The potential harbingers of new physics persist in LHC data

The potential harbingers of new physics just don't want to disappear
Will anomalies observed in the decays of beauty mesons disappear with the new data, as exotic lands disappeared from maps of cartographers? The latest analysis, taking into account long-range interactions, proves that the anomalies are visible not less, but better. Credit: IFJ PAN

For some time now, researchers have noted several anomalies in the decays of beauty mesons in the data coming in from the LHCb experiment at the Large Hadron Collider. Are they more than just statistical fluctuations? The latest analysis, taking into account so-called long-distance effects in the decays of particles, increases the probability that the anomalies are not an error in the measuring techniques.

While scientists seeking direct traces of new physics try to eliminate all potential new signals in particle collisions, revealing the void predicted by the Standard Model, others looking at other phenomena begin to see increasing anomalous signals in the ocean of data that are unresolved.

The Standard Model is a set of theoretical tools developed in the 1970s to describe phenomena occurring at the scale of atomic nuclei and elementary particles. It works very well, but cannot provide answers to some important questions. Why do elementary particles have particular masses? Why do they create families? Why does matter so clearly dominate over antimatter? What does dark matter consist of? There is a well-founded belief among physicists that the Standard Model only describes a fragment of reality, and needs to be extended.

"For a long time in the LHC, there has been an intense hunt for everything that cannot be explained by current physics. At present, the search for new particles or phenomena in a direct way remains fruitless. However, several anomalies have been found in data containing decays of beauty mesons. They are becoming more interesting day by day because the more data we process and the more effects we take into account when describing them, the more they are visible," explains Dr. Marcin Chrzaszcz (IFJ PAN, University of Zurich), co-author of the latest publication in the journal European Physical Journal C. The other three authors are Christoph Bobeth from the Technical University of Munich, Danny van Dyk from the University of Zurich (UZ), and Javier Virto from the PM and the Centre for Theoretical Physics at the Massachusetts Institute of Technology in Cambridge, U.S.

Mesons, made of quark-antiquark pairs, come in many varieties. The B (beauty) mesons contain a down quark, one of the components of protons and neutrons that is common in nature, and a beauty anti-quark. Mesons are unstable systems and quickly disintegrate in ways that are described as channels of decay. One of these anomalies was observed in the decay channel of meson B to another meson (K*; this meson contains a strange quark instead of a beauty quark) and a muon-antimuon pair (muons are with properties similar to electrons, only almost 200 times more massive).

"In previous calculations, it was assumed that when the disintegrates, there are no more interactions between its products. In our latest calculations, we have also included long-distance effects called charm-loops. With a certain probability, the products of decay interact with each other, for example exchanging gluons, the particle responsible for strong interactions, bonding quarks in protons and neutrons," says Dr. van Dyk (UZ).

The effect of measurements in physics is usually described by the value of the sigma standard deviation. An effect differing from predictions by more than three standard deviations (3 sigma) is treated as an observation. A discovery is said to have been made when the accuracy rises above 5 sigma (which means a probability of less than one in 3.5 million that random fluctuation will give the observed result). Analyses of the decays of B mesons to K* mesons and a muon-antimuon pairs showed a tension with Standard Model prediction of 3.4 sigma (in other decay channels, anomalies of a similar nature were observed). Meanwhile, the inclusion of long-range effects in the theoretical description increased this value to 6.1 sigma. The researchers hope that the mathematical methods they proposed, applied to similar decay channels, will also significantly increase the precision of estimates.

"The detected anomalies do not disappear in subsequent analyses. Now that the theoretical description of these processes has been worked out, everything depends only on the statistical precision, which is determined by the number of decays being analyzed. We will probably have a sufficient amount within two or three years to confirm the existence of an with a credibility entitling us to talk about a discovery," says Dr. Chrzaszcz.

The origin of the observed anomalies remains unknown. Many physicists propose that an unknown elementary particle outside the Standard Model may be responsible for their existence. A good candidate, for example, would be the Z' boson, proposed by theoreticians. Direct verification of this hypothesis, however, would require further experiments performed on an accelerator more powerful than the modern LHC configuration.

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More information: Christoph Bobeth et al, Long-distance effects in B→K∗ℓℓ from analyticityThe European Physical Journal C (2018). DOI: 10.1140/epjc/s10052-018-5918-6
Citation: The potential harbingers of new physics persist in LHC data (2018, August 31) retrieved 17 July 2019 from
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Sep 01, 2018
For us laypeople, does the above research point to one or more of the new physics models described at Wikipedia's page on Z' bosons? - https://en.wikipe..._bosons. If so, which one(s)? Or does it point to something else? TIA!

Sep 03, 2018
@ coastaljon, as fellow layman I agree, but the Wiki already gives some information about Z' bosons:

1. Models with a new U(1) gauge symmetry. The Z′ is the gauge boson of the (broken) U(1) symmetry.
2. E6 models. This type of model contains two Z′ bosons, which can mix in general.
3. Topcolor and Top Seesaw Models of Dynamical Electroweak Symmetry Breaking have Z′ bosons to select the formation of particular condensates.
4. Little Higgs models. These models typically include an enlarged gauge sector, which is broken down to the Standard Model gauge symmetry around the TeV scale. In addition to one or more Z′ bosons, these models often contain W′ bosons.
5. Kaluza–Klein models. The Z′ boson are the excited modes of a neutral bulk gauge symmetry.
6. Stueckelberg Extensions (see Stueckelberg action). The Z′ boson is sourced from couplings found in string theories with intersecting D-branes.

Sep 03, 2018

I have no idea if the anomalies would directly confirm the Z boson or with it the Little Higgs models or String theory (probably not), but maybe someone with a background in Physics could help explain.

And with "background in Physics" I don't mean electric universes, dense aether stuff, or whatever that regularly pollutes the comment section!

Sep 03, 2018
The article and paper was a bit impenetrable, but it looks to me like it suggests improved ways to model observed decay modes rather than a particle mass peak as such (Fig. 2 of the paper). The usual particle quality limits are, what I understand, increased by 2 sigma - from 3 to 5, say - to account for "the look elsewhere effect" of matching a peak signature of unknown mass energy along an energy range. (So I would say that 5 sigma is their usual "observation" or "detection" limit, where one can see 3 to almost 5 sigma candidate detections come and go simply from random noise effects.)

Their model improvement by including new methods and a option for new physics looks nice (Fig. 2) but is not that impressive seen by sigma (~ 1 sigma each, Fig. 3), nor conclusive (new method better than simply new physics) and not compared vs number of variables what I can see (given more parameters you can fit anything).

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