day1- determining particle properties Peter Wittich Cornell University

Transcription

1 day1- determining particle properties Peter Wittich Cornell University

2 One view of experiment xkcd, looks like ATLAS! CMS is clearly different. :) 2

3 my goal for these lectures give you a glimpse of experiment difference between what we think about and what you think about give you tools to read an experimental paper give you tools to listen to an experimental seminar maybe help you learn how to talk to your experimental colleagues what do they do all day, if not giving helicopters cancer? all this with the specific case of the LHC energy frontier program 3

4 ask me questions! I can t promise to be able to answer them all but I ll try it is more fun for me when there is give-and-take. 4

5 goals of measurement reconstruct the four-vectors of individual events p T, η, Φ, ET -- the language of exp. HEP particle identification e,γ, μ, jets (b), τ -- all you have missing energy & other event information understand the composition of events sources? known and unknown? extract physics parameter cross section, coupling constant but first we need to collide the beams -- accelerators. 5

6 accelerators two types: linear vs circular Linear: cathode ray TV s to SLC (SLAC) ILC (future) RF accelerators radio-frequency cavities kick beams superconducting one-shot acceleration, no synchrotron radiation 6

7 superconducting RF wall power losses due to heating in normal RF cavities superconducting cavities, high Q ~ exotic materials, often niobium tin compounds 7

8 circular accelerators synchrotrons: many passes at acceleration vary B to accelerate particles on fixed path time structure corresponding to RF frequency synchrotron radiation limits for light particles, but not for e.g. protons ( E ) 4 P m 8

9 Colliding beams modern accelerators are synchrotrons energy frontier machines are colliding beam machines: fixed target: E cm = 2E bean m target colliding beams: E cm =2E beam important parameters in colliders are the energy of the beams and the rate of collisions luminosity R = dn dt = Lσ 9

10 units of luminosity [L]=1/cm 2 1/s 1/(cross section x time) [integrated luminosity] = unit of 1/cross section easy conversion from data size to number of events for a given process in 100/pb of data, LHC will produce this many top quark pairs (σ = 800 pb) some numbers: LHC lumi goal 100/fb N = σ L dt = 800pb 100/pb = 80k what we observe in the experiment is a different story 10

11 Making luminosity L = f n 1n 2 4πσ x σ y = f n 1 n 2 4 ɛ x β xɛ y β y important factors are frequency f number of particles in a bunch (n1,n2) size of the beam in the transverse plane (σx, σy) β*, ε - accelerator language re size β* - beam optics: beta star ε - bunch preparation: emittance Thus, to achieve high luminosity, all one has to do is make high population bunches of low emittance to collide at high frequency at locations where the beam optics provides as low values of the amplitude function as possible. -- Particle Data Group 11

12 LHC machine parameters f = khz Nbunch = 2808 protons/bunch: 1.7 x bunch spacing 25 ns (75 ns) normalized εn = 3.75 μm, β*=0.55 m bunch length 7.5cm all together lead to L = 1.0 x 1034 cm -2 s -1 12

13 hep units and measures GeV, cm, ns, barns ET, PT, MT transverse plane η = -ln tan θ/2 pseudo-rapidity ΔR missing ET -- MET E T = E sin θ η = ln tan θ 2 R φ 2 + η 2 E T 13

14 transverse plane we focus on the transverse plane opposite of forward high q 2 interactions B field: measure (transverse) momentum curvature of charged particle transverse energy, mass analogs E T = E sin θ m 2 T = E 2 T p 2 T 14

15 η - pseudo-rapidity ) y 1 2 ln ( E + pz ( rapidity ) E ( p ) z = tanh 1 p z E η = ln tan θ 2 dn/dy is constant under boost but η = - ln tan(θ/2) ~ y for p m and better yet we can calculate η w/o knowing the mass of the particle 15

16 y z x η = 0 η = 1 η = 2 η = 3 interaction point η = large 16

17 ranges for η Typical η values for general-purpose detectors are η <2 ( central ) 2< η <5 ( forward ) η=1 η=3 D0 17

18 delta R R φ 2 + η 2 used to measure distance in HEP events used to group particles for jet reconstruction used to determine proximity between particles isolation in calorimeter or tracking chambers typical values: 0.5, 0.7 (jet algorithms) 18

19 missing ET E T i E i Tn i one of the most interesting and most difficult quantities in experimental HEP calorimeter towers junk collector hadronic energy scale muons are missing (MIP) 19

20 why transverse? hadron collisions: you don t know the initial state proton is not what scatters pz of partons that are in hard scatter? to good approximation: pt i = 0 momentum conservation in transverse plane final state: only get estimate of vector sum pt f 20

21 example: mt in W μν events / 0.5 GeV 1000 CDF II preliminary $ L dt -1 # 200 pb 500 M W = (80349 ± 54 stat ) MeV 2 " /dof = 59 / m T (µ!) (GeV) E 2T p2t = m T 2p µ T E T (1 cos φ) for pt W =0 edge at mw/2 ( jacobian peak ) 21

22 MET = junk anything going wrong produces MET need careful work to understand samples 22

23 now our protons collide initial state radiation (ISR) hard scattering of partons (parton density func) final state radiation (FSR) underlying event p p 23

24 PDF s σ(pp CX)= ij f p i (x i,q 2 )f p j (x j,q 2 )ˆσ(ij C)dx i dx j Parton model allows factorization of QCD into two parts, long and short distance long: dynamics of hadrons in quark short: describes the hard event - calculable PDF s determined with input experiment neutrino DIS, ep, ppbar, pp scale PDF s from one energy scale to next, allowing us to apply these results to LHC (Q 2 ) MRST, CTEQ collaborations uncertainties 24

25 hadronization Consider: pp t t t Wb lνb t W b q q b consider final state above hadronization: dressing of the colored particles jets! (later: jet algorithms) collimated spray of particles electrons, γ s, and hadrons neutral and charged particles 25

26 underlying event R. Field everything except the hard scatter is called the underlying event includes initial state, final state radiation remnants of the beam particles 26

27 now on to the detector PDG end view layers 27

28 passage of particles PDG same segmentation, but now unrolled need to understand physics underlying these design choices 28

29 bethe-bloch de dx = kz2 Z A 1 β 2 [ 1 2 ln 2m ec 2 β 2 γ 2 T max I 2 β 2 δ 2 ] describes average ionization loss of relativistic particles main source of loss for most particles besides electrons does not include radiative corrections will start to become important at the LHC (π energies above ~10 GeV e.g.) 29

30 30 PDG

31 min at beta γ ~3 example: MIP in silicon de/dx: 1.6 MeV/(g/ cm 2 ) x 2.33 g/cm 3 = 3.7 MeV/cm (not much!) 31

32 electron/photon T. Dorigo radiative losses cannot be ignored (not π and μ) e: mostly bremsstrahlung, γ mostly pair prod. electromagnetic shower, radiation length X 0 (bethe-bloch different too ) 32

33 electrons & γs Positrons Lead (Z = 82) 0.20 PDG de dx (X 0!1 ) 1 E! Electrons Ionization Møller (e! ) Bhabha (e + ) Bremsstrahlung (cm 2 g!1 ) Material X0[cm] Ec[MeV] Pb Fe Positron annihilation E (MeV) characteristic length that describes the energy decay of a beam of electrons X 0 = g cm 2 A Z(Z + 1) ln(287/ Z) ; de dx = E X 0 33

34 muons SNO/ Chris Kyba for HEP purposes muon is stable particle cτ ~ 700 m no strong interaction - only MIP E c scales like m 2 Fe: E c=890 GeV! μ s are very penetrating TeV μs after 5000 mwe muon id 34

35 multiple scattering x/2 x s plane! plane y plane " plane multiple Coulomb scattering off nuclei well approximated by Gaussian with above width depends on 1/beta, x/x0 important for tracking accuracy (large scatters too) θ 0 = 13.6MeV βcp z x/x 0 [ ln(x/x 0 )] 35