LIGO and Gravitational Waves: A Hep-ex perspective

The exciting Twitter rumors have been confirmed! On Thursday, LIGO finally announced the first direct observation of gravitational waves, a prediction 100 years in the making. The media storm has been insane, with physicists referring to the discovery as “more significant than the discovery of the Higgs boson… the biggest scientific breakthrough of the century.” Watching Thursday’s press conference from CERN, it was hard not to make comparisons between the discovery of the Higgs and LIGO’s announcement.



The gravitational-wave event GW150914 observed by the LIGO Collaboration
The gravitational-wave event GW150914 observed by the LIGO Collaboration


Long standing Searches for well known phenomena


The Higgs boson was billed as the last piece of the Standard Model puzzle. The existence of the Higgs was predicted in the 1960s in order to explain the mass of vector bosons of the Standard Model, and avoid non-unitary amplitudes in W boson scattering. Even if the Higgs didn’t exist, particle physicists expected new physics to come into play at the TeV Scale, and experiments at the LHC were designed to find it.


Similarly, gravitational waves were the last untested fundamental prediction of General Relativity. At first, physicists remained skeptical of the existence of gravitational waves, but the search began in earnest with Joseph Webber in the 1950s (Forbes). Indirect evidence of gravitational waves was demonstrated a few decades later. A binary system consisting of a pulsar and neutron star was observed to release energy over time, presumably in the form of gravitational waves. Using Webber’s method for inspiration, LIGO developed two detectors of unprecedented precision in order to finally make direct observation.


Unlike the Higgs, General Relativity makes clear predictions about the properties of gravitational waves. Waves should travel at the speed of light, have two polarizations, and interact weakly with matter. Scientists at LIGO were even searching for a very particular signal, described as a characteristic “chirp”. With the upgrade to the LIGO detectors, physicists were certain they’d be capable of observing gravitational waves. The only outstanding question was how often these observations would happen.


The search for the Higgs involved more uncertainties. The one parameter essential for describing the Higgs, its mass, is not predicted by the Standard Model. While previous collider experiments at LEP and Fermilab were able to set limits on the Higgs mass, the observed properties of the Higgs were ultimately unknown before the discovery. No one knew whether or not the Higgs would be a Standard Model Higgs, or part of a more complicated theory like Supersymmetry or technicolor.


Monumental scientific endeavors


Answering the most difficult questions posed by the universe isn’t easy, or cheap. In terms of cost, both LIGO and the LHC represent billion dollar investments. Including the most recent upgrade, LIGO cost a total $1.1 billion, and when it was originally approved in 1992, “it represented the biggest investment the NSF had ever made” according to France Córdova, NSF director. The discovery of the Higgs was estimated by Forbes to cost a total of $13 billion, a hefty price to be paid by CERN’s member and observer states. Even the electricity bill costs more than $200 million per year.


The large investment is necessitated by the sheer monstrosity of the experiments. LIGO consists of two identical detectors roughly 4 km long, built 3000 km apart. Because of it’s large size, LIGO is capable of measuring ripples in space 10000 times smaller than an atomic nucleus, the smallest scale ever measured by scientists (LIGO Fact Page). The size of the LIGO vacuum tubes is only surpassed by those at the LHC. At 27 km in circumference, the LHC is the single largest machine in the world, and the most powerful particle accelerator to date. It only took a handful of people to predict the existence of gravitational waves and the Higgs, but it took thousands of physicists and engineers to find them.


Life after Discovery


Even the language surrounding both announcements is strikingly similar. Rumors were circulating for months before the official press conferences, and the expectations from each respective community were very high. Both discoveries have been touted as the discoveries of the century, with many experts claiming that results would usher in a “new era” of particle physics or observational astronomy.


With a few years of hindsight, it is clear that the “new era” of particle physics has begun. Before Run I of the LHC, particle physicists knew they needed to search for the Higgs. Now that the Higgs has been discovered, there is much more uncertainty surrounding the field. The list of questions to try and answer is enormous. Physicists want to understand the source of the Dark Matter that makes up roughly 25% of the universe, from where neutrinos derive their mass, and how to quantize gravity. There are several ad hoc features of the Standard Model that merit additional explanation, and physicists are still searching for evidence of supersymmetry and grand unified theories. While the to-do list is long, and well understood, how to solve these problems is not. Measuring the properties of the Higgs does allow particle physicists to set limits on beyond the Standard Model Physics, but it’s unclear at which scale new physics will come into play, and there’s no real consensus about which experiments deserve the most support. For some in the field, this uncertainty can result in a great deal of anxiety and skepticism about the future. For others, the long to-do list is an absolutely thrilling call to action.


With regards to the LIGO experiment, the future is much more clear. LIGO has only published one event from 16 days of data taking. There is much more data already in the pipeline, and more interferometers like VIRGO and (e)LISA, planning to go online in the near future. Now that gravitational waves have been proven to exist, they can be used to observe the universe in a whole new way. The first event already contains an interesting surprise. LIGO has observed two inspriraling black holes of 36 and 29 solar masses, merging into a final black hole of 62 solar masses. The data thus confirmed the existence of heavy stellar black holes, with masses more than 25 times greater than the sun, and that binary black hole systems form in nature (Atrophysical Journal). When VIRGO comes online, it will be possible to triangulate the source of these gravitational waves as well. LIGO’s job is to watch, and see what other secrets the universe has in store.

A New Particle at LHC for Christmas??

Hello particle gobblers and happy new year from my new location at the University of Granada.

In between presents and feasting, you may have heard rumblings over the holidays that the LHC could be seeing hints of a new and very massive particle. The rumors began even before the ATLAS and CMS experiments announced results from analyzing the brand new 13 TeV (in particle physics units!) data which was collected in 2015. At 13 TeV we are now probing higher energy scales of nature than ever before. These are truly uncharted waters where high energy physicists basically have no idea what to expect. So there was a lot of anticipation for the first release of new data from the LHC in early December and it appears a tantalizing hint of new physics may have been left there dangling for us, like a just out of reach Christmas cookie.

Since the announcement, a feeding frenzy of theoretical work has ensued as theorists, drunk from the possibilities of new physics and too much holiday food, race to put forth their favorite (or any) explanation (including yours truly I must confess:/). The reason for such excitement is an apparent excess seen by both CMS and ATLAS of events in which two very energetic photons (particles of light) are observed in tandem. By `excess’ I basically mean a `bump‘ on what should be a `smooth‘ background exactly as discussed previously for the Higgs boson at 125 GeV. This can be seen in the CMS (Figure 1) and ATLAS (Figure 2) results for the observed number of events involving pairs of photons versus the sum of their energies.

Figure 1: CMS results for searches of pairs of photons at 13 TeV.
Figure 1: CMS results for searches of pairs of photons at 13 TeV.
Figure 2: ATLAS results for searches of pairs of photons at 13 TeV.
Figure 2: ATLAS results for searches of pairs of photons at 13 TeV.













The bump in the ATLAS plot is easier to see (and not coincidentally has a higher statistical significance) than the CMS bump which is somewhat smaller. What has physicists excited is that these bumps appear to be at the same place at around 750 GeV^1. This means two independent data sets both show (small) excesses in the same location making it less likely to be simply a statistical fluctuation. Conservation of energy and momentum tells us that the bump should correspond to the mass of a new particle decaying to two photons. At 750 GeV this mass would be much higher than the mass of the heaviest known particle in the Standard Model; the top quark, which is around 174 GeV while the Higgs boson you will remember is about 125 GeV.

It is of course statistically very possible (some might say probable) that these are just random fluctuations of the data conspiring to torture us over the holidays. Should the excess persist and grow however, this would be the first clear sign of physics beyond the Standard Model and the implications would be both staggering and overwhelming. Simply put, the number of possibilities of what it could be are countless as evidenced by the downpour of papers which came out just in the past two weeks and still coming out daily.

A simple and generic explanation which has been proposed by many theorists is that the excess indicates the presence of a new, electrically neutral, spin-0 scalar boson (call it \varphi) which is produced from the fusion of two gluons and which then decays to two photons (see Figure 3) very much like our earlier discussion of the Higgs boson. So at first appearance this just looks like a heavy version of the Higgs boson discovered at 125 GeV. Crucially however, the new potential scalar at 750 GeV has nothing (or atleast very little) to do with generating mass for the W and Z bosons of the Standard Model which is the role of the Higgs boson. I will save details about the many possibilities for a future post^2, but essentially the many models put forth attempt to explain what occurs inside the gray `blobs’ in order to generate an interaction between \varphi with gluons and photons.

Figure 3: Production of a new scalar particle via gluon fusion followed by decay into photons.
Figure 3: Production of a new scalar particle via gluon fusion followed by decay into photons.

It will of course take more data to confirm or deny the excess and the possible existence of a new particle. Furthermore, if the excess is real and there is indeed a new scalar particle at 750 GeV, a host of other new signals are expected in the near future. As more data is collected in the next year the answers to these questions will begin to emerge. In the meantime, theorists will daydream of the possibilities hoping that this holiday gift was not just a sick joke perpetrated by Santa.


1. It is a bit difficult to tell by eye because the ATLAS plot axis is linear while that for CMS is logarithmic. A nice discussion of the two bumps and their location can be found here.

2. For those feeling more brave, a great discussion about the excess and its implications can be found here and here.