ATLAS' scientific exploration uses precision measurement to push the frontiers of knowledge by seeking answers to fundamental questions such as: What are the basic building blocks of matter? What are the fundamental forces of nature? Could there be a greater underlying symmetry to our universe?

In addition, we now know most of the energy content of our universe is unknown to us, and not within the purview of what is an established theory the “Standard Model” (SM). An unknown form of energy called Dark Energy (DE) and an unknown form of matter beyond our knowledge of ordinary matter called Dark Matter (DM) consist of the ~96% of the energy content of the universe. We are also searching for these, in particular, the DM. Generally Supersymmetric theory (SUSY) has been popular for solving some of the mysteries; these are also hunted at the LHC experiments.

atlas detector without upgrade
ATLAS detector

Detector Work

Professor Mallik and her group joined the ATLAS experiment with SLAC at the end of 2006. She and her group were involved in the Silicon Pixel detector (PIX), with a big part of the group residents at CERN. They were responsible for the calibration of the PIX (in DSP), fully involved in the commissioning of the project and cosmic ray data taking and analysis, as well as prepare the databases. They then participated in the liquid Argon Calorimeter (LAr), in Phase 1 trigger upgrade and LAr operation leadership. The group had a central role in the High Granularity Timing Detector (HGTD), as part of the LAr system for the Hi-Lumi upgrade when the luminosity is expected to be 0.75´1035 cm-2 s-1, and typical pile-up will reach 140, with a maximum of 200. The HGTD is a state-of-the-art Si detector with ~30 psec timing resolution. This will add an extra dimension in discrimination with the high pile-up rate and thus make effective use of the high event rate.

sim event Lol ITk layout

Now they have gotten involved in the ITk silicon-strips detector: in the production and test of them at Brookhaven National Laboratory (BNL) and at CERN. BNL is the center for the U.S. contingent of the ATALS experiment, and also the headquarter for the ATLAS ITk-strip project for the barrel part of the detector. Two graduate students spent time at BNL working on the project, helping the experts and learning the state-of-the-art technologies, both soft and hardware, hands-on. This is beneficial to both the project and the students. They take responsibility for independent projects needed for developing quality testing and streamlining of various types of tests as they are produced or arrive at BNL where they are assembled. The group members at CERN will participate in the tests when these loaded “staves” arrive there from various continents before the final assembly of the full ITk-strips sub-detector. 

inner tracker upgrade diagram
inner tracker upgrade diagram

Physics interest and Analyses

The group's physics interests lie in several direction all aimed at understanding what physics lies beyond the SM (BSM), what gave rise to it?

One possible way is to understand the Higgs boson properties very well, after all this is our window into the beyond. The group spearheaded the measurement of the Higgs decay into a b-quark pair, in order to test the Yukawa coupling (Higgs coupling to fermions). They have now established (5s) the H --> b-bbar decay in  V+H (V=W/Z) production.

They continue to improve this area, now dividing up into many finer measurements, like the Simplified template cross section (STXS) to lessen dependence on modeling and high pT Higgs bosons where any deviation from SM is likely to show up.

In another path to BSM physics, they are engaged in search for DM, in mono-Higgs channel as part of the Higgs portal models. They are also involved in combining several DM channels to extend the established boundaries. They already published a join DE and DM paper, the DE part based on a specific DE theoretical model, the first of the kind in a colliding beam experiment. Taking advantage the increased Run2 energy of LHC (13 TeV), they searched for any VH resonance and set limits according to the 2HDM and HVT models. 

They had started some precision measurement of the SM, like measuring Weak mixing angle. In other SM and beyond searches they are looking for Drell-Yan studies, especially at high masses in two-lepton final states.
They have now launched an investigation for any high mass Higgs-like particle following a 2HDMa model where a pseudoscalar heavy boson (A) and a heavy Higgs boson (>125 GeV) is searched for.

Running the ATLAS experiment and data taking

The group has taken leadership roles in several areas of data taking, lately, it has been in the Liquid Argon calorimeter (LAr), as a run-coordinator for past several years. They also maintain the muon quality performance related to all Higgs analysis studies.