High Energy Physics and the ATLAS Detector

Transcription

1 igh Energy Physics and the ATLAS Detector ABSTRACT igh Energy Physics deals with the study of the ultimate constituents of matter and the nature of the interactions between them. This study is fundamental and draws much interest. igh energy is reuired to probe the very small distance scales associated with elementary particles. Since many of the fundamental particles have large masses, high energy is also reuired to create them for study. Modern igh Energy Physics experiments typically reuire giant particle accelerators and sophisticated detectors. Although the Standard Model of particle physics offers a very successful description of the interactions of the fundamental constituents of matter at the smallest scales and highest energies accessible to current experiments, there remain many unanswered and important uestions. What is an electron made of? What is the origin of mass? What is the nature of Dark Matter? Is nature only made of three spatial dimensions? The Large adron Collider (LC), soon to start operation at CERN, Geneva, will produce 14 TeV energy proton-proton collisions, opening a new and unexplored window into the fabric of nature. Canadian physicists are contributing to the construction and operation of ATLAS, one of two multi-purpose particle detectors that will harvest the LC collisions. This talk will present some of the most exciting uests of physics at the energy frontier. The ATLAS detector will also be presented, with an emphasis on the experimental challenges at the LC. Michel Lefebvre CUPC 24, Victoria, BC 1

2 La physiue des hautes énergies et le détecteur de particules ATLAS. ABSTRACT La physiue des hautes énergies étudie les constituants de base de la matière et la nature de leurs interactions. Ce sujet est fondamental et captive beaucoup de scientifiues. De hautes énergies sont reuises pour sonder les petites dimensions associées au monde des particules élémentaires. Plusieurs de ces particules ont une masse importante; de hautes énergies sont donc aussi reuises pour les créer et les étudier. De nos jours, la physiue expérimentale des hautes énergies utilise en général des accélérateurs de particules géants ainsi ue des détecteurs très sophistiués. Le Modèle Standard de la physiue des particules offre une description très précise des interactions entre les particules élémentaires aux plus petites dimensions et aux plus hautes énergies accessibles aujourd'hui. Mais plusieurs uestions importantes demeurent sans réponses. De uoi est fait un électron? Quelle est l'origine de la masse? Quelle est la nature de la matière noire? N'y a-t-il ue trois dimensions spatiales? Le grand collisionneur de hadrons (LC pour Large adron Collider), ui bientôt débutera son opération au CERN, à Genève, produira des collisions proton-proton de 14 TeV d'énergie, ce ui permettra d'atteindre des régions nouvelles et inexplorées de la nature. Plusieurs physiciens et physiciennes du Canada contribuent à la construction et à la mise en opération d'atlas, un des deux principaux détecteurs de particules ui étudieront les collisions produites au LC. Ce colloue présentera uelues uns des sujets de recherche les plus intéressants de la physiue aux plus hautes énergies. Le détecteur ATLAS et les difficultés expérimentales de la physiue au LC seront également traités. Michel Lefebvre CUPC 24, Victoria, BC 2

3 La physiue des hautes énergies et le détecteur de particules ATLAS. igh Energy Physics and the ATLAS Detector CUPC 24, Victoria 6 Nov 24 Michel Lefebvre Physics and Astronomy University of Victoria

4 igh Energy Physics Motivation: Understanding nature s fundamental constituents and forces Michel Lefebvre CUPC 24, Victoria, BC 4

5 Normal Matter 1 1 m 14 1 m 15 1 m electrons uarks NIST Image 7 nm 7 nm from a scanning tunneling microscope showing a single zig-zag chain of Cs atoms (red) on the GaAs(11) surface (blue). Michel Lefebvre CUPC 24, Victoria, BC 5

6 Scattering Experiment Wave detector Source of wave Object to investigate Scattered wave The wave can resolve features about the size of its wavelength Michel Lefebvre CUPC 24, Victoria, BC 6

7 Matter Waves Einstein de Broglie relations particle aspect p = h λ E = hν h is Planck s constant wave aspect Fundamentally, nature is made of particles. Their dynamics is governed by matter waves euations, like Schrödinger, Dirac and Maxwell euations Michel Lefebvre CUPC 24, Victoria, BC 7

8 Rutherford Scattering K 5 MeV m 4 GeV/c c MeV fm α particle λ= h = h = 2π c 6 fm p 2 2mK 2mc K Rutherford was able to discover that most of the atom s mass was in its nucleus Atom and nucleus (not to scale!) The matter wave can resolve features about the size of its wavelength Michel Lefebvre CUPC 24, Victoria, BC 8

9 Relativistic Dynamic E p =γmc =γmv 2 2 E pc E = = ( ) 2 ( 2 ) pc + mc v c massless particles carry momentum!! m= E = pc and v = c E = mc m= E/ c 2 2 euivalence of mass and energy!! 2 Albert Einstein γ 1 ( v ) c 2 1/ 2 relativistic particles 2 E mc E = pc and v = c Natural units: = c = GeV fm = 1 Michel Lefebvre CUPC 24, Victoria, BC 9

10 Particle Accelerators igh energy particle accelerators have been used to probe the structure of the proton. A Proton is found to have a size and an electric charge distribution. Probing deeper, with higher energy probes, it is found to be made mainly of three uarks! 15 1 m Michel Lefebvre CUPC 24, Victoria, BC 1

11 Colliding Particles Fixed target : available energy E m 2mE Collider : available energy 2E E E Michel Lefebvre CUPC 24, Victoria, BC 11

12 Detecting Particles Particle detector: Ideally, identify, for each particle produced in each collision, its type (mass, electrical charge, spin, other uantum numbers), and its 4-vector (energy, p x, p y, p z ) at the interaction point. In practice, a good detector will measure only a subset of all the available information for each event. Data analysis techniues are then reuired to best reconstruct each event. Michel Lefebvre CUPC 24, Victoria, BC 12

13 pp : SppS at CERN ee + - A few Colliders with a maximum beam energy of 315 GeV Probing nature down to m!! : Large Electron Positron collider at CERN with a maximum beam energy of 15 GeV Probing nature down to m!! pp : Tevatron at Fermilab with a maximum beam energy of 98 GeV Probing nature down to m!! W, Z particles discovery!!! 3 families of leptons!!! Precision measurements top uark discovery!!! Michel Lefebvre CUPC 24, Victoria, BC 13

14 Elementary Particles M t M of 76 Os M W M of 38 Sr Michel Lefebvre CUPC 24, Victoria, BC 14

15 Typical Detector Components Michel Lefebvre CUPC 24, Victoria, BC 15

16 Typical Detector Michel Lefebvre CUPC 24, Victoria, BC 16

17 The Standard Model Particles fermions leptons uarks ν e e u d ν µ µ c s ν τ τ t b -1 +2/3-1/3 matter bosons U(1) Y SU(2) L SU(3) C B W 1 W 2 W 3 g 1-8 EW γ W + W - Z g 1-8 electroweak strong radiation iggs doublet ϕ 1 +iϕ 2 ϕ 3 +iϕ 4 Michel Lefebvre CUPC 24, Victoria, BC 17

18 Global Symmetry and Conservation Laws Symmetry homogeneity of space homogeneity of time isotropy of space more abstract symmetry Conservation of momentum energy angular momentum some charge Michel Lefebvre CUPC 24, Victoria, BC 18

19 Local Symmetry and Fundamental Forces To reuire the laws of nature to be invariant under a local symmetry is to invoke a Gauge Principle. All known fundamental interactions are formulated as Gauge Theories Electromagnetic Weak Strong Gravity What about a uantum theory of gravity? See Dr Peet s talk! Michel Lefebvre CUPC 24, Victoria, BC 19

20 Weak Interaction e - disfavoured e - spin µ - spin The weak interaction violates parity!! This is very odd, and crucial to the understanding of the mystery of the origin of mass e - e - spin favoured µ - ν e W - e - νµ muon decay The interaction is weak because it is mediated by massive vector bosons ν e e - Z µ + W - e - d u e + µ d d e + e Z µ + µ u u - n p + e + ν This weak reaction e + controls the star p + p d + e + burning rate!! ν e Michel Lefebvre CUPC 24, Victoria, BC 2

21 Strong Interaction Each uark can have one of three different strong interaction charges, called colours. Quantum ChromoDynamics (QCD) The strong interaction is mediated by gluons between coloured objects. Photons do not carry (electric) charge, but gluons carry (strong) charge, so they can interact with each other!! g g g It is believed that this leads to confinement: only colour singlets can exist in nature. igh energy uarks produced in collisions will result in jets of hadrons, typically many light mesons Michel Lefebvre CUPC 24, Victoria, BC 21

22 iggs Mechanism It turns out that it is not possible to have the reuired gauge symmetries for the electromagnetic, weak and strong forces, AND to have ANY particle with a mass!!! Big problem: We want to have a gauge principle to give us the interactions and we know that most particles have mass The solution: The iggs Mechanism Michel Lefebvre CUPC 24, Victoria, BC 22

23 A fundamental scalar iggs field ϕ(x) is postulated, with an energy density that has a minimum for a non-zero value of the field... Nature chooses one of the possible minima for its vacuum: spontaneous symmetry hiding. 2 2 µ v V ( ϕ ) = min µ ϕ = ϕ = v > 2λ 2 From the measured value of the Fermi coupling constant G F, we obtain v = 246 GeV iggs Mechanism V 2 ( ) ( ) 2 ϕ = µ ϕϕ+λ ϕϕ λ> V ( ϕ) 2 µ < µ 2λ 2 µ > ϕ Michel Lefebvre CUPC 24, Victoria, BC 23

24 iggs Mechanism A room full of physicists chattering uietly is like space filled with the iggs field... a well-known scientist walks in, creating a disturbance as he moves across the room and attracting a cluster of admirers with each step... this increases his resistance to movement, in other words, he acuires mass, just like a particle moving through the iggs field... if a rumor crosses the room... it creates the same kind of ATLAS educational web clustering, but this time among page, adapted from an idea the scientists themselves. In this from Dr D. J. Miller analogy, these clusters are the iggs particles Michel Lefebvre CUPC 24, Victoria, BC 24

25 The Standard Model of Electroweak and Strong Interactions Gauge invariance U(1) Y SU(2) L SU(3) C Glashow Salam Weinberg Spontaneous symmetry hiding in the electroweak sector iggs mechanism: U(1) Y SU(2) L U(1) Q Residual (non-hidden) symmetry: U(1) Q SU(3) C massless photons massless gluons Michel Lefebvre CUPC 24, Victoria, BC 25

26 The Standard Model Particles fermions leptons uarks ν e e u d ν µ µ c s ν τ τ t b -1 +2/3-1/3 matter bosons U(1) Y SU(2) L SU(3) C B W 1 W 2 W 3 g 1-8 EW γ W + W - Z g 1-8 electroweak strong radiation iggs doublet ϕ 1 +iϕ 2 ϕ 3 +iϕ 4 Michel Lefebvre CUPC 24, Victoria, BC 26

27 SM iggs Interactions The Standard Model iggs mechanism generates particle masses and predicts the existence of the iggs Boson and its exact interaction with other particles... but ironically it does not predict its mass!! (direct searches) 114 GeV < M < 219 GeV (indirect searches) 95% CL iggs couplings proportional to mass 2 g = 4 2G M F 2 W f γ charged W f igmf µν 2M coloured igm W g W + W - γ g g W + W - igm Zg cosθ Z W µν Z Z Z Michel Lefebvre CUPC 24, Victoria, BC 27

28 SM iggs Decays (direct searches) 114 GeV < M < 219 GeV (indirect searches) 95% CL Fat! 1 bb _ WW BR() ZZ ZZ opens up τ + τ cc _ gg tt - γγ Zγ WW opens up M [GeV] due to c reduced running mass Michel Lefebvre CUPC 24, Victoria, BC 28

29 New Colliders pp : Large adron Collider at CERN 27- with a beam energy of 7 TeV Probing nature down to m!! ee + - : Linear Collider (site under discussion) 215- with a beam energy of 25 GeV Probing nature down to m!! Michel Lefebvre CUPC 24, Victoria, BC 29

30 Luminosity Let L: Machine luminosity (in cm -2 s -1 ) σ: cross section for the relevant scattering process R: event production rate Then we have R = Lσ 1 barn = 1-28 m 2 Defining the integrated luminosity L = L d t then the number of events is given by N = L Therefore if you want to make a measurement of a rare process (low cross section) with any significance, you need a large integrated luminosity. If you want to achieve this in a reasonable time, you need a large luminosity! σ Michel Lefebvre CUPC 24, Victoria, BC 3

31 Aerial View of CERN Michel Lefebvre CUPC 24, Victoria, BC 31

32 CERN Accelerators LC: 7 TeV on 7 TeV proton-proton Design luminosity of 1 34 cm -2 s -1 Michel Lefebvre CUPC 24, Victoria, BC 32

33 pp Collisions at the LC When protons collide at very high energy, what actually collides are constituents, uarks or gluons. 7 TeV For example - - W γ e ν γ e 7 TeV Michel Lefebvre CUPC 24, Victoria, BC 33

34 LC PP Cross Section σ pp (14TeV) 1mb 15 1 inelastic 12 1 Events for 1 fb -1 (one year at 1 33 cm -2 s -1 ) bb 6.7 times the commissioning luminosity 83% of first year luminosity 1% of nominal luminosity 1µb 1nb 1pb QCD jets (P T > 2 GeV) W eν Z ee t t = 175 GeV) (m t iggs SUSY m = 1 GeV m = 2 GeV m = 8 GeV m(g) m() 1 TeV It is a challenge to fish out events that are more than 1 orders of magnitude rarer than the most common interactions Michel Lefebvre CUPC 24, Victoria, BC 34

35 SM iggs Production at the LC (one year at 1 34 cm -2 s -1 ) g g t t t, W,Z W,Z, g t t g t t W,Z W,Z Michel Lefebvre CUPC 24, Victoria, BC 35

36 ATLAS Detector length: 45 m radius: 12 m weight:7 kt Michel Lefebvre CUPC 24, Victoria, BC 36

37 ATLAS Calorimetry: energy measurement Built in Canada Michel Lefebvre CUPC 24, Victoria, BC 37

38 ATLAS Installation Web Cam Nov 6 2:3 Michel Lefebvre CUPC 24, Victoria, BC 38

39 ATLAS Detector Challenges Particle bunch crossing: 25 ns igh radiation environment, in particular close to the beam pipe Need to trigger efficiently on events of interest Expect over 1 PByte (a million GByte) of data per year! Over 2 collaborators! Michel Lefebvre CUPC 24, Victoria, BC 39

40 Beyond the Standard Model The SM of particle physics is extremely successful at describing ALL experimental results so far. But we know the theory has technical problems. Possible solutions to these problems have been proposed, including SuperSymmetry Should be able to find superparticles with masses less than about 1 TeV. Extra Spatial Dimensions Leads to energy leaking out into other dimensions, escaping detection! Michel Lefebvre CUPC 24, Victoria, BC 4

41 SuperSymmetry Maximal extension of the Poincaré group SUSY actions are invariant under superpoincaré 3D rotations... Lorentz pure boosts... Poincare... superpoincare 4D translations SUSY translations... they are composed of an eual number of bosonic and fermionic degrees of freedom SUSY mixes fermions and bosons exact SUSY there should exist fermions and bosons of the same mass clearly NOT the case SUSY IS BROKEN A solution to the hierarchy problem If the iggs is to be light without unnatural fine tuning, then (softly broken) SUSY particles should have M SUSY < 1 TeV. SUSY can be viable up to M PL AND be natural! GUT acceptable coupling constant evolution The precision data at the Z mass (LEP and SLC) are inconsistent with GUT s using SM evolution, but are consistent with GUT s using SUSY evolution, if M SUSY 1 TeV A natural way to break EW symmetry The large top Yukawa coupling can naturally drive the iggs uadratic coupling negative in SUSY Lightest SUSY particle is a cold dark matter candidate Local SUSY is SUperGRAvity! Michel Lefebvre CUPC 24, Victoria, BC 41

42 SuperSymmetry Maximal extension of the Poincaré group 3D rotations... Lorentz pure boosts... Poincare 4D translations... SUSY translations... superpoincare SUSY mixes fermions and bosons. Exact SUSY predicts superpartners of all particles!!! Is this crazy? Recall Dirac predicted that all fermions would have anti-fermions... Lightest SUSY particle (LSP) is a cold dark matter candidate!!! Michel Lefebvre CUPC 24, Victoria, BC 42

43 Minimal SUSY iggs Sector MSSM: SM + an extra iggs doublet + SUSY partners SUSY breaking g W W W B l l ν g W W W B l l ν L R L u L u R d L d R u u d d L R L u L u R d L d R u u d d g W W γ Z l l ν g χ χ χ χ χ χ χ χ l l ν h A L R l u L u R d L d R l u 1 u 2 d 1 d EW symmetry breaking CP odd CP even 5 massive iggs particles, with M h < 13 GeV At tree level, all iggs boson masses and couplings can be expressed in terms of two parameters only (in constrained MSSM ) vev vev tanβ and m d u A = 2,, l, l l, l χ,, W, B χ, W γ,z W, B 1 R L 2 1 R L 1,2,3,4 d u 1,2 ± ± ± Note that we also have the following mixings with off-diagonal elements proportional to fermion masses Michel Lefebvre CUPC 24, Victoria, BC 43

44 SUSY Signatures The production of SUSY particles often yield events with many hadron jets and significant missing energy. In most SUSY models, sparticles are produced in pair, so that there would always be 2 LSP escaping detection. If SUSY particles exist at the electroweak scale (less than 1TeV), discovery should be easy at the LC g g () () χ 2 χ 1 χ 1 gg gg + χ χ l + l Michel Lefebvre CUPC 24, Victoria, BC 44

45 Grand Unification? Michel Lefebvre CUPC 24, Victoria, BC 45

46 Cosmic Connection physics that was common at 1-12 s! TeV scale Michel Lefebvre CUPC 24, Victoria, BC 46

47 Canada and ATLAS Activities focused on Liuid Argon Calorimetry 4 Major Projects Funded by Major Installation Grants Endcap adronic Calorimeter Forward adronic Calorimeter Frontend-Board Electronics Endcap Signal Cryogenics Feedthroughs Work in close clooaboration with TRIUMF, Canada s national particle and nuclear physics laboratory Many other activities including Computing, software, radiation studies Currently involved in installation, commissioning and getting ready to use ATLAS for physics studies!! Alberta Carleton CRPP Montréal SFU Toronto TRIUMF UBC Victoria York Michel Lefebvre CUPC 24, Victoria, BC 47

48 Conclusions ATLAS at the LC is expecting first collision in 27 Boldly look where no one has looked before, probing nature at the TeV scale Many unanswered uestions... and very likely many surprises! You can be part of it! Michel Lefebvre CUPC 24, Victoria, BC 48

49 Particle Physics at UVic One of the strongest group in Canada! BaBar at SLAC ATLAS at CERN Linear Collider T2K- From Tokai To Kamioka, ν experiment Theory We are recruiting graduate students! Michel Lefebvre CUPC 24, Victoria, BC 49