by Sean Carroll
Notice that this diagram, and several of the ones to follow, describe a particle emitting another particle while remaining itself unchanged. That can never happen all by itself, because it wouldn’t conserve energy. All diagrams of this sort must be embedded as part of some bigger diagram.
Electromagnetism, unlike gravity, is felt only directly by charged particles. An electron can emit a photon, but a neutrino or a Higgs cannot. They can do so indirectly, through more complicated diagrams, but there’s no simple vertex that does the trick.
Likewise, any strongly interacting particles (quarks and gluons) can emit gluons. Note that gluons are strongly interacting, while photons are not electrically charged—there is a three-gluon vertex, but no three-photon vertex.
Now we come to the weak interactions, where things are a bit messier. The Z boson is actually pretty simple; any particle that feels the weak interactions can emit one and go on its merry way. (Again, as part of a bigger diagram.)
Once we get to the W bosons, things are a bit more complicated. Unlike the other bosons we’ve just considered, the Ws are electrically charged. That means they can’t be emitted without changing the identity of the particle emitting them; if they did, charge wouldn’t be conserved. So the W bosons serve to convert between up-type quarks (up, charm, top) and down-type quarks (down, strange, bottom), as well as between the charged leptons (electron, muon, tau) and their associated neutrinos.
The Higgs boson is much like the Z: Any particle that feels the weak interaction can emit one.
Now we turn to bosons coming in. They can emit another boson, or they can split into two fermions. However, since a fermion line can never end, a boson has to split into one fermion and one antifermion; that way the total number of fermions at the end is zero, just like it was at the beginning. Here we have a multitude of examples. Note that these are all related to diagrams we’ve already drawn, just by moving lines around and flipping particles to antiparticles where appropriate. If the entering boson is massless, we once again know that it can only be used as part of a bigger diagram, since massless particles can’t decay into massive ones while satisfying conservation of energy. (One way to see that is that the combination of two massive particles must have a “rest frame” in which the total momentum is zero, while a single massless particle has no state of rest.)
The only remaining fundamental diagram is the Higgs interacting with itself; it can split into two or three copies. Clearly this would violate energy conservation unless it were embedded in a bigger diagram.
The real fun comes from combining these fundamental diagrams to make bigger ones. All we have to do is join lines describing matching particles: We join an electron to an electron, and so forth. Starting from the diagrams above, we might have to flip some lines from right to left and turn particles into antiparticles to make it work.
For example, let’s say we want to ask how a muon can decay. We see that there is a diagram where a muon emits a W- and turns into a muon neutrino; but that can’t happen by itself, since the W is heavier than the muon. Never fear; all is okay as long as the W remains virtual, and decays into something lighter than the muon, such as an electron and its neutrino. All we have to do is glue together the W- lines from two of the previous diagrams in a consistent way.
We can also bend lines back on themselves to form loops. Here is a diagram that contributes in an important way to the search for the Higgs at the LHC: a Higgs decaying into two photons. The loop of virtual particles in the middle could contain any particle that couples both to the Higgs (so that the vertex on the left exists) and to photons (so that the vertices on the right exist). Particles with stronger couplings will contribute the most; in this case, that would be the top quark, which is the most massive particle in the Standard Model, and therefore the one with the strongest coupling to the Higgs.
Finally, here are some of the important ways that Higgs bosons are actually produced at the LHC before they decay. There is “gluon fusion,” where two gluons come together to make a Higgs; because gluons are massless, they must proceed through a virtual massive particle that feels the strong force, namely a quark.
There is also “vector boson fusion,” referring to the fact that the W and Z bosons are sometimes called “vector bosons.” Since they are massive, they can combine directly into a Higgs.
At last there are two different kinds of “associated production,” where the Higgs is made along with something else: either a W or Z boson, or a quark-antiquark pair.
The take-home lesson here isn’t the ins and outs of all the different processes that contribute to Higgs production and decay; it’s simply that both processes are complicated, arising from a collection of different possibilities, and we have definite rules that allow us to figure out what they are. It’s amazing to think that these little cartoons capture something deeply true about the microscopic behavior of the natural world.
JoAnne Hewett, rapping about dark matter at a physics slam in Eugene, Oregon, in 2011.
© JACK LIU
At CERN on July 4, 2012, Fabiola Gianotti, Rolf Heuer, and Joe Incandela, preparing for the big announcement.
© CERN
Leon Lederman, standing outside Fermilab.
FERMILAB NATIONAL ACCELERATOR LABORATORY
Sau Lan Wu of the University of Wisconsin, who has been searching for the Higgs at both LEP and the LHC.
JEFF MILLER/UNIVERSITY OF WISCONSIN
Fabiola Gianotti, spokesperson for ATLAS in 2011 and 2012.
© CERN
Carlo Rubbia, discoverer of the W and Z bosons and advocate for the LHC.
CREATIVE COMMONS
Lyn Evans, the man who built the LHC.
© CERN
An aerial view of CERN and the Large Hadron Collider, with major experiments marked. The actual ring is underground and not visible from above.
© CERN
The Globe of Science and Innovation at CERN, a striking building that serves as a symbol for the laboratory. The Globe houses public-information exhibitions about particle physics and CERN's mission.
© CERN
Inside the LHC tunnel, with dipole magnets installed and ready to go.
© CERN
Damage to LHC magnets after the September 19 accident.
© CERN
All the protons for the LHC beam come from this tiny canister of hydrogen. It contains enough protons to feed the LHC for a billion years.
© CERN
A model of the cross-section through the dipole magnets in the LHC. The two beam pipes carry protons moving in opposite directions.
© CERN
One of the “Ping-Pong balls” equipped with a radio transmitter that was sent down the beam pipe of the LHC to check for obstructions.
LYN EVANS
Joe Incandela, spokesperson for CMS in 2012.
© CERN
A candidate Higgs event at the ATLAS detector. The two long blue lines are muons, and the short blue lines are electrons, so this could represent the decay of the Higgs into two Z bosons.
© CERN
The ATLAS detector under construction. Note the person standing at the middle bottom. The eight giant tubes are magnets used to deflect muons in order to measure their energies.
© CERN
The CMS detector under construction.
© CERN
Yoichiro Nambu, pioneer of symmetry breaking, gluons, and string theory.
CREATIVE COMMONS/BESTYTHEDEVINE
Philip Anderson, leader in condensed-matter physics and thoughtful curmudgeon.
CREATIVE COMMONS/PHILIP WARREN
Left to right: Tom Kibble, Gerald Guralnik, Carl Richard Hagen, François Englert, and Robert Brout, at the 2010 Sakurai Prize ceremony. Peter Higgs shared the award but was absent.
Peter Higgs, visiting the ATLAS experiment.
© CERN
Left to right: Sheldon Glashow, Abdus Salam, and Steven Weinberg, at the 1979 Nobel Prize ceremony.
© BETTMANN/CO
RBIS
The data produced and analyzed in the LHC’s search for the Higgs. These plots show the number of events that produce two high-energy photons, where the total energy of the photons ranges from 100 to 160 GeV, in the 2011–2012 data from ATLAS and CMS. The dotted lines shows the prediction without any Higgs boson; the solid curve includes a Higgs with a mass of 126.5 GeV (ATLAS) or 125.3 GeV (CMS).
© CERN
© CERN
Why we do science.
ZACH WEINERSMITH, SATURDAY MORNING BREAKFAST CEREAL
A flowchart illustrating the elementary particles of the Standard Model. This is the modern version of the periodic table of the elements. Quarks are in blue, leptons in purple, gauge bosons in green, and the Higgs boson in red.
SEAN CARROLL
FURTHER READING
Aczel, Amir. Present at the Creation: The Story of CERN and the Large Hadron Collider. New York: Crown Publishers, 2010.
CERN. CERN faq: LHC, the guide. http://multimedia-gallery.web.cern.ch/multimedia-gallery/Brochures.aspx, 2009.
Close, Frank. The Infinity Puzzle: Quantum Field Theory and the Hunt for an Orderly Universe. New York: Basic Books, 2011.
Crease, Robert P., and Mann, Charles C. The Second Creation: Makers of the Revolution in Twentieth-Century Physics. New York: Collier Books, 1986.
Halpern, Paul. Collider: The Search for the World’s Smallest Particles. Hoboken, NJ: Wiley, 2009.
Kane, Gordon. The Particle Garden: The Universe as Understood by Particle Physicists. New York: Perseus Books, 1995.
Lederman, Leon, with Teresi, Dick. The God Particle: If the Universe Is the Answer, What’s the Question? Boston, MA: Houghton Mifflin, 2006.
Lincoln, Don. The Quantum Frontier: The Large Hadron Collider. Baltimore, MD: Johns Hopkins University Press, 2009.
Panek, Richard. The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality. Boston, MA: Mariner Books, 2011.
Randall, Lisa. Knocking on Heaven’s Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World. New York: Ecco, 2011.
Sample, Ian. Massive: The Missing Particle That Sparked the Greatest Hunt in Science. New York: Basic Books, 2010.
Taubes, Gary. Nobel Dreams: Power, Deceit, and the Ultimate Experiment. New York: Random House, 1986.
Traweek, Sharon. Beamtimes and Lifetimes: The World of High Energy Physicists. Cambridge, MA: Harvard University Press, 1988.
Weinberg, Steven. Dreams of a Final Theory. New York: Vintage, 1992.
Wilczek, Frank. The Lightness of Being: Mass, Ether, and the Unification of Forces. New York: Basic Books, 2008.
REFERENCES
References refer to keywords in the main text. The one exception is Chapter Eleven, “Nobel Dreams,” where I include two lists of additional references: one for the personal reminiscences of the people involved in the 1964 symmetry-breaking papers, and one that includes all of the technical papers alluded to in the discussion.
Prologue
Hewett: http://blogs.discovermagazine.com/cosmicvariance/2008/09/11/giddy-physicists/
Evans: interview, July 4, 2012.
Higgs: http://www.newscientist.com/article/dn22033-peter-higgs-boson-discovery-like-being-hit-by-a-wave.html?full=true
Chapter One: The Point
Faraday: http://bit.ly/ynX3dL
Heuer: http://www.guardian.co.uk/science/2011/dec/13/higgs-boson-seminar-god-particle
Chapter Two: Next to Godliness
Lederman and Teresi: The God Particle, p. xi.
Higgs: http://physicsworld.com/cws/article/indepth/2012/jun/28/peter-higgs-in-the-spotlight
Chapter Four: The Accelerator Story
Janot: V. Jamieson, “CERN Extends Search for Higgs,” Physics World, October 2000.
Watts: private email, April 4, 2012.
Hewett: interview, February 23, 2012.
Schwitters, Bloembergen: quoted in Kelves, preface to the 1995 edition of The Physicists: The History of a Scientific Community in Modern America.
Park: quoted in Weinberg, Dreams of a Final Theory, p. 54.
Anderson: Letter to the Editor, The New York Times, May 21, 1987.
Krumhansl: Sample, Massive, p. 115.
Chapter Five: The Largest Machine Ever Built
Evans, “carnage”: interview, July 4, 2012.
Baguette: http://www.telegraph.co.uk/science/large-hadron-collider/6514155/Large-Hadron-Collider-broken-by-bread-dropped-by-passing-bird.html
Evans: http://www.elements-science.co.uk/2011/11/the-man-who-built-the-lhc/
Evans: http://www.nature.com/news/2008/081217/pdf/456862a.pdf
Giudice: A Zeptospace Odyssey, pp. 103–104.
Evans, summer party: interview, July 4, 2012.
Chapter Six: Wisdom Through Smashing
Anderson: Eugene Cowan, “The Picture That Was Not Reversed,” Engineering and Science 46, 6 (1982).
CERN press release: http://press.web.cern.ch/press/PressReleases/Releases2008/PR10.08E.html
Computing tiers: Brumfield, http://www.nature.com/news/2011/110119/full/469282a.html
Gianotti: interview, May 3, 2012.
Greek Security Team: Roger Highfield, http://www.telegraph.co.uk/science/large-hadron-collider/3351697/Hackers-infiltrate-Large-Hadron-Collider-systems-and-mock-IT-security.html
Chapter Eight: Through a Broken Mirror
Yang and Pauli: Close, The Infinity Puzzle, p. 88.
Chapter Nine: Bringing Down the House
Telegraph: http://www.telegraph.co.uk/science/large-hadron-collider/8928575/Search-for-God-Particle-is-nearly-over-as-CERN-prepares-to-announce-findings.html
viXra log: http://blog.vixra.org/2011/12/01/seminar-watch-higgs-special/
CERN update: http://indico.cern.ch/conferenceDisplay.py?confId=150980
Gianotti: http://www.youtube.com/watch?v=0KOoumH4dYA
Gianotti, “Spirit” and “Bear” quotes: interview, May 15, 2012.
Wu: http://physicsworld.com/cws/article/news/2011/dec/14/physicists-weigh-up-higgs-signals
Ellis, Gaillard, and Nanopoulos: Nuclear Physics B 106, 292 (1976).
Britton: http://www.wired.co.uk/news/archive/2011-09/07/david-britton
ATLAS figure: http://www.atlas.ch/news/2012/latest-results-from-higgs-search.html
CMS figure: http://hep.phys.sfu.ca/HiggsObservation/index.php
Megatek: Taubes, Nobel Dreams, pp. 137–138.
Higgs: http://www.newscientist.com/article/dn22033-peter-higgs-boson-discovery-like-being-hit-by-a-wave.html?full=true
Incandela: interview, July 4, 2012.
Chapter Ten: Spreading the Word
The Daily Show: http://www.thedailyshow.com/watch/thu-april-30-2009/large-hadron-collider
The Daily Mail: http://www.dailymail.co.uk/sciencetech/article-1052354/Are-going-die-Wednesday.html
Appeals court: http://cosmiclog.msnbc.msn.com/_news/2010/08/31/5014771-collider-court-case-finally-closed?lite
Dorigo: http://www.science20.com/quantum_diaries_survivor/where_will_we_hear_about_higgs_first
Conway 1: http://blogs.discovermagazine.com/cosmicvariance/2007/01/26/bump-hunting-part-1/
Conway 2: http://blogs.discovermagazine.com/cosmicvariance/2007/01/26/bump-huning-part-2/
Conway 3: http://blogs.discovermagazine.com/cosmicvariance/2007/03/09/bump-hunting-part-3/
Cirelli and Strumia: http://arxiv.org/abs/0808.3867
Picozza, Cirelli: http://www.nature.com/news/2008/080902/full/455007a.html
Lykken: http://www.nytimes.com/2007/07/24/science/24ferm.html?pagewanted=all
Woit: http://www.math.columbia.edu/~woit/wordpress/?p=3632&cpage=1#comment-88817
Wu: email, May 2012.
Gianotti: http://www.nytimes.com/2012/06/20/science/new-data-on-higgs-boson-is-shrouded-in-secrecy-at-cern.html?_r=1&pagewanted=all
Schmitt: http://muon.wordpress.com/2012/06/17/do-you-like-to-spread-rumors/
Ouellette: http://news.discovery.com/space/
rumor-has-it-120620.html
“Large Hadron Rap”: http://www.youtube.com/watch?v=j50ZssEojtM
Kaplan: interview, May 20, 2012.
Particle Fever: http://www.particlefever.com/index.html
Chapter Eleven: Nobel Dreams
Freund: A Passion for Discovery, World Scientific (2007).
Anderson: P. W. Anderson, “More Is Different,” Science 177, 393 (1972).
Anderson’s biggest contribution: email, 2012.
Higgs on Anderson: P. Rodgers, “Peter Higgs: The Man Behind the Boson,” Physics World 17, 10 (2004).
Lederman: The God Particle.
Lykken: Symmetry, http://www.symmetrymagazine.org/cms/?pid=1000087
Bernardi: Nature, http://www.nature.com/news/2010/100804/full/news.2010.390.html
Anderson on history: email, 2012.