Particle Physics

Lecture 9: Electroweak Model and the Higgs boson March 10th 2008

Electroweak Theory Interactions of W and Z bosons at high energy The Higgs boson

Electroweak Theory

We’ve seen already that wherever a ! boson can be exchanged a Z can also be exchanged: • The weak and electromagnetic force are linked. • At short distances (or high energies) the strength of the electromagnetic force and the weak force are comparable. Can be related by a parameter, sin !W e = gW sin θW The weak and electromagnetic interactions are manifestations of a underlying force: the electroweak force. • Couplings of the !, W (and Z) bosons are related: e = gW sin θW 2 2 2 • Mass of the W and Z bosons are related: mZ = mW /cos θW

Just three fundamental parameters required to describe: • couplings of W, Z and ! to quarks and leptons • masses of the W, Z, ! bosons • interactions of the W, Z, ! bosons with each other

Normally use three most accurately measured parameters e.g. e, GF, mZ

2 6 Summary of Standard Model Vertices

! At this point have discussed all fundamental fermions and their interactions with the force carrying bosons. ! Interactions characterized by SM vertices ELECTROMAGNETIC (QED) - e q Couples to CHARGE - e Q e e q Does NOT change FLAVOUR e2 α = γ γ 4π STRONG (QCD) q Couples to COLOUR g q s Does NOT change FLAVOUR g 2 16 α = s g 2004 s 4π

W and Z boson at WEAKlowChar energiesged Current Lent : ,

νe q $d’µ Changesq FLAVOUR comes the √α At low energies, q m , m : ). Z W " g V q - g w ckm the w µ a ! gW is Q Q The W and Z bosons are virtual: e u For$ e! QUARKS: coupling r • o - BETWEEN generations f or 2 g 2 + state 2 2 2 colour W w W 1 q q = EZ p!Z p!Z = mZ αw = γ W Z 4π

gW ( − · " of 2 2 2 erence q = E p!W p!W = m " ﬁnal , W W WEAK Neutral Current e represent section les W − · " - " " diff µ # e $e! $µ e , ν √α Y

q the

e tic where ( - + -

Annihilation 2 2 .g. e e oss e , ν .

virtual W-bosons are responsible for the decayse of q = m of • W e xistence W Does NOT change par cr " " q ! ONL e the leptons, and the lightest hadrons e.g. µ # e $e! $µ + - FLAVOUR case 0 µ µ

0 pair

Z the the the Z state the r √α masses r o " " Z Drf M.A. Thomson Lent 2004 • interactions of virtual Experimental-boson are not clearly Tests - Le EP e o f then 2 ﬁnal ) the II

evident, as they look similar to ! interactions 1 q ermion γ

Annihilation. Z

(! interactions are much strongerin than Z-boson the

!"#$% !&'()$"*)+,'-.$#-/0,'-.$1-**02)' uons calculating

interactions) vidence of y

!"#$%&'(%)%* +,e --./%#0( 12(34 +.#+546%#%7+% or e

√α $e b

$e neglect + - g t 8%7'#%*,6*4"&&(%7%#$9((!&(:(;<(= 1<<(>%? " " Handout e e Compare $e e # $e e ocess we QCD Thomson om At low energies we see the effect@A%#" 'of.,7 Z"- (boson6#,4(B;C; mainly',(1

D,5#(%EA% #".4%7' &F"(G-%AH0(I%-AH.0(!J0(@KG! q = mZ M.A. scattering involving neutrinos e.g. $e e # $e e Z ! Dr e #e " ! Z 0 ! qq ! hadrons • as ! cannot couple to the neutral neutrinos

34$$5-/-./$&,$!"#

L%&,7"7+%(A#,/5+'.,7("'(!&(:(MN W and Z bosonOP(4.--., 7(at%)(%* " highN< %Q%7'& R%energiesEA' 6! 5-/-./$&,$!"#

Using a collider, we can create K#,/5+'.,7high enough"'( !&(# 1(MS( energies to make real W and Z OC<<<(bosons:%)(%* " S)(S* %Q%7'&R%EA' !"#$7)&/8')9).,/ q mZ , mW ∼ M"&& "7/(T./'H ,6(N< "7/(S! U,&,7& ! • e.g. LEP collider e+e"#Z, eN+e<""#7/W(S+W"U,&,7(+,5A-.7$& ',(V5"#3&("7/(-%A',7&(( S%"3(/%+"9& ,6(H%"Q9(4%&,7& • e.g. Tevatron and SppS! collider pp#! Z+X, pp#! W+X W8I(4%"&5#%4quarks%7'& within p K#%+.&.,7 '%&'&(,6(:,&.2&'2$7-2)*$-;$#&',0+*)$#<=/0+/ Can study the properties of the W and Z and p ! interact to W Z bosons in detail. Nuclear and Particle Physicsmake (orFran z M)uheim 5 • masses, lifetime/width, coupling strengths, decay modes, spin...

Discovery of W and Z bosons at CERN in 1983 at the SppS! collider.

Ep = Ep ! = 270 GeV

# # pp"! W "e $e! event at UA1 experiment

4 Experimental Tests - LEP W and Z boson tests at LEP !"#$Experimental% !&'()$"*)+,'-.$ #-/0,'-.$Tests1-**02)' - LEP !"#$%&'(%)%* +,--./%#0( 12(34 LEP+.#+546%#%7+% - the Large Electron Positron Collider at CERN ! !"#$%8!%7&'()$'#%*,"*)+,'-.$6*4"&&(%7%#-/0,'-.$#$9(( &(:(The;1-**02)'<(= 1world’<<(>%s? highest energy e+e" collider: @A%#"'.,7"-(6#,4(B;C; ',(1<<< !"#$%&'(%)%* +,--./%#0( 12(34 +.#+546%#%7+% D,5#(%EA%#.4%7'&F(G-%AH0(I%-AH.•0(!27J0( @kmKG !circumference, ran from 1989 to 2000 8%7'#%*,6*4"&&(%7%#$9((!&(:(;<(= 1<<(>%? e #e " !EnergyZ 0 ! q qof! masshadrons energy, @A%#"'.,7"-(6#,4(B;C; ',(1<<< • √s = 89 to 206 GeV D,5#(%EA%#.4%7'&F(G-%AH0(I%-AH.0(!•J0Four(@KG !experiments: Aleph, Delphi, L3, Opal e #e " •! MeasurementsZ 0 ! qq ! hadrons corroborates unification of EM & Weak forces Z-boson production • ~4 million e+e"#Z0 events 34$$5-/-./$&,$!"# &s ~ 91 GeV L%&,7"7+%(A#,/5+'.,7("'(!&(:(M N s = (q + q )2 = q2 = m2 )( * < + Z 34$$5-/-./$OP(4.--&,$.,7(!"#% % " N %Q%7'&R%EA' e e− Z ! 6 L5-/-./$%&,7"7+%&,$(A#,!"#/5+'.,7("'(!&(:(MN OPK#,/5+'.,7(4.--.,7(%)(%"*'"(!&N(#< 1%(QM%S7'(&R%EWA'-boson production ! )( * " )( * + " + " 6 5-/-./$OC<<<(&,$% %!"# S S %Q%7'&R%EA•' ~8000 e e #Z#W W events &s ~ 160 GeV !"#$7)&/8')K#,/5+'.,7 "9'().,/!&(# 1(MS( M"&& ")(7/*(T./'H" )(,6(*N< "7/(S! U,&,7& OC<<<(% % S S %Q%7'&R%EA' 2 2 < ! s = (q + q ) = (2m ) N "7/(S U,&,7(+,5A-.7$& ',(V5"#3&("7/(-%A',7&(( + W !"#$7)&/8')9).,/ e e− MS%"3"&& "7(//%+"9&(T./'H,,66((H%"Q9(N< "7/4%&,(S! 7&U,&,7& N

NucleWar8 aInd(4 Pa%r"tic&le5 #Ph%y4sic%s7'& Franz Muheim 5 K#%+.&.,7 '%&'&(,6(:,&.2&'2$7-2)*$-;$#&',0+*)$#<=/0+/

Nuclear and Particle Physics Franz Muheim 5

0 6 40. Plots of cross sections and related quantities 6 40. Plots of cross sections and related quantities + Z resonance σ and R in e e− Collisions

-2 + - 10 ω φ J/ψ e e Annihilation in Feynman+ Diagrams" -3 • Both ! and Z contribute to e e "hadrons 10 ψ(2S) ρ Υ -4 ρ ! + 10 Z In general e+e- annihilation σ and R in e e− Collisions ] -5 b 10 m

involves both photon and [

σ -6 Z exchange : + interference 10

-2 -7 10

10 -8 10 2 • At low ECM mainlyω ! interactionsφ 1 10 10 -3 J/ψ 3 Υ • At $s~mZ, mainly Z-boson: 10 J/ψ ψ(2S) 10 ψ(2S) Z " 1 2 0 + 10 ! nb Υ φ ! ons) σ(e e− Z ff) ρ ω -4 → →ρ ∝ (q2 ! m2 )2 40 R Z had ALEPH 10 ρ! 10 − ! DELPHI Z L3 hadr 30 OPAL 1

ρ " " ] -5 At Z resonance: Z -1 Well below Z: photon 10 e

b 2 exchange dominant " 1 + 10 10 exchange dominant 10For a massive boson, also depends Z e

M (

m • 20 √s [GeV] [ & + + + + Figure 40.6: World data on the total cross section of e e− hadrons and the ratio R(s) = σ(e e− hadrons, s)/σ(e e− µ µ−, s). on total width %Z = /'Z. Shape of %: + → → → + σ(e e− hadrons, s) is the experimental cross section corrected for initial state radiation and electron-positron vertex loops, σ(e e− ℏ measurements (error+ bars→ 2 → µ µ−, s) = 4πα (s)/3s. Data errors are total below 2 GeV and statistical above 2 GeV. The curves are an educative guide: the broken one

σ increased by factor 10) -6 (green) is a naive quark-parton model prediction, and the solid one (red) is 3-loop pQCD prediction (see “Quantum Chromodynamics” section of this Review, Eq. (9.12) or, for more details, K. G. Chetyrkin et al., Nucl. Phys. B586, 56 (2000) (Erratum ibid. B634, 413 (2002)). 2 10 ! from fit Breit-Wigner parameterizations of J/ψ, ψ(2S), and Υ (nS), n = 1, 2, 3, 4 are also shown. The full list of references to the original data and the 10 Γ /4 QED correcteddetails of the R ratio extraction from them can be found in [arXiv:hep-ph/0312114]. Corresponding computer-readable data ﬁles are available 2 Z at http://pdg.lbl.gov/current/xsect/. (Courtesy of the COMPAS (Protvino) and HEPDATA (Durham) Groups, August 2007. Corrections σ(E = q ) = σmax 2 2 2 by P. Janot (CERN) and M. Schmitt (Northwestern U.)) See full-color version on color pages at end of book. (q m ) + Γ /4 M Z Z Z -7 − 86 88 90 92 94 10 E !GeV" • Measurements at LEP: cm 2 -8 • mZ = 91.188 ± 0.002 GeV/c Dr M.A. Thomson 10 Michaelmas 2006 500 • %Z = 2.4953 ± 0.002 GeV 1002 1 10 10ECM (GeV) 6

3 Υ 10 J/ψ ψ(2S) Cross Section Measurements Z At Z resonance mainly observe four types of event: 10 2 φ ω Each has a distinct topologyR in the detectors, e.g. 10 ρ!

1 ρ -1 10 2 1 10 10 √s [GeV]

+ + + + Figure 40.6: World data on the total cross section of e e− hadrons and the ratio R(s) = σ(e e− hadrons, s)/σ(e e− µ µ−, s). To work out cross sections+ first count events of each type → → → + σ(e e− hadrons, s) is the experimental cross section corrected for initial state radiation and electron-positron vertex loops, σ(e e− + → 2 → Then need to knowµ µ“−integrated, s) = 4πα (s)/ luminosity3s. Data errors”aofre t ocollidingtal below 2 GbeamseV and statistical above 2 GeV. The curves are an educative guide: the broken one (green) is a naive quark-parton model prediction, and the solid one (red) is 3-loop pQCD prediction (see “Quantum Chromodynamics” section of this Review, Eq. (9.12) or, for more details, K. G. Chetyrkin et al., Nucl. Phys. B586, 56 (2000) (Erratum ibid. B634, 413 (2002)). Breit-Wigner parameterizations of J/ψ, ψ(2S), and Υ (nS), n = 1, 2, 3, 4 are also shown. The full list of references to the original data and the Dr M.A. Thomson details of the R ratMioicehxatrealmctaiosn 2f0r0o6m them can be found in [arXiv:hep-ph/03121145]0.1Corresponding computer-readable data ﬁles are available at http://pdg.lbl.gov/current/xsect/. (Courtesy of the COMPAS (Protvino) and HEPDATA (Durham) Groups, August 2007. Corrections by P. Janot (CERN) and M. Schmitt (Northwestern U.)) See full-color version on color pages at end of book. 6 Summary of Standard Model Vertices

νe d’ Changes FLAVOUR g V - gw w ckm e u For QUARKS: coupling - BETWEEN generations g 2 W + α = w W

w 4π

Decays of the Z boson M.A. WEAK Neutral Current Thomson - e , ν qf - e Z boson interacts with all quarks and leptons. e , νe

q Does NOT change f e

No change of quark or lepton flavour at Z boson vertex. , WEAK

FLAVOUR-

0 ν

0 e

Z ZZ

Neutral

Dr M.A. Thomson w Lent 2004

Z boson can decay into any pair: fermion!anti-fermion, f f .! =

• 4

, , π

ν e g e

f w

except top quark pair, as mZ < 2mt -

• 2

Current

WEAK

0 g w

Z W 0

+ " s

Char

Z#e e Z#$e$e! Z#uu! Z#dd! = FLAVOUR 4

f ! ν Does NOT change NOT Does g u e s

+ " 2

w Z#µ µ Z#$µ$µ! Z#cc! Z#ss! STR

W Current ckm

ONG

+ " Lent

Z#' ' Z#$'$'! Z#bb! +

2004

BETWEEN generation BETWEEN 4

(QCD)

For QUARKS: couplin QUARKS: For

e e

d’ q

2 -

Lepton Universality ELECTR

⇒ FLAVOUR Changes

g e

s γ g

for e, µ, ' decay is same %(Z#e+e") ( %(Z#µ+µ") ( %(Z#'+'") !

M ⇒ Interactions • and

! OMA

FLAVOUR

their s e

M for $e, $µ, $' decay is same %(Z#$e$e! ) = %(Z#$µ$µ! ) = %(Z#$'$'! ) At

• ⇒ change NOT Does -

GNETIC this

Summar interactions

Q e Q Couples to COLOUR to Couples

7 γ point haracteriz

ha (QED)

FLAVOUR

with

Does NOT change NOT Does ed

discussed o

f b

the

Couples to CHARGE to Couples y Standar

ver

all

tices carr

fundamental

ying

Number of Neutrinos Model

bosons.

Total width of the Z-boson (%Z) is sum of all partial widths: ermions

+ + + r Γ = Γ(Z qq¯) + Γ(Z e e−) + Γ(Z µ µ−) + Γ(Z τ τ −) + N Γ(Z νν¯) tices Z → → → → ν → LEP directly measured: • partial widths: %(Z#e+e"), %(Z#µ+µ") , %(Z#'+'"), %(Z#hadrons) = %(Z#qq)!

total width, %Z • 6 " 2" Cannot measure %(Z #$ $)! directly as nb !

3" neutrinos leave no signal in the detector. had 30 ALEPH ! DELPHI • Can predict %(Z #$ $)! using M(Z #$ $)! L3 4" • Use prediction & measurements to find OPAL 20 number of neutrinos contributing to %Z average measurements, N)=2.999±0.011 error bars increased • by factor 10 10 Consistent with exactly three neutrinos! ⇒ Gives us confidence that there are exactly 0 three generations of quarks and leptons 86 88 90 92 94 Ecm !GeV" 8 24 at LEP

! collisions Ws produced in pairs. ! In StandarWd Model-boson3 possible diagrams for + + e+ W+ e W+ e W+ Colliders may also produce W-bosons γ measure Zproperties0 of the W-boson ν ⇒ + - e W W Pair Produc- tion - - At LEP: W-bosons producede in pairsW- e 24W- e At TWevatron:- W-bosons

] 25 at LEP 0 produced from quarks

!"#$%#&%'()%*+',-#.&#/0Three possible modes:pb σ(Without Z ) [ 20 0 Cross section sensitive to pres- σ(With Z ) and anti-quarks in pp! ! collisions Ws prWW oduced in pairs. ence of the Triple Gauge Boson σ 15 ! In Standard Model 3 possible diagrams for vertex 10 1996-2000, LEP operated above + + + + + + the threshold for pro- e W e 5 W e W Z0 duction ! +γ "0 1 2)0)$ν'e,*3#40 %(e e #150 WW)160 170 measured180 190 200 210 at LEP Predicted!- !"#$%&'()*+-%,- .- /("+'0,/$) *-$1"-2'*3"0 √+45*(3"s [GeV#]'%6'$),*,')- e W e confirmsW eZ -W -W W vertex %(e+e"#WW) + + + + LEP + + 1 Preliminary! ] 25 "#W ( e % e $) "#W ( ' % ' $ ) "#W ( & % & $) 3 "#W ( l% $ 0 20

pb σ(Without+ Z ) + RacoonWW+ / YFSWW+ 1.14 + [ 20 "#W ( d'u$) "#W0 ( s'cCr$ )ossN c"section#W (sensitivee % e $ to"#Wpres-( b't $) 0 σ(With Z ) ]

WW ! !

* " W ( q'pb qence) 2"ofWthe(Triplel% Gauge Boson σ # [ 15$ # $ From measurements at LEP & 15 Cross section agrees WW vertex σ Tevatron: *5'10*6 ! 15'16 #"'789 1996-2000, LEP operated above with Standard Model 10 2 78*90+'":;"'80'(%9'$,the threshold for pro- prediction.mWConﬁrma- = 80.413 ± 0.048 GeV/c 5 18 • , + , + + YFSWW 1.14 tion of the existence of duction RacoonWW e e ( W W ( qqe % e 0 17 %W = 2.141 ± 0.041 GeV 150 160 170 180 190 200 210 5 the • vertex ), s GeV <-"G √ [ ] 16 H = 0*%( 0 LEP Preliminary 160 170 180 190 200 210

20 Ecm [GeV] MeasurementsRacoonWW / YFSWW 1.14 of W and Z bosons validate the Electroweak Model beautifully

] Dr M.A. Thomson Lent 2004 pb

[ 15 9 Cross section agrees WW σ with Standard Model 10 <&)00'prediction.!*3"-)$'Conﬁrma-=0 8$*&.4 18 YFSWW 1.14 tion of the existence of RacoonWW I6('') 2%,J"K<"0('1%L,%/$ 17 the vertex 5 !/$.%(9)"'8%),'$L'"/. (#00'#$%'1-%":');'1! 2)0)$ 16 =M"=N O@ &'(,'8 < >"?@ABCD"! @A@;?""E'F 0 = Nuclear" a160nd> P"Ca170rAtGicCleB P180"h!yAs@icAs@190BG"E200'F 210 Franz Muheim 9 =" Ecm [GeV] Dr M.A. Thomson The HiggsHiggsLent 2004 Boson and Unification The Higgs boson: missing piece of the Standard Model. • The theoretical framework for !the"# $Standard%&'(#) Model*+,- #'&. only works for massless bosons and massless fermions. 6 !"#$%&# '#(&)"#'#*+&,-. !#'/,01/,23/,24 (*5,&%* "4 • Introducing Higgs field give masses to the W and Z bosons Higgs7*89, in :th,%*5e#; #L*a5#g*r+a;("('#+#"&ngian !-<#".)8,$-*&+"(%*+&/,$-""#$+%-*&=,>%?>#",-"5#",5%(?"('& • Two key consequences: • The fermions also get a mass!/0112+3#$-)4025 Peter Higgs The existence of an additional massive,7*89,' neutral%&&%*?, ;("+%$8#,%*,@+(*5("5,2-5#8 • Higgsemeritus couples professor to WW and in ZZ boson: the Higgs boson @$(8("/,%A# @;%*,B/, the School of Physics • Mass of the Higgs is not predicted in CStandard-*DE#" -Model,,F($)) ',DG,H88,;("+%$8#&, we have to search for it. ($I)%"#,'(&& J9,K%??&,%*+#"($+%-*

2gmW Higgs interacts with W and Z bosons, and all massive fermions. !#+#",K%??& gm K%??m&,f$-);8%*? # '(&& ?K.. L,!M!6,01,N,Z'. !"-.,#'#"%+)&, Interaction strength between Higgs∝ O%"#$+,2(&&,P%'%+,2 G,QQR,0#S V*%F -.,[5%*J)"?> and fermions ∝ fermion mass, mf K T#I)%"# 6)&1#+/)7&'4 8'""07#&++96/8:+&+("+&,%*,6BBU 10 ;<=#&2.55#%&. > ;?;@ 4 @V@W,!("+*#"&=,1#"'%-* $ X-&-* .#"'%-* $ &.#"'%-* V*%.%$(+%-* -.,#8#$+"-<#(Y J-&-*,,,,$ Z%*- (*5,&+"-*? %*+#"($+%-*

!Q,L,!#'

!6,L,!4

!:,L,!@,

Nuclear and Particle Physics Franz Muheim 10 The Higgs Mechanism

1. Physicists at a conference reception; all free to move around the room.

2. In comes a noble prize winner; everyone wants to speak to him. The physicists crowd around him. The noble laureate is not free to move around; he has gained inertia by interacting with the crowd.

This is analogous to how the particles acquire mass: by interacting with the Higgs field. Laureates of different popularity gain different masses.

The Higgs Boson

3. The next evening; physicists enjoying another drink. A rumour enters the room: the keynote speaker tomorrow will announce the discovery of a new particle!

4. The physicists gather together to spread the rumour. The group of physicist acquire inertia.

The clustering of the field of physicists is as if a new massive particle has formed. This is the Higgs boson. 12 5

900 DØ Run II Preliminary 800 (0.93 fb-1) 700

600 Events/ 0.01 Simulated Physics Bkg 500 Signal Region Data

400 Instrumental Background

300

200 100 6

-0.30 -0.2 -0.1 0 0.1 0.2 0.3 A(ET,HT) Sample No b-tag Double b-tag ZH(mH = 115 GeV ) 2.46 0.88 ± 0.12 FIG. 2: A(/ET ,/HT ) for data, physics background and instrumental background in the signal region before b-tagging. The selection cut is −0.1 < A(E/T , H/T ) < 0.2, the line at -0.1 indicates the lower edge of this range. W H(mH = 115 GeV ) 1.75 0.61 ± 0.08 W + jets 6750 52.3 → W bb 397 35.4 6 1200 2500 → W cc 1170 9.33 DØ Run II Preliminary DØ Run II PreliminaryZjj Sample No b-tag Double b-tag 1000 (0.93 fb-1) (0.93 fb-1) ZH(mH = 115 GeV ) 2.46 0.88 ± 0.12 2000 Z → τWτH(mH = 115 GeV ) 1.715070.61 ± 0.08 0.25 DATA DATA W + jets 6750 52.3 800 H MC H MCZ → νν → W bb 3297130 35.4 0.63 TOP MC TOP MC 1500 → W cc 1170 9.33 Events/ 5 GeV Z+jets MC Events/ 5 GeV Z+jets MC Zjj W+jets MC W+jets MCZbb 600 Z → ττ 107 0.25 MISC MC MISC MC → Z → νν 2130 0.63 QCD QCDZ ττ 6.39 0.63 1000 Zbb 400 Z → νν Z → ττ 6.32929 0.63 24.9 Z → νν 229 24.9 500 Zcc Zcc 200 Z → ττ 12.8 0.18 Z → ττ Z → νν 41672.8 4.93 0.18 tt 172 29.1 The Search for the 0 Higgs 0 Z → νν 467 4.93 0 20Z40H60 !80 100 120"140"160b180b200# 0 20 40 60 80 100 120 140 Di − boson 228 3.84 PT (GeV) PT (GeV) Total Physics Background 10100 117 ± 17 miss ttInstrumental Background 251607217.2 ± 3.4 29.1 ET + 2 jets Di − bosTootnal Background 12720028 134 ± 18 3.84 Higgs boson has been searched for at LEP, is being searched for at the Tevatron, will be Observed Events 12500 140 1200 Total Physics Background 10100 117 ± 17 2000 TABLE I: Number of events after ﬁnal selection. searched for at LHC. DØ Run II Preliminary DØ Run II Preliminary 1800 Instrumental Background 2560 17.2 ± 3.4 (0.93 fb-1) 1000 (0.93 fb-1) 1600 The Higgs generally likes to decay to the heaviest thing it can. DATA ToDATAtal Background 12700 134 ± 18 • 22 1400 H MC 800 H MC TOP MC 30 DØ Run II PreliminaryTOP MC40 DØ Run II Preliminary 20 DØ Run II Preliminary 50 DØ Run II Preliminary -1 Observed Eve-1nts 12500 -1 140 -1

Events/ 6 GeV 1200 Z+jets MC (0.93 fb ) Z+jets MC (0.93 fb ) 18 (0.93 fb ) (0.93 fb ) 35 mH < ~135 GeV search for H"bb ! decay. Events/ 10 GeV • W+jets MC 600 25 W+jets MC 16 40 1000 MISC MC MISC MC30 14 QCD Events /8 GeV 20 TABLEQCDI: Events /8 GeV Number of eventEvents /8 GeV s after ﬁnal selecEvents /8 GeV tion. + " 0 0 800 25 30 mH > ~135 GeV search for H#W W , H#Z Z decays. (1 boson may be virtual) 400 12 • 15 20 10 600 1 tight b-tag + 15 8 20 400 10 200 6 Lots of other processes look like Higgs events, challenge is to separate background events 10 Ben Kilminster, OSU 10 200 5 14 loose b-tag 5 Moriond QCD ‘07 2 from Higgs events. 0 0 Higgs at the Tevatron0 50 100 150 200 250 300 0 50 100 150 2000 250 300 350 0400 0 0 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 p. 4 E (GeV) Leading Jet P (GeV) Next-to-Leading Jet P (GeV) 22 T Invariant MassT (GeV) T Mjj (GeV) ET (GeV) The Tevatron is currently looking for 30 DØ Run II Preliminary 40 DØ Run II Preliminary 20 DØ Run II Preliminary 50 DØ Run II Preliminary 22 22 -1 -1 -1 -1 FIG. 3: Distributions of the pT for the leading (upper left) and(0.93next- fbto-l)eading jet (upper right50), E/TDØ( lRunow IIe(0.93 Preliminaryr left) fband) 20dijet DØma Runss II Preliminary 20 DØ Run(0.93 II Preliminary fb ) DATA (0.93 fb ) the Higgs boson, no evidence yet -1 -1 18 -1 • For low mass Higgs : 3 main channels 35 (0.93 fb ) 18 (0.93 fb ) 18 (0.93 fb ) (lower right) before b-tagging. TOP MC 25 !" 40 16 16 MET 7 16 40 Events /0.16 14 14 Z+jets MC e.g. pp#! Z#ZH#$$bb! ! Events /8 GeV 30 Events /8 GeV + " WH ! l"bb 30 12 12 ZH ! ""bb 14 W+jets MC

LEP searched for e e #Z#ZH Events /8 GeV Events /8 GeV Events /8 GeV Events /8 GeV ZH ! llbb considered are the invariant mass of the two lead20ing jets in the event (M ), the ∆R between the two jets, 10the p of 10 the leading jet, the pT of the next-to-leading jet, the E/T , the H/T and HT j.jDistributions25of these variables are shoTwn 20 8 128 MISC MC 30 in Fig. 4. The input variables were selected for their ability to separate signal and background, as given 6by their 6 QCD No (conclusive) evidence found impact on the training sample total error reduct15ion. Each NN was trained for 200 epo10c20hs, using 14 hidden4 neurons 104 2 (in a single layer), and one output neuron. The NN outputs for signal, background and data are shown in F2ig. 5. 2 H MC x 20 0 0 80 20 ⇒ mHiggs > 114.4 GeV/c 150 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 0.5 1 1.5 2 2.5 3 3.5 4 H (GeV) H (GeV) 10 Neural Net T T ∆ R DATA DATA 6 H MC FIG10. 4: Distributions of the NNHi nMCput variables, the pT for the leading and next-to-leading jet, dijet mass, E/T , H/T , HT and m = 105 GeV m = 115 GeV H TOP MC H ∆R, from left to right and top toTOPbo MCttom, after neural net b-tagging. 10 Z+jets5 MC Z+jets MC 4 35 W+jets MC 40 5 W+jets MC MISC MC MISC MC 2 DØ Run II Preliminary QCD QCD Picture of event in 30 -1 H x 50 35 H x 50 (0.93 fb ) 0 DØ Run0 II Preliminary 0 0 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 b jet 30 (0.93 fb-1)

Events/ 0.08 25 Events/ 0.08 • For high mass Higgs : 1 mDØain detector channel which Leading Jet P (GeV) Next-to-Leading Jet P (GeV) Mjj (GeV) E (GeV) 25 T T T 20 looks like ZH#$$bb! b jet 20 15 S/B ~ 1.4/134 (but could also be 15 22 22 10 50 DØ Run10 II Preliminary 20 DØ Run II Preliminary 20 DØ Run II Preliminary DATA background) b jet -1 -1 -1 b5 jet (0.93 fb5 ) 18 (0.93 fb ) 18 (0.93 fb ) gg ! H ! WW ! l l TOP MC 0 " " CDF Candid0ate : 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.940 13 0 0.1 0.2 0.3 0.4 0.516 0.6 0.7 0.8 0.9 16

NN output NN output Events /0.16 m i s s 14 14 Z+jets MC Events /8 GeV Events /8 GeV Basic selection: DATAE T = 145 G e V DATA 30H MC 12 H MC 12 mH = 125 GeV TOP MCj e t 1 mH = 135 GeV TOP MC Z+jetsE T MC = 100 G e V Z+jets MC W+jets MC 35 W+jets MCj e t 2 35 10 W+jets MC 10 MISC MC MISC MC DØ Run II Preliminary E T = 5 5 G e V DØ Run II Preliminary 20QCD 8 QCD 8 30 (0.93 fb-1) H x 50 30 (0.93 fb-1) H x 50 - two acoplanar jets M j =j 8 2 G e V MISC MC 6 6

Events/ 0.08 25 Events/ 0.08 25 - ! 1 tagged b-jets (CDF) 20 10 20 4 4 QCD 2 tagged b-jets (DØ) 15 15 2 2 H MC x 20 0 0 0 - E miss > 70 GeV (CDF) 10 0 20 40 60 8010100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 0.5 1 1.5 2 2.5 3 3.5 4 T 5 5 HT (GeV) HT (GeV) ∆ R Search 50 GeGlVo b afor(l DfitØto)a ll StheM data: Higgs0 II 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 NN outputFIG. 4: Distributions of the NN input varNNiab outputles, the pT for the leading and next-to-leading jet, dijet mass, E/T , H/T , HT and [LEPEWWG ’07] m = 144 GeV 6 Limit ∆R, from left to right and top to bottom, after neural net b-tagging. FTheoryIG. 5: N uncertaintyN output distributions for mass points in the range 105 ≤ mH ≤ 135 GeV. +33 (5) As the Higgs boson can interactM with= 76 all otherGeV !# = ⇒ H 24 5 had 15 bosons and fermions, many measurements− are 0.02758±0.00035 0.02749±0.00012 MH < 144 GeV, 95% C.L. 2 slightly sensitive to Higgs mass. 4 incl. low Q data VII. SYSTEMATIC ERRORS 2 • Blue band shows the ! of a fit to data as a 2 Systematic errors associated with the luminosity, trigger efficiencies, jet ID, b-tagging, background MC cross section Higgs Signals at LHC and Q3CD background are estimated and included in the limit setting procedure. All systematic errors are common and !" function of Higgs mass. correlated between signal and background except for the uncertainty on the cross section and the QCD contribution. Table II lists these systematic errors. • Yellow is LEP excluded regionAssump tion for the fit: 2 SM incl. Higgs boson Luminosity Trigger Jet ID b-tagging Background σ QCD multijets +33 1 6.1 5 5 7 6-18 20 • Best fit mH = 76 #24 GeVn.o confirmation of ⇒ TABLE II: Table of systematic errors in %. Higgs mechanism Excluded Preliminary All evidence suggests that (if the Standard 0 • 30 100 300 Model is correct) mass of the Higgs is < 200 GeV Errors are also estimated for the difference in the shape of the NN output when varying the Jet Energy Scale by ± 1σ separately for mallHs ig[nGeVal and] background MC samples at each mass point. The difference in the shape of the NN output because of the uncertainty in the shape of the MC dijet mass spectrum at each mass point is also taken into account. The former source of error was estimated to be ≤ 10% and the latter ≤ 8%; both are included in the Higgs boson seemlismtitosebtteinlgi.ght, MH < 150 GeV ⇒ ∼ Sven Heinemeyer – Higgs–MaManyxwell work sstudieshop (Edinbur gofh) Higgs13.02.20 0bosons8 searches 15 at LHC made e.g. gg#H#Z0Z0#4 charged leptons

Using 2 years of data, if mH < 1 TeV the LHC is predicted should find the Higgs.

... otherwise Standard27 Model is in trouble!

14 Summary

Electromagnetic & weak are Standard Model describes manifestations of a single unified electroweak and QCD. Beautifully electroweak interaction. verified by experiment, apart from (just 3 parameters describe missing Higgs boson. interaction!) At high energies collider produce real W and Z bosons for study. At low energies, virtual W and Z bosons responsible for lepton and Studies of lightest hadron decays and neutrino Z boson decays suggest that only scatterings. three generations of quarks and leptons.

Standard Model predicts a Higgs field Standard Model makes no prediction to account for the masses of the for the Higgs boson mass. bosons. Searches, as yet, have not found any Higgs field also produces a Higgs evidence for the Higgs boson. Should boson. be found at the LHC collider!

Standard Model Interactions 6 Summary of Standard Model Vertices QED QCD Weak Neutral Current Weak Charged Current ! At this point have discussed all fundamental fermions quantum theory of EM quantum theory of and their interactions with the force carrying bosons. quantum theory of weak interactions interactions strong interactions ! Interactions characterized by SM vertices ELECTROMAGNETIC (QED) 6 e- mediated by exchange mediated by exchange mediated by exchangeq SummarmediatedCouples to CHARGEy obyf Standar exchanged Model Vertices - e Q e of virtual photons of gluons e of Z bosonsq ! AtDoesthis NOTpointof change Wha bosonsve discussed all fundamental fermions FLAVOUR e2 6 and their interactions with the force carrying bosons. α = γ γ acts on all chargedSummar y of Standard Model Ve4rπtices ! Interactions characterized by SM vertices acts on quarks only STRONG (QCD)acts on all quarks and leptons particles ELECTROMAGNETIC (QED) ! At this point have discussed all fundamental fermions q Couples to COLOUR- e q Couples to CHARGE and their interactions with the force carrying bosons. gs q Does- NOTe change Q e e Does NOT change couples to electric couples to colour does not change quark FLAVOURchanges quarkq and ! Interactions characterized by SM vertices g 2 = s 2 FLAVOUR charge charge αs or lepton flavour g eleptonsγ flavours ELECTROMAGNETIC (QED) 4π α = γ 6 4π - WEAK Charged Current e q STRONG (QCD) Couples to CHARGEν Summarcouplingy strengthof Standar ∝ de ModelcouplingVer ticesstrength ∝ gS e d’ couplingChanges FLAVOUR strength ∝ gqW - e Q e g g V Couples to COLOUR ∝ !( e ∝ !"S Does NOT- changew w ckm ∝ !"W ! At this point have discussed all fundamentalq fermions e u For QUARKS: coupling gs FLAVOUR q Does NOT change and their interactions with the2force carrying bosons. - BETWEEN generations e 1 γ 1 g 2 W 1 + 1 FLAVOUR propagator: α = propagator: γ α propagator: = w W propagator:g 2 ! Interactions characterized4πby SM vertices w 4π α = s g q2 q2 (q2 m2 ) s 4π 2 2 STRONG (QCD) Z (q mW ) ELECTROMAGNETIC (QED) WEAK Neutral Current− WEAK Charged Current− - q Couples to COLOURe-, e q Couples to CHARGE νe q νe d’ Changes FLAVOUR g e-, ν g V - e Q e s e - gw w ckm e q Does NOTq change Does NOT change q eDoes NOT change u For QUARKS: coupling 2 FLAVOUR FLAVOUR 0 FLAVOUR - BETWEEN generations e g 2 Z 0 g 2 W + α = γ α = s γ g Z α = w W 4π s 4π w 4π Dr M.A. Thomson Lent 2004 STRONG (QCD) WEAK Charged Current WEAK Neutral Current 16 q - νCouples to COLOURd’ e , ν q e Changes FLAVOUR - e gs g V e , ν - gw Does NOTw changeckm e q e For QUARKS: coupling q Does NOT change FLAVOURu g 2 0 FLAVOUR α = s - BETWEEN generations Z 0 s g 2 g W + Z 4π α = w W w 4π WEAK Charged Current Dr M.A. Thomson Lent 2004 νe WEAK Neutrald’ ChangesCurrent FLAVOUR g V - - gw w ckm e , ν q e u - For QUARKS:e coupling e , νe BETWEEN generations 2 - gw W + q Does NOT change αw = W 4π 0 FLAVOUR Z 0 WEAK Neutral Current Z - e , ν q - eDr M.A. Thomson Lent 2004 e , νe q Does NOT change 0 FLAVOUR Z Z0

Dr M.A. Thomson Lent 2004