? Foundations of Particle Physics Workshop University of Michigan 11 - PowerPoint PPT Presentation

This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics. FERMILAB-SLIDES-18-038-T I would like to know … Chris Quigg 
 Fermi National Accelerator Laboratory ? Foundations of Particle Physics Workshop· University of Michigan· 11 March 2018

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Problems of High-Energy Physics (NAL Design Report, January 1968) We would like to have answers to many questions. Among Do the laws of electromagnetic radiation, which are now them are the following: known to hold over an enormous range of lengths and fre- quencies, continue to hold in the wavelength domain char- Which, if any, of the particles that have so far been discov- acteristic of the subnuclear particles? ered, is, in fact, elementary, and is there any validity in the concept of “elementary” particles? What is the connection between the weak interaction that is associated with the massless neutrino and the strong one What new particles can be made at energies that have not that acts between neutron and proton? yet been reached? Is there some set of building blocks that is still more fundamental than the neutron and the proton? Is there some new particle underlying the action of the “weak” forces, just as, in the case of the nuclear force, Is there a law that correctly predicts the existence and na- there are mesons, and, in the case of the electromagnetic ture of all the particles, and if so, what is that law? force, there are photons? If there is not, why not? Will the characteristics of some of the very short-lived par- In more technical terms: Is local field theory valid? A fail- ticles appear to be di ff erent when they are produced at such ure in locality may imply a failure in our concept of space. higher velocities that they no longer spend their entire lives What are the fields relevant to a correct local field theory? within the strong influence of the particle from which they What are the form factors of the particles? What exactly are produced? is the explanation of the electromagnetic mass di ff erence? Do “weak” interactions become strong at su ffi ciently small Do new symmetries appear or old ones disappear for high distances? Is the Pomeranchuk theorem true? Do the total momentum-transfer events? cross sections become constant at high energy? Will new What is the connection, if any, of electromagnetism and symmetries appear, or old ones disappear, at higher energy? strong interactions? 4

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T o-do / wish list for particle physics & friends, from 2005 6

Before LHC Two then-new Laws of Nature + pointlike quarks & leptons We do not know 
 Mendele’ev 
 what the Universe 
 did not know of at large is made of. the noble gases. Interactions: SU(3) c ⊗ SU(2) L ⊗ U(1) Y gauge symmetries 8 gluons 7

Quantum Chromodynamics Dynamical basis for quark model Gluons (vector force particles) mediate interactions among the quarks and experience strong interactions. Contrast photons , which mediate interactions among charged particles, not among themselves. Quark, gluon interactions ➾ nuclear forces 8

Antiscreening evolution of the strong coupling “constant” 12 11 10 9 8 1/ α s 7 6 5 4 ✓ Q ◆ α s ( µ ) + ( 33 − 2n f ) 1 1 α s ( Q ) = ln 3 6 π µ 2 10 0 10 1 10 2 10 3 Q [GeV] 9

The World’s Most Powerful Microscopes 
 nanonanophysics 8.12 T eV 10

sum of parts rest energy Nucleon mass (~940 MeV): exemplar of m = E 0 / c 2 up and down quarks contribute few % χ PT: M N � 870 MeV for massless quarks 11

QCD could be complete, * up to M Planck … but that doesn’t prove it must be Prepare for surprises! How might QCD Crack? (Breakdown of factorization) Free quarks / unconfined color New kinds of colored matter Quark compositeness Larger color symmetry containing QCD – massive gluon partners? * modulo Strong CP Problem 12

Electroweak Symmetry Breaking Interactions: SU(3) c ⊗ SU(2) L ⊗ U(1) Y gauge symmetries 8 gluons W ± · Z 0 · γ 13

Gauge symmetry (group-theory structure) tested in e + e − → W + W − 30 No ZWW vertex Only υ e exchange 20 σ WW (pb) 10 LEP data Standard model 02/17/2005 0 160 180 200 √ s (GeV) 14

Meissner effect Photon has mass in a superconductor 15

Spontaneous symmetry breaking Higgs Kibble † Guralnik † Hagen Englert Brout † 1964– : Goldstone theorem doesn’t apply to gauge theories! 
 Each would-be massless NGB joins with a would-be 
 massless gauge boson to form a massive gauge boson. 16

Simplest example: Abelian Higgs model 
 = Ginzburg–Landau in relativistic notation Yields massive photon 
 + 
 a massive scalar particle 
 “Higgs boson” No mention of weak interactions. No question of origin of fermion masses 
 (not an issue for Yang–Mills theory or QED). 17

An a priori unknown agent hides electroweak symmetry ✴ A force of a new character, based on interactions of an elementary scalar ✴ A new gauge force, perhaps acting on undiscovered constituents ✴ A residual force that emerges from strong dynamics among electroweak gauge bosons ✴ An echo of extra spacetime dimensions 18

The Importance of the 1-T eV Scale EW theory does not predict Higgs-boson mass Thought experiment: conditional upper bound W + W – , ZZ, HH, HZ satisfy s -wave unitarity, _ provided M H ≤ (8 π√ 2/3 G F ) 1/2 ≈ 1 TeV If bound is respected, perturbation theory is “everywhere” reliable If not, weak interactions among W ± , Z , H become strong on 1-TeV scale New phenomena are to be found around 1 TeV 19

Large Hadron Collider CMS LHC b ALICE ATLAS 20

Standard Model Production Cross Section Measurements Status: July 2017 σ [pb] total (x2) ATLAS Preliminary 10 11 Theory inelastic Run 1,2 √ s = 7, 8, 13 TeV LHC pp √ s = 7 TeV 10 6 incl . Data 4.5 − 4.9 fb − 1 dijets 10 5 LHC pp √ s = 8 TeV p T > 25 GeV Data 20.3 fb − 1 10 4 n j ≥ 0 LHC pp √ s = 13 TeV 10 3 n j ≥ 0 n j ≥ 1 p T > 125 GeV Data 0.08 − 36.1 fb − 1 total WW p T > 100 GeV n j ≥ 2 n j ≥ 1 t -chan ~1 Hz 10 2 n j ≥ 1 WW n j ≥ 1 total WW n j ≥ 2 n j ≥ 3 Wt WZ n j ≥ 2 n j ≥ 2 WZ 10 1 ggF WZ n j ≥ 3 ZZ n j ≥ 4 H → WW n j ≥ 3 n j ≥ 4 n j ≥ 3 W γ ZZ ZZ n j ≥ 4 n j ≥ 5 s -chan n j ≥ 5 n j ≥ 4 1 H → ττ n j ≥ 6 Z γ n j ≥ 5 Zt n j ≥ 6 n j ≥ 7 VBF n j ≥ 5 n j ≥ 6 H → WW 10 − 1 n j ≥ 8 n j ≥ 6 n j ≥ 7 H → γγ 10 − 2 n j ≥ 7 H → ZZ → 4 ℓ n j ≥ 7 W ± W ± 10 − 3 WZ pp t¯ V γ Wjj Zjj t W Z VV H WV t¯ γ t γγ t¯ t¯ Z γγ W γγ Z γ jj VVjj t γ Jets WW tW tZ WW γ 21 R =0.4 EWK EWK Excl. EWK EWK fid. fid. fid. fid. tot. tot. fid. fid. fid. fid. tot. tot. fid. fid. fid. tot. fid. fid. fid. fid. fid.


 
 What the LHC has told us about H so far 
 Evidence is developing as it would for 
 a “standard-model” Higgs boson 
 Unstable neutral particle near 125 GeV M H = 125.09 ± 0.24 GeV Motivates HL-LHC, 
 electron–positron Higgs factory decays to γγ , W + W – , ZZ dominantly spin-parity 0 + Hff ̄ couplings 
 not universal evidence for τ + τ – , bb ̄ , tt ̄ ; μ + μ – limited Only third-generation fermions tested 22

Why does discovering the agent matter? Imagine a world without a symmetry-breaking 
 (Higgs) mechanism at the electroweak scale 23

Electron and quarks would have no mass via Higgs QCD would confine quarks into protons, etc. 
 Nucleon mass little changed Surprise: QCD would hide EW symmetry, 
 give tiny masses to W , Z Massless electron: atoms lose integrity No atoms means no chemistry, no stable composite structures like liquids, solids, … 
 … no template for life. arXiv:0901.3958 24

What we expect of the standard-model Higgs sector Hide electroweak symmetry 
 Motivates VLHC Give masses to W, Z, H 
 Regulate Higgs-Goldstone scattering 
 Account for quark masses, mixings Account for charged-lepton masses } Φ BSM A role in neutrino masses? 25

Fully accounts for EWSB ( W , Z couplings)? Couples to fermions? 
 t from production, Htt ̄ 
 need direct observation for b, τ Accounts for fermion masses? 
 Fermion couplings ∝ masses? Are there others? Quantum numbers? ( J P = 0 + ) SM branching fractions to gauge bosons? Decays to new particles? All production modes as expected? Implications of M H ≈ 125 GeV? Any sign of new strong dynamics? 26

Why does the muon weigh? gauge symmetry allows after spontaneous symmetry breaking What does the muon weigh? ς e : picked to give right mass, not predicted fermion mass implies physics beyond the standard model 27

Charged Fermion Masses 10 0 t charged leptons up quarks 10 -1 down quarks Mass / Weak Scale b 10 -2 τ c 10 -3 μ s 10 -4 d u 10 -5 e 10 -6 Running mass m ( m ) … m ( U ) 28

0 … 1 … ∞ 29

The Problem of Identity What makes a top quark a top quark, an electron an electron, a neutrino a neutrino? Why three families? Neutrino oscillations give us another take. 
 Clue to matter excess in the universe? Might new kinds of matter unlock the pattern? 30

More new physics on the TeV scale? WIMP dark matter “Naturalness” Hierarchy problem: EW scale ≪ Unification or Planck scale 
 Vacuum energy problem Clues to origin of EWSB 31