09 July 2010

Friday Physics

Higgs Found?

Rumor has it, from a Fermilab scientist not involved in the experiments who doesn't know which group discovered it or any of the details, that one of the two groups at Fermilab working on the matter (D0 or CDF) has found a signal of a Higgs boson at 115 GeV to 140 GeV of mass with a 99.7% statistical significance level (i.e. three sigma).

Without going into great depth (click the physics tag for prior posts on the subject), the Higgs boson is the last undiscovered particle predicted by the Standard Model of Particle Physics, has been predicted by some to have a mass of 129 GeV (just at the limit of Fermilab's ability to detect), and is central to the Standard Model's explanation of how fundamental particles acquire mass. A Higgs boson is expected to be a elecro-magnetic charge neutral, spin-0 particle unlike quarks, electrons and neutrinos who are spin-1/2 particles, and the force carrying particles (gluons, the photon, the W and the Z) which are spin-1 particles.

Supersymmetric theories (including string theory), unlike the Standard Model, which predicts one Higgs boson, generally predict several Higgs bosons. Supersymmetric theories also predict many other particles some of which should be observable at the LHC if the theories are correct.

The labs are doing their best to get as many conclusions out as possible for the Large Hadron Collider's superior data stream scoops them, so a certain amount of skepticism is in order. As the source of the rumor explains in another post: "CERN experiments at the LHC are still waiting for enough . . . [data] to gain the necessary sensitivity to put CDF (and D0) out of this business."

Also, despite the supposed 99.7% certainty that a result is statistically significant, the conventional wisdom in the field is that three sigma results are wrong about half of the time. A five sigma result (like the recent muonic hydrogen proton size measurement that I discussed yesterday at this blog and also here) is the threshold considered reliable in the field.

Z' Not Found

The labs have also ruled out a light Z' boson (1071 GeV or less). The Z' boson is a hypothetical particle named by analogy to the Z boson of the weak force and predicted in:

Various models of physics beyond the Standard Model predict different kinds of Z' bosons.

* Models with a new U(1) gauge symmetry. The Z' is the gauge boson of the (broken) U(1) symmetry.
* E6 models. This type of model contains two Z' bosons, which can mix in general.
* Topcolor and Top Seesaw Models of Dynamical Electroweak Symmetry Breaking have Z' bosons to select the formation of particular condensates.
* Little Higgs models. These models typically include an enlarged gauge sector, which is broken down to the Standard Model gauge symmetry around the TeV scale. In addition to one or more Z' bosons, these models often contain W' bosons.
* Kaluza-Klein models. The Z' boson are the excited modes of a neutral bulk gauge symmetry.
* Stueckelberg Extensions (see Stueckelberg action). The Z' boson is sourced from couplings found in string theories with intersecting D-branes

Suffice it to say that the theories that predict a Z' particle were already disfavored even before the latest experimental result.

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