Potentialities of the ATLAS detector for studies of highenergy solar cosmic rays S'N'Karpov, Z'M'Kar

1
Potentialities of the ATLAS detector for studies
of high-energy solar cosmic rays S.N.Karpov,
Z.M.Karpova and V.A.Bednyakov Joint Institute
for Nuclear Research (JINR), Dubna

• Physics and Computing at the ATLAS experiment,
  September 16-17, 2008, Protvino, Russia

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Large Hadronic Collider (LHC, CERN).ATLAS
detector.

• LHC is the machine for proton-proton and heavy
  ion collisions. It is under construction at CERN
  of Geneva.
• Length of accelerators ring 27 km
• Energy of proton-proton collisions 7 7 14 TeV

• The design luminosity is 1034 cm2 s1

• The ATLAS is located in underground cavern at the
  depth about 80 m

• Integral luminosity after 1 month will be L 1
  fb1, and after 3 years 300 fb1

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Introduction

• The ATLAS detector is intended to verify the
  standard model and to search for new physics.
• In addition to this primary goal, it also allows
  detection of cosmic ray (CR) muons.
• On the other hand, unusual bursts of the muon
  intensity, which correlate with powerful solar
  flares (GLE events), were recorded and
  investigated earlier at the Baksan Underground
  Scintillation Telescope (BUST, INR, Russia)
  during 21-23 solar activity cycles.
• Similar muon bursts were recorded later in the
  GLE event on 14 July 2000 by the L3C detector
  (CERN, Switzerland) and by the TEMP muon
  hodoscope (MEPhI, Russia).
• The nature of the muon bursts and their probable
  relation to solar cosmic rays (SCR) is still not
  quite clear.

29.09.1989 muon burst at the BUST.
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Solar Cosmic Rays (SCR)

• The most energetic particles of SCR are generated
  on the Sun during powerful flares and processes
  accompanying them.
• The registration of solar particles with greatest
  possible energy achieved on the Sun is one of the
  major observational tasks in the problem of SCR
  generation.

• The ATLAS has the excellent muon system, which
  allows searching for similar muon bursts.
• Nearest years, when the LHC and ATLAS should
  start to operate, an increase of the solar
  activity during the new 24th cycle is expected.
• Therefore, ATLAS has a good opportunity to verify
  the relation of muon bursts to the SCR.

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Baksan Underground Scintillation Telescope (BUST)

• The BUST situated in Baksan Valley, North
  Caucasus.
• Geographic coordinates are 43.28?N, 42.69?E.
• The BUST consists of 3150 detectors with volume
  70?70?30 cm3 filled by liquid scintillator.
• Minimal thickness of rock is about 300 m.
• Effective underground depth makes up 850 m of
  water equivalent.
• Angular resolution on the average is ?2º.
• The telescope has the effective area of 200 m2.

• Minimal energy of muons needed to propagate
  through the rock and to register by the BUST is
  E? 200 GeV.
• Corresponding energy of primary protons is about
  Ep 500 GeV.

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Method of analysis

• The Neutron Monitor (NM) network recorded 36
  Ground Level Enhancements (the GLE events) of
  Solar Cosmic Rays during the BUST operation since
  April 1981 to December 2006. The data of the
  muons registration at the BUST are available in
  34 cases.
• Minimal energy of single muons registered at the
  BUST makes up 200 GeV. Primary protons in this
  case have energy gt500 GeV. It is approximately
  100 times more than energy of SCR, which are
  usually registered by the NM at the Earths
  surface.
• The BUST registers muons as the trajectory events
  and they are summarized in angular distribution
  during each 15-minute. Visible region of sky was
  divided into 680 angular cells of 10?15 in
  size. The cells are mutually overlapped.
• Search for probable signals is carried out
  counting rate of cells during 3-hour interval for
  each flare (1 hour before a maximum of X-ray
  flare and 2 hours after that).
• Only one maximal burst for every GLE event was
  selected from all found excesses above a
  background. Such bursts were considered as
  candidates for the probable signals of SCR.
• The probability P(3h) of random realization of a
  burst due to background fluctuations in any of
  680 angular cells during 3 hours was used then
  for definition of statistical significance of the
  burst

where n 680 ? 12 8160 is total number of
angular cells and time intervals, and w Poisson
probability of the burst with amplitude ?N.
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Integral distribution of the bursts number versus
1/P(3h)

• The total number of bursts having probability not
  exceeding the P(3h) value is shown in each point
  (integration from 1/P up to ?).
• Theoretically the integral distribution of events
  N(1/P) for a purely random process (Poisson,
  Gauss, etc.) in double logarithmic scale presents
  a direct line with a slope k 1 (as a corollary
  of the law of big numbers).
• The 3-hour intervals distanced on 1 day before or
  after corresponding flare were used as a
  background.

• Selection of the bursts within those intervals
  was made by the same method as during GLE events.
  
• The bursts distribution recorded during GLE
  events significantly differs from a background
  and from the theoretically expected distribution.
  Differences of experiment from background and
  theory are beginning from probability P ? 0.1.
• The observed surplus of bursts with large
  amplitude indicates to additional muon flux
  during GLE events.

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Significance of muon bursts recorded at the BUST
during 1981-2006

• Four muon bursts are obviously distinguished by
  the importance 1/P (inverse value to
  probability of random imitation of burst due to
  fluctuations of a background)
• 12 October 1981,
• 29 September 1989,
• 15 June 1991 and
• 28 October 2003.
• These bursts provide difference of integral
  distribution during GLE events from theoretically
  expected and from background distributions
  (previous slide).

• The specified bursts can be considered as
  possible increases of SCR with energy more 500
  GeV.

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Main properties of the most significant muon
bursts, which were found at the BUST during GLE
events

• Main properties
• Short duration ( 15 min )
• Small solid angle ( 10º 15º cell)
• Delay from maximum of X-ray flare on 1-2 hours
• Minimal energy of protons EP 500 GeV

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The bursts distribution inside 3-hour interval of
observation
GLE events

• It is obvious that distribution has kept an
  asymmetry, and the majority of bursts are
  observed within 1-2 hours after a maximum of
  X-ray flare.
• All four most significant bursts are also in this
  time interval.
• Temporal distribution of the bursts for
  background intervals is close to uniform
  distribution.

Background intervals
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The bursts distribution over ecliptic longitude
GLE events

• The majority of bursts are observed from
  directions within the longitudes range from 60
  to 180.
• The background distribution also has similar
  asymmetry. Hence, it is mainly due to orientation
  of the sensitivity diagram of the BUST in these
  directions during GLE events.
• Exception is only interval 0 60 to the West
  from the Sun-Earth direction. Number of events in
  this interval differs appreciably from a
  background event number.
• This interval between 0 60W contains more
  than one third of all bursts, including three out
  of four most significant ones.

Background intervals
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Muon system of the ATLAS detector

• Outer part of muon system of the ATLAS detector
  is a horizontal cylinder with diameter 22 m and
  with length about 30 m.
• Hence, a cross-area of the ATLAS detector for
  vertical flux of cosmic rays will be about 660
  m2. It is 3 times larger than effective area of
  the BUST.
• Therefore, the counting rate of cosmic ray muons
  (and statistics) will be also 3 times more at the
  same muon flux.

• The ATLAS muon system has very high spatial and
  temporal resolution. In combination with very
  strong magnetic field (2 Tesla), it gives high
  precision magnetic spectrometer.
• It allows to measure as initial (ingoing)
  direction of muon track as their momentum
  (energy) up to several tens GeV.

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Additional advantages

• The ATLAS is situated in underground cavern at
  the depth about 80 m. It is 4 times less than
  underground depth of the BUST (320 m).
• As result, the minimal energy, which need to
  propagate muons through the ground and to arrive
  at the ATLAS detector, have to be also 4 times
  less (50 GeV).
• Muon flux at the ATLAS will be 4? times more than
  at the BUST because of energy spectrum of cosmic
  rays is steep decreasing power function with
  exponent ?.

Surface buildings
Shafts
Accelerator
ATLAS cavern

• Exponent ? of integral energy spectrum is
  differed for Galactic Cosmic Rays (GCR, ? 1.7)
  and for Solar Cosmic Rays (SCR, ? 2-6).
• Thus the muon flux at the depth of ATLAS will be
  more, than at the depth of the BUST 10 times for
  GCR and 15-4000 times for SCR (depending on
  spectrum exponent).

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Cosmic ray muons at the ATLAS detector

• Taking into account both larger area and lesser
  underground depth the counting rate of GCR muons
  at the ATLAS will be in 30 times more than at the
  BUST. Increase of counting rate of SCR will be in
  45 times and more.

• Thus, in addition to rise of total counting rate
  the improvement of signal / background ratio
  will take place in case if muon bursts relate to
  SCR.

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Conclusions

• The ATLAS detector can be used for study of solar
  cosmic rays, in particular to search for
  short-term muon bursts during GLE events, which
  has been found earlier at the BUST.
• The prospective signal should have the bigger
  amplitude in comparison with BUST due to smaller
  thickness of a ground above the ATLAS detector
  and due to greater its area. It relate to both
  the total counting rate of muons and to the ratio
  "signal / background".
• It will allow to ascertain a possible relation of
  the muon bursts to the SCR of high energy. On the
  other hand, it can clear up a question about the
  upper limit and form of spectrum of solar cosmic
  rays of high energy.
• Start-up and following some years of operation of
  the LHC and the ATLAS fall on the period of
  increase of solar activity. That raises
  probability to find out the muon bursts from
  powerful flares.
• The continuous registration of the cosmic ray
  muons is necessary with fixation of time and
  direction of the muon arrival to the detector.