Is a window to New Physics about to open?

The Higgs particle was detected by the CERN large particle accelerator in 2012. Now there are hints that CERN’s worldwide unique particle accelerator will help physicists discover a new elementary particle. Excitement is rising.

This illustration shows a proton-proton collision recorded by the CMS detector of the LHC at CERN in September, 2015. Among the collision events, two photons with an energy of a total of 700 to 800 GeV (illustrated by the two green lines) emerged.
Image: Thomas Mc Cauley / CERN

In August 2016 particle physicists from all over the world will gather in Chicago for a large conference: ICHEP 2016. Conferences of this type are the place where physicists exchange ideas, discuss new results and, if such exist, announce new discoveries. In fact, the Chicago meeting could usher in exciting news from the world of elementary particles. “A new heavy elementary particle discovered,” the popular science headline might read. But be warned against hasty sensational reports: It is also possible that the news from Chicago fails to surprise and that the conference comes to an end, unnoticed by the world’s public.

Hope for "New Physic"

Despite the possibility of dissapointment, particle physicists await the conference in Chicago with excitement, for very good reasons. Last December experimental physicists from the European Laboratory for Particle Physics (CERN) in Geneva reported a remarkable observation, which after further reviewing the data, they again reported in March at the Moriond conference. They found in the evaluation of measurement results from the Large Hadron Collider (LHC) at CERN a possible indication of the existence of a new elementary particle; ever since they have been chasing after clues. Scientists around the globe are puzzled over whether the new particle actually exists, and if so, what it would mean. For according to the current theory of particle physics—the so-called Standard Model—all elementary particles that exist have already been found. Should there be another particle, physicists would have to fundamentally review the Standard Model.

“If we really do find a new particle, it would mean a new physics,” says PD Dr. Hans Peter Beck, a particle physicist at the University of Bern, who follows the experiments at CERN closely. Such a discovery would cause a stir quite drifferent than the discovery of the Higgs particle in 2012: The Higgs mechanism and the associated particle were predicted already in 1964 and were therefore integrrated into the Standard Model of physics that was developed in the early 1970s. The experimental proof of the Higgs particle in 2012 was thus a confirmation of the standard model, despite that one could not be sure if the Higgs particle really existed and whether it worked as had been predicted until the end. Should a new particle be found at CERN in the coming months, the implications would be quite different: “That would be a particle that nobody expected,” says Beck. “The standard model would have to be at least extended and possibly reformulated. What if it brought with it a huge expansion of horizons in the understanding of the universe?”

A Particle with Six Higgs Masses?

The question of whether we will arrive at such a point is still open. Because physicists from Geneva have so far reported only a statistical anomaly suggesting the unknown particle’s existence. This anomaly was observed at the LHC, which came into operation in 2010 at CERN. In the LHC, protons are shot at each other and the decay products are then measured. During these measurements, physicists have observed a pair of high-energy, isolated photons appearing strikingly often, which could indicate a new elementary particle with a mass of 750 giga-electronvolts (GeV). With such a mass the new particle would be about six times heavier than the Higgs particle. Both the major LHC experiments ATLAS and CMS confirm independently the occurrence of an unexpected number of photon pairs at this energy.

But it is still unclear whether the data-peak really represents a new particle - or just a statistical blur (fluctuation). The observed peak is based on data collected from spring to autumn 2015 from experiements conducted in the LHC. The data are particularly significant because the LHC has been operating since 2015 with a collision energy increased from 8 to 13 TeV and can thus more easily reveal new heavy particles, if they exist. After a six-month break, the LHC came into operation again in early May. “In the summer of 2016, we will have a clearer picture with the additional data we are now collecting,” says Hans Peter Beck. “The previous measurements are promising, but still not significant enough to be sure.”

Massive Interpretations

A new particle would be the second elementary particle that would have been discovered by the LHC after the Higgs. The discovery would be the direct consequence of the fact that the LHC today generates a collision energy and collision rate like no other previous particle accelerator. But what would the discovery mean? Since an experimental physicist found evidence of such a particle a few months ago, theoretical physicists have flooded the scientific literature with attempts at an explanation. Over 400 articles have appeared in quick succession, all attempting to establish an explanatory framework for the alleged particle. In the foreground are three interpretations: the new particle could be a heavy brother of the Higgs particle; it could be the first expression of dark matter, which makes up almost a quarter of the total energy budget of the universe, according to modern physics, but has so far never been seen; or it could be a Graviton—a particle that mediates the force of gravity but that is currently only theorized to exist.

If physicists were to discover a new particle, they would need additional time to determine whether any of these interpretations are correct, says Hans Peter Beck. Because in order to confirm such a particle, the decomposition pathways of proton-proton collisions must be analyzed much more accurately. So before the work of assigning meaning to the hypothetical particle comes into focus, the general public must now wait anxiously to learn if a particle of 750 GeV really exists. In early August at the ICHEP conference in Chicago, the latest results will be made public.

Author: Benedikt Vogel

Note: The hint of a new particle at an energy of 750 GeV was not confirmed when accumulating more data at the LHC after its restart in May 2016. For more information, please refer to the CERN Press Release of 5 August 2016.

Categories

  • Elementary particles
  • Particle Physics