The ATLAS detector at the Large Hadron Collider: to boldly look where no one has looked before

Transcription

1 The ATLAS detector at the Large adron Collider: to boldly look where no one has looked before UBC 14 July 2006 Michel Lefebvre Physics and Astronomy University of Victoria

2 The ATLAS detector at the Large adron Collider: to boldly look where no one has looked before 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 required to probe the very small distance scales associated with elementary particles. Since many of the fundamental particles have large masses, high energy is also required to create them for study. Modern igh Energy Physics experiments typically require 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 questions. 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 quests 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, UVic UBC Eng Phys, 14 July

3 igh Energy Physics Motivation: Understanding nature s fundamental constituents and forces Michel Lefebvre, UVic UBC Eng Phys, 14 July

4 Normal Matter m m m electrons quarks NIST Image 7 nm 7 nm from a scanning tunneling microscope showing a single zig-zag chain of Cs atoms (red) on the GaAs(110) surface (blue). Michel Lefebvre, UVic UBC Eng Phys, 14 July

5 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, UVic UBC Eng Phys, 14 July

6 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 equations, like Schrödinger, Dirac and Maxwell equations Michel Lefebvre, UVic UBC Eng Phys, 14 July

7 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, UVic UBC Eng Phys, 14 July

8 Relativistic Dynamic E p =γmc =γmv 2 2 E pc E = = ( ) 2 ( 2 ) pc + mc v c massless particles carry momentum!! m= 0 E = pc and v = c equivalence of mass and energy!! E = mc m= E/ c Albert Einstein γ 1 ( v ) c 2 1/ 2 relativistic particles 2 E mc E = pc and v = c Natural units: GeV fm = c = = 1 Michel Lefebvre, UVic UBC Eng Phys, 14 July

9 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 quarks... and also of other quarks and gluons! m Michel Lefebvre, UVic UBC Eng Phys, 14 July

10 Colliding Particles Fixed target : available energy E m 2mE Collider : available energy 2E E E Michel Lefebvre, UVic UBC Eng Phys, 14 July

11 Detecting Particles Particle detector: Ideally, identify, for each particle produced in each collision, its type (mass, electrical charge, spin, other quantum 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 techniques are then required to best reconstruct each event. Michel Lefebvre, UVic UBC Eng Phys, 14 July

12 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 105 GeV Probing nature down to m!! pp : Tevatron at Fermilab with a maximum beam energy of 980 GeV Probing nature down to m!! W, Z particles discovery!!! 3 families of leptons!!! Precision measurements top quark discovery!!! Michel Lefebvre, UVic UBC Eng Phys, 14 July

13 Elementary Particles M t M of 76 Os M W M of 38 Sr Michel Lefebvre, UVic UBC Eng Phys, 14 July

14 Typical Detector Components Michel Lefebvre, UVic UBC Eng Phys, 14 July

15 Typical Detector Michel Lefebvre, UVic UBC Eng Phys, 14 July

16 The Standard Model Particles fermions leptons quarks ν e e u d ν µ µ c s ν τ τ t b /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 0 g 1-8 electroweak strong radiation iggs doublet ϕ 1 +iϕ 2 ϕ 3 +iϕ 4 0 Michel Lefebvre, UVic UBC Eng Phys, 14 July

17 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, UVic UBC Eng Phys, 14 July

18 Local Symmetry and Fundamental Forces To require 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 quantum theory of gravity?? Michel Lefebvre, UVic UBC Eng Phys, 14 July

19 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 0 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, UVic UBC Eng Phys, 14 July

20 Strong Interaction Each quark 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 quarks produced in collisions will result in jets of hadrons, typically many light mesons Michel Lefebvre, UVic UBC Eng Phys, 14 July

21 iggs Mechanism It turns out that it is not possible to have the required 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, UVic UBC Eng Phys, 14 July

22 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 0 = v > 0 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 < 0 µ 2λ 2 µ > 0 ϕ 0 Michel Lefebvre, UVic UBC Eng Phys, 14 July

23 iggs Mechanism A room full of physicists chattering quietly 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 acquires 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, UVic UBC Eng Phys, 14 July

24 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, UVic UBC Eng Phys, 14 July

25 The Standard Model Particles fermions leptons quarks ν e e u d ν µ µ c s ν τ τ t b /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 0 g 1-8 electroweak strong radiation iggs doublet ϕ 1 +iϕ 2 ϕ 3 +iϕ 4 0 Michel Lefebvre, UVic UBC Eng Phys, 14 July

26 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, UVic UBC Eng Phys, 14 July

27 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, UVic UBC Eng Phys, 14 July

28 New Colliders pp : Large adron Collider at CERN with a beam energy of 7 TeV Probing nature down to m!! ee + - : Linear Collider (site under discussion) with a beam energy of 250 GeV Probing nature down to m!! Michel Lefebvre, UVic UBC Eng Phys, 14 July

29 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 = 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, UVic UBC Eng Phys, 14 July

30 Aerial View of CERN Michel Lefebvre, UVic UBC Eng Phys, 14 July

31 CERN Accelerators LC: 7 TeV on 7 TeV proton-proton Design luminosity of cm -2 s -1 Michel Lefebvre, UVic UBC Eng Phys, 14 July

32 LC 7 TeV proton beams Luminosity of 1034 cm-2s-1 Bunch crossing every 25 ns Michel Lefebvre, UVic UBC Eng Phys, 14 July

33 pp Collisions at the LC When protons collide at very high energy, what actually collides are constituents, quarks or gluons. 7 TeV For example - - qq W γ e ν γ e 7 TeV Michel Lefebvre, UVic UBC Eng Phys, 14 July

34 LC PP Cross Section σ pp (14TeV) 1mb inelastic Events for 10 fb -1 (one year at cm -2 s -1 ) bb 6.7 times the commissioning luminosity 83% of first year luminosity 10% of nominal luminosity 1µb 1nb 1pb QCD jets (P T > 200 GeV) W eν Z ee t t = 175 GeV) (m t iggs SUSY m = 100 GeV m = 200 GeV m = 800 GeV m(g) m(q) 1 TeV It is a challenge to fish out events that are more than 10 orders of magnitude rarer than the most common interactions Michel Lefebvre, UVic UBC Eng Phys, 14 July

35 SM iggs Production at the LC (one year at cm -2 s -1 ) g g t t t q q,q W,Z W,Z q q,q 0 g t t q g t t q W,Z W,Z Michel Lefebvre, UVic UBC Eng Phys, 14 July

36 ATLAS Detector length: 45 m radius: 12 m weight:7 kt Michel Lefebvre, UVic UBC Eng Phys, 14 July

37 ATLAS Detector underground Michel Lefebvre, UVic UBC Eng Phys, 14 July

38 ATLAS Calorimetry: energy measurement Built in Canada Michel Lefebvre, UVic UBC Eng Phys, 14 July

39 ATLAS Installation ATLAS detector components have been produced around the world. The detector is in its last phases of assembly. Whole Installation Barrel Toroid Coils Barrel Calorimeter Endcap Calorimeter Michel Lefebvre, UVic UBC Eng Phys, 14 July

40 ATLAS Michel Lefebvre, UVic UBC Eng Phys, 14 July

41 ATLAS Detector in the transverse plane Michel Lefebvre, UVic UBC Eng Phys, 14 July

42 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 2000 collaborators! Michel Lefebvre, UVic UBC Eng Phys, 14 July

43 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, UVic UBC Eng Phys, 14 July

44 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 equal 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 quadratic coupling negative in SUSY Lightest SUSY particle is a cold dark matter candidate Local SUSY is SUperGRAvity! Michel Lefebvre, UVic UBC Eng Phys, 14 July

45 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, UVic UBC Eng Phys, 14 July

46 Minimal SUSY iggs Sector MSSM: SM + an extra iggs doublet + SUSY partners SUSY breaking g W W W B l l ν q q q q g W W W B l l ν q q q q 0 0 L R L u L u R d L d R 0 0 u 0 u 0 d d L R L u L u R d L d R u 0 u 0 d d g W W γ Z l l ν q q q q g χ χ χ χ χ χ χ χ l l ν q q q q h A 0 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 < 130 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,q q,q q l, l l, l χ,, W, B χ, W γ,z W, B 1 R L 2 1 R L 0 1,2,3,4 0 d 0 u 0 0 1, ± ± ± Note that we also have the following mixings with off-diagonal elements proportional to fermion masses Michel Lefebvre, UVic UBC Eng Phys, 14 July

47 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 (q) q q (q) χ 0 2 q q χ 0 1 χ 0 1 gg gg qqqq + χ χ l + l Michel Lefebvre, UVic UBC Eng Phys, 14 July

48 Grand Unification? Michel Lefebvre, UVic UBC Eng Phys, 14 July

49 Cosmic Connection physics that was common at s! TeV scale Michel Lefebvre, UVic UBC Eng Phys, 14 July

50 Canada and ATLAS Activities focused on Liquid 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 collaboration with TRIUMF, Canada s national particle and nuclear physics laboratory Many other activities including Computing, software, calibration, reconstruction Currently involved in installation, commissioning and getting ready to use ATLAS for physics studies!! Alberta Carleton McGill Montréal SFU Toronto TRIUMF UBC Victoria York Michel Lefebvre, UVic UBC Eng Phys, 14 July

51 Conclusions ATLAS at the LC is expecting first collision in 2007 Boldly look where no one has looked before, probing nature at the TeV scale Many unanswered questions... and very likely many surprises! You can be part of it! Michel Lefebvre, UVic UBC Eng Phys, 14 July

52 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, UVic UBC Eng Phys, 14 July