Electroweak precision physics and search for new physics

Search for supersymmetric particles in H (->bb) + γ + MET (+ jets) final states

Supersymmetry (SUSY) is an extension of the Standard Model (SM) of particle physics. It is an appealing theory in a sense that it can provide solutions to problems that the SM does not address. Such as the hierarchy problem, source of dark matter and the gauge coupling unification.

SUSY is a proposed symmetry between fermions and bosons. For each SM particle it introduces a superpartner with identical quantum numbers except for the spin which differs by a half-integer. Therefore all SM bosons have a fermionic superpartner, and vice versa. Since no SUSY particles have been observed with the same mass as their SM partners, this symmetry is evidently broken.

The symmetry breaking can be explained by different mechanisms. There are also many free parameters in the theory, thus numerous SUSY models have been developed. There are several models in which the lightest supersymmetric particle (LSP) is stable and thus provides a viable dark matter candidate.

If a supersymmetric particle is produced in the p-p collisions of the LHC, it would go under a cascade decay until reaching the LSP, which would escape the detector without interacting with any of its material. In such an event, an imbalance of the transverse momentum can be observed which we call missing transverse energy (MET).

The gauge-mediated symmetry breaking (GMSB) models predict the gravitino to be the LSP and the neutralino to be the next-to-lightest supersymmetric particle (NLSP). In these models NLSP would always decay to a gravitino and a SM particle. If the neutralino has the right properties, its SM decay product can be a Higgs boson or a photon.

Our research interest is the case when 2 neutralinos are produced and decay into 2 gravitinos, 1 photon and 1 Higgs boson. With the Higgs boson most probably decaying into bb quarks, our desired final state is photon+H(bb)+MET. With this search we would like to discover a significant excess of events over the SM expectation which would point to New Physics, or in the absence of a signal, determine new limits on SUSY particle masses and other SUSY parameters.

Measurement of the pp -> Zγ -> llγ cross-section and anomalous triple gauge couplings

The study of Zγ production in proton-proton (pp) collisions at TeV energies represents an important test of the standard model (SM), which prohibits direct coupling between the Z boson and the photon.

Within the SM, Zγ production is primarily due to radiation of photons from initial-state quarks (ISR) or final-state leptons (FSR). However, new physics phenomena at higher energies may be manifested as an effective self-coupling among neutral gauge bosons, resulting in a deviation from their predicted zero values in the SM.

Anomalous triple gauge couplings (aTGC) of ZZγ and Zγγ can thus be measured in CMS with the production of two oppositely charged leptons (electrons or muons) with an isolated photon. For the Zγ process the existence of aTGCs would typically lead to an enhancement of photons with high transverse momentum. The observed photon transverse momentum distribution is therefore used to extract limits on ZZγ and Zγγ aTGCs.

Measurement of electroweak llγ cross-section and anomalous quartic gauge couplings

Study of strong interactions in proton-proton and heavy ion collisions

Study of Bose-Einstein correlations of identical hadrons in pp collisions

Exclusive studies

In high energy physics, exclusive processes are special type of collisions, where both colliding particles remain intact and we observe all of the produced particles. These can be either electroweak (two-photon fusion), strong (double pomeron exchange) or mixed electromagnetic and strong (photoproduction) processes. One of the greatest advantage of these collisions is that there are constraints on quantum numbers of final state. This property is useful in a wide variety of high energy physics research, such as search for glueballs or the measurement of anomalous quartic gauge couplings.

Our group is involved in a measurement that focuses on central exclusive production of π+π- final state in 5.02 and 13 TeV LHC data. The silicon tracking detector of CMS is used to select events with two oppositely charged tracks and calorimeters to reject events with particles not detected by the tracker. Pions are identified via their mean energy-loss. The π+π- total and differential cross-sections with respect to invariant mass, transverse momentum and rapidity are calculated. The cross-section of resonant channels is calculated by fitting three resonances, ρ(770), f0(980) and f2(1270) with interfering Breit-Wigner functions.

Technical contributions to the experiment

Electron / photon trigger

Luminosity measurement

CMS uses five detectors to monitor and measure luminosity based on rate measurements of a variety of observables: the CMS silicon pixel detector, the Drift Tubes in the barrel (DT), the Forward Hadronic Calorimeter (HF), the Fast Beam Condtions Monitor (BCM1f) and the Pixel Luminosity Telescope (PLT). The PLT, BCM1f, and HF monitor luminosity via a fast readout system that is asynchronous with the readout of the CMS experimental apparatus. The silicon pixel detector and DT feature very low occupancy and good stability over time. These two detectors utilize the standard CMS trigger and data aquisition systems. Our group is committed to the measurement of calibration constants for the PLT offline luminosity, that has a crucial impact on the final luminosity values published by CMS and used in most physics analyses. We are also contributing to the development of software, mainly dedicated to data acquisition, as well as to studies on the baseline precision for luminosity measurements in Run 2 and beyond.

Zero Degree Calorimeter

Many heavy-ion experiments need to measure the geometry, or centrality, of each collision. One way to do this is by measuring the nucleons that do not participate in the collisions. The SPS, RHIC, and LHC heavy-ion experiments have measured these spectator nucleons with Zero Degree Calorimeters (ZDCs). The CMS ZDCs are two identical small forward calorimeters located between the two LHC beam pipes at approximately z = ± 140 m on each side of the CMS interaction region. They reside in special detector slots in the neutral particle absorber (TAN), which is designed to protect the first superconducting quadrupole from synchrotron radiation. As a subsystem of the CMS Forward HCAL Project, the ZDCs are designed to detect neutral particles, mainly photons and neutrons, with pseudorapidity up to |η| = 8.3. Our group contributes to the commissioning and calibration of the ZDC detectors in proton-lead and lead-lead datatakings.

Condition Database