1 / 39

Roberto Chierici CNRS (Lyon)

Roberto Chierici CNRS (Lyon). Unveiling the top secrets (with the LHC). Nobel symposium – Stockholm – 13-17 May 2013.

lilike
Télécharger la présentation

Roberto Chierici CNRS (Lyon)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Roberto ChiericiCNRS (Lyon) Unveiling the top secrets (with the LHC) Nobel symposium – Stockholm – 13-17May 2013

  2. The top secretsWhy doing top physics is more exciting than ever. Especially after the discovery of a neutral scalar bosonSelected(*) experimental resultsAnalyses at the LHC and the experimental challenge on top physics(focus on) precision physics for constraining the Standard Model(some) searches for new physics coupled to topConclusions and outlookOpen questions in the exciting way forward Outline • (*) Disclaimer: this seminar will not (can not !) give a complete review of results on top physics. The choice made is personal and, by definition, biased. This talk will mostly cover LHC results. Tevatron results are flashed when relevant. For the state of the art of experimental results please go here: • https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsTOP • https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResults • http://www-cdf.fnal.gov/physics/new/top/top.html • http://www-d0.fnal.gov/Run2Physics/top/

  3. Introduction The top secrets Experimental challenges μ νe νμ ντ e τ u d c s b t H Z W

  4. A particle with unique characteristics • Unique because of its enormous mass: it weights like a tungsten atom ! • Still a point-like particle in our understanding • The top and the Higgs are “strongly” coupled • The top mass dramatically affects the stability of the Higgs mass • If we consider the SM valid up to a certain scale  t W, Z H • It is the only quark that does not hadronise • τ(had)~h/QCD~2 10-24 s • τ(top)~h/top~5 10-25 s • Compare with τ(b)~10-12 s • Decays before forming a “dressed” top quarks • No bound tq states, its spin properties are directly passed to its decay products • Flavour and EWK physics at their best !

  5. Constraining the SM  mt2  ln(mH) • Can use the fact that mt, mW, mH are linked at loop level to constrain the SM • The Higgs/symmetry breaking sector can be explored with more insights coming from top physics mW=mW(mt2, log(mH))  Now known at NNLO QCD. Vacuum meta-stability when the minimum of V() is just local arXiv:1205.6497 • The top also provide other direct constraints to the model • Direct access to parameters of the SM (mt, Vtb) • Other stringent tests of SM (QCD in d/dX, couplings, CPT invariance,…)

  6. Direct probe of EWSB and new physics • Top physics is now essential to study the properties of the Higgs boson (and contribute to the measurement of the top Yukawa coupling) • Top is also the best gateway to the searches for new physics: many new models involve the top sector • Many may concern the top sector exclusively • Top partners are present in SUSY, UED, little Higgs, 4th generation models • Final states from new physics involving top partners may be very similar to top-pair production, typically with the associated production of other particles → Interesting to study total cross sections and the “environment” of top production → Precise measurements of differential cross sections are important probes as well (charge asymmetries, spin structure, anomalous couplings) • Do not forget that top physics may be an important background for searches ! t~t~→tt+X g~ g~→tt(tt)+X t’t’→bbWW, ttZZ b’b’→ttWW Four tops

  7. (single) top production at the LHC • Top is produced in pairs (QCD) or singly (EWK) • Single top EWK production happens via three main contributions Vtb~1 • (7 TeV)~64 pb • (7 TeV)~4.6 pb • (7 TeV)~15.6 pb , , , , • Backgrounds coming from W/Z+jets, top pair production, QCD

  8. Top (pair) production at the LHC • Top pair QCD production happens mainly via gluon fusion • Final states depend on the decay of the W bosons , , , • BR~10% • BR~44% • BR~46% • Backgrounds coming from W/Z+jets, single top (tW), QCD

  9. …but still a needle in a haystack

  10. Experimentally challenging e, μ, τ b-tagging • Top pair studies use all parts of HEP detectors… • Charged lepton reconstruction • Jet reconstruction • Missing transverse energy Missing ET Muon resolution b-tagging light jets (energy scale) • Optimal use of the detectors… • Particle Flow reconstruction in CMS • Combine all sub-detector information to reconstruct and identify particles • Exploit excellent calorimetry in ATLAS • … and sophisticated • analysis tools: • B-tagging, τ reconstruction, • kinematic fitting MET resolution in Z→μμ events ε(b-tag, data)/ε(b-tag, MC)

  11. EXPERIMENTAL RESULTS Top pair cross sections Single top Top properties Looking for new physics W, t polarisations Production (diff.) cross-section PDFs Decay modes Spin, couplings, mass, … ~100% Top pair environment

  12. Collected data • Impressive performance of the LHC in 2011(@7TeV)/2012(@8 TeV) • About ~6/fb collected in total at 7 TeV • About ~23/fb collected at 8 TeV • Statistics important for top physics • LHC is the first top factory ever ! • O(1M) tt @7TeV, O(10M) @8TeV • While precision measurements soon limited by systematic errors, many possibilities for other studies open up • Rare processes • Searches for new physics • Constrain of systematic errors and backgrounds by using data • Results presented in this seminar: • Mainly at 7TeV • Not necessarily at full statistics • A few 8TeV result 2012 2011 2010 Example: how the single top signal improves

  13. tt→qqbμνb tW→μνbμν Top pair (differential) production cross sections tt→eνbqqb tt→eνbμνb

  14. Total cross section measurements • Monitoring the total production cross section is the first fundamental step for understanding top physics at the LHC • Test the presence of new production mechanisms • In the frame of the SM, test QCD predictions and help constraining the PDFs (especially gluons) • Important for Higgs production, for instance • Indirect determination of mt or S. • Constrain a very important background for many searches at the LHC • Almost all decay modes are investigated at the LHC • The measurements are performed at different level of complexity: • Counting experiment in acceptance • Fit to data in several portions of phase space with in situ constraining of various backgrounds • Multivariate analyses

  15. Top pair cross section • Di-lepton final states (e, μ) background free • Likelihood fits to the number of reconstructed (b-tagged) jets. DY background data-driven ATLAS-CONF-2012-131 • ℓ+jets final states represent a good compromise between statistics and purity • Multidimensional ML fit to data • Use data themselves to constrain the backgrounds by including regions where they dominate • Hadronic channels (all-jets, τ+jets) are very difficult • Entirely dominated by QCD, need to estimate it directly from data • Use NN to separate signals from backgrounds CMS-PAS-TOP-11-003 CMS-PAS-TOP-11-004

  16. Cross section combination • All top pair final states are (being) investigated • ℓ(e,μ,)+jets, ℓℓ(all but )+jets and fully hadronic final states in the combination. • Best measurements in the di-lepton channels • Combinations performed taking into account correlations between errors • Experimental uncertainty close to 5% • Already challenging the newly available NNLO computations • Current TH errors about 4% Czakon, Fiedler, Mitov, arXiv:1303.6254 Tevatron TOPLHCWG

  17. Top pair differential cross sections • Test top physics in different portions of the phase space • Important test of pQCD, constrain of MC models and systematic effects, sensitive to new physics • Use unfolding techniques on background-subtracted reconstructed distributions for a direct comparison to theory predictions • Propagation of the systematic errors (only shape errors important) • Most relevant coming from background knowledge, radiation and hadronization • Look at lepton, jets, but also at more complex variables involving top quarks • Need a full reconstruction of top kinematics • Compare to reference generators and predictions on differential distribution from theory Data start to challenge NLO predictions? m(tt), di-lepton pT(t), l+jets pT(ℓ), ℓ+jets arXiv:1207.5644 arXiv:1211:2220

  18. A special case: radiation in top pair • At the LHC top quark are often produced with extra jets from initial (or final) state radiation • Higher energy and high scale of the process • Initial state preferentially from gluons (more colour) • Impact in the ability to reconstruct top pair • ~half of the event with a jet with pT >50 GeV! • Jet pairing may be difficult (see following) • Systematic errors due to radiation description in MC can be dominant • Important to monitor and describe jet production • Inclusive jet multiplicities, extra jet pTs, s,… • Constrain generator parameters on radiation • Aim also to look for new physics production in tt+jets pT(tt), ℓ+jets arXiv:1211:2220 t g t Njets, ℓ+jets t t ATLAS-CONF-2012-155

  19. Associated production of top and bosons • tt+W/Z are rare processes in the SM • Monitor couplings between t and Z • Investigate top pair in association with extra leptons • tt+bb. Important also for SM physics • Higgs in association to top. Top Yukawa. • Study N(b-jet) in di-lepton events • MadGraph: 1.2%; POWHEG: 1.3% CMS-PAS-TOP-12-014 Trileptons Dileptons • tt+ important for the direct measurement of the top charge • Look for isolated photons • Need more statistics, results with 5/fb at 7TeV in agreement with NLO predictions CMS-PAS-TOP-12-024 ATLAS-CONF-2011-153 • Not yet sensitive tottH, both ATLAS and CMS ready for new data

  20. Single top: why is that important? • The production cross section gives direct access to the CKM matrix element |V|tb • May also test the presence of a possible 4th generation quark • Check for presence of FCNC • Important background for Higgs searches in associated production W/ZH→qqbb • Investigate t-channel and tW production • s-channel still out of range for an observation • t-channel: 1 isolated eor μ, one b-tagged jet, one forward jet, missing ET • tWchannel: 2 isolatedcharged leptons (e, μ), one b-tagged jet, missing ET • Main backgrounds from top-pair production, W+jets, QCD • Use data whenever possible to constrain the backgrounds

  21. Single top cross sections • Typically use multivariate techniques (NN, BDT) • Optimize S/B separation using full event properties, constrain systematic effects by simultaneously analyzing signal and background dominated regions • Cross sections in agreement with the SM expectations, |Vtb| can be derived by assuming arXiv:1209.4533 ℓ>2 CMS-PAS-TOP-12-011 Phys. Lett. B716 (2012) 142

  22. op properties

  23. Constraining the SM with the top mass • Remember: the top mass, the W mass and the Higgs mass depend on each other

  24. Determination of the top mass b-tag b-tag • Direct reconstruction methods • Full reconstruction by resolving the pairing ambiguities (all channels studied) • Use kinematic constrained fitting to improve the mass resolution • Constrain the light jet energy scale in situ by using the W mass constraint • Fit the mass with MC template fits or event by event likelihood fits • Methods very sensitive to the description of radiation and JES uncertainties • Indirect methods (most of them still in the works at the LHC) • Use the dependence on the top mass on other variables • Top pair cross section • Lepton pT and end-point methods • Invariant mass of the system J/Ψ+lepton from W • Decay length of the b hadron • Main issue: need of a lot of statistics t t t t

  25. Top mass direct reconstruction CDF, ℓ+jets ATLAS, μ+jets CMS, hadronic • Plenty of statistics to reconstruct the resonance • Top mass determination is calibrated by using MCs • Dominated by systematic errors • Dominant sources are JES and TH uncertainties • Improvements from fitting JES in situ and constraining radiation from data CMS-PAS-TOP-11-017 arXiv:1207.6758 Eur. Phys. C72 (2012) 2046 CMS, ℓ+jets CMS arXiv:1209.2319

  26. Top mass combinations • Work ongoing in the TOPLHCWG and TEVEWWG to properly perform combinations • Need to properly assess correlation of systematic sources in between experiments • In both di-lepton and ℓ+jets channels, the measurements at the LHC are now competitive with the corresponding ones at the Tevatron → After 2 years of data the LHC provides an error on the top mass similar to that of the Tevatron after 20 years of run → Both nail down the top mass at O(1GeV), and agree • Towards the first world combination with Tevatron and the LHC • Work ongoing in the TOPLHCWG • In contact with the TEVEWWG → will need a detailed understanding of the correlated systematics errors, especially modelling TEVEWWG TOPLHCWG (*) Best Linear Unbiased Estimator [Lyons, Gibaud, Clifford; Nucl. Instr. Meth. A270 (1988), 16.]

  27. Spin structure of top decays • The spin structure of the top decay is transmitted to its daughters • By investigating the helicity of Ws from top we can test the V-A structure of the coupling • The experimental “analyzers” are typically the decay product of the Ws • Measure d/dcosθ*ℓ, the angle between the lepton and the b direction (in the W rest frame) JHEP 1206 (2012) 088

  28. Constraining anomalous couplings • The polarization fractions can be extracted by a fit to data • Fit performed with and without the assumption of FR=0 • Main systematic errors represented by JES and theory uncertainties/W+jets normalization • Agreement with the expectations in both ATLAS, CMS and combined results TOPLHCWG TOPLHCWG • The helicity fractions can be translated into constraints of anomalous couplings and NP operators • The LHC combination is consistent with the expectation of the SM 0 in the SM

  29. Top polarization and spin correlations • While top quarks are produced individually unpolarized in top pair production… • Can be studied via the angular distributions of the leptons from W decay • Fully leptonic final states particularly well suited • …the spin of the two tops are correlated • Strength depending on the spin quantization axis • Can be measured from angular distributions of the top decay products • A: correlation strength at production • i: amount of spin information from each probe • Δbetween leptons particularly well suited variable • Sensitive to NP in both production and decay ! CMS-PAS-TOP-12-016 5 observation Sensitivity ~25% Phys. Rev. Lett. 108, 212001 (2012)

  30. Looking for signs of physics beyond the SM with top

  31. Charge asymmetries • Tevatron observes anomalous charge asymmetries • May be an indication of new physics mechanisms in the production of top pair, both in s- or t-channel? arXiv:1101.0034 arXiv:0712.0851 • At the LHC it is possible to be conclusive on this, but the asymmetry needs to be defined differently • Initial state charge symmetric • In the SM the asymmetry is not exactly zero • Asymmetry introduced by interferences between ISR and FSR Phys. Lett. B717 (2012) 129

  32. Differential asymmetries at the LHC • AC is determined from the background-subtracted distribution of |yt|-|ytbar| • Detector effects are unfolded • In many new physics scenarios the charge asymmetry depends on phase space • High mass/pT regimes enhance the quark annihilation part of the initial state • Measure Ac differentially as a function of pT, y or mass of the top pair system • Good agreement betweendata and SM expectations within uncertainties • Results also compared with EFT predictions • Anomalous axial coupling of gluons to quarks: capable to explain the Tevatron anomaly CMS Phys. Lett. B717 (2012) 129 Di-leptons, 5/fb ℓ+jets, 5/fb ATLAS ATLAS-CONF-2012-057 ℓ+jets, 1/fb Di-leptons, 5/fb Eur. Phys. J. C72 (2012) 2039 CMS PAS-TOP-12-010 EFT: PRD84:054017,2011 NLO: Rodrigo, Kuehn Eur. Phys. J. C72 (2012) 2039 Phys. Lett. B717 (2012) 129

  33. Resonant top-pair production • Several new physics models predict the presence of new heavy particles decaying into top-pairs • Event’s kinematics strongly depend on the mass of the intermediate state • Low mass analyses: standard tt reconstruction • High mass analyses: no lepton isolation, top tagging techniques are required • Analyze substructure of jets • Fake rates directly determined via QCD data excluded ranges at 95% C.L. 1.3 TeV < mZ′ < 1.5 TeV mZ’(gKK) > 1.7(1.9) TeV tt QCD ℓ+jets hadronics ATLAS-CONF-2012-136 JHEP09 (2012) 029

  34. Semi-leptonic candidate with m(tt)=1.6TeV mZ’(gKK) > 1.7(1.9) TeV ATLAS-CONF-2012-136 mZ’(gKK) > 1.5(1.8)TeV arXiv:1209.4397

  35. Top-like searches • Final states from top production, or top-like • The studies presented so far are only a part of the rich program in ATLAS+CMS • But no sign of new physics yet mt’ > 557 GeV FCNC SUSY Excited W’ Z’ exchange mWR’>1.13 TeV

  36. Conclusions • Top physics is a pillar of the current research program in HEP • Ideal probe for constraining (directly+indirectly) the symmetry breaking of the SM • The top is way heavy → the Higgs scalar mostly couples to tops • Ideal probe for looking for new physics beyond the model itself • Via precision measurements • Via direct searches for new signals • Tevatron has handed the baton to the LHC • From a handful of events to a deep testing of the sector • We are now starting to challenge the theory predictions in many respects • Still the Standard Model looks healthier than ever • No hints of new physics (yet?) • Exciting times ahead of us… what should we do next? Remember: the fate of the universe depends on the Higgs boson and the top quark !

  37. Q1: what is the future of top physics at the LHC? • Top physics will remain the “Swiss knife” of LHC physics • Privileged window for a direct observation of new physics • Best handles for constraining existing physics • What should the priorities be in the next years? Top-pair “environment” Inclusive quantities Differential cross sections Evidence 2010 2011 2012 Wtb vertex structure top properties constrain systematics, PDFs total cross sections top properties top couplings measure systematics +direct searches +direct searches +direct/indirect searches • a. Keep looking for NP in top(like) events • b. Look for tails and “top pair environment” • Access to new physics in association, to top couplings to bosons, ttH • Useful to constrain hard and soft QCD in situ → back to precision measurements • b’. Fight systematic sources directly +direct/indirect searches

  38. Q2: which systematic sources to fight for precision? • In the absence of direct evidence of new physics, precision measurements are more vibrant than ever • All precision QCD/EWK measurements in top physics are dominated (by far) by systematic uncertainties. How much (and where) can we gain in the future? • We have the possibility to use the enormous amount of data we (will) have to directly constrain them. What are the best ways to go? • Modelling uncertainties particularly worrying • May expand measurements as a function of observables • Use data (top) themselves to constrain systematic errors • Radiation, CR/soft QCD effects… • Diversify analyses ! • Exploit different (smaller) region of acceptance, much less sensitive to traditional systematic error sources • Use different techniques with independent systematic sources and combine measurements. Always room for new ideas….

  39. Q3: what is the future of top physics at accelerators? (in the hypothesis of still being in the realm of the SM) • Very precision physics at a HL LHC (14 TeV, 3000/fb) • Establish ttH and measure top couplings to bosons • Couplings of top to Z/ can be improved to 2-10% precision • Searches in top in the focus of the research program • Impressive improvements expected in direct searches like FCNC, resonances,… • Final word on asymmetries and their dependence on kinematics (e.g. m(tt)) • What will be left beyond the LHC, the case of ILC • The keys are the EWK couplings of the top • tZ/ couplings at better than 1%. Left and right handed polarized couplings are accessible via beam polarizations • Access to (g-2)top to 0.1%→constrain compositeness to 100 TeV • Δyt/yt ~10%(500 GeV)-6%(1TeV) • Top mass by threshold scans, extremely precise access to the top quark pole mass 500/fb, 500 GeV 300/fb

More Related