misure di funzioni di struttura ad lhc
DESCRIPTION
Misure di Funzioni di Struttura ad LHC. by Alessandro Tricoli Rutherford-Appleton Laboratory and University of Oxford. Workshop sui Monte Carlo, la Fisica e le Simulazioni ad LHC Frascati, 23 Maggio 2006. Overview. Introduction: What are Parton Distribution Functions (PDFs)? - PowerPoint PPT PresentationTRANSCRIPT
Misure di Funzioni di Struttura ad LHC
by
Alessandro Tricoli
Rutherford-Appleton Laboratory andUniversity of Oxford
Workshop sui Monte Carlo, la Fisica e le Simulazioni ad LHCFrascati, 23 Maggio 2006
Alessandro Tricoli, RAL & Oxford University 2MCWS Frascati, 23 Maggio 2006
Overview Introduction:
What are Parton Distribution Functions (PDFs)? (PDFs)? How they determined How the PDF Uncertainties are estimated
Why the accurate knowledge of PDFs is vital for the LHC
Impact of PDF Uncertainty on the discovery of New Physics signals: Higgs, Extra Dimensions etc.
Impact of PDF Uncertainty on SM measurements sensitive to New Physics: Inclusive Jet cross-section, Drell-Yan cross-section
How to constrain PDF at LHC, some examples: Inclusive jet cross section W and Z rapidity distributions
Conclusions
Alessandro Tricoli, RAL & Oxford University 3MCWS Frascati, 23 Maggio 2006
What are PDF distributions?PDFs are parameterizations of the partonic content of the proton:
i = uv, dv, g and seax = pparton / Ebeam parton momentum fraction Q2 momentum transfer
How are PDFs determined from global Fits? QCD predicts the scale dependence of fi(x,Q2) through the DGLAP evolution equations, BUT does not accurately predict the x-dependence which has non-perturbative origin. x-dependence is parameterised at a fixed scale Q0
2 ~ 1-7 GeV2 :
Valence Quarks: f ~ x (1-x)P(x)
Sea/Gluon: f ~ x- (1-x)P(x)
Different people use different parameterisations with different no. of free parameters
fi(x,Q2) is evolved from Q02 to any other Q2 by numerically solving the
conventional DGLAP equations to various orders (LO,NLO, NNLO) The free parameters are determined by fit to data from exp. observables:
DIS processes (fixed target and HERA), DY lepton pair production High Et jets (CDF, D0), W rapidity asymmetry (CDF)N dimuon (CCFR, NuTeV)
etc.
fi(x,Q2)
Alessandro Tricoli, RAL & Oxford University 4MCWS Frascati, 23 Maggio 2006
PDF uncertainties
The theoretical uncertainties are estimated varying the theoretical assumptions,But only recently the correlated syst. on data points are properly considered: PDF sets after year 2000 provide UNCERTAINTIES: fi(x,Q2) ± δ fi(x,Q2):
use a modified to consider non-gaussian syst. errors and their correlations. T= tolerance
Theoretical Uncertainties
Theoretical Formalism: perturbative calculations, i.e DGLAP approx., higher order truncation, etc. Model Assumptions: non-perturbative parameterisations (x-depedence) i.e. assumptions to limit the no. of free parameters
Experimental Uncertainties
Statistical and Systematic Uncertainties on experimental data inputs Correlated Systematic Uncertainties on data points:
~
Offset Method: the correlated syst. errors affect only the determination of the PDF uncertainty, NOT the best fit (centre value) e.g. ZEUS-S T2~49 Hessian method: the collective effect of the correlated syst. errors can also modify the values of the best fit e.g. CTEQ6 T2=100, MRST01: T2=50
Alessandro Tricoli, RAL & Oxford University 5MCWS Frascati, 23 Maggio 2006
At Hadron Colliders every Cross-Section calculation is a convolution of the cross-section at parton level and PDFs:
PDFs are vital for reliable predictions for new physics signal (Higgs,
Super- Symmetry, Extra Dimensions etc.) and background cross-section at LHC.
Why PDFs are vital at LHC ?
pA
pB
fa
fb
x1
x2
X
Alessandro Tricoli, RAL & Oxford University 6MCWS Frascati, 23 Maggio 2006
A lesson from Tevatron on the importance of PDFs
Today they are considered within Proton Structure Uncertainty band
PDF UncertaintiesPDF Uncertainties must be properly taken into account or SM physics features could be misinterpreted as new physics signal
(Da
ta-T
he
ory
)/T
he
ory
CDF Run I
Tevatron Jet datawere originally takenas evidence of New PhysicsSince Proton Structure UncertaintySince Proton Structure Uncertaintywas not properly consideredwas not properly considered
Alessandro Tricoli, RAL & Oxford University 7MCWS Frascati, 23 Maggio 2006
LHC Kinematic regime
ys
Mx exp2,1 MQ
Kinematic regime for LHC much broader than currently explored
z
z
pE
pEy ln
2
1
Test of QCD: Test DGLAP evolution at small x:
Is NLO DGLAP evolution sufficient at so small x ?
Are higher orders important?
Improve information of high x gluon distribution
xmns log~
At the EW scale cross section predictions for LHC are dominated by low-x gluon uncertainty (i.e. W and Z masses) => see later slides
At TeV scale New Physics cross section predictions are dominated by high-x gluon uncertainty(not sufficiently well constrained by PDF fits) => see later slides
Alessandro Tricoli, RAL & Oxford University 8MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on New Physics: Higgs g
g
H
q
q
W/Z
W/Z
W/Z
H
Djouadi & Ferrag, Phys. Lett. B 586 345:352 (2004)
PDF uncertainties(CTEQ6M, MRST01E, Alekhin02)on Higgs cross-sections:Up to 10% (15%)
g
g
H
t
t_
q
q
W/Z
H_
Alessandro Tricoli, RAL & Oxford University 9MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on New Physics:
Extra Dimensions
Mc= 8 TeV
Pt(GeV)
2XD
4XD
6XD
SM
Mc= 2 TeV
Pt(GeV)
(mb)
An E.D. Model: di-jets cross section in the E.D. regime is a continuity of the Standard Model one with new s running: XD
SJJJJ
XD
dM
d
dM
d
PDF uncertainties decrease discovery reach for E.D. from MC 5 (10) TeV to < 2 (3)TeV
High-x gluon is responsible of the big PDF uncertainties
E.D. are masked by PDF uncertainties:
Standard Model prediction zone:where every measured cross section can be explained by a PDF fit, and every power of discovering new physics is killed and absorbed by the PDF fit
Central value1 limits3 limits
SM predictionCTEQ6M PDFs
Pt(GeV)
Ferrag, hep-ph/0407303 (2004)
Alessandro Tricoli, RAL & Oxford University 10MCWS Frascati, 23 Maggio 2006
10 fb-1
~600 GeV
Impact of PDF uncertainty on SM:
Drell-Yan cross section Sensitive to New Physics:
ppXgq LQxxPDFN ),,( 221,
2?" physics new" ~ Z
High mass dileptons → Uncertainties at high-x important (up to 10%)
Mll [GeV]
40 CTEQ6PDFs
PDF Relative Uncertainty
MC@NLO
Alessandro Tricoli, RAL & Oxford University 11MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on SM:
Inclusive Jet cross-section A SM measurement sensitive to New Physics: compositeness, Black Holes etc.
Experimental and theoretical errors can distort the measurements and predictions creating false signals of new physics:
PDF Uncertainty (gluon), Renormalisation and Factorisation scale uncertainty, Experimental Jet Energy scale uncertainty.
(NLOJET,CTEQ6) Factorisation & Renormalisation scales=pt/2
D. Clements, C. Butter, A. Moraes
Alessandro Tricoli, RAL & Oxford University 12MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on SM:
Inclusive Jet cross-section Current PDF uncertainty: 10% at 1 TeV, 25% at 2 TeV, up to 60% at 5 TeV
Can we constrain PDF with jets at LHC as done by Tevatron?
D. Clements, C. Butter, C. Gwenlan, T. Carli, A. Cooper-Sarkar, M Sutton
UncorrelatedExp. Systematics
LHC jet data are useful to constrain the gluon PDF,Uncertainty on the extracted gluon PDF is dominated by Systematics,Statistical uncertainty negligible even at 1fb-1
ATLAS estimate:
The large PDF uncertainty indicates that we might be able to constrain the high-x gluon with high ET jets up to 1 TeV even with 1 fb-1 luminosity.
Alessandro Tricoli, RAL & Oxford University 13MCWS Frascati, 23 Maggio 2006
How to constrain PDFs at low-x at LHC?
At the EW scale cross sections are dominated by sea and/or gluon interactions at low-x. Furthermore, at Q2~M2
W/Z the sea is driven by the gluon (via gluon splitting) which is far less precisely determined for all x values.
Wud
WduW production:(main contributions)
Zdd
Zuu
Z production:(main contributions)
We can improve our knowledge of PDF’s at LHC measuring the vector boson production: W’s, Z’s (and photons?)
Alessandro Tricoli, RAL & Oxford University 14MCWS Frascati, 23 Maggio 2006
How to constrain low-x PDFs at LHC
single Z and W± Productions
4% MRST02 error
W,Z tot. cross sections
MRST PDF
NNLO corrections small ~ few%NNLO residual scale dependence < 1%
-6 -4 -2 0 2 4 60
1
2
3
4
5
x1 = 0.0003
x2 = 0.12
x1 = 0.12
x2 = 0.0003
x1 = 0.006
x2 = 0.006
yW
MRST2002-NLO LHC
dW
/dy W
. B
l
(nb
)
W±
Symmetric
W+- diff. cross section(rapidity)
PDF Set
ZEUS-S
CTEQ6.1
MRST01
lWWB lWW
B llZZ B
41.007.12
(nb) (nb) (nb)
30.076.8 06.089.1
56.066.11 43.058.8 08.092.1
23.072.11 16.072.8 03.096.1
Theoretical uncert. dominated by PDFs
LHC exp. uncertainty is sufficiently small to distinguish between different PDF sets(LHC dominated by systematic uncertainty)
W, Z very clean signals (bkg on W->e ~1%)
Alessandro Tricoli, RAL & Oxford University 15MCWS Frascati, 23 Maggio 2006
W -> e rapidity distributions (1)Experimentally we detect e+- from W decays: W+- -> e+-
HERWIG MC Simulations with NLO Corrections
At y=0 the total PDF uncertainty is ~ ±5.2% from ZEUS-S ~ ±3.6% from MRST01E ~ ±8.7% from CTEQ6.1MZEUS-S to MRST01E central value difference ~5% ZEUS-S to CTEQ6.1 central value difference ~3.5%
CTEQ61 MRST02 ZEUS-S
CTEQ61 MRST02 ZEUS-S
e- rapidity e+ rapidity
Generator Level
ATLASDetector Levelwith sel. cuts
Error boxesare the Full PDF Uncertainties
GOAL: syst.syst. exp. error ~4%
A. Tricoli hep-ex/0511020,
A. Tricoli, A. Cooper-Sarkar, C. Gwenlan,
CERN-2005-014, hep-ex/0509002
Alessandro Tricoli, RAL & Oxford University 16MCWS Frascati, 23 Maggio 2006
Effect of including the ATLAS W Rapidity “pseudo-data” in global PDF Fits: how much can we reduce the PDF errors when LHC is up and running?
Simulate real experimental conditions:
Generate 1M “data” sample with CTEQ6.1 PDF through ATLFAST detector simulation and then include this pseudo-data (with imposed 4% error) in the global ZEUS PDF fit (with Det.->Gen. level correction).Central value of ZEUS-PDF prediction shifts and uncertainty is reduced:
ZEUS-PDF BEFORE including W data
e+ CTEQ6.1 pseudo-data
low-x gluon shape parameter λ, xg(x) ~ x –λ
BEFORE λ = -0.199 ± 0.046AFTER λ = -0.181 ± 0.030
41% error reduction
NB: in ZEUS-PDF fit the e± Normalisation is left free => no assumption on Luminosity measurement
ZEUS-PDF AFTER including W data
e+ CTEQ6.1 pseudo-data
W -> e rapidity distributions (2)
In few day stat. of LHC at low Luminosity
Alessandro Tricoli, RAL & Oxford University 17MCWS Frascati, 23 Maggio 2006
W Charge Asymmetry measurement (1)
HERWIG MC Simulations with NLO Corrections
In the Asymmetry experimental uncertainties and the gluon/sea PDF Uncertainty mostly cancel out: ~5% PDF error within each PDF set But MRST02 predicts Asym. ~15% lower than the other PDF sets, WHY?
CTEQ61 MRST02 ZEUS-S
)(/)(/
)(/)(/)(
edydedyd
edydedydyA
e+ - e- asymmetry:
Generator Level
ATLASDetector Levelwith selection cuts
Error boxesare the Full PDF Uncertainties
Experimentally we measure e+- charge asymmetry from W decays: W+- -> e+-
Alessandro Tricoli, RAL & Oxford University 18MCWS Frascati, 23 Maggio 2006
W Charge Asymmetry measurement (2)A. Cooper-Sarkar
At LO the Asymmetry is dominated by uv –dv parameter :
uv –dv parameter is not well constrained by data at very low-x current PDFs simply have prejudices as to the low-x valence distributions coming from the input parameterisations.
The small PDF uncertainties at low x do NOT actually reflect the real uncertainty.
uddu
udduyA
)(qdu
At small-x qdu
duyA
VV
VV
2)(
CTEQ6.1
MRST02
uV – dV
Q2=Mw2
x- range affecting W asymmetry in the measurable rapidity range
x
Q2=7 GeV2
at Q2=MW2 and x~0.006 (corresponging to y~0 at LHC):
MRST uV –dV is 25% lower than other PDF setswhich reflects on A(y) measurement.
For the first time with the LHCwe will have valence PDF discrimination measuring valence distributions at x~0.005 on proton targets
Alessandro Tricoli, RAL & Oxford University 19MCWS Frascati, 23 Maggio 2006
W -> e rapidity distributionsto detect new low-x physics scenarios
Comparison between a conventional PDF set, CTEQ61, which INCLUDEs very low-x data (down to x=6 10-5 )and a “toy” PDF set MRST03 which EXCLUDES HERA low-x data (x > 5 10 -3 )
e+ - e- asymmetry
CTEQ6.1MRST2003
CTEQ6.1MRST2003
e- rapidity
e+ rapidity
Shows what would be our predictions w/o low-x HERA data warning against premature extrapolations of our knowledge to new kinematic regions
Shows the impact of low-x data on current LHC predictions: Since the validity of the DGLAP formalism is not certain at such low-x, the LHC should be able to detect new low-x scenarios right in the central rapidity region
Alessandro Tricoli, RAL & Oxford University 20MCWS Frascati, 23 Maggio 2006
The computation of PDF uncertainties on MCs is time consuming To estimate the full PDF uncertainty for MRST01, ZEUS-S, CTEQ61 we have to generate 92 event samples, since
30 sub-sets (15 eigenvectors) provide the full MRST02 PDF uncertainty 22 sub-sets (11 eigenvectors) provide the full ZEUS-S PDF uncertainty 40 sub-sets (20 eigenvectors) provide the full CTEQ61 PDF uncertainty
TOO LONG generation time
The PDF re-weighting technique is useful tool to quickly evaluate the full PDF uncertainties for many PDF sets, saving generation time
only 1 event sample is generated with one PDF set each event is re-weighted off-line with a second PDF set, applying an Event Weight calculated (for the moment) from the hard scatter parameters x1, x2, Q2 only
Can we compute the PDF Uncertainty with MC in a fast way:
PDF Re-weighting
),,(
),,(
),,(
),,(
221.
222.
111.
112.
Qscaleflavxf
Qscaleflavxf
Qscaleflavxf
QscaleflavxftEventWeigh
PDFn
PDFn
PDFn
PDFn
Alessandro Tricoli, RAL & Oxford University 21MCWS Frascati, 23 Maggio 2006
Can we use PDF re-weighting to simulate other PDFs?- OK for RAPIDITY distributions
Events generated with HERWIG & MRST02and re-weighted with CTEQ61are compared toEvents generated with HERWIG & CTEQ61
Accuracy of ~0.5% in rapidity and no evidence of a y-dependent bias. For the PT distribution it’s more complex:
it needs re-weighting of the Parton Shower (Sudakov Form factors)
Relative differencebetween Re-weightedand Generateddistributions
W- W+
CTEQ61 Generated
CTEQ61 Re-weighted from MRST02
CTEQ61 Generated
CTEQ61 Re-weighted from MRST02
W- W+
Weighted meanon whole y-range
Alessandro Tricoli, RAL & Oxford University 22MCWS Frascati, 23 Maggio 2006
Conclusions
Precision Parton Distribution Functions are crucial for new physics discoveries at LHC: PDF uncertainties can compromise the potential for discovery
At LHC we are not limited by statistic but by systematic uncertainties To discriminate between conventional PDF sets we need to reach high experimental accuracy ( ~ few%)
LHC experiments are currently working hard to understand better and improve the detector performances to determine and reduce systematic errors.
The SM processes like Z, W productions, Jet productions (and hopefully Direct Photon) are good candidates to constrain PDF’s at the LHC
LHC can significantly constrain PDF’s, especially the gluon distribution with unprecedented precision The W charge asymmetry can constrain for the first time valence distributions at very low-x New low-x physics scenarios can be easily accessible by the LHC with early data
From now to the LHC start up, 2007, our PDF knowledge might improve HERA-II: substantial increase in luminosity, possibilities for new measurements
Projection: significant improvement to high-x PDF uncertainties (high-scale physics at the LHC) impact on New Physics searches
Alessandro Tricoli, RAL & Oxford University 23MCWS Frascati, 23 Maggio 2006
EXTRAS
Alessandro Tricoli, RAL & Oxford University 24MCWS Frascati, 23 Maggio 2006
How to constrain PDFs at mid-low-x at LHC?
At the EW scale cross sections are dominated by sea and/or gluon interactions at low-x. Furthermore, at Q2~M2
W/Z the sea is driven by the gluon (via gluon splitting) which is far less precisely determined for all x values.
Wud
WduW production:(main contributions) Zdd
Zuu
Z production:(main contributions)
Direct production:(LO contributions)
Compton:(~90%)
Annihilation:(~10%)
We can improve our knowledge of PDF’s at LHC measuring the vector boson production: W’s, Z’s (and photons?)
Alessandro Tricoli, RAL & Oxford University 25MCWS Frascati, 23 Maggio 2006
How to constrain gluon-PDFs at LHCZ + b-jet (1)
Sensitive to b content of the proton:
(J.Campbell et al. Phys.Rev.D69:074021,2004)
Also background to Higgs searches:
(J.Campbell et al. Phys.Rev.D67:095002,2003)
Why do we measure the b-PDF? bb->Z @ LHC is ~5% of entire Z production
Knowing σZ to about 1% requires
a b-PDF precision of the order of 20%
Now we have only HERA measurements, far from this precision
Alessandro Tricoli, RAL & Oxford University 26MCWS Frascati, 23 Maggio 2006
How to constrain gluon-PDFs at LHC
Z + b-jet (PDF Uncertainty) HERWIG with
MRST03CNNLO, CTEQ5M1, Alehkin1000
Nu
mbe
r of
eve
nts
PDF Differences in total Z+b cross-section are of the order of 5% to 10 %
S. Diglio, A. Tonazzo and M. Verducci (2005)
Z+b selected events (10 fb-1)
Signal+Background
Background
Jet PT (GeV)
Event selection: only Z→
Two isolated muons with high PT
inclusive b-tagging of jet
ATLAS
Alessandro Tricoli, RAL & Oxford University 27MCWS Frascati, 23 Maggio 2006
How to constrain gluon-PDFs at LHCdirect production
Typical Jet + eventJet and photon are back to back
Good candidate to constrain PDF’s on a wide pT range:smaller energy scale uncertainty than jets, no jet-finder bias sensitivity to high-x comes with high-pt and high- photons
Problems: Large background (especially at low PT) Can the Theoretical-Experimental Discrepancy in PT distribution be well understood?
ATLAS
I. Hollins (2005)
Kumar et al. Physics Review D 67
In NLO cross-section calculations: discrepancy
between different PDF set predictions up to ~20%
Alessandro Tricoli, RAL & Oxford University 28MCWS Frascati, 23 Maggio 2006
Differences between PDF sets
different data sets in fit different sub-selection of data different treatment of exp. sys. errors
different choices of
tolerance to define fi (CTEQ: Δχ2=100, MRST: Δχ2=50 Alekhin: Δχ2=1)
parametric form Axa(1-x)b[..] etc theoretical assumptions about sea flavour symmetryfactorisation/renormalisation scheme/scale
Q02
αS
treatment of heavy flavours
Alessandro Tricoli, RAL & Oxford University 29MCWS Frascati, 23 Maggio 2006
PDF Re-weighting Implementation I generate a MC event with one specific PDF set, say pdf set n.1
This event happens to have: One hard process scale (Q=MW)
Two primary partons with two specific flavours(flav1,flav2) Momentum fractions x1, x2 of the two primary partons (calculated at the
Hard Process, before the Parton Shower in the backward evolution is applied in the MC) according to the probability (i.e. xf) estimated by the PDF they are generated with (say pdf set n.1)
Offline (with LHAPDFv3) I evaluate the probability, i.e. xf, of picking up the same flavoured partons with the same momentum fractions x1,x2, according to a second PDF set, i.e. PDF set n.2, at the same energy scale, i.e. Q.
Then I perform the Ratio:
),,(
),,(
),,(
),,(
221.
222.
111.
112.
Qscaleflavxf
Qscaleflavxf
Qscaleflavxf
QscaleflavxftEventWeigh
PDFn
PDFn
PDFn
PDFn
Alessandro Tricoli, RAL & Oxford University 30MCWS Frascati, 23 Maggio 2006
PDF scenario at LHC start up (2007)might be different
In most of the relevant x regions accessible at LHC HERA data are most important source of information in PDF determinations (low-x sea and gluon PDFs)
HERA now in second stage of operation (HERA-II)
substantial increase in luminosity
possibilities for new measurements
HERA-II projection shows significant improvement to high-x PDF uncertainties relevant for high-scale physics at the LHC
where we expect new physics !!
- significant improvement to valence-valence-quarkquark uncertainties over all-x all-x
- significant improvement to sea and gluonsea and gluon uncertainties at mid-to-high-xmid-to-high-x
- little visible improvement to sea and gluon uncertainties at low-x
Gluon fractional error
x
C. Gwenlan, A. Cooper-Sarkar,C. Targett-Adams, hep-ph/0509220 (2005)
Alessandro Tricoli, RAL & Oxford University 31MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on New Physics:
Extra Dimensions (Theory) Hierarchy problem:
•EW symmetry breaking scale ~ 102 GeV•GUT scale ~ 1016 GeV•Planck scale ~ 1019 GeV
Alternative: 1 fondamental scale: ~ few tens TeV and 1+3+ time-space structure
Phenomenological aspects: • Possibility to produce Gravitons at LHC: low Planck scale• Kaluza Klein (KK)excitations: compactified extra dimensions
• Violation of the expected (MS)SM evolution behavior of em,w,s
(E. Dudas, R. Dienes, T. Ghergetta, hep/ph9803466 and hep/ph9807522)
Exerimentally: Evolution of s by measuring di-jets cross section on a large energy range
Parameters: number of extra-dimensions compactification scale Mc
MGUT ~30 TeV(4+2)D, R=1/10 Tev-1
MSSM+XD
Alessandro Tricoli, RAL & Oxford University 32MCWS Frascati, 23 Maggio 2006
How to constrain gluon-PDFs at LHC
Z + b-jet (Measurement) Signal: Background:
Event selection: only Z→
Two isolated muons (Pt > 20 GeV/c, opposite charge, invariant mass close to Mz) inclusive b-tagging of jet (total Z+ b selection efficiency ~15%, purity ~53% )
Z+b Z+jet
Di-muons Invariant Mass Di-muons Invariant Mass
GeV
Z+b selected events (10 fb-1)
Signal+Background
Background
Jet PT (GeV)ATLAS
Alessandro Tricoli, RAL & Oxford University 33MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on SM:
Inclusive Jet cross-section
PDF error: 10% at 1 TeV, 25% at 2 TeV, up to 60% at 5 TeV Can we constrain PDF with jets at LHC as done by Tevatron?
The large PDF uncertainty indicates that we might be able to constrain the high-x gluon with high ET jets up to 1 TeV even with 1 fb-1 luminosity.
PDF error is dominant at high pT , but low stat. PDF error negligible at low pT w.r.t. other syst. uncertainty sources :
Fact.&Ren. Scale uncertainty ~14% at 1 TeV 1% (10%) exp. energy scale uncertainty => 6% (70%) incl. jet x-sec. error
Proportional Error
PDF error dominated byEigenv.29,30: high x gluon dominated
PDF set 30
PDF set 29
C. Gwenlan, A. Cooper-Sarkar, C. Targett-Adams, hep-ph/0509220 (2005)
D. Clements, C. Butter, A. Moraes
Alessandro Tricoli, RAL & Oxford University 34MCWS Frascati, 23 Maggio 2006
Higgs production and decays at LHC Higgs production mechanisms
Higgs decays
Alessandro Tricoli, RAL & Oxford University 35MCWS Frascati, 23 Maggio 2006
Parton Luminosity uncertainty
Note: high x gluon should become better determined from Run 2 Tevatron data
PDF uncertainties encoded in parton-parton luminosity functions:
LHC (Alekhin 2002)
Tevatron (Alekhin 2002)
J. Stirling
Alessandro Tricoli, RAL & Oxford University 36MCWS Frascati, 23 Maggio 2006
High ET jet cross section at LHC
(James Stirling)
MRST2001E pdf error band
T
Alessandro Tricoli, RAL & Oxford University 37MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on SM:
Inclusive Jet cross-section (Syst.)
cms energy=14TeV, CTEQ6, NLOJET
Factorisation and Renormalisation scale uncertaintydominant theoretical uncertainty at low pT being overtaken by pdf uncertainties at a leading jet pT of ~1TeV
Jet Energy scale uncertaintythis systematic error is stat. significant, particularly for low pT jets
Alessandro Tricoli, RAL & Oxford University 38MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on SM:
Inclusive Jet cross-section (NLO/LO) The ratio of the inclusive jet cross-section as calculated for pdfs CTEQ6L1(LO) to CTEQ6m (NLO) using PYTHIA
Proportional Error
CTEQ6MEigenv.29,30: high x gluon dominated
ckin(3): PT min of hard scatter
Alessandro Tricoli, RAL & Oxford University 39MCWS Frascati, 23 Maggio 2006
Impact of PDF uncertainty on SM:
D.-Y. production.
CTEQ6.1E
x
% uncertainty
x
-5
+5
x
% uncertainty
x
-5
),( 0Qxux
+5
High mass dileptons → Uncertainties at high x important
),( 0Qxxu
Q2 = 104
Alessandro Tricoli, RAL & Oxford University 40MCWS Frascati, 23 Maggio 2006
Direct production
Pythia v6.221
Photon PT spectrum I. Hollins (2005)
Alessandro Tricoli, RAL & Oxford University 41MCWS Frascati, 23 Maggio 2006
W± Production at LHC
pp -> W± + …W
p p
Wud
Wdu Valence-Sea and Sea-Sea : largest contribution
CabibboSuppressed
Wcs
Wsc Sea-Sea: next largest contribution (Cabibbo dominating), whereas ~5% at Tevatron
(17%)
(23%)
Cabibbo Suppressed Contribution 1-3% at LHC
LHC
Alessandro Tricoli, RAL & Oxford University 42MCWS Frascati, 23 Maggio 2006
W -> e rapidity distributions
Signal:W -> eCTEQ6.1
W ->
Z -> e-e+
Z ->
e- No Cuts e+ No Cuts
e- After Sel. Cuts e+ After Sel. Cuts
Signal vs Background
Small Background contamination: ~1%=> Very clean measurement
Alessandro Tricoli, RAL & Oxford University 43MCWS Frascati, 23 Maggio 2006
•Data on the low-x valence distributions comes only from the CCFR/NuTeV data on Fe targets. The data extend down to x~0.01, but are subject to significant uncertainties from heavy target corrections in the low-x region.
•HERA neutral current data at high-Q2, involving Z exchange, make valence measurements on protons- but data are not yet very accurate and also only extend down to x~0.01
•Current PDFs simply have prejudices as to the low-x valence distributions - coming from the input parametrisations. The PDF uncertainties at low x do not actually reflect the real uncertainty (horse’s mouth- Thorne)
•LHC W asymmetry can provide new information and constraints in the x region 0.0005 < x <0.05
A. Cooper-Sarkar
Alessandro Tricoli, RAL & Oxford University 44MCWS Frascati, 23 Maggio 2006
Z + b-jet
Background and Systematic uncertainties
Efficiency of b-tagging we can expect Δεb/εb = 5%
Background from mistag Check mis-tagging on a sample where no b-quark jets should be
present: we use W + jets. Precision will be dominated by other sources of systematics:
Luminosity measurement Jet reconstruction and energy resolution
It is likely that the overall precision will be some-%, comparable to uncertainty on theoretical prediction
Alessandro Tricoli, RAL & Oxford University 45MCWS Frascati, 23 Maggio 2006
_d – u Asymmetry measurement
at LHC ? (I)
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It has been known that dbar ≠ ubar in the sea, since the violation of the Gottfried sum-rule in 1992. More recently, E866 Drell-Yan data have measured the shape of dbar-ubar.
This difference is usually fitted as a ‘valence-like’ quantity: dbar-ubar→0 as x → 0 e.g. dbar-ubar = 0.24 x0.5 (1-x)9 at Q2
0 ~ 7 GeV2
James Stirling
Alessandro Tricoli, RAL & Oxford University 46MCWS Frascati, 23 Maggio 2006
d – u Asymmetry measurement at LHC ? (II)
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The measured difference between dbar and ubar is actually very small, and is negligible at the Q2 of interest at the LHC. But the question has been raised: what if dbar-ubar ≠ 0 as x → 0 ?
Could this be significant for W production at LHC?x (dbar-ubar) dbar ubarE866
Alessandro Tricoli, RAL & Oxford University 47MCWS Frascati, 23 Maggio 2006
d – u Asymmetry measurement at LHC ? (III)
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Try the parametrisation 0.005x-0.16(1-x)13 (1+100x) at Q20 ~ 7 GeV2 inspired by the
shape of the gluonx (dbar-ubar) dbar ubar
dbar and ubar difference still negligible at the Q2 of interest at the LHC : dbar-ubar DOES NOT EVOLVE in Q2 the way that the singlet quantities gluon and sea do, it doesn’t even evolve in the more modest way that a non-singlet valence quantity does- it is the difference between two quantities which evolve in the same way.
To get it to matter at LHC Q2 it would have to be ‘fine-tuned’ at the starting scale