The End of the Tevatron

Cosmic VarianceBy John ConwayJan 10, 2011 10:48 PM


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It's the beginning of the end for the Tevatron at Fermilab. In the fall, the Department of Energy's High Energy Physics Advisory Panel recommended that the Tevatron be funded to run for three years beyond the planned end in September of 2011, largely in order to provide additional information in the search for the Higgs boson. The recommendation was contingent on there being new funds, about 5% above current levels, in order to staff and operate the machine and the experiments. But in a letter to day to the chair of HEPAP, the head of the Office of Science at the Department of Energy, William Brinkman, wrote that "Unfortunately, the current budgetary climate is very challenging, and additional funding has not been identified. Therefore...operation of the Tevatron will end in FY2011, as originally scheduled."

The dream for a superconducting proton synchrotron at Fermilab goes back to at least 1976, when it began to become clear that the interesting mass range to explore in order to understand the weak interaction would be around 100 GeV. The lab was engaged in a wide range of fixed target experiments, using the Fermilab Main Ring proton synchrotron as its workhorse, and in 1977 the b (or bottom) quark was discovered there. This meant there had to be a top quark, as well as very massive (80-100 GeV) W and Z bosons.

But Europe pulled ahead - it already had the Super Proton Synchrotron, and plans to convert it into a proton-antiproton collider. Whoever did so first would have the energy to produce W's and Z's directly, and nail down their masses. And maybe, whoever managed to create the first high energy proton-antiproton collider would be able to find the top quark, whose mass could be, well, just about anything above the b quark mass of 5 GeV, but probably at least 20 GeV.

Fermilab designed a new generation of superconducting magnets to operate at 3 Tesla, and began construction of the new ring, then dubbed the Saver (short for Energy Saver, since Energy Doubler, the initial name, sounded too expensive). At the same time CERN developed its antiproton source and the techniques for cooling the antiproton bunches for injection to the new SppS, the Super Proton-Antiproton Synchrotron (there's a bar over the second p).

CERN won the race for the W and Z. In early 1983, not long after commissioning the new complex, handfuls of these massive carriers of the weak force were observed by the UA1 experiment, and confirmed by UA2. Carlo Rubbia, who lead UA1, and Simon van der Meer, who designed the antiproton source, shared the Nobel the next year. Would the top quark soon follow?

But in late 1983, the Fermilab Saver turned on, running extracted proton beams to the new raft of fixed target experiments awaiting them. My graduate thesis experiment, E615, was among them, and I count it among some of the most thrilling science moments in my career to have had my face plastered to the viewer of an oscilloscope, eyes dark-adapted, waiting for the first pulses from the counters in our experiment to tell me the secondary pion beam had arrived to us. The chain of injectors, a thousand new superconducting magnets, RF kickers, beam all worked! There were the pions! We went on, over the next year or so, to collect tons (for us) of data on the structure of the pion, still the best measurement to date.

UA1 and UA2 continued on, and CERN commenced deep tunnel drilling for LEP, the Large Electron Positron collider, slated to begin in 1988. It was designed to directly produce thousands of Z bosons by colliding electrons and positrons (antielectrons), and measure the Z as precisely as posible. At SLAC at Stanford, construction was underway to build the SLC, the SLAC Linear Collider, which would challenge LEP in the race for the Z. The age of the great colliders was in full blossom.

Fermilab commissioned its antiproton source in 1985 and 1986, and began collisions, redubbing the machine the Tevatron, as the ultimate goal was to reach 1 TeV per beam of energy - a trillion volts! The first engineering run was 800 GeV per beam, and much work remained to be done to get the beam intensity high enough to produce useful numbers of W's and Z's and surpass the SppS results.

SLC and LEP began operating in 1989 (a year late for LEP) but the Mark II experiment at the SLAC and the four (!) LEP experiments immediately made two important discoveries. First, by measuring the Z production rate, there appeared to be only three species of neutrino to which the Z could decay. This meant a fourth generation of quarks and leptons did not appear to be ready to be discovered. The second important SLC/LEP discovery was that the top quark must be rather massive, 150 GeV or more. This put it out of range of the electron positron machines, and meant that the top could be discovered soon at the Tevatron, if it could collide enough protons and antiprotons.

While Europe put its hopes of discovery in the electron positron machine, the US community was already building the biggest machine of all, the Superconducting Supercollider, or SSC, in Waxahachie, Texas. This mammoth proton-proton collider with 54 miles of superconducting magnets would begin operation in 1999 at an energy 20 times that of the Tevatron. "Throw deep" was Ronald Reagan's exhortation to the physicists when presented with the project, and deep they threw. Too deep perhaps - by 1993, the cost of the project had grown too rapidly, and, in a cost-cutting mood not unlike the present day, in October of that year the US Congress killed the project, which had spent about $2 billion, and dug almost 15 miles of tunnel (now backfilled).

The demise of the SSC was a serious blow to the US high energy community, and left one option for staying on the high energy frontier: an enhanced Tevatron complex. But it took another half decade after the LEP startup to amass the data sample necessary to get the first glimpse of top-antitop production in the CDF and D0 experiments at Fermilab. In early 1995, during "Run 1" of the Tevatron, both experiments announced the discovery of excess production of events consistent with that expected from top production. The mass of the top turned out to be a whopping 175 GeV. Pinning it down, and also measuring the mass of the W would answer the ultimate question: where is the Higgs boson lurking? Can it be seen at the Tevatron? Or would LEP2, with ever-increasing energy in the late 1990's, get it first? Or would the Large Hadron Collider, scheduled to begin colliding protons in the LEP tunnel at CERN in 2004, be the first to see it?

Fermilab completed construction of the new Main Injector and Antiproton Recycler as the millennium turned, and cast its eye upon the Higgs boson. The two new additions to the Fermilab accelerator complex would allow a huge increase in the beam intensities, possibly enough to produce the very weakly interacting Higgs. I helped lead a study which, in late 2000, published the prediction: it would take somewhere around 15 inverse femtobarns of integrated luminosity, plus major upgrades to the experiments, to find the Higgs boson, at least the vanilla Standard Model one.

LEP2, in late 2000, was facing its own imminent shutdown to make way for the LHC, having reached 206 GeV energy and a sensitivity to a Higgs boson of at least 114.5 GeV, but had no evidence for the Higgs boson. The accelerator and detectors began to be removed the next year, 2001, just as the Tevatron began Run 2.

The first years of Run 2 at the Tevatron were plagued by problems, some due to the then-aging infrastructure, some from the years of downtime since 1996, and some due the fact that, well, colliding matter and antimatter in large quantities is just damned hard to do. There was a learning curve, and by 2003 the lab was on track to meet the design goals for the collider. Large new samples began to roll in, and physics papers began to roll out. But getting to even 10 inverse femtobarns before the LHC turned on seemed to be a receding hope.

The LHC was having difficulties of its own. The plan to operate in 2004, became 2005, then 2006. A somewhat snide plot of the projected start date as a function of time showed that the machine would start in late 2008.

Which it did. But then, as faithful readers of CV know, the LHC had its rather spectacular failure triggered by a faulty magnet interconnect, and propagated by a detection/avoidance system that failed to detect the condition and was insufficient to avoid the massive boil-off of six tons of liquid helium. The resulting damage took a year and 35 million Euros to repair.

In these years, the Tevatron marched on, reaching its first inverse femtobarn by mid 2005, then setting new records almost routinely in the following years. The huge new sample provided a treasure trove for us data-starved particle hunters. And it became clear that with the slow turn-on of the LHC, the Tevatron had a chance, albeit a slim one, to discover the Higgs or some other new physics before the LHC. And so in 2009 the run of the Tevatron was extended until late 2011. The machine has kept running, kept improving, and we now hope to have 10 fb-1 by the time we shut down.

Will the Tevatron experiments see the SM Higgs boson before the LHC? It is looking quite doubtful, but there is some chance the run will end with a tantalizing excess. How long will it take the LHC? With the new data collected last year, only a tiny fraction of the Tevatron sample but at much higher energy, we are beginning to see the physics power of the new machine, and I owe you all a lot of posts on the picture beginning to emerge. But the Higgs boson could turn out to be at the hardest-to-discover mass of all at the LHC, around 120 GeV, in which case the LHC will almost certainly need to run well beyond the end of 2012 to get enough data. It may take as much as 10 inverse femtobarns at the LHC to see it at the golden five-sigma level. But of course we'll get excited a long time before that.

And we know that the Higgs boson cannot be the one of the Standard Model, right? More on that tantalizing prospect later. (And three years ago from the Tevatron.)

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