6. Summary and Conclusions 165
7.4. Possibilities for smaller systematic uncertainties
Improvements of systematic uncertainties require significant additional effort and have to fo-cus on the largest contributions. Dominating uncertainties are the corrections for efficiency and systematic uncertainties from cut variations in the whole pT range and thepT resolution correction at large pT. In addition, contributions from particle composition and secondary contamination are relevant mostly at low and intermediatepT.
Possible reduction of systematic uncertainties could arise from new and improved track cuts and changes in the data reconstruction and simulations that lead to better agreement between simulation and data. In particular the description of the detector response in the simulations needs to be improved, e.g. by including ion tail cancellation and cross-talk effects. In ad-dition the discrepancies between Monte Carlo event generators used for simulations and the measurements have to be reduced. The inclusion of measured results for identified particle production, in particular the composition of primary particles, allows more precise corrections and thus lower systematic uncertainties.
Transverse momentum resolution
The pT resolution can be significantly improved with a new reconstruction, that includes a better parameterization of the space point resolution. These improvements have already been incorporated in the reconstruction of the p–Pb data, improving thepT resolution at highpTby about a factor 3 compared to previous pp and Pb–Pb data (see Figure 3.27 in section 3.7.4).
During the long shutdown 1 (LS1) of the LHC (ending early 2015) it is intended to reconstruct most of the data recorded by ALICE again. This reprocessing, with all the improvements in the tracking and reconstruction software that have been made in the past years, will lead to significantly better performance, in particular an improvedpT resolution. Better alignment of the ITS with respect to the TPC also improves thepT resolution.
If hits in the TRD are included in the track fitting procedure the pT resolution will benefit from the larger lever arm. The improvement will depend on the space point resolution in the
172 7. Outlook
TRD and the alignment with respect to the TPC. The fact that the TRD does not have full2π azimuthal coverage to date does not represent a major problem for a single particle analysis.
However, the verification of the pT resolution estimate from the track covariance matrix (see section 3.7.4) remains an issue. The cross check done so far was based on the invariant mass peaks of 2-particle-decays ofK0S and Λ, but this method can not be extended to the highest pT. A trigger for cosmic muons crossing the TPC and ITS, running during ordinary data taking would be the optimal solution in the future. The difference of the pT measured in the upper and lower half of the detector for a cosmic particle is a direct measurement of thepTresolution.
This method has already been applied for tracks in the TPC, where the long integration time allows to use cosmic tracks that are unavoidably part of the collision events.
Inclusion of identified particle measurements
The analysis of charged particle spectra presented in this thesis was completed before several measurements of identified particle spectra, in particular charged pions, proton, kaons [161, 215,216] and (multi-)strange baryons [245–247] were available. Incorporating this additional knowledge into the charged particle analysis helps two improve its precision.
The dominant effects depending on the particle composition are efficiency and secondary cor-rections, and, in the case of p–Pb, also acceptance corrections. Preferably the inclusion of the measurements is achieved via tuning of Monte Carlo generators, as this would directly decrease discrepancies between data and simulation. As a workaround, correction factors for efficiency, contamination and acceptance, that are obtained from MC simulations, can be rescaled properly taking into account measured particle compositions. However, the usage of such effective corrections is not feasible for application to all MC-related quantities. Not only the corrections, but all systematics (for example cut variations or matching efficiency) depend on the primary and secondary particle composition. Including effective corrections to all of them would inflate the number of such effective corrections.
In the analysis of the 2013 p–Pb data, preliminary measurements of π, K, p transverse mo-mentum spectra have already been taken into account for the efficiency and acceptance cor-rections (see section 3.7.1). The analysis of pp and Pb–Pb collisions include a scaling factor for the contamination from secondaries to account for the strangeness production, which is underestimated in the MC generators (see section 3.7.2).
Track cut modifications
A modification of the track selection criteria could help to reduce systematic uncertainties and overcome shortcomings of the Monte Carlo description of the detector.
In particular, a cut on the track length in the active area of the TPC has been recently proposed [248] and is currently under investigation. This cut is designed to remove tracks which cross TPC sector boundaries under small angles resulting in a large fraction of the track located either in the inactive zone between two sectors or in the vicinity of such a sector boundary, where the performance of the TPC is worst. Since the cut variable itself is defined purely by geometry, the description in the simulations is much better compared to cuts which involve also quantities depending on physical processes like energy loss of the particle or detector response.
7.4. Possibilities for smaller systematic uncertainties 173
Another possible improvement could come from a moderate cut on the number of clusters used for tracking, to remove outliers tracks with have an exceptional low number of clusters assigned. Recall that the pT resolution depends also onpncl. A more restrictive cut on the rationrows/nfindable has a similar effect and could be used in addition.
A major part of the track cut systematic uncertainties is due to the rather large cut on the number of crossed rows in the TPC. In the combination with the cut on the active length, the requirement of a large number of crossed rows could be relaxed to reduce systematic uncertainties at intermediate pT. With a pT dependent cut on nrows it is possible to keep the requirement of long tracks at highpT where the momentum resolution is crucial, but improve at low and intermediatepT. Especially at the lowestpT<200 MeV/cthe current cut is on the edge of the kinematic limit, a looser cut would increase the efficiency to a level comparable to that atpT>200 MeV/c.
MC simulations
Improvements in the simulations would also help to reduce systematic uncertainties. In par-ticular the use of improved Monte Carlo generators, that are tuned to reproduce the available measurements, and a more accurate description of detector geometry and response.
Better tuned or different generators need to reproduce the correct composition of primary particles, which are only poorly described by the generators that were used. In particular sim-ulations with a proper baryons/meson ratio and correct production of strangeness will improve the corrections for efficiency and contamination and reduce the discrepancy in the TPC–ITS track matching between simulation and data. The re-weighting procedures for efficiency and contamination would become obsolete.
A better description of the detector geometry, (mis-)alignment, material budget and response (for example the error parameterization of space point resolution) will also result in smaller systematic uncertainties.
Most simulations currently available suffer from low statistics at large pT, especially when obtained differentially also in other variables like pseudorapidity, multiplicity andz-position of the primary vertex. In this case the large statistical uncertainties of the efficiency mask potential systematic effects. This strongly affects the corresponding corrections at high pT, which is averaged over a rather wide interval of pT. For a reliable extraction of correction factors the available statistics of simulated events should be at least as large as in data, in order not to have uncertainties dominated by MC statistics. Another possible solution to this are simulations which have an enhanced contribution at highpT.
Efficiency
The efficiency is the main correction and has a large systematic uncertainty assigned. Improve-ments of the efficiency correction are possible from better detector calibration and improved MC simulations that have the correct particle composition. The reweighting of the efficiencies as described above is also an alternative. This reweighting could in principle also be applied to improve the agreement between data and simulation in the TPC-ITS track matching efficiency which is used to estimate systematic uncertainties on the the tracking efficiency.
174 7. Outlook
Another inaccuracy comes from the efficiencies of strange baryons (Σ±, Ξ−, Ω−) which are non-zero at largepT. For these particles apTdependence of the efficiency and particle fractions extended to largerpT could improve the accuracy especially at large pT.
Generally the description of efficiency at high pT would benefit from larger MC statistics or dedicated high pT enhanced productions. As high pT particles are mostly embedded in a jet, the influence of the local track density on the efficiency needs to be considered.
Contamination
Secondary particles mostly affect the spectrum at the lowestpT. An improved MC description of strangeness would render the effective correction applied to scale the secondaries unneces-sary. This helps to reduce systematic uncertainties at lowpT.
Correlated uncertainties
Another potential improvement of the measurement could come from separation of systematic uncertainties that are correlated between the pT bins. These could be incorporated into the normalization uncertainty and improve the accuracy in the measurement of thepTshape.
Even more benefits will arise from the study of systematic uncertainties correlated between different systems (i.e. pp and p–Pb as well as pp and Pb–Pb), as they partially cancel in the ratiosRAAandRpPb. Similar cancellations have already been exploited for the ratios of spectra at different energies in pp and ratios at different pseudorapidity in p–Pb.