Decay of Higgs boson into bottom quarks observed
Particle Physics (IPA)
On 2 December, the second running period of the Large Hadron Collider (LHC) at CERN has come to an end. The most recent scientific highlight during that period was the observation of the decay of the Higgs boson into a pair of bottom quarks. ETH physicists had a leading role in obtaining this landmark result.
The announcement, on 4 July 2012, that a particle compatible with the properties of the Higgs boson has been discovered at CERN marked a breakthrough achievement in particle physics, completing the picture of the Standard Model. Since then various properties of the new particle have been measured, and all of them are consistent with the theoretical predictions for the Standard Model Higgs boson.
The programme of exploring Higgs physics, and to check whether the experimentally observed behaviour matches the predictions of the Standard Model, is based on studying interactions with, and decay into, other particles. The 2012 Higgs discovery was driven by the analysis of two particular decay channels: the decay into two photons, and the decay into two Z-bosons that in turn decay into two leptons each. These two decay channels are both quite rare (with branching fractions on the order of 10-3 and 10-4, respectively), but they are relatively ‘clean’, in the sense that the background stemming from other processes can be well managed for these processes.
In contrast, the decay channel with the highest branching fraction is plagued by an overwhelmingly strong background. 58% of the time, the Higgs boson decays into a pair of bottom quarks (H→bb̄), but for each pair of bottom quarks created in this way, some 107 pairs are generated through other processes occurring in proton–proton collisions at the relevant energies. The observation of the H→bb̄ decay is, however, an important milestone of the ‘Higgs programme’, not least as the bottom quark is the heaviest fermion into which a Higgs boson can decay. (Decay into the even heavier top quark is forbidden by energy conservation, but the production of Higgs bosons in association with top-quark pairs has been observed earlier this year.)
Separating the wheat form the chaff
That the H→bb̄ decay has now been observed is mainly thanks to the analysis of several channels through which the Higgs boson can be produced. The most dominant processes relevant for the search for H→bb̄ events are those in which the Higgs boson is produced in association with a vector boson (that is, a Z- or a W-boson). This route is known as ‘VH production’, and it is special insofar that the kinematics of the H→bb̄ decay can be combined with that of the decays of the Z- or W-bosons, which proceed in the opposite direction (see figure above).
The vector bosons can decay into a charged lepton and a neutrino (for W) and into a pair of neutrinos or a pair of charged leptons (for Z). The charged leptons provide important cues for VH events. This information in turn helps pinpointing signals stemming from H→bb̄ decays, and to discriminate against background events due to other processes. Still, the task remains highly challenging, even if LHC provided outstanding data. Only with the development of sophisticated analysis tools, among them modern machine-learning techniques such as deep neural networks (DNNs), the analysis sensitivity could now be increased to a point where sufficiently high signal-to-background ratios were obtained.
Key contributions from ETH groups
The VH-production channel has been a main focus of the ETH group of Christoph Grab at the Institute for Particle Physics and Astrophysics since 2010. Within the CMS collaboration — the Compact Muon Solenoid, CMS, experiment is one of the two LHC detectors — they have taken full responsibility for the channel where a Z-boson decays into pairs of charged leptons, hence providing a key contribution to the observation of the H→bb̄ decay. The role of ETH physicists has been emphasised by the fact that one of Grab’s postdocs, Luca Perrozzi, has been asked to present the findings of the CMS collaboration when the H→bb̄ observation was announced at CERN on 28 August 2018.
Over the years, also the ETH groups of Rainer Wallny and Günther Dissertori have similarly made key contributions to the discovery of the Higgs boson and in the subsequent programme to explore the physics of the newly discovered particle, for instance, through work on the channels where the Higgs boson decays into photon pairs and the analysis of the Higgs production in association with a top-quark pair. Moreover, all three groups (as well as the former group of Felicitas Pauss, who retired in 2016) contribute the general work on calorimeters, tracking and software development.
As for the H→bb̄ decay, work is now underway on a ‘legacy paper’, in which the complete data set of the two LHC runs (2009–2013 and 2015–2018) is used in one single analysis. In the meanwhile, LHC remains now shut down until 2021, during which phase major maintenance, upgrades and consolidation are performed.
References
Sirunyan AM et al. (CMS Collaboration). Observation of Higgs boson decay to bottom quarks. Phys. Rev. Lett. 121, 121801 (2018). doi: external page 10.1103/PhysRevLett.121.121801
Haber HE. Higgs decay into bottom quarks seen at last. external page APS Viewpoint article