A Quark Gluon Plasma Primer

Artist's rendition of a proton breaking down into free quarks after a critical temperature. Image credit Lawrence Berkeley National Laboratory.
Figure 1: Artist’s rendition of a proton breaking down into free quarks after a critical temperature. Image credit Lawrence Berkeley National Laboratory.

Quark gluon plasma, affectionately known as QGP or “quark soup”, is a big deal, attracting attention from particle, nuclear, and astrophysicists alike. In fact, scrolling through past ParticleBites, I was amazed to see that it hadn’t been covered yet! So consider this a QGP primer of sorts, including what exactly is predicted, why it matters, and what the landscape looks like in current experiments.

To understand why quark gluon plasma is important, we first have to talk about quarks themselves, and the laws that explain how they interact, otherwise known as quantum chromodynamics. In our observable universe, quarks are needy little socialites who can’t bear to exist by themselves. We know them as constituent particles in hadronic color-neutral matter, where the individual color charge of a single quark is either cancelled by its anticolor (as in mesons) or by two other differently colored quarks (as with baryons). But theory predicts that at a high enough temperature and density, the quarks can rip free of the strong force that binds them and become deconfined. This resulting matter is thus composed entirely of free quarks and gluons, and we expect it to behave as an almost perfect fluid. Physicists believe that in the first few fleeting moments after the Big Bang, all matter was in this state due to the extremely high temperatures. In this way, understanding QGP and how particles behave at the highest possible temperatures will give us a new insight into the creation and evolution of the universe.

The history of experiment with QGP begins in the 80s at CERN with the Super Proton Synchrotron (which is now used as the final injector into the LHC.) Two decades into the experiment, CERN announced in 2000 that it had evidence for a ‘new state of matter’; see Further Reading #3 for more information. Since then, the LHC and the Brookhaven Relativistic Heavy Ion Collider (RHIC) have taken up the search, colliding heavy lead or gold ions and producing temperatures on the order of trillions of Kelvin. Since then, both experiments have released results claiming to have produced QGP; see Figure 2 for a phase diagram that shows where QGP lives in experimental space.

Phases of QCD and the energy scales probed by experiment.
Phases of QCD and the energy scales probed by experiment.

All this being said, the QGP story is not over just yet. Physicists still want a better understanding of how this new matter state behaves; evidence seems to indicate that it acts almost like a perfect fluid (but when has “almost” ever satisfied a physicist?) Furthermore, experiments are searching to know more about how QGP transitions into a regular hadronic state of matter, as shown in the phase diagram. These questions draw in some other kinds of physics, including statistical mechanics, to examine how bubble formation or ‘cavitation’ occurs when chemical potential or pressure is altered during QGP evolution (see Further Reading 5). In this sense, observation of a QGP-like state is just the beginning, and heavy ion collision experiments will surely be releasing new results in the future.


Further Reading:

  1. “The Quark Gluon Plasma: A Short Introduction”, arXiv hep-ph 1101.3937
  2. “Evidence for a New State of Matter”, CERN press release
  3. “Hot stuff: CERN physicists create record-breaking subatomic soup”, Nature blog
  4. “The QGP Discovered at RHIC”, arXiv nucl-th 0403.032
  5. “Cavitation in a quark gluon plasma with finite chemical potential and several transport coefficients”, arXiv hep-ph 1505.06335

How much top quark is in the proton?

We know that protons are made up of two up quarks and a down quark. Each only weigh a few MeV—the rest of the proton mass comes from the strong force binding energy coming from gluon exchange. When we collider protons at high energies, these partons interact with each other to produce other particles. In fact, the LHC is essentially a gluon collider. Recently, however, physicists have been asking, “How much top quark is there in the proton?

Presenting: Top-Quark Initiated Processes at High-Energy Hadron Colliders
Authors: Tao Han, Joshua Sayre, Susanne Westhoff (Pittsburgh U.)
Reference: 1411.2588JHEP 1504 (2015) 145

In fact, at first glance, this is a ridiculous question. The top quark is 175 times heavier than the proton! How does it make sense that there are top quarks “in” the proton?

The proton (1 GeV mass) doesn't seem to have room for any top quark component (175 GeV mass).
The proton (1 GeV mass) doesn’t seem to have room for any top quark component (175 GeV mass).

The discussion is based on preliminary plans to build a 100 TeV collider, though there’s a similar story for b quarks (5 times the mass of the proton) at the LHC.

Before we define what we mean by treating the top as a parton, we should define what we mean by proton! We can describe the proton constituents by a series of parton distribution functions (pdf): these tell us the probability of that you’ll interact with a particular piece of the proton. These pdfs are energy-dependent: at high energies, it turns out that you’re more likely to interact with a gluon than any of the “valence quarks.” At sufficiently high energies, these gluons can also produce pairs of heavier objects, like charm, bottom, and—at 100 TeV—even top quarks.

But there’s an even deeper sense in which these heavy quarks have a non-zero parton distribution function (i.e. “fraction of the proton”): it turns out that perturbation theory breaks down for certain kinematic regions when a gluon splits into quarks. That is to say, the small parameters we usually expand in become large.

Theoretically, a technique to keep the expansion parameter small leads to an interpretation of this “high-energy gluon splitting into heavy quarks inside the proton” process as the proton having some intrinsic heavy quark content. This is called perturbative QCD, the key equation known as DGLAP.

High energy gluon splittings can yield top quarks (lines with arrows). When one of these top quarks is collinear with the beam (pink, dashed), the calculation becomes non-perturbative.
High energy gluon splittings can yield top quarks (lines with arrows). When one of these top quarks is collinear with the beam (pink, dashed), the calculation becomes non-perturbative. Maintaining the perturbation expansion parameter leads on to treat the top quark as a constituent of the proton. Solid blue lines are not-collinear and are well-behaved.

In the cartoon above: physically what’s happening is that a gluon in the proton splits into a top and anti-top. When one of these is collinear (i.e. goes down the collider beamline), the expansion parameter blows up and the calculation misbehaves. In order to maintain a well behaved perturbation theory, DGLAP tells us to pretend that instead of a top/anti-top pair coming from a gluon splitting, one can treat these as a top that lives inside the high-energy proton.

A gluon splitting that gives a non-perturvative top can be treated as a top inside the proton.
A gluon splitting that gives a non-perturvative top can be treated as a top inside the proton.

This is the sense in which the top quark can be considered as a parton. It doesn’t have to do with whether the top “fits” inside a proton and whether this makes sense given the mass—it boils down to a trick to preserve perturbativity.

One can recast this as the statement that the proton (or even fundamental particles like the electron) look different when you probe them at different energy scales. One can compare this story to this explanation for why the electron doesn’t have infinite electromagnetic energy.

The authors of 1411.2588 a study of the sensitivity a 100 TeV collider to processes that are produced from fusion of top quarks “in” each proton. With any luck, such a collider may even be on the horizon for future generations.

More Dark Matter in Dwarf Galaxies

Hi Particlebiters,

This is part 2 in my “series” on dark matter in dwarf galaxies. In my previous post, I explained a bit about WIMP-like dark matter and why we look for its signature in these particular type of small galaxies (dsphs) that are orbiting the Milky Way. About 2 months ago, the Dark Energy Survey (DES) collaboration released its first year of data to the public.  Similar to SDSS, DES is also a surveying the optical-near infrared sky using the 4 m Victor M. Blanco Telescope at Cerro Tololo Inter-American Observatory in Chile.   The important distinction between SDSS and DES is that while SDSS observes the northern galactic latitudes, DES observes southern galactic latitudes. Since this is a whole new region of observation, we expect a lot of new exciting things to come out of the data… And sure enough exciting things came. Eight new dsphs candidates were discovered and published in the first data release orbiting. Dwarf galaxies are very old (> 13 billion years old) and have little gas, dust and star formation. I say candidates, because to confirm that these dsph candidates not something else, follow-up observations with other telescopes have to be done.


However, that doesn’t mean that we (we being everyone since the Fermi data is public) can’t have a look at these potential dark matter targets. On the same day that the new DES candidate dsphs were released, the Fermi-LAT team had a look. Of the eight candidates, most were far away (~100 kpc or ~300k light years). This distance makes looking for dark matter difficult because a signal will be very weak. However, there was one candidate that was only 32 kpc away (DES J0335.6-5403 or Reticulum II), making it the most interesting search target. You can see the counts map of Reticulum II on the right.




The results (on the left) showed that there was no clear WIMP-like dark matter signature coming from any of the candidates (shucks!!). However, the closest target wasn’t totally boring. Another team found a small excess (~2 sigma) in Reticulum II. When the Fermi-LAT team compared analysis methods, we found that there results were optimistic, yet not inconsistent with ours. This got the New York Times’s writer Dennis Overbye excited :).



The good news is that DES is going to continue for at least 4 more years, which means we’ll have many more opportunities to search for dark matter in dsphs. What we need to find is nearby dsphs. And even more exciting, the Large Synoptic Survey Telescope (LSST) will start taking in the 2020s. This telescope will have access to ~half of the sky (more on the LSST in a future post ;)). This will give us many more targets in the years to come, so stay tuned!


Uncovering a Higgs Hiding Behind Backgrounds

Hello particle munchers,

Figure 1: Monsieur Higgs boson hiding behind a background.

Last time I discussed the Higgs boson decay into photons, i.e. `shining light on the Higgs boson‘. This is a followup discussing more generally the problem of uncovering a Higgs boson which is hiding buried behind what can often be a large background (see Figure 1).

Perhaps the first question to ask is, what the heck is a background? Well, basically a background is anything that we `already know about’. In this case, this means the well understood Standard Model (SM) processes which do not involve a Higgs boson (which in this case is our `signal’), but can nevertheless mimic one of the possible decays of the Higgs. For most of these processes, we have very precise theoretical predictions in addition to previous experimental data from the LEP and Tevatron experiments (and others) which previously searched for the Higgs boson. So it is in reference to these non-Higgs SM processes when we use the term `background’.

As discussed in my previous post, the Higgs can decay to a variety of combinations of SM particles, which we call `channels’. Each of these channels has its own corresponding background which obscures the presence of a Higgs. For some channels the backgrounds are huge. For instance the background for a Higgs decaying to a pair of bottom quarks is so large (due to QCD) that, despite the fact this is the dominant decay channel (about 60% of Higgs’ decay to bottom quarks at 125 GeV), this channel has yet to be observed.

This is in contrast to the Higgs decay to four charged leptons (specifically electrons and muons) channel. This decay (mediated by a pair of virtual Z bosons) was one of the first discovery channels of the Higgs at the LHC despite the fact that roughly only one in every 10,000 Higgs bosons decays to four charge leptons. This is because this channel has a small background and is measured with very high precision. This high precision allows LHC experiments to scan over a range of energies in very small increments or `windows’. Since the background is very small, the probability of observing a background event in any given window is tiny. Thus, if an excess of events is seen in a particular window, this is an indication that there is a non background process occurring at that particular energy.

Figure 2: The energy spectrum of a Higgs decaying to four charged leptons (red) and its associated background (blue).

This is how the Higgs was discovered in the decay to four charged leptons at around 125 GeV. This can be seen in Figure 2 where in the window around the Higgs signal (shown in red) we see the background (shown in blue) is very small. Thus, once about a dozen events were observed at around 125 GeV, this was already enough evidence for experiments at the LHC to be able to claim discovery of the long sought after monsieur Higgs boson.

 Further Reading:

Seeking and Studying the Standard Model Higgs Particle

Decays of the Standard Model Higgs