verducci

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: 

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

Large Hadron Collider: 

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

Slide4: 

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

Slide5: 

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…

Slide6: 

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.

Slide7: 

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)

The measurement: Z+jet (b): 

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

Slide9: 

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

Slide10: 

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) Pt b (MeV/c) Number of events

The D0 measurement of Zb/Zj: 

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

Slide12: 

LHC vs Tevatron 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) J.Campbell et al. Phys.Rev.D69:074021,2004

Slide13: 

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.

Slide14: 

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

Slide15: 

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)

How estimate the events…: 

How estimate the events… Backgrounds: Signal: Acceptance Efficiency = 59.6% Trigger Efficiency > 95% Cuts Efficiency ~ 40%

Slide17: 

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

Z+1 jet reconstruction (II): 

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

Slide19: 

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

Slide20: 

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

Slide21: 

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 Full Simulation Rome Sample Diglio 2 Gev per bin 5 Gev per bin 5-2 Gev per bin

Calibration in Situ: 

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

Slide23: 

pT balance = (pT jet – pT boson)/ pT boson

Slide24: 

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

Slide25: 

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%

Backup: 

Backup

Slide27: 

Inclusive b-tagging Algorithm Inclusive jet b-tagging Primary Vertex d Impact Parameter Extrapolated track Secondary Vertex, B-hadron decays 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 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)

Slide28: 

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

Slide29: 

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