Studies of Flavour Production Mechanisms in the pp Interaction

in English  in Romanian

Financing source (agency):    Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI)

Project code:    PN-II-ID-PCE-2011-3-0749; contract no. 56/07.10.2011

Current project director:    Dr. Florin MACIUC (since May 1, 2013)

Former project director:    Dr. Raluca Anca MUREŞAN (until May 1, 2013)

Institution:    "Horia Hulubei" National Research Institute for Physics and Nuclear Engineering (IFIN-HH), 30 Reactorului, P.O. B. MG-6, RO-077125, Bucharest-Măgurele, Romania, EU

E-mail: florin {dot} maciuc [at] cern < dot > ch

Research Team

    Active Members:
  • Dr. Florin Maciuc
    (principal investigator since May 2013)
  • Dr. Alexandru T. Grecu
  • Dr. Bogdan Popovici
  • Drd. Lavinia-Elena Giubega
  • Drd. Lucian Nicolae COJOCARIU
  • Drd. Vlad Mihai PLĂCINTĂ
  • Alexandru Cătălin ENE (Ms.Sc. student)
    Former Members:
  • Dr. Raluca Anca Mureşan
    (principal investigator upto May 2013)
  • Dr. Eliza Teodorescu
    [upto end of February 2013]
  • Cătălin Hanga (Ms.Sc. student)
    [upto end of December 2014]
  • Dr. Bogdan Popovici
    [upto February 2016]

Service Personnel


    Studies of the flavour production mechanisms are important for the understanding of the hadronisation process, a process not well understood and described so far only by phenomenological models. Data recorded by the LHCb experiment at CERN LHC offer an interesting environment for such studies providing an opportunity to measure the prompt particle production not only at an unprecedented interaction energy but also in a phase space region complementary to the other LHC experiments, where measurements from previous experiments are not available and hadronisation models predictions diverge. Understanding the prompt particle production represents also an important ingredient for the New Physics searches as studies of the light hadron production offer information on the underlying event background, essential for putting in evidence the new phenomena, while a knowledge of the number of b-hadrons promptly produced is important for some of the CP measurements in the b-sector. We envisage studies of flavour production mechanisms in the pp interaction, with emphasis on strange and beauty particle production correlation measurements using data recorded at 7 TeV interaction energy with LHCb detector. Such analyses bring an important element of novelty by probing hadronisation models at the highest interaction energy available for accelerator based experiments, in a unique rapidity range and beyond the single particle production level.

Project Stages *

  1. Stage I (2012):
  2. Stage II (2013):
  3. Stage III (2014):
  4. Stage IV (2015):
  5. Stage V (2016):

LHCb External Data and Preliminary Results Access Policy

While most if not all of the scientific output of the LHC collaborations, and LHCb in particular, is published in media which allow open access according to the Common Creative License, members of each collaboration are expected to comply to a specific set of rules and regulations when releasing scientific results to the public domain which involve the collaboration. Most of the scientific team members of the present project are also members of the LHCb collaboration, in which case the policy detailed in the LHCb Publication Procedure applies to their work and preliminary results. Also, the implications of the LHCb External Data Access Policy must be considered by the scientists supported in the frame of the present grant when making public certain LHCb data and results. Therefore, all publicly available reports are backed-up by detailed scientific reports of the studies undergone as part of the project implementation which contain LHCb preliminary results and may be released to evaluation committees only after receiving approval from the collaboration management and under the condition that they are not for public distribution.

Conferences, workshops and talks

Public results - these results/talks are not constrained by the LHCb policies regarding publications which contain the LHCb data and the LHCb official software (see the upper links on the LHCb Publication Policy).

For Stage I (2012):

  1. The organization of an international workshop at Horia Hulubei National Institute of Physics and Nuclear Engineering -- IFIN-HH .
       "LHCb Generators Tuning mini-Workshop in Bucharest (LHCb-MC)" - main organizers: Raluca Muresan from IFIN-HH and Gloria Corti from CERN.
       Workshop timetable displayed at the web page: "LHCb-MC Workshop in Bucharest" or alternatively "LHCb-MC Meeting in Bucharest"
  2. Two contributions from our group members within the previously mentioned workshop:
       A. "Rivet in LHCb"(link on web) talk given by Dr. Alexandru T. Grecu.
       B. "Welcome & interest for MC generators in Bucharest group"(link on web) introduction by Dr. Raluca Muresan.
  3. Within the work-agenda of the same workshop, two sessions were coordinated by two of our colleagues as conveners of the sessions
       A. Monte Carlo generators session on "Pythia8" - convener Dr. Raluca Muresan;
       B. Monte Carlo generators session on "Sherpa" - convener Dr. Alexandru T. Grecu.

For Stage II (2013):

  1. Florin Maciuc, "Xi+- and Omega+- hyperon production ratios", 69th LHCb Week, Sep. 9-13, 2013, Krakow, Poland

For Stage III (2014):

  1. The organisation of a workshop at CERN, Geneva, Switzerland
       "LHCb workshop on quantum interference effects, QCD measurements and generator tuning" - main organizers: Florin Maciuc from IFIN-HH and Marcin Kucharczyk from Universita & INFN, Milano-Bicocca, Italy.
       Workshop overview and agenda can be consulted at this web page
       Within the same workshop agenda, two sessions on QCD physics and generator tuning were coordinated by Dr. Florin Maciuc.
  2. During the previously mentioned workshop, members of our group presented three personal contributions and one general contribution for the LHCb proton-ion physics group:
       "LHCb and Introduction to Tuning and QCD Measurements at LHCb", Florin MACIUC [slides]
       "Generator Tuning with Professor/RIVET at LHCb. Status of PYTHIA8 Optimization", A. T. GRECU[slides]
       "LHCb results in pA", Florin MACIUC on behalf of M. SCHMELLING and the pA Phyics Group[slides]
       "Soft QCD measurements in LHCb", Florin MACIUC[slides]
  3. Preparation and submission of a scientific paper as proceedings to a local conference atended by members of our group ("14th International Balkan Workshop on Applied Physics", IBWAP-2014, 2-4 July, 2014, Constanta, Romania)
    Ana Elena Dumitriu, A. T. Grecu, "RIVET Plug-in for $Z^0 \to e^+ e^-$ Production Cross-Section Measurement in pp Collisions at $\sqrt{s} = 7$ TeV", Rom. J. Phys. vol. 60 (3-4), 415-428 (2015) [link]

For Stage IV (2015):

  1. F. Maciuc, (on behalf of the LHCb Collab.), "QCD and Electroweak Boson Production in the Forward Region in LHCb", Proceedings of International Workshop on LowX Physics, Rehovot and Eilat, Israel, 30 May - 4 Jun 2013; Open Physics Journal 1 (2014) 36-42; DOI:10.2174/1874843001401010036.
  2. A.T. Grecu (on behalf of the LHCb collaboration), "Soft QCD Measurements at LHCb", in Proceedings of 20th Particles & Nuclei International Conference, 25-29 August 2014, Hamburg, Germany, p. 137, DOI:10.3204/DESY-PROC-2014-04/249 (ISBN 978-3-935702-91-1, ISSN 1435-8077, Verlag Deutsches Elektronen-Synchrotron, Hamburg, Germany, 2015)
  3. Talks in various working group meetings at CERN (Geneva, Switzerland) related to the LHCb MC generator tuning programme (access to the meeting agenda may be restricted!):

For Stage V (2016):

  1. Attending the ceremony of raising the Romanian flag at CERN, with the participation of the Romanian president, His Excelency Mr. Klaus Werner IOHANNIS, on the occasion of finalising the admission of Romania as full member of the European Organisation for Nuclear Research. The Romanian LHCb group presented its activity during a dedicated poster session. poster logo
  2. Organising a scientific workshop in collaboration with members of the Romanian LHCb group from University Ştefan cel Mare of Suceava(USV):
    "Workshop on Sensors and High Energy Physics (SHEP 2016)", 21-22 October 2016, University Ştefan cel Mare of Suceava, Suceava, Romania [web page]
  3. Representing IFIN-HH and USV in organising at Suceava, in collaboration with LHCb and CERN, the yearly outreach event "LHCb Masterclasses" which focuses on popularizing among the highschool and fresh University students the high energy physics domain:
    "LHCb Masterclasses 2016", 2 March 2016, Suceava [official program; local web page]
  4. Talks at conferences and workshops:
    • Florin MACIUC, "High Energy Physics Measurements, Status and Prospects", Workshop on Sensors and High Energy Physics (SHEP 2016), Ştefan Cel Mare University of Suceava (USV), Suceava, Romania, 21 – 22 October 2016.
    • "LHCb 63rd Analysis and Software Week", CERN, 9-13 May 2016, Geneva, Switzerland [web page]:
      Alex GRECU, "HEPData mini workshop summary", 09.05.2016 (plenary)
    • "HEPData mini-workshop", CERN, 25.04.2016, Geneva, Switzerland [web page]:
      Alex GRECU ”LHCb and HEPData”, 25.04.2016 (plenary)
    • QCD@LHC2016, UZHÐ, 22-26 August 2016, Zuerich, Switzerland, [web page]:
      Alex GRECU, "Gauge boson physics in the forward region at LHCb", 22.08.2016 (parallel)
      Alex GRECU, "Impact of LHCb results on the tuning of Monte Carlo generators", 22.08.2016 (parallel)
    • Workshop on Sensors and High Energy Physics (SHEP 2016), Ştefan cel Mare University of Suceava, 21-22 October 2016, Suceava, Romania [web page]:
      • Alex GRECU, "Overview of the LHCb Monte Carlo Simulation Framework", 21.10.2016
      • Teodor IVĂNOAICA, "Grid site as a tool for data processing and data analysis, computing performance evolution", 21.10.2016
      • Elena L. Giubega, "Study of the Λ* resonances in Λb → Λ*(p+K-)γ decay, using Helicity Formalism", 21.10.2016
      • Alexandru Cătălin ENE, "Comparison of cosmic ray collisions generators and PYTHIA", 21.10.2016
    • Alexandru Cătălin ENE, "Particle production in pp collisions at LHC energies", The 2016 Annual Scientific Communication Session of the Faculty of Physics, University of Bucharest, June 2016
    • Alexandru Cătălin ENE, "Particle production in pp collisions at LHC energies", The Pentagon of the Physics Faculties 2016, Cluj-Napoca, July 2016, 1st place in Ms.Sc.-PhD. category
    • Alex GRECU, 5 talks on the project topics given during the LHCb working group meetings which are organised monthly at CERN (videoconference attendance).
  5. Papers sent and accepted for publication in field journals:
    • L. E. Giubega on behalf LHCb, "Radiative Decays at LHCb", Physics of Atomic Nuclei, 79(10) (2016) 47-55, ISSN 1063-7788
    • L. E. Giubega, A. I. Jipa, A. C. Ene, "Study of the resonances structure appearance in the Λb → Λ∗ (→ p + K − )γ decay using helicity formalism", accepted for publication in Romanian Journal of Physics, [pre-print]
  6. Papers in preparation to be sent for publication to scientific journals:
    • „Production of hyperons in cosmic ray generators and PYTHIA 8.2” - aimed journal: Romanian Journal of Physics
    • “Comparison of cosmic ray models with PYTHIA at energies near the 2nd ‘knee’ of the cosmic ray spectrum” - aimed journal: Romanian Journal of Physics
    • „Measurement of production and charge ratio of hyperons Ξ+/Ξ- and Ω+/Ω- at LHCb”, LHCb analysis note under preparation (usually shortly followed by publication of a paper containing the main results in prestigous journals for the high energy physics field)

Preliminary results shown during what are considered internal meetings of the LHCb collaboration

Preliminary results: - can not be made public till they are published in Scientific Journals with the LHCb Collaboration author list

   A number of talks - talks outlaying the results of our group - were given within the periodical meetings of the Physics and Analysis subgroups of the LHCb collaboration, especially the "QCD, Electroweak and Exotica" and "Simulation and Monte Carlo" working groups.

I. Strange hadrons production studies

Report dead-line:    15.12.2012

I.1. Identifying the kinematic observables that can be used for studies of strange particles production correlations and of the production models interesting to test

I.2. Studies of strange meson anti-meson and of baryon anti-baryon production correlations using K±, Λ, and anti-Λ samples


Abridged report - PDF (in Romanian)

The in-progress scientific analyses of this project use data recorded by LHCb experiment [1], one of the four large experiments installed at the LHC (Large Hadron Collider) in international laboratory at CERN, Geneva, Switzerland.

The first stage of the project in 2012 was dedicated to building the research team and to the incipient scientific studies for the implementation of further analyses. The initial team of four researchers mentioned in the project proposal was enlarged with an IT engineer offering technical support (on software deployment and networking), a postdoc with solid and long previous scientific experience in the LHCb collaboration as PhD student followed by a postdoc grant from the Heidelberg University and Max Plack Institute in Germany, a Ms. Sc. student who during the year was admitted as student of the doctoral school at the University of Bucharest and another Ms. Sc. student. At present, the recruiting process is continued even if it proved troublesome and time-consuming both from technical and administrative reasons but also because of the degree and quality of physics education of the interviewed candidates. The PhD. and Ms.Sc. students, preliminary selected based on the submitted applications and recommendations, were required to pass a series of tests designed by members of the group to verify their abilities to understand and use the scientific literature and their programming skills. The postdocs and the candidates for the technical support position were only interviewed. The English language skills were tested for both types of candidates. Following repeated announcements of the available positions, 12 candidates were invited to the final interview/test. Two of the candidates who passed the tests subsequently refused the positions. Other two candidates recalled their applications as they found job offers that they considered more advantageous. The two recruitees received a training program to follow in order to familiarize themselves with the software tools used by the LHCb collaboration. During one of the periodic collaboration meetings, they were accompanied by members of the group to CERN where they enlisted as student members of LHCb, they visited the control room at the experiment site and by attending the meeting's plenary sessions they had the opportunity to learn of the physics topics other members of the collaboration are working on as well as meet some of the group's close collaborators from LHCb international institutes.

The scientific activity focused on strange particle production studies in order to offer a better understanding of the strange hadron production mechanisms in the light of the theoretical models that describe them. The ultimate goal is to contribute to the effort of optimizing the parameters which control the existing models and/or help in designing new models. Thus, some of the time spent for working stages at CERN was dedicated to discussions with the theoreticians involved in creating production/hadronisation models in order to identify the best kinematic observables for such studies. The final conclusions were presented to other LHCb members activating in the same working group at CERN to ensure synergy between the proposed measurements and the evolution of the existing software tools and that sufficient statistics for measurement will be available by including the necessary pre-selection and trigger lines to retain it during raw data processing. Taking into account that the PYTHIA [2] generator, based on the string fragmentation model [2, 3], is currently the most used generator in the field, several meetings were scheduled at CERN with authors of this software package. Apart the already contracted studies - the correlated differential particle production as function of rapidity, y, and transversal momentum, pT, further analyses were proposed and their feasibility was tested in the frame of the LHCb experimental constraints.

One such endeavour is the measurement of the correlation between strange mesons and baryons K and Λ, F(y,pT), produced in the same hadronic jet [3,4]. In this case, y and pT will not be defined in the centre-of-mass system of the colliding pp pair but with respect to the main axis of the jet containing the two particles. This approach comes at a good moment when the jet reconstruction procedures at LHCb reached the level of maturity with enables high precision measurements to be made[5].

As an alternative to defining a reference axis one can use instead of the jet central axis the moving direction of the particle with the largest measured pT in the event. This measurement raises some difficulties for the particular setup of the LHCb experiment as a forward spectrometer because the trajectory with the highest pT detected in the detector acceptance has a smaller probability to the associated to the highest pT particle from the primary pp interaction than in the case of the (barrel-like) detectors covering the whole solid angle ($4 \pi$). Yet, not considering the drop in statistics, the benefits would be important. A very particular and optimal case is the situation when a strange hadron proves to be the reference particle. In this case, the strange hadron may represent the „leading particle” [3,4] in a jet and therefore will contain partons from the original string which are directly produced in the pp interaction before the process of string fragmentation/hadronisation started. These ideas lead to conclusions that emphasize the importance of the already intended studies of strange meson to strange baryon correlations and strange baryon correlations. The analysis of (K+ Λ), (K- anti-Λ) and (Λ anti-Λ) correlated production is an adequate environment for testing correlation models which imply the creation of diquarks and the correlated production of quark-diquark and diquark-diquark, all these partons originating from the same string [3,4,6]. The production of baryons containing more than one strange quark, from here on called multistrange, e.g. Ξ and Ω, correlated to another strange baryon or meson can be also analysed in the context of diquark-quark or diquark--anti-diquark correlations. Another class of models that can be tested is based on a "pop-corn"[2] type de-correlation mechanism. The multistrange baryon study allows testing the production of diquarks containing two strange quarks. It is postulated that the production of such diquarks is very low in the fragmentation process and therefore the study requires special reconstruction techniques to be elaborated in order to highlight this behaviour. These conclusions drawn from the private meetings with CERN theoreticians, show that the LHCb measurements foreseen in the frame of the present project will test the physics processes at diquark scale (femto-meter) directly by studying the strange meson-hadron, meson-baryon and baryon-baryon correlations.

Further discussions with the theoreticians and attending theory seminars inspired the planning of other interesting measurements such as the analysis of the correlated production of strange hadrons with respect to the number of particles produced in the event (the multiplicity of the event) and the angular production correlations of two strange hadrons. We had significant progress in the study of K+ K- correlation. This analysis has the advantage that the strange mesons are produced more abundantly than the strange baryons. The difficult aspect comes from the requirement to select a sample of K± of high purity which must contain only kaons created promptly in the pp interaction or by decays of very short lived resonances. In order to discriminate K± from other particle species, the LHCb experiment uses the information provided by two RICH (Ring Imaging Cherenkov) [7] detectors. The default LHCb K± selection was further tunes to obtain the necessary sample purity. The improvements focused on eliminating the contamination of the sample with other particle species, the fake "ghost" trajectories due to detection errors and the kaons produced in subsequent decays of resonances with medium and high life-time. The studies performed on simulated data show an increase in the purity of the K± sample from 27% up to 92%-95%. The kaons produced in decays of other mesons and baryons, e.g. ф, represent under 2.5% of the final sample of daughter particles K+ + K-. Taking into account the high purity of the sample this contamination can be neglected in the further study. However, the strong cuts applied allowed only 39% of the initial K± to be found in the sample after optimization. The measured data used in the preliminary study of the K± correlated production were registered at a center-of-mass energy of 7 TeV. To ensure that the analysed K± pair comes from the same pp interaction, we used data samples measured during the first LHC runs (2010) for which the primary interaction rate is low so that most of the events include only particles originating from a single pp interaction.

An important step of the analysis is to correct the observable distributions obtained for the detected particles taking into account the reconstruction efficiency, the acceptance and the difference in particle identification efficiency between real data and simulated data samples. The reconstruction and acceptance efficiency are determined using simulated data in the hypothesis that the interaction between the particles and the detect material si correctly reproduced in these data. The models for generating the primary particles are tuned using measurements from previous experiments, at low interaction energies and at central (pseudo)rapidity. The predictions of these models, extrapolated in energy and pseudorapidity (η) cannot reproduce satisfactorily the η, y and pT distributions obtained from the experimental data. For LHCb the geometric/kinematic acceptance is frequently expressed in terms of η and pT. The main differences between the simulated and experimental data samples are linked to the identification efficiency of the particle species and to the medium/maximum value of the transversal momentum of the K± mesons. The real data distributions have higher values in pT. To account for these differences various corrections are applied. Two types of analyses of the K+ + K- pairs are of particular interest: the measurement of the production cross-section for a K+ + K- pair and the correlations in the production of kaons with opposite electrical charges (i.e. where the strange quarks may come from a s anti-s pair produced during the fragmentation process). To emphasize this correlation one needs a sample containing uncorrelated K+ + K- pairs. Such a sample can be constructed using K+/- from different events. Such pairs are given weights so that the kinematic distributions from the primary data samples and these artificially generated uncorrelated data samples match. To compare the results obtained using the two data samples, one has to consider that for events with the same multiplicity, the reconstruction and particle identification efficiency depends on η and pT. Thus, the kaon candidates from the simulated data sample get additional weights in order to obtain similar single particle distributions both in real and simulated data. The ratio between two uni-particle distributions as function of η and pT defines the correlation function (and will be called as such from here on). The distributions are not yet corrected for effects due to the K+/- tracking and identification efficiencies. Comparing the uncorrected real data and simulated data distributions one would observe that in general the measured distributions are closely followed the simulated ones yet there are areas in the phase-space where the experimental points diverge in modulus with up to 15% from the correlation functions obtained from the simulation. In the transversal momentum difference plots, the simulated distributions are closer to the values obtained from the real data and one may notice the kinematic anticorrelations between kaons from the same event. These samples as well as the calibration samples described below contain a fraction of kaons generated from decays of ф mesons (as mentioned above). These secondary kaons are correlated and this fact is clearly visible in distributions including or excluding them from the analysed statistics. For these data the ф signal is more accentuated due to imposing filtering conditions at the same time for both kaons. This component is trivial to eliminate using constraints on the invariant mass of the K+ + K- pair, yet it is useful for calibration and systematic uncertainties check-ups. The raw distributions using measured data must be corrected applying the reconstruction and identification efficiencies for various particle species for the LHCb detector. One should keep in mind that an important analysis could also focus on flavour/strangeness correlations between kaons produced by the same string (see the Lund model of hadronic fragmentation [2-4]) as well as other kinematic or dynamical correlations e.g., momentum conservation, jets, etc.

In order to obtain the kaon identification efficiency corrections we apply the "Probe and Tag" method which uses the kaons originating from ф mesons decays to calibrate the procedure for evaluating the particle identity. This method, described in detail in [7], must be applied in a particular way in the present case. "K-probe" is the kaon which fulfilled the filtering conditions applied on the reconstruction and identification parameters while the "K-tag" represents an unfiltered kaon which together with the "probe" kaon have a combined invariant mass near the mass of the ф meson. The phase-space for the "tag" kaons is split in (η, pT) cells and the events from the calibration samples containing ф mesons receive a specific weight so that distribution of the number of reconstructed tracks is identical with the ones corresponding to the events from the analysed samples. The method applied to determine the efficiencies is the "sWeights" method [8] which considers the ф meson mass distribution fits as "sPlots" in each cell from the (η, pT) plane. Evaluating the signal to background ratios associated to the mass fits, one obtains for each phase-space cell the kaon identification efficiencies for the RICH detector. At RICH level, in each cell, the filtering for "tag" kaons is identical to the one used for the kaons used in the correlation study. The events containing ф mesons for calibration are filtered using a pre-selection which is applied similarly to real and simulated data.

The huge amount of data produced in pp collisions prohibits the indiscriminate registration of all the data, so trigger lines are employed to detect and veto on the acquisition of events of interest. Even so, allowing each member of an experiment to process all the raw experimental data (at present the LHCb collaboration has 843 members from 60 institutes spread in 16 countries world-wide) would be a waste of time, work force and especially computing power. Therefore, the user-physicists are offered access only to data filtered by the triggering system which were further reconstructed and pre-selected to ensure obtaining enriched samples containing the particles and/or the events to study. Vast re-processing campaigns are foreseen to take place with low frequency (about twice a year) to allow the results of various studies of the detector resulting in new alignments and better detector description to be applied as part of the reconstruction and preselection algorithms. Due to the large quantity of events needed to produce the simulated data samples, this type of data is produced in campaigns well planned in advance. The models used to produce the simulated data need to be implemented in the collaboration simulation software (GAUSS). In conclusion the processing of the experimental data as well as the production of simulated data cannot be done using the computing resources of one single institute, instead computers from researched institutes and universities from various corners of the world and linked in a super-network called GRID and used for these purposes. Access to these global resources that allow parallel processing of data is essential for fulfilling physics measurements.

Such event enriching is not necessary for the study of kaon correlations since these particles are copiously produced in each event. A dedicated trigger line called Minimum Bias controls the registration of events that contain at least one track in the detector acceptance. From this very abundant type of events only a small fraction is measured and stored, and the subsequent preselection reduces the statistics even more proportional to the requirements of the analysis. Such an approach is not possible for the next stages of this project. Therefore we provided the LHCb collaboration with a set of stripping lines which ensure the production of strange baryon enriched event samples. For the Λ baryons two methods are available: (i) using the selected samples to calibrate the proton identification in the RICH detector or (ii) re-enabling the preselection line previously used by the collaboration for the single particle analyses. For the multistrange baryons Ξ and Ω members of our group joined the effort of testing the stripping lines which were implemented before the last reprocessing campaign, also aiming at enhancing their efficiency - one of the necessary conditions in order to perform correlation studies. At present, a study is being performed with the goal to establish (using simulated data) the number of events needed to obtain a given precision of the measurements performed in the correlation studies involving strange baryons. This kind of planning is also very important for the analyses on hadrons containing the "beauty" quark since these are produced with much lower probability. As the LHCb experiment was designed in principal for the study of such particles, we envisage to use the already existing preselection lines, the challenge being to choose the lines corresponding to as loose as possible constraints on the phase-space (given the complexity of these selections which is far superior to the ones applied to filter simple strange particles, this intermediary goal is essential to be obtained). Such lines would allow us to study correlations between opposite charge B mesons. Therefore it is imperious to find selections which use similar methods so that the baryon/meson samples retain this similitude with respect to the kinematic observables and allow closely related correction algorithms to be applied, thus facilitating the evaluation of the systematic uncertainties. A study to identify such preselection lines was also initiated.

One of the purposes of these measurements is to provide information and data in order to optimize the existing hadronisation models and implicitly the generators implementing them but also to create new models and/or new generators. The first step on this road consists in comparing the existing models and tunes with single particle production measurements performed by the experiments at LHC. We tried to identify a reference tune, defined as being the specific tune that best describes not only the observables for the strange hadrons measured by LHCb but at the same time the experimental data from the other compatible experiments at LHC. Therefore, we tested various PYTHIA tunes after consulting the scientific literature on this subject and applying the conclusions of discussions with the authors of this generator. Two different versions of PYTHIA were used, the old generation PYTHIA 6 coded in FORTRAN and the new evolving version PYTHIA 8 coded in C++. A series of PYTHIA 6 tunes pointed as adequate by the generator authors of members of other LHC collaborations were considered: Perugia0 - the reference tune for pre-LHC experiments, AMBT1 - a first tune proposed by ATLAS in 2010 and obtained on data collected at LHC during runs at 7 TeV centre-of-mass energy and using Leading Order Parton Distribution Functions (LO PDF), CMS Z1 - a further optimization of AMBT1 proposed by members of the CMS experiment using the CTEQ5L set of PDFs, Perugia 2011 - the last PYTHIA 6 proposed in 2011 as an improvement of Perugia 2010 using CTEQ5L[9] PDF set. Additionally the default PYTHIA 8 tune at the LHC energies, called "4C", was tested. The comparison between LHC data and these tunes was subsequently extended to include the specific LHCb MC tune[10]. The experimental data measured by LHCb[11], ALICE[12] and CMS[13] at different collision energies were confronted to the distributions obtained using the PYTHIA tunes above as input for the corresponding RIVET[17] plugins (see Annex 4 for a few such comparisons generated using the stand-alone versions of the PYTHIA generators and the RIVET miniframework). Unfortunately, it was found that no one of the tested tunes matches satisfactorily the specific observables for the considered measurements. As a continuation we'll analyse the distributions associated to the various tunes taking into account the main differences between the values of their control parameters in order to interpret the results and the discrepancies from the experiment. This strangeness production based study will contribute to the interpretation of the foreseen correlated production measurements, once they will be finalised. The research of the scientific literature is under way to identify the models of interest for the correlated production of hadrons containing a b quark. We are determining the feasibility of implementing these models in the LHCb simulation software packages and create new RIVET plugins in order to benefit from the existing methods of comparing independently MC tunes and experimental data.

On the same topic, since we consider advantageous that the results of our measurements can be interpreted and easily used by the theory community and in order to keep up to date with their research as well as to establish new contacts, we organized a workshop in Bucharest at IFIN-HH on 22nd and 23rd of November 2012. We invited both theoreticians and collaborators from the LHCb experiment (or other experiments at the LHC and not only) who are interested in implementing and tuning Monte Carlo generators. The scientific programme (available here) was structured in sessions containing key talks dedicated to different models/generators and the software used to tune and validate them, followed by specific talks about the way these generators are implemented or integrated into the software framework of the experiment and/or used by LHCb collaborators. Generous time was allocated to discussions. These discussions dealt with the way simulation is used by analysts emphasizing their request for ease of access to the software in order to obtain the simulated data, the possibilities one sees in order to optimize the generators making use of the measurements done by the experiments. A clear gain was the opportunity both for the members of the project team and the other experimentalist attendees to be informed of the latest improvements and the plans of development for the main generators used at present in particle physics by the experiments installed at large particle accelerators, namely PYTHIA[2], SHERPA[14], HERWIG[15], EVTGEN[16]. Issues related to generator validation and tuning were also considered and also the current differences between model predictions and experimental points were discussed. The two workshop days were very intense and useful for all the participants. A fruitful exchange of informations took place between theoreticians and experimentalists which we hope will crystalize in future synergy of these topics.

References (Show list...)

II. Continuation of correlation studies between particle with unitary strangeness (mesons and baryons) and their anti-particles

Report dead-line:    15.12.2013

II.1 Patching and re-tuning the final baryon (pre-)selection

II.2 Identifying additional background sources and methods for eliminating them. Including new unfolding methods in the analysis in order to deconvolute the signal from background

II.3 Obtaining the final corrections for acceptance, selection, unfolding and calibration and the final values for the systematic errors

II.4 Interpreting the final results, comparison to similar results from other experiments, disseminating the results in the scientific community

II.5 Comparing the results to Monte Carlo particle generator predictions (e.g PYTHIA) for various generator tunes. Preliminary actions to prepare 1-2 scientific papers to be submitted to physics journals with ISI impact factor

II.6 Development/maintenance and identification of software packages, analysis procedures and data samples for studies to be performed in 2014

Preliminary results

Progress was made in implementing and executing the selection algorithm of baryon particle pairs (Lambda, anti-Lambda) and in charged Kaon pair selection algorithm. These selections were applied on real 2010 data at a total center-of-mass collision energy of 7 TeV, and on the corresponding Monte Carlo samples, samples which are compatible with the 2010 real data. The meson+baryon correlation studies, i.e. charged Kaon plus Lambda hyperon, remain to be implemented in a similar fashion.

In the next two months, we plan to extend the analysis on the 2011 and 2012 data at various LHC proton-proton collision energies: 8 TeV and 2.76 TeV, respectively. In parallel with the previous 2011-2012 data analyses, we will continue the effort for background reduction and reconstruction efficiency estimation. The latter can be obtained directly from the data, by using the calibration decay channels of various particles. With tracking efficiencies close to 100 %, this is especially useful in the case of RICH ("Ring-imaging Cherenkov detectors") detectors particle identification efficiencies, which give the dominant effect on particle reconstruction and identification loss.

The LHCb collaboration gives access to its members at various services, chiefly among which is, besides data access, the Monte Carlo data mass production facility on the GRID. These MC data are generated, digitized and reconstructed by LHCb collaboration experts. Our group has forwarded several requests and is waiting for the finalizing of the corresponding MC production. These requests are mostly for "Minimum Bias" data samples, data which are ideally suited to conduct efficiency and generation studies in case of QCD processes that take place for small parton-parton collision energies when compared to LHC collision energy. These data are also useful in describing the fragmentation/hadronization processes, which are exactly the processes that are directly probed during our correlation studies. Up to now, the MC data used were Monte Carlo data produced for 2010 real data, and small private samples generated and reconstructed by our group using dedicated reconstruction and simulation software packages of the LHCb collaboration. We foresee that we shall have available the new MC samples within the next months, to use in reconstruction efficiency estimation and LHCb detector acceptance unfolding for the 2011-2013 real data.

Given the LHCb policy of data access, which place some restrictions on the access to LHCb reconstructed data, it became imperious to implement and commit a preselection algorithm as part of the official LHCb software. The algorithm will be used on 2011 and 2012 data. This will enable the increase by at least an order of magnitude for the number of correlation candidates (K-,anti-Λ), (Λ,anti-Λ), (K+,K-) which have the total strangeness number equal to zero. Here the work on the preselection algorithms will go forward till the end of July. The work opened a potential collaboration with the members of another LHCb group, which are interested in the Bose-Einstein and Fermi-Dirac types of correlations between particles found in the same data samples.

Concerning the production studies of particles with associated strangeness number higher than unit, i.e. baryons like Xi and Omega, the preselection algorithms mentioned above were implemented and runs were executed already on 2011 and 2012 data. The found candidates - at 7 TeV, 2.76 TeV and 8 TeV proton-proton collision energy - were analysed and the signal was separated from the background. The next phase of this analysis will include a detailed analysis of each event in which Xi and Omega candidates were found and searches will be conduced for strange baryons and mesons: Lambda and charged Kaons, or other Xi/Omega candidates. These studies will help gauge the feasibility of initiating a multi-strangeness correlation study between Omega/Xi particles and other or same strange species of particles.

In addition to the previous program, an alternative program is being implemented. An extra constraint will be imposed in preselection: the events should contained a well defined jet of particles and the strange mesons and baryons have to be part of it. The jet requirement can be replaced alternatively by the requirement of a high transverse momentum particle in conjunction with the strange hadron pair. In this case, the energy cut in the transverse domain will be placed between 3 and 5 GeV. The studies will encompass the kinematic parameters of the strange hadrons with respect to the kinematic parameters of the jet or of the high-pT particle in the event. The jet structure and presence will pinpoint a well defined hadronization event within the LHCb phase space. The observables measured for a jet with two correlated strange particles will provide insights into the fragmentation process which generated them. If the second preselection requiring the presence of a highly energetic particle in the event was already tested, the jet reconstruction with its implementation within the preselection LHCb algorithms together with the strange particle pair constraint is currently being implemented. Given the high complexity in terms of the jet physics and of detector quantities involved, we hope to use the latter preselection algorithm by the end of this year.

At the end of this report on the status of the QCD studies at low energy which generate correlations between strange particles through s-sbar parton correlations, we underline the immediate scope of the analysis being the comparison between the LHCb measurements of correlation observables and the quantities obtained from the PYTHIA Monte Carlo generator. The eventual observed differences have the potential to allow for an optimization/retuning of hadronization processes within the PYTHIA code. In this sense we shall benefit from the the recent addition to the software functionality of interfacing the RIVET package - "Robust Independent Validation of Experiment and Theory" is a toolkit for the validation of Monte Carlo event generators RIVET web link - to the LHCb software simulation code. The latter interface developing and implementing is a main responsibility for one of our team members. Additionally the RIVET package will be used in conjunction with the "PROFESSOR" tuning tool for Monte Carlo event generators PROFESSOR web link to perform various similar tunes of PYTHIA with and without LHCb simulation software. The previous work shall be done in direct collaboration with an external scientific peer, member of the LHCb collaboration. The final outcome of the different tuning scheme will be to reduce the difference between the generator results and real data measurements, with the latter extrapolated back to the generation level form the initial raw detector measurements. This will be done with a proper choice of control parameters for tuning of PYTHIA generator. Since some of the parameters mentioned previously are the ones directly controlling the strange parton correlations and the strange meson versus strange baryon ratios, this will allow to interpret the LHCb correlation results within these theoretical production and hadronization models and parametrizations.

By the end of this year, we expect to have obtained from the 2010, 2011 and 2012 data, the raw measurements of the correlation functions and distributions and extrapolate them to the moment just after the hadronization. In computing the initial distributions of the correlated strange particles, we shall make use of the LHCb reconstruction efficiency and acceptance mapping which are currently being computed for the candidates and the events of interest. E.g., when the magnetic effects on the track curvatures are taken separately, the LHCb geometrical acceptance is typical for a single-arm-spectrometer, with the polar angle limits with respect to beam-axis of about 15 to 300 (250) milliradians - 300 for bending plane limit and 250 for non-bending plane limit, respectively.

Stripping v20r0p2 data stripping lines

The following stripping lines were proposed and implemented by the Electro-Weak(EW) physics group. As members of the EW subgroup working on soft QCD our team corrected, adjusted and released the lines in red frame in the table below (the table was adapted from this restricted access page - a snapshot of the page in .PDF format is available here*).

Line name N evnts Rate, % ms/evt
StrippingLowMultCEP_D2KpPipPim_line 9860 0.1401 1.511
StrippingLowMultCEP_D2KmPipPip_line 5727 0.0814 0.059
StrippingLowMultCEP_ChiC2KK_line 849 0.0121 0.055
StrippingLowMultCEP_ChiC2PiPi_line 2866 0.0407 0.055
StrippingLowMultCEP_ChiC2KpKpPimPim_line 734 0.0104 0.055
StrippingLowMultCEP_ChiC2KpKmPipPim_line 2136 0.0303 0.054
StrippingLowMultCEP_KpKm_line 3226 0.0458 0.041
StrippingLowMultCEP_KpKp_line 468 0.0066 0.038
StrippingLowMultCEP_LMR2KPi_line 4989 0.0709 0.042
StrippingLowMultCEP_LMR2KK_line 1580 0.0224 0.041
StrippingLowMultCEP_LMR2PiPi_line 5847 0.0831 0.041
StrippingWMuJetsLine 2893 0.0411 9.209
StrippingSbarSCorrelationsPhiLine 1052 0.0149 0.048
StrippingSbarSCorrelationsF2Line 26 0.0004 0.105
StrippingSbarSCorrelationsLambdaCplusLine 618 0.0088 0.056
StrippingSbarSCorrelationsLambdaCminusLine 612 0.0087 0.059
StrippingWeJetsLine 4138 0.0588 9.454

Soft QCD

Brief analysis description, dataset Previous LHCb result (PAPER/CONF and dataset used) Groups involved/interested (if any) Active? Priority Energy dependence: priority, active?
Charged particle multiplicities VELO E-dep PAPER-2011-011 CERN Nodone 2 No
Charged particle multiplicities, forw. tracks 2011 paper in review Heidelberg Yes 2 2 No
Incl. particle production 2011 Heidelberg Yes 1 2 No
Forw. energy flow PAPER-2012-034 Heidelberg No done 2 No
Total inel x-section 2011 2011 paper in review Heidelberg Yes 1 1 No
Underlying event No 2 2 No
Properties of diffractive enriched events No 3 3 No
K0 x-section, 2011 PAPER-2010-001 (2010)Heidelberg Yes 1 2No
Xi+-, Omega +- x-section, 2.76,7, 8 TeV Bucharest Yes 1 2 Yes
Xi+-, Omega +- ratios, 2.76,7, 8 TeV EPFL, Bucharest Yes 1 2 Yes
Phi mesons PAPER-2011-007 (2010) No done 2 No
K* mesons No 2 2 No
Open charm LHCB-PAPER-2012-041 (2010) No done 2 No
Baryon/antibaryon and baryon/meson ratio 2.76 TeV LHCB-PAPER-2011-005 (0.9, 7 TeV)BucharestYes 1 2Yes
Positive/negative 5 TeV Rome (Tor Vergata)Yes 2 2 No
pi+-, K+-, ppbar, K/pi, p/pi ratios PAPER-2011-037 (0.9, 7 TeV) OxfordNo 1done 2 No
BE correlations 2011 Milano, KrakowYes 2 3 No
Fermi-Dirac correlations No 2 3 No
Lambda vs antiLambda lifetime (CPT test), 2011 Imperial Yes 2 3 No
Lambda vs antiLambda mass difference (CPT test) No 2 3 No
Lambda polarisation, 2012 NIKHEF, Groningen Yes 2 2 No
Ridge effect, 5 TeV Heidelberg Univ. Yes 2 2 No
Strangeness correlations 7 TeV Bucharest Yes 2 2 No
MC tuning No 2 2 No

Proton Ion

Brief analysis description, dataset Previous LHCb result (PAPER/CONF and dataset used) Groups involved/interested (if any) Active? Priority
Charged particle multiplicities Heidelberg Univ. Yes 1
Inelastic cross section LHCb-CONF-2012-034 Heidelberg MPI Yes 1
Incl. particle production Heidelberg MPI Yes 1
Forward energy flow Heidelberg MPI No 2
Upsilon Beijing Yes 1
K0s, Lambdas, antiLambdas Heidelberg MPI No 1
Xi and Omega Bucharest No 2
Phi mesons No 2
Open charm No 2
Drell-Yan Liverpool, Zurich? No 1
Exclusive analyses: (see electroweak)
AntiLambda/Lambda AntiLambda/K0s Beijing Yes 1
Positive/negative particle ratios Yes 2
pi+/pi-, K+/K-, p/pbar, p,/pi Heidelberg No 2
BE correlations No 2
Ridge effect Heidelberg Univ. Yes 2

MC Sample Generation Requests

EvtGen Decay File EventType Events Requested Request Responsible Request Reason Request Priority (1-10) Request Status
Xi_LambdaPi 35103100 1M MagDown, 2.76 , Reco10, MC11a, half open VELO Florin MACIUC special 2.76 TeV runs from 2011 1 Proposed
Xi_LambdaPi 35103100 1M MagUp, 2.76 TeV, Reco10, MC11a, half open VELO Florin MACIUC special 2.76 TeV runs from 2011 1 Proposed
Omega_LambdaK+ 36103100 1M MagDown,2.76 TeV, Reco10, MC11a, half open VELO

Florin MACIUC special 2.76 TeV runs from 2011 1 Proposed
Omega_LambdaK+ 36103100 1M MagUp, 2.76 TeV, Reco10, MC11a, half open VELO Florin MACIUC special 2.76 TeV runs from 2011 1 Proposed
W_munubjet=TightCuts 42971002 500K MagDown 7TeV Phythia6 Reco14 K.A. Petridis 2012 Analysis 1 Proposed
W_munubjet=TightCuts 42971002 500K MagUp 7TeV Phythia6 Reco14 K.A. Petridis 2012 Analysis 1 Proposed
Z_mumujet=l17 42112022 1.5m MagUp 7TeV Phythia8 Reco14 R. Gauld 2011 Analysis 1 Proposed
Z_mumujet=l17 42112022 2.5m MagUp 8TeV Pythia8 Reco14 M. Sirendi 2012 Analysis 6 Proposed
minbias 30000000 1M MagUp, 2.76 TeV Reco10 MC11a, half open VELO F. Maciuc 2011 special runs analysis 1 Proposed
minbias 30000000 1M MagDown 2.76 TeV, Reco10 MC11a, half open VELO F. Maciuc 2011 Special runs analysis 1 Proposed
minbias 30000000 10M MagDown 2.76 TeV Pythia8 Reco14 F. Maciuc 2013 Analysis 3 Proposed
minbias=HardScattering,pt0,pt20GeV,incl_b 30000041 500K MagDown 7TeV Pythia6 Reco14 K.A. Petridis 2011 Analysis 1 Proposed
gg_Higgs_bb=mH125GeV,2binAcc 40900001 500K MagDown 8TeV Pythia6 K.A. Petridis 2012 Analysis 1 Proposed
minbias 30000000 10 M MagDown 7 TeV Pythia8 F. Maciuc 2011 Analysis 3 Submitted
minbias 30000000 10 M MagUp 7 TeV Pythia8 F. Maciuc 2011 Analysis 3 Submitted
minbias 30000000 10 M MagDown 8 TeV Pythia8 F. Maciuc 2012 Analysis 3 Submitted
minbias 30000000 10 M MagUp 8 TeV Pythia8 F. Maciuc 2012 Analysis 3 Submitted
Xi->LambdaPi 35103100

1M MagDown 7 TeV Pythia8

F. Maciuc 2011 Analysis 1 Submitted
Xi->LambdaPi 35103100 1M MagUp 7 TeV Pythia8 F. Maciuc 2011 Analysis 1 Submitted
Omega->LambdaK 36103100 1M MagDown 7 TeV Pythia8 F. Maciuc 2011 Analysis 1 Submitted
Omega->LambdaK 36103100 1M MagUp 7 TeV Pythia8 F. Maciuc 2011 Analysis 1 Submitted
Xi->LambdaPi 35103100 1M MagDown 8 TeV Pythia8 F. Maciuc 2012 Analysis 1 Submitted
Xi->LambdaPi 35103100 1M MagUp 8 TeV Pythia8 F. Maciuc 2012 Analysis 1 Submitted
Omega->LambdaK 36103100 1M MagDown 8 TeV Pythia8 F. Maciuc 2012 Analysis 1 Submitted
Omega->LambdaK 36103100 1M MagUp 8 TeV Pythia8 F. Maciuc 2012 Analysis 1 Submitted
Xi->LambdaPi 35103100 4M MagDown 7 TeV Pythia8 F. Maciuc 2011 Analysis 4 Proposed
Xi->LambdaPi 35103100 4M MagUp 7 TeV Pythia8 F. Maciuc 2011 Analysis 4 Proposed
Omega->LambdaK 36103100 4M MagDown 7 TeV Pythia8 F. Maciuc 2011 Analysis 4 Proposed
Omega->LambdaK 36103100 4M MagUp 7 TeV Pythia8 F. Maciuc 2011 Analysis 4 Proposed
Xi->LambdaPi 35103100 4M MagDown 8 TeV Pythia8 F. Maciuc 2012 Analysis 4 Proposed
Xi->LambdaPi 35103100 4M MagUp 8 TeV Pythia8 F. Maciuc 2012 Analysis 4 Proposed
Omega->LambdaK 36103100 4M MagDown 8 TeV Pythia8 F. Maciuc 2012 Analysis 4 Proposed
Omega->LambdaK 36103100 4M MagUp 8 TeV Pythia8 F. Maciuc 2012 Analysis 4 Proposed
Z->mumu 42112001 1M MagDown 2.76 TeV M. Sirendi 2013 Analysis 6 Requested
pp->tq' 41900003 800k MagDown 7TeV Pythia8 R. Gauld 2011 Analysis 3 Completed
Z->tautau 42100000 500k MagUp + 500k MagDown M. Sirendi 2011 Analysis 3 Submitted
gamma+jets 42000100 100k MagUp + 100k MagDown M. Sirendi 2011 Analysis 3 Submitted

References (Show list...)

III. (1) Continuation of correlation studies between particle with unitary strangeness (mesons and baryons) and their anti-particles. (2) Studies of multistrange baryon production correlated with production of other strange hadrons

Report dead-line:    December 2014

III.1 Strange particle production and correlation studies using LHCb data

III.2 Monte Carlo data analysis using a software package alternative to LHCb framework: PYTHIA – RIVET

III.3 Production and correlation studies for "beauty" hadrons

Scientific results - 2014

Abridged report - PDF

IV. Feasibility study. Production measurement for correlated beauty particles, mesons and baryons.

Report dead-line:    December 2015

IV.1 Developing trigger and preselection lines maximizing the signal. Re-optimizing the final selection using the previously chosen trigger and preselection lines.

IV.2 Estimation of the required luminosity and processing of already collected data

IV.3 Re-optimizing analysis procedures to apply to larger samples to be collected with the upgraded LHCb detector

IV.4 Using the $b\bar{b}$ inclusive cross-section measurement for Monte Carlo (generator) tuning and in correlation studies on simulated events (cross-section measurement is performed by peers in LHCb collaboration)

Scientific results - 2015

Scientific Report - PDF

V. 2016 Unique Stage

Report dead-line:    December 2016

V.1 Correlated particle production models for "beauty" hadrons: B mesons and Lambda beauty baryons

V.2 Correlated production models for strange baryons and mesons (Monte Carlo) including Xi and Omega baryons. Dissemination of the result in the scientific community and the LHCb collaboration.

Scientific results - 2016

Scientific Report - PDF (temporarily in Romanian only)

The document contains also the final scientific report summarizing the results obtained in each phase of the project implementation.
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