Wednesday, 10 October 2012

A problem with bees

This one is not about the colony collapse disorder but about particle bees, also known as b-quarks. Older readers who still remember the LEP collider may also remember a long-standing anomaly in one of the LEP precision measurement. The observable in question is the forward-backward asymmetry of the b-quark production in electron-positron collisions. In the events with a pair of b-jets in the final state one counts the number of b-quarks (as opposed to b-anti-quarks) going in the forward and backward directions (defined by the electron and positron beam directions), and then defines the asymmetry as:

The observable is analogous to the top forward-backward asymmetry, widely discussed in the context of the anomalous Tevatron measurements, although the origin of the 2 anomalies is unlikely to be directly related. At LEP, the b-quark pair production is mediated mostly by a photon or a Z-boson in the s-channel. The latter has chiral couplings to matter, that is to say, it couples differently to left- and right-handed particles. Thanks to that, a significant b-quark asymmetry of order 10% is predicted in the standard model. However, the asymmetry observed at LEP was slightly smaller than predicted. The anomaly, sitting in the 3 sigma ballpark, has attracted some attention but has never been viewed as a smoking-gun of new physics. Indeed, it was just one anomaly in the sea of LEP observables that perfectly matched the standard model predictions. In particular, another b-quark precision observable measured at LEP  - the production rate of b-quark pairs,  the so-called Rb - seemed to be in perfect agreement with the standard model.  New physics models explaining the data involved a certain level of conspiracy: one had to arrange things such that the asymmetry but not the overall rate was affected. 

Fast forward to the year 2012. The Gfitter group posted an update of the standard model fits to the electroweak precision observables. One good reason to look at the update is that, as of this year, the standard model has no longer any free parameters that haven't been directly measured: the Higgs mass, on which several precision observables depend via loop effects, has been pinpointed by ATLAS and CMS to better than 1%.  But there's more than that. One notices that, although most precision observables perfectly fit the standard model, there are two measurements that stand out above 2 sigma. Wait, two measurements?  Right, according to the latest fits not only the b-quark asymmetry but also the b-quark production rate at LEP deviates from  the standard model prediction at the level of 2.5 sigma.   

The data hasn't changed of course. Also, the new discrepancy is not due to including the Higgs mass measurement, as that lies very close to the previous indirect determinations via electroweak fits. What happened is that the theory prediction has migrated.  More precisely, 2-loop electroweak corrections to Rb computed recently turned out to be significant and moved the theoretical prediction down. Thus, the value  of Rb measured at LEP is, according to the current interpretation, larger than predicted by the standard model. The overall goodness of the standard model fit has decreased, with the current p-value around 7%. 

Can this be a hint of new physics?  Actually, it's trivial to  explain the anomalies in a model-independent way. It is enough to  assume that the coupling of the Z-boson to b-quarks deviates from the standard model value:

In the standard model gLb ≈ -0.4, and gRb ≈ 0.08, and  δgLb = δgRb = 0. Given two additional parameters δgLb and δgRb we have enough freedom to account for both the b-quark anomalies.  The fit from this paper shows that one needs an upward shift of the right-handed coupling by 10-30%, possibly but not necessarily accompanied by a tiny (less than 1%) shift of the left-handed coupling. This sort of  modification is easy to get in some concrete scenarios beyond the standard model, for example in the Randall-Sundrum-type models with the right-handed b-quark localized near the IR brane.

So, maybe, LEP has seen a hint of compositeness of right-handed b-quarks? Well, one more 2.5 sigma anomaly does not make a summer; overall the standard model is still in a good shape.  However it's intriguing that both b-quark-related LEP precision observables do not quite agree with the standard model. Technically, modifying both AFB and Rb is much  more natural from the point of view of new physics interpretations. So I guess it may be worth,  without too much excitement but with some polite interest, to follow the  news on  B' searches at the LHC.

5 comments:

Anonymous said...

How long before we get a "Topological Geometrodynamics" explanation of the anomaly? ;-)

Luboš Motl said...

Strange, Csaki Randall Terning would say that the left-handed bottom (and both tops), not the right-handed one, is composite.

Jester said...

Yes, that would be more natural in RS models (because it's natural for tops to be highly composite, and bL sits in a doublet with tL). However realizing a composite bR is also possible.

Luboš Motl said...

Dear Jester, don't you want to install Mathjax on your blog? Look how wonderful LaTeX looks like with it, including arbtrarily high-resolution, printing etc. Equations are written as\[

E=mc^2

\] and inline maths as \(\sin x\), it just works great everywhere. LM

Anonymous said...

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