Physics topics in photon-proton and
photon-ion interactions at the LHC
Joakim Nystrand
Dept. of Physics and Technology,
University of Bergen, Norway
• Photon-nucleon/nucleus interactions will
occur at the LHC at energies higher than at
any existing accelerator.
• Examples: photoproduction_ of Vector
Mesons, photon+gluon  qq
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
Photon spectrum at Heavy-Ion Colliders
Electromagnetic field An equivalent flux
of photons.
(Fermi 1924, Weizsäcker-Williams 1935)
Equivalent photon spectrum
The photon spectrum
extends to  /R
 Wp  1000 Gev in
Pb+Pb collisions (Q2  0)
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
Two examples
”Elastic”
+p  V+p
or
+A  V+A
 Studied at RHIC
_
”Partonic” +g  q+q
in p+p or A+A
Both processes sensitive to the proton/nucleus gluon
density at low x.
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
Exclusive 0 production - STAR@RHIC
RHIC: Au+Au at s = 200 GeV
Au+Au Au+Au+0
Experimental signature:
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
Coherent coupling  pT  ħ/R
 Impossible to tag the outgoing nuclei
pT spectrum shows clear coherent signal
background, like-sign pairs
Signal+background, unlike-sign pairs
27 - Mar - 2004
HERA and the LHC
Fit to exp(-bt):
b  R = 7.5 ± 2 fm
Joakim Nystrand
First final results from STAR Collaboration at RHIC snn =130 GeV
(C. Adler et al., Phys. Rev. Lett. 89(2002)272302)
Cross sections in agreement with
Glauber model:
(Au+Au  Au+Au+0) [mb]
STAR
Exclusive: 410190100
Theory
350
Possible to have production
in coincidence w/ Coulomb
breakup.
 Easier to trigger on, but
reduces  by factor 10-100.
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
Summary of HERA Results
• Wp up to 200 GeV
• Light mesons:
  Wp 0.22
• J/:
  Wp 0.80
• a few tens of  seen
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
What can one expect at LHC ?
dn
d
k
 (p  Vp)
dy
dk
y=0

J/
LHC PbPb Wp = 130 GeV
x  5 · 10-4
Wp = 220 GeV x  1 · 10-3
LHC pp
Wp = 210 GeV x  2 · 10-4
Wp = 350 GeV x  6 · 10-4
Tevatron
Wp = 80 GeV x  1 · 10-3
Wp = 130 GeV x  5 · 10-3
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
p data from HERA
J/:  = 1.5 Wp 0.80 [nb, W in GeV]
 Extrapolate this to LHC energies
Too little data on , QCD predicts (2 gluon exchange)
d
dt
t 0
2
 s2ee
M
3
2
V

16

[
xg
(
x
,
)]
3M V5
4
Ryskin 1993
Can be parameterized as
(Martin et al. PLB 454 (1999)339, Frankfurt et al. JHEP 02(1999)002
:  = 0.06 Wp 1.7 [pb, W in GeV]
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
S.R.Klein, J. Nystrand
hep-ph/0311164, PRL
in press.
d
k
 (p  Vp)
dy
dk
dn
 in ultra-peripheral Pb+Pb collisions at the LHC
Uncertainty in measured
p cross section (mainly
statistics).
A2 scaling of QCD
prediction
See also Frankfurt, Guzey,
Strikman, Zhalov hep-ph/0304218
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
Can we see these events at LHC?
Rates at design Luminosity:
J/+–, e+e– : 0.8 Hz
+–, e+e– : 8 events/hour
Br.(J/+–, e+e–)  5.9 % , Br.(+–, e+e–)  2.4 %
• The rates should be sufficient.
• The production is centered
around mid-rapidity.
• No other particles in the event.
• The coherence gives a clear
experimental signal through
reconstruction of the total
transverse momentum.
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
(pppp+V)
dn
d
k
 (p  Vp)
dy
dk
S.R.Klein, J. Nystrand
hep-ph/0311164, PRL
in press.
Drees&Zeppenfeld
photon spectrum
point charge, b>1fm
Unlike Pb+Pb, production
not centered at mid-rapidity.
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
J/ at RHIC,  at Tevatron: (”photon”)  10-3 (”hadron”)
 Have to reject hadronic background by a factor 103
Rapidity gaps:
<dnch/dy>  4 - 6 in pp at Tevatron/LHC
Probability of having a gap of width y:
 exp( - <dnch/dy> · y )
 y  2 will be sufficient
Further rejection from pT distribution, pT < 1 GeV/c
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
Example 2: +gluon 
–
cc,
–
bb
S.R.Klein, J.Nystrand,
R.Vogt PRC 66 (2002)
044906.
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand
Summary
• Photon-proton and photonuclear interactions
will occur at the LHC.
• Extends the energy range studied at HERA.
• Large cross section for exclusive production of
vector mesons in p+p and Ultra-peripheral Pb+Pb
collisions.
27 - Mar - 2004
HERA and the LHC
Joakim Nystrand