PDF uncertainties and the W mass: Report from the Workshop “Parton Distributions for the LHC”

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PDF uncertainties and the W mass!
Report from the Workshop “Parton Distributions for the LHC”!
Benasque Center for Science Pedro Pascual, 15-21 Feb 2015
Juan Rojo!
STFC Rutherford Fellow!
Rudolf Peierls Center for Theoretical Physics!
University of Oxford!
!
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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About the Benasque Workshop!
The goal of the workshop was to provide an overview of the field of PDF determinations at the dawn of
the LHC Run II data taking, from the point of view of experimental data, theoretical developments, tools
for PDF fitting and methodological procedures!
Talks were be 20+20 minutes, thus allowing ample time for discussions afterwards!
The agenda was kept deliberately light to allow for free discussions and collaborative work between the
participants of the workshop!
In addition, we had several discussion sessions during the workshop. !
Theoretical uncertainties on PDFs (Wednesday, lead by Pavel)!
PDF systematics in electroweak precision observables (Friday, lead by Stefano Camarda)!
Recap of the workshop and plans for Run II (Friday)!
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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Workshop Program
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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Workshop Program
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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Workshop Program
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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PDF systematics in the W mass measurement
!
A number of recent studies have focused on the PDF systematics for the W mass measurement:!
“PDF uncertainties on the W boson mass measurement from the lepton transverse momentum distribution”, Bozzi,
Citelli and Vicini, arxiv:1501.05587, updating the previous study, “The Impact of PDF uncertainties on the
measurement of the W boson mass at the Tevatron and the LHC” by Bozzi, JR and Vicini, arXiv:1104.2056 !
ATLAS Collaboration, “Studies of theoretical uncertainties on the measurement of the mass of the W boson at the
LHC”, ATL-PHYS-PUB-2014-015!
“Parton distributions and the W mass measurement” , Quackenbush and Sullivan, arXiv:1502.04671!
!
Full summary beyond the scope of this talk, restrict to highlight some topics of the Benasque discussion!
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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Issues with the modelling of Z pt!
One of the main concerns in the discussions was the difficulty in reproducing the Z pt spectrum with the
required accuracy relevant for the W mass measurement!
No calculation seems to do a perfect job, but resumed calculations and tailored Monte Carlo shower tunes
seems to perform best. Can we do better?!
What about the NNLO calculation? Do we need it in the context of the W mass measurement?!
Can we reduce modelling uncertainties by including directly the Z pt distribution into PDF fits, using for
instance aMC@NLO+aMCfast? What about the reweighing of the PDF in the shower?!
Will be interesting to compare as well with the upcoming CMS 8 TeV measurement!
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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Consistency between PDF sets!
The PDF uncertainty from different sets can be kept at the 10 MeV level if an additional acceptance cut of
ptW < 15 GeV is imposed, but larger spread between different groups!
Why CT10 larger PDF errors than other groups? What about CT14?!
Can we understand differences between sets? Ongoing benchmark in the context of an updated PDF4LHC
recommendation might shed some light!
What is the best strategy to reduce PDF uncertainties in this measurement?!
!
ptW < 15 GeVno ptW cut
Template fit to ptlepton
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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Consistency between PDF sets
Juan Rojo Parton Distributions for the LHC, Benasque Center for Science, 16/02/2015
!
The PDF uncertainty from different sets can be kept at the 10 MeV level if an additional acceptance cut of
ptW < 15 GeV is imposed, but larger spread between different groups!
This spread is also reproduced in the ATLAS analysis!
For average W, the spread is substantially reduced when going from a HERA-only fit (first column) to a
global fits: spread and PDF uncertainties should be reduced when more data included!!
!
HERA-only

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Reducing PDF uncertainties
Juan Rojo Parton Distributions for the LHC, Benasque Center for Science, 16/02/2015
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The ideal measurements to reduce PDF uncertainties for the W mass measurement are those which cover
similar kinematics and PDF decomposition: inclusive W and Z production and asymmetries, low and high
mass Drell-Yan, and W and Z pt measurement!
We seem to go in the right direction: NNPDF3.0 much more EWK LHC data than NNPDF2.3, and!
!
Available and upcoming measurements that should be included in PDF fits asap include the ATLAS 7 TeV
5 1/fb W,Z distributions, the CMS 8 TeV Drell-Yan, and the latest LHCb Z and W rapidity distributions!

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Other topics for discussion in Benasque
Stefano Camarda
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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Other topics for discussion in Benasque
Agreed that PDF fitters should provide their latest sets in a range of mcharm, bottom values to estimate
the combined PDF+mc/mb uncertainty!
Addressing the accuracy of NLO+PS reweighting is crucial for W mass measurement studies!
!
Strangeness has an important contribution to the total PDF
errors in the W mass!
Reduce with LHC data on inclusive W,Z and W+charm.
Interplay with low energy neutrino data?!
!
Juan Rojo LPCC Electroweak WG, MW topical meeting, CERN, 23/02/2015

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Detector effects
Juan Rojo Parton Distributions for the LHC, Benasque Center for Science, 16/02/2015
The smearing of the lepton transverse momentum seems to be the least of our worries!
The crucial issue is how well we can model production, from there onwards everything seems to be under
control!
!
Mandy Cooper-Sarkar

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Summary and outlook
Juan Rojo Parton Distributions for the LHC, Benasque Center for Science, 16/02/2015
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The contribution to PDF uncertainties to the W mass determination is one of the dominant sources of
theoretical uncertainties!
The most promising approach seems to be to include as much LHC data as possible in the global PDF
fits, in particular for processes like inclusive W, Z, Drell-Yan, W+charm and Z pt which cover similar range
and flavour decomposition!
PDFs also enter in the modelling of the Z pt via NLO+PS event generators: here tailored tunes or the
inclusion of the Z pt directly into PDF fits might be good solutions. So far, data more accurate that any
theoretical calculation ….!
From our side, PDF fitters need to work not only on reducing the intrinsic PDF uncertainties, but also
understanding the spread of central values between different groups
We are confident of keep doing steady progress here!