Anomalous Soft Photon Production
in Multiple Hadron Processes
Vassili Perepelitsa
ITEP, Moscow/IFIC, Valencia
for WA83, WA91, WA102
and DELPHI Collaborations
Content
• Introduction: The puzzle of anomalous soft
photons
• Experiments with hadronic beams
• LEP, DELPHI observations:
experimental technique;
the signal;
cross-checks;
• Striking behaviour of the signal
• Discussion
Content
• Introduction: The puzzle of anomalous soft
photons
• Experiments with hadronic beams
• LEP, DELPHI observations:
experimental technique;
the signal;
cross-checks;
• Striking behaviour of the signal
• Discussion
Theory: Bremsstrahlung from external lines should
dominate
Soft Photons: having transverse momenta pT << pT of typical
transverse momenta of hadrons in HE interactions=300-400 MeV/c
Low theorem/Gribov extension
ISR
FSR
Radiation from central blob
and
2
2
M ~ 1/[(p – k) – m ] = 1/2pk
Bremsstrahlung calculations
where K and k denote photon four- and three-momenta,
P are the four-momenta of all the charged particles participating
in the reaction. ƞ = +1 for negative incoming and for positive
outgoing particles, ƞ = -1 for positive incoming and negative
outgoing particles, and the sum is extended over all the N + 2
charged particles involved. The last factor in the integrand is a
differential hadron production ratio.
CERN WA27: beginning the puzzle (1983-1984)
• Kp
+
hadrons + gamma
at 70 GeV/c
BEBC
Photons: -0.001<XF<0.008, pT<60 MeV/c
After subtraction of photons coming from all known hadronic decays
the residual signal was found to be similar in shape to the bremsstrahlung, but bigger in size by a factor of about four
= 4.0 ± 0.8
In such a way the effect of anomalous soft photons has
CERN NA22 and WA83: confirmation of the signal
• CERN NA22 (1990-1991), EHS Spectrometer
π+ p
Kp
+
•
hadrons + gamma at 250 GeV/c
hadrons + gamma at 250 GeV/c
pT < 40 MeV/c
= 6.9 ± 1.3 (pion beam)
= 6.3 ± 1.6 (kaon beam)
CERN WA83 (1985-1992), Ω Spectrometer+El.Mag.Cal.
_
π p
hadrons + gamma at 280 GeV/c
p T < 10 MeV/c
= 7.9 ± 1.6
WA91 experiment,
π-p exposure, 280 GeV/c
WA91 event examples
WA91 raw signal
WA91 energy dependence
The spectra were fitted by a form:
dNɣ
E α
— = A (—
), E0=1 GeV
dE
E
0
Signal:
A = (6940±540±1910) 1/GeV
α = ―1.11±0.09±0.04
Bremsstrahlung:
A = (1460±44) 1/GeV
α = ―0.93±0.04
Signal energy dependence agrees with that of the bremsstrahlung
Observed photon rate:
-3
(92±4±15) ˣ 10 ɣ/evt
Predicted hadronic bremsstrahlung:
-3
(17.4±0.3±1.2) ˣ 10 ɣ/evt
= 5.3±0.2±0.9
WA102 experiment,
pp exposure, 450 GeV/c
WA102 soft photon events
WA102 pT and angular distributions
Observed photon rate:
-3
(47.3±1.8±9.1) ˣ 10 ɣ/evt
Predicted hadronic bremsstrahlung:
-3
(11.6±0.2±0.7) ˣ 10 ɣ/evt
=4.1±0.2±0.8
Experiment,
Reaction, Beam momentum
Photon
kinematic range
Signal/Brems
ratio
SLAC, BC
π+ p —› hadrons+ɣ, 10.5 GeV/c
0 < XF < 0.01
Eɣ > 30 MeV,
PT < 20 MeV/c
1.25 ± 0.25
CERN WA27, BEBC
K + p —› hadrons+ɣ,
70 GeV/c
-0.001 < XF < 0.008
Eɣ > 70 MeV,
PT < 60 MeV/c
4.0 ± 0.8
CERN NA22, EHS
< XF < 0.008
Eɣ > 70 MeV,
PT < 40 MeV/c
6.4 ± 1.6
6.9 ± 1.3
2 < yc.m.s. < 5
CERN WA83, OMEGA
π p —› hadrons + ɣ, 280 GeV/c 0.2 < Eɣ < 1 GeV,
PT < 10 MeV/c
7.9 ± 1.4
CERN WA91, OMEGA
1.4 < yc.m.s. < 5
π - p —› hadrons + ɣ, 280 GeV/c 0.2 < Eɣ < 1 GeV,
PT < 20 MeV/c
5.3 ± 0.9
K + p —› hadrons + ɣ, 250 GeV/c
+
π p —› hadrons + ɣ, 250 GeV/c
- 0.001
18 GeV/c
-1.4 < yc.m.s. < 0
15 < Eɣ < 150 MeV, PT < 10 MeV/c
< 2.7
(at 90% CL)
CERN NA34 (HELIOS)
p Be —› hadrons + ɣ, 450 GeV/c
-1.4 < yc.m.s. < 0
15 < Eɣ < 150 MeV, PT < 10 MeV/c
< 1.5 – 3
(at 90% CL)
CERN WA102, OMEGA
p p —› hadrons + ɣ, 450 GeV/c
1.2 < yc.m.s. < 5
0.2 < Eɣ < 1 GeV,
PT < 20 MeV/c
4.1 ± 0.8
BNL
p Be —› hadrons + ɣ,
LEP, DELPHI observations
• Signal discovery
• Check-ups
• Muon inner bremsstrahlung: control
experiment
• Signal dependence on the parent jet
characteristics
• Non-trivial behaviour with the jet neutral
and total multiplicities
LEP, DELPHI observations
• Signal discovery
EPJ C47 (2006) 273
• Check-ups
• Muon inner bremsstrahlung: control
experiment
EPJ C57 (2008) 499
• Signal dependence on the parent jet
characteristics
CERN-PH-EP/2009-14
• Non-trivial behaviour with the jet neutral
and total multiplicities
The DELPHI detector
Typical hadronic event with soft ɣ
+
<— e , p=390MeV/c
e-, p=100MeV/c —›
High neutral flow soft ɣ
Signal observation
=3.4±0.2±0.6
=4.0±0.3±0.8
Observed photon rate:
-3
(69.1±4.5±12.9) ˣ 10 ɣ/jet
Predicted hadronic bremsstrahlung:
-3
(17.1±0.1±1.2) 10 ɣ/jet
=4.0±0.3±0.8
Check-ups
Changing generator
Test with charged particles
Difference/Signal = 1:7
Difference/Signal = 1:11
Test with neutral pions
Two converted photons
Converted + HPC photons
Combined upper limit: RD/MC < 1.015 at 95% CL
DELPHI dimuon event
the same event, the photon region
_
Muon inner bremsstrahlung in μ μ
0
events of Z decays
+
Signal corrected for efficiency:
25.9±4.0±1.4
Muon inner bremsstrahlung:
23.30±0.01±0.93
Signal/muon brems:
1.06±0.12±0.07
Upper limit for an excess:
1.29 at 95% CL
Dead cone of the muon bremsstrahlung
Г = 432
Max at √3/Г
4 mrad
Max at 1/Г
2
Dependences on jet characteristics
Jet momentum
Jet charged multiplicity
SIMILARLY TO BREMSSTRAHLUNG
Dependences on jet mass and hardness
mjet = √ Ejet2 ― pjet2
κj = Ejet sin θ/2
Θ is angle to the closest jet
Similarly to bremsstrahlung
Dependences on Nneu and Npar
What about explanation?
• No theoretical explanation of the
phenomenon still exists,
in spite of the problem being
under active investigation.
Strong dependence on Nneu suggests:
a) either the radiation comes from individual
quarks and/or quark-antiquark pairs;
b) or it comes, due to some collective effects,
from a jet as a whole.
Collective models fail experimental tests:
no dependence on Mjet, neither on jet net charge
2
(collective behaviour predicts Nnet
dependence).
Noncoherent models agree well with linear
dependence on total particle multiplicity
(the radiation ~ sum of quark charges squared)
Modification of noncoherent approach:
consider quark-antiquark pairs as radiating
(electromagnetic) dipoles:
—›
2
d = Ʃq
i=1
i
ri
—›
String fragmentation model
List of (failed) models
• String (Lund) model
• Van-Hove/Lichard model
(cold
quark-gluon
_
plasma, via processes qq->gɣ, qg->qɣ)
• Collective model (Barshay’s pion condensate)
• Armenian model (Unruh-Davies effect)
• Nachtmann’s model (quark synchrotron
radiation in the stochastic QCD vacuum)
• Shuryak’s model (confinement forces)
Models still alive but underdeveloped
• Internal quark loop model (Simonov Yu.A.):
based on nonperturbative
QCD methods
_
applied to the large size systems
(contains a strong enhancement mechanism)
• Gluon dominance model (Kokoulina E.S.)
appeals to a new physics phenomenon:
exitation of physical vacuum leading to
thermal radiation with T ~ 30 MeV
Wong’s model (arXiv:1001.1691)
• Exploits longitudinal dominance and transversal
confinement of fragmentation process in order to
_
use the formalism of 2-dimensional QED (QED2)
for calculation of anomalous soft photon yield
associated with the meson production.
• Production of mesons in the model arises from the
oscillations of colour charge densities of the quark vacuum in
the flux tube when a quark and antiquark pull away from each
other at high energies. Because a quark carries both a colour
charge and an electric charge, the underlying
dynamical motion of quarks will also generate electric
charge oscillations which will lead to ASP production.
Is it a tail of New Physics?
Is it a tail of New Physics?