Jun 12 2020
New results for a physics process termed vector boson scattering were presented by the ATLAS and CMS collaborations at this year’s LHCP conference. In addition, CMS announced the first-ever observation of what is called the massive triboson synthesis.
(Left) CMS event display of a candidate event in which two W bosons and one Z boson are produced and decay into three electrons and a muon. (Right) ATLAS event display of a candidate event in which two Z bosons are produced, along with two jets. The Z bosons subsequently decay into two electrons and two muons. Image Credit: CERN.
It is crucial to investigate these processes to test the Standard Model as it could offer insights into new physics. The study outcomes were presented online at the virtual LHCP conference, which was originally due to be held in Paris.
Several particles, such as photons and W and Z bosons—the carriers of the electroweak force––are produced during proton collisions at the LHC. In the Standard Model, these bosons are simply named as vector bosons, and vector boson scattering is one of the processes that result in their pair production.
Vector boson processes serve as an ideal probe to look for deviation from theoretical predictions. Diboson production through vector boson scattering and triboson production are two rare processes that are of specific interest as they analyze the self-interactions of four-vector bosons.
It is crucial to observe and measure these processes as they explore the electroweak symmetry breaking mechanism, in which the unified electroweak force disintegrates into weak and electromagnetic forces in the Standard Model. These act complementary to the quantification of the production and decay of Higgs boson.
During a vector boson scattering process, a quark in each proton radiates a vector boson, where the vector bosons tend to scatter off each other to create a diboson final state. Instead, the production of tribosons refers to the synthesis of three massive vector bosons.
As part of the LHCP conference, physicists from the CMS and ATLAS collaborations reported new quests for the generation of a pair of Z bosons through electroweak production, which includes the vector boson scattering mechanism. This process was observed at 5.5 sigma by ATLAS and CMS confirmed strong evidence.
In addition, CMS announced the first-ever observation of a W boson generated together with a photon using the vector boson scattering process. Also reported were more accurate measurements of the same-sign WW production, as well as observation of the production of a W and a Z boson through vector boson scattering, where all these complement previous ATLAS observations.
One more technique to investigate four-boson interaction is the analysis of the extremely rare synthesis of three massive bosons or tribosons. In April 2020, a 5.7 sigma result of the triboson phenomenon was achieved by the CMS experiment, demonstrating it as a stable observation, after the first evidence of this process was observed last year in the ATLAS experiment.
A majority of the physics processes involving fundamental particles engage two or more individual particles interacting with one other through an intermediary particle emitted or absorbed during the process.
The more bosons produced, the rarer the event. This new observation of tribosons was very difficult because it is a much rarer process than the one that led to the Higgs boson discovery, and very interesting because it may reveal signs of new particles and anomalous interactions.
Roberto Carlin, Spokesperson, CMS Collaboration, CERN
During the vector boson and triboson scattering processes, Z and W can interact with themselves to produce more Z and W particles, thereby synthesizing two or three bosons. Since Z and W are highly unstable particles, they tend to rapidly decay into quarks or leptons (electrons, taus, muons, and their corresponding neutrinos).
However, these processes are very rare, and since the triboson and diboson events sought by physicists are simulated by background processes, they are highly challenging for physicists to study.
To separate signal from background, physicists have to be ingenious and employ advanced machine learning algorithms. This is a challenging task for such rare processes, and requires meticulous and thorough studies.
Karl Jakobs, Spokesperson, ATLAS Collaboration, CERN
The measurements of triboson production and vector boson scattering exhibited at LHCP 2020 are in agreement with the estimations of the Standard Model, which still offers the best insights into fundamental particles and their interactions. Moreover, the observations offer physicists tools to explore quartic self-interaction that occurs between massive electroweak bosons.
The existing measurements restrict the strength with which such quartic interactions occur. Higher precision achieved by using new datasets could pave the way for new physics at higher energy scales in the LHC and result in potential discoveries of new particles.