December 3, 2012 | 17
You have by now heard about the discovery of the Higgs boson here at CERN – a momentous scientific, technological and human accomplishment. It was the last undiscovered particle of the Standard Model. Particle physicists like to capitalize Standard Model. I suppose capitals make clear that this is not just a standard, as in ordinary, model, but THE Standard Model. (Cue trumpet fanfare.)
Fifty-one years after its basis was explored by Sheldon Glashow, forty-five years after the full theory was proposed by Steven Weinberg (and independently the next year by Abdus Salam), the Standard Model has successfully accounted for or predicted all calculationally tractable experimental results in particle physics. Okay, with one exception — the oscillation of one type of neutrino into another. But that demands only the mildest of extensions to the Standard Model – the inclusion of right-handed neutrinos. And those neutrinos were left out only because in 1967 nobody knew that neutrinos oscillated. “To include right-handed neutrinos or not to include right-handed neutrinos?” makes for an awkward soliloquy, and is really not an existential question for the Standard Model.
But I digress. After 45 years, many billions of dollars, tens – maybe hundreds – of thousands of person years of concerted effort, in hundreds of universities and laboratories around the world, we have finally discovered ALL the particles predicted by the most successful theory of science, as measured by the number of decimal places to which its predictions have been tested. (For the sticklers among you, the particle that was discovered here at CERN this year, and confirmed by scientists at Fermilab, has not officially been confirmed to be the Higgs boson, although many of us would be prepared to eat our hats if it is not the Higgs boson. Especially those of us who don’t wear hats.)
By the way, this final check-off on the Standard Model particle-hunter’s bucket list was a real find. The Higgs boson is to the Higgs field what the photon is to the electromagnetic field – its fundamental excitation; and the Higgs field plays a very special role in the Standard Model – it gives mass to every fundamental particle that has mass, from quarks and electrons to W and Z bosons. So finding the Higgs was crucial. It was also a real challenge. The Standard Model gives no a priori indication of the mass of the Higgs, although precision measurements at past colliders (and careful calculations) suggested that if the Standard Model is the complete story, then the Higgs should have a mass in the neighborhood of 115-170 GeV/c2, about 120-180 times the mass of a proton.
And the mass of this new (apparently Higgs) boson is … (drumroll) … 125 GeV/c2! Chalk up another success for the Standard Model. How … ummm … exciting?! Sure, if you’re over 60 and fondly recall the glory days of the 1960s and early ‘70s when the Standard Model became, well the Standard Model. Or if you’re an experimentalist who has dedicated the last 10-30 years to designing, building and operating the extraordinary Large Hadron Collider and Tevatron accelerators (where the discovery was made and confirmed respectively), or their monumental detectors — ATLAS and CMS, CDF and D0. Celebrate away. Drink champagne, toast your successes, wonder over Nobel and lesser (but more broadly shared) prizes. You’ve earned it. But what about the rest of us?
“If the Standard Model is the complete story.” What a depressing phrase. We were promised that it couldn’t be! The Standard Model is absurdly fine-tuned, we were told – balanced on a knife-edge off which it has no right not to tumble. It has an un-natural hierarchy of scales. It has too many free parameters, and some of them are very, very small. Why, the electron mass is less than 0.00001 times the weak scale (the energy scale governing weak interactions such as the W and Z boson masses), which is itself 10-17 (that 0.0000000000000001) times the Planck scale (the energy scale governing gravity)! And speaking of gravity, the Standard Model can’t accommodate quantum gravity. We need Low-Energy Supersymmetry, or Technicolor, or Large Extra Dimensions, or … One of these MUST be found at the LHC!
Forty years of theoretical work has been based on these expectations. Papers with thousands of citations have been written. Courses taught. Textbooks published.
Prizes awarded! Illustrious careers navigated! And yet despite all this build up of theoretical expectations, there is no experimental hint of anything outside the Standard Model at the LHC. Hence the long faces and worried words wherever theorists gather to drink coffee. Hence the disappointment in the eyes of the young experimentalists looking forward to the next accelerator, the next frontier where their mark will be made. It doesn’t even help to have been one of the few Cassandras belatedly warning that the edifice of Beyond the Standard Model theories might be built on sand. We want there to be more physics – more particles, more interactions. More to discover.
Walk the halls, go to theory seminars, have lunch with a theorist, or an ambitious young experimentalist. Look for the classic symptoms of grief.
Denial. Vigorous debates about whether the fact that the dog did not bark in the night suggests that it is a Chihuahua or a Rottweiler. My friends – at some point if there is no barking, we must conclude there is no dog.
Anger. At those of us “misguided” enough to doubt the imminence or even the necessity of Beyond the Standard Model physics.
Bargaining. Perhaps BSM physics has not been discovered because we’ve been demanding too much explanatory power from science. If we just relax our expectations for the predictivity of science, and introduce a multitude of universes in which we occupy a particular one best suited to our existence, then we can let our extensions to the Standard Model be un-natural, many of their properties unpredictable, and explain why they haven’t been discovered yet!
Depression. Even as the glorious LHC data piles up around us, like platters at a holiday feast, we worry where tomorrow’s meal will come from. Hurry up somebody. Find a superpartner! A mini-black hole! Even a technipion. Discover a dark matter particle! We’ll even settle for astronomical observations confirming that the dark energy is not just a cosmological constant! Better still, surprise us with something completely unexpected. Because if you don’t, its going to be Standard Model for breakfast, Standard Model for lunch and Standard Model for dinner.
We’re not ready for Acceptance! At least, sitting here listening to the LHC hum, I can still hope.
Previously in this series: