Διερεύνηση κινηματικών ιδιοτήτων υπερσυμμετρικών διασπάσεων σε συγκρούσεις πρωτονίων με το πείραμα CMS στον επιταχυντή LHC

Ο Μεγάλος Επιταχυντής Αδρονίων LHC δίνει τη δυνατότητα για την αναζήτηση Φυσικής πέραν του Καθιερωμένου Προτύπου (ΚΠ) σε συγκρούσεις πρωτονίου-πρωτονίου σε ενέργειες 13 TeV. Μία από τις πιο ελκυστικές θεωρίες που δίνουν απάντηση στα ανοιχτά ερωτήματα του ΚΠ είναι η Υπερσυμμετρία. Η υψηλή luminosity (45.14 /fb) που έχει επιτευχθεί μέχρι τώρα στον LHC επιτρέπει την ανίχνευση σήματος σε οριακές περιοχές του φασικού χώρου των μαζών των σωματιδίων της Υπερσυμμετρίας αλλά ταυτόχρονα δημιουργεί την ανάγκη ανάπτυξης μεθόδων που θα πιέσουν ακόμα περισσότερο τα όρια αυτά. Στην παρούσα εργασία, παρουσιάζουμε μια μελέτη για την ανίχνευση νέας Φυσικής σε γεγονότα με δύο χαμηλής ορμής και αντίθετου φορτίου λεπτόνια και ελλείπουσα εγκάρσια ορμή. Τα αποτελέσματα ερμηνεύονται στα πλαίσια παραγωγής ζεύγους charginos και neutralinos με σχεδόν εκφυλισμένες μάζες και παραγωγής ζεύγους top squarks, όταν το ελαφρύτερο neutralino και το top squark έχουν σχεδόν ίση μάζα (μοντέλα φυσικής Υπερσυμμετρίας με συμπιεσμένο φάσμα μαζών). Το κύριο υπόβαθρο στη συγκεκριμένη μελέτη αποτελείται από ζεύγη top quarks που οδηγούν σε τελική κατάσταση με 2 λεπτόνια, 2 νετρίνα και 2 b quarks. Έτσι, για την μείωση του υποβάθρου είναι σημαντική η απόρριψη γεγονότων που περιέχουν b quarks. Η απαίτηση της μελέτης για τελικά προϊόντα χαμηλής ορμής οδηγεί σε b quarks με χαμηλή ορμή, των οποίων η ανίχνευση με τις συμβατικές μεθόδους δεν είναι αποδοτική. Η παρούσα εργασία εξερευνά τη δυνατότητα ανίχνευσης πιδάκων από b quarks με τη χρήση μεταβλητών που σχετίζονται με secondary vertices και ελέγχει την βελτίωση που παρέχει αυτή η μέθοδος στην παραπάνω μελέτη. Αρχικά, επιλέγονται οι κατάλληλες μεταβλητές και στη συνέχεια εφαρμόζονται πάνω τους διαδοχικά cuts με σκοπό την αύξηση της significance στην signal region (SR). Η εργασία κλείνει με την παρουσίαση της βελτίωσης της significance που επιτυγχάνεται ανάλογα με τη διαφορά μαζάς μεταξύ των Υπερσυμμετρικών σωματιδίων που έχουμε επιλέξει στην ερμηνεία των αποτελεσμάτων.The Large Hardron Collider (LHC) gives the opportunity to search for Physics beyond the Standard Model (SM) in proton-proton collisions at energy of 13 TeV. One of the most appealing theories that can resolve the unanswered questions of the SM is Supersymmetry. The high luminosity (45.14 /fb) that has been achieved at the LHC allows the detection of signal at the frontiers of the supersymmetric mass phase space and creates the necessity for methods that push these frontiers even further. In this thesis, we present an analysis searching for new physics in events with two soft oppositely charged leptons and missing transverse momentum. The results have been interpreted in terms of a model of pair production of chargino and neutralino with almost degenerate masses with the lightest neutralino (Lightest Supersymmetric Particle – LSP) and in terms of a model of top squark pair production, when the LSP and the top squark have almost identical masses (Natural Supersymmetry models with compressed mass spectrum). The main background in this analysis is the pair production of top quarks that lead to a final state with two leptons, two neutrinos and two b quarks. Therefore, the rejection of events with b quarks is crucial to achieve a significant background reduction. The requirement of the analysis for soft particles in the final state leads to b quarks with low transverse momentum. However, the detection of soft b quarks with conventional methods is not efficient. This thesis investigates the possibility to detect b quark jets with the utilization of cuts of variables related with secondary vertices (SV) and checks the improvement that this method brings to the aforementioned analysis. First, we choose a set of suitable SV variables and then we try to find sequential cuts with the aim of maximizing the significance in the signal region (SR). The thesis concludes with the presentation of the improvement of the significance as a function of the mass difference between the particles chosen in the interpretation of the results

Generalizing mkFit and its Application to HL-LHC

mkFit is an implementation of the Kalman filter-based track reconstruction algorithm that exploits both thread- and data-level parallelism. In the past few years the project transitioned from the R&D phase to deployment in the Run-3 offline workflow of the CMS experiment. The CMS tracking performs a series of iterations, targeting reconstruction of tracks of increasing difficulty after removing hits associated to tracks found in previous iterations. mkFit has been adopted for several of the tracking iterations, which contribute to the majority of reconstructed tracks. When tested in the standard conditions for production jobs, speedups in track pattern recognition are on average of the order of 3.5x for the iterations where it is used (3-7x depending on the iteration). Multiple factors contribute to the observed speedups, including vectorization and a lightweight geometry description, as well as improved memory management and single precision. Efficient vectorization is achieved with both the icc and the gcc (default in CMSSW) compilers and relies on a dedicated library for small matrix operations, Matriplex, which has recently been released in a public repository. While the mkFit geometry description already featured levels of abstraction from the actual Phase-1 CMS tracker, several components of the implementations were still tied to that specific geometry. We have further generalized the geometry description and the configuration of the run-time parameters, in order to enable support for the Phase-2 upgraded tracker geometry for the HL-LHC and potentially other detector configurations. The implementation strategy and high-level code changes required for the HL-LHC geometry are presented. Speedups in track building from mkFit imply that track fitting becomes a comparably time consuming step of the tracking chain

Line Segment Tracking: Improving the Phase 2 CMS High Level Trigger Tracking with a Novel, Hardware-Agnostic Pattern Recognition Algorithm

Charged particle reconstruction is one the most computationally heavy components of the full event reconstruction of Large Hadron Collider (LHC) experiments. Looking to the future, projections for the High Luminosity LHC (HL-LHC) indicate a superlinear growth for required computing resources for single-threaded CPU algorithms that surpass the computing resources that are expected to be available. The combination of these facts creates the need for efficient and computationally performant pattern recognition algorithms that will be able to run in parallel and possibly on other hardware, such as GPUs, given that these become more and more available in LHC experiments and high-performance computing centres. Line Segment Tracking (LST) is a novel such algorithm which has been developed to be fully parallelizable and hardware agnostic. The latter is achieved through the usage of the Alpaka library. The LST algorithm has been tested with the CMS central software as an external package and has been used in the context of the CMS HL-LHC High Level Trigger (HLT). When employing LST for pattern recognition in the HLT tracking, the physics and timing performances are shown to improve with respect to the ones utilizing the current pattern recognition algorithms. The latest results on the usage of the LST algorithm within the CMS HL-LHC HLT are presented, along with prospects for further improvements of the algorithm and its CMS central software integration

Generalizing mkFit and its Application to HL-LHC

mkFit is an implementation of the Kalman filter-based track reconstruction algorithm that exploits both threadand data-level parallelism. In the past few years the project transitioned from the R&D phase to deployment in the Run-3 offline workflow of the CMS experiment. The CMS tracking performs a series of iterations, targeting reconstruction of tracks of increasing difficulty after removing hits associated to tracks found in previous iterations. mkFit has been adopted for several of the tracking iterations, which contribute to the majority of reconstructed tracks. When tested in the standard conditions for production jobs, speedups in track pattern recognition are on average of the order of 3.5x for the iterations where it is used (3-7x depending on the iteration). Multiple factors contribute to the observed speedups, including vectorization and a lightweight geometry description, as well as improved memory management and single precision. Efficient vectorization is achieved with both the icc and the gcc (default in CMSSW) compilers and relies on a dedicated library for small matrix operations, Matriplex, which has recently been released in a public repository. While the mkFit geometry description already featured levels of abstraction from the actual Phase-1 CMS tracker, several components of the implementations were still tied to that specific geometry. We have further generalized the geometry description and the configuration of the run-time parameters, in order to enable support for the Phase-2 upgraded tracker geometry for the HL-LHC and potentially other detector configurations. The implementation strategy and high-level code changes required for the HL-LHC geometry are presented. Speedups in track building from mkFit imply that track fitting becomes a comparably time consuming step of the tracking chain. Prospects for an mkFit implementation of the track fit are also discussed

Development and validation of HERWIG 7 tunes from CMS underlying-event measurements

This paper presents new sets of parameters (“tunes”) for the underlying-event model of the HERWIG7 event generator. These parameters control the description of multiple-parton interactions (MPI) and colour reconnection in HERWIG7, and are obtained from a fit to minimum-bias data collected by the CMS experiment at s=0.9, 7, and 13Te. The tunes are based on the NNPDF 3.1 next-to-next-to-leading-order parton distribution function (PDF) set for the parton shower, and either a leading-order or next-to-next-to-leading-order PDF set for the simulation of MPI and the beam remnants. Predictions utilizing the tunes are produced for event shape observables in electron-positron collisions, and for minimum-bias, inclusive jet, top quark pair, and Z and W boson events in proton-proton collisions, and are compared with data. Each of the new tunes describes the data at a reasonable level, and the tunes using a leading-order PDF for the simulation of MPI provide the best description of the dat

Search for dark photons in Higgs boson production via vector boson fusion in proton-proton collisions at √s = 13 TeV

A search is presented for a Higgs boson that is produced via vector boson fusion and that decays to an undetected particle and an isolated photon. The search is performed by the CMS collaboration at the LHC, using a data set corresponding to an integrated luminosity of 130 fb−1, recorded at a center-of-mass energy of 13 TeV in 2016–2018. No significant excess of events above the expectation from the standard model background is found. The results are interpreted in the context of a theoretical model in which the undetected particle is a massless dark photon. An upper limit is set on the product of the cross section for production via vector boson fusion and the branching fraction for such a Higgs boson decay, as a function of the Higgs boson mass. For a Higgs boson mass of 125 GeV, assuming the standard model production rates, the observed (expected) 95% confidence level upper limit on the branching fraction is 3.5 (2.8)%. This is the first search for such decays in the vector boson fusion channel. Combination with a previous search for Higgs bosons produced in association with a Z boson results in an observed (expected) upper limit on the branching fraction of 2.9 (2.1)% at 95% confidence level

Search for top squark pair production in a final state with two tau leptons in proton-proton collisions at root s=13 TeV

A search for pair production of the supersymmetric partner of the top quark, the top squark, in proton-proton collision events at s = 13 TeV is presented in a final state containing hadronically decaying tau leptons and large missing transverse momentum. This final state is highly sensitive to high-tan beta or higgsino-like scenarios in which decays of electroweak gauginos to tau leptons are dominant. The search uses a data set corresponding to an integrated luminosity of 77.2 fb(-1), which was recorded with the CMS detector during 2016 and 2017. No significant excess is observed with respect to the background prediction. Exclusion limits at 95% confidence level are presented in the top squark and lightest neutralino mass plane within the framework of simplified models, in which top squark masses up to 1100 GeV are excluded for a nearly massless neutralino.Peer reviewe

Measurement of the t(t)over-barb(b)over-bar production cross section in the all-jet final state in pp collisions at root s=13 TeV

A measurement of the production cross section of top quark pairs in association with two b jets (t (t) over barb (b) over bar) is presented using data collected in proton-proton collisions at root s=13 TeV by the CMS detector at the LHC corresponding to an integrated luminosity of 35.9 fb(-1). The cross section is measured in the all-jet decay channel of the top quark pair by selecting events containing at least eight jets, of which at least two are identified as originating from the hadronization of b quarks. A combination of multivariate analysis techniques is used to reduce the large background from multijet events not containing a top quark pair, and to help discriminate between jets originating from top quark decays and other additional jets. The cross section is determined for the total phase space to be 5.5 +/- 0.3 (stat)(-1.3)(+)(1.6) (syst)pb and also measured for two fiducial t (t) over barb (b) over bar, definitions. The measured cross sections are found to be larger than theoretical predictions by a factor of 1.5-2.4, corresponding to 1-2 standard deviations. (C) 2020 The Author. Published by Elsevier B.V.Peer reviewe

Search for top squark pair production using dilepton final states in pp collision data collected at root s=13TeV

A search is presented for supersymmetric partners of the top quark (top squarks) in final states with two oppositely charged leptons (electrons or muons), jets identified as originating from bquarks, and missing transverse momentum. The search uses data from proton-proton collisions at root s = 13 TeV collected with the CMS detector, corresponding to an integrated luminosity of 137 fb(-1). Hypothetical signal events are efficiently separated from the dominant top quark pair production background with requirements on the significance of the missing transverse momentum and on transverse mass variables. No significant deviation is observed from the expected background. Exclusion limits are set in the context of simplified supersymmetric models with pair-produced lightest top squarks. For top squarks decaying exclusively to a top quark and a lightest neutralino, lower limits are placed at 95% confidence level on the masses of the top squark and the neutralino up to 925 and 450 GeV, respectively. If the decay proceeds via an intermediate chargino, the corresponding lower limits on the mass of the lightest top squark are set up to 850 GeV for neutralino masses below 420 GeV. For top squarks undergoing a cascade decay through charginos and sleptons, the mass limits reach up to 1.4 TeV and 900 GeV respectively for the top squark and the lightest neutralino.Peer reviewe

Measurement of the azimuthal anisotropy of Y(1S) and Y(2S) mesons in PbPb collisions at root s(NN)=5.02 TeV

The second-order Fourier coefficients (v(2)) characterizing the azimuthal distributions of Y(1S) and Y(2S) mesons produced in PbPb collisions at root s(NN) = 5.02 TeV are studied. The Y mesons are reconstructed in their dimuon decay channel, as measured by the CMS detector. The collected data set corresponds to an integrated luminosity of 1.7 nb(-1). The scalar product method is used to extract the v2 coefficients of the azimuthal distributions. Results are reported for the rapidity range vertical bar y vertical bar < 2.4, in the transverse momentum interval 0 < pT < 50 GeV/c, and in three centrality ranges of 10-30%, 30-50% and 50-90%. In contrast to the J/psi mesons, the measured v(2) values for the Y mesons are found to be consistent with zero. (C) 2021 The Author(s). Published by Elsevier B.V.Peer reviewe