## Posts Tagged ‘oscillation’

### Mixing it up

Wednesday, November 14th, 2012

One of the other results presented at the Hadron Collider Physics Symposium this week was the result of a search for $$D^{0}–\bar{D}^{0}$$ mixing at LHCb.

Cartoon: If a $$D^0$$ is produced, at some time t later, it is possible that the system has "oscillated" into a $$\bar{D}^0$$. This is because the mass eigenstates are not the same as the flavor eigenstates.

Neutral meson mixing is predicted for any neutral meson system, and has been verified for the $$K^0–\bar{ K}^0$$, $$B^0–\bar{B}^0$$ and $$B_s^0–\bar{B_s}^0$$ systems. However, for the $$D^0–\bar{D}^0$$ system, no one measurement has provided a result with greater than $$5\sigma$$ significance that mixing actually occurs, until now.

The actual measurement is of $$R(t)=R$$, which is effectively the Taylor expansion of the time dependent ratio of $$D^0 \rightarrow K^+ \pi^-$$ (“Wrong Sign” decay) to $$D^0\rightarrow K^- \pi^+$$ (“Right Sign” decay). Charge conjugates of these decays are also included. There is a “Wrong Sign” and a “Right Sign” because the Right Sign decays are much more probable, according to the standard model.

The mixing of the $$D^0–\bar{D}^0$$ system is described by the parameters $$x = \Delta m /\Gamma$$ and $$y = \Delta \Gamma / 2\Gamma$$, where $$\Delta m$$ is the mass difference between the $$D^0$$ and $$\bar{D}^0$$, $$\Delta \Gamma$$ is the difference of widths of the mass peaks, and $$\Gamma$$ is the average width. What appears in the description of $$R$$, however, is $$x’$$ and $$y’$$, which give the relations between the $$x$$ and $$y$$ with added information about the strong phase difference between the Right Sign and Wrong Sign decays. The important part about $$x’$$ and $$y’$$ are that they appear in the time dependent terms of the Taylor expansion of $$R$$. If there were no mixing at all, then we would expect the ratio to remain constant, and the higher order time dependence to vanish. If mixing does occur, however, then a clear, non-flat trend should be seen, and hence a measurement of $$x’$$ and $$y’$$. That is why the time dependent analysis is so important.

Fit of ratio of WS and RS decays as a function of decay time of the D meson. Flat line would be no mixing, sloped line indicates mixing. From http://arxiv.org/pdf/1211.1230.pdf

Result of the mixing parameter fit of the neutral D meson system. 1,3 and 5 standard deviation contours are shown, and the + represents no mixing. From http://arxiv.org/pdf/1211.1230.pdf

The result is a 9.1 $$\sigma$$ evidence for mixing, which is also in agreement with previous results from BaBar, Belle and CDF. On top of confirming that the neutral D meson system does mix, this result is of particular importance because, coupled with the result of CP violation in the charm system, it begs the question whether or not there is much more interesting physics beyond the standard model involving charm just waiting to be seen. Stay tuned!

### Ghost Hunters: an international team tracks a disappearing particle

Friday, March 9th, 2012

Born in the hearts of stars and nuclear reactors, almost undetectable, nearly as fast as light, able to pass unhindered through everything from planets to people, and confirmed shapeshifters. That role call describes what makes the particles known as neutrinos both exciting and perpetually challenging for physicists on the hunt.

A series of brilliant experiments designed and executed since the 1950s have managed to detect these slippery subatomic wonders, revealing much about their origins, travels, and presence as one of the most abundant particles in the cosmos.

Earlier this week, an international collaboration led by China and the United States at the Daya Bay Reactor Neutrino Experiment in the south of China pinpointed the action behind one of the neutrino’s signature magic tricks: its ability to seemingly vanish entirely. The disappearing act is the product of neutrino oscillations, and the Daya Bay team calculated the final unknown transformation type. The 5-sigma discovery not only helps demystify the neutrino, but it will also guide future experiments in exposing more fundamental mysteries – such as how we exist.

Sensitive photomultiplier tubes line the Daya Bay detector walls, designed to amplify and record the faint flashes that signify an antineutrino interaction. (Courtesy of Roy Kaltschmidt, Lawrence Berkeley National Laboratory)

“It’s surprising and exciting that this result came so quickly and precisely,” said Brookhaven Lab’s Steve Kettell, who is Chief Scientist for the U.S. at Daya Bay. “It has been very gratifying to be able to work with such an outstanding international collaboration at the world’s most sensitive reactor neutrino experiment.” (more…)