Multivariate analysis in the search for the higgs boson

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Multivariate analysis in the search for the higgs boson

No document with DOI "10.1.1.870.201"

This media update is part of a series related to the Large Hadron Collider Physics conferencetaking place from 25 to 30 May These include searches for rare transformations of the Higgs boson into a Z boson — which is a carrier of one of the fundamental forces of nature — and a second particle. Observing and studying transformations that are predicted to be rare helps advance our understanding of particle physics and could also point the way to new physics if observations differ from the predictions.

However, similar signatures may be produced by other Standard-Model processes. Scientists must therefore first identify the individual pieces that match this signature and then build up enough statistical evidence to confirm that the collisions had indeed produced Higgs bosons.

When it was discovered inthe Higgs boson was observed mainly in transformations into pairs of Z bosons and pairs of photons. Other transformations are predicted to occur only very rarely, or to have a less clear signature, and are therefore challenging to spot. The Z thus produced, itself being unstable, transforms into pairs of leptons, either electrons or muons, leaving a signature of two leptons and a photon in the detector.

multivariate analysis in the search for the higgs boson

While these limits are much greater than the predictions from the Standard Model, they demonstrate the ability of the detectors to make inroads in the search for physics beyond the Standard Model. Scientists believe that the Higgs boson could hold clues as to the nature of dark-matter particles, as some extensions of the Standard Model propose that a Higgs boson could transform into dark-matter particles. The Higgs boson continues to prove invaluable in helping scientists test the Standard Model of particle physics and seek physics that may lie beyond.

When data volumes are not high enough to claim a definite observation of a particular process, physicists can predict the limits that they expect to place on the process. In the case of Higgs transformations, these limits are based on the product of two terms: the rate at which a Higgs boson is produced in proton—proton collisions production cross-section and the rate at which it will undergo a particular transformation to lighter particles branching fraction.

ATLAS expected to place an upper limit of 1. News News Topic: Physics. Technical note When data volumes are not high enough to claim a definite observation of a particular process, physicists can predict the limits that they expect to place on the process. Related Articles. CERN experiments announce first indications o Searching for matter—antimatter asymmetry in Also On Physics. CMS sees evidence of top quarks in collisions LHCb sees new form of matter—antimatter asymm A roadmap for the future.

NA63 makes crystal-clear study of radiation r NA62 sees first significant evidence of rareDue to the small production cross section for the Higgs boson, only a substantial statistics can offer the chance to study this particle properties. In order to perform these searches it is desirable to avoid the contamination of the signal signature by the number and variety of the background processes produced in pp collisions at LHC. Much account assumes the study of multivariate methods which, compared to the standard cut-based analysis, can enhance the signal selection of a Higgs boson produced in association with a top quark pair through a dileptonic final state ttH channel.

The statistics collected up to is not sufficient to supply a significant number of ttH events; however, the methods applied in this thesis will provide a powerful tool for the increasing statistics that will be collected during the next LHC data taking.

Gestione del documento:. Salva citazione. Dublin Core. Tipologia del documento. Semprini Cesari, Nicola. Fisica [LM-DM]. Altri metadati Tipologia del documento. Vedi altre statistiche.During the hunt for the Higgs boson, scientists had to investigate and study a number of predicted processes. Science proceeds step by step, looking for the unknown and the unexplored.

Higgs production is rare, and as many different processes contribute to the background against which the Higgs signal must be distinguished, physicists have to reduce that background piece by piece to bring it to an acceptable level.

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The left plot show the distribution of the dijet mass. The right plot shows the neural network output. Both plots are for the one-tag candidates where events from all lepton categories are added together.

The best fit to the data is shown. One of the ways in which the Higgs was hunted is through its associated production with W bosons. W boson properties are well known and provide a way to select events in which a Higgs boson can be searched through its decays into two b quarks. Unfortunately there are processes that can mimic our signal. One of them is the production of a W and a Z boson together, with the Z boson decaying into two heavy flavor quarks two charm or two beauty quarks and the W decaying into leptons one charged and one neutral.

A second is the production of a WW boson pair, in which the second W decays into heavy flavor quark. These processes are predicted in the Standard Model, but until now, they escaped a clear observation in this final state. The separation of signal and background is shown in the left plot of the above figure. Among the techniques used as part of this analysis was that of a neural network.

Its output is shown on the right side of the figure. This technique is modeled on the central nervous system. These physicists were responsible for this analysis. CDF also looked for events containing a W or a Z decaying into heavy quarks, and we used the know-how developed in the Higgs experiment.

The evidence for the Higgs boson was obtained in part thanks to an earlier version of this study. CDF measured a production cross section for WW of 9. Navbar Toggle. News at work COVID resources for employees Calendar — Note cancellations and changes Search all laboratory news From lab leadership Submit content — login required Provide feedback Subscribe to our newsletter — login required.Many questions in particle physics are related to the existence of particle mass.

In a sense, this mass is the essential quantity, which defines that at this place there is a particle rather than nothing. In the early s, physicists had a powerful theory of electromagnetic interactions and a descriptive model of the weak nuclear interaction — the force that is at play in many radioactive decays and in the reactions which make the Sun shine. They had identified deep similarities between the structure of these two interactions, but a unified theory at the deeper level seemed to require that particles be massless even though real particles in nature have mass.

Intheorists proposed a solution to this puzzle.

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The peculiarity of this mechanism is that it can give mass to elementary particles while retaining the nice structure of their original interactions. Importantly, this structure ensures that the theory remains predictive at very high energy. Particles that carry the weak interaction would acquire masses through their interaction with the Higgs field, as would all matter particles.

The photon, which carries the electromagnetic interaction, would remain massless. In the history of the universe, particles interacted with the Higgs field just 10 seconds after the Big Bang.

Brian Cox Higgs Boson Particle Discovery BBC

Before this phase transition, all particles were massless and travelled at the speed of light. After the universe expanded and cooled, particles interacted with the Higgs field and this interaction gave them mass. The BEH mechanism implies that the values of the elementary particle masses are linked to how strongly each particle couples to the Higgs field.

These values are not predicted by current theories. However, once the mass of a particle is measured, its interaction with the Higgs boson can be determined. The BEH mechanism had several implications: first, that the weak interaction was mediated by heavy particles, namely the W and Z bosons, which were discovered at CERN in Second, the new field itself would materialize in another particle. The mass of this particle was unknown, but researchers knew it should be lower than 1 TeV — a value well beyond the then conceivable limits of accelerators.

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This particle was later called the Higgs boson and would become the most sought-after particle in all of particle physics. Though LEP did not find the Higgs boson, it made significant headway in the search, determining that the mass should be larger than GeV. Ina few physicists and engineers at CERN were exploring the possibility of installing a proton-proton accelerator with a very high collision energy of TeV in the same tunnel as LEP.

This accelerator would probe the full possible mass range for the Higgs, provided that the luminosity [1] was very high. However, this high luminosity would mean that each interesting collision would be accompanied by tens of background collisions. Given the state of detector technology of the time, this seemed a formidable challenge.

On the theory side, the s saw much progress: physicists studied the production of the Higgs boson in proton-proton collisions and all its different decay modes. As each of these decay modes depends strongly on the unknown Higgs boson mass, future detectors would need to measure all possible kinds of particles to cover the wide mass range. Each decay mode was studied using intensive simulations and the important Higgs decay modes were amongst the benchmarks used to design the detector.

multivariate analysis in the search for the higgs boson

Meanwhile, at the Fermi National Accelerator Laboratory Fermilab outside of Chicago, Illinois, the Tevatron collider was beginning to have some discovery potential for a Higgs boson with mass around GeV. Inafter a long and intense period of construction, the LHC and its detectors were ready for the first beams.

The machine worked beautifully and we had very high hopes.

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Alas, ten days later, a problem in the superconducting magnets significantly damaged the LHC. A full year was necessary for repairs and to install a better protection system. The incident revealed a weakness in the magnets, which limited the collision energy to 7 TeV. When restarting, we faced a difficult decision: should we take another year to repair the weaknesses all around the ring, enabling operation at 13 TeV?

Or should we immediately start and operate the LHC at 7 TeV, even though a factor of three fewer Higgs bosons would be produced?

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Detailed simulations showed that there was a chance of discovering the Higgs boson at the reduced energy, in particular in the range where the competition of the Tevatron was the most pressing, so we decided that starting immediately at 7 TeV was worth the chance.

The LHC restarted in at 7 TeV with a modest luminosity — a luminosity that would increase in Muons have even been used via decays of intermediate weak bosons in the detection of new particles, such as the Higgs boson — now the centerpiece of its own extremely rich field of research. In the Standard Model, elementary particles acquire mass through interaction with the Higgs field: the stronger the interaction, the larger the mass of the particle.

So far, physicists have collected conclusive evidence of the Higgs boson interacting with bosons and the heaviest elementary fermions belonging to the third fermion generation tau-lepton as well as top and bottom quarks.

Yet to date, there is no indication whether the Higgs boson interacts with the next lighter fermions, muon or charm quark, belonging to the second fermion generation. Despite a simple experimental signature, spotting this rare decay continues to be a challenge. This is due to the low probability of the Higgs boson decaying to muons predicted to be just 0. Only 0. Fortunately, a signal can be distinguished from background processes by looking at the shape of the mass distribution of the precisely measured muon pairs.

Higgs-boson events will cluster around the Higgs-boson mass of GeV, producing a narrow peak that can be distinguished from the smoothly-falling distribution of background events. By fitting the invariant-mass spectrum, ATLAS physicists are able to directly constrain the background and extract a possible signal.

Inside these categories, events were further split using dedicated multivariate discriminants Boosted Decision Trees. As a result of this complex division, ATLAS physicists could separate out the few Higgs-boson-like events from the more common, but less Higgs-boson-like, events.

In addition, ATLAS physicists developed a robust and ambitious background-modelling strategy using a variety of simulation techniques to create more than 10 billion simulated events.

Detailed ATLAS detector simulations totalling about five times the Run-2 dataset were complemented by dedicated fast simulation samples more than times the dataset. The fast simulation samples were crucial to ensure that the overwhelming backgrounds could not mimic a false signal, while maximising the analysis sensitivity to a real signal. The new ATLAS result gives a first hint of the Higgs boson decaying to a muon pair; the significance of the observed signal amounts to 2.

The data, together with the signal-plus-background fit, are shown in Figure 2, where data events are weighted to reflect the signal-to-background ratio of their respective categories. More data to be collected in Run 3 — and during the operation of the High-Luminosity LHC will help close in on this first hint. Physics Briefing. Tags: Higgs bosonHiggs groupPhysics Results.

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The wheat from the chaff Despite a simple experimental signature, spotting this rare decay continues to be a challenge. Events are weighted according to the expected signal-to-background ratio of their category.

multivariate analysis in the search for the higgs boson

In the top panel, the signal-plus-background fit is visible in blue, while in the lower panel the fitted signal in red is compared to the difference between the data and the background model. Let the die be cast The new ATLAS result gives a first hint of the Higgs boson decaying to a muon pair; the significance of the observed signal amounts to 2. Physics Briefing - 7 Oct Leptons at a distance: a new search for long-lived particles.

Physics Briefing - 25 Aug Z bosons zoom through quark—gluon plasma as jets quench.HEP Experiments.

multivariate analysis in the search for the higgs boson

Learn more. Jung Chang Chonnam Natl.

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Tsing Hua U. Jae Sik Lee Chonnam Natl. Jubin Park Chonnam Natl. Published in: Phys. D 1 DOI: Citations per year 0 2. Abstract: APS. Note: 18 pages, 6 figures, 4 tables; improved results presented.

Phenomenological aspects of field theory, general methods CERN LHC Coll: upgrade background top mass dependence Higgs particle: production Monte Carlo higher-order: 1 data analysis method numerical methods. References Figures Georges Aad Freiburg U. B Serguei Chatrchyan Yerevan Phys. Po-Yan Tseng Taiwan, Natl. JHEP 05 Higgs precision analysis updates Kingman Cheung Taiwan, Natl. D 90 Central U. JHEP 09 Dicus Texas U. Chung Kao Texas U. Scott S. Willenbrock Wisconsin U.

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Pair production of neutral Higgs particles in gluon-gluon collisions T. Plehn DESY.Toss in rookie Swiss Army Knife Christian McCaffery and a stout front seven, and there should be a significant level of interest.

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No document with DOI "10.1.1.870.201"

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