In the reaction K − +p→ Λ +X at 4.25 GeV/ c both the target fragmentation of the proton into lambda and the beam fragmentation of kaon into lambda have been studied. The sample consists of 21 000 Λ events with t >−1.0 (GeV/ c ) 2 and 9000 Λ events with u > −1.0 (GeV/ c ) 2 . Abundant π 0 and meson resonance production for residual masses M x <1.1 GeV/ c 2 is observed. In the t -channel of am effective exchange trajectory is deduced. which lies between the K and K(890) trajectories. In the u -channel the effective exchange trajectory is in good agreement with a nucleon trajectory. The extrapolated average pion multiplicities of the residual mass system in the t -channel are in very good agreement with the values measured in antiproton-proton annihilations at rest.
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Emission of intermediate mass fragments (IMFs) (Z>~3) from central collisions of 40Ar+45Sc (E/A=35–115 MeV), 58Ni+58Ni (E/A=35–105 MeV), and 86Kr+93Nb (E/A=35–95 MeV) was studied. For each system, the average number of IMFs per event increased with beam energy, reached a maximum, and then decreased. The beam energy of peak IMF production increased linearly with the combined mass of the system. The number of IMFs emitted at the peak also increased with the system mass. Percolation calculations showed a weaker dependence of the peak beam energy and the number of IMFs on the total mass of the system.
A search for narrow resonances in the dijet mass spectrum is performed using data corresponding to an integrated luminosity of 2.9 inverse pb collected by the CMS experiment at the LHC. Upper limits at the 95% confidence level (CL) are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark-quark, quark-gluon, or gluon-gluon pairs. The data exclude new particles predicted in the following models at the 95% CL: string resonances, with mass less than 2.50 TeV, excited quarks, with mass less than 1.58 TeV, and axigluons, colorons, and E_6 diquarks, in specific mass intervals. This extends previously published limits on these models.
A search for the central exclusive production of top quark-antiquark pairs ($\mathrm{t\bar{t}}$) is performed for the first time using proton-tagged events in proton-proton collisions at the LHC at a centre-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 29.4 fb$^{-1}$. The $\mathrm{t\bar{t}}$ decay products are reconstructed using the central CMS detector, while forward protons are measured in the CMS-TOTEM precision proton spectrometer. An observed (expected) upper bound on the production cross section of 0.59 (1.14) pb is set at 95% confidence level, for collisions of protons with fractional momentum losses between 2 and 20%.
The study of global event shape variables can provide sensitive tests of predictions for multijet production in proton-proton collisions. This paper presents a study of several event shape variables calculated using jet four momenta in proton-proton collisions at a centre-of-mass energy of 13 TeV and uses data recorded with the CMS detector at the LHC corresponding to an integrated luminosity of 2.2 fb$^{-1}$. After correcting for detector effects, the resulting distributions are compared with several theoretical predictions. The agreement generally improves as the energy, represented by the average transverse momentum of the two leading jets, increases.
The first measurements of the Fourier coefficients ($V_{n\Delta}$) of the azimuthal distributions of charged hadrons emitted from photon-proton ($\gamma$p) interactions at the LHC are presented. The data are extracted from 68.8 nb$^{-1}$ of ultra-peripheral proton-lead (pPb) collisions at $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV using the CMS detector. The high energy lead ions produce a flux of photons that can interact with the oncoming proton. This $\gamma$p system provides a set of unique initial conditions with multiplicity lower than in photon-lead collisions but comparable to recent electron-positron and electron-proton data. The $V_{n\Delta}$ coefficients are presented in ranges of event multiplicity and transverse momentum ($p_\mathrm{T}$) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean $p_\mathrm{T}$ of charged particles is smaller in $\gamma$p than in pPb collisions. For both the $\gamma$p and pPb samples, $V_{1\Delta}$ is negative, $V_{2\Delta}$ is positive, and $V_{3\Delta}$ consistent with 0. For each multiplicity and $p_\mathrm{T}$ range, $V_{2\Delta}$ is larger for $\gamma$p events. The $\gamma$p data are consistent with model predictions that have no collective effects.
We have investigated the particle production and fragmentation of nuclei participating in the interactions of 10.6 GeV/n gold nuclei in nuclear emulsions. A new criterion has been found to distinguish between the interactions of these gold nuclei with the light (H,C,N,O) and heavy (Ag, Br) target nuclei in the emulsion. This has allowed separate analyses of the multiplicity and pseudo-rapidity distributions of the singly charged particles emitted in Au-(H,C,N,O) and Au-(Ag,Br) interactions, as well as of the modes of breakup of the projectile and target nuclei. The pseudo-rapidity distributions show strong forward asymmetries, particularly for the interactions with the light nuclei. Heavy target nuclei produce a more severe breakup of the projectile gold nucleus than do the lighter targets. A negative correlation between the number of fragments emitted from the target nuclei and the degree of centrality of the collisions has been observed, which can be attributed to the total destruction of the relatively light target nuclei by these very heavy projectile nuclei.
Attenuation measurements of reaction and total cross sections have been made for π− beams at 410, 464, and 492 MeV on targets of CD2, 6Li, C, Al, S, Ca, Cu, Zr, Sn, and Pb. These results are assisted by and compared to predictions from a recent eikonal optical model. Calculations with this model, which does not include pion absorption, agree with recent elastic scattering data, but are significantly below our measured reaction and total cross sections.
The energy loss spectrum of 150 GeV muons has been measured with a prototype of the ATLAS hadron calorimeter in the H8 beam of the CERN SPS. The differential probability dP/dv per radiation length of a fractional energy loss v = ΔEμ/Eμ has been measured in the range v = 0.01 ÷ 0.95; it is compared with the theoretical predictions for energy losses due to bremsstrahlung and production of electron—positron pairs or of energetic knock-on electrons. The integrated probability \(\int_{0.01}^{0.95}({\rm d}P/{\rm d}v){\rm d}v\) is (1.610 ± 0.015stat ± 0.105syst) · 10−3 in agreement with the theoretical predictions 1.556 · 10−3 and 1.619 · 10−3. Agreement with theory is also found in two intervals of v where production of electron-positron pairs and knock-on electrons dominates. In the region of bremsstrahlung dominance (v = 0.12 ÷ 0.95) the measured integrated probability (1.160 ± 0.040stat ± 0.075syst) · 10−4 is in agreement with the theoretical value of 1.185 · 10−4, obtained using the Petrukhin and Shestakov description of the bremsstrahlung process. The same result is about 3.6 standard deviations (defined as the quadratic sum of statistical and systematic errors) lower than the theoretical prediction of 1.472 · 10−4, obtained using Tsai’s description of bremsstrahlung.
Angular distributions of the α-particle production differential cross section from the breakup of 6Li and 7Li projectiles incident on a 208Pb target have been measured at seven projectile energies between 29 and 52 MeV. The α-breakup cross section of 6Li was found to be systematically greater than that of 7Li across the entire energy range. These data have been compared with previously reported results and with the predictions of continuum-discretized coupled channels (CDCC) calculations including resonant and nonresonant projectile breakup. The present data compare well with previous measurements, while the CDCC calculations provide a reasonable prediction of the relative α-breakup cross sections but underpredict their absolute values. The calculations confirm that a major factor in the enhancement of the 6Li to 7Li α-breakup cross section is the difference between the α-breakup thresholds of the two isotopes. These results have implications for structural studies of light exotic nuclei based on elastic scattering.