PROVIDENCE, R.I. [Brown University] —On Tuesday, Aug. 28, researchers with the two large-scale experiments at the Large Hadron Collider, CMS and ATLAS, announced the observation of the Higgs boson decaying to two bottom quarks.
That process is critical in understanding the properties of the Higgs boson, the manifestation of a field that enables other fundamental particles to acquire their mass. Its existence had been predicted for years, but it wasn’t until 2012 that the particle was first observed at the Large Hadron Collider (LHC).
When a Higgs boson is produced in an LHC collision, it lasts only a septillionth of a second before it decays into a cascade of other particles. The Standard Model — the scientific theory that describes the behavior of subatomic particles — predicts that transformation to two bottom quarks is the most common Higgs decay pathway, so confirming it experimentally has been a priority for CMS and ATLAS researchers.
The CMS measurement of the Higgs boson decay to bottom quarks is consistent with the expectations from the Standard Model. It strengthens the case that the Higgs boson couples to a class of particles called fermions, including the up and down quarks, which make up the familiar protons and neutrons.
Brown researchers David Cutts, Ulrich Heintz, Greg Landsberg and Meenakshi Narain have been active in the CMS experiment for years. The Brown team contributed significantly to the discovery of the Higgs boson with Landsberg as physics coordinator, directing the overall physics analysis effort of CMS. Narain contributed to b-quark identification and the search for Higgs decays to Z boson pairs. She was recently named chair of the collaboration board of U.S. CMS scientists. Heintz led the internal review of one of the discovery analyses.
Xavier Coubez, a postdoctoral researcher working with Narain, played a key role in this latest Higgs decay discovery. Here, he discusses the new findings.
Q: Why is it important for physicists to understand how the Higgs decays?
The Higgs discovery in 2012 opened a new era in particle physics. The Standard Model, which describes our understanding of the universe based on a set of fundamental particles, is now complete. However, theoretical considerations as well as experimental observations indicate that the Standard Model is only an effective theory from which a more general theory could be constructed. It is then important to test the validity of the model, for example, by measuring the coupling of the Higgs boson to various particles, either through the associated production of the Higgs boson with these particles or through the decays of the Higgs boson.
Q: What's so special about the particular decay pathway described in today’s announcement?
Generally speaking, particles in the Standard Model fall into two categories: bosons and fermions. The Higgs boson was discovered through its decays into other bosons. While these decays have a clean signature inside the CMS detector, they represent only a small fraction of Higgs boson decays. In the Standard Model, the Higgs boson can also couple to fermions (leptons and quarks), with a coupling strength proportional to the fermion mass.
The rate of Higgs decays to fermions is related to the coupling strength squared, and the study of these decays is the focus of our research. Although the decay of the Higgs boson to two bottom quarks (H→bb for short) is actually the most frequent one of all possible Higgs decays — accounting for about 60 percent of Higgs decays — it has been a real experimental challenge to observe it. This is because there is an overwhelmingly large number of other Standard Model processes, which we refer to as background, that can mimic the experimental signature characterized by the appearance of a bottom and an anti-bottom quark. The observation announced today, based on the study of 2017 data together with previous years, is therefore both an impressive technical success and a confirmation of the validity of the Standard Model.
Q: Can you describe what kind of work goes into making a discovery like this?
Such a discovery involves the work of a collaborative team, and a lot of actors deserve credit. First, the accelerator needs to provide collisions at a rate that allows us to study rare processes. The LHC accelerator performance exceeded expectations and provided the experiments the large data sets needed to perform an ambitious physics program.
The next step is the operation of the detector, which images the proton-proton collisions. Based on the information recorded by the detector, the particles are identified and their momentum and energy are measured. The imaging of the bottom and anti-bottom quarks from the decays of the Higgs boson produced in the collision relies in particular on the precision of the tracker, the central part of the detector, which is used to reconstruct the trajectories of charged particles in the magnetic field and the hadron calorimeter, which records the energy of the particles. The Brown University group is involved in the operations of both the tracker and the hadron calorimeter.
Finally, analysis teams use the events reconstructed following the above steps and try to isolate the small signals from the physics process of interest — in our case H→bb — from the billions of background events. Many of the techniques used to identify the particles and extract the small signal rely on innovative algorithms, for example, deep machine learning methods.
Q: What was the biggest challenge the research team had to overcome?
The main challenge of the H→bb analysis was to overcome the overwhelming background and extract the signal from the many events that have a similar signature. One way to proceed is to use the associated production of the Higgs boson with a vector boson, which leaves a clean signature in the detector. Beyond the ability to reconstruct this boson, the main ingredient of the analysis is the identification of the jets of particles originating from the decay of the b-quarks produced in the Higgs boson decays — the so-called b-jets. Since we do not identify the charge of the b-quarks, we do not distinguish between b-quark and anti-bquark from the Higgs boson and generically label them both as b-jets.
Q: What, specifically, was your role in the research?
I worked on the identification and calibration of the b-quarks. Once we identify two b-jets produced in the collision, the next step is to establish that they are from the decay of the Higgs boson, as opposed to other particles such as Z boson decays, or top quark pair production or directly produced in proton-proton collisions. There could be also be more than two b-jets in the event due to other sources. I worked on optimization of the algorithm to associate b-jets to Higgs boson candidates, defining the selection of the base set of events for this study.
The b-quarks are identified through their physical properties — large mass, characteristic lifetime —translated to physical observables. New techniques have been developed recently in order to identify b-jets inside the CMS detector using deep neural networks. Professor Narain has led the decade-long involvement of Brown University in the CMS heavy flavor identification group and the study of the performance of such algorithms. In addition to my contributions towards heavy flavor (b and c-quark) identification, graduate students Rizki Syarif and Mary Hadley (Narain group), and Eric Scotti and Jangbae Lee (Heintz group) have been working on various aspects of b-jet studies. Martin Kwok and David Yu (Landsberg group) contributed to a previous search using a different production mode that was included in the final result.
Q: Where does the work go from here? What’s the next big question you’ll be working on?
To date, the Higgs boson couplings that have been measured are to third-generation fermions (tau lepton, top and b-quarks). The next step is to study the coupling of the Higgs boson to c-quarks second-generation fermions, which are lighter in mass and have shorter lifetimes than the b-quarks. Together with Brown graduate student Jangbae Lee, we are collaborating with RWTH Aachen in Germany in setting up the analysis for a first CMS measurement of the coupling of the Higgs boson to c-quarks. The much shorter lifetime of the c-quarks compared to the b-quarks makes their identification in the detector more complicated, as much higher precision is needed to measure their short flight from the point where they are produced.
While this study is technically more challenging than studying the coupling of the Higgs to b-quarks, the observation announced today shows what can be achieved with the current state-of-the-art techniques and give us hope that meaningful results are within our reach. We will greatly benefit from the experience acquired working on the H→bb analysis.
In parallel, the study of the coupling of the Higgs boson to b-quarks will continue, in order to provide measurements with better precision and to probe possible deviations from the Standard Model. The continuing work by the Brown team will benefit the H→bb analysis by improving the handling of b- and c-jet identification and by probing a challenging signature closely related to the H→bb signature.