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 Analysis on fully reconstructed MC samples Possible checks on systematics from data b-tagging efficiency background Jet energy scale Conclusions and Outlook 23th October 2006 Monica Verducci

Large Hadron Collider stot(pp) = 70 mb proton-proton 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 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 23th October 2006 Monica Verducci

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 23th October 2006 Monica Verducci

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… pA pB fa fb x1 x2 X 23th October 2006 Monica Verducci

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. 23th October 2006 Monica Verducci

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-2005-01 (A.C.Sarkar, hep-ph/0512228, Les Houches) Compton ~90% Annihilation ~10% 23th October 2006 Monica Verducci

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 23th October 2006 Monica Verducci

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 23th October 2006 Monica Verducci

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) 23th October 2006 Monica Verducci

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 23th October 2006 Monica Verducci

LHC vs Tevatron J.Campbell et al. Phys.Rev.D69:074021,2004 Cross Section (pb) TEVATRON LHC Processes ZQ inclusive 13.40.9 0.8 0.8 6.83 49.2 13.8 89.7 Zj inclusive 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) 23th October 2006 Monica Verducci

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. 23th October 2006 Monica Verducci

Impact on New Physics Susy Background: Z(->nn) +jet Black: ISAJET 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 23th October 2006 Monica Verducci

Z+jet(b) Analysis (70 <Mmm<110 GeV) Pt > 20 GeV/c 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) 23th October 2006 Monica Verducci

How estimate the events… Backgrounds: Signal: Acceptance Efficiency = 59.6% Trigger Efficiency > 95% Cuts Efficiency ~ 40% 23th October 2006 Monica Verducci

Z+1 jet reconstruction (I) # events (Rome) = 516550 (Layout for Rome Atlas Physics Workshop 2005) # events (CSC) = 139400 (Computing System Commissioning 2006) CSC cuts # event Rome CSC Eff 2 mu 306129 67737 59.3% 48.6% pt>20 Eta cut 182628 35.4% M range 157764 50721 30.5% 36.4% ROME 23th October 2006 Monica Verducci

Z+1 jet reconstruction (II) CSC5145 Algo # event Rome Eff Rome CSC Eff CSC Cone 0.7 51487 10% 16600 11.9% Cone 0.4 51864 16137 11.6% Kt 20118 3.9% 8808 6.3% ATLANTIS DISPLAY (Rφ) Three different algorithms to select the jets with different radius. Jet: pT > 15 GeV,|η|< 2.5 23th October 2006 Monica Verducci

BTagging Soft Muon Tagging All Jets B Jets BTagging Efficiency 59.5% 30 fb-1 b jet other # events 176642 204265 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% 23th October 2006 Monica Verducci

Systematic Effects Efficiency of b-tagging Background from mistag 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 23th October 2006 Monica Verducci

Jets will cover the whole Pt range 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 23th October 2006 Monica Verducci

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 23th October 2006 Monica Verducci

(pT jet – pT boson)/ pT boson pT balance = (pT jet – pT boson)/ pT boson Jet Z Df 23th October 2006 Monica Verducci

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 23th October 2006 Monica Verducci

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% 23th October 2006 Monica Verducci

Backup 23th October 2006 Monica Verducci

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 23th October 2006 Monica Verducci

Calibration in Situ (II) Cone DR=0.7 Et> 15 GeV Et(cell)=1.5 GeV E,m,g: pt>5GeV ISR Correction 23th October 2006 Monica Verducci

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 23th October 2006 Monica Verducci