The Impact of Misalignment on Physics

Transcription

1 The Impact of Misalignment on Physics G. Steinbrück a for the ATLAS, CMS, LHCb Collaborations a Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, Hamburg, Germany Abstract georg.steinbrueck@desy.de In this article we summarize the impact of misalignment of tracking detectors and the muon system on physics measurements.studies from the ATLAS, CMS, and LHCb collaborations are presented. 1 Introduction To exploit the enormous physics potential of the high luminosity environment at the LHC, highly segmented tracking detectors have been built by the LHC collaborations. In order to unambiguiously identify particles in a dense environment and to measure their momenta and spatial coordinates precisely, these tracking devices have to be aligned well with respect to each other. In the following article we summarize the impact misalignment has on benchmark physics measurements and basic physics quantities. Fig. 1: Expected misalignment after a few months of data. Fig. 2: Expected misalignemt after a few months of data taking, substructures. 2 Simulation of Tracker Misalignment ATLAS and CMS have defined several scenarios which describe the amount of misalignment expected in various stages of the experiment. ( > reference goes here.). Simulated data is producd with the detectors being misaligned according to these scenarios, and the impact on physics observables is studied. For CMS, two levels of misalignment are studied: The first data taking scenario describes the amount of misalignment expected after a few months of data taking. The precision of the strip tracker alignment is determined by the mechanical precision during construction and the laser alignment system, yielding unceratinties in the 100µm range, while the pixel detector is already assumed to be aligned to the 10/mum-range using tracks. The misalignment after first data taking is summarized in Table 1 and 2, whereas the amount of misalignment for the CMS muon system is summarized in Table 3. The situation after more than six months of data taking is discribed in the long term alignment scenario. The alignment of the CMS strip tracker improves by a factor of 10, while the pixel tracker is assumed to already be aligned in the first step. Hence it s alignment does not change between the two scenarios. The alignment of the muon system improves by a factor of 5 between the two scenarios. Fig. 3: Expected misalignment for the CMS muon system after a few months of data. Atlas has introduced correlated misalignment in three levels in their simulation: At the first level, the whole detector is misaligned to a few mm/ a few tenths of a mrad. At the second level, large detector structures (layers and discs) are misaligned to µm and 0.5 1mrad. At the third level, detector modules are misaligned to µm and 1mrad. 3 Impact of Misalignment on tracking and vertex finding. A misaligned tracking device can compromise physics observables through a deteriation in track quality and tracking efficiencies. Before looking at specific physics channels, the impact on tracks is summarized here. Fig. 4 shows the relative transverse momentum resolution for tracks in the CMS tracker as a function of pseudorapidity for three different levels of alignment: The first data scenario described

2 earlier, the longterm scenario, and without any misalignment of the tracker. A clear degradation can be seen. Likewise, the Fig.?? shows the impact of tracker alignment on the transverse impact parameter for the same CMS scenarios. The tracking efficiency is shown in Fig.?? as a function of pseudorapidity for several misalignment scenarios. It can be seen that if the errors are taken into account correctly, there is no visible impact of the levels of misalignment studied here on the tracking efficiency. CMS has also studied the impact of misaligment of both the tracking detectors and the muon system on high transverse momentum muons (1 TeV), an energy range where both the tracker and the muon system contribute to the measurement. Fig. 7 shows the relative resolution in 1/p T as a function of pseudorapidity for three levels of combined misalignment. A clear degradation of the p T resolution can be observed if the tracking detectors and the muon system are misaligned. Fig. 6: Track efficiencies for various levels of misalignment (CMS).Also shown is the tracking efficiency for the short term misalignment scenario if increased hit position errors are not taken into consideration. Fig. 4: Track pt resolution as a function of pseudorepidity. Fig. 7: Muon 1/pT resolution as a function of pseudorepidity. Fig. 5: Transverse impact parameter resolution as a function of pseudorepidity. 4 Vertex finding/ fitting. CMS has studied the impact of a misaligned tracker on vertex finding and fitting Table. 8 compares vertex resolutions and biases for the vertex finding step for three benchmark physics processes, B S J/ψφ, t th, and Drell-Yan production. Again, the three reference scenarios described earlier are used. While the impact of vertex finding efficiencies (not shown here) is small, a clear degradation in the vertex resolution in x and y can be seen when comparing a perfectly aligned tracker with the short term scenario. The difference between the short term scenario and the longterm scenario is small for low pt tracks as present in the B s sample. This is due to the fact that the misalignment of the pixel tracker is the same for both scenarios, while the strip tracker alignment improves by a factor of 10. For higher tracks with higher momentum as present in the Drell-Yan and the Higgs samples, the stip tracker contributes and some improvement can be seen. Apart from the degradation of vertex resolutions caused by random misalignment, systematic misalign-

3 ment of larger substructures can cause biases in the vertex position that can be decremental (word) to many physics measurements... After the vertex finding step, vertex positions are fit using a Kalman filter algorithm. Table. 9 again compares benchmark observables for fitted vertices for the same scenarios and physics channels. Here, the results are shown separately for the primary and the secondary vertices for the B s sample. For secondary vertices, a difference of 12µm can be seen for all coordinates. Fig. 10: Relative pt resolution as a function of pt for muons in H ZZ µµee decays (left). Z µµ mass peak (right) for the same sample. Compared are the three aforementioned misalignment scenarios. Fig. 8: Expected impact of misalignment on vertex finding. Fig. 11: Reconstruction efficiency as a function of dimuon mass. Fig. 9: Expected impact of misalignment on vertex fitting. 5 Impact of Misalignment on Physics Measurements CMS has studied the impact of misalignment on various benchmark physics channels. Fig. 10 shows the pt resolution of muons in H ZZ µµee events and the di-muon mass resolution for the three benchmark scenarios. Excellent alignment is required to observe no effect. Fig. 11 shows the reconstruction efficiency as a function of dimuon invariant mass, while the dimuon mass resolution is shown in Fig. 12. Again, the impact on the offline reconstruction efficiency is small, while the mass resolution suffers strongly, specially at high mass: With respect to a perfectly aligned tracker, the mass resolution at 1 TeV is enlarged by a factor of 1.3 and 3 for the longterm and first data scenario, respectively. At 3 TeV, the corresponding factors are 1.4 and 5, respectively. Fig. 12: Dimuon invariant mass resolution as a function of dimuon mass.

4 Fig. 16: Dimuon mass resolution for 3 TeV RS graviton, various levels of misalignment. 5.1 W mass Fig. 13: Integrated luminosity needed for 5 sigma discovery for different Z models. Fig. 17: W mass uncertainties, CMS. Fig. 18: W mass uncertainties, ATLAS. Fig. 14: Integrated luminosity for 5 sigma discovery for RSgravitons. Fig. 15: Dimuon mass resolution for 1 TeV RS graviton, various levels of misalignment. 5.2 Supersymmetry In some regions of msugra parameter space higher mass neutralinos are produced from the decay of squarks and gluinos. These can decay into a lighter neutralino and same flavor opposite sign lepton pairs, according to χ 0 2 χ 0 1ll. The dilepton invariant mass spectrum exibits a characteristic endpoint given by the mass difference of the two neutralinos. CMS has analyzed ability to observe this endpoint in reconstructed events, comparing results for a misaligned tracker to perfect alignment. For a tracker misaligned to a level assumed in the first data scenario the end point is still visible, with a shift of about 1 GeV. However, the event selection efficiencies are reduced significantly: For the dielectron channel they decrease by 10the first data (long term) scenarios, respectively. For the di-muon channel the effect is even larger with 13Fig. 19 shows the dimuon mass spectrum with and without misalignment.

5 Fig. 19: Dimuon invariant mass for χ 0 2 χ 0 1µµ, CMS, with and without misalignment. 5.3 H 4µ ATLAS has studied the effect of muon system misalignment on the channel H ZZ µµµµ. Fig.?? shows the widths of the Higgs resonance obtained by fitting a Breit- Wigner function to the four-muon spectrum, as a function of displacements and rotations. Likewise, in Fig?? the dependence of the acceptance for these events is shown as a function of displacement and rotation. A linear dependance of the width and the acceptance on the muon system misalignment can be observed. As an example, the resolution is increased by 10% at an angle of 5 mrad, while the acceptance is specially sensitive to rotations around the x-axis. Fig. 20: Higgs width (Breit-Wigner) for displacements (above) and rotations (below). Fig. 21: Acceptance for H 4µ events as a function of displacements (above) and rotations (below). 5.4 B Physics Precise tracking is specially important in B physics. LHCb has studied misalignment of the VELO detector (vertex locator). The alignment of the VELO is potentially an issue due to its special buildup: The two halves of the VELO are retractable from the beam during fills (see Fig. 22 LHCb has studied uncorrelated misalignment of modules and misalignment of a whole box (VELO half). The impact parameter resolution for different levels of box misalignment is shown in Fig. 23. The impact parameter resolution deteriorates with increasing misalignment, specially for large track momentum. The impact parameter is directly related to the proper time, hence the proper time measurement would be degraded. LHCb uses its silicon tracking detector in the Level 1 trigger. Fig. 24 shows the trigger efficiency for B S KK events and the rejection for minimum bias tracks as a function of rotation around the y-axis. The level 1 trigger efficiency is reduced by 50% for a rotation of 0.7 mrad around y. // As a benchmark B-physics channel, CMS has also studied the effect of misalignment on B S rightarrowµµ events. Fig. 29. The B S mass resolution increases from 45 MeV for a perfectly aligned detector to 49 and 54 MeV for the longterm and shortterm scenario, respectively.

6 Fig. 25: B S π + K mass resolution as a function of momentum resolution. Fig. 22: The LHCb VELO detector. Fig. 26: Background/Signal for B S considered is B d K + π. KK. The background Fig. 23: Impact parameter resolution for LHCb VELO box misalignment in x (10 100µm. Fig. 24: Trigger efficiency for B S KK events as a function of VELO rotation. Fig. 27: Signal/ p (Signal + Background) for B S µµ.

7 Fig. 30: (Degradation of b tagging performance with misalignment (ATLAS). Fig. 28: Background/Signal for B S KK. The background considered is B d K + π. 5.6 Subsection about bibliographic references All bibliographical references are numbered and listed at the end of the paper in a section called References, which is generated by a thebibliography environment. References in the text to a bibliographic entry are generated with a \cite{key} command, while inside the thebibliography environment the associated bibliographic information is entered with the help of \bibitem{key} entry. After two runs of L A TEX on your file you should get reference numbers in square brackets. The order of appearance in the text and in the bibliography should be identical. This is a simple bibliographic citation [1]. When part of the text add Ref., e.g., see Ref. [1] and Refs. [2 4] Subsubsection about equations Equation (1) is Einstein s famous ennergy mass relation. Fig. 29: Dimuon invariant mass for B S levels of misalignment (CMS). µµ events for three E = mc 2 (1) On the other hand Eq. (2), derived by Euler, connects several basic mathematical constants. 1 + e iπ = 0 (2) A range of equations is referenced thus: Eqs. (1) (2) Example of a subsubsubsection Here we make a reference to Section B Tagging The performance of a b-tagging algorithm is directly affected by misalignment due to the loss in separation to distinguish displaced tracks from tracks originating from the primary vertex. ATLAS has studied the rejection power of their 3-D b-tagging algorithm for three levels of misalignment. Compared are the absolute rejection power with respect to light jets for two reference b-tagging efficiencies (50%and60%), and the relative rejection power normalized to the values for no misalignment. The results sumarized in Table 30 show a strong dependence of the b-tagging performance on misalignment. 1 Footnotes are to be used only when absolutely necessary. 6 More detailed instructions 6.1 Title of paper and authors The article title as well as the section headings should be entered in lowercase letters, only capitalizing proper names and acronyms, as needed. The initial(s) followed by the last name(s) of the author(s), their organizations, mailing addresses and addresses should appear on separate lines. The title and authors section ends the initial one-column part. The rest of the document should be typeset, unless locally expressly overridden, in two-column mode (see Section 6.2). The first entry in the body is the abstract (implemented with the abstract environment).

8 6.2 Margins and two-column format The text of the article is typeset in two columns. This is achieved by using the multicols environment: \begin{multicols}{2}... \end{multicols} Note that you can at any time go into one-column mode by ending the environment by the \end{multicols}, typeset a table or figure in full-width mode, then return on twocolumn mode by \begin{multicols}{2}. Table 2 summarizes available L A TEX commands and environments. More information about rules and procedures can be found at the Web page asp, where you can also download the class files needed to compose your document ( DTP/cernrep.zip). 6.3 Figures and tables For maximum flexibility figures and tables in the current cernyrep2 class do not float. Authors should place them as close as possible to where they are mentioned in the text Typesetting a table Tables are identified by the table environment. Table captions are specified as the argument of a \caption command that has to be placed above the table body (see, Table 1), which is an example of a small table. Table 1: A small table Heading row Column 1 Column 2 Column 3 Row 2 Column 1 a entry (2,2) entry (2,3) Row 3 Column 1 entry (3,2) entry (3,3) a Notes in tables can be constructed by hand. Tables and figures may span the whole page. To achieve this you can exit the multicols environment, typeset the table or figure, before entering the multicols environment again. An example is Table 2, where we interrupt 2-column mode to typeset it full-width at the current position. Jumping between one- and two-column mode should only be used when really necessary. Table 2: Summary table of required L A TEX commands Style Command or environment Title Authors names Output title Abstract Main heading Subheading Subsubheading Subsubsubheading Table Figure Caption Reference (definition) Reference (use) Bibliography Bibliographic citation \title{title of article} \author{list of authors, affiliation, etc.} \maketitle \begin{abstract}... \end{abstract} \section{section title} \subsection{subsection title} \subsubsection{subsubsection title} \subsubsubsection{subsubsubsection title} \begin{table}... \end{table} \begin{figure}... \end{figure} \caption{text of caption} \label{mykey} \ref{mykey} \begin{thebibliography}... \end{thebibliography} \cite{mybibkey} Bibliographic definition \bibitem{mybibkey} Typesetting a figure Figures are identified by the figure environment. Figure captions are specified as the argument of a \caption command that is placed below the figure body (see Fig.??). Photographs and figures can be included in the final PostScript (or PDF) file with L A TEX s includegraphics command. They must be one of the types EPS, PDF, PNG or JPEG. In any case check their quality on your local printer at the final size you want to reproduce them in your article. Pay particular attention to colour images. Since the printed proceedings will be produced in black and white, please ensure that the readability of the text and patterns of your image is still guaranteed when printed in black and white and at the envisaged scale. Acknowledgements We wish to thank A.N. Colleague for enlightening comments on the present topic.

9 References [1] J.M. Raby, Biophysical aspects of radiation quality, International Atomic Energy Agency, Technical Reports Series No. 58 (1966). [2] H. Appleman et al., J. Med. Biol. 8 (1959) 911. [3] E. van Berg, D. Johnson and J. Smith, Rad. Res. 5 (1965) 215. [4] M.A. Allen et al., IEEE Trans. Nucl. Sci. NS 24 (1977) 1780.