Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 298.882 LHC PhysicsExperimental Methods and Measurementsb Hadron Lifetimes and Other Essentials[Lecture 18, April 13, 2009]C.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 2Organization General question●any? Project 2●nobody handed in yet :-) Project 3●is available on the Web, lectures will enlighten the subject more, but you can get a full prototype going●due May 2.... Final project: the conference●May 19, 12:00 Kolker Room, is that ok?C.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 3Lecture Outline b hadron lifetimes and other essentials●motivation and theoretical introduction●methodology and experimental challenges●existing measurements●project 3: outline Convention used in the community●b – quark of down-type pertaining to the third quark family●B – meson containing one b antiquark and a non b quark●b hadron – B meson or baryon containing a b quark●charge conjugate states are generally implicitC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 4Motivation Gain access to CKM matrix elements●B mesons have access to Vcb, Vub, Vtd, Vts●remember the CKM matrix Conventionally: down types primed●d' is mixture of d, s and b as described by CKM matrixlifetimesmixingC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 5Quarks and Hadrons Lowest order: bare b quark width – spectator picturereceives modifications by quark andgluon cloudsC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 6Quarks and Hadrons Hadronic effects on the width (Γ)●Pauli interference (PI), causes τu > τd (not obvious)destructive interferenceC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 7Quarks and Hadrons Hadronic effects on the width (Γ)●weak annihilation (WA)●obviously opens channel for u,c●shortens τu,c●weak exchange (WE) (baryons: W scattering)●not obvious: spin conservation●shortens τbaryonC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 8Some Theory Heavy Quark Expansion (HQE) Details●HQE assumes that quark is heavy: little influence from light quark●PI, WA, and WE correction systematically included●perturbative calculations give cn(f) ~ 1/mQn-3●lattice QCD (or sum rules): <HQ|On|HQ> ~ ΛQCD●measurements are so precise, theory is well behind.... so we do not measure CKM matrix but test HQEC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 9Test Heavy Quark Expansion Measure all b hadron lifetimes●form ratios for comparisons●HFAG pages (http://www.slac.stanford.edu/xorg/hfag)C.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 10Motivation to Measure Δm, sin2β, γ, α Gain direct access to CKM matrix elements●4 fundamental parameter Standard Model parameters●problematic is always the non-perturbative QCD part●works for Bd and Bs: frequency direct function of |Vtq|2 Full calculation of above diagrams possible but●large number of corrections because of quark/gluon cloud●find quantities which are least affected●find ways to intrinsically correct for hadronic effectsC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 11Motivation to Measure Δm, sin2β, γ, αMeasure triangle in all possible independent ways and confirm its closureC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 12Theory Uncertainties - ΔmIain Stewart (MIT) is an expert of this, ask himC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 13Bag Model and Form Factor The MIT bag model●3 non-interacting quarks inside spherical cavity●condition: vector current disappears on boundary●non-interacting → asymptotic freedom●vector current zero on boundary → confinement●model adds a factor to the predictions which depends on the particular b hadron type Bh Form factors●describe more general the shape of the wave functions●hadronic particles: wave function of the contained quarks Non-trivial to derive here, remember their meaningC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 14Experimentally .... Various ways to measure lifetimes are possible●inclusively reconstructed decays, ex. B → l X●semi-exclusively reconstructed decays, ex. B → l D X●exclusively reconstructed decays, ex. B0 → D-- π+(π+π--)●lots of data: exclusive modes (theory uncertainties small)from PDG web siteC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 15First Lifetime Measurement Sample of high pT leptons●lepton track impact parameter●sign determined by jet direction●155 muon events●113 electron events Experiments●MAC: 1.8±0.8±0.4 ps●Mark II: 1.2±0.4±0.3 ps Experimental details●e+e-- at 29 GeV●109 pb--1●3500 bbbar pairsPRL51(1983)1022PRL51(1983) 1316C.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 16Experimental Ingredients Decays of particle follow simple exponential●N(t) = N0 exp(-t/τ) Need to measure●proper time of B decay●geometric displacement in r-phi plane:●momentum in r-phi plane: pT(B)●proper time: ct = m(B) Lxy(B) / pT(B)●distributions of number of candidates per proper timeC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 17Experimental Ingredients In the data see signal and background●background has no lifetime (some accidental lifetime)●mostly prompt, some mis-reconstructed, some real displaced●proper time is smeared with detector resolution●use event by event vertex resolution as measuredC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 18Experimental Ingredients What we see in the reconstructed mass ●flat background: is mostly short lived●Gaussian peak of signal Significance of mass plot●mass does not determine lifetime●independent handle on signal and background:●sideband represent background●signal area can be cleanly corrected●likelihood does it all in one shot●for given m probability of signal or background is easily derivedC.Paus, LHC Physics: b Hadron Lifetimes and Other Essentials 19Experimental Ingredients What we see in the proper time distribution●plot shown in logarithmic scale (y): expect exponential●expect long exponential tail consistent with signal lifetime (linear in log scale)●expect a pronounced peak at zero ct: prompt background●left side