1 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Measurement of the charge ratio of atmospheric muons with the CMS detector A. Fanfani (Bologna University & INFN) on behalf of the CMS Collaboration ICATPP Conference on Cosmic Rays for Particle and Astroparticle Physics October 2010 (Villa Olmo, Como, Italy)

2 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Charge ratio of atmospheric muons  Atmospheric muons are produced via interactions of high-energy cosmic-ray particles with air nuclei in the upper layers of the atmosphere  mainly from  s and Ks decays C. Amsler et al., Physics Letters B667, 1 (2008) PDG review 2010 at Earth surface R =  Primary cosmic radiation is essentially positively charged  positive meson production is favored, thus more positive muons are expected  Ratio of μ + over μ - reflects excess of  + /K + vs  - /K -  Above critical momentum of 115 GeV/c pions start to interact before they decay (850 GeV/c for Kaons) → predicts a change in charge ratio between 0.1 and 1 TeV  Muon charge ratio R is the ratio of positive to negative atmospheric muons

3 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 The CMS detector  The Compact Muon Solenoid (CMS) is one of the detectors at the LHC  located 89 m below Earth`s surface (420 m above sea level)  The charge ratio measurement relies on curvature of the  used to measure both the charge sign and momentum of the   CMS has powerful muon tracking capability  uniform B field at 3.8T inside the solenoid  muon chambers embedded in steel return yokes (≈2T) muon chambers tracker

4 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Charge ratio measurement in CMS  Collecting a large sample of cosmic muons was very important for the commissioning and the performance study of the CMS detector  Documented in 23 papers on detector performance: J. Inst. 5 (2010)J. Inst. 5 (2010)  Developed dedicated triggers and special settings for reconstruction, taking into account difference wrt LHC muons Three analyses combined to obtain a single charge ratio measurement as function of momentum at the Earth’s surface  Combine information from Tracker and Muon systems  Use only information from Muon system  On surface Magnet Test and Cosmic Challenge (MTCC 2006)  underground Cosmic Run At Four Tesla (CRAFT 2008) with different analysis approach:

5 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Analysis on surface data (MTCC 2006)  Small fraction of the detector in the assembly hall on surface  No material above the detector  High rate of muons  Acces to low momentum region  60º slice of the muon detector  Very charge asymmetric detector  Define perfectly left-right symmetric fiducial volume  equal acceptance for μ+ and μ- (trigger and reconstruction)  Only hits inside the fiducial volume are accepted (symmetric illumination plot) Run 4406, Oct K events

6 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Charge ratio results on surface data (MTCC)  1 st CMS Physics measurement!  Applying correction for  energy loss traversing CMS (few GeV)  charge mis-assignment estimated from simulation (small at low p) Muons with a measured momentum below 200GeV/c are considered

7 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Underground analysis (CRAFT 2008)  Full detector in the underground cavern  270 M events collected with B=3.8T during 4 weeks (Oct-Nov 2008)  two legs global-muon analysis: 245 K events  Combine Tracker and Muon information  Reconstructing 2 muons allows a fully data driven procedure to estimate momentum resolution and charge confusion  one leg standalone-muon analysis: 1.6 M events  larger acceptance, good p resolution due to long lever arm  more reliance on simulation

8 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Analysis Components  correcting for detector effects  Momentum resolution  charge mis-assignment  estimating systematic uncertainties  alignment, momentum scale, trigger, charge confusion, material model, muon rate losses, event selection, B field....  correcting for energy losses in Earth  correct momentum of each muon for expected energy loss based on straight line extrapolation from CMS to Earth surface through the material map (same used in cosmic simulation)  random fluctuations taken into account Measuring the charge ratio as a function of true momentum at Earth’s surface requires: energy loss

9 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Momentum resolution and charge mis-assignment  Unfolding technique is applied to charge-signed inverse momentum (q/p) at Earth surface to obtain the true momentum at Earth’s surface  resolution derived from data using global-muons  standalone-muon resolution derived from simulation  data-driven handle comparing standalone track to tracker track, available for 40% of events

10 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Alignment/Momentum scale  Alignment is crucial for reconstruction of high p T muons and it is a complex process  parameters at CMS !  Effects of residual misalignment on charge ratio are estimated using different misalignment scenarios different slope due to charge asimmetry  Determine muon momentum scale at high momentum(~ 1 TeV/c) from minimum of q/p T  Correction applied and uncertainty is assigned as systematic  Possible global deformation of detector (  2 invariant) affecting the momentum scale for cosmic muons of opposite charge in opposite directions → constant offset in q/pT

11 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Systematic uncertainties  Surface analysis (MTCC):  main systematic uncertainty from the understanding of the detector alignment (up to 8%)  less important: charge mis-assignment uncertainty, knowledge of B-field precision  Underground analyses: No capability to reverse the magnetic field → challenging to keep systematic effects under control  knowledge of absolute momentum scale up to 4% and residual alignment (up to 1% for the global analysis and up to 4% for the standalone)  charge mis-assignment: corrected in the unfolding procedure. In the standalone analysis differences in the charge mis-assignment between data and simulation quoted as a systematic (up to 3% at high momenta)  <50GeV: possible bias in muon rate due to asymmetry in detector acceptance (~1%)  less important contribution from: knowledge of B-field, trigger efficiency, uncertainty on material overburden, event selection Low p High p

12 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 CMS results  Combine all measurements below 100 GeV/c:  p : R = ± (stat) ± (syst), with χ2/ndf = 7.3/11  p·cosθz : R = ± (stat) ± (syst), with χ2/ndf = 15.3/11  Fitting just the region p·cosθz < 70 GeV/c:  R = ± (stat) ± (syst) with χ2/ndf = 4.0/8 Good agreement with previous measurement Lower probability to be consistent with flat R Consistent with flat R hypothesis  Good agreement between analysis. Results combined taking into account correlations of systematic uncertainties between bins and analysis

13 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Pion-Kaon fit model  A rise in the charge ratio at high momentum is observed -> due to the increase of Kaon contribution  Pion-Kaon model  R extracted from a parametrized muon spectrum  η: ratio of K to  rates contributing to μ ± rate =  fk (f  ): fraction of K(  ) rate decaying to positive muons  Fit of the Pion-Kaon model to the CMS data in the entire p·cosθz region  f  =0.553 ± fK=0.66 ± 0.06  2/ndf=7.8/7  Compare to fit to L3+C, Minos data far & near:  f  =0.551 ± fK= ± Astropart. Phys. 32, Issue 1 (2009) 61,Schreiner, Reichenbacher & Goodman  CMS results compared with previous measurements → good agreement

14 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Conclusions  First physics measurement using muons at CMS! Published in Physics Letters B: Phys. Lett. B 692, 83–104 (2010)Phys. Lett. B 692, 83–104 (2010)  This measurement implies a good understanding of  reconstruction, trigger efficiencies and  alignment  Precise measurement of the muon charge ratio below 100 GeV  and the first to span the “interesting” range 10 GeV –1 TeV  Emphasis is now on muons from LHC collisions!

15 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010 Backup slides

16 A.Fanfani – Measurement of Charge Ratio of atmospheric  with CMS – ICATPP 2010  Global-muon and Standalone-muon charge ratio:  Combined results (MTCC, Global-muon,standalone-muon): p range Runcertainty > p·cosθz range Runcertainty >