Recent results from CERN NA59

1
Recent results from CERN NA59
GlueX collaboration meeting May 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
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
  60s
• 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
z0
81.3m
4
The NA59 experiment
Si crystal gonimeter
crystal polarimeter
pair spectrometer
lead glass
178 GeV e- beam
beam dump
z0
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 ltlt 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
• qx45mr
• qy35mr
• CB spectrum has many overlapping low-energy peaks
  (x lt 0.2)
• Ltyp 5 mm
• l 1 mm gtgt 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 (x0.66)
• 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.