2014: New results of Higgs boson study in the ATLAS experiment (CERN)

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Physicists from Lomonosov Moscow State University (the group headed by Prof. Lidia Smirnova) in ATLAS collaboration (CERN) received new research results on Higgs boson in ATLAS experiment.

The Nobel Prize in Physics 2013 was awarded jointly to Francois Englert and Peter W. Higgs following the discovery in summer 2012 of new scalar particle in ATLAS and CMS experiments at the Large Hadron Collider in CERN. During 2012, the effective operation with the registration of proton-proton collisions at the centre mass energy 8 TeV, the largest world achievement, was carried on by these experiments. At the end of collider run the amount of detected proton collisions was in approximately 2.5 times larger than at a moment of the new particle discovery announcing. Results of the full data set analysis for the first run of Large Hadron Collider operation, completed at the beginning of 2013, confirm that the properties of discovered particle with mass 125 GeV corresponds to Standard Model Higgs boson [Phys. Lett. B 716, 1-29 (2012); 726, 120-144 (2013); 726, 88-119 (2013)]. The main arguments for this statement are the determination of spin and parity of the particle (JP = 0+), the correspondence of measured production cross sections and branching ratios for different decay channel to Standard Model predictions. The elaborate analysis allowed even to define the roles of different production mechanisms of new particle. It is shown, that in addition to gluon fusion as the main production mechanism of Higgs boson, the mechanism of vector boson (W, Z) fusion contributes also. The last contribution is at the level of 7% of the total Higgs boson production cross section at 8 TeV, which is equal to 22 pb. The observation of the new particle in different decay channels gives the possibility to extract the estimations of Higgs boson couplings with gauged bosons and fermions. It is shown that the ratio of couplings with W and Z bosons is equal to unity, as expected in Standard Model. Non-zero coupling with fermions is observed mainly from measurements of Higgs boson decays to tau-leptons.

The study, established the correspondence of a discovered particle with mass 125 GeV to Higgs boson, responsible for breaking of electroweak symmetry in the Standard Model, is actively continuing with existing experimental data. A question is whether this state is in fact the Higgs boson of the Standard Model (SM), or part of an extended Higgs sector (such as that of the minimal super-symmetric Standard Model, MSSM), a composite Higgs boson, or a completely different particle with the Higgs-like couplings. New results of such study are published by ATLAS experiment in February this year [Phys. Rev. D 89, 032002 (2014)]. This paper reports the first search at the Large Hadron Collider data for the new heavier states of Higgs boson, decaying by the cascade with observed particle with mass of 125 GeV as a final state. Many beyond-the-SM models introduce a heavy neutral Higgs partner H0 and additional Higgs doublet with non-zero electric charge, H±. From the experimental data one can conclude that there are no events in addition to SM predictions. But, the obtained limits do not exclude completely existing of heavy partners of h0 particle.

New result is obtained in ATLAS experiment in search of resonance in Zγ system, H → Zγ [Phys. Lett. B 732, 8-27 (2014)]. Diagrams of such decay are similar to diagrams for decay H → γγ, which is reliably registered in an experiment, and the expected event number in SM for such decay is well known. The resonance excess of events is possible to observe if H particle is a new neutral scalar of a different origin or a composite state. The observed limits for resonance signals in mass region mH from 120 to 150 GeV are at the level from 3.5 to 18 times larger than SM predictions (Fig.3). The expected limit for resonance signals assuming the existence of a SM Higgs with mH = 125.5 GeV is 10 times SM prediction. The results are dominated by statistical uncertainties, so increase of data is necessary.

The research group of Prof. Lidia Smirnova (Facult of Physics, MSU) that participates in the ATLAS collaboration (CERN), includes also diploma and graduate students from the Faculty of Physics who develop methods for reconstruction and identification of the electrons and muons. The results of the above studies have been published in Phys. Rev. D 89, 032002 (2014) and in Phys. Lett. B 732, 8-27 (2014).