Data Analysis Group

Higgs Boson Measurements

The most important achievement of the Run 1 LHC data taking period is the discovery of the Higgs boson by the ATLAS and CMS collaborations in 2012 [publications: ATLAS, CMS]. Subsequent measurements of properties of this boson were, so far, in agreement with the expectation for an SM Higgs boson. Is the observed particle indeed the SM Higgs boson, and where can we expect to see deviations from the SM Higgs boson predictions if there is physics beyond the SM (BSM)?

Because the Higgs sector is less constrained by symmetries than other SM sectors, short-term, an intriguing answer lies in the Higgs boson decay to a pair of τ leptons. During the LHC Run 2, the group lead the CMS H→ττ observation and subsequent measurements of the H→ττ decay mode. Although BSM signals are strongest in this channel, the achievable precision is still insufficient to provide evidence for, or rule out, the majority of scenarios of the best-motivated BSM candidates. Thus, the quest continues in the LHC Run 3 from 2022-25.

Displaced signatures

Despite the beautiful experimental results in the SM Higgs sector, fundamental questions in particle physics remain unanswered. Uncovering the nature of dark matter, the SM’s most prominent shortcoming, is a strong motivation to pursue the search for new particles at the LHC.

Are the LHC experiments covering the whole possible BSM parameter space? Are there well-motivated experimental signatures that were not sufficiently studied? Hypothetical ''dark sectors'' of particles and forces that account for the dark matter provide an intriguing answer. Messenger particles between the SM and the dark sector can have a macroscopic lifetime and produce displaced decay vertices in ways not possible in the SM. A plausible mechanism of connection between the dark sector and the SM is via the Higgs portal, where pairs of dark sector mediators known as dark photons originate in rare decays of the Higgs boson.

The group probes a conspicuous, but scarcely studied, signature: muon pairs emerging from a common vertex up to several meters from the collision point. Such muon pairs can arise from decays of long-lived dark photons. A sensitivity to an extremely wide range of particle lifetimes is achieved by combining the information from the innermost and outermost CMS detectors: inner tracker and muon spectrometer. Three complementary categories of muon pairs are built from muons detected in both tracker and muon spectrometer (TMS) and in muon spectrometer only (STA). Latest results of the group [link], provide the strongest constraints on the decay branching ratio of the Higgs boson to pairs of dark photons for broad ranges of lifetimes and masses.  Currently, the group is working on a novel trigger strategy which will improve the search sensitivity and give access to a new uncharted BSM territory for the upcoming LHC Run 3 data taking period (2022-2025). 

Physics of the top quark

The heaviest known elementary particle, the top quark, was discovered at the previous hadron collider, the Tevatron [publications: CDF, D0]. The LHC experiments rediscovered the top quark, and the first measurements of its properties enter the precision regime. Another salient feature of the data is the absence, so far, of BSM signals. Can we be sure that nature is well described by the SM, even at the highest achievable energies?

Searches for BSM often focus on high energetic events, neglecting the possibility that fainter signals are concealed in the subtle properties of differential distributions. Owing to the SM top quark’s unique role, many BSM models, however, predict anomalous couplings of the top quark to the electroweak gauge bosons. The group aims to extract these hypothetical modifications from the differential cross section data on top quark pair production in conjunction with a gauge boson, e.g., a Z boson or a photon. These processes probe fundamental electroweak phenomena, such as the top quark’s dipole moment, that has never been directly targeted before. A measurement of the Z boson’s transverse momentum in events with a top quark pair [link] provides a precise determination of these essential parameters.

A close cousin of the Z boson in the standard model is the photon, the particle of light. It governs everyday life, for example when depositing energy on the retinas in your eyes we're provided with eyesight. A unified understanding of these seemingly unrelated phenomena accounts for the fascination with physics. At 10 billion times the energy of visible light, photons originating from the top quark cannot be seen directly. Instead, the CMS electromagnetic calorimeter indirectly provides eyesight. When photons impact the detector material, their energy showers into large numbers of electrons and more photons.

In a new measurement of the photon spectrum of top quarks [link, with an accessible explanation here], we test the standard model by exploiting a bizarre quantum mechanical property, the “spin”: although the top quark has no dimensions, it has angular momentum as if it were rotating. The measured distributions allow us to determine the spin axis’ reaction to an electric or magnetic field. These quantities are called the top quarks electric and magnetic dipole moments and the standard model predicts that they are tiny. If the top quarks’ dipole moments differed from the standard model’s prediction, we would observe more events at high photon momentum as indicated in the figure (colored lines). This way, we can test phenomena beyond the standard model, irrespective of the specifics. The measurement is the currently most precise determination of the top quark's dipole moments.

Missing energy

What if a particle does not interact with the detector? Indeed, the SM neutrinos can travel light-years through matter without interaction, and hence their detection at collider experiments is hopeless. The hypothetical particles constituting the dark matter share this property. We can nevertheless obtain strong indirect signals of invisible particles. The underlying event property is momentum conservation of all decay products from a collision in the plane orthogonal to the initial protons’ movement. Invisible particles seemingly violate it. The precise determination of the transverse momentum imbalance (p_T^miss) is critical for measuring final states with neutrinos, such as those containing leptonic decays of the W boson. Besides, p_T^miss is a critical observable in searches for supersymmetry (SUSY). The performance of the p_T^miss measurement is affected by additional proton-proton interactions in the same or nearby bunch crossings (pileup). A detailed understanding of pileup and detector effects in all CMS components allows the computation of the likelihood a given event is compatible with the zero-p_T^miss, termed the p_T^miss significance. The groups’ involvement in the commissioning of this observable [link] facilitates the SUSY searches discussed below.

Supersymmetry

The SM incorporates our best understanding of nature at its most fundamental level and is undoubtedly amongst the most extensively tested scientific theories in history. Yet, the theory appears to be incomplete when confronted with astronomical measurements of the matter content of the universe because it cannot accommodate 73% of the total, which we attribute to dark matter [Planck 2018]. Furthermore, the Higgs boson poses challenges too: the famous instability of the Higgs boson mass under quantum corrections is known as the hierarchy problem. Supersymmetry (SUSY), a yet undiscovered symmetry of nature, can naturally solve both problems by predicting partner particles that stabilize the Higgs boson mass. Under reasonable theoretical assumptions (e.g., R-parity conservation), it also predicts that the lightest supersymmetric particle (LSP) has all the properties of a dark matter candidate and allows the reconciliation of particle physics with the universe at its largest scales. The LHC at the energy frontier is the unique place to tackle these questions experimentally. The group’s focus in searches for TeV scale superpartners is twofold. 

Firstly, searches for top squarks and gluinos in the single- and dilepton channels aim to detect spectacular signal events. The p_T^miss significance observable allows a stable selection efficiency despite varying pileup conditions during the LHC Run 2. Aggressive background suppression techniques feed into the quantity M_T2(ll) that exploits the SM particles’ kinematic properties in the decay chain. It is the first result from the group using the full LHC Run 2 dataset.

The second approach to SUSY asks whether special SUSY mass configurations can evade the previous limits. Suppose the mass difference between the next-to-lightest SUSY particle and the LSP is small (compressed). In that case, the signal events will escape classical search strategies because the decaying particles provide too little energy. The decisive idea is to exploit an SM feature that’s otherwise considered a nuisance. Requiring the presence of a high-p_T jet from initial-state radiation (ISR) leads to a boost of the SUSY particle pair system and enhances the amount of p_T^miss and the momentum of the final state lepton. Sensitive searches use this technique to target BSM models that can not be tackled otherwise.

Interpretation

So far, the searches at the ATLAS and CMS experiments at the LHC show no signs of physics beyond the Standard Model. Models with SUSY and other BSM phenomena are subject to tight constraints, and a plethora of SM measurements in various final states characterize the TeV scale. How can this diversity of results be compared to an equally complex landscape of theoretical models? The SModelS software package takes on this problem. An arbitrary BSM model, not restricted to SUSY, is decomposed into ``simplified model’’ topologies and tested against all the existing LHC bounds. This decomposition allows an extensive survey of models and enormously simplifies identifying the regions of parameter space that are still allowed by the current searches.

The next step is the protomodels algorithm. It performs a random walk in the SModelS database to identify dispersed signals that can not be identified by a single search strategy. After all, the next standard model is probably  just around the corner.

Quarkonia

Quantum chromodynamics (QCD), an integral part of the SM, describes the strong force dynamics. The surprising absence of a smooth classical limit at low energies sets QCD apart. ``Quarkonia`` bound states, i.e., systems of heavy charm or beauty quarks, mitigate the lack of predictivity at low energies. Production and decay can be treated separately. Despite intractable nonperturbative effects, the separation of short-distance and long-distance behaviour allows calculable and measurable observables. This framework is called the non-relativistic QCD (NRQCD) and is our studies’ target. We use the cross section and polarization of the charm bound states J/Psi and chi_c and of the bottom quark bound state Psi(2S) to verify NRQCD predictions. The project concluded in 2016.