Descent into the Descent into the Descent into the Descent into the ProtonProtonProtonProtonA Journey Inside an A Journey Inside an A Journey Inside an A Journey Inside an Elementary ParticleElementary ParticleElementary ParticleElementary ParticleSaturday Morning Physics Saturday Morning Physics ----Texas A&M UniversityTexas A&M UniversityDr. Rainer J. FriesMarch 22, 2008Descent into the Proton2Zooming in on the World around usDescent into the Proton3AtomsDemocritus, Greek philosopher ~ 400 B.C:“All matter is made up of very small indivisible elements”He called them ‘atomos’.19thcentury chemistry confirms:there are only 92 different ‘elements’, from hydrogen H to uranium U. Everything around us is built from combinations of these elements.Descent into the Proton4AtomsSandia National LabU of Oregon ChemistryToday: we can make atoms visibleSize of the smallest atom (hydrogen):0.000 000 000 1 m (meter) = 10-10m = 1 AngstromHow is it possible to see such tiny structures?Descent into the Proton5Scattering ExperimentsOur vision: the eye collects light reflected from objects and our brain processes the informationUse this principle:Shoot a ray of light or particles at an object. Measure the scattered rays with a detector.Resolution of the probe (light, particle) is important:The wavelength must be smaller than the size of the structure to probe.Light: wavelength 4000 – 7000 Angstrom, too large to see an atom.Better: X-rays, electrons Descent into the Proton6ElectronsWhat is electric current?In wires there seems to be a flow of very small quantities of negative electric charge carried by tiny particles. They are called electrons e–.In fact these ‘quanta’ can be extracted from metals by heating them up → cathode rays.Basic properties of electrons, measured around 1900:Electric charge is –e. e = 1.6 × 10-19C is called the fundamental charge.Mass = 1/2000 u = 511 keV. 1 u is the mass of the hydrogen atom.J. J. Thomson (1897):Electrons are small parts of atoms.The first ‘subatomic’particle was discovered.Descent into the Proton7Taking a Look inside an AtomAtoms are neutral. If they contain electrons there must be an equal amount of positive charge. How does an atom look on the inside?1) Shooting at a bag of beans2) Bag of equal weight but stuffed withcotton and a few small lead beadsCompare the following two “scattering experiments”: Professional scientist, closed lab, do not attempt!Obviously the possible scattering angles of the bullets are different in both cases. 1) Only small angles possible.2) Some bullets are scattered at large angles.Descent into the Proton8Taking a Look inside an AtomIn 1911 E. Rutherford did this famous experiment with α-particles instead of bullets. His target were gold atoms.The positive charge in an atom and most of its mass is concentrated in a tiny, very dense center, the nucleus.Rutherford’s result was similar to the second scenario! Gold atomsDescent into the Proton9The NucleusMore than 99% of the mass of an atom is in the nucleus, which is more than 10,000 times smaller than the atom, about 1 – 10 fm (Fermi).1 fm = 10-5Angstrom = 10-15m.A cloud of electrons orbits the nucleus, held in placeby the mutual attraction of the electric charges.Most of the atom is just empty space! But with a strong electromagnetic field present.Nuclei are made up of two particles:Protonsp: positive charge +e, mass ≈ 1uNeutronsn: neutral, roughly the same mass as pProtons and neutrons are kept together by a new force: the strong force.Descent into the Proton10Atoms: A Modern ViewQuantum mechanics tells us that electrons can not be localized. Only a probability can be given for their current position.⇒ Atomic orbitals.Rutherford, BohrHeisenbergSchroedingerDescent into the Proton11ParticlesWe distinguish particles by their …participation in strong interactionsYES: they are called hadronse.g. proton, neutronNO: they are called leptonse.g. electronspin= Quantized angular momentum (can take values 0ℏ, ½ ℏ, 1 ℏ, 3/2 ℏ, 2 ℏ, etc)Electrons, protons, neutrons: spin ½ ℏParticles with integer spin are called bosons.Particles with half-integer spin are called fermions.massusually measured in electronvolts (eV)1 u ≈ 0.939 GeV (Gigaelectronvolts,Giga = Billion)Electrons, protons and neutrons are fermions.electric chargepositive or negativeusually in multiples of eDescent into the Proton12The Hadron ZooIn 1940 only 5 elementary particles were known: proton, neutron,electron, muon and positron. With the advent of accelerators at the end of the decade a big ‘zoo’ of hadrons was discovered. Remember: hadrons are subject to the strong force, like the proton They could be grouped into one of two categories:  Heavier baryons, whose total number is always conserved.E.g. protons, neutrons Lighter mesons, which can decay into particles which are not hadrons.E.g. pions, kaonsToo many! Maybe hadrons are not elementary particles after all?Descent into the Proton13The Hadron ZooGell-Mann & Zweig (1964): the systematicsof hadrons could be understood if hadrons consisted of combinations of smaller, more fundamental particles. Those must be fermions (spin-½) and have fractional charges.Gell-Mann called them quarks. Nobody believed them.Eventually it was found that hadrons with similar properties can be grouped into multiplets.Compare: periodic table of the elements for atoms.Descent into the Proton14Deep-Inelastic Scattering (DIS)γE’e-Q2How could this hypothesis be tested? A new Rutherford experiment with better resolution!Introducing: deep-inelastic scattering (DIS),shoot electrons at protons with Ecm> 1 GeVMeasurement: deflection angle θfinal electron energy E’θ and E’ can be rewritten as two quantities known as x and Q2.x = fraction of the proton energy carried by what is hit inside the proton.Q2= resolution of the photon.pxEE.g. proton as a whole: x=1.If it consisted of three equal parts with the same energy, each of those would have x = 1/3.Descent into the Proton15Deep-Inelastic Scattering (DIS)+=Ω′)2/(cos),(2)2/(sin),(2)2/(sin2222222122θθθασQQxMxFMQxFEdEdd h“Structure functions”F1and F2know about the structure of the proton. DIS scattering formula:(cross section as function of θ and E’)Different predictions had been made.For the quark model (i.e. proton is a loose collection of point-like spin-½ fermions):1) F1, F2don’t depend on