arXiv subjects

Hannu Paukkunen

Publications and source records attributed to Hannu Paukkunen.

At least 55 records · Page 3Linked to original sources

arXiv2020

Can we fit our nuclear PDFs with the high-$x$ CLAS data?

Nuclear parton distribution functions (nuclear PDFs) are non-perturbative objects that encode the partonic behaviour of bound nucleons. To avoid potential higher-twist contributions, the data probing the high-$x$ end of nuclear PDFs are sometimes left out from the global extractions despite their potential to constrain the fit parameters. In the present work we focus on the kinematic corner covered by the new high-$x$ data measured by the CLAS/JLab collaboration. By using the Hessian re-weighting technique, we are able to quantitatively test the compatibility of these data with globally analyzed nuclear PDFs and explore the expected impact on the valence-quark distributions at high $x$. We find that the data are in a good agreement with the EPPS16 and nCTEQ15 nuclear PDFs whereas they disagree with TuJu19. The implications on flavour separation, higher-twist contributions and models of EMC effect are discussed.

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arXiv2020

Impact of dijet and D-meson data from 5.02 TeV p+Pb collisions on nuclear PDFs

We discuss the new constraints on gluon parton distribution function (PDF) in lead nucleus, derivable with the Hessian PDF reweighting method from the 5.02 TeV p+Pb measurements of dijet (CMS) and $D^0$-meson (LHCb) nuclear modification ratios. The impact is found to be significant, placing stringent constraints in the mid- and previously unconstrained small-$x$ regions. The CMS dijet data confirm the existence of gluon anti-shadowing and the onset of small-$x$ shadowing, as well as reduce the gluon PDF uncertainties in the larger-$x$ region. The gluon constraints from the LHCb $D^0$ data, reaching down to $x \sim 10^{-5}$ and derived in a NLO perturbative QCD approach, provide a remarkable reduction of the small-$x$ uncertainties with a strong direct evidence of gluon shadowing. Furthermore, we find a good description of the data even down to zero $D^0$-meson transverse momentum within a purely DGLAP-based approach without a need for imposing any non-linear effects. Importantly, the constraints obtained from the dijet and $D^0$ data are mutually fully consistent, supporting the universality of nuclear PDFs in hard-scattering processes.

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arXiv2019

Saturation and forward jets in proton-lead collisions at the LHC

We investigate the forward-jet energy spectrum within the Color Glass Condensate framework at 5 TeV center-of-mass energy. In particular, we focus on the kinematic range covered by the CMS-CASTOR calorimeter. We show that our saturation-model calculations are compatible with the CASTOR measurements and that to optimally reproduce the data, effects of multi-parton interactions need to be included. We predict a significant nuclear suppression - reaching down to 50% at the lowest considered jet energies $E_{\rm jet} \sim 500 \, {\rm GeV}$.

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arXiv2019

Non-quadratic improved Hessian PDF reweighting and application to CMS dijet measurements at 5.02 TeV

Hessian PDF reweighting, or "profiling", has become a widely used way to study the impact of a new data set on parton distribution functions (PDFs) with Hessian error sets. The available implementations of this method have resorted to a perfectly quadratic approximation of the initial $χ^2$ function before inclusion of the new data. We demonstrate how one can take into account the first non-quadratic components of the original fit in the reweighting, provided that the necessary information is available. We then apply this method to the CMS measurement of dijet pseudorapidity spectra in proton-proton (pp) and proton-lead (pPb) collisions at 5.02 TeV. The measured pp dijet spectra disagree with next-to-leading order (NLO) theory calculations using the CT14 NLO PDFs, but upon reweighting the CT14 PDFs, these can be brought to a much better agreement. We show that the needed proton-PDF modifications also have a significant impact on the predictions for the pPb dijet distributions. Taking the ratio of the individual spectra, the proton-PDF uncertainties effectively cancel, giving a clean probe of the PDF nuclear modifications. We show that these data can be used to further constrain the EPPS16 nuclear PDFs and strongly support gluon nuclear shadowing at small $x$ and antishadowing at around $x \approx 0.1$.

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arXiv2019

Double D-meson production in proton-proton and proton-lead collisions at the LHC

We consider the simultaneous production of two heavy-flavoured hadrons - particularly D mesons - at the LHC. We base our calculations on collinearly factorized QCD at next-to-leading order, using the contemporary parton distribution functions and D-meson fragmentation functions. The contributions of double-parton scatterings are included in the approximation of independent partonic interactions. Our framework benchmarks well with the available proton-proton data from the LHCb collaboration giving us confidence to make predictions for proton-lead collisions. Our results indicate that the double D-meson production in proton-lead collisions should be measurable at the LHCb kinematics with the already collected Run-II data, and should provide evidence for double-parton scattering at perturbative scales with a nuclear target.

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arXiv2019

Constraints for nuclear PDFs from the LHCb D-meson data

We quantify the impact of LHCb D-meson measurements at $\sqrt{s}=5 \, {\rm TeV}$ on the EPPS16 and nCTEQ15 nuclear PDFs. In our study, the theoretical description of D-meson production is based on the recently developed SACOT-$m_{\rm T}$ variant of the general-mass variable-flavour-number formalism, and the impact on PDFs is estimated via reweighting methods. We pay special attention on the theoretical uncertainties known to us, and are led to exclude the $p_{\rm T}<3 \, {\rm GeV}$ region from our main analysis. The LHCb data can be accommodated well within EPPS16 and nCTEQ15, and the data provide stringent constraints on the gluons in the shadowing/antishadowing regions. No evidence of non-linear effects beyond standard DGLAP evolution is found even if the full kinematic region down to zero $p_{\rm T}$ is considered.

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arXiv2019

Nuclear gluons at RHIC in a multi-observable approach

We explore the possibility of measuring nuclear gluon distributions at the Relativistic Heavy-Ion Collider (RHIC) with $\sqrt{s}=200 \, {\rm GeV}$ proton-nucleus collisions. In addition to measurements at central rapidity, we consider also observables at forward rapidity, consistent with proposed upgrades to the experimental capabilities of STAR and sPHENIX. The processes we consider consist of Drell-Yan dilepton, dijet, and direct photon-jet production. The Drell-Yan process is found to be an efficient probe of gluons at small momentum fractions. In order to fully utilize the potential of Drell-Yan measurements we demonstrate how the overall normalization uncertainty present in the experimental data can be fixed using other experimental observables. An asset of the RHIC collider is its flexibility to run with different ion beams, and we outline how this ability could be taken advantage of to measure the $A$ dependence of gluon distributions for which the current constraints are scarce.

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arXiv2019

Hadroproduction of open heavy flavour for PDF analyses

Due to the large masses of the charm and bottom quarks, their production cross sections are calculable within the perturbative QCD. This makes the heavy-quark mesons important observables in high-energy collisions of protons and nuclei. However, the available calculations for heavy-flavored-meson hadroproduction have been somewhat problematic in reliably describing the cross sections across the full kinematic range from zero to very high $p_{\rm T}$. This has put some question marks on the robustness of LHC heavy-flavored-meson measurements in studying the partonic structure of the colliding hadrons and nuclei. Here, we introduce SACOT-$m_{\rm T}$ - a novel scheme for open heavy-flavour hadroproduction within the general-mass variable-flavour-number formalism that solves this problem. The introduced scheme is an analogue of the SACOT-$χ$ scheme often used for deeply-inelastic scattering in global analyses of PDFs.

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arXiv2018

Impact of CMS dijets in 5.02 TeV pPb and pp collisions on EPPS16 nuclear PDFs

The CMS measurement of dijet pseudorapidity distributions in pPb versus pp collisions at 5.02 TeV provides a direct probe on nuclear gluon PDFs. We show that while the predicted pPb pseudorapidity distributions suffer from sizable free-proton PDF uncertainties, the ratios of the pPb and pp distributions are practically insensitive to scale and free-proton PDF choices. We find the CMS data on pPb to pp ratios to be in good agreement with the EPPS16 nuclear modifications. Using a non-quadratic extension of the Hessian PDF reweighting method, we study the impact of these data on the EPPS16 nuclear PDFs. Relative to EPPS16, we find stronger evidence for mid-x gluon antishadowing as well as indication for larger gluon shadowing at small x. The data are also able to further constrain the gluon PDF in the EMC region.

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arXiv2018

Nuclear PDFs Today

I review the current status of global analysis of nuclear parton distributions and discuss some near-future prospects. I also advocate proton-nucleus (p-$A$) measurements at the LHC with $A\ll 208$ from the viewpoint of nuclear parton distributions.

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arXiv2018

Heavy flavour production in the SACOT-$m_{\rm T}$ scheme

The hadroproduction of heavy-flavoured mesons has recently attracted a growing interest e.g. within the people involved in global analysis of proton and nuclear parton distribution functions, saturation physics, and physics of cosmic rays. In particular, the D- and B-meson measurements of LHCb at forward direction are sensitive to gluon dynamics at small $x$ and are one of the few perturbative small-$x$ probes before the next generation deep-inelastic-scattering experiments. In this talk, we will concentrate on the collinear-factorization approach to inclusive D-meson production and describe a novel implementation - SACOT-$m_{\rm T}$ - of the general-mass variable flavour number scheme (GM-VFNS). In the GM-VFNS framework the cross sections retain the full heavy-quark mass dependence at $p_{\rm T}=0$, but gradually reduce to the ordinary zero-mass results towards asymptotically high $p_{\rm T}$. However, the region of small (but non-zero) $p_{\rm T}$ has been somewhat problematic in the previous implementations of GM-VFNS, leading to divergent cross sections towards $p_{\rm T} \rightarrow 0$, unless the QCD scales are set in a particular way. Here, we provide a solution to this problem. In essence, the idea is to consistently account for the underlying energy-momentum conservation in the presence of a final-state heavy quark-antiquark pair. This automatically leads to a well-behaved GM-VFNS description of the cross sections across all $p_{\rm T}$ without a need to fine tune the QCD scales. The results are compared with the LHCb data and a very good agreement is found. We also compare to fixed-order based calculations and explain why they lead to approximately a factor of two lower D-meson production cross sections than the GM-VFNS approach.

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arXiv2018

Impact of CMS 5.02 TeV dijet measurements on gluon PDFs - a preliminary view

We discuss the implications of the preliminary CMS dijet data from 5.02 TeV pp and pPb collisions for gluon PDFs of the proton and nuclei. The preliminary pp data show a discrepancy with NLO predictions using for example the CT14 PDFs. We find that this difference cannot be accommodated within the associated scale uncertainties and debate the possible changes needed in the gluon PDF. A similar discrepancy is found between the CMS pPb data and NLO predictions e.g. with the EPPS16 nuclear modifications imposed on the CT14 proton PDFs. When a nuclear modification ratio of the pp and pPb data is constructed, the uncertainties in the scale choices and in proton PDFs effectively cancel and a good agreement between the data and EPPS16 is found, except in some bins at backward rapidities corresponding to large x of the nucleus. To assess the impact of these data on the EPPS16 nuclear PDFs, we use a non-quadratic extension of the Hessian PDF reweighting method. A significant reduction in EPPS16 uncertainties is obtained with the fit supporting strong nuclear shadowing and valence-like antishadowing for gluons. We also indicate the possible extensions needed in the EPPS16 parametrization at large x.

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arXiv2018

Revisiting the D-meson hadroproduction in general-mass variable flavour number scheme

We introduce a novel realization of the open heavy-flavour hadroproduction in general-mass variable flavour number scheme at next-to-leading order in perturbative QCD. The principal novelty with respect to the earlier works is in the treatment of small-transverse-momentum limit, which has been a particularly challenging kinematic region in the past. We show that by a suitable choice of scheme, it is possible to obtain a well-behaved description of the open heavy-flavour hadroproduction cross sections from zero up to asymptotically high transverse momentum. We contrast our calculation with the available D$^0$-meson data as measured by the LHCb and ALICE collaborations at the LHC, finding a very good agreement within the theoretical and experimental uncertainties. We also compare our framework with other theoretical approaches.

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arXiv2018

EPPS16 - Bringing nuclear PDFs to the LHC era

We report on EPPS16, the first global analysis of nuclear parton distribution functions (nPDFs) to include LHC data. Also for the first time, a full flavour dependence of nPDFs is allowed. While the included Z and W data are found to have insufficient statistics to yield stringent constraints, the CMS 5.02 TeV proton-lead dijet data prove crucial in setting the shape of nuclear gluon modifications. With these and other observables being measured in proton-lead runs, we are experiencing a shift of nPDFs to the LHC precision era.

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arXiv2017

Pion-nucleus Drell-Yan data as a novel constraint for nuclear PDFs

We have studied the prospects of using the Drell-Yan dilepton process in pion-nucleus collisions as a novel input in the global analysis of nuclear parton distribution functions (nPDFs). In a NLO QCD framework, we find the measured nuclear cross-section ratios from the NA3, NA10 and E615 experiments to be largely insensitive to the pion parton distributions and also compatible with the EPS09 and nCTEQ15 nPDFs. These data sets can thus be, and in EPPS16 have been, included in global nPDF analyses without introducing significant new theoretical uncertainties or tension with the other data. In particular, we explore the constraining power of these data sets on the possible flavour asymmetry in the valence-quark nuclear modifications. Moreover, using the COMPASS kinematics we present predictions for pion charge-difference ratio, a new measurable which could help to further constrain this asymmetry.

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arXiv2017

EPPS16 - First nuclear PDFs to include LHC data

We present results of our recent EPPS16 global analysis of NLO nuclear parton distribution functions (nPDFs). For the first time, dijet and heavy gauge boson production data from LHC proton-lead collisions have been included in a global fit. Especially, the CMS dijets play an important role in constraining the nuclear effects in gluon distributions. With the inclusion of also neutrino-nucleus deeply-inelastic scattering and pion-nucleus Drell-Yan data and a proper treatment of isospin-corrected data, we were able to free the flavor dependence of the valence and sea quark nuclear modifications for the first time. This gives us less biased, yet larger, flavor by flavor uncertainty estimates. The EPPS16 analysis indicates no tension between the data sets used, which supports the validity of collinear factorization and universal nPDFs for nuclear hard-collision processes in the kinematical range studied.

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arXiv2017

An update on nuclear PDFs at the LHeC

The prospects for a measurement of nuclear parton distribution functions (PDFs) at the Large Hadron--Electron Collider are discussed in the light of recent progress made in the front of global analysis of nuclear PDFs.

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arXiv2017

The EPPS16 nuclear PDFs

We report on EPPS16 - the first analysis of NLO nuclear PDFs where LHC p-Pb data (Z, W, dijets) have been directly used as a constraint. In comparison to our previous fit EPS09, also data from neutrino-nucleus deeply-inelastic scattering and pion-nucleus Drell-Yan process are now included. Much of the theory framework has also been updated from EPS09, including a consistent treatment of heavy quarks in deeply-inelastic scattering. However, the most notable change is that we no longer assume flavour-blind nuclear modifications for valence and sea quarks. This significantly reduces the theoretical bias. All the analysed data are well reproduced and the analysis thereby supports the validity of collinear factorization in high-energy collisions involving heavy nuclei. However, flavour by flavour, the uncertainties are still rather large.

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