Beyond the God Particle

by Leon M. Lederman

  As of this writing (March 2013), the CERN LHC is down for upgrades. It will come back online around January 2015 at the full design energy, whence the protons will each have 7 TeV, giving a total collision energy of 14 TeV. The increase in total collision energy from 8 TeV to 14 TeV causes almost twenty times as many gluon-gluon collisions that produce Higgs bosons and new particles, and will yield much more data. It will also nearly double the discovery reach for new particles that are unanticipated in the Standard Model.

  The run of the LHC starting in January 2015 and extending through to around 2018 will be one of the most important voyages the human species has ever taken into an unknown wilderness. Only this run of the LHC with the large detectors—the experiments called ATLAS and CMS—has a chance of seeing if there is anything “beyond the Higgs boson.”

  THE DETECTORS

  Of course, as in the case of microscopy, the art of particle physics is not simply a matter of accelerating and colliding beam particles. It involves the eyepiece of the microscope, i.e., how to see the debris of these collisions and how to detect any new and unexpected particles produced within the collisions. We have said very little about the behemoth detectors of particle physics in this book. Alas, the detectors of particle physics necessitate another whole book, and we must suffice to point you to the Internet for more. Just search on the keywords “ATLAS CERN” and “CMS CERN,” and you're on your way. You'll find the web addresses http://atlas.ch/ and http://cms.web.cern.ch/; you can see the remarkably short URLs these two experiments enjoy, perhaps in part because CERN was the origin of the World Wide Web in the era of Tim Berners-Lee. (You can also search for other detectors such as “CDF” and “D-Zero,” formerly at Fermilab, and the LEP detectors mentioned above.)

  But we do want to take a moment to salute an old late friend and colleague, and a hero of the resistance movement in France during World War II: Georges Charpak.27

  In the 1960s through 1970s, particle detection involved mainly taking photographs of collisions within large and cumbersome devices called “bubble chambers,” followed by the examination of these photographs by a human eyeball. This is a slow, non-automated, and labor-intensive method. It could not possibly deal with the very high statistics demanded of particle physics experiments that seek to find something like a Higgs boson, a needle in a haystack of trillions of collisions.

  To make progress in detection, automation was required along with more advanced detectors. In 1968, the French physicist Georges Charpak developed the “multiwire proportional chamber.” This is a gas-containing box with a large number of parallel wires, much like a piano harp, each connected to individual electronic amplifiers. Electrically charged particles passing through this array of wires ionize the gas and leave a small electric charge on the nearest wires. Amplifiers can turn this into an electrical signal, and this can be directly coupled to a computer. The system can automatically perform the task of particle detection at rates that are thousands of times greater than previously existing detectors.

  Georges Charpak was later awarded the 1992 Nobel Prize in Physics for his work on the development of these automated particle detectors. Charpak's work has also significantly contributed to the use of this technology in many other fields that use ionizing radiation, such as biology, radiology, and nuclear medicine. Here again, we see knowledge driving the wealth of nations in real time.

  BEYOND THE LHC?

  The problem of designing higher-energy proton (and antiproton) colliders is solved: just build a bigger tunnel and a lot of magnets, etc., and scale up the LHC in size to whatever energy you desire.

  Circular electron beam accelerators of ultra-high energy must overcome the problem of severe energy loss due to synchrotron radiation. This generally requires either a circular machine (a synchrotron) of extremely large radius or a return to the linear accelerator, but with a machine significantly longer and more expensive than those currently in use.

  There is, at this writing, an active discussion under way to form a world collaboration with Japan for the construction of the International Linear Collider (ILC), which would be hosted on Japanese soil.28 The ILC would essentially consist of two long lines of RF cavities. To make the Higgs boson we need to collide an electron head-on with an antielectron (positron) at a total energy of about 245 GeV. This production process makes a Higgs boson with a mass of about 125 GeV plus a Z0 boson with a mass of 90 GeV. We would need another 30 GeV or so to make the collision rate for this process maximal, so we need a total energy of our electron plus positron to be about 125 + 90 + 30 = 245 GeV. To make collisions means we need one linac for the electrons of about 122.5 GeV and another for the positrons of 122.5 GeV, and then we must arrange to have the two beams collide head-on.

  The proposed ILC is a machine approximately 22 kilometers in length, mainly due to the very long string of RF cavities required for acceleration. There are also a lot of additional systems needed to prepare the beams of electrons and antielectrons, and a complex “final focus system” to bring the beams into collision.

  Some of the challenges with the ILC are: (1) Each pulse must contain about a hundred trillion electrons and a hundred trillion positrons, out of which only one pair of electrons collides; the pulses are not re-circulated and re-collided many times, as in a synchrotron, so the system uses a lot of energy. (2) The ILC system, due to power considerations, cannot be scaled upward to arbitrarily higher energies and is ultimately limited to a highest collision energy of about 1 TeV; thus far the LHC has given no hints of new physics in this energy range, so this can only serve as a Higgs factory. (3) The cost of constructing the required string of 8,000 RF cavities is very high, previously estimated by the DOE to be over $16 billion dollars fully loaded, but the number we keep hearing now is $7 billion, and we don't understand what changed the arithmetic over the past decade. In any case, this project will have to draw upon a lot of contributing government resources.

  Yet another possibility would be to build a large circular e+e– Higgs factory. The energy loss per turn in an electron synchrotron goes as E4/R, where E is the beam energy and R is the radius. The synchrotron energy loss rate rapidly rises with energy, E, for fixed radius. But we can make the machine larger to somewhat reduce the energy loss, i.e., increase R. A detailed estimate suggests that such a machine operating under ideal LEP-like conditions could be built with an 80-kilometer-circumference tunnel. The technical challenges with this option are not quite as severe as for a linear collider, mainly because we only need one RF station with perhaps 100 RF cavities that the electrons and positrons pass through many times, but the idea of an 80 kilometer-circumference ring is daunting—we've already had a bad experience going down that path toward the SSC.

  Such a machine could ultimately be converted to a very high-energy hadron collider, the Very Large Hadron Collider (VLHC), with energies three to ten times those of the LHC. This would bring the “energy frontier” back to the US, if it were ever to be built here. It would, alternatively, be an ideal machine for a growing country to invest in to get into the business of particle physics. Indeed, the Gobi Desert in Asia, or Siberia might be ideal places for such a machine.

  Another option is to find a particle that is much heavier than the electron so it doesn't lose energy to synchrotron radiation but that should also be point-like, unlike the spongy proton and more like the point-like electron, so many of the advantages of using electron beams are available. That leads us to the Muon Collider, which we discuss later. To make the Higgs boson directly in a head-on collision, without the slower associated process involving the Z0 requires a Muon Collider and would take only 125 GeV in total muon + anti-muon energy. This has a number of other physics advantages.

  Far and away the biggest issues and challenges for all Higgs factories will be “What will we gain from such a Higgs factory that we won't already have learned at the LHC?” The LHC will do a superb job on improving the detailed understanding of the Higgs boson—will it really be worth ten billio
n dollars to squeeze that sponge a little more once we have all the LHC data in hand? Or should we do something else? Shouldn't we simply upgrade the LHC? What if the LHC runs very long and hard and finds no new particles beyond the Higgs boson? Shouldn't we improve our capability to indirectly access higher-energy scales in nature? Moreover, if the all-important LHC run starting in 2015 GeV does discover new physics at still higher energies than the Higgs boson mass scale, will we really want to have all of our limited resources diverted into an expensive and relatively lower-energy Higgs factory? Shouldn't we wait for LHC at 14 TeV, then think about the next big collider way beyond the Higgs boson? It is simply too early to chart the future course of colliders until about 2017 or so.

  We can also explore new high-energy physics scales using indirect methods. It's called Project X, and it's a pathway beyond to a Muon Collider. Project X is smaller, less expensive, yet offers enormous discovery potential, and is something we could begin to build now.

  PRELUDES TO PARTICLE PHYSICS

  From our modern vantage point, one of a deep understanding of fundamental symmetries governing all forces and space and time, and a wealth of experimental results from accelerators that reach way down to distance scales of a billionth of a billionth of an inch (10-18 centimeters), it is hard to fathom what it was like in the earlier, quaint era, the dawn of particle physics.

  Today, modern, ultra-fast electronics and computers, and the development of strong superconducting magnets and radio-frequency cavities, which led to the great particle colliders, are the essential enabling technologies of physics. Producing W+ and W– bosons, Z0 bosons, and top quarks is now the bread and butter of the LHC. The newly discovered Higgs boson, too, will soon become a familiar landmark along the trail to the shortest distance scales in nature. The LHC experiments will map out this newly discovered realm in great detail. We're now contemplating “Higgs factories” and the push deeper into the details of what the Higgs boson really is, and what may lie beyond.

  A hundred and twenty years ago none of this could have been imagined. No one had yet noticed the “weak interactions” that involve the W+ and W– bosons flickering into existence for a miniscule instant in time as a “quantum fluctuation,” according to Heisenberg's uncertainty principle. The weak interaction processes are so rare that we could seemingly have flipped a switch and turned them off altogether and no one would notice a thing. Yes, the weak interactions had created all the matter out of which we are made and had triggered the supernova explosions that redistributed it throughout the galaxies, so that solar systems with earth-like planets, and life, could form. The weak interactions had stylized the universe for us, giving us our home and the materials that are our being. But that story was hidden, deeply and completely, out of sight.

  In a sudden burst of major scientific breakthroughs at the end of the nineteenth century the faint hints of the “weak interactions” were first noticed by humans. New enabling technologies had come into existence in the late 1800s, the technologies of vacuum pumps, high-voltage coils, and “electrical discharge tubes,” the chemistry of fluorescence and phosphorescence, and photography. Particle physics emerged from the discovery of X-rays and radioactivity in 1895, while accelerators were still a long way off. This burst of discoveries came from careful study and analysis of very rare processes that nature only displays to the most patient and meticulous observers—and often, they came as mere serendipity.

  THE FIRST RAYS OF DAWN

  Wilhelm Conrad Röntgen was a German scientist, born on March 27, 1845, who received a PhD in mechanical engineering from the University of Zurich in 1869. He began research in physics at the University of Strasbourg, then took on a number of academic positions at various universities in Europe. Röntgen had once accepted an appointment at Columbia University in New York City, even purchased his transatlantic tickets, but he changed his plan with the imminent outbreak of World War I. He remained at Munich and died on February 10, 1923.1

  In 1895 Röntgen was investigating the properties of “cathode rays”—produced by high voltage as an “electrical discharge,” like a big spark—that passed through an evacuated glass tube. The tubes, when internally coated with fluorescent material, much like a modern fluorescent light, gave off visible light when the high voltage was applied. The high voltage was generated by opening a circuit containing a large coil that had been “charged” with current from a battery. This is exactly how a spark-coil works in an automobile, stepping up the lowly 12 volts of the car battery to many thousands of volts to cause the spark that ignites the fuel mixture. (This is also the phenomenon that caused the catastrophic magnet explosion at CERN as the LHC was first ramping up.) A large coil can be made to discharge its current into a vacuum tube, producing the motion of “cathode rays” (soon to be understood by J. J. Thompson to be fundamental particles, called “electrons,” ripped out of their atoms by the electric force).

  Röntgen noticed that he was able to get some “rays” from the powerful electric current to apparently exit the tube itself. The rays caused a shimmering of light on a small cardboard screen on which he had painted a known fluorescent compound, barium platinocyanide, close to the tube. But were these rays simply visible light coming from the tube, or were they some stray “cathode rays”? There seemed to be nothing exotic going on here.

  To understand this phenomenon, Röntgen completely covered the tube with a cardboard blind so that no visible light could emerge. He then allowed the coil to discharge into the tube, and at that moment he just happened to see an observable shimmer on a fluorescent screen that was serendipitously placed on a table at a distance of about 6 feet from the tube. This was too far from the tube to be “stray cathode rays” (electrons), and there was no light coming from the cloaked tube itself. What was causing the mysterious distant shimmer?

  This is a gorgeous example of an “accidental discovery,” since Röntgen was simply trying to establish that the system was light-tight. In so doing he discovered something invisible that was getting out and producing the faint fluorescent shimmering light some distance away. He found that the experiment could be reliably repeated and modified, and he could consistently establish that there were mysterious ghost-like penetrating “rays” emerging from the tube as it discharged (scientific observations, unlike a séance that calls back the departed souls only when the spiritual weather conditions are right, are always reproducible, or else they're false). This new type of “ray” that was emerging from the electrical activity in the tube could penetrate the materials that blocked the pathway. Röntgen named these determined escapees from the discharge tube “X-rays.”

  Röntgen threw himself into to the detailed study of the penetrating power of his X-rays. Within a few weeks he had produced a ghostly photograph of the bones in his wife's hand and even saw his own skeleton as an X-ray shadow cast on a fluorescent card, to which he declared, “I have seen my own death!” Röntgen later discovered that ordinary lead was an effective barrier to X-rays.

  Röntgen had single-handedly discovered a marvelous new phenomenon of nature and then quickly developed most of the ingredients of the first effective medical/dental-imaging technology. It is no surprise that Wilhelm Conrad Röntgen received the first Nobel Prize in Physics in 1901. Today we know that X-rays are a very high-energy, ultra-short-wavelength form of light—they are very energetic and invisible photons.

  INSPIRATION

  Inspired by Röntgen's discovery of X-rays, a French scientist, Henri Becquerel, began to rethink “phosphorescence.” This is the stimulated emission of light from a material, following the material's exposure to an external source of light, where the resulting emitted light from the phosphorescent material is generally of a color different than that of the source light (for example, a clock dial that glows in the dark after the room lights are turned off is phosphorescent). Becquerel had reasoned that phosphorescent materials, such as uranium, a fairly common mineral that was found in a black, otherwise seemingly useless gravel-
like material called “pitchblende,” might actually be coaxed to emit the newly discovered X-rays after being exposed to a source of bright sunlight. He did an experiment of placing the sunlight-exposed pitchblende onto a photographic plate, developing it, and finding that the plate had become “fogged,” indicating that the pitchblende was indeed emitting something. Becquerel had discovered that X-rays came from uranium salts found in pitchblende. However, his raison d’être, i.e., his initial hypothesis of phosphorescence, was wrong. Upon performing subsequent experiments, he found that the X-rays were spontaneously coming from the uranium and needed no sunlight exposure to stimulate their emission!

  Becquerel had discovered natural “radioactivity.” Working with two brilliant doctoral students, Marie Curie and her husband Pierre, Becquerel subsequently discovered radioactivity in other “heavy” elements, such as thorium, polonium, and radium (the latter two elements were actually discovered by the Curies in the course of this research in Becquerel's lab). Together these three scientists shared one of the early Nobel Prizes for their discoveries of radioactivity and the new elements. In these experiments, they had observed three types of “rays” emitted by radioactive substances.2

  With one of these newfound rays, the so-called “beta rays” (“beta” is the Greek letter β), they had witnessed, unknowingly and for the first time, the weak interactions. We've told you this story because it is through the weak interactions that we have, today, uncovered the celebrated Higgs boson. The Higgs boson, in giving masses to the force carriers of the weak interaction, mainly the W+ and W– bosons, causes them to become “weak” and hard to detect. It is a rare quantum fluctuation that causes a beta ray to be emitted from a decaying atomic nucleus. Yet, our century-long ascent into the physical world of the weak interactions began with the first steps of Becquerel and the Curies, way back in 1896 with the discovery of radioactivity. What came next was the grandest revolution in our understanding of nature, the development of the quantum theory.3