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Today, scientists in the Dark Energy Survey released results that have been five years in the making. Researchers used the world’s most complete census of dwarf galaxies around our Milky Way galaxy to probe the nature of dark matter, an invisible form of matter that dominates the universe. These new measurements provide information about what dark matter can and cannot be made of.

In particular, the new results constrain the minimum mass of the dark matter particles, as well as the strength of interactions between dark matter and normal matter.

According to these new results, a dark matter particle must be heavier than a zeptoelectronvolt, which is 10^-21 electronvolts. That’s one trillionth of a trillionth of the mass of an electron. This study also shows that dark matter’s interactions with normal matter must be roughly 1,000 times weaker than the weak nuclear force. Of the known forces, only gravity is weaker.

This shows the result of two numerical simulations predicting the distribution of dark matter around a galaxy similar to our Milky Way. The left panel assumes that dark matter particles were moving fast in the early universe (warm dark matter), while the right panel assumes that dark matter particles were moving slowly (cold dark matter). The warm dark matter model predicts many fewer small clumps of dark matter surrounding our galaxy and thus many fewer satellite galaxies that inhabit these small clumps of dark matter. By measuring the number of satellite galaxies, scientists can distinguish between these models of dark matter. Image: Bullock and Boylan-Kolchin (2017); simulations by V. Robles, T. Kelley and B. Bozek, in collaboration with Bullock and Boylan-Kolchin

These novel measurements used data from the Dark Energy Survey, a cosmological survey designed to study dark energy, the mysterious force driving the accelerated expansion of the universe. In contrast, dark matter is gravitationally attractive, resisting the expansion of the universe and gravitationally binding astronomical systems such as galaxies. The smallest “dwarf” galaxies can have hundreds to thousands of times more dark matter than normal matter. Over the past five years, the Dark Energy Survey has combined with other surveys to more than double the known population of these tiny galaxies. The current total is now over 50.

“The Dark Energy Survey analysis has added a stringent new test to the standard cosmological paradigm and places tight constraints on several alternative dark matter models,” said Dark Energy Survey spokesperson Rich Kron, a scientist at the University of Chicago and Fermilab. “This is a fantastic example of how cosmological observations of the very large can inform particle physics experiments studying the very small.”

Dark matter makes up 85% of the matter in the universe, but we have yet to detect it directly in the laboratory. The gravitational effects of dark matter are indirectly but clearly visible in the motions of stars in galaxies, the clumpy distribution of galaxies in the universe, and even in the amount of lightweight elements. The robust astronomical evidence for the existence of dark matter has motivated many experimental searches here on Earth, using instruments ranging from cryogenic detectors buried deep underground to energetic particle colliders.

“The faintest galaxies are among the most valuable tools we have to learn about dark matter because they are sensitive to several of its fundamental properties all at once,” said Ethan Nadler, the study’s lead author and graduate student at Stanford University and SLAC.

By combining the observed census of dwarf galaxies with advanced cosmological simulations of the distribution of dark matter around the Milky Way, scientists were able to predict how the physical properties of dark matter would affect the number of small galaxies. Small galaxies form in regions where the dark matter density in the early universe is very slightly above average. Physical processes that smooth out these regions of higher density (if dark matter moves too quickly or gains energy due to interactions with normal matter) or prevent density variations from collapsing to form galaxies (thanks to quantum interference effects) would reduce the number of galaxies observed by the Dark Energy Survey.

“It’s exciting to see the dark matter problem attacked from so many different experimental angles,” said Fermilab and University of Chicago scientist Alex Drlica-Wagner, Dark Energy Survey collaborator and one of the lead authors on the paper. “This is a milestone measurement for DES, and I’m very hopeful that future cosmological surveys will help us get to the bottom of what dark matter is.”

The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries. For more information about the survey, please visit the experiment’s website.

Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Funding and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science and Technology, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.

Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

Editor’s note: The following press release was issued today by CERN. It announces that the ATLAS and CMS collaborations at the Large Hadron Collider have seen evidence of a new type of decay not yet observed: the Higgs boson decaying into a pair of muons.

US CMS — the United States contingent of the global CMS collaboration — played a crucial role in this result, contributing to the excellent performance of CMS detector. US CMS members have been instrumental in the design, construction and upgrades of detector components that capture the particle tracks and help filter potential signals from the background noise: the tracker detector, the muon detectors, the muon trigger system and the computing system. They continue to lead the successful maintenance and operations of these systems.

“US CMS is very proud to acknowledge the significant impact made by its members in deploying innovative analysis techniques, including cutting-edge AI methods, which were critical in establishing the evidence for Higgs boson decays into a muon and antimuon pair,” said Brown University physicist Meenakshi Narain, chair of the US CMS collaboration. “This is a rare process, and finding evidence for it is a vital step toward understanding the Higgs particle and the Standard Model.”

CMS is an international collaboration with members from 238 institutes across 55 countries. US CMS, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, makes up about a third of the CMS collaboration.

“The achievement, reached significantly ahead of what was expected, relies on the excellent performance of our detector, on the large data set provided by LHC and on advanced analysis techniques,” said Roberto Carlin, spokesperson for the CMS experimental collaboration.

The ATLAS and CMS experiments at CERN have announced new results that show that the Higgs boson decays into two muons. The muon is a heavier copy of the electron, one of the elementary particles that constitute the matter content of the universe. While electrons are classified as a first-generation particle, muons belong to the second generation. The physics process of the Higgs boson decaying into muons is a rare phenomenon as only about one Higgs boson in 5,000 decays into muons. These new results have pivotal importance for fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.

Physicists at CERN have been studying the Higgs boson since its discovery in 2012 to probe the properties of this very special particle. The Higgs boson, produced from proton collisions at the Large Hadron Collider, disintegrates – referred to as decay – almost instantaneously into other particles. One of the main methods of studying the Higgs boson’s properties is by analyzing how it decays into the various fundamental particles and the rate of disintegration.

A candidate of a Higgs boson decays into two muons as recorded by CMS. Image: CMS collaboration, CMS collaboration, Thomas McCauley

CMS achieved evidence of this decay with 3 sigma, which means that the chance of seeing the Higgs boson decaying into a muon pair from statistical fluctuation is less than one in 700. ATLAS’ two sigma result means the chances are one in 40. The combination of both results would increase the significance well above 3 sigma and provides strong evidence for the Higgs boson decay to two muons.

“CMS is proud to have achieved this sensitivity to the decay of Higgs bosons to muons and to show first experimental evidence for this process. The Higgs boson seems to interact also with second-generation particles in agreement with the prediction of the Standard Model, a result that will be further refined with the data we expect to collect in the next run,” says Roberto Carlin, spokesperson for the CMS experiment.

The Higgs boson is the quantum manifestation of the Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. By measuring the rate at which the Higgs boson decays into different particles, physicists can infer the strength of their interaction with the Higgs field: the higher the rate of decay into a given particle, the stronger its interaction with the field. So far, the ATLAS and CMS experiments have observed the Higgs boson decays into different types of bosons such as W and Z, and heavier fermions such as tau leptons. The interaction with the heaviest quarks, the top and bottom, was measured in 2018. Muons are much lighter in comparison, and their interaction with the Higgs field is weaker. Interactions between the Higgs boson and muons had, therefore, not been seen at the LHC.

A candidate ATLAS event display of a Higgs boson decay to two muons. Image: ATLAS collaboration

“This evidence of Higgs boson decays to second-generation matter particles complements a highly successful Run 2 Higgs physics program. The measurements of the Higgs boson’s properties have reached a new stage in precision and rare decay modes can be addressed. These achievements rely on the large LHC data set, the outstanding efficiency and performance of the ATLAS detector, as well as the use of novel analysis techniques,” says Karl Jakobs, ATLAS spokesperson.

What makes these studies even more challenging is that, at the LHC, for every predicted Higgs boson decaying to two muons, there are thousands of muon pairs produced through other processes that mimic the expected experimental signature. The characteristic signature of the Higgs boson’s decay to muons is a small excess of events that cluster near a muon-pair mass of 125 GeV, which is the mass of the Higgs boson. Isolating the Higgs boson to muon-pair interactions is no easy feat. To do so, both experiments measure the energy, momentum and angles of muon candidates from the Higgs boson’s decay. In addition, the sensitivity of the analyses was improved through methods such as sophisticated background modelling strategies and other advanced techniques such as machine-learning algorithms. CMS combined four separate analyses, each optimized to categorize physics events with possible signals of a specific Higgs boson production mode. ATLAS divided their events into 20 categories that targeted specific Higgs boson production modes.

The results, which are so far consistent with the Standard Model predictions, used the full data set collected from the second run of the LHC. With more data to be recorded from the particle accelerator’s next run and with the High-Luminosity LHC, the ATLAS and CMS collaborations expect to reach the sensitivity (5 sigma) needed to establish the discovery of the Higgs boson decay to two muons and constrain possible theories of physics beyond the Standard Model which would affect this decay mode of the Higgs boson.

Scientific materials

CMS

Physics analysis summary: https://cds.cern.ch/record/2725423/files/HIG-19-006-pas.pdf

Physics briefing: https://cmsexperiment.web.cern.ch/news/cms-sees-evidence-higgs-boson-decaying-muons

Physics event displays :

https://cds.cern.ch/record/2720665?ln=en

Physics plots http://cds.cern.ch/record/2725728

ATLAS

ATLAS paper on arXiv: https://arxiv.org/abs/2007.07830

ATLAS Physics briefing: https://atlas.cern/updates/physics-briefing/new-search-rare-higgs-decays-muons

Event display:

https://cds.cern.ch/record/2725717?ln=en