Observation of ttH production by ATLAS
After the discovery of the Higgs boson in 2012, a major new step has just been taken in the understanding of the mechanism that confers mass to the elementary particles constituting matter. The CPPM ATLAS team played a major and pioneering role in this outcome.
The ATLAS collaboration, which operates one of the giant detectors scanning proton collisions produced by CERN's Large Hadron Collider (LHC) in Geneva, announced on Monday 4 June 2018 at the LHCP conference in Bologna that it had observed the simultaneous production of the Higgs boson with two top quarks, colloquially called "ttH production" by experts. "This is a fundamental step in the exploration of the Higgs mechanism," comments Karl Jakobs, the spokesman for the 3,000 or so physicists worldwide who form the ATLAS collaboration. This result firmly establishes that the Higgs boson interacts directly with the quarks, which make up the protons and neutrons of matter. Moreover, the quark top being the most massive elementary particle, this measurement is a precise probe of the nature of this interaction.
"This research is one of the most difficult ever conducted in the experiment," says Fabio Cerutti, who coordinates all studies on the Higgs boson in ATLAS. Indeed, this result, corroborated by CMS, the LHC's other great experience, is a feat in more ways than one. First of all because the phenomenon is extremely rare: at the current energy of the LHC, it takes more than 150 billion proton collisions to produce a single event of this type! In addition, as the Higgs boson disintegrated in multiple forms, a wide range of different and innovative analyses were put in place to make maximum use of the events produced, and the results were combined by advanced statistical methods. This variety of disintegrations, combined with the fact that the top quarks themselves disintegrate into many particles, leads to complex signatures in the detector, simultaneously comprising a large number of objects of different nature that must be measured accurately.
The result announced today is the result of a tiered analysis strategy led by numerous collaboration teams, leveraging data collected between 2015 and 2017 at an energy of 13 TeV. At first, it was the disintegration of the Higgs boson into two bottom quarks that was sought. This is indeed the most likely disintegration, but it is very difficult to observe in the detector. With the accumulation of more data, it became possible to search for Higgs boson disintegrations in particular a pair of W bosons, rarer mode but producing leptons, less ambiguous to identify in the detector. Finally, more recently, even rarer but experimentally very clear modes of disintegration, which allowed the discovery of the Higgs boson when it was produced alone, have also been combined. This is notably the case of the disintegration of the Higgs boson into two photons which occurs once for 500 disintegrations. The combination of all these measures led to the observation of ttH production with a statistical sensitivity above the threshold of 5 standard deviations traditionally required for a discovery. This threshold means that if ttH production did not actually exist, the experiment had at most about one chance in 3.5 million to observe it nevertheless by mistake. With future data collected, the coupling between the Higgs boson and the quark top will be measured more precisely, which could lead to new discoveries.
The CPPM ATLAS team has played a major role in this long-standing study, training some 15 PhD students on this subject. In the early 2000s, the group conducted the first detailed studies on the feasibility of research in the channel with two bottom quarks, and proposed the channel with two W bosons and introduced for this purpose lepton signatures (two leptons of the same sign and three leptons). Since 2015, ttH production research has been one of the group's two main themes, and a group physicist coordinated this activity in ATLAS in 2015-2016. For the first mode with two bottom quarks, the team used its widely recognized expertise in the experimental identification of these quarks, built on the exploitation of very precise information from the pixel detector, partially designed and built in the laboratory. To this end, the group has implemented identification methods based on automatic learning techniques, and has also developed similar techniques for the analysis itself to discriminate the signal from background noise. Similarly, relying on local expertise for electron identification, linked to the design and construction of part of the electromagnetic calorimeter by the laboratory, the team also played a leading role in lepton channel analysis, including electron identification and signal discrimination by automatic learning techniques. For these two stages of analysis, the team had a key role in the final statistical adjustment. Two members of the group finally had editorial responsibility for the collaboration of the two publications related to these channels, published in April in Physical Review D.
- https://arxiv.org/abs/1712.08895, Phys. Rev D 97 (2018) 072016
- https://arxiv.org/abs/1712.08891, Phys. Rev D 97 (2018) 072003
Contact : Laurent VacavantDernière modification: Sep 3, 2018, 10:08:46 PM
Have we identified the first source of high energy astronomic neutrinos ?
On September 22, 2017, IceCube detected a neutrino with an energy of 290 TeV whose arrival direction corresponds to that of a known gamma blazar, TXS 0506+056, observed in an eruptive state. A blazar is an active galaxy nucleus housing a super-massive black hole in the center accompanied by jets of high-energy particles pointing towards the Earth. Large multi-wavelength observation campaigns, covering the entire electromagnetic spectrum, were quickly triggered by the detection of the neutrino.
In addition, IceCube also detected an excess of high-energy neutrino events compared to atmospheric background noise in the direction of the blazar TXS 0506+056 between September 2014 and March 2015, which constitutes evidence of 3.5 sigma (99.98% probability of signal observation) for the emission of neutrinos, before the eruptive episode of this blazar in 2017.
ANTARES performed a similar analysis for high energy neutrinos from this source. A slight excess is also observed in the data taken between 2007 and 2017.
These observations suggest that blazars are the first identifiable sources of high energy astrophysical neutrino flux. These results were publicly announced by the IceCube Collaboration at a press conference on July 12th.
PhD prize for Thomas Calvet
Thomas Calvet has just been laureate of the 2017 Aix-Marseille University PhD Prize, after having been awarded the thesis prize of the doctoral school.
This award recognizes the excellence of his research work on the associated production of the Higgs boson with a pair of top quarks in the channel where the Higgs boson decays into two bottom quarks, called the ttH (bb) channel, and the b quark jetting with the ATLAS experiment at the Large LHC Hadron Collider. The result is part of the June 4, 2018 announcement of the observation of the production of the Higgs boson with a pair of top quarks at the LHC.
This prize will be officially awarded by the end of the year by the University.