Searching for the Triplet Higgs at the LHC

The type II seesaw model is a scientific theory that tries to explain why Neutrinos, which are very small particles, have mass. It also attempts to clarify why there is more matter than antimatter in our universe and how our universe started expanding after the Big Bang. The Large Hadron Collider (LHC), a powerful particle accelerator, offers scientists a chance to test this theory by searching for a particular kind of particle called the triplet Higgs.

In this study, we look at how we can find the triplet Higgs at the LHC by studying events where multiple particles called leptons are produced. Our analysis shows that we can improve the chances of detecting these particles thanks to certain processes that create them. Particularly, we focus on how the future LHC, which will have increased brightness, could find a triplet Higgs particle that weighs about 1.2 trillion electron volts (TeV).

One of the big questions in physics is where neutrinos get their mass. In the standard model of particle physics, neutrinos have no mass at all, but experiments have shown that they do have very small masses. This issue has prompted scientists to look for new models that can extend the standard model. One popular idea is the seesaw mechanism, which comes in three types. The type I and III seesaws add additional particles, while the type II seesaw adds a different particle called a triplet scalar.

In the type II seesaw model, the triplet Higgs particle can interact directly with leptons. If this Higgs gets a certain kind of value (known as vacuum expectation value), it can create a mass for neutrinos. This model is interesting because it not only suggests a way to give mass to neutrinos but also proposes a way to explain the imbalance between matter and antimatter. Moreover, if this triplet Higgs also serves a role in the early expansion of the universe (inflation), it presents a simpler solution to multiple significant problems in physics.

Previous models that explain the production of matter after the Big Bang (leptogenesis) often require special particles to have very high mass. The type II seesaw, however, allows the triplet Higgs to be much lighter, making it more likely to be found in experiments at the LHC. Current searches have already set mass limits for the Doubly Charged Higgs, related to the triplet Higgs, at a few hundred GeV depending on how it decays.

The way the doubly charged Higgs decays depends on how much value the triplet Higgs has. If the triplet Higgs has a large value, it will mostly decay into two other particles called gauge bosons. On the other hand, if its value is lower, it will mainly decay into pairs of leptons. For our leptogenesis model to work, we prefer values on the order of 1 keV to avoid interfering with the production of leptons. This makes searching for the triplet Higgs through leptonic decay channels a clear option.

We conduct a detailed look at how we might find the triplet Higgs in future experiments at the LHC. There have been many previous studies on this topic. We find that the type II seesaw model can introduce additional particles that lead to more lepton production events compared to the standard model.

Inside the type II seesaw model, the scalar sector consists of both the regular Higgs and the triplet scalar field. Once the regular Higgs field is "turned on," it creates various additional scalar particles. These include several charged Higgs particles and different neutral Higgs particles. The charged Higgs and doubly charged Higgs can be paired together in specific interactions, which gives us a pathway to look for these new particles.

The ATLAS collaboration at the LHC has already begun searching for the doubly charged Higgs, particularly focusing on its decay into pairs of leptons. They have found that the mass of doubly charged Higgs particles can go up to about 800 GeV. The search strategies are usually grouped into three types of lepton channels: four-lepton, three-lepton, and two-lepton channels. Each of these channels has its own level of sensitivity to the signals produced during collisions.

As the triplet Higgs can also produce charged Higgs particles, we analyze how this could enhance the search. The charged Higgs decays into a lepton and a neutrino, creating a missing energy signal that could be useful in searches.

Our study includes simulations of how these particles would behave in different types of collisions. We incorporate various methodologies, including adding in the triplet Higgs model and adjusting for potential backgrounds to the events. Our simulations help verify our methods and allow us to determine the effectiveness of different search strategies.

In our analysis, we require that the events we look at contain at least one lepton pair with the same charge. We also impose certain restrictions on the total energy and momentum of the particles involved. This helps us filter out background noise caused by other processes, ensuring that we focus more on the events we want to detect.

When we combine our results from various search channels, we can derive limits on how massive the triplet Higgs can be. The first results show that our combined search strategies can yield tighter limits on the mass of the triplet Higgs compared to previous studies.

Our simulations indicate that we should look for missing energy signals in some types of collisions. The background processes are mostly from other particle interactions, but by focusing on missing energy, we can better isolate the events we think are caused by the triplet Higgs.

In the future, as the LHC becomes more advanced, we anticipate that searches for the triplet Higgs could reach masses as high as 1.2 TeV. Nevertheless, our analysis also reveals that for masses lower than 800 GeV, searches through multiple leptons may still yield better sensitivity.

In conclusion, the type II seesaw model presents an exciting avenue for understanding neutrino masses and the makeup of our universe. The LHC's potential to verify the existence of the triplet Higgs could lead to significant advancements in particle physics. By refining our search strategies and improving our understanding of how these particles behave, we take crucial steps closer to solving some of the most profound mysteries in physics.