Antiproton Collisions Research Articles

Scaling behaviour of charged hadron distributions in pp and collisions

We report on a scaling behaviour in the transverse momentum () distributions for charged hadrons produced in proton–proton (pp) collisions with different centre of mass energies ( = 0.9, 2.76 and 7 TeV) at the Compact Muon Solenoid detector. This scaling behaviour appears when the is replaced by , where K is a parameter and depends on . A similar scaling behaviour is observed in the pT distributions of charged hadrons produced in proton–antiproton () collisions with = 0.63, 1.8 and 1.96 TeV at the Collider Detector at Fermilab. The particle production mechanism behind the scaling behaviour in the pp or collisions could be explained by the model of percolation of strings.

Measurement of the differential [formula omitted]-jet cross section and the ratio [formula omitted] in [formula omitted] collisions at [formula omitted

We present the first measurements of the differential cross section dσ/dpTγ for the production of an isolated photon in association with at least two b-quark jets. The measurements consider photons with rapidities |yγ|<1.0 and transverse momenta 30<pTγ<200 GeV. The b-quark jets are required to have pTjet>15 GeV and |yjet|<1.5. The ratio of differential production cross sections for γ+2b-jets to γ+b-jet as a function of pTγ is also presented. The results are based on the proton–antiproton collision data at s=1.96 TeV collected with the D0 detector at the Fermilab Tevatron Collider. The measured cross sections and their ratios are compared to the next-to-leading order perturbative QCD calculations as well as predictions based on the kT-factorization approach and those from the sherpa and pythia Monte Carlo event generators.

Measurement of B(t→Wb)/B(t→Wq) in top-quark-pair decays using dilepton events and the full CDF Run II data set.

We present a measurement of the ratio of the top-quark branching fractions R=B(t→Wb)/B(t→Wq), where q represents any quark flavor, in events with two charged leptons, imbalance in total transverse energy, and at least two jets. The measurement uses proton-antiproton collision data at center-of-mass energy 1.96TeV, corresponding to an integrated luminosity of 8.7 fb^{-1} collected with the Collider Detector at Fermilab during Run II of the Tevatron. We measure R to be 0.87±0.07, and extract the magnitude of the top-bottom quark coupling to be |V_{tb}|=0.93±0.04, assuming three generations of quarks. Under these assumptions, a lower limit of |V_{tb}|>0.85(0.87) at 95% (90%) credibility level is set.

Close-coupling approach to antiproton-impact breakup of molecular hydrogen

An ab initio time-dependent convergent close-coupling approach to describing antiproton collisions with molecular hydrogen or the hydrogen molecular ion has been developed. The approach accounts for all possible orientations of the molecular target. Averaging over the molecular orientations is performed fully analytically. For the molecular hydrogen target calculated orientation-averaged total cross sections for single ionization and proton production are compared with several experiments over the energy range of 1–2000 keV. Results for single ionization are in good agreement with experiment, except for the region around the experimental maximum. For proton production reasonable agreement with experiment is observed above 40 keV.

Confronting current NLO parton fragmentation functions with inclusive charged-particle spectra at hadron colliders

The inclusive spectra of charged particles measured at high transverse momenta (pT≳2 GeV/c) in proton–proton and proton–antiproton collisions in the range of center-of-mass energies s=200–7000 GeV are compared with next-to-leading order perturbative QCD calculations using seven recent sets of parton-to-hadron fragmentation functions (FFs). Accounting for the uncertainties in the scale choices and in the parton distribution functions, we find that most of the theoretical predictions tend to overpredict the measured LHC and Tevatron cross sections by up to a factor of two. We identify the currently too-hard gluon-to-hadron FFs as the probable source of the problem, and justify the need to refit the FFs using the available LHC and Tevatron data in a region of transverse momenta, pT≳10 GeV/c, which is supposedly free from additional non-perturbative contributions and where the scale uncertainty is only modest.

Antiproton helium collisions in time-dependent density functional theory

We have calculated total cross sections for single and double ionization in antiproton helium collisions over a range of impact energies. Several models for the incorporation of correlation have been explored in the context of time-dependent density functional theory.

Combination of measurements of the top-quark pair production cross section from the Tevatron Collider

We combine six measurements of the inclusive top-quark pair (ttbar) production cross section (sigma_ttbar) from data collected with the CDF and D0 detectors at the Fermilab Tevatron with proton anti-proton collisions at sqrt{s} = 1.96 TeV. The data correspond to integrated luminosities of up to 8.8 fb^{-1}. We obtain a value of sigma_ttbar = 7.60 \pm 0.41 pb for a top-quark mass of m_t=172.5 GeV. The contributions to the uncertainty are 0.20 pb from statistical sources, 0.29 pb from systematic sources, and 0.21 pb from the uncertainty on the integrated luminosity. The result is in good agreement with the standard model expectation of 7.35^{+0.28}_{-0.33} pb at NNLO+NNLL in pertubative QCD.

Standard Model Higgs boson searches at the Tevatron

We give an overview of Standard Model (SM) Higgs boson studies performed at the CDF and D∅ experiments at the Tevatron proton–antiproton collider. Combining the results of many individual analyses, most of which use the full data set available, an excess with a significance of 3.0 standard deviations with respect to the SM hypothesis is observed at a Higgs boson mass of 125 GeV /c2. At that mass, the combined best-fit cross-section is consistent with the SM prediction. Constraints are also placed on the Higgs boson couplings with fermions and electroweak vector bosons and are consistent with the SM predictions within the uncertainties.

The performance and long term stability of the D0 Run II forward muon scintillation counters

The performance of the D0 experiment forward muon scintillation counters system during Run II of the Tevatron from 2001 to 2011 is described. The system consists of 4214 scintillation counters in six layers. The long term stability of the counters amplitude response determined using LED calibration system and muons produced in proton–antiproton collisions is presented. The averaged signal amplitude for counters of all layers has gradually decreased over ten years by 11%. The reference timing, determined using LED calibration, was stable within 0.26ns. Averaged value of muon timing peak position was used for periodic D0 clock signal adjustments to compensate seasonal drift caused by temperature variations. Counters occupancy for different triggers in physics data collection runs and for minimum bias triggers are presented. The single muon yields versus time and the luminosity dependence of yields were stable for the forward muon system within 1% over 10 years.

Asymmetries in the angular distributions in the $t\bar t$ system

We review the status of the forward–backward asymmetry measurements in the pair production of top and antitop in proton–antiproton collisions. We also comment on the charge asymmetry in the production of the [Formula: see text] system in the proton–proton collisions.

Proton–Proton and Proton–Antiproton Colliders

In the last five decades, proton–proton and proton–antiproton colliders have been the most powerful tools for high energy physics investigations. They have also deeply catalyzed innovation in accelerator physics and technology. Among the large number of proposed colliders, only four have really succeeded in becoming operational: the ISR, the SppbarS, the Tevatron and the LHC. Another hadron collider, RHIC, originally conceived for ion–ion collisions, has also been operated part-time with polarized protons. Although a vast literature documenting them is available, this paper is intended to provide a quick synthesis of their main features and key performance.

TOP AND HIGGS PHYSICS AT THE HADRON COLLIDERS

This review summarizes the recent results for top quark and Higgs boson measurements from experiments at Tevatron, a proton–antiproton collider at a center-of-mass energy of [Formula: see text], and the Large Hadron Collider, a proton–proton collider at a center-of-mass energy of [Formula: see text]. These results include the discovery of a Higgs-like boson and measurement of its various properties, and measurements in the top quark sector, e.g. top quark mass, spin, charge asymmetry and production of single top quark.

Formation of antihydrogen in antiproton positronium inelastic scattering

Possibility of producing antihydrogen during inelastic collisions of antiproton with atoms and ions suggests the investigation of multi-channel scattering processes in which antihydrogen formation is considered. Present work deals with inelastic collisions of antiproton with positronium atoms at energies between 10 and 1000 eV. Coupled static approximation and Green’s function iterative numerical technique is employed for calculating partial and total cross sections corresponding to seven values of total angular momentum l (0 ≤ l ≤ 6). It is assumed that elastic and antihydrogen formation channels are open and all other channels are closed. Effects of “switching on” various polarization potentials on antiproton positronium inelastic collisions are investigated in detail within the framework of coupled static model. Our calculations have shown that at low energies, target polarization plays an important role and must be taken into account. A comparison made between our results and those determined by previous authors gives reasonable agreement.

Unified treatment of hadronic annihilation and protonium formation in slow collisions of antiprotons with hydrogen atoms

Antiproton ($\overline{p}$) collisions with hydrogen atoms, resulting in the hadronic process of particle-antiparticle annihilation and the atomic process of protonium ($\overline{p}p$) formation (or $\overline{p}$ capture), are investigated theoretically. As the collision energy decreases, the collision time required for the $\overline{p}$ capture becomes necessarily longer. Then, there is the possibility that the $\overline{p}$-$p$ annihilation occurs significantly before the $\overline{p}$ capture process completes. In such a case, one can no longer consider the annihilation decay separately from the $\overline{p}$ capture process. The present study develops a rigorous unified quantum-mechanical treatment of the annihilation and $\overline{p}$ capture processes. For this purpose, an $R$-matrix approach for atomic collisions is extended to have complex-valued $R$-matrix elements allowing for the hadronic annihilation. Detailed calculations are carried out at low collision energies ranging from ${10}^{\ensuremath{-}8}$ to ${10}^{\ensuremath{-}1}$ eV, and the annihilation and the $\overline{p}$ capture (total and product-state selected) cross sections are reported. Consideration is given to the difference between the direct annihilation occurring during the collision and the annihilation of $\overline{p}p$ occurring after the $\overline{p}$ capture. The present annihilation process is also compared with the annihilation in two-body $\overline{p}+p$ collisions.

Measurement ofR=B(t→Wb)/B(t→Wq)in top-quark-pair decays usinglepton+jetsevents and the full CDF run II dataset

We present a measurement of the ratio of the top-quark branching fractions $R=\mathcal{B}(t\rightarrow Wb)/\mathcal{B}(t\rightarrow Wq)$, where $q$ represents quarks of type $b$, $s$, or $d$, in the final state with a lepton and hadronic jets. The measurement uses $\sqrt{s}$ = 1.96 TeV proton--antiproton collision data from 8.7 fb$^{-1}$ of integrated luminosity collected with the Collider Detector at Fermilab during Run II of the Tevatron. We simultaneously measure $R=0.94 \pm 0.09$ (stat+syst), the $t\bar{t}$ production cross section $\sigma_{t \bar t} = 7.5 \pm 1.0$ (stat+syst) pb. The magnitude of the Cabibbo-Kobayashi-Maskawa matrix element, $|V_{tb}| = 0.97 \pm 0.05$ (stat+syst) is extracted assuming three generations of quarks, and a lower limit of $|V_{tb}|>0.89$ at 95% credibility level is set.

TOP QUARK PHYSICS AT THE TEVATRON

The heaviest known elementary particle, the top quark, was discovered in 1995 by the CDF and D0 collaborations at the Tevatron proton–antiproton collider at Fermilab. Since its discovery, a large program was set in motion by the CDF and D0 collaborations to characterize the production and decay properties of top quarks, and investigate their potential for searches of new phenomena beyond the standard model. During the past 20 years, new methods were developed and implemented to improve the measurements and searches for new physics in the top quark sector. This paper reviews the achievements and results obtained through studies of the top quark at the Tevatron.

John Adams and CERN: Personal Recollections

By any standards, John Adams had a most remarkable career. He was involved in three important, emerging technologies, radar, particle accelerators and controlled fusion, and had an outstanding impact on the last two. Without a university education, he attained hierarchical positions of the highest level in prestigious national and international organizations. This article covers the CERN part of his career, by offering some personal insights into the different facets of his contributions to major accelerator projects, from the first strong-focusing synchrotron, the PS, to the SPS and its conversion to a proton–antiproton collider. In particular, it outlines his abilities as a leader of an international collaboration, which has served as an example for international initiatives in other disciplines.

The cos2ϕ azimuthal asymmetry of unpolarized dilepton production in $p\bar{p}$ collisions around Z-resonance

We calculate the cos2ϕ azimuthal asymmetry of the unpolarized \(p\bar{p}\) dilepton production process in the Z-resonance region at the Tevatron kinematic domain. Compared to the pp process, such an azimuthal asymmetry can provide additional information about a spin-related three-dimensional parton distribution function of the proton, the Boer–Mulders function. It plays a significant role in separating the contributions of the valence quarks and of the sea quarks. Besides, the Z production process is much different from the virtual photon mediate Drell–Yan process as well as the J/ψ and ϒ production processes. Our estimation indicates that this asymmetry is sizable to be detected. Therefore the available data of unpolarized proton–antiproton collision at Tevatron can contribute to our study on the spin-related structure of the nucleon.

Measurement of the [formula omitted] production cross section at [formula omitted

We present a measurement of the cross section for W boson production in association with at least one b-quark jet in proton–antiproton collisions. The measurement is made using data corresponding to an integrated luminosity of 6.1 fb−1 recorded with the D0 detector at the Fermilab Tevatron pp¯ Collider at s=1.96 TeV. We measure an inclusive cross section of σ(W(→μν)+b+X)=1.04±0.05(stat.)±0.12(syst.) pb and σ(W(→eν)+b+X)=1.00±0.04(stat.)±0.12(syst.) pb in the phase space defined by pTν>25 GeV, pTb-jet>20 GeV, |ηb-jet|<1.1, and a muon (electron) with pTℓ>20 GeV and |ημ|<1.7 (|ηe|<1.1 or 1.5<|ηe|<2.5). The combined result per lepton family is σ(W(→ℓν)+b+X)=1.05±0.12(stat.+syst.) pb for |ηℓ|<1.7. The results are in agreement with predictions from next-to-leading order QCD calculations using mcfm, σ(W+b)⋅B(W→ℓν)=1.34−0.34+0.41(syst.) pb, and also with predictions from the sherpa and madgraph Monte Carlo event generators.

Search for a Higgs boson in the diphoton final state using the full CDF data set from [formula omitted] collisions at [formula omitted

A search for a narrow Higgs boson resonance in the diphoton mass spectrum is presented based on data corresponding to 10 fb−1 of integrated luminosity collected by the CDF experiment from proton–antiproton collisions at s=1.96 TeV. To increase the sensitivity of the search, we employ a multivariate discriminant technique for the first time in this channel at CDF. No evidence of signal is observed, and upper limits are set on the cross section times branching ratio of the resonant state as a function of the Higgs boson mass. The limits are interpreted in the context of the standard model with an expected (observed) limit on the cross section times branching ratio of 9.9 (17.0) times the standard model prediction at the 95% credibility level for a Higgs boson mass of 125 GeV/c2. Moreover, a Higgs boson with suppressed couplings to fermions is excluded for masses below 114 GeV/c2 at the 95% credibility level.