A Higgs boson pair candidate event with four particle showers (jets) which were initiated by b-quarks in 13 TeV proton-proton collisions. This collision event was recorded with the ATLAS detector at the Large Hadron Collider at CERN in 2016. The tracks of the produced particles are displayed as green lines. The energy deposits in the calorimeters are shown as yellow, orange, green or blue cuboids, where the length of the cuboid is proportional to the deposited transverse energy.
Searching for Higgs boson pair production at the Large Hadron Collider
The Standard Model of particle physics predicts that the Higgs boson should couple to itself, and produce for example Higgs boson pairs. Researchers in the HiggsSelfCoupling ERC project are analysing data from the Large Hadron Collider and developing tools to look for evidence of di-Higgs production, as Professor Çi ğdem İ şsever explains.
The existence of the Higgs boson was experimentally verified at the Large Hadron Collider in 2012, and researchers continue to study its properties and are trying to find experimental evidence that it interacts with itself. As the Principal Investigator of the HiggsSelfCoupling project, Professor Çi ğdem İşsever is investigating the process by which pairs of Higgs bosons are produced, research which could shed new light on whether the shape of the Higgs potential matches theoretical predictions. “By understanding Higgs selfcoupling, through the production of pairs of Higgs bosons, we will be able to see if the shape of the Higgs potential is really as predicted by theorists,” she outlines. “The shape of the Higgs potential has important implications on structures in the universe and the fate of the universe.”
New insights into the strength of the Higgs self-coupling could help scientists
learn more about the Higgs mechanism, a mechanism which explains how elementary particles acquire mass. “The Higgs potential is a scalar field which has been the focus of a lot of attention in research. But what we haven’t yet probed is whether the shape of this potential is really as earlier theorists predicted, or if
HiggsSelfCoupling project
This topic is central to Professor İşsever’s work in the HiggsSelfCoupling project, in which she and her colleagues are analysing data from particle collisions at the LHC recorded by the ATLAS experiment, looking to identify collisions where pairs of Higgs bosons were produced. This involves
“We try to identify what kinds of particles were produced in collisions at the Large Hadron Collider, what energies they had, and the directions they were flowing in from the point of collision.”
it is actually different in some way,” says Professor İ şsever. “The way to probe this is through measuring the strength of the Higgs coupling to itself, through the production of Higgs pairs, because the production rate of Higgs pairs depends on the strength of the Higgs self-coupling.”
studying the debris from the vast number of collisions generated within the ATLAS detector and working through the available data. “We try to identify what kinds of particles were produced in collisions, what energies they had, and the directions they were flowing in from the point of collision,”
explains Professor şsever. Based on that information, Professor İşsever says it’s possible to then essentially reconstruct these particles and identify Higgs pair candidates.
“If the signature fits, then candidate particles in the final state actually have the mass of the Higgs boson, and fulfil the right kinetic characteristics,” she continues. “However, we always talk about candidates here. We can compare the characteristics of the Higgs pair candidates that we have identified with our predictions, in order to judge if this was likely to have been the signal that we were looking for.”
The Standard Model of particle physics predicts that the Higgs boson should couple to itself and produce Higgs pairs, while other models and theories predict that heavier particles could also decay into Higgs pairs. So alongside looking for evidence of the Standard Model di-Higgs process, Professor İşsever’s team is also looking for resonances or signatures of new particles which could decay into Higgs pairs, which could have very different characteristics.
The production of Higgs pairs from a proton-proton collision is pretty rare, significantly less common than
the production of a single Higgs, and Professor İ şsever says it can be difficult to gather sufficient evidence. “We have not yet collected enough data or statistics to see these events in significant numbers,” she explains. “These di-Higgs processes are so rare that, in order to achieve the project’s goals, we need a lot of data. We want to analyse all the data from run 2 and run 3 of the LHC, in order to have the highest possible number of di-Higgs events in my dataset.”
- is in the 4b state, roughly 33 percent of the cases. Most of the time Higgs pairs would be expected to decay into a pair of b-quarks,” she outlines. While this approach generates a relatively large amount of signal, there’s also a lot of background noise that needs to be identified and removed, which affects the sensitivity. “There are also final states with not such a large amount of statistics but less background. For example, one of a pair of Higgs bosons may decay into a b-quark pair, and the other into two photons (γγ), or
“If the signature fits, then candidate particles in the final state actually have the mass of the Higgs boson, and fulfil the right kinetic characteristics.”
This work has been disrupted by the pandemic and Professor İşsever is still in the process of gathering and analysing data, with run 3 of the LHC ongoing. As the rate of diHiggs production is low, Professor şsever says it’s important to work with a final state that offers the highest possible chance of seeing them, which is the 4b-quark (4b) final state. “If you have two Higgs bosons, the maximum branching ratio - the decay fraction
two tau leptons (ττ),” continues Professor İşsever. “The main channels used to investigate the production of Higgs pairs are 4b, bb-ττ and bb- γγ gamma.”
Candidate particles
Researchers are currently sifting through the available data and looking to identify candidate particles with the characteristics of Higgs boson pairs. This involves the use
of sophisticated new methods and analytical tools, including machine learning algorithms and H-bb taggers, algorithms which identify certain features typical for Higgs bosons decaying into a pair of b-quarks. “For example, H-bb taggers are used to analyze traces of particle showers in the detector and trying to identify features that are typical of b-quarks or b-jets,” explains Professor İşsever. The algorithm then generates a score variable or probability of a particular particle shower being a Higgs boson that decayed into a b-quark pair or not. Then it can be subjected to further, more detailed analysis.
A further important consideration in the project is distinguishing Higgs pair events from quantum chromodynamics (QCD) interactions that form a background to this signature. While some QCD events are relatively easy to identify and remove, others are more difficult. “There are also QCD events which produce in their final state 4b-quarks. Those are so-called indistinguishable backgrounds, and we can only try to get rid of these events by studying their kinematics and utilizing more sophisticated methods.”
Run 3 of the LHC is currently ongoing, running at a collision energy of 13.6 trillion electronvolts (TeV) after which the LHC will shut down for a few years in order to perform upgrades to the machine and the experiments, so Professor İ şsever is very keen to make the most of the data that it will generate until then. “This is the key source of data that we will be able to work with for a while, so I will want to analyse it in great depth,” she stresses. This work is currently in progress, and Professor İ şsever’s team hopes to uncover fresh insights that could also open up new avenues of investigation beyond the Standard Model. “If we see new signals that are not predicted by the Standard Model in the data which we are now analysing then that would be very exciting,” she says. “The Standard Model is not able to explain quite a few of the major unanswered questions in physics, like what dark matter is and why there is an asymmetry between matter and anti-matter in our universe. Hence searches for signatures that point beyond the Standard Model are also immensely important and exciting.”
HiggsSelfCoupling
Uncovering the Origins of Mass:
Discovery of the di-Higgs Process and Constraints on the Higgs Self-Coupling
Project Objectives
This project aims to discover the diHiggs process and to set stringent bounds on the strength of the Higgs coupling to itself that provides unique information on the underlying nature of how particle acquire mass.
Project Funding
The research for this work has received funding from the European Research Council (ERC) project HiggsSelfCoupling (Grant agreement no 787331).
ERC-ADG - Advanced Grant EU Contribution € 2 262 897,00
Contact Details
Project Coordinator, Professor Çiğdem İşsever Humboldt-Universität zu Berlin Institut für Physik
T: +49 30 2093 82270
E: isseverc@hu-berlin.de
W: https://www.physik.hu-berlin.de/de/ eephys/ag-c-issever
Çi ğdem İşsever is Professor of Physics at Humboldt University of Berlin, where she is head of the Experimental High Energy Physics Group. She is also lead scientist at the DESY synchrotron, and a visiting Professor at the University of Oxford.
