Particle Physics Handout 7 the Strong Force: Quantum Chromodynamics

Subatomic Physics: Particle Physics Handout 7 The Strong Force: Quantum Chromodynamics

QCD Colour quantum number Gluons The parton model Colour confinement, hadronisation & jets

2 QCD 2 Quantum ChromodynamicsQCD (QCD) QUANTUM ELECTRODYNAMICS: is the quantum •QUQCDANTUM isELECTR thetheor quantumODyYNAMICS:of the descriptionelectris theomaquantumgnetic ofinteraction. the strong force. theory of the electr!omamediatedgnetic interaction.by massless photons ! mediatedQEDby massless! photonphotonscouples to electric charge,QCD ! photon couples! toStrengthelectricofcharinteractionge, : . quantum! Strength theoryof interaction of the : quantum. theory of the strong electromagnetic interactions interactions QUANTUM CHROMO-DYNAMICS: is the quantum QUANTUM CHROMO-DYNAMICS: is the quantum mediated by the exchangetheory of the ofstr ong interaction.mediated by the exchange of theory of the strong interaction. virtual photons! mediated by massless gluons, i.egluons. ! mediated by massless gluons, i.e. propagator propagator acts on all charged! particles acts on quarks only ! gluon couples togluon“strong”coupleschargtoe “strong” charge ! Only quarks ha!veOnlnon-zy quarksero “strhaong”ve non-zchargereo, “strong” charge, couplestheref to electricalore only quarkstheref chargeforeeel stronlongy quarksinteractioncouplesfeel strong tointeraction colour charge Basic QCD interactionBasic QCDlooksinteractionlike a stronglookser like a stronger coupling strength ! e ! !! coupling strength gS !!S version of QED, version of QED, ! ! QED QEDq QCD q QCDq q √α Q √α Qq√α √αS S q q q q q

2 α = e /4π ~ 1/137 2 α = g 2/4π ~ 1 2 α = e /4γ π ~ S1/137S γ αS = gS /4π ~ 1 g

( subscript em is sometimes( subscriptusedemtoisdistinguishsometimestheused to distinguish the 2 of electromagnetismoffrelectrom oma). gnetism from ).

Dr M.A. Thomson Lent 2004 Dr M.A. Thomson Lent 2004 Colour • Colour charge is the charge associated with QCD interactions. • Three colours: red, blue, green. • Like electric charge, it is a conserved quantum number.

• Quarks always have a colour charge: r , g or b • Anti-quarks always have an anti-colour charge: r ̅ , b ̅ or g̅

• Leptons and bosons for other forces (", W, Z) don’t carry colour charge.

• Mesons are colour neutral; colour charges are: (r r)̅ , (b b)̅ or (g g)̅ • Baryons are colour neutral; colour charges are: ( r g b ) • Anti-baryons have: ( r ̅ g ̅ b ̅ ) Formally different colours of quarks are different fundamental particles.G luons # 1st generation is: e $e ur ub ug dr db dg

!"#$%&'($)*(+,*- 4 4 3 F F

!"#$%&* '()** +'&&")&& &,-%./*0$&$%& * * E E

D D 8 Glu!o% 1n2s3*% ,($,'4'5$( /67 $=5)5D $=5)5D $ $ 5 5 4())%* 4())%* =$%&)(?)@ =$%&)(?)@ 7#'(<&* 7#'(<&* 9+-&&-$%*$(*&5'5)&* '0&$(,5-$%*&5'5)&* $:*4"#$%&*0;*7#'(<&* 5 5 -&* -&* 0;* 0;* ) ) " " 7C,>$ 7C,>$ Gluons -%5$ -%5$ C=$"$#(* C=$"$#(*

=>'%4)&*=$"$#(*4 $:*4 7#'(<& . 2$"$#(*-&*=$%&)(?)@ !"#$%&'($)*(+,*- m m i i % % 5>)+&)"?)& 5>)+&)"?)& e e - - 0 0 h h & & u u 0 0 Gluons are massless, spin-1ℏ bosons. * * • M M 2$"$#(* 2$"$#(* 0$&$%& 0$&$%& / / B B !"#$%& '()*+'&&")&& &,-%./*0$&$%& 193B* 193B* 4"#$%&* 4"#$%&* z z 7#'(<* 7#'(<* 4 4 . . n n * * a a 4 4 r r @ @

s s 8 s s G'5#() G'5#() F F $:* $:* B B 8 8 They propagate the strong force: !exchange momentum ) ) • 123*,($,'4'5$( /67 '%@* '%@* ( ( 7 7 n n n n (( (( 6 6 :($+* :($+* &,-%./* &,-%./* between quarks. 0;* 0;* B B / / & & o o o o 9+-&&-$%*$(*'0&$(,5-$%*$:*4"#$%&*4 4 0;*7#'(<&* ) ) 7#'(<& 7#'(<& 0 0 4"#$%&* 4"#$%&* u u 4 4 u u $=5)5 $=5)5 B B l l l l % % I* I* $:* $:* ( ( ' ' =>'%4)&*=$"$#(*$:*7#'(<& . 2$"$#(*L* -&*L* =$%&)(?)@ 0 0 * * &&")&& &&")&& G G > > G G B B s s

We draw gluons as curly-wurly lines: = = c c

• '0&$(,5-$%* '0&$(,5-$%* * * i i ( ( ()'"-&)@* ()'"-&)@* .K* .K* s s +' +' % % 4 4 @-::)()%5* @-::)()%5* y y $,'4'5$( $,'4'5$( - - $ $ h h B B ( ( $(* $(* 2"#$%- 2"#$%- % % P P =$"$#(* =$"$#(* # #

0 0 , , )H,)=5* )H,)=5* " "

! $ $ !"#$%&*='((;*=$"$#(*=>'(4)e 5>)+&)"?)&e * * l l '()* '()* 4 4 4 4 # # c c i i " " 3 3 t t B B $1& $1& ?)(;* ?)(;* Gluons also carry colour charge. r r • 4 4 2 2 4 4 a a ( ( &2 &2 P P

)A4A*(4 4"#( $%*=>'( %4)& ()@*7#'(<*-%5$ 4())%**

'($)*(+,*- '($)*(+,*- 1 1 3 3 !"#$%&*='((;*=$"$#(*=>'(4) !"#$%&*='((;*=$"$#(*=>'(4) d d B B

Colour charged is always conserved. n n 0 0

• a a

! ! J;++)5(; J;++)5(; ! ! ! )A4A* ! )A4A* !"#$%& 9+-&&-$%* !"#$%& =>'%4)&* 9+-&&-$%* =>'%4)&* G'-?)"; ( G'-?)"; ( 123* 123* 123*?)(;*@-::)()%5*:($+*193B*r 7C,>$r 5$%D E*F a a e e l l c c u u

!"#$%& !"#$%& .#/0*(&.#/0*(& $1&2"#$%-.#/0*(& N N • Number of gluons: there are eight different gluons. G'-?)"; )H,)=5*I*4"#$%&* • Symmetry of the strong interaction! tell!"#$%&*='((;*=$"$#(*=>'(4) us these are: 5>)+&)"?)& (0B (4B 40B 4(B 0(B 04B ((B 44B 00 rb ̅ rg ̅ bg ̅ br ̅ gr ̅ gb ̅ (rr ̅ − gg)A4A*)/̅ !2 ((rr4 ̅4+" gg#$% ̅ −*= 2>bb'%)/̅ 4!)6& ()@*7#'(<*-%5$ 4())%** J;++)5(; .K**L*$=5)5 '%@*/*&-%4")5 &5'5)&* 123*?)(;*@-::)()%5*:($+*193B*7C,>$5$%D E*F One big difference between QED and QCD .#/0*(&$1&2"#$%- ! QED propagated by photons: photons no electric charge ! QCD propagated by gluons: gluonsG'-?)"; have colour)H,)=5* chargeI*4"#$%&* (0B (4B 40B 4(B 0(B 0!4B 3((&2B "4#4$B%-00()'"-&)@*0;*G'5#() C=$"$#(*$=5)5D 4 J;++)5(; .K**L*$=5)5 '%@*/*&-%4")5 &5'5)&*

Nuclear and Particle Physics Franz Muheim 4

! 3&2"#$%- ()'"-&)@*0;*G'5#() C=$"$#(*$=5)5D

Nuclear and Particle Physics Franz Muheim 4 Quark & Gluon Interactions Quark-anti-quark scattering q2 q1 α VQED(r) = = −4π0r − r 6 6 Short distance potential: SELF-INTERASELF-INTERACTIONSCTIONS 4 αs VQCD(r) = %" = duhhhljkhsdlkh̅ describes a meson: e.g. hhhljkhsdlkh −3 r AtAtthisthispoint,point,QCDQCDlookslookslikelikea stra strongongererverversionsionofofQEDQED. . strong force is responsible for holding meson together. ThisThisis istruetrueupuptotoa point.a point.HoHoweweverver, in, ipracticen practiceQCDQCD behabehavesvesververy diffy differentlerently tyotQEDo QED. The. Thesimilaritiessimilaritiesarisearisefromfrom • Gluons carrythe thecolourfactfactthat charge.thatbothbothin vinolvevolvethetheexecxhangchange oefoMASSLESSf MASSLESS spin-1 bosons. The big difference is that GLUONS carry • They also feelspin-1 the strongbosons. forceThe big ! diffgluonserence canis thatinteractGLUONS withcarr othery gluons! colourcolour“char“charge”.ge”. 12 GLUONSGLUONS3-gluonCANCAN vertexINTERAINTERACTCTWITHWITH4-gluonOTHEROTHER vertexGLUONS:GLUONS: g g Running of g g g g ! g g specifies the strength of the strong interaction g g g g g g 3 GLUON VERTEX 4 GLUON VERTEX ! BUT just as in QED, isn’t a constant, it 3 GLUON VERTEX34 4 GLUON VERTEX “runs” 5 EXAMPLE: Gluon-Gluon Scattering EXAMPLE: Gluon-Gluon Scattering ! In QED the bare electron charge is screened by a cloud of virtual electron-positron pairs. In the limit the propagator for a DECAYING+ + state becomes: The Parton+ Model+ ! In QCD a similar effect occurs. • The parton model proposes that, in high energy interactions,In QCD hadronsquantum interactfluctuations lead to a ‘cloud’ of EXAMPLE: ( NON-EXAMINABLE)as if they were made of their constituentEle parts.ctro ne.g.-Proton: vir tualScatteringpairs q • (Problem sheet 2, Q3) J/'&µ+µ# decay • (Handout 4, p11) ep&ep scattering. - - !"#$%&'()*+*Proton interacts,# as if it were !" #$! !" # e one of many (an infinite set) J/ψ e.g. rµ g+gb→r r%&'()*+*,($+r b -.#/*+01! q q γ c γ e.g. r g+gb→threer r+ independentr b quarks. q 0)!$2!3(.'( &0/!) such diagrams analogous to + + Electron scatters off one quark e c µ )*.#/!$*4$(!5/!6+*(5$)#*() q those for QED. q In centre-of-mass system ( ) would expect cross 1 e 2 4!" M $ e e # # q Dr M.A. Thomson 2 2 2 Lent 2004 section to be of form: Dr M.A. Thomson q q q Lent 2004 • At higher energies, proton consist of more-$.//' than/*0#%., three quarks:In QCD the gluon self-interactions ALSO lead to a quarks are- constantly+ exchanging gluons. Gluons7&,8'8 */can*+3$4 ,convert&$)6'++!&* (5$ 4 2 2‘cloud’ of virtual gluons d3 2 e 16! " First, note partialintowidth quarkfor !anti-quark :pairs. $ M $ # d2 q4 q4 g one of many (an infinite set) • Proton consists of three “valance quarks9”" .plus,.! (gluons+0.$+&'( )and4!& such diagrams. Here there is no 2 g “sea quarks”. q2 # q 1 q # p1 - p1 # p2 / p2 - 2 p . p g 1 % f i & f i f i 1 q ! analogy in QED, photons don’t giving p f # %E f , p f & - + 2 ! ! The sea quarks are exactly balanced between# 2me quarks- 2%E f Ei - andp f pi cos0 & 1 • p # %E , p! & have self-interactions since they anti-quarks. g i i i Partial width for : 2 , 0 ) q don’t carry the charge of the in- # -4E f Ei sin * ' :;!($(!5/!6+*(5$. + 2 ( q ! • Net quark content of proton is u, u, d. teraction.

12#3*$4.$5'60"##*$%,712#3*$4.$5'12#3*$4.$5'60"##*$%,7'60"##*$%,7'Dr M.A. Thomson Lent 2004 with $$$$(!5/!6+$#&,+,($&!6,*/ d3 " 2 Dr M.A. Thomson Lent 2004 # d2 2 4 , 0 ) Lab 4E sin * ' 6 + 2 (

2 2 d3 " E f , , 0 ) q , 0 )) # *cos2 * ' - sin2 * '' * 2 ' d2 Lab 2 4 , 0 ) E + 2 ( 2M + 2 ( 4E sin * ' i + p ( + 2 (

Nuclear and Particle Physics Franz Muheim 8 Parton Distributions in the Proton • Positron-proton scattering measurements (at HERA accelerator) have measured the valance and sea quarks and gluon in the proton. pparton • Key parameter is “Feynman x”: x = | | p | proton| • Graph shows measured fraction, f, of each parton (u-, d-, s-, c-quarks & gluons) as a function of x. • f also depends on momentum, Q 2 = ( q ) 2 transferred by the boson ("). − ! The higher the Q2, the more energetic 7 the partons. ! At LHC energies,CONFINEMENT proton contain lots of gluons! NEVER OBSERVE: single FREE quarks/gluons • LHC collisions will be a mixture of: quark-quark,! quarks quark-gluon,are always gluon-gluon,confined within hadrons anti-quark! This!quark,is a consequence anti-quark!gluonof the etc.strong • One challenge:self-interactions for each individualof gluons. collision we doQualitativel not know ythe, picture flavourthe or colourmomentumfield between two of the interacting partons! quarks. The gluons mediating the force act as 7 additional sources of the colour field - they attract each other. The gluon-gluon interaction pulls the lines of colour force into a narrow tube or Colour ConfinementSTRING. In this model the string has a ‘tension’ and as the quarks separate the string stores Experimentally we do not see free quarks:potential quarks enerare confinedgy. within hadrons 9 • Gluons attract each other: they self interact JETS • Gluon-gluon interaction pulls the colour field lines into a narrow tube. Consider the pair produced in : • Potential increases linearly with distance: V(r) = kr + Infinite energy is required to separate two quarks.e q • Colourγ Electric COLOUR CONFINEMENTq q Energy storedfieldper unitlineslengthfield linesconstant. e- q Colour confinement is a direct consequence of gluon self-interactions ! Requires infinite energy to separate two Tot al pot ent ial: 1 -4 quarks. Quarks alwaV =ysαcomes+kr in combinations As4 theαs quarks separate, the 3r VQCD(r) = + kr with zero net (GeV) colour0 charge: CONFINEMENT. energy stored in the colour -4 −3 r V = αs

QCD 3r ﬁeld (‘string’) DrstarM.A.tsThomsonto in- Lent 2004

V -1 Force required to separate quarks:crease linearly with separa- α =0.2 dVtion. When4 αs -2 s F = = + k k=1 GeV/fm QCD − dr new3 r2 pairs can be created. -3 At large distances F ( k # 100 GeV/fm = 160,000 N !!! 0 0.2 0.4 0.6 0.8 1 r(fm) 8 q q

q q

q q q q

as energy decreases... hadrons freeze out

Dr M.A. Thomson Lent 2004 9 9 JETS

Consider the pair produced in : Consider the pair produced in : + e+ q e γ q γ q q - q q e- q Hadronisatione q 1 -4α 9 1 V =-4 s+kr As the quarks separate, the V = 3rαs+kr • What happensAs the whenquarks weseparate try ,tothe pull apart(GeV) 0 two quarks?3r energy stored in the colour (GeV) JETS 0 -4αs energy stored in the colour V =-4 QCD α • At LHC productionﬁeld (‘string’) of energeticstarts to in- quarks is common Ve.g. = 3r qqs $̅ g$qq.̅

ﬁeld (‘string’) starts to in- QCD 3r V -1 crease linearly with separa-

V -1 • qq ̅ producedcrease linearlat samey with pointConsidersepara- in space.the pair produced in : tion. When αs=0.2 tion. When -2 α+=0.2 • q and q ̅ havene veryw largepairs canmomentumbe -2! they fly eapart.k=1s GeV/fm q new pairs can be k=1 GeV/fm created. -3 γ created. -3 0 0.2 0.4 0.6 0.8 1 q q 0 0.2 0.4 0.6 r(fm)0.8 1 e- r(fm) q • The energy between the qq ̅ increasesq q as they move apart E(V(r)(kr q q

1 -4αs 2 q V = +kr When E > 2 mqc ... As the quarks separateq, the 3r 10

• (GeV) q q 0 energy stored in the colour -4α As quarks separate, more pairs areVpr = oduceds ﬁeldq (‘string’)q qstarts toqin- QCD 3r

q q q q V -1 from the potentialcrease enerlinearly withgysepara-of the colour ﬁeld. This as energytion. decreases...When hadrons freeze out αs=0.2 • As the kinetic energy decreasesas energy decreases...... the hadrons hadrons freeze freeze out -2 out process is called HADRnew pairONIZAs can be TION. Stark=1t GeV/fmout with created. -3 0 0.2 0.4 0.6 0.8 1 quarks and end up with narrowly collimatedr(fm)JETS • This process is known as hadronisation. Dr M.A. Thomson Lent 2004 of HADRDr M.A. ThomsonONS q q Lent 2004 9 q q q + q π (ud) q q etc... q qq q q q

SPACE Jets q as energyq decreases... hadrons freeze+ out 0π - q q π +π • A collision produces energetic quarks, whichq hadronise. π- K + q 0 0 π • The produced hadrons decay... (into more hadrons andπ maybeπ 0 leptons) TIME p π • In the detector this appears as a collimated “jet” of particles. Dr M.A. Thomson Lent 2004 e+ q q γ Feynman e- e+ Diagram - e q q CoM Frame

Typical Event Event from LEP collider ECM = 91 GeV e+e#$qq" The hadrons in a 2 jets in detector quark(anti-quark) jet follow the direction of

the original quark(anti-10 quark). Consequently is observed as a pair of back-to-back jets of hadrons

Dr M.A. Thomson Lent 2004 Running of !

!"#$%&'()*"'+,-',)! 24 !"#$%&"'()*($+$,"#)-.&%$"/,(/%"$#.,"/)% Running of 24 Running of ! 0($123" /4(%)"(.(,)%4".%"(."(.++(5/4".%,$4 ! specifies.-*##/the strength of the interaction ! specifies the strength of the interaction Review:between an electr QEDon6and)"photon.($- 7"Coupling89(.#):%5(*#$$($+$," #Constant)%( between an electron!andBUphoton.T isn’t a constant 2 ! BUT isn’t a constant ,#$."/)%(.%5(.%%/'/+."/)% e 1 • StrengthConsider of interactiona free electron between: Quantum electronfluctuations andlead photonto a α = Consider a freeHowever,electron‘cloud’: Quantum ! ofs virnottualfluctuations reallyelectron/positr a)lead *constant...(;/to#on"a:.pair+($s+$,"#)%<7)4/"#)%(7∝./#4 4π0 ≈ 137 ‘cloud’ of vir•tual electron/positron paire-s • eAn- electron is never0*122'&'( alone: - γ e e+ γ =.#$(,'.this#&$is .just%5(-one.44 of)*($+$,"#)%()%+8(;/4/>+$( - γ - e e•+ itγ emits virtualthis isphotons,juste one theseof can convert to e- many (an infinite set) electron positronmany (an pairs...infinite."(;set)$#8(4')#"(5/4".%,$4 e+ such diagrams. e+ γ γ sucγ h diagrams.e+ !γ /%,#$.4$4(?/"'(?/"' +.#&$#(-)-$%":-("#.%4*$# γ + γ e e- e- • Any test charge will feel the e+e# pairs: true charge of the electron! The vacuum is screenedacts like a.dielectric medium ! The vacuum acts like a dielectric medium ! variese$e as# % a& function$ & # of ! The virtualAt higherpair! energysThearevirpolariztual (shortered pair distances)s are polariz theed test 12 • ! ! At large distancescharge canthe Atbare seelarelectrg ethedistanceson “bare”chargthee is chargebarescreened.electr ofon thechar electron.ge is screened. energy and distance - - At large R Runningtest charge of + At large R test+ charge - - - sees screened- e- charge- sees screened e charge + + + + - + - ! + + + specifiesTest Charge the strength of the strong - + + - Test- Charge - + + - - interaction - - ! BUT just as in QED, isn’t a constant, it + At small R test+ charge At small R test charge - - sees -bare e- charge - sees bare e- charge + + + + “runs” - + Test Charge @+.44/,.+(- + Test+/-/" Charge 12 + + + +! In QED the bare electron charge is screened - - - -1 ! ! + + A 0(B(((((((((((((( 0(C2CDE - RunningF-"(4')#"(bofy5/4".%,$4a cloud of virtual electron-positron pairs. 11 A!1 0In(GHQCDB(I$JaKsimilar1 ! ! 0(effC2Cect1Loccurs. Dr M.A. Thomson Lent 2004 ! Dr M.A. Thomsonspecifies the strength of the strong Lent 2004 NucleaInr anQCDd Particquantumle Physics fluctuationsFranz Mleaduheimto a ‘cloud’ of 12 interaction virtual pairs ! BUT just as in QED, isn’t a constant, it QCD Couplingq Constant “runs” one of many (an infinite set) q q In! QCDIn QED the interactionthe bare electr strengthon c haris !gSe - isalsoscreenedq not really a constant. • such diagrams analogous to • Quarkby emita cloud gluons:of vir whichtual electr can formon-positrq virtualon quarkpair s.-those anti-quarkfor QED .pairs. q ! In QCD a similar effectq occurs. However the gluons themselves also carry colour charge, which effects the • In QCD the gluon self-interactions ALSO lead to a Inscreening.QCD quantum fluctuations lead to a ‘cloud’ of virtual pairs ‘cloud’ of virtual gluons q g one of many (an infinite set) such diagrams. Here there is no one ofg many (an infinite set) q q g q q analogy in QED, photons don’t such diagrams analogous to have self-interactions since they q those forgQED. q q don’t carry the charge of the in- q q teraction.

In QCD the gluon self-interactionsDr M.A. Thomson ALSO lead to a Lent 2004

• ‘c!Sloud’ decreasesof vir tualat highgluons energies! ⇔ !S increases at large distances! g one of many (an inﬁnite set) At low energies the coupling constant becomes large, !S ~1. We cannot use • such diagrams. Here there is no perturbationg g theory to calculate cross sections! q analogy in QED, photons don’t • The understanding of thisha phenomenave self-interactions won sincethe Nobelthey prize in 2004. g q don’t carry the charge of the in- q 12 teraction.

Dr M.A. Thomson Lent 2004 22 Evidence for Gluons

In QED, electrons can radiate photons. In QCD quarks can radiate gluons.

e+ q √α γ S g √α Qq√α 1 e- q2 q 1 giving an extra factor of in the matrix element, i.e. an extra factor of in cross ParticEvidencele Physics section.for Gluons In QED we can detect the photons. In QCD we 2 • !S is large at high energy (high q )ne quarksver see arefree verygluons likelydue totoconﬁnement emit a gluon.. Experimentally detect gluons as an additional jet: High energyDr gluonsM.A. alsoThomson hadronise, and also form jets. • 3-Jet Events. e+ q g √α γ S g √α q Feynman Qq√α Diagram 1 q e- q2 q ! Angular distribution of gluonCoMjet Framedepends on gluon spin 3 jets event: e+e#&q q ̅ g Dr M.A. Thomson Lent 2004

Event from LEP collider at CERN ECM = 91 GeV Event from PETRA collider at DESY 6 E =35 GeV 6 CM SELF-INTERACTIONS SELF-INTERACTIONS13 hhhljkhsdlkh At this point, QCDhhhljkhsdlkhlooks like a stronger version of QED. Part II, Lent Term 2004At this point, QCD looks like a stronger version of QED. This is true up to a point. However, in practice QCD This is true up to a point. However, in practice QCD behaves very differently to QED. The similarities arise from HANDOUT IIIbehaves very differently to QED. The similarities arise from QCD Summarythe fact that both involve the exchange of MASSLESS the fact that both involve the exchange of MASSLESS spin-1 bosons. The big difference is that GLUONS carry spin-1 bosons. The big difference is that GLUONS carry Dr M.A. Thomson colourLent 2004“cQuarksharge”. and gluons carry QCD: Quantum Gluonscolour are“c theharg e”. colour charge. GLUONS CAN INTERACT WITH OTHER GLUONS: Chromodymanics is the propagatorGLUONS of theCAN INTERACT WITH OTHER GLUONS: quantum description of strong force Gluons self-interact:g g the strong force. g gg g g Only quarks feel the g g g strong force. g gg g 3 GLUON VERTEX 4 GLUON VERTEX 3 GLUON VERTEX 4 GLUON VERTEX Hadrons can be Electromagnetic coupling constant ! decreasesEXAMPLE: Gluon-Gluon Scattering • EXAMPLE: Gluon-GluondescribedScattering as consisting as a charged particles get further apart. of partons: quarks and Strong coupling constant ! increases as • S gluons, which+ interact + further apart quarks become. + independently+ Quarks and gluons produced in Colour Confinement collisions hadronise: hadrons are energy required to separate produced. quarks $ % The decay products of the hadrons quarks are confined to hadrons appear e.g.in ther g detector+e.g.gb→r gr as+r+g jetsrbb→.r r+r b 14

Dr M.A. Thomson Lent 2004 Dr M.A. Thomson Lent 2004 Notes

Notes