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

Vector Bosons with Jets @ ATLAS 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 Monica Verducci CERN

Vector Bosons with Jets @ ATLAS LHC Energy per proton 7 TeV Bunch spacing 25 ns Bunch size 15 m  12 cm Protons per Bunch 1011 Bunches per ring 2835 Lifetime 10 hours Luminosity 1034 cm-2 s-1 Lenth of the ring 27 Km Number of collisions per bunch 25 Vector Bosons with Jets @ ATLAS stot(pp) = 70 mb proton-proton event rate R = s L = 109 eventi\sec (ad alta luminosità) Monica Verducci CERN

ATLAS@LHC Muon Spectrometer: Pt measurements and muon identification Mounted on an air-core toroid with B field 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 Monica Verducci CERN

Importance of PDFs at LHC 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… Vector Bosons with Jets @ ATLAS pA pB fa fb x1 x2 X Monica Verducci CERN

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. Monica Verducci CERN

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->en rapidity 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) Compton ~90% Annihilation ~10% Vector Bosons with Jets @ ATLAS Monica Verducci CERN

Photons and W Boson Analysis 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% Vector Bosons with Jets @ ATLAS CTEQ61 MRST02 ZEUS02 e- rapidity e+ rapidity Generated Monica Verducci CERN

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 Monica Verducci CERN

Vector Bosons with Jets @ ATLAS 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 Monica Verducci CERN

Vector Bosons with Jets @ ATLAS 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 Monica Verducci CERN

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. Monica Verducci CERN

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

Vector Bosons with Jets @ ATLAS 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 Monica Verducci CERN

Vector Bosons with Jets @ ATLAS Backgrounds: Cross Section (pb) ZQ inclusive 13.40.9 0.8 0.8 Zj inclusive 89.7 13.8 49.2 6.83 Processes LHC TEVATRON 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% Monica Verducci CERN

Vector Bosons with Jets @ ATLAS BTagging All Jets 30 fb-1 b jet other # events 176642 204265 Vector Bosons with Jets @ ATLAS B Jets BTagging Efficiency 59.5% Purity 60.7% Soft Muon Tagging All Muons 30 fb-1 b jet other # events 22630 68088 B Muons Soft MuonTagging Efficiency 7.2% Purity 37.2% Monica Verducci CERN

Vector Bosons with Jets @ ATLAS 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 Monica Verducci CERN

Vector Bosons with Jets @ ATLAS 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 Diglio 2 Gev per bin Vector Bosons with Jets @ ATLAS 5 Gev per bin 5-2 Gev per bin Full Simulation Rome Sample Monica Verducci CERN

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 Monica Verducci CERN

Vector Bosons with Jets @ ATLAS 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 processes like Direct 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 Monica Verducci CERN

Vector Bosons with Jets @ ATLAS 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 Monica Verducci CERN

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

Vector Bosons with Jets @ ATLAS Backup Monica Verducci CERN

Event Selection Criteria for W+- ->l+- nl (TDR selection cuts) Backgrounds sums: QCD 2->2 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 Z -> t+t- Vector Bosons with Jets @ ATLAS Z -> e-e+ W -> tn - Monica Verducci CERN

Vector Bosons with Jets @ ATLAS W Analysis (I) What is effect of including ATLAS W rapidity “pseudo-data” into global PDF fits. Created 1M “data” sample, generated using CTEQ6.1 PDF and simulate ATLAS detector response using ATLFAST. Correct back to generator level using ZEUS-S PDF and use this “pseudo-data” in a global ZEUS-S PDF fit. Central value of ZEUS-S PDF prediction shifts and uncertainty is reduced: Vector Bosons with Jets @ ATLAS ~1day of data-taking at low Lumi xg(x) ~ x –λ : 35% error reduction BEFORE λ = -0.199 ± 0.046 AFTER λ = -0.181 ± 0.030 low-x gluon shape parameter λ: Monica Verducci CERN

Vector Bosons with Jets @ ATLAS 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) Monica Verducci CERN

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 Monica Verducci CERN

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 Monica Verducci CERN

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 Monica Verducci CERN