Atoms are made of electrons, protons and neutrons. Protons and neutrons are in turn made up of quarks. These are just some of the elementary particles that make up the foundation of modern particle physics. But how do we know about these particles when we can’t see atoms directly, much less their constituents? One of the early methods was through a device known as a cloud chamber, and it is quite a clever invention.

The basic idea of a cloud chamber is a sealed container filled with alcohol vapor. The vapor is cooled to the point that it is supersaturated. Because of this, small disturbances can trigger the formation of small alcohol droplets, which appear as a streak of mist or “cloud” (hence the name). When the container is made with transparent sides, then the cloud trails can be easily seen, and even photographed.

Photograph of a cloud chamber showing the first discovered positron. Credit: Carl D. Anderson

What makes this device useful for particle physics is that droplets can be triggered by charged particles passing through the chamber. A trail of droplets forms along the path of the particle, and by observing these trails we know that a charge particle has passed by. Cloud chambers provided the first observation of cosmic rays, appearing as random streaks in an isolated chamber. They also demonstrated that radioactive materials emit charged particles, since trails appeared when radioactive materials were placed near a chamber.

By itself, a cloud chamber can only show that a charged particle has passed by. It can’t distinguish the type of charged particle, or even the sign of its charge. Thus it can’t distinguish between protons and electrons, for example. To distinguish particles you also need a magnetic field.

When a charged particle moves through a uniform magnetic field, it experiences a force perpendicular to its motion. This means the magnetic field doesn’t affect the speed of the particle, but instead changes its direction. As a result, the charged particle will move in a circular path. The direction the particle circles depends upon the sign of its charge, and the size of the circle depends upon the particle’s mass. By placing a magnetic field through a cloud chamber, one determine the charge and mass of particles by their circular trails. This is how the first positron (anti-electron) was discovered. It produced a circular path that was the same size as an electron’s, but in the opposite direction.

While cloud chambers gathered the first crucial evidence for particle physics, they had the downside of having fuzzy cloud trails. This meant that getting precise observations was often difficult. The trails also diffused quickly, making them hard to study. In 1952 a similar device known as a bubble chamber was invented. This used critically heated liquid rather than vapor, and charged particles would produce a trail of vapor bubbles. This was particularly useful for early high-energy particle physics. Since then a range of other particle detectors were developed, and now particle physics is typically done with sensitive detector arrays.

As our understanding of particle physics increased, so did our understanding of the universe. Black holes, neutron stars, supernovae and the big bang are just a few of the phenomena we now understand thanks to particle physics. And the cloud chamber is what allowed us to make crucial breakthroughs in the early 1900s.

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When the Large Hadron Collider was frantically searching for the Higgs boson, you could hear murmuring speculation about the possibility of the LHC creating black holes that could destroy the Earth. There was a lawsuit filed against Cern in an effort to prevent such a catastrophe.  Part of this was fueled by our love of a good doomsday scenario, but part of it was driven by the idea that the LHC was producing the highest energy collisions ever known. While the LHC is humanity’s highest energy particle collider, it doesn’t produce the highest energy collisions known. Much higher collisions are produced by cosmic rays.

Cosmic rays are high energy particles, typically protons, seen to impact the Earth’s atmosphere.  The energy of the LHC can accelerate a proton to an energy of about 7 TeV, or 7 trillion electron volts. An electron volt is the amount of energy a proton would get moving across an electric potential of one volt.  Seven trillion is a lot of energy for a proton, and means it it traveling at about 99.9999991% of the speed of light. That’s just 3 m/s shy of the ultimate speed limit.  Cosmic rays at TeV energies strike the Earth all the time. In a given square kilometer, one will strike every few seconds.  These cosmic rays aren’t nearly as efficient at producing exotic particles as the LHC, but they do produce a particle cascade as they strike our atmosphere.

The most powerful cosmic ray was observed in 1991. It was a proton with an energy of 400 million TeV. That’s such an astounding amount of energy that it has been nicknamed the Oh-My-God! particle.  At that energy, the proton was moving so close to the speed of light that if it trailed a photon for a year it would only be about 50 nanometers behind.  Because of time dilation, its experience of time was extraordinarily slowed relative to us. In traveling a million light years, it would only experience a couple minutes of time.

We still aren’t sure what produces these ultra-high energy cosmic rays. The leading candidates are active galactic nuclei and supernova remnants.  The Oh-My-God particle had so much energy that it was likely undeflected by things like galactic magnetic fields, so it was hoped that it could provide a clue as to its source. However in the direction of the OMG particle there isn’t anything that could produce such a high energy particle.

In 2015 the LHC plans to double particle energies to 14 TeV. That is still far less than cosmic particle accelerators, so there’s no need to worry about creating black holes or destroying the Earth through particle research.

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In an earlier post I wrote about the solar neutrino problem, and how it was resolved by recognizing neutrinos can change flavor.  The fact that neutrinos can do this means they cannot be traveling at the speed of light, and so they have mass.  Earlier theories of particle physics had assumed that neutrinos were massless, but we now know this assumption was wrong.  But while we now know the three flavors of neutrinos have mass, we don’t know what their masses are.  Now a new paper published in Physical Review Letters has given us an idea of their mass, and it does so by alleviating a bit of cosmic tension.

Galaxy clusters put an upper limit on neutrino mass.
Credit: Sloan Digital Sky Survey.

Moments after the big bang, the initial electrons, photons and neutrinos of the universe were produced.  This means the number of neutrinos in the universe is related to the number of photons.  We can calculate the photon number by observing the cosmic microwave background, so we have a good idea of the number of neutrinos in the universe.  If the neutrinos had a larger mass their gravity would affect the distribution of galaxies, specifically what is known as the Baryon Acoustic Oscillation, or BAO.  Galaxy surveys see no such effect, which suggests the total mass of all three neutrino types can be no more than 0.23 eV, or less than 2 millionths the mass of an electron.  Observations of neutrino oscillations also require that at least one flavor of neutrino has a mass of at least 0.04 eV.

Different measures of the universe give slightly different values for dark matter and dark energy. Credit: ESO.

The BAO is one way we can determine the structure of the universe, in particular the amount of dark matter and dark energy it has.  There are two other main ways, which are the cosmic microwave background (CMB) which is a measure of the remnant heat of the big bang, and the gravitational lensing of distant galaxies.  Each of these gives slightly different results for the various cosmological parameters that describe the universe.  Based on the cosmic microwave background, we would expect to see slightly less galaxy clustering than we actually observe.  The amount of gravitational lensing we see is less than expected from the two other measures.

Because neutrinos have mass and weakly interact with other particles, they are often brought up as a possible solution to dark matter.  However neutrinos have such a small mass that they cannot be the solution to the dark matter problem.  But neutrinos could ease some of the tension between our different cosmological measures.

In this new paper, the authors looked at how massive neutrinos would affect the cosmological measures we observe.  What they found was that massive neutrinos would work to increase the amount of galaxy clusters we see, and they would decrease the amount of gravitational lensing we observe.  So the presence of neutrinos would bring our different measures into better agreement.  The best agreement between these three measures is found when the total mass of all three neutrino types is  0.320 +/- 0.081 eV.

So it seems the mystery of neutrino mass can solve the observational tension that has been bugging cosmologists.

Paper: Battye RA, Moss A. Evidence for Massive Neutrinos from Cosmic Microwave Background and Lensing Observations. Phys. Rev. Lett. 2014;112(5)

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Now that the CERN results have been announced, what do we know?

1. We’ve discovered a particle at a level of about 5 sigma.
2. It has an energy (mass) of about 125 GeV (about 125 times heavier than a proton), which is in the range expected for the Higgs.
3. It appears to be a Higgs-type particle, but more work needs to be done to confirm it is indeed the Higgs.

Now that some of the graphs are being released you can get an idea of how hard it is to pin down the Higgs.  Below is a graph plotting the number of events per unit energy as a function of expected mass.  The dotted line is what we would expect if there  were no Higgs.  The small bump at around 126.5 GeV is the signature of the Higgs.  That small bump is (part of) what tells us the Higgs exists.

This is what modern “big science” looks like folks, and it’s pretty awesome.

So what now?

Now that we know the mass range of this particle, experiments can be tuned  specifically to that range to study the overall properties.  Efforts will focus on measuring those properties to confirm it is the Higgs as theory predicts.  Work will also focus on determining if there is only one type of Higgs (as the standard model predicts) or if there are multiple types of Higgs (as predicted by things such as string theory).

Note: This was written 4 July 2012 on Google+.  I’m posting it here just to have a copy.

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Tomorrow CERN is expected to announce the results of the search for something called the Higgs boson.  So just what is the Higgs boson, and why would finding (or not finding it) be such a big deal?  For that we have to cover a bit of background on particle physics.

You are likely familiar with the basic concept of atoms.   Atoms were once thought to be the building blocks of everything, and it was assumed that atoms were indivisible.  We now know that atoms are made up of a nucleus of protons and neutrons, surrounded by a cloud of electrons.  Often the electrons are pictured as orbiting the nucleus like planets, but that’s not really how it works.

As far as we know, electrons aren’t made up of anything smaller.  They are a type of elementary particle.  Protons and neutrons however do have parts, known as quarks.  Protons and neutrons are made of 3 quarks each, so they are not elementary particles.  The quarks are the elementary particles.

It turns out the electron and the quarks that make up protons and neutrons (known as up and down quarks) are not the only elementary particles there are.  There is also the neutrino, which has similarities to the electron but doesn’t have electric charge.  Electrons and neutrinos are part of a group of elementary particles known as leptons, and it turns out there are six known leptons.  It also turns out that there are six types of quarks (up, down, charm, strange, top and bottom).

So it seems that everything in the universe is built out of the 6 types of quarks and the 6 types of leptons.  Twelve elementary particles that are the building blocks of the universe.  Of course this raises the question of how these particles interact.  In Newton’s gravity, masses interact through the gravitational field, however in particle physics a field is produced by a type of particle known as a gauge boson.  There are four types of fields in the universe:  gravity, electromagnetism, weak nuclear and strong nuclear, which means there are four types of gauge bosons:  the graviton (which has never been observed directly), the photon, the W boson, and the gluon.  The basic idea is that particles move because of boson interactions.  So, for example, if an elementary particle has charge, then the gauge photons interact with it, which is why two charges attract or repel each other.

Given all that, what about mass?  You might say that since gravity is due to the graviton boson, gravitons must produce the effect of mass just like photons produce the effect of charge, and you’d be partly right.  However there are actually two properties of mass.  One is that masses attract each other gravitationally.  The other is that a mass resists changing its motion (known as inertia).  It is this second property that makes larger masses harder to move than smaller ones.  Gravitons determine how masses interact gravitationally, but gravitons don’t determine a particle’s inertia.  So there must be a fifth type of gauge boson that determines a particles inertial mass.  This new type of particle is known as the Higgs boson.

So why is it so important to find the Higgs boson?  It turns out that all the quarks, leptons and bosons are part of a theory known as the Standard Model.  If the standard model is correct, then the 12 types elementary particles are the only ones that exist, the four gauge bosons tell them how to move, and the Higgs boson determines their inertial masses.  The Higgs must exist, otherwise the standard model is wrong.  The theory also predicts that if the Higgs boson exists, the Large Hadron Collider at CERN must be able to observe its effects.  That means that either we find the Higgs boson (and the standard model is right) or we don’t find it (and the standard model is wrong).  Since the standard model is the foundation of all particle physics, proving it or disproving it is a huge deal.  As in Nobel prize big deal.  That is why everyone is so keyed up about the announcement tomorrow.

Of course there is a third possibility for tomorrow.  It could be that CERN announces the discovery of something, but something that isn’t the Higgs.  We’ll just have to wait and see.  Either way, once they announce things, I’ll try to write a post talking about what it means.

Note: This post was written on 3 July 2012.  It is posted here just to have a copy.

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When the Higgs result hits the internet tomorrow, look for the phrase “five sigma”.  The reason is that if the Higgs is observed at five sigma it will be officially discovered.  But what is a sigma and why is five the magic number?

Suppose you suspected a coin was not evenly balanced, say instead of an even chance of heads and tails, it was 55% heads and 45% tails.  If you only tossed a coin a few times, you might not notice anything amiss.  But if you did it 100 times you might see 53 heads and 47 tails, which makes you suspicious.  It is possible that the coin is fair, and by random chance you got slightly more heads than tails.  But if you keep tossing the coin (a thousand times, a million), and you continue to see about 55% heads and 45% tails, then it is more likely that the coin isn’t fair.

But the coin tosses are still random, so it is possible that the coin is fair, and you’ve just had bad luck in your results.  The question is how likely is it that the coin is fair.  For that you need statistics.  The sigma is a measure of how likely your 55/45 spit is due to random chance.  The bigger the sigma, the less likely your observation is to be random.

So if your result had a measure of 1 sigma, then there is about a 30% chance that your coin is still fair.  At 2 sigma there is only a 4% chance of being fair.  In particle physics the threshold for observation is 5 sigma.  At that level there is only 1 chance in 1.7 million that you’ve observed a false positive.  So if the Higgs is observed at 5 sigma, that means we are 99.99994% certain that it exists.

Which in science terms is “close enough”.

Note: This was written 3 July 2012 on Google+.  I’m posting it here just to have a copy.

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