arXiv subjects

Ilkka Helenius

Publications and source records attributed to Ilkka Helenius.

66 records · Page 4Linked to original sources

arXiv2014

LHC data challenges the contemporary parton-to-hadron fragmentation functions

We discuss the inclusive high-pT charged-particle production in proton-proton collisions at the LHC. The experimental data are compared to the NLO perturbative QCD calculations employing various sets of parton-to-hadron fragmentation functions. Most of the theoretical predictions are found to disastrously overpredict the measured cross sections, even if the scale variations and PDF errors are accounted for. The problem appears to arise from the presently too hard gluon-to-hadron fragmentation functions.

hep-ph↗

arXiv2014

Zeroing in on the initial state -- tomography using bulk, jets and photons

One of the unsolved problems in the current 'standard model' of heavy ion physics is the apparent rapid thermalization of QCD matter in the pre-equilibrium stage. While it is challenging to probe this mechanism directly, there are now several observables available which allow tomographic imaging of the initial state geometry, which is expected to carry remnant information of the equilibration mechanism. On the fluid dynamics side, scaled fluctuations in the momentum space anisotropy parameters v_n image the initial eccentricity fluctuations epsilon_n almost directly with only a weak dependence on the details of the fluid dynamical evolution. From a different direction, due to the strong non-linear dependence of their emission rates on temperature, thermal photons and their v_n are very sensitive to the initial state graininess. Finally, the v_2 and v_3 of high P_T hadrons coming from hard processes reflect the attenuation pattern of partons propagating through the inhomogeneous matter density after some fluid dynamical evolution. Combining information from all these channels does not yet lead to a fully consistent picture, however intriguing trends pointing towards non-trivial initial state dynamics emerge.

hep-ph↗

arXiv2014

Probing the small-$x$ nuclear gluon distributions with isolated photons at forward rapidities in p+Pb collisions at the LHC

Inclusive direct photon production in p+Pb collisions at the LHC is studied within the NLO perturbative QCD. Our aim is to quantify the dominant $x$ regions probed at different rapidities and to identify the best conditions for testing the nuclear gluon parton distribution functions (nPDFs) at small $x$. A comparison to the inclusive pion production reveals that from these two processes the photons carry more sensitivity to the small-$x$ partons and that this sensitivity can be further increased by imposing an isolation cut for the photon events. The details of the isolation criteria, however, seem to make only a small difference to the studied $x$ sensitivity and have practically no effect on the expected nuclear modifications. We consider also the yield asymmetry between forward and backward rapidities which can be used to probe the nPDFs irrespectively of whether an accurate p+p baseline is available.

hep-ph↗

arXiv2014

Confronting current NLO parton fragmentation functions with inclusive charged-particle spectra at hadron colliders

The inclusive spectra of charged particles measured at high transverse momenta ($p_T\gtrsim$2GeV/c) in proton-proton and proton-antiproton collisions in the range of center-of-mass energies $\sqrt{s}=200-7000$GeV are compared with next-to-leading order perturbative QCD calculations using seven recent sets of parton-to-hadron fragmentation functions (FFs). Accounting for the uncertainties in the scale choices and in the parton distribution functions, we find that most of the theoretical predictions tend to overpredict the measured LHC and Tevatron cross sections by up to a factor of two. We identify the currently too-hard gluon-to-hadron FFs as the probable source of the problem, and justify the need to refit the FFs using the available LHC and Tevatron data in a region of transverse momenta, $p_T\gtrsim$10GeV/c, which is supposedly free from additional non-perturbative contributions and where the scale uncertainty is only modest.

hep-ph↗

arXiv2014

Constraining nPDFs with inclusive pions and direct photons at forward rapidities in p+Pb collisions at the LHC

In this talk, we present NLO pQCD predictions for inclusive pion and direct photon nuclear modifications in p+Pb collisions at mid- and forward rapidities at the LHC. In addition to the minimum bias predictions, we also address the centrality dependence with spatially dependent nuclear PDFs. To understand which regions of the nuclear momentum fraction $x_2$ these observables predominantly probe, we present also the underlying $x_2$ distributions at different rapidities. We are led to conclude that the isolated photons at forward rapidities are more sensitive to the small-$x_2$ dynamics than the inclusive pions.

hep-ph↗

arXiv2014

Centrality and rapidity dependence of inclusive pion and prompt photon production in p+Pb collisions at the LHC with EPS09s nPDFs

The centrality dependencies of the inclusive neutral pion and prompt photon nuclear modification factors for p+Pb collisions at the LHC are studied using a spatially dependent set of nuclear PDFs, EPS09s. The calculations are performed at mid- and forward rapidities searching for an observable which would optimally probe the spatial dependence of the nuclear PDFs. In addition, we discuss to which $x$ values of the nucleus the different observables are sensitive.

hep-ph↗

arXiv2013

Elliptic flow of thermal photons from event-by-event hydrodynamic model

Elliptic flow of direct photons in relativistic heavy ion collisions is believed to be dominated by contribution from thermal radiation of quark gluon plasma up to $p_T$ $\sim 5$ GeV/$c$, although other sources start outshining the thermal contribution at already smaller values of $p_T$ in the direct photon spectrum. The elliptic flow of thermal photons from ideal hydrodynamics considering a smooth initial density distribution under-predicts the PHENIX direct photon data from 200A GeV Au+Au collisions at RHIC by a large margin in the range $1 < p_T < 5$ GeV/$c$. However, a significant enhancement of thermal photon production due to fluctuations in the initial QCD matter density distributions is expected. We show that such fluctuations result in substantially larger photon elliptic flow for $p_T > 2.5 $ GeV/$c$ compared to a smooth initial-state-averaged density profile. The results from event-by-event hydrodynamics are found to be sensitive to the fluctuation size parameter. However, the effects of initial state fluctuations are insufficient to account for the discrepancy to the PHENIX data for direct photon elliptic flow. Furthermore, the photon $v_2$ is reduced even more when we include the NLO pQCD prompt photon component. We also calculate the spectra and elliptic flow of thermal photons for 2.76A TeV Pb+Pb collisions at LHC and for the 0--40% centrality bin. Thermal photons from event-by-event hydrodynamics along with prompt photons from NLO pQCD calculations explain the ALICE preliminary direct photon data well in the region $p_T \ge 2.5$ GeV/c. Similar to RHIC, the elliptic flow results at LHC are again found to be much smaller than the ALICE preliminary $v_2$ data.

hep-ph↗

arXiv2013

Centrality dependence of inclusive prompt photon production in d+Au, Au+Au, p+Pb, and Pb+Pb collisions

We calculate the centrality dependence of the midrapidity nuclear modification for inclusive prompt photon production in d+Au and Au+Au collisions at RHIC and in p+Pb and Pb+Pb collisions at the LHC. Our results, using the recent spatially dependent nuclear PDF set EPS09s, are consistent with the existing high-pT data from the PHENIX and CMS collaborations. The good agreement even in the case of nucleus+nucleus collisions suggests that the high-pT direct photon production is not significantly altered by the strongly interacting medium produced in such collisions. We find the centrality dependence of the nuclear modifications generally rather weak but perhaps measurable at low pT.

hep-ph↗

arXiv2012

EPS09s and EKS98s: Impact parameter dependent nPDF sets

In our recent study we have determined two new spatially dependent nuclear PDF (nPDF) sets, EPS09s and EKS98s. With these, the hard-process cross-sections can be calculated in different centrality classes consistently with the globally analyzed nPDFs for the first time. The sets were determined by exploiting the $A$-systematics of the globally fitted nPDF sets, EPS09 and EKS98. For the spatial dependence of the nPDFs we used a power series ansatz in the nuclear thickness function $T_A$. In this flash talk we introduce the framework, and present our NLO EPS09s-based predictions for the nuclear modification factor in four centrality classes for inclusive neutral pion production in p+Pb collisions at the LHC and for inclusive prompt photon production in d+Au collisions at RHIC at midrapidity.

hep-ph↗

arXiv2012

Extracting the impact parameter dependence of the nPDFs from the EKS98 and EPS09 global fits

As all the globally fitted nuclear PDFs (nPDFs) have been so far impact parameter independent, it has not been possible to calculate the hard process cross sections in different centrality classes consistently with the global analyses. In \cite{Helenius:2012wd} we have offered a solution to this problem by determining two spatially dependent nPDF sets, \texttt{EPS09s} and \texttt{EKS98s}, using the $A$-systematics of the earlier global fits EPS09 and EKS98 and an assumption that the spatial dependence can be written as a power series of the nuclear thickness function. For a data comparison, we have calculated the nuclear modification factor of inclusive neutral pion production in d+Au collisions at RHIC in four centrality bins at midrapidity and compared these to a PHENIX measurement. In addition, we have also performed a similar calculation for inclusive photon production in d+Au collisions at RHIC.

hep-ph↗

arXiv2012

Modeling the Impact Parameter Dependence of the nPDFs With EKS98 and EPS09 Global Fits

So far the nuclear PDFs (nPDFs) in the global DGLAP fits have been taken to be spatially independent. In this work, using the $A$-dependence of the globally fitted sets EPS09 and EKS98, we have determined the spatial dependence of the nPDFs in terms of powers of the nuclear thickness functions. New spatially dependent nPDF sets EPS09s (NLO, LO, error sets) and EKS98s (LO) are released. As an application, we consider the nuclear modification factor $R_{dAu}^{π^0}$ at midrapidity for neutral pion production in deuteron-gold collisions at RHIC in NLO. Comparison with the PHENIX data in different centrality classes is also shown. In addition, predictions for the corresponding nuclear modification factor $R_{pPb}^{π^0}$ in proton-lead collisions at the LHC are discussed.

hep-ph↗

arXiv2012

Impact-parameter dependent nuclear parton distribution functions: EPS09s and EKS98s and their applications in nuclear hard processes

We determine the spatial (impact parameter) dependence of nuclear parton distribution functions (nPDFs) using the $A$-dependence of the spatially independent (averaged) global fits EPS09 and EKS98. We work under the assumption that the spatial dependence can be formulated as a power series of the nuclear thickness functions $T_A$. To reproduce the $A$-dependence over the entire $x$ range we need terms up to $[T_A]^4$. As an outcome, we release two sets, EPS09s (LO, NLO, error sets) and EKS98s, of spatially dependent nPDFs for public use. We also discuss the implementation of these into the existing calculations. With our results, the centrality dependence of nuclear hard-process observables can be studied consistently with the globally fitted nPDFs for the first time. As an application, we first calculate the LO nuclear modification factor $R^{1jet}_{AA}$ for primary partonic-jet production in different centrality classes in Au+Au collisions at RHIC and Pb+Pb collisions at LHC. Also the corresponding central-to-peripheral ratios $R_{CP}^{1jet}$ are studied. We also calculate the LO and NLO nuclear modification factors for single inclusive neutral pion production, $R_{dAu}^{π^0}$, at mid- and forward rapidities in different centrality classes in d+Au collisions at RHIC. In particular, we show that our results are compatible with the PHENIX mid-rapidity data within the overall normalization uncertainties given by the experiment. Finally, we show our predictions for the corresponding modifications $R_{pPb}^{π^0}$ in the forthcoming p+Pb collisions at LHC.

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