The Plot Of The Week - News On CP Violation
    By Tommaso Dorigo | May 18th 2010 04:54 AM | 21 comments | Print | E-mail | Track Comments
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    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    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!


    Can you say something about the prospects for CDF to make a similar measurement? I heard Nth-hand that the prospects are not good, but I don't understand why that is so. Thanks!

    Hi Matt,

    it depends of which measurement we are talking about. CDF does measure phi_s and Delta_Gamma_s and our precision is, I believe, better than DZERO's on those (need to check).
    If you talk about the recent asymmetry, instead, then DZERO has a clear advantage: they can invert the polarity of their magnet, so that the trajectory once made by a positive particle becomes that of a negative one, and vice versa.

    I do not believe CDF can match DZERO in this particular measurement. But I will be happy to be proven wrong...

    "...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!"

    That connection is always cited, but it's a rather fragile one. Can you explain, why this source of CP violation constitutes a very good shot, or do you just assume that any CP violation beyond the SM could
    have something to do with the matter-antimatter asymmetry? There's million explanations and mechanisms for the matter-antimatter asymmetry, many of which don't require low-energy CP violation at all (e.g. some variations of Leptogenesis).

    At the end it is just one number (eta_B) and there's allways a very model-dependent path to calculate it. I wouldn't overstate it therefore. Much more fascinating is the fact that we might have discovered the first sign of physics beyond the SM in a collider. That would be a revolution.

    Hi anon,

    why don't you ask this to the DZERO folks ? They wrote it in their article, now public. In any case, the asymmetry is in the direction of creating more matter (negative muons), which is the right one at least.

    Historically, how many times has a 3.2 sigma signal vanished on obtaining further statistics? (Not based on probability, but on actual particle collider experience?).


    There's a well-known saying in particle physics: "Half of all three sigma results are wrong".

    Hi Andrew, nice to see you here. Indeed, the central limit theorem is a long way away from typical HEP search results! :)

    This is precisely why we have LHCb!

    How will the LHC be a spectator when LHCb is designed for exactly these sort of measurements?! With the LHC operating at half energy then it will soon surely be the time for the LHC to begin to confirm the Fermilab results. With a higher luminosity you would expect these findings to be further refined as well.

    Congratulations to DZERO, but yeah ... go LHCb! My QI calculation of 0.0388 for 2beta_s is right on the SM mark, which makes this very, very interesting. Since we have only looked at the complex number case so far, it is not surprising that we only get the SM value. Looking for this potential real world non SM value could make for some fun work ...

    LHCb can find stuff like this much faster than the other experiments, that is true. I am not sure whether the data they will collect in the next few months will be enough...


    Before jumping the gun on the contribution of SUSY and exotic physics, it is worth recalling that a similar CP violation in cosmology (namely baryon-antibaryon) require the underlying reactions to be out of equilibrium.

    If the D0 prediction stands, it seems to me that the most natural explanation of charge asymmetry is that oscillations of B mesons fall out of equilibrium. Providing that this is true, there is a plethora of surprising implications regarding physics beyond SM. These discoveries are most likely around the corner.



    A collider experiment is a pretty out of equilibrium setup.
    I'm not sure whether something as an equilibrium can be defined for B mesons, since this is usually a property of many body systems

    Anon 11,

    Statistical physics of many-body systems may either comply with Boltzmann-Gibbs distributions at thermodynamic equilibrium or fail to comply. A typical example of non-equilibrium thermodynamics is provided by Tsallis theory. It deals with the behavior of non-extensive systems for which entropy is no longer an additive quantity. There is a large body of research papers dealing with applications of Tsallis statistics in high-energy physics, the astrophysics of cosmic rays and inflationary cosmology.


    Is this level of CP violation unexplained by the standard model. I thought that the standard model, allow for up to three complex parses in the CKM quark mixing matrix representing CP violation, as ad hoc parameters to be measured. I wasn't aware that any part of the standard model explained why. Was my knowledge out of date/incorrect.

    The CKM matrix contains four free parameters -- three mixing angles that describe flavor changing decays, and one phase that describes CP violation. Once you've measured the CP violation in K0 decay, the violation in B decay is a firm prediction, and therefore a consistency test, of the Standard Model.

    Then this is a sexy and new result, needing a fern explanation, looking forward to reading all those new phenomenology papers on ArXiv with generational groups and new particles/forces to explain the violation.


    It is important to emphasize that Standard Model is unable to explain the physical origin of the four free parameters entering the CKM matrix. Measuring the complex phase in the decay or K mesons does not tell anything about why CP violation occurs in the first place.


    Actually the phase violating parameter (delta) in the standard parameterization of the CKM matrix changes also the mixing angles. In fact, there are only four parameters, but there are ten obvious observables in the CKM matrix; nine transition probabilities and one CP violation parameter (Jarlskog invariant J_CP). So when you parameterize the matrix, it's inevitable that there will be interactions in their effects. The net result of this is that when you're looking at CP violation in the CKM matrix for different processes, all the various CKM measurements contribute to the error bars, not just measurements of CP violation for the other processes. Additionally, J_CP is zero (and so there is no CP violation) when delta is zero, but J_CP is also zero when one or more of the mixing angles theta_jk are zero. Thus CP violation is always also a function of the mixing angles. It's probably best to just leave the stuff up to the computers and let them run tests over huge numbers of parameters to see which are consistent with measurements of observables.

    My fellow experimentalists keep telling me that I should not believe any measurement of a charge asymmetry that is supposed to be better than half a percent due to systematic uncertainties. The Standard Model prediction for this thing is 10^-3 -- so if your (D_0's) systematic uncertainties are of the order of a percent, that's what they would measure...

    Alexey, I concur. And in fact, it seems that the Bs parameters have now returned in good agreement to the standard model. There is a link to slides of a CDF talk with new results, see Peter Woit's blog.