Minimum Bias Events Research Articles

Overview of Energy Reconstruction, and Electron and Photon Performances with the CMS ECAL in Run II

The electromagnetic calorimeter (ECAL) of the Compact Muon Solenoid (CMS) Experiment is crucial for achieving high resolution measurements of electrons and photons. Maintaining and possibly improving the excellent performance achieved in Run I is vital for measurements of the Standard Model Higgs boson and searches for new higher mass resonances in final states with electrons and photons. Since spring 2015, the ECAL has operated with proton-proton collisions at 13 TeV center-of-mass energy and at a reduced bunch spacing of 25 ns. The instantaneous luminosity delivered by the LHC during Run II is expected to exceed the levels previously attained. The average number of concurrent proton-proton collisions per bunch-crossing (pileup) is expected to reach up to 40 interactions. In this summary we present new crystal energy reconstruction algorithms and clustering techniques that have been developed to maintain the excellent performance of the CMS ECAL throughout Run II. We will show first performance results from 2015 data, achieved through energy calibrations using electrons from W and Z boson decays, photons from π0/η decays, and the azimuthally symmetric energy distribution of minimum bias events. Lastly, we present an outlook on the expected Run II performance in the next years.

Precision crystal calorimetry in LHC Run II with the CMS ECAL

The electromagnetic calorimeter (ECAL) of the Compact Muon Solenoid (CMS) Experiment, based on lead tungstate scintillating crystals, is crucial for achieving high-resolution measurements of electrons and photons. Maintaining and possibly improving the excellent performance achieved in Run I is vital for measurements of the Standard Model Higgs boson and searches for new higher mass resonances in final states with electrons and photons. In Spring 2015 the LHC started “Run II”, colliding protons at 13 TeV centre-of-mass energy and with 25 ns bunch spacing. This is very close to the original design specifications of the LHC (14 TeV at 25 ns). At this higher energy, and with the rapidly growing dataset, the performance for higher electron and photon energies becomes crucial. At the same time, the instantaneous luminosity has increased and, over the coming years, is expected to surpass the design value, possibly by a factor of two to about 2×1034 cm−2 s−1. The average number of concurrent proton-proton collisions per bunch crossing (pileup) is expected to reach about 40. This pileup is a major challenge, both for calibration and for ultimate energy reconstruction. The CMS ECAL design ensures that its superb performance extends over a very wide range of energies up to electron and photon energies of 1 TeV and beyond. We present new energy reconstruction algorithms and clustering techniques, developed to maintain the excellent performance of the CMS ECAL throughout Run II. We will show first performance results from 2015 and 2016 data, including triggering efficiency, event reconstruction and calibration precision. The latter has been achieved through the measurements of electrons from W and Z boson decays, photons from π0/η decays, and the azimuthally-symmetric energy distribution of minimum bias events. We also present an outlook on the expected Run II performance in the coming years, including the impact of the ECAL on resonance searches in the mass range up to 1 TeV.

Perspetives of study the direct photon production process at FAIR energy

The modeling of high energy photons production in collisions of antiproton beam having E beam = 15 GeV with the proton target pp→ γ + {ptX} is done using the event sample simulated by PYTHIA6 generator. Such energy is high enough to consider this collision as a relativistic one and being caused by parton-parton scattering. The distribution of the set of kinematic variables and cuts which can be useful for getting the information about proton structure in the available kinematic region is obtained. The contributions of fake photons which can appear from the hadron decays as well as of the background caused by the minimum bias events and other QCD processes are estimated. The set of cuts which can be useful for separation of signal events containing the direct photons from background events is proposed.

Centrality and rapidity dependence of inclusive jet production in [formula omitted] proton–lead collisions with the ATLAS detector

Measurements of the centrality and rapidity dependence of inclusive jet production in sNN=5.02 TeV proton–lead (p+Pb) collisions and the jet cross-section in s=2.76 TeV proton–proton collisions are presented. These quantities are measured in datasets corresponding to an integrated luminosity of 27.8 nb−1 and 4.0 pb−1, respectively, recorded with the ATLAS detector at the Large Hadron Collider in 2013. The p+Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval −4.9<η<−3.2 in the direction of the lead beam. Results are presented for the double-differential per-collision yields as a function of jet rapidity and transverse momentum (pT) for minimum-bias and centrality-selected p+Pb collisions, and are compared to the jet rate from the geometric expectation. The total jet yield in minimum-bias events is slightly enhanced above the expectation in a pT-dependent manner but is consistent with the expectation within uncertainties. The ratios of jet spectra from different centrality selections show a strong modification of jet production at all pT at forward rapidities and for large pT at mid-rapidity, which manifests as a suppression of the jet yield in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton–nucleus collisions. Furthermore, the modifications at forward rapidities are found to be a function of the total jet energy only, implying that the violations may have a simple dependence on the hard parton–parton kinematics.

Multilevel Workflow System in the ATLAS Experiment

The ATLAS experiment is scaling up Big Data processing for the next LHC run using a multilevel workflow system comprised of many layers. In Big Data processing ATLAS deals with datasets, not individual files. Similarly a task (comprised of many jobs) has become a unit of the ATLAS workflow in distributed computing, with about 0.8M tasks processed per year. In order to manage the diversity of LHC physics (exceeding 35K physics samples per year), the individual data processing tasks are organized into workflows. For example, the Monte Carlo workflow is composed of many steps: generate or configure hard-processes, hadronize signal and minimum-bias (pileup) events, simulate energy deposition in the ATLAS detector, digitize electronics response, simulate triggers, reconstruct data, convert the reconstructed data into ROOT ntuples for physics analysis, etc. Outputs are merged and/or filtered as necessary to optimize the chain. The bi-level workflow manager - ProdSys2 - generates actual workflow tasks and their jobs are executed across more than a hundred distributed computing sites by PanDA - the ATLAS job-level workload management system. On the outer level, the Database Engine for Tasks (DEfT) empowers production managers with templated workflow definitions. On the next level, the Job Execution and Definition Interface (JEDI) is integrated with PanDA to provide dynamic job definition tailored to the sites capabilities. We report on scaling up the production system to accommodate a growing number of requirements from main ATLAS areas: Trigger, Physics and Data Preparation.

Review of physics results from the Tevatron: QCD physics

We present a summary of results from studies of quantum chromodynamics at the Fermilab Tevatron collider by the CDF and the D0 experiments. These include Run II results for the time period up to the end of Summer 2014. A brief description of Run I results is also given. This review covers a wide spectrum of topics, and includes measurements with jet and vector boson final states in the hard (perturbative) energy regime, as well as studies of soft physics such as diffractive and elastic scatterings, underlying and minimum bias events, hadron fragmentation, and multiple parton interactions.

Ballistic protons in incoherent exclusive vector meson production as a measure of rare parton fluctuations at an electron-ion collider.

We argue that the proton multiplicities measured in Roman pot detectors at an electron ion collider can be used to determine centrality classes in incoherent diffractive scattering. Incoherent diffraction probes the fluctuations in the interaction strengths of multiparton Fock states in the nuclear wave functions. In particular, the saturation scale that characterizes this multiparton dynamics is significantly larger in central events relative to minimum bias events. As an application, we study the centrality dependence of incoherent diffractive vector meson production. We identify an observable which is simultaneously very sensitive to centrality triggered parton fluctuations and insensitive to details of the model.

The CMS electromagnetic calorimeter calibration during Run I: progress achieved and expectations for Run II

The CMS ECAL is a high-resolution, hermetic, and homogeneous electromagnetic calorimeter made of 75,848 scintillating lead tungstate crystals. It relies on precision calibration in order to achieve and maintain its design performance. A set of inter-calibration procedures is carried out to normalize the differences in crystal light yield and photodetector response between channels. Different physics channels such as low mass di-photon resonances, electrons from W and Z decays and the azimuthal symmetry of low energy deposits from minimum bias events are used. A laser monitoring system is used to measure and correct for response changes, which arise mainly from the harsh radiation environment at the LHC. The challenges of the different calibration techniques are discussed along with the performance evolution during Run I. The impact on physics performance is illustrated through the successful quest for the Higgs boson via its electromagnetic decays, and the subsequent mass measurement of the newly discovered particle. Conclusions are drawn for the performance to be expected from 2015 onwards, following the start of the LHC Run II.

Measurement of electrons from semileptonic heavy-flavor hadron decays inppcollisions ats=2.76 TeV

The $p_{\rm T}$-differential production cross section of electrons from semileptonic decays of heavy-flavor hadrons has been measured at mid-rapidity in proton-proton collisions at $\sqrt{s} = 2.76$ TeV in the transverse momentum range 0.5 < $p_{\rm T}$ < 12 GeV/$c$ with the ALICE detector at the LHC. The analysis was performed using minimum bias events and events triggered by the electromagnetic calorimeter. Predictions from perturbative QCD calculations agree with the data within the theoretical and experimental uncertainties.

Hard Diffraction with Proton Tagging at the LHC

The main parts of the LHC diffractive physics programme possible to be measured using a proton tagging technique are presented. The geometric acceptance of the ATLAS forward proton detectors: ALFA and AFP for various LHC optics settings are shown. The probabilities of observing a proton originating from a minimum-bias event in ALFA and AFP stations are given. The main properties of single diffractive and double Pomeron exchange production of dijets, photon+jet, jet-gap-jet and W/Z bosons are discussed. The possibility of measuring the jet production in exclusive (double proton tag) and semi-exclusive (single tag) mode is evaluated.

Hydrodynamics and jets in dialogue

Energy and momentum loss of jets in heavy ion collisions can affect the fluid dynamic evolution of the medium. We determine realistic event-by-event averages and correlation functions of the local energy-momentum transfer from hard particles to the soft sector using the jet-quenching Monte-Carlo code Jewel combined with a hydrodynamic model for the background. The expectation values for source terms due to jets in a typical (minimum-bias) event affect the fluid dynamic evolution mainly by their momentum transfer. This leads to a small increase in flow. The presence of hard jets in the event constitutes only a minor correction.

Characteristics of disintegration of different emulsion nuclei by relativistic 28Si nuclei at 3.7 A GeV

An analysis of the data based on 924 inelastic interaction events induced by 28Si nuclei in a nuclear emulsion is presented. The nuclear fragmentation process is studied by analysing the total charge (Q) distribution of the projectile spectators for different emulsion target groups along with the comparison of Monte Carlo Glauber model results. Probability distributions for total disintegrated events as a function of different projectile masses are shown and compared with cascade evaporation model results at same energy per nucleon. Further, mean multiplicities of different charged secondaries for different classes of events are presented and for each event, variation of mean multiplicities as a function of total charge (Q) is also presented. The pseudorapidity distributions and normalized pseudorapidity distributions of the produced charged particles in nucleus–nucleus collisions at 3.7 A GeV are analysed for total disintegration (TD) as well as minimum-bias events.

Performance of the LHCb Vertex Locator

The Vertex Locator (VELO) is a silicon microstrip detector that surrounds the proton-proton interaction region in the LHCb experiment. The performance of the detector duringthe first years of its physics operation is reviewed. The system isoperated in vacuum, uses a bi-phase CO2 cooling system, and thesensors are moved to 7 mm from the LHC beam for physics data taking. The performance and stability of these characteristic features of the detector are described, and details of the material budget are given. The calibration of the timing and the data processing algorithms that are implemented in FPGAs are described.The system performance is fully characterised. The sensors have asignal to noise ratio of approximately 20 and a best hit resolution of4 μm is achieved at the optimal track angle. The typical detectoroccupancy for minimum bias events in standard operating conditions in2011 is around 0.5%, and the detector has less than 1% of faultystrips. The proximity of the detector to the beam means that the innerregions of the n+-on-n sensors have undergone space-charge signinversion due to radiation damage. The VELO performance parametersthat drive the experiment's physics sensitivity are also given. Thetrack finding efficiency of the VELO is typically above 98% and themodules have been aligned to a precision of 1 μm for translations inthe plane transverse to the beam. A primary vertex resolution of 13 μm in thetransverse plane and 71 μm along the beam axis is achieved forvertices with 25 tracks. An impact parameter resolution ofless than 35 μm is achieved for particles with transverse momentumgreater than 1 GeV/c.

Centrality and energy dependence of elliptic flow of light nuclei and hadrons in STAR

We present the measurement of azimuthal anisotropy in the distributions of deuteron ( d ), anti-deuteron ( d ¯ ) , triton ( t ) and helium ( 3 He) nuclei produced at midrapidity ( | η | < 1.0 ) in heavy-ion collisions at s NN = 200 , 62.4 , 39 , 27 , 19.6 , 11.5 and 7.7 GeV . We also present systematic study of the difference between particle and anti-particle elliptic flow ( Δ v 2 ) for identified hadrons ( π ± , K ± , p , p ¯ , Λ , Λ ¯ and Ξ ± ) at mid-rapidity for Au+Au collisions at RHIC Beam Energy Scan (BES) energies. Elliptic flow coefficient v 2 of nuclei has been compared with those for p , p ¯ . We observe mass ordering in nuclei v 2 ( p T ) for low transverse momenta ( p T < 2.0 GeV / c ) and a clear centrality dependence. The triton and helium, having almost the same mass, show similar magnitude of v 2 within the available event statistics. Mass number ( A ) scaling of nuclei v 2 ( p T ) has been observed, which indicates the possibility of coalescence mechanism to be the underlying physics behind nuclei formation in heavy ion collisions. New results on Δ v 2 for all particles, studied in 10–40% centrality, show similar trend as was observed for minimum bias (0–80%) events. The proton and anti-proton Δ v 2 for three different collision centralities is presented as a function of s NN .

Measurement of charged particle multiplicities and densities in [Formula: see text] collisions at [Formula: see text]TeV in the forward region.

Charged particle multiplicities are studied in proton–proton collisions in the forward region at a centre-of-mass energy of sqrt{s} = 7;TeV with data collected by the LHCb detector. The forward spectrometer allows access to a kinematic range of 2.0<eta <4.8 in pseudorapidity, momenta greater than 2;text{ GeV/ }c and transverse momenta greater than 0.2;text{ GeV/ }c. The measurements are performed using events with at least one charged particle in the kinematic acceptance. The results are presented as functions of pseudorapidity and transverse momentum and are compared to predictions from several Monte Carlo event generators.

Thermal neutron flux measurements in the STAR experimental hall

We report measurements of thermal neutron fluxes at different locations in the STAR experimental hall during RHIC Run 13 with proton–proton collisions at s=510GeV. We compare these measurements to calculations based on PYTHIA as a minimum bias event generator, detailed GEANT3 simulation of the STAR detector and experimental hall, and with GCALOR as the neutron transport code. A fairly good agreement was found between simulation and measurements.

Production ofKS0,K*±(892)andϕ0(1020)in minimum bias events andKS0andΛ0in jets inpp¯collisions ats=1.96 TeV

We report measurements of the inclusive transverse momentum pT distribution of centrally produced kshort, kstar(892), and phi(1020) mesons up to pT = 10 GeV/c in minimum-bias events, and kshort and lambda particles up to pT = 20 GeV/c in jets with transverse energy between 25 GeV and 160 GeV in pbar p collisions. The data were taken with the CDF II detector at the Fermilab Tevatron at sqrt(s) = 1.96 TeV. We find that as pT increases, the pT slopes of the three mesons (kshort, kstar, and phi) are similar, and the ratio of lambda to kshort as a function of pT in minimum-bias events becomes similar to the fairly constant ratio in jets at pT ~ 5 GeV/c. This suggests that the particles with pT >~ 5 GeV/c in minimum-bias events are from soft jets, and that the pT slope of particles in jets is insensitive to light quark flavor (u, d, or s) and to the number of valence quarks. We also find that for pT <~ 4 GeV relatively more lambda baryons are produced in minimum-bias events than in jets.

Multiplicity distributions in the forward rapidity region in proton-proton collisions at the Large Hadron Collider

Measured multiplicity distributions of primary charged particles produced in the forward rapidity region of the proton-proton ($pp$) collisions at the center-of-mass energy, $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$, at the LHC have been analyzed in terms of the negative binomial distribution function. Like the multiplicity distributions in the midrapidity region for the $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$, the distributions for the minimum bias events in the forward region are also better described with the superposition of two negative binomail distributions, as proposed by a two-component model of particle production from two processes, the soft and the hard. However, the multiplicity distribution for the ``hard-QCD'' events in a large pseudorapidity window does not oblige the two-component model.

Monte-Carlo simulation of lepton pairs production in "pp̄ → μ+μ− + X" events at Ebeam = 5 GeV

Muon pairs production via the quark-antiquark annihilation process in collisions of antiproton beam with Ebeam = 5 GeV (which corresponds to the center-of-mass energy of the pp̄ system Ecm = 3.363 GeV) with the proton target in PANDA experiment is modeled by using of PYTHIA6.4 generator. The considered quark level subprocess goes through the production of virtual photon which converts into lepton pair (qq̄ → γ* → μ+μ−). Quark-antiquark annihilation process of hadron-hadron collision may provide an interesting information about the quark dynamics inside the hadron. The measurement of the total transverse momentum of a lepton pair as a whole system may provide an important information about the intrinsic transverse momentum kT that appears due to the Fermi motion of quarks inside the nucleon. The distributions of different kinematical variables of final state muons are presented. The problems due to the presence of fake leptons that appear from meson decays, as well as due to the background caused by minimum bias events are also discussed. The set of cuts which allows to separate the signal events with lepton pairs from the backgrounds is proposed.

SOFT QCD AND DIFFRACTIVE PHYSICS AT LHC

After a short introduction on the importance of the soft and of the diffractive studies in the understanding of minimum bias events, the main results obtained at LHC are discussed. This overview includes identified particle and inclusive measurements, minimum bias and underlying events, all of them shedding light on the soft process production mechanisms. The results of the inelastic cross-section measurements obtained by the LHC experiments and their compatibility are discussed together with the models used to extrapolate the data at low diffractive masses. A review of the most recent diffraction results is presented, showing the different approaches used by the LHC experiments, relying on different experimental techniques. The combination of the results obtained by ALICE, ATLAS, CMS, LHCb and TOTEM provides a wide sample of informations, covering an unprecedented pseudorapidity range. A detailed comparison between the obtained results is shown, followed by a critical discussion on the still existing discrepancies between the experimental data and the Monte Carlo used at LHC to simulate soft and diffractive physics.