C. Veelken

Cross sections for elastic production of J/Psi mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb^{-1}. Results are presented for photon virtualities Q^2 up to 80 GeV^2. The dependence on the photon-proton centre of mass energy W_{gamma p} is analysed in the range 40 < \Wgp < 305 GeV in photoproduction and 40 < \Wgp < 160 GeV in electroproduction. The \Wgp dependences of the cross sections do not change significantly with Q^2 and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q^2. Differential cross sections d\sigma/dt, where t is the squared four-momentum transfer at the proton vertex, are measured in the range |t|<1.2 GeV^2 as functions of \Wgp and Q^2. Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/Psi production and decay angular distributions are consistent with s-channel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q^2 and is found to be described by perturbative QCD based models.

When using machine learning (ML) techniques, users typically need to choose a plethora of algorithm-specific parameters, referred to as hyperparameters. In this paper, we compare the performance of two algorithms, particle swarm optimisation (PSO) and Bayesian optimisation (BO), for the autonomous determination of these hyperparameters in applications to different ML tasks typical for the field of high energy physics (HEP). Our evaluation of the performance includes a comparison of the capability of the PSO and BO algorithms to make efficient use of the highly parallel computing resources that are characteristic of contemporary HEP experiments.

An algorithm for reconstruction of the Higgs mass in H → ττ decays is presented. The algorithm computes for each event a likelihood function P(Mττ) which quantifies the level of compatibility of a Higgs mass hypothesis Mττ with measured momenta of the visible tau decay products plus the missing transverse energy reconstructed in the event. The algorithm is used in the CMS H → ττ analysis, where it is found to improve the sensitivity to discover the Standard Model Higgs boson in this decay channel by about 30%.

Abstract The analysis of vast amounts of data constitutes a major challenge in modern high energy physics experiments. Machine learning (ML) methods, typically trained on simulated data, are often employed to facilitate this task. Several choices need to be made by the user when training the ML algorithm. In addition to deciding which ML algorithm to use and choosing suitable observables as inputs, users typically need to choose among a plethora of algorithm-specific parameters. We refer to parameters that need to be chosen by the user as hyperparameters. These are to be distinguished from parameters that the ML algorithm learns autonomously during the training, without intervention by the user. The choice of hyperparameters is conventionally done manually by the user and often has a significant impact on the performance of the ML algorithm. In this paper, we explore two evolutionary algorithms: particle swarm optimization and genetic algorithm, for the purposes of performing the choice of optimal hyperparameter values in an autonomous manner. Both of these algorithms will be tested on different datasets and compared to alternative methods.

Identifying and reconstructing hadronic τ decays (τh) is an important task at current and future high-energy physics experiments, as τh represent an important tool to analyze the production of Higgs and electroweak bosons as well as to search for physics beyond the Standard Model. The identification of τh can be viewed as a generalization and extension of jet-flavour tagging, which has in the recent years undergone significant progress due to the use of deep learning. Based on a granular simulation with realistic detector effects and a particle flow-based event reconstruction, we show in this paper that deep learning-based jet-flavour-tagging algorithms are powerful τh identifiers. Specifically, we show that jet-flavour-tagging algorithms such as LorentzNet and ParticleTransformer can be adapted in an end-to-end fashion for discriminating τh from quark and gluon jets. We find that the end-to-end transformer-based approach significantly outperforms contemporary state-of-the-art τh reconstruction and identification algorithms currently in use at the Large Hadron Collider.

We present a feasibility study to probe quantum entanglement and Bell inequality violation in the process ${\mathrm{e}}^{+}{\mathrm{e}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ at a center-of-mass energy of $\sqrt{s}=10.579\text{ }\text{ }\mathrm{GeV}$. The sensitivity of the analysis is enhanced by applying a selection on the scattering angle $\ensuremath{\vartheta}$ in the ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ center-of-mass frame. We analyze events in which both $\ensuremath{\tau}$ leptons decay to hadrons, using a combination of decay channels ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\nu}}_{\ensuremath{\tau}}$, ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}{\ensuremath{\nu}}_{\ensuremath{\tau}}$, and ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\nu}}_{\ensuremath{\tau}}$. The spin orientation of the $\ensuremath{\tau}$ leptons in these decays is reconstructed using the polarimeter-vector method. Assuming a dataset of 200 million ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ events and accounting for experimental resolutions, we expect the observation of quantum entanglement and Bell inequality violation by the Belle-II experiment will be possible with a significance well in excess of five standard deviations.

The polarimeter vector of the τ represents an optimal observable for the measurement of the τ spin. In this paper we present an algorithm for the computation of the τ polarimeter vector for the decay channels τ−→π−π+π−ντ and τ−→π−π0π0ντ. The algorithm is based on a model for the hadronic current in these decay channels, which was fitted to data recorded by the CLEO experiment [1]. Program Title: PolarimetricVectorTau2a1, version 1.0.1 CPC Library link to program files: https://doi.org/10.17632/z986tk5pyv.1 Developer's repository link: https://github.com/TTauSpin/PolarimetricVectorTau2a1 Licensing provisions: MIT Programming language: C++ 11 Nature of problem: The polarimeter vector h of the τ can be used to measure the τ spin orientation. The vector h is a function of the momenta of the particles produced in the τ decay and needs to be computed in the restframe of the τ lepton. While for the decay channels τ−→π−ντ and τ−→π−π0ντ expressions for h exist in the literature, no corresponding expressions exist for the channels τ−→π−π+π−ντ and τ−→π−π0π0ντ. Solution method: In this paper, we present an algorithm for the computation of the τ polarimeter vector h for the decay channels τ−→π−π+π−ντ and τ−→π−π0π0ντ. The algorithm is based on a model for the dynamics of hadronic interactions in these decay channels. The parameters of the model have been determined by a fit to data recorded by the CLEO experiment.

We present an algorithm for the reconstruction of the Higgs mass in events with Higgs bosons decaying into a pair of τ leptons. The algorithm is based on matrix element (ME) techniques and achieves a relative resolution on the Higgs boson mass of typically 15–20%. A previous version of the algorithm has been used in analyses of Higgs boson production performed by the CMS collaboration during LHC Run 1. The algorithm is described in detail and its performance on simulated events is assessed. The development of techniques to handle τ decays in the ME formalism represents an important result of this paper.

Abstract Monte Carlo (MC) simulations are extensively used for various purposes in modern high-energy physics (HEP) experiments. Precision measurements of established Standard Model processes or searches for new physics often require the collection of vast amounts of data. It is often difficult to produce MC samples containing an adequate number of events to allow for a meaningful comparison with the data, as substantial computing resources are required to produce and store such samples. One solution often employed when producing MC samples for HEP experiments is to partition the phase space of particle interactions into multiple regions and produce the MC samples separately for each region. This approach allows to adapt the size of the MC samples to the needs of physics analyses that are performed in these regions. In this paper we present a procedure for combining MC samples that overlap in phase space. The procedure is based on applying suitably chosen weights to the simulated events. We refer to the procedure as “stitching”. The paper includes different examples for applying the procedure to simulated proton-proton collisions at the CERN Large Hadron Collider.

Identifying and reconstructing hadronic $\tau$ decays ($\tau_{\textrm{h}}$) is an important task at current and future high-energy physics experiments, as $\tau_{\textrm{h}}$ represent an important tool to analyze the production of Higgs and electroweak bosons as well as to search for physics beyond the Standard Model. The identification of $\tau_{\textrm{h}}$ can be viewed as a generalization and extension of jet-flavour tagging, which has in the recent years undergone significant progress due to the use of deep learning. Based on a granular simulation with realistic detector effects and a particle flow-based event reconstruction, we show in this paper that deep learning-based jet-flavour-tagging algorithms are powerful $\tau_{\textrm{h}}$ identifiers. Specifically, we show that jet-flavour-tagging algorithms such as LorentzNet and ParticleTransformer can be adapted in an end-to-end fashion for discriminating $\tau_{\textrm{h}}$ from quark and gluon jets. We find that the end-to-end transformer-based approach significantly outperforms contemporary state-of-the-art $\tau_{\textrm{h}}$ reconstruction and identification algorithms currently in use at the Large Hadron Collider.

The polarimeter vector of the $\tau$ represents an optimal observable for the measurement of the $\tau$ spin. In this paper we present an algorithm for the computation of the $\tau$ polarimeter vector for the decay channels $\tau^{-} \rightarrow \pi^{-}\pi^{+}\pi^{-}\nu_{\tau}$ and $\tau^{-} \rightarrow \pi^{-}\pi^{0}\pi^{0}\nu_{\tau}$. The algorithm is based on a model for the hadronic current in these decay channels, which was fitted to data recorded by the CLEO experiment.

We present a feasibility study to probe quantum entanglement and Belle inequality violation in the process $\textrm{e}^{+}\textrm{e}^{-} \rightarrow \tau^{+}\tau^{-}$ at a center-of-mass energy of $\sqrt{s} = 10.579$ GeV. The sensitivity of the analysis is enhanced by applying a selection on the scattering angle $\vartheta$ in the $\tau^{+}\tau^{-}$ center-of-mass frame. We analyze events in which both $\tau$ leptons decay to hadrons, using a combination of decay channels $\tau^{-} \rightarrow \pi^{-}\nu_{\tau}$, $\tau^{-} \rightarrow \pi^{-}\pi^{0}\nu_{\tau}$, and $\tau^{-} \rightarrow \pi^{-}\pi^{+}\pi^{-}\nu_{\tau}$. The spin orientation of the $\tau$ leptons in these decays is reconstructed using the polarimeter-vector method. Assuming a dataset of $200$ million $\tau^{+}\tau^{-}$ events and accounting for experimental resolutions, we expect the observation of quantum entanglement and Bell inequality violation by the Belle-II experiment will be possible with a significance well in excess of five standard deviations.

The CMS pixel detector measures the direction and momentum of charged particles close to the nominal interaction point of the CMS experiment at the CERN Large Hadron Collider (LHC). The pixel detector is a rather complex device, consisting of 1444 detector modules, in total providing about 66 million pixels of size 100 × 150 μm and an active area of about 1 m2 for the measurement of charged particles traversing its fiducial volume. The control systems of the CMS pixel detector will be described in this document.

The algorithm used for reconstruction and identification of hadronic tau decays by the CMS experiment at the LHC is presented. The tau reconstruction in CMS takes advantage of the particle-flow algorithm which allows to reconstruct individual hadronic decay modes. The performance of the algorithm in terms of tau identification efficiency and in terms of rates for jets to be misidentified as hadronic tau decays is studied in pp collision data recorded in 2012 at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb−1.

Searches for charged and neutral Higgs bosons in the context of the Minimal Supersymmetric Standard Model (MSSM) and in the context of the Next–to–Minimal Supersymmetric Standard Model (NMSSM) are presented. The analyses are based on proton–proton collision data recorded by the CMS experiment at s=7 TeV and s=8 TeV center–of–mass energy in 2011 and 2012, respectively, corresponding to integrated luminosities of up to 4.9 fb-1 and 20.7 fb-1. No evidence for further Higgs bosons in addition to the discovered SM–like Higgs boson of mass ≈ 125 GeV is found and stringent exclusion limits are derived.

A search for the Higgs boson in the H→ ZZ four-lepton decay channel, with each Z boson decaying to an electron, or a muon pair, is reported. The search covers Higgs boson mass hypotheses in the range 110<mH < 600 GeV. The analysis uses pp collision data recorded by the CMS detector at the LHC, corresponding to integrated luminosities of 5.05 fb−1 at √ s = 7 TeV and 5.26 fb−1 at √ s = 8 TeV. The four-lepton invariant-mass distributions m4` is found to be consistent with the standard model expectations for background ZZ production for invariant masses above 180 GeV. Upper limits at 95% confidence level exclude the standard model Higgs boson in the range 131–162 and 172–525 GeV, while the expected exclusion range is 121–570 GeV. An excess of events is observed in the low m4` mass range, making the observed limits weaker than expected in the absence of a signal. These events cluster around a mass m4` ' 125.5 GeV, giving rise to a local excess with respect to the background expectation, with a significance of 3.2σ . This result constitutes evidence for a new massive state.

A search for the Higgs boson in the H → ZZ four-lepton decay channel, with each Z boson decaying to an electron, or a muon pair, is reported.The search covers Higgs boson mass hypotheses in the range 110 < m H < 600 GeV.The analysis uses pp collision data recorded by the CMS detector at the LHC, corresponding to integrated luminosities of 5.05 fb -1 at √ s = 7 TeV and 5.26 fb -1 at √ s = 8 TeV.The four-lepton invariant-mass distributions m 4 is found to be consistent with the standard model expectations for background ZZ production for invariant masses above 180 GeV.Upper limits at 95% confidence level exclude the standard model Higgs boson in the range 131-162 and 172-525 GeV, while the expected exclusion range is 121-570 GeV.An excess of events is observed in the low m 4 mass range, making the observed limits weaker than expected in the absence of a signal.These events cluster around a mass m 4 125.5 GeV, giving rise to a local excess with respect to the background expectation, with a significance of 3.2σ .This result constitutes evidence for a new massive state.

A search for charged and neutral Higgs boson production in the context of the minimal supersymmetric standard model is presented.The search is based on proton-proton collision data recorded at √ s = 7 TeV center-of-mass energy by the CMS experiment in 2011.The production of charged Higgs bosons is analyzed in the decay channel to tau lepton and neutrino.The presence of neutral Higgs bosons in the CMS data is tested in three channels: in decays to pairs of bottom quarks, taus and muons.The analyzed dataset corresponds to an integrated luminosity of 2.0-5.0 fb -1 , depending on analysis channel.

We apply the matrix element method (MEM) to the search for non-resonant Higgs boson pair (HH) production in the channel HH → bb̄WW* at the LHC and study the separation between the HH signal and the large irreducible background, which arises from the production of top quark pairs (tt̄). Our study focuses on events containing two leptons (electrons or muons) in the final state. The separation between signal and background is studied for experimental conditions characteristic for the ATLAS and CMS experiments during LHC Run 2, using the DELPHES fast-simulation package. We find that the tt̄ background can be reduced to a level of 0.26% for a signal efficiency of 35%.

When using machine learning (ML) techniques, users typically need to choose a plethora of algorithm-specific parameters, referred to as hyperparameters. In this paper, we compare the performance of two algorithms, particle swarm optimisation (PSO) and Bayesian optimisation (BO), for the autonomous determination of these hyperparameters in applications to different ML tasks typical for the field of high energy physics (HEP). Our evaluation of the performance includes a comparison of the capability of the PSO and BO algorithms to make efficient use of the highly parallel computing resources that are characteristic of contemporary HEP experiments.

Recent results of analyses investigating the production of tau leptons in ep collisions at HERA are reviewed. In total, four different types of analyses are presented: the production of τ+τ− pairs, the production of single, isolated tau leptons in events with large missing transverse momentum (in addition to that resulting from the tau decay); plus two searches for specific new phenomena - for doubly charged Higgs bosons with subsequent decay H++→e+τ+ and for tau leptons produced in lepton flavor violating processes ep→τX.

Various theories beyond the Standard Model predict the existence of heavy resonances decaying to Higgs (H) boson pairs. In order to maximize the sensitivity of searches for such resonances, it is important that experimental analyses cover a variety of decay modes. The decay of H boson pairs to four $\tau$ leptons (HH$ \to \tau\tau\tau\tau$) has not been discussed in the literature so far. This decay mode provides a small branching fraction, but also comparatively low backgrounds. We present an algorithm for the reconstruction of the mass of the H boson pair in events in which the H boson pair decays via HH$ \to \tau\tau\tau\tau$ and the $\tau$ leptons subsequently decay into electrons, muons, or hadrons. The algorithm achieves a resolution of $7$-$22\%$ relative to the mass of the H boson pair, depending on the mass of the resonance.

Various theories beyond the Standard Model predict the existence of heavy resonances decaying to Higgs (H) boson pairs. In order to maximize the sensitivity of searches for such resonances, it is important that experimental analyses cover a variety of decay modes. The decay of H boson pairs to four $τ$ leptons (HH$ \to ττττ$) has not been discussed in the literature so far. This decay mode provides a small branching fraction, but also comparatively low backgrounds. We present an algorithm for the reconstruction of the mass of the H boson pair in events in which the H boson pair decays via HH$ \to ττττ$ and the $τ$ leptons subsequently decay into electrons, muons, or hadrons. The algorithm achieves a resolution of $7$-$22\%$ relative to the mass of the H boson pair, depending on the mass of the resonance.

Events with isolated leptons and large missing transverse momentum are searched for in the e+p and e−p data recorded by the H1 experiment at HERA. In previous analyses of the HERA I e+p data (L = 104.7 pb−1), an excess of events with isolated electrons or muons and large missing transverse momentum was found. No significant excess was found in the HERA I e−p data (L = 13.6 pb−1). In the following, new results of the search for events with isolated leptons and large missing transverse momentum in the HERA II e+p and e−p datasets are presented. The analysed HERA II data correspond to integrated luminosities of 53 pb−1 and 21 pb−1 in e+p and e−p collisions, respectively.

We apply the matrix element method (MEM) to the search for non-resonant Higgs boson pair ($\textrm{HH}$) production in the channel $\textrm{HH} \to \textrm{b}\bar{\textrm{b}}\textrm{W}\textrm{W}^{*}$ at the LHC and study the separation between the $\textrm{HH}$ signal and the large irreducible background, which arises from the production of top quark pairs ($\textrm{t}\bar{\textrm{t}}$). Our study focuses on events containing two leptons (electrons or muons) in the final state. We find that our work has the potential to reduce the $\textrm{t}\bar{\textrm{t}}$ background by three orders of magnitude for a signal efficiency of $35\%$.