Why is there mass?

1. Why is there mass? Tony Liss Saturday Physics for Everyone November 9, 2013 (With debts to Chris Quigg, Leonard Susskind, Hitoshi Murayama)

2. What is mass? Mass is stuff Mass is the stuff that gravity acts on r Mass is stuff that resists you when you push it Mass is rest energy

3. What’s the Matter in the Universe?

4. Ordinary Matter

5. The periodic table is made up of “quarks” and “leptons” A proton is made of u ud Add an electron to make a hydrogen atom A neutron is made of d d u Now you have all the building blocks of the periodic table.

6. The Masses ~173000 Mev/c2 ~4.8 Mev/c2 ~2.3 Mev/c2 .511 MeV/c2 According to theory, the Higgs boson gives these particles their mass.

7. The periodic table: M=Smi ? Helium: Two protons, two neutrons, two electrons MHec2 =3,728.398128 MeV MHe/ (2MP+2MN+2Me) = 0.9927 Iron: 26 protons, 30 neutrons, 26 electrons MFec2= 52,019 MeV MFe/(26MP+30MN+26Me )=0.9909 Hydrogen: One proton, one electron MHc2 = 938.890124 MeV MH/(MP+Me) = 0.999999986

8. Are the proton and neutron masses like atomic masses? ~173000 Mev/c2 No! Proton: Two up quarks and a down quark (2Mu+Md)c2< 10 MeV MPc2 = 938 MeV ! Neutron: Two down quarks and an up quark (2Md+Mu)c2 ~12 MeV MNc2= 940 MeV ! ~4.8 Mev/c2 ~2.3 Mev/c2 .511 MeV/c2 The mass of neutrons and protons is due (mostly) to the strong force (QCD) that holds them together:

9. Let’s Review • The mass of atoms is the sum of the mass of their parts (with very small corrections) – protons+neutrons+electrons • The mass of protons and neutrons is not the sum of the mass of their parts (quarks) • It is mostly from the energy in the strong force (QCD) binding the quarks together. • Therefore, the mass of periodic table and the visible universe comes not from the masses of the fundamental particles, but from QCD!

10. So who needs the Higgs Boson? Consider the proton & neutron masses The proton is electrically charged & therefore surrounded by its own electric field. The neutron is electrically neutral. You would guess that the proton is heavier than the neutron. But

11. Thanks to the Higgs… Mdown > Mup Mneutron > Mproton The heavier neutron “beta decays” in about 15 minutes into a proton plus an electron and a neutrino

12. Otherwise If the quarks had no mass, then MP > MN The proton would beta decay into a neutron No hydrogen atom If the electron had no mass… No atoms at all No me No you

13. How does it work? The Higgs ‘potential’

14. The Standard Model Electroweak force Force carriers Interactions Atoms

15. Weak Interactions Beta decay http://www.philica.com/display_article.php?article_id=126 d u u d u d e– W–

16. Electroweak Interactions In the 1960s, the electromagnetic & weak interactions were unified into a single framework based on electroweak symmetry. The symmetry principle requires the carriers of the electromagnetic and weak forces to have zero mass. Quarks and leptons also have no mass. But this isn’t true. Only the photon has no mass. Weak interactions are very short-ranged, implying that the carriers, W & Z bosons, are very massive. Electroweak symmetry must be hidden.

17. Hidden Symmetry I first saw this demo in a talk by Chris Quigg

18. ?

19. The Higgs Field is Special All other fields Energy of field 0 Strength of field The Higgs field The minimum energy has a non-zero Higgs field! http://commons.wikimedia.org/wiki/File:Mexican_hat_potential_polar_with_details.svg

20. Energy & Fields + Water molecules: Electrically neutral, but with a ‘dipole moment’ - O O O O H H H H H H H H + - (Thanks to Leonard Susskind)

21. How to Find the Higgs

23. Is it THE Higgs Boson? It is certainly A Higgs boson Therefore we know It is responsible for at least some of the W and Z mass (strong evidence). It is probably responsible for at least some of the fermion masses (evidence getting stronger). We don’t yet know: If it is the ONLY Higgs boson. Why it is so light.

24. The Electron Mass The energy in the electric field adds to the electron mass Electric field lines around an electron We know the mass of the electron: This observed mass is an unobservable bare mass plus a correction due to the field energy We know , then Dmec2 ~ 10 GeVand 0.511 MeV = (-9999.489 + 10000.000) MeV This is fine tuning Hitoshi Murayama http://arxiv.org/abs/hep-ph/0002232

25. The Solution: Antiparticles - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - Electron-positron pairs ‘pop’ out of the vacuum and shield the bare charge of the electron

26. The Mass of the Higgs Boson The problem is, is very very large (~1038GeV) if there is nothing else going on. This requires to be equally large in magnitude to cancel it and give

27. Supersymmetry (or something) SUSY includes a partner to the top quark, the ‘stop quark’ or ‘top squark’ that nearly cancels the top quark loop But we haven’t found the stop quark yet. The more we don’t find it, the heavier it must be – if it exists at all…

28. What if there is nothing else? Nature could be fine tuned. There is no law that forbids It’s just not very pleasing.

29. The Anthropic Principle Since we are here, the fundamental constants must have values in the narrow range compatible with conscious life.

30. One Universe, or many?

31. What We Don’t Know yet (partial list) • Is there more than one Higgs boson? • Supersymmetry, and many other ideas, include a family. • Does the Higgs boson give dark matter it’s mass? • Does the Higgs boson have anything to do with neutrino mass? • Is the anthropic principle correct? • What in the Universe is dark energy?