Endgame for the Tevatron

By John Conway | September 21, 2009 12:27 pm

With little fanfare, last week the Tevatron at Fermilab, and the two experiments CDF and D0, emerged from an 11-week shutdown for what will likely be the final run of the collider, which is over 20 years old. In the past year, the machine has regularly set new records for luminosity (essentially the number of collisions per second) and delivered over 2 fb-1 (inverse femtobarns) of proton-antiproton collisions at a center of mass energy of 1.96 TeV to the experiments, still the highest in the world. The startup has gone very smoothly, and the Tevatron delivered a solid load of data to the experiments last week.

IntegratedLuminosity

This funny unit, inverse femtobarns, allows us to calculate how many collision events of a certain type to expect. Take top quark pair production, for example. For protons colliding with antiprotons at Tevatron energies, we can calculate (or measure) what we call the production cross section. This cross-section is in fact expressed as an area, a very small area, since protons and antiprotons are so small. One “barn” is 10-28 m2, and the cross section for top pair production is about 7 x 10-12 barns, or 7 pb. By multiplying the cross-section times the integrated luminosity we can get the number of top quark pairs events produced. To get the number we actually observe in the detector, we need to take into account the efficiency for reconstructing them.

The plot here shows the history of what we call Run 2 at the Tevatron. After a very slow start in 2001, following a five-year shutdown to upgrade the whole complex, the collider set new luminosity records year after year, and has nearly delivered 7 fb-1. This final run is expected to last two years, until the end of 2011, by which point we hope to have recorded another 5 fb-1, nearly doubling the present sample.

This past year had been expected to be the year of the LHC at CERN. But the magnet quench incident of one year ago caused a delay of over a year in repairs and retrofits. It is still expected that the LHC will return to commissioning in November of this year, possibly colliding protons on protons before the end of the calendar year, albeit at low energies. When it does come online at higher energies, probably early next year, it is expected that the LHC will deliver no more than about 0.2 fb-1 at a collision energy of 10 TeV, five times that of the Tevatron. Even with such a small sample, there could be striking discoveries at the LHC which are out of reach for the Tevatron, simply because the LHC energy is so much larger.

The media love this sort of race, and have portrayed it as a race to discover the particle the media has heard the most about – the Higgs boson. With 12 fb-1, even combining all the search modes and channels, and combining the data from both experiments, a standard model Higgs boson might be seen at the three standard deviation level, but almost certainly not the five standard deviation level, which is the gold standard in the field. The LHC won’t be able to see a standard model Higgs boson with the initial sample either. It will take a year or two at higher luminosity, probably starting in 2012, to get there.

To my mind, if there is a race, it is a race for the unknown. What I worry about, what I literally lie awake thinking about, is whether we are looking at the Tevatron data exactly the right way. People have searched for many different new physics signals at the Tevatron, but there has been no unambiguous observation of anything beyond the standard model. To get a five sigma discovery with the remainder of the Tevatron data, it would have to be the case that there is already about a three sigma excess in the data we have. But have we looked at everything?

Nevertheless, a hard-core of dedicated, talented, and very new-physics-hungry physicists will continue to operate the detectors and analyze the data to come, myself among them. Like many, I am playing both sides: when LHC data come we’ll analyze that, too.

CATEGORIZED UNDER: Science
  • Sili

    Of course, once something if discovered at the LHC, someone’ll go back and find it in the Tevatron data. Let’s just hope that it’s only at the 2σ level.

    I’m excited, though, to be living in this age – even if I don’t know, much less understand, everything that’s been discovered in the past fity years.

  • Brian137

    Bye bye, Big T. It’s been great.

  • Shantanu

    John,
    Has Tevatron ruled out any Beyond standard model theories so far? Does any one know of any Tevatron
    bets?

  • http://www.funky49.com funky49

    @Shantanu

    I’m sure there are bookies who will take bets on this. What odds would you give Tevatron versus the LHC in the next 3 years? Maybe we should pick a charity and collect bets. If you ‘win’ the losers money is donated to the charity?

    funky49
    Fresh Prince of Batavia

  • pilate

    Quid est veritas stephen? Eam audis, eam cognoscis quando dicitur?
    Ita audio. Num et tu?
    Quomodo? Potes mihi dicere stephen?
    Si non vis veritatem audire, nemo tibi dicere potest.
    Veritas. Vis meam veritatem cognoscere? Einstein me monuit, Stephen. Bis monuit, vice proxima ipse iuravit sanguis erit meus. Ecce est mea veritas: morituri te salutant…
    (on the dumb holes that didnt evaporate any sound at Haifa, hence proving that Hawking radiation did not exist)

  • Xenophage

    Under what conditions and how long must one search for the Higgs if there is no Higgs? The Standard Model arrives massless. The Higgs is a jury rig to reconcile elegant theory with empirical observation. As with all interpolations, psychology to economics, theorists boast promiscuity while empiricists pay child support.

  • John

    Xenophage: If there is no *Standard Model* Higgs boson, we’ll rule it out experimentally, possibly even at the Tevatron if there are further improvements to the analyses. But if the Tevatron does not rule it out, the LHC will after getting a few inverse femtobarns. This is because a Standard Model Higgs boson cannot have a mass much larger than about 200 GeV at the outside, and we already know the SM Higgs mass must be greater than about 114 GeV. So this is a falsifiable hypothesis, experimentally.

    If it is ruled out that leaves open the possibility that the Higgs boson or whatever plays the role of the Higgs boson is more complicated, and there is no guarantee that the LHC or the Tevatron can discover it…

    In a sense you are right that the Higgs is a jury rigged solution, but the Standard Model has been eerily successful in explaining a huge range of observations. Neverthelss, nature may have something completely different in store for us.

  • shantanu

    funky49: I only wanted to get a sense of how optimistic people were that Tevatron
    would find evidence for physics beyond SM.
    BTW this maybe a naive question. Can Tevatron tell us anything about or against anthropic principle or multiverse? I have seen papers in literature arguing that all standard model parameters are anthropically selected ?
    thanks

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