Measurement of Muon Energy Loss in ATLAS C. Kourkoumelis, D. Fassouliotis, K. Nikolopoulos University of Athens A. Poppleton CERN

Muon Spectrometry in ATLAS The ATLAS Muon Spectrometer is designed to perform precise muon reconstruction up to PT =1 TeV. To achieve this goal, the spectrometer is equipped with Precision Chambers, Optical Alignment Systems and 3 independent Toroidal Magnetic Fields. Oct 2005 K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Calorimeter correction Matching with the inner tracks Muon Reconstruction in ATLAS First measured point Calorimeter correction Refit @ vertex Matching with the inner tracks and combined fit Track Reconstruction @ Muon Spectrometer Using the hits in the Muon Spectrometer detectors. Back-tracking @ vertex Accounting for energy loss and multiple scattering. Combination with Inner Detector Track re-fit using the hits from the Muon Spectrometer and the Inner Detector, accounting for energy loss and multiple scattering. K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

MS only for identification Muon momentum resolution Standalone Reconstruction Energy Loss fluctuations are significant. Especially in low momentum region. ID dominates MS only for identification MS dominates Cross-over region K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Parametrization of the Energy Loss The energy loss of muons in matter is Landau distributed. However, radiation effects become significant for high momenta (relevant for LHC). Traditionally, for the Eloss correction a parametrization of the energy loss (function of the material traversed and P of μ) is used.There are 2 possibilities: Mean energy loss parametrization, Most Probable loss parametization. Each of them has advantages and disavantages. The mop being most appropriate for low Momentum (PT<300. GeV/c) and the Mean being most appropriate for high mass multi-lepton objects However, the parametrization being a statistical quantity cannot account for the event-by-event fluctuations of the energy loss. σMOP K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Measuring the energy deposition in the calorimeters The idea is to use the calorimeter measurement to correct on an event-by-event basis for the muon energy loss fluctuations. The ATLAS calorimeter, which accounts for more than 80% of the material traversed by the μ before reaching the Muon Spectrometer, provide energy deposition measurements! Naturally, most of the energy loss fluctuations will occur inside the calorimeters. The μ track from the Muon Spectrometer is backtracked to the calorimeter and then an area around the position of the track is considered. The calorimeter cells (inside this area) exceeding the noise threshold criterion, are kept and summed. The size of the measurement area has been optimized by testing different scenarios. K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Calorimeter Noise The calorimeter noise is a very significant factor for the success of the method. Low noise levels permit the measurement of small energy depositions Noise Mean = 0 MeV Sigma = 22 MeV Signal Mop ≈ 500 MeV K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Inert Material Correction PT = 50GeV/c Muons deposit energy to the inert material of the detector as well. We call “Inert Material” the material which is not sampled by the calorimeters For this correction good-knowledge of the material distribution and of the Mean/Mop energy loss of the muon is essential. K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Is the energy loss of the muon mop compatible? Combination the Energy Measurement with the MoP parametrization Note that: The Calorimeter resolution scales The MoP parametrization describes accurately muons in the MoP region (86% @ PT =10 GeV to 59% @ PT=1TeV) The calorimeter can offer an important piece of information: Is the energy loss of the muon mop compatible? Use Mop Parametrization Mop+2*σMop The obtained method is much less affected by possible deterioration of the calorimeter measurement resolution (e.g. forward region, pileup). This procedure provides a stable and robust method of accurate energy loss reconstruction. Use Measurement K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Significant improvement in energy loss estimation K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

We need to discriminate between Isolated and Non-Isolated muons. Isolation When using the calorimeter information we should always have in mind that muons are not alone in the event. We need to discriminate between Isolated and Non-Isolated muons. K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Calorimeter Isolation Calorimeter and Tracker Isolation Calorimeter Isolation Cut on the energy deposition around the measurement area. μ track Tracker Isolation Cut on the number of charged tracks around the μ track Isolated μ Non Isolated μ K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Isolation Muons from W Muons from Quarks Mop Only Measurement/Mop Ιsolation study based on ttbar sample Measurement/Mop No Isolation criteria Muons arising from W are isolated most of the time Muons arising from Quark jets are generally not isolated K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Isolation (cont.) Muons from W Muons from Quarks Calo Isolation + PT>15GeV/c Positive tail O(1/100) Calo +InDet Isolation + PT>15GeV/c Positive tail O(1/1000) K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Strategy for muon Eloss reconstruction Muon Track Perform measurement (Applying e/mip and dead material corrections) Is energy measurement “feasible”? (Significant signal and isolated muon) Use parameterization with mean or mop value. No Yes Is the energy measurement “significant” w.r.t the mop? (EMeas > MoP + 2. * σMoP) Yes Use energy measurement No Use the most probable value parameterization K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Improvement in resolution is observed. Muon momentum reconstruction: Resolution Improvement in resolution is observed. K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Muon momentum reconstruction resolution Fraction of events within 3 σ of Energy Loss resolution is higher and constant vs PT when the new method is used. K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

10 % Improvement in mass resolution Performance with Physics Samples: Z μμ Mop Parametrization 10 % Improvement in mass resolution Measurement/Mop Tail due to large energy loss. Tail almost disappeared! K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Performance with Physics Samples: Z’μμ Mop Parametrization ΜΖ’ = 1 TeV/c2 Measurement/Mop K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

The new method is promising, even at the hard LHC environment Conclusions A method using the calorimeter measurement for accurate muon energy loss reconstruction has been developed. The method uses both the calorimeter measurement and the energy loss parametrization in a combined way and includes safety measures against non isolated muons. The results were found to be stable under different conditions (including pile-up), at least for |η|<2. Already, part of the Official ATLAS Muon Reconstruction Chain. In terms of performance: The method significantly reduces the tails w.r.t the standard parametrization method. Improved Z mass resolution by 10 % (Standalone Muon Reconstruction) The new method is promising, even at the hard LHC environment The project is co-funded by the European Social Fund and National Resources – (EPEAEK II) PYTHAGORAS II K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Back-Up Slides K. Nikolopoulos University of Athens – ATLAS Physics Workshop – Rome, June 10th 2005

Muon momentum reconstruction resolution Cross-over momentum PT ≈ 50 GeV Energy Loss fluctuations significant in low momentum region. ID dominates MS only for identification MS dominates K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Muon Energy Loss Muons are considered minimum ionizing particles, however the can lose energy in various other ways (this is especially true when the muon momentum is high) Ionization Bremstrahlung Knock-on electrons K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Calorimeter Noise Electronic Noise Only K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Measurement Cone Optimization |η|<2. Due to software problems We varied the size of the cones both in the Hadronic and the E/M calorimeters. The results for the measured energy were found to be consistent. The choice of the narrow cones will be made obvious once isolation is discussed K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Forward Correction The Most Probably muon signal in LArHEC (≈ 500 MeV )  very close to the noise threshold at zero luminosity. The mop like region of the muon energy loss cannot be measured, but we can still measure the landau tails. (where usually the large fraction of the energy loss is located in one calorimeter sampling) Correction for the introduced bias (cell with signal below noise cut) should be applied. Else a muon with Eloss in the landau tail will be considered to have a mip – like behaviour and the muon energy loss will be systematically underestimated in the forward region. When no muon signal is observed in one sampling, we can rather accurately guess that it had a mop-like behaviour – in other case we would have measure some signal. Using this assumption the expected energy loss at that sample can be estimated. The performed correction is based on the knowledge of the material in each sampling and of the MoP energy loss. K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Significant improvement in energy loss estimation K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Muons from W are isolated most of the time Isolation: Muons From W Measurement/Mop No Isolation criteria Ιsolation study based on ttbar sample Calo Isolation + PT>15GeV/c Muons from W are isolated most of the time Mop Only RMS = 4.7 GeV Events ±3σ=81% Calo +InDet Isolation + PT>15GeV/c RMS = 3.8 GeV Events ±3σ=86% 19% measurement K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Muons arising from Quark jets are generally not isolated Isolation: Muons From Quarks Measurement/Mop No Isolation criteria Calo Isolation + PT>15GeV/c Positive tail O(1/100) Muons arising from Quark jets are generally not isolated Mop Only Positive tail O(1/1000) Calo +InDet Isolation + PT>15GeV/c 0.6% measurement K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Combining Measurement & Mop Toy MC – Resolution The slope of the Meas + Mop line is ≈ ¼ (28%) of the slope of the Meas Only K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Performance with Physics Samples: Z PT resolution for Moore muons. In the following we use only the muons inside the fit region. For those muons the resolution is 3.53% . This resolution is the lower limit for the resolution that can be achieved by Muid. Univ. of Athens / CERN – Muon Software and Performance Meeting – CERN, April 11th 2006

Performance with Physics Samples: Z When using the measurement we get Muid resolution for the muons of 3.59%. Very close to the ideal one (3.53% moore resolution) K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Effects of Pile – Up I |η|<2. In order to assess the effect of Pile in the energy loss procedure a PT=100GeV/c single μ sample in different luminosity scenarios was used. Important: For the following results the settings of the method were those of zero luminosity. Worst case scenario and checking the method’s robustness. |η|<2. Due to software problems Luminosity 0  5*1033 /cm2/s Measured Energy Mean ≈ + 160 MeV Measured/Mop Energy = Mean ≈ + 50 MeV Almost unaffected These small shifts can be corrected by subtracting the Mean Noise contribution. (not available yet) K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006

Effects of Pile - Up |η|<2. Luminosity 0  5*1033 /cm2/s Measured Energy Sigma : ≈ + 21% Measured/Mop Energy = Sigma : ≈ + 8 % Small effect |η|<2. Using higher thresholds will reduce part of the effect. We are testing the worse case scenario. K. Nikolopoulos University of Athens – HEP 2006 – Ioannina, April 16th 2006