Tuesday, 3 July 2012

The Tevatron Higgs Announcement

The Tevatron today announced stronger evidence for the Higgs from its own searches.  There had been rumours that such an announcement would be made before the LHC likely claims discovery on Wednesday.

The Tevatron results are not enough, by the standards of the particle physics community, to claim discovery.  They are not even enough to claim "evidence", a lower standard where people tend to start getting excited.  But the results are very interesting and useful, even so.  A big part of this is because the Tevatron search channels are different to the ones currently used by the LHC.

The LHC results from last year primarily come from looking for the Higgs decaying into either two photons, or two Z bosons each of which decays to either two electrons or two muons.  These channels have the advantage they are "clean": they have low backgrounds from unrelated processes.  Other expected decay modes of the Higgs produce quarks, and these are produced by the bucketload at the LHC, so that the Higgs becomes the needle in a haystack.

That's not to say that the LHC won't be able to see those Higgs decays, by the way.  It will just take longer.

The Tevatron, in contrast, looks for the Higgs decaying to two bottom quarks.  It can do this for several reasons.  One is simply that fewer top quarks are produced at the Tevatron; tops decay to bottoms essentially all the time.  Another is that bottoms are easier to find at the Tevatron.  To understand why, let me make a brief digression on how bottoms are identified.

Bottom quarks are interesting because they have an intermediate lifetime.  Muons, for example, have a long lifetime of two microseconds.  That might not sound like lot of time, but it's an eternity in a high energy experiment; all muons produced at the Tevatron or LHC leave the experiments without decaying.  Tops quarks, W and Z bosons, and the Higgs all have very short lifetimes.  Even if they are produced near the speed of light, they decay after travelling distances much less than millimetre.

Bottoms, in contrast, decay within an experiment but after travelling a measurable distance, typically several millimetres.  Thus if you can measure that distance, you can identify a the bottom quark.  The problem is that the actual detectors you have are sitting several centimetres from the collision point.  So what you must do is measure the things the bottom decayed into, track back their flight paths and figure out where the started from.
The bottom identification problem: each of the green particles must be measured accurately in the grey region.  Then we trace their flight paths back to see that they did not originate from the black cross, due to the bottom lifetime.

This is easier to do at the Tevatron because each event has less stuff going on.  It is common to speak of the Tevatron as hitting protons and anti-protons together, or the LHC smashing protons into protons.  But what you actually have are beams of protons and/or antiprotons, which pass through each other at the detectors.  At each beam crossing event, you can have several interactions; indeed, the average number at the LHC is 20 to 30.  The Tevatron averages less than 10 interactions per event.  With less other stuff to worry about, the Tevatron can identify bottoms better.  Also, with over a decade of running, the Tevatron experimenters understand their experiment very well, which allows them to tune their search algorithms to squeeze every last drop of information possible from their data, another factor that should not be overlooked.

So, the Tevatron results are useful because they look at a different channel to the LHC.  This different channel can help rule out alternative explanations of the data.  For example, one popular idea is the fermiphobic Higgs.  This is a Higgs that couples to the W and Z with the Standard Model strength, but has suppressed couplings to quarks and leptons.  The Tevatron search disfavours such a model, since it would not decay to bottom quarks.

Lastly, I want to make an important comment on statistics.  Several sources, including the official press release, have said that there is
 only a 1-in-550 chance that the signal is due to a statistical fluctuation.

This is Inaccurate.

The probability that is quoted is the probability, if there is no Higgs, that this data could arise by chance.  What we want is the probability, given that we have a signal, that it arouse by chance.  The difference is analogous to the difference between the probability that it is cloudy, given that it is raining; versus the chance that it is raining, given that it is cloudy.  To answer that second question, we would need to use Bayes' Theorem.  Still, I think most particle physicists would agree that the stated odds are conservative estimates of the true probabilities.

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