Reflections from the LHC Physics Center on observing the Higgs-to-bb decay

This event display from CMS shows a proton-proton collision inside the Large Hadron Collider that has characteristics of a Higgs decaying into two bottom quarks. Image courtesy of CMS

This event display from CMS shows a proton-proton collision inside the Large Hadron Collider that has characteristics of a Higgs decaying into two bottom quarks. Image courtesy of CMS

On Aug. 28, the CMS collaboration announced the observation of the Higgs-to-bb decay mode with the significance of 5 sigma. This is a major achievement for the collaboration, as the bb decay mode is among the most challenging ones to observe, despite having the largest branching ratio.

At the dawn of the Large Hadron Collider run, many deemed a 5-sigma observation in this channel to be extremely difficult — borderline impossible. However, the excellent performance of the LHC coupled with major advancements in the analysis techniques made the result possible way ahead of the expectations. What is perhaps less known is that the roots of the CMS Higgs-to-bb analysis lie at the Fermilab LHC Physics Center where, many years ago, former LPC Distinguished Researchers Jacobo Konigsberg and Jim Olsen initiated the effort and, seven years later, brought it to the finish line.

Below you can find their reflections as well as comments from some of the lead analyzers.

For CMS, the search for this decay mode was a seven-year quest. It was one of those pursuits for which success was very far from guaranteed. We started at the LPC, in collaboration with Princeton, and built together a team of several institutions that continued through LHC Run 1 and Run 2. We always had very talented young people working on the team, and it is to their credit that we succeeded. We needed to implement new techniques and sophisticated analysis strategies to get his done, be patient, gather more data, and keep the team together working coherently through such a long period.

It is a highly satisfying accomplishment, and now it’s a very popular endeavor. It happens like that in particle physics: When you establish the existence of a new process, it quickly becomes a whole new area of research. It’s very exciting.

Jacobo Konigsberg (University of Florida), former LPC distinguished researcher, former convener of the Higgs-to-bb group

This is pretty satisfying. I became involved in the search for the Higgs-to-bb decay in 2008, after reading a paper that proposed an idea to search for this channel in a new way that had not been previously emphasized at the LHC. The effort in CMS started to ramp up in early 2011, when we still didn’t have enough data to even begin. I started to travel to Fermilab to visit the LPC and collaborate with the University of Florida group, and in that process, our two Higgs-to-bb groups started to solidify our efforts. We basically decided we were going to go for a first result with this little data we had to get us on the board, and we built the effort from there.

That was exciting to finally see it, after a decade of hard work. We had this collaboration that came out of the LPC, and having that venue, that incubator of ideas, where we could sit there and brainstorm together — we got a lot of mileage out of it.

James Olsen (Princeton University), former LPC distinguished researcher, former CMS Higgs convener

I’m super happy about this result. Yesterday [Aug. 28] we made the announcement, and I’m still smiling. My interest in this topic started in 2009 when I was a summer student at Fermilab, when I didn’t know anything about physics. I eventually moved to CMS as a grad student in 2012, around the time of the Higgs discovery. We realized that the Higgs decay into two bottom quarks was the largest branching fraction. It’s extremely challenging to identify. It suffered from the largest background: seven orders of magnitude greater than the signal. But I was excited to study it.

I’ve been working on different aspects of this complex result over the years, learned from and collaborated with many talented physicists. Now we’ve got it, and I was part of the team — in fact responsible for the CMS Higgs-to-bb group — that found it.

And it’s not the end of the story. We can exploit this Higgs boson decaying to bb as a tool, probing new physics processes.

Caterina Vernieri (Fermilab), LPC distinguished researcher, convener of the Higgs-to-bb group

This was the result of a lot of hard work from many different groups. It came together thanks to people pushing each other and collaborating in a unique way, which happens only at these large experiments. To get this result, you have the work of literally hundreds of people on this particular result, and then thousands of other people contributing in other small ways.

I’ve been on CMS since 2011; I joined as Ph.D. student. I’ve been working on Higgs-to-bb searches since 2016 with Caterina Vernieri. She and I worked on related but different analyses, different from the mainstream Higgs-to-bb analysis. Our result came out in spring of 2017. It was looking for the Higgs-to-bb decay through gluon fusion. It was a different channel, but we showed that we had some sensitivity in that channel. The really cool part was that, in the end, this cool analysis — it was just a curiosity in the beginning — was necessary to get our big Higgs-to-bb result. You needed our our additional, nonmainstream statistical analysis to add to the main analysis to get over the 5 sigma threshold. The whole is greater than the sum of its parts.

Javier Duarte (Fermilab), LPC distinguished researcher, lead analyst

Getting that result felt great. We’d been watching all the pieces of the Higgs fall into place, from the Higgs-top-quark coupling in June to the major decays and production modes that had been previously confirmed. Now that the dust is finally starting to settle, I can work on finishing my dissertation on the data collected over the past two years related to the Higgs-to-bb decay.

All of this happened quite fast. We’d just completed the 2016 analysis earlier this year, then on September 18, the 2017 analysis was officially published. In retrospect, the enormous amount of work we did was quite surprising. It took a lot of teamwork to get to where we are today, and I’m thankful that we were able to work together to make it happen.

Sean-Jiun Wang (University of Florida), lead analyst for Zvv channel

Cecilia Gerber and Sergo Jindariani are co-coordinators of the LHC Physics Center at Fermilab.