Inpp Collisions Research Articles

Ψ AND ϒ PRODUCTION IN pp COLLISIONS AT 7.0 TeV

This is an extension of recent studies for ϒ(nS) and Ψ(1S, 2S) production at the LHC in pp collisions, [Formula: see text], with the ALICE detector.

Search for narrow-widtht-t resonances inp-p collisions at √s = 1.8 TeV

We present a preliminary result on a search for narrow-width resonances that decay intot-t pairs using 130 pb−1 of lepton + jets data inp-p collisions at √s = 1.8 TeV. No significant deviation is observed from prediction of the standard model, and upper limits at 95% confidence on the product of the production cross-section and its branching fraction tot-t are presented for narrow-width resonances, as a function of resonance massMx. We also use these limits to exclude the existence of a leptophobic top-color particle,X, withM x < 560 GeV/c2 and width Γx = 0.012M x .

Prospects for the extraction of |gn cb| fromB c →J/ψμ + ν decays at LHC

We discuss the possibility of extracting |ν cb| from the future measurement at LHC of theB c→J/ψμ + ν decay rate followed by theleptonic decayJ/ψ→μ + μ −. We examine in detail the influence of the measurement of the muons’ momenta and vertex reconstruction in the final uncertainty of |ν cb|, as well as the required theoretical inputs at both the production and weak decay level, and their reliability in this regard. We stress the relevance of a rigorous description of the fragmentation contributions to inclusive production at large transverse momentum of heavy quarkonia inpp collisions.

HIGH pT JET PRODUCTION IN pp COLLISIONS

Production rates of large pT jets inpp collisions at RHIC and LHC energies are studied using the next-to-leading order calculation of S.D. Ellis, Z. Kunszt and D. Soper. The computed inclusive one-jet cross sections are compared against the CERN and Fermilab jet data from [Formula: see text] and pp collisions. The dependence of the results on the choice of parton distributions and renormalization/factorization scales is investigated.

Similarity of hadron production inpp and heavy-ion collisions

The characteristic features of pseudorapidity distributions of hadron production are investigated in terms of a geometrical parameterh≡(dn/dη)0/Φ, Φ being the phase space covered by the η distribution. It is found thath is about the same for the heavy-ion (HI) andpp collisions at the same energy, the same for π andK production and behaves ash∼Ecm−1/3. This similarity property yieldsK−/π−=0.041±0.004 andK+/K−=4.5±0.6 for HI collisions at 14.6 GeV/A in agreement with BNL experiments.

Bose-Einstein correlations inpp collisions at 400 GeV/c

Bose-Einstein correlations among identically charged pions produced inpp collisions at 400 GeV/c are studied using the EHS spectrometer. Empoying the Kopylov-Podgoretskii parametrization, the average size of the emitting regionr k and its lifetime τ for pion production were determined to ber k=1.71±0.04 fm andcτ=0.89±0.05fm. The average sizer g in terms of the Lorentz invariant Goldhaber parametrization was determined to ber g=1.20±0.03fm. A decrease of the size with increasing momentum of the pions was observed. The size and the incoherence parameter of the pion emitting region were determined as a function of the charged particle multiplicity and the momentum of the pions. Identified charged kaons were used to study Bose-Einstein correlations among identically charged kaonsK ±K±. The average size of the emitting region for kaon production was determined to ber k=1.87±0.33fm in terms of the Kopylov-Podgoretskii parametrization. A study of the influence of a reference sample is presented.

Pair production of charged Higgs particles through two-photon process inpp collisions

In order to obtain model-independent estimates for the production of charged Higgs particles atpp colliders, in particular at the SSC, we investigate their pair production through electromagnetic processes. It turns out that the dominant contribution comes from photon-photon annihilation. For a Higgs mass of a few hundred GeV the two-photon process is thus an appropriate candidate to look for charged Higgs particles.

Tagging diquarks by protons of high transverse momentum inpp collisions at the ISR

Events are analyzed in which a high transverse momentum proton was produced at polar angles of 10°, 20° and 45°. The experiment was performed with the Split Field Magnet detector at the CERN ISR at\(\sqrt s \)=62 GeV. A 4-jet structure of these events is found [1]. The measured charge structure of spectator jets is compatible with proton production from hard diquark scattering. This is supported by a study of baryon number compensation in the towards jets. The observed charge compensation in the towards jets suggests dominance of hard (ud) scattering. Evidence forΔ ++ production at high transverse momentum indicates the presence of an additional (uu) scattering component. The properties of the recoiling away jets are compatible with the fragmentation of a valence quark and/or of a gluon as in the case of meson triggers.

Pion interferometry in jet events at the CERN Intersecting Storage Rings

We have observed an enhanced production of pairs and triplets of identical charged particles at small momentum differences in events with two central jets produced inpp collisions at the CERN Intersecting Storage Rings. The size and structure of this ‘Bose-Einstein’ enhancement, shown by the particles within a jet, resembles that observed in minimum-bias events. The correlation function can be well parametrized by the single variableQ — the absolute value of the four-momentum transfer — and is more peaked than a single Gaussian.

Direct-photon plus away-side jet production inpp collisions at $$\sqrt s = 63$$ GeV and a determination of the gluon distribution

We have determined the double inclusive cross-section for opposite-side high-p T photons and away-side jets withθ γ ≈θjet≈90° produced inpp collisions at the CERN Intersecting Storage Rings at\(\sqrt s = 63\) GeV. Under the assumption that these events arise predominantly from the QCD gluon Compton process we have calculated the gluon structure function in the range 0.15≦x≦0.30 at an average square of the four-momentum transfer of 40 GeV2/c2. The data favour a soft gluon distribution in the proton.

A study of the production of two direct photons inpp collisions at the CERN ISR

We have studied the processpp→γγ+X at\(\sqrt s = 63 GeV\) GeV in the central rapidity region. We report a positive signal at 96% C.L., a ratio γγ/e + e −=4.0±3.0 when the transverse momentum of each photon is above 2 GeV/c, and a cross-sectiondσ/dydM γγ=(5.5±2.7)×10−34 cm2/GeV when |y|<0.5,4<M γγ<6 GeV.

Three-jet production inpp collisions at $$\sqrt s = 63 GeV$$ and a determination of ? s

We have studiedpp collisions at the CERN ISR at\(\sqrt s = 63 GeV\) using a jet trigger. We demonstrate that a sample of well-separated triple high-p T jet events can be described as due to the QCD bremsstrahlung process. The energy flow of these 3-jet events is well described by an independent fragmentation model, with a gluon-jet transverse width 33% greater than the quark-jet width. From the ratio of the 3- and 2-jet production crosssections the strong coupling is determined to lowest order in QCD. We find α s =0.18±0.03 (stat.) ±0.04 (stst.). The error does not include the theoretical uncertainty in the fragmentation scheme.We compare our results with measurements at the SPS Collider and discuss the different methods used.

Dijet production cross-section and fragmentation of jets produced inp p collisions at $$\sqrt s $$ =63 GeV

We present stdies of events triggered on two high-pT jets, produced inpp collisions at the CERN Intersecting Storage Rings (ISR) at\(\sqrt s \)=63 GeV, using a large solid angle calorimeter. The cross-section for producing two jets is measured in the dijet mass range 17–50 GeV/c2. A high-statistics sample of dijet events, where each jet has transverse energy above 10 GeV, is used to study the structure of jets and the associated event. We find the longitudinal fragmentation function to be similar to that of jets emerging frome+e− collisions but considerably harder than that observed at the Super Proton Synchrotron (SPS)\(p\bar p\) Collider. A steepening of the fragmentation function is observed when increasing the jet energy. Studies of the charge distribution in jets show that these predominantly originate from fragmenting valence quarks. The transverse energy and particle flows are presented as functions of the azimuthal distance from the jet axis.

Observation of double-Pomeron exchange in alpha-alpha collisions at the CERN intersecting storage rings

We observed the process αα→ααX in which the α′s were emitted uncorrelated in the forward direction and the charged component of the clusterX was confined to a limited portion (|η|≲2) of the central region. We identified such reactions as being due to double-Pomeron exchange, for which we found a cross-section of (720±140)μb. The raw mean charged multiplicity of the clusterX was found to be 6.76±0.07 with a dispersionD=4.8. The measurements were performed at the CERN ISR at a centre-of-mass energy of\(\sqrt s = 126\) GeV. Similarities are drawn between double-Pomeron exchange in αα and inpp collisions.

A diquark scattering model for highp T proton production inpp collisions at the ISR

The relative yield of high transverse momentum protons measured inpp interctions at the CERN ISR is calculated in the framework of simple parton models. Whereas models based exclusively on quark and gluon scattering fail to describe the data, the hypothesis of diquark scattering provides a quantitative understanding of the measured dependences on transverse momentum, polar angle, and centre-of-mass energy. Different assumptions on diquark structure functions are tested.

Are recombination and fragmentation models able to describe two-pion spectra?

We compare new data on lowp T inclusive π± and π±π± production inpp collisions at\(\sqrt s\) = 62 GeV with predictions of the valon recombination and the two-chain fragmentation models. We discuss the original version of the models which are based on the assumption of universality of valon structure and jet fragmentation, respectively. Within the limitation of present analysis, both models agree well with π− and π−π− data, but none of them is able to provide a satisfactory description of both π+ and π+π+ distributions.

Hyperon production in hadronic collisions

A simple fragmentation model of hadronic collisions is used to study hyperon production. The fragmentation is visualised with the aid of the string model and string breaking is modelled using a simple Monte Carlo procedure. The strings are created via a Drell-Yan type quark-diquark fusion mechanism with another string being created among the spectator partons. With the fusion cross-sections assumed to have a logarithmics behaviour, a fair description ofdσ/dx F for λ production inpp and πp collisions over a wide energy range is obtained. The available Σ production data is modelled by assuming, in addition, that some Σ′s are the products of Λp* decay. Some observations are made on the differences betweenuu andud diquarks and on hyperon polarisations.

Properties of jets in high-E T events produced inpp collisions at $$\sqrt s $$ =63 GeV

The properties of high-E T events produced inpp collisions at\(\sqrt s \)=63 GeV have been studied at the CERN Intersecting Storage Rings. The fragmentation of the jets is found to be very similar to that of jets produced ine + e − annihilation and at the SPS\(p\bar p\) collider. We measure an excess of soft particles compared withe + e − events and this underlying event is found to be sensitive to the trigger. Thep T balance between the jets is measured. The distribution of charges in the jets indicates that a substantial fraction of the jets originate from valence quarks.

Direct photon production beyond leading log approximation—q+q→γ+q+q contribution

Theq+q→γ+q+q contribution (for incoming quarks of equal and different flavours) to the next-to-leading correction for the direct photon production inpp collisions is calculated, analyzed in terms of double moments and compared with the data. The dimensional regularization method is used and the final state Bremsstrahlung infinities are regularized by an angular cut-off. The correction is found to be 30–40% of the Born contribution providing a better description of the data.

Production of π0 andη 0 at 11° inpp Collisions at $$\sqrt s = 63 GeV$$

The inclusive π0 production cross-section and the η/π0 ratio have been measured inpp collisions at\(\sqrt s = 63 GeV\) at the CERN Intersecting Storage Rings in the rapidity range 2.00<y<2.75. The π0 cross-section exhibits a strongy-dependence and falls more steeply as a function ofpT, compared with the cross-section measured aty∼0. We find a value of 0.46±0.07 for the η/π0 ratio with no significantpT dependence over the range 2.0<pT<4.0 GeV/c.