The CMS analysis group at HEPHY uses data from proton-proton collisions produced by CERN's Large Hadron Collider (LHC) at the highest achievable energies of 7, 8 (2010-12, LHC Run I), and 13 TeV (2015-18, LHC Run II). The particles generated in these collisions are recorded by the CMS experiment and deepen our knowledge of elementary particles and their interactions  [LHC long-term plan]

The discovery of a Higgs boson at the LHC has completed the “standard model” (SM) of particle physics. Measurements in the Higgs sector are an essential part of our profile. Moreover, the SM has several shortcomings. The lack of an explanation for "dark matter" is a prominent example. 

Using two main approaches, we engage in searches for extensions of the SM that can provide solutions: First, direct searches for signals of new particles, either supersymmetric or from so-called ``dark bosons``, could be found in rare and spectacular events. Second, indirect searches test the SM by comparing precision measurements with highly accurate SM predictions. We focus on the field of top quark physics and Higgs boson measurements. Among the other activities of the group are the study of quarkonium states, detector related measurements, and new approaches for the interpretation of LHC results on a global level. In more detail:

Higgs Boson Measurements

The most important achievement of the Run I 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 II, 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 III from 2021-24.

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. That’s not possible in the SM. The group is involved in the most prominent, yet uncovered, signature: muon pairs emerging from a common vertex up to several meters from the collision point. Such muons can be detected and reconstructed using muon detectors located in the outermost layers of CMS [link].

 

Physics of the top quark

The heaviest known SM elementary particle, the top quark, was discovered at the previous hadron collider, the Tevatron [original 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. The processes probe fundamental electroweak phenomena, such as the top quark’s dipole moment, that has never been targeted directly before. A measurement of the Z boson’s transverse momentum in events with a top quark pair [link] provides the currently most precise determination of these essential parameters.

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 (pTmiss) is critical for measuring final states with neutrinos, such as those containing leptonic decays of the W boson. Besides, pTmiss is a critical observable in searches for supersymmetry (SUSY). The performance of the pTmiss 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-pTmiss, termed the pTmiss 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 pTmiss significance observable allows a stable selection efficiency despite varying pileup conditions during the LHC Run II. Aggressive background suppression techniques feed into the quantity MT2(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 II dataset of 137 fb-1.

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-pT jet from initial-state radiation (ISR) leads to a boost of the SUSY particle pair system and enhances the amount of pTmiss 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 chic and of the bottom quark bound state Psi(2S) to verify NRQCD predictions. The project concluded in 2016.

Staff members

  • Jochen Schieck

    Director
    Rare Event Searches
    CMS Physics Analysis,  COSINUS,  CRESST,  DANAE
  • Claudia Wulz

    Group Leader CMS Trigger
    CMS Trigger
    CMS Physics Analysis