How and Why to go Beyond the Discovery of the Higgs Boson
John Alison University of Chicago
http://hep.uchicago.edu/~johnda/ComptonLectures.html Lecture Outline
April 1st: Newton’s dream & 20th Century Revolution April 8th: Mission Barely Possible: QM + SR April 15th: The Standard Model April 22nd: Importance of the Higgs April 29th: Guest Lecture May 6th: The Cannon and the Camera May 13th: The Discovery of the Higgs Boson May 20th: Problems with the Standard Model May 27th: Memorial Day: No Lecture June 3rd: Going beyond the Higgs: What comes next ?
2 Reminder: The Standard Model
Description fundamental constituents of Universe and their interactions Triumph of the 20th century Quantum Field Theory: Combines principles of Q.M. & Relativity
Constituents (Matter Particles) Spin = 1/2 Leptons: Quarks:
νe νµ ντ u c t ( e ) ( µ) ( τ ) ( d ) ( s ) (b ) Interactions Dictated by principles of symmetry Spin = 1 QFT ⇒ Particle associated w/each interaction (Force Carriers) γ W Z g Consistent theory of electromagnetic, weak and strong forces ...... provided massless Matter and Force Carriers Serious problem: matter and W, Z carriers have Mass ! 3 Last Time: The Higgs Feild
New field (Higgs Field) added to the theory Allows massive particles while preserve mathematical consistency Works using trick: “Spontaneously Symmetry Breaking”
Zero Field value Symmetric in Potential Energy not minimum Field value of Higgs Field Ground State
0 Higgs Field Value
Ground state (vacuum of Universe) filled will Higgs field Leads to particle masses: Energy cost to displace Higgs Field / E=mc 2 Additional particle predicted by the theory.
Higgs boson: H Spin = 0 4 Last Time: The Higgs Boson What do we know about the Higgs Particle: A Lot Higgs is excitations of v-condensate ⇒ Couples to matter / W/Z just like v X
matter: e µ τ / quarks W/Z
h h ~ (mass of matter) ~ (mass of W or Z)
matter W/Z
Spin: 0 1/2 1 3/2 2
Only thing we don’t (didn’t!) know is the value of mH 5 History of Prediction and Discovery
Late 60s: Standard Model takes modern form. Predicts massive W/Z bosons 1983: W/Z discovered at CERN: u e u ν Z W
u e d e Early 90s: W/Z used to predict top mass t t Z Z W W t b 1995: top quark discovered at fermilab 2000s: W/top quark and used to predict the higgs: h h t t W W 50 < mH < 150 GeV (95%) 6 Today’s Lecture
The Cannon and the Camera
7 Particle Physics for 3rd Graders
No Body Knows Protons Quarks ? u What’s in the Lunch Box ?
Look inside.
No Fun! What’s in the Lunch Box ?
Bang! What’s in the Proton ? Protons are Too small to look inside.
SMASH THEM!!!
Bang! The Worlds Biggest Cannon The Worlds Biggest Cannon Really Big Camera!!! Really Big Camera!!! 3. Reconstruction and Commissioning 20
17
Figure 3.2: Event display of a tt¯ di-lepton candidate in the eµ-channel with two b-tagged jets. The electron is shown by the green track pointing to a calorimeter cluster, the muon by the long red track miss intersecting the muon chambers, and the ET direction is indicated by the blue dotted line in the x-y view. The secondary vertecies of the two b-tagged jets are indicated by the orange ellipses in the upper right. 3rd Grade Explanation is Essentially Correct
Some Caveats: - More sophisticated analog to “what are quarks made of ?” - What comes out of the lunch box is there to begin with…
However, basic concepts/methods something that anyone understands One of the great things about this business !
Rest of lecture: - Refine basic notions of camera and cannon - Discuss challenges in collecting/analyzing pictures - Talk about how we use picture to compare to test SM 3. The ATLAS ExperimentA Toroidal LHC ApparatuS 17
Figure 3.1: Cut-away view of the ATLAS detector. 19
signed to measure the energy of electrons, photons, and hadrons. They are sensitive to both charged and neutral particles. The Muon Spectrometer (MS) surrounds the calorimeters. All particles except muons and neu- trinos are stopped by the calorimeter system. The MS is designed to measure the trajectories of muons leaving the calorimeter. The MS is composed of muon chambers operating in a magnetic field, provided by the toroid magnetics. A common coordinate system is used throughout ATLAS. The interaction point is defined as the origin of the coordinate system. The z-axis runs along the beam line. The x-y plane is perpendicular to the beam line, and is referred to as the transverse plane. The positive x-axis points from the interaction point to the center of the LHC ring; the positive y-axis points upward to the surface of the earth. The detector half at positive z-values is referred to as the “A-side”, the other half the “C-side”. The transverse plane is often described in terms of r-⇤ coordinates. The azimuthal angle ⇤ is measured from the x-axis, around the beam. The radial dimension, r, measures the distance from the beam line. The polar angle ⇥ is defined as the angle from the positive z-axis. The polar angle is often reported in terms of pseudorapidity, defined as = ln tan(⇥/2). The distance R is defined Not reviewed, for internal circulation only 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 td ftesi fteHgslk atcei the in particle Higgs-like the of spin the of Study hne ih2. fb 20.7 with channel ⇤ erdcino hsatceo at fi salwda pcfidi h CB-. license. CC-BY-3.0 the in specified as allowed is it of parts or article this of Reproduction c rdcinfato agrta 5 o pn2pril,ada 1 ofiec ee fone if level confidence 91% a at assumes and one particle, if spin-2 level pure a confidence assumes for 95% 25% at and than signal signal, larger spin-2 spin-0 fraction the a production a of favours in exclusion detector excess ATLAS the the observed in with results the 2012 of in collected compatibility Data the disfavours charge-parity. the on which on photons, reports two constraint note to This only mode decay the 125 hypothesis. observed Presently, about the spin-1 the of from a unknown. mass of stems a particle are rate this with properties production of boson physics measured spin Higgs other The Model its Standard boson. the but GeV, Higgs with consistent Model is Standard particle the new for search the in H oyih 03CR o h eeto h TA Collaboration. ATLAS the of benefit the for CERN 2013 Copyright The Basic Outputs: ⌅ eety h TA olbrto eotdteosraino e eta particle neutral new a of observation the reported collaboration ATLAS the Recently, Inner Tracking WW ( ⇥ ) System ⌅ Text Text gg production. e A lot goes into ofwork making/understanding these basic outputs. ⇥ µ ⇥ erhaiigfo ihrasi- rasi- atcewt positive with particle spin-2 a or spin-0 a either from arising search 1 of h TA Collaboration ATLAS The TA NOTE ATLAS ⇧ s eray2,2013 26, February = detector Abstract 8 e aacletdwt h ATLAS the with collected data TeV Electro-Magnetic Calorimeter H ! WW ( ⇤ rf eso 1.0 version Draft ) q ! q ⌅ e ⌫ X µ ⌫ Calorimeter Hadronic Muon Muon Tracking System + +
ν ν ν e µ τ u c t e µ τ d s b
21 + + g γ W Z
22 Higgs decays
H → bb: ~60% H → WW: ~20%
+ + +
H → ττ: ~5% H → ZZ: ~2% H → γγ: 0.2%
+ + + + + +
23 3. Reconstruction and Commissioning 20
Jet (b-tag) Muon
MeT
Jet (b-tag)
Electron
24
Figure 3.2: Event display of a tt¯ di-lepton candidate in the eµ-channel with two b-tagged jets. The electron is shown by the green track pointing to a calorimeter cluster, the muon by the long red track miss intersecting the muon chambers, and the ET direction is indicated by the blue dotted line in the x-y view. The secondary vertecies of the two b-tagged jets are indicated by the orange ellipses in the upper right. Experimental Challenges
25 b-jet Identification (b-Tagging)
Critical as b-jet ubiquitous in Higgs final states.
b-jet Displaced - b-lifetimes ~pico-seconds Tracks - Typical speed (time dilation) ⇒ travel O(mm) Secondary Vertex ~mm Jet Lxy ~10 µm
d0 InteractionPrimary Drives specs. on detector resolution Vertex Point (Must know detector positions to ~µm)
Jet Detectors size apartment buildings, measure to accuracy of something barely visible to human eye. Major cost driver 26
1 Triggering
- LHC provides orders of magnitude more collisions than can save to disk - Can only keep 1 out of 40,000 events / Discarded data lost forever - Interesting physics is incredibly rare. ~1 Higgs per billion events
Triggering: Process of selecting which collisions to save for further analysis.
Triggering in ATLAS: - Custom Electronics + Commodity CPU - Fast processing of images (micro-seconds / seconds) - Events rate from 40 MHz → 1kHz. - Data rate from 80 TBs (!) → 2 GB/s Another major cost driver
27 The Cannon
Most of the time protons miss one another: Cant aim with enough precision to ensure a direct hit each time Need to collide bunches of tightly packed protons to ensure hit LHC: 10^11 protons per bunch
28 The Cannon
Most of the time protons miss one another: Cant aim with enough precision to ensure a direct hit each time Need to collide bunches of tightly packed protons to ensure hit LHC: 10^11 protons per bunch
Can in principle be calculated in theory In practice, calculations extremely hard (αS so big) So extract from measurements in data 21 29 What We Measure Probability for process to happen given in terms of an area: Cross-section Event Rate = Cross-Section × Particle Flux
Rate certain pictures Known input from LHC Infer “Cross Section” (Directly measured) Protons /area / time Units area / Probability
Cross-Section (σ) can be calculated from theory Output of the Theory (Details) 2 (x1,x2) f(x1)f(x2) Feynman Diagrams: Pictures of what happens Invaluable⇠ Tool for calculation| | e e Re e p↵
ψ = + e- e+ + … p↵
q q q q
- Theory give prescription for assigning numerical value to diagram. 21 Other rules associated to the lines / Sum overall possible configurations 30 - Sum of diagrams (# associated with diagrams) is ψ - Really infinite sum. In practice, only the first few terms dominate 46 What We Measure Probability for process to happen given in terms of an area: Cross-section Event Rate = Cross-Section × Particle Flux
Rate certain pictures Known input from LHC Infer “Cross Section” (Directly measured) Protons /area / time Units area / Probability
Cross-Section (σ) can be calculated from theory (x ,x ) 2f(x )f(x ) ⇠ | 1 2 | 1 2 Quote σ (areas) in funny units: barns 1 barn = cross section forR neutron to interact with Uranium (E. Fermi)
σ(n,U(238)) ~ 1 barn ~ (238)^2/3 × σ(p,p) σ(p,p) ~ 0.03 barn (30 milli-barn) ~ GeV^-2 238 (n+p) 31 What We Measure Count pictures (“Events”) Compare events selected w/particular signature to prediction from theory Probability of process to happen SM Prediction: (SM calculation) Signal N = Size of the dataset Events ⇥ L (Total number of events)
Measurement: Observed Signal Background NEvents =NEvents +NEvents
Report measured probabilities σ “cross sections” / Compare directly to theory
Observed Background NEvents NEvents Measured = L 32 Standard Model Total Production Cross Section Measurements Status: August 2016
1 11 500 µb− 10 1 80 µb− ATLAS Preliminary Theory [pb] s TeV Run 1,2 √ =7,8,13 √s TeV σ LHC pp =7 6 10 1 Data 4.5 4.9 fb− −
1 LHC pp √s =8TeV 5 81 pb− 10 1 20.3 1 35 pb− Data fb− 1 81 pb− 1 35 pb− LHC pp √s =13TeV 4 10 1 Data 0.08 13.3 fb− −
103
102
total 1 1 10 2.0 fb−
VBF 1 VH
t¯tH 1 10−
pp W Z t¯t t WW H Wt WZ ZZ t t¯tW t¯tZ t-chan s-chan 33 Advantage of Higher Energy
N13 TeV 13 TeV 13 TeV N = Events = L Events N8TeV 8TeV ⇥ 8TeV ⇥ L Events L
WJS2013 100 ratios of LHC parton luminosities: 13 TeV / 8 TeV Now ~ 10 (linear w/dataset size)
gg_ Σ 13 TeV 8TeV qq NEvents 100 NEvents qg ⇠ ⇥ 10
q/g new luminosity ratioluminosity physics
q/g MSTW2008NLO 1 100 1000 M (GeV) 1.5 TeV X 34 Advantage of Higher Energy
N13 TeV 13 TeV 13 TeV N = Events = L Events N8TeV 8TeV ⇥ 8TeV ⇥ L Events L
WJS2013 100 ratios of LHC parton luminosities: 13 TeV / 8 TeV Now ~ 10 (linear w/dataset size)
gg_ Σqq 13 TeV 8TeV NEvents 500 NEvents qg ⇠ ⇥ 10
q/g new
luminosity ratioluminosity physics
q/g
MSTW2008NLO 1 100 1000 3 TeV M (GeV) X 35 Advantage of Higher Energy
N13 TeV 13 TeV 13 TeV N = Events = L Events N8TeV 8TeV ⇥ 8TeV ⇥ L Events L
WJS2013 100 ratios of LHC parton luminosities: 13 TeV / 8 TeV Now ~ 10 Pick up next time discussing how these tools (linear w/dataset size) used to discover and study the Higgs Boson
gg_ Σqq N13 TeV 20 N8TeV qg Events ⇠ ⇥ Events 10
g luminosity ratioluminosity h 2.3
g MSTW2008NLO 1 100 1000 M (GeV) mH X 36