Lots of particle physics news from the Tevatron the past two
At some level the first two have a bearing on the last one,
which has started to get some attention in the media, including
a nice interview
with Prof. Heidi Schellman from
Northwestern today on Science Friday, and a piece
in Scientific American to
which I contributed quotes.
The observation of the production of single top quarks (rather
than the easier-to-see production of top-antitop quark pairs)
has been a goal of the Tevatron experiments for years. The
success of this analysis demonstrates that extremely complex
dissections of the data like this can be undertaken, and reveal
faint signals like that of single top. The search for the Higgs
boson is more difficult still.
But is the race for the Higgs boson heating up? Is there a race at
all? Can the Tevatron see it before the LHC, given that the LHC
has been delayed
a year due to the
quench incident last September?
It all hinges on the mass of the Higgs boson. In different mass
ranges it decays to different final states, changing the
experimental approach and changing the sensitivity of the
Tevatron and LHC experiments. The new measurement of the W mass
by D0 adds a bit more knowledge about what the Higgs boson mass
might be, since it depends on the mass of the W boson (the
carrier of the weak force, which governs nuclear beta decay) and
the mass of the top quark directly.
The plot at left demonstrates where we are right now. I call it
the “billion dollar plot” but it probably cost a lot more than
that to produce, because it shows the results of decades of
experimentation at the Tevatron at Fermilab, LEP at CERN, the
SLC at SLAC, and other measurements. As you can see (I hope) we
are in a very interesting situation: the world’s data seem to
indicate that the best Higgs boson mass is deep into the
territory already excluded by LEP 2 in 2000! They set a 95%
confidence bound on the Higgs mass at about 114 GeV. Their limit
does not extend very far beyond that mass at all; it was limited
by the energy of the LEP accelerator. The mass range above 114
GeV is open experimentally, but if you take the W mass/top mass
constraint seriously (in the context of the Standard Model you
must) then it would certainly appear very likely that the Higgs
must lie in the range 114-185 GeV, with a strong preference for
the lower end.
The new Tevatron result takes a new bite out of the upper end of
the range, excluding from 160-170 GeV the “sweet spot” where the
Higgs can decay to two W bosons. This is in some sense “first
blood” for the Tevatron: at last the two experiments can exclude
a Standard Model Higgs boson somewhere it hasn’t already been
But, to my mind, the interesting end of the range is at the low
end. The data favor it, and theory favors it in the sense that
if nature is more complicated, and supersymmetry is manifest,
then one would expect that the light Higgs boson in
supersymmetry exists in the range 120-130 GeV or so. In this
picture there would be heavier Higgs bosons lying in wait for
either the LHC or the Tevatron, though the LHC has the edge here
with higher energy.
Assuming that the improvements in the analyses continue to
outpace the data, as shown in the plot below, it is not
impossible that the Tevatron could start to extend the region
excluded by LEP, by this summer. But discovery? A three sigma
result is possible with a good deal more data, but a five-sigma
discovery looks very hard, and always
For a gold-plated, five-sigma-significance discovery, my money
is on the LHC, I have to say. But the LHC will initially see the
Higgs boson decaying to two photons, and we really need to see
it decaying to two quarks or two leptons to really know its
nature. I think the Tevatron could do that before the LHC,
measuring the decay of the Higgs boson to two b quarks, and that
alone is reason enough to keep the machine running until it
does, to my mind, provided there are sufficient personnel to run
the detectors and analyze the data. That decision, though, is
above my pay grade…
The LHC will likely be able to see the Higgs boson decaying to
two tau leptons before the Tevatron can see it decaying to two b
quarks. Is that a race? I view the two observations as
complementary, and they both add to the scientific picture.
Without the Tevatron, it will just take longer.
One last word…the Higgs boson is damned hard to see, and when
the LHC turns on, a ton of other new physics may pour out first.
It is a very interesting year, that’s for sure!