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Alexander Mitov

Measurements that point to deviation from the established theoretical models

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The most intriguing measurements at particle accelerators are the ones that deviate from the theoretical predictions. Such deviations can signify:

  1. The discovery of fundamental new physics (think Nobel prize)
  2. A mis-interpretation of the ongoing physics
  3. Incorrect measurement or theoretical prediction
I will not dwell on 3. Clearly everyone hopes for 1. But in reality it is very hard to eliminate 2., i.e. to get a clear shot at 1.
Here are few of the most recent examples and what we have learned from them.


A very interesting neutrino Nucleon scattering experiment in the late 1990's. With reasonable (though somewhat simplified) assumptions, it measured directly the weak (a.k.a. Weinberg) angle, i.e. the ratio of the two gauge couplings of the Electroweak model (of Weinberg and Salam). A deviation from the theoretical expectation was found at the very intriguing (3+)σ level. The resolution turned out to be what many would call a "mundane physics" - namely the proton. In my opinion the most important legacy of this experiment was the experimental confirmation of flavor asymmetry in the strange quark sector. That agrees with theoretical prediction in QCD. It is worthwhile pointing out that this asymmetry is generated in QCD starting from the 3-loop level, i.e. it is a deep quantum effect. The moral of the story is: do not underestimate the proton: we do need to know its structure when we measure proton collisions. Think LHC.

Dimuon asymmetry observed by the DØ experiment (Tevatron)

This is a few months old result with no clear explanation yet. The DØ experiment measured a large (around 1%) relative difference between events containing two positively charged muons and events containing two negatively charged muons. The expectations within the Standard Model are that such CP breaking effect arises from intermediate B-meson oscillations. The problem is that the number we expect is much smaller that the measurement, i.e. there appears to be a 3.2σ discrepancy between the two. It is tempting to attribute that discrepancy to New Physics in the B-meson sector. But have we learned from NuTeV and are we finally able to control all proton-related effects?
Last Updated ( Wednesday, 24 November 2010 00:19 )