Regular Series

Vol. 52 (2021), No. 8, pp. 855 – 1098

XXVII Cracow Epiphany Conference on Future of Particle Physics

Cracow, Poland; January 7–10, 2021


Comparison of Machine Learning Approach to Other Commonly Used Unfolding Methods


Unfolding in high energy physics represents the correction of measured spectra in data for the finite detector efficiency, acceptance, and resolution from the detector to particle level. Recent machine learning approaches provide unfolding on an event-by-event basis allowing to simultaneously unfold a large number of variables and thus to cover a wider region of the features that affect detector response. This study focuses on a simple comparison of commonly used methods in RooUnfold package to the machine learning package OmniFold.

Angular Observables in the \(B^0 \rightarrow K^{*0} \mu ^+ \mu ^-\) Decay at LHCb


This contribution summarizes the most recent results of measurements of the angular observables in \(B^0 \rightarrow K^{*0} \mu ^+ \mu ^-\) decays using data samples from the LHCb experiment. Measurement of \(P^\prime _5\) observable shows 2.5 and 2.9\(\sigma \) deviation from the Standard Model prediction in two \(q^2\) bins. Also for the value of \(\mathcal {R}e(C_9)\) parameter 3.3\(\sigma \) deviation was observed.

Simplified Differential Equations for Master Integrals at N\(^3\)LO


We present the application of the Simplified Differential Equations approach for the computation of three-loop families of master integrals. More specifically, we apply our method to compute the ladder-box-type integrals with up to one massive leg.

Overview of CMS Results — Recent Highlights


In these proceedings, highlights of the CMS experiment are presented, with a focus on the latest results obtained when analysing the whole LHC Run 2 dataset. The presentation of the CMS results is divided into three main parts. The first one is dedicated to Standard Model measurements as a probe for new physics, with, in particular, the presentation of multiboson production results. The second part covers the latest Higgs boson measurements in various decay channels, with a focus on the recent CMS observation of the Higgs boson decay to a pair of muons. The last part presents a selection of few results on direct searches for new physics.

Detector Simulation for a Potential Upgrade of the Vertex Detector of the Belle II Experiment


We present the results of the first detailed simulations done for an upgraded Belle II vertex detector with a fully pixelated detector and different geometries. We observed that the new designs have better tracking performances compared to the current one, are more robust to the beam-induced background level, and have a lower occupancy. All these improvements will be greatly beneficial as the instantaneous luminosity increases.

Measurement of the Associated Production of a Higgs Boson Decaying into \(b\)-quarks with a Vector Boson at High Transverse Momentum with the ATLAS Detector


The observation of the decay of the Higgs boson to a \(b \bar {b}\) pair in 2018 provided a confirmation of the Yukawa interaction and the first measurement of a Yukawa coupling to down-type quarks. The experimental challenges that the \(H \rightarrow b \bar {b}\) decay entails at hadron colliders can be addressed by studying the associated Higgs boson production with a vector boson \(V\) decaying leptonically. Following the observation, the main focus is now the study of the Higgs boson production cross section as a function of its transverse momentum (\(p_{\mathrm {T}}\)). These measurements are well motivated by a wide range of Beyond the Standard Model (BSM) theories predicting an enhancement of \(VH(H \rightarrow b \bar {b})\) events at high transverse momentum with respect to the SM expectations. At such high energy regimes, the Higgs boson is highly Lorentz-boosted and the \(H \rightarrow b\bar {b}\) decay is reconstructed in the detector as a single object with a two-prong substructure, using novel techniques. The paper will illustrate the measurement of \(VH(H \rightarrow b \bar {b})\) in the boosted regime performed inclusively, as well as in bins of the vector boson transverse momentum, for \(p_{\mathrm {T}}^V \in [250,400)\) GeV and \(p_\mathrm {T}^V \in [400,\infty )\) GeV. Their interpretation using the Effective Field Theory (EFT) framework will also be discussed. The results are based on the full LHC Run 2 dataset collected by the ATLAS experiment, corresponding to an integrated luminosity of 139 fb\(^{-1}\).

Measurement of the Higgs Simplified Template Cross Sections using \(H \to \gamma \gamma \) Decays with the ATLAS Experiment


The measurement of Higgs Simplified Template Cross Sections (STXS) in the \(H \to \gamma \gamma \) decay channel by the ATLAS experiment at the LHC is presented. The analysis relies on categorizing events with a machine learning approach according to their topology. In addition, strategies for the interpretation of the results in terms of an Effective Field Theory are discussed.

Theory Needs for Future \(e^+e^-\) Colliders


New high-luminosity \(e^+e^-\) colliders have been proposed to perform precision measurements of electroweak and Higgs physics to scrutinize the mechanism of electroweak symmetry breaking and search for signs of new physics. The interpretation of the data from such a machine is only possible with the help of accurate theoretical calculations of the Standard Model expectations, including higher-order radiative corrections. This contribution provides an overview of the current knowledge and required improvements for our theoretical understanding of a range of key observables. Furthermore, it gives a short summary of the calculational techniques that can be leveraged to realize these improvements.

Sector Decomposition Scheme for N\(^3\)LO Beam Function


This article presents preliminary results for the calculation of quark-to-quark \(\mathrm {N^3LO}\) beam function. To this end, we employ the techniques of sector decomposition and selector functions, along with other techniques to supplement the calculation. Our results show agreement with available predictions from the renormalization group.

Long-lived Particles at Future Colliders


Long-lived particles have significant enough lifetimes as to, when produced in collisions, leave a distinct signature in the detectors. Driven by increasingly higher energies, trigger and reconstruction algorithms at particle colliders are optimized for increasingly heavier particles, which in turn, tend to be short-lived. This makes searches for long-lived particles difficult, usually requiring dedicated methods and sometimes dedicated hardware to spot them. However, taking upon the challenge brings enormous potential, since new, long-lived particles feature in a variety of promising new physics models that could answer most of the open questions of the Standard Model, such as: neutrino masses, dark matter, or the matter–antimatter unbalance in the Universe. Currently, the international high energy physics community is planning future post-LHC facilities, and various particle colliders have been proposed. Crucial physics cases connected to long-lived particles will be accessible then, and in this presentation, three interesting examples are highlighted: heavy neutral leptons, hidden sectors connected to dark matter, and exotic Higgs boson decays. This is followed by a small review of the preliminary studies assuming different future colliders, exploiting the complementary advantages that different colliding particles and accelerator types provide.

On the Role of the Precision Monte Carlo Generators in Future Electron Colliders


In this note, we argue that in the future electron–positron colliders with up to two orders higher event rates, the role of the Standard Model precision calculations in the data analysis performed using Monte Carlo (MC) event generators will be dominant, much more important than in the past LEP era experiments. This will require designing, constructing and testing an entirely new class of the precision Monte Carlo event generators for all important processes such as production and decay of the \(Z\) and Higgs boson, \(W\) pairs and the Bhabha process. We are going to outline challenges on the way to construction of these new MC tools and foresee possible lines of technical developments which will be necessary.

Solving the Puzzle of Nuclear Stopping Power


This paper reviews our recent findings on the dynamics of transport of baryon number in proton-induced reactions at the CERN SPS. These are put in a more general context of the present understanding of baryon stopping phenomena up to the LHC and cosmic ray energies. The implications of our studies, advantages to be provided by modern and high-quality experimental data, and perspectives of new measurements are shortly discussed.

M-theory and the Birth of the Universe


In this review article, we first discuss a possible regularization of the Big Bang curvature singularity of the standard Friedmann cosmology, where the curvature singularity is replaced by a spacetime defect. We then consider the hypothesis that a new physics phase gave rise to this particular spacetime defect. Specifically, we set out on an explorative calculation using the IIB matrix model, which has been proposed as a particular formulation of nonperturbative superstring theory (M-theory).

all authors

P. Fernandez Declara, G. Ganis, B. Hegner, C. Helsens, M. Petric, A. Sailer, V. Volkl, F. Gaede, T. Madlener, W. Fang, W. Li, T. Lin, X. Zhang, J. Zou, X. Huang, T. Li, S.H. Ko, J. Wang

Key4hep — Turnkey Software for Future Colliders


Future collider experiments rely on a well-maintained software for their physics performance studies and detector optimisation. The Key4hep project aims to design and provide a common set of software tools that can be used by future or even present-day high-energy physics projects. It unites the communities of all current future collider projects. These proceedings give an overview of the goals of the Key4hep project and briefly describe the main components that are currently under development: the common event data model, EDM4hep, the detector description toolkit, DD4hep, the interfaces to Delphes, a brief description of the core framework and, finally, the infrastructure to deploy and build the whole software stack using spack. They also include some details about how different communities plan to adapt their software stacks to the Key4hep project. Overall, they show that the Key4hep software stack can be used already for first physics studies and highlight its potential as a baseline for future high-energy physics experiments.

Exclusive Dilepton Production in Ultraperipheral Lead–Lead Collisions in the ATLAS Experiment


Exclusive dilepton pairs are produced via electromagnetic interactions in ultraperipheral heavy-ion collisions (UPC). Electromagnetic fields associated with relativistic lead nuclei can be considered as fluxes of quasi-real photons. The dilepton photoproduction, \(\gamma \gamma \rightarrow \ell ^{+}\ell ^{-}\), is one of the fundamental processes in UPC and, therefore, can provide a reference for other processes. Given the large theoretical uncertainty of photon flux modelling, its precise measurement with exclusive dilepton pairs can improve predictions for other photoproduction processes. The results for the \(\gamma \gamma \rightarrow \mu ^{+}\mu ^{-}\) process using 2015 Pb+Pb data collected by the ATLAS experiment at the Large Hadron Collider are presented along with control distributions for the \(\gamma \gamma \rightarrow e^{+}e^{-}\) process in 2018 Pb+Pb data.

Spectroscopy in Beauty Decays at the LHCb Experiment


The beauty hadron decays provide a unique laboratory to study charmonium and charmonium-like states, such as the \(\chi _{c1}(3872)\) meson, other exotic states, and the tensor \(D\)-wave \({\psi }_2(3823)\) states. However, the nature of many exotic charmonium-like candidates is still unknown. The most recent LHCb results related to \(b\)-hadron decays to charmonium states and obtained using large data samples collected during the Run 1 and Run 2 periods are presented. This includes the most precise determination of the mass and width of the \(\chi _{c1}(3872)\) state using the \(B^+ \to J/\psi \pi ^+ \pi ^-K^+\) decays, observation of a resonant structure denoted as \(X(4740)\) in the \(J/\psi \phi \) mass spectrum from \(B^0_s \to J/\psi \pi ^+\pi ^-K^+K^-\) decays and the precise measurement of the \(B^0_s\) meson mass.

Extended Scalar Sectors at Future Colliders


After the discovery of the Higgs boson in 2012, particle physics has entered an exciting era. An important question is whether the Standard Model of particle physics correctly describes the scalar sector realized by nature, or whether it is part of a more extended model, featuring additional particle content. A prime way to test this is to probe models with extended scalar sectors at future collider facilities. We here discuss such models in the context of high-luminosity LHC, a possible proton–proton collider with 27 and 100 TeV center-of-mass energy, as well as future lepton colliders with various center-of-mass energies.

Mixing and CP Violation in Charm Mesons at LHCb


In 2019, LHCb reported the first observation of direct CP violation in the charm sector. As of yet indirect CP violation has not been observed but measurements of this and mixing is essential to our understanding of the Standard Model. We report on the latest measurements of CP violation and mixing in charm mesons from LHCb and present prospects for future measurements.

Conformal Transformations of Conservation Equations in Spin Hydrodynamics


We study and analyse the conformal transformations of different conservation laws in the spin hydrodynamics framework.

Analytic Representation of All Planar Two-loop Five-point Master Integrals with One Off-shell Leg


In this contribution, we present analytic expressions in terms of polylogarithmic functions for all three families of planar two-loop five-point master integrals with one off-shell leg, recently published in arXiv:2009.13917 [hep-ph]. The calculation is based on the Simplified Differential Equations approach. The results are relevant to the study of many \(2\to 3\) scattering processes of interest at the LHC, especially for the leading-color \(W+2\) jets production.

List of Participants


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