Transcript CLAS Simulations for the E5 Data Set Introduction CEBAF The Continuous Electron Beam Accelerating Facility(CEBAF) is the central particle accelerator at JLab.

CLAS Simulations for the E5 Data Set
Introduction
CEBAF
The Continuous Electron Beam Accelerating Facility(CEBAF) is the
central particle accelerator at JLab. CEBAF is capable of producing
electron beams up to 6 GeV. The accelerator is about 7/8 of a mile
around and is 25 feet underground. The electron beam is accelerated
through the straight sections and magnets are used to make the beam
travel around the bends(See Fig. 1). An electron beam can travel
around the accelerator up to five times
near the speed of light. The
beam is sent to one of three
halls where it collides with a
target and causes particles
Hall B
to scatter into the detectors.
Physics Motivation
The CLAS detector is a large (10-m diameter, 45-ton) spectrometer designed to measure and identify the debris from a nuclear collision.
The first three layers of CLAS, the drift chambers, consist of high-voltage wires that send signals when a charged particle scatters near
them to capture a ‘snapshot’ of the particle’s trajectory. Wire misalignments and sag can effect the quality of our data. To understand the
response of CLAS, we simulate its performance with a software package called GSIM. We can see how much of what we observe is
from real physics or artifacts of the detector. We can then correct for these artifacts.
Asymmetries
Perl Scripts and Simulating CLAS
In a D(e,e’p)n reaction, we look to measure the out-of-plane
components of the nuclear cross section which have never
been determined in this energy region. An essential
observable is fpq, the angle between the scattering plane
and the reaction plane (see Figure 3).
In order to separate real physics results from artifacts of the detector we
simulate the performance of CLAS. A Perl script executes a sequence of
commands to run different programs, manage files, etc. The scripts are
executed on the 34-node supercomputing cluster in the University of
Richmond nuclear physics laboratory. An outline of this script is below.
Fig.4. Track vertex
geometry
We measure the vertex shifts from the beam line along the
z-axis from real data and obtain the results shown below.
We are able to see how far from the beam axis each
sector is located. We then insert these shifts into our
simulation.
1. QUEEG- (Quasi-Elastic Electron Generator) creates electron 4vectors (events).
Fig. 1 JLab Accelerator and
Halls A, B, and C
Fig 3. Kinematic
quantities.
CLAS
The CEBAF Large Acceptance Spectrometer(CLAS), located in Hall B,
is used to detect electrons, protons, pions and other subatomic
particles. CLAS is able to detect most particles created in a nuclear
reaction, because it covers a large solid angle. The particles go
through each region of CLAS leaving behind information that is
collected and stored on tape. The event rate is high (about 3000 Hz),
so the initial data analysis is done at JLab, and we analyze more deeply
those results at the University of Richmond. There are six different
layers of CLAS that
produce electrical signals
and provide information on
velocity, mass, and energy,
allowing us to identify and
separate different subatomic
particles. The drift chambers
make up the first three layers,
which measures the path of
different particles. The paths
of the particles are bent in a
large, toroidal magnet to
measure momentum. The
data here were collected during
the E5 running period at beam
energies 2.6 GeV and 4.2 GeV
on deuterium and hydrogen.
Fig. 2. The CLAS
detector at Jefferson
Lab.
Misalignments of the components of CLAS can create
false asymmetries in our data. To investigate this issue, we
simulate our data without true asymmetries (ALT=ATT=0),
but include small shifts in the positions of the CLAS
components. The geometry of the track vertex is shown in
the figure below.
R.Burrell, K.Gill, G.P.Gilfoyle
University of Richmond, Physics Department
The Thomas Jefferson National Accelerator Facility (JLab) in
Newport News, Virginia, is used to understand the fundamental
properties of matter in terms of quarks and gluons. We describe
here how we simulate the performance of one of the detectors to
better understand its response.
Track Vertex Shifts
2. txt2part – converts QUEEG output to BOS files (part bank 4vectors).
The differential cross section is given by:
ds
 s ( , f )  s L  s T  s LT cos(f pq ) 
dd e d p
3. GSIM – CLAS simulation program (main program).
3
s TT cos(2f pq )  hs 'LT sin(f pq )
(1)
where h is the beam helicity (±1). To extract the different fdependent terms in the cross section we take advantage of
the orthogonality of sines and cosines. For example, to
extract sLT consider the following.
2
cosf pq 
 s ( ,f
0
pq
) cosf pq df pq
2
 s ( ,f
pq
)df pq
s LT
ALT


2(s L  s T )
2
(2)
The asymmetry ALT is proportional to sLT and less sensitive
to acceptance corrections and other experimental effects
because we are using a ratio. We examine this asymmetry
ALT and another one ATT which is proportional to the sTT
term in the cross section. Equation 3 shows the expressions
we use to determine the asymmetries in a kinematic bin
where N is the total number of events.
ATT
N
pq
)
ALT
Simulation
Analysis
5. RECSIS – event reconstruction program (reconstructs tracks).
6. nt10maker – convert EVNT and PART BOS banks to hbook
ntuples (software package for physics analysis)
7. h2root – convert hbook ntuples to root ntuples
0
cos(2f


4. gppjlab – removes dead components.
cos(f


N
(3)
pq
)
Figure 5. Plots of the y-component of the electron track
vertex position for each CLAS sector taken from real
data and fitted with a gaussian curve.
8. eod5root – local code for final analysis to extract histograms,
asymmetries, etc.
Data Banks
In the simulation and analysis process, two event banks are used: the
“PART” bank contains the thrown events generated with QUEEG before
they are passed through the CLAS simulation package GSIM (see
above). This lets us know exactly what goes into the detector. A second
“EVNT” bank contains events that were processed in the CLAS
simulation and represent what actually made it through the detector and
our analysis codes.
The effect of the vertex
shifts in the
simulation is shown in
Figure 6. The shifts
create a significant false
asymmetry that will have
to be corrected in the
final analysis. If the
vertex shifts are
removed the false
Figure 6. False ALT and ATT
asymmetries
asymmetries created by vertex
disappear.
shifts in the simulation.
Conclusions
• We have developed scripts to control and execute the
CLAS simulation package GSIM on the Richmond
supercomputing cluster.
• We have found small shifts in the electron track vertex
position observed in our data can create false asymmetries
in the simulation.
• We are studying how to correct for these shifts.