Experimental Evidence for an Attractive p−ϕ Interaction

The inclusive production cross sections of the strange vector mesons K * 0 , K * 0 , and φ have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to pT = 3.5 GeV/c. The atomic number dependence is parametrised as σpA = σpN * A α , where σpN is the proton-nucleon cross section. Within the phase space accessible, α(K * 0 ) = 0.86 ± 0.03, α(K * 0 ) = 0.87 ± 0.03, and α(φ) = 0.96 ± 0.02. The total proton-nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are σ pN→K * 0 = (5.06 ± 0.54) mb, σ pN→K * 0 = (4.02 ± 0.45) mb, and σ pN→φ = (1.17 ± 0.11) mb. For all resonances the Cronin effect is observed; compared to the measurements of Cronin et al. for K ± mesons, the measured values of α for φ mesons coincide with those of K + mesons for all transverse momenta, while the enhancement for K * 0 /K * 0 mesons is smaller.

Nature, 2020

The strong nuclear interaction between nucleons (protons and neutrons) is the effective force that holds the atomic nucleus together. This force stems from fundamental interactions between quarks and gluons (the constituents of nucleons) that are described by the equations of Quantum Chromodynamics (QCD). However, as these equations cannot be solved directly, physicists resort to describing nuclear interactions using effective models that are well constrained at typical internucleon distances in nuclei [1-5] but not at shorter distances. This limits our ability to describe high-density nuclear matter such as in the cores of neutron stars [6]. Here we use high-energy electron scattering measurements that isolate nucleon pairs in short-distance, high-momentum configurations [7-9] thereby accessing a kinematical regime that has not been previously explored by experiments, corresponding to relative momenta above 400 MeV/c. As the relative momentum between two nucleons increases and their separation thereby decreases, we observe a transition from a spin-dependent tensor-force to a predominantly spin-independent scalar-force. These results demonstrate the power of using such measurements to study the nuclear interaction at short-distances and also support the use of point-like nucleons with two-and three-body effective interactions to describe nuclear systems up to densities several times higher than the central density of atomic nuclei. Electron Scattering SRC Studies Electron scattering is well described by single-photon exchange [7-14], where electrons scatter from the nucleus by transferring a single virtual photon carrying momentum and energy ω. In the high-resolution one-body view of QE scattering at large momentum transfer, this virtual photon is absorbed by a single off-shell nucleon with initial energy ϵand momentum-. If the nucleon does not re-interact as it leaves the nucleus, it will emerge with momentum " =-+ and energy ϵ " = +-. Outgoing-nucleon rescattering from other nucleons can change the detected momentum and energy. However, we can still approximate the initial momentum and energy of that nucleon as the measured missing momentum 2-33 ≡ " − ≈-and missing energy 2-33 ≡ − " ≈ " − ϵ-(where " = " − " is the detected-nucleon kinetic energy). Unlike nucleons in SRC pairs, almost all non-SRC nucleons in atomic nuclei occupy momentum states up to the nuclear Fermi momentum : (~ 250 MeV/c). Therefore, when 2-33 > : , the knockout nucleon should predominantly originate from an SRC pair and should be accompanied by the simultaneous emission of the other (recoil) nucleon with momentum <=>?-@ ≈ −-[10-13, 15, 16], see Extended Data Fig. 1. Previous A(e,e'p) studies observed that more complicated (non-QE) reaction mechanisms can lead to high 2-33 events that are not due to the knockout of nucleons from SRC pairs. To minimize such contributions, our measurement was performed at kinematics where these non-SRC contributions were shown to be suppressed [8, 17-19], namely: % ≡ % − ω % ≳ 1.5 (GeV/c) 2 and H ≡ % 2m K ω ⁄ ≥ 1.2, so that 2-33 was almost anti-parallel to , and % grows with 2-33. See Methods for details. Previous measurements of A(e,e'pN) reactions off 4 He and 12 C, performed at similar kinematics, have shown that proton-neutron (pn) SRC pairs predominate over protonproton (pp) SRC pairs for 300 < 2-33 < 600 MeV/c by a factor of almost 20 [10-13, 15]. This is due to the dominance of the tensor part of the NN interaction in this momentum range. The tensor force only operates on spin-1 NN pairs. As spin-1 pp-SRC pairs are suppressed by the Pauli exclusion principle, there are far more pn-than pp-SRC pairs [7, 8, 17]. At higher missing-momentum where the repulsive core of the NN interaction is expected to become dominant, the interaction should be predominantly scalar; i.e. one that operates on both spin-0 and spin-1 pairs. This transition should therefore lead to an increased fraction of pp-SRC pairs. Previous work [11] saw initial evidence for such an increase, but its data were statistically limited. Here we extend these studies by measuring the A(e,e'p) and A(e,e'pp) reactions for 400 ≤ 2-33 ≤ 1000 MeV/c for C, Al, Fe and Pb nuclei. The measurements were performed at the Thomas Jefferson National Accelerator Facility using a 5.01 GeV electron beam. The CEBAF Large Acceptance Spectrometer, CLAS, (Fig. 1) [20] was

2011

Physical Review C - Nuclear Physics, 2011

Nuclear Physics A, 2000

The pion-production reactions π + A → π + π ± A ′ were studied on 2 H, 12 C, 40 Ca, and 208 P b nuclei at an incident pion energy of T π + =283 MeV. Pions were detected in coincidence using the CHAOS spectrometer. The experimental results are reduced to differential cross sections and compared to both theoretical predictions and the reaction phase space. The composite ratio C A ππ between the π + π ± invariant masses on nuclei and on the nucleon is also presented. Near the 2m π threshold pion pairs couple to (ππ) I=J=0 when produced in the π + → π + π − reaction channel. There is a marked near-threshold enhancement of C A π + π − which is consistent with theoretical predictions addressing the partial restoration of chiral symmetry in nuclear matter. Furthermore, the behaviour of C A π + π − is well described when the restoration of chiral symmetry is combined with standard Pwave renormalization of pions in nuclear matter. On the other hand, nuclear matter only weakly influences C A π + π + , which displays a flat behaviour throughout the energy range regardless of A.

PROCEEDINGS OF THE 15TH INTERNATIONAL CONFERENCE ON MESON-NUCLEON PHYSICS AND THE STRUCTURE OF THE NUCLEON

Recent developments in studying the φ meson in nuclear matter are summarized and reviewed from a theoretical point of view. Special emphasis is laid on the most recent results dealing with the momentum dependence of the φ meson properties in medium and a first attempt at simulating the pA reactions employed in experimental studies of the φ meson in nuclear matter, making use of a transport model approach.

Nuclear Physics A, 1990

Measurements of the spin-dependent left-right asymmetries for n-p elastic scattering at E, = 183 MeV have been obtained by scattering polarized neutrons from polarized protons. If charge symmetry holds in the n-p system, the two analyzing powers A,, and A, must be equal. Our preliminary experimental result for the average value of A,-A, in the range 82.2' < B Em < 116.1' is (32.1 f 6.1 f 6) x 1O-4 where the first error is the statistical uncertainty and the second is the systematic uncertainty. This result provides clear evidence of charge symmetry breaking in the nuclear force, and agrees well with meson-exchange predictions based on the Bonn nucleon-I~ucleon potential.

2019

Results on φ meson production in inelastic p+ p collisions at CERN SPS energies are presented. They are derived from data collected by the NA61/SHINE fixed target experiment, by means of invariant mass spectra fits in the φ → K+K− decay channel. They include the first ever measured double differential spectra of φ mesons as a function of rapidity y and transverse momentum pT for proton beam momenta of 80 GeV/c and 158 GeV/c, as well as single differential spectra of y or pT for beam momentum of 40 GeV/c. The corresponding total φ yields per inelastic p + p event are obtained. These results are compared with existing data on φ meson production in p + p collisions. The comparison shows consistency but superior accuracy of the present measurements. The emission of φ mesons in p + p reactions is confronted with that occurring in Pb + Pb collisions, and the experimental results are compared with model predictions. It appears that none of the considered models can properly describe all th...

The European Physical Journal C, 2007

The inclusive production cross sections of the strange vector mesons K * 0 , K * 0 , and φ have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to pT = 3.5 GeV/c. The atomic number dependence is parametrised as σpA = σpN * A α , where σpN is the proton-nucleon cross section. Within the phase space accessible, α(K * 0 ) = 0.86 ± 0.03, α(K * 0 ) = 0.87 ± 0.03, and α(φ) = 0.96 ± 0.02. The total proton-nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are σ pN→K * 0 = (5.06 ± 0.54) mb, σ pN→K * 0 = (4.02 ± 0.45) mb, and σ pN→φ = (1.17 ± 0.11) mb. For all resonances the Cronin effect is observed; compared to the measurements of Cronin et al. for K ± mesons, the measured values of α for φ mesons coincide with those of K + mesons for all transverse momenta, while the enhancement for K * 0 /K * 0 mesons is smaller.

Physical Review Letters, 2014