Low Invariant Mass Research Articles

Dilepton production in proton-proton collisions at top RHIC energy

We study dielectron production in proton-proton collisions at top RHIC beam energy within an extended statistical hadronization model. The invariant mass spectrum of correlated dielectron pairs is evaluated in the low invariant mass region and calculated results are compared with the PHENIX experiment. The model is found to be able to describe the data very well up to invariant masses of 1 GeV with few adjustable parameters.

On the background in the $ \gamma$p $ \rightarrow$ $ \omega$($ \pi^{0}_{}$ $ \gamma$)p reaction and mixed event simulation

In this paper we evaluate sources of background for the $\gamma p \to \omega p$, with the $\omega$ detected through its $\pi^0 \gamma$ decay channel, to compare with the experiment carried out at ELSA. We find background from $\gamma p \to \pi^0 \pi^0 p$ followed by decay of a $\pi^0$ into two $\gamma$, recombining one $\pi^0$ and one $\gamma$, and from the $\gamma p \to \pi^0 \eta p$ reaction with subsequent decay of the $\eta$ into two photons. This background accounts for the data at $\pi^0 \gamma$ invariant masses beyond 700 MeV, but strength is missing at lower invariant masses which was attributed to photon misidentification events, which we simulate to get a good reproduction of the experimental background. Once this is done, we perform an event mixing simulation to reproduce the calculated background and we find that the method provides a good description of the background at low $\pi^0 \gamma$ invariant masses but fakes the background at high invariant masses, making background events at low invariant masses, which are due to $\gamma$ misidentification events, responsible for the background at high invariant masses which is due to the $\gamma p \to \pi^0 \pi^0 p$ and $\gamma p \to \pi^0 \eta p$ reactions.

Charge asymmetries in [formula omitted] at the ϕ resonance

We consider the forward–backward pion charge asymmetry for the e+e−→π+π−γ process. At tree level we consider bremsstrahlung and double resonance contributions. Although the latter contribution is formally sub-leading, it is enhanced at low dipion invariant mass due to ρ resonant effects. We consider also four alternative models to describe the final state radiation at the loop level: Resonance Chiral Perturbation Theory, Unitarized Chiral Perturbation Theory, Kaon Loop Model and Linear Sigma Model. The last three models yield results compatible with experimental data. The Kaon Loop Model requires an energy dependent phase to achieve the agreement.

γγ interactions with KLOE

Studies of γγ interactions are performed with the KLOE detector, without tagging of the outgoing e+e−. The data sample is from an integrated luminosity of 240 pb−1. It was collected at the ϕ-factory DAΦNE with e+e− beams colliding at √s ≃ 1 GeV, below the ϕ resonance peak. We present preliminary results concerning the observation of the γγ → η process and the evidence for γγ → π0π0 production at low π0π0 invariant masses.

Spin correlation effects in top quark pair production at the LHC

At a 14 TeV proton-proton collider, the Large Hadron Collider (LHC), we show that top quark pair production is dominated at low invariant mass by the fusion of two like-helicity gluons, producing top quark pairs in the left-left or right-right helicity configurations. Whereas, at higher invariant mass the production is dominated by the fusion of unlike-helicity gluons, producing top quark pairs in the up-down or down-up off-diagonal configurations, identical to top quark pair production via quark-antiquark annihilation. We study in detail the low invariant mass region, and show that the spin correlations can be easily observed in this region by looking at the distribution of the difference in the azimuthal angles, $\ensuremath{\Delta}\ensuremath{\phi}$, of the dileptons decay products of the top quarks in the laboratory frame. Because of the large cross section for top pair production at the LHC, even with a cut requiring that the invariant mass of the top quark pair be less than 400 GeV, the approximate yield would be ${10}^{4}$ dilepton $(\mathrm{e},\ensuremath{\mu})$ events per ${\mathrm{fb}}^{\ensuremath{-}1}$ before detector efficiencies are applied. Therefore, there is ample statistics to form the $\ensuremath{\Delta}\ensuremath{\phi}$ distribution of the dilepton events, even with the invariant mass restriction. We also discuss possibilities for observing these spin correlations in the lepton plus jets channel.

TeV scale seesaw model and a flavorfulZ′at the LHC

Small neutrino masses and their large mixing angles can be generated at the TeV scale by augmenting the standard model with an additional generation-dependent, anomaly-free $U(1{)}_{\ensuremath{\nu}}$ symmetry, in the presence of three right-handed neutrinos. The ${Z}^{\ensuremath{'}}$ gauge boson associated with the breaking of the $U(1{)}_{\ensuremath{\nu}}$ symmetry can be produced at the LHC. The flavorful nature of the ${Z}^{\ensuremath{'}}$ can be established by measuring its nonuniversal couplings to the charged leptons as determined by the lepton's $U(1{)}_{\ensuremath{\nu}}$ charges, which also govern the neutrino flavor structure. While the LHC has the potential of discovering the ${Z}^{\ensuremath{'}}$ up to ${M}_{{Z}^{\ensuremath{'}}}=4.5\text{ }\text{ }\mathrm{TeV}$ with $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ data at the center of mass energy $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$, to establish the flavorful nature of the ${Z}^{\ensuremath{'}}$ requires much higher integrated luminosity. For our benchmark parameters that are consistent with neutrino oscillation data, at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$, a $5\ensuremath{\sigma}$ distinction between the dielectron and dimuon channels for ${M}_{{Z}^{\ensuremath{'}}}=3\text{ }\text{ }\mathrm{TeV}$ requires $500\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data. We find that the forward backward asymmetry distributions can also be useful in distinguishing the dielectron and dimuon channels in the low invariant mass and transverse momentum regions.

Exclusive measurements of [formula omitted]: Double-pionic fusion without ABC effect

Exclusive measurements of the reaction pp→dπ+π0 have been carried out at Tp=1.1 GeV at the CELSIUS storage ring using the WASA detector. The isovector π+π0 channel exhibits no enhancement at low invariant ππ masses, i.e. no ABC effect. Therefore this most basic isovector double-pionic fusion reaction qualifies as an ideal test case for the conventional t-channel ΔΔ excitation process. Indeed, the obtained differential distributions reveal the conventional t-channel ΔΔ mechanism as the appropriate reaction process, which also accounts for the observed energy dependence of the total cross section.

Forward-backward asymmetry, branching ratio and rate difference between electron and muon channels of B → K 1(K*)ℓ+ℓ− transition in supersymmetric models

The mass eigen states K 1(1270) and K 1(1400) are mixture of the strange members of two axial-vector SU(3) octet, 3 P 1(K A 1) and 1 P 1(K B 1). Taking into account this mixture, the forward-backward asymmetry(A FB ), branching ratio(Br) and rate difference of electron channel to muon channel(R) of B → K 1(1270, 1400)ℓ+ℓ− transitions are studied in the framework of different supersymmetric models. MSSM with R parity is considered because considerable deviation from the standard model predictions can be obtained in B → X s ℓ−ℓ+. Taking C Q1 and C Q2 about one which is consistent with the B → K*μ+μ− rate at low dileptonic invariant mass region(1 ≤ q 2 ≤ 6GeV2), we obtain a size able deviation for A FB , Br and R with respect to the Standard Model results. Any measurement of physical observables and their comparison with the results obtained in this paper can gives useful information about the nature of interactions beyond the standard model.

A hadronization model for few-GeV neutrino interactions

We present a detailed description of a new hadronic multiparticle production model for use in neutrino interaction simulations. Its validity spans a wide invariant mass range starting from the pion production threshold. This model focuses on the low invariant mass region which is probed in few-GeV neutrino interactions and is of particular importance to neutrino oscillation experiments using accelerator and atmospheric fluxes. It exhibits reasonable agreement with a wide variety of experimental data. We also describe measurements that can be made in upcoming experiments that can improve modeling in areas where uncertainties are currently large.

Charge asymmetries of top quarks: A window to new physics at hadron colliders

With the next start of LHC, a huge production of top quarks is expected. There are several models that predict the existence of heavy colored resonances decaying to top quarks in the TeV energy range. A peak in the differential cross section could reveal the existence of such a resonance, but this is experimentally challenging, because it requires selecting data samples where top and antitop quarks are highly boosted. Nonetheless, the production of such resonances might generate a sizable charge asymmetry of top versus antitop quarks. We consider a toy model with general flavour independent couplings of the resonance to quarks, of both vector and axial-vector kind. The charge asymmetry turns out to be a more powerful observable to detect new physics than the differential cross section, because its highest statistical significance is achieved with data samples of top-antitop quark pairs of low invariant masses.

Top pair invariant mass distribution: a window on new physics

We explore in detail the physics potential of a measurement of the ttbar invariant mass distribution. First, we assess the accuracy of the best available predictions for this observable and find that in the low invariant mass region, the shape is very well predicted and could be used to perform a top mass measurement. Second, we study the effects of a heavy s-channel resonance on the ttbar invariant mass distribution, in a model independent way. We provide the necessary Monte Carlo tools to perform the search and outline a simple three-step analysis.

Massive color-octet bosons and the charge asymmetries of top quarks at hadron colliders

Several models predict the existence of heavy colored resonances decaying to top quarks in the TeV energy range that might be discovered at the CERN LHC. In some of those models, moreover, a sizable charge asymmetry of top versus antitop quarks might be generated. The detection of these exotic resonances, however, requires selecting data samples where the top and the antitop quarks are highly boosted, which is experimentally very challenging. We asses that the measurement of the top quark charge asymmetry at the LHC is very sensitive to the existence of excited states of the gluon with axial-vector couplings to quarks. We use a toy model with general flavor independent couplings, and show that a signal can be detected with relatively not too energetic top and antitop quarks. We also compare the results with the asymmetry predicted by QCD, and show that its highest statistical significance is achieved with data samples of top-antitop quark pairs of low invariant masses.

NLO QCD corrections toZbb¯production with massive bottom quarks at the Fermilab Tevatron

We calculate the next-to-leading order (NLO) QCD corrections to $Zb\overline{b}$ production in hadronic collisions including full bottom-quark mass effects. We present results for the total cross section and the invariant mass distribution of the bottom-quark jet pair at the Fermilab Tevatron $p\overline{p}$ collider. We perform a detailed comparison with a calculation that considers massless bottom quarks, as implemented in the Monte Carlo program MCFM. We find that neglecting bottom-quark mass effects overestimates the total NLO QCD cross section for $Zb\overline{b}$ production at the Tevatron by about 7%, independent of the choice of the renormalization and factorization scales. Moreover, bottom-quark mass effects can impact the shape of the bottom-quark pair invariant mass distribution, in particular, in the low invariant mass region.

Coupled-channel effects in the [formula omitted] reaction

The cross sections for the pp→ppK+K− reaction were measured at three beam energies 2.65, 2.70, and 2.83 GeV at the COSY-ANKE facility. The shape of the K+K− spectrum at low invariant masses largely reflects the importance of KK¯ final state interactions. It is shown that these data can be understood in terms of an elastic K+K− rescattering plus a contribution coming from the production of a K0K¯0 pair followed by a charge-exchange rescattering. Though the data are not yet sufficient to establish the size of the cusp at the K0K¯0 threshold, the low mass behaviour suggests that isospin-zero production is dominant.

Logarithmically enhanced corrections to the decay rate and forward–backward asymmetry in [formula omitted

We study logarithmically enhanced electromagnetic corrections to the decay rate in the high dilepton invariant mass region as well as corrections to the forward–backward asymmetry (FBA) of the inclusive rare decay B ¯ → X s ℓ + ℓ − . As expected, the relative effect of these corrections in the high dilepton mass region is around −8% for the muonic final state and therefore much larger than in the low dilepton mass region. We also present a complete phenomenological analysis, to improved NNLO accuracy, of the dilepton mass spectrum and the FBA integrated in the low dilepton mass region, including a new approach to the zero of the FBA. The latter represents one of the most precise predictions in flavour physics with a theoretical uncertainty of order 5%. We find ( q 0 2 ) μ μ = ( 3.50 ± 0.12 ) GeV 2 . For the high dilepton invariant mass region, we have B ( B ¯ → X s μ μ ) high = ( 2.40 −0.62 +0.69 ) × 10 −7 . The dominant uncertainty is due to the 1 / m b corrections and can be significantly reduced in the future. For the low dilepton invariant mass region, we confirm previous results up to small corrections.

The antikaon–nucleus interaction and alternative views to deeply bound antikaonic nuclear systems

We present an overview of the latest theoretical studies on the antikaon properties in the nuclear medium, in connection with the recent experimental claims of very deeply bound antikaon nuclear states. We argue that proper many-body formulations using modern realistic antikaon–nucleon interactions are not able to generate such systems. Instead, a simple two-nucleon antikaon absorption mechanism where the remaining nucleus acts as spectator explains the enhancement observed in semi-inclusive proton momentum spectra, seen as a bump in the KEK PS-E549 experiment on a 4He target or as a peak in the FINUDA experiment on a 6Li target. This signal is clearly visible in another FINUDA experiment measuring the invariant mass of Λ-proton pairs after two-nucleon kaon absorption. We also show that another peak of this experiment, seen at lower invariant masses and interpreted as a bound K−pp state, is simply generated by the same two-nucleon absorption mechanism followed by final-state interactions of the produced particles with the residual nucleus. Our conclusion is that all the experimental claims for the formation of very deeply bound antikaonic nuclear systems receive an alternative explanation in terms of conventional nuclear processes.

Drell-Yan production and Lam-Tung relation in the color glass condensate formalism

We study the Drell-Yan production cross section and structure functions in proton (deuteron)-nucleus collisions using the color glass condensate formalism. The nucleus is treated in the color glass condensate framework which includes both higher twist effects due to the inclusion of multiple scatterings and leading twist pQCD shadowing due to the small $x$ resummation, while the proton (or deuteron) is treated within the DGLAP improved parton model. In particular, the Drell-Yan structure functions are used in order to investigate the Lam-Tung relation at small $x$, which is known to be identically zero at leading twist up to next-to-leading order, and is thus a good playground for studying higher twist effects. In agreement with this, we find that violations of this relation are more important for low momentum and invariant mass of the Drell-Yan pair, and also in the region of rapidity that corresponds to smaller values of $x$ in the nucleus.

High momentum lepton pairs from jet–plasma interactions

We discuss the emission of high momentum lepton pairs (pT > 4 GeV) with low invariant masses (M ≪ pT) in central Au+Au collisions at RHIC ( GeV). The spectra of dileptons produced through interactions of quark and antiquark jets with the quark-gluon plasma (QGP) have been calculated. Annihilation and Compton scattering processes, as well as processes benefitting from collinear enhancement, including Landau–Pomeranchuk–Migdal (LPM) effects, are calculated and convolved with a one-dimensional hydrodynamic expansion. The jet-induced contributions are compared to thermal dilepton emission and Drell–Yan processes, and are found to dominate around pT = 4 GeV.

Study of leading hadrons in gluon and quark fragmentation

The study of quark jets in e(+)e(-) reactions at LEP has demonstrated that the hadronisation process is reproduced well by the Lund string model. However. our understanding of gluon fragmentation is less complete. In this study enriched quark and gluon jet samples of different purities are selected in three-jet events from hadronic decays of the Z collected by the DELPHI experiment in the LEP runs during 1994 and 1995. The leading systems of the two kinds of jets are defined by requiring a rapidity gap and their sum of charges is studied. An excess of leading systems with total charge zero is found for gluon jets in all cases, when compared to Monte Carlo simulations with JETSET (with and without Bose-Einstein correlations included) and ARIADNE. The corresponding leading systems of quark jets do not exhibit such an excess. The influence of the gap size and of the gluon purity on the effect is studied and a concentration of the excess of neutral leading systems at low invariant masses (less than or similar to 2 GeV/c(2)) is observed, indicating that gluon jets might have an additional hitherto undetected fragmentation mode via a two-gluon system. This could be an indication of a possible production of gluonic states as predicted by QCD. (c) 2006 Elsevier B.V. All rights reserved.

Exclusive measurements of pd→He3ππ: The ABC effect revisited

Abstract Exclusive measurements of the reactions p d → He 3 π + π − and p d → He 3 π 0 π 0 have been carried out at T p = 0.893 GeV at the CELSIUS storage ring using the WASA detector. The π + π − channel evidences a pronounced enhancement at low invariant ππ masses—as anticipated from previous inclusive measurements of the ABC effect. This enhancement is seen to be even much larger in the isoscalar π 0 π 0 channel. The differential distributions prove this enhancement to be of scalar–isoscalar nature. ΔΔ calculations give a good description of the data, if a boundstate condition is imposed for the intermediate ΔΔ system.