## Cosmic Microwave Background: The Role of Particles in Astrophysics

Over the past decade, a new trend has been emerging in physics, one that is motivated by several key questions: what do we know about the origin of our universe? What do we know about its composition? And how will the universe evolve from here? To delve into these questions naturally requires a thorough examination of the universe via the astrophysics lens. But studying the universe on a large scale alone does not provide a complete picture. In fact, it is just as important to see the universe on the smallest possible scales, necessitating the trendy and (fairly) new hybrid field of particle astrophysics. In this post, we will look specifically at the cosmic microwave background (CMB), classically known as a pillar of astrophysics, within the context of particle physics, providing a better understanding of the broader questions that encompass both fields.

Essentially, the CMB is just what we see when we look into the sky and we aren’t looking at anything else. Okay, fine. But if we’re not looking at something in particular, why do we see anything at all? The answer requires us to jump back a few billion years to the very early universe.

Immediately after the Big Bang, it was impossible for particles to form atoms without immediately being broken apart by constant bombardment from stray photons. About 380,000 thousand years after the Big Bang, the Universe expanded and cooled to a temperature of about 3,000 K, allowing the first formation of stable hydrogen atoms. Since hydrogen is electrically neutral, the leftover photons could no longer interact, meaning that at that point their paths would remain unaltered indefinitely. These are the photons that we observe as CMB; Figure 1 shows this idea diagrammatically below. From our present observation point, we measure the CMB to have a temperature of about 2.76 K.

Since this radiation has been unimpeded since that specific point (known as the point of ‘recombination’), we can think of the CMB as a snapshot of the very early universe. It is interesting, then, to examine the regularity of the spectrum; the CMB is naturally not perfectly uniform, and the slight temperature variations can provide a lot of information about how the universe formed. In the early primordial soup universe, slight random density fluctuations exerted a greater gravitational pull on their surroundings, since they had slightly more mass. This process continues, and very large dense patches occur in an otherwise uniform space, heating up the photons in that area accordingly. The Planck satellite, launched in 2009, provides some beautiful images of the temperature anisotropies of the universe, as seen in Figure 2. Some of these variations can be quite severe, as in the recently released results about a supervoid aligned with an especially cold spot in the CMB (see Further Reading, item 4).

So what does this all have to do with particles? We’ve talked about a lot of astrophysics so far, so let’s tie it all together. The big correlation here is dark matter. The CMB has given us strong evidence that our universe has a flat geometry, and from general relativity, this provides restrictions on the mass, energy, and density of the universe. In this way, we know that atomic matter can constitute only 5% of the universe, and analysis of the peaks in the CMB gives an estimate of 26% for the total dark matter presence. The rest of the universe is believed to be dark energy (see Figure 3).

Both dark matter and dark energy are huge questions in particle physics that could be the subject of a whole other post. But the CMB plays a big role in making our questions a bit more precise. The CMB is one of several pieces of strong evidence that require the existence of dark matter and dark energy to justify what we observe in the universe. Some potential dark matter candidates include weakly interacting massive particles (WIMPs), sterile neutrinos, or the lightest supersymmetric particle, all of which bring us back to particle physics for experimentation. Dark energy is not as well understood, and there are still a wide variety of disparate theories to explain its true identity. But it is clear that the future of particle physics will likely be closely tied to astrophysics, so as a particle physicist it’s wise to keep an eye out for new developments in both fields!

1. The Cosmic Cocktail: Three Parts Dark Matter”, Katherine Freese
2. “Physics of the cosmic microwave background anistropy”, from the arXiv:astro-ph
3. Summary of dark matter vs. dark energy and other resources from NASA
4. Summary of the supervoid aligned with a cold spot in the CMB, Royal Astronomical Society monthly notices

## A Tau Neutrino Runs into a Candy Shop…

We recently discussed some curiosities in the data from the IceCube neutrino detector. This is a follow up Particle Bite on some of the sugary nomenclature IceCube uses to characterize some of its events.

As we explained previously, IceCube is a gigantic ultra-high energy cosmic neutrino detector in Antarctica. These neutrinos have energies between 10-100 times higher than the protons colliding at the Large Hadron Collider, and their origin and nature are largely a mystery. One thing that IceCube can tell us about these neutrinos is their flavor composition; see e.g. this post for a crash course in neutrino flavor.

When neutrinos interact with ambient nuclei through a W boson (charged current interactions), the following types of events might be seen:

I refer you to this series of posts for a gentle introduction to the Feynman diagrams above. The key is that the high energy neutrino interacts with an nucleus, breaking it apart (the remnants are called X above) and ejecting a high energy charged lepton which can be used to identify the flavor of the neutrino.

• Muons travel a long distance and leave behind a trail of Cerenkov radiation called a track.
• Electrons don’t travel as far and deposit all of their energy into a shower. These are also sometimes called cascades because of the chain of particles produced in the ‘bang’.
• Taus typically leave a more dramatic signal, a double bang, when the tau is formed and then subsequently decays into more hadrons (X’ above).

In fact, the tau events can be further classified depending on how this ‘double bang’ is resolved—and it seems like someone was playing a popular candy-themed mobile game when naming these:

In this figure from the TeVPA 2 conference proceedings, we find some silly classifications of what tau events look like according to their energy:

• Lollipop: The tau is produced outside the detector so that the first ‘bang’ isn’t seen. Instead, there’s a visible track that leads to the second (observable) bang. The track is the stick and the bang is the lollipop head.
• Inverted lollipop: Similar to the lollipop, except now the first ‘bang’ is seen in the detector but the second ‘bang’ occurs outside the detector and is not observed.
• Sugardaddy: The tau is produced outside the detector but decays into a muon inside the detector. This looks almost like a muon track except that the tau produces less Cerenkov light so that one can identify the point where the tau decays into a muon.
• Double pulse: While this isn’t candy-themed, it’s still very interesting. This is a double bang where the two bangs can’t be distinguished spatially. However, since one bang occurs slightly after the other, one can distinguish them in the time: it’s a “double bang” in time rather than space.
• Tautsie pop: This is a low energy version of the sugardaddy where the shower-to-track energy is used to discriminate against background.

While the names may be silly, counting these types of events in IceCube is one of the exciting frontiers of flavor physics. And while we might be forgiven for thinking that neutrino physics is all about measuring very `small’ things—let me share the following graphic from Francis Halzen’s recent talk at the AMS Days workshop at CERN, overlaying one of the shower events over Madison, Wisconsin to give a sense of scale:

## The Glashow Resonance on Ice

Are cosmic neutrinos trying to tell us something, deep in the Antarctic ice?

Presenting:

“Glashow resonance as a window into cosmic neutrino sources,”
by Barger, Lu, Learned, Marfatia, Pakvasa, and Weiler
Phys.Rev. D90 (2014) 121301 [1407.3255]

Related work: Anchordoqui et al. [1404.0622], Learned and Weiler [1407.0739], Ibe and Kaneta [1407.2848]

The IceCube Neutrino Observatory is a gigantic neutrino detector located in the Antarctic. Like an iceberg, only a small fraction of the lab is above ground: 86 strings extend to a depth of 2.5 kilometers into the ice, with each string instrumented with 60 detectors.

These detectors search ultra high energy neutrinos by looking for Cerenkov radiation as they pass through the ice. This is really the optical version of a sonic boom. An example event is shown above, where the color and size of the spheres indicate the strength of the Cerenkov signal in each detector.

IceCube has released data for its first three years of running (1405.5303) and has found three events with very large energies: 1-2 peta-electron-volts: that’s ten thousand times the mass of the Higgs boson. In addition, there’s a spectrum of neutrinos in the 10-1000 TeV range.

These ultra high energy neutrinos are believed to originate from outside our galaxy through processes involving particle acceleration by black holes. One expects the flux of such neutrinos to go as a power law of the energy, $\Phi \sim E^{-\alpha}$ where $\alpha = 2$ is a estimate from certain acceleration models. The existence of the three super high energy events at the PeV scale has led some people to think about a known deviation from the power law spectrum: the Glashow resonance. This is the sharp increase in the rate of neutrino interactions with matter coming from the resonant production of W bosons, as shown in the Feynman diagram to the left.

The Glashow resonance sticks out like a sore thumb in the spectrum. The position of the resonance is set by the energy required for an electron anti-neutrino to hit an electron at rest such that the center of mass energy is the W boson mass.

If you work through the math on the back of an envelope, you’ll find that the resonance occurs for incident electron anti-neutrinos with an energy of  6.3 PeV; see figure to the leftt. This is “right around the corner” from the 1-2 PeV events already seen, and one might wonder whether it’s significant that we haven’t seen anything.

The authors of [1407.3255] have found that the absence of Glashow resonant neutrino events in IceCube is not yet a bona-fide “anomaly.” In fact, they point out that the future observation or non-observation of such neutrinos can give us valuable hints about the hard-to-study origin of these ultra high energy neutrinos. They present  six simple particle physics scenarios for how high energy neutrinos can be formed from cosmic rays that were accelerated by astrophysical accelerators like black holes. Each of these processes predict a ratio of neutrino and anti-neutrinos flavors at Earth (this includes neutrino oscillation effects over long distances). Since the Glashow resonance only occurs for electron anti-neutrinos, the authors point out that the appearance or non-appearance of the Glashow resonance in future data can constrain what types of processes may have produced these high energy neutrinos.

In more speculative work, the authors of [1404.0622] suggest that the absence of Glashow resonance events may even suggest some kind of new physics that impose a “speed limit” on neutrinos propagating through space that prevents neutrinos from ever reaching 6.3 PeV (see top figure).