The Higgs Discovery Adventure

Narei Lorenzo Martinez

17 June 2022

Narei Lorenzo Martinez

I started my phD in 2010, and I arrived at the LAL laboratory (France) the exact same week ATLAS was taking its first data after the LHC incident of 2008 that delayed the data-taking.

The ambiance in the laboratory was already very special, everybody was so excited to see the first data arriving, and I remember having « seen » the first Z bosons in my life on a histogram I made with the first data.

My supervisor, Marumi Kado, was a visionary person, he was absolutely convinced (and he convinced me !) that we will discover the Higgs boson in its two-photon decay channel in the next few years. However, to see this decay, we needed a good precision in the photon energy scale, and this was not yet the case since the collisions were just starting and we did not know yet well our detector. My task the first year of my phD was to improve the knowledge of the photon energy scale and resolution. This was a passionating and crucial task for the discovery of the Higgs boson. During this task, many non-trivial effects were found and corrected [1], and this helped improving for long our electromagnetic energy scale knowledge (that was then useful for the W mass measurement). To give a flavor of the improvement, the systematic uncertainties on the photon energy scale was reduced by more than a factor two thanks to this work.

In 2011, I started to focus more on the search for the Higgs boson in the two-photon decay channel: I computed theoretical effects (interference between signal and background diagrams that lead to a reduction of the Higgs boson production cross-section), worked on the signal modeling, computed systematic uncertainties on the Higgs boson yield, especially the ones coming from the knowledge of photon energy and resolution, and participated to all meetings where the results were discussed. Months after months, we were seeing this incredible thing, an « excess » over the background continuum appearing and continuously increasing. In 2012, during the last months before the announcement of the discovery, we worked very hard to get robust results. All possible, imaginable and crazy tests were performed to be sure we were not seing something spurious, that this was not an effect of background fluctuation or wrong calibration of our detector or an artefact of the way we were selecting events. I remember one of the last days before the announcement, Fabiola and Marumi called me very late in the night (or very early in the morning ;-) ), while I was still awake, to ask me for a final verification. The day D, my collaborators and I did not even got a sit in the auditorium because we worked until the very last moment to finalise the results that were going to be shown and we could not make the queue in advance. We watched the announcement on another room at CERN. When Fabiola presented all of our work, we were so proud and we felt so lucky to have participated to this unforgettable and historical experience.

This was not the end of the story, rather the beginning of a new chapter. During the rest of 2012 and during 2013, before my phD defense in September 2013, I focused on the precise measurement of the Higgs boson mass, and on the two-photon and four-leptons combination [2] as well as ATLAS-CMS combined mass measurement [3]. This measurement was integrating all new developments on the photon energy scale that I discussed above, and this was considerably improving the mass measurement precision. One of the questions that motivated a precise measurement of the Higgs boson mass at that moment, was to understand if we were in a stable or metastable universe [4] (crucial question isn’t ? ;-)). We don’t have the answer yet, but we are getting closer and closer to it, as we improve the precision on the Higgs boson and top masses.

After my phD defense in 2013, I got a postdoc position at Indiana University (USA), and changed my research topic to work on quartic gauge boson couplings. This is another story that I can tell you another day :-)

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[1] https://arxiv.org/abs/1407.5063 (photon calibration)
[2] https://journals.aps.org/prd/pdf/10.1103/PhysRevD.90.052004 (Hgg and H4l mass ATLAS mass measurement)
[3] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.191803 (ATLAS and CMS mass measurement combination)
[4] https://arxiv.org/abs/1205.6497 (stable vs metastable)