Jet structures in New Physics and Higgs searches

Transcription

1 Jet structures in New Physics and Higgs searches Gavin P. Salam LPTHE, UPMC Paris 6 & CNRS LHC New Physics Forum, IWH Heidelberg, Germany, February 2009 Part based on work with Jon Butterworth, Adam Davison (UCL) & Mathieu Rubin (LPTHE)

2 Jets, G. Salam, LPTHE (p. 2) Intro LHC will (should...) span two orders of magnitude in p t : m EW 2 50m EW That s why it s being built In much of that range, EW-scale particles are light [a little like b-quarks at the Tevatron] This talk: about reconstructing high-p t EW-scale particles

3 Jets, G. Salam, LPTHE (p. 3) Intro Boosted massive particles, e.g.: EW bosons Hadronically decaying EW boson at high p t two jets boosted X z single jet R m p t 1 z(1 z) (1 z) Rules of thumb: m = 100 GeV, p t = 500 GeV R < 2m p t : always resolve two jets R < 0.4 R 3m : resolve one jet in 75% of cases ( 1 p 8 < z < 7 8 ) R 0.6 t

4 Jets, G. Salam, LPTHE (p. 4) Intro Relevant in (at least) two ways New heavy particles can decay to W, Z, top hadrons Need taggers for boosted hadronic SM particles To help extract new-physics signals; help identify their decays Continue here: top-quark ID New EW-scale particles may be easier to discover at high-p t Some relevant fraction produced at high-p t ( s m) Jet combinatorics are easier at high p t cleaner events Easier to organise cuts so as not to sculpt backgrounds Start here: light Higgs-boson search

5 Jets, G. Salam, LPTHE (p. 4) Intro Relevant in (at least) two ways New heavy particles can decay to W, Z, top hadrons Need taggers for boosted hadronic SM particles To help extract new-physics signals; help identify their decays Continue here: top-quark ID New EW-scale particles may be easier to discover at high-p t Some relevant fraction produced at high-p t ( s m) Jet combinatorics are easier at high p t cleaner events Easier to organise cuts so as not to sculpt backgrounds Start here: light Higgs-boson search

6 Jets, G. Salam, LPTHE (p. 4) Intro Relevant in (at least) two ways New heavy particles can decay to W, Z, top hadrons Need taggers for boosted hadronic SM particles To help extract new-physics signals; help identify their decays Continue here: top-quark ID New EW-scale particles may be easier to discover at high-p t Some relevant fraction produced at high-p t ( s m) Jet combinatorics are easier at high p t cleaner events Easier to organise cuts so as not to sculpt backgrounds Start here: light Higgs-boson search

7 Jets, G. Salam, LPTHE (p. 5) Intro Low-mass Higgs LHC: complex because dominant decay channel, H bb, often swamped by backgrounds. Various production processes gg H ( γγ) WW H... gg t th feasible feasible v. hard q q WH,ZH small; but gives access to WH and ZH couplings Currently considered impossible

8 Jets, G. Salam, LPTHE (p. 6) Intro WH/ZH search LHC Signal is W lν, H b b. Backgrounds include Wb b, t t lνb bjj,... Studied e.g. in ATLAS TDR Difficulties, e.g. Poor acceptance ( 12%) Easily lose 1 of 4 decay products p t cuts introduce intrinsic bkgd mass scale; gg t t lνb b[jj] has similar scale small S/B Need exquisite control of bkgd shape H b W e,µ b ν

9 Jets, G. Salam, LPTHE (p. 6) Intro WH/ZH search LHC Signal is W lν, H b b. Backgrounds include Wb b, t t lνb bjj,... Studied e.g. in ATLAS TDR pp WH lνb b + bkgds ATLAS TDR Difficulties, e.g. Poor acceptance ( 12%) Easily lose 1 of 4 decay products p t cuts introduce intrinsic bkgd mass scale; gg t t lνb b[jj] has similar scale small S/B Need exquisite control of bkgd shape H b W e,µ b ν

10 Jets, G. Salam, LPTHE (p. 6) Intro WH/ZH search LHC Signal is W lν, H b b. Backgrounds include Wb b, t t lνb bjj,... Studied e.g. in ATLAS TDR pp WH lνb b + bkgds ATLAS TDR Conclusion (ATLAS TDR): Difficulties, e.g. The extraction of a signal from H b b decays in the WH channel will be very difficult at the LHC, even under the most optimistic assumptions [...] Poor acceptance ( 12%) Easily lose 1 of 4 decay products p t cuts introduce intrinsic bkgd mass scale; gg t t lνb b[jj] has similar scale small S/B Need exquisite control of bkgd shape W e,µ H b b ν

11 Jets, G. Salam, LPTHE (p. 7) Intro Study subset of WH/ZH with high p t b H b At high p t : Higgs and W/Z more likely to be central high-p t Z ν ν becomes visible Fairly collimated decays: high-p t l ±,ν,b Good detector acceptance e,µ W ν Backgrounds lose cut-induced scale t t kinematics cannot simulate bkgd Gain clarity and S/B Cross section will drop dramatically By a factor of 20 for p th > 200 GeV Will the benefits outweigh this?

12 Jets, G. Salam, LPTHE (p. 8) Boosted object finding Finding a boosted Higgs? How do we find a boosted Higgs inside a single jet? Special case of general (unanswered) question: how do we best do jet-finding? Various people have looked at boosted objects over the years Seymour 93 [heavy Higgs WW νljets] Butterworth, Cox & Forshaw 02 [WW WW νljets ] Agashe et al. 06 [KK excitation of gluon t t] Butterworth, Ellis & Raklev 07 [SUSY decay chains W, H] Skiba & Tucker-Smith 07 [vector quarks] ETC. Lillie, Randall & Wang 07 [KK excitation of gluon t t]

13 Jets, G. Salam, LPTHE (p. 9) Boosted object finding Boosted ID strategies q q Select on the jet mass with one large (cone) jet Can be subject to large bkgds [high-p t jets have significant masses] q q Choose a small jet size (R) so as to resolve two jets Easier to reject background if you actually see substructure [NB: must manually put in right radius] q q Take a large jet and split it in two Let jet algorithm establish correct division

14 Jets, G. Salam, LPTHE (p. 10) Boosted object finding Past methods Use k t alg. s distance measure (rel. trans. mom.) to cut out QCD bkgd: Use k t jet-algorithm s hierarchy to split the jets Y-splitter d kt ij = min(p 2 ti,p2 tj ) R2 ij only partially correlated with mass

15 Jets, G. Salam, LPTHE (p. 10) Boosted object finding Past methods Use k t alg. s distance measure (rel. trans. mom.) to cut out QCD bkgd: Use k t jet-algorithm s hierarchy to split the jets Y-splitter d kt ij = min(p 2 ti,p2 tj ) R2 ij only partially correlated with mass

16 Jets, G. Salam, LPTHE (p. 11) Boosted object finding #1: Our tool The Cambridge/Aachen jet alg. Dokshitzer et al 97 Wengler & Wobisch 98 Work out Rij 2 = y2 ij + φ2 ij between all pairs of objects i,j; Recombine the closest pair; Repeat until all objects separated by R ij > R. [in FastJet] Gives hierarchical view of the event; work through it backwards to analyse jet

17 Jets, G. Salam, LPTHE (p. 11) Boosted object finding #1: Our tool The Cambridge/Aachen jet alg. Dokshitzer et al 97 Wengler & Wobisch 98 Work out Rij 2 = y2 ij + φ2 ij between all pairs of objects i,j; Recombine the closest pair; Repeat until all objects separated by R ij > R. [in FastJet] Gives hierarchical view of the event; work through it backwards to analyse jet k t algorithm Cam/Aachen algorithm Allows you to dial the correct R to keep perturbative radiation, but throw out UE

18 Jets, G. Salam, LPTHE (p. 12) Boosted object finding #2: The jet analysis b R b g Start with high-p t jet 1. Undo last stage of clustering ( reduce R): J J 1,J 2 2. If max(m 1,m 2 ) 0.67m, call this a mass drop [else goto 1] Automatically detects correct R R bb to catch angular-ordered radn. 3. Require y 12 = min(p2 t1,p2 t2 ) R 2 m min(z 1,z 2 ) max(z 1,z 2 ) > 0.09 [else goto 1] dimensionless rejection of asymmetric QCD branching 4. Require each subjet to have b-tag [else reject event] Correlate flavour & momentum structure

19 Jets, G. Salam, LPTHE (p. 12) Boosted object finding #2: The jet analysis b R b R bb g mass drop R bb Start with high-p t jet 1. Undo last stage of clustering ( reduce R): J J 1,J 2 2. If max(m 1,m 2 ) 0.67m, call this a mass drop [else goto 1] Automatically detects correct R R bb to catch angular-ordered radn. 3. Require y 12 = min(p2 t1,p2 t2 ) R 2 m min(z 1,z 2 ) max(z 1,z 2 ) > 0.09 [else goto 1] dimensionless rejection of asymmetric QCD branching 4. Require each subjet to have b-tag [else reject event] Correlate flavour & momentum structure

20 Jets, G. Salam, LPTHE (p. 13) Boosted object finding #3: jet filtering R b b R bb UE g mass drop R bb At moderate p t, R bb is quite large; UE & pileup degrade mass resolution δm R 4 p Λ t UE M [Dasgupta, Magnea & GPS 07] Filter the jet Reconsider region of interest at smaller R filt = min(0.3,r b b/2) Take 3 hardest subjets b, b and leading order gluon radiation

21 Jets, G. Salam, LPTHE (p. 13) Boosted object finding #3: jet filtering R b b R bb R filt UE g mass drop R bb filter At moderate p t, R bb is quite large; UE & pileup degrade mass resolution δm R 4 p Λ t UE M [Dasgupta, Magnea & GPS 07] Filter the jet Reconsider region of interest at smaller R filt = min(0.3,r b b/2) Take 3 hardest subjets b, b and leading order gluon radiation

22 Jets, G. Salam, LPTHE (p. 14) Boosted object finding pp ZH ν νb m H =115GeV Herwig Jimmy FastJet 2.3 SIGNAL Zbb BACKGROUND Cluster event, C/A, R=1.2 arbitrary norm.

23 Jets, G. Salam, LPTHE (p. 14) Boosted object finding pp ZH ν νb m H =115GeV Herwig Jimmy FastJet 2.3 SIGNAL Zbb BACKGROUND Fill it in, show jets more clearly arbitrary norm.

24 Jets, G. Salam, LPTHE (p. 14) Boosted object finding pp ZH ν νb m H =115GeV Herwig Jimmy FastJet SIGNAL 200 < p tz < 250 GeV m H [GeV] Zbb BACKGROUND < p tz < 250 GeV Consider hardest jet, m = 150 GeV m H [GeV] arbitrary norm.

25 Jets, G. Salam, LPTHE (p. 14) Boosted object finding pp ZH ν νb m H =115GeV Herwig Jimmy FastJet SIGNAL 200 < p tz < 250 GeV m H [GeV] Zbb BACKGROUND < p tz < 250 GeV split: m = 150 GeV, max(m 1,m2) m = 0.92 repeat m H [GeV] arbitrary norm.

26 Jets, G. Salam, LPTHE (p. 14) Boosted object finding pp ZH ν νb m H =115GeV Herwig Jimmy FastJet SIGNAL 200 < p tz < 250 GeV m H [GeV] Zbb BACKGROUND < p tz < 250 GeV split: m = 139 GeV, max(m 1,m2) m = 0.37 mass drop m H [GeV] arbitrary norm.

27 Jets, G. Salam, LPTHE (p. 14) Boosted object finding pp ZH ν νb m H =115GeV Herwig Jimmy FastJet SIGNAL 200 < p tz < 250 GeV m H [GeV] Zbb BACKGROUND < p tz < 250 GeV check: y 12 p t2 p t1 0.7 OK + 2 b-tags (anti-qcd) m H [GeV] arbitrary norm.

28 Jets, G. Salam, LPTHE (p. 14) Boosted object finding pp ZH ν νb m H =115GeV Herwig Jimmy FastJet SIGNAL 200 < p tz < 250 GeV m H [GeV] Zbb BACKGROUND < p tz < 250 GeV R filt = m H [GeV] arbitrary norm.

29 Jets, G. Salam, LPTHE (p. 14) Boosted object finding pp ZH ν νb m H =115GeV Herwig Jimmy FastJet SIGNAL 200 < p tz < 250 GeV m H [GeV] Zbb BACKGROUND < p tz < 250 GeV R filt = 0.3: take 3 hardest, m = 117 GeV m H [GeV] arbitrary norm.

30 Jets, G. Salam, LPTHE (p. 15) Boosted object finding Compare with standard algorithms Check mass spectra in HZ channel, H b b, Z l + l pp HZ, H b-jets 100% b-tagged (a) C/A MD-F, R=1.2 k t, R=1.0 anti-k t, R=1.0 SISCone, R= pp Zj(b in event) b-tagged (b) C/A MD-F, R=1.2 k t, R=1.0 anti-k t, R=1.0 SISCone, R= pp Zj no b-tagging (c) C/A MD-F, R=1.2 k t, R=1.0 anti-k t, R=1.0 SISCone, R= < p tz /GeV < < p tz /GeV < < p tz /GeV < /N dn/dm /N dn/dm /N dn/dm m [GeV] m [GeV] m [GeV] Cambridge/Aachen (C/A) with mass-drop and filtering (MD/F) works best

31 Jets, G. Salam, LPTHE (p. 16) Results The full analysis (scaled to 30 fb 1 ) Consider HW and HZ signals: H b b, W lν, Z l + l and Z ν ν, 3 channels: l ± + /E T ; l + l ; /E T Common cuts p tv,p th > 200 GeV η Higgs jet < 2.5 l = e,µ, p t,l > 30 GeV, η l < 2.5 No extra l, b s with η < 2.5 Channel-specific cuts: see next slide Assumptions Real/fake b-tag rates: 0.7/0.01 S/ B from 16 GeV window optimistic, but not inconceivable ATLAS jet-mass resln half this? Tools: Herwig 6.510, Jimmy 4.31 (tuned), hadron-level FastJet 2.3 Backgrounds: VV, Vj, jj, t t, single-top, with > 30 fb 1 (except jj)

32 Jets, G. Salam, LPTHE (p. 17) Results Leptonic channel combine HZ and HW, p t > 200 GeV Common cuts p tv,p th > 200 GeV η H < 2.5 [p t,l > 30 GeV, η l < 2.5] No extra l, b s with η < 2.5 Real/fake b-tag rates: 0.7/0.01 S/ B from 16 GeV window Leptonic channel Z µ + µ, e + e 80 < m l + l < 100 GeV At 5.9σ for 30 fb 1 this looks like a possible new channel for light Higgs discovery. Deserves serious exp. study!

33 Jets, G. Salam, LPTHE (p. 17) Results combine HZ and HW, p t > 200 GeV Missing E T channel Common cuts p tv,p th > 200 GeV η H < 2.5 [p t,l > 30 GeV, η l < 2.5] No extra l, b s with η < 2.5 Real/fake b-tag rates: 0.7/0.01 S/ B from 16 GeV window Missing-E t channel Z ν ν, W ν[l] /E T > 200 GeV At 5.9σ for 30 fb 1 this looks like a possible new channel for light Higgs discovery. Deserves serious exp. study!

34 Jets, G. Salam, LPTHE (p. 17) Results combine HZ and HW, p t > 200 GeV Semi-leptonic channel Common cuts p tv,p th > 200 GeV η H < 2.5 [p t,l > 30 GeV, η l < 2.5] No extra l, b s with η < 2.5 Real/fake b-tag rates: 0.7/0.01 S/ B from 16 GeV window Semi-leptonic channel W νl /E T > 30 GeV (& consistent W.) no extra jets η < 3,p t > 30 At 5.9σ for 30 fb 1 this looks like a possible new channel for light Higgs discovery. Deserves serious exp. study!

35 Jets, G. Salam, LPTHE (p. 17) Results combine HZ and HW, p t > 200 GeV 3 channels combined Common cuts p tv,p th > 200 GeV η H < 2.5 [p t,l > 30 GeV, η l < 2.5] No extra l, b s with η < 2.5 Real/fake b-tag rates: 0.7/0.01 S/ B from 16 GeV window 3 channels combined Note excellent VZ, Z b b peak for calibration NB: q q is mostly t t At 5.9σ for 30 fb 1 this looks like a possible new channel for light Higgs discovery. Deserves serious exp. study!

36 Jets, G. Salam, LPTHE (p. 18) Results Rough impact of going to high-p t How can we be doing so well despite losing factor 20 in X-sct? Signal Background Eliminate t t, etc. 1/3 [very approx.] p t > 200 GeV 1/20 1/60 [bkgds: Wb b,zb b] improved acceptance 4 4 twice better resolution 1/2 add Z ν ν total much better S/B; better S/ B [exact numbers depend on analysis details]

37 Jets, G. Salam, LPTHE (p. 19) Results Impact of b-tagging, Higgs mass Significance 7 6 (a) 200GeV R = 1.2 Eff = 70% 300GeV R = 0.7 Eff = 70% 200GeV R = 1.2 Eff = 60% 300GeV R = 0.7 Eff = 60% Significance 7 6 (b) 200GeV R = 1.2 Eff = 70% (1%) 300GeV R = 0.7 Eff = 70% (1%) 200GeV R = 1.2 Eff = 60% (2%) 300GeV R = 0.7 Eff = 60% (2%) b Mistag Probability Higgs Mass (GeV) Most scenarios above 3σ For it to be a significant discovery channel requires decent b-tagging, lowish mass Higgs [and good experimental resolution] In nearly all cases, looks feasible for extracting WH, ZH couplings

38 Jets, G. Salam, LPTHE (p. 19) Results Impact of b-tagging, Higgs mass Significance 7 6 (a) 200GeV R = 1.2 Eff = 70% 300GeV R = 0.7 Eff = 70% 200GeV R = 1.2 Eff = 60% 300GeV R = 0.7 Eff = 60% Significance 7 6 (b) 200GeV R = 1.2 Eff = 70% (1%) 300GeV R = 0.7 Eff = 70% (1%) 200GeV R = 1.2 Eff = 60% (2%) 300GeV R = 0.7 Eff = 60% (2%) b Mistag Probability Higgs Mass (GeV) Most scenarios above 3σ For it to be a significant discovery channel requires decent b-tagging, lowish mass Higgs [and good experimental resolution] In nearly all cases, looks feasible for extracting WH, ZH couplings

39 Jets, G. Salam, LPTHE (p. 20) t t Boosted top [hadronic decays]

40 Jets, G. Salam, LPTHE (p. 21) t t X t t resonances of varying difficulty RS KK resonances t t, from Frederix & Maltoni, NB: QCD dijet spectrum is 500 times t t

41 Jets, G. Salam, LPTHE (p. 22) t t Boosted top Tagging boosted top-quarks High-p t top production often envisaged in New Physics processes. high-p t EW boson, but: top has 3-body decay and is coloured. 4 papers on top tagging in 08 (at least). All use the jet mass + something extra. Questions What efficiency for tagging top? What rate of fake tags for normal jets? Rough results for top quark with p t 1 TeV Extra eff. fake [from T&W] just jet mass 50% 10% Brooijmans 3,4 k t subjets, d cut 45% 5% Thaler & Wang 2,3 k t subjets, z cut + various 40% 5% Kaplan et al. 3,4 C/A subjets, z cut + θ h 40% 1% Almeida et al. predict mass dist n, use jet-shape

42 Jets, G. Salam, LPTHE (p. 23) t t Boosted top Efficiency v. p t with calo (0.1x0.1)

43 Jets, G. Salam, LPTHE (p. 24) t t Boosted top Fair assumptions for detector? Theory t t studies use η φ segmentation of 0.1. Limiting when R 0.1 But charged tracks and EM-calo provide much better angular resolution. two-body mass m E total z(1 z) R For z 0.2: 25% error on z 10% error on mass h cal e cal tracker for total energy E total for angular distribution of energy? z, R Even rough info from tracks & e-cal very valuable

44 Jets, G. Salam, LPTHE (p. 25) t t Boosted top Efficiency v. p t (ideal detector) without detector segmentation

45 Jets, G. Salam, LPTHE (p. 26) t t Boosted top Using (coloured!) boosted top-quarks If you want to use the tagged top (e.g. for t t invariant mass) QCD tells you: the jet you use to tag a top quark the jet you use to get its p t t b jet for top p t jet for top tag Within inner cone 2mt p t (dead cone) you have the top-quark decay products, but no radiation from top ideal for reconstructing top mass Outside dead cone, you have radiation from top quark essential for top p t Cacciari, Rojo, GPS & Soyez 09

46 Jets, G. Salam, LPTHE (p. 27) t t Boosted top 1/N dn/dbin / Use small cone qq, M = 4000 GeV Cam/Aachen, R=0.4 Q w f=0.24 = GeV Impact of using small cone angle arxiv: /N dn/dbin / Use large cone qq, M = 4000 GeV C/A-filt, R=1.2 Q w f=0.24 = GeV arxiv: dijet mass [GeV] dijet mass [GeV] Figure actually from (Cacciari, Rojo, GPS & Soyez) for light q q resonance but t t will be similar How you look at your event matters:

47 Jets, G. Salam, LPTHE (p. 28) Closing Conclusions General Boosted EW-scale particles can be found in jets Cambridge/Aachen alg. is very powerful (flexible, etc.) tool for this General two-body eff/fake is 60% v. 3 7% Higgs discovery high-p t limit recovers WH and ZH channel at LHC Separately see WH, ZH couplings Deserves & needs in-depth experimental study ongoing/starting within ATLAS/CMS Top Efficiencies/fake rates: up to 40/1(2)% Only get this if detector can resolve fine structure Top-quark at decay (the one you tag) and top-quark at production are different objects need different R for them

48 Jets, G. Salam, LPTHE (p. 29) Extras EXTRAS

49 Jets, G. Salam, LPTHE (p. 30) Extras Jet-alg comparison Cross section for signal and the Z+jets background in the leptonic Z channel for 200 < p TZ /GeV < 600 and 110 < m J /GeV < 125, with perfect b-tagging; shown for our jet definition (C/A MD-F), and other standard ones close to their optimal R values. Jet definition σ S /fb σ B /fb S/ B fb C/A, R = 1.2, MD-F k t, R = 1.0, y cut SISCone, R = anti-k t, R =

50 Jets, G. Salam, LPTHE (p. 31) Extras K-factors Analysis shown without K factors. What impact do they have? Determined with MCFM, MC@NLO Signal: K 1.6 Vbb backgrounds: K t t backgrounds: K 2 for total; not checked for high-p t part Conclusion: S/ B should not be severely affected by NLO contributions

51 Jets, G. Salam, LPTHE (p. 32) Extras Worsen b-tagging: 60%/2% Events / 8GeV / 30fb 16 (a) qq S/ B = 2.1 V+jets 14 in GeV VV V+Higgs Events / 8GeV / 30fb (b) q S/ B = 3.1 in GeV q V+jets VV V+Higgs Mass (GeV) Mass (GeV) Events / 8GeV / 30fb (c) S/ B = 2.9 in GeV qq V+jets VV V+Higgs (d) 140 S/ B = in GeV Events / 8GeV / 30fb qq V+jets VV V+Higgs Mass (GeV) Mass (GeV)

52 Jets, G. Salam, LPTHE (p. 33) Extras Raise p t cut to 300 GeV -1 Events / 30fb Leptonic Z Channel Events / 30fb qq V+jets VV Higgs S/ B = 2.1 Range GeV Mass (GeV) Leptonic W Channel qq V+jets VV Higgs S/ B = 4.1 Range GeV -1 Missing Et Channel Events / 30fb -1 Events / 30fb qq V+jets VV Higgs S/ B = 3.4 Range GeV Mass (GeV) All Leptonic Channels qq V+jets VV Higgs S/ B = 5.5 Range GeV NB: kills t t background Mass (GeV) Mass (GeV)