Search

The proton is made from three quarks, and mesons are made from a quark and antiquark. But what other configurations of quarks and antiquarks can form a particle? From a theoretical standpoint, this question is difficult to answer. Yet new particles with different combinations of quarks keep popping up in experiments. The latest of these, reported by the Large Hadron Collider Beauty (LHCb) Collaboration at CERN, is the pentaquark, a never-before-seen combination of five quarks that consists of a proton and a pair of heavy charm-anticharm quarks [1]. The detection of this particle comes as somewhat of a surprise: although the existence of the pentaquark doesn’t require a revision of the standard model of particle physics, earlier searches for a pentaquark composed of lighter quarks came up empty or detected candidate particles that could not be reproduced [2]. From studying this new form of quark matter, physicists stand to learn more about the complex forces between bound quarks. The concept of multiquark particles is not new [3]. But for decades after the quark model was conceived, only two categories of strongly interacting particles were known: baryons (such as protons and neutrons), consisting of three quarks, and mesons. Only within the past decade have scientists started to observe new particles that do not fit into the quark model, such as the X(3872) [4] and the Zc(3900) (see 17 June 2013 Viewpoint), which are about 4 times heavier than the proton. The Zc(3900) decays into a charm-anticharm meson (called the J/ψ), plus a light meson, suggesting the particle is made from two quarks and two antiquarks. The observation of this four-quark state—or tetraquark—implied that other multiquark states, such as a heavy pentaquark, might exist [5]. The challenge has been looking for them. Rare particles are the proverbial needle in the haystack, as scientists must hunt for them in the copious debris from high-energy collisions. In the LHCb experiment, two protons collide at very high energy, sometimes producing composite particles that have a heavy bottom quark and which stay close to the beam line. The bottom-quark particles go a short distance and then decay into other particles, such as a pentaquark plus a K meson, which are then picked up by a series of detectors. The key step that allowed LHCb’s detector to identify the candidate pentaquark was to only look for products of the bottom-quark particle Λb, which has a lifetime long enough for it to move away from the proton-proton collision point before it decays. Using the Λb decay as a filter, the researchers were able to see two new pentaquarks—or, rather, the products of their decays—sufficiently far from other confounding particle debris. Both pentaquarks that LHCb has observed are resonant-energy states of the proton and the J/ψ particle and are therefore referred to as “charmonium pentaquarks.” More formally, they are named Pc(4450) and Pc(4380), where the numbers, according to convention, indicate the particles’ masses in mega-electron-volts. A natural question to ask is what other combination of quarks—besides the resonant state of a proton and J/ψ that the LHCb researchers assume—could form a pentaquark resonance? For example, over a decade ago, theorists predicted [6] that a pentaquark could be made from a proton and a φ meson—a pair of strange-antistrange quarks. But experimental searches never uncovered this type of pentaquark. Similarly, a resonant combination of the neutron and a K+ meson, called the Θ+pentaquark, was predicted almost two decades ago. But again, the particle’s existence wasn’t confirmed, despite extensive experimental searches. (The exception was a claim by the Laser Electron Photon experiment in Japan that has been hotly contested [2].) In fact, there are many combinations of quarks that can form a pentaquark, but the only one seen so far is the charmonium pentaquark found by LHCb. What makes it so special? The short answer is we don’t know. The forces between quarks are complex, and accurately describing a manyquark system using the theory of the strong force, quantum chromodynamics (QCD), is an extremely difficult

We have made simultaneous measurements of singles ($\stackrel{\ensuremath{\rightarrow}}{p}, {\stackrel{\ensuremath{\rightarrow}}{p}}^{\ensuremath{'}}$) spin-transfer observables and coincident ($\stackrel{\ensuremath{\rightarrow}}{p}, {p}^{\ensuremath{'}}\ensuremath{\gamma}$) yields and asymmetries for the 15.11 MeV, ${1}^{+}$, $T=1$ state in $^{12}\mathrm{C}$, at an incident proton bombarding energy of 200 MeV. Data were taken at four proton scattering angles ranging from ${\ensuremath{\theta}}_{p}^{\mathrm{c}.\mathrm{m}.}=5.5\ifmmode^\circ\else\textdegree\fi{} \mathrm{to} 16.5\ifmmode^\circ\else\textdegree\fi{}$. Both vertical (normal to the reaction plane) and horizontal (in-plane) incident beam polarizations were used, which allowed us to extract 16 different observables for this transition. In particular, using the ($\stackrel{\ensuremath{\rightarrow}}{p}, {p}^{\ensuremath{'}}\ensuremath{\gamma}$) reaction, we measured the sideways and longitudinal analyzing powers, ${D}_{0S}$ and ${D}_{0L}$, which vanish identically in the ($\stackrel{\ensuremath{\rightarrow}}{p}, {p}^{\ensuremath{'}}$) reaction. Detailed comparisons of all observables to calculations done in both relativistic and nonrelativistic formalisms are presented. Surprisingly, a relativistic description in which knock-on exchange processes are not explicitly included provides the best overall agreement with the data.

Differential cross sections were measured and compared to distorted-wave impulse-approximation calculations for the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${4}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{d}}_{5/2}$,\ensuremath{\pi}${\mathit{p}}_{3/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{12}\mathrm{C}$(n,p${)}^{12}$B (4.52 MeV) reaction, the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${6}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu},${\mathit{f}}_{7/2}$\ensuremath{\pi}${\mathit{d}}_{5/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{28}\mathrm{Si}$(n,p${)}^{28}$Al (5.175 MeV) reaction, and the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${10}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{h}}_{11/2}$,\ensuremath{\pi}${\mathit{g}}_{9/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{120}\mathrm{Sn}$(n,p${)}^{120}$In (\ensuremath{\simeq}0.0 MeV) reaction at 300 MeV. We obtained a normalization factor of 0.68 for the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${10}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{h}}_{11/2}$,\ensuremath{\pi}${\mathit{g}}_{9/2}^{\mathrm{\ensuremath{-}}1}$) state excited in $^{120}\mathrm{Sn}$(n,p${)}^{120}$In (\ensuremath{\simeq}0.0 MeV) reaction; this is the first ``1\ensuremath{\Elzxh}\ensuremath{\omega}'' stretched state known in this mass region.

The {sup 70,72}Ge({ital n},{ital p}){sup 70,72}Ga reactions have been measured at a beam energy of 200 MeV in order to study the behavior of Gamow-Teller (GT) strength near the {ital N}=40 subshell closure. The GT quenching factor extracted for {sup 70}Ge is consistent with results for {sup 54}Fe and with calculations done on the quasiparticle random-phase-approximation model if they are renormalized by a factor of about 0.6. The quasiparticle random-phase approximation predicts a drastic reduction in concentrated GT strength from {sup 70}Ge to {sup 72}Ge which is also seen in the data. These results are relevant to models of supernovae.

The ratios of inclusive electron scattering cross sections of {sup 4}He, {sup 12}C, and {sup 56}Fe to {sup 3}He have been measured at 1 1.4 GeV{sup 2}, the ratios exhibit two separate plateaus, at 1.5 2.25. This pattern is predicted by models that include 2- and 3-nucleon short-range correlations (SRC). Relative to A=3, the per-nucleon probabilities of 3-nucleon SRC are 2.3, 3.1, and 4.4 times larger for A=4, 12, and 56. This is the first measurement of 3-nucleon SRC probabilities in nuclei.

The fission of nuclei with $159\ensuremath{\le}A\ensuremath{\le}232$ induced by the bombardment of 19.1 to 44.5 MeV $^{3}\mathrm{He}$ ions has been measured using solid-state detectors with time-of-flight measurements. Analysis with statistical fission theory, including precompound nucleon emission, indicates fission barriers which decrease only slightly relative to the liquid-drop model values with decreasing $Z$, approaching about 90% of the liquid-drop model barrier for $A\ensuremath{\approx}160$. These results are in contrast with measurements at higher angular momenta which indicate much lower fission barriers (60 to 70% of the liquid-drop model) for this mass region. The angular correlations indicate complete, or slightly greater than complete momentum transfer to the compound system. This is in opposition to that observed using heavier projectiles where incomplete momentum transfer is probable.NUCLEAR REACTIONS, FISSION $^{159}\mathrm{Tb}$, $^{165}\mathrm{Ho}$, $^{169}\mathrm{Tm}$, $^{\mathrm{nat}}\mathrm{Ta}$, $^{\mathrm{nat}}\mathrm{Re}$, $^{194,195}\mathrm{Pt}$, $^{197}\mathrm{Au}$, $^{202}\mathrm{Hg}$, $^{205}\mathrm{Tl}$, $^{206,208}\mathrm{Pb}$, $^{\mathrm{nat}}\mathrm{Bi}$, $^{\mathrm{nat}}\mathrm{Th}$($^{3}\mathrm{He}$,f), $E(^{3}\mathrm{He})=19.1 \mathrm{to} 44.5$ MeV; measured ${\ensuremath{\sigma}}_{F}(\ensuremath{\theta}, E)$, angular correlations, coincident fragment kinetic energies; deduced relative mass division, total kinetic energy release, statistical-model fission parameters.

Differential cross sections and analyzing powers for the /sup 7/Li(p-arrow-right,..pi../sup +- /) reactions leading to discrete states of /sup 8/B and /sup 8/Li, as well as to the continuum, are presented at T/sub p/ = 280, 354, and 489 MeV. These results are compared with previously published results at 200 and 250 MeV. At fixed four-momentum transfers squared in the region 0.6>t>0.4 GeV/sup 2//c/sup 2/, the energy dependence of the (p,..pi../sup +/) and (p,..pi../sup -/) differential cross sections is quite similar, in sharp contrast to earlier results from the /sup 13/C(p,..pi../sup +- /) reactions. The observed variations in the (p,..pi../sup -/) energy dependence are presumed to reflect differences in the relative importance of resonant and nonresonant amplitudes in the (p,..pi../sup -/) reaction mechanism. The analyzing powers of the (p-arrow-right,..pi../sup -/) and (p-arrow-right,..pi../sup +/) reactions are qualitatively very similar except for a difference in sign.

The interactions between a K/sup ..pi../ = 5/sup -/ aligned two-quasineutron band and two octupole bands have been observed in /sup 164/Er. Band crossings occur in both even- and odd-spin sequences. The members of all three bands are observed from the bandheads (L/sup ..pi../ = 1/sup -/, 2/sup -/, and 5/sup -/) to about 12/sup -/.

Proton-induced pion production to the continuum was investigated via the reactions /sup 13/C(p,..pi../sup -/)X, /sup 13/C(p,..pi../sup +/)X, and /sup 16/O(p-arrow-right,..pi../sup +/)X at bombarding proton energies of 250, 354, and 489 MeV, in the kinematic region where the pion has nearly its maximum allowable kinetic energy. Results from the /sup 16/O(p-arrow-right,..pi../sup +/)X reaction indicate that at small missing momenta the analyzing powers can be adequately described by a quasi-free p-arrow-rightp..-->..d..pi../sup +/ process, but as the missing momentum is increased this description becomes less accurate. The energy dependence of the cross section for the inclusive (p,..pi../sup +/) reactions is very similar to that of previously published exclusive (p,..pi../sup +/) data from the same target nuclei. The double differential cross sections for the ..pi../sup -/ data, however, exhibit an energy dependence different from that of the exclusive (p,..pi../sup -/) reaction.

Inclusive measurements of the linear momentum transfer distribution and exclusive studies of light-charged particles as a function of linear momentum transfer have been performed for the reaction of 150-MeV /sup 6/Li ions with /sup 238/U. Linear momentum transfer properties and an upper limit for the complete fusion cross section for this system were deduced from the folding angle between binary fission events and found to be consistent with data for heavier complex projectiles. The exclusive light-charged-particle data demonstrate the existence of fission following the transfer of all possible subdivisions of the /sup 6/Li projectile, as well as inelastic scattering. Absorptive breakup involving /sup 4/He-/sup 2/H cluster structure is found to be the dominant reaction mechanism. In addition, substantial yields of energetic protons associated with full linear momentum transfer are observed in the backward hemisphere. However, the temperature inferred from these spectra is nearly three times as large as that for a fully equilibrated compound nucleus, suggesting a nonequilibrium source for these particles.

Angular correlations between binary fission fragments have been measured in coincidence with light charged particles for 64, 129, 199, and 269 MeV {sup 3}He-induced reactions on a {sup 238}U target. Linear momentum transfer and missing linear momentum have been determined for a broad range of light-charged-particle energies {ital E}{sub LCP} and emission angles {theta}{sub LCP}. Average momentum transfer increases from a fraction of beam momentum for small detection angles to nearly full momentum transfer at backward angles. The missing momentum depends primarily on available energy'' {ital E}{sub av}=E{sub beam}{minus}E{sub LCP}, but neither on beam energy {ital E}{sub beam} nor light particle detection angle. Backward proton and deuteron emission is incompatible with statistical evaporation following complete or incomplete fusion.

Differential cross section measurements have been made for 250 MeV proton inelastic scattering from /sup 28/Si. Hadronic deformation lengths for the states at 1.78 (2/sup +/), 4.62 (4/sup +/), 6.8 (3/sup -/ and 4/sup +/), 9.70 (5/sup -/), and 10.2 MeV (3/sup -/) have been extracted using a first-order vibration model distorted-wave Born approximation description. Deformation lengths for the 2/sup +/ and 4/sup +/ states are found to be independent of proton energy from 40 to 500 MeV. Cross sections for the excitation of the giant resonance region are found to be highly fragmented between 16 to 25 MeV. This region of excitation is found to contain an energy weighted sum rule depletion of 70% for the isovector giant dipole resonance, 26% for the isoscalar giant quadrupole resonance, and 5% for the isoscalar hexadecapole resonance. No structure was observed in the inelastic continuum region of 25 to 45 MeV of excitation energy.

Fission of /sup 238/U, /sup 209/Bi and /sup 197/Au induced by both positive and negative pion beams at incident energies of 60 to 100 MeV is examined. Energy and mass spectra and angular correlations of coincident fission fragments are compared to theoretical calculations and to other data. Excitation energies of the fissioning systems in excess of 100 MeV are deduced, implying that the fission examined in this study is caused primarily by true absorption of the pion. The masses of the coincident fission fragments are found to have a wider distribution than those from fissioning systems of similar excitation energy induced by nucleon beams. The fission cross sections are close to the reaction cross sections for /sup 238/U and scale with the fissility for the other nuclei studied.

Differential cross sections and analyzing powers are presented for the /sup 16/O(p-arrow-right,..pi../sup +/)/sup 17/O* reaction at T/sub p/ = 250, 354, and 489 MeV and compared with preliminary 200 MeV data as well as previously published data taken at other energies above and below the ..delta../sub 1232/ resonance region. The differential cross sections for each final state exhibit an energy dependence similar to that of the pp..-->..d..pi../sup +/ reaction at equivalent center of mass energy and four-momentum transfer, as has been seen previously for (p,..pi../sup +/) reactions with other target nuclei. The shape of the analyzing power angular distributions varies with the /sup 17/O/sup */ nuclear final state. Some states exhibit broad similarities with that of the p-arrow-rightp..-->..d..pi../sup +/ reaction, while others are markedly different.