The Baryon Asymmetry Problem

1. The Baryon Asymmetry Problem Daniel Molina

2. What is a baryon? • Technical definition - massive particles made up of three quarks • Quarks? – small particles that make up other particles, such protons and neutrons • Have charge in fractions of electron charge • Baryons are the particles that make up matter • Protons and neutrons are both baryons • These are the building blocks of atoms • While non-baryonic matter does exist, it is not commonly encountered in normal life

3. What about anti-baryons? • There are also anti-baryons which comprise anti-matter • Anti-particles are like their normal particle counterparts, but with opposite charge • The anti-particle of a proton (+ charge) is the antiproton (- charge) and the anti-particle of the electron (-charge) is the positron (+charge) • Some particles are their own anti-particles, such as photons • Charge can mean more than just electric charge, but this talk is mainly concerned with electric charge

4. Origin of Baryons • Baryons were created during the Big Bang, starting after about seconds. • The Standard model tells us that the same number of baryons and anti-baryons should have developed • This means we expect there to be equal amounts of matter and anti-matter in the universe

5. Baryon Asymmetry Observations • Our everyday observations tell us that there is much more matter than anti-matter • Everything we see around us is made of normal matter • These everyday observations are corroborated by observations in physics • No significant amount of anti-matter has been detected in any survey of regions of the universe • Density of anti-matter on Earth is ~1-100 particles per square meter • People are made up of ~ normal matter particles • This observed imbalance is unexplained by any models of physics that are generally accepted, such as the Standard Model • This makes the baryon asymmetry problem extremely interesting for modern physics, as new models must be able to predict the asymmetry

6. Baryon Asymmetry Parameter • The baryon asymmetry parameter, , is a measure of the difference in number of baryons and anti-baryons (there is a scaling factor, but this is not important) • The observed baryon asymmetry parameter is approximately 8 orders of magnitude larger than the Standard Model predicts • 100 million times larger – quite significant

7. Anti-matter Detection • How do we know far-away galaxies are made of normal matter and not of anti-matter? • Far away anti-matter would look the same as matter • Anti-matter produces light in the same way as matter • Matter and anti-matter annihilate – the collision of a particle and its antiparticle results in the destruction of these particles and the creation of new particles • One of the products of annihilation is gamma radiation, a type of radiation made up of highly energized photons • Gamma radiation would be detectable at boundaries between anti-matter and matter galaxies, but the predicted amount of radiation has not been observed

8. Other Explanations? • Some argue that the presence of matter in much larger quantities than anti-matter was inherent in the development of baryons • The universe “favored” matter since the beginning • Nothing we know about the early universe points to this being true, and it is not a widely accepted assumption • There are a few other explanations that rely on very exotic physics and assumptions that are unlikely and outside the scope of this talk • We can assume that the asymmetry developed over time rather than being present from the beginning

9. Sakharov Conditions • In 1967, Andrei Sakharov determined three conditions necessary for baryon asymmetry of any amount • Baryon number violation • C and CP symmetry violation • Interactions out of thermal equilibrium • The combination of these three conditions during the creation of baryons and anti-baryons would have allowed them to be produced at different rates, leading to one or the other becoming more prevalent

10. 1. Baryon Number Violation • Baryon number is a measure of the number of quarks and anti-quarks in a system • Quarks contribute +1/3 and anti-quarks contribute -1/3 • The baryon number of baryons is +1, and the baryon number of anti-baryons is -1 • In typical particle physics, this number has been shown to be conserved – the baryon number of a system does not change • We believe the universe initially had an equal amount of matter and anti-matter (baryons and anti-baryons), so the baryon number of the universe has since increased • Must allow for processes which violate baryon number conservation • For those interested, sphalerons and some supersymmetric particles allow for baryon number violation

11. 2. C and CP Symmetry Violation - Definitions • Symmetry – a symmetry exists when changing some property of a system does not change any of the physics of the system • C-symmetry – When reversing the charges of the system preserves the physics • P-symmetry – When reversing the “parity” of the system preserves the fixes • Parity refers to the spatial directions of the system • P-symmetry is satisfied when changing all 3 spatial directions does not change the physics • CP-symmetry – Symmetry when BOTH charge and parity are flipped

12. 2. C and CP Symmetry Violation • The vast majority of processes preserve C and CP symmetries • C symmetry is commonly violated in the Standard Model • CP-symmetry violation was not directly observed until 1999 • Very few systems have been proven to violate CP-symmetry • More instances of CP-violation must be found to satisfy this Sakharov condition – motivation for several modern experiments, such as permanent electric dipole moment experiments

13. 3. Interactions Out of Thermal Equilibrium • Because particles and their anti-particles have the same energy, thermal physics tells us thermal equilibrium would work to keep the number of baryons and anti-baryons equal, and they would simply annihilate • Reactions that produce an asymmetry would happen in reverse just as often, resulting in no net asymmetry • Our understanding of the Big Bang tells us that rapid expansion of the universe during baryon development would prevent thermal equilibrium

14. Recap • Baryons and anti-baryons – comprise matter and anti-matter. Originated shortly after the Big Bang • Accepted models of physics predict equal numbers of baryons and anti-baryons – not the case, baryons much more prevalent • Sakharov conditions must be met for development of more matter than anti-matter • 1: Boson number violation – Boson number is a measure of the number of quarks vs. anti-quarks in a system. Processes must exist that allow boson number to change. • 2: CP-symmetry violation – charge and parity (direction) reversal must change the physics of some systems • 3: Lack of Thermal Equilibrium – early boson/anti-boson interactions must have been out of equilibrium, or asymmetries would have been eliminated as quickly as they were made

15. CP-Symmetry Violating Processes • Evidence of 2nd Sakharov Condition (C and CP symmetry violation) is being sought out • The Standard Model accounts for a small amount of CP-symmetry violation, but not enough to explain the observed baryon asymmetry • Need to prove existence of proposed sources of CP-symmetry violation not predicted by the Standard Model • Provides motivation for large scale, high precision experiments • The most prevalent of these experiments is a set of experiments involving permanent electric dipole moments in subatomic particles, such as neutrons and protons

16. What is a permanent electric dipole moment? • Electric dipole moments (EDMs) are a measure of the separation of positive and negative charges in a system • The most basic electric dipole system is a single negative charge and a single positive charge separated by a distance • Subatomic particles, according to classical physics, do not have EDMs • An electron is one negative charge, not a distribution of positive and negative charges in classical physics • A neutron is neutral!

17. Permanent Electric Dipole Moments and CP-Symmetry Violation • However, quarks have charge, and each subatomic particle being studied has a distribution of positively charged quarks and negatively charged quarks • Measurement of quark level interactions – must be very precise • It is hard to explain directly why permanent EDMs violate CP-symmetry • However, we use the fact that CPT-symmetry (T for time) always holds • It can be shown that if P-symmetry and T-symmetry are violated and CPT-symmetry holds, then CP-symmetry is violated, as desired • P- and T- symmetry violation occurs if there is an EDM

18. Neutron Electric Dipole Moment (nEDM) Experiment • Standard Model prediction: • Supersymmetry prediction: • Current experimental precision: • First nEDM experiment in 1951 at Harvard • Expected to meet supersymmetry precision within 3-5 years • Precision over time appears to be rolling off, but the latest experiments should break this trend and increase sensitivity much more than previous experiments

19. nEDM Experiment – Basic Procedure • Because neutrons are overall neutral, can easily use electric and magnetic fields without causing the neutrons to move • The experiment starts by cooling down neutrons until they can be stored • These cold neutrons are then placed in parallel electric and magnetic fields • If there is an EDM in the neutron, the direction of the electric field will affect the spin of the neutron • Spin is a quantum mechanical effect, but can be thought of like normal spin here • Can extract the value of the EDM by tracking the change in spin due to flipping the electric field

20. nEDM Experiment Precision • The biggest factor on precision in the nEDM experiment is the strength of electric field used • Stronger electric fields are better, but this can cause heating • Can warm up neutrons and shorten the amount of time they can be stored, negatively affecting precision • The experiments rely on superconducting lead, which also must be kept at a very low temperature • Difficult to produce very large and uniform fields over several meters • If the electric and magnetic field strength are not extremely stable during the length of the measurement, systematic error is introduced, because the EDM calculation requires these values not change

21. Electron Electric Dipole Moment (eEDM) Experiment • Standard Model prediction: • Technicolor theory prediction: • Current experimental precision: • ACME experiment at Yale is the main contributor to the eEDM experiment • The ACME experiment works the same way as the nEDM experiments, but the electric fields are generated by embedding electrons in Thorium Monoxide, which has large natural electric fields when placed in an external electric field

22. Proton Electric Dipole Moment (pEDM) Experiment • Standard Model prediction: • Technicolor theory prediction: • Proposed experimental precision: • Experiments have been proposed, but none of reasonable precision have been completed

23. Other EDMs and CP-Symmetry Violating Processes • Measurements of permanent EDMs can be obtained from other subatomic particles with quarks of different charges • Atoms and molecules are also candidates for EDM measurements • Experimental design is harder, as atoms and molecules have EDMs due to both quark interactions and electron-proton interactions • Many extensions of the Standard Model introduce new particles that are free to violate CP-symmetry, but these particles in general have not been discovered

24. Summary • In order to meet the Sakharov Conditions, sources of CP-symmetry violation must be found • The best current candidates for CP-symmetry violation experiments are subatomic particles that may have permanent electric dipole moments • EDMs due to distribution of quarks within a particle • Neutron electric dipole moment (nEDM) and electron electric dipole moment (eEDM) experiments are a few years from reaching the level of precision predicted by modern models • These experiments will also contribute to placing constraints on new models of physics – they have to get the predictions right

25. Summary • If permanent EDMs are detected, their magnitudes can be incorporated into early universe models of baryon creation, and the results can be compared to the matter-antimatter asymmetry we see now • If permanent EDMs are not detected at larger magnitudes than predicted by the Standard Model, we will be pushed to look for more substantial CP-violating processes

26. Resources Canetti, Laurent, et al. “Matter and Antimatter in the Universe.” New Journal of Physics, vol. 14, no. 9, 2012, p. 095012., doi:10.1088/1367-2630/14/9/095012. Lamoreaux, S K, and R Golub. “Experimental searches for the neutron electric dipole moment” iopscience.iop.org/article/10.1088/0954-3899/36/10/104002/pdf. “The ACME EDM Experiment” Electronedm.info, electronedm.info/.