11. The Top Quark and the Higgs Mechanism

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1 11. The Top Quark and he Higgs Mechanism Paricle and Nuclear Physics Dr. Tina Poer Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 1

2 In his secion... Focus on he mos recen discoveries of fundamenal paricles The op quark predicion & discovery The Higgs mechanism The Higgs discovery Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 2

3 Third Generaion Quark Weak CC Decays 173 GeV log(mass) 1.3 GeV 2.3 MeV c u Cabibbo Allowed V b ~1, V cs ~ V ud ~0.975 Cabibbo Suppressed V cd ~ V us ~0.22 V cb ~ V s ~0.05 b s d 4.8 GeV 95 MeV 4.8 MeV Top quarks are special. m() m(b) (> m(w )) τ s decays before hadonisaion V b 1 BR( W + b)=100% Boom quarks are also special. b quarks can only decay via he Cabbibo suppressed Wcb verex. V cb is very small weak coupling! τ(b) τ(u, c, d, s) Je iniiaed by b quarks look differen o oher jes. b quarks ravel furher from ineracion poin before decaying. b-je races back o a secondary verex b-agging. Ineracion poin Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 3 b b-je

4 The Top Quark The Sandard Model prediced he exisence of he op quark ( )( )( ) e u c 1 d s b 3 e which is required o explain a number of observaions. Example: Non-observaion of he decay K 0 µ + µ B(K 0 µ + µ ) < 10 9 The op quark cancels he conribuions from he u and c quarks. Example: Elecromagneic anomalies This diagram leads o infiniies in he heory unless Qf = 0 where he sum is over all fermions (and colours) Q f = [3 ( 1)] + f [ ] + [3 3 ( 13 ) ] d = 0 s γ u/c/ W W + Requires quark Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 4 f f ν µ f µ µ + γ γ

5 The Top Quark The op quark is oo heavy for Z or W + b so no direcly produced a LEP. However, precise measuremens of m Z, m W, Γ Z and Γ W are sensiive o he exisence of virual op quarks: b W + W + Z Z Z W b b Example Sandard Model predicion Also depends on he Higgs mass Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 5

6 The Top Quark The op quark was discovered in 1994 by he CDF experimen a he worlds (hen) highes energy pp collider ( s = 1.8 TeV), he Tevaron a Fermilab, US. q q g b W + W b Final sae W + W b b Mass reconsruced in a similar manner o m W a LEP, i.e. measure je/lepon energies/momena. V b 1, so decay of op quark is 100% bw + m m W, so he W + is real. The weak decay is jus as fas as a srong decay ( s), so he quark has no ime o hadronise here are no -hadrons Possible op quark decays are bq q or blν l In hadron collisions, mulije final saes are he norm for rare processes i s much easier o look for leponic decays, accompanied by b-quark jes. Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 6

7 Firs observaion of op (1995) CDF collaboraion published firs PRL (1995) b W + b W Final sae l ν + qq + bb Curren saus Resuls from LHC as well as Tevaron. All consisen, and in agreemen wih indirec expecaion from LEP daa. daa signal background Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 7

8 Higgs mechanism and he Higgs Boson Recall he Klein-Gordon equaion for massive bosons is: 2 ψ 2 = ( 2 m 2) ψ However, he erm m 2 ψ (or 1 2 m2 ψ 2 in he Lagrangian formulaion), is no gauge invarian. So in gauge field heories, he gauge bosons should be massless. OK for QED and QCD, bu plainly no for W ± and Z. The Higgs mechanism ries o fix his. Imagine inroducing a scalar Higgs field φ, which has ineracions wih he W ± and Z fields, wih coupling srengh y, giving a erm in Lagrangian yφψψ. Looks like a mass erm ( ψ 2 ). Mass of he bosons becomes effecively relaed o heir coupling o he Higgs field. Requires he vacuum (lowes energy sae of space) o have a non-zero expecaion value for he Higgs field. How can his come abou? Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 8

9 Higgs poenial Suppose he Higgs field φ (acually a complex double) has self ineracions yielding V (φ) = aφ 4 bφ 2 The equilibrium poin, φ = 0, respecs he symmery, bu is unsable. The sable equilibrium poin is a φ 2 GS = b/2a. The symmery is sponaneously broken. A weak boson propagaing in he Higgs field will appear o have a mass yφ GS. Expanding abou he ground sae V (φ GS + x) = V min + 2bx 2 So can ge exciaions of he Higgs field abou he minimum. These form he physical Higgs scalar boson, H he observable physical manifesaion of he operaion of he Higgs mechanism. Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 9

10 Classical analogue of he Higgs mechanism Maxwell s equaions lead o waves ravelling a velociy c, hence o massless phoons. Consider waves propagaing in a charged plasma, wih elecron densiy n per uni volume. Plasma: J = ne v; me v Maxwell: E = 2 E = = µ 0ne 2 E m e ( B = e E ) J = ne2 E m e = B = 1 c 2 2 E 2 2 E 1 c 2 2 E 2 = µ 0ne 2 E m e ( µ 0J 1 E + c 2 Compare wih Klein-Gordon. Phoon propagaes wih effecive mass meff 2 = µ 0ne 2 m e c 2 Noe m eff e, he coupling. Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 10 )

11 Higgs heory summary Gauge bosons (and also fermions) are inrinsically massless, and need o be so o saisfy Gauge Invariance. Neverheless, ineracions wih he Higgs field make paricles look like hey have mass. Apparen masses are conrolled by free parameers called Yukawa Couplings (he srengh of he coupling o he Higgs field) A Higgs Boson arises as an exciaion of he Higgs field. I mus be a scalar paricle o make everyhing work. The Higgs Boson has a mass, bu he mass is no prediced by he heory we have o find i experimenally. The Higgs Boson has couplings o all he paricles o which i gives mass (and so has many ways i could decay), all fully calculable and deermined by he heory as a funcion of is (a priori unknown) mass. Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 11

12 Higgs boson decays Higgs Boson ineracs via couplings which are proporional o masses. Higgs boson herefore decays preferenially o he heavies paricles ha are kinemaically accessible, depending on is mass. Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 12

13 Higgs decay mechanisms Direcly o wo fundamenal fermions or bosons, coupling o mass, e.g. b Z H H b Z Indirecly o massless paricles (phoons or gluons) via massive loops γ γ g W H γ H W W γ H g Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 13

14 Higgs a LEP Higgs Producion a LEP (Large Elecron Posiron Colider 1990s): If m H < s m Z e + Z Z Higgssrahlung mechanism e In 2000, LEP operaed wih s 207 GeV, herefore had he poenial o discover Higgs boson if m H < 116 GeV. Searches were conduced in many possible final saes (differen decays for Z and H). All negaive. Ulimaely, LEP excluded a Higgs Boson wih a mass below 114 GeV. H Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 14

15 Higgs a Large Hadron Collider Higgs Producion a he LHC The dominan Higgs producion mechanism a he LHC is g Higgs Decay a he LHC H g γ γ H H b b Gluon fusion Low mass Medium mass High mass One Z may be virual Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 15 H Z Z

16 The Large Hadron Collider The LHC is a new proon-proon collider now running in he old LEP unnel a CERN. In TeV; in TeV; ulimaely TeV Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 16

17 ATLAS a general purpose LHC deecor Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 17

18 Higgs Observaions (Augus 2014) Indirec indicaions from LEP ha Higgs mass should be no far above 115 GeV. Dominan decay modes are all difficul: b b, c c (swamped by QCD jes) W + W, τ + τ (missing neurinos) Bes opions are he rare decays: ZZ l + l l + l γγ Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 18

19 H ZZ 4l Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 19

20 Higgs Boson Discovery Convincing signal consisen wih m(h) = 126 GeV seen in muliple decay modes & in wo experimens. Is i he Higgs boson of he SM? Need o check is quanum numbers (should be J P = 0 + ). Check branching raios and couplings. Look ok so far... Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 20

21 Higgs spin+pariy? Sudied using angular disribuions of decay producs Boh 2 + and 0 seem disfavoured, bu currenly he amoun of daa is meagre. So far i looks like he SM Higgs. Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 21

22 Summary Top quark observed, and compaible wih oher precise elecroweak measuremens. Elecroweak heory depends on he Higgs mechanism o endow paricles wih mass. This is a non-sandard feaure, which needs experimenal verificaion. Higgs boson deeced in 2012 a 126 GeV. Work coninues o deermine he properies of he boson and check wheher i is he Higgs boson of he Elecroweak Sandard Model. Up nex... Secion 12: Beyond he Sandard Model Dr. Tina Poer 11. The Top Quark and he Higgs Mechanism 22