arXiv subjects
Publications and source records attributed to A. D. Martin.
I present results on updates on PDFs which are obtained within the general framework which led to the MSTW2008 PDF sets. There are some theory and procedural improvements and a variety of new data sets, including many relevant up-to-date LHC data. A new set of PDFs is very close to being finalised, with no significant changes expected to the preliminary PDFs shown here.
We show how the photon input parton distribution function (PDF) may be calculated with good accuracy, and used in an extended DGLAP global parton analysis in which the photon is treated as an additional point-like parton. The uncertainty of the input photon PDF is relatively small, since the major part of the distribution (which is produced by the coherent emission of the photon from a proton that remains intact) is well known. We present the expected photon PDFs, and compare the predictions with ZEUS data for isolated photon electroproduction at negative rapidities.
The cross section for exclusive psi(2S) ultraperipheral production at the LHC is calculated using gluon parametrisations extracted from exclusive J/psi measurements performed at HERA and the LHC. Predictions are given at leading and next-to-leading order for pp centre-of-mass energies of 7, 8 and 14 TeV, assuming the non-relativistic approximation for the psi(2S) wave function.
We study the behaviour of elastic and diffractive proton dissociation cross sections at high energy. First, we describe what would be expected to be observed at the LHC based on conventional Regge theory. We emphasize the tension between these expectations and the recent LHC measurements, and we discuss the possibilty to modify the classic Reggeon Field Theory in a physically-motivated way so as to accommodate the tendencies observed at the LHC. As a result, we show that we are able to achieve a `global' description of the wide variety of high energy elastic and diffractive data that are presently available, particularly from the LHC experiments. The model is based on only one pomeron pole, but includes multi-pomeron interactions and, significantly, includes the transverse momentum dependence of intermediate partons as a function of their rapidity, which provides the rapidity dependence of the multi-pomeron vertices. We give predictions for diffractive observables at LHC, and higher, energies.
We present a `global' description of the wide variety of high energy elastic and diffractive data that are presently available, particularly from the LHC experiments. The model is based on only one pomeron pole, but includes multi-pomeron interactions and, significantly, includes the transverse momentum dependence of intermediate partons as a function of their rapidity, which provides the rapidity dependence of the multi-pomeron vertices. We give predictions for diffractive observables at LHC energies.
We introduce the physical factorisation scheme, which is necessary to describe observables which are `not completely inclusive'. We derive the formulae for NLO DGLAP evolution in this scheme, and also for the `rotation' of the conventional MSbar PDFs into the physical representation. Unlike, the MSbar prescription, where, for example, the gluon PDF at NLO obtains an admixture of the quark-singlet PDF, and vice-versa, the physical approach does not mix parton PDFs of different types. That is, the physical approach retains the precise quantum numbers of each PDF. The NLO corrections to DGLAP evolution in the physical scheme are less than those in the MSbar case, indicating a better convergence of the perturbative series.
We show that the conventional prescription used for DGLAP parton evolution at NLO has an inconsistent treatment of the contribution from the infrared (IR) region. We illustrate the problem by studying the simple example of QED evolution, treating the electron and photon as partons. The deficiency is not present in a physical approach which removes the IR divergency and allows calculation in the normal 4-dimensional space.
We show that to correctly describe the effects of the heavy-quark mass, m_h, in DGLAP evolution, it is necessary to work in the so-called `physical' scheme. In this way, we automatically obtain a smooth transition through the heavy-quark thresholds. Moreover, we show that to obtain NLO accuracy, it is sufficient to account for the heavy-quark mass, m_h, just in the LO (one-loop) splitting function. The use of the MS-bar factorisation scheme is not appropriate, since at NLO we deal with a mixture of quarks and gluon (that is, the mass of the heavy parton is not well-defined). The formulae for the explicit m_h dependence of the splitting functions, and for alpha_s, are presented.
New, more precise data on diffractive J/psi photoproduction and on exclusive J/psi production in the process pp -> p + J/psi + p at the LHC allow a better constraint on the low-x gluon distribution in the domain of relatively low scales Q^2 ~ 2-6 GeV^2. We account for the `skewed' effect and for the real part of the amplitude, as well as for the absorptive corrections in the case of the exclusive process pp -> p + J/psi + p. The predictions for exclusive J/psi production at \sqrt{s}=8 and 14 TeV and for exclusive Upsilon(1S) production at 7, 8 and 14 TeV are also given. We present results at leading and next-to-leading order.
We show that the diffractive pp (and p\bar{p}) data (on σ_tot, dσ_el/dt, proton dissociation into low-mass systems, σ^D(low M), and high-mass dissociation, dσ/d(Δη)) in a wide energy range from CERN-ISR to LHC energies, may be described in a two-channel eikonal model with only one `effective' pomeron. By allowing the pomeron coupling to the diffractive eigenstates to depend on the collider energy (as is expected theoretically) we are able to explain the low value of σ^D(low M) measured at the LHC. We calculate the survival probability, S^2, of a rapidity gap to survive `soft rescattering'. We emphasize that the values found for S^2 are particularly sensitive to the detailed structure of the diffractive eigenstates.
We study how unintegrated parton distributions in nuclei can be calculated from the corresponding integrated partons using the EPS09 parametrization. The role of nuclear effects is presented in terms of the ratio $R^A=\text{uPDF}^A/A\cdot \text{PDF}^N$ for both large and small $x$ domains.
We investigate the effect of extending the standard MSTW parameterisation of input parton distribution functions (PDFs) using Chebyshev polynomials. We find evidence that four powers in the polynomial are sufficient for extremely high precision. Applying this to valence and sea quarks we find an improvement in the global fit, but a significant change only in the small-$x$ valence up-quark PDF, $u_V$. We investigate the effect of also extending, and making more flexible, the `nuclear' correction to deuteron structure functions. The extended `Chebyshev' parameterisation results in an improved stability in the deuteron corrections that are required for the best fit to the `global' data. The resulting PDFs have a significantly, but not dramatically, altered valence down-quark distribution, $d_V$. For the extended set of MSTW PDFs, their uncertainties can be obtained using 23, rather than the usual 20, orthogonal `uncertainty' eigenvectors. Since the dominant effect is on the valence quarks, we present a detailed study of the dependence of the valence--sea separation on the predictions for the decay lepton charge asymmetry which results from $W^\pm$ production at the LHC, illustrating the PDFs and the $x$ range probed for different experimental scenarios. We show that the modified MSTW PDFs make significantly improved predictions for these data at the LHC, particularly for high values of the $p_T$ cut of the decay lepton. However, this is a special case, since the asymmetry is extremely sensitive to valence--sea details, and in particular to the combination $u_V-d_V$ of valence PDFs for $x \sim M_W/\sqrt{s}$ at low lepton rapidities. We show that the predictions for a wide variety of total cross sections are very similar to those obtained using the MSTW2008 PDFs, with changes being much smaller than the PDF uncertainties.
We show that the observation of the Drell-Yan production of low-mass lepton-pairs (M < 20 GeV) at high rapidities (Y > 3) at the LHC can make a direct measurement of parton distribution functions (PDFs) in the low x region, x < 10^{-4}. We describe a procedure that greatly reduces the sensitivity of the predictions to the choice of the factorization scale and, in particular, show how, by imposing a cutoff on the transverse momentum of the lepton-pair, the data are able to probe PDFs in the important low scale, low x domain. We include the effects of the Sudakov suppression factor.
First, we show how to reduce the sensitivity of the NLO predictions of the Drell-Yan production of low-mass, lepton-pairs, at high rapidity, to the choice of factorization scale. In this way, observations of this process at the LHC can make direct measurements of parton distribution functions in the low x domain; x < 10^{-4}. Second, we find an inconsistency in the conventional NLO treatment of the infrared region. We illustrate the problem using the NLO coefficient function of Drell-Yan production.
We study theoretically Bose-Einstein condensates with polarized dipolar interactions in anisotropic traps. We map the parameter space by varying the trap frequencies and dipolar interaction strengths and find an irregular-shaped region of parameter space in which density-oscillating condensate states occur, with maximum density away from the trap center. These density-oscillating states may be biconcave (red-blood-cell-shaped), or have two or four peaks. For all trap frequencies, the condensate becomes unstable to collapse for sufficiently large dipole interaction strength. The collapse coincides with the softening of an elementary excitation. When the condensate mode is density-oscillating, the character of the softening excitation is related to the structure of the condensate. We classify these excitations by linear and angular characteristics. We also find excited solutions to the Gross-Pitaevskii equation, which are always unstable.
We discuss stochastic phase-space methods within the truncated Wigner approximation and show explicitly that they can be used to solve non-equilibrium dynamics of bosonic atoms in one-dimensional traps. We consider systems both with and without an optical lattice, and address different approximations in the stochastic synthesization of quantum statistical correlations of the initial atomic field. We also present a numerically efficient projection method for analyzing correlation functions of the simulation results, and demonstrate physical examples of non-equilibrium quantum dynamics of solitons and atom number squeezing in optical lattices.
We discuss the contribution coming from the infrared region to NLO matrix elements and/or coefficient functions of hard QCD processes. Strictly speaking, this contribution is not known theoretically, since it is beyond perturbative QCD. For DGLAP evolution all the infrared contributions are collected in the phenomenological input parton distribution functions (PDFs), at some relatively low scale Q_0; functions which are obtained from a fit to the `global' data. However dimensional regularization sometimes produces a non-zero result coming from the infrared region. Instead of this conventional regularization treatment, we argue that the proper procedure is to first subtract from the NLO matrix element the contribution already generated at the same order in α_s by the LO DGLAP splitting function convoluted with the LO matrix element. This prescription eliminates the logarithmic infrared divergence, giving a well-defined result which is consistent with the original idea that everything below Q_0 is collected in the PDF input. We quantify the difference between the proposed treatment and the conventional approach using low-mass Drell-Yan production and deep inelastic electron-proton scattering as examples; and discuss the potential impact on the `global' PDF analyses. We present arguments to show that the difference cannot be regarded as simply the use of an alternative factorization scheme.
`Soft' high-energy interactions are clearly important in pp collisions. Indeed, these events are dominant by many orders of magnitude, and about 40% are of diffractive origin; that is, due to elastic scattering or proton dissociation. Moreover, soft interactions unavoidably give an underlying component to the rare `hard' events, from which we hope to extract new physics. Here, we discuss how to quantify this contamination. First we present a brief introduction to diffraction. We emphasize the different treatment required for proton dissociation into low- and high-mass systems; the former requiring a multichannel eikonal approach, and the latter the computation of triple-Pomeron diagrams with multi-Pomeron corrections. Then we give an overview of the Pomeron, and explain how the QCD (BFKL-type) Pomeron is the natural object to continue from the `hard' to the `soft' domain. In this way we can obtain a partonic description of soft interactions. We introduce the so-called KMR model, based on this partonic approach, which includes absorptive multi-Pomeron corrections that become increasingly important as we proceed further into the soft domain. This model is able to describe total, elastic and proton dissociation data, and to predict the survival probability of large rapidity gaps to soft rescattering --- in terms of a few physically-motivated parameters. However, more differential phenomena, such as single particle p_t distributions, can only be satisfactorily described if hadronization effects are included. This is achieved by incorporating the KMR analytic approach into the SHERPA Monte Carlo framework. It allows a description of soft physics and diffraction, together with jet physics, in a coherent, self-consistent way. We outline the structure, and show a few results, of this Monte Carlo, which we call SHRiMPS, for reasons which will become clear.