Physics at Very Small Angles
with CASTOR at CMS
E. Norbeck, Y. Onel
A. Panagiotou
M. Murray
U. of Iowa
U. of Athens, Greece
U. of Kansas
For the 22st Winter Workshop On Nuclear Dynamics, March 13, 2006
What is CASTOR?
•Tungsten-quartz Čerenkov calorimeter surrounding
beam pipe 15 to 16.5 m from interaction point, just
beyond HF at the end of CMS
•16 azimuthal segments (22.5o)
•18 longitudinal segments
•Additional position resolution by
a thin tracking detector in front.
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Note location and size of HF at the ends of CMS
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CASTOR Integration in CMS
H
CASTOR
A
L
T2
HF
IP
CASTOR (~ 10 λI)
EM: 5.3 < η < 6.45, 99% containment
H: 5.2 < η < 6.3, 95% containment
at η=5.2  ~ 5 λI
FH half
depth @ η '06= 5.25
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The Conceptual Design
Collar
Table
T2 GEM
CASTOR
Both shield and CASTOR are split vertically at beam line
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3D model of CASTOR
Crossbeams
Longitudinal beams
J. Blocki
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HCAL
CMS with CASTOR has almost complete angular coverage
Forward angles important for pp and essential for heavy ions
Extreme radiation levels will require the removal of CASTOR
during high luminosity pp runs.
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Need CASTOR for accurate measurement of missing transverse energy
Red shows η range
of CASTOR at one
end of CMS
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Limiting Fragmentation
Note importance of |η| > 5 for LHC
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A
..\..\Proposals\CASTORMRI\CASTOR_MRI_011606.doc
LHC will allow studies of low x physics to
x values 200 times smaller than at RHIC
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Colored Glass Condensate
• The number of gluons increases with 1/x as expected
from the ΔEΔt uncertainty principle.
• Most cross sections are dominated by gluon induced
processes.
• At very low x, cross sections would become excessive.
• Gluon density saturates (by recombination).
• Can use a classical effective theory for the saturation
regime.
• Reduced cross sections seen at RHIC.
• Much larger effects at small angles with CMS
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Ultra-peripheral nucleus-nucleus collisions
Useful for studying many processes unattainable via
any other experimental arrangement.
Photo production involving gluons from one nucleus
and gammas generated by the intense electric field of
the other nucleus.
c bb‾, W+W- …
Will see photo production gg → l+l-, c‾,
Photo production of J/ψ … has been seen at RHIC
Rate for Higgs photo production is predicted to be too
small to observe.
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FLOW
With 16 fold azimuthal (22.5o) symmetry CASTOR
should be good for determining the reaction plane.
Solenoidal magnetic field may rotate the plane
relative to the silicon tracker.
Depends on pt
Needs simulation calculations.
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Connection to cosmic ray studies
Cosmic ray events often show anomalous energy-loss
profiles for particles in the cores of showers.
(See many papers by Ewa Gładysz-Dziaduś,
Institute of Nuclear Physics, Krakow, Poland)
Even in colliding beam experiments these cores would be
at small angles, the angles covered by CASTOR.
If such events represent new processes, they should be
observable with much higher statistics in CASTOR data.
Study of “normal” events in CASTOR will provide the first
good energy calibration for cosmic ray experiments.
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Trigger
CASTOR data will be a normal part of CMS data at ~100 events/s.
Some remarkable events in CASTOR may not show anything
remarkable in the rest of CMS.
•About 3% of high-energy cosmic rays (Fe + N) are “exotic.”
PbPb should produce events that are even more exotic!
It is the longitudinal segmentation that allows exotic events to be recognized
•Trigger on exotic CASTOR events and read out only CASTOR data.
•CMS has more than 105 times as many data channels as CASTOR.
•Reading out 1000 or 10000 CASTOR-only events/second would add
almost nothing to the data stream.
Magnetic Monopole
(Back of the envelope calculations)
B between passing Pb ions of 2 X 1020 G will separate two Dirac
monopoles if they are 0.128 fm apart.
Without B field separation would require 26 GeV.
Monopole “atom” with MMc2 = 90 MeV has diameter 0.128 fm.
If MMc2 = 9 GeV, separation would require 2.6 TeV.
Would make track like a heavy ion with Z = 68.5
In CMS the track would bend toward the direction of the beam line
and pick up 82 GeV/m from the 4 T magnetic field.
A monopole with small pt could gain about 1000 GeV in the
solenoidal field on its way to CASTOR.
Easily recognized in CASTOR by its distinctive energy-loss profile
(if it is included in the trigger)
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Virtual Magnetic Monopoles
No (spin ½ point monopoles) seen at FNAL with Mc2 < 900 GeV
Many types of magnetic monopoles in the literature
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Large magnetic fields
Passing Pb ions at 20 fm make 2 x 1020 G midway between.
Since B ~ gZ, simple scaling gives 2 x 1020 G midway between
passing e+, e- at 20 fm for only 124 GeV e+ on e- (~LEP energies)
For e+e- collider, field lasts for a shorter time but can be made
arbitrarily large by decreasing the distance between them (~r2)
For Pb, total energy is 1144 TeV, for e+e- it is 0.248 TeV
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Equivalent photons for relativistic charged particles
Black Pb
Red electrons
Blue gamma photons
From David d'Enterria "CMS ZDC: physics case" (LEMIC meeting)
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Status
•Have been two beam tests with prototypes
•Final prototype to be tested in October 2006
•Proposal to NSF for construction funds
•Will be ready for first lead beams
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