Quark asymmetries hint at physics beyond the Standard Model

Aug 26, 2013 by Lisa Zyga feature
This is the first plot showing the predicted bottom-quark forward-backward asymmetry (in percentage) plotted against the energy (in GeV units) of the bottom quark-antiquark pair produced in the proton-antiproton collision at the Tevatron. Orange is the prediction of the Standard Model. The other colors correspond to predictions from proposed extensions of the Standard Model that add a new particle called an “axigluon,” which was proposed to explain the anomaly in the observed top-quark forward-backward asymmetry. The different colors correspond to different parameters assumed for the axigluon (the parameters are the mass of the axigluon, MG, and the strength of the interaction of the axigluon with the top quark, g). The blue plot deviates strongly from the orange, so experiments should be able to tell the difference. The other colors do not deviate that much from the orange, so more work or a different method would be needed if the axigluon has parameters like those (but the blue line is for parameters that best explain the top-quark case). Credit: Grinstein and Murphy. ©2013 American Physical Society

(Phys.org) —While scientists have become increasingly convinced that the Standard Model of particle physics is incomplete, it's still unclear exactly how the Standard Model needs to be extended. Experiments have shown that the Standard Model cannot explain certain top quark observations, but a variety of extensions of the Standard Model have been proposed to explain them, and it's unclear which extension is correct. In a new paper published in Physical Review Letters, physicists Benjamín Grinstein and Christopher W. Murphy at the University of California, San Diego, have explained how upcoming data on the bottom quark can be used to distinguish between competing new physics explanations of unexpected top quark data.

"There has been much excitement the last couple of years precipitated by reports by the two experimental collaborations working at the Tevatron (at Fermilab, outside Chicago) that a much larger-than-expected top-quark forward-backward is seen," Grinstein told Phys.org. "Several models have been proposed to explain this unexpected result. Our paper suggests a way to distinguish among the various models that have been proposed, since these models give very different bottom-quark forward-backward asymmetries. When a sufficiently of the bottom-quark forward-backward asymmetry is performed, we will be able to narrow down significantly the that the Tevatron experiments seem to have uncovered.

"But perhaps more importantly, observations of the bottom-quark forward-backward asymmetry in disagreement with expectations from the Standard Model, when put together with the top-quark forward-backward asymmetry, would demonstrate fairly conclusively that there is new physics in the form of new particles and interactions not included in the Standard Model, and would point the way toward its direct experimental confirmation. So, as you can see, this would go to the heart of the question in particle physics."

As the explain, a quark's forward-backward asymmetry refers to the likelihood that the quark is moving in the forward or backward direction after it is produced in a proton-antiproton collision.

"The Tevatron is a large circular particle accelerator in which protons and antiprotons travel in opposite directions," Murphy said. "The direction of travel of a proton at the point it collides with an antiproton is called the 'forward' direction. Often a b-quark and an anti-b-quark are produced as a result of the proton-antiproton collision. There are several ways to define a 'bottom-quark forward-backward asymmetry,' but they all are a measure of how more (or less) likely it is for the produced b-quark to be moving preferentially in the forward direction. For example, one may count the number of collisions with a forward-moving b-quark, subtract the number of collisions with a backwards-moving b-quark, and divide this by the total number of collisions that produce b-quarks. It should be noted that the asymmetry is not just a single number because it can be determined for various values of the energies of the produced b-quarks. So in fact the asymmetry is a function of the energy of the b-quarks."

This is the second plot showing the predicted bottom-quark forward-backward asymmetry (in percentage) plotted against the energy (in GeV units) of the bottom quark-antiquark pair produced in the proton-antiproton collision at the Tevatron. Again, orange is the prediction of the Standard Model. Now the other colors are for a model of "scalar weak doublets." The particles in this model are also characterized by two parameters: a mass and an interaction strength. Interestingly, on the bin that includes the Z-boson, the sign of the asymmetry is reversed for the scalar weak doublets model. The effect is large enough that the experiment should easily distinguish the new physics, if present. Credit: Grinstein and Murphy. ©2013 American Physical Society

If new physics is involved, as the physicists expect, then the bottom-quark forward-backward asymmetry might be larger than predicted by the Standard Model, or the asymmetry may even be reversed.

"The Standard Model of electroweak and strong interactions predicts a very small bottom-quark forward-backward asymmetry at the Tevatron, of the order of a few percent," Grinstein said. "What we have shown in our work is that new physics can change this number dramatically. One of the interesting features we discovered is that when the energy of the b-quark and b-antiquark sum to the rest energy of the Z-boson (one of the responsible for weak interactions), the asymmetry is enhanced. We furthermore showed that, at this particular energy, the effects of new physics can be greatly amplified. For example, in one popular class of models the sign of the asymmetry is reversed, relative to that predicted by the Standard Model, in the energy region corresponding to the Z-boson's rest energy."

The plots in the physicists' paper tell a more detailed story of the possibilities for new physics. The two plots included here show the predicted bottom-quark forward-backward asymmetry (in percentage) plotted against the energy (in GeV units) of the -antiquark pair produced in the proton-antiproton collision at the Tevatron.

In both plots, the orange line represents the Standard Model prediction, while the other colors correspond to predictions from proposed extensions of the Standard Model. The plots are not continuous, but instead they are bar graphs in which the quark pairs of energies in given "bins" are collected together. The black vertical bars indicate what the CDF experiment at Fermilab predicts as its sensitivity, meaning they will be able to distinguish between colored lines that are separated by more than the size of the black bar.

Explore further: The unifying framework of symmetry reveals properties of a broad range of physical systems

More information: Benjamín Grinstein and Christopher W. Murphy. "Bottom-Quark Forward-Backward Asymmetry in the Standard Model and Beyond." PRL 111, 062003 (2013). DOI: 10.1103/PhysRevLett.111.062003

Related Stories

Precision measurements using top quarks at CMS

Nov 05, 2012

Amongst all known elementary particles, the top quark is peculiar: weighing as much as a Tungsten atom, it completes the so-called 3rd generation of quarks and is the only quark whose properties can be directly ...

12 matter particles suffice in nature

Dec 13, 2012

How many matter particles exist in nature? Particle physicists have been dealing with this question for a long time. The 12 matter particles contained in the standard model of particle physics? Or are there ...

Recommended for you

What time is it in the universe?

Aug 29, 2014

Flavor Flav knows what time it is. At least he does for Flavor Flav. Even with all his moving and accelerating, with the planet, the solar system, getting on planes, taking elevators, and perhaps even some ...

Watching the structure of glass under pressure

Aug 28, 2014

Glass has many applications that call for different properties, such as resistance to thermal shock or to chemically harsh environments. Glassmakers commonly use additives such as boron oxide to tweak these ...

Inter-dependent networks stress test

Aug 28, 2014

Energy production systems are good examples of complex systems. Their infrastructure equipment requires ancillary sub-systems structured like a network—including water for cooling, transport to supply fuel, and ICT systems ...

User comments : 7

Adjust slider to filter visible comments by rank

Display comments: newest first

Q-Star
4.3 / 5 (7) Aug 26, 2013
@ Zephyr, there ya are, I was beginning to worry that Doc Brown fellow had sent the tipstaffs after ya.
Torbjorn_Larsson_OM
4 / 5 (5) Aug 27, 2013
"While scientists have become increasingly convinced that the Standard Model of particle physics is incomplete, it's still unclear exactly how the Standard Model needs to be extended. Experiments have shown that the Standard Model cannot explain certain top quark observations,".

Zyga is blatantly lying yet again, on a science web site about science. The Standard Model was just completed, but it is known to be a not complete account for particles. (E.g. doesn't predict neutrino masses, Higgs masses, DM or the inflaton.) That is different.

And those "top quark observations" are known to be ~ 2 sigma from combined data fishing, or as the paper itself confesses not even 3 sigma if cherry picked. 2 sigma effects come and go. and there is as of yet nothing new for the SM to predict.

Yikes, and more lies, now from an anti-science troll. There is no such predictive, established theory.
ant_oacute_nio354
1 / 5 (11) Aug 27, 2013
The standard model is all wrong, because mass is an intrinsic property.
The Higgs doesn't exist.
The mass is an electric dipole moment.

Antonio Jose Saraiva
mytwocts
not rated yet Aug 27, 2013
Whether asymmetry has been observed or not I cannot decide from the plots and teh (lack of) information in the text.
What are the black points in the plots? Are these perhaps experimental values? If so than NO asymmetry has been observed.
Author, please clarify since I have no access to the research paper.
mytwocts
not rated yet Aug 27, 2013
I have no access to the research paper
Be my guest http://arxiv.org/....6995...

Thanks. There seems to be an inaccuracy the first paragraph of the paper. The value of Att_FB at Mtt=500 GeV is quoted as 0.295, but from the graphs there and in the reference it is more likely to be 0.259. Thus it is not >3 sigma but 2.5 sigma away from the standard model. Anyway, it is statistically questionable to pick one of the 4 values with the largest deviation/sigma ratio. For the values at the two largest Mtt the deviation is 2 sigma.
mytwocts
not rated yet Aug 27, 2013
Since Abb_FB is so small and its deviation from the standard model is less significant the best strategy is still to improve the Att_FB statistics.
baudrunner
not rated yet Sep 07, 2013
I always thought that quark asymmetries were necessary for differentiation to occur. If all the color change combinations in quantum chromodynamics were realized, then reality would not necessarily be skewed the way it is, and existence of the universe would have been over in a relatively short time. Sort of like comparing a complex fractal to a simple triangle. Creation is, after all, a fractal expression.