The CMS collaboration presented its most successful current measurement of the Higgs boson properties in the two-photon decay channel at the ICHEP conference in August 2020. The results are based on the complete LHC Run 2 data recorded between 2016 and 2018 and show a level of accuracy never before achieved.
Thanks to this increased sample size, sophisticated analysis methods using artificial intelligence and developed in part by the CMS group at Irfu, previously unimaginable measurements are now possible: the study of rare modes of production becomes possible. This work of ants has made it possible to carry out increasingly precise measurements of the properties of the Higgs boson, enabling the Standard Model of particle physics to be tested ever more thoroughly. The latter once again emerged triumphant from this confrontation.
But with the restart of the LHC collider in 2022, and then its brightness increase in 2027, the amount of data will increase significantly, allowing the Standard Model to be examined from every angle.
The Higgs boson in one word (or almost...)
Since the first observation of the Higgs boson by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) in 2012, the precise measurement of its properties has become one of the priorities of the LHC physics program. The Higgs boson disintegrates instantaneously in the detector and is therefore studied thanks to its decay products. A gold decay channel for studying the properties of the Higgs boson is the two-photon channel. Indeed, despite its rarity (less than 0.3% of the Higgs boson decays), it allows to measure the mass of the Higgs boson with an excellent resolution thanks to the precise measurement (within about 1%) of the photon energy in the CMS electromagnetic calorimeter (ECAL). This decay channel, which was one of the two discovery channels in 2012, also allows access to all modes of Higgs boson production because the background noise is moderate. Finally, the background noise can be estimated directly from the data, which significantly reduces the sources of uncertainty compared to other channels. The IRFU CMS group played a central role in the construction of ECAL and continues to play a crucial role in its calibration. It has therefore naturally also played an important role in the discovery and measurement of the properties of the Higgs boson in its two-photon decay and in particular in recent years a leading role in the study of the production of the Higgs boson in association with a pair of top anti-top quarks ttH.
The Higgs boson plays a central role in the theory that describes particles and their interactions, the Standard Model of Particle Physics (SMP). This model, developed during the 20th century, acquired its present final form around the 1970s. It predicts the existence of a particle, the Higgs boson, as a consequence of the breaking of electroweak symmetry, a mechanism that allows elementary particles to acquire mass. The MS has been intensively tested and can account for almost all the phenomena observed in the laboratory to date, its latest success being the observation of the Higgs boson in 2012. However, the MS is only an incomplete description of our Universe. It does not explain many properties of elementary particles such as the number of their families or the differences between their mass scales. From a more general point of view, it does not explain either the origin of dark matter or dark energy necessary to describe astrophysical and comological observations. Physics beyond the Standard Model, or "New Physics" (NP) has been actively sought directly at the LHC, i.e. in the form of new particles not predicted by the MS. Unfortunately, this research has been unsuccessful and large areas of mass and effective sections have been excluded by this research. The lack of direct observation of new particles at the LHC has led to a paradigm shift: precision measurements are now of primary importance, in order to highlight inconsistencies in the model. Indeed, physics beyond the MS could interact and interfere with known particles and leave its imprint on the MS particle properties. Thus, accurate measurement of the properties of particles and the strength of their interactions could provide clues about the LOC when these measurements deviate from the MS predictions. Many properties of the Higgs boson have not yet been studied in detail. Does the Higgs boson interact with other particles as predicted by the MOH? In the MS, the strength of the interaction between the Higgs boson and any other particle is directly related to the mass of the latter. Changes in these interactions are predicted by many new models, extensions of the MS. Thus, the precise study of the properties of the Higgs boson is a gateway to NP. For example, the strength of the interaction could be different from that predicted by the MS.
In the past, measures have been limited to the efficient sections of the main modes of production. The four main modes of production of the Higgs boson at the LHC, whose diagrams are shown below, are:
With the large data sample available at Run 2, 137 fb-1 of proton-proton collisions at an energy in the center of mass of 13 TeV, and experiments that develop new methods of analysis, previously impossible measurements are coming to light: rare modes of production or decay become accessible, measurements with a finer granularity can also be made to test different corners of the phase space, some of which may be more sensitive to the New Physics. The CMS collaboration presented its most successful current measurement of the properties of the Higgs boson in the diphoton channel at the ICHEP conference in August 2020.
Characterize the different modes of production of the Higgs boson, an agreement between theoreticians and experimenters!
Theorists and experimentalists have in recent years jointly defined a common framework for ATLAS and CMS that allows precision measurements in the Higgs boson sector in different regions of the space of predefined phases and finer and finer over time. This common framework is called "Simplified template cross section" (STXS). In this framework, the main modes of production of the Higgs boson are themselves subdivided into different regions, defined by certain properties of the events, such as the transverse impulse of the Higgs boson or the number of additional jets. We propose to measure the effective production cross sections in each of these regions and to compare them with the MS predictions. These regions are chosen to maximize experimental sensitivity while decreasing as much as possible the dependence on the theory of results. This framework also allows an easy combination of results in the different decay channels and between experiments as well as an easier reinterpretation of the results by the theorists. The complete STXS measurement framework is presented below, with the different colors corresponding to the different modes of production of the Higgs boson!
The goal is to measure the production rates, or effective sections, for each of these boxes or "bin" in order to get a detailed picture of the properties of the Higgs boson. Some of these regions are more sensitive to physics beyond the MS, for example those with a Higgs boson produced with a very large transverse pulse.
Surgical tools
First, events with two well identified photons are selected. They are then classified according to their production mechanism (by requesting the presence of additional objects in the event that signs the production mode). For example, for the mechanism of Higgs boson production by vector boson fusion ("VBF"), one expects to find 2 jets in the forward regions of the detector. Thus, by labeling events with forward jets, one can construct event categories enriched in VBF production. In the same way, categories are constructed for the different modes of production: production associated with a vector boson (VH), production associated with top quarks (ttH, tH) or production by gluon fusion (ggF).
These categories are subdivided into sub-categories, enriched with events of different "STXS bins". In doing so, it becomes possible to measure the effective sections in each bin in order to construct the desired fine description of the Higgs boson. Machine learning algorithms are used extensively to increase the purity of the categories and to reject background noise, with the ultimate goal of minimizing measurement uncertainties. This part of the work requires fine optimization. Each algorithm has a specific goal, for example the identification of photons, or the rejection of a dominant background noise for a particular process, or the rejection of another mode of production of the Higgs boson that would pollute the category. The purer the categories are in events corresponding to the targeted STXS bin, the greater the accuracy because the uncertainty on the contaminations decreases. Likewise, the lower the background noise, the greater the uncertainty on it, and therefore the uncertainty on the final measurement, decreases. These algorithms are trained using simulated data and sometimes also real data when possible or desirable. These algorithms use many input variables to discriminate background noise or other modes of signal production. Each type of category uses for its selection one or more of these algorithms dedicated and optimized for that particular type of category.
For the first time, for example, a Deep Neural Network is used to separate events where the Higgs boson is produced with a single quark top (tH) from those where it is produced with a pair of quarks top (ttH). Separating these events that have very close topologies in the detector is difficult and the production with a single quark top is a very rare process and never yet observed at the LHC. The image below shows a possible tH event, recorded by the CMS detector in August 2018.
The measured quantities are used to assign a "bin STXS" to each event. For some "bins", this choice is made through a self-learning algorithm called "boosted decision tree". In the case of ttH production, the measured transverse pulse is used to classify the events, in order to make for the very first time measurements based on the Higgs boson transverse pulse.
Thanks to the excellent performance of the CMS electromagnetic calorimeter, it is possible to measure the photon energy very precisely and thus to determine the mass of the Higgs boson with a good resolution. Thanks to this high accuracy, photon pairs originating from the Higgs boson appear as a narrow peak corresponding to the mass of the Higgs boson (125 GeV) in the diphoton invariant mass distribution, whereas the background noise, dominated by photon pairs not originating from a Higgs boson, has a continuous and slowly decreasing invariant mass spectrum, as shown in the figure below. In order to extract the rms sections, a model describing the shapes of the mass distributions of the Higgs boson ("signal") events and the background noise are determined through simulation, after various corrections have been applied to the simulation to make it faithful to the real data. These corrections and calibrations are determined on the real data using known processes, such as the two-electron Z-boson decay used to calibrate the properties of the photons resulting from the decay of the Higgs boson into two photons, the two signatures being very similar in the detector. These models are then fitted to the data in the different categories to determine the effective cross sections for each STXS bin. The figure below shows the model (in red) and data (in black) for all categories (88 categories) of the analysis combined.
Precision above all else
Many measurements can be made thanks to the defined categories, depending on the parameters that one decides to leave free in the adjustment and which are therefore determined by it. First of all, the total production rate of the Higgs boson as well as the production rates for the different main production modes are measured by determining the "signal strength", µ. The "signal strength" is defined as the ratio between the measured production rate and the theoretically predicted production rate, where 1 means that the production rate is exactly as predicted by the MS. The measured values and their uncertainties are shown opposite, where the colored dots are the measurements by production mode and the black dot represents the measurement of the total production. Here, the tH production is measured together with the ttH production, via the µtop parameter. Although fluctuations are observed with respect to the unit, the results are compatible with the MS within the uncertainties. Note that the uncertainty on the total production rate is about ten percent, this measurement being dominated by the main gluon fusion production mode, while it is about thirty percent for the VBF, VH and ttH modes.
Dissection of the Higgs boson
Then, to go further, the effective cross-sections in the different STXS "bins" are measured. The figure below shows the measured values as well as their uncertainties for 24 "bins", simultaneously determined when fitting the model to the data. Here, some "bins" have been grouped together to avoid too large measurement uncertainties. The color scheme corresponds to the one used in the previous figures: the gluon fusion is in blue, the vector boson fusion and the production associated with a vector boson decaying into quarks are in orange, the production associated with a vector boson decaying into leptons is in green, and finally the ttH and tH productions are in pink and yellow respectively. The MS predictions for each point, with their theoretical uncertainties, are represented by the grey dot boxes.
This result provides the very first dedicated study of tH production, a rare mode of production that was previously inaccessible. It is also the first measurements of ttH production in different regions of the Higgs boson cross-pulse. The measured cross sections are in very good agreement with the predictions of the Standard Model, which is still triumphant and leaves an increasingly narrow margin to the New Physics. However, statistical uncertainties are still the dominant uncertainties in these measurements, meaning that new data would increase their accuracy. At the next run of the LHC, which should start in 2022, then at the High-Luminosity LHC, starting around 2027, the available data will increase very significantly (up to 20 times more data). In addition, the combinations of these measurements in different Higgs boson decay channels and in the different experiments will also increase the accuracy, in order to test the Standard Model in these smallest corners.
Contact: Julie Malclès
To go further:
http://cms-results.web.cern.ch/cms-results/public-results/preliminary-results/HIG-19-015/index.html
