Missing Piece to the Higgs Production

23 January 2025

Photo: CMS Collaboration: A. Cardini, S. Hurst, N. Stathak

To bring more clarity to the Higgs boson production, Quantum Universe postdoc Andrea Cardini together with doctoral researchers looked deeper into the Higgs production associated with bottom quarks. The study relies on data from proton-proton collisions at the Large Hadron Collider (LHC) at CERN recorded by the CMS detector.

The discovery of the Higgs boson was one of the biggest achievements in the recent history of particle physics. The following decade has been brimming with further advancements in the study of its properties. Scientists part of the international CMS collaboration, including some from Hamburg, have methodically studied how the Higgs boson is produced at the LHC, adding more and more pieces to the puzzle of the Higgs production.

First was the fusion of gluons, mediated by a loop of quarks, since the Higgs doesn’t couple directly to these massless bosons. Then the fusion of vector bosons, the Z and W weak bosons, where the Higgs is left in the middle of the detector with two quarks going to the most forward regions of CMS. Sometimes the Higgs is also radiated by a particularly energetic vector boson in what we refer to as Higgs-Strahlung (radiation in German). Finally, sometimes the Higgs is produced together with the heaviest fermion, the top quark. Scientists have by now discovered or found evidence for these processes, but could there be more pieces left to be added to this puzzle?

Scientists decided to find out and probe the process where the Higgs boson is produced in association with bottom quarks (bbH), a crucial yet elusive process in the realm of particle physics. The process comprises only 3% of the Higgs bosons produced at the LHC.

Adding a missing piece is not easy, though, since its shape must be known well fit into the puzzle. Quantum Universe postdoc Andrea Cardini, together with doctoral researchers P. R. Bartschi (Zurich University) and M. Bayat Makou (DESY) took on the challenge by studying the proton-proton collisions recorded by CMS at the LHC.

A key feature of this production mechanism is how it involves different Higgs couplings: as shown in Fig. 1, there is more than one way for the Higgs to be generated together with bottom quarks, and they interfere with each other. This makes the bbH process sensitive to specific quantitative relations in the way the Higgs boson couples to various particles, which means that it is particularly interesting to see how a measurement of the bbH production probability in LHC collisions fits together with other Higgs-related processes.

Fig. 1  Feynman diagrams for the H+bb production process.

Measuring the total cross section of the bbH process, σbbH, is the first step to better understand how the Higgs couplings intertwine. However, as the process is rare and challenging to identify, the available data only provides an upper limit, rather than a precise value. The analysis reports the first direct bound on σbbH to be 3.7 times the standard model expectation, as shown in Fig. 2.

Fig. 2 Upper limits on the signal strength of the bbH process, the ratio between the measured cross section and the one expected in the standard model.

This milestone not only establishes constraints on a fundamental process but also highlights the need for more precise theoretical inputs, as experimentalists chase the goal of bbH observation. The interference with the other pieces, the presence of a gluon splitting into two b-quarks, and the higher-order quantum corrections to the process, all currently limit the accuracy of theoretical predictions to about 40%. There is room for more theoretical advancements, and scientists are looking forward to seeing some progress soon.

A unique feature of this measurement is the ability to simultaneously constrain the strength and the relative sign of Higgs boson couplings to bottom and top quarks. In the Standard Model, we expect the relative sign to be positive, and a significant asymmetry would hint at new phenomena. While our current measurement shows compatibility with the Standard Model expectations (as shown in Fig. 3), more precise measurements in the future can potentially change the picture.

Fig. 3: Allowed ranges for the Higgs couplings to top quarks (Kt) and bottom quarks (Kb), according to previous measurements in red and in combination with our analysis, in green for simulation and in blue for the measurement.

While the puzzle is still unsolved, we will persist, so that one day we can finally have bbH among the known and understood production mechanisms of the Higgs boson. As our curiosity pushes us to advance, we pursue our quest to one day watch the completed puzzle and admire the patterns, symmetries, and beauty.

(News adapted from CMS Physics briefing, Missing Piece to the Higgs Production, 25 March 2024.)