In July 2012, the ATLAS and CMS collaborations at CERN's Large Hadron Collider announced the discovery of a Higgs-like boson, a new heavy particle at a mass more than 130 times the mass of a proton. Since then, further data have revealed its properties to be strikingly similar to those of the Standard Model Higgs boson, a particle expected from the mechanism introduced almost 50 years ago by six theoreticians including British physicists Peter Higgs from Edinburgh University and Tom Kibble from Imperial College London. The discovery is the culmination of a truly remarkable scientific journey and undoubtedly the most significant scientific discovery of the twenty-first century so far. Its experimental confirmation turned out to be a monumental task requiring the creation of an accelerator and experiments of unprecedented capability and complexity, designed to discern the signatures that correspond to the Higgs boson. Thousands of scientists and engineers, in each of the ATLAS and CMS teams, came together from all four corners of the world to make this massive discovery possible.
The Standard Model (SM) of particle physics has emerged through both theoretical and experimental discoveries spanning the last five decades. It comprises the building blocks of visible matter, the fundamental fermions (quarks and leptons) and the bosons (photons, W and Z bosons, and gluons) that mediate three of the four fundamental interactions: photons for electromagnetism, the W and Z bosons for the weak interaction and gluons for the strong interaction . The photon remains massless while the W and Z bosons acquire mass through a spontaneous symmetry-breaking mechanism proposed by three groups of physicists (Englert and Brout; Higgs; and Guralnik, Hagen and Kibble) [2–7]. This is achieved through the introduction of a complex scalar field leading to an additional massive scalar boson, labelled the SM Higgs boson. The fermions acquire mass through a Yukawa interaction of the Higgs boson. Only the gravitational interaction remains outside the SM.
The year 2013 marked the 30th anniversary of the discovery of the W and Z bosons by the UA1 and UA2 experiments at the proton–antiproton collider at CERN, Geneva, Switzerland. The discovery of the W and Z bosons focused efforts, and set the stage, for the search for the Higgs boson. In the following year, 1984, a workshop was held in Lausanne, Switzerland, where the first ideas were discussed about a possible high-energy proton–proton collider and associated experiments for this search. Among the leading protagonists were the scientists from the UA1 and UA2 experiments. The aim was to re-use the Large Electron Positron collider (LEP) tunnel after the end of the electron–positron programme. An exploratory machine was required to cover the wide range of mass, the diverse signatures and mechanisms thought to be effective for the production of new particles at a centre-of-mass energy 10 times higher than previously had been probed. A hadron (proton–proton) collider is such a machine as long as the proton energy is high enough and the instantaneous proton–proton interaction rate is sufficiently large. The centre-of-mass energy was set at 14 TeV and the rate at 1 billion pairs of protons interacting every second (corresponding to an instantaneous luminosity L=1034 cm−2 s−1). The hadron colliders can provide these conditions, though at the expense of ‘clean’ experimental conditions owing to multiple interactions in every bunch crossing overlaying on the one of interest (labelled pile-up).
Some of the physics questions the particle physics community was pondering over are listed below.
A key aim was to clarify symmetry breaking in the electroweak sector, most likely requiring a search for the SM Higgs boson, among other possibilities.
In the absence of a Higgs boson or equivalent, at very high energies such as at the Large Hadron Collider (LHC), the probability of some fundamental processes such as WL−WL scattering violates unitarity, i.e. the probability of occurrence becomes greater than one—which obviously would not make sense. A process involving the exchange of a Higgs boson would be able to ‘regulate’ the process and give a finite answer.
Furthermore, it was known that the discovery of a fundamental scalar (spin 0) Higgs boson would raise another deep question—why would the mass of such a Higgs boson lie in the range probed by the LHC. With known physics, quantum corrections make the mass of a fundamental scalar particle float up to the next highest physical mass scale that, in the absence of extensions to the SM, could be as high as 1016 GeV. It is widely believed that the answer to this question would lie in new physics beyond the SM (BSM). One appealing hypothesis, much discussed at the time, and still being investigated, predicts a new symmetry labelled supersymmetry. For every known SM particle there would be a partner with spin differing by half a unit; fermions would have boson partners and vice versa, thus doubling the number of fundamental particles. The contributions from the boson and fermion superpartners, and vice versa, would lead to cancellations and allow the existence of a low mass for the Higgs boson. In the simplest forms of supersymmetry, five Higgs bosons are predicted to exist with one resembling the SM Higgs boson with a mass below approximately 140 GeV. The lightest of this new species of superparticles could be the candidate for dark matter in the Universe that is around five times more abundant than ordinary matter.
Also it was clear that a search had to be made for new physics at the TeV energy scale as the SM is logically incomplete; it does not incorporate gravity. Superstring theory is an attempt towards a unified theory with dramatic predictions of extra space dimensions and supersymmetry.
The LHC and its experiments  were designed to find new particles, new forces and new symmetries, among which could be the Higgs boson(s), supersymmetric particles, Z′ bosons, or evidence of extra space dimensions. An experiment that could cover the detection of all these ‘known’ but yet undiscovered particles, or phenomena, would also allow discovery of whatever else Nature has in store at the LHC energies.
2. The standard model higgs boson and the large hadron collider
The mass of the Higgs boson is not predicted by theory but for a given mass all of its other properties are precisely predicted. From general considerations MH<1 TeV while precision electroweak constraints imply that MH<152 GeV at the 95% confidence level (CL) [9,10]. The lower limit on the mass of the Higgs boson from the LEP experiments was 114.4 GeV [10,11].
Once produced the Higgs boson disintegrates in one of several ways (decay modes) into known SM particles, depending on its mass. A search had to be envisaged not only over a large range of masses but also many possible decay modes: into pairs of photons, Z bosons, W bosons, τ leptons and b quarks.
The predicted cross sections and the branching ratios into the various decay modes of the SM Higgs boson as a function of mass are illustrated in figure 1a and b, respectively ; 13. The dominant Higgs boson production mechanism, for masses up to approximately 700 GeV, is gluon–gluon fusion. The W–W or Z–Z fusion mechanism, known as vector boson fusion (VBF), becomes important for the production of higher mass Higgs bosons. Here, the quarks that emit the W/Z bosons end up in the final states with transverse momenta of the order of W and Z masses. The detection of the resulting high-energy jets in the forward regions, 2.0<|η|<5.0,1 can be used to tag the reaction, improving the signal-to-noise ratio and extending the mass range over which the Higgs boson can be discovered. These tagging jets turned out also to be very important in the measurements of the properties of the newly found boson.
3. Timeline of the large hadron collider project
In the late 1980s and early 1990s [14–16], several workshops and conferences took place where the formidable experimental challenges  at a high-energy, high-luminosity Hadron collider started to appear manageable, provided that enough research and development work could be carried out, especially on detectors.
The search for the SM Higgs boson played a vital role in the design of the general-purpose detectors. A search had to be made across the entire allowed range of masses; from around a mass of approximately 50 GeV, the lower limit at the time, up to its largest possible value of approximately 1000 GeV.
In 1990, at a seminal meeting in Aachen, Germany, discussions focused on the physics potential, the detector technologies and magnetic field configurations in possible experiments. The natural width of the SM Higgs boson in the low-mass region is very small (less than 10 MeV⇒ΓH/MH∼10−4). Hence the width of any observed peak would be entirely dominated by instrumental mass resolution. Considerable emphasis was therefore put on the value of the magnetic field strength, on the precision charged particle tracking systems and on high-resolution electromagnetic calorimeters.
In 1992, four experiment designs were presented at a meeting in Evian, France: two deploying toroids (one superconducting) and two deploying superconducting high-field solenoids. In June 1993, CERN's scientific peer review committee, the LHC Committee (LHCC), recommended that the ATLAS and CMS experiments proceed further to the next phase of technical proposals.
In the 1990s, the two collaborations grew most rapidly in terms of people and institutes. Finding new collaborators was high on the ‘to do’ list of the leaders of the experiments.
The formal approval for construction was given in July 1997 by the then director-general, Chris Llewellyn Smith, imposing a material cost ceiling of 475 MCHF. Later two dedicated experiments were given the go-ahead: ALICE to study heavy ion collisions and LHCb to study matter–antimatter asymmetry.
The magnitude of the task is captured by a saying prevalent in the late 1980s and early 1990s—‘We think we know how to build a high energy, high luminosity hadron collider—but we don’t have the technology to build a detector for it’. Many technical, financial, industrial and human challenges lay ahead which were all overcome to yield experiments of unprecedented complexity and power. A flavour can be attained from ; 19; .
After the formal approval, an intense 10 year period of construction ensued. In 2008, the LHC experiments were ready for proton–proton collisions.
(a) The ATLAS and CMS experiments
Arguably the most important aspect of experiment design and layout at hadron colliders is the choice of the configuration of the magnetic field for the measurement of the momentum of muons . Large bending power is needed to measure, with sufficient precision, the momentum of charged particles. This forces a choice of superconducting technology for the magnets.
The ATLAS design (figure 2) [21–23] centred on three very large superconducting air-core toroids for the measurement of muons, supplemented by a superconducting 2 T solenoid to provide the magnetic field for inner tracking and by a liquid-argon/lead electromagnetic calorimeter with a novel ‘accordion’ geometry. The CMS design (figure 3) , in a complementary design, was based on a single large-bore, long, high-field solenoid for analysing muons, together with powerful silicon microstrip-based inner tracking and an electromagnetic calorimeter of scintillating crystals.
The CMS and ATLAS detectors have performed very well, and according to the ambitious design specifications laid down in the mid-1990s. They are the most sophisticated general-purpose particle physics experiments ever built. They were rapidly commissioned and started producing publishable physics results within a few months of recording the first proton–proton collisions.
In order to discover the new phenomena mentioned above, protons have to collide head on; in fact, the partons inside the protons (quarks and gluons) have to collide head-on, in what is termed a ‘hard interaction’ (figure 4), as opposed to a glancing collision where less energy is involved in the physics of the interaction. Any new particles produced as a result of these collisions will manifest themselves through disintegration into the well-known particles of the SM mentioned above. The photons, electrons and muons can emerge into the detectors directly from the hard interaction, whereas quarks and gluons, never visible as free particles, appear in the detectors as collimated bunches of stable or quasi-stable particles labelled ‘jets’.
Figure 5 shows the distribution of invariant masses of all di-muon pairs detected in CMS upon examination of the first 3 trillion proton–proton interactions. For example, as can be seen in figure 5, the observed width of particles such as J/ψ or Y is dominated by the instrumental resolution while that of the Z by its natural width. The background can also be seen and the clarity (high signal over background) of the signals is evident. It is remarkable that the ATLAS and CMS experiments, arguably the most technologically challenging scientific instruments ever built, achieved their design mass resolutions after the first few months of data taking.
4. First data and first physics results
After a short low-energy run in 2009 at a centre-of-mass energy of √s=0.9 TeV, the LHC physics journey started in earnest in April 2010, when the first proton–proton collisions at an unprecedented centre-of-mass energy of √s=7 TeV, 3.5 times larger than at the previous most powerful hadron collider, the Tevatron at Fermilab, Chicago, IL, USA), inaugurated the exploration of a new energy scale. The collision energy was raised to √s=8 TeV in 2012. The first LHC data-taking period (so-called ‘Run 1’) covered about 3 years, from April 2010 to February 2013.
A large amount of data, about 5 billion events, from the examination of some 2000 trillion proton–proton interactions, was recorded in Run 1 by each of the two experiments, ATLAS and CMS. They include, for each experiment and after the main selections of the analysis, about 100 million W→lν events, 10 million Z→ll decays, where, unless otherwise stated, l stands for an electron or a muon, half a million top-quark pair events with at least one lepton in the final state, etc. Such datasets exceed, in some cases by orders of magnitude, the size of the samples recorded by the CDF and D0 experiment at the Tevatron during the whole lifetime of that project. They have enabled detailed measurements of a large variety of SM processes in a new energy regime, searches for physics beyond the SM, with null results so far, and the discovery of a new particle compatible with the SM Higgs boson. By the time of the Royal Society meeting (January 2013), ATLAS and CMS had each submitted about 270 articles for publication in peer-reviewed journals.
These extraordinary accomplishments on a short time scale are the result of the competence and dedication of the scientists involved in the ATLAS and CMS experiments, and of the excellent performance, right from the beginning, of the LHC accelerator, detectors and computing infrastructure. These key ‘ingredients’ are discussed briefly later.
The ATLAS and CMS consist of about 3000 scientists each, from about 40 countries. Figure 6 shows the age distribution of the ATLAS collaboration: about half of the physicists are below age 35, and about 30% are PhD students. The fraction of women is 20% on average, larger in the younger age ranges.
In 2012, the LHC  achieved the record instantaneous luminosity of L=7×1033 cm−2 s−1 by circulating high-intensity beams consisting of about 1300 bunches separated by 50 ns, with 150 billion protons per bunch, and a transverse size of about 20 μm when colliding in ATLAS and CMS. The integrated luminosity delivered to each experiment during Run 1 was approximately , most of which was at √s=8 TeV. Since the number of events, N, produced by a specific physics process with cross section σ is given by , the physics reach, in particular the possibility of observing rare processes with small cross sections such as a heavy (new) particle, demands large integrated luminosities, implying high instantaneous luminosities and minimization of the accelerator downtime. Although essential for physics, the high luminosities achieved in Run 1 gave rise to a harsh experimental environment, and ATLAS and CMS had to deal with a large number of simultaneous proton–proton interactions (so-called ‘pile-up’) produced, at each beam crossing every 50 ns, by the very dense colliding bunches. The challenge is illustrated in figure 7.
In 2010, the instantaneous luminosity was L∼1032 cm−2 s−1, and typically two interactions were produced at each bunch crossing: the resulting events were pretty clean, as shown in figure 7a. In 2011, the event complexity increased, due to the number of overlapping interactions reaching approximately 10 per crossing (figure 7b). Finally in 2012, with luminosities above 5×1033 cm−2 s−1 and an average of 20 pile-up collisions per crossing, extraction and measurements of the interesting hard interactions required the full power of the detectors and a lot of inventiveness at the analysis level.
Concerning the detectors, examples of their excellent operational performance include: a data-taking efficiency (this is the fraction of delivered luminosity recorded by the experiments) well above 90%; a fraction of non-operational channels between a few per mille and a few per cent (figure 8); and a data quality (the fraction of recorded data good enough to be used for physics studies) of about 95%. High efficiencies are crucial because some of the interesting processes, for instance Higgs boson production followed by decay into four leptons (H→4l), are extremely rare: only a handful of such events had been recorded by each experiment at the time of the announcement of a Higgs-like boson in July 2012. Such a remarkable operation performance for experiments of the complexity of ATLAS and CMS testifies to the excellent quality of the construction and the powerful control and calibration systems.
The LHC computing infrastructure is based on a network of 135 sites distributed in 35 countries across the globe, the so-called worldwide LHC Computing Grid The Grid provides resources for data storage and analysis, integrated into a single system accessible in a seamless way by all scientists involved in the LHC. The challenges are daunting also in this case, as about 200 PB of disc space are required to store the huge amount of LHC data, and 350 000 CPU cores to process and analyse them. In 2012, the LHC was ranked among the top 10 producers of big data in the world. Since the beginning of the LHC operation, the Grid has allowed users from all over the world to access the data shortly after they were recorded and to do analysis in an effective manner. Today the grid is used also in other fields, from archaeology to finance and life sciences.
5. Standard model measurements
Owing to the excellent performance described in the previous section, ATLAS and CMS were able to ‘reproduce’ 50 years of particle physics in less than 1 year of operation, as illustrated in figure 9. The W and Z bosons and the top quark had already been detected in the first half of 2010, and since then have been measured with increasing precision. For instance, the top-quark mass is known today with an uncertainty of a few per mille from the Tevatron and the LHC, as shown in figure 10. The top quark is the heaviest elementary particle ever observed, with a mass comparable to that of a gold atom. It decays before hadronizing, and plays a special role in radiative corrections . It is therefore a very intriguing particle, which could offer a doorway to physics beyond the SM, and needs to be measured with the highest precision. The results reported in figure 10 are a further demonstration of the excellence of the LHC detectors and software tools (simulation, reconstruction, etc.). Indeed, events due to the production and decay of top-quark pairs contain all main physics objects (leptons, jets, b-quark jets, missing transverse energy), all of which must be measured extremely well, and with an excellent control of the related systematic uncertainties, to be able to achieve such an exquisite precision on the determination of the top-quark mass.
The ATLAS and CMS have also measured the production cross sections for the main SM processes, both for inclusive final states as well as for topologies where the main particle is accompanied by jets (figure 11). As another example, figure 12 shows the production of jets over a transverse energy range from 80 GeV to 2 TeV and in different angular regions of the detector . The SM predictions are in excellent agreement with the data, in some cases over cross-section ranges of several orders of magnitude. These results are also very useful to improve the theoretical description of the physics (including parton distribution functions, underlying event, initial- and final-state jet radiation) in the Monte Carlo generators used to simulate the various processes.
In conclusion, the SM works beautifully also in the (new) energy range explored by the LHC. Indeed, no significant deviations from its predictions have been observed so far. Furthermore, because most of the known processes are important backgrounds to searches for new particles, including the Higgs boson, SM measurements are an essential prerequisite for the experiments to be able to observe an unmistakeable signal. Armed with these solid foundations, ATLAS and CMS were ready to undertake the path towards the Higgs boson discovery.
6. The discovery of a higgs boson
Undoubtedly, the most striking result to emerge from the ATLAS  and CMS  experiments is the discovery of a new heavy boson with a mass of approximately 125 GeV. The analysis was carried out in the context of the search for the SM Higgs boson.
The predicted rate of production of the SM Higgs boson, its decay modes and its natural width vary widely over the allowed mass range (100–1000 GeV). It couples to the different pairs of particles in a proportion that is precisely predicted by the SM, i.e. for fermions (f) proportional to and for bosons (V) proportional to m4V/v2, where v is the vacuum expectation value of the scalar field (v=246 GeV). For example, at MH=125 GeV the SM boson is predicted to decay into pairs of photons with a branching ratio BR=2.3×10−3 (note that this decay proceeds via a quantum loop), into Z bosons and then four electrons or muons or two muons and two electrons with BR=1.25×10−4, into a pair of W bosons and then into llvv with BR approximately 1%, a pair of τ-leptons with BR=6.4%, and into a pair of b-quarks with BR=54%.
For a given Higgs boson mass hypothesis, the sensitivity of the search depends on:
— the mass of the Higgs boson;
— the Higgs boson production cross section (figure 1a);
— the decay branching fraction into the selected final state (figure 1b);
— the signal selection efficiency;
— the expected Higgs boson experimental mass resolution; and
— the level of backgrounds with the same or a similar final state.
To improve sensitivity events are separated into categories with different S/B and analysed independently. For many analyses all relevant information on signal versus background discrimination (aside from mass itself) is encoded into a multi-variate analysis independent of mass.
In 2011, the ATLAS and CMS experiments recorded data corresponding to an integrated luminosity of approximately 5 fb−1 at √s=7 TeV. In December 2011, the first ‘tantalizing hints’ of a new particle from both the CMS and ATLAS experiments were shown at CERN. The general conclusion was that both experiments were seeing an excess of unusual events at roughly the same place in mass (in the mass range 120–130 GeV) in two different decay channels. That set the stage for data taking in 2012.
In January 2012, it was decided to slightly increase the energy of the protons from 3.5 to 4 TeV, giving a centre-of-mass energy of 8 TeV. By June 2012, the number of high-energy collisions examined had doubled and both CMS and ATLAS had greatly improved their analyses. It was decided to look at the region that had shown the excess of events but only after all the algorithms and selection procedures had been agreed, in case a bias was inadvertently introduced. These data led to the discovery of a Higgs boson, independently in both the ATLAS and CMS experiments in July 2012 (see §6a).
In what follows, we shall concentrate on the region of low mass (114<MH<150 GeV) where the two channels particularly suited for unambiguous discovery are the decays to two photons and to two Z bosons, where one or both of the Z bosons could be virtual, subsequently decaying into four electrons, four muons or two electrons and two muons. These are particularly suited as the observed mass resolution (approx. 1% of MH) is the best and the backgrounds manageable or small.
By the end of 2012 (LHC Run 1), the total amount of data that had been examined corresponded to approximately 5 fb−1 at √s=7 TeV and approximately 20 fb−1 at √s=8 TeV, equating to the examination of some 2000 trillion proton–proton collisions, potentially producing 600 k SM Higgs bosons in each of the two experiments. Using these data the first measurements of the properties of the new boson were also made (see §6b).
All kinematic distributions shown below have been produced after all the selection cuts have been applied.
(a) The discovery of a Higgs boson: results from 2011 to partial 2012 datasets
In this section, we discuss the analyses that led to the discovery of a new heavy boson around a mass of 125 GeV, using the data accumulated up to June 2012.
(i) The H→γγ decay mode
In the H→γγ analysis, a search is made for a narrow peak in the diphoton invariant mass distribution in the mass range 110–150 GeV, on a large predominantly irreducible background from quantum chromodynamics (QCD) production of two photons (via quark–antiquark annihilation and ‘box’ diagrams). There is also a reducible background where one or more of the reconstructed photon candidates originate from misidentification of jet fragments, with the process of QCD Compton scattering dominating.
A candidate event recorded in the CMS detector is shown in figure 13.
The event selection requires two isolated photon candidates satisfying pT and photon identification criteria. A photon candidate is considered to be isolated if the energy in a defined region is found to be less than a certain value or threshold. In CMS , a pT threshold of mγγ/3 (mγγ/4) is applied to the photon leading (sub-leading) in pT, where mγγ is the diphoton invariant mass. Scaling the pT thresholds in this way avoids distortion of the shape of the mγγ distribution. The background is estimated from data, without the use of MC simulation, by fitting the diphoton invariant mass distribution in a range (100<mγγ<180 GeV). A polynomial function is used to describe the shape of the background.
(ii) The H→ZZ→4l decay mode
In the H→ZZ→4l decay mode, a search is made for a narrow four-charged lepton mass peak in the presence of a small continuum background. The background sources include an irreducible four-lepton contribution from direct ZZ production via quark–antiquark and gluon–gluon processes. Reducible background contributions arise from Z+ (b–anti-b quarks) and (t–anti-t quarks) production where the final states contain two isolated leptons and two b-quark jets producing secondary leptons. A candidate event recorded in the ATLAS detector is shown in figure 15.
The event selection requires two pairs of the same flavour, oppositely charged leptons. Since there are differences in the reducible background rates and mass resolutions between the sub-channels 4e, 4μ and 2e2μ, they are analysed separately. Electrons are typically required to have pT>7 GeV. The corresponding requirements for muons are pT>5–6 GeV. Both electrons and muons are required to be isolated. The pair with invariant mass closest to the Z boson mass is required to have a mass in the range 40–120 GeV and the other pair is required to have a mass in the range 12–120 GeV. The ZZ background, which is dominant, is evaluated from Monte Carlo simulation studies.
The M4l distribution from the ATLAS experiment is shown in figure 16 . A clear peak is observed at approximately 125 GeV in addition to the one at the Z mass. The latter is due to the conversion of an inner bremstrahlung photon emitted simultaneously with the di-lepton pair. A similar result was obtained by the CMS experiment .
A search was also made in other decay modes of a possible Higgs boson and combined to yield the final results published in August 2012 by ATLAS  and CMS . The observed (expected) local significances were 6.0σ (5.0σ) and 5.0σ (5.8σ) in ATLAS and CMS, respectively. It was clear that both ATLAS and CMS independently discovered a new heavy boson at approximately the same mass, clearly evident in the same two different decay modes, γγ and ZZ.
The decay into two bosons (two γ; two Z bosons; two W bosons) implied that the new particle is a boson with spin different from one and its decay into two photons that it carries either spin-0 or spin-2.
The results presented by both ATLAS and CMS collaborations were consistent, within uncertainties, with the expectations for a SM Higgs boson. Both noted that collection of more data would enable a more rigorous test of this conclusion and an investigation of whether the properties of the new particle imply physics beyond the SM.
(b) Results from the full 2011 and 2012 dataset: properties
We present here the results from the full dataset corresponding to an integrated luminosity of approximately 5 fb−1 at √s=7 TeV and approximately 20 fb−1 at √s=8 TeV. This larger dataset allowed confirmation of the discovery of the new boson, a better examination of the decay channels other than the H→γγ and the H→ZZ→4l decay modes and the first substantial investigations of the boson's properties.
(i) The H→γγ and the H→ZZ→4l decay modes
The results from the ATLAS experiment are shown for the H→γγ decay mode (figure 17)  and those from the CMS experiment for the H→ZZ→4l mode (figure 18) . The signal is unmistakable and the significances have increased (see §7). The data show an even clearer excess of events above the expected background around 125 GeV. The complementary results from the two experiments can be found in [30,32].
(ii) H→WW→2l2ν decay mode
The search for H→W+W− is based on the study of the final state in which both W bosons decay leptonically, resulting in a signature with two isolated, oppositely charged, high pT leptons (electrons or muons) and large missing transverse momentum, EmissT, due to the undetected neutrinos. The signal sensitivity is improved by separating events according to lepton flavour into e+e−, μ+μ− and eμ samples and according to jet multiplicity into 0-jet and 1-jet samples. The dominant background arises from irreducible non-resonant WW production.
The mll distribution in the 0-jet and different-flavour final state is shown for CMS in figure 19 . The expected contribution from a SM Higgs boson with MH=125 GeV is also shown. The transverse mass, mT, distribution and the background-subtracted mT distribution are shown in figure 20 for the ATLAS experiment . The hatched areas represent the total uncertainty on the sum of the signal and background yields from statistical, experimental and theoretical sources. Both show a clear excess of events compatible with a Higgs boson with mass approximately 125 GeV. The observed (expected) significance of the excess with respect to the background-only hypothesis at a mass of 125.5 is 3.8 (3.8) σ in the ATLAS experiment  and 4.3 (5.4) σ in the CMS experiment .
(iii) The H→ττ decay mode
The H→ττ search is performed using the final-state signatures ee, eμ, μμ, eτh, μτh, τhτh, where electrons and muons arise from leptonic τ-decays and τh denotes a τ lepton decaying hadronically. Each of these categories is further divided into two exclusive sub-categories based on the number and the type of the jets in the event: (i) events with one forward and one backward jet, consistent with the VBF topology, and (ii) events with at least one high pT hadronic jet but not selected in the previous category (to give boosted candidate Higgs bosons). In each of these categories, a search is made for a broad excess in the reconstructed ττ mass distribution. The main irreducible background, Z→ττ production, and the largest reducible backgrounds (W+jets, multijet production, Z→ee) are evaluated from various control samples in the data.
Figure 21 shows the combined observed and expected di-tau mass distributions from the CMS experiment , weighting all distributions in each category of each channel by the ratio between the expected signal and background yields for the respective category in a di-tau mass interval containing 68% of the signal. Figure 22 shows a similar distribution from the ATLAS experiment . In both experiments, a small excess of events is seen around MH=125 GeV. The plots also show the difference between the observed data and expected background distributions, together with the expected distribution for a SM Higgs boson signal with MH=125 GeV. The observed (expected) significance of the excess with respect to the background only hypothesis at this mass is 3.2 (3.7) σ in the CMS experiment and 4.2 (3.2) σ in the ATLAS experiment. The results include the search for a SM Higgs boson decaying into a τ pair and produced in association with a W or Z boson decaying leptonically.
(iv) decay mode
The decay mode has by far the largest expected branching ratio (54%). However, as σbb (QCD) is approximately , the search concentrates on Higgs boson production in association with a W or Z boson which decay leptonically, i.e. W→eν/μν and Z→ee/μμ/νν. The Z→νν decay is identified by the requirement of a large missing transverse energy. The Higgs boson candidate is reconstructed by requiring two b-tagged jets. The search is divided into events where the vector bosons have medium or large transverse momentum and recoil away from the candidate Higgs boson.
Figure 23 shows the weighted di-jet invariant mass distribution in CMS  when all backgrounds, except di-boson production, are subtracted. The expected signal for a Higgs boson with a mass of 125 GeV is also shown. The data are consistent with the presence of a di-boson signal (ZZ and WZ, with Z→bb), with a small excess consistent with that originating from the production of a 125 GeV SM Higgs boson. For a Higgs boson of mass 125 GeV the excess corresponds to an observed (expected) local significance of 2.1 (2.1) σ.
7. Higgs boson measurements
The detection of significant excesses of events by both experiments around a mass of 125 GeV in several different final states leaves no doubt that a new particle was discovered. Combining all channels discussed in the previous section, the probability that the observed signal is instead due to upward fluctuations of the backgrounds is about 10−24, corresponding to a total signal significance of more than 10Σ per experiment.
Following the announcement of the discovery in July 2012, ATLAS and CMS have been measuring the properties of the new particle with increasing precision and scope. For example, using the highest resolution channels, H→γγ and the H→4l, the mass is determined to be: MH=125.36±0.37 (stat.)±0.18 (syst.) GeV by ATLAS  and MH=125.03±0.27 (stat.)±0.14 (syst.) GeV by CMS .
Besides the precise measurement of the mass, the two primary questions to ask are: Is the new particle a Higgs boson? If this is the case, is it the SM Higgs boson or a Higgs boson belonging to a more general theory?
Concerning the first question, a particle of the type ‘Higgs boson’ is a very special one, intrinsically different from all others observed so far; it is neither a matter particle nor a force carrier. Two ‘fingerprints’ distinguish it. First, to accomplish its job, a Higgs boson has to interact with the other elementary particles with strengths proportional to their masses. The LHC measurements (figure 24) indicate that the couplings of the new particle are indeed proportional to mass over a broad range, spanning from the τ-lepton (mass about 1.8 GeV) to the top quark (mass about 100 times larger). Figure 25 further shows that, within the present 15% uncertainties, the measured production rates of the new particle in the various decay channels are in agreement with the predictions for a SM Higgs boson. The combination over all channels of the ratios between measurements and SM expectations gives μ=1.00±0.13 from CMS  and μ=1.30±0.18 from ATLAS .
The second fingerprint of a Higgs boson is its spin. Unlike matter particles, which are spin-1/2 fermions, and force carriers, which are spin-1 bosons, a Higgs boson is a scalar, i.e. a spin-0 positive-parity (JP=0+) particle. The spin-parity of a particle can be inferred from the angular distributions of its decay products in the final state, which carry the imprint of the parent particle's spin. Figure 26 presents the distribution of the polar angle θ* of the di-photon system with respect to the z-axis of the Collins–Soper frame  for events in the signal peak (122<mγγ<130 GeV) of the spectrum shown in figure 18. The data are compared with the expectations for a spin-0 and a spin-2 particle. Although the statistical uncertainties on the experimental points are large, it is nonetheless evident that the data favour the spin-0 over the spin-2 hypothesis. A quantitative likelihood analysis, based on more information and on the combination of the H→γγ, H→4l and H→lνlν final states, shows that the spin-0 hypothesis is strongly preferred by both experiments, with the alternative JP=0−, 1+, 1−, 2+ hypotheses rejected with confidence levels larger than 97.8% [41,42]; 43; 44.
Hence, within the present experimental uncertainties, the new particle is an elementary object matching both fingerprints expected of a Higgs boson. These results have dramatic consequences also for our understanding of the evolution of the Universe. Indeed, according to cosmology, supported also by several experimental observations, the initial, exponential expansion of the Universe, called ‘inflation’, is conjectured to be triggered by a scalar field . The discovery of a Higgs boson is the first experimental evidence that scalar fields do exist in Nature.
(a) The remaining questions
Although enormous progress has already been made by ATLAS and CMS to pin down the properties of the newly discovered particle, several outstanding questions remain. They will be addressed by the LHC Run 2, starting in Spring 2015, by the LHC luminosity upgrades  and by possible future colliders.
Improved measurements of the properties of the new particle, including the observation of rare decays such as H→μμ, will provide more definitive information about its nature (e.g. whether it is elementary or composite), and offer a doorway to new physics. Indeed, physics beyond the SM is expected to modify the Higgs boson couplings to fermions and bosons by up to a few per cent, depending on the energy scale of the new physics; hence experimental precision from a few per mille to a few per cent is required to detect significant deviations from the SM expectations.
Higgs boson self-couplings, which would give access to the scalar potential in the SM Lagrangian, may be observed with the full luminosity of the upgraded LHC (3000 fb−1 per experiment).
In parallel, searches for new physics may clarify whether or not the (light) Higgs boson mass is stabilized by a new symmetry. Discovery of additional Higgs bosons will indicate the existence of a more complex Higgs sector.
Finally, a definitive exploration of the electroweak symmetry-breaking mechanism requires studies of WW, ZZ and WZ production at high masses of the boson pairs (mVV). Such studies will also provide a powerful ‘closure test’ of the SM. As mentioned in §1, in the SM without a Higgs boson, the cross section for the scattering of two gauge bosons diverges with energy, becoming unphysical for mVV∼ TeV. It is therefore crucial to verify that the newly discovered particle restores the good behaviour of the theory or, else, unravel any additional dynamics contributing to electroweak symmetry breaking .
The discovery of a Higgs boson by the ATLAS and CMS experiments at the LHC represents a giant leap for science, as also recognized by The Economist in its issue of 7 July 2012. Such a superb accomplishment is the result of the ingenuity, vision and perseverance of the high-energy physics community, and of more than 20 years of talented, dedicated work of those involved in the LHC projects.
After decades of superb theoretical and experimental efforts, and 3 years of LHC operation, the SM is now complete. However, the SM is not the ultimate theory of particle physics, as many crucial questions remain unanswered. They include the composition of the Universe, especially the identity of dark matter, the source of the asymmetry between matter and antimatter, the origin of neutrino masses, the motivation for the light mass of the Higgs boson, and the extreme feebleness of gravity compared with the other forces.
In the 10–20 years to come, the (upgraded) LHC and possibly new accelerators will help address some of these questions. Perhaps most importantly, the LHC may tell us what are the right questions to ask, and how to move ahead.
One contribution of 12 to a Discussion Meeting Issue ‘Before, behind and beyond the discovery of the Higgs boson’.
↵1 The pseudorapidity, η, is defined as where θ is the polar angle measured from the positive z-axis (along the anticlockwise beam direction).
- © 2014 The Author(s) Published by the Royal Society. All rights reserved.