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Discovery of Higgs particle throws up new mysteries
- Waseda students contribute to research, looking ahead to the future

Kohei Yorita
Associate Professor, Faculty of Science and Engineering, Waseda University

Nobel Prize in Physics 2013

The Nobel Prize in Physics 2013 was awarded jointly to theoretical physicists Francois Englert and Peter Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle by the ATLAS and CMS experiments at CERN's Large Hadron Collider (LHC)." As this reason for their award shows, this time the Nobel Prize in Physics was given for great achievements in both theoretical discovery and practical discovery. The award was all the more impressive to me (being a particle experimental physicist) because it truly demonstrated how “a theory first makes sense when proved by an experiment” (Figure 1).

Figure 1: Researchers of the ATLAS and CMS experiments when the Nobel Prize in Physics was announced on October 8, 2013 (at Building 40, CERN. Front left is Rolf Heuer, Director General of CERN.)

The Higgs particle has had a varied history in the fifty years since its prediction by Peter Higgs and others in 1964 and discovered at CERN in Europe in 2012 by experiments (ATLAS and CMS) using the LHC, a state-of-the-art accelerator. For example, the LHC’s predecessor was the LEP, which was used to conduct experiments in the same underground tunnel ring from 1989 to 2000 by an international research team including members of the University of Tokyo and others who produced significant results in the search for the Higgs particle. Meanwhile, the Tevatron energy frontier experiments (CDF and D0 experiments) conducted between 1985 and 2010 at the Fermi National Accelerator Laboratory in the US, with the participation of members from University of Tsukuba, Waseda University and others, discovered the top quark (in 1994), which moved the search for the Higgs particle forward. However, both of these big experiments were shut down before the Higgs particle was found. In the end, it was the LHC (ATLAS and CMS), launched in 2010, that was handed the baton and crossed the finish line in the race to discover the Higgs particle. I have research acquaintances that took part in all three of these experiments. I myself have also worked on some of this research, having been involved in the search for the Higgs particle in the Tevatron’s CDF experiment from 1999 and taken part in the ATLAS experiment at LHC from 2005 as a member of the University of Chicago where I was enrolled at the time. The Higgs particle has brought together thousands of particle physicists from around the world fascinated by the world of elementary particles, in an intense pursuit transcending national borders.

Theoretical prediction: The papers of Peter Higgs and others

Dr. P. Higgs with some others turned around the existence problem of weightless particles that appear upon the spontaneous breakdown of symmetry (Nambu-Goldstone bosons), successfully explaining the physical phenomenon of the generation of mass as being the mass term (longitudinal component) of weakly interacting medium particles called W and Z bosons. This is the Higgs mechanism in the standard model of particle physics. In 1964, three papers on this topic were published independently of each other. The order in which they were published was: (1) a joint paper by Francois Englert and the late Robert Brout in August, (2) an independent paper by Higgs in October, and (3) a joint paper by Gerald Guralnik, Carl Hagen and Tom Kibble in November. Considering that this year’s Nobel Prize was awarded to the above (1) and (2), Brout would no doubt have been the third recipient had he still been alive. (There had been a lot of talk about whether CERN, or LHC, would be the first to receive the award as an organization, but this did not happen.) In fact the series of events in 1964 is extremely interesting with all kinds of dramatic inside stories, but it is already part of scientific history and so I will not say anything else about it here.

The discovery of the Higgs particle and the outlook for the future

The LHC in the CERN laboratory which straddles the Franco-Swiss border is a massive experiment that has been conducted with the international cooperation of more than 70 countries, and the contribution of Japanese companies and researchers was crucial to the construction of its accelerator facility and ATLAS detector. To give one example, CERN told the Japanese company that produced its superconducting cables, “Your failure is our failure, your success is our success.” There is a strong sense that the contribution of Japanese companies and their supporting technologies in the field of particle experimentation are truly world-class. Meanwhile, the analysis of the Higgs particle was not achieved overnight but was the result of stiff international competition. Revolving around a strong sense of professionalism that does not permit any mistakes, there were detailed assessments of background phenomena and extremely thorough verifications of errors, as well as the rule of blind analysis that states, “The important part of observational data must never be looked at until everyone in ATLAS agrees.” Furthermore, although there is a tendency to appreciate only those researchers who conducted the physical analysis, I want to emphasize that in fact this huge discovery was brought about by the mobilization of many talented people working behind the scenes, including those who developed, manufactured, operated and maintained the detector and the data collectors who worked 24-hour shifts on-site, in particular, doctoral students in their 20s and young researchers in their 30s competed against their counterparts around the world while devoting themselves around the clock and losing sleep in order to obtain more refined results.

Figure 2: Detector reactive display of a (highly probable) event of Higgs particle decaying to a tau lepton pair (released in November 2013). The event itself was observed on November 5, 2012.

Figure 3: Enhanced case of an event in the ATLAS experiment at the LHC (simulation). This disturbance event, called a pile-up, is when several concurrent events occur with one collision, making data collection and physical analysis difficult.

Just the other day, at the end of November, the ATLAS experiment officially reported its observation of the mechanism of a Higgs particle decaying to a tau lepton pair (actually confirmation on a 4s level) (Figure 2). This analysis resulted from on-site research led by doctoral students from Waseda University. The LHC experiment will be restarted in spring 2015 with nearly double the center-of-mass energy. The experimenters will continue to try to explain the properties of the Higgs particle, but the discovery of new physical phenomena (beyond the standard model) is also worth attention. In fact the recently discovered Higgs particle is a completely new type of particle and full of mystery. It is the only scalar elementary particle (spin 0), for instance, and the reason for its own mass being 125GeV/c2 is a complete mystery with no guiding principles. Turning our attention to supersymmetry particles, there are at least five in the “Higgs particle family,” of which the youngest child (or eldest son?) may happen to be the recently discovered Higgs particle. One of the supersymmetry particles could very well be a fusion of particles and space, a new paradigm creation, in the sense that it could be the dark matter that fills the universe. The LHC experiment from 2015 is expected to lead to more such exciting new discoveries, although experimental conditions will be taken to even greater extremes (Figure 3). The Waseda University group is therefore already looking to the future by developing and constructing a system to facilitate high quality data acquisition even in such extreme conditions.

The Higgs particle discovery and its Nobel Prize is an indicator of the way forward.
- New discoveries create new mysteries –
Particle physics is approaching a critical phase for theoreticians and experimentalists alike.

Finally, as another contributor to this Science Opinion website, I wrote in October 20, 2008, “Finding the Higgs particle is the primary goal of the LHC and its first step.” This first step was far greater than imagined. The next discovery means finding a completely new phenomenon that fully exceeds the standard model. At the same time that researchers have taken the first step in discovering the Higgs particle, they are already rushing toward the next “new first step.”

Kohei Yorita
Associate Professor, Faculty of Science and Engineering, Waseda University

2003 – 2004: Visiting scholar at Fermi National Accelerator Laboratory, US
March 2005: Completed doctoral program at the Faculty of Science and Engineering, Waseda University, gained PhD in Science.
April 2005 – September 2008: Fermi fellow and research scientist at the University of Chicago, US
October 2008 – present: Associate professor, Department of Physics, School of Advanced Science and Engineering, Waseda University
He has conducted studies on Higgs particle detection with the Tevatron experiment in the US and the LHC (Large Hadron Collider) experiment in Europe.
Waseda University officially joined the ATLAS experiment at the LHC in 2009, after he had taken up his new post at the university, and since then he has worked on the construction of a trigger circuit, the search for the Higgs particle and other new particles, and so on.