An essay about the Higgs boson ten years after its discovery? Already ten years? This unique adventure has left indelible memories to the thousands of people who have been instrumental in giving birth to this wonderful discovery. There have been also Higgs-boson adventures at LEP and at the Tevatron, probably with equal excitement to that experienced by all of us at the LHC, but without the fulfilment of the discovery itself. This has to be mentioned with humility, because it was nature’s choice to reward one adventure rather than another.
A few words are required perhaps about myself since my personal experience colours undoubtedly this essay differently than that of others who have written or will write about this period in particle physics. I have been a member of the experiment called ATLAS today, from its very first glimmerings in the late 1980s before it had a name. As most of my colleagues, I worked by necessity on both the physics potential (as de facto physics coordinator until 1998) and on the detector development and construction (in the RD6 project in the early days and as project leader of the transition radiation tracker from 1996 to 2003). I then worked on the preparation of tackling real data as electron/photon performance co-convener from 2005 to 2009. From then on, I worked on many different topics with real data, both as a participant in mostly Standard Model analyses and as a reviewer of some key Higgs boson analyses.
My perception today is that time has flowed somehow differently in the different phases of the Higgs-boson adventure at the LHC:
Time flowed too fast and in many directions: workshops comparing the potentials of LHC versus SSC versus a linear collider of the CLIC type, the sad demise of the SSC project, many difficult milestones for the experimental projects, internal deadlines, but also very important formal and national/international deadlines through external reviews. This was accompanied by intensive research and development and test-beam activities in areas hitherto little probed by the community. One can quote prominent examples such as radiation-hard components, in particular micro-electronics chips and printed circuit boards, and complex multi-level trigger and data acquisition systems.
This phase, certainly the most exciting and breath-taking years of my life as an experimentalist, lasted from end of 2009 to summer 2012 with the joint discovery of the Higgs boson by both experiments.
Time flowed very slowly in this phase, and differently, somewhat secretly between two competing experiments: every day brought potentially new exciting plots, numbers, mishaps, breakthroughs, false alerts, and sometimes an almost unbearable stress.
In this phase, time flowed more normally, although the pressure of conferences and of the competition between the two experiments remained very high, too high in my opinion. Many of us were very eager to follow the numerous important analyses in the Higgs working group, but this turned out to be most often an impossible challenge due to the time pressure of upcoming conferences and the healthy but very real race with the rival sister experiment.
I have wondered a lot about what to write at this moment in time. Certainly not an accurate scientific account of what occurred over these almost forty years of my life. During all four phases, the expected experimental signatures of this elusive and unique particle guided the steps taken by the ATLAS and CMS collaborations and by many theory colleagues to optimise our ability to extract any sign of its presence from the data. It so happens that nature has chosen a value of 125 GeV for the mass of the Higgs boson, a wonderful coincidence, hinting perhaps at a deeper scientific significance. Why wonderful? Because only in this mass region can the Higgs boson be observed in basically all of its production modes and in all of its decay modes with a non-negligible branching ratio. This has extended the search period for specific Higgs-boson channels over the whole of Phase 4, while at the same time bringing the more abundant channels into the mature period of systematic and ever more precise differential measurements, together with their comparisons to increasingly accurate theoretical predictions and to possible deviations expected from new physics.
A number of books have been written about the discovery of the Higgs boson, so this essay focuses mostly on Phases 1 and 2, which are now part of the prehistory of the ATLAS and CMS projects as they are today and therefore less well known than Phase 3 about which a lot of literature exists and Phase 4 which is still vivid in the memories of most of us. I must also here put in a disclaimer sign in bold if I dared: this short essay is of course heavily biased by my own personal experience, memories, lack thereof, and can therefore in no way be claimed to be close to an objective historical narrative.
Phase 1 - Exploration
Quite surprisingly, at least to me, the Higgs boson was not discovered at the LEP electron-positron collider, at a time when there were only very weak limits on its mass, as explained for example in 1976 in Reference , where the authors “apologise to experimentalists for having no idea what is the mass of the Higgs boson unlike the case with charm, and for not being sure of its couplings to other particles, except that they are probably all very small. For these reasons we do not want to encourage big experimental searches for the Higgs boson, but we do feel that people performing experiments vulnerable”, sic!, “to the Higgs boson should know how it may turn up.” Twenty years later, I heard an eminent theory colleague during a lecture on Standard Model physics in Crimea jokingly make the following analogy: “The Higgs boson is like the Communist Party: it controls the masses.” Its own mass however is a fundamental parameter about which the Standard Model of particle physics has basically nothing to say except that it should not be too heavy, leading to an upper limit of about 1 TeV, which was completely out of reach for LEP and TeVatron, and would have been difficult to reach even at the high-luminosity LHC or its defunct rival collider the SSC.
I mention on purpose previous colliders or collider projects, because at one time or other in the past these were perceived by many of us in the LHC experimental community as our most dangerous competitors. They represented competition in a very real sense to the whole project, while the rivalry between experiments or even experimental projects before their approval was seen as something local and inherent to the overall LHC project itself. I choose the word “dangerous” because after the fact it certainly appears impossible to imagine that the SCC with the SDC and GEM detectors and the LHC with the ATLAS and CMS detectors would have been sustainable contemporaneously from the point of view of overall financial and human resources available in the world for such projects. The unitarity argument is not applicable to field theory alone, as humanity is beginning (sadly only now) to realise and act upon on a global scale.
Chronologically, the SSC project  was a very strong rival of the LHC for a decade from 1984 to 1993. It was even leading the competition since it was already well structured and funded in the late 80s. It was however under a rather rigid oversight: during four years of scientific review while I served on its Scientific Advisory Committee (from 1989 to 1993), no discussion was ever allowed of possibly increasing the design luminosity of the accelerator. This would have raised significant funding issues but also the question of the ability of the experiments to deal with a large number of overlapping collisions occurring at the same time, a phenomenon called pile-up. The European community of particle physics built on this bizarre situation and on its rapidly growing ability to compare totally different projects on a sound quantitative basis. It launched a workshop to compare the physics potentials of two proton-proton colliders, the SSC at 40 TeV centre-of-mass energy and the LHC at 20 TeV centre-of-mass energy with a luminosity up to ten times higher than the SSC, and of an electron-positron linear collider (CLIC) at 2 TeV centre-of-mass energy [6,7]. The bulk of the comparison effort was devoted to the searches for the Higgs boson and for supersymmetry. The initial study in 1987 concluded readily that the LHC could not compete with the SSC unless operating at much higher luminosity. The second study in 1988 updated in particular the Higgs boson studies for the LHC operating at luminosities of up to 1034 cm-2 s-1. More importantly perhaps, it focused for the first time on a preliminary assessment of the impact of pile-up and radiation damage on the detector performance and the physics potential of the experiments, and on the design requirements for the tracking detectors, the front-end electronics, and the trigger system. This succession of workshops led to the creation of a dedicated funding for research and development work towards building detectors able to operate at these high luminosities.
A major undertaking in the nascent experimental collaborations at that time was the gradual building of the simulation/reconstruction/trigger software backbone and algorithms. The description of the detectors in all their gory details would eventually comprise tens of millions of volumes spread over 30000 cubic metres. These software projects described in minute detail the geometry and composition of the materials in the detector together with their response to the interactions of particles emerging from the interaction point forty million times per second. They were essential at that stage to define the scope and main parameters of the experiments and to answer the numerous questions raised in the review process. They had to adapt constantly to the rapid evolution of the detector project (some crucial technological choices were made long after the overall approval for construction of the experiments in December 1994). And the Higgs boson played a major role throughout this process, but with an unknown mass ranging from 100 to 1000 TeV.
The choice of one of the main design parameters of the experiments is illustrated in Figure 1, from which the overall geometrical coverage required for the calorimetry in the experiment was determined. The concept of providing hermetic coverage such that no interacting stable particle could escape unseen by the detector was indeed not new since it dated back to the design of the UA1 experiment with the purpose of somehow “observing” a neutrino from W-boson decay through a large energy imbalance measured in the transverse plane. At the much higher energies produced in the interactions at the LHC, such measurements turned out to be much more demanding because they required measuring particle energies down to very small distances, centimetres typically, from the vacuum pipe enclosing the circulating beams and therefore into regions where the highest radiation doses were expected.
Figure 1. Example of major detector design choice in ATLAS at the time of the letter of intent (1992). The expected rates as a function of the missing transverse energy MET for signatures containing neutrinos with high transverse momentum, whether from ZZ to llνν continuum production (dashed curve) or from H to ZZ to llνν production (solid curves shown for Higgs boson masses of 500 and 800 GeV) are compared to the rates from Z+jet production for which large values of MET would be observed because of jets escaping outside the calorimeter coverage in pseudorapidity ηmax(dashed bands).
The physics studies were all based in those years on leading-order QCD calculations. This led in many cases to largely underestimated rates for the overwhelming backgrounds to the very small expected signals from a Higgs boson. For the Higgs-boson signal itself, theoretical progress was rapid and higher-order predictions came very quickly, and, happily, they led to significantly higher expected rates for the Higgs boson signal itself, a factor of two for the dominant gluon-gluon fusion production mode. However, the ATLAS and CMS experiments tacitly continued to define the discovery potential for the Higgs boson signal at the so-called 5σ threshold for any specific channel in terms of its probability to be present when compared to a background-only prediction, where both predictions were always leading-order QCD. This is actually the only significant difference between the expectations from the 90s for Higgs-boson discovery at the LHC (as extrapolated to the lower centre-of-mass energy and appropriate integrated luminosity) and the real results published in 2012 at the time of discovery for the two most sensitive channels in the intermediate mass range, namely H to γγ and H to ZZ* to 4 lepton decays.
The LEP-2 project, even though approved and about to be launched, was also compared in the early 90s to the LHC in terms of its potential for the search for a Higgs boson in the mass range between 80 GeV and 150 GeV, favoured by minimal supersymmetric models. Later, at a time when LEP-2 was approaching its maximal centre-of-mass energy and considering seriously how to increase this maximum even further given hints of a Higgs boson signal at a mass of 114 GeV, the upgraded Tevatron machine and its upgraded CDF and D0 detectors became a focus of attention. A large-scale effort was launched to assess the potential to find a Higgs boson in the same intermediate mass range, motivated both by the results from the searches at LEP-2 and by the global electroweak fit which was able to provide quite significant bounds on the Higgs-boson mass after the top quark was discovered at the TeVatron.
Phase 2 - Preparation
The very strict 5σ discovery paradigm was even applied until the late 90s, and the change of paradigm only arrived when the competition with the upgraded TeVatron project was launched. The Tevatron experiments reasonably argued that they could combine channels and use sophisticated and optimistically extrapolated analysis techniques since these were based on experience from well understood detectors at an operational collider. They however vitally needed to combine every single shred of potential signal to approach the discovery sensitivity required for the projected maximum integrated luminosity of 10 fb-1.
In the years 2000-2008, three developments occurred, all of them critical to the future success of the Higgs boson search at the LHC:
Phase 3 - Revelation
The very short duration of Phase 3, barely two and a half years from the first recorded inelastic collisions at 900 GeV energy shortly before Christmas 2009 to the joint announcement of the discovery of the Higgs boson by ATLAS and CMS in July 2012, was a huge surprise to me and most of my friends and collaborators who had been intensely involved in ATLAS for twenty years or so. Several aspects of the beginning of the data-taking phase in ATLAS and CMS came out as rather unexpected to me.
A first and very pleasant surprise was the extremely rapid turn-on of the accelerator luminosity over 2010 and 2011. This performance was in stark contrast to the initial cautious expectations given to the experiments by the accelerator experts. Many of the particles in the Standard Model were “rediscovered” and measured in 2010 with an initial dataset, which was 5000 times smaller than that accumulated to-date. This was done at a speed defying my imagination. Cassandra-like predictions from colleagues who had experienced the far more complicated turn-on of previous machines did not occur. The machine backgrounds were indeed very small, so trigger signatures using missing transverse energy, characteristic perhaps of possible dark matter candidates (such as the lightest supersymmetric neutralino), became operational very rapidly and led to searches for new physics exceeding the sensitivity of similar searches at the Tevatron already by summer 2010.
During 2011, the luminosity delivered by the machine increased so rapidly that the flexibility of the trigger system was taxed to the utmost: I remember nights on shift when the trigger coordination was forced to change the trigger menu on the fly to adjust the rate of selected events sent to permanent storage within the limits of the capacity of the data acquisition system.
A second, almost immediate, and this time rather unpleasant surprise was that the unprecedented huge size of our collaborations meant that a single individual could not possibly follow at any deep level of understanding the details of all the analyses going on in parallel in our community. The most immediate and dire (to me) consequence was that I was never able to even read quickly all the paper drafts sent out for comments before publication even though I was one of the hundreds of signatories. Clearly, the incredible diversity and high quality of most of the publications of our huge experiments outweigh these unexpected individual drawbacks.
Another consequence of the sheer size of the collaboration was that no one was ever able to participate in all important analysis approval meetings. These were the appropriate meetings where one was required to give significant feedback, which might possibly affect the analysis outcome. Otherwise, at a later stage, e.g. at the time of publication, the overall schedule of the publication was bound to be affected, conference deadlines might be missed, so the resistance and inertia, unwilling or not, of the collaboration publication juggernaut could not be swayed. The most important analysis approval meetings, such as those concerning the hunt for the Higgs boson which began in earnest in summer 2011, had an attendance of hundreds of people, many of them squeezed in person in a meeting room and many more usually sitting frustrated at times at the far end of a video-connection.
Far more expected were the inevitable hiccoughs in the unveiling of the results of the hunt at the equilibrium point, which turned out to be summer and fall 2011 (see Figure 2). I would define this equilibrium point as the moment in time when the expected sensitivity of the experiments to the three major decay channels was close to the 3σ threshold for declaring evidence for a signal. The hiccoughs occurred in part because of human nature: certain small groups of people jumped the gun by either adjusting their selections to maximise, perhaps unwittingly, a certain upward fluctuation of the observed data with respect to expectations in the desired configuration. They also occurred because of the overall tension at all levels of the experiment, including management, which is not invulnerable to the vagaries of human nature.
These rare upheavals were however contained fairly easily by the procedures in place in the collaborations. These included blinding of the signal regions, detailed cross-checks of the individual expected level of each relevant background process in the signal region together with its systematic uncertainties, and more often than not gruelling review by editorial boards and other bodies in the collaborations. It was therefore quite difficult for important results to be made public in a matter of days.
This is perhaps the main reason (in stark contrast to my experience at the time of the UA1/UA2 rivalry in their successful search for the W/Z bosons and unsuccessful searches for the top quark and for new physics) why the Higgs boson discovery jointly by ATLAS and CMS can truly be claimed to have happened as one might dream it could happen when writing a textbook about experimental particle physics . The evolution of the (downward) probability peak in Figure 2 as a function of the Higgs-boson mass hypothesis considered illustrates this very well. The probability curve displays insignificant fluctuations in summer 2011. It shows a clear trend at the end of 2011, at which point most of my theory colleagues were convinced that this was the real thing and that, perhaps unfortunately, it looked very much like the expected Standard Model Higgs boson rather than like a more exotic and thereby more exciting beast. Finally, the two bottom curves, produced at the time of discovery and six months later based on the full run-1 dataset, show without doubt that a Higgs boson with a mass of about 125 GeV, as observed jointly by ATLAS and CMS, could now be entered into the Particle Data Group booklet. In my mind, there can be no finer justification in the history of particle physics that laboratory management should always do the utmost to provide for two general-purpose experiments of equivalent potential at any large-scale new accelerator project.
Figure 2. Evolution of the combined significance of the Higgs-‐boson signal in the ATLAS and CMS experiments from exclusion limits in summer 2011 to discovery in summer 2012 and consolidation at the end of 2012.
Phase 4 - Consolidation
After the discovery, measurements of the properties of the Higgs boson were performed in successive stages, first focusing on its spin, then on its couplings to bosons and fermions and on possible non-SM contributions to its width. At the end of run-1, ATLAS and CMS produced a combined paper on the Higgs-boson couplings , leading to the conclusion that in all production modes and decay channels which had been measured at the time, the Higgs-boson properties were compatible with what one would expect from the SM.
In contrast to the two most prominent channels used for the discovery, H to γγ and H to ZZ* to 4 leptons, the vast majority of the signals explored in the other channels led to much more difficult Higgs-boson measurements due to the diverse and potentially large backgrounds and/or to the fact that the signal does not yield a narrow peak above the background. One impressive example of such complex channels is the H to WW to llνν channel, which was included for ATLAS in the discovery paper published in September 2012 but not quite ready for the announcement of the discovery a couple of months earlier. Figure 3 shows how clearly the excess of events from Higgs boson decays observed over a wide kinematic region can be seen above the background despite the fact that the Higgs boson mass cannot be measured directly on an event-by-event basis in this channel because of the escaping neutrinos in the final state. One of the most beautiful ATLAS publications, in my view, explains very pedagogically the analysis of the full run-1 data in this channel .
Figure 3. Distribution of the transverse mass of the Higgs boson candidates in the H to WW decay channel, as observed by ATLAS at the time of discovery in summer 2012. The expected signal for mH = 125 GeV is shown stacked on top of the overall background prediction.
This essay portrays these years as perceived by an experimentalist and thus allows the reader to perhaps get a more complete view of the range of efforts and technical challenges faced by the community to build, commission and operate what was required to discover the Higgs boson over a very broad range of masses. What about the interplay with theory? Well, my experience is that our field thrives and buzzes with excitement when large amounts of measurements beyond the previous energy frontier appear. This happened at least four times during my lifetime and was always a time of very productive interactions between theorists and experimentalists. I cannot describe here even briefly the huge amount of theoretical work done over the beginning of this century to provide the best possible tools to describe the thousands of measured data distributions published by the experiments, in particular in the Higgs sector.
However, in terms of novel ideas, not yet existing at the time of Phase 1 as denoted in this essay, I can personally come up with one such in the Higgs sector, namely that the natural width of the Higgs boson resonance which is much too small to be directly measured experimentally at the LHC has a measurable impact on off-shell events measured in H to WW/ZZ decays . This theoretical insight has led to what was not believed possible until then: the LHC experiments have reached a sensitivity to the Higgs-boson width in the 10 MeV range rather than the 1000 MeV range!
Quite recently, each experiment has produced updated results based on the whole run-2 data. This is illustrated in Figure 4 based on the most recent run-2 ATLAS Higgs combination results  and shows that the strength of the measured Higgs-boson couplings to fermions and bosons follows the expectations from the SM, in which for example the Yukawa fermion coupling is expected to be proportional to the fermion mass.
Figure 4. Reduced coupling strength modifiers κF mF/v for fermions (F = t, b, c, τ, μ) and sqrt κV mV/v for weak gauge bosons (V = W, Z) as a function of their masses mF and mV, respectively, where the vacuum expectation value of the Higgs field v = 246 GeV. The results are obtained from the full ATLAS 13 TeV data and the SM prediction is also shown (straight line). The coupling modifiers κF and κV are measured assuming that there are no beyond Standard Model contributions to the Higgs boson decays or production processes. The lower inset shows the ratios of the measured values to their Standard Model predictions.
Sadly, no sign of any physics beyond the Standard Model has been seen yet in the Higgs sector as explored to-date at the LHC, nor has there been any sign of such physics in any of the searches performed over more than a decade, whether pursuing supersymmetry or more exotic models of new physics. The few results shown here, together with, for example, the very active ongoing searches for dark matter or long-lived particles, demonstrate that there are many areas still to be covered in the search for new physics at the LHC, including in the Higgs-boson sector (this is covered concretely in Ref. ). The accelerator and all its experiments will remain for many years to come a wonderful provider of new data in this quest for physics beyond the Standard Model, however elusive it may be.
The formidable challenge related to the design, construction, installation, and commissioning of the ATLAS and CMS experiments lasted 25 years and reached a successful conclusion at the end of 2009 with the beginning of data-taking. At the time, the next challenge was as daunting and even more exciting for all the people participating in the exploitation phase: understand the performance of these unprecedented detectors as precisely as possible and extract the bountiful harvest of physics, which would undoubtedly show up very soon after the LHC machine began operation.
Ten years later, after taking large amounts of data at centre-of-mass energies of 7, 8 and 13 TeV and operating successfully at luminosities exceeding even the design goals of the machine and the experiments, our community can look back with tremendous pride and respect at what has been achieved by the thousands of people involved in the accelerator and the experiments. But we have also been very lucky and should feel huge gratitude towards nature which has offered the ATLAS and CMS experiments the possibility to first observe and later measure the Higgs boson in the somehow miraculous variety of production processes and decay channels with which it manifests itself at the LHC.
The searches for new physics at this new frontier have, however, unfortunately not yielded yet any sign of where the explanations of some of the remaining mysteries of nature might lie. Nevertheless, the physics harvest already available from this wonderful tool for fundamental research is already rich beyond belief and the ongoing analyses in the experiments continue to probe the Standard Model predictions to the utmost of our current capabilities. Might new physics emerge nevertheless from the expected thirty times larger datasets to be collected over the coming ten to fifteen years from the upgraded machine and experiments? Might the Higgs boson be the key to unlock the door leading us to find answers to some of the fundamental questions in particle physics and cosmology today? The hopes remain high, yet only nature knows.
Looking back, I feel as strongly as in July 2012 a sense of fulfilment in my professional life, which I never imagined could be so profound. To you, all the people who have made this incredible adventure possible, my deepest and heartfelt thanks together with my best wishes for the future, to those who have left us before 2012, to those who were there from the beginning, and to those, often much younger, who have joined the adventure perhaps only a few years ago or who will join it in the years to come.