J/ψ µµ: sidebands subtraction Stefano Recanatesi Università degli studi di Pisa Summer Student CERN 2010 stefano.recanatesi@gmail.com August 27, 2010 Supervisors: Sara Bolognesi, Marco De Mattia 1 Introduction At the startup of LHC the dimuon resonances (J/ψ, Y, Z), well known in the standard model, are playing a crucial role for commissioning studies. These events can be exploited to have a deeper understanding of the behavior of the detector. In particular it is possible to understand the precision in the measurment of the track resolution and scale. For instance in the MuscleFit [1] algorithm the constrain on the J/ψ mass is used to calibrate the muon momentum scale and the J/ψ lineshape is exploited to extract the muon momenutm resolution. In this framework I have considered the J/ψ decay into two muons and I have studied the kinematic distributions of this process. The aim of this work is to figure out which kind of kinematic variables are well reconstructed in the detector and which of them can be used to select J/ψ signal. This report, after a short review of the samples (data and MC) used, explains firstly the method I have used to select the J/ψ µµ events from the background, the so called sidebands subtraction method, afterwords I report a deep analysis of the kinematic distributions of J/ψ events with the help of this tool. At the end I will draw my conclusions. 2 Technical details I use data in PAT format from the official production of the quarkonia group which runs on a dedicated central skim. At least two muons with M(µµ) 2 GeV are requested at this stage, then I have applied additional selection cuts. In CMS muons can be reconstructed from a combined fit to the inner silicon track with the outer track in the muon chambers (global muons). Very low P t muons can cross only one muon station, in this case the inner track must be compatible with the muon hits but a full fit to those hits is not attempted (tracker muons). In my analysis I search for a pair of global muons. If I couldn t find it, I look??? for a global-tracker pair. In the case that even such pair is not found I ask for a tracker-tracker muons pair. After this search I impose some additional cuts for both tracker and global muons which are reported in Table:1. The cuts for a global muon selection have to be added to those for a tracker muon selection. All the cuts are very loose and their aim is to have a well renconstructed track that you can use in your analysis for commissioning studies. I analyzed 226.5 nb 1 of data, selecting good runs and lumisections from central CMS validation. I have considered three MonteCarlo samples: prompt J/ψ production (75nb 1 ) /JPsiToMuMu 2MuPEtaFilter 7TeV-pythia6-evtgen/Spring10-START3X V26-v1, 1
T selection: G selection: inner track hits > 11; global track reduced χ 2 < 20.0; inner track reduced χ 2 < 4.0; number of valid muon hits > 0; number of pixel layers with measurement 1; transverse impact parameter < 3.0 cm; longitudinal impact parameter < 15.0 cm; good matchint track-muon hits (TMLastStationAngTight); Table 1: Cuts on tracker and global muons. background (122 nb 1 ) /ppmux/spring10-start3x V26 S09-v1. B J/ψ X (120 nb 1 ) /BpToJPsiMuMu 2MuPtEtaFilter 7TeV-pythia6-evtgen/Spring10-START3X V26-v1 Here I will show the results only for the signal sample with prompt J/ψ. In this MC sample the following cuts at generator level are applied: P µ > 2.5 GeV; η µ < 2.5. These cuts make the MC slightly different from the data as will be discussed in the following. Even if I should have used the same trigger for data and MC, that is HLT L1 MuOpen, I couldn t because this trigger was not available in simulation. More strict triggers can be applied, nevertheless in a commissioning study you are not so interested in having the right normalization while is more important to maximize the statistic. 3 Sidebands subtraction method The method of the sidebands subtraction aims to have the kinematic distributions for those events in which a J/ψ is present. To achieve this aim it is necessary to eliminate the events of background under the mass peak. The starting assumption is that the kinematic variable distributions for the background in the peak region of the J/ψ are extremely similar to the average of the same distributions for events in the sidebands regions. In practice the strategy can be explained in three easy steps: You define the sidebands of the peak as in fig:1a paying attention to the following issues: the sidebands have not to be in a region in which another resonance is present (e.g. ψ 2s with mass 3.65 GeV); they must be a bit far from the peak beacause you don t want events from the peak in the sidebands and the final state radiation makes the peak tail longer on the left side; they must be as equal as possible in dimension but sufficently large. You take a given kinematic distribution for the three regions. These distributions can be called 1 Φ left, Φ peak, Φ right. You fit the sidebands with a curve like k 1 + k x 2 2 (fig:1b) and the area under the fit in the peak region is an estimation of the number of events due to the background. The areas under the fit for the three regions can be called A left, A peak, A right. A You procede in doing the subtraction calculating Φ sub = Φ peak peak A left +A right (Φ left + Φ right ) that is the distribution you want. As it is possible to see the sidebands I chose are different one from the other. To understand why this doesn t introduce an error it is possible to look at the ratio between the sidebands subtracted distribution for the transverse momentum choosing in one case the sidebands as in fig:1a and in the other equally far from the peak and of the same size (the one of the right sideband in the previous case). This ratio is represented in fig:2. 2
(a) The sidebands defined for the sidebands subtraction(b) The fit to the sidebands to estimate the number of and the region of the peak events of background in the peak region Figure 1: J/ψ peak analysis Figure 2: ratio between sidebands subtracted P t for equal sidebands and different sidebands 4 Application of the method Now it is possible to apply this tool to many variables and to confront the subtracted distributions with the MC. 4.1 Transverse momentum P t of the J/ψ In fig:4a it is possible to see the distribution for the sidebands and the peak normalized to have the same area (MC is normalized to Data), the same normalization is applied to all the following histograms if not otherwise specified. It is possible to see on this plot that the transverse momentum of the peak region is harder and its shape is driven by the acceptance in η of the detector. It is also possible to see the difference between the two sidebands and the fact that the right one is a bit harder in P t. In fig:5b the data before and after the subtraction are compared with MC. The MC is not in agreement with the data because of the cut P µ 2.5 GeV that makes the MC spectrum harder. It is possible to have a better agreement between MC and data asking for the reconstructed momentum of the J/ψ to point in the barrel so that the muons must have a larger momentum. The plot for the P t distributions of J/ψ pointing in the barrel ( η(j/ψ) < 1.1) or endcap ( η(j/ψ) > 1.9) are represented in fig:4. To explain the kinematics of every single events it is worthy to look also at the P z of the J/ψ and at the φ and 3
η between the two muons. Before moving in this direction I will explain why the acceptance of the detector is so relevant in these plots. (a) P t for the three regions (b) P t sidebands subtraction Figure 3: P t analysis (a) P t for J/ψ in the barrel (b) P t for J/ψ in the endcap Figure 4: P t for J/ψ in barrel and endcap respectevly 4.2 Detector acceptance The pair of muons recontructed in the detector can be of four different kinds, where the fact of being reconstructed from a endcap or from the barrel makes the difference. In fig:5 it is possible to see the four configurations. The second is not allowed by the kinematic for a muon pair coming from a J/ψ. The configurations with at least one muon in the barrell are suppressed because for a muon to go through the barrel a transverse momentum of at least about 3 GeV is needed. You can see these characteristics looking to fig:6a and fig:6b where the fact that most of the muons goes to the endcaps is evident in the first plot and the little bump at 3 GeV in the second is an estimation of the P t needed by a muon to go through the barrel. 4.3 η between two muons In fig:7a it is possible to see the correlation between the η of the two muons and the P t of the reconstructed J/ψ. In this plot is visible the acceptance of the detector. In the plot after the subtraction 4
(a) endcap-same endcap (b) forward endcapbackward endcap (c) endcap-barrel (d) barrel-barrel Figure 5: Di-muons events in CMS (a) η muons distribution (b) P t muons distribution Figure 6: Muons kinematic variables (fig:7b) is evident how a large region of the plot is kinematically suppressed. Such kind of suppression is easy explained if you look at the distibution of η vs M of the reconstructed J/ψ in fig:8: an invariant mass of at least 5 GeV is needed by the two muons to have a η 2.5. It is interesting to look also at the plots in one dimension for η (fig:??). In particular fig:9b describes the ratio between MC and Data which is close to one. (a) η vs P t for all the data (b) η vs P t sidebands subtraction for peak region Figure 7: η vs P t study 5
Figure 8: η vs M for al the data (a) η for data and MC (b) Ratio betwee η for data and MC Figure 9: η analysis 4.4 φ between two muons In fig:10a there is the φ of the muons against the P t of the J/ψ and in fig:10b there is the distribution after the subtraction. Once again the behavior of the acceptance of the detector is visible thanks to the shape of the white region in the down left part of the plot. The correlation between the two variable is evident and this plot is in good agreement with MC. When the P t is equal to zero the φ can be π, increasing the J/ψ boost in the transverse plane the φ of the two muons decreases. It is interesting to look also at the plots in one dimension for φ. In particular the second plot describe the ratio between MC and Data. 4.5 P z of the J/ψ To have a complete view of the event I report here the distribution of the J/ψ P z both for MC and data before and after the subtraction in fig:12a and their ratio in fig:12b. The distribution of the MC agrees with data in a fairly good way. 6
(a) φ vs P t for all the data (b) φ vs P t sidebands subtraction for peak region Figure 10: φ vs P t study (a) φ for data and MC (b) Ratio between φ for data and MC Figure 11: φ analysis (a) P z for data and MC (b) P z ratio between data and MC Figure 12: Distribution of J/ψ P z 5 Conclusion In the analysis of the new data at CMS it is worthy to use as many tool as possible to extract the largest possible amount of information. The sidebands subtraction is one of that tool that allows you to investigate the phisics of the kinematic distibutions of the signal. For commissioning studies it would be 7
a good thing to investigate also those variables that are related with detector performaces like the number of hits in the tracker. This could improve the reconstruction of the momentum scale and resolution of CMS. References [1] The CMS collaboration, CMS PAS TRK-10-004 8