1. Recent results from CERN NA59 Richard Jones, University of Connecticut GlueX collaboration meetingMay 20-22 2004, Bloomington 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

2. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 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. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 3 The NA59 experiment z=081.3m

4. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 4 The NA59 experiment z=081.3m Si crystal gonimeter beam dump pair spectrometer crystal polarimeter lead glass 178 GeV e - beam

5. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 5 The NA59 experiment

6. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 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. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 7 What is meant by “coherent bremsstrahlung”? q p k p’ q p k  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. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 8 Is it coherent over the entire crystal? … not really, it depends on the relative value of k q p p’ electron is temporarily off-shell k L

9. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 9 What happens as k goes to zero? L  As long as L << the treatment of sequential one-photon processes is correct  Multi-photon processes are present, but suppressed by powers of  QED 

10. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 10 What happens as k goes to zero? L. “ channeling ” soft photons

11. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 11 now back to NA59  special crystal orientation: “String-of-strings”  x =45  r  y =35  r  CB spectrum has many overlapping low-energy peaks (x < 0.2) L typ = 5  m  = 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. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 12 What the paper actually says:  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  /e  97% of all photons are in bin 0 1/k 0.5  /e (D. Sober)

13. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 13 What the paper actually says:  Key to understanding Fig. 4: What is E tot ?  “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. Richard Jones, GlueX meeting, Bloomington, May 20-22, 2004 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.