Femtoscopy with identified charged pions in proton-lead collisions at sNN=5.02 TeV with ATLAS

Abstract

Femtoscopy with identified charged pions in proton-lead collisions at√ s NN = 5.02 TeV with ATLASThe ATLAS Collaboration Bose-Einstein correlations between identified charged pions are measured for p+Pb collisions at √ s NN = 5.02 TeV using data recorded by the ATLAS detector at the LHC corresponding to a total integrated luminosity of 28 nb −1 . Pions are identified using ionization energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of collision centrality as well as the average transverse momentum (k T ) and rapidity (y ππ ) of the pair. Pairs are selected with a rapidity −2 < y ππ < 1 and with an average transverse momentum 0.1 < k T < 0.8 GeV. The effect of jet fragmentation on the two-particle correlation function is studied, and a method using oppositecharge pair data to constrain its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair k T . A correlation of the radii with the local chargedparticle density is demonstrated. The scaling of the extracted radii with the mean number of participating nucleons is also used to compare a selection of initial-geometry models. The cross-term R ol is measured as a function of rapidity, and a nonzero value is observed with 5.1σ combined significance for −1 < y ππ < 1 in the most central events.1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the center of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). P 8) minimum-bias events are generated at a center-of-mass energy per nucleon-nucleon pair of √ s NN = 5.02 TeV. Hp+Pb The energy and boost settings are the same as in the nominal p+Pb reconstructed simulation, except that the minimum hard-scattering transverse momentum is adjusted as described in Section 4.2. This boost is applied only in the p+Pb sample. Hpp The generator is run with all settings the same as in the p+Pb sample, except that both incoming particles are protons. P8 pp [54] The set of generator parameters from ATLAS "UE AU2-CTEQ6L1" [55] is used with P 8.209, which utilizes the CTEQ 6L1 [56] parton distribution function (PDF) from LHAPDF6 [57]. Herwig ++ pp [58]The NNLO MRST PDF [59] is used with Herwig ++ 2.7.1.

Signatures of New Physics Versus the Ridge Phenomenon in Hadron–Hadron Collisions at the LHC

“…Remarkably, similar long-range ridge structures show up in proton-nucleus [8,10] and even protonproton [10,11] collisions, under several conditions on events, like high multiplicity and a given transverse momentum range of charged particles. The interpretation of a positive V 2 in these small systems is currently highly debated, and different observables have been proposed to probe new dynamical effects related to large hadronic densities [12].…”

Hard processes in small systems

“…In particular, event-to-event color fluctuations in the configuration of the proton, which in the context of geometric modeling is implemented in the so-called Glauber-Gribov Color Fluctuation (GGCF) model [26,27], may play an important role. In fact, some measurements, such as that of femtoscopic radii in centrality-selected p+Pb collisions presented by ATLAS at this conference [28], suggest a preference for the broader N part distribution in the GGCF model over that in the traditional Glauber model. When geometric parameters are estimated under the GGCF model, hard-process yields (such as Z bosons in Ref.…”

Elliptic flow coefficients from transverse momentum conservation

“…[8,[31][32][33][34][35][36][37][38][39][40][41], indicating that collective flow also exists in small but dense systems. The more detailed experimental measurements find that the elliptic flow coefficient is smaller in p+p and p+Pb than in Pb+Pb for a given multiplicity at the LHC energy, but it does not seem to turn off at low multiplicities [42,43]. 1 These features seem to challenge our current understanding of collectivity based on the hydrodynamical picture, since it is expected that hydrodynamics should not be applicable to low multiplicity events, where the initial state effects are expected to be well visible, see, e.g., [45][46][47][48][49][50][51].…”