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Andrei Gritsan

Overview of Research Activities

Prof. Gritsan's research is focussed on the observation and study of the new form of matter-energy, such as a Higgs boson. Together with his colleagues on the CMS experiment on the Large Hadron Collider, he worked on the Higgs boson discovery [1], study of its spin-parity quantum numbers [2, 3], measurement of its production mechanism and deeper understanding of its properties [4], constraints on the Higgs boson width [5, 6] and lifetime [7], comprehensive study of the Higgs boson quantum numbers and anomalous interactions [8, 9], and taking a number of these measurements to a new level with Run II data from LHC [10, 11, 12]. He worked with a group of experts to develop new methods (also known as MELA technique and JHU generator) [13, 14, 15, 16] for the angular and statistical analysis of the decay products and associated particles of the Higgs boson. Read more about this effort.

All the above measurements point to the property of vacuum, which is filled with the all-penetrating Higgs field, where the boson is simply its excitation created in the laboratory (see also Nobel Prize Physics-2013). Our past, present, and future depend on the properties of this field, and we are still to understand all the implications of this grand discovery and to study in detail this new form of matter-energy never known before. However, it is likely that the discovered Higgs boson is just a tip of an iceberg of new states of matter-energy. The undiscovered symmetries of nature which unify the fundamental forces and particles, the puzzle of dark matter and energy, and the apparent lack of antimatter in our Universe, all these mysteries point to something new that could be uncovered at an unprecedented energy scale with the Large Hadron Collider. Prof. Gritsan's team also pursues both direct searches [17, 18, 19] for such new states and indirect constraints through precision measurements of the known states, such as the Higgs boson [1-16] and Z boson [20, 21, 22].

Prior to LHC, direct access to new fundamental particles (such as the Higgs boson or new states) was beyond the energy reach of operating accelerators. Gritsan worked with the heavy flavor quarks, such as the "beauty" or b quark, which were produced in electron-positron annihilations at the BABAR and CLEO experiments. He was looking for new ways to search for new fundamental particles that could exist briefly as heavy virtual states in the decays of b quarks. The Heisenberg uncertainty principle in quantum physics allows such non-trivial effects, called "penguin" loops, to occur for short instants. On CLEO, he discovered [23, 24] this process as part of his Ph.D. research. It was the first observation of the gluonic penguin transition b->s+gluon. On the BABAR experiment, he discovered [25] a surprisingly large transverse polarization of the vector mesons produced in a penguin decay, which contradicted all expectations and may become evidence for new particles and interactions, and took this approach of angular analysis to a new level [26, 27, 28, 29].

Another important aspect of heavy quark decays is that they include the only known example of Charge-Parity (CP) reversal symmetry violation, which is equivalent to Time-reversal symmetry violation and which follows from the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing model (see also Nobel Prize Physics-2008). CP violation is necessary to produce our matter-dominated Universe and has important cosmological consequences. The Standard Model mechanism that leads to CP violation is represented graphically by the so-called "Unitarity Triangle." On the BABAR experiment, Gritsan discovered the B decays to two rho mesons, showing that an analysis of this system could determine one of the angles alpha of the "Triangle" precisely by constraining penguin effects [30, 31, 32].

Prof. Gritsan is also an expert on various aspects of high-energy physics detectors, such as silicon tracking detectors, electromagnetic calorimeters, tracking drift chambers, triggers. He had been leading the tracking and silicon detector alignment groups at the BARAR and CMS experiments [33, 34]. An essential element of the LHC program is the alignment of thousands of silicon detectors that track the particles' paths which must be understood to micron precision. Prof. Gritsan led the CMS team to successful commissioning of its silicon tracker alignment at the time of data-taking startup [35].

Further references may be found in [36].