I remember when …

As time passes, we remember the glory days of yesterday, like the time when we found the Higgs boson. Nowadays, some scientists take the existence of the Higgs boson as a given and have moved on, using it as a way to find something new.

Readers of this column almost certainly remember the announcement of the discovery of the Higgs boson on July 4, 2012. It was a heady time, as it is rare that one confirms the final missing piece of a wildly successful theory. For some, the observation of the Higgs boson was the crowning achievement of a lifetime of effort.

That was then, and this is now. While there is no doubt that physicists will continue to carefully measure the properties of the Higgs boson to see if there any surprises, some scientists have moved beyond searching for the Higgs boson and have started using it as a means to search for even deeper insights into the rules that govern the universe. As an example, while the Standard Model of particle physics successfully describes a broad range of phenomena, it has some deep and fundamental structural mysteries. For instance, it is hard to understand why the mass of the Higgs boson is as low as it is. Naively, its mass should be much higher — perhaps as much as a hundred quadrillion times higher.

Theoretical physicists have devised possible solutions to answer this conundrum, and a number of them predict new phenomena accessible by the LHC. While many ideas are bandied about, today’s analysis investigates two broad classes. One of them predicts a cousin of the familiar Z boson (unimaginatively called the Z’, pronounced “Z prime”), and the other predicts that the Higgs boson consists of smaller building blocks.

To search for evidence for these ideas, scientists sifted through their data, looking for events in which both a Z boson and a Higgs boson were created. The idea is that these two particles would be the decay products of a heavier particle, for example a Z’ or a Higgs boson whose building blocks are in an excited state. A complication of this approach is that both the Z and H bosons are unstable, and we therefore can’t observe them directly. Instead, we must search for the decay products of the Z and H bosons. To reduce the likelihood of copycat events, scientists looked for a single decay signature, specifically with events which contain two quarks and two tau leptons.

While scientists observed some events with these properties, the number they found was consistent with the rate from known and familiar physics processes. Finding no excess, scientists were able to set a limit on the production of a heavier particle in the range of 64 to 200 times the mass of the Higgs boson. However, with the newly refurbished LHC about to resume operations, scientists will be able to revisit this idea and make more precise measurements. The story of the investigation into events in which a Z and H boson are produced is only beginning.

Don Lincoln

University of Rochester physicist Roberto Covarelli contributed to this analysis.
These people helped organize and support the recent U.S. CMS Phase I upgrade workshop at Caltech. Top, from left: Lupe Llamas, Jason Trevor, Jean-Roch Vliman, Harvey Newman, Si Xie, Marelene Fouché. Bottom row, from left: Javier Duarte, Samir Curry, Artur Apresyan, Maria Spiropulu (workshop organizer), Dorian Kcira. Not pictured: Cristian Peña, Alex Mott, Ann Wang.