Stories about string theory and antimatter caught my eye in the last couple of weeks. In the story about the former, the possible discovery of a new particle using the Large Hadron Collider (LHC) has led physicists to question string theory. In the story about the latter, the question of why there is far more matter than antimatter may finally have an explanation.
Is String Theory Dead?
In a paper appearing in the May 27, 2025, edition of Physical Review Research, physicists at the University of Pennsylvania and Arizona State University decided to stress test string theory by searching for particles that are conspicuously absent from string-based calculations.
The particles in question are a 5-plet, five related particles tied together by symmetry, important in the explanation of the Standard Model of Physics. One of the 5-plet particles is the Majorana fermion, first postulated by Italian physicist Ettore Majorana in 1937. A more familiar fermion is the electron, which we learned about in high school physics and chemistry.
String theory and the Standard Model are competing theories to describe the structure of the Universe.
String theory proposes that the fundamental particles of the Universe are tiny vibrating strings. The theory’s goal is to unite the four fundamental forces of the Universe, gravity, electromagnetism, the strong nuclear force and the weak nuclear force into a single framework.
The Standard Model describes electromagnetic, weak and strong nuclear forces as fundamental to the classification of all elementary particles. It does not include gravity. The Standard Model states there is no known particle, theoretically called a graviton, to incorporate into the explanation of the theory and that the other three forces explain almost all known phenomena in the Universe observed to date. Instead of a graviton, the explanation for gravity is provided by Einstein’s theory of general relativity.
While the Standard Model has been highly successful in explaining the observable Universe, it is said to be incomplete without incorporating gravity as the fourth fundamental force. Meanwhile, string theory, with its 10 or 11-dimensional mathematical requirements, remains untested.
Enter the LHC, where protons are slammed together to create light and heavy particles, including fermions. These particles rapidly decay, with only the heavier ones visible for short periods. The search for the 5-plet is on. It can help explain dark matter while at the same time disproving string theory. So far, the search had produced no 5-plets which would be five times heavier than the famous Higgs-Boson particle, first detected by the LHC in 2012. Future searches with upgraded tests may lead to the discovery, and if so, the stress testing of string theory will have snapped the strings.
Why Is There More Matter Than Antimatter?
Nobel Prize-winning physicist, Tsung-Dao Lee, has stated, “Symmetry reveals the beauty of the Universe, while asymmetry generates its substance.” What did he mean?
Symmetry in cosmology refers to fundamental laws of nature where matter and antimatter coexist. At the subatomic level, the particle interactions are similar, so why isn’t there an equal amount of matter and antimatter in the Universe, or is it that we have just not found the nooks and crannies where the latter may exist?
Asymmetry can be seen in the visible Universe. Galaxies, stars, planets and all the other visible space stuff are made of matter. Antimatter cannot be seen. The ratio of matter to antimatter is asymmetric. Yet we know that for every matter particle, an antimatter counterpart exists exhibiting the same mass but an opposite charge. For example, electrons have an antiparticle is the positron. When the two meet, they annihilate each other.
Today, we use particle accelerators like the LHC to produce antimatter, but in the Universe, the imbalance is notable. There is very little antimatter out there.
That’s why a new study by Xiao-Gang He and Chia-Wei Liu from the Shanghai Jiao Tong University provides new insights into the fundamental mechanisms of the matter-antimatter asymmetry in the Universe, testing the Standard Model and possibly leading to new physics. The study looks at anti-triplet charmed baryon decays into octet baryons and pseudoscalar mesons using flavoured symmetry. The decay rates are analyzed for their CP-violating effects.
What are CP violations? They occur when the laws of physics differentiate between particles and their antiparticles. CP stands for Charge conjugation (C) and parity in mirror coordinates (P). When both are violated, it produces particles and antiparticles that are not perfect opposites.
The presence of observed and abundant CP violations may explain why there is far more matter than antimatter in the Universe with these violations occurring from the moment of the Big Bang.
