Proton-antiproton Collisions Research Articles

Search for Higgs-like particles produced in association with bottom quarks in proton-antiproton collisions

We report on a search for a spin-zero non-standard-model particle in proton-antiproton collisions collected by the Collider Detector at Fermilab at a center-of-mass-energy of 1.96 TeV. This particle, the $\phi$ boson, is expected to decay into a bottom-antibottom quark pair and to be produced in association with at least one bottom quark. The data sample consists of events with three jets identified as initiated by bottom quarks and corresponds to $5.4~\text{fb}^{-1}$ of integrated luminosity. In each event, the invariant mass of the two most energetic jets is studied by looking for deviations from the multijet background, which is modeled using data. No evidence is found for such particle. Exclusion upper limits ranging from 20 to 2 pb are set for the product of production cross sections times branching fraction for hypothetical $\phi$ boson with mass between 100 and 300 GeV/$c^2$. These are the most stringent constraints to date.

Inclusive production of charged pion pairs in proton-antiproton collisions

In this study, we have considered the contribution of the higher-twist (HT) effects of the subprocesses to inclusive pion pair production cross section in the high energy proton-antiproton collisions by using various pion distribution amplitudes (DAs) within the frozen coupling constant approach and compared them with the leading-twist contributions. The feature of the HT effects may help the theoretical interpretation of the future PANDA experiment. The dependencies of the HT contribution on the transverse momentum $p_T$, the center of mass energy $\sqrt s$, and the variable $x_T$ are discussed numerically with special emphasis put on DAs. Moreover, the obtained analytical and numerical results for the differential cross section of the pion pair production are compared with the elastic backward scattering of the pion on the proton. We show that the main contribution to the inclusive cross section comes from the HT direct production process via gluon-gluon fusion. Also, it is strongly dependent on the pion DAs, momentum cut-off parameter $\triangle p$ and $<{q_{T}^2}>$ which is the mean square of the intrinsic momentum of either initial parton.

Energy Dependence of the Ratio of the Elastic-to-Total Cross Sections for Proton—Proton and Antiproton—Proton Collisions

The paper presents the phenomenological analysis of the energy dependence for the ratio of elastic to total cross section in proton-proton and antiproton-proton scattering. The analytic functions based on the study of low- and high-energy experimental data for various scattering parameters provide the quantitative description of energy dependence of the ratio with statistically acceptable qualities in wide range of collision energy $\sqrt{s} \geq 3$ GeV in the case of the separate datasets for $pp$, $\bar{p}p$ collisions and at $\sqrt{s} \geq 5$ GeV for the joined experimental data ensemble. Based on the fit results the estimations are derived for the ratio of elastic to total cross section in $pp$ scattering at various $\sqrt{s}$ up to energy frontier $\sqrt{s}=10$ PeV which can be useful for present and future hadron colliders as well as for cosmic ray measurements at ultra-high energies. The indication is observed for onset of the asymptotic region at $\sqrt{s} \gtrsim 5-10$ PeV for the ratio of cross sections under consideration.

DyTER – Dynamic Track and Event Reconstruction

The future PANDA experiment at FAIR experiment aims to cover a wide range of processes in antiproton-proton collisions at event rates of up to 20 MHz. Such event rates make reconstruction a challenging task for the purely software-based event filter. Investigating complex event topologies with displaced vertices increases the difficulty even further. Here we present two attempts to meet these future challenges: an algorithm for track reconstruction based on pattern matching with pre-determined look-up tables, and as a continuation of this approach a system of neural networks for identifying specific particle track candidates and predicting their momentum.

Measurement of the differential cross sections for W -boson production in association with jets in pp¯ collisions at s=1.96 TeV

This paper presents a study of the production of a single $W$ boson in association with one or more jets in proton-antiproton collisions at $\sqrt{s}=1.96$ TeV, using the entire data set collected in 2001-2011 by the Collider Detector at Fermilab at the Tevatron, which corresponds to an integrated luminosity of $9.0$ fb$^{-1}$. The $W$ boson is identified through its leptonic decays into electron and muon. The production cross sections are measured for each leptonic decay mode and combined after testing that the ratio of the $W(\rightarrow \mu\nu)+$jets cross section to the $W(\rightarrow e\nu)+$jets cross section agrees with the hypothesis of $e$-$\mu$ lepton universality. The combination of measured cross sections, differential in the inclusive jet multiplicity ($W+\geqslant N$ jets with $N=1,\,2,\,3, \textrm{or }4$) and in the transverse energy of the leading jet, are compared with theoretical predictions.

Search for standard-model Z and Higgs bosons decaying into a bottom-antibottom quark pair in proton-antiproton collisions at 1.96 TeV

The Collider Detector at Fermilab collected a unique sample of jets originating from bottom-quark fragmentation ($b$-jets) by selecting online proton-antiproton ($p\bar{p}$) collisions with a vertex displaced from the $p\bar{p}$ interaction point, consistent with the decay of a bottom-quark hadron. This data set, collected at a center-of-mass energy of $\sqrt{s}=$1.96 TeV, and corresponding to an integrated luminosity of $5.4~\rm{fb}^{-1}$, is used to measure the $Z$-boson production cross section times branching ratio into $b\bar{b}$. The number of $Z\rightarrow b\bar{b}$ events is determined by fitting the dijet-mass distribution while constraining the dominant $b$-jet background, originating from QCD multijet events, with data. The result, $\sigma(p\bar{p} \rightarrow Z) \times \mathcal{B}(Z \rightarrow b\bar{b})= 1.11\pm 0.08(\text{stat}) \pm 0.14(\text{syst})~\text{nb}$, is the most precise measurement of this process, and is consistent with the standard-model prediction. The data set is also used to search for Higgs-boson production. No significant signal is expected in our data and the first upper limit on the cross section for the inclusive $p\bar p \rightarrow H\rightarrow b\bar b$ process at $\sqrt{s}=$1.96 TeV is set, corresponding to 33 times the expected standard-model cross section, or $\sigma = 40.6$ pb, at the 95\% confidence level.

The production of charged-kaon pairs in proton–antiproton collisions: The role of higher-twist mechanism

The higher-twist (HT) contribution to the charged-kaon pair production in the high energy proton–antiproton collisions at large transverse momentum [Formula: see text] is investigated by using the frozen coupling constant approach for various kaon distribution amplitudes (DAs), which are predicted by light-cone formalism, the light-front quark model, the nonlocal chiral quark model and the light-front holographic AdS/CFT approach. In the numerics, the dependencies of the HT contribution on the transverse momentum [Formula: see text], the rapidity [Formula: see text], and the variable [Formula: see text] are discussed with special emphasis put on DAs. The HT contribution is also compared with the leading-twist ones. It is shown that the HT contributions are dependent on the kaon DAs and also some other phenomenological parameters such as momentum cutoff parameter [Formula: see text]. Inclusive kaon pair production presents a remarkable test case in which HT terms dominate those of LT in certain kinematic regions. The HT direct production process via gluon–gluon fusion contributes significantly to the inclusive cross-section at large [Formula: see text].

Single top-quark production at the Tevatron and the LHC

This paper provides a review of the experimental studies of processes with a single top quark at the Tevatron proton-antiproton collider and the LHC proton-proton collider. Single top-quark production in the t-channel process has been measured at both colliders. The s-channel process has been observed at the Tevatron, and its rate has been also measured at the center-of-mass energy of 8 TeV at the LHC in spite of the comparatively harsher background contamination. LHC data also brought the observation of the associated production of a single top quark with a W boson as well as with a Z boson. The Cabibbo-Kobayashi-Maskawa matrix element |Vtb| is extracted from the single-top-quark production cross sections, and t-channel events are used to measure several properties of the top quark and set constraints on models of physics beyond the Standard Model. Rare final states with a single top quark are searched for, as enhancements in their production rates, if observed, would be clear signs of new physics.

Study of the X±(5568) state with semileptonic decays of the Bs0 meson

We present a study of the $X^\pm(5568)$ using semileptonic decays of the $B_s^0$ meson using the full Run II integrated luminosity of 10.4 fb$^{-1}$ in proton-antiproton collisions at a center of mass energy of 1.96\,TeV collected with the D0 detector at the Fermilab Tevatron Collider. We report evidence for a narrow structure, $X^\pm(5568)$, in the decay sequence $X^\pm(5568) \to B_s^0 \pi^\pm$ where $B_s^0 \rightarrow \mu^\mp D_s^\pm \, \mathrm{X}$, $D_s^\pm \rightarrow \phi \pi^{\pm}$ which is consistent with the previous measurement by the D0 collaboration in the hadronic decay mode, $X^\pm(5568) \to B^0_s \pi^\pm$ where $B^0_s \to J/\psi\phi$. The mass and width of this state are measured using a combined fit of the hadronic and semileptonic data, yielding $m = 5566.9 ^{+3.2}_{-3.1} \thinspace {\rm (stat)} ^{+0.6}_{-1.2} {\rm \thinspace (syst)}$\,MeV/$c^2$, $\Gamma = 18.6 ^{+7.9}_{-6.1} {\rm \thinspace (stat)} ^{+3.5}_{-3.8} {\rm \thinspace (syst)} $\,MeV/$c^2$ with a significance of 6.7$\,\sigma$.

Extraction of Polarization Parameters in the p̄p → Ω̄Ω Reaction

A method to extract the polarization of Ω hyperons produced via the strong interaction is presented. Assuming they are spin 3/2 particles, the corresponding spin density matrix can be written in terms of seven non-zero polarization parameters, all retrievable from the angular distribution of the decay products. Moreover by considering the full decay chain Ω → ΛK → pπK the magnitude of the asymmetry parameters βΩ and γΩ can be obtained. This method, applied here to the specific Ω case, can be generalized to any weakly decaying hyperon and is perfectly suited for the PANDA experiment where hyperon-antihyperon pairs will be copiously produced in proton-antiproton collisions. The aim is to take a step forward towards the understanding of the mechanism that reigns strangeness production in these processes.

Enhancement in p–Pb collision as an indication of quark–gluon plasma formation at the Large Hadron Collider

Proton–nucleus collisions serve as an important baseline for the understanding and interpretation of the nucleus–nucleus collisions. These collisions have been employed to characterize the cold nuclear matter effects at SPS and Relativistic Heavy-Ion Collider energies for the past several years, as it was thought that quark–gluon plasma (QGP) is not formed in such collisions. However, at the Large Hadron Collider (LHC), there seems a possibility that QGP is formed during proton–lead (p–Pb) collisions. In this work, we have derived an expression for gluon induced excitation of to , using pNRQCD, and show that the relative enhancement of vis-à-vis , especially at high pT, gives further indication that the QGP is indeed formed in p–Pb collisions at the most central collisions at LHC energy. and suppression effects seen at ALICE are also qualitatively explained.

Combined Forward-Backward Asymmetry Measurements in Top-Antitop Quark Production at the Tevatron.

The CDF and D0 experiments at the Fermilab Tevatron have measured the asymmetry between yields of forward- and backward-produced top and antitop quarks based on their rapidity difference and the asymmetry between their decay leptons. These measurements use the full data sets collected in proton-antiproton collisions at a center-of-mass energy of sqrt[s]=1.96 TeV. We report the results of combinations of the inclusive asymmetries and their differential dependencies on relevant kinematic quantities. The combined inclusive asymmetry is A_{FB}^{tt[over ¯]}=0.128±0.025. The combined inclusive and differential asymmetries are consistent with recent standard model predictions.

Charmed-baryon production in antiproton-proton collisions within an effective Lagrangian model

We study the productions of charmed baryons ${\bar \Lambda}_c^- \Lambda_c^+$, ${\bar \Lambda}_c^- \Sigma_c^+$, and ${\bar \Sigma}^- \Sigma_c^+$ in the antiproton-proton collisions within an effective Lagrangian model that has only the baryon-meson degrees of freedom and involves the physical hadron masses. The baryon production proceeds via the $t$-channel exchanges of $D^0$ and $D^{*0}$ mesons in the initial collision of the antiproton with the target proton. The distortion effects in the initial and final states are accounted for by using an eikonal approximation-based procedure. We find that the reaction amplitudes of all the production channels are dominated by the $D^{*0}$ meson-exchange diagrams. We discuss the relative roles of tensor and vector components of the $D^{*0}$ coupling in the $D^{*0}$ meson-exchange component of the total production cross sections. The magnitudes of the cross sections are predicted for each final state for the range of beam momenta of relevance to the ${\bar P}ANDA$ experiment.

Measurement of the inclusive-isolated prompt-photon cross section in pp¯ collisions using the full CDF data set

A measurement of the inclusive production cross section of isolated prompt photons in proton-antiproton collisions at center-of-mass energy s=1.96 TeV is presented. The results are obtained using the full Run II data sample collected with the Collider Detector at the Fermilab Tevatron, which corresponds to an integrated luminosity of 9.5 fb−1. The cross section is measured as a function of photon transverse energy, ETγ, in the range 30<ETγ<500 GeV and in the pseudorapidity region |ηγ|<1.0. The results are compared with predictions from parton-shower Monte Carlo models at leading order in QCD and from next-to-leading-order perturbative QCD calculations. The latter show good agreement with the measured cross section.Received 1 March 2017DOI:https://doi.org/10.1103/PhysRevD.96.092003© 2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle phenomenaParticles & Fields

Minimal NCSM Direct Photon Production in Proton-antiproton Collisions

In this paper, we present the results of our calculations for the direct photons with proton-antiproton collision in Minimal Non-Commutative Standard Model at a center-of-mass energy √ s = 1.96 Tev. The analytical expression of the differential cross section of the two subprocesses is deduced after applying a series of rotation from the equatorial coordinates system to the laboratory frame. Subsequently, we have calculated the inclusive cross section in both space-space and space-time cases independently, by averaging over all unknown angles except the scattering angle and the colatitudes γ of electric-like vector $\vec {\theta }_{E}$ and magnetic-like vector $\vec {\theta }_{B}$ . Thus, comparison with the experimental CDF data of Tevatron allowed us to deduce two larges bounds on the non-commutativity parameter: $$ {\Lambda}_{bound}^{space-space} =449.93\pm 29.20\text{GeV} $$ and ${\Lambda }_{bound}^{space-time} =459.83\pm 16.40\text { GeV}$ . Moreover, the analysis between the difference experiment-theory and the purely non-commutative contribution, for γ varying between 0 ∘ to 90 ∘, allowed us to deduce a Pearson correlation coefficient: 0,58 ≤ ρ s p a c e−s p a c e ≤0,64 and 0,55 ≤ ρ s p a c e−t i m e ≤0,61. We deduce that the perpendicular to the earth rotation axis, given by γ = 90∘, is the preferred orientation of $\vec {\theta }_{E}$ and $\vec {\theta }_{B}$ .

Measurement of the D+ -meson production cross section at low transverse momentum in pp¯ collisions at s=1.96 TeV

We report on a measurement of the $D^{+}$-meson production cross section as a function of transverse momentum ($p_T$) in proton-antiproton ($p\bar{p}$) collisions at 1.96 TeV center-of-mass energy, using the full data set collected by the Collider Detector at Fermilab in Tevatron Run II and corresponding to 10 fb$^{-1}$ of integrated luminosity. We use $D^{+} \to K^-\pi^+\pi^+$ decays fully reconstructed in the central rapidity region $|y|<1$ with transverse momentum down to 1.5 GeV/$c$, a range previously unexplored in $p\bar{p}$ collisions. Inelastic $p\bar{p}$-scattering events are selected online using minimally-biasing requirements followed by an optimized offline selection. The $K^-\pi^+\pi^+$ mass distribution is used to identify the $D^+$ signal, and the $D^+$ transverse impact-parameter distribution is used to separate prompt production, occurring directly in the hard scattering process, from secondary production from $b$-hadron decays. We obtain a prompt $D^+$ signal of 2950 candidates corresponding to a total cross section $\sigma(D^+, 1.5 < p_T < 14.5~\mbox{GeV/}c, |y|<1) = 71.9 \pm 6.8 (\mbox{stat}) \pm 9.3 (\mbox{syst})~\mu$b. While the measured cross sections are consistent with theoretical estimates in each $p_T$ bin, the shape of the observed $p_T$ spectrum is softer than the expectation from quantum chromodynamics. The results are unique in $p\bar{p}$ collisions and can improve the shape and uncertainties of future predictions.

Measurement of the direct CP violating charge asymmetry in B±→μ±νμD0 decays

We present the first measurement of the CP violating charge asymmetry in $B^\pm \to \mu^\pm \nu_\mu {D}^0$ decays using the full Run II integrated luminosity of 10.4 fb$^{-1}$ in proton-antiproton collisions collected with the D0 detector at the Fermilab Tevatron Collider. We measure a difference in the yield of $B^-$ and $B^+$ mesons in these decays by fitting the reconstructed invariant mass distributions. This results in an asymmetry of $A^{\mu D^{0}} =\left[ -0.14 \pm 0.20 \right]\%$, which is consistent with standard model predictions.

Measurement of low-pTD+-meson production cross-section at CDF II

In this paper I report on a measurement of the low-pTD+-meson production cross-section in proton-antiproton collisions at 1.96 TeV center-of-mass energy, using the full data set collected by the CDF experiment at the Tevatron collider during Run II. The measurement is performed in a yet unexplored low transverse momentum range, down to 1.5 GeV/c. The actual QCD theory cannot predict the behavior of the strong interactions in the low transferred-four-momentum region because in these kinematic conditions the strong coupling constant is of the order of the unity. Thus, a perturbative expansion is not useful. At present, several phenomenological models have been proposed, but they are able to describe only a few aspects of the observed physical quantities and not the full complexity. Experimental results in these conditions are then crucial to test new QCD models. The measurement of the differential cross section at low pT plays an important role in this context allowing refinement of current knowledge. While these results lie within the band of theoretical uncertainty, differences in shape suggest that theoretical predictions can benefit from further refinement taking account of them.

Hard Diffraction at Colliders

The phenomenon of hard diffraction is presented. The discussion includes: the present understanding of the process, experimental techniques, past measurements performed at the SPS, HERA and Tevatron colliders, the first LHC data and prospects for new results.

Reconstructing Hyperons with the ANDA Detector at FAIR

Hyperon production and the study of their properties is an important part of the physics programme of the future ANDA experiment at FAIR. Antihyperon-hyperon pairs will be produced in antiproton-proton collisions through the annihilation of at least one light antiquark-quark (u, d) pair and the creation of a corresponding number of antiquark-quark s pairs. By measuring the decay products of the hyperons, spin observables such as the polarisation can be measured.Many hyperons have a long life-time which gives rise to final state particles originating from displaced vertices. A pattern recognition algorithm using information from the ANDA Straw Tube Tracker is extended to reconstruct not only the transversal, but also the longitudinal components of charged tracks. A Hough transform and a path finding method as tools to extract the longitudinal components are being developed.