Examination of Crystal-Enhanced Coherent Bremsstrahlung in GlueX from CERN NA59 Results

1. GlueX collaboration meeting May 20-22 2004, Bloomington Recent results from CERN NA59 Does the enhancement seen in bremsstrahlung from a crystal oriented in near-alignment of the symmetry axis with the beam offer interesting possibilities for GlueX? as reported in preprint nuhep-exp/2-007 A. Apyan, R.O. Avakian, et.al reviewed by Richard Jones, University of Connecticut

2. Motivation • coherent bremsstrahlung is a well-understood process • abundant data over energy range from 100 MeV to 100 GeV • theoretical CB description established since late 60’s • analytic formulas give good agreement with data • for certain configurations CB description has problems • multi-photon processes become important • quasi-classical picture sometimes useful: “channeling” • “channeling” is not a different process, just a different picture • channeling is usually associated with production of low-energy photons • BUT channeling effects are still an area of active experimental study • various sightings have been reported of large enhancements extending up to the end-point, including high degrees of polarization. • recent results from CERN seem to suggest that enhancements of the high-energy end of the photon spectrum might occur, albeit with low polarization.

3. The NA59 experiment z=0 81.3m

4. The NA59 experiment Si crystal gonimeter crystal polarimeter pair spectrometer lead glass 178 GeV e- beam beam dump z=0 81.3m

5. The NA59 experiment

6. Interpretation of NA59 results • The experimental conditions described in the article fall well within the coherent bremsstrahlung regime. • A generic result for coherent bremsstrahlung is that the gain goes to zero at the end-point. • When read carefully, the paper actually makes no claim of enhancement in the hard part of the photon spectrum. • The photon flux in the paper is much worse for the purposes of GlueX than a simple amorphous radiator.

7. What is meant by “coherent bremsstrahlung”? k k p p p’ p’ q q • general approach to QED problem of radiation by high-energy electron moving in a matrix of massive charges. • dominated by one-photon exchange diagrams above • integration over q splits into two distinct pieces: • small q: reduces to a discrete sum over allowed reciprocal lattice vectors of virtual Compton scattering terms with no recoil shift • large q: probability that crystal recoils coherently goes to zero, amplitude reverts to form for incoherent process (individual atoms).

8. Is it coherent over the entire crystal? … not really, it depends on the relative value of k k p electron is temporarily off-shell p’ q L

9. What happens as k goes to zero? L l • As long as L << l the treatment of sequential one-photon processes is correct • Multi-photon processes are present, but suppressed by powers of a(QED)

10. What happens as k goes to zero? L l soft photons . “channeling”

11. now back to NA59 • special crystal orientation: “String-of-strings” • qx=45mr • qy=35mr • CB spectrum has many overlapping low-energy peaks (x < 0.2) • Ltyp = 5 mm • l = 1 mm >> L (far from channeling regime) • beam emittance, mozaic spread, multiple scattering • blur out individual peaks in the low energy (10 GeV) region • probably some channeling from angular divergence • no coherent enhancement above 120 GeV (x=0.66) • very thick radiator: 15 mm ! (many photons per electron)

12. What the paper actually says: (D. Sober) • single-photon spectrum taken with pair spectrometer • spectrum drops faster than atomic bremsstrahlung • Monte Carlo (red histogram) also predicts rapid drop • paper says lowest data point “saturated” the detector • lowest energy bin is missing from the plot, but… • text says average yield is 14 g/e • 97% of all photons are in bin 0 1/k 0.5 g/e

13. What the paper actually says: • Key to understanding Fig. 4: What is Etot? • “total radiated energy” • It is the sum of the energy of some dozen photons • This is not the intensity spectrum of individual photons in the beam. • An electron radiates away ~2/3 of its energy in 15 mm which is only 0.7 rad. len. • Most of this energy goes into low-energy (~10 GeV) photons.

14. Summary • Using crystal radiators aligned near an axis can produce large yields of low-energy photons. • Claims are sometimes heard that these enhancements extend all the way to the end-point of the photon spectrum. • CERN experiment NA59 was carried out to look for such enhancements and check the polarization of hard photons. • Carefully interpreted, the NA59 results show neither an enhancement of the hard component of the photon spectrum nor any evidence of polarization there. • Hard bremsstrahlung is associated with large momentum exchange and is not enhanced by many soft interactions.