Early top physics with ATLAS Richard Hawkings CERN

Early top physics with ATLAS Richard Hawkings (CERN) LHC top physics workshop, Lisbon 13/3/09 § Introduction: top physics environment at LHC § What can we expect in the first years. . ? § Experimental ingredients for top physics at LHC § Electron and muon identification, jets, ET miss and b-tagging § Top-pair cross-section measurements § Initial ‘rediscovery’ and beyond § Single top cross-section measurements § Reconstruction of top pair events and the top mass § Conclusions § Concentrating on ‘early’ measurements and not on couplings and rate decays - covered later… § All results shown can be found in the ATLAS ‘CSC’ book CERN-OPEN-2008 -020 13 th March 2009 Richard Hawkings 1

Why top quark physics? § Top quark within Standard Model: § It exists! Measure its fundamental parameters (production cross-section, mass, charge, couplings, etc. ) § Electroweak corrections typically mt 2 – interesting for model builders § Heaviest known quark, least studied, some peculiar properties § Decays involve real rather than virtual W § Decays before hadronises – spin/polarisation information is preserved § Top quark beyond the Standard Model: § Top may be produced in new particle decays (t-tbar resonances, heavy H …) § Top quarks may decay in peculiar ways, e. g. t H+b § Top is a ‘template’ for many new physics topologies § Complex decay signatures involving leptons, missing energy, multi-jets, b-jets § Understand the detectors, develop the tools needed for hunting for exotic things § Top production will be a background to many new physics processes § H leptons, SUSY, more exotic things § Understanding top physics is essential in many searches 13 th March 2009 Richard Hawkings 2

Top quark production at LHC strong t-tbar pair production electroweak single top production (13%) (dominant) (87%) stt(th)=830± 100 pb @ 14 Te. V st(th) 320 pb @ 14 Te. V § At 1033 cm-2 s-1 (‘nominal’ low luminosity), get 1 top pair/second, or 8 M/year § 30% of these decay to {e/ } b jjb or {e/ } b final states, good trigger efi § At 1034 cm-2 s-1 (full LHC design luminosity before SLHC upgrade), get 10 Hz top pairs … O(100 M)/year § But background from pileup at 1034 will make precision studies difficult - analyses like top mass cocentrate on 1033 scenario 13 th March 2009 Richard Hawkings 3

Data samples in the first year of running § LHC will start with lower performance § Initial luminosity 1031 -1032 cm-2 s-1 § CM energy ≤ 10 Te. V in 2009 -10 § What can we expect…? Events in 50 pb-1 (ATLAS commissioning selection) § Current plan is to start in October 2009, run through to autumn 2010 § Then shutdown to complete consolidation and prepare 14 Te. V(? ) § Tt cross-section ~400 pb at 10 Te. V § Hope for O(100) pb-1 analysable data from 2009 -10 run § Very competitive with final Tevatron ttbar sample +good W mass, 100 pb-1 § Results here assume 14 Te. V! § ~factor 2 more L needed at 10 Te. V 13 th March 2009 Richard Hawkings 4

Accessing top physics at LHC § Typical analysis - semileptonic top decay § Hard (ET> ~20 Ge. V) isolated lepton § Electron and/or muon identification § Missing energy ET>~20 Ge. V § Both lepton and ET miss similar to W production § Hadronic W-decay: § 2 hard (ET>20 -40 Ge. V) light quark jets § Known mjj=mw – calibration of jet energy scale § 2 hard (ET>20 -40 Ge. V) b quark jets § b-tagging essential tool in top identification § Some opportunities for b-tag calibration t t § Explore/commission full range of detector capabilities (leptons, ET miss, jets, b-tagging) § Main background is W+multijet production § Multijet with fake lepton will also be important § Both difficult to simulate, need data normalisation § In dilepton channel, Z+jets and diboson prodn § Single top: similar ingredients, typically study semileptonic decay ({e/ } b ) 13 th March 2009 Richard Hawkings 5

Ingredients - electron identification § Leptons are key handle on top decays Electron candidates/100 pb-1 § Detector optimised for electron/muon ID § Huge background (~105) from jet production § Electrons identified by: § Shower shape in EM calorimeter § Matching high p. T track in tracking detector § Transition radiation in TRT (e/ separation) § Sources of electrons § Fakes from jets (especially at low ET) § Electrons from heavy flavour (b, c e) in jets § ‘Prompt’ electrons from W/Z (and top) § Top (and W/Z) physics requires isolation § Low activity in a cone around electron Rejection of non-isolated e in top events Endcap, 17 Ge. V Barrel, 30 Ge. V § E. g. ET<6 Ge. V in cone R<0. 2 § Effectively removes heavy flavour, leaving W, Z 13 th March 2009 Richard Hawkings 6

Ingredients - electron trigger and efficiency § Electron also used to trigger event p. T(e) in top decay § L 1 calorimeter trigger with fast sums § L 2 and EF based on full detector info § Good efi for 25 Ge. V threshold - around 85% of offline selected electrons Electron trigger efficiency (wrt offline) § Majority of t W e have p. T>20 Ge. V § Need to measure trigger and selection efficiency for electrons from data § Use copious Z ee decays to measure ‘generic’ efficiency with ‘tag and probe’ § Select one electron with standard cuts § Look for a second electron which makes Z mass, see how many pass ID cuts § Corrections needed to subtract fake b/g Probe e Z § Method can be used for trigger/offline § Corrections needed for Z top § Different isolation performance due to presence of extra jets § Eventually take from data, need statistics 13 th March 2009 1 -2% error for 100 pb-1 Tag e Richard Hawkings 7

Ingredients - muon identification and efficiency § Muons reconstructed in external muon spectrometer with toroid field § Matching to inner detector tracks reduces fake rate and improves p. T resolution § Fake rate for p. T>20 Ge. V ~10 -3 in top-pair events - K/ decays in flight and punch-through § Further improvements by cutting on match 2 § Efficiency ~90% but losses due to coverage § As for electrons, need isolation to remove muons from heavy flavour decays Muon trigger § Muon trigger: § Dedicated chambers (RPC, TGC) at L 1 § Full info at L 2/EF efficiency Efficiency p. T( )>10 Ge. V #fake muons/tt event § Measure efi using tagand-probe Z § As for electrons 13 th March 2009 Richard Hawkings 8

Ingredients - jet reconstruction and calibration § Quarks from t/W decay give jets in detector § Want to reconstruct energy and direction… § Fragmentation and hadronisation § Complex detector response to different particles; different technologies, dead material, cracks § Effects of clustering, jet algorithms, out of cone § Effect of ISR/FSR, underlying event and pileup § Jet efficiency (ET threshold), resolution, scale § § Just calculating acceptance (N jets > threshold) will take time and data /Z-jet balance to determine ET scale Particle types in jets photons K, p, n PT balance vs energy Z ee jet Z+1 jet event § Few % statistical precision with 100 pb-1 § Ultimate goal ~1% 13 th March 2009 Richard Hawkings 9

Ingredients - B-jet energy scale and missing ET § Complication for b-jets (important in top) b-jet energy scale, Jets with § 19% of b-jets contain a muon + neutrino § b , b c decays accompanied by § Effect does not occur in light jets - systematic underestimate of b-jet ET scale § If muon found in jet, can correct (O(10%)) § Similar effect also for electrons… § Stages of ET-miss calculation/refinement: § Calorimeter cells sum (noise suppression, calib) § Addition of identified muons (fake , ID/calo info) § Refine calo info for jets; estimate energy in dead regions tt {e, } bjjb § Genuine ET-miss in top events ( from W) § Remove b/g from QCD multijets with fake e/ ET-miss resolution evolution W {e, } Z § Missing ET measurement - sensitive to energy carried off by neutrinos in transverse plane 500 Ge. V dijets § Painstaking work to commission… § Can also use phsyics processes to help with calibration (including semileptonic top decays) 13 th March 2009 Richard Hawkings 10

Ingredients - b-tagging using top-pair events § Top-pair events offer source of b-jets for use in b-tagging efficiency calibration § Method I: Count number of events with 0, 1, 2, 3 b-tagged jets in cross-section seln b-vetos § Can determine b and c along with tt § Get b to ~5% (incl. syst) in 100 pb-1 § Method II: Exploit the topology / kinematics of the top-pair event to select the leptonic top b-jet, without using its b-tag info § Selection methods exploiting mass info, kinematic fit or jet/lepton kinematics § Jet samples ~70 -90% pure in b flavour, have to estimate and subtract background b-tag unbiased b-jet leptonic top mass § Can then study distributions of b-tagging input and output variables, calculate b § Get b to 5 -10% in 200 pb-1, can also look at distributions (e. g. b vs jet ET and ) § b determined in-situ, complementary to dijetbased methods also used at Tevatron 13 th March 2009 Richard Hawkings Abs stat error, 200 pb-1 11

Rediscovering the top quark § Detector performance will take data and time to understand § For initial top quark ‘rediscovery’… emphasis on simple selections § Select events with high ET lepton, missing ET and 4 jets (ET>20 -40 Ge. V) § Backgrounds from W+jets events, QCD multi-jet with fake lepton, single top § W+jet and multijet events can be reduced by requiring 1 or 2 b-tagged jets ( b~ 50 -70%) § But also interesting to look at simplified analyses without b-tagging: § Find combination of 3 jets which represents the hadronic top decay § Criteria such as largest sum p. T, good W mass between 2 of the jets Largest sum p. T, 100 pb-1 13 th March 2009 Richard Hawkings +good W mass, 100 pb-1 12

Top cross section in semileptonic channel § Extract signal using two methods: § Event counting (need background estimate) § Fit to 3 -jet hadronic top mass distribution +jets § Reduces sensitivity to W+jets background normalisation and jet energy scale § ISR/FSR (acceptance) modelling also important 100 pb-1 (e+ ) Stat (%) Syst (%) PDF (%) Lumi (%) Count 3 16 3 5 Likelihood 7 15 3 5 Statistical signficance vs L § Once b-tagging is available, use it: § With 100 pb-1, expect to commission b-tag and understand efficiency to ~5% § Helps with W+jets background reduction and combintorial background 13 th March 2009 e+jets § 5 sensitivity from ~ 20 pb-1 § Very clear signal at 100 pb-1, even with x 2 background Richard Hawkings 13

Top cross-section in dilepton channel § Di-lepton signal (l b l b) potentially very clean: § Background from Z ee, can be removed with mass cut, leaving Z , lepton+jets ttbar and diboson (WW/ZZ/WZ) Dilepton mass 100 pb-1 § e channel particularly clean (only Z is from ) § Require ET-miss + 2 jets… no need for b-tag § Cross-section extracted using counting, likelihood or template (ET-miss vs njet) 100 pb-1 Stat (%) Syst (%) PDF (%) Lumi (%) Count 4 5 2 5 Template 4 4 2 5 Likelihood 5 8 0. 2 5 Jet multiplicity § Systematics dominated by jet ET Stat+syst scale, ISR/FSR significance § Clear signal even with 10 -20 pb-1 § Probably the ‘discovery’ channel 13 th March 2009 Richard Hawkings 14

Properties of initial top-pair events § Once top-pair signal established, look at properties of events for non-SM behaviour Hadronic top p. T § Use semileptonic selection - study p. T and distribution of top… can also start to look for extra jets etc § Study top-antitop invariant mass distribution § Constrained fit to t-tbar system using known W and top masses (without b-tagging - could be used to reduce combinatorics) § t-tbar mass resolution 5 -9% from 200 -850 Ge. V t-tbar invariant mass 13 th March 2009 Richard Hawkings Hadronic top 15

Top cross-section using tau decays § Expect top-pair states involving same as e, b-tag weight of highest p. T jet § Branching ratio could be enhanced: t H+b § Important background for e. g. SUSY searches § Decay tt {e, } b had b (‘leptonic’) § Similar to semileptonic selection, but with identified hadronic tau replacing jets from W qq § Get additional rejection using 1 or 2 b-tagged jets § S/B 1: 10 without b-tagging, get 1: 1 with 1 b-tag § ~50 events, in 100 pb-1, b/g W+jets and single top § Can also use this channel to study -identification § Decay channel tt qqb had b (‘hadronic’) § No electron or muon - need to trigger on some combination of , N-jets and ET-miss § Require ≥ 4 jets, of which 2 must be b-tagged, plus identified hadronic § Expect about 300 candidates in 100 pb-1, with S/B~20 § Use to study identification and trigger (for events triggered on jets/ET-miss) § Both analyses are challenging - requiring well understood ID, jets, b-tagging, … § Will come later than the ‘discovery’ analyses with electrons and muons 13 th March 2009 Richard Hawkings 16

Single top production at LHC § Electroweak top quark production - contrast to pair production § Sensitive to new particles (e. g. H+, W’) and flavour changing neutral currents § Important background to many new physics searches (lepton, missing energy) § Overall cross section is large (c. f Tevatron), can distinguish contributions: s-channel: t=11 1 pb t-channel: t=247 12 pb Wt-channel: t=66 2 pb ( c. f. top-pair tt=830 50 pb ) § Large backgrounds from top pair production, also W+multijet and QCD jet events § At LHC - attempt to measure all production modes (s-chan & Wt challenging) § Can then extract |Vtb| and study polarisation, charge asymmetries, searches … § Basic event signatures: high ET lepton, missing ET, restricted number of (b) jets § Plenty of events, but large backgrounds which will have to be understood from data 13 th March 2009 Richard Hawkings 17

Single top production: t-channel cross-section § Require lepton, missing ET and one b-jet from the top quark decay b-jet p. T § Infer top quark mass using missing ET § Jet from light quark is forward, can require this jet and/or veto additional central jets § Second b-jet is usually soft - below ET cut § O(1 k) events per fb-1, similar size tt background large systematics (jet E scale, b, b/g) § Can be reduced by multivariate techniques - e. g. Boosted Decision Tree with event shape variables § Measurement to ~10% precision possible with 10 fb-1 § Then get |Vtb| to ~5% 13 th March 2009 BDT value ATLAS Mtop BDT>0. 6 Int. L Method Stat(%) Syst(%) Lumi (%) 1 fb-1 count 5 41 18 1 fb-1 BDT 6 19 9 10 fb-1 BDT 2 8 5 Richard Hawkings 18

Single top production: W-t and s-channel § Much smaller signal cross-sections, very large background § Especially from top-pair events where some particles are missed § W-t: channel: Single b-jet; look for two light jets consistent with W decay (l-j channel), or second lepton from leptonic W decay § Can use control region with similar kinematics but rich in top-pairs (e. g. require extra b-jet) to estimate background, cancel systematics W-t chan s chan § s-channel: Two b-jets, lepton + missing ET, no other high ET jets § In both cases, multivariate techniques can be used to enhance signal significance § Some representative analysis results - note small S/B and large systematics § Mainly from background - b-tagging/vetos, jet energy scales, PDFs, . . Channel Int. L Nsignal S/B Stat(%) Syst(%) Lumi (%) W-t (l+j) 1 fb-1 86 0. 38 21 43 20 W-t (l+j) 10 fb-1 860 0. 38 7 18 8 s-chan 10 fb-1 150 0. 19 20 44 18 § Need O(10) fb-1 of data and careful background studies to establish 5 signals 13 th March 2009 Richard Hawkings 19

Top reconstruction - for mass and more § Top mass, angular analysis need reconstruction of ttbar final state § 4 -vectors of all or some particles § Kinematic fits, exploiting constraints § E. g. W-mass in light jets from W qq should give known W mass Energy scale correction Reconstructed W-mass § Extra ‘in-situ’ calibration of light jet energy scale for the top-pair topology § Essential for top mass systematics § Analyses typically require ≥ 1 fb-1 § Make tight selections (4 jets ET>40 Ge. V) + b-tags § Various other cuts on reconstructed W and top § E. g. extract hadronic top mass from mjjb distribution Hadronic top mass § Statistical error 0. 3 Ge. V from 1 fb-1 data § Other techniques, e. g. global 2 fit also involving leptonic side of event, under development § Important for spin correlations, polorisation etc … 13 th March 2009 Richard Hawkings 20

Top mass measurement § Top mass quickly systematics dominated Uncertainty (1 fb-1) Value Light jet energy scale 0. 2 Ge. V / % b-jet energy scale 0. 7 Ge. V / % ISR/FSR ~0. 3 Ge. V b quark fragmentation ≤ 0. 1 Ge. V Background negligible Method 0. 1 -0. 2 Ge. V Light jet multiplicity § C. f. Tevatron current average 1. 2 Ge. V § Need to control light and especially b-jet JES to O(1%) to be competitive and exploit statistics (0. 3 Ge. V stat error in 1 fb-1) § This will take time and data… ISR/FSR changes Hadronic top mass, 1 -btag events § Control ISR/FSR from data distributions, e. g. jet multiplicity § For early data … can extract top mass with relaxed b-tag requirements § Start to learn about techniques and data 13 th March 2009 Richard Hawkings 21

Conclusions § LHC now has a firm ‘recovery’ schedule and expects to start in late 2009 § Hope for 100 -200 pb-1 data, mainly at 10 Te. V, by autumn 2010 § This gives a top pair data sample very comparable to that of the Tevatron § But we have to achieve a mature understanding of the detector. . . will take time § On the other hand, can make first observations with first ~20 pb-1 § Top analysis requires and exercises much of ATLAS object reconstruction § § § Electron and muon trigger and identification Jet reconstruction and energy scale Missing ET reconstruction Tagging of b-jets … all this will also help with understanding top as a background for searches § Initial measurements § Focus on top-pair cross section in dilepton, semileptonic and then other channels § Interesting measurements can already be done with 100 -200 pb-1 § Start to develop tools to go further - top reconstruction, single top backgrounds § Then move on to ‘new’ territory: single top, top properties, rare decays … 13 th March 2009 Richard Hawkings 22