Charged-particle multiplicity fluctuations in Pb–Pb collisions at $$\sqrt{s_{\mathrm {NN}}}$$ = 2.76 TeV

Measurements of event-by-event fluctuations of charged-particle multiplicities in Pb–Pb collisions at $$\sqrt{s_{\mathrm {NN}}}$$ s NN $$=$$ = 2.76 TeV using the ALICE detector at the CERN Large Hadron Collider (LHC) are presented in the pseudorapidity range $$|\eta |<0.8$$ | η | < 0.8 and transverse momentum $$0.2< p_{\mathrm{T}} < 2.0$$ 0.2 < p T < 2.0 GeV/c. The amplitude of the fluctuations is expressed in terms of the variance normalized by the mean of the multiplicity distribution. The $$\eta $$ η and $$p_{\mathrm{T}}$$ p T dependences of the fluctuations and their evolution with respect to collision centrality are investigated. The multiplicity fluctuations tend to decrease from peripheral to central collisions. The results are compared to those obtained from HIJING and AMPT Monte Carlo event generators as well as to experimental data at lower collision energies. Additionally, the measured multiplicity fluctuations are discussed in the context of the isothermal compressibility of the high-density strongly-interacting system formed in central Pb–Pb collisions.

Study of theoretical luminosity precision for electron colliders at higher energies

We present an estimation of the theoretical precision of low angle Bhabha scattering at the proposed future ILC collider at 500 GeV. The analysis is an extension of the previous analysis done for the FCCee collider at $$\sqrt{s}=M_Z$$ s = M Z . As the state-of-the-art and the reference point we use the Monte Carlo event generator. Based on the current precision status of for LEP analysis, we estimate how various error components evolve from the LEP to ILC setups. The conclusion of our work is that for the ILC the precision of the current version of 4.04 deteriorates to 0.5%, by more than an order of magnitude w.r.t. the present precision for LEP. With the expected future improvements, the precision of can change to 0.016%, nearly as good as for the FCCee at the $$M_Z$$ M Z setup (0.01%). Based on the developed methodology we present also results for ILC$$_{1000}$$ 1000 , FCCee$$_{350}$$ 350 and CLIC$$_{3000}$$ 3000 setups.

Optimising simulations for diphoton production at hadron colliders using amplitude neural networks

Machine learning technology has the potential to dramatically optimise event generation and simulations. We continue to investigate the use of neural networks to approximate matrix elements for high-multiplicity scattering processes. We focus on the case of loop-induced diphoton production through gluon fusion, and develop a realistic simulation method that can be applied to hadron collider observables. Neural networks are trained using the one-loop amplitudes implemented in the NJet C++ library, and interfaced to the Sherpa Monte Carlo event generator, where we perform a detailed study for 2 → 3 and 2 → 4 scattering problems. We also consider how the trained networks perform when varying the kinematic cuts effecting the phase space and the reliability of the neural network simulations.

The Z lineshape challenge: ppm and keV measurements

The FCC-ee offers powerful opportunities for direct or indirect evidence for physics beyond the standard model, via a combination of high-precision measurements and searches for forbidden and rare processes and feebly coupled particles. A key element of FCC-ee physics program is the measurement of the Z lineshape from a total of $$5\times 10^{12}$$ 5 × 10 12 Z bosons and a beam-energy calibration with relative uncertainty of $$10^{-6}$$ 10 - 6 . With this exceptionally large event sample, five orders of magnitude larger than that accumulated during the whole LEP1 operation at the Z pole, the defining parameters—$$m_\mathrm{Z}$$ m Z , $$\Gamma _\mathrm{Z}$$ Γ Z , $$N_\nu $$ N ν , $$\sin ^2\theta _\mathrm{W}^\mathrm{eff}$$ sin 2 θ W eff , $$\alpha _\mathrm{S}(m_\mathrm{Z}^2)$$ α S ( m Z 2 ) , and $$\alpha _\mathrm{QED}(m^2_\mathrm{Z})$$ α QED ( m Z 2 ) —can be extracted with a leap in accuracy of up to two orders of magnitude with respect to the current state of the art. The ultimate goal that experimental and theory systematic errors match the statistical accuracy (4 keV on the Z mass and width, $$3\times 10^{-6}$$ 3 × 10 - 6 on $$\sin ^2\theta _\mathrm{W}^\mathrm{eff}$$ sin 2 θ W eff , a relative $$3\times 10^{-5}$$ 3 × 10 - 5 on $$\alpha _\mathrm{QED}$$ α QED , and less than 0.0001 on $$\alpha _\mathrm{S}$$ α S ) leads to highly demanding requirements on collider operation, beam instrumentation, detector design, computing facilities, theoretical calculations, and Monte Carlo event generators. Such precise measurements also call for innovative analysis methods, which require a joint effort and understanding between theorists, experimenters, and accelerator teams.

Probing QCD with LHCb

While the LHCb detector was specifically designed for measuring heavy-flavor physics, it has also proven itself as a forward general purpose detector with flexible data acquisition, excellent lifetime resolution and charged particle identification. This makes LHCb an ideal laboratory for exploring phenomena related to quantum chromodynamics (QCD), particularly with heavy-flavor content. This review explores some of the novel QCD measurements from LHCb, with an emphasis placed on comparisons to predictions from the Pythia 8 Monte Carlo event generator.

Hadronic rescattering in pA and AA collisions

In a recent article we presented a model for hadronic rescattering, and some results were shown for $$\mathrm {p}\mathrm {p}$$ p p collisions at LHC energies. In order to extend the studies to $$\mathrm {p}\mathrm {A}$$ p A and $$\mathrm {A}\mathrm {A}$$ A A collisions, the Angantyr model for heavy-ion collisions is taken as the starting point. Both these models are implemented within the general-purpose Monte Carlo event generator Pythia, which makes the matching reasonably straightforward, and allows for detailed studies of the full space–time evolution. The rescattering rate is significantly higher than in $$\mathrm {p}\mathrm {p}$$ p p , especially for central $$\mathrm {A}\mathrm {A}$$ A A collisions, where the typical primary hadron rescatters several times. We study the impact of rescattering on a number of distributions, such as $$p_{\perp }$$ p ⊥ and $$\eta $$ η spectra, and the space–time evolution of the whole collision process. Notably rescattering is shown to give a significant contribution to elliptic flow in $$\mathrm {XeXe}$$ XeXe and $$\mathrm {PbPb}$$ PbPb , and to give a nontrivial impact on charm production.

Challenges in Monte Carlo Event Generator Software for High-Luminosity LHC

We review the main software and computing challenges for the Monte Carlo physics event generators used by the LHC experiments, in view of the High-Luminosity LHC (HL-LHC) physics programme. This paper has been prepared by the HEP Software Foundation (HSF) Physics Event Generator Working Group as an input to the LHCC review of HL-LHC computing, which has started in May 2020.

CASCADE3 A Monte Carlo event generator based on TMDs

The Cascade3 Monte Carlo event generator based on Transverse Momentum Dependent (TMD) parton densities is described. Hard processes which are generated in collinear factorization with LO multileg or NLO parton level generators are extended by adding transverse momenta to the initial partons according to TMD densities and applying dedicated TMD parton showers and hadronization. Processes with off-shell kinematics within $$k_{{t}}$$ k t -factorization, either internally implemented or from external packages via LHE files, can be processed for parton showering and hadronization. The initial state parton shower is tied to the TMD parton distribution, with all parameters fixed by the TMD distribution.

Measurement of differential cross sections for Z bosons produced in association with charm jets in pp collisions at $$ \sqrt{s} $$ = 13 TeV

Measurements are presented of differential cross sections for the production of Z bosons in association with at least one jet initiated by a charm quark in pp collisions at $$ \sqrt{s} $$ s = 13 TeV. The data recorded by the CMS experiment at the LHC correspond to an integrated luminosity of 35.9 fb−1. The final states contain a pair of electrons or muons that are the decay products of a Z boson, and a jet consistent with being initiated by a charm quark produced in the hard interaction. Differential cross sections as a function of the transverse momentum pT of the Z boson and pT of the charm jet are compared with predictions from Monte Carlo event generators. The inclusive production cross section 405.4 ± 5.6 (stat) ± 24.3 (exp) ± 3.7 (theo) pb, is measured in a fiducial region requiring both leptons to have pseudorapidity |η| < 2.4 and pT> 10 GeV, at least one lepton with pT> 26 GeV, and a mass of the pair in the range 71–111 GeV, while the charm jet is required to have pT> 30 GeV and |η| < 2.4. These are the first measurements of these cross sections in proton-proton collisions at 13 TeV.