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Precision SM tests at the LHC using ATLAS and CMS Peter R Hobson School of Engineering & Design Brunel University Talk given at RAL on 13 June 2005.

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Presentation on theme: "Precision SM tests at the LHC using ATLAS and CMS Peter R Hobson School of Engineering & Design Brunel University Talk given at RAL on 13 June 2005."— Presentation transcript:

1 Precision SM tests at the LHC using ATLAS and CMS Peter R Hobson School of Engineering & Design Brunel University Talk given at RAL on 13 June 2005

2 Contents ATLAS & CMS Jets Drell-Yan B physics Top physics Electroweak (TGC) Single photons

3 ATLAS

4 CMS

5 Day 1 of LHC p+p From F Gianotti, LHC Physics, La Thuile 2005

6 Year 1 at the LHC From F Gianotti, LHC Physics, La Thuile 2005

7 Year 1 at the LHC From F Gianotti, LHC Physics, La Thuile 2005

8 Effects on physics reach

9 From G Polisello, Les Houches 2005 b-tagging in ATLAS

10 Jet Physics Atlas Measure jet E T spectrum, rate varies over 11 orders of magnitude Test QCD at the multi-TeV scale E T of jet Event s > 1 TeV 4 10 6 > 2 TeV 3 10 4 > 3 TeV 400 Inclusive jet rates for 300 fb -1 : From J Mnich, Physics at the LHC, Vienna 2004

11 Jet signatures Test of pQCD in an energy regime never probed!Test of pQCD in an energy regime never probed! The measurement of di-jets and their properties (E T and η 1,2 ) can be used to constrain p.d.f.s Inclusive jet cross section: α s measurement with 10% accuracy Multi-jet productionMulti-jet production is important for several physics studies: –Top-pair production with hadronic final states –Higgs production in association with tt and bb –Search for R-parity violating SUSY (8 – 12 jets). Systematic uncertaintiess (statistical will be small): –luminosity (dominant uncertainty 5% -10% ) –jet energy scale –calorimeter response (linearity) –jet trigger efficiency –knowledge of p.d.f.s –value of strong coupling constant, α s –uncertainties in parton shower modeling - - E T Jet [GeV] From VA Mitsou, QCD Conference Montpellier 2004

12 LHC 1 fb -1 Drell-Yan Lepton-Pair Production p T > 6 GeV | | < 2.5 Total cross section pdf parton lumi search for Z, extra dim.,... Much higher mass reach as compared to Tevatron /Z q q e, e +, + Inversion of e + e qq at LEP Z pole From J Mnich, Physics at the LHC, Vienna 2004

13 Forward-backward asymmetry estimate quark direction assuming x q > x q Measurement of sin 2 W effective 2004: LEP & SLD sin 2 W = 0.23150 0.00016 A FB around Z-pole large cross section at the LHC (Z e+e ) 1.5 nb stat. error in 100 fb -1 incl. forward electron tagging (per channel & expt.) sin 2 W 0.00014 Systematics (probably larger) PDF Lepton acceptance Radiative corrections Drell-Yan Lepton-Pair Production Atlas [%] From J Mnich, Physics at the LHC, Vienna 2004

14 Drell-Yan processes QCD effects enter DY production in initial state only predictions less uncertain Reconstruction of leptons (e, μ) unambiguous identification ( opposed to jets ) Di-lepton productionDi-lepton production constrains proton structure at Q 2 m 2 W and Z production: huge statistical samples ~10 5 events containing W (p T W > 400 GeV, L=30 fb -1 ) ~10 4 events containing Z (p T Z > 400 GeV, L = 30 fb -1 ) W ± productionW ± production: –higher cross-section for W + than for W - –different y W -distributions: W + forward; W - central –constrain quark and anti-quark densities in the proton [ud(bar) W + ; u(bar) d W - ] W+jet productionW+jet production study colour coherence Z productionZ production provides accurate reconstruction of final state (no neutrino!) Pair production (WW, ZZ, WZ)Pair production (WW, ZZ, WZ) study triple gauge boson constants p T miss >20 GeV |η |<2.5 NLO calculation Represent background sources to many new phenomena searches From VA Mitsou, QCD Conference Montpellier 2004

15 B Physics at ATLAS & CMS From VM Ghete Physics at LHC Vienna, 2004

16 B Physics at ATLAS & CMS From VM Ghete Physics at LHC Vienna, 2004

17 B Physics at ATLAS & CMS From VM Ghete Physics at LHC Vienna, 2004

18 c & b production Dominant production mechanism for heavy quarks (b and t) is gg fusion Cross-section calculation: pQCD processes leading to QQ state non-pQCD to transform into colour-singlets tuning with Tevatron data Measurements of heavy quark production will provide constraints on the gluon density Jet-flavour identification (c-jet or b-jet): –high-p T muons (ε 85%, σ=39 MeV) –b-tagging (vertexing detectors) b-quark –lower-p T mesons are experimentally accessible compared to charm-quarks –10 -4 { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/3/794557/slides/slide_18.jpg", "name": "c & b production Dominant production mechanism for heavy quarks (b and t) is gg fusion Cross-section calculation: pQCD processes leading to QQ state non-pQCD to transform into colour-singlets tuning with Tevatron data Measurements of heavy quark production will provide constraints on the gluon density Jet-flavour identification (c-jet or b-jet): –high-p T muons (ε 85%, σ=39 MeV) –b-tagging (vertexing detectors) b-quark –lower-p T mesons are experimentally accessible compared to charm-quarks –10 -4

19 Top production Cross section determined to NLO precision –Total NLO (tt) = 834 ± 100 pb –Largest uncertainty from scale variation Compare to other production processes: –Top production cross section approximately 100x Tevatron Opposite @ FNAL ~90% gg ~10% qq ProcessN/sN/year Total collected before start LHC W e 1510 8 10 4 LEP / 10 7 FNAL Z ee 1.510 7 10 7 LEP tt110 7 10 4 Tevatron bb10 6 10 12-13 10 9 Belle/BaBar ? H (130)0.0210 5 ? LHC is a top factory! From S Bentvelsen, QCD Conference Moriond 2004

20 Golden-plated M Top channel Lepton side Hadron side Br(tt bbjjl )=30% for electron + muon Golden channel –Clean trigger from isolated lepton The reconstruction starts with the W mass: –different ways to pair the right jets to form the W –jet energies calibrated using m W Important to tag the b-jets: –enormously reduces background (physics and combinatorial) –clean up the reconstruction Typical selection efficiency: ~5-10%: Isolated lepton P T >20 GeV E T miss >20 GeV 4 jets with E T >40 GeV >1 b-jet ( b 40%, uds 10 -3, c 10 -2 ) Background: <2% W/Z+jets, WW/ZZ/WZ

21 Lepton + jet: reconstruct top Hadronic side –W from jet pair with closest invariant mass to M W Require |M W -M jj |<20 GeV –Assign a b-jet to the W to reconstruct M top Kinematic fit –Using remaining l+b-jet, the leptonic part is reconstructed |m l b - | < 35 GeV Kinematic fit to the tt hypothesis, using M W constraints j1j1 j2j2 b-jet t W-mass Selection efficiency 5-10% From S Bentvelsen, QCD Conference Moriond 2004

22 Top mass systematics –Method works: Linear with input M top Largely independent on Top P T –Biggest uncertainties: Jet energy calibration FSR: out of cone give large variations in mass B-fragmentation –Verified with detailed detector simulation and realistic calibration Source of uncertainty Hadronic M top (GeV) Fitted M top (GeV) Light jet scale0.90.2 b-jet scale0.7 b-quark fragm0.1 ISR0.1 FSR1.90.5 Comb bkg0.40.1 Total2.30.9 Challenge: determine the mass of the top around 1 GeV accuracy in one year of LHC From S Bentvelsen, QCD Conference Moriond 2004

23 Use exclusive b-decays with high mass products (J/ ) –Higher correlation with M top –Clean reconstruction (background free) –BR(tt qqb +J/ ) 5 10 -5 – ~ 30% 10 3 ev./100 fb-1 (need high lumi) Top mass from J/ Different systematics (almost no sensitivity to FSR) Uncertainty on the b- quark fragmentation function becomes the dominant error M(J/ +l) M top M(J/ +l) M lJ/ From S Bentvelsen, QCD Conference Moriond 2004

24 Top During Commissioning Determination M Top in initial phase –Use Golden plated lepton+jet Selection: –Isolated lepton with P T >20 GeV –Exactly 4 jets ( R=0.4) with P T >40 GeV Reconstruction: –Select 3 jets with maximal resulting P T –Signal can be improved by kinematic constrained fit Assuming M W 1 =M W 2 and M T 1 =M T 2 Period Stat M top (GeV) Stat / 1 year0.10.2% 1 month0.20.4% 1 week0.42.5% No background included Calibrating detector in comissioning phase Assume pessimistic scenario: -) No b-tagging -) No jet calibration -) But: Good lepton identification From S Bentvelsen, QCD Conference Moriond 2004

25 Top During Commissioning Signal plus background at initial phase of LHC Most important background for top: W+4 jets –Leptonic decay of W, with 4 extra light jets Alpgen, Monte Carlo has hard matrix element for 4 extra jets (not available in Pythia/Herwig) ALPGEN: W+4 extra light jets Jet: P T >10, | | 0.4 No lepton cuts Effective : ~2400 pb With extreme simple selection and reconstruction the top- peak should be visible at LHC L = 150 pb -1 (2/3 days low lumi) measure top mass (to 5-7 GeV) give feedback on detector performance From S Bentvelsen, QCD Conference Moriond 2004

26 Direct |V tb | extraction: single top / single W Moreover, in principle, many theoretical errors would disappear by normalising s-channel events over single W events: (with care in choosing coherent cuts for the two processes, to avoid the reintroduction of the same errors in a subtler way) R(|V tb |)= From A Giammanco, Les Houches 2005

27 Single top: how to General strategy (both s/t-ch.): 1 isolated lepton 2 high E t jets at least 1 tagged b-jet missing E t l+MET: M T compatible with W H t (scalar sum of all E t s) M(lb) in a window around M t s/t-channel separation: 2(b-t-b)/1 tagged b-jets 0/1 jets in the forward calo 2/1 central jets angular distance between the reco top and the remaining jet 1 st jet: b from t2 nd jet: recoil 3 rd jet: b (mostly undetectable) T-channel For MET and H t, single top lies in the middle between non-top and ttbar bkgs. S-channel: S/B 2l (1 lost), Wbb, t-channel. T-channel is much easier to select, due to higher cross section and unique topology. CMS note 1999/048 From A Giammanco, Les Houches 2005

28 TGC From M Dobbs, Hadron Collider Physics 2004

29 TGC From M Dobbs, Hadron Collider Physics 2004

30 QGC From M Dobbs, Hadron Collider Physics 2004

31 TGC CMS studies W (Kate Mackay, Peter Hobson, Karlsruhe Group) –CMSJET studies with BAUR generator (Phys Rev D41 1476 (1990)) –Full background study –CMS Notes: 2000/017, 2001/052, 2001/056, CMS Thesis 1999/019 Z (Kate Mackay, Peter Hobson, Davy Machin, Karlsruhe Group) –CMSJET studies with BAUR Z generator –Full background study –CMS notes: 2000/017, 2002/028, CMS Thesis 2005 WZ –No CMS specific study W (Richard Croft) –CMSJET study with W2GRAD generator

32 Status of CMS W Analysis Signal –BAUR NLO MC –Used in CMSJET studies Backgrounds –W+jet – main background –Radiative W decay –Quark-Gluon fusion Cuts: isolated high p t photon, lepton and missing energy. p T ( )> 100 GeV p T ( l )> 25 GeV p T ( )> 50 GeV M T (,l, ) > 90 GeV R(,l ) > 0.7 p T 2 nd Jet < 25 GeV | | < 2.5 Peter Hobson, Kate Mackay

33 Status of CMS W Analysis Peter Hobson, Kate Mackay

34 Direct photon Two main contributions: –qg q QCD Compton scattering (dominating) –qq g annihilation process Information on gluon density in the proton ( requires good knowledge of α s ) Background: jets with a leading π 0 Isolation cut Isolation cut: low hadronic activity in a cone around the photon ATLAS: high granularity calorimeters ( |η| < 3.2 ) allow good γ/jet separation Di-photon production: m γγ and Δφ γγ sensitive to soft gluon emission Understanding irreducible background from fragmentation in gg fusion: crucial for H γγ searches - From VA Mitsou, QCD Conference Montpellier 2004 LO γγ production


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