interact with light very much, and they are really heavy (up to 1000 GeV!).
The second image shows the Standard Model of Elementary Particles - a list of all the different types of particles we have found so far. If WIMPs do exist then they will have to fit into this model somewhere, which is why it has been suggested that they are a type of neutrino (see the bottom row). This is why the hunt for dark matter particles is really on, and scientists at CERN are hopeful that they may be able to find something when the accelerators are turned on later this year.
But if a dark matter particle can’t be found, what happens then? Well dark matter might not exist and we might have to find another theory to try and explain it all! More to come in the next post!
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