1 Constitunts of th Atom To b know th constitunts of th atom with thir masss and chargs To b abl to calculat th spcific charg of th constitunts To b abl to xplain what isotops and ions ar Th Nuclar Modl (Also sn in GCSE Physics 1 and 2) W know from Ruthrford s xprimnt that th structur of an atom consists of positivly chargd protons and nutral nutrons in on plac calld th nuclus. Th nuclus sits in th middl of th atom and has ngativly chargd lctrons orbiting it. At GCSE w usd chargs and masss for th constitunts rlativ to ach othr, th tabl abov shows th actual chargs and masss. Constitunt (C) Mass (kg) Proton 1.6 x 10-19 1.673 x 10-27 Nutron 0 1.675 x 10-27 Elctron - 1.6 x 10-19 9.1 x 10-31 Almost all of th mass of th atom is in th tiny nuclus which taks up practically no spac whn compard to th siz of th atom. If w shrunk th Solar Systm so that th Sun was th siz of a gold nuclus th furthst lctron would b twic th distanc to Pluto. If th nuclus was a full stop it would b 25 m to th first lctron shll, 100 to th scond and 225 to th third. Notation (Also sn in GCSE Physics 2) W can rprsnt an atom of lmnt X in th following way: A Z X Z is th proton numbr. This is th numbr of protons in th nuclus. In an unchargd atom th numbr of lctrons orbiting th nuclus is qual to th numbr of protons. In Chmistry it is calld th atomic numbr A is th nuclon numbr. This is th total numbr of nuclons in th nuclus (protons + nutrons) which can b writtn as A = Z + N. In Chmistry it is calld th atomic mass numbr N is th nutron numbr. This is th numbr of nutrons in th nuclus. Isotops (Also sn in GCSE Physics 1 and 2) Isotops ar diffrnt forms of an lmnt. Thy always hav th sam numbr of protons but hav a diffrnt numbr of nutrons. Sinc thy hav th sam numbr of protons (and lctrons) thy bhav in th sam way chmically. 35.5 Chlorin If w look at Chlorin in th priodic tabl w s that it is rprsntd by 17 Cl. How can it hav 18.5 35 37 nutrons? It can t! Thr ar two stabl isotops of Chlorin, 17Cl which accounts for ~75% and 17Cl which 35.5 accounts for ~25%. So th avrag of a larg amount of Chlorin atoms is 17 Cl. Spcific Spcific charg is anothr titl for th charg-mass ratio. This is a masur of th charg pr unit mass and is simply workd out by workd out by dividing th charg of a particl by its mass. You can think of it as a how much charg (in Coulombs) you gt pr kilogram of th stuff. Constitunt (C) Mass (kg) -Mass Ratio (C kg -1 ) or (C/kg) Proton 1.6 x 10-19 1.673 x 10-27 1.6 x 10-19 1.673 x 10-27 9.58 x 10 7 Nutron 0 1.675 x 10-27 0 1.675 x 10-27 0 Elctron (-) 1.6 x 10-19 9.1 x 10-31 1.6 x 10-19 9.11 x 10-31 (-) 1.76 x 10 11 W can s that th lctron has th highst charg-mass ratio and th nutron has th lowst. Ions (Also sn in GCSE Physics 2) An atom may gain or los lctrons. Whn this happns th atoms bcoms lctrically chargd (positivly or ngativly). W call this an ion. If th atom gains an lctron thr ar mor ngativ chargs than positiv, so th atom is a ngativ ion. Gaining on lctron would man it has an ovrall charg of -1, which actually mans -1.6 x 10-19 C. Gaining two lctrons would man it has an ovrall charg of -2, which actually mans -3.2 x 10-19 C. If th atom loss an lctron thr ar mor positiv chargs than ngativ, so th atom is a positiv ion. Losing on lctron would man it has an ovrall charg of +1, which actually mans +1.6 x 10-19 C. Losing two lctrons would man it has an ovrall charg of +2, which actually mans +3.2 x 10-19 C.
2 Particls and Antiparticls To know what is th diffrnc btwn particls and antiparticls To b abl to xplain what annihilation is To b abl to xplain what pair production is Antimattr British Physicist Paul Dirac prdictd a particl of qual mass to an lctron but of opposit charg (positiv). This particl is calld a positron and is th lctron s antiparticl. Evry particls has its own antiparticl. An antiparticl has th sam mass as th particl vrsion but has opposit charg. An antiproton has a ngativ charg, an antilctron has a positiv charg but an antinutron is also unchargd lik th particl vrsion. Amrican Physicist Carl Andrson obsrvd th positron in a cloud chambr, backing up Dirac s thory. Anti particls hav opposit, Numbr, Lpton Numbr and. If thy ar mad from quarks th antiparticl is mad from antiquarks Annihilation Whnvr a particl and its antiparticl mt thy annihilat ach othr. Annihilation is th procss by which mass is convrtd into nrgy, particl and antiparticl ar transformd into two photons of nrgy. Mass and nrgy ar intrchangabl and can b convrtd from on to th othr. Einstin linkd nrgy and mass with th quation: 2 E mc You can think of it lik mony; whthr you hav dollars or pounds you would still hav th sam amount of mony. So whthr you hav mass or nrgy you still hav th sam amount. Th law of consrvation of nrgy can now b rfrrd to as th consrvation of mass-nrgy. Th total mass-nrgy bfor is qual to th total mass-nrgy aftr. Photon Max Planck had th ida that light could b rlasd in chunks or packts of nrgy. Einstin namd ths wav-packts photons. Th nrgy carrid by a photon is givn by th quation: hc E hf Sinc c f w can also writ this as: E How is thr anything at all? Whn th Big Bang happnd mattr and antimattr was producd and snt out xpanding in all dirctions. A short tim aftr this thr was an imbalanc in th amount of mattr and antimattr. Sinc thr was mor mattr all th antimattr was annihilatd laving mattr to form protons, atoms and vrything around us. Pair Production Pair production is th opposit procss to annihilation, nrgy is convrtd into mass. A singl photon of nrgy is convrtd into a particl-antiparticl pair. (This happns to oby th consrvation laws) This can only happn if th photon has nough mass-nrgy to pay for th mass. Lt us imag mass and nrgy as th sam thing, if two particls ndd 10 bits and th photon had 8 bits thr is not nough for pair production to occur. If two particls ndd 10 bits to mak and th photon had 16 bits th particl-antiparticl pair is mad and th lft ovr is convrtd into thir kintic nrgy. If pair production occurs in a magntic fild th particl and antiparticl will mov in circls of opposit dirction but only if thy ar chargd. (Th dflction of chargs in magntic filds will b covrd in Unit 4: Forc on a d Particl) Pair production can occur spontanously but must occur nar a nuclus which rcoils to hlp consrv momntum. It can also b mad to happn by colliding particls. At CERN protons ar acclratd and fird into ach othr. If thy hav nough kintic nrgy whn thy collid particl-antiparticl pair may b cratd from th nrgy. Th following ar xampls of th ractions that hav occurrd: p p p p p p p p p p p p p p n n In all w can s that th consrvation laws of particl physics ar obyd.
3 Quarks To know what quarks ar and whr thy ar found To b abl to xplain how thy wr discovrd To know th proprtis of ach typ of quark Ruthrford Also sn in GCSE Physics 2 Ruthrford fird a bam of alpha particls at a thin gold foil. If th atom had no innr structur th alpha particls would only b dflctd by vry small angls. Som of th alpha particls wr scattrd at larg angls by th nucli of th atoms. From this Ruthrford dducd that th atom was mostly mpty spac with th majority of th mass situatd in th cntr. Atoms wr mad from smallr particls. Smallr Scattring In 1968 Physicists conductd a similar xprimnt to Ruthrford s but thy fird a bam of high nrgy lctrons at nuclons (protons and nutrons). Th rsults thy obtaind wr vry similar to Ruthrford s; som of th lctrons wr dflctd by larg angls. If th nuclons had no innr structur th lctrons would only b dflctd by small angls. Ths rsults showd that protons and nutrons wr mad of thr smallr particls, ach with a fractional charg. Quarks Ths smallr particls wr namd quarks and ar thought to b fundamntal particls (not mad of anything smallr). Thr ar six diffrnt quarks and ach on has its own antiparticl. W nd to know about th thr blow as w will b looking at how largr particls ar mad from diffrnt combinations of quarks and antiquarks. Quark Anti Quark d -⅓ +⅓ 0 d +⅓ -⅓ 0 u +⅔ +⅓ 0 u -⅔ -⅓ 0 s -⅓ +⅓ -1 s +⅓ -⅓ +1 Th othr thr ar Charm, Bottom and Top. You will not b askd about ths thr Quark No. Charmnss Bottomnss Topnss d -⅓ +⅓ 0 0 0 0 u +⅔ +⅓ 0 0 0 0 s -⅓ +⅓ -1 0 0 0 c +⅔ +⅓ 0 +1 0 0 b -⅓ +⅓ 0 0-1 0 t +⅔ +⅓ 0 0 0 +1 Th Lon Quark? Nvr! Quarks nvr appar on thir own. Th nrgy rquird to pull two quarks apart is so massiv that it is nough to mak two nw particls. A quark and an antiquark ar cratd, anothr xampl of pair production. A particl calld a nutral pion is mad from an up quark and an antiup quark. Moving ths apart crats anothr up quark and an antiup quark. W now hav two pairs of quarks. Trying to sparat two quarks mad two mor quarks. Particl Classification Now that w know that quarks ar th smallst building blocks w can sparat all othr particls into two groups, thos mad from quarks and thos that arn t mad from quarks. Hadrons Havy and mad from smallr particls Lptons Light and not mad from smallr particls
4 Hadrons To know what a hadron is and th diffrnc btwn th two typs To know th proprtis common to all hadrons To know th structur of th common hadrons and which is th most stabl Mad from Smallr Stuff Hadrons, th Grk for havy ar not fundamntal particls thy ar all mad from smallr particls, quarks. Th proprtis of a hadron ar du to th combind proprtis of th quarks that it is mad from. Thr ar two catgoris of Hadrons: s and Msons. s Mad from thr quarks Proton Nutron u +⅔ +⅓ 0 d -⅓ +⅓ 0 u +⅔ +⅓ 0 u +⅔ +⅓ 0 d -⅓ +⅓ 0 d -⅓ +⅓ 0 p +1 +1 0 n 0 +1 0 Th proton is th only stabl hadron, all othrs vntually dcay into a proton. Msons Mad from a quark and an antiquark Pion Plus Pion Minus u +⅔ +⅓ 0 u -⅔ -⅓ 0 d +⅓ -⅓ 0 d -⅓ +⅓ 0 π + +1 0 0 π - -1 0 0 Pion Zro Pion Zro u +⅔ +⅓ 0 d -⅓ +⅓ 0 u -⅔ -⅓ 0 d +⅓ -⅓ 0 π 0 0 0 0 π 0 0 0 0 Kaon Plus Kaon Minus u +⅔ +⅓ 0 u -⅔ -⅓ 0 s +⅓ -⅓ +1 s -⅓ +⅓ -1 K + +1 0 +1 K - -1 0-1 Kaon Zro AntiKaon Zro d -⅓ +⅓ 0 d +⅓ -⅓ 0 s +⅓ -⅓ +1 s -⅓ +⅓ -1 K 0 0 0 +1 0 K 0 0-1 Anti Hadrons Anti hadrons ar mad from th opposit quarks as thir Hadron countrparts, for xampl a proton is mad from th quark combination uud and an antiproton is mad from th combination u u d W can s that a π + and a π - ar particl and antiparticl of ach othr. Anti Proton Anti Nutron u -⅔ -⅓ 0 d +⅓ -⅓ 0 u -⅔ -⅓ 0 u -⅔ -⅓ 0 d +⅓ -⅓ 0 d +⅓ -⅓ 0 p -1-1 0 n 0-1 0 You nd to know all th quark combination shown on this pag as thy may ask you to rcit any of thm!!!!
5 Lptons To b abl to xplain what a lpton is To know th proprtis common to all lptons To b abl to xplain th consrvation laws and b abl to us thm Fundamntal Particls A fundamntal particl is a particl which is not mad of anything smallr. s and Msons ar mad from quarks so thy ar not fundamntal, but quarks thmslvs ar. Th only othr known fundamntal particls ar Bosons (s 6: Forcs and Exchang Particls) and Lptons. Lptons Lptons ar a family of particls that ar much lightr than s and Msons and ar not subjct to th strong intraction. Thr ar six lptons in total, thr of thm ar chargd and thr ar unchargd. Th chargd particls ar lctrons, muons and tauons. Th muon and tauon ar similar to th lctron but biggr. Th muon is roughly 200 tims biggr and th tauon is 3500 tims biggr (twic th siz of a proton). Each of th chargd lptons has its own nutrino. If a dcay involvs a nutrino and a muon, it will b a muon nutrino, not a tauon nutrino or lctron nutrino. Th nutrino is a charglss, almost masslss particl. It isn t affctd by th strong intraction or EM forc and barly by gravity. It is almost impossibl to dtct. Lpton Lpton Numbr (L) Anti Lpton Lpton Numbr (L) Elctron - -1 +1 Anti Elctron + +1-1 Elctron Nutrino ν 0 +1 Anti Elctron Nutrino ν ē 0-1 Muon μ - -1 +1 Anti Muon μ + +1-1 Muon Nutrino ν μ 0 +1 Anti Muon Nutrino ν μ 0-1 Tauon τ - -1 +1 Anti Tauon τ + +1-1 Tauon Nutrino ν τ 0 +1 Anti Tauon Nutrino ν τ 0-1 Consrvation Laws For a particl intraction to occur th following laws must b obyd, if ithr is violatd th raction will nvr b obsrvd (will nvr happn): : Must b consrvd (sam total valu bfor as th total valu aftr) Numbr: Must b consrvd Lpton Numbr: Must b consrvd : Consrvd in EM and Strong Intraction. Dosn t hav to b consrvd in Wak Intraction Exampls In pair production a photon of nrgy is convrtd into a particl and its antiparticl γ - + + Q 0-1 + +1 0 0 Consrvd B 0 0 + 0 0 0 Consrvd L 0 +1 + -1 0 0 Consrvd S 0 0 + 0 0 0 Consrvd Lt us look at bta plus dcay as w knw it at GCSE. A nutron dcays into a proton and rlass an lctron. n p + - Q 0 +1 + -1 0 0 Consrvd B +1 +1 + 0 +1 +1 Consrvd L 0 0 + +1 0 +1 Not Consrvd S 0 0 + 0 0 0 Consrvd This contributd to th sarch for and discovry of th nutrino. Numbr Rmindrs Thr may b a clu to th charg of a particl; π +, K + and + hav a positiv charg. It will only hav a baryon numbr if it IS a baryon. Msons and Lptons hav a Numbr of zro. It will only hav a lpton numbr if it IS a lpton. s and Msons hav a Lpton Numbr of zro. It will only hav a strangnss if it is mad from a strang quark. Lptons hav a strangnss of zro.
6 Forcs and Exchang Particls To know th four fundamntal forcs, thir rangs and rlativ strngths To know what ach forc dos and what it acts on To b abl to xplain what xchang particls ar Th Four Intractions Thr ar four forcs in th univrs, som you will hav com across alrady and som will b nw: Th lctromagntic intraction causs an attractiv or rpulsiv forc btwn chargs. Th gravitational intraction causs an attractiv forc btwn masss. Th strong nuclar intraction causs an attractiv (or rpulsiv) forc btwn quarks (and so hadrons). Th wak nuclar intraction dos not caus a physical forc, it maks particls dcay. Wak mans thr is a low probability that it will happn. Intraction/Forc Rang Rlativ Strngth Strong Nuclar ~10-15 m 1 (1) Elctromagntic ~10 2 (0.01) Wak Nuclar ~10-18 m ~10 7 (0.0000001) Gravitational ~10 36 (0.000000000000000000000000000000000001) Exchang Particls In 1935 Japans physicist Hidki Yukawa put forward th ida that th intractions/forcs btwn two particls wr causd by virtual particls bing xchangd btwn th two particls. H was working on th strong nuclar forc which kps protons and nutrons togthr and thorisd that thy wr xchanging a particl back and forth that carrid th forc and kpt thm togthr. This is tru of all th fundamntal intractions. Th gnral trm for xchang particls is bosons and thy ar fundamntal particls lik quarks and lptons. Ic Skating Analogy Imagin two popl on ic skats that will rprsnt th two bodis xprincing a forc. If A throws a bowling ball to B, A slids back whn thy rlas it and B movs back whn thy catch it. Rpatdly throwing th ball back and forth movs A and B away from ach othr, th forc causs rpulsion. Th analogy falls a littl short whn thinking of attraction, but bar with it. Now imagin that A and B ar xchanging a boomrang (bar with it), throwing it bhind thm pushs A towards B, B catchs it from bhind and movs towards A. Th forc causs attraction. Which Particl for What Forc Each of th intractions/forcs has its own xchang particls. Intraction/Forc Exchang Particl What is acts upon Strong Nuclar Gluons btwn quarks Pions btwn s Nuclons (Hadrons) Elctromagntic Virtual Photon d particls Wak Nuclar W + W Z 0 All particls Gravitational Graviton Particls with masss Borrowing Enrgy to Mak Particls Th xchang particls ar mad from borrowd nrgy, borrowd from whr? From nowhr! Yukawa usd th Hisnbrg Uncrtainty Principl to stablish that a particl of mass-nrgy ΔE could xist for a tim Δt as long as E. t h whr h is Planck s constant. This mans that a havy particl can only xist for a short tim whil a lightr particl may xist for longr. h is Planck s Constant, h = 6.63 x 10-34 J s. In 1947 th xchang particl of th strong nuclar intraction wr obsrvd in a cloud chambr. Lnding Mony Analogy Think of making xchang particls in trms of lnding sombody som mony. If you lnd sombody 50 you would want it paid back fairly soon. If you lnd sombody 50p you would lt thm hav it for longr bfor paying you back.
7 Th Strong Intraction To know why a nuclus dosn t tar itslf apart To know why a nuclus dosn t collaps in on itslf To know why th nutron xists in th nuclus Th Strong Intraction Th strong nuclar forc acts btwn quarks. Sinc Hadrons ar th only particls mad of quarks only thy xprinc th strong nuclar forc. In both s and Msons th quarks ar attractd to ach othr by xchanging virtual particls calld gluons. On a largr scal th strong nuclar forc acts btwn th Hadrons thmslvs, kping thm togthr. A pi-mson or pion (π) is xchangd btwn th hadrons. This is calld th rsidual strong nuclar forc. Forc Graphs Nutron-Nutron or Nutron-Proton Hr is th graph of how th forc varis btwn two nutrons or a proton and a nutron as th distanc btwn thm is incrasd. W can s that th forc is vry strongly rpulsiv at sparations of lss than 0.7 fm ( x 10 15 m). This prvnts all th nuclons from crushing into ach othr. Abov this sparation th forc is strongly attractiv with a pak around 1.3 fm. Whn th nuclons ar sparatd by mor than 5 fm thy no longr xprinc th SNF. Proton-Proton Th forc-sparation graphs for two protons is diffrnt. Thy both attract ach othr du to th SNF but thy also rpl ach othr du to th lctromagntic forc which causs two lik chargs to rpl. Graph A Graph B Graph C Graph A shows how th strong nuclar forc varis with th sparation of th protons Graph B shows how th lctromagntic forc varis with th sparation of th protons Graph C shows th rsultant of ths two forcs: rpulsiv at sparations lss than 0.7 fm, attractiv up to 2 fm whn th forc bcoms rpulsiv again. Nutrons Nuclar Cmnt In th lightr lmnts th numbr of protons and nutrons in th nuclus is th sam. As th nuclus gts biggr mor nutrons ar ndd to kp it togthr. Adding anothr proton mans that all th othr nuclons fl th SNF attraction. It also mans that all th othr protons fl th EM rpulsion. Adding anothr nutron adds to th SNF attraction btwn th nuclons but, sinc it is unchargd, it dos not contribut to th EM rpulsion.
8 Th Wak Intraction To b abl to writ th quation for alpha and bta dcay To know what a nutrino is and why is must xist To b abl to stat th changs in quarks during bta plus and bta minus dcay Alpha Dcay Whn a nuclus dcays in this way an alpha particl (a hlium nuclus) is jctd from th nuclus. A X 4 4 A Y A Z Z2 2 or X A 4 4 Z Z 2Y2H All th mittd alpha particls travlld at th sam spd, maning thy had th sam amount of nrgy. Th law of consrvation of mass-nrgy is mt, th nrgy of th nuclus bfor th dcay is th sam as th nrgy of th nuclus and alpha particl aftr th dcay. Alpha dcay is NOT du to th wak intraction but Bta dcay IS Bta Dcay and th Nutrino In bta dcay a nutron in th nuclus changs to a proton and rlass a bta particl (an lctron). Th problm with bta dcay was that th lctrons had a rang of nrgis so th law of consrvation of massnrgy is violatd, nrgy disappars. Thr must b anothr particl bing mad with zro mass but variabl spds, th nutrino. W can also s from th particl consrvation laws that this is a forbiddn intraction: n p Q: 0 +1 1 0 0 is consrvd Numbr B: +1 +1+0 1 1 numbr is consrvd Lpton Numbr L: 0 0+1 0 1 Lpton numbr is NOT consrvd Bta Minus (β ) Dcay In nutron rich nucli a nutron may dcay into a proton, lctron and an anti lctron nutrino. n p Q: 0 +1 1+0 0 0 is consrvd Numbr B: +1 +1+0+0 1 1 numbr is consrvd Lpton Numbr L: 0 0+1 1 0 0 Lpton numbr is consrvd In trms of quarks bta minus dcay looks lik this: dud uud d u which simplifis to: Q: ⅓ +⅔ 1+0 ⅓ ⅓ is consrvd Numbr B: +⅓ +⅓+0+0 ⅓ ⅓ numbr is consrvd Lpton Numbr L: 0 0+1 1 0 0 Lpton numbr is consrvd Bta Plus (β + ) Dcay In proton rich nucli a proton may dcay into a nutron, positron and an lctron nutrino. p n Q: +1 0+1+0 1 1 is consrvd Numbr B: +1 +1+0+0 1 1 numbr is consrvd Lpton Numbr L: 0 0 1+1 0 0 Lpton numbr is consrvd In trms of quarks bta plus dcay looks lik this: uud dud u d which simplifis to: Q: +⅔ ⅓+1+0 ⅔ ⅔ is consrvd Numbr B: +⅓ +⅓+0+0 ⅓ ⅓ numbr is consrvd Lpton Numbr L: 0 0 1+1 0 0 Lpton numbr is consrvd Th wak intraction is th only intraction that causs a quark to chang into a diffrnt typ of quark. In bta dcay up quarks and down quarks ar changd into on anothr. In som ractions an up or down quark can chang into a strang quark maning strangnss is not consrvd. During th wak intraction thr can b a chang in strangnss of ±1.
9 Fynman Diagrams To know what a Fynman diagram shows us To b abl to draw Fynman diagrams to rprsnt intractions and dcays To b abl to stat th corrct xchang particl Fynman Diagrams An Amrican Physicist calld Richard Fynman cam up with a way of visualising forcs and xchang particls. Blow ar som xampls of how Fynman diagrams can rprsnt particl intractions. Th most important things to not whn daling with Fynman diagrams ar th arrows and th xchang particls, th lins do not show us th path that th particls tak only which com in and which go out. Th arrows tll us which particls ar prsnt bfor th intraction and which ar prsnt aftr th intraction. Th wav rprsnts th intraction taking plac with th appropriat xchang particl lablld. Exampls Diagram 1 rprsnts th strong intraction. A proton and nutron ar attractd togthr by th xchang of a nutral pion. Diagram 2 rprsnts th lctromagntic intraction. Two lctrons rpl ach othr by th xchang of a virtual photon. Diagram 3 rprsnts bta minus dcay. A nutron dcays du to th wak intraction into a proton, an lctron and an anti lctron nutrino Diagram 4 rprsnts bta plus dcay. A proton dcays into a nutron, a positron and an lctron nutrino. Diagram 5 rprsnts lctron captur. A proton capturs an lctron and bcoms a nutron and an lctron nutrino. Diagram 6 rprsnts a nutrino-nutron collision. A nutron absorbs a nutrino and forms a proton and an lctron. Diagram 7 rprsnts an antinutrino-proton collision. A proton absorbs an antinutrino and mits a nutron and an lctron. Diagram 8 rprsnts an lctron-proton collision. Thy collid and mit a nutron and an lctron nutrino. Gtting th Exchang Particl Th aspct of Fynman diagrams that studnts oftn struggl with is lablling th xchang particl and th dirction to draw it. Look at what you start with: If it is positiv and bcoms nutral you can think of it as throwing away its positiv charg so th boson will b positiv. This is th cas in lctron captur. If it is positiv and bcoms nutral you can think of it as gaining ngativ to nutralis it so th boson will b ngativ. This is th cas in lctron-proton collisions. If it is nutral and bcoms positiv w can think of it ithr as gaining positiv (W+ boson) or losing ngativ (W boson in th opposit dirction). Work out whr th charg is going and labl it.