Post on 14-Dec-2015
transcript
Low-x and PDF studies at LHCSept 2008
A M Cooper-Sarkar, Oxford
At the LHC high precision (SM and BSM) cross section predictions require precision Parton Distribution Functions (PDFs)
How do PDF Uncertainties affect SM physicsW/Z production, Higgs profuction
How do PDF uncertainties affect BSM physics?-sometimes it will only affect precision e.g. Z’ in high-mass Drell-Yan-sometimes it will compromise discovery e,g, contact interactions in highET jet
production
What measurements can we make at LHC to improve the PDF uncertainty?
Do we even understand QCD a small-x?
• First pp collisions in Sep 2008 s = 10 TeV • Spring ’09 s = 14 TeV at Low luminosity • L= 1 fb-1/year (1032cm-2s-1)• End ’09 s = 14 TeV at High luminosity • L= 10 fb-1/year (1033 cm-2s-1)
Process (nb) Ev./10fb-1
W e 15 ~108
Z e+ e 1.5 ~107
t tbar 0.8 ~107
jets
(pT>200 GeV)
100 ~109
LHC is W, Z, top … factory
Large statistics for SM processes
• SM precision physics (EW, top-,b-physics, multijets…)
• Big potential for new physics (Higgs, Extra Dimensions, SUSY…)
So when is it all going to happen?
The Standard Model is not as well known as you might think
pA
pB
fa
fb
x1
x2
XThe central rapidity range for W/Z production AT LHC is at low-x
(5 ×10-4 to 5 ×10-2)
particularly in the QCD sectorand particlarly in the non-perturbative part of the QCD sector
At the LHC high precision (SM and BSM) cross section predictions require precision Parton Distribution Functions (PDFs
Pre HERA Post HERA
ZEUS data
Look at predictions for W/Z rapidity distributions Pre- and Post-HERA
Why such an improvement
?
It’s due to the improvement in the low-x gluon
At the LHC the q-qbar which make
the boson are mostly sea-sea
partons at low-x And at Q2~MZ2 the sea is driven
by the gluon
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-xAnd at Q2~MZ2 the sea is driven by the gluon- which is far less precisely determined for all x values
Not just statistical improvement. Each experiment can be used to calibrate the other since they have rather different sources of experimental systematics
• Before combination the systematic errors are ~3 times the statistical for Q2< 100
• After combination systematic errors are < statistical
• → very consistent data input HERAPDFs use Δχ2=1
Recent development: Combining ZEUS and H1 data sets
This is post HERA but just
one experiment
(ZEUS)
This is post HERA using the
new (2008) HERA combined
PDF fit
However there is still the possibility of trouble with the
formalism at low-x
MRST PDF
NNLO corrections small ~ few%NNLO residual scale dependence < 1%
W/Z production have been considered as good standard candle processes with small theoretical uncertainty.
PDF uncertainty is THE dominant contribution and most PDF groups quote uncertainties <~5% (but note HERAPDF ~1-2%)
BUT the central values differ by more than some of the uncertainty estimates. Experimental errors are now very small- but how about model uncertainty?Some differences are not just choices, massless heavy quark treatments won’t do.
PDF set σW+ BW→lν
(nb)σW- BW→lν (nb) σz Bz→ll (nb)
ZEUS-2005 11.87±0.45 8.74±0.31 1.97±0.06
MRST01 11.61±0.23 8.62±0.16 1.95±0.04
HERAPDF 12.13±0.13 9.13±0.15 2.01±0.025
CTEQ65 12.47±0.47 9.14±0.36 2.03±0.07
MRST04 11.74 8.71 1.97
CTEQ61 11.61±0.56 8.54±0.43 1.89±0.09
Can we improve the situation 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 (actually use the decay lepton spectra) Central value of prediction shifts and uncertainty is reduced
e+ rapidity spectrum and gluon PDF BEFORE these data are included in the PDF fit
BEFORE including W data AFTER including W data
e+ rapidity spectrum and gluon PDF AFTER these pseudodata are included in the PDF fit
Gluon PDF uncertainties are reduced
The uncertainty on the W+ W- and Z rapidity distributions are all dominated by gluon PDF uncertainty BUT there is cancellation of this uncertainty in the ratio
ZW = Z/(W+ + W-)
the PDF uncertainty on this ratio is ~1% and there is agreement between PDFsets
cteq66 HERAPDF0.1 Mrst04(1)
But the same is not true for the W asymmetryAw = (W+ - W-)/(W+ + W-)the PDF uncertainty on this ratio is reduced compared to that on the W rapidity spectra within any one PDF setBUT there is not good agreement between PDF sets- a difference in valence PDFs is revealed
Dominantly, at LO Aw= (u dbar – d ubar) (u dbar + d ubar)
And ubar = dbar = qbar at small x So Aw~ (u – d) = (uv – dv) (u + d) (uv + dv + 2 qbar )
Actually this pretty good even quantitatively
The difference in valence PDFs you see here explains the difference in AW
x- range affecting W asymmetry in the measurable rapidity range
uv – dvcteq65
mrst04
mrst04cteq65
Generate data with 4% error using MRST04 PDF and then include this pseudo-data in the global ZEUS PDF fit (actually use the lepton asymmetry data)
The PDF uncertainty of the valence distributions is improved by the input of such data and the central value can be changed
MRST04pseudodata ZEUS-S prediction
BEFORE including Ae pseudo-data
AFTER including Ae pseudo-data
ATLAS/CMS LHC Aw data can measure valence distributions at x~0.005
CTEQ6.5MSTW08
But what about the formalism at LOW-X ?: LHC will be a low-x machine (at least for the early years of running)
And note: even conventionally our knowledge of PDFs is decreases as x < 10-4
Q2=10
Q2=10000
Note CTEQ in general bigger uncertainties…but NOT for low-x gluon where more flexible parametrization of MSTW08 gives larger errors
But what about the formalism at LOW-X ?: LHC will be a low-x machine (at least for the early years of running)
Is NLO (or even NNLO) DGLAP good enough?
The QCD formalism may need extending at small-x
MRST03 is a toy PDF set produced without low-x data
MRST02
MRST03
200k events of W+- -> e+- generated with MC@NLO using MRST03 and MRST02
Reconstructed Electron Pseudo-Rapidity Distributions (ATLAS fast simulation)
6 hours running
Reconstructed e-Reconstructed e+
If something is very different about low-x behaviour it will show up in the our measurable rapidity range
But the TOY PDF is unlikely to be realistic - a better way cold be to look at pt spectra for W and Z production
Pt spectra show PDF differences, but also show differences in modelling – e.g. PYTHIA/HERWIG differences
Probably needs more sophisticated treatment e.g. RESBOS.
There has been an interesting recent calculation of how lack of pt ordering at low-x may affect the pt spectra for W and Z production at the LHC (See hep-ph/0508215)
MW(fit)
< pT(W) >
Same pattern
Pt spectra are also used to measure MWRaw dMW from PDF uncertainties as of today, when using pt(e), is ~20 MeV
So we’d better be sure we’ve got the calculations for Pt spectra right
0 1 2 3 4 5 60.0
0.2
0.4
0.6
0.8
1.0
MRST2002NLO ALEKHIN02NLO
d
(W- )/
dy /
d(W
+)/
dy
yW
x1=0.52 x2=0.000064
x1=0.006 x2=0.006
How to probe down to really low-x experimentally? Well the kinematics are such that it’s bound together with probing to higher-x.
We had hoped to learn about valence PDFs (d/u )at high-x by looking at the W-/W+ ratio at large rapidity, but this is bound up with our knowldege of low-x
LHCB kinematics
So it’s better to look at the PDF uncertainty on the total Drell-Yan cross-sections
Improvement with LHCb measurements
And this will feed into PDFs
Moving on to BSM physics
Tevatron jet data were originally taken as evidence for new physics—
i
These figures show inclusive jet cross-sections compared to predictions in the form (data - theory)/ theory
Theory CTEQ6M
Today Tevatron jet data are considered to lie within PDF uncertaintiesAnd the largest uncertainty comes from the uncertainty on the high x gluon
Theory MRST2002
SM + structure function uncertainty band
Mc = 2 TeV
2XD + structure function uncertainty band
4XD + structure function uncertainty band
Up to ~50% at high mass :
Enough to lose sensitivity to higher compactification scales
S.Ferrag
MJJ (GeV)
d/dM (a.u)
Such PDF uncertainties in the jet cross sections compromise the LHC potential for discovery of any new physics which can written as a contact interaction E.G. Dijet cross section has potential sensitivity to compactification scale of extra dimensions (M
c)
And what consequences might this have?
Can we know the high-x gluon better?
mrst01mstw08
There is newer Tevatron Run-II jet data in the latest PDF fits but no very striking improvement in the high-x gluon uncertainty- is there further hope from HERA jets?
Note there is now new Tevatron Run-II jet data
But it does not make MUCH difference to the
level of PDF uncertainties
Can we know the high-x gluon better?
And how might this impact on LHC high-ET jet cross-sections?
HERA now completed second stage of operation (HERA-II)HERA-II projection shows significant improvement to high-x PDF uncertainties
And will we be able to use LHC data itself to improve the situation?
Recently grid techniques have been developed to NLO cross-sections in PDF fits (e.g ZEUS-JETs fit)
This technique can be used for LHC high-ET jet cross-sections
Use data at lower PT and higher η-where new physics is not expected
Pseudo-data has been generated up to PT= 3 TeV for pseudo rapidity ranges
And then used in a global PDF fit to assess the impact of ATLAS data on PDFs
Impact of increasing statistics
Impact of decreasing experimental
systematic uncertainty
Impact of decreasing experimental correlated systematic uncertainty
Challenging!
Can we decrease Jet Energy Scale systematic
to 1%?
1 year (10 fb-1)
ATLAS TDR
But not all BSM physics is strongly compromised: e.g PDF Uncertainty in High-mass Drell-Yan- won’t stop us seeing Z’s
dominant
Gluons dominant
7 – 9 % Uncertainty
d-Valence dominant
Sea dominant
Different mass ranges have different contributions to the PDF uncertainty
And how do PDF uncertainties affect the Higgs discovery potential?- not too badly
g
g
Ht
q
q
W/Z
W/Z
W/Z
H
S Ferrag
Summary
PDF uncertainties impact significantly on
Precise W/Z cross-sections, hence on use of these as luminosity monitor
(however Z/W ratio is a golden calibration measurement)
High Et jet cross-sections, hence on discovery of new physics which can be written in terms of contact interactions
PDF uncertainties should not obscure discovery of
Higgs in mass range 100-1000 GeV
High mass Z’ in mass range 150-2500 GeV
Measurements from LHC itself may improve knowledge of
Gluon PDF at low-x (W prodn) and high-x (high ET jets/direct photon)
Low-x / high-x valence PDFs ( W asymmetry)
Low-x partons/ Low-x theory (low-mass Drell-Yan)
extras