E. Bergeaas Kuutmann. Uppsala University

Transcription

1 LHC Physics conference, New York, 2014 Physics with boosted top Elin Bergeaas Kuutmann Uppsala University Outline: - Why care about boosted top? Searches for new physics - How to handle boosted top? Top-taggers and substructure - Results with boosted top tt resonances and top partners 1

2 Why boosted top? 2

3 Why top quarks? Top is special: heaviest (known) elementary particle, large contribution to loop corrections The Hierarchy problem: why does the Higgs boson have the (low) mass we observe? Fine-tuning? or Hints of new physics? Proposed solutions (without fine-tuning): Supersymmetry Strong dynamics (Technicolor, ) Extra dimensions (Randall-Sundrum, ) Fourth generation / vector-like quarks... Image: Sandbox Studio / symmetrymagazine.org All these theories tend to come with top partners or particles decaying to top quarks. No such particles observed yet => they are likely heavy (if they exist). 3

4 The importance of boosted top LHC: top factory. At 8 TeV: σtt 250 pb, L = pb1 >10 million top quark pairs produced. Rule of thumb: R 2m/pT pt = 350 GeV, R=1.0 => top decay in one jet! Reconstruction as 3 separate jets fails. b-jet light jets Top decay: t Wb W q q or W ℓ ν one jet CMS PAS JME boost resolved 3 separate jets boosted 1 massive jet 4

5 First boosted top results Boosted top in CDF: jet mass as top ID 5

6 CDF boosted top search Note 10234, Jan 2011 Tevatron, 6 fb1, s = 1.96 TeV. all-had Utilising jet mass as top ID. Selection: 1 jet trigger ET>100 GeV 1 jet p >400 GeV, η <0.7 T 2 massive jets (all-had) or 1 massive jet, 4 < ETmiss / ET < 10 ( lepton+jets ). Midpoint cone jets, R=1.0. tt signal region: 130<mjet<210 GeV. CDF note 10234, Jan 2011 Candidate events (both channels): observed 58 events expected 44 ± 8 (stat) ± 13 (syst). 5 SM tt events expected. Upper 95% CL limit on tt prod, pttop>400 GeV : σ < 40 fb. 6

7 Boosted top at the LHC Handling t b ℓ ν : loosely isolated leptons Handling t b q q: Jet substructure variables Declustering jets Concepts of top-tagging...with examples from physics analyses 7

8 Boosted top at the LHC Large statistics: opportunity to try top taggers Boosted top overviews: - CMS PAS JME ATLAS-CONF BOOST workshop reports 2012: EPJC 74:2792, : JP.G G39 (2012) : EPJC71:1661,2011 I will not cover - Shower deconstruction (ATLAS-CONF ) - Jet grooming (ATLAS-CONF , CMS JHEP05 (2013) 090) =>Marco Peruzzi QCD plenary Tuesday - N-subjettiness (CMS PAS JME , ATLAS-CONF ) - b-tagging at high pt (CMS PAS BTV ) tt e + jets candidate, mtt = 2.6 TeV ATLAS-CONF

9 Resonances: X tt searches test bench for boosted top studies at the LHC Two benchmark theories with X tt: Leptophobic topcolor Z' Topcolor: new strong dynamics. A new Z-like boson, Zt', couples strongly to top quarks. Zt' tt (BR ~30%). Narrow (Γ / m = 1.2%), colour Z' Z' singlet. ATLAS and CMS use the same model. Kaluza-Klein gluons (gkk). Randall-Sundrum scenario I: extra spatial dimension. excitation Kaluza-Klein gluon. gkk tt (~90% BR). Broad (Γ / m = 15-20%), colour octet. CMS-PAS-B2G slightly different models in ATLAS and CMS. Large pttop => boost 9

10 t bℓν: (non-)isolation of the leptons 10

11 Loosely isolated leptons handle merging of decay products in t b ℓ ν Standard isolation: ptcone ptℓ < 2-4 GeV or (ptcone ptℓ) / ptℓ < fixed cone size (typically R= ) b-jet e or µ ATLAS boosted approach: mini-isolation (K. Rehermann and B. Tweedie, JHEP 1103 (2011) 059) Mini-isolation definition: low pt b-jet Isolation cone KT =10 GeV. Cut on Iℓmini/pTℓ < 0.05 R(e,j)>0.4, R(µ,j)>0.1 CMS boosted approach: No isolation. Kinematic cuts: R(ℓ, j) > 0.5 OR p (ℓ, j) > 25 GeV ETmiss + ptℓ > 150 GeV high pt e or µ 11

12 t b q q: Background understanding; massive QCD jets Jet substructure Searches for boosted top in BSM models 12

13 Jet algorithms and substructure Sequential clustering jet algorithms: List of protojets (i, j, ) For all protojets, define: di = pt, i2n, dij = min(pt, i2n, pt, j2n) Rij2 / R2 Find minimum of all di, dij Min = di? ==> jet. Min = dij? ==> Merge i and j, reiterate. n = 1: n = 0: n = 1: kt algorithm Cambridge-Aachen (C-A) algorithm anti-kt algorithm Note: - kt algorithm: last protojets added to final jets are the hardest. - C-A clusters by angular ordering only. Substructure Clustering history holds information about the jet substructure - kt split scales d12 and d23. - Use clustering history of C-A (HEPTopTagger, CMS tagger). - Use structure of clusters (Top Template method). Recluster any jet with kt or C-A. 13

14 ATLAS tt l+jets resonances 8 TeV, 14 fb1. Pre-selection: ==1 e or µ, mini-iso ETmiss, 1b-tags. Boosted hadronic top trimmed anti-kt jet, R=1.0 cut-based tagging: mjet > 100 GeV, ptjet > 300 GeV d12 > 40 GeV, φ(l,jet)>2.3. Boosted and resolved reconstruction - pass boosted: boosted mass reco - if not pass boosted, test resolved selection. Systematic uncertainties boosted: large-r JES (17%), total 22% resolved: b-tag, PDF, lumi (2-4%), tot 14% Mass exclusions: Z': 0.5 <m< 1.8 TeV (obs) 0.5 <m< 1.9 TeV (exp) gkk: 0.5 <m< 2.0 TeV (obs) 0.5 <m< 2.1 TeV (exp). ATLAS-CONF

15 Declustering C-A (1): CMS top-tagger CMS-PAS-JME Based on Johns-Hopkins top-tagger (Kaplan et al. PRL101, (2008)). Start with C-A R=0.8 jets, pt>350 GeV, decluster C-A 0.8 jet Decluster in 2 steps up to 4 subjets Subjet momentum criterion: ptsubjet > 0.05 ptjet. Adjacency criterion: ΔR(s1, s2) > pt(parent). Additional background suppression through mass of original jet ~ mtop. number of subjets after 2nd declustering step >= 3. minimum pairwise mass of leading 3 subjets mmin = min(m12, m13, m23) ~ mw. 15

16 CMS tt resonances (1) PRL 111, (2013) 8 TeV, 20 fb1. Combination of tt l+jets (boosted and resolved) and tt all-had (boosted). Resolved l+jets: ==1 e or µ, isolated, ETmiss 4 jets, pt> 70, 50, 30, 30 GeV anti-kt jets R=0.5 ==1 or 2 b-tags. Boosted Boosted l+jets: ==1 e or µ, no iso, ETmiss 2 jets, pt> 150, 50 GeV anti-kt jets R=0.5 ==0 or 1 b-tags. Dominant non-top background: W+jets 16

17 CMS tt resonances (2) PRL 111, (2013) Boosted all-had: 2 CMS-tagged tops - C-A jets R=0.8, pt>400 GeV - φ(jet,jet) > π/2 - y(jet,jet) < 1.0. Dominant non-top background: non-top multijets (NTMJ). Estimated from data. Boosted and resolved combination through mass cut-off. Mass exclusions Z': m<2.1 TeV (obs & exp) gkk: m<2.5 TeV (obs) m<2.4 TeV (exp). 17

18 Declustering C-A (2): HEPTopTagger (Plehn et al, JHEP 1010 (2010) 078) CambridgeAachen (grooming) Look at triplets G. Kasieczka BOOST2012 R= min(0.3, R(a,b)) 18

19 ATLAS tt resonances all-hadronic decays with HEPTopTagger JHEP 1301 (2013) TeV, 5 fb1. Selection: 2 C-A 1.5 jets, pt>200 GeV, HEPTopTagged 2 b-tagged anti-kt R=0.4 jets embedded in top-tagged jets. Background dominated by tt. Main systematics: JES, JER, b-tag. Pile-up stable. Typically performs best for moderately boosted top. 19

20 Cluster structure: Top Template Tagger Almeida et al, PRD Take all topological clusters in an anti-kt R=1.0 jet. Find best 3-parton template that matches parton energy Ei with Etopo. JHEP 1301 (2013) 116 Templates τn pt step 100 GeV Combine OV3 cut with cut on invariant mass of the topo clusters (close to mtop). ATLAS tt jets resonances example. Slightly better performance than HEPTopTagger for high tt masses. 20

21 tt resonances overview BOOST2012, EPJ.C74:2792,2014 Update by Vos & Villaplana TTbarResonanceSearches.pdf + Preliminary ATLAS-CONF TeV 14/fb l+jets combined May '13, m>1.8 TeV => See also BSM 4 session, Thursday Ivan Marchesini (CMS) Barbara Alvarez (ATLAS) 21

22 Other top-tagging applications: searches for massive top partners 22

23 Top partner searches: T5/3 PRL 112 (2014) Searching for heavy vector-like quarks, exotic charge +5/3 T5/3T5/3 W+t Wt 2 same-sign leptons top-tag W-tag Resolved / boosted transition: NC 7 (Nc = # constituents) Lepton or anti-kt 0.5 jet = 1 NC W-tag = 2 Nc Top-tag = 3 Nc Backgrounds: - VV, ttv, ttww, and WWW - non-prompt leptons Exclusion: m<800 GeV (obs) m<810 GeV (exp) => See also Tulika Bose, BSM 1 plenary, Wednesday 23

24 T'T' HtHt 6b4q CMS-PAS-B2G T': vector-like top-like quark T' Ht ; H bb, t tqq (Limits on tz and bw decays also set) Signature: 2 C-A 1.5 jets 1 HEPTopTagged jets pt>200 GeV, 1 b-tagged subjet 1 Higgs-tagged jet pt>150 GeV, 2 b-tagged subjets, mbb>60 GeV Discriminator: combination of HT and mbb Limits: m>747 GeV (obs), >701 GeV (exp) Comparable to a l+jets study, CMS, PLB 729 (2014) : m>706 GeV (obs), m>770 GeV (exp) HEPTopTagger-like tagger also used in a CMS SUSY analysis, PAS SUS (=> H. Weber BSM 1 session Tuesday) ATLAS direct stop search, arxiv: (=>C. Wanotayaroj, poster session) 24

25 Summary and outlook Many physics results from LHC include boosted top Stronger limits with substructure techniques PRL 112 (2014) Plenty of tt resonance searches... First top partner searches, e.g. T' Ht tth studies next? Boosted boson studies gearing up. Run II coming soon TeV. Pile-up! The boosted era has just begun. My personal thanks to Lily Asquith, Giorgio Chiarelli, Kyle Cranmer, Dominik Duda, Robin Erbacher, Johannes Erdmann, James Ferrando, Arnaud Ferrari, Sebastian Fleischmann, Simon Head, Anna Henrichs, Andreas Jung, Yvonne Peters, Sal Rappoccio, Pekka Sinervo, Sara Strandberg, Marcel Vos, Horst Wahl, Chaowaroj Wanotayaroj, Jiahang Zhong. 25

26 Backup extra material for discussion 26

27 tt reconstruction tt Wb + Wb lνb + qqb Resolved reconstruction 3 jets Boosted reconstruction One big jet Hadronic side tt ν tt Leptonic side e or µ e or µ ν b-jet b-jet 27

28 Visibility of boosted tops Simulation of gkk tt at various masses, leading jet. boost If mjet >~ 60 GeV: massive QCD jet? W qq? t bqq? s=10 TeV Jet merging starts if pttop > 2mtop / R => mtt > 1 TeV (R=1). ATL-PUB-PHYS Single Jet Mass [GeV] Identifying boosted top quarks: top-tagging. 28

29 Boosted hadronically-decaying top: A few words about jet formation Make a jet (sequential clustering algorithm) Start with a list of input components i (e.g. calorimeter energy clusters) 1) For each component, define di = pt, i2n For each pair (i, j) define dij = min(pt, i2n, pt, j2n) Rij2 / R2 n is algorithm-dependent 2) Find dmin = minimum of all di, dij 3) If dmin is a dij, merge clusters to a new cluster dk If dmin is a di, the object is a jet. Remove cluster from list. ( next page) Rij2 = (ϕi ϕj)2 + (ηi - ηj )2 R is the radius parameter (typically ) Repeat until all clusters are in jets. 29

30 Jet substructure: Split scale d12 Last kt merging step: d12 = min(pt, 12, pt, 22) R122 / R2. If jet contains top decay (t Wb qqb) d12 (mtop / 2)2 (heavily smeared). anti-kt, R=1.0, reclustered with kt. Light jets: d12 0. Cut: d12 > 40 GeV tt l + jets. PRD 88, (2013) 30

31 The power of boosted top tt ℓ ν bb qq Compare mtt spectra. Resolved Resolved reconstruction: ==1 lepton (e or µ), ETmiss (from ν) 4 jets*, 1 b-tags. Boosted reconstruction: ==1 lepton (e or µ), ETmiss (from ν) 1 small-r jet near the lepton 1 b-tags (anywhere) 1 massive large-r jet, far away from the lepton. EPJC 72 (2012) 2083 Better S/B for boosted reconst. at high tt masses despite worse statistics! Upper expected cross section limits Z' mass Resolved Boosted 0.6 TeV 6.0 pb 10.4 pb 0.8 TeV 2.1 pb 1.6 pb 1.3 TeV 0.62 pb 0.39 pb Boosted *or 3 jets if mjet>60 GeV (minimally boosted). JHEP (2012) 042 LHCP New York 6 June 31

32 HEPTopTagger vs TopTemplateTagger JHEP 1301 (2013) 116 Comparison, gkk limits (7 TeV, 5 fb1) TopTemplateTagger: slightly stronger limits at high masses 32

33 tt resonances summary Mass range exclusions for Z' tt (width 1.2%) 7 TeV 5 fb1 comparison, tt all-had Exp. Tagger exclusion (obs) (exp) CMS CMS/JHU 1.3 <m< 1.5 TeV 1.0 <m <1.3 TeV ATLAS HEPTopTagger 0.70<m<1.00 TeV 0.68<m<1.16 TeV 1.28<m<1.32 TeV ATLAS TopTemplate 7 TeV 5 fb1 comparison, tt l+jets Exp. Tagger exclusion (obs) CMS CMS (JHU) 0.50<m<1.49 TeV ATLAS cut-based 0.50<m< 1.74 TeV 8 TeV summary Exp. Tagger exclusion (obs) CMS CMS/cuts m<2.1 TeV ATLAS cut-based 0.5<m<1.8 TeV 8 TeV Exp. CMS ATLAS (exp) m<1.53 TeV m<1.63 TeV (exp) m<2.1 TeV m<1.9 TeV Ref JHEP09(2012)029 JHEP01(2013)116 JHEP01(2013)116 Ref JHEP12(2012)015 PRD (2013) Dataset 20 fb1 14 fb1 Ref PRL 111, CONF gkk exclusions (ATLAS and CMS not directly comparable) Tagger exclusion (obs) (exp) Dataset Ref 1 CMS/cuts m<2.5 TeV m<2.4 TeV 20 fb PRL 111, cut-based 0.5<m<2.0 TeV m<2.1 TeV 14 fb1 CONF

34 T'T' searches, T' W+b or Zt or Ht PLB 729 (2014) T': vector-like top-like quark T' Ht, Zt or W+b Final states with 2 W, at least one W l ν Signature l+jets: ==1 isolated lepton 3 anti-kt 0.5 jets 1 W-tag or a 4th jet ETmiss Number of top-tags and W-tags to BDT Multileptons: 2 SS leptons or 2 OS leptons or 3 leptons 2 or 3 jets Limits for BR(T' Ht) = 100% : m>706 GeV (obs), >770 GeV (exp) 35

35 Jet trimming Krohn et al: JHEP 1002 (2010) 084 Pile-up protection Take the fat jet ATLAS: a-kt 1.0 R=1.0 Recluster into thin jets inclusive kt jets, R=Rsub Rsub = 0.3 Remove the softest thin-jets if ptsubjet / ptfatjet < fcut fcut = 0.05 Krohn, Thaler, Wang 36

36 Effect of jet trimming on mass From ATLAS-CONF Ungroomed Trimmed 37