1 / 47

Misure di Funzioni di Struttura ad LHC

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)?

casey
Download Presentation

Misure di Funzioni di Struttura ad LHC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. Overview • Introduction: • What are Parton Distribution Functions(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

  3. What are PDF distributions? • PDFs are parameterizations of the partonic content of the proton: • i = uv, dv, g and sea • x = pparton / Ebeam parton momentum fraction • Q2momentum 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 Q02 ~ 1-7 GeV2 : • Valence Quarks:f ~xl(1-x)hP(x) • Sea/Gluon:f ~x-l (1-x)hP(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) • nN dimuon (CCFR, NuTeV) • etc. fi(x,Q2) Alessandro Tricoli, RAL & Oxford University

  4. PDF uncertainties • 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: 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 provideUNCERTAINTIES:fi(x,Q2)± δ fi(x,Q2): use a modified c2 -> c2 + DT2to consider non-gaussian syst. errors and their correlations. T= tolerance ~ • Offset Method: the correlated syst. errors affect only the determination of the PDF uncertainty, NOT the best fit (centre value) e.g. ZEUS-S DT2~49 • Hessian method: the collective effect of the correlated syst. errors can also modify the values of the best fit e.g. CTEQ6 DT2=100, MRST01: DT2=50 Alessandro Tricoli, RAL & Oxford University

  5. fa pA x1 pB x2 fb X Why PDFs are vital at LHC ? • 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. Alessandro Tricoli, RAL & Oxford University

  6. Today they are considered within Proton Structure Uncertainty band A lesson from Tevatron on the importance of PDFs PDF Uncertainties must be properly taken into account or SM physics features could be misinterpreted as new physics signal Tevatron Jet data were originally taken as evidence of New Physics Since Proton Structure Uncertainty was not properly considered (Data-Theory)/Theory CDF Run I Alessandro Tricoli, RAL & Oxford University

  7. LHC Kinematic regime Kinematic regime for LHC much broader than currently explored 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 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

  8. g q H H W/Z q _ g q t g W/Z H W/Z q g t W/Z PDF uncertainties (CTEQ6M, MRST01E, Alekhin02) on Higgs cross-sections: Up to 10% (15%) Impact of PDF uncertainty on New Physics:Higgs _ H Djouadi & Ferrag, Phys. Lett. B 586 345:352 (2004) Alessandro Tricoli, RAL & Oxford University

  9. Standard Model prediction zone: where every measured cross section can be explained by a PDF fit, and everypower of discovering new physics is killed and absorbed by the PDF fit SM prediction CTEQ6M PDFs Central value E.D. are masked by PDF uncertainties: 1 s limits Pt(GeV) 3 s limits High-x gluon is responsible of the big PDF uncertainties PDF uncertainties decrease discovery reach for E.D. from MC 5 (10) TeV to < 2 (3)TeV Impact of PDF uncertainty on New Physics: Extra Dimensions Ferrag, hep-ph/0407303 (2004) An E.D. Model: di-jets cross section in the E.D. regime is a continuity of the Standard Model one with new as running: (mb) Mc= 2 TeV Mc= 8 TeV SM 2XD 4XD 6XD Pt(GeV) Pt(GeV) Alessandro Tricoli, RAL & Oxford University

  10. 10 fb-1 ~600 GeV Impact of PDF uncertainty on SM: Drell-Yan cross section Sensitive to New Physics: PDF Relative Uncertainty MC@NLO 40 CTEQ6 PDFs Mll [GeV] High mass dileptons → Uncertainties at high-x important (up to 10%) Alessandro Tricoli, RAL & Oxford University

  11. (NLOJET,CTEQ6) Factorisation & Renormalisation scales=pt/2 Impact of PDF uncertainty on SM: Inclusive Jet cross-section A SM measurement sensitive to New Physics: compositeness, Black Holes etc. D. Clements, C. Butter, A. Moraes • Experimentaland theoretical errorscan distort the measurements and predictions • creating false signals of new physics: • PDF Uncertainty (gluon), • Renormalisation and Factorisation scale uncertainty, • Experimental Jet Energy scale uncertainty. Alessandro Tricoli, RAL & Oxford University

  12. Uncorrelated Exp. Systematics D. Clements, C. Butter, C. Gwenlan, T. Carli, A. Cooper-Sarkar, M Sutton Current PDF uncertainty: 10% at 1 TeV, 25% at 2 TeV, up to 60% at 5 TeV 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: Impact of PDF uncertainty on SM: Inclusive Jet cross-section • 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 jetsup to 1 TeV even with 1 fb-1 luminosity. Alessandro Tricoli, RAL & Oxford University

  13. W production: (main contributions) Z production: (main contributions) How to constrain PDFs at low-x at LHC? We can improve our knowledge ofPDF’s at LHCmeasuring the vector boson production:W’s, Z’s (and photons?) At the EW scale cross sections are dominated by sea and/or gluon interactions at low-x. Furthermore, at Q2~M2W/Z the sea is driven by the gluon (via gluon splitting) which is far less precisely determined for all x values. Alessandro Tricoli, RAL & Oxford University

  14. W,Z tot. cross sections W+- diff. cross section (rapidity) W± MRST PDF Symmetric NNLO corrections small ~ few% NNLO residual scale dependence < 1% 4% MRST02 error PDF Set Theoretical uncert. dominated by PDFs (nb) (nb) (nb) ZEUS-S W, Z very clean signals (bkg on W->e ~1%) CTEQ6.1 LHC exp. uncertainty is sufficiently small to distinguish between different PDF sets MRST01 (LHC dominated by systematic uncertainty) How to constrain low-x PDFs at LHCsingle Z and W± Productions Alessandro Tricoli, RAL & Oxford University

  15. e-rapidity e+ rapidity CTEQ61 CTEQ61 MRST02 MRST02 ZEUS-S ZEUS-S Generator Level Error boxes are the Full PDF Uncertainties ATLAS Detector Level with sel. cuts GOAL: syst. exp. error ~4% W -> e n rapidity distributions (1) Experimentally we detect e+- from W decays: W+- -> e+- A. Tricoli hep-ex/0511020, A. Tricoli, A. Cooper-Sarkar, C. Gwenlan, CERN-2005-014, hep-ex/0509002 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 CTEQ6.1M ZEUS-S to MRST01E central value difference ~5% ZEUS-S to CTEQ6.1 central value difference ~3.5% Alessandro Tricoli, RAL & Oxford University

  16. ZEUS-PDFBEFORE including W data ZEUS-PDFAFTER including W data e+CTEQ6.1 pseudo-data e+CTEQ6.1 pseudo-data |h| low-x gluon shape parameter λ,xg(x) ~ x –λ BEFORE λ = -0.199 ± 0.046 AFTERλ = -0.181 ± 0.030 41% error reduction W -> e n rapidity distributions (2) 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: |h| In few day stat. of LHC at low Luminosity NB: in ZEUS-PDF fit the e± Normalisation is left free => no assumption on Luminosity measurement Alessandro Tricoli, RAL & Oxford University

  17. CTEQ61 MRST02 ZEUS-S Generator Level Error boxes are the Full PDF Uncertainties ATLAS Detector Level with selection cuts W Charge Asymmetry measurement (1) Experimentally we measure e+- charge asymmetry from W decays: W+- -> e+- e+ - e-asymmetry: HERWIG MC Simulations with NLO Corrections • In the Asymmetryexperimental uncertaintiesand thegluon/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? Alessandro Tricoli, RAL & Oxford University

  18. At small-x uV – dV Q2=7 GeV2 CTEQ6.1 MRST02 Q2=Mw2 x x- range affecting W asymmetry in the measurable rapidity range 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. at Q2=MW2 and x~0.006 (corresponging to y~0 at LHC): MRST uV –dV is 25% lower than other PDF sets which reflects on A(y) measurement. For the first timewith the LHC we will have valence PDF discrimination measuring valence distributions at x~0.005 on proton targets Alessandro Tricoli, RAL & Oxford University

  19. W -> e n 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-rapidity CTEQ6.1 MRST2003 e+ - e- asymmetry CTEQ6.1 MRST2003 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

  20. Can we compute the PDF Uncertainty with MC in a fast way:PDF Re-weighting • 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 LONGgeneration 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 Alessandro Tricoli, RAL & Oxford University

  21. CTEQ61 Generated CTEQ61 Generated CTEQ61 Re-weighted from MRST02 CTEQ61 Re-weighted from MRST02 Can we use PDF re-weighting to simulate other PDFs?- OK for RAPIDITY distributions Events generated with HERWIG & MRST02 and re-weighted with CTEQ61 are compared to Events generated with HERWIG & CTEQ61 W- W+ W- W+ Relative difference between Re-weighted and Generated distributions Weighted mean on whole y-range • Accuracy of ~0.5% in rapidityand 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) Alessandro Tricoli, RAL & Oxford University

  22. Conclusions • Precision Parton Distribution Functionsare crucial for new physicsdiscoveries 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 processeslike 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

  23. EXTRAS Alessandro Tricoli, RAL & Oxford University

  24. Direct g production: (LO contributions) Compton: (~90%) Annihilation: (~10%) W production: (main contributions) Z production: (main contributions) How to constrain PDFs at mid-low-x at LHC? We can improve our knowledge ofPDF’s at LHCmeasuring the vector boson production:W’s, Z’s (and photons?) At the EW scale cross sections are dominated by sea and/or gluon interactions at low-x. Furthermore, at Q2~M2W/Z the sea is driven by the gluon (via gluon splitting) which is far less precisely determined for all x values. Alessandro Tricoli, RAL & Oxford University

  25. Why do we measure the b-PDF? How to constrain gluon-PDFs at LHCZ + b-jet(1) • Also background to Higgs searches: • Sensitive • to b content of the proton: • bb->Z @ LHC is ~5% of entire Z production Knowing σZ to about 1% requires a b-PDF precision of the order of 20% (J.Campbell et al. Phys.Rev.D67:095002,2003) • (J.Campbell et al. Phys.Rev.D69:074021,2004) Now we have only HERA measurements, far from this precision Alessandro Tricoli, RAL & Oxford University

  26. Z+b selected events (10 fb-1) Signal+Background Background Jet PT (GeV) How to constrain gluon-PDFs at LHCZ + b-jet(PDF Uncertainty) HERWIG with MRST03CNNLO, CTEQ5M1, Alehkin1000 S. Diglio, A. Tonazzo and M. Verducci (2005) Event selection: only Z→m+m- • Two isolated muons with high PT • inclusive b-tagging of jet • PDF Differencesin total Z+b cross-section are of the order of 5% to 10 % ATLAS Number of events Alessandro Tricoli, RAL & Oxford University

  27. How to constrain gluon-PDFs at LHCdirect g production Typical Jet +  event Jet and photon are back to back In NLO cross-section calculations: discrepancy between different PDF set predictions up to~20% ATLAS Kumar et al. Physics Review D 67 • 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-h photons • Problems: • Large background (especially at low PT) • Can theTheoretical-Experimental Discrepancy in PT distribution be well understood? I. Hollins (2005) Alessandro Tricoli, RAL & Oxford University

  28. 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 symmetry factorisation/renormalisation scheme/scale Q02 αS treatment of heavy flavours Alessandro Tricoli, RAL & Oxford University

  29. 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: Alessandro Tricoli, RAL & Oxford University

  30. Gluon fractional error x 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-quark uncertainties over all-x - significant improvement to sea and gluon uncertainties at mid-to-high-x - little visible improvement to sea and gluon uncertainties at low-x C. Gwenlan, A. Cooper-Sarkar,C. Targett-Adams, hep-ph/0509220 (2005) Alessandro Tricoli, RAL & Oxford University

  31. 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+d time-space structure Parameters: number of extra-dimensionsd compactification scale Mc Phenomenological aspects: • Possibility to produce Gravitons at LHC: low Planck scale • Kaluza Klein (KK)excitations: d compactified extra dimensions • Violation of the expected (MS)SM evolution behavior of aem,w,s • (E. Dudas, R. Dienes, T. Ghergetta, hep/ph9803466 and hep/ph9807522) MSSM+XD MGUT ~30 TeV (4+2)D, R=1/10 Tev-1 Exerimentally: Evolution of as by measuring di-jets cross section on a large energy range Alessandro Tricoli, RAL & Oxford University

  32. Z+b selected events (10 fb-1) Di-muons Invariant Mass Z+b Z+jet Signal+Background Background Jet PT (GeV) GeV How to constrain gluon-PDFs at LHCZ + b-jet(Measurement) Signal: Background: • Event selection: only Z→m+m- • 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% ) ATLAS Alessandro Tricoli, RAL & Oxford University

  33. Proportional Error PDF error: 10% at 1 TeV, 25% at 2 TeV, up to 60% at 5 TeV PDF set 30 PDF set 29 PDF error dominated by Eigenv.29,30: high x gluon dominated Impact of PDF uncertainty on SM: Inclusive Jet cross-section D. Clements, C. Butter, A. Moraes C. Gwenlan, A. Cooper-Sarkar, C. Targett-Adams, hep-ph/0509220 (2005) • 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 jetsup 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 Alessandro Tricoli, RAL & Oxford University

  34. Higgs production and decays at LHC Higgs production mechanisms Higgs decays Alessandro Tricoli, RAL & Oxford University

  35. Parton Luminosity uncertainty J. Stirling PDF uncertainties encoded in parton-parton luminosity functions: Note:high x gluon should become better determined from Run 2 Tevatron data LHC (Alekhin 2002) Tevatron (Alekhin 2002) Alessandro Tricoli, RAL & Oxford University

  36. MRST2001E pdf error band T High ET jet cross section at LHC (James Stirling) Alessandro Tricoli, RAL & Oxford University

  37. cms energy=14TeV, CTEQ6, NLOJET Impact of PDF uncertainty on SM: Inclusive Jet cross-section (Syst.) Jet Energy scale uncertainty this systematic error is stat. significant, particularly for low pT jets Factorisation and Renormalisation scale uncertainty dominant theoretical uncertainty at low pT being overtaken by pdf uncertainties at a leading jet pT of ~1TeV Alessandro Tricoli, RAL & Oxford University

  38. Proportional Error CTEQ6M Eigenv.29,30: high x gluon dominated 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 ckin(3): PTmin of hard scatter Alessandro Tricoli, RAL & Oxford University

  39. Impact of PDF uncertainty on SM: D.-Y. production. % uncertainty % uncertainty +5 +5 -5 -5 CTEQ6.1E x x Q2 = 104 x x High mass dileptons → Uncertainties at high x important Alessandro Tricoli, RAL & Oxford University

  40. Direct g production Photon PT spectrum I. Hollins (2005) Pythia v6.221 Alessandro Tricoli, RAL & Oxford University

  41. Valence-Sea and Sea-Sea: largest contribution (17%) Sea-Sea: next largest contribution (Cabibbo dominating), whereas ~5% at Tevatron (23%) W± Production at LHC LHC pp -> W± + … W p p Cabibbo Suppressed Cabibbo Suppressed Contribution 1-3% at LHC Alessandro Tricoli, RAL & Oxford University

  42. e- No Cuts e+ No Cuts Z -> t+t- Z -> e-e+ W -> tn e- After Sel. Cuts e+ After Sel. Cuts Signal:W -> en (CTEQ6.1) W -> e n rapidity distributions Signal vs Background Small Background contamination: ~1% => Very clean measurement Alessandro Tricoli, RAL & Oxford University

  43. A. Cooper-Sarkar • 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 Alessandro Tricoli, RAL & Oxford University

  44. Z + b-jetBackground 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

  45. _ _ d – u Asymmetry measurement at LHC ? (I) 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)9at Q20 ~ 7 GeV2 James Stirling Alessandro Tricoli, RAL & Oxford University

  46. _ _ d – u Asymmetry measurement at LHC ? (II) 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? ubar x (dbar-ubar) dbar E866 Alessandro Tricoli, RAL & Oxford University

  47. _ _ d – u Asymmetry measurement at LHC ? (III) Try the parametrisation 0.005x-0.16(1-x)13 (1+100x) at Q20 ~ 7 GeV2 inspired by the shape of the gluon x (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 Alessandro Tricoli, RAL & Oxford University

More Related