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Vector Boson Production associated with jets @LHC (Atlas)

Vector Boson Production associated with jets @LHC (Atlas). Monica Verducci CERN/CNAF On behalf of Atlas Collaboration IFAE 2006 Pavia. Summary. Introduction @ LHC (ATLAS Detector) Parton Density Function (PDFs) measurements @ LHC

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Vector Boson Production associated with jets @LHC (Atlas)

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  1. Vector Boson Production associated with jets @LHC (Atlas) Monica Verducci CERN/CNAF On behalf of Atlas Collaboration IFAE 2006 Pavia

  2. Summary • Introduction @ LHC (ATLAS Detector) • Parton Density Function (PDFs) measurements @ LHC • Vector Boson Production analysis as a possible constrain for PDFs: /W/Z+jets • Potential of the Z+jet analysis • Systematics of the measurement • First data: calibrations • Conclusions Vector Bosons with Jets @ ATLAS

  3. LHC Vector Bosons with Jets @ ATLAS stot(pp) = 70 mb proton-proton event rate R = s L = 109 eventi\sec (ad alta luminosità)

  4. Muon Spectrometer: Pt measurements and muon identification Mounted on an air-core toroid with B field ATLAS@LHC Vector Bosons with Jets @ ATLAS Inner Tracker: Pt Measurements and charge of the particles with a solenoidal magnetic field of 2 T. Calorimeters: electromagnetic and hadronic

  5. At a hadron collider, cross sections are a convolution of the partonic cross section with the PDFs. PDFs are important for Standard Model physics, which will also be backgrounds to any new physics discovery: Higgs, Extra Dimensions… fa pA x1 pB x2 fb X Importance of PDFs at LHC Vector Bosons with Jets @ ATLAS

  6. Parton Kinematic Regime@LHC • The kinematic regime at the LHC is much broader than currently explored. • At the EW scale (ie W and Z masses) theoretical predictions for the LHC are dominated by low-x gluon uncertainty Vector Bosons with Jets @ ATLAS • At the TeV scale, uncertainties in cross section predictions for new physics are dominated by high-x gluon uncertainty The x dependence of f(x,Q2) is determined by fits to data, the Q2 dependence is determined by the DGLAP equations. Fits and evaluation of uncertainties performed by CTEQ, MRST, ZEUS etc.

  7. Compton ~90% Annihilation ~10% Constraining PDFs at LHC • Direct photon production Studies ongoing to evaluate experimental uncertainties (photon identification, fake photon rejection, backgrounds etc.) (I.Dawson - Panic05,proc.) • W and Z rapidity distributions Impact of PDF errors on W->enrapidity distributions investigated using HERWIG event generator with NLO corrections. Systematics < 5% (A.Tricoli, hep-ex/0511020,PHOTON05) (A.Tricoli, Sarkar, Gwenlan CERN-2005-014) (A.C.Sarkar, hep-ph/0512228, Les Houches) • Z+b-jet (Diglio,Tonazzo,Verducci- ATL-COM-PHYS-2004-078 AIP Conf 794:93-96, 2005, hep-ph/0601164, CERN-2005-014) Vector Bosons with Jets @ ATLAS

  8. Photons production Photon couples only to quarks, so potential good signal for studying underlying parton dynamics. g Selection Efficiency~80% CTQE6L-MRST01E ~ 16-18% W Production 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 difference ~5% ZEUS-S to CTEQ6.1 difference~3.5% CTEQ61 CTEQ61 MRST02 MRST02 ZEUS02 ZEUS02 e-rapidity e+ rapidity Generated Generated Photons and W Boson Analysis Vector Bosons with Jets @ ATLAS

  9. The measurement: Z+jet (b) • Measurement of the b-quark PDF • Process sensitive to b content of the proton (J.Campbell et al. Phys.Rev.D69:074021,2004) • Tuning of the MonteCarlo tools for Standard Model • Background of new physics signatures • Calibration Tool (clean and high statistics signature) (Santoni, Lefevre ATL-PHYS-2002-026) (Gupta et al. ATL-COM-PHYS-2005-067) • Luminosity Monitor Vector Bosons with Jets @ ATLAS

  10. Why measure 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% Vector Bosons with Jets @ ATLAS Now we have only HERA measurements, far from this precision

  11. PDF Uncertainty HERWING: MRST03CNNLO – CTEQ5M1 – Alehkin1000 • Differences in total Z+b cross-section are of the order of 5-10% • The D0 collaboration has measured the ratio: (Z+b)/ (Z+jet) with Z→mm and Z → ee Phys.Rev.Lett.94:161801,2005 • Fitted values for selected sample in 184 pb-1 Vector Bosons with Jets @ ATLAS #Events Pt b-jet (MeV) NLO (J.Campbell et al.): 0.018 +/- 0.004

  12. Z+jet: Impact to other measurements • Background to Higgs search • In models with enhanced (h+b) and BR(h->mm) (J.Campbell et al. Phys.Rev.D67:095002,2003) • Background to MS Higgs search • In models where pp -> ZH con H -> bb Vector Bosons with Jets @ ATLAS Simple spread of existing PDFs gives up to 10% uncertainty on prediction of Higgs cross section.

  13. Susy Background: Z(->nn) +jet Effective Mass distribution for No-Leptons Mode after standard event selection M(g)≈M(q)≈1TeV Impact on New Physics Black: ISAJET Red: PYTHIA Vector Bosons with Jets @ ATLAS Susy Atlas meetings T.S.S.Asai U. of Tokyo Event Topology

  14. Z+jet(b) Analysis Event selection: taking into account only Z→mm • Two isolated muons with • Pt > 20 GeV/c • opposite charge • invariant mass close to Mz (70 <Mmm<110 GeV) • Two different b-tagging algorithms have been considered: • Soft muon • Inclusive b-tagging of jets Vector Bosons with Jets @ ATLAS Analysis presented @ ATLAS Physics Workshop 2005

  15. TEVATRON LHC Processes ZQ inclusive 13.40.9 0.8 0.8 6.83 49.2 13.8 89.7 Zj inclusive Cross Section (pb) • Backgrounds: Vector Bosons with Jets @ ATLAS • Signal: • (J.Campbell et al. Phys.Rev.D69:074021,2004) Z Mass RECO MC Acceptance Efficiency = 59.6% Trigger Efficiency > 95% Cuts Efficiency ~ 40%

  16. BTagging All Jets Vector Bosons with Jets @ ATLAS B Jets BTagging Efficiency 59.5% Purity 60.7% Soft Muon Tagging All Muons B Muons Soft MuonTagging Efficiency 7.2% Purity 37.2%

  17. Systematic Effects • Efficiency of b-tagging • To check b-tagging efficiency, we can use b-enriched samples. Experience at Tevatron & LEP indicates that we can expect: • Δεb/εb = 5% • Background from mistag • Check mistagging on a sample where no b-quark jets should be present Vector Bosons with Jets @ ATLAS

  18. Diglio 2 Gev per bin • We use W+jet events, where there are not b jet • Jets will cover the whole Pt range • Statistics 30x Z+j (after selection of decays to muons) • The relative error on background from mistagging can be kept at the level of few-% in each bin of the Pt range Vector Bosons with Jets @ ATLAS 5 Gev per bin 5-2 Gev per bin Full Simulation Rome Sample

  19. First Data Z+jet: Calibration • Calibration in situ of the jet energy scale -> jet energy absolute scale within 1% • This means calibrate the calorimeters using jets reconstructed in the exp. • Z+jet (b 5%) high statistic -> 380pb • pjetT = pZT balance criteria on transverse plan Vector Bosons with Jets @ ATLAS Truth Reco Gupta,et al. Atlfast Santoni,Lefevre

  20. Conclusions I • Precision Parton Distribution Functions are crucial for new physics discoveries at LHC and to tune MonteCarlo studies: • PDF uncertainties can compromise discovery potential (HERA-II: significant improvement to high-x PDF uncertainties) • At LHC the major source of errors will not be statistic but systematic uncertainties • To discriminate between conventional PDF sets we need to reach high experimental accuracy ( ~ few%) and to improve the detector performance and resolution • Standard Model processeslikeDirect Photon, Z and W productions are good processes: • to constrain PDF’s at LHC, especially the gluon • to calibrate the detector Vector Bosons with Jets @ ATLAS

  21. Conclusions II • Z+b measurement in ATLAS will be possible with high statistics and good purity of the selected samples with two independent tagging methods • We will have data samples to control systematic errors related to b-tagging at the few-% level over the whole jet Pt distribution • b-tagging efficiency • Mistagging: from W+jet • Jet Calibration in situ: error within 1% Vector Bosons with Jets @ ATLAS

  22. Many Thanks to • Patrizia Azzi & Fulvio Piccinini • Giacomo Polesello • Fabiola Gianotti • Alessandro Tricoli • Ada Farilla & Sara Diglio • Chiara Roda Vector Bosons with Jets @ ATLAS

  23. Backup Vector Bosons with Jets @ ATLAS

  24. W -> tn Z -> t+t- Z -> e-e+ QCD 2->2 Event Selection Criteria for W+- ->l+-nl(TDR selection cuts) Backgrounds sums: • Electrons: |η| < 2.4 • Pt > 25 GeV • Missing Et > 25 GeV • To reject QCD bkg & high Pt W and Z due to I.S.R. : • No reconstructed jets in the event with Pt > 30 GeV • Recoil on transverse plane should satisfy |u|< 20 GeV Vector Bosons with Jets @ ATLAS -

  25. low-x gluon shape parameter λ: BEFOREλ = -0.199 ± 0.046 AFTERλ = -0.181 ± 0.030 xg(x) ~ x –λ: 35% error reduction W Analysis (I) • What is effect of including ATLAS W rapidity “pseudo-data” into global PDF fits. Created 1M “data” sample, generated usingCTEQ6.1 PDFand simulate ATLAS detector response using ATLFAST. Correct back to generator level usingZEUS-S PDFand use this “pseudo-data” in a globalZEUS-S PDF fit. Central value ofZEUS-S PDFprediction shifts and uncertainty is reduced: Vector Bosons with Jets @ ATLAS ~1day of data-taking at low Lumi

  26. W Analisys (II) • To improve on current PDF uncertainties: • Study of Rapidity distributions W+- -> e+-n Vector Bosons with Jets @ ATLAS Cross section for pp→W+X with W→lν, l=e,μ is ~30 nb (10 time larger than Tevatron) 300M evts/y at low Luminosity Cuts acceptance ~25% Assuming Lepton reconstruction efficiency ~ 90% & identification efficiency ~ 80% Total Selection Efficiency ~20% 60 M W’s/y al low Luminosity (10 fb-1)

  27. BTagging Algorithm Inclusive jet b-tagging Identification of a single jet in the event with b flavour Vector Bosons with Jets @ ATLAS • pT > 15 GeV • |η|< 2.5 • Number of tracks > 0 • Secondary vertex >3 (weight) Primary Vertex Secondary Vertex, B-hadron decays d Impact Parameter Extrapolated track Life time of a bottom hadron is about t ~ 1.5 ps long enought to permit to a hadron of 30 GeV of energy to do a distance of L ~ 3 mm before decaying

  28. Calibration in Situ (II) • Cone DR=0.7 • Et> 15 GeV • Et(cell)=1.5 GeV • E,m,g: pt>5GeV Vector Bosons with Jets @ ATLAS ISR Correction

  29. Calibration in Situ (III) • BiSector Method • Measurement of the resolution via estimation of the ISR contribution • Transverse plane: • η depends only on ISR •  depends on both resolution and ISR Vector Bosons with Jets @ ATLAS

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