1 / 29

Z Production associated with jets @LHC (ATLAS)

Z Production associated with jets @LHC (ATLAS). Monica Verducci CERN/INFN On behalf of Atlas Collaboration MCWS Frascati (Rome). Summary. Introduction @ LHC (ATLAS Detector) Parton Density Function (PDFs) measurements @ LHC Physics motivations for Z+jet measurement

allene
Télécharger la présentation

Z Production associated with jets @LHC (ATLAS)

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. Z Production associated with jets @LHC (ATLAS) Monica Verducci CERN/INFN On behalf of Atlas Collaboration MCWS Frascati (Rome)

  2. Summary • Introduction @ LHC (ATLAS Detector) • Parton Density Function (PDFs) measurements @ LHC • Physics motivations for Z+jet measurement • Analysis on fully reconstructed MC samples • Possible checks on systematics from data • b-tagging efficiency • background • Jet energy scale • Conclusions and Outlook Monica Verducci

  3. Large Hadron Collider stot(pp) = 70 mb proton-proton event rate R = s L = 109 eventi\sec (high luminosity) Bunch-crossing frequency: 40 MHz ~ 20 collisions p-p per bunch crossing Hierarchical trigger system ~MB/sec ~PB/year raw data 109 events/s =>1GHz 1 event~ 1MB (~PB/s) • Z(ll)+jet (~2Hz) • γ+jet (~ 0.1 Hz) • At low luminosity Monica Verducci

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

  5. fa pA x1 pB x2 fb X 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… Monica Verducci

  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 • 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

  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-2005-01 (A.C.Sarkar, hep-ph/0512228, Les Houches) Monica Verducci

  8. The measurement: Z+jet (b) • Measurement of the b-quark PDF • Process sensitive to b content of the proton (Diglio,Tonazzo,Verducci- ATL-COM-PHYS-2004-078 AIP Conf 794:93-96, 2005, hep-ph/0601164, CERN-2005-014) (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,Mehdiyev, Vichou, ATL-COM-PHYS-99-054) • Luminosity Monitor Monica Verducci

  9. 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% Now we have only HERA measurements, far from this precision Monica Verducci

  10. Z+b with different PDF sets MRST5NLO, CTEQ5M1, Alehkin1000 (with LHAPDF in Herwig) • Differences in total Z+b cross-section are of the order of 5% • Some sensitivity from differential distributions: jet energy calibration crucial • Other PDF sets predict larger differences (e.g., MRST5NNL0 >10%) • New studies are undergoing with different sets of PDFs function using NLO generators (Diglio, Farilla, Verducci) Number of events Pt b (MeV/c) Monica Verducci

  11. The D0 measurement of Zb/Zj • The D0 collaboration has recently measured: • (Z+b)/ (Z+jet) with Z→mm and Z → ee • → Phys.Rev.Lett.94:161801,2005 • Analysis flow: • select events with Z→ee or • Z→mm + jet • apply b-tagging • extract content of b, c and light quarks • (assuming Nc/Nb from theory) • Fitted values for selected sample in 184 pb-1 NLO (J.Campbell et al.): 0.018 +/- 0.004 Monica Verducci

  12. LHC vs Tevatron J.Campbell et al. Phys.Rev.D69:074021,2004 The measurement of Z+b should be more interesting at LHC than at Tevatron: • Signal cross-section larger (x80), and more luminosity • Relative background contribution smaller (x5) Monica Verducci

  13. 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 Simple spread of existing PDFs gives up to 10% uncertainty on prediction of Higgs cross section. Monica Verducci

  14. 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 Susy Atlas meetings T.S.S.Asai U. of Tokyo Event Topology Monica Verducci

  15. 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 Analysis presented @ ATLAS Physics Workshop 2005 ATL-COM-PHYS-2006-051 (Verducci, Diglio, Farilla, Tonazzo) Monica Verducci

  16. How estimate the events… • Backgrounds: • Signal: Acceptance Efficiency = 59.6% Trigger Efficiency > 95% Cuts Efficiency ~ 40% Monica Verducci

  17. Z+1 jet reconstruction (I) # events (Rome) = 516550 (Layout for Rome Atlas Physics Workshop 2005) # events (CSC) = 139400 (Computing System Commissioning 2006) CSC ROME Monica Verducci

  18. Z+1 jet reconstruction (II) ATLANTIS DISPLAY (Rφ) CSC5145 • Three different algorithms to select the jets with different radius. • Jet: pT > 15 GeV,|η|< 2.5 Monica Verducci

  19. BTagging All Jets B Jets BTagging Efficiency 59.5% Purity 60.7% Soft Muon Tagging All Muons B Muons Soft MuonTagging Efficiency 7.2% Purity 37.2% Monica Verducci

  20. 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 Monica Verducci

  21. 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 5 Gev per bin 5-2 Gev per bin Full Simulation Rome Sample Monica Verducci

  22. Calibration in Situ •  and Z0 are well calibrated objects at EM scale balancing the recoiling hadronic system potentially large statistics available: L=1033cm-2s-1 pT range from 20 GeV to ~60 GeV • 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 -0.16 ± 0.01 (pT jet – pT zeta)/ pT zeta Monica Verducci

  23. Df Jet Z pT balance = (pT jet – pT boson)/ pT boson Monica Verducci

  24. 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 processeslike Direct Photon, Z and W productions are good processes: • to constrain PDF’s at LHC, especially the gluon • to calibrate the detector Monica Verducci

  25. 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% Monica Verducci

  26. Backup Monica Verducci

  27. Inclusive b-tagging Algorithm Inclusive jet b-tagging Identification of a single jet in the event with b flavour • 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

  28. Calibration in Situ (II) • Cone DR=0.7 • Et> 15 GeV • Et(cell)=1.5 GeV • E,m,g: pt>5GeV ISR Correction Monica Verducci

  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 Monica Verducci

More Related