2 The ATLAS Experiment at the Large Hadron Collider at CERN

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1 Studies of the Di-Muons Resonances at the ATLAS Experiment at CERN PhD Detailed Research Project PhD Candidate: Camilla Maiani Supervisor: Prof. Carlo Dionisi, Dott. Stefano Giagu Università di Roma La Sapienza Dottorato in Fisica, XIV Ciclo 1 Introduction The Large Hadron Collider (LHC) at the CERN laboratories saw its first proton-proton collisions in November 2009 at a center-of-mass energy of 900 GeV. In March 2010, after the winter annual stop, energy was brought up to 7 TeV, 3.5 TeV per proton beam. Since then the LHC instantaneous luminosity has improved rapidly: beams reached in the past weeks L = cm 2 s 1, and work is going on to reach cm 2 s 1 by the end of The LHC plan is to have by the summer of 2011 between 0.5 and 1 fb 1 of integrated luminosity. In the following I will briefly present the phd thesis work and project which is evolving within the ATLAS collaboration at LHC. I will begin with a brief overview of the ATLAS detector, with a special reference to the muon and tracking systems and reconstruction. I will then present the J/Ψ prompt non-prompt production cross-section ratio measurement performed at ATLAS with the first 17.5 nb 1 collected. The last section of this project will be centered on the B s J/Ψ + Φ lifetime measurement. 2 The ATLAS Experiment at the Large Hadron Collider at CERN ATLAS is a multi-purpose experiment located at one of the LHC interaction points. An almost full coverage in the solid angle around the collision point is requested to perform the detailed physics studies overseen in its scientifical program. The ATLAS detector, going from its innermost to its outermost layers, consists of: an Inner Detector tracking device (ID), for the reconstruction of charged particles produced in the interaction; an electromagnetic calorimeter, for electron and photon identification and measurements; a hadronic calorimeter for jet and missing transverse energy reconstruction; a Muon Spectrometer system (MS) for muon reconstruction. The Inner Detector and Muon Spectrometer devices are of particular interest for the studies presented in this project, and will be briefly described in the next paragraphs. 2.1 The ATLAS Tracking System The ATLAS Tracking System [3] provides a high-precision transverse momentum and position measurement for charged particles coming from the interaction point. It occupies 1

2 the volume of a cylinder 7 meters long and with a radius of 1.15 meters around the beam line. It is immersed in a 2 Tesla axial magnetic field provided by a superconductive solenoid that contains the tracking system. The ID provides full coverage in the azimuthal angle coordinate (φ), and covers the range η < 2.5 in the longitudinal plane where η is the pseudo-rapidity coordinate, defined as: η = ln (tan θ 2 ). It consists of three sub-detectors: a silicon Pixel detector, a silicon strip detector (SCT) and a Transition Radiation Tracker (TRT). The silicon detectors are the nearest to the beam line and provide higher precision measurements. The TRT detector supplies a much higher number of measurements along the track, providing an almost continuous track reconstruction. A track that passes through the ATLAS Inner Detector will on average have three pixel hits, eight SCT hits and thirty-six TRT points reconstructed. The determination of the transverse impact parameter of a track is of particular interest for the measurements presented in the sections 3 and 4. The resolution on this parameter with the ATLAS tracking system is expected to be: σ(d 0 ) = p T sin(θ) (µm) 2.2 The ATLAS Muon System The ATLAS Muon Spectrometer [4] (MS) is the farther sub-detector from the interaction point, it is used to identify muons and measure their parameters with high precision. The whole system is built in-air to minimize multiple scattering effects. It is constituted by high precision chambers and trigger chambers enveloped by the superconductive toroidal magnet system. The muon precision measurement in ATLAS is performed using two different types of chambers. For η < 2 Monitored Drift Tubes (MDT) are employed, whereas in the forward region, at 2 < η < 2.7 where a more radiation-resistant technology is needed, the measurement is performed by Cathode Strip Chambers (CSC). The drift tubes have a resolution of 80 µm per point on the plane transverse to the beam line, the CSC chambers perform a higher precision measurement, with a resolution of 60 µm. The spectrometer is designed so that a muon coming from the interaction vertex passes through three layers of precision chambers both in the barrel region, where the chambers are organized in cylindrical layers around the beam line, and in the end-cap regions, where the chambers form three wheels closing the barrel cylinder on the two sides. The muon transit is also detected by the trigger chambers. In the barrel ATLAS uses Resistive Plate Chambers (RPC), which are organized in three layers. In the end-cap regions three layers of Thin Gap Chambers (TGC) provide a trigger up to η < 2.4. For a more detailed description of the ATLAS muon trigger system see [5]. The transverse momentum of the muons is measured, in the muon system, thanks to the large toroidal magnets. The MS magnet system consists of eight toroids in the central part of the detector, at η < 1.05 (Barrel region), and two at 1.05 < η < 2.7 (End-Cap regions). The toroids provide a peak magnetic field of, respectively, 3.9 Tesla in the barrel region and of 4.1 Tesla in the end-cap regions, which bends the muon in the longitudinal plane of the detector. 2.3 Muon Reconstruction at ATLAS Muon identification and reconstruction in ATLAS provide a coverage of η < 2.7 in a p T range that goes from 1 GeV up to more than 1 TeV. Different types of muon reconstruction 2

3 are employed in ATLAS: Tagged Muons are formed by an ID track associated to segments in the first layer of the muon spectrometer. This type of reconstruction is used in particular for low p T muons, that don t get to pass through the three layers of the muon spectrometer. Due to ID coverage, tagged muons reconstruction is effective up to η < 2.5. Stand-Alone Muons are reconstructed using measurements performed by the Muon System only. The position and transverse momentum of the muon are measured in the MS reference system and back-extrapolated to the primary vertex. The extrapolation, using a Geant4 simulation of the detector, takes into account the multiple scattering and energy losses suffered by the muon during the transit in the ID and calorimeters. The stand-alone reconstruction covers η < 2.7. Combined Muons are a statistical combination of a Stand-Alone Muon and an Inner Detector track. A tight match is performed to associate each muon to a track. When the matching is successful the parameters of the two objects are combined according to their error matrices. As for tagged muons, combined muons reconstruction is effective up to η < J/Ψ µ + + µ Studies with Early Data J/Ψ can be produced either in prompt decays and decays of heavy c c states or in b-hadron decays. In the first two cases the J/Ψ production vertex coincides with the primary vertex of the interaction, whereas in the b-hadron decay production the vertex is displaced due to the long life of the parent. This characteristic can be exploited to separate the promptly and non-promptly produced J/Ψs, and compute the ratio: R = σ(pp b b + X J/Ψ + X ) σ(pp J/Ψ + X ) The measurement of this ratio can be performed very early as, in principle, the acceptances of the detector and the reconstruction and trigger efficiencies, which should be taken into account in any cross-section estimation, cancel out in the formula. The prompt non-prompt ratio measurement has an interest both for the commissioning of the detector and reconstruction and physics wise. It is the first step towards the B inclusive lifetime measurement, which will give an important contribution to the understanding and commissioning of the track and muon reconstruction. The ratio measurement is also very useful in view of an exclusive decay channel lifetime measurement, such as the B s J/Ψ + Φ described in the next section. 3.1 Measurement of the J/Ψ Prompt to Non-Prompt Production Cross- Section Ratio The determination of the ratio is performed exploiting two different observables: the invariant mass of the J/Ψ candidate, and the displacement of its decay vertex with respect to the position of the primary vertex of the interaction. The invariant mass distribution is expected to present a peak in correspondence to the resonance mass 1. Fitting this distribution allows us to separate the signal, hence both prompt and non-prompt J/Ψs, from the combinatorial background, represented by any pair of muons coming from pions, kaons or b b decays produced in the interaction. 3

4 ψ Candidates / (0.04 GeV) ATLAS Preliminary N J/ψ = 6820 ± 90 m J/ψ = ± GeV = 57 ± 1 MeV σ mj/ψ s = 7 TeV 1 L dt = 290 nb Data 2010: Opposite Sign Data 2010: Same Sign Fit projection Fit projection of background Tight Selection m µµ [GeV] m µµ Figure 1: J/Ψ candidates invariant mass distribution fitted with a gaussian for the signal and a linear function for the combinatorial background. In red is the total mass fit, in blue the background component of the mass fit. The distribution in green represents the same sign muon pairs of the data sample as a comparison with the opposite sign candidates pairs. A displacement of the dimuon candidate vertex is distinctive of the b-hadron produced J/Ψs, and can be used to separate prompt from non-prompt candidates. In this measurement the pseudo-proper time of the candidate is used for this purpose. The pseudo-proper time is defined as: τ = L xym J/Ψ p dimuon T where m J/Ψ is the J/Ψ mass, and p dimuon T is the measured transverse momentum of the J/Ψ candidate. L xy is the distance in the transverse plane between the candidate production vertex and the primary vertex of the event projected in the direction of p dimuon T. If a J/Ψ is produced prompt, the pseudo-proper time, in principle, should be zero, and its distribution a delta function. When it is measured, a smearing effect has to be taken into account because of the limited detector resolution. So for prompt J/Ψ we represent the pseudo-proper time distribution as a gaussian convoluted with a delta function. When a J/Ψ is produced non-prompt, instead, one expects the distance L xy, hence the pseudoproper time, to be greater than zero. This behaviour can be described convolving the resolution gaussian with an exponential function e t/τ L. A cartoon representation of the prompt and non-prompt component description is provided in figure 2. An analogous but more complicated parametrization is used for describing the background pseudo-proper time distributions. This can have too a prompt and one or more non-prompt components, but it can also present un-physical negative tails. A total pdf is thus defined to describe both the signal and background components of the invariant mass and the pseudo-proper time distributions: F(τ, δ τ, m µµ, δ m ) = G sig (τ, δ τ )H sig (m µµ, δ m ) + G bkg (τ, δ τ )H bkg (m µµ, δ m ) where G and H represent respectively the pseudo-proper time and mass PDF functions, and sig and bkg indicate if they correspond to the signal or background component. τ, δ τ, m µµ, δ m are the observables, respectively the pseudo-proper time and its per-event error and the mass and its per-event error. A maximum likelihood fit is performed on data in five p T ranges of the J/Ψ candidates, the pseudo-proper time fit in one of the p T bins is represented in figure 3. 4

5 Figure 2: In green is the functional description of the J/Ψ prompt ct distribution, a delta function convoluted with a gaussian. In blue you can see the non-prompt component, given by a delta function plus an exponential both convoluted with the resolution gaussian. In red is the sum of the two. Figure 3: Pseudo-proper time fit result in the J/Ψ candidates p T range [10, 15] GeV. In red the total fit function, in dark green the signal component, in blue the background component. One of the parameters of the fit model is the non-prompt J/Ψ fraction, which is the ratio between the number of non-prompt and the total number of J/Ψ observed. From this parameter one can extract the prompt non-prompt cross-section ratio of the J/Ψ. The results are represented as a function of the p T of the J/Ψ candidate in figure 4. A good agreement within the statistical and systematic error is found with the Pythia Monte Carlo expected values. 4 Study of B s J/Ψ + Φ Decay And Determination of Γ s and Φ s at ATLAS The experimental measurements related to particle anti-particle mixing are of great interest for the determination of the elements and phases of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. These measurements will also have an impact on the search of new physics beyond the Standard Model (SM) at the LHC experiments. Particle anti-particle mixing is observed whenever the mass and flavor eigenstates, for a given particle and its anti-particle, differ. This is the case for two neutral B 0 B 0 systems: the Bd 0 B d 0 and the B0 s B s 0. In both cases we have a light and a heavy mass eigenstate, with a mass difference m s/d = m B 0 s/dh 5 m B 0 s/dl, and a total decay width

6 B ± mass fit - no Lxy cut Figure 4: Ratio measurement results. The black dots represent the measured ratio values in the p T bins, the red and the black error lines are the statistical and systematic uncertainties on the points. The blue stars are the Monte Carlo expected values in each bin, and the blue band is the theoretical uncertainty on these values.! = 36 +/- 5 MeV B ± PDG mass ±0.29 MeV 8 Figure 5: B + mass peak fit performed at ATLAS. In red you have the total fitting function, in blue its background component. difference between these two states, Γ s/d = Γ s/dh Γ s/dl. The time evolution of the oscillation between the two flavor eigenstates is determined by these two quantities. Furthermore, if the measured Γ s is larger than a few percents of the mean decay rate Γ s = 1 2 (Γ sl + Γ sh ), a time-dependent analysis of the B s decays would become sensitive to the CKM CP violating phase φ s. The Standard Model prediction is that φ s should be very small, φ s = ± [6], hence if a measurement finds a significant deviation of its value from zero, this would be a strong indication of the existence of new physics. B-hadrons decays in exclusive channels are already under study at ATLAS. The B + mass peak has been observed with 3.4 pb 1 of integrated luminosity 5 in the decay channel B + J/Ψ + K +, with the J/Ψ decaying in two muons. 4.1 Analysis on the B s J/Ψ + Φ As a first step in the determination of Γ, the measurement of the lifetime distribution of the B s is performed. As the lifetime distribution has little separation power to distinguish the light and the heavy mass eigenstates, we need some additional information to perform 6

7 the measurement. A possibility is to use the CP parity of the two mass states. In the case when Φ s = 0, hence when there is no CP violation, the heavy and light states have opposite CP parity: B sl is CP even and B sh is CP odd. So, to be able to perform a measurement of both B sl and B sh lifetimes separately, a decay which is a composition of both odd and even states is needed. This is the case for the B s J/Ψ + Φ decay, with the J/Ψ decaying in two muons and the Φ decaying in (K + + K ). As both J/Ψ and Φ are vector mesons, while the B s is a pseudo-scalar, the angular distributions of the decay products allow to separate the CP odd from the CP even component. In order to measure Γ it is thus necessary to perform a simultaneous fit on data on different variables. The invariant mass of the B s candidates will provide us separation between the signal and background component of our data. A fit on the pseudo-proper time will help us estimate a combined lifetime of both B sh and B sl. The fit on the three angles of the decay products will help us separate the heavy from the light component. Experimental constraints on Γ s have been provided by the CDF [7] and D0 [8] experiments. At the LHC, with a higher luminosity, we will be able to collect B 0 s J/Ψ+Φ events in larger number, and provide a more precise measurement of these parameters. References [1] The ATLAS Collaboration, Detector and Physics Performance Technical Design Report Vol. I, CERN/LHCC/ (1999) [2] The ATLAS Collaboration, Detector and Physics Performance Technical Design Report Vol. II, CERN/LHCC/ (1999) [3] The ATLAS Collaboration, Inner Detector Technical Design Report Vol. I and II, CERN/LHCC/ (1997) [4] The ATLAS Collaboration, Muon Spectrometer Technical Design Report, CERN/LHCC/ (1997) [5] The ATLAS Collaboration, First-Level Trigger Technical Design Report, CERN/LHCC/ (1998) [6] U.C.M. Bona, M. Ciuchini, E. Franco, V/ Lubicz, G. Martinelli, F. Parodi, M. Pierini, P. Rodeau, C. Schiavi, L. Silvestrini, A. Stocchi and V. Vagnoni, Constraints on new physics from the quark mixing unitary triangle, Phys.Rev.Lett. 97 (2006) [7] The CDF Collaboration, T. Aaltonen et al., Measurement of Lifetime and Decay- Width Difference in B 0 s J/ΨΦ Decays, Phys.Rev.Lett. 100 (2008) , arxiv: [hep-ex] [8] The D0 Collaboration, V. M. Abazov et al., Measurement of B 0 s mixing parameters from the flavor tagged decay B 0 s J/ΨΦ, arxiv: [hep-ex] 7