ROLE OF CHARM AT ULTRAHIGH ENERGY

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ROLE OF CHARM AT ULTRAHIGH ENERGY
CRIS 2006 UHECR Status and Perspectives

• I.M.Dremin, V.I.Yakovlev
• Lebedev Physical Institute of RAS

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Recent cosmic ray results at ultrahigh
energies reveal new exciting features. They
can be related to charm production and allow to
make some predictions for further studies at
Pierre Auger installation.
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The energy spectrum of cosmic rays decreases
with energy very fast. Moreover, it has been
pre- dicted 1, 2 that due to interactions of
primary protons with the Background relic
radiation their spectrum should be drastically
cut off at energies exceeding 1.51019 eV
(GZK-effect). For primary nuclei, this drop down
starts at even lower energies about 1018 eV due
to their photo- disintegration.
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Knowing the density of the background photons and
the photoproduction cross section one can
estimate the interaction length and show that the
distance to the sources of such particles can not
exceed 40 - 100 Mps. Thus the ultrahigh energy
cosmic rays are important not only from the point
of view of hadron interactions at these energies
but also for studies of the cosmic rays origin.
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Recent studies of extensive air showers (EAS)
showed however that some excess of events at
energies above 51019 eV over the predicted
cutoff persists even though there is some
disagreement between the data obtained at
different installations 3, 4, 5.
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ENERGY SPECTRUM MEASURED WITH AGASA
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New surprises came out from the energy region
just below the cutoff threshold 1018 lt E lt 1019
eV. At the ICRC-2005 it was reported 6 that
at Pierre Auger installation a significant
number of very horizontal events are detected,
offering a novel view of EAS. (X.Bertou) Earlier
such result was pointed in papers of Yakutsk
group.
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Below we demonstrate that charm production
can explain present situation and predict some
new unexpected phenomena. Let us turn to short
history of charm in cosmic rays investigations.
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SHORT HISTORY OF LFC

• 1971 K.Niu published the very 1-st event with
  charm.
• 1972 V.Yakovlev presented several events with
  slow attenuated component (later Long Flying
  Component) in calorimeter (Tokyo).
• 1974 J / y discovery. First publication on
  LFC.
• 1975 E.Feinberg assumed that charmed particles
  could be in charge on LFC production.
• Late 70s Dc-mesons detected. Estimated lifetime
  18 times less than real. Idea of Feinberg
  abandoned for several years.
• 1984 I.Dremin, V.Yakovlev - Estimation of charm
  production
• cross-section at 10-20 TeV (Tien-Shan
  effect).
• 1987 PAMIR collaboration LFC confirmed.
• 2005 RHIC confirmed Tien-Shan data 1984 on charm
• production cross-section.

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• To manifest itself in the cascade charmed
  particles
• should possess the following properties
• Large enough production cross-section
• They should carry out significant share of
  energy
• They should lose less energy at interaction.
• It was shown by Boreskov and Kaidalov that at
• associative production Lc and D-bar take off
• the overwhelming part of interacting particle
  energy.
• Owing to presence of heavy quark charmed
  particles
• lose less energy at interaction and thus keep the
  
• energy until decay and carry it deep into
  cascade.

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Long Flying Component in Air If the role of
charmed particles is significant than, at
energies above 1017 eV, they should manifest
themselves deeply in the atmosphere. This idea
was formulated for the first time by Stodolsky
and McLerran in 1982.
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If one assumes that charm production
cross section saturates at 5 mb/nucl at energies
above 100 Tev and sc(A) A1sc p-p then for
nucleon-air interactions fraction of EAS (at E gt
3.1018 eV) with charmed particles produced in
the first interaction will be 5x16/456 0,175.
Here 16 is the average atomic weight of air
and 456 mb is the p-air inelastic cross
section at this energy. Possibly, charm
production cross-section in A-A interactions is
even larger.
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We should stress, that because the
secondary particles multiplicity at Egt1017 eV
is high, above 0,175 of total secondaries in
each EAS will produce charmed particles.
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EASs with charm delay in their development in
comparison with normal showers. Thus with
increasing of EAS energy their maximum shifts
very fast toward the observation level. It was
observed experimentally.
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The ratio of the EAS size at sea level to those
in shower maximum as a function of the primary
energy was investigated. This ratio steeply
increases starting from the shower energy E gt
1017 eV, that indicates the rapid shift of the
shower maximum toward the sea level.
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What happens with cascade in atmosphere in
this case?
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We can see that in delayed cascade number of
EAS particles (E0) can be overestimated area
under cascade (E0) can be underestimated.
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Preliminary studies based on comparing real
data with simulation data give energies that are
systematically higher than the FD-normalized
energies by approximately 25. This number has
some dependence on the hadronic model, the
primary mass and the shower propagation code that
are assumed. 29-th ICRC P. Sommers for The
Pierre Auger Collaboration
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Thus our 3 expectations for Pierre Auger array
are 1) Ground array will detect more events in
comparison with fluorescent array at the
energy which looks as if being equal 2) Ground
array can demonstrate seeming absence of GZK
cutoff. 3) Underestimation of fluorescence light
flux can increase with energy.
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Let us turn now to very inclined
showers. Because they cross very thick
atmosphere, their electron-photon component is
absorbed, so they contain only muons. Muons with
positive and negative electric charge diverge in
opposite directions in geomagnetic field and
thus form the pattern similar to the number 8.
EAS should become assymetric. In addition the
lateral distribution of muons should become
flatter in comparison with vertical showers.
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However, according to data of Yakutsk
group shower asymmetry decreases at very large
zenith angles that can indicate on increase of
muon transverse impulse or increase of muon
generation depth. This group has obtained also
data on increase of muon lateral distribution
steepness for Inclined showers at highest
energies .
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As is seen the muon lateral distribution for
inclined EASs becomes at high energies as steep
as for vertical showers. It means that these
showers were produced close to the observation
level, that is they were produced by unstable
particles.
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CONCLUSION The large role of charm production
changes EAS parameters at ultrahigh energies.
Showers delay in their development. This
can result in underestimation of energy released
in the atmosphere and overestimation of particle
flux at the observation level (seeming absence
of GZK cutoff !!!). Parameters of inclined EASs
at ultrahigh energies can become the same as for
vertical ones without charm, including presence
of hadronic component.
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