1 Cosmic Muon Analysis: Current Status Stuart Mufson, Brian Rebel Argonne March 18, 2005

(Mufson&Rebel) Data 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 FractionRemaining cutdataMC no cuts plane cut length cut passed fit cut uv asymmetry cu5. t reduced χ 2 cut end points cut fiducial dz,dr cut track-like cut double-ended strip cut consistent timing, dcos(y) cut MC tracks less noisy than in the data

(Mufson&Rebel) The 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 reconstruction charge ratio “bump” “bump” present in data not found in MC

(Mufson&Rebel) The Problem Is this a real physical effect? Almost certainly not. When normal & reverse field data are co-added, the effect vanishes: charge ratio Normal and reverse field data weighted by live time Fit Momentum charge ratio 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) The Problem 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 120field map 201 Fit Momentum charge ratio data MC data/MC

(Mufson&Rebel) The Problem No significant differences from map 120  map 201  map 202 field map 201 field map 202 Fit Momentum charge ratio data MC data/MC

(Mufson&Rebel) Cos(Zenith Angle) Cut An effective cut that minimizes the “bump” is cos(zenith) = cos(  ) 31 o ) charge ratio cos(  ) zenith  Fit Momentum field map 202 “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(  ) charge ratio field map 202 cos(  ) charge ratio Cos(Zenith Angle) Cut data MC data/MC

(Mufson&Rebel) Recent Progress Define the quantity: track strip use fraction = (# double ended strips in the track)/(total # of strips in the track) charge ratio 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 dataMC 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 Fit Momentum charge ratio all cuts through cos(  ) cut + track strip use fraction cut

(Mufson&Rebel) Recent Progress Fit Momentum charge ID purity 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

(Mufson&Rebel) Data Sets – effect of final cuts FractionRemaining cutdataMC no cuts plane cut length cut passed fit cut uv asymmetry cut reduced χ 2 cut end points cut fiducial dz,dr cut track-like cut double-ended strip cut consistent timing, dcos(y) cut FractionRemaining cutdataMC 11. track strip use fraction impact parameter cos(  ) higher quality MC tracks

(Mufson&Rebel) field map 202 Recent Progress Fit Momentum charge ratio data cuts: 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 Rays NOT-SO-GOOD-NEWS: we still do not completely understand charge reconstruction for cosmics (and therefore atmospheric neutrino events) Fit MomentumAzimuth charge ratio field map 202, all cuts EXCEPT cos(  ) cut

(Mufson&Rebel) Cosmic 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