Proton-Proton Physics with ATLAS Detector
Standard Model is a physics theory that governs the subatomic world of fundamental constituents of the Universe and their interactions and has proved to describe most of the experimental results over the last 40 years. However, the current state of knowledge indicates that some processes from beyond the SM should occur at very high energies, although their expected rates are low and easy to miss in the overwhelming spectrum of phenomena that are currently experimentally accessible.
To verify the Standard Model predictions and to search for a New Physics the Large Hadron Collider (LHC) at CERN, the largest experimental device ever, was constructed. It consists of a 27-km long underground tunnel which contains a ring of superconducting magnets with a number of accelerating structures. Its main goal is to accelerate proton and heavy ion beams to unprecedented energies (6.5 TeV in case of protons) and to collide them in four designated points, instrumented with devices measuring basic properties of outgoing particles. Data collected by these detectors (one of them is the ATLAS detector) give a unique opportunity to search for processes occurring with very low probabilities.
The Higgs boson postulated by the Standard Model was discovered in 2012 by the ATLAS and CMS experiments. However, theories that go beyond the SM typically require an extended Higgs sector implying existence of additional scalar Higgs-like bosons. Within our ATLAS group we search for those additional Higgs bosons which can be electrically neutral or charged.
The searches performed by our group concentrate on New Physics processes with heavy fermions (t-quarks, b-quarks and tau leptons) in the final states. In many models of non-SM physics the decay chains of New Physics particles contain one or more of them and they are predicted to be produced frequently enough to be detected by the ATLAS experiment. In particular, searches for a heavy neutral and a charged Higgs bosons will be performed with a pair of tau leptons or a t-quark and tau lepton in the final state, respectively.
Heavy, electrically neutral Higgs boson can also decay into two b-quarks. An additional b-quark is expected in the process, coming from the so-called associated production. The quarks cannot be directly observed in the detector, but rather jets originating from them are recorded. The jets resulting from b-quark hadronization have certain properties (such as displaced secondary vertex) that could allow to distinguish them from other, so-called light jets. The number of events similar to searched process, but resulting from other phenomena, the so-called background, will be enormous and many sophisticated techniques will have to be employed to efficiently reduce the background and increase sensitivity for finding the possible signal process.
Another interesting New Physics process, which is search by our group, is the production of pairs of SM Higgs bosons decaying into two W-bosons and two tau leptons. Such events may suggest the existence of an exotic object, such as graviton, which would decay into Higgs boson pair and enhance the production rate over the very small rate predicted by SM.
Finding an evidence for an extended Higgs sector would make an outstanding discovery. On the other hand, excluding ever larger areas of parameter space available to BSM scenarios is of paramount importance. In either case, the boundaries of our knowledge are extended.
The ATLAS experiment plans an upgrade which encompass a number of detector, trigger, software and computing developments that will be required to continue the exploitation of ATLAS throughout and beyond the next decade. The entire new Inner Detector will be built. Upgrades are required to cope with the anticipated increase in the beam luminosity bringing an increase to 10x1034 cm-2 s-1 (10 times design luminosity) after 2026. The larger luminosity will allow to perform precise measurements of the Higgs boson and other Standard Model processes and to continue searching for new physics beyond the Standard Model. Our group is involved in the ATLAS detector upgrade, both in the design of the new Inner Detector and the new tracking software.