arXiv subjects

A. D. Martin

Publications and source records attributed to A. D. Martin.

At least 55 records · Page 3Linked to original sources

arXiv2016

Exclusive $J/ψ$ process tamed to probe the low $x$ gluon

We address the question as to whether data for J/ψmesons produced exclusively in the forward direction at the LHC can be used in global parton analyses (based on collinear factorization) to pin down the low x gluon PDF. We show that it may be possible to overcome the problems that (i) the process is described by a skewed or Generalized Parton Distribution (GPD), (ii) it is very sensitive to the choice of factorization scale and (iii) there is bad LO, NLO,... perturbative stability to the predictions. However, we start by briefly explaining how the alternative k_T factorization approach has been used to describe the process.

hep-ph↗

arXiv2016

Exclusive J/psi and Upsilon photoproduction and the low x gluon

We discuss the potential to constrain the small-$x$ PDFs using the exclusive production of heavy vector mesons. The calculation of $J/ψ$ and $Υ$ photoproduction at NLO in collinear factorisation is described. The different behaviour of the NLO corrections for $J/ψ$ and $Υ$ is highlighted and we outline what might be expected from the inclusion of these processes in a PDF fit.

hep-ph↗

arXiv2016

Advantages of exclusive γγproduction to probe high mass systems

We recall that the exclusive production of high mass objects via γγfusion at the LHC is not strongly suppressed in comparison with inclusive γγfusion. Therefore it may be promising to study new objects produced by the γγsubprocess in experiments with exclusive kinematics. We list the main advantages of exclusive experiments. We discuss the special advantage of observing $γγ\to X \to γZ$ exclusive events.

hep-ph↗

arXiv2016

Two scales in Bose-Einstein correlations

We argue that the secondaries produced in high energy hadron collisions are emitted by small size sources distributed over a much larger area in impact parameter space occupied by the interaction amplitude. That is, Bose-Einstein correlation of two emitted identical particles should be described by a `two-radii' parametrization ansatz. We discuss the expected energy, charged multiplicity and transverse momentum of the pair (that is, $\sqrt{s},~N_{\rm ch}, k_t$) behaviour of both the small and large size components.

hep-ph↗

arXiv2016

Charm and beauty quark masses in the MMHT2014 global PDF analysis

We investigate the variation in the MMHT2014 PDFs when we allow the heavy quark masses $m_c$ and $m_b$ to vary away from their default values. We make PDF sets available in steps of $Δm_c =0.05~{\rm GeV}$ and $Δm_b =0.25~{\rm GeV}$, and present the variation in the PDFs and in the predictions. We examine the comparison to the HERA data on charm and beauty structure functions and note that in each case the heavy quark data, and the inclusive data, have a slight preference for lower masses than our default values. We provide PDF sets with 3 and 4 active quark flavours, as well as the standard value of 5 flavours. We use the pole mass definition of the quark masses, as in the default MMHT2014 analysis, but briefly comment on the $\overline{\rm MS}$ definition.

hep-ph↗

arXiv2016

Inclusive prompt $χ_{c,b}(1^{++})$ production at the LHC

We study the prompt production of the $χ_c(1^+)$ and $χ_b(1^+)$ mesons at high energies. Unlike $χ(0^+,2^+)$ production, $χ(1^+)$ mesons cannot be created at LO via the fusion of two on-mass-shell gluons, that is $gg\toχ_{c,b}(1^+)$ are not allowed. However, the available experimental data show that the cross sections for $χ_c(1^+)$ and $χ_c(2^+)$ are comparable. We therefore investigate four other $χ(1^+)$ production mechanisms: namely, (i) the standard NLO process $gg\toχ_{c,b}(1^+)+g$, (ii) via gluon virtuality, (iii) via gluon reggeization and, finally, (iv) the possibility to form $χ_{c,b}(1^+)$ by the fusion of three gluons, where one extra gluon comes from another parton cascade, as in the Double Parton Scattering processes.

hep-ph↗

arXiv2015

Exclusive $J/ψ$ and $Υ$ photoproduction and the low $x$ gluon

We study exclusive vector meson photoproduction, $γp \to V + p$ with $V=J/ψ$ or $Υ$, at NLO in collinear factorisation, in order to examine what may be learnt about the gluon distribution at very low $x$. We examine the factorisation scale dependence of the predictions. We argue that, using knowledge of the NLO corrections, terms enhanced by a large $\ln(1/ξ)$ can be reabsorbed in the LO part by a choice of the factorisation scale. (In these exclusive processes $ξ$ takes the role of Bjorken-$x$.) Then, the scale dependence coming from the remaining NLO contributions has no $\ln(1/ξ)$ enhancements. As a result, we find that predictions for the amplitude of $Υ$ production are stable to within about $\pm 15\%$. This will allow data for the exclusive process $p p \to pΥp$ at the LHC, particularly from LHCb, to be included in global parton analyses to constrain the gluon PDF down to $x\sim 10^{-5}$. Moreover, the study of exclusive $J/ψ$ photoproduction indicates that the gluon density found in the recent global PDF analyses is too small at low $x$ and low scales.

hep-ph↗

arXiv2015

Uncertainties on $α_S$ in the MMHT2014 global PDF analysis and implications for SM predictions

We investigate the uncertainty in the value of the strong coupling $α_S(M_Z^2)$ when allowing it to be a free parameter in the recent MMHT global analyses of deep-inelastic and related hard scattering data that was undertaken to determine the parton distribution functions (PDFs) of the proton. The analysis uses the standard framework of leading twist fixed--order collinear factorisation in the ${\overline {\rm MS}}$ scheme. We study the constraints on the value of $α_S(M_Z^2)$ coming from the individual data sets by repeating the NNLO and NLO global analyses at various fixed values of $α_S(M_Z^2)$ about its optimum values, spanning the range $α_S(M_Z^2)=0.108$ to $0.128$ in units of $0.001$, and making all PDFs sets available. The inclusion of the measurements of the cross section for inclusive $t\bar{t}$ production in the global fit allows us to explore the correlation between the values taken for the mass $m_t$ of the top quark and $α_S(M_Z^2)$. We find that the best fit values are $α_S(M_Z^2)=0.1201\pm0.0015$ and $0.1172\pm 0.0013$ at NLO and NNLO respectively, with the central values changing to $α_S(M_Z^2)=0.1195$ and $0.1178$ when the world average of $α_S(M_Z^2)$ is used as a data point. We investigate the interplay between the uncertainties on $α_S(M_Z^2)$ and the uncertainties on the PDFs. In particular we calculate the cross sections for key processes at the LHC and show how the uncertainties coming from the standard MMHT2014 PDFs and from $α_S(M_Z^2)$ can be provided independently and can be combined.

hep-ph↗

arXiv2015

The Effect of Final HERA inclusive Cross Section Data on MMHT2014 PDFs

We investigate the effect of including the HERA run I + II combined cross section data on the MMHT2014 PDFs. We present the fit quality within the context of the global fit and when only the HERA data are included. We examine the changes in both the central values and uncertainties in the PDFs. We find that the prediction for the data is good, and only relatively small improvements in $χ^2$ and changes in the PDFs are obtained with a refit at both NLO and NNLO. PDF uncertainties are slightly reduced. There is a small dependence of the fit quality on the value of $Q^2_{\min}$.

hep-ph↗

arXiv2015

Parton distributions in the LHC era: MMHT 2014 PDFs

We present LO, NLO and NNLO sets of parton distribution functions (PDFs) of the proton determined from global analyses of the available hard scattering data. These MMHT2014 PDFs supersede the `MSTW2008' parton sets, but are obtained within the same basic framework. We include a variety of new data sets, from the LHC, updated Tevatron data and the HERA combined H1 and ZEUS data on the total and charm structure functions. We also improve the theoretical framework of the previous analysis. These new PDFs are compared to the `MSTW2008' parton sets. In most cases the PDFs, and the predictions, are within one standard deviation of those of MSTW2008. The major changes are the $u-d$ valence quark difference at small $x$ due to an improved parameterisation and, to a lesser extent, the strange quark PDF due to the effect of certain LHC data and a better treatment of the $D \to μ$ branching ratio. We compare our MMHT PDF sets with those of other collaborations; in particular with the NNPDF3.0 sets, which are contemporary with the present analysis.

hep-ph↗

arXiv2015

Automated weighing by sequential inference in dynamic environments

We demonstrate sequential mass inference of a suspended bag of milk powder from simulated measurements of the vertical force component at the pivot while the bag is being filled. We compare the predictions of various sequential inference methods both with and without a physics model to capture the system dynamics. We find that non-augmented and augmented-state unscented Kalman filters (UKFs) in conjunction with a physics model of a pendulum of varying mass and length provide rapid and accurate predictions of the milk powder mass as a function of time. The UKFs outperform the other method tested - a particle filter. Moreover, inference methods which incorporate a physics model outperform equivalent algorithms which do not.

physics.data-an↗

arXiv2015

Collision-induced frequency shifts in bright matter-wave solitons and soliton molecules

A recent experiment has detected collision-induced frequency shifts in bright matter-wave solitons for the first time [Nat. Phys. 10, 918 (2014)]. Using a particle model, we derive the frequency shift for two solitons in a harmonic trap, and compare it to the results of the recent experiment and reported theoretical approximation. We find regimes where the frequency shift is much smaller than previously predicted, and propose experiments to test these findings. We also predict that reducing the experimental trap frequency will reveal for the first time soliton molecules or soliton bound states in a cold atoms system. The dynamics of such bound states are found to be both highly phase-dependent, and sensitive to the residual 3D nature of the experiment.

cond-mat.quant-gas↗

arXiv2014

t dependence of the slope of the high energy elastic pp cross section

We consider the main factors which cause the variation of the value of the local slope of the elastic pp cross section B(t)=d[\ln(dσ_el(pp)/dt]/dt with t. Namely, we discuss the role of the pion-loop insertion in the pomeron trajectory, the t-dependence of the pomeron-nucleon coupling and the role of the eikonalization of the proton-proton amplitude in both the one- and two-channel eikonal models.

hep-ph↗

arXiv2014

Measuring the performance of sensors that report uncertainty

We provide methods to validate and compare sensor outputs, or inference algorithms applied to sensor data, by adapting statistical scoring rules. The reported output should either be in the form of a prediction interval or of a parameter estimate with corresponding uncertainty. Using knowledge of the `true' parameter values, scoring rules provide a method of ranking different sensors or algorithms for accuracy and precision. As an example, we apply the scoring rules to the inferred masses of cattle from ground force data and draw conclusions on which rules are most meaningful and in which way.

physics.data-an↗

arXiv2014

Lessons from LHC elastic and diffractive data

In the light of LHC data, we discuss the global description of all high energy elastic and diffractive data, using a one-pomeron model, but including multi-pomeron interactions. The LHC data indicate the need of a $k_t(s)$ behaviour, where $k_t$ is the gluon transverse momentum along the partonic ladder structure which describes the pomeron. We also discuss tensions in the data, as well as the $t$ dependence of the slope of $dσ_{el}/dt$ in the small $t$ domain.

hep-ph↗

arXiv2014

Evolution in opening angle combining DGLAP and BFKL logarithms

We present an evolution equation which simultaneously sums the leading BFKL and DGLAP logarithms for the integrated gluon distribution in terms of a single variable, namely the emission angle of the gluon. This form of evolution is appropriate for Monte Carlo simulations of events of high energy pp (and p\bar{p}) interactions, particularly where small x events are sampled.

hep-ph↗

arXiv2014

Partonic transverse momenta in soft collisions

The partonic transverse momentum, k_t distribution plays a crucial role in driving high-energy hadron interactions. If k_t is limited we have old fashioned Regge Theory. If k_t increases with energy the interaction may be described by perturbative QCD. We use BFKL diffusion in ln k_t, supplemented by a stronger absorption of low k_t partons, to estimate the growth of the mean transverse momenta with energy. This growth reveals itself in the distribution of secondaries produced at the collider energies. We present a simple, BFKL-based, model to demonstrate the possible size of the effect. Moreover, we propose a way to evaluate experimentally the shape of the parton transverse momenta distribution by studying the spectra of the (D or B) mesons which contain one heavy quark.

hep-ph↗

arXiv2014

BFKL equation for an integrated gluon density

We show how it is possible to rewrite the BFKL equation for the unintegrated gluon distribution, in terms of integrated gluons, similar to that used in DGLAP. We add to our equation the next-to-leading log terms which provide exact energy-momentum conservation and account for the kinematic constraint in real gluon emission. In this way the equation includes the major part of the higher-order corrections to BFKL evolution. We discuss the possibility to obtain a unified BFKL-DGLAP evolution equation relevant to processes at the LHC where both log(1/x) and logQ^2 are large simultaneously.

hep-ph↗