Why are PDF’s important for ATLAS
Durham, Sep 18th 2006
A M Cooper-Sarkar, Oxford
SM CSC notes
UK effort
•Min bias
Glasgow, Sheffield
•W/Z cross-section
RAL, Oxford, Liverpool(?)
•W mass
•Asymmetry in Z
•W/Z +jets
•Dibosons
RAL
Birmingham, Oxford
•Gamma + jet
Birmingham(?)
•PDFs
Oxford, Glasgow, UCL, RAL
PDF CSC note depends on all the rest
(plus the jet notes)
Couple of examples of physics processes which could feed into the
CSC note.. Think of more during this meeting?
How do PDF uncertainties affect BSM physics?
high ET jets..contact interactions/extra dimensions
Dan Clements
How do PDF uncertainties affect SM physics ?
Investigate ‘standard candle’ processes- W/Z production
which are insensitive to PDF uncertainties to
calibrate experiment
measure machine luminosity?
Alessandro Tricoli
What measurements can we make at ATLAS to improve the PDF
uncertainty? Think of more during this meeting?
HERA and the LHC- transporting PDFs to hadron-hadron cross-sections
QCD factorization theorem for shortdistance inclusive processes
where X=W, Z, D-Y, H, high-ET jets,
^prompt-γ
and  is known
• to some fixed order in pQCD and EW
• in some leading logarithm
approximation (LL, NLL, …) to all orders
via resummation
pA
fa
x1
ˆ
pB
x2
fb
X
Knowledge of the PDFs is vital
The general trend of PDF uncertainties is that
The u quark is much better known than the
d quark
The valence quarks are much better known
than the gluon and sea at high-x
The valence quarks are poorly known at
small-x but it is the sea which matters in
this region
The sea and the gluon are well known at
low-x
The sea is poorly known at high-x, but the
valence quarks are more important in this
region
The gluon is poorly known at high-x
And it can still be very important for
physics e.g.– high ET jet xsecn- need to
tie down the high-x gluon
Example of how
PDF uncertainties
matter for BSM
physics– Tevatron
jet data were
originally taken as
evidence for new
physics--
These figures show inclusive jet cross-sections
i
compared to predictions in the
form (data - theory)/ theory
Something seemed to be going on at the highest E_T
And special PDFs like CTEQ4/5HJ were tuned to describe it better- note the
quality of the fits to the rest of the data deteriorated.
But this was before uncertainties on the PDFs were seriously considered
Today Tevatron jet data are considered to lie
within PDF uncertainties. (Example from CTEQ
hep-ph/0303013)
We can decompose the uncertainties into
eigenvector combinations of the fit parametersthe largest uncertainty is along eigenvector 15 –
which is dominated by the high x gluon
uncertainty
We can translate the current level of PDF uncertainty into the uncertainty on
LHC jet cross-sections.
Such PDF uncertainties on the jet cross sections compromise the potential
for discovery.
E.G. Dijet cross section potential sensitivity to compactification scale of extra
dimensions (Mc) reduced from ~6 TeV to 2 TeV. (Ferrag et al)
Mc = 2 TeV,
no PDF error
SM
2XD
4XD
6XD
Mc = 6 TeV,
no PDF error
Mc = 2 TeV,
with PDF error
Can we improve our knowledge of PDFs using ATLAS data
itself?
Use data at lower ET ( < ~ 1 GeV) and higher η (|η| >~ 1)
-where new physics is not expected
………..talk of Dan Clements
Note that inputting data to a PDF fit needs a prediction for the cross-section
which can be easily obtained analytically –true for DIS inclusive crosssection. But NLO jet cross-sections can only be computed by MC and can
take 1-2
days tohas
compute.
This cannot
be done
for every
iteration of a
ThisCPU
technique
been extended
to LHC
high-ET
jet cross-sections
PDF fit.
Recently grid techniques have been developed to include DIS jet crosssections in PDF fits (ZEUS-JETs fit). This has been extended to p-p jet crosssections within ATLAS ( Carli et al).
Example of how PDF uncertainties matter for SM physics
W/Z production have been considered as good standard candle processes insensitive to
PDF uncertainties……? This is true WITHIN a PDFset
But how about comparing PDFsets?
generator level
We actually measure the decay
lepton spectra in limited rapidity
range. SO..
electron
positron
ATLFAST
electron
positron
Generate with HERWIG+k-factors
(checked against MC@NLO) using
CTEQ6.1M ZEUS_S MRST2001
PDFs with full uncertainties
from LHAPDF eigenvectors
At y=0 the total uncertainty is
~ ±6% from ZEUS
~ ±4% from MRST01E
~ ±8% from CTEQ6.1
ZEUS to MRST01 central value
difference ~5%
Can we improve our knowledge of PDFs using ATLAS data?
To improve the situation we NEED to be more accurate than ~4%
Study of the effect of including the LHC W Rapidity distributions in global PDF fits
by how much can we reduce the PDF errors with early LHC data?
Generate data with 4% error using CTEQ6.1 PDF, pass through ATLFAST detector
simulation and then include this pseudo-data in the global ZEUS PDF fit
Central value of prediction shifts and uncertainty is reduced
BEFORE including W data
AFTER including W data
AMCS, A. Tricoli
(Hep-ex/0509002)
Lepton+ rapidity spectrum
data generated with CTEQ6.1
PDF compared to predictions
from ZEUS PDF
Lepton+ rapidity spectrum
data generated with CTEQ6.1
PDF compared to predictions
from ZEUS PDF AFTER these
data are included in the fit
→ the low-x gluon PDF is improved
Ongoing work on W/Z for CSC note – see talk of Alessandro Tricoli
The uncertainty on the W/Z rapidity distributions is dominated by –- gluon PDF
dominated eigenvectors and there could be cancellation in the ratios.
Within the SM ZW = Z/(W+ + W-) IS very well known ~1% within a PDF set AND
comparing PDF sets
BUT AW = (W+ - W-)/(W+ + W-) has uncertainty ~13% from valence PDF related
eigenvectors
Within each PDF set uncertainty in AW, the
lepton asymmetry, IS LESS than in the lepton
rapidity spectra, e.g about 2% for the
asymmetry at y=0, as opposed to about 4%
for the lepton rapidity spectra themselves
(using MRST2001 PDFerrors)
However the PDF sets differ from each
other more strikingly- MRST04 and
CTEQ6.1 differ by about 13% at y=0!
But this is an opportunity to use ATLAS
measurements to increase knowledge of
the valence PDFs at x~0.005 - see AMCS
February06 SM session
generator level
ATLFAST
Prompt photon cross-sections may also be useful to improve the gluon PDF
–compare photon pt and η distributions for Cteq61 up and down eigenvector 15 emphasizing the uncertainties in the high-x gluon (PT > 330 GeV)
Ivan Hollins, Birmingham (no talk)
New ideas..
Strange quark PDFs and strange/antistrange asymmetry..
See talk of Laura Gilbert
SUMMARY
•Work hard for the CSC notes
•Think of more new ideas
extras
And how do PDF uncertainties affect the Higgs discovery potential?
q
g
W/Z
t
H
W/Z
g
S Ferrag
q
W/Z
H
The uncertainty on the W/Z rapidity distributions is
dominated by –- gluon PDF dominated eigenvectors
It may at first sight be surprising that W/Z
distns are sensitive to gluon
parameters BUT our experience is
based on the Tevatron where Drell-Yan
processes can involve valence-valence
parton interactions.
At the LHC we will have dominantly
sea-sea parton interactions at low-x
And at Q2~MZ2 the sea is driven by
the gluon- which is far less precisely
determined for all x values
Because the uncertainties in W+,Wand Z spectra are all coming from
the gluon PDF there is cancellation
in the ratios
AW = (W+ - W-)/(W+ + W-)
ZW = Z/(W+ + W-)
Remaining uncertainty comes from
valence PDF related eigenvectors
Well Known? Gold plated?
We will actually measure the lepton asymmetry
Uncertainty in the lepton asymmetry IS LESS than in
the lepton rapidity spectra, e.g about 4% for the
asymmetry at y=0, as opposed to about 8% for the
lepton rapidity spectra themselves (using CTEQ6.1M
PDFS)