arXiv subjects
Publications and source records attributed to A. D. Martin.
We use the BFKL equation to calculate the rate of deep inelastic scattering events containing two forward jets (adjacent to the proton remnants) at HERA. We compare the production of two forward jets with that of only one forward jet (the "Mueller" process). We obtain a stable prediction for this two to one jet ratio, which may serve as a measure of the BFKL vertex function.
We study the transverse momentum ($p_T$) spectrum of charged particles produced in deep inelastic scattering (DIS) at small Bjorken $x$ in the central region between the current jet and the proton remnants. We calculate the spectrum at large $p_T$ with the BFKL $\ln (1/x)$ resummation included and then repeat the calculation with it omitted. We find that data favour the former. We normalize our BFKL predictions by comparing with HERA data for DIS containing a forward jet. The shape of the $x$ distribution of DIS + jet data are also well described by BFKL dynamics.
We introduce a coupled pair of evolution equations for the unintegrated gluon distribution and the sea quark distribution which incorporate both the resummed leading $\ln (1/x)$ BFKL contributions and the resummed leading $\ln (Q^2)$ DGLAP contributions. We solve these unified equations in the perturbative QCD domain. With only two physically motivated parameters we obtain an excellent description of the HERA $F_2$ data.
We show that the hadroproduction of a pair of jets with large transverse energy in the central region bounded by rapidity gaps is an ideal process to see important double logarithmic QCD suppression effects. We compute the cross sections for both exclusive and inclusive double-diffractive dijet production at Tevatron and LHC energies for a range of rapidity configurations.
We compare the WW and pomeron-pomeron fusion mechanisms for the double-diffractive production of a Higgs boson. We determine the suppression of the 'rapidity gap' pomeron-pomeron fusion events due to QCD radiative effects. In particular we use leading log techniques to estimate the cross sections for both exclusive and inclusive double-diffractive Higgs production at LHC energies. The same approach can be applied to the double-diffractive central production of large E_T dijets. These two processes provide one of the most justified applications of various aspects of leading logarithm QCD techniques.
We argue that the use of the universal unintegrated gluon distribution and the $k_T$ (or high energy) factorization theorem provides the natural framework for describing observables at small x. We introduce a coupled pair of evolution equations for the unintegrated gluon distribution and the sea quark distribution which incorporate both the resummed leading $ln (1/x)$ BFKL contributions and the resummed leading $ln (Q^2)$ GLAP contributions. We solve these unified equations in the perturbative QCD domain using simple parametic forms of the nonperturbative part of the integrated distributions. With only two (physically motivated) input parameters we find that this $k_T$ factorization approach gives an excellent description of the measurements of $F_2 (x,Q^2)$ at HERA. In this way the unified evolution equations allow us to determine the gluon and sea quark distributions and, moreover, to see the x domain where the resummed $ln (1/x)$ effects become significant. We use $k_T$ factorization to predict the longitudinal structure function $F_L (x,Q^2)$ and the charm component of $F_2 (x,Q^2)$.
The study of an intermediate mass Higgs boson, via the process gamma-gamma -> H -> bbbar from an initiallypolarized J_z = 0 state, has been advocated as an important feasible goal of a future photon linear collider. The crucial argument was the m_b^2/s suppression of the background process gamma-gamma (J_z = 0) -> bbbar. We critically review the contribution of the radiative background processes (in which the m_b^2/s suppression is absent) to the quasi-two-jet-like events with at least one, but preferably two, tagged energetic b jets. Within a complete study of the radiative processes, we find that a sizeable background contribution can come from the helicity-violating gamma-gamma (J_z = 0) -> bbbar process accompanied by soft gluon emission. These latter radiative corrections contain a new type of double logarithmic (DL) terms. We clarify the physical nature of these novel DL corrections. Despite the fact that the one-loop DL terms are comparable or even larger than the Born term, fortunately we find that the calculation of the cross section in the two-loop approximation is sufficient for a reliable evaluation of the background to the Higgs signal.
We present a formulation which allows heavy quark (c, b, ...) mass effects to be explicitly incorporated in both the coefficient functions and the splitting functions in the parton evolution equations. We obtain a consistent procedure for evolution through the threshold regions for c anti-c and b anti-b production in deep inelastic scattering, which allows the prediction of the charm and bottom quark densities. We use the new formulation to perform a next-to-leading order global parton analysis of deep inelastic and related hard scattering data. We find that the optimum fit has alpha_S(M_Z^2) = 0.118. We give predictions for the charm components of the proton structure functions F_2 and F_L as functions of x and Q^2 and, in particular, find that F_2^c is in good agreement with the existing measurements. We examine the Q^2 range of validity of the photon-gluon fusion model for c anti-c electroproduction. We emphasize the value of a precision measurement of the charm component F_2^c at HERA.
We use perturbative QCD and parton-hadron duality to estimate the cross sections for the diffractive electroproduction of rhoprime(1-) and rho(3-) resonances which occur in the 1.3-1.8 GeV mass interval. We present the cross sections and the ratios sigma_L/sigma_T as a function of Q^2. We compare the predictions with those for the diffractive electroproduction of rho mesons. We show how such diffractive electroproduction measurements at HERA can probe features of the perturbative QCD 'Pomeron'.
We predict the rate of deep inelastic scattering (DIS) events containing an identified forward $π^0$ that is expected in the experiments at the HERA electron-proton collider, in order to see if this process can be used as an indicator of the underlying small $x$ dynamics. We determine the background due to deep inelastic events containing forward photons which are fragments of the forward jet. We compare the DIS + $π^0$ cross section with that of the DIS + parent jet process.
We critically review the QCD predictions for the cross sections sigma_L and sigma_T for diffractive rho meson electroproduction in longitudinally and transversely polarised states in the HERA energy region. We show that both perturbative and non-perturbative approaches which involve convolution with the rho meson wave function predict values of sigma_T which fall-off too quickly with increasing Q^2, in comparison with the data. We present a perturbative QCD model based on the open production of light qqbar pairs and parton-hadron duality, which describes all features of the data for rho electroproduction at high Q^2 and, in particular, predicts a satisfactory Q^2 behaviour of sigma_L/sigma_T. We find that precise measurements of the latter can give valuable information on the Q^2 behaviour of the gluon distribution at small x.
We use perturbative QCD to calculate the cross sections $σ^{L,T}$ for the diffractive production of open charm $(c\overline{c})$ from longitudinally and transversely polarised photons (of virtuality $Q^2$) incident at high energy $(\sqrt{s})$ on a proton target. We study both the $Q^2$ and $M^2$ dependence of the cross sections, where $M$ is the invariant mass of the $c\overline{c}$ pair. Surprisingly, the result for $σ^T$, as well as for $σ^L$, is perturbatively stable. We estimate higher-order corrections and find a sizeable enhancement of the cross sections. The cross sections depend on the {\it square} of the gluon density $g (x, K^2)$, and we show that the observation of open charm at the HERA electron-proton collider can act as a sensitive probe of the gluon distribution for $x = (Q^2 + M^2)/s$ and scale $K^2 = (m_c^2 + \langle k_T^2 \rangle) (1 + Q^2/M^2)$ where the average quark transverse momentum squared $\langle k_T^2 \rangle \sim m_c^2$. As compared to diffractive $J/ψ$ production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the $J/ψ$ wave function. We estimate the fraction of diffractive events that arise from $c\overline{c}$ production.
We derive a modified form of the BFKL equation which enables the structure of the gluon emissions to be studied in small $x$ deep inelastic scattering. The equation incorporates the resummation of the virtual and unresolved real gluon emissions. We solve the equation to calculate the number of small $x$ deep-inelastic events containing 0,1,2 ...resolved gluon jets, that is jets with transverse momenta $q_{T} > μ$. We study the jet decomposition for different choices of the jet resolution parameter $μ$.
New data, especially HERA measurements of the proton structure function at small $x$, allow the opportunity to improve our knowledge of the gluon and quark distribution functions. We perform a global analysis which incorporates these new precise data, and which extends down to $Q^2 = 1.5\; GeV^2$. We discuss the sensitivity to the value of $α_s$ and the improvement in the determination of the gluon. We compare the predictions of the single jet inclusive cross section with recent measurements from the Fermilab Tevatron.
We review the latest information that is available about the parton distributions of the proton, paying particular attention to the determination of the gluon. We briefly describe the various processes that have been advocated to be a measure of the gluon. We discuss the importance of the gluon to the description of the structure function F2 at small x, with emphasis on the ln 1/x resummations.
We summarize the developments on the structure of the proton that were studied at the Workshop on "HERA Physics" that was held in Durham in September 1995. We survey the latest structure function data; we overview the QCD interpretations of the measurements of the structure functions and of final state processes; we discuss charm production and the spin properties of the proton.
We calculate the rate of producing deep inelastic events containing an energetic isolated forward photon at HERA. We quantify the enhancement arising from the leading $\log 1/x$ gluon emissions with a view to using such events to identify the underlying dynamics.
The recent CDF single jet inclusive measurements at Fermilab are incorporated in a global next-to-leading order parton analysis of the available deep inelastic and related data. We find that it is impossible to achieve a simultaneous QCD description of both the CDF jet distribution for transverse energies $E_T > 200$~GeV and the deep inelastic structure function data for $x > 0.3$. However, the CDF data for $E_T < 200$~GeV and the deep inelastic data are adequately described provided that the QCD coupling $α_S(M_Z^2)$ is increased from its preferred deep inelastic value to $α_S (M_Z^2) \simeq 0.116 - 0.120$.