CERN, the LHC, its experiments, CMS and the Higgs boson in perspective – a personal and not strictly professional view

by Professor Daniel Denegri, founding member and former Physics Coordinator of the CMS Collaboration

[N.B.: The views expressed by the author are those of the author alone and are not necessarily endorsed by the CMS Collaboration.]

Simple was the world of Aristotle and the prevalent philosophies and religions of our Middle Ages, a world with the Earth at its centre, with the immutable seven celestial spheres, and humankind, the ultimate masterpiece, at the centre of creation. Of course, many centuries before, some ancient Greek philosopher-scientists, Thales and the Ionians first among them, in the years from ~550 BCE to ~300 BCE, had a very different and “modern” vision, most notably Aristarchus of Samos or Democritus of Abdera, and these were followed by the Alexandrine school (~300 BCE to ~400 CE) and more generally the scientist-engineers of the Hellenistic period, Archimedes the most famous among them¹. But this classical rationalism gave way gradually to a more religious and mystical approach to Nature, to Natural Philosophy, and by the 3rd—4th centuries CE, it was essentially eliminated. And things remained largely like that for more than a millennium, at least in the Christian West. However, as classical science and thought gradually faded away and Europe fell into darkness, there was a survival, even a revival and flourishing, of scientific thought in the Muslim world. While the most powerful ruler of the West, Charlemagne (~800 CE), was illiterate, in the Abbasid capital of Bagdad – probably the largest city of the time – in the “House of Knowledge” and at Grand Caliph Harun-al-Rachid’s court, scholars were discussing Plato’s ideas, Aristotelian and Ptolemy’s world schemes, even questioning it².

It can even be argued that Bologna is not Europe’s first university, but rather Cordoba in Umayyad/Almoravid al-Andalous, where, in the 10th and 11th centuries, theology, astronomy, medicine, mathematics and philosophy (philosophy then meant natural sciences – it survives in our PhD titles) were already taught, and the study of classics and of Persian scholars pursued. Cordoba, a city where Al Rushd-Averroès and Maïmonides coexisted, and where stood the largest Library of the time (not in Constantinople!); the scientific “light-house” was not in Alexandria any more.

Image 1a: Engraving appearing in a book by a 19th century French astronomer-writer C. Flammarion.

To peer behind the seven spheres of Aristotle (Image 1a) and understand in a rational way the workings of our world was without doubt a strong incentive and motivation. Finally, with Copernicus (~1540), Tycho Brahe (~1590), Kepler, Galileo (~1600), Descartes, Huygens (~1650), Hook, Newton (~1690) and others, the West, and with it all of humanity, entered a new era. Note that the Ulugh Beg Observatory in Timurid Samarkand - unfortunately largely destroyed – precedes Tycho’s observatory by ~150 years, and Jaipur’s well-preserved great observatory, the Jantar Mantar (Image 1b), built by a Hindu king in Islamic Mughal India, follows Tycho Brahe’s observatory in Uraniborg by about 100 years…

Image 1b: The Jantar Mantar observatory in Jaipur, India. Instruments used are sundials; meridians; various instruments for precision measurements of time, positions of celestial bodies; and astrology instruments [Image by user jo.schz on Flickr (CC BY-NC-SA 2.0): https://www.flickr.com/photos/joschz/7863156902/]

Central to this change of emphasis, change of spirit and perspective, was Galileo’s experimental approach and his invention of the telescope, together with Leeuwenhoek’s invention/application of the microscope. Even if Galileo’s development of the telescope was most likely financed by the Arsenale of Venice allowing Venetian admirals to observe and gauge from greater distance the menacing Turkish ships and fleets, the essential step was that Galileo turned his instrument towards the skies to observe the Moon with its mountains, discovered the phases of Venus, Jupiter and its satellites – a Copernican world by itself, mathematised the kinematics of free fall and ballistics etc. This was the onset (physicists would say “the trigger”) of an unstoppable scientific revolution that brought us where we are today³. Observe Nature with tools that exceed the capabilities of our sensory organs, ask questions to Nature and provoke answers, try to interpret observations and mathematise observations, theorise - this is at the heart of physics and the experimental method and theoretical framework that led to the tremendous progress of our civilisation over the past four hundred years. Newton, co-inventing calculus with Leibniz, initiated “theoretical physics”. Our present-day scientific instruments, the LHC – the Large Hadron Collider at CERN - and experiments operating at the LHC, can be seen as direct descendants of Galileo’s and Leeuwenhoek’s instruments and methods - the most recent undertakings on humanity’s path in this quest for a rational explanation to natural phenomena.

The way towards the LHC, the largest existing scientific instrument in the world today, was not simple. Scientific Europe emerged from World War II totally ruined, continent-states like the USA or the USSR dominated the scene. To find and regain at least some elements of its past scientific expertise and fame, in the years 1953—1954 it was decided to establish CERN (Conseil Européen pour la Recherche Nucléaire) – the European Organization for Fundamental Research in Geneva – a place, where putting their money in a “common pot”, European researchers could build and operate accelerators and eventually detectors/experiments beyond the financial and technical capabilities of any individual European state. The promoters of CERN, the founding figures of this unique organisation, were physicists like Eduardo Amaldi from Italy, Pierre Auger from France, Isidore Rabbi from the USA, renowned theorists like Niels Bohr from Denmark, Werner Heisenberg from Germany, Victor Weisskopf from Austria/USA, the Swiss humanist De Rougemont and many others. But fruits and great successes were slow to come; the training of physicists and especially development of intuition – where to look for new and key discoveries – is a long process. For many years, even decades after WWII and the founding of CERN, despite accelerators and detectors performing very well, practically all the major discoveries in elementary particle physics and related Nobel Prizes were going to the USA. I remember vividly the disappointment of my professors at the University of Zagreb when the observation of two distinct neutrino species (ν_e vs. ν_μ) – a major discovery in the sixties – was made at Brookhaven National Laboratory, despite the fact that CERN had the adequate accelerator…

A change occurred in the ’70s with the first major discovery at CERN – the one of weak neutral currents in 1973 in the Gargamelle bubble chamber, and especially at the end of that decade, when electronic detectors built at CERN reached and even exceeded the sophistication and performances of those at American research labs. This change became obvious with the building and operation of the antiproton-proton collider at CERN, the construction of the UA1 and UA2 detectors, followed by the discovery with these detectors of the W and Z bosons in 1982—1983, arguably the most decisive discovery of the second half of 20th century. What we call today the Standard Model – the theory describing particles and their interactions at the most fundamental level – was in the making since the mid-’60s. Some of the key steps were the proposition for a unified electroweak interaction (by Glashow, Weinberg, Salam, Ward… ~1965—1967) with the theoretical prediction of a hypothetical Z boson, and the suggestion by Brout, Englert, Higgs and few others (1964) of a mechanism providing masses to the W and Z whilst keeping the photon massless, followed by the proof by ’t Hooft, Veltman, B. Lee, Zinn–Justin (1971) of the mathematical consistency of gauge theories which were the framework of the electroweak unification and of the theory of strong interactions (Gross, Appelquist, Politzer, Wilczek…) developing through the ’70s. The discovery of the W and Z at CERN, clearly the most coveted prize in physics in those years, showed beyond doubt that gauge theories⁴ are the proper approach to the description of fundamental interactions at this scale of energies or particle elementarity. All other theoretical approaches practiced for years, such as S-matrix theory, Regge poles, axiomatic field theory, Bootstrap etc., vanished from the scene; quantum field theory with gauge invariance won the day.

With the discovery of the W and Z, the barycentre of research in particle physics crossed the Atlantic once more. This must have hurt US physicists in their pride; they dominated the scene so outrageously and for so long a time. Their reaction was to propose a colossal project, the SSC – the Superconducting Super Collider – a 40TeV machine dwarfing CERN’s antiproton-proton collider (540 GeV and 630 GeV), CERN’s LEP – Large Electron Positron collider (90 GeV and ~200 GeV) – in construction at the time, and the Fermilab Tevatron (~2000 GeV), which was just beginning operation. After the discovery of the W and Z, the next obvious goal in particle physics was to uncover the mechanism that provides masses to particles and is thus responsible for electroweak symmetry breaking, be it the Brout-Englert-Higgs mechanism or some alternative model. The SSC should entirely sweep the field, bring the answer to this, and possibly discover Supersymmetry, or Technicolor, possible new extra gauge bosons etc.

Already in 1984, CERN had discussed the possibility of constructing a proton-proton collider in the LEP tunnel, once its scientific potential were exhausted. This collider, which eventually would become the LHC, could not exceed most optimistically a proton-proton collision energy of 17 TeV, provided ~10 tesla bending magnets could be developed (ultimately ~8 tesla magnets had to be used). In view of its limited transverse size, the space available in the LEP tunnel, one of the key elements that would make the project feasible was the “two-in-one” magnet scheme proposed by Robert Palmer from Brookhaven National Laboratory. However the approval and start-up of SSC construction in 1987 was making the LHC hopelessly outdistanced and outmatched in energy. But in 1989 Carlo Rubbia, fresh from his W and Z success, was elected Director General of CERN. It must be recognised that it is thanks to him that the LHC project was not abandoned altogether; instead work started in earnest. It took the immense self-confidence and daring of Rubbia to try to match a collider of a type never built or tested before – the two-in-one magnet scheme, with superconducting magnets at the very limit of technology, and with a machine having to operate at 10³⁴ cm^-2s^-1 luminosity (machines of the time were operating maximally at ~10³⁰ cm^-2s^-1!!), which also meant at an order of magnitude larger luminosity than planned for the more conventional SSC design with two independent beam pipes. The idea here was to try to compensate with ten-times larger luminosity the factor of at least three-times smaller LHC energy when compared to the SSC. The key argument used to promote LHC, first in Europe and then outside, was that the LHC could be much cheaper to build, thanks to the already existing LEP tunnel and all the infrastructure and accelerators present at CERN, and the two-in-one scheme halving the number of magnets required. The estimate was that it would cost altogether ~2.5 billion CHF and could be ready by ~1998(!) at worse by 2000, i.e. before the SSC in the trans-Atlantic race for the Higgs boson. In the early ’90s, a number of important trips were made with the DG and CERN representatives to Japan, India, USA, Russia etc. to present the LHC project, its potential physics programme, the technical challenges and opportunities in joining the project⁵. The problem for the SSC was that the costs of the project escalated continuously, from an initial estimate of 6 billion USD in 1987 then to 8 billion and, by 1993, it reached 11 billion USD, at which point the project was cancelled by the US administration, despite the ~1.5 billion already spent on civil engineering works and development of magnets and detectors. Exit the SSC. Countries like Japan, USA, Russia, India, Canada, soon committed themselves to the project, to the construction of specific LHC components where their financial contribution represented about 5% of the overall machine cost but much more on the experiments, the detectors, reaching up to ~40%.

With no SSC any more, a large number of US researchers after 1993—1994 gradually joined LHC experiments, then still being in the design-optimisation or prototypes-testing phase. The LHC thus became the first fully international, global, scientific project. Today, for example in CMS, the largest contingent of scientists by nation is from the USA (~600 scientists), followed by Italy, Switzerland, Germany, France, Russia and so on down to the smallest contingents from Estonia, Ireland, soon to be joined by Montenegrins.

By 1994 the number of general-purpose experiments initially proposed was reduced by CERN management to two, the ATLAS and CMS experiments, with in addition two more large, but specialised, detectors LHCb and ALICE. From the very beginning of the LHC project in 1989—1990 there was the intention to have an experiment dedicated to the study of “beauty physics” and related aspects of CP violation – what would become LHCb – and another one optimised for the study of heavy-ion collisions and the produced quark-gluon plasma – ALICE. Early in the LHC project a decision was taken to have also a smaller-scale experiment dedicated to forward and diffractive physics studies, what subsequently became TOTEM.

The general-purpose detectors ATLAS and CMS were designed with the same research goals in mind, namely uncovering and studying the mass-generation mechanism, studying the top quark, looking for Supersymmetry, Technicolor, searching for new gauge bosons, for signs of possible substructure, manifestations of possible extra dimensions of space, quantum mini-black-holes etc. The goals are the same but the configurations of the two detectors and the detection techniques implemented are very different, very much complementary. Never before were detectors of such complexity and performances designed and built. This sophistication was required to match the luminosity and functioning mode of the LHC, with extreme demands on spatial granularity and energy/momentum resolution, rapidity of response, radiation hardness of sub-detectors etc. Due to the smallness of the Higgs boson’s production cross-section, less than a billionth of the proton-proton interaction probability, an immense quantity of data must be produced and sifted through, at a rate of a billion proton-proton collisions per second, requiring to be triggered upon, selected and finally recorded at most at a rate of a few hundred potentially interesting collisions per second in view of final physics analysis. A very sophisticated and innovating triggering and data-handling system was thus required. In the following we shall talk essentially only about the CMS detector – the most beautiful one! There are hidden beauties to CMS – not only in the Vasarely-type colouring scheme, but rather in the very elegance and simplicity of its basic design, resulting in a very effective detector, greatly simplifying its exploitation as a scientific instrument.

Image 2: Left – a view of the Grande Rosace on the southern transept of the Notre Dame cathedral in Paris [Image by Brian Chiger on Flickr (CC BY-NC 2.0): https://www.flickr.com/photos/bchiger/22432673728/]. Right – view of the barrel part of the CMS detector, view transverse to the beam line. [Image by Max Brice and Michael Hoch: https://cds.cern.ch/record/1474902]

The CMS detector, opened-up so as to show its inner structure with sub-detectors inside each other like Russian dolls, is shown in Image 2, where the harmony of its design is compared to the celestial beauty of the Grande Rosace of the Notre Dame cathedral in Paris. The sizes are similar, the choice of colours too (accidentally? – maybe not, physicists do have a sense for beauty and symmetry, not only for abstract mathematical ones…) and the elevated spiritual quest is the same in both cases. Even if the approaches used over the span of 850 years are somewhat different between the Medieval world with its tall gothic cathedrals and aspirations towards God and the modern technological world in its search for understanding the basic laws of Nature. The height of the CMS detector is about 15 metres, the overall length is 28 metres, and overall weight is about 14,000 tonnes (the Eiffel Tower, for comparison, is about 8,000 tonnes). The main contributing countries to the construction, operation or maintenance of sub-detectors in the barrel part of CMS are shown in Image 3.

Image 3: Components of the CMS detector in its barrel part, with main countries contributing to their construction

Image 4: The modular design of CMS – it allowed the detector to be mounted and tested on surface before lowering it module by module into the experimental hall 100 metres underground, straddling the LHC beam. Lowering the central module weighing about 2500 tonnes, equipped with muon chambers, carrying the coil cryostat and the hadronic calorimeter was particularly demanding. The various sub-detectors and detection technologies employed in each of them are indicated. [Image by Tai Sakuma: https://cms-docdb.cern.ch/cgi-bin/PublicDocDB/ShowDocument?docid=11514] N.B.: The new Pixel Tracker installed in 2017 has more channels than depicted in this image.

An exploded view of CMS and its 13 main structural elements is shown in Image 4, with, in Image 5, a photograph of CMS in October 2013 with the barrel and endcap still separated. The heart of the CMS detector, the real backbone around which the entire experiment was designed, is the huge superconducting solenoidal magnet operating at 4 kelvin, the largest of its sort in the world: 13 metres long, 6 metres in diameter, producing a uniform 4 tesla field, twice the field in usual detector magnets. The energy stored in the magnetic field is nearly 3 Gigajoules (this is equivalent to 700 kilograms of TNT – the largest classical bombs used during World War II were of ~1 tonne)! Image 4 shows its location, in the coil cryostat – the vacuum and thermal enclosure for the coil – supported by the central wheel. In the barrel end-on view of Image 3 the end of the cryostat is clearly visible and indicated explicitly. The concept for the magnet was introduced by Saclay engineers, with a number of technical innovations, like four-layer coil winding, three-component reinforced superconductor and a thermosyphon cooling system. The construction proper was managed by the Magnet Collaboration, regrouping experts from CERN, ETH-Zurich, INFN-Genova, Saclay and the University of Wisconsin. The magnet was built with components from the all over the world – as indicated in Image 3 – with parts for the superconducting cable from Finland, Japan, Switzerland, USA; cable construction/welding done in Switzerland and France; coil winding in Italy; and final assembly and tests at CERN. Many counties were involved: for example, the bus-bars feeding the magnet with 20 kiloamperes came from Croatia etc.

Image 5: Fish-eye view of CMS in October 2013. The endcap structure (left), with the “nose” carrying the forward hadronic and electromagnetic calorimeters designed to fit into the barrel (right), is clearly visible, as is the beam pipe through which the LHC beams circulate.

Outside the cryostat is the magnet’s return-yoke made of iron - in red in Images 3 and 4 - allowing magnetic field lines to close upon themselves. This structure, consisting in its central barrel part of five huge “wheels” of ~2000 tonnes each, clearly visible in Image 4, was designed at CERN and built in Germany, with iron blocks cut in Russia. It was finally assembled at CERN and held with elements (orange elements in Image 3) from the Czech Republic and tie-bars from France, everything resting on “feet” built in Pakistan. The two endcaps of the CMS detector, visible in Images 4 and 5, consist each of three disc-like walls of iron completing the flux return path and holding a variety of detectors. The endcap discs themselves were designed at the University of Wisconsin and produced in Japan; they are resting on feet built in China, and are secured by special anti-seismic support-bars built in the USA, the final assembly done at CERN.

Inside the iron yoke are interleaved four layers of muon detection chambers – the silvery boxes clearly visible in Image 3. These detectors were designed and constructed in Germany, Spain and Italy for the Drift Tube (DT) chambers, and in Belgium, Bulgaria, Italy, South Korea, Pakistan, Egypt etc. for the Resistive Plate Chamber (RPC) detectors. This central barrel-part beyond the coil also houses scintillator detectors built in India and complementing the inner calorimetry. The muon detectors covering the endcap discs (Images 4 and 5) are also organised in four layers, using Cathode Strip Chamber (CSC) techniques designed and built in the USA, Russia and China, as well as the RPC-type detectors already mentioned. The muon detection system is central to the CMS detector design, as visible from its very name “Compact Muon Solenoid”, being at the beginning the main one on which CMS “founding figures” counted on in the search for the Higgs boson through a process like H→ZZ→μ⁺μ^–μ⁺μ^–, a mode effective over a very broad mass range and immune to possible difficulties at high luminosity. The muon detection system of CMS is thus very robust and versatile in triggering, the DTs and CSCs providing precision in track position measurements and the RPCs a better time resolution.

Looking now inwards from the cryostat in Images 3 and 4, we find three types of detectors, first the Hadron Calorimeter, followed by the Electromagnetic Calorimeter and, at the centre, surrounding the accelerator-collider beam pipe is the central Tracker. A few words about each of these sub-detectors.

The Hadron Calorimeter (HCAL) is of conventional design, made of alternating layers of absorber (brass) and scintillator plates. It is used to measure particle “jets”, groups of particles that are the macroscopic manifestations of quarks and gluons. A curiosity concerning this detector is that the 1600 tonnes of brass required for the absorber were recovered by our Russian colleagues from the cartridges of naval artillery from the discarded Russian Black-Sea Navy cruisers! The brass then went to Bulgaria for processing and was ultimately cut and engineered on the design from Fermilab/USA colleagues in a shipyard in Spain. Again a good example of international cooperation! On the HCAL components in barrel, the end-cap and the very forward regions, there is again a collaboration of institutes from USA, Russia, Ukraine, Turkey, Iran, Hungary…

The next layer we encounter in the CMS detector as we go inwards is the Electromagnetic Calorimeter (ECAL) – see Images 3 and 4. It is one of the most original parts of CMS, made of ~76000 scintillating crystals (PbWO₄ or lead tungstate), each crystal of size ~22×2×2 cm³, in shape very much like the obelisk from Luxor at the Place de la Concorde in Paris. These crystals are organised in the barrel part as a cylindrical shell, complemented with two end-caps, with all crystals (almost) pointing towards the interaction point at the centre of the CMS detector. The crystals were produced over several years in Russia and China after a five-year-long research-and-development programme (reminiscent of the five-year plans of socialist economies!), an interesting scientifico-politico-industrial saga in itself in those post-USSR collapse years… Countries or institutions contributing to various aspects of mechanical design and construction, read-out elements, electronics, calibration etc. of this very sophisticated and high-performance detector, include CERN, France, Greece, Italy, Japan, Switzerland, UK, USA, Taiwan.

Finally, the innermost detection system within the CMS experiment is the central Tracker. It is its most sophisticated and technologically advanced element. In a cylindrical volume of about 6 metres length and 2.3 metres diameter are organised, in cylindrical layers for the central part and in flat disks towards the two ends, 10 million silicon microstrip detectors, typically 6 cm long, 100 microns to 400 microns wide and 300 microns thick. The overall surface of Si- microstrips is ~200 square metres, and at the time of its conception and design no Si-detector in the world exceeded 2 square meters! This silicon microstrip sub-detector is complemented in its central part, closest to the beam pipe and interaction point, by a pixel detector, originally 70 million pixels altogether of size 100 microns by 150 microns, organised in three cylindrical barrel layers (at 4 cm, 7 cm and 11 cm from the beam line) and by endcap disks, three on each side, providing precision track measurements of ~15 microns. Eighty million individual electronic read-out channels altogether! Producing this tracking detector of extreme requirements concerning mechanical construction, precision, electronics, radiation hardness etc. was a collaborative effort from a number of countries: Austria, Belgium, CERN, Germany, Italy, France, Switzerland and the USA.

The CMS detector also has a highly innovative, powerful and flexible system for data acquisition, triggering, monitoring, control and processing, with both hardware and software components. For example, the first hardware selection level has less than 3 microseconds to take a decision on whether to retain or reject a collision event, and the second software level implemented on a farm of ~5000 processors analyses up to 100,000 events per second selecting about a thousand for final data storage and analysis. This system is for the detector the equivalent of the nervous system of an organism, transforming and making sense from the ultimately electrical signals produced by the sub-detectors into physical quantities and variables amenable to physics analysis, that led, for example, to the discovery of the Higgs boson in 2012. The countries or institutions contributing to this system, among others, are Austria, CERN, France, Germany, Greece, Italy, Portugal, Spain, Taiwan, UK, USA and more. The overall cost of CMS is 600 million CHF; the ATLAS detector is of comparable cost.

The reader may wonder how this quest for what is sometimes called “the God particle”, the Higgs boson - surely not more God-like than the W or Z for that matter! – with all these institutions, laboratories or universities distributed all over the globe, about 200 of them, from close to 45 countries, altogether more than 3000 scientists and engineers in 2016, how this global endeavour with people of different cultures, religions, languages did not suffer from the “Tower of Babel syndrome” (Image 6)? How did God let humans peer into one of his deepest secrets, the origin of mass, without confounding them with a multitude of languages? Well, the secret is that there was a common purpose, physics and mathematics are universal, we all used broken-English to communicate among ourselves and give talks and seminars all over the planet, and - to give satisfaction to French pride – for construction we used the metric system of units; no inches, feet, yards, pounds, imperial gallons etc.!

Image 6: The Tower of Babel by Bruegel the Elder, Kunsthistorisches Museum in Vienna and the “camembert of countries/nationalities” participating in CMS, situation in 2012; in the meantime many new countries have joined CMS, Columbia, Lithuania, Egypt, Thailand, Malaysia, Indonesia, Equator, Oman, Saudi Arabia, and CMS is still growing in 2017, with Lebanon and the Montenegro joining most recently.

Finis coronat opus, Image 7a shows a beautiful example of a Higgs boson candidate event for a Higgs decaying to 4 leptons H→ZZ→μ⁺μ^–μ⁺μ^–as seen in the CMS detector, and another decaying to two photons H→γγ in Image 7b. According to quantum mechanics you can never be sure on a particular event – it could still be part of the background population – this is why we say “a candidate event”, the proof is ultimately statistical and was obtained beyond doubts in 2012 (Image 8). The lobster pot was well designed and the beast caught! It must be said that the ATLAS detector was equally successful in this hunt.

Image 7a: Candidate event for Higgs boson production followed by a decay into four leptons according to H → ZZ → e+e-μ+μ-, data of 2012; the red sticks indicate the energy depositions by e+ and e- in the crystal-calorimeter cells and the white-contour boxes indicate muon chambers traversed by the muons. [Image by Thomas McCauley and Lucas Taylor: https://cds.cern.ch/record/1606502]

Image 7b: Candidate event for a H → γγ decay as seen in CMS, the energy depositions by the two photons are in green. [Image by Thomas McCauley and Lucas Taylor: https://cds.cern.ch/record/1606503]

Image 8: The four-lepton spectrum with full statistics accumulated in 2012 in CMS at colliding energies of 7 TeV and 8 TeV, data points are in black, in blue is the expected ZZ* and ZZ background contribution – including the Z-boson-to-four-leptons decay mode peaking at a mass of ~90 GeV, in green is the Zbb-bar background, and the Higgs signal in orange peaking at ~125 GeV. On the right is the low-mass part of the spectrum for ~12fb⁻¹ of integrated luminosity of 13 TeV data accumulated and analysed in view of scientific conferences held in summer 2016.

CERN is an excellent example – probably even the best! – of what Europe can achieve uniting its forces⁶. It is good to think of this success in the present – not so optimistic times – when, beyond the essentially utilitaristic and mercantilistic Anglo-Saxon point of view, the ideal of an (asymptotically) united Europe is questioned. This recent discovery of the Higgs boson justifies CERN’s reputation as the world’s leading laboratory in particle physics. It is also worth reminding that during the “cold war”, CERN had been a place of contacts and meetings for scientists from both sides. CERN is somehow reminiscent of Alexandria’s Museion (Urania was the Muse of astronomy-sciences) and Library; it can be thought as being their heir, with scientists the world over being attracted to it and visiting it.

One last thought: these discoveries, of the W, Z, H, can also be viewed in the context of the present-day cosmological model of the Big Bang – sketched on the left-hand side of Image 9. At ~10⁻¹⁵ to 10⁻¹² seconds after the “initial event”, the Big Bang, the expanding Universe, matter has gradually cooled to reach temperatures of order 1000 GeV to 100 GeV (energy and temperature often used interchangeably, are related through E = kT, k is Boltzmann’s constant), the scale of the electroweak phase transition. The W, Z, H acquire their masses (electromagnetic and weak interactions thus separate), they are almost in thermal equilibrium, but they decay very rapidly in 10⁻²⁴ to 10^−-21 seconds and disappear from the cosmological scene.

At the LHC we recreate them – right-hand side of Image 9 – by provoking collisions of two droplets of partonic matter, heating it up to very high and comparable energies or temperatures, provoking “mini Big Bangs”, but out of equilibrium, local and short-lived. In these collisions sometimes, very rarely, once in about 10¹⁵ proton-proton collisions, two gluons fuse into a Higgs (through a top-quark loop), followed immediately (in ~10⁻²⁴ sec) by its decay into two Z bosons, each decaying in turn into a μ⁺μ⁻ or e⁺e⁻ pair (in ~10⁻²³ sec) – decay products we capture in our detectors, as sketched on the right-hand side of Image 9. Few tens of such captures per year of LHC functioning.

Image 9: Left-hand side: thermal history of the Universe in the Big Bang model, with the Universe expanding and cooling through various stages (phases). In the electroweak era (~10^-15 to 10^-12 sec) the entire Universe is populated with our present-day quarks, leptons, gluons, photons, W, Z and Higgs bosons (but other particles, like super symmetric ones, might be present too). [Image by Particle Data Group at Lawrence Berkeley National Lab: http://www.particleadventure.org/history-universe.html] The right-hand side sketches a proton-proton collision in the LHC as a “mini Big Bang”, with an occasional formation and subsequent decay of a Higgs boson in a decay mode (ZZ, with Z to electrons or muons) for whose detection the CMS detector has been designed and optimised.

With the discovery of the Higgs boson, the LHC story is not over however. Many years of studies are ahead of us to understand the exact nature of the discovered Higgs boson, its couplings, self-couplings etc., as well as many highly interesting studies and searches with possibly other discoveries in store: Supersymmetry, dark matter, signs of extra space dimensions, quantum mini-black-holes etc. It may also be that no new “spectacular” discoveries (à la W, Z, H) will be made at the LHC, but rather that indications of what the new horizon in physics is will emerge from detailed studies finally revealing the limitations of the present-day paradigm, the Standard Model. We all know that, despite the fact that there is at present no physics result, no uncontroversial measurement, really in contradiction with the Standard Model within its realm, this theory is surely incomplete. Many questions come to mind, like the origin of the Higgs potential, an explanation for neutrino masses, their mixing, CP violation, whether neutrinos are Dirac or Majorana particles. There is no place or explanation in the Standard Model for either dark matter or dark energy, for the cosmic matter-antimatter asymmetry etc. and the accelerator-collider physics may not be the only or even the most appropriate way to attack these issues. The LHC will continue with a high-luminosity phase in some eight-to-ten years from now (and possibly doubling the energy is also considered, a much more exciting perspective, but also more costly) and in about 20 years from now, when this research programme will be completed, probably a new more powerful machine, with still more complex detectors, will take over to carry-on this incessant quest towards deeper understanding of Nature, the essence of Natural Philosophy – allowing to pursue the pleasure of discovery This is why physics is so fascinating intellectually and even purely aesthetically. The search for a rational explanation of natural phenomena must go on, this is particularly important in view of the present resurgence of various forms of obscurantism, superstition and religious fanaticism.

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I would like to thank Gabriele Kogler and Jasmine Yazgan for a careful reading of the text and for their comments on its “readability” for non-professional physicists, remarks I took at least in part into account.

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Footnotes