Cosmic Muon Analysis: Current Status

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Cosmic Muon Analysis:Current Status

Stuart Mufson, Brian Rebel

Argonne

March 18, 2005

(Mufson&Rebel)

Data SetsData Sets

•Run files processed at Indiana with R1.14

forward field data: December 2003 – April 2004 analysis with field map 202 3,931,684 events

• MC files processed at Indiana

field map 201 2,640,752 events

Fraction Remaining

cut data MC

no cuts 1.0 1.0

1. plane cut 0.759 0.763

2. length cut 0.755 0.758

3. passed fit cut 0.752 0.757

4. uv asymmetry cu5. t

0.746 0.755

5. reduced χ2 cut 0.706 0.737

6. end points cut 0.704 0.736

7. fiducial dz,dr cut 0.688 0.720

8. track-like cut 0.687 0.720

9. double-ended strip cut

0.687 0.720

10. consistent timing, dcos(y) cut

0.687 0.720

MC tracks less noisy than in the data

(Mufson&Rebel)

The ProblemThe Problem

There are many manifestations of the cosmic muon problem. The most physically obvious:

When we plotted the +/- ratio for cosmic muons as a function of their reconstructed momentum, we found the following structure:

Fit Momentum

data

MC

data/MC

field map 120 & standard reconstructionch

arge

rat

io

“bump”

“bump” present in data not found in MC

/

2.5qp

q p

(Mufson&Rebel)


Is this a real physical effect? Almost certainly not. When normal & reverse field data are co-added, the effect vanishes:

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Normal and reverse field data weighted by live time

Fit Momentum

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Normal and reverse field data weighted by live time

Azimuth

Even the azimuthal distribution (another manifestation of the problem) of the charge ratio becomes flat when normal/reversefield data are combined

(Mufson&Rebel)


Since reversing the field makes the effect go away, Mufson/Rebel suspect that the magnetic field plays an important role. So far, map improvements have not had a dramatic effect.

field map 120 field map 201

Fit Momentum Fit Momentum

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data

MC

data/MC

(Mufson&Rebel)


No significant differences from map 120 map 201 map 202

field map 201 field map 202


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data

MC

data/MC

(Mufson&Rebel)

Cos(Zenith Angle) CutCos(Zenith Angle) Cut

An effective cut that minimizes the “bump” is cos(zenith) = cos( ) < 0.85 ( > 31o)

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cos()

zenith

Fit Momentum Fit Momentumfield map 202

“bump” “bump”

cut

Events coming from near the zenith contribute substantially to the “bump”

(Mufson&Rebel)

Interestingly, even though the field map does not change the charge ratio significantly, it does flatten the cos(zenith) distribution

field map 202

field map 201

field map 120

cos()

cos()

cos()

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field map 202

cos()

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Cos(Zenith Angle) CutCos(Zenith Angle) Cut

data

MC

data/MC

(Mufson&Rebel)

Recent ProgressRecent Progress

Define the quantity:

track strip use fraction = (# double ended strips in the track)/(total # of strips in the track)

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data

MC

cut

data and MC show asymmetry in track strip use fraction

for low track strip use fraction – • data: systematically incorrect charge ID• MC: coin flip

Track Strip Use Fraction

Cut on events with low track strip use fraction > 0.55

(Mufson&Rebel)

Track Strip Use Fraction -- Events

Track Strip Use Fraction -- Events

data MC

no obvious differences between data events and MC events

Charge ID: coin flip

Charge ID: systematic error

(Mufson&Rebel)

although very suggestive, the track strip use fraction cut only improves low momentum reconstruction somewhat

Track Strip Use Fraction Cut

Track Strip Use Fraction Cut

Fit Momentum

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all cuts through cos() cut

all cuts through cos() cut + track strip use fraction cut

(Mufson&Rebel)


Fit Momentum

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From NorthFrom South

While studying MC events, we noticed an asymmetry in the charge ID purity between events coming from the north and events coming from the south

all cuts through cos() cut + track strip use fraction cut

With very high probability, these are all events in the outer part of the detector.

Cut on impact parameter < 3.0 m

(Mufson&Rebel)

Asymmetry in Charge ID Purity – MC Events

Asymmetry in Charge ID Purity – MC Events

(Mufson&Rebel)

Data Sets – effect of final cuts

Data Sets – effect of final cuts

Fraction Remaining

cut data MC

no cuts 1.0 1.0

1. plane cut 0.759 0.763

2. length cut 0.755 0.758

3. passed fit cut 0.752 0.757

4. uv asymmetry cut 0.746 0.755

5. reduced χ2 cut 0.706 0.737

6. end points cut 0.704 0.736

7. fiducial dz,dr cut 0.688 0.720

8. track-like cut 0.687 0.720

9. double-ended strip cut

0.687 0.720

10. consistent timing, dcos(y) cut

0.687 0.720

Fraction Remaining

cut data MC

11. track strip use fraction

0.503 0.499

12. impact parameter 0.406 0.402

13.

0.151 0.178

14. cos() 0.104 0.131

qp

qp

higher quality MC tracks

(Mufson&Rebel)

field map 202



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datadata

cuts: 1-10

• 1-10 +• cos() cut• track strip fraction cut• impact parameter cut

cuts:

GOOD NEWS: for beam events, charge reconstruction works!• beam events do not come from near the zenith (zenith angle cut)• need a track quality cut like the track strip fraction cut (optimized for lower momentum events with showers)• need a fiducial volume cut (impact parameter cut)

And it will get better ....

(Mufson&Rebel)

Cosmic RaysCosmic Rays

NOT-SO-GOOD-NEWS: we still do not completely understand charge reconstruction for cosmics (and therefore atmospheric neutrino events)

Fit Momentum Azimuth

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field map 202, all cuts EXCEPT cos() cut

(Mufson&Rebel)

Cosmic RaysCosmic Rays

• Jeff Nelson conjecture: we know the field most poorly in the outer parts of the detector; the tracker gives all track points equal weight

tracker gives points equal weight, even though field not known as accurately in outer reaches of detector

we know these events are trouble

• Argonne conjecture: to be discussed by Maury (Erik?)

• Sergei conjecture: discussed at last collaboration meeting; problems with tracker

Documents

Cosmic Muon Analysis: Current Status