Want to know the latest research on the Higgs boson? Or the current findings on the search for dark energy?

You could search the internet, or even the latest scientific literature. Or you could find all your answers in one spot: The Review of Particle Physics, an 1800-page doorstopper compendium of measurements, tables and review articles that includes everything we currently know about the building blocks of the universe and the fundamental forces that govern it.  

In an era of overwhelming information, the 60-year-old publication serves as a continually updated, curated hierarchy of research results. “In a field as large as particle physics, it’s good to have a central place to find authoritative answers and information you might need,” says Juerg Beringer, group leader of the Particle Data Group, or PDG, at Lawrence Berkeley National Laboratory, which oversees the publication. In addition to research results, PDG also covers the tools of the HEP trade, such as detectors, accelerators, probability and statistics.

And though it’s the most-cited publication in particle physics, it’s not just for scientists. The book is distributed free of charge, around the world, to anyone.

“A fair fraction of our audience is students or the general public who are interested in learning about the field,” Beringer says. 

First, a wallet card reference 

The book’s beginnings were much humbler—in fact, it was originally designed to fit in your pocket. Physicists Murray Gell-Mann and Art Rosenfeld published a Particle Properties Table in the 1957 Annual Review of Nuclear Science, which then was produced as a wallet card showing easy-to-access experimental and theoretical information on the few particles known at that time.

By 1964, thanks to an explosion of experiments, the number of known particle measurements had grown so much that the wallet card became a small book (though wallet cards could still be requested—smaller ones to fit American wallets, and larger, more readable ones for European wallets).

“Enrico Fermi once said, ‘If I could remember the names of all these particles, I would have been a botanist,’” says Michael Barnett, former head of the Particle Data Group. “And now we have many, many more particles.”

The book has continued to grow since then—usually around 10 percent per year. The Particle Data Group, which updates the Review, is led by a small team but involves an international collaboration of 223 authors from 148 institutions in 24 countries. Every year, team members scan newly published scientific articles to determine what new information should be included, and how. It might be a new measurement of a particle, or a new review of a field, like inflation. Though there may be a discussion about which information to include and when, the process is generally conflict-free, and any quibbles are placed in the footnotes. 

In print, serendipity

The publication is updated yearly online, and a print version is updated every two years. The Review was put on the internet in 1995, and since then the website has had more than 130 million hits. But the printed publications remain popular: For recent editions, the group distributed 14,000 copies of the book and 32,000 copies of the booklet. 

They say you don’t feel like a particle physicist until you get your first booklet.

“Graduate students use it like a textbook,” Barnett says. “They write in the margins, bookmark pages, underline things they want to remember.” Having a physical book also encourages experiences searching on the internet can’t provide: serendipitous exploration of other topics in physics.

The smaller, spiral-bound booklet—which was originally also meant to fit into your pocket, but now at 348 pages will at least fit into your bag—is often used in classrooms as an introduction to the field. 

“They say you don’t feel like a particle physicist until you get your first booklet,” Barnett says. 

New information added to the book ebbs and flows with the startup and shutdown of high-energy physics facilities and experiments. In 2012 the group was about to send an edition to print when the Higgs boson discovery was announced. They stopped the presses, commissioned an addendum to the Higgs review article summarizing the discovery and were able to include it in the final manuscript before it went to press.

“That was very, very exciting for everyone,” Beringer says. 

A more searchable future

The latest edition includes more than 3000 new measurements from 721 papers and reviews on everything from the Higgs boson to Grand Unified Theories. But all the new information pushed the book’s size to 1800 pages, and it began to look more like a telephone book than a textbook.

“It became a health hazard to carry it around with you,” Beringer says. So the group took out the data tables and particle measurements; now those are just listed online. Beringer says online usage is becoming increasingly important, and they are working to make the information more easily searchable. 

Budgetary constraints and the general trend toward online publishing have made it increasingly difficult to offer a free printed version of the PDG book, Beringer says. So the group is considering alternatives such as offering print-on-demand for a fee. 

But the group hopes to find a way to continue printing the book for the 2018 edition, slated to publish this summer. A recent survey showed that 80 percent of respondents still want a printed booklet, and more than two-thirds want a printed book. The group also wants to continue to make the book accessible to readers who may not be able to afford the cost of printing and shipping. 

Printed information, it seems, still has its place in a digital world. In fact, Barnett recently received an email from someone asking for a new booklet because their copy had been stolen. 

“I’m happy to see that it’s still so valuable that someone will steal it,” he says. 

This week, four particles finished what they'd started.

In February, the proton, photon, electron and neutron threw their hats into the ring for the Subatomic Smackdown. This month, they sparred at the open house for the National High Magnetic Field Laboratory in Florida. (The electron won.) They even held a press conference at a meeting of the American Physical Society in Los Angeles.

 On Monday, the countdown to the final event began. #TeamProton, #TeamPhoton, #TeamElectron and #TeamNeutron each had a day to pump up their particle.

Then today, a poll opened on Twitter to determine the winner. The electron and the photon quickly emerged as the favorites, but midway through the day, the proton came out of nowhere to overtake the electron. (This might have had something to do with the surprise restart of the Large Hadron Collider, which collides protons.) It wasn't enough to put the proton in the lead, though, and the photon powered through to victory.

It was a good, clean fight, and the fans showed us the worth of each of the particle participants.

Fermi National Accelerator Laboratory declared their support for protons, which will help them create the world's most intense beam of neutrinos—particles too aloof to participate in the Smackdown themselves—for the upcoming Deep Underground Neutrino Experiment. Protons also play a starring role at Brookhaven National Accelerator Laboratory, which collides them in the Relativistic Heavy Ion Collider, or RHIC. Clara Nellist explained that scientists use protons in the LHC because they have a magnetic charge, which scientists can use to steer them, and because they’re heavy enough not to lose most of their energy as they loop around the accelerator complex.

Amanda Solliday threw her support behind the photon for its achievements in X-ray science at SLAC National Accelerator Laboratory. The University of Wisconsin-Milwaukee also cheered on the photon, which it uses to answer fundamental bio-molecular questions in partnership with the BioXFEL Science and Technology Center. David J. Gillcrist pointed out that photons carry the electromagnetic force that allows electrons to interact. Stephanie Keys supported the photon for its achievements at the Canadian Light Source. Alan Fry gave a shout-out to photons for helping scientists study black holes. @SandHillScienceMill mentioned that photons will be essential to the upcoming LUX-ZEPLIN, or LZ, dark matter experiment. And Karl Gumerlock thanked photons for cohering to create frickin’ laser beams.

The Mag Lab championed the electron, which it uses to study everything from qubits to cancer. They pointed out the particle’s role in operating electronics, including the cell phones and computers readers used to vote in the Twitter poll. Kristen Coyne cheered the electron for being the particle behind superconductivity. Brookhaven lent support to #TeamElectron; they accelerate the particles to nearly the speed of light at their National Synchrotron Light Source II to “see” the atomic structures of proteins, battery materials and catalysts in action. Electrons also help them cool the particle beams at RHIC.

William Ratcliff pointed out that neutrons allow scientists to see through steel. Rob Dimeo touted the neutron’s ability to help scientists watch atoms and molecules move in materials and to see magnetism at the nanoscale. Oak Ridge National Laboratory supported the neutron for its role in a variety of areas of research, from studying 3D printing for rocket science; to searching for ways to build better vehicle armor and safer suspension bridges; to working to make better medicines by studying crystals grown in space; to gaining insights into aquatic biochemistry through the study of sturgeon ear bones.

The photon earned the most votes, but in the end, the real winner is science.

Mike Lamont, the deputy head of the Beams Department at CERN, home to the Large Hadron Collider, turns and looks toward the entrance of the accelerator control center. “There she is,” he says.

Fabiola Gianotti, the Director General of CERN, strolls towards the crowd of engineers, operators and physicist huddled around the wall of screens that form the control panel for LHC.

“Who is the operator on shift?” Gianotti asks, and a woman perched in front waves her over. Gianotti hands her a large Colomba Gocce di Gioccolato, a traditional Italian Easter cake.

For most of CERN, today is a holiday. But for the people who operate the LHC, it’s time to wake up the world’s most powerful particle accelerator after a regularly scheduled three-month shutdown for repairs. The last several weeks have been a gradual build-up to this moment.

“We’ve done 10,000 tests,” says Rende Steerenberg, the leader of the Operations Group, the team that makes sure CERN’s entire accelerator complex is running smoothly.

The morning started as most mornings do in the CERN Control Centre. The person responsible for the LHC’s pre-beam checkout gave a short presentation about everything that had happened during the previous 24 hours and gave an update on the plan for the day. The 10-minute presentation ended with a pithy statement in a big yellow text box: LHC ready for beam injection. Normally, only a handful of people show up to this daily briefing. But today, every seat at the table was filled.

The CCC is a large, open room with long, vertical windows looking toward the snow-capped Jura mountains. It’s divided into four circular islands, each responsible for one part of CERN’s accelerator complex. Today the LHC island is buzzing with life and energy.

Jörg Wenninger, who is responsible for the LHC operation, switches between English, French and German as he briefs various colleagues. “What’s the intensity for today?” asks one of the specialists for the radio-frequency cavities, the machinery that accelerates the proton beam. Each team settles in front of their control panels as the well-choreographed process begins.

The restart doesn’t happen all at once. Rather, engineers send single packets of protons partway into the LHC and intentionally absorb them with collimators before sending the next packets slightly further.

“We adjust the steering and make sure that there’s nothing blocking the proton’s path,” Steerenberg says.

Sector by sector, the trajectory of the proton beam is measured and fine-tuned. After about two hours, the beam makes it to Point 5—halfway around the accelerator. Then there’s an electrical trip near the beam dump. The relevant expert gets in a car and makes his way through the French countryside to check on the hardware, which is stored above ground near the access point.

In addition to holding the title for “World’s Single Largest Machine,” the LHC is also perhaps the most complicated contraption ever built. Every part needs to work flawlessly for the overall accelerator to function. The people responsible are no strangers to troubleshooting problems as they arise.

“It’s really variable,” Steerenberg says. “Sometimes we can run for many days without any faults, but other times it’s one after the other.”

Whenever there’s a problem, the LHC’s operators and experts tackle it with the efficiency of a well-oiled machine.

“At the very beginning, when the LHC first started up, we were more anxious,” Steerenberg says. “After seven years of operation, we’re all really used to it.”

The expert returns. “He’s back so we’ll restart in a few minutes,” Wenninger announces to the room.

At 12:17 p.m., the operators and machine experts laugh and cheer.

“C’est bon, ça circule!” applauds Frédérick Bordry, CERN’s Director for Accelerators and Technology, in French. The first proton beam has made it all the way around the LHC and is circulating clockwise at close to 11,000 times every second.

“On attend le beam deux,” he says.

Within 20 minutes, beam two is circulating in the opposite direction.

“Bravo, mesdames et messieurs!” Bordry cheers. “Fantastique.”

Liquid helium is constantly pulsing through sophisticated plumbing that runs both inside and outside of the Large Hadron Collider. Thanks to this cryogenic cooling system, the LHC is colder than interstellar space. 

But why does it need to be kept at these intensely frigid temperatures?

“Because if not, the magnets would not work,” says Serge Claudet, the deputy head of CERN’s cryogenics group.

The cable that is coiled to make the LHC’s powerful electromagnets carries 11,800 amperes of current—roughly as much as a small bolt of lightning. The average toaster, for reference, uses only 9 amperes. 

For a cable the width of a finger to carry this much current and not burn up, it must be a superconductor. A superconductor is a type of material that carries an electrical current with zero electrical resistance. You see evidence of electrical resistance every time you turn on a light. If a lightbulb filament were made from a superconducting wire, it would give off no heat and no light—the electricity would pass straight through.

Most industrial superconductors gain the magical property of superconductivity only at extremely low temperatures—a few degrees above absolute zero. 

So perhaps surprisingly, the LHC lives in a pleasantly warm tunnel, about 80 degrees Fahrenheit. To insulate the superconducting magnets from this temperate climate, engineers nestled layers of insulation inside one another like a matryoshka doll, each colder than the last, protecting the magnetic core. 

On the very outside is a vacuum chamber, which acts like the walls of a thermos. On the very inside, the magnets are submerged in a static bath of 1.9-Kelvin superfluid liquid helium, which seeps into every nook and cranny of the LHC’s magnetic coils and supports.

If engineers had to worry only about protecting the LHC from the warmth of the tunnel, two feet of protection swollen with liquid helium might be enough. But their most formidable foe lies within.

“Most heating is internal,” says Gareth Jones, a CERN cryogenic operator. “It comes from the proton beam and the magnets.”

Heat is a measurement of how much particles jostle, and the 3.5 quintillion protons that stream through the heart of the LHC certainly create a stir. Every time a proton rounds a corner, it releases quick bursts of light, which are absorbed by the surrounding material and awaken sleeping molecules. 

Meanwhile, the loosely bound electrons of the copper-coated beampipe flow through the metal in pursuit of the positively charged proton beam, generating an electrical current. Some electrons will even leave their atomic confines and leap into the vacuum, only to crash and liberate even more electrons. These electrons move like water down a river gorge, bouncing off obstacles and swirling in eddies. All of this generates more and more heat, which threatens the sensitive conditions required to keep the magnets superconducting.

“If the magnets get above 2.17 Kelvin, they start to lose their superconducting properties,” says Guy Crockford, an LHC operator. “When this happens, what was originally just a little bit of internal heating quickly escalates into a lot of heat.”

To keep these magnets cool, engineers designed a complex cryogenic system that takes advantage of a very simple principle: When a liquid transforms into a gas, it absorbs heat. This is why we feel cold after a shower; it’s not because the water is cold, but because it carries away our heat as droplets evaporate off our skin. 

A long and thin pipe pierces the magnet support structure and delivers a stream of pressurized, ultra-cold liquid helium. As the liquid helium absorbs the excess heat, it evaporates and is quickly pumped out. 

Another cooling pipe runs through the inside of the beampipe and sops up energy right at the source. These internal capillaries are fed by a highway of five pipes running alongside the LHC: Two transport cold helium for injection; two carry warm helium back for re-cooling; and one is the main artery that helps maintain the pressure and temperature of the entire circuit. The LHC cycles about 16 liters of liquid helium every second to keep the entire system operational.

Despite all of these efforts, LHC magnets do sometimes heat up enough to lose their superconductivity in an event called a magnet quench. 

“It’s normally just one concentrated point that warms up, and it happens so fast,” Crockford says. 

Sensors detect the change in voltage and trigger a system that fires quench heater strips, which distribute the heat throughout the entire magnet and divert the electrical current away from the magnet. At the same time, the LHC beam is automatically rerouted into a concrete block called a beam dump, and the entire accelerator takes a pause for a few hours while the magnet recovers back to its super-cooled state.

“This has happened only about once every two years,” Crockford says. “We want to protect our magnets at all costs, and cryogenics is always on our mind.”

Curious how scientists can deliver particles to particle physics experiments? Two new animations from Fermilab and CERN will help you visualize how it works.

This animation from the Department of Energy’s Fermi National Accelerator Laboratory shows the path particles take through the accelerator complex.

It all starts at the proton source. The beam of particles moves through various systems such as the linear accelerator, booster and main injector. The beams can generate a variety of particles, including protons, neutrons, muons, pions and neutrinos, which are then studied in experiments and in research programs.

You can learn about the components in even more detail here.

Then there’s CERN’s animation, which focuses on their newest linear accelerator: Linac4. It’s scheduled to be connected to the next accelerator in the chain, the Proton Synchrotron Booster, in 2019, and should supply all of the protons at CERN starting in 2021.

The Standard Model of particle physics has been developed over several decades to describe the properties and interactions of elementary particles. The model has been extended and modified with new information, but time and again, experiments have bolstered physicists’ confidence in it.

And yet, scientists know that the model is incomplete. It cannot predict the masses of certain particles, nor can it explain what most of the universe is made of. To discover what lies beyond the Standard Model, scientists are searching for its flaws—untenable assumptions and phenomena that it does not predict. A growing set of results from the study of bottom quarks may offer physicists a welcome chance to do just that.

“The Standard Model is very rigid,” says Marco Nardecchia, a theorist from Italy, “so the best way to break it is by precisely testing its predictions.”

The Standard Model makes many detailed predictions about how particles should interact or decay. Some subatomic processes are so complicated that even theorists aren’t quite sure exactly how they are supposed to work. For one: quarks—the constituents that make up elementary particles—should interact in the same way with the electron as with its heavier cousins, the muon or tau lepton. 

There are six types of quarks. The lightest and most common are the up and down quarks, which together make up protons and neutrons. Particles carrying a bottom quark—which is much heavier—are short-lived. In their decays, the bottom quark transitions into a lighter quark, preferentially a charm quark and rarely an up quark, forming another known particle.  

The remaining energy is carried by a charged lepton: an electron, a muon or a tau, each accompanied by its associated neutrino. According to the Standard Model, the rates of producing electrons, muons and taus differ only due to the very different masses of these three charged leptons. (The tau mass, for example, exceeds the electron mass by a factor of about 3500.)

“These predictions are straightforward and precise,” says Vera Lüth, a scientist on the Babar experiment, “which is why we decided to pursue these measurements in the first place.”

Scientists working on three different experiments are testing these predictions by examining specific decays of particles that carry a bottom quark. 

The first hint of an unexpected tau enhancement appeared in 2012 at the BaBar experiment at SLAC National Accelerator Laboratory, which studied close to 500 million events produced in electron-position collisions, and reconstructed less than 2000 decays involving taus. In 2015, the Belle experiment in Japan reported a similar enhancement in the tau rate in data collected from electron-position collisions at the same energy.

“A friend working on another experiment was sure that we had done something wrong,” Lüth says. “Then they observed the same effect.”

In 2015, scientists working on the LHCb experiment operating at CERN saw signs of the same phenomenon in very large samples of proton-proton collisions at much higher energy and collision rates. 

“All these results point in the same direction,” says Hassan Jawahery, a professor at the University of Maryland working on LHCb. “That’s what puzzles everyone.”

On their own, these individual results have a significance below the level that would raise an eyebrow. But together, they are “intriguing,” according to Tom Browder, the spokesperson of the Belle experiment and its successor, Belle II. “We are pretty sure that something new is out there. Proving even a tiny deviation from the  Standard Model could lead to a revolution in our field.”

The results accumulated so far have already inspired theorists to speculate about what kind of new physics processes might cause these enhancements. 

Some theories suggest that perhaps there is a yet undiscovered charged Higgs boson which favors the heavy tau over the much lighter muon and electron. Other models predict the existence of at least one new particle outside the Standard Model. “We may need something which interacts with quarks and leptons simultaneously,” Nardecchia says. 

Scientists won’t know what’s happening without further study, and gathering enough data to allow more detailed and precice studies will be a crucial step toward to find out.  

Scientists at the LHCb experiment are only at the beginning of this study. They plan to analyze about four times as many events in the next few years. They hope to complete new and updated measurements by this summer. The LHC accelerator complex program foresees major upgrades that will enlarge the experiments’ datasets over the next decade. In parallel, Belle II is scheduled to start collecting data in 2019 and is expected to record enough to shed light on this query in a few years. 

Physicists around the globe are eagerly waiting to compare notes.

Major science laboratories from around the world today announced a Global Physics Photowalk competition that will be open to amateur and professional photographers. Physics facilities in Asia, Australia, Europe and North America will open their doors for a rare opportunity to see behind the scenes of some of the world’s most exciting and ground-breaking science.

The photowalk will involve local and national competitions, with the winning national photos submitted to a global judging panel. The program is organized by the Interactions Collaboration and supported by the Royal Photographic Society (RPS). The global shortlist will be announced in August, followed by a public vote.

Confirmed locations include CERN, the home of the Large Hadron Collider; underground laboratories in the US, Australia and the UK; and labs and facilities in Italy, the UK, the US, Canada, and—for the first time—China.

“This is a fantastic celebration of the stunning beauty of science on an international scale," says Mark Richardson, Chair of the RPS Science Committee. "The world’s best scientific research is based on international collaboration, a worldwide melting pot of expertise and technologies, each working for the benefit of our global society and economy. The photowalk is a rare opportunity to capture work behind the scenes at the world’s best international laboratories and capture it, frame by frame.

The international competition will include the following laboratories:

• Boulby Underground Laboratory (UK)
• Brookhaven National Laboratory (US)
• CERN (France/Switzerland)
• Chilbolton Observatory (UK)
• Daresbury Laboratory (UK)
• Fermilab (US)
• Frascati National Laboratories (Italy)
• Gran Sasso National Laboratory (Italy)
• Institute of High Energy Physics (China)
• Lawrence Berkeley National Laboratory (US)
• Legnaro National Laboratories (Italy)
• Rutherford Appleton Laboratory (UK)
• SLAC National Accelerator Laboratory (US)
• Southern National Laboratories (Italy)
• Stawell Underground Physics Laboratory (Australia)
• Sanford Underground Research Facility (US)
• TRIUMF (Canada)
• UK Astronomy Technology Centre (UK)

Places for each photowalk are limited and are strictly by registration only.

Details about the facilities and local photowalks can be found by clicking on the links above, and you can follow along on social media with #PhysPics18. A selection of winning images from previous photowalks is available here. 

Editor's note: this article was adapted from a press release by the Interactions Collaboration.

The next two years are pivotal for the Deep Underground Neutrino Experiment, the international particle physics experiment hosted by the US Department of Energy’s Fermi National Accelerator Laboratory.

In a vote earlier this month, the DUNE collaboration elected Stefan Soldner-Rembold, professor of particle physics at the University of Manchester, as its new co-spokesperson to help guide the experiment through these next two years. Soldner-Rembold has experience leading a large collaboration—he was co-spokesperson of the 500-member DZero experiment at Fermilab from 2009 to 2011—and has been working in neutrino physics for more than a decade.

Soldner-Rembold has served in several leadership positions within the DUNE collaboration, including chair of the Speakers Committee, and was elected as a member of the DUNE Executive Committee in 2016.

Two prototype detectors for DUNE are scheduled to be completed at CERN in Switzerland later this year, and technical design on the experiment’s full-size detector will be worked out over the next 18 months. The DUNE collaboration continues to grow—it currently includes more than 1,000 members from 31 countries—and continues to attract young minds from around the world, eager to contribute to this global-scale neutrino experiment.

“This is a formative period for DUNE,” Soldner-Rembold says. “What we decide now will shape the detectors and the way the collaboration works for the next 10 to 20 years. I’m thrilled to be stepping in as co-spokesperson during such an exciting time.”

It’s also a time in which the UK’s contributions to DUNE are ramping up. The UK has committed $88 million to the construction of the experiment (including the facility that will house it and the accelerator upgrades that will power it), and Soldner-Rembold is currently leading the UK-US consortium designing and constructing vital components of the DUNE detector. Prototypes of these components are currently being installed in the ProtoDUNE detectors under construction at CERN, another major partner in DUNE.

“To build the world’s best neutrino detector, we need to attract further international partners,” Soldner-Rembold says. “The election of an international co-spokesperson sends a signal to other countries that this is an interesting and exciting project that they should join and commit to.”

Over the next few years, Soldner-Rembold says, it will be important to continue to encourage young scientists to participate in DUNE.

“In order to create a vibrant and strong collaboration, we need to encourage the next generation of young physicists to be engaged with the project,” he says.

Soldner-Rembold will take over the position from Mark Thomson of the University of Cambridge and will join Edward Blucher of the University of Chicago as co-spokesperson.

“I look forward to working closely with Stefan,” Blucher says. “His wealth of experience will prove invaluable as the DUNE collaboration navigates the exciting years ahead.”