Hadron physicsChallenges and Achievements

Mikhail Bashkanov

University of Edinburgh

UK Nuclear Physics Summer School

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OUTLINE OF THE COURSE• Lecture 1: Hadron Physics. Experiments: new toys –

new knowledge (progress in particle detector systems). Research areas: Hadron spectroscopy, meson rare decays (physics beyond SM), structure of hadrons.

• Lecture 2: Baryon spectroscopy, naïve quark model and beyond, molecular states, new horizons with precise measurements.

• Lecture 3: Using EM probes to learn about the nucleon. Nucleon form factors. Radius of the proton.

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HADRON PHYSICS

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ELECTROMAGNETIC INTERACTIONS

Ze

Ze

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ELECTROMAGNETIC INTERACTIONS

Ze

Ze

2

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EM -> STRONG INTERACTIONS

2qq

q q

g

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QUARKS• Fermions (spin )• 3 colors (red, green, blue)• Parity +1

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ENERGY DEPENDENCE OF THE COUPLING CONSTANT

q

Bare quark

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ENERGY DEPENDENCE OF THE COUPLING CONSTANT

q

Dressed quark

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ENERGY DEPENDENCE OF THE COUPLING CONSTANT

q

Dressed quarkΔ𝑝 ∙ Δ𝑥 ≥h

Low energy probe

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ENERGY DEPENDENCE OF THE COUPLING CONSTANT

q

Dressed quarkΔ𝑝 ∙ Δ𝑥 ≥h

High energy probe

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ELECTRON MICROSCOPYde Broglie wavelength of probe particle must be ~size of the object you wish to study

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STRONG COUPLING CONSTANT

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STRONG COUPLING CONSTANT

Perturbative QCDParticle Physics

Nonperturbative QCDNuclear Physics

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NUCLEAR VS PARTICLE PHYSICS

Nuclear Physics Particle PhysicsBelow charm threshold Above charm threshold

Nucleon structure Mesons with mass > 1.2 GeV

Light quark baryons (without c/b quarks)

anticolor

Meson Baryon

color

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MAJOR DIRECTIONS• Hadron spectroscopy:

• Hadron properties (mass, with, decay branching…)• Hadron structure (, , , meson-baryon molecule…)

• Precision tests of SM:

• Neutron magnetic moment• Neutron electric dipole moment• Muon/electron magnetic moment (g-2)• Rare decays of mesons• …

• Size and structure of nucleon

• Nucleon form factor• Nucleon radius

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RECENT PROGRESS IN NUCLEAR PHYSICS

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BUBBLE CHAMBERS

Gargamelle Bubble Chamber

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MAGNETIC SPECTROMETERS

𝐸2=𝑝2+𝑚2

Time Of Flight->velocity

𝑝=𝑚𝑣

√1−𝑣2

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MODERN DETECTORS• Large acceptance (close to 4 coverage)

• Charge and neutral particles

• Magnetic field, drift chambers• Calorimeters

• High luminosity

• High rate, fast triggering• Polarized beams/targets

• Polarimeters

High precision

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MODERN DETECTORS

KLOE

WASA

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PHOTONSBasics

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WHY DO WE USE E/M PROBES?

Pros:• Interaction is understood

(QCD)• Beams are clean• Beams can be polarized• Targets can be polarized

and dense

Cons:• Cross-sections are small• Photon beams were(!)

challenging• Polarized targets are

challenging• Nucleon polarimetry is

complicated

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TYPES OF PHOTON POLARIZATION

Linear polarization:

(Electric field vector oscillates in plane)

Circular polarization:

(Electric field rotates Clockwise or anticlockwise)

• Both real and virtual photons can have polarization• Determining azimuthal distribution of reaction products

around these polarization directions gives powerful information.

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HOW DO WE GENERATE INTENSE ELECTRON BEAMSMicrotron: (MAMI, JLab)• Electron beam accelerated by RF cavities.• Tune magnetic field to ensure path through

magnets multiple of Wavelength of accelerating field - electrons arrive back in phase with the accelerating field.

• Gives “continuous” beam(high duty factor)

Stretcher ring: (ELSA, Spring8)• Electron beams fed in from linac.

Then accelerated and stored in ring.Useable beam bled off slowly

• Many stretcher rings built for synchrotron radiation – can exploit infrastructure for multiuse (e.g. Spring8)

• Tend to have poorer duty factors, less stable operation and poorer beam properties than microtrons.

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REAL PHOTON BEAMS FROM ELECTRON BEAMS

Wide range of photon energies

Good time/position resolution for the tagger

Small radiator-target distance

Bremsstrahlung spectra

Θ𝑐=𝑚𝑒[𝑀𝑒𝑉 ]𝐸𝑒[𝑀𝑒𝑉 ]

[𝑟𝑎𝑑]

E e=855MeV→Θ𝑐=0.6𝑚𝑟𝑎𝑑

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POLARIZATION IN REAL PHOTON BEAM

Linear polarization:• crystalline radiator,

e.g. thin diamond.

• orient diamond to give polarised photons in certain photon energy ranges.

𝐸𝑒=1600𝑀𝑒𝑉

Circular polarization:• helicity polarised

electrons.

• bremsstrahlung in amorphous radiator, e.g. copper.

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COHERENT BREMSSTRAHLUNG

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LINEAR POLARIZATION

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COHERENT BREMSSTRAHLUNG

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FROZEN SPIN TARGET• available (Mainz) since

05.2010

• Butanol() or D-Butanol

• 3He/4He dilution refrigerator (50mK)

• Superconducting holding magnet

• Longitudinal or transverse polarizations are possible

• Maximal polarization for protons ~90%, for deuterons ~75%

• Relaxation time ~2000 hours

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THE POLARIZED TARGET

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NUCLEON POLARIMETER𝑛 (Θ ,𝜙 )=𝑛0(Θ)(1+𝐴(Θ) [𝑃 𝑦 cos (𝜙 )−𝑃𝑥sin (𝜙)])

𝐀𝐲

𝚯�⃗�

𝝓

Number of nucleons scattered in the direction

Polar angle distribution for unpolarized nucleons

Analysing powerPolarization

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HADRON SPECTROSCOPY

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REAL EXPERIMENT

𝑒−

�⃗�

Diamond

𝜸

Target

𝝅+¿ ¿𝝅−𝒑

Θ ,𝜙 ,𝐸

𝚯′ ,𝝓 ′

�⃗� �⃗�→ �⃗� 𝜋+¿𝜋 −¿ Polarimeter

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INTERFERENCE

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DECAY WIDTH

Mean life time

Decay width

Typical “strong” decay width

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NUCLEON EXCITED STATES

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DOUBLE POLARIZATION EXPERIMENTS

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POLARIZATION OBSERVABLES

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RESONANCE HUNTING

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MESON PHOTOPRODUCTION CROSS SECTIONS

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RARE EVENTSThe Standard Model and beyond

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PRECISION IS POWER

• Neutron electric dipole moment

• Muon magnetic moment (g-2)

• rare decays

• ….

Testing Standard Model with precise measurements

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ELECTRIC DIPOLE MOMENT

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NEUTRON EDM

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NEUTRON EDM

SM

SUSY

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RARE DECAYS

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: CP VIOLATION

𝐵𝑟 ¿

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UNIVERSE CONTENT

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SEARCH FOR DARK PHOTONDark force:

Dark photon

𝜼 /𝝅𝟎

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CONCLUSION

• Enormous progress in nuclear physics

• Precision is a new motto

• Acceptance• Luminosity• Polarization

• Photons are the best

• Experimentally clean• Well understood theoretically