If you follow the blogosphere as a source of information on cutting-edge high-energy-physics results, you certainly by now know that the DZERO collaboration has produced a new exciting result. They find a 3.2 standard deviation effect in a study of charge asymmetry of muon pairs, which can be due to a unexpected, large source of CP violation -one which constitutes a very good shot at explaining the origin of the matter-antimatter asymmetry of the Universe!
I got news of this by Resonaances, who asked me about the analysis (I had no information for him), and later wrote an excellent blog posting on it. I refer you to his blog posting there, and to the links he publishes (yes I am not repeating them here, so you are forced to visit his site!), for more information and details which are however only understandable by insiders.
Instead, what I will do here is just to publish a figure from the DZERO paper, which combines all the information on a couple of parameters governing the CP violation asymmetry in the physics of Bs mesons. I am not in the mood of explaining in detail the physics behind the plot you find below, but you might still like to see where we stand in a graphical representation. Suffices here to say that Bs mesons are neutral hadrons containing a b- and an s-quark. They may "oscillate" [a meson may turn into a ], and the mechanism that turns the particle into the antiparticle is sensitive to the intervention of new physics, such a new supersymmetric body exchanged in the process. and , which stand on the axis labels of the plot, are just two meaningful parameters that describe the amount of violation of the symmetry of nature called "CP" that the physical system of Bs mesons exhibit.
Okay, a simple explanation before the plot...
If the above sounds utterly meaningless to you, I apologize -this is really hard physics to explain. What I might be able to have you get away with is the following: the law of nature called "conservation of CP" states that if you take a physical system and you invert all particles with their antiparticles, and place the result in front of a "mirror" (which exchanges left with right), then the physics of the CP-inverted system should be the same as the original one: you should measure the same production and decay rates, for instance. A small amount of CP violation signals that there is "something" (a new physics mechanism which intervenes in a small fraction of cases, or with a weak effect) that distinguishes the two systems.
CP violation -at the level of a few parts per mille- has been discovered in 1964 by Christenson, Cronin, Fitch, and Turlay in a system of neutral K mesons, particles quite similar to B mesons which are composed of a strange and one down quark. Again, neutral hadrons with different quarks. Of course, ever since 1964 we have tried very hard to get to know all we can about K mesons and their CP violation asymmetries, but ever since we discovered B particles in the late seventies, we have started to fantasize about doing the same CP investigations with B mesons: that is because in B mesons the possible effects of new physics (ones producing CP asymmetries) should be in principle easier to detect -a result of the larger mass of the b-quark with respect to the s-quark.
The focus on CP asymmetry is clearly due to the fact that we have no clear understanding of why our Universe seems to be primarily composed of matter: how can the Big Bang have produced matter instead of antimatter, if the processes yielding one and the other are related by a CP symmetry inversion and no violation of CP exists in the standard model ? Or to put it in another way: what is the source of the huge CP asymmetry we observe in the Universe today ?
And without further ado...
Okay, now for the plot of the week. It is shown below.
The two parameters governing the CP violation in the Bs sector are shown on the two axes. The vertical black bar labeled "SM" shows where we should be: the standard model prediction for the strong phase is zero. For instead the prediction is small and positive, with a uncertainty which correspond to the length of the bar.
And where are we, after the measurements of Bs mesons performed by DZERO, and after the recent 3.2 sigma asymmetry they found in dimuons ? We are where the blue crosses lay. The blue contours around them show 1-sigma regions -area of the parameter space which are compatible with the measurement within their uncertainties. The red hatches instead encircle the two-sigma region -reality is outside it only 5% of the times. Two further regions are encircled by the green hatches: only 1% of the time the experiment may have measured the two parameters where the crosses lay while the real value is outside that region.
There are two crosses, and two regions allowed by measurements, because the measurements are insensitive to the product of the signs of the two parameters, modulo a certain phase shift. What matters, however, is that the standard model prediction is well outside the 95% region. Is this the first true sign of new physics in the Bs sector ? We will need more investigations by DZERO and CDF, and maybe further input from other experiments too. But this field of research is definitely exciting.
One further thing to note: it would be ironic if new physics required a deep study of B mesons (particles which do not require the highest energy to be produced) rather than pushing to higher and higher energy our investigations. The LHC might turn out to be a spectator in this race for a while... Of course, the ATLAS and CMS experiments will one day have more data than CDF and DZERO and will surpass by far the sensitivity of their Fermilab cousins even in B-physics measurements; but it will take years!
The Plot Of The Week - News On CP Violation
By Tommaso Dorigo | May 18th 2010 02:54 AM | Print | E-mail