Science & Technology

Detecting Higgs Boson Pairs

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Six years ago physicists at the Large Hadron Collider announced the discovery of the Higgs Boson. A particle which essentially completed and confirmed the standard model of particle physics. Since that discovery, researchers have been somewhat disappointed that the study of the particle has not lead to new avenues in physics.

But that may be about to change.

Some researchers at the LHC now hope to narrow their ‘Higgs search’ down to searching for collisions that create pairs of the particle rather than single examples. A conference was held at the Fermilab, Illinois, last week in which over 100 physicists met to discuss the conceptual requirements of detecting double-Higgs events.

In doing so they hope to unlock new aspects of particle physics including new particles it may also answer one of the most fundamental questions in cosmology; why is there more matter in the universe than antimatter?

The Higgs Boson. Completing the Standard Model of Particle Physics

The Standard Model of Particle Physics explains how various particles interact via three of the four fundamental forces of nature; electromagnetism, the strong-nuclear-force and the weak-nuclear-force. As yet the model doesn’t include any interactions driven by gravity.

The mathematics that governs the Standard Model relies on fundamental symmetries that determine which interactions are possible (or impossible). There’s just one hitch. This mathematical background relies on the principle that particles begin their existence without mass. Mass must emerge latter in particle creation as a result of an interaction with a field that exists in space, much like an electromagnetic field.

This means there must be a particle that acts as a ‘messenger’ for this interaction. Much in the way that the electron is the messenger in interactions involving the electromagnetic field, physicists needed a particle to act in the same way for this mass-granting- field, later named the Higgs field as a result of the work of British theoretical physicist, Peter Higgs.

The search was on for the Higgs boson. The latter half of the name resulting from the fact that all force-carrying particles are bosons, particles made up of an even number of Fermions.

The CMS detector at CERN

The CMS detector at CERN

The Higgs mechanism was confirmed in 2012 by the discovery of the Higgs boson by scientists at the LHC using the Toroidal LHC Apparatus (ATLAS) and the Compact Muon Solenoid (CMS). The particle had the predicted mass of Higgs boson, 133 times that of the proton, and produced the required daughter particles as a result of its decay.

What distinguishes the Higgs field from an electromagnetic field is that fact that in the presence of no electrons, an electromagnetic field disappears. The Higgs field, on the other hand, cannot disappear, as particles are continuing to appear with mass.

The Standard Model assumes that the Higgs field’s staying power is directly related to the fact that it interacts with itself and can, therefore, take a non-zero strength. This means that the collisions that create the Higgs boson must occasionally produce double particles.

Unfortunately, such occurrences would be extremely rare, approximately 6 in 10,000 collision events.

Detecting the Double Higgs bosons.

A further difficulty in detecting such pairs, aside from their scarcity, is the fact that such an event would produce a multitude of other particles, thus making it extremely difficult for researchers to pick to the signal from the double-Higgs. It’s likely that the LHC has already produced a multitude of such events, but ALTAS and CMS are not yet powerful enough to detect the required signal.

A 2016 collision seen by the ATLAS detector implies the decay of two Higgs bosons into bottom quarks (ATLAS Experiment, CERN)

A 2016 collision seen by the ATLAS detector implies the decay of two Higgs bosons into bottom quarks (ATLAS Experiment, CERN)

But techniques used to identify Higgs bosons are improving and in August researchers at the LHC announced that they had detected the required ‘messy decay’ had would indicate the production of a double-Higgs. The decay involved the production of two bottom quarks, which are produced in approximately 60% of such events. This suggests a pair with at least one of the Higgs particles decaying in this way.

A Feynman diagram of the collision which produces two bottom quarks from the Higgs boson (PhysRevD.93.014019)

A Feynman diagram of the collision which produces two bottom quarks from the Higgs boson (PhysRevD.93.014019)

Unfortunately, scientists will have to wait to search further. The LHC is set to shut down for upgrades for at least two-years before 2019. After opening again in 2020, the upgrade process will be repeated again at some point in 2026 with LHC lying idle for another two years.

These improvements will lead to the high-luminosity LHC producing vastly increased collision rates until 2034. After these improvements, researchers would hope to discover the creation of double-Higgs pairs at a higher rate than that predicted by the standard model as such an increase would indicate a strongly-interacting Higgs field.

This increase could also give an indication of particles beyond the Standard Model. A strongly interacting Higgs field may also imply why there is vastly more matter than anti-matter in the Universe.

Maria Carena, a theorist at Fermilab, suggests an answer for this conundrum. The sudden emergence of a strongly interacting Higgs-field may have “locked-in” such an imbalance in the early universe.


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