Transcript Slide 1
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 2 Large Hadron Collider stot(pp) = 70 mb proton-proton event rate R = s L = 109 eventi\sec (high luminosity) Energy per proton 7 TeV Bunch spacing 25 ns Bunch size 15 m 12 cm Bunch-crossing frequency: 40 MHz ~ 20 collisions p-p per bunch crossing Protons per Bunch 1011 Bunches per ring 2835 109 events/s =>1GHz 1 event~ 1MB (~PB/s) Hierarchical trigger system ~MB/sec ~PB/year raw data Z(ll)+jet (~2Hz) γ+jet (~ 0.1 Hz) At low luminosity 23th October 2006 Lifetime Luminosity Lenth of the ring Number of collisions per bunch Monica Verducci 10 hours 1034 cm-2 s-1 27 Km 25 3 ATLAS@LHC Muon Spectrometer: Pt measurements and muon identification Mounted on an air-core toroid with B field Calorimeters: electromagnetic and hadronic 23th October 2006 Monica Verducci Inner Tracker: Pt Measurements and charge of the particles with a solenoidal magnetic field of 2 T. 4 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… fa pA x1 s X x2 23th October 2006 p Monica Verducci fb ˆ B 5 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 6 Constraining PDFs at LHC Direct photon production Studies ongoing to evaluate experimental uncertainties (photon identification, fake photon rejection, backgrounds etc.) (I.Dawson - Panic05,proc.) Compton ~90% Annihilation ~10% ud W e n du W e n 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) 23th October 2006 Monica Verducci ud W e n uu Z du W e n dd Z 7 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 8 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 9 Z+b with different PDF sets MRST5NLO, CTEQ5M1, Alehkin1000 Differences in total Z+b cross- section are of the order of 5% Some sensitivity from differential distributions: jet energy calibration crucial Number of events (with LHAPDF in Herwig) 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) 23th October 2006 Monica Verducci Pt b (MeV/c) 10 The D0 measurement of Zb/Zj The D0 collaboration has recently measured: s(Z+b)/ s(Z+jet) with Z→ and Z → ee → Phys.Rev.Lett.94:161801,2005 Analysis flow: – select events with Z→ee or Z→ + 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 0.005 s Z b = 0.024 0.005( stat) ( syst ) 0.004 s Z j 23th October 2006 Monica Verducci NLO (J.Campbell et al.): 0.018 +/- 0.004 11 LHC vs Tevatron Cross Section (pb) J.Campbell et al. Phys.Rev.D69:074021,2004 TEVATRON Processes gb Zb gb Zbb gc Zc gc Zcc LHC ZQ inclusive 13.40.9 0.8 0.8 70 30 1040 70 60100 50 6.83 49.2 20.3 11..85 0.111..32 40 1390 100 60 70 80 13.8 89.7 Zj inclusive qq Zg , gq Zq 9 7 1010 44 40 2 12 60 300 15870900 600 300 500 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 12 Z+jet: Impact to other measurements Background to Higgs search In models with enhanced s(h+b) and BR(h-> (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 13 Impact on New Physics Black: ISAJET Red: PYTHIA Susy Background: Z(->nn jet Effective Mass distribution for No- Leptons Mode after standard event selection M(g)≈M(q)≈1TeV Susy Atlas meetings T.S.S.Asai U. of Tokyo 23th October 2006 Event Topology Monica Verducci 14 Z+jet(b) Analysis Event selection: taking into account only Z→ Two isolated muons with • • • Pt > 20 GeV/c opposite charge invariant mass close to Mz (70 <M<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 15 How estimate the events… Backgrounds: Signal: Acceptance Efficiency = 59.6% Trigger Efficiency > 95% Cuts Efficiency ~ 40% Pythia sother = 2.6 106 mb Nb = s table b BR ( Z ) L t acc cuts b Pythia N other = s other L t acc cuts other 23th October 2006 Monica Verducci 16 Z+1 jet reconstruction (I) # events (Rome) = 516550 (Layout for Rome Atlas Physics Workshop 2005) CSC # events (CSC) = 139400 (Computing System Commissioning 2006) cuts 2 mu # event Rome 306129 # event CSC 67737 Eff Rome 59.3% Eff CSC 48.6% pt>20 Eta cut 182628 67737 35.4% 48.6% M range 157764 50721 30.5% 36.4% 23th October 2006 Monica Verducci ROME 17 Z+1 jet reconstruction (II) CSC5145 Algo # event Rome Eff Rome # event CSC Eff CSC Cone 0.7 51487 10% 16600 11.9% Cone 0.4 51864 10% 16137 11.6% Kt 20118 3.9% 8808 6.3% Three different algorithms to select the jets with different radius. Jet: pT > 15 GeV,|η|< 2.5 23th October 2006 Monica Verducci ATLANTIS DISPLAY (Rφ) 18 BTagging All Jets 30 fb-1 b jet other # events 176642 204265 B Jets BTagging Efficiency 59.5% Purity 60.7% Soft Muon Tagging 30 fb-1 b jet other # events 22630 68088 All Muons B Muons Soft MuonTagging Efficiency 7.2% Purity 37.2% 23th October 2006 Monica Verducci 19 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 23th October 2006 Monica Verducci 20 Diglio We use W+jet events, where there are not b jet 2 Gev per bin Jets will cover the whole Pt range Statistics 30x Z+j (after selection of decays to muons) 5 Gev per bin The relative error on background from mistagging can be kept at the level of few-% in each bin of the Pt range 5-2 Gev per bin Full Simulation Rome Sample 23th October 2006 Monica Verducci 21 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 23th October 2006 Monica Verducci -0.16 ± 0.01 (pT jet – pT zeta)/ pT zeta 22 pT balance = (pT jet – pT boson)/ pT boson (p TRaw p TZ ) Cal 0 Raw ~ 1 = p = p T T (1 1 ) Raw pT 0 (p Cal p TZ ) Cal Cal 0 T ~ ) 2 = p = p ( 1 T T 2 0 p Cal T p TZ bi n 0 p TZ p Cal T bi n 2 Df Jet Z 23th October 2006 Monica Verducci 23 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 24 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 25 Backup 23th October 2006 Monica Verducci 26 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 Primary Vertex d •Number of tracks > 0 •Secondary vertex >3 (weight) Impact Parameter Secondary Vertex, B-hadron decays 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 27 Calibration in Situ (II) Cone DR=0.7 Et> 15 GeV Et(cell)=1.5 GeV E,,: pt>5GeV (p TRaw p TZ ) Cal 0 Raw ~ ) 1 = p = p ( 1 T T 1 p TRaw 2 = (p 23th October 2006 p ) Cal 0 T Cal 0 T p Z T p Cal T =p Cal 0 T ~ ) (1 2 Monica Verducci p TZ bi n 0 p TZ p Cal T bi n 2 ISR Correction 28 Calibration in Situ (III) BiSector Method Measurement of the resolution via estimation of the ISR contribution Transverse plane: 1. η depends only on ISR 2. depends on both resolution and ISR K T = ( p jet T f jet f Z p ) sin ( ) 2 K T = ( p jet T f jet f Z p ) cos( ) 2 Z T Z T s D = s 2 s 2 s(pT ) = a pT b pT c 23th October 2006 Monica Verducci 29