Anomalous Soft Photons in Hadronic Decays of Z

1
Anomalous Soft Photons in Hadronic Decays
of Z
o

• Vassili Perepelitsa
• ITEP, Moscow/IFIC, Valencia
• for DELPHI Collaboration

2
Content

• Introduction The puzzle of anomalous soft
  photons
• DELPHI observations
• experimental technique
• the signal
• cross-checks
• Striking behaviour of the signal
• Discussion

3
Content

• Introduction The puzzle of anomalous soft
  photons
• DELPHI observations
• experimental technique
• the signal
• cross-checks
• Striking behaviour of the signal
• Discussion

4
Theory Bremsstrahlung from external lines should
dominate

• Soft Photons having transverse momenta pT ltlt pT
  of typical transverse momenta of hadrons in HE
  interactions300-400 MeV/c
• Low theorem/Gribov extension

?
?
?
and
Enhanced
Suppressed
2
2
M 1/(p k) m
1/2pk
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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. ? 1 for negative incoming
  and for positive
• outgoing particles, ? -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.

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CERN WA27 beginning the puzzle (1983-1984)

• K p hadrons gamma at 70 GeV/c
• BEBC
• Photons -0.001ltX lt0.008, p lt60 MeV/c
• After subtraction of photons coming from all
  known hadronic decays
• the residual signal was found to be similar
  in shape to the brems-
• strahlung, 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

T
F
Signal/Brems
arrived
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WA91 raw signal
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WA91 energy dependence
dN?
a

A (),
E
E 1 GeV
dE
E
0
0
Signal energy dependence agrees with that of the
bremsstrahlung
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Fixed target experiments



-
-
10
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

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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

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The DELPHI detector
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Typical hadronic event with soft ?

lt e , p390MeV/c
-
e , p100MeV/c
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High neutral flow soft ?
E?630MeV
Neutral jet energy40GeV
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Signal observation
Signal/Brems
Signal/Brems
3.40.20.6
4.00.30.8
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DELPHI results
-3
-3
Signal/Brems
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Check-ups
Changing generator
Test with charged particles
Difference/Signal 17
Difference/Signal 111
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Test with neutral pions
Two converted photons
Converted HPC photons
Combined upper limit RD/MC lt 1.015 at 95 CL
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DELPHI dimuon event
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the same event, the photon region
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Muon inner bremsstrahlung in µ µ events of Z
decays

_
0
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Dead cone of the muon bremsstrahlung
? 432
Max at
v3/?
4 mrad
2
Max at 1/?
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Dependences on jet characteristics
Jet momentum
Jet charged multiplicity
SIMILARLY TO BREMSSTRAHLUNG
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Dependence on jet mass and hardness
?j Ejet sin ?/2
mjet v Ejet ? pjet
2
2
T is angle to the closest jet
Similarly to bremsstrahlung
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Dependences on Nneu and Npar
SURPRISE!
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What about explanation?

• No theoretical explanation of the
• phenomenon still exists,
• in spite of the problem being under active
  investigation.

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Testing theoretical models

• 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
• (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
• d ? q i r i
  

2
---gt but fail to explain stronger
dependence on Nneu
example LUND string model of fragmentation needs
2-order enhancement!
2


i1
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String fragmentation model
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List of (failed) models

• String (Lund) model
• Van-Hove/Lichard model (cold quark-gluon plasma,
  via processes qq-gtg?, qg-gtq?)
• Collective model (Barshays pion condensate)
• Armenian model (Unruh-Davies effect)
• Nachtmanns model (quark synchrotron radiation in
  the stochastic QCD vacuum)
• Shuryaks model (confinement forces)

_
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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

_
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Is it a tail of New Physics?
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Is it a tail of New Physics?