Synergy of studies of

unpolarized (sea) quark

PDFs

Pavel Nadolsky

With contributions from CTEQ-TEA: S. Dulat, J. Gao,

J. Huston, M. Guzzi, T.J. Hou, H-L Lai, J. Pumplin,

T.-J. Hou, D. Stump, C.-P. Yuan

+Slides by my co-conveners at 2013 workshops

QCD Frontier workshop, Jefferson Lab, Nov. 22, 2013

1. A windfall of new data to compare with

(LHC, HERA, Tevatron, JLab, RHIC,…)

2. Tests of QCD factorization at new √�,

targeting 1% precision

3. New tools (HERA Fitter, APPLGRID, FastNLO,…)

and methods (MC sampling, PDF reweighting)

revolutionize the PDF analysis

It is a payoff decade for the PDF analysis efforts!

A large fraction of hadronic experiments relies

on PDFs in theoretical simulations

Figure: G. Salam, 2012

Arrows: PDF publications

PQCD based on pre-LHC PDFs successfully predicted early

LHC cross sections within 3-10% accuracy

LHC 7 TeV data vs CT10 NNLO PDFs

We now wish to know PDFs to within 1% to realize the

LHC program of EWSB studies

C. Brock, Snowmass’2013

Various QCD effects can produce

non-trivial sea PDFs.

1% accuracy can distinguish

between these effects

Uncertainties in unpolarized

�� �integrated PDFs feed into

determination of polarized and TMD

PDFs, fragmentation functions

SU(2) and charge symmetry breaking

�̅ � � � , ��� ����

May be caused by

• DGLAP evolution

• Fermi motion

• Electromagnetic effects

• Nonperturbative meson

fluctuations

• Chiral symmetry breaking

• Instantons

• …

E866 Drell-Yan pair production:

�̅ � � � � 0 at � � 0.1

(large difference)

E866 constraints will

be strengthened by SeaQuest

LHC �/� production:

�̅ � � � � 0 at � � 0.1

(a few percent)

?

SU(2) and charge symmetry breaking

At a few-percent accuracy, charge symmetry violation and nuclear corrections must be explicitly estimated in the future if the data on the neutron/nuclei are used

Extrinsic and intrinsic sea PDFs

“Extrinsic” sea(maps on disconnected diagrams of lattice QCD for both

heavy and light flavors?)

“Intrinsic” sea (excited Fock

nonpert. states, maps on connected

diagrams of lattice QCD?)

x0.1

����

Intrinsic

Extrinsic

�

p

�

p

(Dis)connected topologies in lattice QCD

Liu, Chang, Cheng, Peng, 1206.4339

Extrinsic and intrinsic sea PDFs

Liu, Chang, Cheng, Peng, 1206.4339

Smooth � � ��parametrizations can

hide existence of two

components

Intrinsic charm (IC) can carry up to 1%

of the proton momentum

CT10 IC NNLO PDFs, S. Dulat et al.,

1309.0025

1% accuracy on PDFs demands major

leaps beyond the present level• Experiment: finding new clean measurements to

probe unconstrained PDF combinations

• Theory: Computation of (N)NNLO QCD + NLO EW

corrections and resummation. Total revision of

computer codes for fits to bring their numerical

accuracy from the current ~1% to >0.1%.

• Statistics: fast multivariate fits for unbiased PDF

parametrizations with 100+ fitted parameters and

multiple correlated nuisance parameters of

experimental and theoretical origin

=> MC sampling, neural networks, PDF reweighting, meta-PDFs, …

Where should future PDFs come from?

• From a combination

of BIG, medium,

and small experiments

• Complementarity in

– kinematical ranges

– systematics

LHC RHIC, JLab,

EIC COMPASS,

HERMES,…

+ lattice QCD

2013 status of the unpolarized PDFs

At NNLO QCD, general-purpose PDF

parametrizations are available from

ABM, CT, HERA, MSTW, NNPDF groups

Typical PDFs are constrained by • DIS at HERA

• Vector boson production at low �,

Tevatron

• Inclusive jet production

• early LHC data

Parton distribution functions in a nucleon

• Any PDF set makes assumptions about poorly constrained PDF

combinations, e.g., sea PDFs at � � 0.01 and � � 0.3. Photon PDF

is largely unknown.

• Fixed-target data sets are critical at � � 0.01, but may be

replaced in the future by collider measurements in ��̅, direct-�,��, … production

Our latest PDFs: CT10 and CT1X NNLO

• CT10 NNLO [arXiv:1302.6246] is an NNLO counterpart

either to CT10 NLO or CT10W NNLO

– In good agreement with early LHC data

• CT1X NNLO – a preliminary extension of CT10 NNLO

that includes latest HERA data on FL(x,Q) and

F2c(x,Q), LHC 7 TeV data (ATLAS W & Z, ATLAS jets,

CMS W asymmetry)

• The new data provide only minor improvements

compared to the CT10 data set. We investigate its

agreement with the CT10 data sets and await for

more precise LHC data to be included in the CT1X

public release

2010->2012: changes in the PDF luminosities

improvements

from 2010 to

2012…

…and from NLO

to NNLO

so Higgs PDF

uncertainty under

good control

αs uncertainty

still +/-0.002J. Huston

LHC and future non-LHC experiments

side by side

Kinematical ranges

Aschenauer et al. arXiv:1309.5327

LHC can access PDFs in most

of the x range at Q>10 GeV,

provided the experimental

and theoretical systematics is

controlled

Kinematical reach

Aschenauer et al. arXiv:1309.5327

EIC/fixed-target experiments can

compete at x>0.1, lower Q;

-probe excited Fock states and charge

asymmetry violation in the nucleon

-must rigorously control theory and

experim. uncertainties as in the LHC

analyses

K. Lipka, DIS’13 WG1 summary

Charge asymmetry at the LHC

� ! " #

�$ �%

�" ��$ �&

�"

�$ �%

�" ��$ �&

�"

K. Lipka, DIS’13 WG1 summary

Constraints on strangeness in LHC

W, Z, and Wc production

CCFR (inclusive '( DIS) and NuTeV (SIDIS dimuon

production): constraints on strangeness

Constraining strangeness PDF by LHC W and Z cross

sections

Correlation between $)/$* and f(x,Q=85 GeV)

2008, CTEQ6.6 (arXiv:0802.0007):the ratio $)/$* at LHC must

be sensitive to the strange

PDF ���, +�

The uncertainty on ���, +�limits the accuracy of the

W boson mass measurement

at the LHC

Correlation cosine cos/ 0 11:

�Measurement of X

imposes tight constraints

on Y

Constraining strangeness PDF by LHC W and Z cross

sections

2012: the ATLAS analysis

(arXiv:1203.4051) of W and Z

production suggests

�̅��, +�/�̅ �, + = 1.00&2.34%2.35

at � # 0.023 and +3 # 1.9GeV2

Similarly, $��%��/$��&�� cross section

ratios show preference for 8̅ 9,:

;� 9,:~1,

larger than in most pre-LHC PDF sets

W charge asymmetry at RHIC

Approximate kinematic range

at RHIC: -0.06 < x < 0.4 for

� 2 � " � 2Sensitivity to �̅��,=)�/� �,=) at � 0 0.1In the central rapidity region

Hadron multiplicities in SIDIS (COMPASS, EIC,…)

The uncertainty on the fragmentation functions >?!�@�

is larger than the PDF uncertainty. The combined

analysis of PDF+FF uncertainties is feasible with PDF

reweighting methods (Aschenauer, Stratmann, et al.)

� � � �̅��� can be determined much better than in

'( DIS, if the knowledge (anti)-kaon FFs is improved

Example: LO analysis of strangeness

at HERMES

S(x) ≈ 0 with the measurement error for x ≥ 0.15, as reported in PLB666, 446 (2008). Compatible with NNPDF2.1 in magnitude, not detailed shape. Monotonous behavior, no obvious valence-like features

Example: LO analysis of strangeness

at HERMES

Difficulty: NNLO QCD corrections are large; dependence on FFs; higher twists?

Example: A1 production at the EIC

Aschenauer, Stratmann, in 1108.1713

Intrinsic charm at the EIC

Guzzi, Nadolsky, Olness, Sec. 1.9 in 1108.1713

The ultimate test of the intrinsic charm mechanism is possible in charm

SIDIS at the EIC with modest luminosities

Important for EW precision physics (W mass measurements),

require deep revisions in the PDF analysis

NNPDF (1308.0598) published NNLO QCD+LO QED PDFs with error sets

• An important step despite substantial limitations

CTEQ pursues a similar effort

The only previous existing QCD+QED PDF set is MRST’2004 QED, not

updated for detailed studies

NNPDF

CTEQ

x

xf(

x,Q

)

Photon PDFs: include � as a new parton

The current quark (sea) PDFs are obtained

• using isoscalar targets• under the assumption �B �, + # C��, +�, etc.

But quark electric charges are DE # 2D/3, D; # �D/3⇒ charge symmetry is violated

Direct measurements of �B��, +� in CC SIDIS, etc. on a

proton eventually feeds into the constraints onto

�̅B �, + � �B��, +�

Fixed-target experiments targeting charge symmetry

violation will improve reliability of photon PDFs

x

Charge symmetry is violated by EM couplings

Main messages• Flavor separation of unpolarized quark PDFs at 1% accuracy is

important for the understanding of the nucleon structure and

electroweak precision experiments at the LHC; provides the

baseline input for determination of polarized and TMD PDFs

• While ATLAS, CMS, LHCb will be the main information source

about the sea quark flavor, EIC/RHIC and fixed-target

experiments can be valuable in constraining �/�̅, � 1 �̅ at

� � 0.01, studying charge symmetry violation and intrinsic heavy-

flavor production

• Rigorous control of NNLO QCD, NLO EW, nuclear contributions

and their correlations is a must in all these measurements.

Correlated factors of this kind can be implemented across all

experiments using open-source frameworks such as HERA Fitter.

Backup slides

Charm quark mass dependence in a global QCD analysisJ. Gao, M. Guzzi, P. Nadolsky, arXiv:1304.3494

• The assumed value of mc and the implementation of a

particular general mass scheme affects precision LHC

variables

• Constraints on the =G mass mc(mc) from the CT10 NNLO data

set were found to be 1.18&2.I5%2.24GeV, including both PDF and

theoretical errors

• The best-fit value of mc(mc) is consistent with the world

average 1.275+/-0.025 GeV within errors

• It has a significant dependence on the form of the rescaling

parameter (controlled by a parameter l in the generalized re-

scaling prescription by Nadolsky and Tung, 2009)

Preferred regions for mc(mc) vs. the rescaling parameter . The best-fit values

and confidence intervals are shown for two alternative methods for

implementation of correlated systematic errors.

68% and 90%CL contours for mc(mc) from the

CT10 NNLO analysis

Error ellipses: CT10 NNLO

PDF errors

Yellow-red scattered points:

both J and K are varied

1 L J J L 1.36 GeV

0 L K L 0.2

Black squares: only K is varied

J J # 1.275PDQ0 L K L 0.2

Using a fixed world-average

J �J � reduces the total

uncertainty of the fit

Uncertainty in LHC total cross sections due to mc(mc)

and rescaling parameter λ

LHC data => new PDFsNNPDF2.3: the first published PDF set

that includes LHC 7 TeV data sets:

• ATLAS inc. jets and �1/� rapidity

distributions

• LHCb �1 rapidity distributions

• CMS W asymmetry

Some reduction in the PDF uncertainty

compared to pre-LHC measurements

Reduced error on strangeness PDF

Large constraint for “collider only” PDFs