CMS Improves Tau Decay Measurement to Probe Origin of Matter

The CMS experiment has refined its measurement of how Higgs bosons decay into tau leptons, bringing scientists closer to understanding why matter dominates the universe over antimatter. Since the Higgs boson’s 2012 discovery, researchers have sought to explain the observed imbalance; the laws of physics suggest matter and antimatter should have formed in equal amounts after the Big Bang, yet nearly all antimatter appears to have vanished. This new analysis, utilizing data from 2022, 2023, improves upon previous attempts by more accurately reconstructing the direction of elusive neutrinos produced in tau decays. “What makes this result exciting is that CP violation in the Higgs sector remains possible,” remarks Dr. Océane Poncet, who earned her PhD from the University of Strasbourg for her work on this measurement, “and the data show a slight preference for a CP-violating scenario—in other words, the Higgs boson may exhibit characteristics of CP-odd behavior.”

Higgs Boson Decays Probe Matter-Antimatter Asymmetry

A subtle asymmetry in the behavior of matter and antimatter may be within reach of detection, thanks to refined analysis of Higgs boson decays by the CMS experiment. This imbalance suggests a violation of charge-parity (CP) symmetry, a fundamental principle governing particle interactions, and current observations of CP violation in other processes are insufficient to explain the observed matter-antimatter discrepancy. Researchers are now focusing on the Higgs boson as a potential source of this asymmetry, specifically examining how it decays into pairs of tau leptons, heavier relatives of electrons. The latest CMS analysis, incorporating data from 2022, 2023, employs an updated method to more accurately reconstruct the decay paths of these taus, even accounting for elusive neutrinos.

This precision depends on estimating the direction of the neutrinos by analyzing the deflection of charged pions produced in the decay. Stepan Zakharov, a PhD student at DESY, notes that “in the previous analysis, the decay planes could only be approximated because of the missing neutrinos.” While the current measurement of the CP-violating angle, α Hττ, remains consistent with zero, indicating no violation, the data exhibit a slight preference for a CP-odd scenario.

Tau Decay Plane Reconstruction with Three-Pion Trajectories

The search for answers to the matter-antimatter imbalance has led physicists to increasingly precise measurements of the Higgs boson’s decay patterns, and the CMS experiment is refining its techniques for observing these subtle effects. A key challenge lies in reconstructing the decay of tau leptons, heavier cousins of electrons, which quickly disintegrate into other particles; these decays are crucial because the Higgs boson can transform into pairs of them. Previous analyses approximated tau decay planes due to undetectable neutrinos, but a new method focuses on reconstructing these planes using three charged pions and a neutrino, offering a more detailed view of the process. This updated method determines the tau’s direction by pinpointing the intersection of three pion trajectories and estimates the neutrino’s path by observing how the pions deflect at the decay point.

This reconstruction allows for a more precise measurement of the angle ϕ CP between the tau decay planes, a critical observable for detecting CP-violation. The resulting data, when combined with previous LHC “Run-2” data, continues to refine the measurement of α Hττ, an angle parameterizing the decay’s CP-nature; a value of 0° indicates no CP-violation, while other values suggest the presence of this asymmetry.

CP-Violation Analysis Yields αHττ = 36° with Ongoing Data Collection

Lucas Russell, a PhD student at Imperial College London, is among the researchers investigating a fundamental asymmetry in the universe, the apparent dominance of matter over antimatter. The CMS experiment is focused on whether the Higgs boson exhibits CP-violation, a phenomenon where physical laws behave differently under a combined transformation of charge, parity, and time reversal. In the new measurement, decay planes are estimated with an updated method. This improved technique reconstructs the direction of the tau from its decay products, providing a better estimate of the elusive neutrino’s trajectory. The resulting measurement of the CP-violating angle, α Hττ , is (36 +33 -30 )°, though still consistent with zero within current uncertainties. Despite not yet confirming CP-violation, the data hint at a possible deviation from CP-symmetry. Dr. Océane Poncet, who earned her PhD from the University of Strasbourg for her work on this measurement, notes that with CMS continuing to collect data, a more definitive result is anticipated in the near future.