studies of pdf uncertainties for the measurement of the ... · october 21, 2014 stefano camarda 2....

Studies of PDF uncertainties for the measurementof the mass of the W boson at the LHC

Stefano Camarda (DESY)

October 21, 2014

October 21, 2014 Stefano Camarda 1

PDF uncertainties on mW

ATL-PHYS-PUB-2014-015“Studies of theoretical uncertainties for the measurement ofthe mass of the W boson at the LHC”

IntroductionTheory predictions and PDF uncertaintiesEvent selection and methodologyW polarization and PDF uncertaintiesCharm-initiated W production and PDF uncertaintiesDetector effects and summary of PDF uncertaintiesParton shower uncertaintiesConclusions

Points of discussion for a coherent treatment of PDFuncertainties between ATLAS and CMS


https://cds.cern.ch/record/1956455

Introduction

The extraction of mW from the p`T spectrum, is likely to belimited by theoretical uncertainties.

This study addresses PDF uncertainties, and non-perturbativeQCD uncertainties related to the parton shower model

We need not only a precise estimation of PDF and othertheoretical uncertainties, but also a roadmap to control andreduce them, by mean of precise experimental measurementsof alternative observables

The idea is to perform a breakdown of the physicalmechanisms behind the PDF uncertainties, and estimatewhich are the most relevant sources of uncertainties.

By pointing out the largest uncertainties, the idea is toprovide a pattern to reduce them, rather than a preciseestimation to be used in our measurement


Introduction

PDF uncertainties for the extraction MW from p`T at theTevatron are 9 MeV (CDF) and 11 MeV (D0)

It has been suggested in Eur. Phys. J. C 69 (2010) 379397[Krasny, Dydak, Fayette, Placzek, Siodmok, ’10] that PDFuncertainties in proton-proton collisions could be larger thanin proton-antiproton collisions for

1st quark generation effect: u and d PDF uncertainties on Wboson polarisation2nd quark generation effect: strange-quark PDF uncertaintyon charm-initiated W boson production3rd quark generation effect: bottom quark mass uncertainty inthe extraction of non pertubative parameters from pZT

The idea of this study is to estimate such effects withstandard tools for theory predictions and Monte Carlo, andget feedback from the theory community

Are we missing something important?Do we need better, additional theoretical predictions toestimate these effects?


http://arxiv.org/abs/1004.2597


Introduction - disclaimer

The emphasis is on tracking the physical sources of theuncertainties, we are not estimating the uncertainties to beused in the measurementThis is not the ATLAS final word on PDF and PSuncertainties for the W mass, and on theory uncertaintiesWe are considering in this study:

u and d valence and sea PDF uncertaintiesStrange PDF uncertaintiesPS uncertainties, assuming they can be extrapolated from themeasurement of pZT to the modelling of pWTDetector effects on the muon momentum resolution

We are not considering in this study:Gluon PDF uncertainties in all-order resummation (gluon PDFis varied only at NLO)Heavy flavour masses in the matrix-element calculationsDifferences in the heavy flavour content of W and Zproduction when propagating PS uncertainties from pZT to pWTAny QED FSR, and NLO EW effectDetector effects on the measurement of the hadronic recoil


Theory predictions

EW schemeGµ-scheme: with GF , MZ , MW as inputs from PDG 2012, αand θW calculated at tree levelΓZ and ΓW measured value from PDG 2012CKM from PDG 2012, but Vtx = 0, no top in the initial state

Theory predictions and toolsMCFM

W+j production at LO O(αs), which is the real part of Winclusive calculation at NLOfinite width, leptonic decay, spin correlations

CuTe

differential W pT at NNLL with matching corrections atO(αs) (NLO+NNLL)zero width approximationno decay of the W

APPLGRID: Fast PDF convolution

Need to combine the two codes, MCFM and CuTe, to get arealistic prediction of the lepton pT spectrum


CuTe

Infrared Safety from the Collinear Anomaly [Becher, Neubert,Wilhelm ’11]

The factorisation scale is set to µ = q∗ + qT , withq∗ ∼ 1.82 GeV

The non-perturbative scale q∗ ∼ e−C/αs(mV ) protects theprocesses from receiving large long-distance hadroniccontributions

Allows to calculate the derivative of pW ,ZT for pT → 0 with

perturbative QCD

One additional non perturbative parameter ΛNP = 0.6 GeVintroduce a gaussian smearing for hadronic non-pQCD effects

Public C++ code, very fast




Theory predictions - benchmark

0 20 40

[p

b/G

eV

] +

W T/d

pσ

d

10

210

310

410

510

610 = 7 TeV s; +

W→pp

NLO (fixed order)CuTe CT10nloMCFM CT10nlo

[GeV] +

W

T p

0 20 40

Ra

tio

0.98

1

1.02 0 20 40

[p

b/G

eV

]

W T/d

pσ

d

10

210

310

410

510

610 = 7 TeV s;

W→pp

NLO (fixed order)CuTe CT10nloMCFM CT10nlo

[GeV]

W

T p

0 20 40

Ra

tio

0.98

1

1.02

Perfect agreement between the two codes, at fixed order O(αs),zero width, no decay.


Theory predictions - combination

Reweighting function defined as r(pT ) = NLO+NNLLNLO

The reweighing is applied, in the range 0.1 < qT < 150 GeV,outside this range the weight is set to 0

[GeV]+

W

Tp

0 20 40 60 80 100 120 140

[p

b/G

eV

] T

/dp

σd

10

210

310

410

510

+ W→CuTe; pp

NLO (fixed order)NLO+NNLL

[GeV]

W

Tp

0 20 40 60 80 100 120 140

[p

b/G

eV

] T

/dp

σd

10

210

310

410

510

W→CuTe; pp

NLO (fixed order)NLO+NNLL

[GeV]+

W

Tp

0 20 40 60 80 100 120 140

Ra

tio

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 + W→CuTe; pp

NLONLO+NNLL

[GeV]

W

Tp

0 20 40 60 80 100 120 140

Ra

tio

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

W→CuTe; pp

NLONLO+NNLL


CKM decomposition of the reweighting function

The NLO+NNLL/NLO ratio has a significant dependence onthe flavour of the quarks initiating the W -boson productionprocess: heavy quarks result in a harder pWT spectrum and aharder ratio between resummed and fixed order predictionsThe reweighting function is decomposed in terms of the CKMmatrix: 6× 2 functions are the NLO+NNLL/NLO ratiosevaluated by setting to 0 all the CKM matrix except the Vxy

term, separately for W+ and W−

[GeV]+

W

Tp

0 20 40 60 80 100 120 140

Rew

eig

hting function

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

+ W→CuTe; pp

udV

usV

ubV

cdV

csV

cbV

[GeV]

W

Tp

0 20 40 60 80 100 120 140

Rew

eig

hting function

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

W→CuTe; pp

udV

usV

ubV

cdV

csV

cbV


Theory predictions - benchmark

0 20 40

[p

b/G

eV

] +

W T/d

pσ

d

210

310

410 = 7 TeV s;

+ W→pp

NLO+NNLL

CuTeMCFM+CuTe

[GeV] +

W

T p

0 20 40

Ra

tio

0.95

1

1.05 0 20 40

[p

b/G

eV

]

W T/d

pσ

d

210

310

410 = 7 TeV s;

W→pp

NLO+NNLL

CuTeMCFM+CuTe

[GeV]

W

T p

0 20 40

Ra

tio

0.95

1

1.05

Good agreement at NLO+NNLL after reweighting.PDF uncertainties reproduced at high pT , at low pT CuTe haslarger PDF uncertainties.In CuTe the factorisation scale is set to µ = q∗ + qT , withq∗ ∼ 1.82 GeV, → CuTe is sensitive to larger PDF uncertaintiesassociated with the low x region. In MCFM prediction, thefactorisation scale is set to 80 GeV.


Dedicated PDF set

A dedicated PDF set has been produced to study the PDFuncertainties for MW extraction from plTSimple setup which allows breakdown of uncertainties, notintended for final estimate of PDF uncertainties

NLO Fit to HERA I data

Starting scale Q20 = 1.7 GeV2

charm mass mc = 1.38 GeV

bottom mass mb = 4.75 GeV

top mass mt = 3.5 TeV → 5 flavour

strange fraction rs = s/d = 1

13p parametrisation

26 hessian variations

4 model variations: mc = 1.32, 1.44, rs = 0.72, 1.25

Total of 30 variations


PDF at the starting scale

x 410

310 210 110 1

)2

(x,Q

V x

u

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2

= 1.7 GeV2QMWNLO exp+modMWNLO exp

x 410

310 210 110 1

)2

(x,Q

V x

d

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

2


x 410

310 210 110 1

)2

xg

(x,Q

1

0.5

0

0.5

1

1.5

2

2.5

3

2


x 410

310 210 110 1

)2

(x,Q

Σ x

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

2


x 410

310 210 110 1

)2

(x,Q

u x

0

0.1

0.2

0.3

0.4

0.5

2


x 410

310 210 110 1

)2

(x,Q

d x

0

0.1

0.2

0.3

0.4

0.5

2


x 410

310 210 110 1

)2

xs(x

,Q

0

0.1

0.2

0.3

0.4

0.5

0.6

2


x 410

310 210 110 1

)2

)(x,Q

d+

u)/

(s

x(s

+

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

2 = 1.7 GeV2Q

MWNLO exp+modMWNLO exp

Blue band: experimental (hessian) uncertainties

Red band: experimental uncertainties plus model variations, rsand mc


PDF uncertainties at the scale of MW

x 410

310 210 110 1

ref

)2

(x,Q

V)/

xu

2(x

,QV

xu

0.8

1

1.2

2 = 6464 GeV2Q


x 410

310 210 110 1

ref

)2

(x,Q

V)/

xd

2(x

,QV

xd

0.8

1

1.2

2 = 6464 GeV2Q


x 410

310 210 110 1

ref

)2

)/xg

(x,Q

2 x

g(x

,Q

0.8

1

1.2

2 = 6464 GeV2Q


x 410

310 210 110 1

ref

)2

(x,Q

Σ)/

x2

(x,Q

Σ x

0.8

1

1.2

2 = 6464 GeV2Q


x 410

310 210 110 1

ref

)2

(x,Q

u)/

x2

(x,Q

u x

0.8

1

1.2

2 = 6464 GeV2Q


x 410

310 210 110 1

ref

)2

(x,Q

d)/

x2

(x,Q

d x

0.8

1

1.2

2 = 6464 GeV2Q


x 410

310 210 110 1

ref

)2

)/xs(x

,Q2

xs(x

,Q

0.8

1

1.2

2 = 6464 GeV2Q


x 410

310 210 110 1

ref

)2

)(x,Q

d+

u)/

(s

)/x(s

+2

)(x,Q

d+

u)/

(s

x(s

+

0.8

1

1.2

2 = 6464 GeV2Q


Valence PDF dominated by experimental uncertainties fromthe HERA I data

u, d and strange PDF dominated by model variations, rs andmc


Methodology and event selection

Event selection|ηl | < 2.4pνT > 30 GeVMT > 60 GeV30 < plT < 50 GeV

Generate a test sample with MW = 80.385Estimate only statistical experimental uncertainty in 5 fb−1,assuming no efficiency lossConsider normalised plT distribution, in bins of 0.5 GeVCalculate a χ2 profile as a function of MW , in the region±100 MeV (80.286 < MW < 80.484), in steps of 2 MeVFit the χ2 profile with a parabolic function to find minimumand sigma at ∆χ2 = 1Compute the minimum for each PDF variation of a given PDFset, calculate PDF uncertainty as the difference between theminimum for the central PDF and each PDF variationRepeat the analysis for W+ only, W− only and both W+ andW−


Methodology and event selection - Example of χ2 profile

[MeV]WM80300 80350 80400 80450 80500

2χ

200

0

200

400

600

800

+W

W

and W

+W

5.0±80385.2 5.8±80385.1

3.8±80385.2

The procedure gives us a rough estimate of the statisticaluncertainty expected from the 2011 data set at 7 TeV.Detector resolution effect are not yet included in this plot

The statistic of the MC sample is much higher, the statisticalbias on the central value of MW = 80.385 is below 0.2 MeV


W polarization and PDF uncertainties

The production of W by u and d quarks at LO, at a given rapidityy can be decomposed in two terms corresponding to λ = +1,−1helicity states:

σW+(y) ∝ u(x1) · d(x2) + d(x1) · u(x2) (1)

σW−(y) ∝ d(x1) · u(x2) + u(x1) · d(x2) (2)

x1,2 = MW√s· e±y

At central rapidity y = 0, x1 = x2, the two terms are equal, →unpolarised W

For y 6= 0, the two terms are different, the W is polarised onaverage

The uncertainty in the u and d valence and sea PDF determinesan uncertainty in the average polarisation of the W , which in turnspropagates into an uncertainty on the plT spectrum.



•

λ = −1

zlab

`+

(WRF)

ν(WRF)

θ∗

•λ = 0

zlab

`+

(WRF)

ν(WRF)

θ∗

•

λ = +1

zlab

`+

(WRF)

ν(WRF)

θ∗

1

)*

θcos(

-1 -0.8-0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 10

0.5

1

1.5

2

2.5

3

3.5

4=-1λ=+1λ

=0λ=-1,0,+1λ=-1,1λ

The λ = +1,−1, and 0 helicity states correspond to differentdistribution of the polar angle θ∗ between the direction of themomentum of the incoming quark in the laboratory frame,assumed to be parallel to the beam axis, and the direction of theleptons momenta in the W -boson rest frame



The uncertainty on the u and d valence and sea PDFtranslates into an uncertainty in the average W polarisation,which affects the plT distribution

Simple setup to disentagle the effect of W polarisation

Keep only Vud 6= 0 and set all the other terms of the CKMmatrix to 0

Apply a random rotation to the decay angle of the leptons inthe W rest frame → No spin correlations

Apply a sign flip to the lepton momentum in W rest frame →λ = ±1 symmetric

Switch on spin correlations and compare PDF uncertainties

Thanks to Paolo Nason for the idea of randomising the leptonmomenta to switch off spin correlations



*) θ cos(1 0.5 0 0.5 1

*) [

pb

]θ

/dco

s(

σ d

1000

2000

3000

4000

5000

6000

7000

= 7 TeV s; +

W→pp

MCFM+CuTe

No spin correlations

1 symmetric± = λ

Spin correlations

*) θ cos(1 0.5 0 0.5 1

*) [

pb

]θ

/dco

s(

σ d

500

1000

1500

2000

2500

3000

3500

4000 = 7 TeV s;

W→pp

MCFM+CuTe


1 symmetric± = λ

Spin correlations

cos θ∗ distributions in the 3 samples

The dashed bands show PDF uncertainties, only in the samplewith spin correlations the PDF uncertainties affect theaverage W polarisation



30 35 40 45 50

[p

b /

Ge

V]

l T/d

pσ

d⋅ σ

1/

210

110 = 7 TeV s;

+ W→pp

Spin correlationsNo spin correlations

[GeV/c] l

T p

30 35 40 45 50

Ra

tio

0.995

1

1.005 30 35 40 45 50

[p

b /

Ge

V]

l T/d

pσ

d⋅ σ

1/

210

110 = 7 TeV s;

W→pp

Spin correlationsNo spin correlations

[GeV/c] l

T p

30 35 40 45 50

Ra

tio

0.995

1

1.005

Dramatic shrink of the PDF uncertainties on p`T distributionwhen spin correlations are switched off



PDF member

1 6 11

16 21

26

[M

eV

]W

m

80360

80370

80380

80390

80400

80384.8 +3.1 3.0

80386.2 +3.1 3.3

80385 +17 21

No spin correlations+

W

1 symmetric± = λ +

W

Spin correlations+

W

PDF member

1 6 11

16 21

26

[M

eV

]W

m

80365

80370

80375

80380

80385

80390

80395

80400

80405

80384.5 +5.6 5.5

80386.4 +6.8 6.1

80385 +28 27


W

1 symmetric± = λ

W

Spin correlations

W

Sets 1-26, hessian variations

Set 27 mc = 1.32, Set 28 mc = 1.44

Set 29 rs = 0.72, Set 30 rs = 1.25

∼ 20 MeV effect in W+, mostly due rs variations

∼ 25 MeV effect in W−, spread across eigenvector variations



PDF member

1 6 11

16 21

26

[M

eV

]W

m

80365

80370

80375

80380

80385

80390

80395

80400

80384.7 +3.4 3.5

80386.3 +3.8 3.7

80385 +15 17

No spin correlations±

W

1 symmetric± = λ ±

W

Spin correlations±

W

Since PDF variations are different between W+ and W−,when W+ and W− spectra are used simultaneously, theuncertainty is reduced to ∼ 15 MeV



Why rs variations affects the W -boson polarisation, when onlyu- and d- initiated W production is considered?

x 410

310 210 110 1

ref

)2

)(x

,Qd /

V)/

x(d

2)(

x,Q

d/V

x(d

0.8

1

1.2

2 = 6464 GeV2QMWNLO exp+modMWNLO exp

varsMWNLO r

DIS data constrains the sum of d and strange PDFIn the medium and high x region, variations of the ratio of thestrange-quark PDF over the d PDF, corresponding tors = 0.72 and rs = 1.25, give a significant contribution to thetotal PDF uncertainty of the dV /d ratio.


Charm quark in the initial state and strange PDF

A charm in the initial state is expected to alter the kinematicof the W productions

Roughly, a kick of the order of mc = 1.4 GeV in the pWTspectrum is expected

Randomise decay angle of the leptons in the W rest frame →unpolarised W

Switch between a setup with only Vud term in the CKMmatrix, and a setup with Vud and Vcs terms

However, the amount of charm initiated W production willalso alter the balance between valence quark initiated and seaquark initiated production, which in turns affect the Wpolarisation and the W plT spectrum


Charm mass effect

0 20 40

] 1

[G

eV

+W T

/dp

σ d⋅

σ 1

/

310

210

110 = 7 TeV s; +

W→pp

CuTe NLO+NNLL

udV

csV

cs and VudV

[GeV] +

W

T p

0 20 40

Ra

tio

0.60.8

11.2 0 20 40

] 1

[G

eV

W T

/dp

σ d⋅

σ 1

/

310

210

110 = 7 TeV s;

W→pp

CuTe NLO+NNLL

udV

csV

cs and VudV

[GeV]

W

T p

0 20 40

Ra

tio

0.60.8

11.2

As expected, the pWT spectrum acquires a kink fromcharm-initiated production

Notice that the charm is treated as massless in the MEcalculation, the effect of the charm mass we see is encoded inthe PDF, as a threshold between 3 and 4 flavours



30 35 40 45 50

[p

b / G

eV

] l T

/dp

σ d⋅

σ 1

/

210

110 = 7 TeV s;

+ W→pp

cs and VudV

udV

[GeV/c] l

T p

30 35 40 45 50

Ra

tio

0.995

1

1.005 30 35 40 45 50

[p

b / G

eV

] l T

/dp

σ d⋅

σ 1

/

210

110 = 7 TeV s;

W→pp

cs and VudV

udV

[GeV/c] l

T p

30 35 40 45 50

Ra

tio

0.995

1

1.005

Significant shrink of the PDF uncertainties on p`T distributionwhen charm-initiated W production is switched off



PDF member

1 6 11

16 21

26

[M

eV

]W

m

80376

80378

80380

80382

80384

80386

80388

80390

80392

80394

80384.8 +3.1 3.0

80384.9 +8.4 6.9

ud V+

W

cs and Vud V+

W

PDF member

1 6 11

16 21

26

[M

eV

]W

m

80375

80380

80385

80390

80395

80384.5 +5.6 5.5

80384.6 +11.6 9.5

ud V

W

cs and Vud V

W

Sets 1-26, hessian variations

Set 27 mc = 1.32, Set 28 mc = 1.44

Set 29 rs = 0.72, Set 30 rs = 1.25

∼ 5 MeV effect in W+ and W− due to rs variations



PDF member

1 6 11

16 21

26

[M

eV

]W

m

80375

80380

80385

80390

80395

80384.7 +3.4 3.5

80384.8 +9.4 7.7ud V

pmW

cs and Vud V±

W

Since the rs variations are correlated between W+ and W−,there is no significant reduction of the uncertainty when W+

and W− p`T spectra are used simultaneously


Total PDF uncertainties

Consider altogether polarisation and charm-initiated effects,by using normal spin correlations and SM CKM matrix

Observed a partial cancellation of the two effects

PDF member

1 6 11

16 21

26

[M

eV

]W

m

80375

80380

80385

80390

80395

80385 +12 11

80385 +20 19

80385 +11 10

+W

W

and W

+W


Detector effects - Muon pT smearing

Implemented detector pT smearing for muons fromarXiv:1404.4562, to asses the impact of detector smearing on PDFuncertainties



Detector effects - Muon pT smearing

[MeV]Wm80300 80350 80400 80450 80500

2χ

0

100

200

300

400

+W

with detector effects+

W

5.0±80385.2

5.7±80384.9

PDF member

1 6 11

16 21

26

[M

eV

]W

m

80375

80380

80385

80390

80395

80385 +12 11

80385 +13 12

+W

with detector effects+

W

[MeV]Wm80300 80350 80400 80450 80500

2χ

50

0

50

100

150

200

250

300

350

W

with detector effects

W

5.8±80385.1

6.5±80385.2

PDF member

1 6 11

16 21

26

[M

eV

]W

m

80375

80380

80385

80390

80395

80400

80385 +20 19

80385 +22 21

W

with detector effects

W

Small effect, increase of the PDF uncertainties by ∼ 10%

However, detector effects on the hadronic recoil are notconsidered


Summary of PDF uncertainties

MW-NLO CT10nlo MSTW2008CPdeutnlo NNPDF30 nlo as 118

W+ +13 -12 +18 -22 +11 -10 +8 -10

W− +22 -22 +18 -23 +11 -10 +8 -9

W± +11 -11 +14 -18 +7 -7 +6 -5

CT10nlo scaled to 68 cl

The dedicate PDF set used for the study has a reasonableuncertainty compared to the global PDF set

The larger W− uncertainty is due to missing constraints fromadditional dataset like Tevatron (and LHC) W asymmetry


Summary of PDF uncertainties

Difference between the input value of mW = 80385 MeV and theextracted value of mW , when using CT10nlo as PDF for thereference p`T spectrum, and another PDF for the test spectra withvarious values of mW

MW-NLO CT10nlo MSTW2008CPdeutnlo NNPDF30 nlo as 118

W+ -9 -0.1 -20 -1.2

W− +48 +0.2 +13 +12

W± +16 0.0 -6 +5

Large, unrealistic shift for the dedicated PDF set MW-NLOfor W− production

Known issue related to the limited number of parameters (13parameters), and not enough constraints from HERA data onthe d valence PDF


Low pWT modelling uncertainty

Exploit the pZT data to constrain the non perturbative QCDparameters at low pT of Pythia8: Tune AZ

Propagate the data uncertainty to the pWT by mean ofeigentunes hessian uncertainties

[GeV] ZT

p1 10 210

Pre

dict

ion/

Dat

a

0.8

0.9

1

1.1

Data uncertainty

PYTHIA8 4C

PYTHIA8 AZ

ATLAS

-1 Ldt = 4.7 fb∫ = 7 TeV; s

[GeV] ZT

p1 10 210

Pre

dict

ion/

Dat

a

0.8

0.9

1

1.1

Data uncertainty

POWHEG+PYTHIA8 4C

POWHEG+PYTHIA8 AZNLO

ATLAS

-1 Ldt = 4.7 fb∫ = 7 TeV; s

Tune AZ AZNLO 4C

Primordial kT [GeV] 1.71 ± 0.03 1.75 ± 0.03 2.0

αISRs (mZ ) 0.1237 ± 0.0002 0.118 (fixed) 0.137

ISR cut-off [GeV] 0.59 ± 0.08 1.92 ± 0.12 2.0

χ2min/dof 45.4/32 46.0/33 -



[GeV]µ

Tp

30 32 34 36 38 40 42 44 46 48 50

Rat

io to

AZ

0.995

0.996

0.997

0.998

0.999

1

1.001

1.002

1.003

1.004

1.005Central tune

Variation 1+

Variation 2+

Variation 3+

[GeV]µ

Tp

30 32 34 36 38 40 42 44 46 48 50

Rat

io to

AZ

0.995

0.996

0.997

0.998

0.999

1

1.001

1.002

1.003

1.004

1.005Central tune

Variation 1-

Variation 2-

Variation 3-

Tune Variation Positive Negative

1± 3 −42± 3 −43± 3 −4

Total 5 7



[GeV]+

W

Tp

0 10 20 30 40 50

]1

[G

eV

T/d

pσ

d⋅ σ

1/

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07CuTe NLO+NNLL

+ W→pp

udV

usV

ubV

cdV

csV

cbV

[GeV]

W

Tp

0 10 20 30 40 50

]1

[G

eV

T/d

pσ

d⋅ σ

1/

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07CuTe NLO+NNLL

W→pp

udV

usV

ubV

cdV

csV

cbV

[GeV]Z

Tp

0 10 20 30 40 50

]1

[G

eV

T/d

pσ

d⋅ σ

1/

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07 CuTe NLO+NNLL Z→pp

uu

ddss

cc

bb

Heavy flavour production results in different distorsions of theW and Z pT spectrum

Differences in the heavy flavour content of W and Zproduction when propagating PS uncertainties from pZT to pWTare not accounted


Summary and conclusions

Performed a breakdown of most important PDF uncertaintiesfor the MW extraction from p`T , by tracking the physicalmechanism which give rise to PDF uncertainties

Large contribution of polarisation, of the order of 10− 20MeV, can be improved by better knowledge of u, d valenceand sea PDF

Strange-quark PDF uncertainty due to charm-initiated Wproduction is at the level of 5 MeV, but partially cancel theeffect of strange-quark PDF on polarisation

Preliminary and partial estimation of parton showeruncertainties at the level of 5− 7 MeV


POINTS FOR DISCUSSION

With inputs from Luca Perozzi and Maria Rosaria D’Alfonso


Coherent treatment of PDF uncertainties between ATLASand CMS

Various theory predictions can be used to evaluate PDFuncertainties: MCFM, RESBOS, DYRES, POWHEG,[email protected] we expect such tools to provide the same estimation ofPDF uncertainties? Should we agree on which prediction(s)can be safely used?

Various PDF set are available on the market, at least CT10,MSTW, NNPDF, but also ABM, JR, HERAPDF, and recentlyalso ATLAS and CMS PDF sets. Do we want to agree on acommon PDF set? We should at least be sure that ATLASand CMS results can be combined accounting for thecorrelation of PDF uncertainties

Prescription for evaluating PDF uncertainties: single set?envelope as in PDF4LHC prescription? META-PDF?


How to reduce PDF uncertainties for mW

Is the measurement of mW at different collider energies, 7, 8,13 TeV, beneficial for the reduction of PDF uncertainties?

Which LHC measurements can further constrain the PDFuncertainties for mW ?


BACKUP


Choice of ptsqmin 4.0 in POWHEG+PYTHIA8

ptsqmin 0.2ptsqmin 0.8ptsqmin 2.0

0 1 2 3 4 50

10

20

30

40

50

60

70

80

PZ

T [GeV]

dσfid./dPZ T[pb/GeV

]


studies of pdf uncertainties for the measurement of the ... · october 21, 2014 stefano camarda 2....

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