Research activities

Flavor Physics

(A. Bragagnolo, T. Dorigo, H. Yarar, E. Lusiani, M. Margoni, P. Ronchese, F. Simonetto, G. Strong)

Besides being a powerful handle to improve our knowledge of QCD and electroweak interactions, the study of the production and decay properties of hadrons with a b-quark provides a valuable tool in the search for New Physics, which would reveal itself in discrepancies between precision measurements of rare b decays and the standard model predictions. This approach, also known as the intensity frontier, is complementary to, and in some cases even more far-reaching than, the direct search of heavy new particles (energy frontier).
Thanks to excellent tracking and vertex reconstruction, and superb muon detectors, the omnipurpose CMS spectrometer competes on equal grounds with detectors designed for Flavor Physics, like LHCb and Belle2. Our group has a well recognized reputation in b-Physics, consolidated over years of research dating back to previous DELPHI and BABAR experiences. Previous results in CMS include:

  • the measurement of the pp → bbX → μ+ μ- X' inclusive cross section at √s = 8 TeV
  • the first evidence of the rare Bs → μ+ μ- decay
  • the measurement of the controversial P'5 parameter in the B0 → K* μ+ μ- decays
  • the study of CP violation in Bs → J/Ψ φ decays at 8 and 13 TeV collisions using a muon tag
We are now involved in the analysis of about 130 fb-1 data collected by CMS aimed at:
  • an improved measurement of CP violation in Bs → J/Ψ φ based on a wider data set and improved flavor tagging exploiting deep machine learning algorithms.
  • a search for the rare decay Bs → τ+ τ-. Although helicity enhanced with respect to the Bs → μ+ μ- decay, this process is more challenging because the τ lepton is instable, and very few of its final states can be profitably used for its identification. A very complex analysis is ongoing on the large CMS muon-parked dataset, which includes secondary vertex reconstruction and regression of the observed particle momenta (an isolated muon, or else three charged pions from a common vertex) to the τ mass, tackling a sizable background produced by several different sources.
Our long-term plans include:
  • a measurement of the pp → bbX → μ+ μ- X' and the pp → gg → (bb)(bb) → 4 μ X' cross sections at √s=13TeV
  • A measurement of the CP-induced dimuon asymmetry ACP = N(μ+ μ+ - μ- μ-) / N(μ+ μ+ + μ- μ-) using the aforementioned parked data. A sizable deviation from the standard model, claimed by the D0 collaboration, was not confirmed by indirect measurements at the B-factories and LHCb. The large size of the parked data will allow a direct check of the D0 claim, provided that systematic uncertainties are controlled with few per-mille precision.

The Physics of the Higgs boson and the Standard Model

(R. Ardino, U. Gasparini, G. Grosso, S. Y. Hoh, J. Pazzini, M. Zanetti, A. Zucchetta)

The discovery of the Higgs boson at the CMS and ATLAS experiments in 2012 represents a milestone in the understanding of the electroweak sector of the Standard Model (SM) of elementary particles. The discovery started a new era of measurements at CMS aiming to determine to the highest possible accuracy the couplings of the Higgs boson to the other particles in the SM. A discrepancy between the experimental measurement and the theoretical expectation from the SM would imply a "new physics", hinting to new particles yet to be discovered. The CMS PAdova group is currently focused on two channels involving the Higgs boson:

Measurement of the associated Higgs boson production with a vector boson and the decay to a pair of W bosons

Although the Higgs boson decay to a piar of W bosons is now consolidated, there is a particular interest in the observation of a process involving the same coupling in the production and decay in order to determine or constrain the natural width of the new particle. With this goal in mind, the CMS PAdova group is currently involved in the measurement of the WH → W(WW*) channel, and specifically in the final state where two leptons with the same electrical charge are present. This channel represents a good tradeoff between the number of expected events and the limited background from SM processes. The key points of the analysis include the lepton identification, in the rejection of the backgrounds originating by non-genuine leptons, and the SM background estimation. By combining the results WH → W(WW*) → 3l e ZH → Z(WW*) → 4l the Higgs boson production rate is measured to be μ = 1.85+0.33-0.32(stat)+0.27-0.25(exp)+0.10-0.07(theo) times the SM expectation, equal to a significance of 4.7 standard deviations. The results are also interpreted in the STXS theoretical framework to provide multi-differential Higgs boson cross sections.

Higgs and Z bosons rare decays to J/Ψ meson and a photon

Studying the couplings of the Higgs boson to the second generation quarks represents the real challenge of High Luminosity-LHC. A very rare but promising channel is the decay of the Higgs boson in an energetic photon (γ) and a J/Ψ meson, the latter decaying to a pair of muons. The H → J/Ψ γ takes place also through charm quark loops, and an accurate measurement would allow to determine its couplings with the Higgs boson. The channel is experimentally very clean, thanks to the very low SM backgrounds and the excellent resolution on the invariant mass peak, but also very rare, thus requiring large amounts of data. The observation of the Z → J/Ψ γ, whose rate is 100 times larger than the similar process with the Higgs boson and has never been observed before, is an intermediate objective that can be achieved with a reasonable amount of data.

Vector Boson Scattering and Anomalous Couplings

(P. Azzi, M. Presilla)

The discovery of the Higgs boson and the measurements of its properties are a great success, but they do not complete the picture of the electroweak sector of the SM; interaction vertices with three (triple gauge coupling) or four (quartic gauge couplings) bosons still remain unxplored. Within this context, our group is pursuing the very first implementation of the analysis for the O(α6) electroweak VZ boson pairs production mediated by a Vector Boson Scattering (VBS) mechanism. Besides the pure EW measurement, it is well known that the study of the VBS process gives primary access to the non-Abelian gauge structure of the electroweak interactions through quartic gauge couplings. Therefore, VBS analyses are sensitive to new physics at energy scales beyond the collider direct reach through an effective field theory (EFT) description of the standard model (SM).
The analysis is focused on the measurement with the full Run 2 data, 137 fb-1 of the electroweak ZVjj semileptonic process, collected with the CMS detector, which is characterized by the presence of two high-pT same-flavour leptons, one large-cone jet, and two forward jets. The VBS process can be modified by anomalous quartic gauge couplings (AQGCs), resulting in an enhancement of the production cross-section at large masses of the di-boson system and high-transverse momenta.

Advanced statistical methods

Optimization of design of experimental apparata and measurement techniques

(T. Dorigo, L. Layer, G. Strong)

The machine learning technique called INFERNO uses differentiable programming methods to optimize the measurement of physical parameters in analyses affected by non negligible systematic uncertainties, with the creation of summary statistics that make the measurement robust to the effect of the systematics. This technique is being applied to measurements of top quark properties produced with data collected by the CMS experiment, to demonstrate the potential of INFERNO.
In parallel to the above described activity, the use of deep learning techniques, and more in general of differentiable programming, is being considered to construct differentiable models of even the intrinsically stochastic of the information extraction chain which from a detector and a physical process of interest produces statistical inference on the parameters of interest, through the reconstruction of electronic signals and the creation of summary statistics. More in detail, the specification of a loss function that includes the true objectives of the experiment (e.g. the minimum uncertainty on the measurement of a parameter, or the linear combination of a series of such outputs, optionally constrained by external demands such as detector cost) and the composition of a differentiable pipeline including a modeling of the interaction of radiation with matter, detector geometry, pattern recognition of signals, and data analysis, allows the gradient descent toward minimal loss, to obtain a full end-to-end optimization of the entire experimental apparatus and measurement procedure.
The nascent MODE collaboration is starting studies for now focused on simplified use cases that involve e.g. muon tomography apparata for border control applications, and proton therapy apparata, which will in the future move to the optimization of a hybrid calorimeter for a future collider.

Search for new physics signals with unsupervised anomaly detection techniques

(T. Dorigo, M. Fumanelli, C. Maccani)

The search for new physics in CMS data uses theoretical models that predict the distinctive characteristics of the involved physical processes. However, these models do not exhaust the totality of possible experimental signatures that may allow the manifestation of new processes. The large number of possible signatures requires automated tools for the scanning of high-dimensional parameter spaces. An algorithm of recent design (RanBox) allows to operate an advantageous transformation of the feature space, which makes the density of standard model processes in the space almost uniform. In such a space a search for multidimensional intervals with high density of surrounding regions allows to evidence anomalies that potentially are the result of new physical processes. The algorithm is being finalized, with the prospects of an application to CMS data.