Richard MilnerHERA symposium June 30, 20071 HERMES physics, a historical * perspective Why was HERMES proposed? How did the concept of HERMES evolve? What

Richard Milner HERA symposium June 30, 2007 1

HERMES physics, a historical* perspective

•Why was HERMES proposed?• How did the concept of HERMES evolve?• What were the physics goals?

* and personal

Outline

• The physics which motivated HERMES• My personal path to HERMES





This worthless equation…


HERMES originated in 1987 from a number of sources

• EMC spin results => quarks carry only a fraction of the proton’s spin

• Significant development in polarized internal gas target technology

• HERA had the potential for longitudinally polarized electron/positron beams

• Common interests of scientists in Europe and North America

• Seek to understand proton structure in terms of the quarks (and gluons) of QCD

• Desire to be technically innovative


The structure of the proton ~ 1985

• Structure and dynamics understood in terms of quarks: gluons assumed to carry half of the proton’s momentum but essentially `invisible’

• Theoretical QCD inspired models based on quarks and confinement, e.g. the MIT bag model

• EMC Effect => quark momentum modified in the nucleus

• Violation of Gottfried Sum Rule => flavor asymmetry of the sea established

• No lattice QCD


The EMC Effect

Geesaman, et al., Ann. Rev. Nucl. Part. Sc. 45, 337 (1995)

J.J. Aubert et al. Phys. Lett. B 123, 275 (1983)


The MIT Bag Model

• Quarks are treated as massless particles inside a bag of finite dimension

• The quarks are infinitely massive outside the bag

• Confinement results from the balance of pressure on the bag walls from the outside and the pressure resulting from the kinetic energy of the quarks inside

• The bag pressure constant, B, is related to the equilibrium radius of the bag R as B ~ R-4

• Inside the bag perturbative QCD applies

• The total color charge of the matter inside the bag must be colorless => valid hadronic bags only contain qqq and q

A. Chodos et al., Phys. Rev.

D 9, 3471 (1974)

q Models of the NucleonBhaduri


Constituent quark model of proton

• All pairs of colored quarks are antisymmetric under interchange• Pairs must be symmetric under interchange of other quantum numbers• Suppose symmetric in momentum space => identical favors couple symmetrically, i.e. to S=1• Coupling S=1 with the S=½ of the remaining valence quark to form J=½ yields


Proton spin in constituent quark model


Quark fraction of proton spin

ExperimentallygA/gV ~ 1.25


Asymmetries


Nucleon magnetic moments


Integrals (2·Z in Bj’s paper)

isovectoroctetsinglet

Isovector, SU(3)flavor octet and singlet axial charges


Sum RulesBjorken Sum Rule (1966)

Ellis Jaffe Sum Rules (1974) when αs -> 0, and Δs = 0.


Ellis-Jaffe Sum Rules

Ellis-Jaffe (1974) predicts Ip = 0.175 ± 0.018


SLAC E80, E130• Data taken in late 1970’s

• 22.7 GeV polarized electron beam pB ~ 80%, I ~ 10 nA

• Polarized beam generated from 6Li atomic beam• 8 GeV spectrometer and custom larger acceptance

spectrometer used

• DNP butanol polarized proton target pT ~ 60%

• Asymmetry measured in region 0.1 < x < 0.7, 1 < Q2 < 10• Asymmetries independent of Q2

• Proton integral = 0.165 ± 0.05 • Consistent with Ellis-Jaffe Sum Rule • Assuming the BSR, this implied that the neutron integral

should be small


E130 SLAC Experiment


E80 and E130 Data

5/9

x-dependence of A1

in the valence regionconfirms the quarkmodel predictions


Expected neutron asymmetry after E130

Hughes and KutiAnn. Rev. Nucl. and Part. Sc. 33,611 (1983)0


EMC Experiment

• Data taken in mid 1980’s• 100-200 GeV muon beam, flux ~ 2 x 107 s-1

• Muon beam naturally polarized pB ~ 80%

• DNP 1 meter long ammonia polarized target pT ~ 80%

• Asymmetries measured in the range 0.01 < x < 0.7 and 3 < Q2 < 30 (GeV/c)2

• I first became aware of the results of the EMC experiment from Terry Sloan at the SLAC Workshop on Electronuclear Physics with Internal Targets on January 5-8, 1987



EMC CERN Experiment


EMC Polarized Target


Dilution Factor


EMC Data


Violation of Ellis-Jaffe Sum Rule!

• Surprise results from effects of sea quarks/gluons at low x • Sizable neutron integral• Sizable, negative polarization of strange quarks!


Polarized lepton beams

• In ~ 1987 available beams included- low intensity, highly polarized muon beams- high intensity, low polarization electron

beams • Interest in studying the spin structure of the nucleon

drove the polarized electron beam technology in a major way

• At SLAC, ~ 80% polarized electron beams were developed

• At DESY, HERMES drove the development of ~65% polarized electron/positron beams using the Sokolov-Ternov effect

• Precision beam polarimetry was developed


SLAC Polarized Electron Source


See talk by Erhard Steffens


Spinoff I: Strangeness in the Proton

• From EMC data and assuming SU(3)flavor

symmetry, it was concluded that the polarization

of the strange quarks was negative Δs ~ -0.10±0.04

• When taken together with other data from pion

and neutrino scattering, this led to a major effort to observe sizable effects of strange quarks

• For example, a worldwide program of parity violating electron scattering from the nucleon was carried out at Bates, Mainz, and Jefferson Lab

• No clear evidence for the effects of strange quarks has been observed


G. Cates


Spinoff II: GDH Sum Rule Measurements

Causality, unitarity, Lorentz and gauge invariance => low Q2 sum rule

This has been a major focus of effort at ELSA, Mainz, JLab with electron beams and at GRAAL, Spring-8 and LEGS/BNL with photon beams


My personal path to

HERMES



OF PEGASYS


PEGASYS Experimental Layout


OF PEGASYS


PEGASYS projection for An1


- Terry Sloan gave a talk on the semi-inclusive unpolarized measurements with the EMC experiment

- Privately, he told us of the unexpected results on the proton spin asymmetries

- Terry encouraged the idea of an internal target experiment to measure A1

n using polarized 3He

- Bob Jaffe went off to the SLAC library to write a paper



Richard Milner HERA symposium June 30, 2007 44Meeting at DESY, September 30th 1987





HERMES became the official nameof the collaboration about June 1989

Costas Papanicolas, then at the University of Illinois, came up withthe name



Semi-inclusive deep inelastic scattering

F. Close and R.M., Phys. Rev. D44, 3691 (1991)


Rates for SIDIS



August 1992





HERMES Physics Goals

• Measure the valence quark spin distribution in the nucleon

• Test the Bjorken Sum Rule• Provide a stringent test for models of the nucleon• Measure semi-inclusive deep inelastic scattering

to probe the polarization of the sea • Transverse target polarization was to constrain

the magnitude of g2!


HERMES

• Was a technically innovative experiment• Owes its existence to the DESY laboratory being

open to new ideas• Has had a physics scope and impact far beyond

that which motivated its beginnings• Has been (together with ZEUS and H1) the major

source of new insight into the fundamental structure of matter over the last decade

• Has educated and trained new generations of hadron physicists

• Was conceived, realized and carried out in a truly international spirit of cooperation by physicists largely from institutions outside of DESY

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Richard MilnerHERA symposium June 30, 20071 HERMES physics, a historical * perspective Why was HERMES proposed? How did the concept of HERMES evolve? What