Searching in the deep for rare, charming Higgs decays at the LHC

Fermilab scientists are digging deep into data from the CMS experiment at CERN’s Large Hadron Collider, searching for very rare decays of the Higgs boson, the special particle discovered 10 years ago by the CMS and ATLAS collaborations.

Higgs bosons decay in many different ways, with probabilities calculated by theory and represented in this pie chart. Experimental results so far agree with these values, but the gg (gluon + gluon) and cc (charm + anticharm) modes have not yet been observed, because of very high strong interaction backgrounds and poor mass resolution. The much rarer 2-photon (gamma + gamma) mode did not have those limitations and actually was a discovery channel. Image: Anadi Capena, Fermilab

Why is this particle, called H(125) (its mass being 125 GeV), so special? While its mass is 133 times that of a proton, it is more than a billion times smaller (by volume) and has no charge and no spin. The top quark, discovered at Fermilab in 1995, is 38% more massive than the Higgs but has electric charge and spin, like other particles of matter.

The Higgs boson is the quantum of a field that pervades the universe, having a value that is the same everywhere but no direction. That is very different from electromagnetic and gravity fields that differ everywhere in strength and direction. Finding the Higgs boson in 2012 proved that the Higgs field exists, nearly 50 years after it had been proposed as a way by which elementary particles like electrons, quarks and W and Z-bosons acquired mass. If the Higgs field were switched off, its value made zero, these would all be massless, and atoms (and people) would be impossible.

A unique feature of the Higgs boson is that the strength of its interaction to other particles scale with their mass, coupling most strongly to the more massive particles. The H(125) can decay in many different ways, the most common (58% probability) being to a bottom(b)-quark pair. (The top quark is too heavy at 173 GeV for the H(125) to decay to top-antitop.)

The H -> b + anti-b decay took six years after the Higgs discovery to be observed. That was difficult because the b-quark decays almost immediately to jets (or sprays) of particles, and their energy cannot be precisely measured; also there is an overwhelming background from strong production (gluon + gluon -> b + anti-b). These difficulties are even more severe for the Higgs decay to charm + anti-charm.

The charm-quark mass is only about 30% of the b-quark mass (1.27 GeV/4.18 GeV), and the decay probability being proportional to the square of the masses, H(125) decay to a charm-quark pair is more than 10 times rarer and has not yet been observed.

A paper recently submitted by the CMS collaboration to Physical Review Letters describes a possible way to address that challenge. Massive particles, such as the H(125) and the Z-boson (91.2 GeV), can be produced with large momenta at large angles (large “transverse momentum,” pT), so that may be a place to look.

Nhan Tran and Nicholas Smith provided analysis on this study. Photo: CMS collaboration

The Z-boson also decays to a charm pair (with 12% probability). Z production and decay are very well known, and they were used to test the sophisticated data analysis in search of Higgs to charm-quark pairs. “Fat jets” with pT above 450 GeV were selected, and a deep neural network was applied to distinguish jets likely to contain charm-quark pairs from background.

For experts, the procedures are described in the paper. Nicholas Smith, a Fermilab research associate, and scientist Nhan Tran have been analysts on this study, applying deep neural network methods to discriminate H(cc) events from the background events. This important progress in achieving sensitivity beyond earlier expectations to rare processes has been made possible by advances in machine learning and artificial intelligence, as well as in detectors and event selection (triggers).

While the Z -> c + cbar decay was reconstructed with the expected strength, the H -> c + cbar decay remains elusive, and a 95% confidence level upper limit on its probability was placed at 47 times larger than theoretical expectation. We are not there yet, but it is a step in the right direction. With further refinement of the method and more data, we may eventually show that the Higgs coupling to the charm quarks is — or is not (!) — what the theory predicts.

Mike Albrow is a scientist emeritus in the CMS department at Fermilab.

CMS communications are coordinated by Fermilab distinguished scientist Pushpa Bhat.