1. Masses, Forces, Higgs and Gluons
Elton S. Smith, Jefferson Lab
Constituents and forces
Electromagnetic + weak interaction
Color forces between quarks
Why is the glue interesting

2. All the mass is contained in the small nucleus
Components of an atom
Atom contains
negative electrons
positive protons
neutral neutrons
All the mass is contained in the small nucleus

3. Elementary constituents
charge p
The Standard Model +1 u d
+2/3
−1/3
charge e- −1 n
electron
neutrino d u
+2/3
−1/3 n
And all their anti-particles
Quarks are confined inside
the proton and neutron

4. Forces and force carriersp e
The electrons are bound to atoms by the electromagnetic force g p e
But there are two other forces at play in the atomic nucleus
the “color force” keeps the quarks bound inside nucleons (more later)
“Weak” -decays change neutron into a proton and emit an electron d n u d u e g W± u d

5. Differences: electromagnetic and weak forcesp e
Electromagnetic force
One carrier
Charge = 0
Mass = 0
Weak force n p e W±
Two carriers
Charge = +1 and -1
Mass = heavy

6. Similarities: electromagnetic and weak forcesp e
Electromagnetic n p e W±
Weak interactions
The weak force behaves just like the
electromagnetic force at high energies
The connection is found in a symmetry of nature

7. Symmetries and conservation laws
Symmetry
Conservation
translation
momentum
time invariance
energy
rotations
angular momentum
Gauge symmetry
charge
The E&M force obeys a U(1) Gauge symmetry

8. “Massive” gauge symmetry
Gauge symmetry and the conservation of
charge is intimately related to the fact that
the mass of the photon is zero.
Theory of
massless particles
satisfying Gauge Symmetry +
New particle (Higgs Particle) =
Interactions of
massive particles
Portrait of Peter Higgs by Ken Currie

9. Unification: gauge symmetry for weak forces
To put the electromagnetic and weak forces on the same footing, we assume the weak force has
SU(2) Gauge symmetry,
This symmetry requires a neutral force carrier
(or gauge boson), call it the Z0
CM energy (GeV)
s (nb)
~ 1990
But to make the force carriers massive, the Higgs boson was still missing until...

10. Discovery of Higgs Particle
Higgs particle decays to two jets and e+ e- pair
(and many other particles)
On July 4, 2012, CMS and ATLAS experiments at CERN announce evidence for the Higgs boson at a mass of 126 GeV
Mechanism for generation of quark masses is confirmed

11. Carriers of the force between quarksd g
Color force
Quantum Chromodynamics
(QCD)
Gluons are massless, so why is the interaction short range?

12. Asymptotic Freedom
The 2004 Nobel Prize was awarded for work that lead to our understanding of the theory of one of Nature's fundamental forces, the force that ties together the smallest pieces of matter – the quarks.

13. Strong coupling constant as vs energy
Confinement
Asymptotic Freedom
Large distance
small distance
Coupling constant
Energy (GeV)

14. Electromagnetic and color forces
+/- charges g
3 “color” charges

15. Mass without mass Mass of u quark ~ 0.003 GeV
Mass of d quark ~ GeV
Mass of the proton ~ 1 GeV p u d
+2/3
−1/3 d u
+2/3
−1/3
Most of the mass of the proton and neutron comes from the energy stored in the gluon fields that bind the quarks inside. n
Note that 99% of the mass of ordinary matter is in the form of protons and neutrons

16. But... signs of gluons are hard to find
Puzzle
Essentially all properties of protons, neutrons and mesons are described by quarks alone. q
mesons
Passive gluons
Our investigations aim and understanding the nature of gluonic fields inside these particles
glueball meson
hybrid meson q
Search for states that
must exist
Active gluons

17. JPC quantum numbers for normal mesons
Families of particles with definite JPC
are generated by taking combinations of the 3 lightest quarks (u,d,s and u,d,s) K+ p− K− p+ p0 h’ h K0 L q q
JPC = 0– – – 1+ – 2++ …
Allowed combinations
JPC = 0– – 0+ – 1– – …
Not-allowed: exotic

18. QCD calculations on the lattice
Calculations in QED are perturbative expansions in the (small) coupling constant a
At high energies, QCD calculations be performed by expanding in the (small) coupling constant as
At low energies, calculations in QCD are extremely challenging and are computed at discrete points on a space-time grid (lattice) to avoid singularities. The coupling constant as as can be arbitrarily large.

19. Meson Spectroscopy from LQCD
exotics
normal mesons
Isovector mesons, mp~700 MeV
Dudek PRD 83 (2011)
Dudek PRD 84 (2011)

20. Search for QCD exotics 1 1 ’1 b2 h2 h’2 b0 h0 h’0
1−+
2+−
0+−
Mass scale ~ 2 GeV
h1 → a+1p- → (o+)(-) → +-+-
h0 → bo1po → (o)gg → +-gggggg
h’2 → K+1K− → o K+ K− → +−K+K−
all charged
many photons
strange particles
The GlueX Detector Design
has been driven by the need
to carry out Amplitude analysis.  p X
,K,g
n,p

21. Areal view of accelerator

22. Photon beam and experimental area
Tagger
area
Hall D
North linac
Electron
Beam dump
Top View
75 m
Tagger Area
Experimental
Hall D
Electron beam
Coherent Bremsstrahlung
photon beam
Solenoid-
Based detector
Collimator
Photon
Beam dump
Counting House
Radiator
East arc

23. Pb-glass detector (Fcal) Superconducting 2 T solenoid
Hall D – GlueX detector
Pb-glass detector (Fcal)
Time-
of-flight
(tof)
Hermetic detection
of charged and
neutral particles in
solenoid magnet
Barrel
Calorimeter
(Bcal)
Target
(LH2)
Initial Flux 107 g/s
18,000 FADCs
4,000 pipeline TDCs
20 KHz L1 trigger
300 MB/s to tape
Tagger Spectrometer
(Upstream)
Tracking
Cathode strips
Drift chambers
Straw tubes
Superconducting 2 T solenoid

24. “typical” gp p+2p- p

25. Detector status

26. Summary
The symmetry of electro-weak interactions is responsible for the masses of quarks through their coupling to the Higgs particle.
But most of the mass of protons and neutrons comes from gluonic fields, not quarks
The gluonic fields are poorly understood
The GlueX experiment at Jefferson Lab is being built in the new Hall D to search for new particle states that have clear signatures of gluonic excitations.