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 Antimatter sounds like science fiction, and it has certainly powered its fair share of imaginary space ships and interplanetary blasters. But antimatter itself is fact, not fantasy. Antimatter is the opposite of matter: Bring the two of them into contact and they annihilate each other, generating energy according to Einstein’s famous equation E = mc
 
 2
 
 . For every kind of matter particle we know about, there is an antimatter “opposite.” Physicists have observed antimatter versions of electrons, protons and neutrons. We’ve even made a few antimatter atoms, including anti-hydrogen and anti-helium. In fact, the only thing stopping us from making an entire anti-universe—with an anti-you, anti-me, anti-everything—is that it’s very hard to make enough antimatter. And that’s a mystery too. Scientists think that, when the universe was young, antimatter and matter were made in equal quantities, yet in the universe we see only matter. Why is that? Nobody knows the answer, but it is one of the most pressing questions of modern physics.
 
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 Don Lincoln Explains Antimatter | NOVA’s Nature of Reality
 </a>
 

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 Go Deeper
 
 

 Editor’s picks for further reading
 


 BigThink:
 <a href="http://www.youtube.com/watch?v=Oo8a9FCgnfk&amp;feature=fvwrel">
 The Search for Antimatter
 </a>
 
 In this video, Michio Kaku answers questions about antimatter.
 

 CERN: Antimatter:
 <a href="http://livefromcern.web.cern.ch/livefromcern/antimatter/">
 Mirror of the Universe
 
</a>
 Everything you want to know about antimatter.
 

 Fermilab:
 <a href="http://www.youtube.com/watch?v=en2S1tBl1_s&amp;feature=plcp">
 What Is Antimatter?
 
</a>
 In this video, Fermilab scientist Don Lincoln describes antimatter and its properties.
 

 NOVA scienceNOW:
 <a href="http://www.youtube.com/watch?v=XCkw0Hvp1SA">
 Antimatter Engines
 </a>
 
 In this video, Neil deGrasse Tyson answers questions about antimatter propulsion.
 
</section>

Antimatter 101

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 Astronomers have a pretty sweet job. They are paid to stare at the heavens and wonder. Some of their observations are pretty ordinary, but some observations are revolutionary—like the measurements of galaxy rotation that convinced astronomers that our universe is studded with invisible mass called dark matter. In this pencast, I will explain how that apparently simple observation led astronomers to such an extraordinary conclusion.
 
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 Don Lincoln Explains Dark Matter
 </a>
 

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 When astronomers watch rotating galaxies and compare their observations with predictions based on Newton’s laws of gravity, they find something strange. Stars near the center of galaxies are well behaved and move as expected. However stars farther from the center are rebellious. They move far faster than the laws of physics predict they should; so fast, in fact, that these galaxies shouldn’t exist: They should be ripped apart. Since we know that galaxies have existed for billions of years, this is a glaring paradox.
 

 This conundrum nagged at scientists for over half a century. Astronomers proposed many solutions, from suggestions that our understanding of inertia is wrong to new ideas of how gravity works. But the likeliest explanation is that galaxies contain more matter than we see.
 

 When I say “see,” I don’t mean just “seeing” with our eyes or even with the familiar telescopes that are sensitive to visual light. I mean “seeing” with any and every kind of telescope in our arsenal, including the huge antennas that pick up radio emission from the vast clouds of hydrogen that typically make up most of the mass of galaxies.
 

 To acknowledge the fact that this proposed extra matter is invisible to our ordinary methods of detection, we call it “dark matter.” We know it’s out there, but what is it? Come back next week for more about the quest to capture traces of dark matter here on Earth.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 American Museum of Natural History:
 <a href="http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_rubin.html">
 Vera Rubin and Dark Matter
 
</a>
 In this profile, learn how astronomer Vera Rubin’s galaxy observations helped establish the presence of dark matter.
 

 NOVA scienceNOW:
 <a href="http://www.pbs.org/wgbh/nova/physics/dark-matter-mystery.html">
 The Dark Matter Mystery
 </a>
 
 In this video, explore the evidence for dark matter.
 

 TED:
 <a href="http://www.ted.com/talks/patricia_burchat_leads_a_search_for_dark_energy.html">
 Patricia Burchat sheds light on dark matter
 </a>
 
 In this talk, physicist Patricia Burchat explores dark matter and dark energy.
 
</section>

Dark Matter 101

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 A new eye is now open to the cosmos. The
 <a href="http://www.darkenergysurvey.org/DECam/camera.shtml">
 Dark Energy Camera
 </a>
 , which saw first light on September 12, 2012, is a spectacular new scientific facility with the grandest of goals: no less than understanding the evolution and fate of the entire universe.
 

 For every telescope, “first light” is the moment when the optics and camera are assembled into a single instrument and turned to the night sky for the first time. But first light is just the beginning. While it often yields a spectacular photo or two, single photographs rarely lead to substantive results. Modern measurements require a subtle understanding of the equipment’s idiosyncrasies and the operators and scientists must spend a while familiarizing themselves with their instrument’s performance. After the facility has been put through its paces, real research begins. On January 9, Joshua Frieman, leader of the
 <a href="http://www.darkenergysurvey.org/index.shtml">
 Dark Energy Survey
 </a>
 (DES) collaboration,
 <a href="http://astro.uchicago.edu/~frieman/Talks/Frieman-DES-AAS-press-briefing-1-2013.pptx">
 announced
 </a>
 at the 221st meeting of the American Astronomical Society in Long Beach, California, that the team is wrapping up this getting-to-know-you phase, known as the commissioning period. They have already made interesting scientific observations, including discovering distant supernovae and clusters of galaxies.
 

 The 570 megapixel Dark Energy Camera is hooked up to the venerable four-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory, located in the Chilean Andes. Together, they will complete a study of the sky called the
 <a href="http://www.darkenergysurvey.org/">
 Dark Energy Survey
 </a>
 , which may bring us closer to an answer to one of the deepest mysteries in cosmology: What is dark energy?
 

 This question has been vexing astronomers since 1998, when astronomers discovered that, contrary to their expectations, the expansion of the universe wasn’t slowing down—
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html">
 it was speeding up
 </a>
 ! Cosmologists accounted for this surprising behavior by invoking a form of repulsive gravity first imagined by Einstein. But Einstein abandoned the idea when Hubble’s observation of the expanding universe made it seem unnecessary. Today, in the absence of a specific explanation, astronomers describe it with the generic term “dark energy.”
 

 The Dark Energy Survey will help scientists probe the nature of dark energy. Over the course of 525 nights over five years, astronomers will survey a quarter of the southern sky to a depth of billions of light years, revealing the how the cosmic expansion rate has changed over nearly nine billions of years.
 

 The Dark Energy Survey studies the universe in four distinct ways:
 
<ol>
<li>
 It looks for 4,000 distant supernovae. By comparing their distance (determined by simultaneously observing their brightness and their redshift, the change in their color due to the expansion of the universe, and comparing these two numbers), astronomers will get a good handle on the cosmic expansion history.
 </li>
<li>
 The camera will also study patterns in the spatial distribution of galaxies that are set by a phenomenon called baryonic acoustic oscillations. When the universe was smaller and hotter, the explosion of the Big Bang caused the universe to ring like a bell as the sound of the Big Bang rippled across the cosmos. About 370,000 years after the Big Bang, the universe cooled below a critical temperature, freezing these vibrations into patterns we can still see in microwave radiation and distribution of galaxies that are blazoned across the sky. This process is analogous to flash freezing the ripples on the surface of a pond. By comparing the apparent size of the ripples with their initial size, which can be calculated using information about the conditions that prevailed in the early cosmos, astronomers can provide crucial data on the shape of space itself: whether it is flat or curved and, if curved, exactly how.
 </li>
<li>
 The camera will also have the capacity to study the size and makeup of vast clusters of galaxies. Because the properties of dark energy help determine how and when these clusters formed, by studying their history, we can gain new insight into dark energy.
 </li>
<li>
 Finally, the Dark Energy Camera will see how light from distant clusters of galaxies is being bent by mass between those clusters and our telescopes. This information will tell us more about how dark energy has shaped the distribution of matter throughout the universe by studying the size and shape of clusters of galaxies over time. In total, the camera will be able to track three hundred million galaxies!
 </li>
</ol>

 Through these four distinct strategies—each with different strengths and weaknesses—the survey will provide independent measurements of the dark energy of the universe.
 

 The portion of the sky that the DES will study in detail is observable from Chile from September to February. Since first light, the collaboration has put their equipment through its paces. To get an early glimpse at a complete set of data, the DES collaboration will spend the rest of the 2012-2013 observation season studying a little under 5% of the region they will eventually explore. Using this strategy, they will have as good a measurement on a small portion of the sky after just a few nights of observation as they will over their entire target after five years. This will allow a relatively quick analysis of a subset of the sky and the caliber of this small study will already be world-class. The final shakedown is expected to be completed in February and in September of 2013, the survey will start in earnest, hopefully leading to new insights into the nature of dark energy.
 

 Stay tuned. This is a very exciting time.
 
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 src="https://www.pbs.org/wgbh/nova/media/images/reidar1.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 

 The center of the Milky Way galaxy lends its awesome beauty to the skyline above the telescope domes at Cerro Tololo. The Greater and Lesser Magellanic Clouds grace the upper left corner of the photo. (Photo credit: Reidar Hahn)
 
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 Go Deeper
 
 

 Editor’s picks for further reading
 


 Baryon Oscillation Spectroscopic Survey:
 <a href="http://astro.berkeley.edu/~mwhite/bao/">
 Baryon acoustic oscillations and dark energy
 </a>
 
 On this web page, learn more about how baryon acoustic oscillations can be used to study the history of dark energy.
 

 Minute Physics:
 <a href="http://www.youtube.com/watch?v=v6o2bUPdxV0">
 2011 Nobel Prize: Dark Energy
 </a>
 
 In this video explainer, guest narrator Sean Carroll explains dark energy and cosmic expansion in honor of the 2011 Nobel Prize in physics.
 

 NOVA scienceNOW:
 <a href="http://www.youtube.com/watch?feature=player_embedded&amp;v=6ypfFz38Lg4#!">
 Cosmic Perspective: Dark Matter
 </a>
 
 In this short video, astrophysicist Neil deGrasse Tyson muses on just how much we don’t know about the mysterious components of the universe, dark energy and dark matter.
 
</section>

The Dark Energy Survey: What Happens When You Can See Forever?

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 Beneath the ground, just west of Geneva, Switzerland, a leviathan is stirring, slowly rousing from a two-year-long nap. The surrounding area has hosted giants before; the nearby Jura Mountains lend their name to the Jurassic period. However, this colossus isn’t the familiar Diplodocus or Stegosaurus, but rather a wonder of technology. The Large Hadron Collider is back.
 

 <figure class="richtext-image full-width"><img alt="It’s Back! The LHC Prepares for Round Two-cern_620.jpg" class="richtext-image full-width" height="348" width="620"
 src="https://www.pbs.org/wgbh/nova/media/images/cern_620.width-800.jpg"/><figcaption>
 A worker helps prepare the LHC for its next phase of operations. Image credit: Anna Pantelia/CERN. Copyright CERN
 <div class="credit"></div></figcaption>
 </figure>
 


 The LHC was a quarter-century in making when it began operations in early 2010. From 2010 to 2012, the world’s largest particle accelerator smashed together beams of protons traveling nearly at the speed of light. The temperatures in those collisions were a hundred thousand times hotter than the center of the sun and ten times hotter than the center of a raging supernova; the last time the universe was that hot was just a tenth of a trillionth of a second after the Big Bang. This first working phase of the LHC experiment led to the
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/07/higgs-fireworks-on-july-4/">
 discovery
 </a>
 of the
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/tag/higgs-boson/">
 Higgs boson
 </a>
 , the final missing piece of the Standard Model of particle physics.
 

 At the end of 2012, the LHC was ready for a well-deserved rest and was shut down for refurbishments, upgrades, and retrofits. That process is now complete, and this spring, the LHC will power up with substantially enhanced capabilities. The beam energy has been raised from eight to 13 trillion electron volts and the collision rate is expected to nearly triple. The increase in collision energy will raise the temperature inside the collisions by 60%. All of the detectors surrounding the ring have had their capabilities enhanced, with faster electronics and more modern detector technologies. The four experiments are ready to receive the onslaught of data that will soon arrive. With the increased beam intensity and anticipated multi-year running period, the data rate will triple and the total recorded data will be more than ten times what the instruments had gathered previously.
 

 The new LHC won’t just do more of what it did before. In accelerators, energy means discovery: Higher-energy collisions make more, and potentially more exotic, particles. For instance, depending on exactly how the Higgs boson is produced, the LHC’s energy boost will make three to five times as many Higgs bosons per second from beams of the same intensity. However, given that the beam intensity is expected to triple, we should see between nine and 15 times as many Higgs bosons as we did back in 2012. In fact, while Higgs bosons haven’t quite yet become pedestrian, they will soon turn into either a pesky background that must be understood and suppressed so that we can study even rarer physical phenomena, or we will turn to events in which the Higgs boson is produced in conjunction with another particle so that we can investigate more mysterious corners of the Standard Model.
 

 However, before that happens, scientists will first “rediscover” the Higgs boson in the new accelerator and detector environment and then make precision measurements of its properties. This is extremely important, as the Higgs boson and the associated Higgs field offer a promising window into new physics beyond the standard model. Indeed, one of the most distressing outcomes of the first measurements from the LHC is just how well they agree with the predictions of the Standard Model. This is comforting, but it doesn’t point us in the right direction—or any direction at all, really—to deepen our understanding of the universe. If the data agrees with your model, you’re kind of stuck. When you finally discover a measurement that disagrees with your prediction, you have a crucial thread at which to tug. This could lead to the entire sweater of the Standard Model being unraveled before we knit a new model that does a better job of describing the data.
 

 The Higgs discovery raises one glaring question that completely vexes scientists. The mass of the Higgs boson
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2013/02/why-is-the-higgs-so-light/">
 is much smaller than seems “natural”
 </a>
 from the theory. In the most naïve reading of the Standard Model, the mass of the Higgs boson should be in the vicinity of the Planck energy (~10
 
 19
 
 GeV), rather than the 125 GeV that was measured. There are many proposed explanations of this mystery, ranging from supersymmetry to extra spatial dimensions to a Higgs boson consisting of even smaller objects. Hopefully the data taken over the next few years will shed light on and perhaps answer that nagging conundrum.
 

 While it’s hard to make predictions, especially about the future, we hope that the first circulating beams will occur in early March, with first collisions occurring in May and the full data-taking program commencing in June. While these dates might change a little due to the teething pains that accompany the commissioning of such a huge instrument, there is one thing we know: The LHC is about to plow ahead into the unknown and maybe, just maybe, reveal of mystery of the universe that causes us to rewrite the textbooks.
 


 Go Deeper
 
 

 Our picks for further reading
 


 CERN:
 <a href="http://press.web.cern.ch/backgrounders/lhc-season-2-stronger-machine">
 LHC Season 2: A stronger machine
 </a>
 
 This backgrounder from CERN’s press office details the LHC’s equipment upgrades.
 

 Fermilab:
 <a href="https://www.youtube.com/watch?v=NgFaJMnd3FQ">
 #Hashtag3 #RestartLHC
 </a>
 
 In this video parody, two US LHC scientists give a Twitter-style preview of the LHC restart.
 

 NOVA:
 <a href="http://www.pbs.org/wgbh/nova/physics/big-bang-machine.html">
 Big Bang Machine
 </a>
 
 Watch NOVA’s in-depth look at the discovery of the Higgs.
 
</section>

It’s Back! The LHC Prepares for Round Two

Physicists are getting ready for new discoveries from the upgraded Large Hadron Collider.

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 What do you get when you combine four quarks and an antiquark?
 

 If you think this sounds like the opening of a particle physicists’ riddle, you aren’t too far off. Hypothetically, this particular quark combo makes a “pentaquark.” Despite decades of searching, physicists haven’t been able to actually find a pentaquark. Now, though, there’s a hint that two pentaquarks have unexpectedly come out of hiding.
 

 <figure class="richtext-image full-width"><img alt="pentaquark_CERN_620" class="richtext-image full-width" height="349" width="620"
 src="https://www.pbs.org/wgbh/nova/media/images/pentaquark_cern_620.width-800.jpg"/><figcaption>
 Illustration of a possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. © CERN
 <div class="credit"></div></figcaption>
 </figure>
 


 If the new result holds up—a big if—the unexpected discovery would add a new species of particle to the standard model’s menagerie. But the
 <a href="http://arxiv.org/abs/1507.03414">
 measurements
 </a>
 , recently
 <a href="http://press.web.cern.ch/press-releases/2015/07/cerns-lhcb-experiment-reports-observation-exotic-pentaquark-particles">
 announced
 </a>
 by the team collaborating on the
 <a href="http://lhcb-public.web.cern.ch/lhcb-public/en/detector/Detector-en.html">
 LHCb experiment
 </a>
 , are truly perplexing. While the results were submitted for publication a couple of days ago, the first discussion in a large public conference occurred on July 23 at the 2015 meeting of the high energy physics division of the European Physical Society, where I had the opportunity to hear Sheldon Stone, who led the analysis, talk about the result. It’s certainly a topic of both excited and skeptical discussion here at the conference.
 

 Pentaquarks were first predicted in 1964 by Murray Gell-man and George Zweig in the separate and competing papers in which they first hypothesized the existence of quarks. (Gell-man’s name “quark” has stood the test of time, while Zweig independently proposed the now-defunct “aces.”) Physicists have looked for pentaquarks for a long time, unsuccessfully. We don’t know why there has been no evidence for their existence for so long. Maybe they don’t exist. Or maybe they do and the LHCb experiment has finally found them.
 

 Quarks are the building blocks of protons and neutrons and, as far as we know, they are the smallest basic units of matter. Quarks combine with other quarks according to the rules of quantum chromodynamics (QCD), which is the theory describing the behavior of the strong nuclear force, which is the strongest of the known subatomic forces. Pair a quark with an antiquark, and you’ve got a particle called a meson; three quarks make a baryon, like a proton or neutron. The new pentaquark—if it really is a pentaquark—seems to be made up of two up quarks, a down quark, and a charm quark/antiquark pair.
 

 The announcement is the latest chapter in a somewhat dubious story of now-you-see-now-you-don’t discovery. In 2002, scientists in Japan announced the discovery of a particle with a mass about 1.5 times that of a proton. They called it the Θ
 
 +
 
 , and argued that it was a kind of pentaquark. This announcement triggered a flurry of searches by other groups of experimenters, with some groups confirming the Θ
 
 +
 
 and finding other particles that were claimed to be different pentaquark candidates, while other researchers found no evidence for any new particles at all. The excitement continued for three years until 2005, when the community decided that the original announcement was wrong. The death knell of the Θ+ sounded when a group of scientists at the Thomas Jefferson National Accelerator Facility (TJNAF) in Newport News, Virginia, repeated the initial Japanese measurement with far more data. The TJNAF scientists saw no evidence for the existence of the Θ
 
 +
 
 , and the community consigned it to the dustbin of history as one of many particle “discoveries” that ultimately didn’t pan out.
 

 The particles recently announced by the LHCb experiment aren’t the Θ+. Instead, the new particles have a mass of about 4.5 times that of the proton. The LHCb team wasn’t actually searching for pentaquarks when they made their measurements. Instead, they were studying how a particle called the Λ
 
 b
 
 baryon decays. To their surprise, they found that a fraction of the time, some of the “daughter” particles left behind by the decay seemed to be coming from an unknown parent particle. So what the heck was it?
 

 <figure class="richtext-image full-width"><img alt="LHCb_figure" class="richtext-image full-width" height="649" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/lhcb_figure.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 


 The LHCb team was unable to reconcile their measurements with any of the known or predicted particles of the Standard Model. They seemed to need something new. After testing out lots of hypotheses, they considered the discredited pentaquarks. (Remember that pentaquarks are a prediction of the theory of QCD, they’ve just never been seen before.) One pentaquark wasn’t enough to fit their data, but two did the trick. When they included two new pentaquark particles in their calculations, the data and theory agreed.
 

 The two new particles have an unusual amount of quantum mechanical spin, specifically 3/2 and 5/2. (Protons, neutrons and electrons are all spin ½.) Like all particles that are bound by the strong nuclear force and decay under its rules, they live for a very short time, specifically about 10
 
 -23
 
 seconds.
 

 Given the checkered history of previous pentaquark searches, physicists are naturally skeptical. So it is worth dissecting the claim. The first question is whether scientists are confident that they’ve discovered some kind of new particle. Here, the claim is on firmer ground: the two detections have significance of nine and 12 standard deviations respectively. (The usual standard in particle physics to claim the discovery of a phenomenon is five standard deviations, and larger numbers mean more certainty. Nine and 12 are very strong numbers.)
 

 It’s less certain whether the new particles are really pentaquarks. There are good reasons for skepticism: For one thing, the makeup of the new pentaquarks—two ups, a down, and a charm quark/antiquark pair—seems improbable. It should be easier to make a pentaquark consisting of only up and down quarks, which are lighter than charm quarks, and such a particle has never been discovered. Discovering a charm pentaquark first feels like going fishing and pulling up two sharks and no trout. A second possibility is that the new discovery is actually a sort of “molecule”: a particle called a J/ψ attached to a proton, roughly similar to how a deuteron is a proton and neutron bound together. Both have the same quark content, but only “five things in a bag” qualifies as a “real” pentaquark.
 

 When I caught up with Sheldon Stone during the coffee break after his talk at the conference, he speculated that the higher mass of the charm quarks could make the resulting pentaquark more stable or perhaps somehow makes this sort of pentaquark more likely to form. He cautioned, however, that this was speculation on his part and more work would be required to substantiate these ideas.
 

 Theoretical physicists are likewise skeptical. Frank Wilczek, professor of physics at MIT and winner of the Nobel Prize in physics for his contributions to the development of the theory of QCD was
 <a href="https://www.newscientist.com/article/mg18324545-200-the-power-of-five/">
 excited about the possibility of the existence of the pentaquark
 </a>
 , but cautious about the measurement.
 

 So what will it take for the community to embrace this exciting development? Well, as Carl Sagan is famous for noting, extraordinary claims require extraordinary evidence. It is also true that independent confirmation is key. Accordingly, other LHC experiments will try to repeat the analysis approach reported by the LHCb collaboration in order to see if their measurement can be replicated. In addition, theorists will try to see if they can find a mechanism within QCD that will explain why pentaquarks containing charm quarks are more likely to form than ones with lighter quarks.
 

 Now, taking a more personal perspective, what do I think? First, Sheldon Stone made a persuasive and thorough case at his talk. I think the LHCb experiment is a world class collaboration, with some of the finest minds on the planet and ample experience in the subject matter. Further, they are well aware of the history of the pentaquark and would not lightly propose this hypothesis without adequate care. However, I am very cautious of claims of this nature, especially without confirmation from other experiments. I think the only sensible approach is to view the claim charitably, but critically. Taking a phrase from President Ronald Reagan, I “trust, but verify.” I think the next few months will be very interesting.
 


 Go Deeper
 
 

 Author’s picks for further reading
 


 arXiv:
 <a href="http://arxiv.org/abs/1507.03414">
 Observation of J/ψp resonances consistent with pentaquark states in Λ
 
 0
 

 b
 
 →J/ψK
 
 −
 
 p decays
 </a>
 
 Read a pre-print of the scientific paper describing the pentaquark observation.
 

 Inside Science:
 <a href="http://www.insidescience.org/blog/2015/07/21/party-five-physicists-discover-long-sought-%E2%80%98pentaquark%E2%80%99-stroke-luck">
 Party of Five! Physicists Discover Long-Sought ‘Pentaquark’ In Stroke of Luck
 </a>
 
 Science writer Michael Greshko on the “accidental” discovery of the pentaquark.
 

 Nature:
 <a href="http://www.nature.com/news/forsaken-pentaquark-particle-spotted-at-cern-1.17968">
 Forsaken pentaquark particle spotted at CERN
 </a>
 
 Nature News reports on the latest chapter in the pentaquark story.
 
</section>

An unexpected discovery might be the first detection of a new particle called a pentaquark.

What the Heck is a Pentaquark?

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 In the early hours of October 7, 2014, while most of America slumbered, members of the Swedish Academy of Sciences were meeting to decide to whom they should award the Nobel Prize in Physics. The Nobel Committee often recognizes grand discoveries, like the prediction of the Higgs boson and the observation of the accelerating expansion of the universe. Thus many were surprised to see the 2014 prize honor a very practical invention: blue light emitting diodes. Yet this year’s awardees, Shuju Nakamura, Isamu Akasaki and Hiroshi Amano, richly deserve the prize.
 

 <figure class="richtext-image full-width"><img alt="led_lamp_620" class="richtext-image full-width" height="348" width="620"
 src="https://www.pbs.org/wgbh/nova/media/images/led_lamp_620.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 



 The technical achievement
 


 LEDs are one of many useful spinoffs from the basic science of quantum mechanics. In the standard quantum cartoon, electrons orbit the center of atoms, kind of like planets around the Sun. (The planetary model is not completely accurate, but it has many good features that help visualize what is going on in the quantum realm.) Imagine a simple atom with a single electron. According to quantum mechanics, the electron can be in a series of discrete orbits, as if a single planet around our Sun could be in the orbit of Mercury or Venus or any of the planets, but nowhere in between. Electrons near the nucleus have lower energy, while ones farther away have more energy.
 

 When an electron moves from a high-energy orbit to a lower-energy one, it emits energy in the form of light. The color of the emitted light depends on the energy difference between the old orbit and the new one. The bigger the energy difference, the bluer the light.
 

 With a few tweaks, these basic ideas also apply to groups of atoms, like those which make up molecules and solid materials. Which brings us to LEDs. To make an LED, you must fuse together two different types of semiconductor, one with electrons to spare (“n-type”) and one with extra room to hold those electrons (“p-type”). When the proper voltage is placed across the diode, electrons can move and fall into the holes, releasing energy. Early diodes only released a little energy, so they emitted infrared light. To make a diode that could produce visible light, scientists had to widen the energy gap. By the early 1970s, scientists had discovered how to make bright red, yellow, and green light by choosing the proper semiconductor materials and judiciously spiking, or “doping,” them with impurities.
 

 But scientists struggled to make blue light, which requires a very high energy gap. Finally, in 1994, Shuji Nakamura, then employed by the Nichia Corporation, developed high-brightness blue LEDs using indium gallium nitride (InGaN), a mix of gallium nitride and indium nitride. By adjusting the amount of indium in the semiconductor, he tuned the energy gap to produce blue light.
 

 Nakamura wasn’t the first to attempt to use gallium nitride to create LEDs, however most solid state physicists of the era had moved on to different materials. First, nobody knew how to prepare a surface on which gallium nitride crystals could grow, and further, nobody knew how to make p-type layers of GaN. Isamu Asaki and Hiroshi Amano showed it could be done using sapphire as a substrate and were eventually able to create the required p-layer of the material. Quite by accident, they also discovered that a scanning beam microscope increased the brightness of the light emitted by LED.
 

 Nakamura grew his own GaN crystals and developed a simpler method for making the p-type layers using careful heating. He was also the first to understand why the electron beam boosted the light output of LEDs: it was removing hydrogen, just as his own heating technique did.
 

 Modern blue LEDs require a more sophisticated approach, including varying the amount of indium and gallium, although the basic technique is the same as Nakamura’s. Starting with a sapphire substrate, several alternating layers of gallium nitride are added, some doped with indium and others doped with aluminum. These extra elements are key to increasing the efficiency and brightness of the blue LEDs. Further, with the introduction of aluminum, it is possible to make even bluer LEDs—even ultraviolet ones.
 


 The greatest benefit on mankind
 


 To understand why this seemingly mundane development warrants such recognition, one must return to Alfred Nobel’s will, in which he provided the seed money to start the prizes that bear his name. When Nobel’s brother died, a French newspaper mistakenly published Alfred’s obituary. Alfred Nobel was horrified to see himself called a “merchant of death” and a man “who became rich by finding ways to kill more people faster than ever before.” He resolved to repair his legacy by bequeathing a prize to be awarded to “those who…have conferred the greatest benefit on mankind.” Nobel wanted to be remembered as a man who helped make the world a better place.
 

 This year’s award clearly fits the bill. “I really think that Alfred Nobel would be very happy about this prize,” said Per Delsing, head of the Nobel Committee for Physics, in announcing the prize. ”It’s really an invention and it’s really something that will benefit most people.”
 

 The invention of bright blue LEDs brought entirely new industries into existence. Now, blue, red and green LEDs could be combined to make white—or any other color—light. This development led to the power-efficient screens for cell phones, TVs, computers, iPads, and many other electronic miracles of the modern world.
 

 However the real impact of blue LEDs goes well beyond our rainbow-colored gadgets. Today, LEDs are bright enough to use as light sources. Just as Thomas Edison’s original incandescent light bulb revolutionized the early years of the 20
 
 th
 
 century, LEDs are poised to revolutionize the 21
 
 st
 
 .
 

 LEDs can now emit far more light using far less power than incandescent and fluorescent bulbs. For instance, an incandescent light bulb can emit about 16 lumens per watt of electrical power, and a fluorescent light manages about 70 lumens per watt. In comparison, a modern white LED can emit 300 lumens per watt, meaning that it consumes only about 5% of the power of an incandescent. Given that about a quarter of the world’s electricity is used to generate light, the invention of efficient lighting can have considerable economic and environmental impact, potentially reducing the greenhouse gas emissions that are driving anthropogenic global warming. In the developing world, bright, efficient LEDs could provide solar-powered, off-grid energy for homes, hospitals, and more.
 

 This year’s Nobel Prize in physics is very well deserved and reflects Alfred Nobel’s desired legacy of recognizing discoveries and developments that have greatly benefited mankind.
 


 Go Deeper
 
 

 Author’s picks for further reading
 


 bockphysics:
 <a href="https://www.youtube.com/watch?v=ksS-FJ3B8Og">
 Primary Colors of Light – Mixing of Colors
 </a>
 
 Watch as blue, green and red light combine to make white light in this video.
 

 HowStuffWorks:
 <a href="http://electronics.howstuffworks.com/led.htm">
 How Light Emitting Diodes Work
 </a>
 
 Discover more about the physics and chemistry of LEDs in this step-by-step introduction.
 

 Nobelprize.org:
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/popular-physicsprize2014.pdf">
 Blue LEDs – Filling the world with new light
 </a>
 
 Detailed information on the honorees, their work, and its impact, from the Royal Swedish Academy of Sciences.
 
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Discover the physics behind blue LEDs, honored with the 2014 Nobel Prize in Physics.

How Blue LEDs Work, and Why They Deserve the Physics Nobel

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 From
 
 The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind
 
 , an insider’s account of the LHC’s operational history and the search for the Higgs boson, by Don Lincoln. Published by Johns Hopkins University Press. Reprinted by permission of the publisher.
 


 The Victorian era mathematician Augustus de Morgan wrote:
 
<blockquote>

 Great fleas have little fleas upon their backs to bite
 
 And little fleas have lesser fleas, and so ad infinitum.
 
 And the great fleas themselves, in turn, have greater fleas to go on,
 
 While these again have greater still, and greater still, and so on.
 
</blockquote>

 This oft-quoted passage is a parody of Jonathan Swift’s 1733’s
 
 On Poetry: A Rhapsody
 
 , which was written about poetry. However, scientists have taken those lines as a metaphor for the natural world. As one learns about the microworld, one is quickly faced with the observation that all matter is made of molecules. Molecules are in turn made of atoms which are themselves made of electrons and atomic nuclei. The nuclei are made of protons and neutrons and these are composed of quarks.
 

 <figure class="richtext-image full-width"><img alt="ruler_620" class="richtext-image full-width" height="348" width="619"
 src="https://www.pbs.org/wgbh/nova/media/images/ruler_620.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 


 However, as far as we know, quarks and electrons are it. That’s the end of the line as far as structure goes. Unlike the atom or proton, which have a rich structure with complex interactions between their components, the quarks and electrons are currently believed to have no internal structure at all. Both theoretically and physically, they are considered to be mathematical points.
 

 Of course anyone with an ounce of imagination can’t help saying, “Now just hold on a minute. Why couldn’t the quarks and leptons themselves have an internal structure?” Well there’s only one possible answer and that is “they could.” The quarks and electrons (and, by extension, all leptons) could be made of even smaller objects. Or they (rather improbably) may indeed be fundamental (i.e., have no smaller parts, in other words, structureless).
 

 Before we proceed further, let’s consider the sizes involved. Everything in the microworld is small. A single molecule is so small that you could place a million of them side by side in a single millimeter. They are so small that you can’t use ordinary light to see them. And yet, such smaller objects are enormously large: a billion times larger than the research frontier.
 

 Molecules are composed of atoms, which are about a tenth the size of molecules. The mental picture of an atom as a little solar system, with the sun as a nucleus and planetary electrons, is flawed and yet it is not without merit. It highlights the fact that an atom consists of mostly empty space, with the electrons swirling frenziedly far from a small, dense nucleus. The radius of the nucleus is about 10,000 times smaller than the atom and takes up but a trillionth of the volume.
 

 The nucleus of the atom consists of protons and neutrons, packed tightly together. My mental picture of the nucleus is a mass of frog eggs or marbles after being handled by a toddler with very sticky fingers. Each proton or neutron is about 10
 
 -15
 
 meters wide, and you would need a trillion laid end-to-end to span a single millimeter. That’s small.
 

 Protons and neutrons contain within them quarks and gluons. The simplest way to think of a proton is that there are two up quarks and one down quark stuck in a force field of gluons. Think of three numbered plastic balls in one of those air-blown lottery machines and you get the basic idea.
 

 But the mental picture of quarks as plastic balls has one major flaw. The balls are not much smaller than a lottery machine. Quarks are small. Maybe a better mental picture of the proton is three little flecks of Styrofoam in the same machine.
 

 So what do we know of the size of quarks? Earlier I said that they have no size, and that’s certainly how the current theory treats them. However, as an experimenter, I’m more concerned with measurements. You the reader must be curious as to what measurements have revealed the size of a quark to be. And now the answer . . . a drum roll please . . . they haven’t. This doesn’t mean we know nothing about their size. We’ve studied this question rather thoroughly, and we know precisely how good our equipment is. If quarks (and electrons) were larger than about ten thousand times smaller than a proton, we’d have seen that they have a size. In all of our experiments, we’ve never seen even the slightest believable hint of a size. We therefore conclude that, while we can’t say what the size of a quark or electron actually is, we can safely say that if quarks have size at all, they are smaller than one ten-thousandth the size of a proton.
 

 If this idea is hard to understand, let’s consider how small an object you can see with your eyes. You can easily see a grain of sand. With very considerable effort, you might be able to see the smallest bit of flour in your cupboard. But that’s about it. With your bare eye, you can’t see anything smaller. Thus when you decide to look at a germ with your eye, you could conclude that it has no size, but the strictly correct conclusion you should draw is that germs are smaller than a tiny fleck of flour.
 

 With better equipment, say a powerful microscope, one can see that germs actually do have a measurable size. So once you’ve hit the limitation of your equipment, you simply need to get a more powerful microscope. The microscope that is the LHC and its two primary detectors will observe the size of quarks if they are no less than 20 or 30 thousandths of the size of a proton . . . or they will set a limit that is about two or three times smaller than currently thought.
 

 While observations, intuition, and de Morgan’s ditty may be enough to support a casual suspicion that other levels of matter may occur at ever smaller sizes—a whole new layer or set of layers in the cosmic onion—there are more scientific reasons as well. For instance, consider the periodic table. While Mendeleev intended it to be an organizational scheme, with the formulation of the theory of the nuclear atom and quantum mechanics in the first few decades of the twentieth century, it became clear that the periodic table was actually the first indication of atomic structure, half a century before we truly understood the table’s message.
 

 While the story told by the periodic table clearly hinted at atomic structure, the story of nuclear radiation also suggests the structure of the nucleus. For instance, cesium (
 
 137
 

 55
 
 Cs, with fifty-five protons and eighty-two neutrons) emits an electron and becomes barium (
 
 137
 

 56
 
 Ba, with fifty-six protons and eighty-one neutrons).
 

 Let’s take these historical examples and apply the reasoning to the modern world. We realize that historical lessons do not always apply. But sometimes they do.
 

 <figure class="richtext-image full-width"><img alt="12-0193-03D" class="richtext-image full-width" height="855" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/12-0193-03d.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 


 Our “periodic table” of particles is shown in the image above. Its organization is different from the chemical periodic table. In the figure, there are six types of quarks. The up, charm, and top quarks all have +⅔ charge (in a system where the charge of a proton is +1) and the mass of the charm quark exceeds that of the up quark, which in turn is surpassed by the top quark. Similarly, the down, strange, and bottom quarks all have electric charge -⅓, with the mass increasing as one goes toward the right.
 

 In the modern periodic table, the “chemically similar” units are the rows, in contrast to the columns of Mendeleev’s table. We see that there are three “generations” or carbon copies of the same quark and lepton pattern. This is highly reminiscent of the hints that the chemical periodical table was giving us in the latter half of the nineteenth century.
 

 There is another historical similarity to consider. Just like the various atomic nuclei could decay into other nuclei, so too can the quarks and leptons. A top quark can decay into a bottom quark and a W boson. Likewise, the muon can decay into an electron and two neutrinos. Other types of quark and lepton decay are also possible. In fact, all particles in the second and third generations eventually decay into the particles of the first generation. One crucial clue is that the only force that can change one quark or lepton into another (we say “change the quark or lepton’s ‘flavor’”) is the weak force. Further, specifically only the electrically charged W boson can do the job.
 

 There is no hard evidence that the presence of quark and lepton generations indicate that quarks and leptons are themselves composed of smaller (thus far undiscovered) particles. However, the historical analogy is powerfully suggestive and certainly warrants closer attention. The fact that, by emitting a W boson, one can change the quark or lepton flavor is an extremely valuable clue that is screaming something important at physicists.
 

 I just wish that I had the wits to understand what it was saying.
 

 However, even without the crucial insight that cracks the conundrum wide open, we can speculate intelligently on the subject and (much more important) sift through our mounds of data, looking for additional clues. As with all searches for new physical phenomena, you have to make an educated guess about what to look for and then look for it. So, what are the likely experimental signatures of quark structure?
 

 Historically, one of the best places to look has been the most violent collisions. You smash two objects together and see how many collisions there are at each level of violence. Specifically, you look at the amount of “sideward violence.” Technically we call this
 
 transverse momentum
 
 , which means perpendicular to the beam. There are technical reasons for this choice, but mostly it is because you have to hit something hard for it to go sideways from its original direction.
 

 Today, most physicist take a “wait and see” attitude, preferring to see what hints the universe will give us. Even so, names have been proposed for these objects smaller than quarks, with the most popular being “preon” (for pre-quark). However, each theoretical physicist who has devised a theory has invented his or her own name, with subquarks, maons, alphons, quinks, rishons, tweedles, helons, haplons, and Y-particles all having been suggested. I kind of like the names quinks or tweedles myself.
 

 What will be the next big discovery? I have no idea. It may well be one of the topics mentioned here. Or, even more exciting, it may be something utterly unexpected; something that just hits us out of the blue. As they say, time will tell.
 
</section>

In this excerpt from his new book, Don Lincoln describes the quest for new fundamental particles. 

Don Lincoln

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