Measurement of Strange Particle Production in Underlying Events in proton-proton collisions at

Abstract

Measurements are presented of the production of primary and particles in proton-proton collisions at = 7 TeV in the region transverse to the leading charged-particle jet in each event. The average multiplicity and average scalar transverse momentum sum of and particles measured at pseudorapidities || < 2 rise with increasing charged-particle jet in the range 1-10 GeV/c and saturate in the region 10-50 GeV/c. The rise and saturation of the strange particle yields and transverse momentum sums in the underlying event are similar to those observed for inclusive charged particles, which confirms the impact-parameter picture of multiple parton interactions. The results are compared to recent tunes of the PYTHIA Monte Carlo event generator. The PYTHIA simulations underestimate the data by 15-30% for mesons and by about 50% for baryons, a deficit similar to that observed for the inclusive strange particle production in non-single-diffractive proton-proton collisions. The constant strange- to charged-particle activity ratios and the similar trends for mesons and baryons indicate that the multiparton-interaction dynamics is decoupled from parton hadronization, which occurs at a later stage.

Kinematic fit input variables

Figure		Caption
		Distributions of invariant mass and invariant-mass pull for the most probable particle type hypothesis determined by the kinematic fit. The accepted and mass values are denoted as . The black points indicate the data. The histograms show the backgrounds (hatched: misidentified ; green: non-primary from and cascade decays; grey: other sources) and the signal (yellow) as predicted by PYTHIA 6 D6T. The PYTHIA prediction is normalized to the data.
		Distributions of the signed impact parameter with respect to the primary vertex and the corresponding pull distributions, for the most probable particle type hypothesis determined by the kinematic fit. Backgrounds (hatched: misidentified ; green: non-primary from and cascade decays; grey: other sources) and signal (yellow) are shown as predicted by PYTHIA6 D6T. The sum of the simulated signal and background distributions is normalized to data.

Uncorrected rates

Figure		Caption
		Uncorrected average rate of selected candidates per event, per degree and per unit pseudorapidity within -2 < < 2, as a function of the difference in azimuthal angle between the candidate and the leading charged-particle jet. Data and detailed simulation of minimum-bias events with different PYTHIA tunes are shown for reconstructed charged-particle jet > 1 GeV/c. Top: candidates with > 600MeV/c; bottom: candidates with > 1.5 GeV/c.

Results

Figure		Caption
		Fully corrected average multiplicity per unit of pseudorapidity and per radian in the transverse region (-2 < < 2,  < < ), as a function of the of the leading chargedparticle jet: (left) with > 0.6 GeV/c; (right) with > 1.5 GeV/c. Predictions of PYTHIA tunes are compared to the data and the ratios of simulations to data are shown in the bottom panels. For the data, the statistical uncertainties (error bars) and the quadratic sum of statistical and systematic uncertainties (error band) are shown, while for simulations the uncertainty is only shown for PYTHIA 6 tune Z2*, for clarity.
		Fully corrected average scalar sum per unit of pseudorapidity and per radian in the transverse region (-2 < < 2,  < < ), as a function of the of the leading trackjet: (left) with > 0.6 GeV/c; (right) with > 1.5 GeV/c. Predictions of PYTHIA tunes are compared to the data. For data, the statistical uncertainties (error bars) and sum of statistical and systematic uncertainties (error band) are distinguished, while for simulations the total uncertainty is only shown for PYTHIA6 tune Z2* for clarity.
		distributions corrected for selection efficiency and background without correction to the leading charged hadron jet, in the region transverse to a leading reconstructed charged-particle jet with > 3 GeV/c, compared to predictions from different PYTHIA tunes (left: ; right: ). Error bars indicate the quadratic sum of the statistical and systematic uncertainties. Simulations are normalized to the first bin in the data, with normalization factors given in parentheses.
		Ratios of the average multiplicities and scalar pT sums for primary V0 in the transverse region to the same quantities for primary charged particles as a function of charged-particle jet . The error bars indicate the quadratic sum of the statistical and systematic uncertainties.