Rare Higgs boson decay could indicate physics beyond the Standard Model

Particle physicists have detected a new decay of the Higgs boson for the first time, revealing a slight deviation in the predictions of the Standard Model and perhaps pointing to new physics beyond it. The results are published in the journal Physical Examination Letters.

The Higgs boson, predicted theoretically since the 1960s, was finally detected in 2012 at the CERN laboratory in Europe. As a quantum field it permeates all space, through which other particles move, acquiring mass via their interaction with the Higgs field which can be roughly envisioned as a kind of resistance to their movement.

Many properties of the Higgs boson, including how it interacts with other particles and their associated fields, have already been measured to be consistent with Standard Model predictions.

But one mode of Higgs decay that had yet to be studied was a theoretical prediction that a Higgs boson would occasionally decay and produce a photon, the quantum of light, and a Z boson, which is an uncharged particle that, along with the two W bosons, transmits the weak force.

Scientists from CERN’s ATLAS and CMS collaborations used proton-proton collision data collected during Run 2 from 2015 to 2018 to search for this particular Higgs decay of the Z+ photon. CERN’s Large Hadron Collider (LHC) is a high-energy particle accelerator near Geneva, Switzerland, that circulates protons in opposite directions while colliding at specific detection points, million times per second.

For this operation, the energy of the collision of the two protons was 13 trillion electron volts, just below the machine’s current maximum, which, in more comparable units, is 2.1 microjoules. This is equivalent to the kinetic energy of an average mosquito, or grain of salt, moving one meter per second.

Theory predicts that about 15 times every 10,000 decays, the Higgs boson should decay into a Z boson and a photon, the rarest decay in the Standard Model. To do this, it first produces a pair of top quarks, or a pair of W bosons, which themselves then decay into Z and a photon.

The Atlas/CMS collaboration, the result of the work of more than 9,000 scientists, found a “branching ratio,” or fraction of decays, of 34 times per 10,000 decays, plus or minus 11 per 10,000, or 2.2 times the theoretical value.

The measured fraction is too large: 3.4 standard deviations above the theoretical value, a number still too small to exclude statistical chance. Nevertheless, the relatively large difference raises the possibility of a significant deviation from theory that could be due to physics beyond the Standard Model: new particles that are intermediates other than the top quark and W bosons .

One possibility for physics beyond the Standard Model is supersymmetry, the theory that posits a symmetry (relationship) between particles of half a spin, called fermions, and a whole spin, called bosons, with each known particle having a partner with a different spin. by a half integer.

Many theoretical physicists have long been proponents of supersymmetry because it would solve many of the puzzles that plague the Standard Model, such as the big difference (10²⁴) between the forces of the weak force and gravity, or why the mass of the Higgs boson, about 125 gigaelectronvolts (GeV), is much smaller than the large unification energy scale of about 10¹⁶ GeV.

In the experiment, the massive Z boson decays to approximately 3 × 10^-25 a few seconds, long before it hits a detector. The experimenters therefore compensated by looking at the energy of the two electrons or two muons that the Z decay would produce, requiring their combined mass to be greater than 50 GeV, a significant fraction of the Z mass of 91 GeV.

“This very nice result obtained with the CMS collaboration. This is, according to the prediction of the Standard Model, the final state of the rarest Higgs boson, for which we have seen the first evidence,” said Andreas Hoecker , spokesperson for the ATLAS collaboration.

“The decay occurs via quantum loops and is therefore sensitive to new physics in the same way, but not quite the same way, as two-photon decay, which contributed to the discovery of the Higgs boson by ATLAS and CMS in 2012.”

“This result is impressive for several reasons,” added Monica Dunford of the CMS collaboration. “We are able to measure these very rare processes experimentally with such precision. They provide a powerful test of the Standard Model and possible theories beyond it.”

Dunford adds that the groups acquired new data during Run 3 at CERN, which began in July 2022, with 13.6 TeV of total energy. Even more data will come from the High-Luminosity Large Hadron Collider, which will produce about five times more proton-proton collisions per second. The HL-LHC is expected to be commissioned in 2028.

“These results are a preview of what we can continue to achieve,” Dunford said.

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