Wednesday, April 06, 2011
UNEXPECTED PARTICLE(?) SHOWS UP AT FERMILAB
In a paper just released, a huge team of scientists at Fermilab has studied particle interactions and noted that the results suggest a discrepancy with the predictions of the "Standard Model."
There seems to be a "bump" or excess of data at a certain energy. This may correspond to the existence of a particle -- different from any particles that are known or expected to exist.
Invariant Mass Distribution of Jet Pairs
Produced in Association with a W boson in p p Collisions
At sqrt(s) = 1.96 TeV
Produced in Association with a W boson in p p Collisions
At sqrt(s) = 1.96 TeV
The importance of this result is that over the last few decades a model of elementary particles - believed to be the foundation for all the physical processes in the universe - has been developed, and is called "the Standard Model."
The model is known to be fundamentally incomplete. It has many unexplained parameters that determine the actual values of many of its predictions. It does not take into account General Relativity (the theory of gravity). It says nothing about Dark Energy or Dark Matter (which together seem to constitute 96% of the energy around us.) And there is at least one particle (the Higgs Boson) which is expected, but has not been seen.
Despite these large-scale issues that show the incompleteness of the Standard Model, essentially all the actual results of particle experiments have been consistent with this model. When the results of an experiment can be predicted, and the experiment is carried out, the results agree.
Until now.
This is the first time that an experimental result seems to be different in an important way from the predictions of the Standard Model.
Of course, it is possible that the calculation of what to expect was wrong. Or that something was missed in describing the results of the experiment. So, then, the correct calculation from the Standard Model would agree with the correct result from the experiment.
But, if the calculation was correct, and the experimental result is correct and different from the theoretical prediction, then the Standard Model may have to be adjusted, altered, or scrapped for a better theory. (A "better theory" would give all the same predictions as the old theory whenever experiments agreed with the Standard Model, but give the correct result for this experiment.)
The experimental results are highly dependent on extremely difficult measurements, and on estimations of exactly what each measurement means. (The measurements describe the momentum, energy, mass, charge, and other properties of the incoming and outgoing particles when two particles collide and "smash up".)
In the paper, and in a talk at Fermilab, the measurements -- and the assumptions that go into combining all the measurements -- were discussed. (The theoretical calculations that go into predicting what should be expected were not the focus of the talk or the lecture and are available from the references.)
Here's the paper -- (this is raw experimental physics):
http://arxiv.org/PS_cache/arxiv/pdf/1104/1104.0699v1.pdf
Here's the archived video of the talk. Note that the charts are shown next to a small window of video. Do not zoom the video, because that will obscure the charts. There is little visual information in the video -- you need to view the charts as the speaker is talking.
Here's the video:
http://vmsstreamer1.fnal.gov/Lectures/WC/110406Cavaliere/index.htm#
The speaker seems to be in complete command of the subject matter. She is Italian and speaks with a strong accent, but after listening for a while to her rolling rrrr's, you do get acclimated to the sound, and have no trouble understanding what she is saying. (You may need to understand modern experimental physics to understand the details of what she is saying, however.)
Here's the home page for Fermilab activities:
http://www.fnal.gov/pub/today/
Labels: Fermilab, high energy physics, particle physics, Standard Model
