When was the higgs boson particle discovered

Now, don't get too excited. This paper "does not give us new information yet about the Higgs portal into the 'dark sector,'" Beacham said. But "this paper proves that we can look for very rare things like this, quite handily," he said, which pushes the search forward overall.

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Higgs boson candidates are sought by selecting two pairs of isolated leptons, each of which comprises two leptons with the same flavour and opposite charge. In addition, two further leptons may originate from the decays of the fragmentation products of the heavy b-quarks or jets might be misidentified as leptons.

These leptons are less isolated and, when arising from b-quarks, do not originate from the primary interaction point where decays of the Higgs and Z bosons take place. These backgrounds are referred to as reducible backgrounds in the following. The events were categorised according to their lepton-flavour combination. Relative mass resolutions of approximately 1. After final selection requirements, the signal-to-background ratio is found to be about two in the signal mass window GeV.

The observed and expected mass distributions for events after the full selection are displayed in Figure 8. The reconstructed location of the proton-proton interaction is referred to as the primary vertex — charged particle tracks reconstructed in the tracking devices converge at this point. A secondary vertex occurs when a particle originating from the interaction decays after travelling some distance from the primary vertex. The impact parameter of a reconstructed charged particle track is its distance of closest approach to the primary vertex.

The magnitude of the impact parameter can be used to discriminate between the tracks of particles which originated in the interaction and those of particles produced from the decay of a relatively long-lived product of the interaction for example a b-hadron even if it is not possible to reconstruct a secondary vertex.

Leptons and photons originating from a hard interaction, or from the decay chain of a particle, such as a Z boson or Higgs boson, directly produced in a hard interaction are not except by chance closely accompanied by other particles — they are isolated. Many lepton and photon candidates reconstructed in the detector do not originate directly from the hard interaction, but come from the constituents of the jets of particles formed by the hadronization of gluons or quarks.

These candidates tend not to be isolated — they are surrounded by other particles from the jet. A measure of how many particles, or how much energy, surrounds a candidate lepton or photon can be used to determine whether it is probable that it emerged directly from the hard interaction, or more likely that it originated in a jet fragment.

Events selected so as to have negligible probability of containing signal events constitute a control sample , and the selection requirements define a control region in parameter space. The control sample in the control region contains only background events and can be used to estimate the number of background events expected in the region of the signal, provided the effect of whatever distinguishes the control region from the signal region is sufficiently well understood. For both experiments, an excess of events is present in the mass region around GeV.

The corresponding numbers for the CMS experiment in the mass window More details about the observed and expected numbers of events, their uncertainties and the split in the various four-lepton channels are given in Table 1 for the ATLAS and in Table 2 for the CMS experiment. In order to quantify these excesses, the probabilities for the background-only hypotheses were calculated. The local p-value is defined to give the probability of a background fluctuation.

The probability that an excess of events observed anywhere within an extended search range is due to a background fluctuation is termed the global p-value , and is larger than the local p -value — this fact is often referred to as the look-elsewhere effect. The search for the Standard Model Higgs boson decaying to two photons involves detecting a narrow peak above a background diphoton invariant mass spectrum. The Higgs boson decay to two photons proceeds via loop diagrams containing charged particles.

The W boson loop and the top quark loop diagrams dominate the decay amplitude, but contribute with opposite sign. The Standard Model branching fraction is small, having a maximum value of 0. Events containing diphotons are collected by both experiments using diphoton triggers. Further selection requirements are applied to suppress the reducible background contribution from photons originating in jets. Requirements are made on the shapes of the showers in the calorimeter cells or crystals, and on the isolation of the photons from other activity in the detectors.

There are many interactions for each bunch crossing, whose longitudinal position along the beam axis has a RMS spread of about 5 cm. The diphoton interaction vertex is assigned using multiple sources of information combined in a global likelihood ATLAS or using a boosted decision tree BDT Hoecker et al.

The analysis of the ATLAS experiment additionally considers the directions of the photon showers reconstructed in the calorimeter using its longitudinal granularity. That of the CMS experiment additionally examines the correlation between the kinematic properties of the charged particle tracks associated with the reconstructed vertices, with those of the diphoton system.

Both ATLAS and CMS enhance the sensitivity of their analyses by subdividing the selected diphoton events into mutually exclusive categories, where the categorization is based upon criteria sensitive to the diphoton mass resolution, and to the probability that an event is signal rather than background. The ATLAS classification is based on the location of the photons in the calorimeter, whether they are tagged as having converted in the material in front of the calorimeter, by the presence of a reconstructed electron track, and p Tt , the component of the diphoton transverse momentum that is orthogonal to the axis defined by the difference between the photon momenta.

The CMS classification is based on the output of a BDT which is trained to give a high value for signal-like events and for events with good diphoton invariant mass resolution. Events in which a dijet is present, in addition to the diphoton, and satisfies selection criteria chosen to be consistent with the characteristics of signal events produced by the vector-boson fusion process, are placed in dijet categories.

For the analysis of the 8 TeV dataset CMS uses two dijet categories with differing levels of selection stringency. Both analyses published tables listing the categories and indicating the numbers of events expected from a Standard Model Higgs boson signal in each category, the mass resolution expected in each category, and information about the numbers of events present in the data ATLAS Collaboration , CMS Collaboration Some important numbers from these tables are reproduced in Table 3 and Table 4.

For the statistical analysis of the data, the sum of a signal mass peak and a background distribution is fitted to the diphoton invariant mass distribution. The shape of the invariant mass distribution of the signals is obtained from detailed simulation for each of the categories. The background in the different categories is modelled by parametric functions. Various tests are made to determine the potential bias resulting from the choice of background fit functions.

These tests involve either pseudo-experiments, or large samples of simulated events complemented by data-driven estimates. CMS uses polynomials in the Bernstein basis with degree ranging from 3 to 5 depending on category.

ATLAS uses Bernstein polynomials of degree 4, exponentials of a polynomial of degree 2, and plain exponential functions, depending on the category. Based on searches in this channel, mass regions could be excluded by both the Tevatron and the LHC experiments already in summer The presence of neutrinos makes the reconstruction of a narrow mass peak impossible, and evidence for a signal must be extracted from an excess of events above the expected backgrounds.

Usually, the WW transverse mass m T , computed from the leptons and the missing transverse momentum,. Typical selection requirements are the presence of two isolated high- p T leptons with a small azimuthal angular separation and a significant missing transverse energy. The weak decays of the W-bosons imply a correlation between the directions of the charged leptons, which can be exploited to reject the WW background. As quickly as the data arrived it was analysed and, sure enough, the significance of that small bump around GeV increased further.

Seats at the seminar were so highly sought after that only the people who queued all night were able to get into the room. The ATLAS Collaboration celebrated the discovery with champagne and by giving each member of the collaboration a t-shirt with the famous plots.

Incidentally, only once they were printed was it discovered that there was a typo in the plot. After discovery, we began to study the properties of the newly-discovered particle to understand if it was the Standard Model Higgs boson or something else. In fact, we initially called it a Higgs-like boson as we did not want to claim it was the Higgs boson until we were certain.

The mass, the final unknown parameter in the Standard Model, was one of the first parameters measured and found to be approximately GeV roughly times larger than the mass of the proton. It turned out that we were very lucky — with this mass, the largest number of decay modes are possible. In the Standard Model, the Higgs boson is unique: it has zero spin, no electric charge and no strong force interaction. The spin and parity were measured through angular correlations between the particles it decayed to.

Sure enough, these properties were found to be as predicted. The discovery of the Higgs boson relied on measurements of its decay to vector bosons. In the Standard Model, different couplings determine its interactions to fermions and bosons, so new physics might impact them differently. Therefore, it is important to measure both. During Run 2, the increase in the centre-of-mass energy to 13 TeV and the larger dataset allowed further channels to be probed. Over the past year, the evidence has been obtained for the Higgs decay to bottom quarks and the production of the Higgs boson together with top quarks has been observed.

Perhaps one of the neatest ways to summarise what we currently know about the interaction of the Higgs boson with other Standard Model particles is to compare the interaction strength to the mass of each particle, as shown in Figure 4.

This clearly shows that the interaction strength depends on the particle mass: the heavier the particle, the stronger its interaction with the Higgs field. For example, constraining the rate that the Higgs boson decays to invisible or unobserved particles provides stringent limits on the existence of new particles with masses below that of the Higgs boson. We also look for decays to combinations of particles forbidden in the Standard Model.

During this run, 13 TeV proton-proton collisions have been producing approximately 30 times more Higgs bosons than those used in the Higgs boson discovery. As a result, more and more results have been obtained to study the Higgs boson in greater detail.

Over the next few years, analysis of the large Run 2 dataset will not only be an opportunity to reach a new level of precision in previous measurements, but also to investigate new methods to probe Standard Model predictions and to test for the presence of new physics in as model-independent a way as possible.

This new level of precision will rely on obtaining a deeper level of understanding of the performance of the detector, as well as the simulations and algorithms used to identify particles passing through it. It also poses new challenges for theorists to keep up with the improving experimental precision.

Among other measurements, this will open the possibility to investigate a very peculiar property of the Higgs boson: that it couples to itself. Events produced via this coupling feature two Higgs bosons in the final state, but they are exceedingly rare. Thus, they can only be studied within a very large number of collisions and using sophisticated analysis techniques.

Looking more generally, the discovery of the Higgs boson with a mass of GeV sets a new foundation for particle physics to build on. Many questions remain in the field, most of which have some relation to the Higgs sector. For example:. He has worked both as a theoretical and experimental physicist, and is a founding member of ATLAS where he performed, among others, combined Higgs boson analyses and led the Higgs working group.

Both enjoy communicating particle physics to non-specialists. The higher the luminosity, the more events per second.