Year 2016 in particle physics

Working together, particle physicists from the United States and around the world have made exciting progress this year in our understanding of the universe at the smallest and largest scales.

The LIGO experiment enabled the first detection of gravitational waves, originally predicted by Albert Einstein in 1916 in his general theory of relativity. And scientists have gotten closer to the next big discovery in experiments like those at the Large Hadron Collider and ultra-sensitive underground neutrino detectors.

The pursuit of particle physics is a truly international effort. It takes the combined resources and expertise of partner nations to develop and operate unique world-class facilities and state-of-the-art detectors.

Particle physics efforts can be divided into five intertwined fields of investigation: explorations of the Higgs boson, neutrinos, dark matter, cosmic acceleration and the unknown. Following this community vision enabled physicists to achieve major scientific breakthroughs in 2016 and set the stage for a fascinating future.

Artwork by Sandbox Studio, Chicago with Ana Kova

Using the Higgs boson as a new discovery tool

The discovery of the Higgs boson in 2012 at CERN’s Large Hadron Collider opened a new door to understanding the universe. In 2016, the LHC produced roughly the same number of particle collisions as in all its previous years of operation combined. At its current collision rate, it produces a Higgs boson about once per second.

Although it will take time for the collaborations of the ATLAS and CMS experiments to digest this deluge of data, the first results are already probing for any signs of unexpected behavior of the Higgs boson. In August, the ATLAS and CMS collaborations used data from the highest energy LHC collisions to ‘rediscover’ the Higgs boson and confirm that it conforms to the predictions of the Standard Model of particle physics, up to here. Deviations from predictions would signal new physics beyond the Standard Model.

With the LHC aiming to continue operating at its record pace for the next two years and to more than double the collisions of particles delivered to experiments, this window on the universe is only beginning to open. The latest theoretical calculations of all the major ways a Higgs boson can be produced and decay will allow for rigorous new testing of the Standard Model.

American scientists are also stepping up efforts with international partners to develop future improvements for a high-luminosity LHC that would provide 10 times more collisions and launch an era of high-precision Higgs boson physics. Scientists have made significant progress this year in developing more powerful superconducting magnets for HL-LHC, including producing a successful prototype that is currently the strongest accelerator magnet ever created.

Neutrinos moving through a diamond-shaped striped field, bubbles in the field

Artwork by Sandbox Studio, Chicago with Ana Kova

Pursuing neutrino mass physics

In 2016, several experiments continued to study ghostly neutrinos – particles so ubiquitous and distant that 100 trillion of them pass through you every second. In the late 1990s and early 2000s, experiments in Japan and Canada found evidence that these particular particles have a certain mass and can transform between neutrino types as they travel.

A worldwide program of experiments aims to answer many outstanding questions about neutrinos. Long-running experiments study the particles as they pass through the earth between Tokai and Kamioka in Japan or between Illinois and Minnesota in the United States. These experiments aim to discern neutrino masses and whether there are differences between neutrino transformations and those of their antimatter partners, antineutrinos.

In July, the T2K experiment in Japan announced that its data showed a possible difference between the rate at which a muon neutrino turns into an electron neutrino and the rate at which a muon antineutrino turns into an electron antineutrino. The T2K data suggests a combination of neutrino properties that would also give the NOvA experiment in the United States its best chance of making a neutrino discovery in the next few years.

In China, construction is underway for the Jiangmen Underground Neutrino Observatory, which will study the mass of neutrinos with the aim of determining which neutrino is the lightest.

In the longer term, particle physicists aim to definitively determine these answers by hosting the world-class Long Baseline Neutrino Facility, which would send a high-intensity neutrino beam 800 miles from Illinois to South Dakota. There, the international Deep Underground Neutrino Experiment, a mile below the surface, would enable precision neutrino science.

dark matter cube

Artwork by Sandbox Studio, Chicago with Ana Kova

Identify new dark matter physics

Overwhelming circumstantial evidence indicates that more than a quarter of the mass and energy of the observable universe is made up of an invisible substance called dark matter. But the nature of dark matter remains a mystery. Little is known about it except that it interacts by gravity.

To guide the experimental search for dark matter, theorists have studied the possible interactions that known particles might have with a wide variety of potential dark matter candidates with possible masses spanning more than a dozen orders of magnitude. .

Huge, sensitive detectors, such as the Large Underground Xenon, or LUX, experiment located a mile below the Black Hills of South Dakota, directly search for dark matter particles that may continuously pass through Earth. This year, LUX completed the world’s most sensitive search for direct evidence of dark matter, improving the world’s own previous search by a factor of four and shrinking the hidden space for an important class of theoretical dark matter particles.

Additionally, data from the Fermi Gamma-Ray Space Telescope and other facilities have continued to tighten the constraints on dark matter through indirect searches.

This paves the way for a suite of complementary next-generation experiments, including LZ, SuperCDMS-SNOLAB and ADMX-G2 in the US, which aim to dramatically improve sensitivity and reveal the nature of dark matter.

Flower shaped black shapes with cosmos inside and pink and blue arrows around it

Artwork by Sandbox Studio, Chicago with Ana Kova

Understanding Cosmic Acceleration

Particle physicists are looking skyward in their efforts to investigate a different mystery: our universe is expanding at an accelerating rate. Scientists seek to understand the nature of dark energy, responsible for overcoming the force of gravity and breaking up our universe.

Large-scale cosmic ground surveys aim to measure the long-term expansion history of the universe and improve our understanding of dark energy. This year, scientists from the Baryon Oscillation Spectroscopic Survey used their final dataset, including 1.5 million galaxies and quasars, to improve measurements of the cosmological scale of the universe and the growth rate of the structure. cosmic. These measurements will allow theorists to test and refine models aimed at explaining the origin of the current era of cosmic acceleration.

Through efforts that include partnerships with the private sector and international collaborations, American physicists aim to rapidly usher in the era of precision cosmology – and shed light on dark energy – with the Dark Energy Survey in course and the upcoming Dark Energy Spectroscopic Instrument and Large Synoptic Survey Telescope.

Community efforts are also underway to develop a next-generation cosmic microwave background experiment, CMB-S4. Precision measurements from CMB-S4 will not only advance studies of dark energy and provide cosmic constraints on the properties of neutrinos, but also offer a way to probe the first era of cosmic acceleration known as the inflation, which has occurred at energies far beyond those attainable in an accelerator. on earth.

Black, gray and white striped door with curvy lines and question marks sticking out

Artwork by Sandbox Studio, Chicago with Ana Kova

Explore the unknown

Often the results of an experiment show a clue to something new and unexpected, and scientists must devise new technology to determine if what they saw is real. But between 2015 and 2016, LHC scientists both asked and answered their own question.

In late 2015, LHC scientists found an unexpected bump in their data, a possible first hint of a new particle. Theorists were on the case; in early 2016, they framed possible interpretations of the data and explored how this might impact the Standard Model of particle physics. But by August, experimenters had gathered enough new data to consider the hint a statistical fluctuation.

Spurred on by the discovery of the pentaquark and tetraquark states, some theorists have predicted that the bound states of four b quarks should soon be observable at the LHC.

Experimenters continue to test theorists’ predictions against the data by making high-precision measurements or by studying extremely rare particle decays in experiments such as the LHCb experiment at LHC, the upcoming Belle II experiment in Japan, and the Muon g-2 and muon-to-electron conversion experiments at the Fermi National Accelerator Laboratory.

Illustration of a scientist working at the control panel of a large gray and red machine

Artwork by Sandbox Studio, Chicago with Ana Kova

Investing in the future of discovery science

The world-class facilities and experiments that enable the global particle physics program rely on state-of-the-art technology. Continuous research and development of accelerator and particle detector technology opens up the prospect of long-term discovery.

In 2016, scientists and engineers continued to make advancements in particle accelerator technology to prepare for building next-generation machines and possible facilities of the far future.

Advances in the efficiency of superconducting radiofrequency cavities will lead to cost savings in the construction and operation of machines such as the Linac Coherent Light Source II. In February, researchers at the Berkeley Lab Laser Accelerator, or BELLA, demonstrated the first multistage accelerator based on “tabletop” laser-plasma technology. This key step is needed to push towards far-future particle colliders that could be thousands of times shorter than conventional accelerators.

These results reflect only a small portion of the particle physics community’s total scientific output in 2016. The stage is set for exciting discoveries that will advance our understanding of the universe.

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