Pionska femtoskopija sudara protona i protona na energiji 7 TeV u centru mase u ovisnosti o obliku događaja

We present a measurement of event shape dependent two-particle momentum space correlations for identical pions in pp collisions at √s = 7 TeV obtained by the ALICE detector. Homogeneity radii are extracted and studied as a function of pair kT = | p1T+ p2T | /2 in a wide range of event multiplicities for spherical and jet-like events. Spherical or jet-free events show a large reduction in (non-femtoscopic) background contributions to two-particle correlations which expands the kT range of the analysis and allows for a bias-free data driven femtoscopic analysis. For jet-like events the non-femtoscopic contributions are measured to be several times larger than the observed Bose-Einstein signal. By comparing ALICE measurements to PYTHIA and PHOJET generated data, a background signal removal method is developed and in-jet HBT radii are obtained. For both spherical and jet-like events, the extracted 1D homogeneity radii show a diminished kT dependence, differing to previously observed behavior in minimum bias events. After constraining the λ parameter, spherical event radii continue to have no kT dependence, which suggests a lack of radial flow, while jet-like event radii develop a clear kT and multiplicity dependence similar to the one observed in published sphericity integrated results.Prikazana su mjerenja dvočestičnih korelacija impulsa za identične pione u protonskim sudarima na energiji od 7 TeV u centru mase sa ALICE detektorom. Određena je ovisnost radijusa homogenosti o srednjem transverzalnom valnom vektoru para kT = | p1T+ p2T | /2 za širok spektar multipliciteta sudara visoke i niske sferičnosti. Sferični sudari pokazuju smanjenje nefemtoskopskih doprinosa dvočestičnim korelacijama što dopušta analizu podataka, znatno šireg dosega u kT, bez potrebe za dodatnim pretpostavkama. Za sudare sa mlazovima, nefemtoskopski doprinosi sadržavaju signal višestruko veći od oni induciranih Bose-Einsteinovim korelacijama. Usporedbom mjerenja ALICE detektora te PYTHIA i PHOJET generatora razvijena je metoda uklanjanja pozadinskih signala te su izmjereni HBT radijusi unutar mlazova. Korištene su Gausova i eksponencijalna funkcija prilagodbe te su prokomentirane razlike. Izmjereni 1D radijusi homogenosti za obje ekstremne vrijednost sferičnosti pokazuju nisku ovisnost o kT. Nakon ograničavanja λ parametra, sferični događaji i dalje ne pokazuju kT ovisnost, što sugerira manjak radijalnog toka čestica, dok događaji sa mlazovima pokazuju jasnu ovisnost o kT i multiplicitetu, vrlo slično već objavljenim rezultatima

Multiplicity dependence of jet-like two-particle correlation structures in p-Pb collisions at root s(NN)=5.02 TeV

Two-particle angular correlations between unidentified charged trigger and associated particles are measured by the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV. The transverse-momentum range 0.7<pT,assoc<pT,trig<5.0 GeV/c0.7<pT,assoc<pT,trig<5.0 GeV/c is examined, to include correlations induced by jets originating from low momentum-transfer scatterings (minijets). The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range |η|<0.9|η|<0.9. The near-side long-range pseudorapidity correlations observed in high-multiplicity p–Pb collisions are subtracted from both near-side short-range and away-side correlations in order to remove the non-jet-like components. The yields in the jet-like peaks are found to be invariant with event multiplicity with the exception of events with low multiplicity. This invariance is consistent with the particles being produced via the incoherent fragmentation of multiple parton–parton scatterings, while the yield related to the previously observed ridge structures is not jet-related. The number of uncorrelated sources of particle production is found to increase linearly with multiplicity, suggesting no saturation of the number of multi-parton interactions even in the highest multiplicity p–Pb collisions. Further, the number scales only in the intermediate multiplicity region with the number of binary nucleon–nucleon collisions estimated with a Glauber Monte-Carlo simulation

Event-by-event mean pT fluctuations in pp and Pb-Pb collisions at the LHC

Event-by-event fluctuations of the mean transverse momentum of charged particles produced in pp collisions at s√ = 0.9, 2.76 and 7 TeV, and Pb–Pb collisions at sNN‾‾‾‾√ = 2.76 TeV are studied as a function of the charged-particle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations indicative of correlated particle emission are observed in all systems. The results in pp collisions show little dependence on collision energy. The Monte Carlo event generators PYTHIA and PHOJET are in qualitative agreement with the data. Peripheral Pb–Pb data exhibit a similar multiplicity dependence as that observed in pp. In central Pb–Pb, the results deviate from this trend, featuring a significant reduction of the fluctuation strength. The results in Pb–Pb are in qualitative agreement with previous measurements in Au–Au at lower collision energies and with expectations from models that incorporate collective phenomena

J/ψ production and nuclear effects in p-Pb collisions at √sNN=5.02 TeV

Inclusive J/ψ production has been studied with the ALICE detector in p-Pb collisions at the nucleon–nucleon center of mass energy √ sNN = 5.02 TeV at the CERN LHC. The measurement is performed in the center of mass rapidity domains 2.03 &lt; ycms &lt; 3.53 and −4.46 &lt; ycms &lt; −2.96, down to zero transverse momentum, studying the µ +µ − decay mode. In this paper, the J/ψ production cross section and the nuclear modification factor RpPb for the rapidities under study are presented. While at forward rapidity, corresponding to the proton direction, a suppression of the J/ψ yield with respect to binary-scaled pp collisions is observed, in the backward region no suppression is present. The ratio of the forward and backward yields is also measured differentially in rapidity and transverse momentum. Theoretical predictions based on nuclear shadowing, as well as on models including, in addition, a contribution from partonic energy loss, are in fair agreement with the experimental results

Forward-backward multiplicity correlations in pp collisions at root s=0.9, 2.76 and 7 TeV

The strength of forward-backward (FB) multiplicity correlations is measured by the ALICE detector in proton-proton (pp) collisions at = 0.9, 2.76 and 7 TeV. The measurement is performed in the central pseudorapidity region (|eta| &lt; 0.8) for the transverse momentum p (T) &gt; 0.3 GeV/c. Two separate pseudorapidity windows of width (delta eta) ranging from 0.2 to 0.8 are chosen symmetrically around eta = 0. The multiplicity correlation strength (b (corr)) is studied as a function of the pseudorapidity gap (eta (gap)) between the two windows as well as the width of these windows. The correlation strength is found to decrease with increasing eta (gap) and shows a non-linear increase with delta eta. A sizable increase of the correlation strength with the collision energy, which cannot be explained exclusively by the increase of the mean multiplicity inside the windows, is observed. The correlation coefficient is also measured for multiplicities in different configurations of two azimuthal sectors selected within the symmetric FB eta-windows. Two different contributions, the short-range (SR) and the long-range (LR), are observed. The energy dependence of b (corr) is found to be weak for the SR component while it is strong for the LR component. Moreover, the correlation coefficient is studied for particles belonging to various transverse momentum intervals chosen to have the same mean multiplicity. Both SR and LR contributions to b (corr) are found to increase with p (T) in this case. Results are compared to PYTHIA and PHOJET event generators and to a string-based phenomenological model. The observed dependencies of b (corr) add new constraints on phenomenological models