Searches for new phenomena with heavy quarks at the Large Hadron Collider

Transcription

1 Searches for new phenomena with heavy quarks at the Large Hadron Collider Rencontres de Moriond: QCD and High Energy Interac4ons La Thuile, Italy March 26 th, 24 Mark Cooke (LBNL) on behalf of the ATLAS and CMS collabora4ons

2 The Mo>va>on: Heavy quarks and the naturalness problem The discovery of a 25 GeV Higgs boson is established. Ques>on: do other new states exist at the electro- weak scale that could render it natural? Top quark s>ll the heaviest par>cle; largest correc>on to the Higgs mass- squared. Mo>va>on: To search for new phenomena at the LHC that couple preferen>ally to the top quark. ATLAS & CMS searches covered in this talk:. Vector- like quarks (VLQs) that mix with 3 rd genera>on quarks 2. New vector- bosons (Z, W, g KK ) w/ preferen>al couplings to 3 rd genera>on quarks Predicted in models where the Higgs is a pseudo Nambu- Goldstone boson, or a 5- d gauge- field component: Li[le / Composite Higgs, Extra- Dimensions (EDs). LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 2

3 What are vector- like quarks (VLQs)? Fermions in the Standard Model are chiral: Right- handed states: are singlets under the SM SU(2) L gauge group. RH Lea- handed states: form doublets under the SM SU(2) L gauge group. LH (massless fermion case) Vector- like fermions : lea- and right- handed components transform the same way: Singlets T L,R, B L,R Doublets X T B T B Y Interes>ng proper>es: Ø Gauge invariant mass term independent of Higgs. Ø Decoupling proper>es weaken constraints (Higgs produc>on, precision EW). Ø A vector- like top can play a similar role as the stop in regula>ng Higgs mass divergence. Ø Vector- like quarks appear in many BSM models: GUTs, EDs, li[le Higgs, composite Higgs, L,R L,R L,R LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 3

4 Vector- like quark produc>on at the LHC & decay modes Heavy Quark Produc>on Vector- like Top Branching Ra>os (BRs) ( pp QQ ) [pb] 3 2 HATHOR 2 g s = 8 TeV s = 7 TeV Branching Ratio.6 PROTOS SU(2) Singlet T Wb T Zt T Ht (T,B) or (X,T) Doublet T Wb T Zt T Ht (8 Tev) (7 Tev) s = 8 TeV s = 7 TeV 2 g m Q QCD pair produc>on targeted thus far. [GeV] X- sec independent of EW charges / couplings [GeV] VLQs break GIM: tree- level FCNCs. Assume mixing is en>rely w/ 3 rd gen. Diverse final states (several t & b quarks, W, Z, H bosons) ) Comprehensive Search Programs LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 4

5 Vector- like Top: ATLAS searches in one- lepton final states [ A: ATLAS- CONF ] Search targets charged- current: pp! T T! W + bw b! (l )b(q q) b Require a high p T hadronically decaying W boson candidate. Events / 5 GeV Data ( s = 8 TeV) TT (6) Chiral TT (6) Singlet tt Non-tt Total BG uncert. tight ATLAS Preliminary - L dt = 4.3 fb Reconstruct WbWb system. Test m(wb) had. 2 5 Events / GeV All BR sets tested. Best exclusion: < 74 (77) GeV, obs. (exp.) [% WbWb] ATLAS Preliminary e+µ 6 jets, 4 b-tags - L dt = 4.3 fb Data ( s = 8 TeV) t t (6) t t+light jets 8 tt+hf jets tth ttv W+jets Z+jets 6 Diboson Single top Multijet Tot bkg unc. 4 [ A2: ATLAS- CONF ] Search targets neutral- current decay to a Higgs boson: Data / BG pp! T T! HtH t m reco [GeV] Data / MC H T [GeV] Emiss T H T = X p jets T + p lep T Events w/ high jet & b- tagged jet mul>plici>es selected. Test H T distribu>on. Best exclusion: < 85 (85) GeV, obs. (exp.) [% HtHt] LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 5

6 Vector- like Top: CMS searches in mul>- lepton final states [ C: CMS PLB 729 (24) 49 ] A search in the single lepton channel is complemented by searches in mul>- lepton final states; separated into 4 exclusive channels. Opposite- sign dileptons (off- Z) [2 ch.] Events must contain a b- tagged jet; require min(m lb ) > 7 GeV to suppress t t background. Two signal regions (SRs) targe>ng WbWb & HtHt/ZtZt modes, resp. Separate requirements on H T and S T for each SR. H T = X p jets T, S T = H T + X p leptons T + E miss T Same- sign dileptons and Trileptons Requirements on H T and S T as well. LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 6

7 Vector- like Top: ATLAS searches in mul>- lepton final states [ A3. ATLAS- CONF ] Search targets neutral- current decay to a Z boson: pp! T T! ZtZ t Require a Z boson candidate reconstructed from opposite- charge electrons or muons. Entries / 2 GeV ATLAS Preliminary Ldt = 4.3 fb s = 8 TeV ee + µµ - data 22 Z+light Z+bottom tt Other bkg BB (6 GeV) T T (6 GeV) Uncertainty 2 b-tags Events must also contain at least 2 b- tagged jets Entries / 5 GeV 2 ATLAS Preliminary 8 Ldt = 4.3 fb s = 8 TeV ee + µµ - data 22 Z+light Z+bottom tt Other bkg BB (6 GeV) T T (6 GeV) Uncertainty Data/bkg p (Z) [GeV] T b-tags Z candidate required to sa>sfy p T > 5 GeV, and events must contain significant jet ac>vity. 2 Final discriminant is m(zb) using highest p T b- jet. Data/bkg m(zb) [GeV] m(zb) [GeV] Analysis tests both TT and BB hypotheses under all BR assump>ons. Example T exclusion: < 75 (75) GeV, obs. (exp.) [% ZtZt] LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 7

8 .6 = 45 GeV Status: Lepton-Photon SU(2) (T,B) 23 doub. orbidden = 85 GeV SU(2) singlet - Vector- like Top Summary Same-Sign Ht+X [ATLAS-CONF-23-5] [ATLAS-CONF-23-8].6 = 35 GeV 7 GeV.6 m = 6 GeV Zb/t+X Same-Sign [ATLAS-CONF-23-56] [ATLAS-CONF-23-5] T = 65 GeV = 4 GeV Wb+X Zb/t+X [ATLAS-CONF-23-6] [ATLAS-CONF-23-56] 2.6 m = 8 A2 GeVWb+X [ATLAS-CONF-23-6].6 T.6 SU(2) (T,B) doub. SU(2) singlet m = 85 GeV.6 T SU(2) (T,B) doub. SU(2) singlet.6 = 6 GeV.6 = 65 GeV m T = 6 GeV m T = 65 GeV.6 A A3 orbidden BR(T Ht) s = 8 TeV, L dt = 4.3 fb m 95% CL exp. excl. 95% CL obs. excl. T ATLAS Ht+X [ATLAS-CONF-23-8] Preliminary Status: ATLAS Lepton-Photon Same-Sign.6 [ATLAS-CONF-23-5] Preliminary 23 Status: Lepton-Photon 23- s = Zb/t+X 8 TeV, [ATLAS-CONF-23-56] L dt = 4.3 fb - 95% scl = Wb+X exp. 8 TeV, excl. [ATLAS-CONF-23-6] L dt 95% = 4.3 CL obs. fb excl. = 6 GeV 95% CL SU(2) (T,B) Ht+X exp. excl. doub. [ATLAS-CONF-23-8] 95% SU(2) CL obs. singlet excl. = 65 GeV.6.6 = 8 GeV BR(T Wb) orbidden Vector- like top masses below 7 GeV are excluded for a large frac4on of possible BR sets. = 45 GeV = 45 GeV = 8 GeV = 8 GeV ATLAS Vector- like T BR Hypothesis Limit on [GeV]: obs (exp) ref Limit on [GeV]: obs (exp) ref ATLAS Preliminary BR(T Wb) = 85 GeV = 85 GeV % Wb (chiral, Y) 74 (77) A 7 (79) C % Zt 75 (75) A3 78 (8) C.6 BR(T Wb) BR(T Wb).6 % Ht 85 (85) A2 7 (77) C SU(2) singlet T 67 (68) A (comb w/ A2) 7 (77) C T in a (T,B) doublet 79 (75) A2 73 (78) C BR(T W LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 8.6 C = 45 GeV CMS ATLAS Prelim Status: Lepton-Photo s = 8 TeV, L dt = 95% CL exp. excl. 95 Ht+X [ATLAS- Same-Sign [ATLAS- Zb/t+X [ATLAS- Wb+X [ATLAS- SU(2) (T,B) doub.

9 Charge 5/3 Top Partner: CMS search w/ same- sign leptons [ C2: CMS arxiv: ] The X (T 5/3 ) quark can only decay via the charged current. pp! X X! tw + tw! W + W + W W b b Same charge W s mo>vates a search in the low BG same- sign dilepton final state. Employs W- jet & top- jet tagging. Counts cons>tuents (N c ): W- jets & top- jets count as 2 & 3, resp., leptons and normal jets as Signal region: N c 7 and H T (jets +leptons) > 9 GeV. Exclude: m X < 8 (83) GeV, obs. (exp.) [ A4: ATLAS- CONF ] Similar search interpreted in vector- like B context (see back- up) LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 9

10 Vector- like Bo[om: CMS searches in 3 and 4- lepton final states [ C3: CMS B2G- 3-3 ] A search for vector- like B pair produc>on in events with 3 or 4 leptons. Events categorized according to: o Number of OSSF pairs o If OSSF pair, whether on/off Z o Presence of hadronic tau candidates o Presence of b- tagged jets o Magnitude of S T All branching ra>os considered: example exclusions: < 79 (77) GeV, obs. (exp.) [% WtWt] < 52 (5) GeV, obs. (exp.) [% HbHb] Comparable sensi>vity to % ZbZb as a dedicated analysis [C4: CMS B2G- 2-2 ] < 7 (7) GeV, obs. (exp.) [% ZbZb] LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide

11 = 45 GeV.6 Status: Lepton-Photon 23 orbidden.6 A3 = 8 GeV.6 m = 5.6 GeV B = 85 GeV SU(2) (B,Y) doub. SU(2) singlet - Vector- like Bo[om Summary L dt = 4.3 fb Status: Same-Sign Lepton-Photon [ATLAS-CONF-23-5].6 Status: Lepton-Photon 23- s = 8 TeV, L dt = 4.3 fb Zb/t+X [ATLAS-CONF-23-56] - s = 8 TeV, L dt = 4.3 fb 95% CL exp. excl. 95% CL obs. excl. Ht+X [ATLAS-CONF-23-8] Same-Sign m [ATLAS-CONF-23-5] B = 6 GeV.6 m B = m35 B = GeV 65 7 GeV GeV.6 = 4 GeV Zb/t+X [ATLAS-CONF-23-56] Wb+X [ATLAS-CONF-23-6].6m = B 8 GeV.6m B = 85 GeV SU(2) (T,B) doub. SU(2) singlet.6.6 = 6GeV = 65 GeV A orbidden orbidden Vector- like borom masses below 7 GeV excluded for many BR sets; weakest in Hb corner. = 45 GeV BR(B Hb) = 8 GeV.6 ATLAS Vector- like B BR Hypothesis Limit on [GeV]: obs (exp) ref.6 Limit on [GeV]: obs (exp) ref % Wt (chiral, X) 72 (76) A4 (back- up) 79 (77) C3 ATLAS Preliminary 95% CL exp. excl. 95% CL obs. excl. = 85 GeV % Zb 77 (76) A3 7 (7) C4.6.6 % Hb 35 (35) A4 (back- up) 52 (5) C3 BR(T Wb) SU(2) singlet B 64 (64) A3 7 (7).6 = 6 GeV ATLAS Preliminary s = 8 TeV, = 7 GeV B in a (B,Y) doublet 73 (72) A3 65 (63) C3.6 95% CL exp. excl. 95% CL obs. excl. SU(2) (B,Y) doub..6 BR(B Wt) BR(B Wt) SU(2) singlet = 65 GeV = 55 GeV = 75 GeV.6.6 C3,5 (back- up) BR(B W LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide = 45 GeV C3 = 6 GeV CMS = 8 GeV ATLAS Prelimin Status: Lepton-Photon s = 8 TeV, L dt = 95% CL exp. excl. 95% SU(2) (B,Y) doub. Same-Sign [ATLAS-CO Zb/t+X [ATLAS-CO.6 = =

12 Search for resonances decaying to t t : Strategies [ A5. ATLAS- CONF ] Search performed in single lepton final state. Exclusive resolved and boosted categories. Resolved: WbWb system reconstructed w/ small radius jets and χ 2 minimiza>on. Boosted: WbWb system reconstructed using large radius jet to iden>fy hadronic top decay. Main backgrounds described by MC predic>ons. Events / TeV Data/Bkg 8 Data 5 Z (.5 TeV) ATLAS Preliminary - L dt = 4.2 fb - lepton: resolved+boosted t t 5 g (2. TeV) KK Multi-jets W+jets Other Backgrounds s = 8 TeV m [TeV] reco tt [ C6: CMS PRL (23) 284 ] Similar resolved & boosted strategies for single lepton events. Resolved (m [ < TeV) uses smooth BG fit. Boosted (m [ > TeV) complemented by all- hadronic channel: 25% improvement in x- sec 2 TeV. LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 2

13 Search for resonances decaying to t t : Results Ø Narrow top- color Z (.2% width) Same benchmark used by ATLAS & CMS. Mass limits: ATLAS.8 (.9) TeV, obs. (exp.) CMS 2. (2.) TeV, obs. (exp.) Ø Narrow scalar resonance Increased acceptance w.r.t. Z Resolved Boosted Narrow Resonances CMS resolved x- sec limits: 5 GeV.3 75 GeV Ø Wide heavy gluon, g KK (5-2% width) Slightly different benchmarks (~4% diff x- secs) Mass limits: tt) [pb] KK BR(g g KK 3 2 s = 8 TeV - L dt = 4.3 fb Resolved+ Boosted Obs. 95% CL upper limit Exp. 95% CL upper limit Exp. uncertainty Exp. 2 uncertainty Kaluza-Klein gluon (LO) ATLAS Preliminary Wide Resonances ATLAS 2. (2.) TeV, obs. (exp.) CMS 2.5 (2.4) TeV, obs. (exp.) g KK mass [TeV] LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 3

14 Search for resonances decaying to t b : Strategies [ C7. CMS arxiv:42.276] Search for a new charged vector boson with the decay mode: W! t b Selected events contain lepton, missing E T, 2 jets, b- tagged jet. Best top candidate formed first, paired with highest p T remaining jet to form m(tb). and 2 b- tagged jet categories. [ A6. ATLAS- CONF ] Similar selec>on and m(tb) calc. Requires 2 b- tagged jets. Boosted decision tree trained using m(tb), p T (t), and other kinema>c variables. LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 4

15 Search for resonances decaying to t b : Results W mass limit in the case of right- handed couplings to fermions and m( R ) >m(w R) (i.e. no decays to SM leptons) ATLAS.8 (.7) TeV, obs. (exp.) CMS 2. (2.) TeV, obs. (exp.) Mass limit in the case of right- handed couplings to fermions and m( R ) m(wr), or lea- handed w/ no interference: ATLAS.7 (.6) TeV, obs. (exp.) CMS 2. (2.) TeV, obs. (exp.) Lea- handed mass limits including interference: CMS.8 (.8) TeV, obs. (exp.) Both ATLAS and CMS also present limits on non- SM couplings. LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 5

16 Conclusions Strong mo>va>on to search for new phenomena at the LHC that couple preferen>ally to 3 rd genera>on quarks. This is the mandate & exper>se of the ATLAS Heavy Quarks and Top (HQT) and CMS Beyond 2 nd Genera>on (B2G) working groups. Complementary and comprehensive search programs using 8 TeV LHC data: Ø Exclude vector- like top and bo[om quarks with masses below 7 GeV for nearly all possible decay modes, as well as exo>c charge +5/3 and - 4/3 partners. Ø Exclude bosonic resonances decaying to t t or t b, for low masses ( > 5 GeV), high masses ( < 2.5 TeV ), and narrow or large widths. Ø Other results on exo>c top pheno: SS- and 4- tops, excited tops, BNV top decays (see back- up) Thank you! LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 6

17 References to Search Results ATLAS # Short Title Reference A Search for vector- like top pair produc>on (- lepton / charged- current) ATLAS- CONF A2 Search for vector- like top pair produc>on (- lepton / neutral- current) ATLAS- CONF A3 Search for vector- like top and bo[om pair produc>on (dilepton Z) ATLAS- CONF A4 Search vector- like quarks and 4- top produc>on (SS dileptons+b- jets) ATLAS- CONF A5 Search for resonances decaying to t t ATLAS- CONF A6 Search for resonances decaying to t b ATLAS- CONF h[ps://twiki.cern.ch/twiki/bin/view/atlaspublic/exo>cspublicresults CMS # Short Title Reference C Inclusive search for vector- like top pair produc>on arxiv: C2 Search for pair produc>on of a charge 5/3 top partner arxiv: C3 Search for vector- like bo[om pair produc>on (mul>ple leptons) PAS B2G- 3-3 C4 Search for vector- like bo[om pair produc>on (dilepton Z) PAS B2G- 2-2 C5 Search for vector- like bo[om pair produc>on (- lepton) PAS B2G- 2-9 C6 Search for resonances decaying to t t arxiv:39.23 C7 Search for resonances decaying to t b arxiv: h[ps://twiki.cern.ch/twiki/bin/view/cmspublic/physicsresultsb2g LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 7

18 CMS Beyond 2 nd Genera>on (B2G) Summary CMS Searches for New Physics Beyond Two Generations (B2G) 95% CL Exclusions (TeV) T'(5/3)(dilep,ss) g (combined) KK T' tz(combined) T' th(combined) Vector-like T' Z'(.2%)(combined) tt Resonances T' bw(combined) g (semilep) KK B' bz(multilep) B' bh(multilep) Z'(.2%)(semilep) B' tw(multilep) Vector-like B' g (all-had) KK B' bz(dilep) Z'(.2%)(all-had) B' bz(semilep) B' bh(semilep) W'(lep) tb Resonances B' tw(semilep) t+met,vectorial(had) t+met,scalar(had) Dark matter t*(dilep) t*(semilep) Excited tops LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 8

19 ATLAS vector- like top summary BR(T Ht).6 = 35 GeV.6.6 = 4 GeV.6.6 = 45 GeV.6 ATLAS Preliminary Status: Lepton-Photon 23 s = 8 TeV, - L dt = 4.3 fb 95% CL exp. excl. 95% CL obs. excl. Ht+X [ATLAS-CONF-23-8] Same-Sign [ATLAS-CONF-23-5] Zb/t+X [ATLAS-CONF-23-56] Wb+X [ATLAS-CONF-23-6] SU(2) (T,B) doub. SU(2) singlet = 5 GeV = 55 GeV = 6 GeV = 65 GeV = 7 GeV = 75 GeV = 8 GeV = 85 GeV BR(T Wb) LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 9

20 ATLAS vector- like top summary BR(B Hb).6 = 35 GeV.6 = 4 GeV.6 = 45 GeV ATLAS Preliminary Status: Lepton-Photon 23 s = 8 TeV, - L dt = 4.3 fb 95% CL exp. excl. 95% CL obs. excl. Same-Sign [ATLAS-CONF-23-5] Zb/t+X [ATLAS-CONF-23-56] SU(2) (B,Y) doub. SU(2) singlet = 5 GeV = 55 GeV = 6 GeV = 65 GeV = 7 GeV = 75 GeV = 8 GeV = 85 GeV BR(B Wt) LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 2

21 Vector- like Top: CMS search in one- lepton final states [ C: CMS PLB 729 (24) 49 ] Aims to cover all vector- like top decay modes contribu>ng to single lepton final states. Boosted decision trees (BDT) employed to separate signal and background events. Trained for signal w/ 2:: (W:Z:H) BRs, and separately for events w/ and w/o W had cand. Input variables include # of b- jets and top- jets, and p T of various reconstructed objects. LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 2

22 Vector- like Bo[om: ATLAS search w/ same- sign leptons [ A4: ATLAS- CONF ] A chiral- like B, or B in a (T,B) doublet, can only decay via the charged- current. pp! B B! tw tw +! W + W W W + b b Same charge W s mo>vates a search in the low BG same- sign dilepton final state. Events ATLAS Preliminary Ldt = 4.3 fb N b-jets - eµ channel, b ->Wt 8GeV b ->Wt 65GeV Data Q Mis-id Fakes tt+z/w(w) WZ/ZZ ± ± W W s=8 TeV Search requires a b- tagged jet. Signal regions contain 2 jets and H T > 65 GeV. Analysis tests both TT and BB hypotheses under arbitrary BR assump>ons. Example B exclusion: < 72 (76) GeV, obs. (exp.) [% WtWt] BR(b tw) [pb] Expected limit at 95% CL Observed limit Expected limit ± Expected limit ± 2 Theory approx. NNLO ATLAS Preliminary - Ldt = 4.3 fb, Observed Expected s = 8 TeV m b [TeV] LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 22

23 Vector- like Bo[om: CMS searches - lepton final states [ C5: CMS B2G- 2-9 ] A search for vector- like B pair produc>on in events with a single lepton. 4 small radius jets are required, with also b- tagged. Events categorized by the number of V- tags : large radius jets with sub- jets consistent w/ hadronic W, Z, or H decay. S T distribu>on tested for each category. Arbitrary BR assump>ons considered. Example B exclusions: < 7 (7) GeV, obs. (exp.) [singlet B] < 75 (8) GeV, obs. (exp.) [% WtWt] LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 23

24 Vector- like Top Projec>ons [ Snowmass Whitepaper arxiv:39.26 ] Sensi>vity study for pair produc>on of a singlet vector- like top at 4 and 33 TeV. Single and mul>ple lepton final states considered. Assessed the mass range for evidence, discovery, and 95% C.L. limits. 5σ Signal Significance LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 24

25 CMS Limits on W Couplings LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 25

26 Other Searches for New Phenomena w/ Top Quarks [ C8: CMS arxiv: ] Search for excited (spin 3/2) top quarks: pp! t t! tgtg Bump hunt for top- jet resonances in [+jets final states (l+jets). [ C9: CMS arxiv:3.68 ] Search for baryon number viola>ng top decays: t! b cµ + t! būe + [- like final states, but low ME T BR <.6 BR <.7 [ A4: ATLAS- CONF ] VLQ search in SS dilepton+b- jets also interpreted in context of SS and 4- top produc>on. Cross- sec>on limits on contact interac>ons and also specific BSM models. LHC Searches for New Phenomena w/ Heavy Quarks : M. Cooke (LBNL) Slide 26