Quantum Diaries

Right now the LHC is about to start a short run with proton-proton collisions at a center of mass energy 2.76 TeV.  This is lower than what we ran last year and is a special request from the heavy ion physicists.  So you’ve heard a lot about why the particle physicists want to go to higher energy.  But why do we heavy ion physicists want to go to lower energy?

We want a reference for our lead-lead collisions.  If nucleus-nucleus collisions were nothing but a bunch of proton-proton collisions, what we measure in lead-lead collisions should be just some constant times what we measure in proton-proton collisions.  This is a bit simplistic, but it’s a pretty good start.  A lot of our measurements use proton-proton collisions as a reference and look for differences between proton-proton collisions and lead-lead collisions.  For instance, in the paper I discussed here we looked at the distribution of particles as a function of their momenta in lead-lead collisions and compared that to what we observed in proton-proton collisions.  For this paper we used the data from proton-proton collisions at 900 GeV and at 7 TeV to extrapolate to what we’d expect at 2.76 TeV, the same energy per nucleon as our lead-lead collisions.  As discussed here our models for proton-proton collisions are pretty good but they get some of the details wrong – and miss some features like this.   Since we depend on models to extrapolate to 2.76 TeV, we have greater uncertainty in our measurements than we would have if we had data at 2.76 TeV.  The LHC can go down to 2.76 TeV and what we need 2.76 TeV proton-proton data doesn’t require as many statistics (as many total proton-proton collisions) as what the particle physicists need to look for things like the Higgs.  So we’re having a short run with proton-proton collisions at a lower energy because it will significantly help the heavy ion physics program.  (We’ll also get some core physics measurements out of the 2.76 TeV proton-proton data, but like the paper I discussed here, these will refine our understanding but not dramatically change our understanding of proton-proton collisions.)  I hope you’re as excited as I am!

I am overdue for a blog post because I have been way too busy lately.  I got an email from an elementary schooler, Jacob, asking about the QGP so I thought instead of replying privately I’d reply here since it may be of general interest.  The questions are from Jacob.

What is QGP going to be used for in the future when it is better controlled?

Right now we don’t think the QGP has any practical applications.  We’re studying it because we want to understand the universe in general and nuclear matter in particular.  Shortly after the Big Bang, we think that the universe went through a Quark Gluon Plasma phase.  By understanding the QGP better, we may understand how the universe expanded better.  When we do basic research, we don’t usually know what impact it will have.  What we know by looking at history is that basic research eventually leads to benefits to humanity – but we’re very bad at predicting what those benefits will be.  When Mendel studied genetics of plants, he never imagined that genetic studies would lead to all of the improvements in medical care we have now.  Einstein developed his theory of gravity not so that we could send satellites into space or so that we could all have GPS in our cars or get better TV reception – he was motivated by simple curiosity and a desire to understand our universe better.  We are still reaping new benefits from quantum mechanics, developed in the early 20th century – we now have light emitting diodes (LEDs) in traffic lights and flashlights and while LEDs existed when I was your age, they weren’t nearly as widespread, as cheap, or available in so many colors.  So it takes a long time to see the benefits of basic research.

So we don’t know what applications this research will have in the future.  That said, there are a lot of spin off benefits to this research.  In high energy physics, we are always building the fastest and most precise detectors possible.  To do this we often have to develop and test new detector technologies.  Once we’ve developed the technology, these detectors can be used elsewhere too.  Particle detectors are used in hospitals in x-ray and MRI machines.  They are used in chemical and biomedical research to study the images of proteins and the structures of solids.  They are used in national security for detecting radioactive materials.

Basic research moves the boundary of what is possible.  Once we have done that, there are a lot of benefits.  But since we’re working on doing things that have never been done and studying things never studied before, we can’t predict exactly how it will be useful.  Put another way, if we knew what would happen, we wouldn’t call it an experiment.

What attributes does it have that other matter does not have?

This is a difficult question to answer as worded – it depends on what you mean by “attributes”.  When I think of the properties of a particular form of matter, I think about its density, its opacity to different probes (like if you shine light through it does the light come out the other side?)…  All forms of matter have a density.  So I’m going to answer a slightly different question – what makes a QGP unique?  What makes the QGP unique (among the forms of matter we’ve studied in the laboratory) is that the quarks and gluons interact through the strong force.  There are four fundamental forces in nature

1. Gravitation
2. Electromagnetism
3. Weak interaction
4. Strong interaction

The first two are the most familiar.  Gravity is the reason why you stay on the ground instead of floating through the air.  It’s also the reason the Earth orbits the Sun.  The electromagnetic force is ultimately responsible for basically every other force you feel or see.  When you sit in a chair, the reason you don’t fall through the chair is ultimately due to interactions between your atoms and the atoms of the chair.  It’s also behind light and electricity.  It’s how your microwave and your TV work.  The most familiar thing we can attribute to the electroweak decay is beta decay – a particular kind of decay of a nucleus.   The strong force is what holds nuclei together.  If we only had the electromagnetic force, the protons in the nucleus would not be bound.

So a QGP is a liquid of quarks and gluons bound together by the strong force.  Water molecules, for instance, primarily interact through the electromagnetic force.  The properties of water are determined by the way water molecules interact through the electromagnetic force.  To understand the QGP, we have to understand how quarks and gluons interact through the strong force.  This turns out to be a very difficult computational problem.  But by studying the QGP, we can try to calculate what we would expect and then compare what we expect from our theories to what we see in the laboratory.

In addition to that, it is the hottest, densest form of matter ever created in the laboratory.  And it appears to have the lowest viscosity of any form of matter ever created in the laboratory.  Viscosity is a way of measuring how much a fluid resists flowing.  Honey, for instance, is much more viscous than water.

How will QGP affect modern or future physics?

I don’t know exactly.  It depends on what we learn.  Already we’ve learned a lot about relativistic fluids – where the individual particles in the fluid are traveling close to the speed of light.  As I said in the first answer, we don’t know exactly what we’ll learn – because if we did, we wouldn’t call it an experiment. One thing I hope – and maybe you can help me out here – is that we’ll inspire the next generation to go into science, math and engineering.

Also, what state of matter is it?  I know that it is called plasma but I’ve also read that it is very similar to both liquid and gas.

A QGP is a new state of matter.  We believe it is a liquid – indeed, a liquid that probably has the lowest viscosity of anything we’ve ever measured.  We thought it’d be a gas, but it turned out to be a liquid.  Here I have a post describing what we know about the QGP and its phase diagram.

I also could not verify what temperature it occurs at because there is so much different information on the internet.

The reason what you find on the internet is somewhat unclear is that the answer is somewhat unclear.  First, it doesn’t exist at just one temperature.  Think about water.  Water can be cold, warm, hot, etc.  It depends.  There’s a temperature where ice melts and becomes water and below that you can’t have water.  That temperature is called the melting point.  But then once you have water, you can heat it up and you have to heat it up a lot before it boils and becomes a gas.  That also occurs at a special temperature – the boiling point.  The problem is, these temperatures depend on pressure and volume.  Water boils at a lower temperature at high altitude.  Analogously, we have a melting point and a boiling point for the QGP.  We think the melting point at the baryochemical potential at RHIC is about 170 MeV – but there’s a fairly large uncertainty in that number.  We think we’re well above that at RHIC and we’ll be even further above it at the LHC (but we haven’t yet had enough time to analyze the data at the LHC to say how hot it is). This gets to a crucial issue – we don’t have a thermometer to measure a QGP.  If you put a thermometer like the one you have in your house into a vat of QGP (if we could ever create that much of it) it’d melt.  So we have to come up with other ways of measuring the temperature.  We can look at the energies of particles created in the collision, for instance.  But it takes more work than just using a thermometer.

Many thanks to Jacob for the great questions!