Pair Cross Section Research Articles

Higgs phenomenology in the Minimal Dilaton Model after Run I of the LHC

The Minimal Dilaton Model (MDM) extends the Standard Model (SM) by a singlet scalar, which can be viewed as a linear realization of general dilaton field. This new scalar field mixes with the SM Higgs field to form two mass eigenstates with one of them corresponding to the 125 GeV SM-like Higgs boson reported by the LHC experiments. In this work, under various theoretical and experimental constrains, we perform fits to the latest Higgs data and then investigate the phenomenology of Higgs boson in both the heavy dilaton scenario and the light dilaton scenario of the MDM. We find that: (i) If one considers the ATLAS and CMS data separately, the MDM can explain each of them well, but refer to different parameter space due to the apparent difference in the two sets of data. If one considers the combined data of the LHC and Tevatron, however, the explanation given by the MDM is not much better than the SM, and the dilaton component in the 125-GeV Higgs is less than about 20% at 2 sigma level. (ii) The current Higgs data have stronger constrains on the light dilaton scenario than on the heavy dilaton scenario. (iii) The heavy dilaton scenario can produce a Higgs triple self coupling much larger than the SM value, and thus a significantly enhanced Higgs pair cross section at hadron colliders. With a luminosity of 100 fb^{-1} (10 fb^{-1}) at the 14-TeV LHC, a heavy dilaton of 400 GeV (500 GeV) can be examined. (iv) In the light dilaton scenario, the Higgs exotic branching ratio can reach 43% (60%) at 2 sigma (3 sigma) level when considering only the CMS data, which may be detected at the 14-TeV LHC with a luminosity of 300 fb^{-1} and the Higgs Factory.

Search for the production ofZWandZZboson pairs decaying into charged leptons and jets inpp¯collisions ats=1.96 TeV

We present a measurement of the production cross section for ZW and ZZ boson pairs in final states with a pair of charged leptons, from the decay of a Z boson, and at least two jets, from the decay of a W or Z boson, using the full sample of proton-antiproton collisions recorded with the CDF II detector at the Tevatron, corresponding to 8.9 fb^(-1) of integrated luminosity. We increase the sensitivity to vector boson decays into pairs of quarks using a neural network discriminant that exploits the differences between the spatial spread of energy depositions and charged-particle momenta contained within the jet of particles originating from quarks and gluons. Additionally, we employ new jet energy corrections to Monte Carlo simulations that account for differences in the observed energy scales for quark and gluon jets. The number of signal events is extracted through a simultaneous fit to the dijet mass spectrum in three classes of events: events likely to contain jets with a heavy-quark decay, events likely to contain jets originating from light quarks, and events that fail these identification criteria. We determine the production cross section to be 2.5 +2.0 -1.0 pb (< 6.1 pb at the 95% confidence level), consistent with the standard model prediction of 5.1 pb.

Hadronic interactions and cosmic rays at ultra high energies

Observations of ultra high energy cosmic rays allow in principle to obtain information about the properties of hadronic interactions in an energy range not accessible with particle accelerators. This task is however complicated by the fact that the composition of the primary CR is not known and must be estimated from the same data set. Solving the ambiguity between composition and hadronic interaction modeling is a central problem for UHECR observations. Recently the Pierre Auger collaboration has presented an estimate of the p-air interaction length at a laboratory energy E0 � 10 18.24 eV. The interaction length is inferred from the shape of the tail of the Xmax distribution of the observed showers, following a method pioneered by the Fly's Eye group. In this work we want to analyse critically this method, and discuss its potential for further studies. composition of these particles, and at the same time to obtain information about the properties of hadronic interaction in an energy range not accessible to accelerators. These two goals are unfortunately in conflict with each other. Uncertainties in the modeling of the properties of hadronic interactions at high energy limit the precision that can be achieved in the measurement of the composition of the primary radiation; viceversa, uncertainties in the composition of the primary particles do not allow an easy determination of the properties of the showers generated by protons (or other nuclear species). The best strategy to solve this ambiguity is the object of an active field of investigations. At the present moment there is still a significant uncertainty both on the composition of cosmic rays in the UHE range, and on the quality of the description of their showers by the existing montecarlo codes. The analysis of the current situation is difficult also because the observations made by the Pierre Auger (1, 2), HiRes (3) and Telescope Array (4) collaborations are not in agreement. The HiRes and Telescope Array results appear to be consistent with a composition that is proton dominated in the entire energy range, while the data of the Pierre Auger Observatory seem to imply a composition that becomes gradually enriched in heavy nuclei with increasing energy. This disagreement is a serious obstacle in the progress of the field, since a consistent interpretation of all data is currently impossible. It is clear that a satisfactory resolution of these discrepancies is urgent and important. Recently the Pierre Auger collaboration (5) has presented a measurement of the interaction length for protons with energy E0 � 10 18.24 eV (that corresponds to a c.m. energy √ s � 57TeV for nucleon- nucleon interactions, that corresponds to a p-air cross section 505 ± 23(stat) + 28 − 36(sys)mb (this must be understood as the production cross section, that refers to interactions where at least one additional pion is produced, excluding the elastic and target fragmentation processes). Converting the

Top quark forward-backward asymmetry and W ′-boson with general couplings

The measured forward-backward asymmetry in top pair events at the Fermilab Tevatron collider deviates significantly from the standard model expectation. Several models have been proposed to describe the observed asymmetry which grows with the rapidity difference of the top pairs and also with the $t\bar{t}$ invariant mass. The presence of a heavy charged gauge boson $W'$ with the coupling $W'-t-d$ (left-handed, right-handed, and a mixture of left and right-handed couplings) could generate the desired top forward-backward asymmetry keeping the top pair cross section consistent with the standard model prediction. Such $W'$-boson makes contribution to the electric dipole moment of the neutron through contribution to the $d$-quark electric dipole moment, recently measured charge asymmetry ($A_{C}$) by the LHC experiments, and the total cross section of top pair at the LHC and Tevatron. We show that the upper bounds on neutron and top electric dipole moments disfavour any $W'$ with a mass below 240 GeV which could explain the Tevatron forward-backward asymmetry. It is shown that the charge asymmetry provides an allowed region in the parameters space with no overlap with the allowed region where the asymmetry could be described.

Measurement of the top quark pair cross section with ATLAS in pp collisions at [formula omitted] using final states with an electron or a muon and a hadronically decaying τ lepton

A measurement of the cross section of top quark pair production in proton–proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb−1. Events with an isolated electron or muon and a τ lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, σtt¯=186±13(stat.)±20(syst.)±7(lumi.) pb, is in good agreement with the Standard Model prediction.

LHC phenomenology of generalSU(2)×SU(2)×U(1)models

General SU(2)xSU(2)xU(1) models represent a well-motivated intermediate step towards the unification of the Standard Model gauge groups. Based on a recent global analysis of low-energy and LEP constraints of these models, we perform numerical scans of their various signals at the LHC. We show that total cross sections for lepton and third-generation quark pairs, while experimentally easily accessible, provide individually only partial information about the model realized in Nature. In contrast, correlations of these cross sections in the neutral and charged current channels may well lead to a unique identification.

$t\bar t$ and single top cross sections at the Tevatron

We present a summary of the latest measurements of the top pair and single top cross sections performed by the CDF and D0 collaborations at the Fermilab Tevatron collider.

Measurement of the production cross section for pairs of isolated photons in pp collisions at $ \sqrt {s} = 7\;TeV $

AbstractThe integrated and differential cross sections for the production of pairs of isolated photons is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 36 pb−1is analysed. A next-to-leading-order perturbative QCD calculation is compared to the measurements. A discrepancy is observed for regions of the phase space where the two photons have an azimuthal angle difference$ \Delta \varphi \lesssim 2.8 $rad.

Implications of the measurement of ultramassive boosted jets at CDF

The CDF Collaboration recently reported an upper limit on boosted top pair production and noted a significant excess above the estimated background of events with two ultramassive boosted jets. We discuss the interpretation of the measurement and its fundamental implications. In case new physics is involved, the most naive contribution is from a new particle produced with a cross section that is a few times higher than that of the top quark and a sizable hadronic branching ratio. We quantify the resulting tension of a possible larger top pair cross section with the absence of excess found in events with one massive boosted jet and missing energy. The measured planar flow distribution shows deviation from CDF's Pythia QCD prediction at high planarity, while we find a somewhat smaller deviation when comparing with other Monte Carlo tools. As a simple toy model, we analyze the case of a light gluino with R-parity violation and show that it can be made consistent with the data.

Top quark asymmetry from a non-Abelian horizontal symmetry

Motivated by the persistence of a large measured top quark forward-backward asymmetry at the Tevatron, we examine a model of non-Abelian flavor gauge symmetry. The exchange of the gauge bosons in the $t$-channel can give a large ${A}_{\mathrm{FB}}^{t}$ due to the forward Rutherford scattering peak. We address generic constraints on non-Abelian $t$-channel physics models including flavor diagonal resonances and potentially dangerous contributions to inclusive top pair cross sections. We caution on the general difficulty of comparing theoretical predictions for top quark signals to the existing experimental results due to potentially important acceptance effects. The first signature at the Large Hadron Collider can be a large inclusive top pair cross section, or like-sign dilepton events, although the latter signal is much smaller than in Abelian models. Deviations of the invariant mass distributions at the LHC will also be promising signatures. A more direct consistency check of the Tevatron asymmetry through the LHC asymmetry is more likely to be relevant at a later stage.

Short Range Correlations and the EMC Effect

This Letter shows quantitatively that the magnitude of the EMC effect measured in electron deep inelastic scattering at intermediate x(B), 0.35≤x(B)≤0.7, is linearly related to the short range correlation (SRC) scale factor obtained from electron inclusive scattering at x(B)≥1. The observed phenomenological relationship is used to extract the ratio of the deuteron to the free pn pair cross sections and F(2)(n)/F(2)(p), the ratio of the free neutron to free proton structure functions. We speculate that the observed correlation is because both the EMC effect and SRC are dominated by the high virtuality (high momentum) nucleons in the nucleus.

Neutral Higgs boson pair production at Linear Colliders

AbstractWe compute the one‐loop production cross section for neutral Higgs boson pairs at linear colliders from a generic two‐Higgs‐doublet model (2HDM). We identify important quantum effects associated to the triple‐Higgs boson self‐interactions – a genuine feature of the 2HDM, which is impossible to achieve in the Minimal Supersymmetric Standard Model (MSSM).

Measurement of the proton-air cross section with ARGO-YBJ

In this work the results of p-air cross section measurements with the ARGO-YBJ experiment are reported. The analysis is based on the different flux attenuation for different atmospheric depths (i.e. zenith angles), by exploiting the detector capabilities in selecting the shower development stage by means of the size, hit density and lateral profile measurements. The systematic errors introduced by fluctuations in shower development and heavier primaries have been taken into account. The results can give useful insights on p-air interactions at energies where hadronic interaction models start to give significantly different expectations and allow to infer the p-p total cross section in the energy region ( s = 80 GeV − 500 GeV ) not covered by experiments at accelerators.

Theoretical top quark cross section at the Fermilab Tevatron and the CERN LHC

We present results for the top quark pair cross section at the Tevatron and the LHC. We use the resummed double differential cross section, employing the fully kinematics-dependent soft-anomalous-dimension matrices, to calculate the soft-gluon contributions at next-to-next-to-leading order (NNLO). We improve and update our previous estimates by refining our methods, including further subleading terms, and employing the most recent parton distribution function sets. The NNLO soft corrections significantly enhance the NLO cross section while considerably reducing the scale dependence. We provide a detailed discussion of all theoretical uncertainties in our calculation, including kinematics, scale, and parton distribution uncertainties, and clarify the differences between our work and other approaches in the literature.

The Elusive p-air cross section

For the \(\bar pp\) and pp systems, we have used all of the extensive data of the Particle Data Group [K. Hagiwara et al. (Particle Data Group), Phys. Rev. D 66, 010001 (2002)]. We then subject these data to a screening process, the “Sieve” algorithm [M. M. Block, physics/0506010], in order to eliminate “ outliers” that can skew a χ2 fit. With the “Sieve” algorithm, a robust fit using a Lorentzian distribution is first made to all of the data to sieve out abnormally high Δχ 2 i , the individual ith point’s contribution to the total χ2. The χ2 fits are then made to the sieved data. We demonstrate that we cleanly discriminate between asymptotic ln s and ln2 s behavior of total hadronic cross sections when we require that these amplitudes also describe, on average, low energy data dominated by resonances. We simultaneously fit real analytic amplitudes to the “sieved” high energy measurements of \(\bar pp\) and pp total cross sections and ρ-values for \(\sqrt s \) ≥ GeV, while requiring that their asymptotic fits smoothly join the the σ pp and σpp total cross sections at \(\sqrt s \) = 4.0 GeV—again both in magnitude and slope. Our results strongly favor a high energy ln2 s fit, basically excluding a ln s fit. Finally, we make a screened Glauber fit for the p-air cross section, using as input our precisely-determined pp cross sections at cosmic ray energies.

Electron–positron pair production by photons: A historical overview

This account briefly traces the growth of our theoretical and experimental knowledge of electron–positron pair production by photons, from the prediction of the positron by Dirac [1928a. The quantum theory of the electron. Proc. R. Soc. (London) A 117, 610–624; 1928b. The quantum theory of the electron. Part II. Proc. R. Soc. (London) A 118, 1928b, 351–361] and subsequent cloud-chamber observations by Anderson [Energies of cosmic-ray particles. Phys. Rev. 43, 491–494], up to the present time. Photons of energies above 2 m e c 2 (1.022 MeV) can interact with the Coulomb field of an atomic nucleus to be transformed into an electron–positron pair, the probability increasing with increasing photon energy, up to a plateau at high energies, and increasing with increasing atomic number approximately as the square of the nuclear charge (proton number). This interaction can also take place in the field of an atomic electron, for photons of energy in excess of 4 m e c 2 (2.044 MeV), in which case the process is called triplet production due to the track of the recoiling atomic electron adding to the tracks of the created electron–positron pair. The last systematic computations and tabulations of pair and triplet cross sections, which are the predominant contributions to the photon mass attenuation coefficient for photon energies 10 MeV and higher, were those of Hubbell et al. [Pair, triplet, and total atomic cross sections (and mass attenuation coefficients) for 1 MeV–100 GeV photons in elements Z=1–100. J. Phys. Chem. Ref. Data 9, 1023–1147], from threshold (1.022 MeV) up to 100 GeV, for all elements Z = 1 –100. These computations required some ad hoc bridging functions between the available low-energy and high-energy theoretical models. Recently (1979–2001), Sud and collaborators have developed some new approaches including using distorted wave Born approximation (DWBA) theory to compute pair production cross sections in the intermediate energy region (5.0–10.0 MeV) on a firmer theoretical basis. These and other recent developments, and their possible implications for improved computations of pair and triplet cross sections, are discussed.

Cross section data for electron–positron pair production by photons: a status report

Photons of energies primarily above 2 m e c 2 (1.022 MeV) can interact with the Coulomb field of an atomic nucleus to be transformed into an electron–positron pair, the probability increasing with increasing photon energy, up to a plateau at high energies, and increasing with increasing atomic number approximately as the square of the nuclear charge (proton number). This interaction can also take place in the field of an atomic electron, for photons of energy in excess of 4 m e c 2 (2.044 MeV), in which case the process is called triplet production due to the track of the recoiling electron adding to the tracks of the created electron–positron pair. The last systematic computations and tabulations of pair and triplet cross sections, which are the predominant contributions to the photon mass attenuation coefficient for photon energies 10 MeV and higher, were those of Hubbell, Gimm and Øverbø in 1980, from threshold (1.022 MeV) up to 100 GeV, for all elements Z=1–100. These computations required some ad hoc bridging functions between the available low-energy and high-energy theoretical models. Recently, Sud and others have developed some new approaches including using DWBA (distorted wave Born approximation) theory to compute pair production cross sections in the intermediate energy region (5.0–10.0 MeV) on a firmer theoretical basis. Also, Bergstrom et al. [Phys. Rev. A 53 (1996) 2865], Agger and Sørensen [Phys. Rev. A 55 (1997) 402] and others discuss bound-free pair production below the above mentioned 1.022 MeV threshold. These and other recent developments are discussed with an eye toward new computations, if warranted, to update and improve the accuracy of the 1980 Hubbell et al. tabulations.

Boson pair cross sections at LEP

In five years of running at energies above WW and ZZ production thresholds, LEP-II provided each experiment an integrated luminosity of ∼ 700 pb −1. The collected statistics allows to measure with high precision the total e +e − → WW, ZZ, and γγ cross sections. Derived quantities as leptonic and hadronic branching fractions of the W boson, differential distributions of the polar scattering angle and constraints on new physics are also herein reported. All results, unless differently stated, have to be considered preliminary.

Obtaining the nuclear gluon distribution from heavy quark decays to lepton pairs in p A collisions

We have studied how lepton pairs from decays of heavy-flavoured mesons produced in p A collisions can be used to determine the modifications of the gluon distribution in the nucleus. Since heavy-quark production is dominated by the gg channel, the ratio of correlated lepton pair cross sections from D D and B B decays in p A and pp collisions directly reflects the ratio R g A ≡ f g A / f g p. We have numerically calculated the lepton-pair cross sections from these decays in pp and p A collisions at SPS, RHIC and LHC energies. We find that ratio of the p A to pp cross sections agrees quite well with the input R g A . Thus, sufficiently accurate measurements could be used to determine the nuclear modification of the gluon distribution over a greater range of x and Q 2 than presently available, putting strong constraints on models.

Sneutrino mass measurements ate+e−linear colliders

It is generally accepted that experiments at an ${e}^{+}{e}^{\ensuremath{-}}$ linear collider will be able to extract the masses of the selectron as well as the associated sneutrino with a precision of $\ensuremath{\sim}1%$ by determining the kinematic end points of the energy spectrum of daughter electrons produced in their two body decays to a lighter neutralino or chargino. Recently, it has been suggested that by studying the energy dependence of the cross section near the production threshold, this precision can be improved by an order of magnitude, assuming an integrated luminosity of $100 {\mathrm{fb}}^{\ensuremath{-}1}.$ It is further suggested that these threshold scans also allow the masses of even the heavier second and third generation sleptons and sneutrinos to be determined to better than 0.5%. We reexamine the prospects for determining sneutrino masses. We find that the cross sections for the second and third generation sneutrinos are too small for a threshold scan to be useful. An additional complication arises because the cross section for sneutrino pairs to decay into any visible final state(s) necessarily depends on an unknown branching fraction, so that the overall normalization is unknown. This reduces the precision with which the sneutrino mass can be extracted. We propose a different strategy to optimize the extraction of $m({\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}}_{\ensuremath{\mu}})$ and $m({\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}}_{\ensuremath{\tau}})$ via the energy dependence of the cross section. We find that even with an integrated luminosity of $500 {\mathrm{fb}}^{\ensuremath{-}1},$ these can be determined with a precision no better than several percent at the 90% C.L. We also examine the measurement of $m({\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}}_{e})$ and show that it can be extracted with a precision of about 0.5% (0.2%) with an integrated luminosity of $120 {\mathrm{fb}}^{\ensuremath{-}1} (500 {\mathrm{fb}}^{\ensuremath{-}1}).$