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Understanding the Origin of Mass with the ATLAS experiment at the Large Hadron Collider

Final Report Summary - HIGGS-ZAP (Understanding the Origin of Mass with the ATLAS experiment at the Large Hadron Collider)

Higgs-ZAP is a research programme that explored the origin of mass for fundamental particles through searching for the Higgs particle in the early data from the ATLAS experiment at CERN's Large Hadron Collider (LHC). We prepared analysis tools and procedures necessary for the discovery of a light Higgs through its associated production with a highly energetic Z boson and its subsequent decay into two b-jets (a jet is a collimated splash of particles going through the detector).

The short-lived Higgs particle decays in a number of different sets of final particles (decay “channels”) depending on its mass, which physicists detect and infer the properties of the initial Higgs particle. For a light Higgs of about 120 GeV, as it was discovered in the summer of 2012 to be, identifying its decay products from the overwhelming background of already known processes is extremely challenging. In this regime, it is of crucial importance to exploit all possible decay channels.

Higgs-ZAP's contribution to this challenging search was preparing the studies of the cases where the Higgs was produced in association with a Z boson and subsequently decayed into two b-jets. Z bosons are well-understood particles with a clear decay signature of two energetic leptons (electrons or muons), thus offering a very useful way to pick Higgs-Z events from the background. The Higgs particles were required to decay into two b-jets, which themselves give another characteristic signature to search for: B-hadrons contained in the b-jets are relatively long-lived particles, and they travel a measurable length before they decay. Thus the misplacement of their decay vertex with respect to the interaction point is a way to identify such special jets, usually referred to as “b-tagging”. What was the innovative element in Higgs-ZAP's approach, though, was the requirement that the Higgs and the associated Z be very energetic. This requirement forces the particles in the jets to be more collimated and closer to each other, causing two or more jets to merge into single “fat” jets. This feature makes such events really stand out from the background, at the expense of all the potential Higgs events that do not satisfy this requirement, nearly 95% of them.

In order for the analysis to reach the mature level of searching for Higgs events, the relatively new ATLAS detector had to be understood and commissioned, and the data collected checked for their quality and optimized. The activities of the Higgs-ZAP project were concentrated in three areas: the performance study and development of on-line tracking algorithms, the measurement of known Standard Model (SM) processes in the new energy regime provided by the LHC data, and the study of b-tagging algorithms in the environment of highly energetic fat jets.

Most of the processes happening during the LHC collisions are well-understood SM processes that constitute overwhelming backgrounds to extremely rare event searches like the production of a Higgs particle. Therefore, a 3-tier online system (the “trigger”) is being used to filter these events in real time, so that the limited recording capacity of any computer farm, including CERN's, is not wasted on useless information. A crucial component of this trigger system for Higgs-ZAP's search is the algorithms that reconstruct particle tracks through ATLAS's inner tracker, the part of the detector closest to the interaction point. These are the tracks used to pick b-jets online, as well as forming part of the online lepton selection. With the start of the data taking in 2009, as well as in 2010, with higher collision energy and denser collisions, the performance of these algorithms was studied and optimised, leading to 2 external ATLAS publications, a poster and an oral presentation at international conferences by the project fellow. Building on the expertise gained in these studies, the fellow played a leading role in the re-structuring of these algorithms later in 2010 and 2011, which led to a much better framework for the study of these algorithms and the development of new ones. This framework has been in use for the 2012 data taking and has been instrumental in the online identification of the leptons consisting the final state of the channel where the Higgs was discovered in the summer of 2012, the H->4leptons.

In order to verify that the ATLAS detector is performing as expected, and the data collected by it are well understood, known SM processes involving the objects that are also used for discoveries have to be studied. Two such measurements were performed by Higgs-ZAP members: the total number of single and pairs of b-jets produced in LHC collisions, which is the most up-to-date measurement of this quantity into a new energy regime, and the first observation of highly energetic Z bosons decaying into two b-jets merged into a fat jet. The result of the former study is being used in subsequent studies performed by the ATLAS collaboration as a source of better understanding of the colliding protons. Measuring this quantity proves understanding of the basic jet reconstruction and b-tagging. The latter measurement exercises the whole machinery for observing an energetic Higgs decaying into two b-jets, and is a proof of principle that such an observation is possible. These results have been published in major peer-reviewed journals and have been presented at an international conference by the project fellow.

One of the main challenges in identifying highly energetic particles decaying into b-jets is quantifying the performance of the b-tagging algorithms in the dense environment of the fat jet resulting from the merging of two or more normal jets. Such algorithms depend on reliable reconstruction of particle tracks and their correct assignment to individual jets, which are both far from optimal inside fat jets. Higgs-ZAP performed the first systematic study to quantify such effects with real data, which showed a tolerable 5% increase in the uncertainty of any b-jet-dependent measurement caused by them. This result has been crucial input to all the searches for Higgs decaying into two b-jets, as well as the energetic Z decaying into two b-jets search, and has been documented in an internal ATLAS publication. The fellow also initiated an in-depth measurement of the b-jet related uncertainty in fat jets with a student from Edinburgh University, which will eventually become the reference for the whole collaboration.

Due to the timing of the project and the LHC data taking, an observation of the highly energetic Higgs-Z associated production and subsequent decay into two b-jets has not been possible. However, Higgs-ZAP has contributed a number of necessary tools and studies for a future observation with the next run data and, due to these contributions, the fellow has been included in all the internal and external publications of searches for Higgs decaying into two b-jets,. Observing a Higgs to b-jets decay in the coming years will be an important step towards understanding the nature of the Higgs particle and its interactions with other particles. To this end, the channel that Higgs-ZAP concentrated on and the advances it provided in its pursuit will be a crucial component of this understanding.