Squark pair production mechanisms in pp collisions

Through a phenomenological analysis in the transverse plane of a $\text{p}\text{p}$ collision, we study different production mechanisms of squark pairs, decaying into a quark and a photino (missing energy): excitation of squarks inside the proton, fusion of light partons into squark pairs, and decay of heavier gluinos into squarks. The rates for missing transverse momentum are calculated and confronted with the monojet events observed at the CERN collider. In the framework of “minimal” supergravity models the excitation and fusion of squark pairs are disfavoured, whereas the dominant processes involve heavier gluinos and squarks cascading into photinos through intermediate squarks. The agreement with experimental data is better if heavier squarks and gluinos decay through an intermediate b-squark. We remark that energy dissipating effects during the cascade towards the final state tend to improve the comparison between theory and experiment.     

Supersymmetric enhancement factor for the 1-jet cross-section inp - \bar p reactions

We compare the 1-jet inclusive cross-section at highp _⊥ in proton-antiproton reaction atSPS collider predicted by standard QCD and by its simplest supersymmetric extension (SQCD). We first compute the total enhancement factorK between QCD and SQCD jets as a function ofp _⊥. Then we compute the observable enhancement factor which is smaller thanK since the transverse momentum of supersymmetric particles is not fully observable. We have analyzed two cases (i)p _⊥ is small compared to the masses of squarks and only light gluinos (2 GeV) are considered (ii)p _⊥ is large compared to the masses of squarks (17 GeV in our analysis) and both gluinos and squarks are taken into account. The observable enhancement factor between QCD and SQCD is found to be small (of order 1.3 to 1.5 forp _⊥=100 GeV). Missingp _⊥ events with one ordinary jet and one jet due to the production of a supersymmetric particle are found to be non negligible with respect to those with two supersymmetric jets. We also display some interesting supersymmetric relations among parton cross-sections.     

Missing transverse energy events and the standard model

The processes $\text{p+}\text{p}\text{→ W}{}^{\mathrm{\pm }}$, Z⁰+0,1,2 jets are studied in the light of recent observations of events with large missing transverse momentum at the CERN p$\text{p}$ collider. We find that a nontrivial number of such events can arise from the leptonic decays $\text{Z}{}^0\text{→ ν}\text{ν}$ and W^± → ?^±ν where the charged lepton escapes identification. In both cases the number of events with one jet and with two jets are comparable. By correlating these decay modes with those where the leptons are observed we can, in principle, eliminate the theoretical uncertainties inherent in such perturbative QCD calculations.     

Jets and correlations in large pT reactions

Recent experimental data on correlations in large transverse momentum reactions are analyzed in a two-jet picture. Scaling in jet fragmentation is shown to account well for the existing data. The formulae that are given can be used as a basis for a more detailed analysis of the dynamics. A particular result is that we predict that a two-jet structure can also be produced in events where it is not required that any single hadron has large p_T, with a cross section as large as $\text{1}\text{10}$ mb or more (at ISR energies with the total transverse momentum of each jet ?2.5 GeV/c).     

Squark production at the pp collider

The recently observed $\text{p}\text{p}$ collider events with large missing P_T are analysed in terms of a supersymmetric model with a light photino and scalar quarks with mass in the range 20 to 35 GeV. A lower bound of O(60 GeV) on the gluino mass in this scenario is found.     

Production of Z (W) in association with a jet at unusually large transverse momentum

We point out an unusual energy-independent component of the elementary two-body cross section for quark-antiquark annihilation into an intermediate vector boson in association with a jet which arises from a hypothetical spin-zero quantum. This leads to an unexpected extension of the transverse momentum distribution, with possible relevance to some of the novel events seen at the CERN Sp$\text{p}$S collider.     

A measurement of the transverse momenta of partons,and of jet fragmentation as a function of √s in p-p collisions

A large solid-angle apparatus consisting of a superconducting solenoid magnet, cylindrical drift chambers and two arrays of lead-glass counters was used to examine particles associated with a high transverse momentum trigger in p-p collisions with three √s values at the CERN ISR. The trigger was given by energy deposition in lead-glass arrays centred at 90°. The trigger transverse momentum range covered was 3 < $\text{p}{}_{\mathrm{<mtext>T\; trig</mtext>}}\text{< 11}\text{GeV}\text{c}$. Results are given for p_out for both individual charged particles, and also for the sum of charged particle momenta in the hemisphere opposite to the trigger. Mean values are then deduced for the parton transverse momentum k_T, and for the jet fragmentation momentum j_T.     

Vorticity in nuclear fluid dynamics

We consider a model for gloun jet fragmentation based on QCD in which the fast hadrons in the jet are produced by the sequential reaction gluon $\text{→}\text{q}\text{q}\text{?}\text{q}\text{→}$ hadrons. The resulting jet shows an oblate transverse momentum structure, with a major axis preferential oriented normally to the direction of linear polarization of the gluon. We discuss jet-jet oblateness angular correlations in decays of heavy QQ? pseudoscalar and vector systems.     

Hadronic jets with a large Q⊥ lepton pair trigger

We propose to observe hadron jets in correlation with large Q⊥ transverse momentum lepton pairs. This would allow to test the theoretical idea that the large transverse momentum of the pair is mainly produced through a scattering subprocess $\text{a}\text{+}\text{b}\text{→}\text{c}\text{+γ}{}^1$. The quantum number content of these jets is a specific signature of the subprocess, especially of those involved in perturbative QCD.     

Extracting the wavefunction of the LSP at the LHC

We consider associated production of squarks and gluinos with the lightest supersymmetric particle (LSP), or states nearly degenerate in mass with it. Though sub-dominant to pair production of color SU(3)-charged superpartners, these processes are directly sensitive to the wavefunction composition of the lightest neutralinos. Exploiting event-shape variables - including some introduced here for the first time - we are able to identify the composition of the LSP by selecting events involving a single high-pT jet recoiling against missing transverse energy. We illustrate the proposed technique on a set of benchmark cases and propose methods for applying these results in more realistic experimental environments.     

Inclusive V0 production in pp interactions at 12 GeV/c

Results are presented on the inclusive reactions $\text{p}\text{p}\text{→}\text{K}{}^0\text{X}\text{,}\text{p}\text{p}\text{→ Λ}\text{X and}\text{p}\text{p}\text{→}\text{Λ}\text{X}$ at an incident antiproton momentum of 12 GeV/c in BEBC. The cross sections are studied as functions of the Feynman scaling variable x, the rapidity, the transverse momentum of the V⁰ and the missing mass squared. The dependence of the Λ and $\text{Λ}$ polarization on x are also studied. Comparisons with proton-proton data at 12 GeV/c are also made. Finally, events with two detected V⁰ are analyzed in order to study correlations arising from the production of two strange neutral particles.     

Heavy-quark production by hadrons: Gluon jet fragmentation versus fusion

We show that at $\text{p}\text{p}$ collider energies heavy quarks are dominantly produced by the fragmentation of gluon jets into $\text{Q}\text{Q}$ with Q = c,b. This is because the underlying gg → gg jet cross section exceeds the fusion process $\text{gg → Q}\text{Q}$ by over two orders of magnitude. We compute the perturbative contribution to the gluon fragmentation from 2 → 3 processes such as gg $\text{→ gQ}\text{Q}$. We point out the observable consequences for single-lepton production and same-opposite charge dileptons. In particular, the observed production of dileptons with large p_T and low effective mass is explained.     

Charge asymmetry at large pT in inelastic pp reactions at 22.4 GeV/c

Charge asymmetry in $\text{p}\text{p}$ reactions at 22.4 GeV/c is investigated as a function of transverse momentum in the c.m. rapidity interval |y¹| < 1. The charge asymmetries in the forward and backward directions are increasing with increasing transverse momentum. The increase seems to be present both in annihilation and non-annihilation processes.     

Heavy-flavor tagging and the supersymmetry reach of the CERN Large Hadron Collider

The branching fraction for the decays of gluinos to third generation quarks is expected to be enhanced in classes of supersymmetric models where either third generation squarks are lighter than other squarks, or in mixed-higgsino dark matter models constructed so as to be in concordance with the measured density of cold dark matter. In such scenarios, gluino production events at the CERN Large Hadron Collider should be rich in top and bottom quark jets. Requiring b jets in addition to E _T ^miss should, therefore, enhance the supersymmetry signal relative to Standard Model backgrounds from V + jet, VV and QCD backgrounds (V=W,Z). We quantify the increase in the supersymmetry reach of the LHC from b-tagging in a variety of well-motivated models of supersymmetry. We also explore “top tagging” at the LHC. We find that while the efficiency for this turns out to be too low to give an increase in reach beyond that obtained via b-tagging, top tagging can indeed provide a confirmatory signal if gluinos are not too heavy. We also examine c jet tagging but find that it is not useful at the LHC. Finally, we explore the prospects for detecting the direct production of third generation squarks in models with an inverted squark mass hierarchy. This is signaled by b jets + E _T ^miss events being harder than in the Standard Model, but softer than those from the production of gluinos and heavier squarks. We find that while these events can be readily separated from the SM background (for third generation squark masses ∼300–500 GeV), the contamination from the much heavier gluinos and squarks remains formidable if these are also accessible.     

Momentum spectra of secondary particles produced in proton-proton collisions at 14.2, 19.2 and 24.0 GeV/c

Results on charged particle yields in proton-proton collisions at 14.2, 19.2 and 24.0 GeV/c for secondary momenta above 4 GeV/c and lab angles between 10 and 150 mrad are reported. Cross sections for inclusive reactions involving the production of π^± K^±, p and $\text{p}$ are presented. The general features of the dependence of the data on transverse and longitudinal momentum, missing mass, and four-momentum transfer are described and the approach to limiting distributions in the 10 to 30 GeV energy range is discussed.     

On the QCD perturbative predictions for the signal ofW± production in hadronic collisions

Evidence for the intermediate bosons in high energy pp and p?p collisions as signal in the cross section of large p_T charged lepton pp, $\text{p}\text{p}\text{→}\text{W}{}^{\mathrm{\pm }}\text{+}\text{X}$, W^±→?^±ν_? is considered for CM energy range $\text{s}\text{=100?800}\text{GeV}$. Besides the usual Drell-Yan term which gives a very sharp peak in the lepton p_T distribution, the first order QCD contribution is included thus producing the intermediate boson with transverse momentum and smearing the lepton distribution. Scaling violation of the structure functions is incorporated and compared with the case when it is absent.     

A measurement of the energy dependence of the inclusive yield of high-Pt events triggered by a large solid angle calorimeter

Large aperture calorimeter measurements of high transverse momentum events from 100, 200 and 340 GeV pp collisions are reported. Yields are higher by one to two orders of magnitude than the yield expected from uncorrelated statistical fluctuations in multiparticle events. The exponential slopes of the yields versus x_t exhibited a systematic change with $\text{s}\text{(x}{}_{\mathrm{<mtext>t</mtext>}}\text{= 2P}{}_{\mathrm{<mtext>t</mtext>}}\text{/}\text{s}\text{)}$. The events are shown to be more diffuse than pure jets as produced in e⁺e^? collisions. However, these events are consistent with a QCD-inspired 4-jet model.     

Process dependence of the target jet remnants in the photoproduction of large transverse momentum jets

We use the quark recombination model of Das and Hwa to predict the inclusive meson spectra (π⁺, π^?,K ⁺,K ^?) in the target fragmentation region for processes in which a large transverse momentum jet is produced by a nearly real photon. We find that the antiparticle ratios of such target jet mesons are sensitive to the type of process which has produced the largep _t jet. By comparing the ratios found in photoproduction to those in deep inelastic scattering we point out that it is in principle possible to identify the presence of the Bethe-Heitler photoproduction subprocess. We conclude that the target jet remnants can contribute to separating the subprocesses responsible for the photoproduction of largep _t jets.     

A comparison of b and uds quark jets to gluon jets

Symmetric three-jet events are selected from hadronic Z⁰ decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momentum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets, which are produced with the same energy of about 24 GeV and under the same conditions. We observe b jets and gluon jets to have similar properties as measured by the angular distribution of particle energy around the jet directions and by the fragmentation functions. In contrast, gluon jets are found to be significantly broader and to have a markedly softer fragmentation function than uds jets. For the k _⊥ jet finder with y _cut=0.02, we find $${«ngle n^{? ch.}»ngle {? gluon}?er «ngle n^{? ch.}»ngle {? b} {? quark}}=1.089pm 0.024 ({? stat.})pm0.024 ({? syst.})$$ $${«ngle n^{? ch.}»ngle {? gluon}?er «ngle n^{? ch.}»ngle {? uds} {? quark}}=1.390pm 0.038 ({? stat.})pm0.032 ({? syst.})$$ as the ratios of the mean charged particle multiplicity in the gluon jets compared to the b and uds jets. Results are also reported using the cone jet finder.