## The CMB sheds light on galaxy clusters: Observing the kSZ signal with ACT and BOSS

Article: Detection of the pairwise kinematic Sunyaev-Zel’dovich effect with BOSS DR11 and the Atacama Cosmology Telescope
Authors: F. De Bernardis, S. Aiola, E. M. Vavagiakis, M. D. Niemack, N. Battaglia, and the ACT Collaboration
Reference: arXiv:1607.02139

Editor’s note: this post is written by one of the students involved in the published result.

Like X-rays shining through your body can inform you about your health, the cosmic microwave background (CMB) shining through galaxy clusters can tell us about the universe we live in. When light from the CMB is distorted by the high energy electrons present in galaxy clusters, it’s called the Sunyaev-Zel’dovich effect. A new 4.1σ measurement of the kinematic Sunyaev-Zel’dovich (kSZ) signal has been made from the most recent Atacama Cosmology Telescope (ACT) cosmic microwave background (CMB) maps and galaxy data from the Baryon Oscillation Spectroscopic Survey (BOSS). With steps forward like this one, the kinematic Sunyaev-Zel’dovich signal could become a probe of cosmology, astrophysics and particle physics alike.

### The Kinematic Sunyaev-Zel’dovich Effect

It rolls right off the tongue, but what exactly is the kinematic Sunyaev-Zel’dovich signal? Galaxy clusters distort the cosmic microwave background before it reaches Earth, so we can learn about these clusters by looking at these CMB distortions. In our X-ray metaphor, the map of the CMB is the image of the X-ray of your arm, and the galaxy clusters are the bones. Galaxy clusters are the largest gravitationally bound structures we can observe, so they serve as important tools to learn more about our universe. In its essence, the Sunyaev-Zel’dovich effect is inverse-Compton scattering of cosmic microwave background photons off of the gas in these galaxy clusters, whereby the photons gain a “kick” in energy by interacting with the high energy electrons present in the clusters.

The Sunyaev-Zel’dovich effect can be divided up into two categories: thermal and kinematic. The thermal Sunyaev-Zel’dovich (tSZ) effect is the spectral distortion of the cosmic microwave background in a characteristic manner due to the photons gaining, on average, energy from the hot (~107 – 108 K) gas of the galaxy clusters. The kinematic (or kinetic) Sunyaev-Zel’dovich (kSZ) effect is a second-order effect—about a factor of 10 smaller than the tSZ effect—that is caused by the motion of galaxy clusters with respect to the cosmic microwave background rest frame. If the CMB photons pass through galaxy clusters that are moving, they are Doppler shifted due to the cluster’s peculiar velocity (the velocity that cannot be explained by Hubble’s law, which states that objects recede from us at a speed proportional to their distance). The kinematic Sunyaev-Zel’dovich effect is the only known way to directly measure the peculiar velocities of objects at cosmological distances, and is thus a valuable source of information for cosmology. It allows us to probe megaparsec and gigaparsec scales – that’s around 30,000 times the diameter of the Milky Way!

### Measuring the kSZ Effect

To make the measurement of the kinematic Sunyaev-Zel’dovich signal, the Atacama Cosmology Telescope (ACT) collaboration used a combination of cosmic microwave background maps from two years of observations by ACT. The CMB map used for the analysis overlapped with ~68000 galaxy sources from the Large Scale Structure (LSS) DR11 catalog of the Baryon Oscillation Spectroscopic Survey (BOSS). The catalog lists the coordinate positions of galaxies along with some of their properties. The most luminous of these galaxies were assumed to be located at the centers of galaxy clusters, so temperature signals from the CMB map were taken at the coordinates of these galaxy sources in order to extract the Sunyaev-Zel’dovich signal.

While the smallness of the kSZ signal with respect to the tSZ signal and the noise level in current CMB maps poses an analysis challenge, there exist several approaches to extracting the kSZ signal. To make their measurement, the ACT collaboration employed a pairwise statistic. “Pairwise” refers to the momentum between pairs of galaxy clusters, and “statistic” indicates that a large sample is used to rule out the influence of unwanted effects.

Here’s the approach: nearby galaxy clusters move towards each other on average, due to gravity. We can’t easily measure the three-dimensional momentum of clusters, but the average pairwise momentum can be estimated by using the line of sight component of the momentum, along with other information such as redshift and angular separations between clusters. The line of sight momentum is directly proportional to the measured kSZ signal: the microwave temperature fluctuation which is measured from the CMB map. We want to know if we’re measuring the kSZ signal when we look in the direction of galaxy clusters in the CMB map. Using the observed CMB temperature to find the line of sight momenta of galaxy clusters, we can estimate the mean pairwise momentum as a function of cluster separation distance, and check to see if we find that nearby galaxies are indeed falling towards each other. If so, we know that we’re observing the kSZ effect in action in the CMB map.

For the measurement quoted in their paper, the ACT collaboration finds the average pairwise momentum as a function of galaxy cluster separation, and explores a variety of error determinations and sources of systematic error. The most conservative errors based on simulations give signal-to-noise estimates that vary between 3.6 and 4.1.

The ACT and BOSS results are an improvement on the 2012 ACT detection, and are comparable with results from the South Pole Telescope (SPT) collaboration that use galaxies from the Dark Energy Survey. The ACT and BOSS measurement represents a step forward towards improved extraction of kSZ signals from CMB maps. Future surveys such as Advanced ACTPol, SPT-3G, the Simons Observatory, and next-generation CMB experiments will be able to apply the methods discussed here to improved CMB maps in order to achieve strong detections of the kSZ effect. With new data that will enable better measurements of galaxy cluster peculiar velocities, the pairwise kSZ signal will become a powerful probe of our universe in the years to come.

### Implications and Future Experiments

One interesting consequence for particle physics will be more stringent constraints on the sum of the neutrino masses from the pairwise kinematic Sunyaev-Zel’dovich effect. Upper bounds on the neutrino mass sum from cosmological measurements of large scale structure and the CMB have the potential to determine the neutrino mass hierarchy, one of the next major unknowns of the Standard Model to be resolved, if the mass hierarchy is indeed a “normal hierarchy” with ν3 being the heaviest mass state. If the upper bound of the neutrino mass sum is measured to be less than 0.1 eV, the inverted hierarchy scenario would be ruled out, due to there being a lower limit on the mass sum of ~0.095 eV for an inverted hierarchy and ~0.056 eV for a normal hierarchy.

Forecasts for kSZ measurements in combination with input from Planck predict possible constraints on the neutrino mass sum with a precision of 0.29 eV, 0.22 eV and 0.096 eV for Stage II (ACTPol + BOSS), Stage III (Advanced ACTPol + BOSS) and Stage IV (next generation CMB experiment + DESI) surveys respectively, with the possibility of much improved constraints with optimal conditions. As cosmic microwave background maps are improved and Sunyaev-Zel’dovich analysis methods are developed, we have a lot to look forward to.

## Monojet Dark Matter Searches at the LHC

Now is a good time to be a dark matter experiment. The astrophysical evidence for its existence is almost undeniable (such as gravitational lensing and the cosmic microwave background; see the “Further Reading” list if you want to know more.) Physicists are pulling out all the stops trying to pin DM down by any means necessary.

However, by its very nature, it is extremely difficult to detect; dark matter is called dark because it has no known electromagnetic interactions, meaning it doesn’t couple to the photon. It does, however, have very noticeable gravitational effects, and some theories allow for the possibility of weak interactions as well.

While there are a wide variety of experiments searching for dark matter right now, the scope of this post will be a bit narrower, focusing on a common technique used to look for dark matter at the LHC, known as ‘monojets’. We rely on the fact that a quark-quark interaction could actually produce dark matter particle candidates, known as weakly interacting massive particles (WIMPs), through some unknown process. Most likely, the dark matter would then pass through the detector without any interactions, kind of like neutrinos. But if it doesn’t have any interactions, how do we expect to actually see anything? Figure 1 shows the overall Feynman diagram of the interaction; I’ll explain how and why each of these particles comes into the picture.

The answer is a pretty useful metric used by particle physicists to measure things that don’t interact, known as ‘missing transverse energy’ or MEt. When two protons are accelerated down the beam line, their initial momentum in the transverse plane is necessarily zero. Your final state can have all kinds of decay products in that plane, but by conversation of momentum, their magnitude and direction have to add up to zero in the end. If you add up all your momentum in the transverse plane and get a non-zero value, you know the remaining momentum was taken away by these non-interacting particles. In our case, dark matter is going to be the missing piece of the puzzle.

Now our search method is to collide protons and look for… well, nothing. That’s not an easy thing to do. So let’s add another particle to our final state: a single jet that was radiated off one of the initial protons. This is a pretty common occurrence in LHC collisions, so we’re not ruining our statistics. But now we have an extra handle on selecting these events, since that radiated single jet is going to recoil off the missing energy in the final state.

An actual event display from the ATLAS detector is shown in Figure 2 (where the single jet is shown in yellow in the transverse plane of the detector).

No results have been released yet from the monojet groups with the 13 and 14 TeV data. However, the same method was using in 2012-2013 LHC data, and has provided some results that can be compared to current knowledge. Figure 3 shows the WIMP-nucleon cross section as a function of WIMP mass from CMS at the LHC (EPJC 75 (2015) 235), overlaid with other exclusions from a variety of experiments. Anything above/right of these curves is the excluded region.

From here we can see that the LHC can provide better sensitivity to low mass regions with spin dependent couplings to DM. It’s worth giving the brief caveat that these comparisons are extremely model dependent and require a lot of effective field theory; notes on this are also given in the Further Reading list. The current results look pretty thorough, and a large region of the WIMP mass seems to have been excluded. Interestingly, some searches observe slight excesses in regions that other experiments have ruled out; in this way, these ‘exclusions’ are not necessarily as cut and dry as they may seem. The dark matter mystery is still far from a resolution, but the LHC may be able to get us a little bit closer.

With all this incoming data and such a wide variety of searches ongoing, it’s likely that dark matter will remain a hot topic in physics for decades to come, with or without a discovery. In the words of dark matter pioneer Vera Rubin, “We have peered into a new world, and have seen that it is more mysterious and more complex than we had imagined. Still more mysteries of the universe remain hidden. Their discovery awaits the adventurous scientists of the future. I like it this way.“

• Links to the CMS and ATLAS 8 TeV monojet analyses
• “Dark Matter: A Primer”, arXiv hep-ph 1006.2483
• Effective Field Theory notes
• “Simplified Models for Dark Matter Searches at the LHC”, arXiv hep-ph 1506.03116
• “Search for dark matter at the LHC using missing transverse energy”, Sarah Malik, CMS Collaboration Moriond talk

## Dark Photons from the Center of the Earth

Presenting: Dark Photons from the Center of the Earth
Author: J. Feng, J. Smolinsky, P. Tanedo (disclosure: blog post is by an author on the paper)
Reference: arXiv:1509.07525

Dark matter may be collecting in the center of the Earth. A recent paper explores way to detect its decay products here on the surface.

Our entire galaxy is gravitationally held together by a halo of dark matter, whose particle properties remain one of the biggest open questions in high energy physics. One class of theories assumes that the dark matter particles interact through a dark photon, a hypothetical particle which mediates a force analogous to how the ordinary photon mediates electromagnetism.

These theories also permit the  ordinary and dark photons to have a small quantum mechanical mixing. This effectively means that the dark photon can interact very weakly with ordinary matter and mediate interactions between ordinary matter and dark matter—this gives a handle for ways to detect dark matter.

While most methods for detecting dark matter focus on building detectors that are sensitive to the “wind” of dark matter bombarding (and mostly passing through) the Earth as the solar system zooms through the galaxy, the authors of 1509.07525 follow up on an idea initially proposed in the mid-80’s: dark matter hitting the Earth might get stuck in the Earth’s gravitational potential and build up in its core.

These dark matter particles can then find each other and annihilate. If they annihilate into very weakly interacting particles, then these may be detected at the surface of the Earth. A typical example is dark matter annihilation into neutrinos. In 1509.07525, the authors examine the case where the dark matter annihilates into dark photons, which can pass through the Earth as easily as a neutrino and decay into pairs of electrons or muons near the surface.

These decays can be detected in large neutrino detectors, such as the IceCube neutrino observatory (previously featured in ParticleBites). In the case where the dark matter is very heavy (e.g. TeV in mass) and the dark photons are very light (e.g. 200 MeV), these dark photons are very boosted and their decay products point back to the center of the Earth. This is a powerful discriminating feature against background cosmic ray events.  The number of signal events expected is shown in the following contour plot:

While similar analyses for dark photon-mediated dark matter capture by celestial bodies and annihilation have been studied—see e.g. Pospelov et al., Delaunay et al., Schuster et al., and Meade et al.—the authors of 1509.07525 focus on the case of dark matter capture in the Earth (rather than, say, the sun) and subsequent annihilation to dark photons (rather than neutrinos).

1. The annihilation rate at the center of the Earth is greatly increased do to Sommerfeld enhancement: because the captured dark matter has very low velocity, it is much more likely to annihilate with other captured dark matter particles due to mutual attraction from dark photon exchange.
2. This causes the Earth to quickly saturate with dark matter, leading to larger annihilation rates than one would naively expect in the case where the Earth were not yet dark matter saturated such that annihilation and capture occur at equal rates.
3. In addition using directional information to identify signal events against cosmic ray backgrounds, the authors identified kinematic quantities—the opening angle of the Standard Model decay products and the time delay between them—as ways to further discriminate signal from background. Unfortunately their analysis implies that these features lie just outside of the IceCube sensitivity to them.

Finally, the authors point out the possibility of large enhancements coming from the so-called dark disk, an enhancement in the low velocity phase space density of dark matter. If that is the case, then the estimated reach may increase by an order of magnitude.

## How to Turn On a Supercollider

After two years of slumber, the world’s biggest particle accelerator has come back to life. This marks the official beginning of Run 2 of the LHC, which will collide protons at nearly twice the energies achieve in Run 1. Results from this data were already presented at the recently concluded European Physical Society (EPS) Conference on High Energy Physics. And after achieving fame in 2012 through observation of the Higgs boson, it’s no surprise that the scientific community is waiting with bated breath to see what the LHC will do next.

The first official 13 TeV stable beam physics data arrived on June 5th. One of the first events recorded by the CMS detector is shown in Figure 2. But as it turns out, you can’t just walk up to the LHC, plug it back into the wall, and press the on switch (crazy, I know.) It takes an immense amount of work, planning, and coordination to even get the thing running.

The machine testing begins with the magnets. Since the LHC dipole magnets are superconducting, they need to be cooled to about 1.9K in order to function, which can take weeks. Each dipole circuit then must be tested to ensure functionality of the quench protection circuit, which will dump the beam in the event of sudden superconductivity loss. This process occurred between July and December of 2014.

Once the magnets are set, it’s time to start actually making beam. Immediately before entering the LHC, protons are circling around the Super Proton Synchroton, which acts as a pre-accelerator. Getting beam from the SPS to the LHC requires synchronization, a functional injection system, beam dump procedure, and a whole lot of other processes that are re-awoken and carefully tested. By April, beam commissioning was officially underway, meaning that protons were injected and circulating, and a mere 8 weeks later there were successful collisions at the safe energy of 6.5 TeV. As of right now, the CMS detector is reporting 84 pb-1 total integrated luminosity; a day-by-day breakdown can be seen in Figure 3.

But just having collisions does not mean that the LHC is up and fully functional. Sometimes things go wrong right when you least expect it. For example, the CMS magnet has been off to a bit of a rough start—there was an issue with its cooling system that kept the magnetic field off, meaning that charged particles would not bend. The LHC has also been taking the occasional week off for “scrubbing”, in which lots of protons are circulated to burn off electron clouds in the beam pipes.

This is all leading up to the next technical stop, when the CERN engineers get to go fix things that have broken and improve things that don’t work perfectly. So it’s a slow process, sure. But all the caution and extra steps and procedures are what make the LHC a one-of-a-kind experiment that has big sights set for the rest of Run 2. More posts to follow when more physics results arrive!

References:

1. LHC Commissioning site
2. Cyrogenics & Magnets at the LHC
3. CERN collisions announcement
4. CMS Public Luminosity results

## Prospects for the International Linear Collider

Title: “Physics Case for the International Linear Collider”
Author: Linear Collider Collaboration (LCC) Physics Working Group
Published: arXiV hep-ex 1506:05992

For several years, rumors have been flying around the particle physics community about an entirely new accelerator facility, one that can take over for the LHC during its more extensive upgrades and can give physicists a different window into the complex world of the Standard Model and beyond. Through a few setbacks and moments of indecision, the project seems to have more momentum now than ever, so let’s go ahead and talk about the International Linear Collider: what it is, why we want it, and whether or not it will ever actually get off the ground.

The ILC is a proposed linear accelerator that will collide electrons and positrons, in comparison to the circular Large Hadron Collider ring that collides protons. So why make these design differences? Hasn’t the LHC done a lot for us? In two words: precision measurements!

Of course, the LHC got us the Higgs, and that’s great. But there are certain processes that physicists really want to look at now that occupy much higher fractions of the electron-positron cross section. In addition, the messiness associated with strong interactions is entirely gone with a lepton collider, leaving only a very well-defined initial state and easily calculable backgrounds. Let’s look specifically at what particular physical processes are motivating this design.

1. The Higgs. Everything always comes back to the Higgs, doesn’t it? We know that it’s out there, but beyond that, there are still many questions left unanswered. Physicists still want to determine whether the Higgs is composite, or whether it perhaps fits into a supersymmetric model of some kind. Additionally, we’re still uncertain about the couplings of the Higgs, both to the massive fermions and to itself. Figure 1 shows the current best estimate of Higgs couplings, which we expect to be proportional to the fermion mass, in comparison to how the precision of these measurements should improve with the ILC.

2.The Top Quark. Another thing that we’ve already discovered, but still want to know more about its characteristics and behaviors. We know that the Higgs field takes on a symmetry breaking value in all of space, due to the observed split of the electromagnetic and weak forces. As it turns out, it is the coupling of the Higgs to the top that provides this value, making it a key player in the Standard Model game.

3.New Physics. And of course there’s always the discovery potential. Since electron and positron beams can be polarized, we would be able to measure backgrounds with a whole new level of precision, providing a better image of possible decay chains that include dark matter or other beyond the SM particles.

Let’s move on to the actual design prospects for the ILC. Figure 2 shows the most recent blueprint of what such an accelerator would look like.  The ILC would have 2 separate detectors, and would be able to accelerate electrons/positrons to an energy of 500 GeV, with an option to upgrade to 1 TeV at a later point. The entire tunnel would be 31km long with two damping rings shown at the center. When accelerating electrons to extremely high energies, a linear collider is needed to offset extremely relativistic effects. For example, the Large Electron-Positron Collider synchrotron at CERN accelerates electrons to 50 GeV, giving them a relativistic gamma factor of 98,000. Compare that to a proton of 50 GeV in the same ring, which has a gamma of 54. That high gamma means that an electron requires an insane amount of energy to offset its synchrotron radiation, making a linear collider a more reasonable and cost effective choice.

In any large (read: expensive) experiment such as this, a lot of politics are going to come into play. The current highest bidder for the accelerator seems to be Japan, with possible construction sites in the mountain ranges (see Figure 3). The Japanese government is pretty eager to contribute a lot of funding to the project, something that other contenders have been reluctant to do (but such funding promises can very easily go awry, as the poor SSC shows us.) The Reference Design Reports report the estimated cost to be $6.7 billion, though U.S. Department of Energy officials have placed the cost closer to$20 billion. But the benefits of such a collaboration are immense. The infrastructure of such an accelerator could lead to the creation of a “new CERN”, one that could have as far-reaching influence in the future as CERN has enjoyed in the past few decades. Bringing together about 1000 scientists from more than 20 countries, the ILC truly has the potential to do great things for future international scientific collaboration, making it one of the most exciting prospects on the horizon of particle physics.

1. The International Linear Collider site: all things ILC
2. ILC Reference Design Reports (RDR), for the very ambitious reader

## A New Solution to the Hierarchy Problem?

Hello particle Chompers,

Today I want to discuss a slightly more advanced topic which I will not be able to explain in much detail, but goes by the name of the gauge Hierarchy problem or just the the Hierarchy Problem‘. My main motivation is to simply make you curious enough that you will feel inspired to investigate it further for yourself since it is one of the outstanding problems in particle physics and one of the main motivations for the construction of the LHC. A second motivation is to bring to your attention a recent and exciting paper which proposes a potentially new solution to the hierarchy problem.

The hierarchy problem can roughly be stated as the problem of why the vacuum expectation value (VEV) of the Higgs boson, which determines the masses of the electroweak W and Z bosons, is so small compared to the highest energy scales thought to exist in the Universe. More specifically, the masses of the W and Z bosons (which define the weak scale) are roughly $\sim 10^{2}$ GeV (see Figure 1) in particle physics units (remember in these units mass = energy!).

On the other hand the highest energy scale thought to exist in the Universe is the planck scale at $\sim 10^{19}$ GeV which is associated with the physics of gravity. Quantum field theory tells us that the Higgs VEV should get contributions from all energy scales (see Figure 2) so the question is why is the Higgs VEV, and thus the W and Z boson masses, a factor of roughly $\sim 10^{17}$ smaller than it should be?

In the Standard Model (SM) there is no solution to this problem. Instead one must rely on a spectacularly miraculous numerical cancellation among the parameters of the SM Lagrangian. Miraculous numerical coincidences’ like this make us physicists feel uncomfortable to the point that we give it the special name of fine tuning’. The hierarchy problem is thus also known as the fine tuning problem.

A search for a solution to this problem has been at the forefront of particle physics for close to 40 years. It is the aversion to fine tuning which leads most physicist to believe there must be new physics beyond the SM whose dynamics are responsible for keeping the Higgs VEV small. Proposals include supersymmetrycomposite Higgs models, extra dimensions, as well as invoking the anthropic principle in the context of a multiverse. In many cases, these solutions require a variety of new particles at energies close to the weak scale ($\sim 100-1000$ GeV) and thus should be observable at the LHC. However the lack of evidence at the LHC for any physics beyond the SM is already bringing tension to many of these solutions. A solution which does not require new particles at the weak scale would thus be very attractive.

Recently a novel mechanism, which goes by the name of $\emph{cosmological relaxation of the electroweak scale}$, has been proposed which potentially offers such a solution. The details (which physicists are currently still digesting) are well beyond the scope of this blog. I will just mention that the mechanism incorporates two previously proposed mechanisms known as inflation$^1$ and the QCD axion$^2$ which solve other known problems. These are combined with the SM in a novel way such that the weak scale can arise naturally in our universe without any fine tuning and without new particles at the weak scale (or multiple universes)! And as a bonus, the axion in this mechanism (referred to as the relaxion’) makes a good dark matter candidate!

Whether or not this mechanism turns out to be a solution to the hierarchy problem will of course require experimental tests and further theoretical scrutiny, but its a fascinating idea which combines aspects of quantum field theory and general relativity so I hope it will serve as motivation for you to begin learning more about these subjects!

$\bf{Footnotes:}$

1. Inflation is a theorized period of exponential accelerated expansion of our Universe in the moments just after the big bang. It was proposed as a solution to the problems of why our Universe is so flat and (mostly) homogenous while also explaining the structure we see throughout the Universe and in the cosmic microwave background.

2. Axions are particles proposed to explain why the amount of CP violation in the QCD sector in the SM is so small, which is known as the strong CP problem‘.

## A Quark Gluon Plasma Primer

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.

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.

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 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.

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.

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.

## Uncovering a Higgs Hiding Behind Backgrounds

Hello particle munchers,

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.

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.

Seeking and Studying the Standard Model Higgs Particle

Decays of the Standard Model Higgs

## 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!