# Krzysztof Sliwa: research activities

email : Krzysztof Sliwa

conference presentations and invited talks : Krzysztof Sliwa's talks and presentations

# Tufts ATLAS Group

The Tufts University Elementary Particles Group joined ATLAS in 1994. Together with Boston University, Brandeis University, Harvard University and Massachusetts Institute of Technology we decided to work as a single group, the Boston Muon Consortium (BMC), on the ATLAS muon system. After deciding to focus on the ATLAS end-cap muon system, the Tufts Group was very active in initial physics simulations and in the construction and testing of the first MDT end-cap muon chamber, with Prof. Mann leading the latter effort. The Tufts Group's high precision computer-controlled machine shop served as a R&D lab for BMC, and later manufactured thousands of small, very precise and custom-designed pieces needed for the MDT chambers and for the entire muon alignment system. At the same time, the Tufts ATLAS Group was very active in studying and planning the world-wide data analysis and reconstruction, needed for the world-wide collaboration like ATLAS. MONARC, a very successful LHC project created jointly by Tufts and Caltech, provided the first realistic modelling of such a world-wide, distributed computer system.

The Tufts ATLAS Group currently consists of two faculty - Pierre-Hugues Beauchemin, who joined the group in 2011, and Krzysztof Sliwa; a research-associate - Vincent Croft, two graduate students - Alec Drobac and Colette Kaya, and a part-time consultant - Benjamin Whitehouse, who received his Ph.D. from Tufts in 2010. The Tufts Group performed the first analyses of the muon test-beam data, and continued to work on the muon reconstruction software. The Tufts ATLAS Group developed the MuonTrackingGeometry, a completely new description of the entire muon system, including the active and the passive elements, which allows the use of ATLAS common tracking tools in muon reconstruction. The MuonTrackingGeometry is also an essential component of FatRas, a new, fast ATLAS track simulation which models the detector response at the hit level. Tufts ATLAS Group was responsible for simulation of the entire muon system in FatRas, which became part of the Integrated Simulation Framework, a new fast simulation of the entire ATLAS detector.

In physics studies, The Tufts ATLAS Group is interested in W+jets and Z+jets analyses (prof. Beauchemin), top physics, Higgs boson studies (prof. Sliwa), and physics beyond the Standard Model - non-standard Higgs searches, SUSY, extra dimensions et cetera. After discovering the Higgs boson in 2012, the main objective of the LHC analyses is to find out whether the new particle is the Minimal Standard Model Higgs, or some other kind. It would be really exciting if the latter were true. There is also a possibility that, with increased energy of the proton-proton collisions, completely new particles will be found. Here, studies of top quarks are extremely important, as top quarks will constitute the most important background for almost any final states due to “new physics” and have to be understood very well.

The Tufts ATLAS Group has developed a new multidimensional and multiclass analysis technique - an event classifier based on Support Vector Machines (SVM). The method, originally developed by Dr. Ben Whitehouse when working towards his Ph.D. with Prof. Sliwa, allows to take into account simultaneously a large number of physics observables, including correlations between them. Although SVM were used in other research fields in the past, Tufts was the first group to apply it in collider physics. In contrast with other multivariate methods, SVM has a sound mathematical foundation - Mercer Theorem. Prof. Sliwa, together with Dr Whitehouse applied their new analysis technique in measurements of the production rate of top quarks in proton-proton collisions at 7 TeV and 8 TeV at LHC, and their ratio. Understanding top quark production is interesting on its own, but it is also crucial in studies of the Higgs boson in a number of difficult to analyse final states involving leptons, jets and missing transverse energy. Top quark is the main background in Higgs bosons production via a WW-fusion mechanism, and in many Higgs decay modes.

I have decided not to take part in Run 3 at the LHC, and my involvement with the ATLAS Collaboration and the LHC Collider program will not extend beyond Fall 2021.

faculty: Pierre-Hugues Beauchemin, Krzysztof Sliwa

past research associates: Dr Marcin Wolter, Dr Simona Rolli, Dr Sarka Todorova, Dr Federico Sforza

current research associates: Dr Vincent Croft

past graduate students: Samuel Hamilton (Ph.D. 2014, with Prof. Sliwa), Jeffrey Wetter (Ph.D. 2015, with Prof. Sliwa), Hyungsuk Son (Ph.D. 2019, Prof. Beauchemin)

current graduate students: Alec Drobac, Colette Kaya (with Prof. Beauchemin)

Prof. Sliwa won Tufts Faculty Research Awards in 2012 and 2013, and Prof. Beauchemin in 2014. They also jointly won in 2014 the Tufts International Research Award, which allowed both faculty and four students to spend 6 weeks at CERN in the summer of 2014.

# other activities

I am also interested in topology, differential geometry and other areas of modern mathematics, which I studied to gain a deeper insight into the meaning of quantum gauge theories, the origin of mass and the structure of space-time, matter and all interactions, including gravity.

Recently, I embarked on studies of geometry and topology of the Universe. In the Standard Cosmological Model (SCM), the starting point is an interpretation of the observed increase of redshifts with the distance of far-away objects as a "Doppler" effect due to expansion of the Universe. Acceptance of this hypothesis led to the ideas of Big Bang and the Standard Model of Cosmology. Universe locally looks flat, M_o=R1 x R3 - the flat Minkowski "world". However, interestingly, there is another possibility. As shown by Irving Ezra Segal, a mathematician and a mathematical physicist, the same axioms of natural physical symmetries - global isotropy and homogeneity of space and time, and causality properties - are satisfied not only by a flat Minkowski spacetime, but also in a model in which space is not flat. In Segal's model, the geometry of the Universe is M=R1 x S3, with the spacial part being a surface of a 4-dim (4D) sphere. Locally, it is indistinguishable from a flat Minkowski spacetime M_o. It is the geometry of Einstein static Universe, which he abandoned when the interpretation of the increase of redshift with distance was universally accepted as evidence for expanding Universe. The redshift in Segal's model arises in a natural way as a consequence of a distortion analogous to distortions which appear when making maps using stereographic projection from a curved surface of a sphere in 3D, S2, onto a flat surface, R2. Segal's model provides a verifiable prediction for the dependence of this geometric redshift on the propagation time, or geodesic distance on S3. In 2017, I've decided to take a closer look at the newest redshift catalogues available online and compare the data with predictions of Segal's cosmology and SCM. Surprisingly, although the data on supernovae prefer SCM, quasars agree better with Segal's model. Also, the number of observed galaxies as a function of redshift is also in good agreement with Segal's model, at least as good if not better than with SCM. This puzzling result is, of course, preliminary. The comparison, although in principle very simple, is highly non-trivial. For distant objects, one can only use proxies for the distance, it cannot be measured directly. We are now carefully analyzing the updated redshift data again. I was working on this fascinating study of the shape of the Universe in 2019-2020 with Maxwell Kaye, an undergraduate student at Tufts, who was majoring in mathematics, computer science and physics. The topic became the subject of his Senior Thesis, for which he earned Highest Honors. I am continuing with these studies with Eoghan Downey, a senior at Tufts, and with Max Kaye.\

I am also a member of the MoEDAL Collaboration at CERN, a small, dedicated, experiment looking for magnetic monopoles. All searches so far were negative.