Mojtaba Mohammadi Najafabadi School of Particles and Accelerators, IPM Aban 22- IPM Workshop on Electroweak and Higgs at the LHC

Transcription

1 Electroweak studies for the LHC Mojtaba Mohammadi Najafabadi School of Particles and Accelerators, IPM Aban 22- IPM Workshop on Electroweak and Higgs at the LHC 1

2 Why accelerator? We live in a cold and empty universe: only the stable relics and left overs of the Big Bang remain. The unstable particles have decayed away with time, and the symmetries have been broken as the universe has cooled. But every kind of particle that ever existed is still there, in the equations that describe the particles and forces of the universe. The vacuum knows about all of them. We can use accelerators to make the equations come alive, by pumping sufficient energy into the vacuum to create the particles and uncover the symmetries that existed in the earliest universe. 2

3 The LHC is installed in a tunnel 3.8 m in diameter, buried 50 to 175 m below ground. Lake Geneva Geneva French Jura Mts The proton beams are injected at 450 GeV and then accelerated to 7 TeV 3

4 The accelerator Electric waves speed particles up Magnets bend them in a circle 4

5 Collision points At four places the beams intersect 5

6 The Large Hadron Collider (LHC) at CERN Proton-proton collider in the former LEP tunnel at CERN (Geneva) Highest ever energy per collision 14 TeV in the pp-system Conditions as s after the Big Bang 4 experiments: ATLAS CMS LHC-B specialised on b-physics ALICE specialised for heavy ion collisons Constructed in a worldwide collaboration 6

7 The Large Hadron Collider LHC CMS ATLAS 7

8 Physics at Proton Colliders Protons are composite, complex objects - partonic substructure - quarks and gluons Interesting hard scattering processes quark-(anti)quark quark-gluon qluon-gluon However, hard scattering (high momentum transfer) processes are only a small fraction of the total cross section - total inelastic cross section 70 mb (huge!) - dominated by events with small momentum transfer 8

9 Proton-Proton Collisions Proton beam can be seen as beam of quarks and gluons with a wide band of energies The proton constituents (partons) carry only a fraction 0 x 1 of the proton momentum The effective centre-of-mass energy ŝ is smaller than s of the incoming protons To produce a particle of mass mass LHC Tevatron 100 GeV x x TeV x Note: the component of the parton momentum parallel to the beam can vary from 0 to the proton momentum (0 x 1) the variation of the transverse component is much smaller (of order the proton mass) 9

10 Parton Density Functions How do the distributions of the x-values look like? Measured at HERA in ep-scattering, e.g.: u- and d-quarks at large x-values gluons dominate at small x large uncertainties for gluons 10

11 Hard Sub-processes Three possible hard scattering processes: qq: quark-quark, quark-antiquark, antiquarkantiquark qg: quark-gluon, antiquark-gluon gg: gluon-gluon at the Tevatron (2 TeV) quark-antiquark is dominant at the LHC (14 TeV) gluon-gluon is dominant the LHC is really a gluon-gluon collider! 11

12 Parton Density Functions at the LHC LHC is a proton-proton collider But fundamental processes are the scattering of Quark Antiquark Quark Gluon Gluon Gluon y = rapidity Examples: qq W l gg H need precise PDF(x,Q 2 ) + QCD corrections (scale) 12

13 Proton-Proton Collisions at the LHC proton bunches separated by 7.5 m collisions every 25 ns = 40 MHz crossing rate protons per bunch at 10 34/ cm 2 /s 25 pp interactions per crossing pile-up 10 9 pp interactions per second!!! in each collision 1600 charged particles produced enormous challenge for the detectors 13

14 Experimental Signatures 1. Hadronic final states, e.g. quark-quark no high p T leptons or photons in the final state holds for the bulk of the total cross section 2. Lepton/photons with high p T, example Higgs production and decay Important signatures for interesting events: - leptons and photons - missing transverse energy 14

15 A typical (interesting) event For EWK physics: Try to extract the information about the sub-process 15

16 Detector Design Aspects good measurement of leptons (high p T ) muons: large and precise muon chambers electrons: precise electromagnetic calorimeter and tracking good measurement of photons good measurement of missing transverse energy (E T miss ) requires in particular good hadronic energy measurements down to small angles, i.e. large pseudo-rapidities (η 5, i.e. θ 1 ) in addition identification of b-quarks and τ-leptons precise vertex detectors (Si-pixel detectors) Very important: radiation hardness e.g. flux of neutrons in forward calorimeters n/cm 2 in 10 years of LHC operation 16

17 Online Trigger Trigger of interesting events at the LHC is much more complicated than at e + e - machines interaction rate: 10 9 events/s max. record rate: 100 events/s event size 1 MByte 1000 TByte/year of data trigger rejection 10 7 collision rate is 25 ns (corresponds to 5 m cable delay) trigger decision takes a few µs store massive amount of data in front-end pipelines while special trigger processors perform calculations 17

18 Jets Initial quark Jet The force between two colored objects (e.g. quarks) is ~independent of distance Therefore the potential energy grows (~linearly) with distance When it gets big enough, it pops a quark-antiquark pair out of the vacuum These quarks and antiquarks ultimately end up as a collection of hadrons We can t calculate how often a jet s final state is, e.g. ten p s, three K s and a L. Fortunately, it doesn t matter. We re interested in the quark or gluon that produced the jet. Summing over all the details of the jet s composition and evolution is A Good Thing. Two jets of the same energy can look quite different; this lets us treat them the same What makes the measurement possible & useful is the conservation of energy & momentum. 18

19 The CMS Detector Inner Detector: Silicon pixels and strips Preshower: Lead and silicon strips EM Calorimeter: Lead Tungstate E E 5 % ( GeV ) 2 % Hadron Calorimeters: Barrel & Endcap: Cu/Scintillating sheets E 65 % E ( GeV Forward: Steel and Quartz fibre ) 5 % Muon Spectrometer: Drift tubes, cathode strip chambers and resistive plate chambers Magnet: 4T Solenoid

20 Polar Angle: θ Detector Coordinates Detector Coordinates psudeorapidity ln tan 2 axial angle: : p CMS z-axis φ p 20

21 Some definitions We never know total longitudinal momentum in any event. Total transverse momentum of all particles is zero. transverse momentum p T = p sin transverse enery E T = E sin pseudo-rapidity = -ln tan(/2) missing transverse energy E miss T = E Distance in pseudorapidity - azimuthal angle space(used in jet cone algorithm) DR=(D ) 2 +(D) 2 Existence of minimum bias events. LHC: inelastic, non-diffractive 70mb 23 pile-up/crossing@10 34 Tevatron RUN-II: 6 pile-up/crossing(poisson) 21

22 dn/d distribution rapidity y 1 2 E ln E p p pseudo-rapidity = -ln tan(/2) z z cf. ATLAS detector tracker < 2.5 calorimeter <

23 How They Work Particles curve in a central magnetic field Measures their momentum r p qb Particles then stop in the calorimeters Measures their energy Except muons, which penetrate and have their momenta measured a second time. Different particles propagate differently through different parts of the detector; this enables us to identify them. 23

24 Question An electron and a positron were produced when a particle and its antiparticle collided head-on, perpendicular to this screen. What conservation law APPEARS to have been broken? electron positron 24

25 Transverse Quantities Colliding partons have small momentum transverse to beam We detect all interactions transverse to the beam part p x 0 p 0 part y Any missing momentum in x,y plane is attributed to the neutrino Or other non-interacting particles eg neutralinos Transverse momentum: p p p 2 2 T x y Missing E T direction 25

26 b-tag Vertex detector b-quarks have a long lifetime: t(b) ~ 1.5ps (ct~450mm) B-tagging using displaced vertices CDF RUN2a: b = 60%, c = 25%, j = 0.2% RUN2b: b = 70%, c = 10%, j = 0.02% Soft lepton tagging identifies lepton in semi-leptonic b(or c) decays leptons are softer less isolated than from W/Z decay. ATLAS: b = 60(50)% for low (high) lumi. c = 10%, j = 1% 26

27 Cross Section of Various SM Processes Low luminosity phase /cm 2 /s = 1/nb/s approximately 10 8 pp interactions 10 6 bb events 200 W-bosons 50 Z-bosons 1 tt-pair will be produced per second and 1 light Higgs per minute! The LHC is a b, W, Z, top, Higgs, factory! The problem is to detect the events! 27

28 Electroweak Physics 28

29 Electroweak Physics (W and Z Bosons) W and Z bosons were discovered in proton-antiproton collisions 1983: UA1 & UA2 at the SppS collider at CERN Examples of early W/Z events How do W/Z events look like at proton colliders? Use leptonic decays (electrons & muons) (hadronic decay can not be extracted from Bkgs.) W lν high p T lepton + missing E T Z ll 2 oppositely charged, high p T leptons 29

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31 W/Z Physics at the LHC Very clean selection of W and Z boson possible e.g. CMS study of W eν and Z ee Recall rates (initial phase /cm 2 /s): 200 W/s 20 W eν /s 50 Z/s 1.5 Z ee /s plus the same rates for muon decays! W and Z events will provide an excellent tool for detector calibration 31

32 Mass of the W precision measurement at proton colliders possible results competitive to LEP experiments Latest results on m W define transverse mass from missing E T main challenge: electron/muon energy scale use Z ee, µµ events and precise m Z from LEP rel. precision on m W Tevatron results will improve with increasing Run II statistics 32

33 W-mass Cuts: For EW fits: Isolated charged lepton p T > 25 GeV < 2.4 Missing transverse energy E Miss T > 25 GeV No jets with p T > 30 GeV Recoil < 20GeV DM W Dm t Sources of Uncertainty: Statistical uncertainty pp W + X = 30 nb (l= e,m) W l l 3 x 10 8 events < 2MeV for 10 fb-1 Systematic Error Detector performance Physics The selection efficiency is about 15% for the electron channel and 25% for the muon channel Relies on good modelling of detector and physics in Monte Carlo 33

34 Detector energy scale E measured = GeV for all calorimeter cells perfect calibration To measure M w to ~ 20 MeV need a energy scale to 0.2%, ( E electron = 100 GeV then GeV < E measured < GeV ) 34

35 W Mass at the LHC CMS: detailed study of statistical and systematic errors 1 fb-1: early measurement 10 fb-1: asymptotic reach, best calibrated & understood detector, improved theory etc. 35

36 Top Physics Why is the top quark so interesting special? - by far the heaviest fermion - could provide window to New Physics (mass generation) - discovered 1995 at the Tevatron O(100) events observed in Run I - still we know very little about it (mass) would like to measure all other properties - top has a very short lifetime the only quark that decays before forming hadrons can determine spin, polarisation from ist decay products 36

37 Electroweak Precision Measurement = ±

38 Electroweak Precision Measurement Motivation to improve: M W p 2 G EM F 1/2 sin W 1 1 Dr f (m top2, log m H ) c 3G W mmt 11GmMW Dr sw 8 2p 12 2p Using the measured M top and M W Δr (M 2 top)=-0.031±0.002 Dm W Dm top to get similar errors Dm top < 2 GeV (LHC) requires Dm W 15 MeV M log M 2 H 2 W constrains m H to 25% -- if/when Higgs found: check consistency of theory 38

39 A calculation leads to: M W ( GeV ) m t 0.542[( 178 ) 2 1] m H ln( 100 )... 39

40 Di-Boson Production at the LHC very interesting: WW,WZ,ZZ final states not yet observed at the Tevatron test triple gauge boson couplings (TGC) γww and ZWW precisely fixed in SM γzz and ZZZ do not exist in SM! SM New physics deviations from SM are amplified with E also Wγ and Zγ final states can be used ZZ e+e e+e WZ 3 leptons 1 fb -1 sufficient to observe both processes 40

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