Phenomenological study of charm photoproduction at HERA

We present predictions for single inclusive distributions of charmed mesons, relevant to the HERA experiments. Our results are based upon a computation that correctly incorporates mass effects up to the next-to-leading order level, and the resummation of transverse momentum logarithms up to next-to-leading-logarithmic level. We apply the same acceptance cuts as the H1 and Zeus experiments, and compare our results to their data. We perform a study of the sensitivity of our predictions on the charm mass, \LambdaQCD, factorization scale, renormalization scale, and fragmentation parameters.Comment: 15 pages Latex; 25 figures include

The \pt spectrum of heavy quarks in photoproduction

We illustrate a formalism that allows to match the next-to-leading order cross section for the photoproduction of heavy quarks to the cross section obtained by resumming logarithms of \pt/m to the next-to-leading accuracy, thus giving a sensible prediction for any value of \pt. We present a comparison between our predictions and H1 and ZEUS data.Comment: 4 pages Latex; 3 figures included. Talk given at DIS01, 27 April - 1 May 2001, Bologna, Ital

The p_T Spectrum in Heavy-Flavour Photoproduction

We consider the transverse-momentum distribution of heavy flavours in photon-hadron collisions. We present a formalism in which large transverse-momentum logarithms are resummed to the next-to-leading level, and mass effects are included exactly up to order alpha_em alpha_s^2, so as to retain predictivity at both small and large transverse momenta. Phenomenological applications relevant to charm photoproduction at HERA are given.Comment: 26 pages, Latex, epsfig, 15 figures. Submitted to JHE

Pileup subtraction using jet areas

One of the major challenges for the LHC will be to extract precise information from hadronic final states in the presence of the large number of additional soft pp collisions, pileup, that occur simultaneously with any hard interaction in high luminosity runs. We propose a novel technique, based on jet areas, that provides jet-by-jet corrections for pileup and underlying-event effects. It is data driven, does not depend on Monte Carlo modelling and can be used with any jet algorithm for which a jet area can be sensibly defined. We illustrate its effectiveness for some key processes and find that it can be applied also in the context of the Tevatron, low-luminosity LHC and LHC heavy-ion collisions.Comment: 12 pages, 7 figures. Revised version with many changes and additions, small and larger. No changes in conclusions. Version published in Physics Letters

SoftKiller, a particle-level pileup removal method

Existing widely-used pileup removal approaches correct the momenta of individual jets. In this article we introduce an event-level, particle-based pileup correction procedure, SoftKiller. It removes the softest particles in an event, up to a transverse momentum threshold that is determined dynamically on an event-by-event basis. In simulations, this simple procedure appears to be reasonably robust and brings superior jet resolution performance compared to existing jet-based approaches. It is also nearly two orders of magnitude faster than methods based on jet areas.Comment: 26 pages, 16 figures (2 appendices with further checks added

Is There a Significant Excess in Bottom Hadroproduction at the Tevatron?

We discuss the excess in the hadroproduction of B mesons at the Tevatron. We show that an accurate use of up-to-date information on the B fragmentation function reduces the observed excess to an acceptable level. Possible implications for experimental results reporting bottom quark cross sections, also showing an excess with respect to next-to-leading order theoretical predictions, are discussed.Comment: 5 pages, Latex, 4 figures. Submitted to Phys. Rev. Let

On the use of charged-track information to subtract neutral pileup

The use of charged pileup tracks in a jet to predict the neutral pileup component in that same jet could potentially lead to improved pileup removal techniques, provided there is a strong local correlation between charged and neutral pileup. In Monte Carlo simulation we find that the correlation is however moderate, a feature that we attribute to characteristics of the underlying non-perturbative dynamics. Consequently, `neutral-proportional-to-charge' (NpC) pileup mitigation approaches do not outperform existing, area-based, pileup removal methods. This finding contrasts with the arguments made in favour of a new method, "jet cleansing", in part based on the NpC approach. We identify the critical differences between the performances of linear cleansing and trimmed NpC as being due to the former's rejection of subjets that have no charged tracks from the leading vertex, a procedure that we name "zeroing". Zeroing, an extreme version of the "charged-track trimming" proposed by ATLAS, can be combined with a range of pileup-mitigation methods, and appears to have both benefits and drawbacks. We show how the latter can be straightforwardly alleviated. We also discuss the limited potential for improvement that can be obtained by linear combinations of the NpC and area-subtraction methods.Comment: 27 pages, 10 figures; v2 adapts the discussion of cleansing to the trimming parameter choice clarified in v2 of the cleansing paper (arXiv:1309.4777) and identifies and analyses the origin of differences with Np

A note on the CDF high-p_t charged particle excess

It has recently been pointed out that CDF data for the cross section of high-p_t charged particles show an excess of up to three orders of magnitude over QCD predictions, a feature tentatively ascribed to possible violations of factorisation. We observe that for p_t > 80 GeV the measured charged-particle cross sections become of the same order as jet cross sections. Combining this information with data on charged particle distributions within jets allows us to rule out the hypothesis that the CDF data could be interpreted in terms of QCD factorisation violation. We also comment on the difficulty of interpreting the excess in terms of new physics scenarios.Comment: 8 pages, 4 figure

D^* production from e^+e^- to ep collisions in NLO QCD

Fragmentation functions for D mesons, based on the convolution of a perturbative part, related to the heavy quark perturbative showering, and a non-perturbative model for its hadronization into the meson, are used to describe D^* production in e^+e^- and ep collisions. The non-perturbative part is determined by fitting the e^+e^- data taken by ARGUS and OPAL at 10.6 and 91.2 GeV respectively. When fitting with a non perturbative Peterson fragmentation function and using next-to-leading evolution for the perturbative part, we find an epsilon parameter sensibly different from the one commonly used, which is instead found with a leading order fit. The use of this new value is shown to increase considerably the cross section for D^* production at HERA, suggesting a possible reconciliation between the next-to-leading order theoretical predictions and the experimental data.Comment: 20 pages, LaTeX2e, 8 Postscript figure

Measuring helium abundance difference in giants of NGC 2808

Multiple populations have been detected in several globular clusters (GC) that do not display a spread in metallicity. Unusual features of their CMD can be interpreted in terms of differences in the Helium content of the stars belonging to the sub-populations. Differences in He abundance have never been directly observed. We attempt to measure these differences in two giant stars of NGC 2808 with very similar parameters but different Na and O abundances, hence that presumably belong to different sub-populations, by directly comparing their He I 10830 {\AA} lines. The He 10830 {\AA} line forms in the upper chromosphere. Our detailed models derive the chromospheric structure using the Ca II and H$\alpha$, and simulate the corresponding He I 10830 line profiles. We show that, at a given value of He abundance, the He I 10830 equivalent width cannot significantly change without a corresponding much larger change in the Ca II lines. We have used the VLT-CRIRES to obtain high-resolution spectra in the 10830 {\AA} region, and the VLT-UVES to obtain spectra of the Ca II and H$\alpha$ lines of our target stars. The two target stars have very similar Ca II and H$\alpha$ lines, but different appearances in the He region. One line, blueshifted by 17 km s$^{-1}$ with respect to the He 10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas the Na-poor star spectrum is consistent with a non-detection. The difference in the spectra is consistent and most closely explained by an He abundance difference between the two stars of $\Delta Y \ge$ 0.17.We provide direct evidence of a significant He line strength difference in giant stars of NGC 2808 belonging to different sub-populations, which had been previously detected by other photometric and spectroscopic means.Comment: Accepted for publication in A&