Pp Interactions Research Articles

Overview of recent ALICE results

ALICE is the LHC experiment devoted to the study of heavy-ion collisions. While results from Pb-Pb collisions at √ sNN = 2.76 TeV provide insight on the properties of the plasma of quarks and gluons, formed in nucleus-nucleus interactions, the study of p-Pb collisions at √ sNN = 5.02 TeV allows a deeper understanding of cold nuclear matter effects. Therefore, p-Pb results turn out to be a powerful tool to provide a baseline for Pb-Pb, to correctly quantify how the various observables are affected by genuine hot medium effects. In this proceeding, a selection of the most recent ALICE results on the medium global properties and on heavy-flavour and quarkonium production will be discussed. ALICE (A Large Ion Collider Experiment) is one of the four large LHC experiments and its main objective is the study of the properties of the QGP. The ALICE experiment complements the experi- mental heavy-ion program, started about thirty years ago at the Brookhaven Alternating Gradient Syn- chrotron (AGS) and continued at the CERN Super Proton Synchrotron (SPS) up to √ sNN =20 GeV and finally at the Brookhaven Relativistic Heavy-Ion collider (RHIC) up to √ sNN =200 GeV. In the last five years ALICE has collected a large wealth of data, studying Pb-Pb collisions at √ sNN = 2.76 TeV and to complement its heavy-ion program, also p-A and pp interactions, to be used as references, have been investigated. In particular, since in the simpler p-A collision system the conditions which should lead to the formation of the QGP are not fullfilled, hot matter related effects are not expected to set in. Therefore, the study of the behaviour of several probes in the cold nuclear matter provides a fundamental reference to understand the Pb-Pb results, where hot matter effects are present on top of the cold matter ones. One way to investigate the global properties of the QCD medium is via the collective behaviour of the bulk of particles produced in the collisions. In the hot and strongly interacting medium collective behaviours can arise and flow of particles, generated by the presence of pressure gradients, can be

KS0andΛProduction in Pb-Pb Collisions atsNN=2.76 TeV

The ALICE measurement of KS0 and Λ production at midrapidity in Pb-Pb collisions at sNN=2.76 TeV is presented. The transverse momentum (pT) spectra are shown for several collision centrality intervals and in the pT range from 0.4 GeV/c (0.6 GeV/c for Λ) to 12 GeV/c. The pT dependence of the Λ/KS0 ratios exhibits maxima in the vicinity of 3 GeV/c, and the positions of the maxima shift towards higher pT with increasing collision centrality. The magnitude of these maxima increases by almost a factor of three between most peripheral and most central Pb-Pb collisions. This baryon excess at intermediate pT is not observed in pp interactions at s=0.9 TeV and at s=7 TeV. Qualitatively, the baryon enhancement in heavy-ion collisions is expected from radial flow. However, the measured pT spectra above 2 GeV/c progressively decouple from hydrodynamical-model calculations. For higher values of pT, models that incorporate the influence of the medium on the fragmentation and hadronization processes describe qualitatively the pT dependence of the Λ/KS0 ratio.Received 29 July 2013DOI:https://doi.org/10.1103/PhysRevLett.111.222301This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2013 CERN, for the ALICE Collaboration

An exploration of hadronic interactions in blazars using multi-messenger data

High energy photon and neutrino emissions are expected from the interactions of high-energy protons with matter and/or radiation in the blazars environment. In these proceedings, we use the sensitivity of the IceCube neutrino detector estimated for different neutrino spectral indices (derived in Tchernin et al. (2013) [12]) to show that already in its 40-string configuration (IC-40), the sensitivity of the IceCube neutrino telescope is already at the level of constraining the parameter space of purely hadronic scenarios of blazar activity. As a result, using the full detector sensitivity, those constraints should be about a factor of 3 better than what discussed here. Assuming that the γ-ray flux observed by Fermi can be explained with purely hadronic interactions, we estimate the expected neutrino flux from blazars based on the combination of the γ-ray flux observed by Fermi, with the simultaneous IC-40 observations. We consider separately the cases where the proton–proton or proton–γ interactions are dominant. In both cases, we set some constraints on the primary proton spectrum. In the case where pp interactions dominate, the tightest constraints are set for the source 3C 454.3, for which the high energy part of the spectrum is constrained to be harder than E−2 and the cut-off energy larger than 1018eV. Alternatively, in the case where the dominant channel is the interaction of the high-energy protons with the soft radiation fields, the magnitude of the constraints depends on the radiation field energy distribution. For the source 3C 273, the most constrained source of our sample for the pγ interaction, the cut-off energy is constrained to be Ecut≳1018eV for any spectral index and for different soft photon fields, including the radiation from the accretion disk (Big Blue Bump), the broad line region or the synchrotron radiation from the jet.

Modeling of exclusive parton distributions and long-range rapidity correlations in proton-proton collisions at the LHC energies

Soft pp interactions are considered in the framework of the phenomenological model with color strings formation. Under the assumption, that the elementary collision is realized as interaction of two color dipoles, the total inelastic cross section and the multiplicity of the charged particles are estimated and used for the parameters fixing. The special attention is given to the modelling of exclusive parton distributions taking into account the energy conservation and fixing the mass center that is necessary for the description of correlations, corresponding Monte Carlo procedures are developed for an arbitrary number of partons in a hadron. The string fusion algorithm in a cross-section plane is developed taking into account the finite rapidity width of strings. In this framework the influence of string fusion effects on long-range correlations of charged particles are estimated. The proposed algorithms are planned to be used for studying of heavy ion collisions.

Transverse momentum of protons, pions and kaons in high multiplicity pp and pA collisions: Evidence for the color glass condensate?

The CMS experiment at the Large Hadron Collider has recently presented data on the average transverse momentum of protons, kaons and pions as a function of particle multiplicity. We relate the particle multiplicity to saturation momentum using recently published computations of the interaction radius determined from the theory of the Color Glass Condensate. We show that the pp and the pA experimental data scale in terms of these saturation momenta. Computing transverse momentum spectra for identified particles using Boltzmann-type distributions and relating different associated multiplicities using geometric scaling, these simple distributions reproduce the observed dependence of the mean transverse momentum on particle multiplicities seen in both pp and pA interactions for pions to good accuracy, and to fair agreement for protons and kaons.

W, Z and photon production in CMS

The production of electroweak bosons (photons, W and Z particles) in PbPb and pp collisions at sqrt(s) = 2.76 TeV per interacting nucleon pair has been measured with the CMS detector at the LHC. Direct photon production is studied using samples of isolated photons. W and Z bosons are reconstructed through their leptonic decay into muons. Their production rate in PbPb data is studied as a function of the centrality of the collision and compared to that in pp interactions, once normalized by the number of binary nucleon-nucleon interactions. The results are also compared to next-to-leading-order perturbative QCD calculations.

An exploration of hadronic interactions in blazars using IceCube

Context: Hadronic models, involving matter (proton or nuclei) acceleration in blazar jets, imply high energy photon and neutrino emissions due to interactions of high-energy protons with matter and/or radiation in the source environment. Aims: This paper shows that the sensitivity of the IceCube neutrino telescope in its 40-string configuration (IC-40) is already at the level of constraining the parameter space of purely hadronic scenarios of activity of blazars. Methods: Assuming that the entire source power originates from hadronic interactions, and assuming that the models describe the data, we estimate the expected neutrino flux from blazars based on the observed gamma-ray flux by Fermi, simultaneously with IC-40 observations. We consider two cases separately to keep the number of constrainable parameters at an acceptable level: proton-proton or proton-gamma interactions are dominant. Comparing the IC-40 sensitivity to the neutrino flux expected from some of the brightest blazars, we constrain model parameters characterizing the parent high-energy proton spectrum. Results: We find that when pp interactions dominate, some constraints on the primary proton spectrum can be imposed. For instance, for the tightest constrained source 3C 454.3, the very high energy part of the spectra of blazars is constrained to be harder than E^-2 with cut-off energies in the range of Ecut >10^18 eV. When interactions of high-energy protons on soft photon fields dominate, we can find similarly tight constraints on the proton spectrum parameters. [abridged]

Photon and electroweak boson production in PbPb collisions

The production of electroweak bosons (photons, W and Z particles) in PbPb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV has been measured with the CMS detector at the LHC. Direct photons are selected by applying isolation criteria, while W and Z bosons are reconstructed through their muonic decay. The production rates in PbPb data are studied as a function of the collision centrality and compared to that in pp interactions (or next-to-leading-order calculations), once normalised by the number of binary nucleon–nucleon interactions. Apart from an expected asymmetry between W + and W − due to the different quark content of protons and Pb nuclei, no deviation from binary-scaled production is observed. The isolated-photon energy is further used as a reference for away-side jets, which are found to be less energetic in PbPb than in pp collisions.

The LHCb trigger and its performance in 2011

This paper presents the design of the LHCb trigger and its performance on data taken at the LHC in 2011. A principal goal of LHCb is to perform flavour physics measurements, and the trigger is designed to distinguish charm and beauty decays from the light quark background. Using a combination of lepton identification and measurements of the particles' transverse momenta the trigger selects particles originating from charm and beauty hadrons, which typically fly a finite distance before decaying. The trigger reduces the roughly 11 MHz of bunch-bunch crossings that contain at least one inelastic pp interaction to 3 kHz. This reduction takes place in two stages; the first stage is implemented in hardware and the second stage is a software application that runs on a large computer farm. A data-driven method is used to evaluate the performance of the trigger on several charm and beauty decay modes.

The nuclear transparency effect of π−-mesons in p+12C- and d+12C-interactions at 4.2A GeV/c

The use of the nuclear transparency effect of π−-mesons in a proton and deuteron-induced interactions with carbon nuclei at 4.2A GeV/c, to obtain information on the properties of nuclear matter, is presented in this work. The half angle (θ1/2) technique is used to extract information on the nuclear transparency effect. θ1/2 divides the multiplicity of charged particles into two equal parts depending on their polar angle in the laboratory frame in pp interactions. Particles with an angle smaller than (incone particles) and greater than (outcone particles) θ1/2 are considered separately. The average values of multiplicity, momentum and transverse momentum of the π−-mesons are analyzed as a function of a number of identified protons in an event. We observed evidences in the data which could be considered as the transparency effect. For quantitative analysis, the results are compared with the cascade model. The observed effects are categorized into leading effect transparency and medium effect transparency. The transparency in the latter case could be the reason for the collective interactions of grouped nucleons with the incident particles.

Multiple Partonic Interactions in Herwig++

We review the implementation of a model for multiple partonic interactions in Herwig++. Moreover, we show how recent studies on the colour structure of events in Herwig++ led to a significant improvement in the description of soft inclusive observables in pp interactions at the LHC.

Diffraction studies with ALICE

The measurement of Single and Double Diffraction cross-sections in pp collisions by the ALICE Collaboration at √ s = 0.9, 2.76 and 7 TeV will be presented. The relevance of diffraction to the understanding of inelastic pp interactions will be discussed and the measurement of the inelastic pp cross-section will be presented. A brief status of ALICE’s studies of centrally produced systems, selected with a two-pseudorapidity gap topology, will also be given.

Pattern recognition in a high-rate GEM-TPC

A pattern recognition software for a continuously operating high-rate Time Projection Chamber with Gas Electron Multiplier amplification (GEM-TPC) has been designed and tested. Space points are delivered by a track-independent clustering algorithm. A true 3-dimensional track follower combines them to helical tracks, without constraints on the vertex position. Fast helix fits, based on a conformal mapping on the Riemann sphere, are the basis for deciding whether points belong to one track. To assess the performance of the algorithm in a high-rate environment, pp interactions at a rate of 2 × 107 s−1, the maximum rate foreseen for PANDA, have been simulated. The pattern recognition is capable of finding different kinds of track topologies with high efficiency and provides excellent seed values for track fitting or online event selection. The feasibility of event deconvolution has been demonstrated: Different techniques to retain the tracks from an event with known time from other tracks in the TPC are presented in this paper.

Role of Collins effect in the single spin asymmetryANinp↑p→hXprocesses

The much debated issue of the transverse single spin asymmetry A_N observed in the inclusive large P_T production of a single hadron in pp interactions, p(transv. polarized) p --> pion X, is considered in a TMD factorization scheme. A previous result [1,2] stating that the maximum contribution of the Collins effect is strongly suppressed, is revisited, correcting a numerical error. New estimates are given, adopting the Collins functions recently extracted from SIDIS and e+e- data, and phenomenological consequences are discussed.

Formula omitted] meson production at central rapidity in proton–proton collisions at [formula omitted

The pT-differential inclusive production cross section of the prompt charm-strange meson Ds+ in the rapidity range |y|<0.5 was measured in proton–proton collisions at s=7 TeV at the LHC using the ALICE detector. The analysis was performed on a data sample of 2.98×108 events collected with a minimum-bias trigger. The corresponding integrated luminosity is Lint=4.8 nb−1. Reconstructing the decay Ds+→ϕπ+, with ϕ→K−K+, and its charge conjugate, about 480 Ds± mesons were counted, after selection cuts, in the transverse momentum range 2<pT<12 GeV/c. The results are compared with predictions from models based on perturbative QCD. The ratios of the cross sections of four D meson species (namely D0, D+, D⁎+ and Ds+) were determined both as a function of pT and integrated over pT after extrapolating to full pT range, together with the strangeness suppression factor in charm fragmentation. The obtained values are found to be compatible within uncertainties with those measured by other experiments in e+e−, ep and pp interactions at various centre-of-mass energies.

A Study of Diffractive Production in ALICE

The ALICE collaboration is investigating single, double and central diffractive processes in pp interactions. The single and double diffractive cross-sections are presented at 900 GeV 2.76 TeV and 7 TeV using an analysis method based on the classification of diffractive gaps. Promising early results on central diffractive processes have motivated the use of additional diffractive counters, which are currently being installed.

Fluctuations of the number of neutral pions at high multiplicity in pp interactions at 50 GeV

Results obtained by measuring fluctuations of the number of neutral pions in the SERP-E-190 Experiment (Thermalization Project) upon irradiating a liquid-hydrogen target of the SVD-2 setup with a beam of 50-GeV protons are presented. A simulation of the detection of photons from the decay of neutral pions with the aid of an electromagnetic calorimeter revealed a linear relation between the number of detected photons and the mean number of neutral pions in an event. After the introduction of corrections for the loss of charged tracks because of a limited acceptance of the setup, trigger operation, and the efficiency of the data-treatment system, distributions of the number of neutral pions, N0, were obtained for each value of the total number of particles in an event, Ntot = Nch + N0. The fluctuation parameter ω = D/〈N0〉 was measured. In the region Ntot > 22, fluctuations of the number of neutral pions increase, which, within statistical models (GCE, CE, MCE), indicates that the system involving a large number of pions approaches the pion-condensate state. This effect was observed for the first time.

Study of W boson production in PbPb and pp collisions at [formula omitted

A measurement is presented of W-boson production in PbPb collisions carried out at a nucleon–nucleon (NN) centre-of-mass energy sNN of 2.76 TeV at the LHC using the CMS detector. In data corresponding to an integrated luminosity of 7.3 μb−1, the number of W→μνμ decays is extracted in the region of muon pseudorapidity |ημ|<2.1 and transverse momentum pTμ>25 GeV/c. Yields of muons found per unit of pseudorapidity correspond to (159±10(stat.)±12(syst.))×10−8W+ and (154±10(stat.)±12(syst.))×10−8W− bosons per minimum-bias PbPb collision. The dependence of W production on the centrality of PbPb collisions is consistent with a scaling of the yield by the number of incoherent NN collisions. The yield of W bosons is also studied in a sample of pp interactions at s=2.76 TeV corresponding to an integrated luminosity of 231 nb−1. The individual W+ and W− yields in PbPb and pp collisions are found to agree, once the neutron and proton content in Pb nuclei is taken into account. Likewise, the difference observed in the dependence of the positive and negative muon production on pseudorapidity is consistent with next-to-leading-order perturbative QCD calculations.

Age-related differences in episodic memory: A synergistic contribution of genetic and physiological vascular risk factors.

Vascular risk is associated with impairments in age-sensitive cognitive functions. However, age-associated differences in vascular risk and cognitive functioning can be explained in part by genetic factors, such as the presence of ε4 allele of the Apolipoprotein E (APOE) gene. Although the links between these factors and cognitive deficits are frequently reported, their joint impact on healthy adults is rarely investigated. We hypothesized that phenotypic indicators of vascular risk (increased pulse pressure and high blood cholesterol levels) and genetic (APOE ε4 allele) risk factors would exert a synergistic negative influence on episodic memory in healthy rather than typical adults. We measured blood pressure, blood cholesterol levels, recognition memory, and free recall in a life span sample of normotensive adults 18-77 years of age. APOE genotype was determined from buccal cultures. A general linear model analysis showed that elevated pulse pressure was associated with poorer memory but only in the carriers of ε4 allele--APOE Status × PP interaction, F(1, 110) = 4.82, η²(p) = .042, p = .03-whereas advanced age was associated with lower memory scores only in ε3 homozygotes: APOE Status × Age, F(1, 110) = 4.92, η²(p) = .043, p = .029. A joint influence of relatively mild risk factors is associated with reduced memory performance, even in healthy adults.

Forward-backward correlations and charged-particle azimuthal distributions in pp interactions using the ATLAS detector

Abstract Using inelastic proton-proton interactions at $ \sqrt {s} = 900 $ GeV and 7 TeV, recorded by the ATLAS detector at the LHC, measurements have been made of the correlations between forward and backward charged-particle multiplicities and, for the first time, between forward and backward charged-particle summed transverse momentum. In addition, jet-like structure in the events is studied by means of azimuthal distributions of charged particles relative to the charged particle with highest transverse momentum in a selected kinematic region of the event. The results are compared with predictions from tunes of the pythia and herwig++ Monte Carlo generators, which in most cases are found to provide a reasonable description of the data.