Flavor Tagging Research Articles

The time-dependent angular analysis of Bd \u2192 KS\u2113\u2113, a new benchmark for new physics

We consider the time-dependent analysis of Bd→ KSℓℓ taking into account the time-evolution of the Bd meson and its mixing into {overline{B}}_d . We discuss the angular conventions required to define the angular observables in a transparent way with respect to CP conjugation. The inclusion of time evolution allows us to identify six new observables, out of which three could be accessed from a time-dependent tagged analysis. We also show that these observables could be obtained by time-integrated measurements in a hadronic environment if flavour tagging is available. We provide simple and precise predictions for these observables in the SM and in NP models with real contributions to SM and chirally flipped operators, which are independent of form factors and charm-loop contributions. As such, these observables provide robust and powerful cross-checks of the New Physics scenarios currently favoured by global fits to b → sℓℓ data. In addition, we discuss the sensitivity of these observables with respect to NP scenarios involving scalar and tensor operators, or CP-violating phases. We illustrate how these new observables can provide a benchmark to discriminate among the various NP scenarios in b → sμμ. We discuss the extension of these results for Bs decays into f0, η or η′.

Deep learning jet substructure from two-particle correlations

Deciphering the complex information contained in jets produced in collider events requires a physical organization of the jet data. We introduce two-particle correlations (2PCs) by pairing individual particles as the initial jet representation from which a probabilistic model can be built. Particle momenta, as well as particle types and vertex information are included in the correlation. A novel, two-particle correlation neural network (2PCNN) architecture is constructed by combining neural network based filters on 2PCs and a deep neural network for capturing jet kinematic information. The 2PCNN is applied to boosted boson and heavy flavor tagging, and it achieves excellent performance by comparing to models based on telescoping deconstruction. Major correlation pairs exploited in the trained models are also identified, which shed light on the physical significance of certain jet substructure.

Jet charge modification in finite QCD matter

Jet production and jet substructure modification in heavy-ion collisions have played an essential role in revealing the in-medium evolution of parton showers and the determination of the properties of strongly-interacting matter under extreme conditions. It is imperative to extend these studies to include flavor tagging and to devise observables that are sensitive to the partonic origin of jets. The average jet charge, defined as the momentum-weighted sum of the electric charges of particles inside the jet, is a proxy of the electric charge of the quark or gluon that initiates the jet. We demonstrate how the factorization framework of soft-collinear effective theory can be generalized to evaluate the jet charge in a dense strongly-interacting matter environment, such as the one produced in nuclear reactions at collider energies. Observables that can separate the contribution of in-medium branching from the trivial isospin effects are identified and their connection to established jet quenching effects is elucidated. We present predictions for the transverse momentum dependence of the jet charge distribution in nucleus-nucleus collisions and its modification relative to the proton case.

Hardware Accelerated ATLAS Workloads on the WLCG Grid

In recent years the usage of machine learning techniques within data-intensive sciences in general and high-energy physics in particular has rapidly increased, in part due to the availability of large datasets on which such algorithms can be trained, as well as suitable hardware, such as graphic or tensor processing units, which greatly accelerate the training and execution of such algorithms. Within the HEP domain, the development of these techniques has so far relied on resources external to the primary computing infrastructure of the WLCG (Worldwide LHC Computing Grid). In this paper we present an integration of hardware-accelerated workloads into the Grid through the declaration of dedicated queues with access to hardware accelerators and the use of Linux container images holding a modern data science software stack. A frequent use-case in the development of machine learning algorithms is the optimization of neural networks through the tuning of their Hyper Parameters (HP). For this often a large range of network variations must be trained and compared, which for some optimization schemes can be performed in parallel – a workload well suited for Grid computing. An example of such a hyper-parameter scan on Grid resources for the case of flavor tagging within ATLAS is presented.

Jet charge in heavy-ion collisions

Jet quenching effects have been widely used to study the properties of strongly-interacting matter, quark-gluon plasma, in heavy-ion collisions. Flavor tagging in heavy-ion collisions plays an important role to reveal the medium parton showers for quark and gluon evolution. Combining with kinematic information, the average jet charge can be used to separate the contribution of different jet flavors, which is defined as the momentum- weighted sum of the charges of hadrons inside a given jet. Using soft-collinear effective theory with medium interactions, we investigate the factorization of the jet charge in QCD medium. We provide predictions for jet charge distributions and their modifications compared to the ones in proton-proton collisions.

The Belle II flavor tagger

Belle II is a particle-physics experiment at the intensity frontier focused on probing non Standard Model physics through precision measurements of quark-flavor and τ-lepton dynamics. Determining the flavor of neutral B mesons, i.e. their quark composition, is a crucial task which is addressed using flavor tagging algorithms. Due to the novel high-luminosity conditions and the increased beam backgrounds at Belle II, an improved flavor tagging algorithm had to be developed to ensure the success of the Belle II physics program. The new Belle II flavor tagger exploits the flavor-specific signatures of B 0 decays employing boosted decision trees and neural networks. It identifies B 0-decay products providing flavor-specific signatures and combines the information from all possible signatures into a final output. The algorithm has been validated by comparing its performance on simulated events with its performance on collision events collected by the predecessor experiment Belle. To explore the advantages of state-of-the-art deep-learning techniques, the Belle II collaboration developed a deep-learning-based flavor tagger. This algorithm tags the flavor of B 0 mesons without identifying flavor specific signatures using a deep-learning neural network. The validation on Belle data of this algorithm is currently ongoing.

Neutral D→KK^{*} Decays as Discovery Channels for Charm CP Violation.

We point out that the CP asymmetries in the decays D^{0}→K_{S}K^{*0} and D^{0}→K_{S}K[over ¯]^{*0} are potential discovery channels for charm CP violation in the standard model. We stress that no flavor tagging is necessary, the untagged CP asymmetry a_{CP}^{dir}(D[over (-)]→K_{S}K^{*0}) is essentially equal to the tagged one, so that the untagged measurement comes with a significant statistical gain. Depending on the relevant strong phase, |a_{CP}^{dir,untag}| can be as large as 0.003. The CP asymmetry is dominantly generated by exchange diagrams and does not require nonvanishing penguin amplitudes. While the CP asymmetry is smaller than in the case of D^{0}→K_{S}K_{S}, the experimental analysis is more efficient due to the prompt decay K^{*0}→K^{+}π^{-}. One may further search for favorable strong phases in the Dalitz plot in the vicinity of the K^{*0} peak.

Quark flavour-violating Higgs decays at the ILC

Flavour-violating Higgs interactions are suppressed in the Standard Model such that their observation would be a clear sign of new physics. We investigate the prospects for detecting quark flavour-violating Higgs decays in the clean ILC environment. Concentrating on the decay to a bottom and a light quark j, we identify the dominant Standard Model background channels as coming from hadronic Standard Model Higgs decays with mis-identified jets. Therefore, good flavour tagging capabilities are essential to keep the background rate under control. Through a simple cut-based analysis, we find that the most promising search channel is the two-jet plus missing energy signature . At 500 GeV, the expected 95% CL upper limit on mathrm{mathcal{B}}left(hto bjright) is of order 10−3. Correspondingly, a 5σ discovery is expected to be possible for branching ratios as low as a few 10−3.

Disentangling heavy flavor at colliders

We propose two new analysis strategies for studying charm and beauty quarks at colliders. The first strategy is aimed at testing the kinematics of heavy-flavor quarks within an identified jet. Here, we use the SoftDrop jet-declustering algorithm to identify two subjets within a large-radius jet, using subjet flavor tagging to test the heavy-quark splitting functions of QCD. For subjets containing a $J / \psi$ or $\Upsilon$, this declustering technique can also help probe the mechanism for quarkonium production. The second strategy is aimed at isolating heavy-flavor production from gluon splitting. Here, we introduce a new FlavorCone algorithm, which smoothly interpolates from well-separated heavy-quark jets to the gluon-splitting regime where jets overlap. Because of its excellent ability to identify charm and beauty hadrons, the LHCb detector is ideally suited to pursue these strategies, though similar measurements should also be possible at ATLAS and CMS. Together, these SoftDrop and FlavorCone studies should clarify a number of aspects of heavy-flavor physics at colliders, and provide crucial information needed to improve heavy-flavor modeling in parton-shower generators.

Sensitivity of CLIC at 380 GeV to the top FCNC decay t → cH

In the Standard Model (SM), flavour changing neutral current (FCNC) top decays, possible at loop level only, are very strongly suppressed. Observation of any such decay would be a direct signature of physics beyond the SM. Large enhancements are possible in many “new physics” scenarios and the largest enhancement is in most cases expected for the t → cH decay. A full study for CLIC was based on the WHIZARD simulation of FCNC top decays within the 2HDM(III) model. Beam polarization and beam-induced background were taken into account. Top pair production events with the FCNC decay t → cH can be identified based on kinematic constrains and flavour tagging information. Due to a large overlap in the kinematic space with standard top pair events, the final signal selection-efficiency is small, at the 10% level. Expected limits on BR(t → cH) × BR(H → ) are compared with earlier results based on parton level simulation.

Performance of the ATLAS Detector in Run-2

The second LHC run has started in June 2015 with a p-p center-of-mass collision energy of 13 TeV and ATLAS has taken 3.2 fb-1 of data at the new energy in 2015. In this talk the improvements made to the ATLAS experiment during the two-year shutdown 2013/2014 will be discussed. Furthermore, reconstruction algorithms of tracks, e/γ, muons, taus, jets and flavour tagging have been improved for Run-2. Reconstruction efficiency, isolation performance, transverse momentum resolution and momentum scales are measured in various regions of the detector and in momentum intervals enlarged with respect to those measured in the Run-1. The performance of the upgraded detector and the new reconstruction algorithms measured using these data taken in 2015 will be discussed.

Tracking and flavour-tagging performance for HV-CMOS sensors in the context of the ATLAS ITK pixel simulation program

The HV-CMOS (High Voltage - Complementary Metal-Oxide Semiconductor) pixel technology has recently risen interest for the upgrade of the pixel detector of the ATLAS experiment towards the High Luminosity phase of the Large Hadron Collider (LHC) . HV-CMOS sensors can be employed in the pixel outer layers (R >15 cm), where the radiation hardness requirements are less stringent, as they could instrument large areas at a relatively low cost. In addition, smaller pixel granularity can be achieved by exploiting sub-pixel encoding technology. Therefore, the largest impact on physics performance, tracking and flavour tagging, could be reached if exploited in the innermost layer (in place of the current IBL) or in the next-to-innermost layer. This proceeding will present studies on tracking and flavour-tagging performance in presence of HV-CMOS sensors in the innermost layer of the ATLAS detector.

Inclusive Flavour Tagging Algorithm

Identifying the flavour of neutral B mesons production is one of the most important components needed in the study of time-dependent CP violation. The harsh environment of the Large Hadron Collider makes it particularly hard to succeed in this task. We present an inclusive flavour-tagging algorithm as an upgrade of the algorithms currently used by the LHCb experiment. Specifically, a probabilistic model which efficiently combines information from reconstructed vertices and tracks using machine learning is proposed. The algorithm does not use information about underlying physics process. It reduces the dependence on the performance of lower level identification capacities and thus increases the overall performance. The proposed inclusive flavour-tagging algorithm is applicable to tag the flavour of B mesons in any proton-proton experiment.

LHC benchmarks from flavored gauge mediation

We present benchmark points for LHC searches from flavored gauge mediation models, in which messenger-matter couplings give flavor-dependent squark masses. Our examples include spectra in which a single squark - stop, scharm, or sup - is much lighter than all other colored superpartners, motivating improved quark flavor tagging at the LHC. Many examples feature flavor mixing; in particular, large stop-scharm mixing is possible. The correct Higgs mass is obtained in some examples by virtue of the large stop A-term. We also revisit the general flavor and CP structure of the models. Even though the A-terms can be substantial, their contributions to EDM's are very suppressed, because of the particular dependence of the A-terms on the messenger coupling. This holds regardless of the messenger-coupling texture. More generally, the special structure of the soft terms often leads to stronger suppression of flavor- and CP-violating processes, compared to naive estimates.

Enhancement of new physics signal sensitivity with mistagged charm quarks

We investigate the potential for enhancing search sensitivity for signals having charm quarks in the final state, using the sizable bottom-mistagging rate for charm quarks at the LHC. Provided that the relevant background processes contain light quarks instead of charm quarks, the application of b-tagging on charm quark-initiated jets enables us to reject more background events than signal ones due to the relatively small mistagging rate for light quarks. The basic idea is tested with two rare top decay processes: i) t -> c h -> c b bbar and ii) t -> b H+ -> b bbar c where h and H+ denote the Standard Model-like higgs boson and a charged higgs boson, respectively. The major background source is a hadronic top quark decay such as t -> b W+ -> b sbar c. We test our method with Monte Carlo simulation at the LHC 14TeV, and find that the signal-over-background ratio can be increased by a factor of O(6-7) with a suitably designed (heavy) flavor tagging algorithm and scheme.

Measurement of the two track separation capability of hybrid pixel sensors

Large Hadron Collider experiments face new challenges in Run-2 conditions due to the increased beam energy, the interest for searches of new physics signals with higher jet pT and the consequent longer decay length of heavy hadrons. In this new scenario, the capability of the innermost pixel sensors to distinguish tracks in very dense environment becomes crucial for efficient tracking and flavour tagging performance. In this work, we discuss the measurement in a test beam of the two track separation capability of hybrid pixel sensors using the interaction particles out of the collision of high energy pions on a thin copper target. With this method we are able to evaluate the effect of merged hits in the sensors under test due to tracks closer than the sensor spatial granularity in terms of collected charge, multiplicity and reconstruction efficiency.

Decay properties of b-hadrons with the ATLAS experiment

Recent results of the ATLAS experiment at LHC on decay properties of b-hadrons are reviewed. The time-dependent CP asymmetry parameters have been measured in Bs0→J/ψϕ decays using flavour tagging. The parity-violating decay asymmetry parameter αb and the helicity amplitudes have been measured for the decay Λb0→J/ψΛ0. The branching fraction B(B+→χc1K+) has been measured with χc1 reconstruction in the decay χc1→J/ψγ. An excited Bc± state has been observed through its decays to the ground Bc± state and two oppositely charged pions. The mass and decay of this state are consistent with expectations for the second S -wave state of the Bc± meson, Bc±(2S).

Probing CP violation in [formula omitted] decays

The three-body charmless hadronic decay Bs0→KS0π+π− provides a number of novel possibilities to search for CP violation effects and test the Standard Model of particle physics. These include fits to the Dalitz-plot distributions of the decay-time-integrated final state, decay-time-dependent (but without initial state flavour tagging) fits to the Dalitz-plot distribution, as well as full decay-time-dependent and flavour tagged fits. The relative sensitivities of these different approaches are investigated.

Real-time flavor tagging selection in ATLAS

In high-energy physics experiments the online selection is crucial to reject the overwhelming uninteresting collisions. The ATLAS experiment includes b-jet selections in its trigger system, in order to select final states with significant heavy-flavor content. Dedicated selections are developed to timely identifing fully hadronic final states containing b-jets and maintaining affordable trigger rates. ATLAS successfully operated b-jet trigger selections during both 2011 and 2012 Large Hadron Collider data-taking campaigns. Work is on-going now to improve the performance of online tagging algorithms to be deployed in Run 2 in 2015. An overview of the Run 1 ATLAS b-jet trigger strategy along with future prospects is presented in this paper. Data-driven techniques to extract the online b-tagging performance, a key ingredient for all analyses relying on such triggers, are also discussed and preliminary results presented.

Effects of the FCNC Couplings in Production of New Heavy Quarks withinZ′Models at the LHC

We study the flavor changing neutral current couplings of new heavy quarks through theZ′models at the LHC. We calculate the cross sections for the signal and the corresponding standard model background processes. Considering the present limits on the mass of new heavy quarks and theZ′boson, we performed an analysis to investigate the parameter space (mixing and mass) through differentZ′models. For FCNC mixing parameterx=0.1and theZ′massMZ′=2000 GeV and new heavy quark massmt′=700 GeV at the LHC withs=13 TeV, we find the cross section for single production of new heavy quarks associated with top quarks as5.8 fb,3.3 fb,1.5 fb, and1.2 fb within theZη′,Zψ′,ZLP′, andZχ′models, respectively. It is shown that the sensitivity would benefit from the flavor tagging.