The atomic nucleus is composed of two different kinds of fermions: protons and neutrons. If the protons and neutrons did not interact, the Pauli exclusion principle would force the majority of fermions (usually neutrons) to have a higher average momentum. Our high-energy electron-scattering measurements using (12)C, (27)Al, (56)Fe, and (208)Pb targets show that even in heavy, neutron-rich nuclei, short-range interactions between the fermions form correlated high-momentum neutron-proton pairs. Thus, in neutron-rich nuclei, protons have a greater probability than neutrons to have momentum greater than the Fermi momentum. This finding has implications ranging from nuclear few-body systems to neutron stars and may also be observable experimentally in two-spin-state, ultracold atomic gas systems.

Abstract:We have studied the transverse-momentum (p T ) dependence of the inclusive J/ψ production in p-Pb collisions at √ s NN = 5.02 TeV, in three center-of-mass rapidity (y cms ) regions, down to zero p T . Results in the forward and backward rapidity ranges (2.03 < y cms < 3.53 and −4.46 < y cms < −2.96) are obtained by studying the J/ψ decay to µ + µ − , while the mid-rapidity region (−1.37 < y cms < 0.43) is investigated by measuring the e + e − decay channel. The p T dependence of the J/ψ production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/ψ mean p T values. Forward and mid-rapidity results show a suppression of the J/ψ yield, with respect to pp collisions, which decreases with increasing p T . At backward rapidity no significant J/ψ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/ψ production in Pb-Pb collisions are also discussed. JHEP06(2015)055The suppression of charmonia, bound states of c andc quarks, and in particular of the J/ψ state, has long been proposed as a signature for the formation of a plasma of quarks and gluons (QGP) [1] in ultrarelativistic nucleus-nucleus collisions. However, it was soon realized that charmonium production can also be modified by nuclear effects not necessarily related to QGP formation [2]. These so-called cold nuclear matter (CNM) effects can be investigated by studying charmonium production in proton-nucleus (p-A) collisions as confirmed by the analysis of results obtained by several fixed-target (SPS [3, 4], HERA [5] and Tevatron [6]) and collider (RHIC [7] and LHC [8, 9]) experiments.Theoretical models have studied the production of charmonium in p-A collisions and the effects of the surrounding cold nuclear medium by introducing various mechanisms which include nuclear shadowing, gluon saturation, energy loss and nuclear absorption. Models [10][11][12] inspired by Quantum ChromoDynamics (QCD) describe charmonium production as a two-step process, with the cc pair created in a hard parton scattering, followed by its evolution into a bound state with specific quantum numbers. The pair creation is sensitive to the Parton Distribution Functions (PDFs) in both colliding partners and, at high energy, occurs mainly via gluon fusion. Although PDFs are known to be modified in a nuclear environment, information on the dependence of such modifications on the fraction x (Bjorken-x) of the nucleon momentum carried by the gluons and on the four-momentum squared Q 2 transferred in the scattering is still limited [13][14][15]. Charmonium production measurements can therefore provide insight into the so-called nuclear shadowing, i.e., on how the nucleon gluon PDFs are modified in a nucleus.Modifications of the initial state of the ...

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The f1(1285) meson with mass 1281.0 ± 0.8 MeV/c 2 and width 18.4 ± 1.4 MeV (FWHM) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the ηπ + π − , K +K 0 π − , and K − K 0 π + decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the ηπ + π − final-state Dalitz distribution are consistent with the axial-vector J P = 1 + f1(1285) identity, rather than the pseudoscalar 0 − η(1295). The production mechanism is more consistent with s-channel decay of a high-mass N * state, and not with t-channel meson exchange. Decays to ηππ go dominantly via the intermediate a ± 0 (980)π ∓ states, with the branching ratio Γ(a0π (noKK))/Γ(ηππ (all)) = 0.74±0.09. The branching ratios Γ(KKπ)/Γ(ηππ) = 0.216±0.033 and Γ(γρ 0 )/Γ(ηππ) = 0.047 ± 0.018 were also obtained. The first is in agreement with previous data for the f1(1285), while the latter is lower than the world average.

Beam-Line FIG. 3. Implementation of detectors in the πM1 area in a Geant4 [10] simulation. The beam strikes the thin scintillator beam hodoscope and three GEM chambers, passes through a hole in the annular veto scintillator, enters the cryotarget vacuum chamber and strikes one of the targets, then exits the vacuum chamber and goes through the beam monitor. Scattered particles are detected by two symmetric spectrometers, each with two straw chambers wrapped in RF shielding and two planes of scintillator paddles.