Is There Need for a New Particle Physics Model?
December 16, 2019
High energy particle physics experiments in the recent past have brought into question certain parts of the model currently used in particle physics. The Standard Model describes how particles interact and provides a framework of conservation rules that give insight into what decays are possible. It has been relied on for decades, but it may be time for an update. Experiments involving decays of the bottom quark show transitions occurring 10% to 20% beyond the predictions of the Standard Model and decays that violate certain rules of the Standard Model. The research article entitled Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark,1 uses data from former experiments and finds supporting evidence for some New Physics (physics beyond the Standard Model). This theoretical work uses some previously suggested particles, such as the leptoquark described below, to model and explain experimental results that the Standard Model cannot, and it does so very well. The researchers have since reported new results2 that support their theoretical model, from the Large Hadron Collider (LHC) at CERN. The model and experimental results hint at New Physics.
The Standard Model of Particle Physics
Scientists love to have a way to explain empirical evidence (measurements from experiments) as well as predict what measurements they might get under certain conditions. They develop models to do this, and the better the model, the better it fits the experimental measurements and the better it can predict future results. Models provide insights into understanding the physics beneath interactions.
For example, an early model of the atom, often known as the plum pudding model, considered a mixture of protons and electrons. This model predicted that alpha particles (helium nuclei) fired at a thin foil of silver or aluminum would result in none being scattered directly backward. Experiments showed that it happened about one in a thousand times. Scientists ended up throwing out the plum pudding model and introducing a nuclear model for the atom.
Physics tries to understand nature and its interactions. Everything around us is made up of atoms, which are made up of electrons orbiting a nucleus. The nucleus of an atom is made of protons and neutrons.
Electrons are an example of what is currently understood to be a fundamental particle – a particle that cannot be further broken down. Electrons are about 2000 times less massive than a proton. Electrons are in the special category of fundamental particles called leptons, as are its more massive analogs, muons and tauons. Electrons, muons, and tauons are charged particles. Neutrinos are also a type of lepton that when formed are paired with an electron, muon, or tauon and have different masses. The electron neutrino is of the order of 250,000 times less massive than the electron. For example, if a proton was about the same size and weight as an elephant (8500 lbs. or 4 cars), the electron would weigh about 4.5 lbs. or about half a gallon of water. On this scale the electron neutrino would weigh about 18 millionths of a pound or 8 milligrams. The muon neutrino is about 600 times less massive than the muon and about 100,000 times more massive than the electron neutrino, the tao neutrino is about 100 times less massive than the tao particle and about 9 billion times more massive than the electron neutrino. Neutrinos are electrically neutral particles.
Protons and neutrons that comprise the nucleus of atoms are each made of three quarks of the up or down variety. Quarks are fundamental particles; as far as we know, they are not made of smaller parts. Up and down quarks are known as first generation quarks. There are other types of quarks as well, the strange and charm quark are known as second generation quarks, and the top and bottom are third generation quarks. Quarks carry electric charge.
These particles are shown in Figure 1.
FIG. 1. Fundamental particles and force carriers in the Standard Model.
Image Credit: By MissMJ - Own work by uploader, PBS NOVA [1], Fermilab, Office of Science, United States Department of Energy, Particle Data Group, Public Domain,
https://commons.wikimedia.org/w/index.php?curid=4286964
The standard model explains every day matter well, but what about the forces of nature? There are four fundamental forces: gravity (what holds us to Earth), the weak force (involved in radioactive decay), the electromagnetic force (involving charged particles), and the strong force (involved in holding atomic nuclei together). The Standard Model does well in explaining the weak, electromagnetic, and strong force, and it does so with force carriers known as gauge bosons that get exchanged between the interacting particles. For example, when charged particles are interacting photons (bundles of electromagnetic energy) are passed back and forth. When a weak interaction occurs, W and Z particles are involved. The strong force involves quarks exchanging gluons. The gravitational interaction is believed to have mass exchanging gravitons. Gravitons have never been observed and there is no accepted quantum description of gravity. The gravitational force is not described in the Standard Model because it is so weak, however it is well described by Einstein’s General Theory of Relativity.
The big bang theory predicts that the universe started off as pure energy, with reason to believe that the forces were unified. However, it is not clear if and when the gravitational force was unified with the other three. The Grand Unified Theory predicts the unification of the strong, weak, and electromagnetic forces during the first 10-38 seconds of the universe. During this short time the universe is estimated to start off with a temperature of about 100 million quadrillion quadrillion degrees Celsius and drop to about to a mere 1 million quadrillion quadrillion degrees. This is too hot to imagine, but hopefully you can recognize that it would have a lot of energy. At this cooler, but still extremely hot temperature, the strong force separates from the electroweak force, the universe expands dramatically (inflation), and particles, along with their antiparticles, begin to pop into and out of existence. By 10-10 s the universe cooled to a mere 1 quintillion degrees, the electromagnetic and weak force separated, while the elementary particles (quarks and electrons) remained. This is the temperature where quarks begin to combine to form particles such as protons.
To look inside a proton, and study particles such as quarks, you cannot easily dissect it - conditions of the early universe need to be constructed – the environment needs to be very hot, with a lot of energy.
Exploring the Standard Model Experimentally
One of the key ways to explore quarks and the predictions of the Standard Model is to make particles go very fast and then smash these very energetic particles together. Scientists have designed machines to accelerate particles to near the speed of light and have them undergo head on collisions. From these experiments they are trying to understand the fundamental building blocks of our world and our universe. Some questions are: Why do things have mass? Why is our world predominantly matter rather than anti-matter? Were the fundamental forces ever unified?
They have certainly found a number of answers using the Standard Model. One of the big discoveries at CERN was in 2015 with the Higgs Boson. The more a particle interacts with the Higgs field, the more massive it is. There have been discoveries at CERN of the W and Z particles which carry the electroweak force and support the Grand Unified Theory. The Large Hadron Collider beauty experiment at CERN is designed to investigate the bottom quark and reveal some of the consequences of the earlier universe.
What’s Wrong with the Standard Model
Experiments have shown a number of decays involving bottom quarks going into other quarks and leptons, to a very high degree of certainty in the measurements. According to the Standard Model these transitions can happen, but rarely. Experimental observations deviate from the Standard Model by 10 to 20 percent. That deviation is too much for scientists. It means either the model has to be revised or an entirely new model is needed.
The Standard Model does not allow for lepton flavor universality violation. That is, the lepton number for each kind of lepton (electron, muon, tao) is to be conserved in the Standard Model, but evidence, for example from the LHCb, Belle, and BaBar experiments, has consistently shown this not the case. In fact, the Standard Model deviates from experimental results by 4 standard deviations (this is a lot!).
All measurements have uncertainty associated with them, so all experimental results are not exact values. The standard deviation is a way to describe the spread of the data points about an average value that is measured. For a theory to be good, it should not deviate much from the mean experimental results.
Something New: The Transitions of Bottom Quarks
The theoretical model explored by Crivellin and his colleagues is based on a theory proposed in the 1970’s by Salem and Pati.4 Salem and Pati’s model considers more particles than the Standard Model. One set of particles not in the Standard Model are called leptoquarks. A leptoquark is a particle composed of quarks and leptons (see FIG. 1). Crivellin’s work focuses on the third generation leptoquark (LQ3), the one that is associated with top and bottom quarks. Their theoretical approach models very well the anomalies observed, explaining all deviations from the standard model, such as lepton flavor, in experiments at three different colliders: BaBar, Belle, and LHCb. Although the leptoquark itself has not been observed, modeling the experimental system with leptoquarks works much better than modeling with the Standard Model. Further, there are no contradictions with this theoretical model to the Standard Model.
When asked about his current research, Andreas Crivellin of the Paul Scherr Institute explained it this way: “In 1974 the two famous theoretical physicists, Abdus Salam and Jogesh Pati, suggested that the fundamental constituents of matter (quarks and leptons) are unified. This means that there would be only one single elementary particle in nature at high energies. Recently, precision experiments at the Large Hadron Collider (LHC) at CERN revealed discrepancies from the predictions of the Standard Model (SM) of particle physics in decays of heavy (bottom) quarks to leptons. These deviations point towards the existence of physics beyond the SM, i.e. towards new unobserved particles and interactions, called New Physics (NP). In fact, these hints for NP can be explained by a hypothetical particle called leptoquark which is predicted by the Pati-Salam model. In our research, we refined existing calculations by taking into account the effects of quantum corrections and showed that the Pati-Salam leptoquark can successfully explain all observed deviations from the SM predictions. Furthermore, by explaining one class of observables, the hints for NP in the second class of observables are even predicted by the quantum corrections. This gives new actuality to the old idea of Pati-Salam that the fundamental particles of nature are unified and the predicted leptoquarks could even be directly observed at the LHC in the near future.”
This is exciting! Evidence is being gathered that supports New Physics, that supports a new fundamental particle, and that supports the unification of the electroweak and strong forces. Even more exciting were the new results being released during the writing of this article that further support a New Physics model. The results updated the fit in the recent paper to be about 7 standard deviations better than the Standard Model – which means this new model fits the data very well.
Scientists are often eager to discuss their work. The author is very grateful for many discussions with Dr. Andreas Crivellin and for resources he provided. Perhaps this will inspire you to ask a scientist what they are doing! And perhaps check out CERNs outreach site – and the LHCb!
—Heide M. Doss
References & Resources
[1] A. Crivellin, et al., Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark, Phys. Rev. Lett. 122, 011805 (2019)
[2] M. Algueró, et al., Addendum: “Patterns of New Physics in b→sl+l- transitions in the light of recent data” and “Are we overlooking Lepton Flavour Universal New Physics in b→sll?” Submitted to JHEP (2019)
https://arxiv.org/pdf/1903.09578.pdf
[3] To learn more about penguin diagrams read the article about Feynman Diagrams on Physics Central:
D. Lindley, Feynman Diagrams: The Science of Doodling,
https://www.physicscentral.com/explore/action/feynman.cfm
[4] J.C. Pati and A.Salam, Phys. Rev. D 10, 275 (1974); Erratum Phys. Rev. D 11 703 (1975).
https://doi.org/10.1103/PhysRevD.10.275;
https://journals.aps.org/prd/abstract/10.1103/PhysRevD.11.703.2
[5] Sirunyan, A.M., Tumasyan, A., Adam, W. et al. Eur. Phys. J. C (2018) 78: 707.
https://doi.org/10.1140/epjc/s10052-018-6143-z
[6] A. Crivellin, Explaining the Flavour Anomalies with the Pati-Salam Vector Leptoquark, Proc. Of Sci. LHCP (2018)
https://pos.sissa.it/
