PhD research projects

Searches for the production of Higgs boson pairs at the LHC.
  
The discovery of a Higgs-boson particle drives the LHC program towards precision measurements of its couplings and properties, the search for processes to clarify the origin of the electroweak symmetry breaking mechanism or to verify the existence of phenomena not included in the standard model (SM) of fundamental interactions. 
The search for pair-produced Higgs bosons allows one to probe the existence of massive resonances decaying into final states with scalar bosons, which are expected in the frame of Two-Higgs Doublet Models (2HDM) or Extra-Dimension Models. At the same time, the non-resonant production of Higgs boson pairs is suppressed, in the SM, by the negative interference of different types of diagrams. This characteristic renders this search very sensitive to any new physics generating imbalance among these various terms, providing the only direct access to the Higgs trilinear coupling.
The Milano-Bicocca research team is actively involved in the searches for di-Higgs final states with the CMS detector, focussing on mixed topologies. One of the Higgs bosons is searched for in the decay into two b quarks, as it is the final state with the largest branching ratio. The other one is searched for in its decays into two photons or two tau leptons.
These studies capitalize on the detailed knowledge of the CMS electromagnetic calorimeter and endcap pixel detector, the longstanding experience in the algorithms for online and offline photon and tau lepton reconstruction and identification, and the close collaboration with international partners, such as the École Polytechnique in Paris.
To extract at best the information from the data of the ongoing LHC Run 2, the analyses will have to feature the characteristics of the new CMS pixel detector to improve the online and offline identification of b quarks and tau leptons, to identify with the largest efficiency is possible the decay into photons. At the same time, it will be necessary to exploit advanced analysis techniques such as multi-variate discriminants or boosted objects reconstruction to improve the background rejection and enlarge the energy domain of the searches.
This study is well suited and timely for the analysis section of a PhD thesis, about the final state with two b quarks and two tau leptons, or for one about the final state with two b quarks and two photons.

Search for Vector Boson Scattering at the LHC.

The discovery of a Higgs boson at the LHC corresponds, within the experimental uncertainties, to the expectations of the Standard Model (SM) of Particle Physics. The mass of the resonance of 125 GeV, though, leaves unresolved a number of open issues in the theory, such as the need of a high degree of theoretical fine tuning to justify such a small mass with respect to the GUT scale, the lack of candidates for dark matter, the missing explanation of the inflation period or matter-antimatter asymmetry in the Universe.
To find answers for these questions, a precise characterization of the electroweak symmetry breaking mechanism, which is the core of the SM, is needed. Besides introducing the need of Higgs bosons, this mechanism also normalizes the longitudinal component of the vector boson scattering (VBS) processes, that would have a diverging cross-section at high energy otherwise. At the LHC, the analyses look at final states with two vector bosons and two jets: the latter are the residual of the proton quarks, which irradiated the bosons that subsequently interact in the VBS process.
The Milano-Bicocca Physics Department is actively involved in the VBS studies, leading the VBSCan international collaboration, involving more than twenty institutes with phenomenological and experimental expertise. The longstanding experience in the search for Higgs bosons and high mass resonances decaying into vector boson pairs, as well as in the reconstruction of hadronically-decaying boosted vector bosons, puts the group in the position of leading the analysis of the events that will be collected by the CMS experiment during the Run 2 of the LHC.
The analyses will feature all possible vector boson combinations in the final state, measuring the cross-section of the vector boson scattering and searching for possible deviations from the SM predictions. The final states will either be very rare, or contaminated by background processes: advanced analysis techniques will be necessary to exploit at best the angular correlations in the final state stable particles, development of on-line selections will be necessary to maximize the statistics collected in the semi-leptonic channels, and a theoretically-sound treatment of the signal and background models will be developed to cope with interferences and non-resonant components in the calculations.
The dataset that will be delivered by the LHC in the forthcoming years will allow for the first time ever for a systematized study of VBS across final states and experiments, a topic which is well suited and timely for the analysis section of a PhD thesis on any of the most promising final states.

Search for long lived particles at the LHC.

While the discovery of the Higgs boson at the LHC produces a landscape which is formally complete from the theoretical point of view, several questions in Nature remain unresolved: the Standard Model (SM) of Particle Physics does not provide any candidates for dark matter, it does not describe the inflation of the Universe in its early development after the Big Bang, nor does it justify the matter-antimatter asymmetry observed nowadays.
Answers to some or all of these questions can be found within theories that go beyond the Standard Model (BSM), which are actively searched for at the LHC. In some specific configurations of the parameters of these BSM theories, the expected phenomenology can be hard to identify, for example in cases when new particles with very long life time would be produced at the interaction vertex, and travel a long path before decaying inside the detector volume. For these cases, dedicated reconstruction algorithms, specific studies of the theoretical models and advanced analysis techniques are necessary to interpret the data collected by the experiments.
The Milano-Bicocca Physics Department is starting a collaboration with the DESY laboratories on the subject, where the expertise of the two groups will be merged to exploit the whole response of the CMS detector at best for the analysis, and plan for future improvements in the analyses after the CMS detector upgrade in view of the future high-luminosity LHC (HL-LHC) running. The expertise of the DESY group on multivariate data analysis techniques and its longstanding activity on SUSY searches, together with the experience of the Milano-Bicocca group on the calorimetry and on future timing detectors puts the collaboration in the position of playing a leading role in the analysis for the forthcoming years of data taking and for the long term HL-LHC plans.
The analysis will feature the track reconstruction capabilities of the new four-layer pixel detector of the CMS experiment with customized algorithms, and will improve backgrounds rejection by using event-shape variables built on the energy flow of the event and exploiting discriminants usually employed in other searches. The various elements will be best combined by using advanced data mining techniques, for example based on deep learning algorithms. For long term studies, the measurement of the new Barrel Timing Layer element of the CMS detector, which has been proposed to the collaboration by the Milano-Bicocca group, will be included in prospective studies.
This analysis is very timely with the current LHC programme and well suits a PhD thesis program, covering a physics use case from the short and long term point of view, encompassing the development of a new detector in its framework.

Measurement of the Higgs boson properties in the diphoton decay channel and optimization and calibration of the CMS electromagnetic calorimeter.

After the discovery of the Higgs boson by the ATLAS and CMS collaborations in 2012, the priority of the LHC physics program is the precise measurement of the properties of this new particle to test the consistency of the standard model of particle physics. 
Analyses performed so far using proton-proton collisions data at sqrt(s) = 7, 8 and 13 TeV have explored a wide variety of Higgs production and decay modes and showed full consistency of the experimental results with the SM predictions. The LHC is expected to deliver more than 100 fb-1 by the end of Run 2, allowing a substantial improvement on the precision of the measurements of the Higgs boson couplings to SM particles and of its properties.
One of the golden channels for the detailed characterization of the Higgs boson is represented by its decay to a pair of photons. Despite the small branching ratio (BR ∼ 0.23% for mH=125 GeV), the H→ γγ channel is distinguished by a clean experimental signature, with two high transverse momentum isolated photons resulting in an invariant mass peak that can be reconstructed with high precision.
The analyses of interest are the study of exclusive production modes (Vector Boson Fusion, VH and ttH associated production), which allow to directly explore the couplings to vector bosons and fermions, and the measurement of the differential Higgs production cross section, as an additional probe of consistency with the standard model.
For these analyses, since the H→γγ decay channel is characterised by the presence of photons in the final state, the stability and the performance of the CMS electromagnetic calorimeter (ECAL) play a fundamental role. It is, in fact, crucial to maintain the current excellent energy resolution (about 2%, for photons in the energy range of those produced in the Higgs boson decay) throughout the entire data taking to be able to efficiently discriminate the resonant signal from the continuum background. The analysis for precision measurements will be complemented therefore by the work on the ECAL calibration and on the monitoring of the ECAL response stability using known physics processes, such as exploiting isolated electrons from W and Z decays.

Precision timing for the CMS phase II upgrade.

The discovery of the Higgs boson by the ATLAS and CMS collaborations drives the LHC physics program towards precision measurements of the properties of this new particle, to test the consistency of the standard model and the search for new phenomena at higher mass scales. To meet these goals, an upgrade of the accelerator (known as HL-­LHC, High Luminosity LHC) is planned around 2025 to achieve a peak luminosity of about 1035 cm-2s-1 and to deliver 3000 fb-1 within 10 years of operation.
Along with radiation and rate conditions, one of the outstanding challenges posed by the HL­-LHC will be the increased interaction multiplicity in the events: 140÷200 simultaneous interactions per beam crossing (pileup) are expected, spread over a few centimeters along the beam axis and about 180 ps in time. In such conditions, the reconstruction of the most interesting collision event becomes difficult and the overlap of energy deposits from different interactions deteriorates the performance of particle identification and energy measurement. The measurement of the production time of the outgoing particles with a precision of a few tens of picoseconds would allow to distinguish particles from different interactions, and thus it would reduce the effective pileup in the HL-­LHC environment by a factor of 10, resulting in a level similar to the one of the current LHC. In particular, an excellent measurement of the time of each single charged particle would significantly improve the reconstruction of the interaction vertex and of the precise measurement of the time of the collision.

The Milano Bicocca group is involved in the design and optimization of a dedicated timing layer for the precise timing of minimum ionizing particles for the CMS Phase II upgrade. In particular, the activity of the group is focused on the following aspects:
Optimization and characterization of the Barrel Timing Layer sensors (LYSO crystals + SiPM)
Study of the detector modules design and assembly
Development of dedicated algorithms to integrate precision time information in the CMS event reconstruction software and evaluation of the impact of precision timing on benchmark physics channels (e.g.: Higgs boson properties measurements, searches for new phenomena, measurements of rare decays of the standard model bosons).

Search for rare exclusive decays of the W boson with the CMS detector.


In the Standard Model (SM) of elementary particles, the mass of the W boson (mW) can be expressed at tree level as a function of the three fundamental SM parameters only, i.e. the Z boson mass (mZ), the fine-structure constant (αem) and the Fermi constant GF. Higher order corrections introduce a dependence of the W boson mass on the masses and gauge couplings of the heavy particles of the SM (such as the top quark and the Higgs boson), and potentially of new particles and interactions predicted by theories extending the Standard Model. The comparison of the predicted and measured values of mW is then a probe of loop diagrams and could reveal the presence of new Physics phenomena.
The precision on the W boson mass measurement (mW = 80.385 ± 0.015 GeV) is currently the leading uncertainty in the SM consistency test, which would require an experimental uncertainty on mW at the level of ~6 MeV. A precise measurement of the W boson mass is therefore of fundamental high importance.
At hadron colliders, the W boson mass measurement is traditionally carried out using its leptonic decays (W→lν, with l being an electron or a muon). The presence of an escaping neutrino, whose transverse momentum (pT) can be inferred from the energy imbalance in the detector plane perpendicular to the beam direction and whose longitudinal momentum remains instead unkown, makes it impossible to fully reconstruct a mass peak. The mass of the W boson is then extracted from the Jacobian edges of the final-state kinematic distributions measured in the transverse plane, such as the lepton pT or the lepton-neutrino transverse mass. The precision attainable with this approach is currently limited by systematic effects, the dominant ones coming from the limited knowledge of the probability density functions (PDFs) of quarks in the protons and the modelling of the strong interaction, both affecting the dynamics of the boson production and decay.
An alternative technique is to use fully reconstructed exclusive hadronic decays of the W boson. These final states, which have never been directly observed, are attractive as they allow the W boson mass to be measured in a way that is much less limited by systematic effects, as the proton PDFs as well as the QCD modelling play no role in the reconstruction of the invariant mass. The main challenges of this approach consist in the extremely low branching ratio of the decay modes accessible at the LHC, i.e. those with a clean signature providing a trigger information and a sufficient rejection of the QCD background, and the reconstruction of a multi-track final state in the harsh radiation environment of high-luminosity proton-proton collisions. A promising channel is the W→J/ψDs decay mode, where the J/ψ decaying into a muon pair provides the trigger signature, and the Ds is searched for in its KKπ decay mode. The branching ratio of this process is expected to be at the level of 10-8, therefore with a W boson production cross section of 200 nb at 13 TeV center-of-mass energy, more than 10 events are expected in each fb-1 of integrated luminosity.
The analysis of the data collected by the CMS experiment up to the end of the LHC Phase I (300 fb-1 expected by 2022) will allow the observability of the rare W boson exclusive decays to be assessed, and pave the way towards a ~20 MeV-precise mW measurement exploiting the full 3000 fb-1 dataset of the High-Luminosity phase of LHC.