LHC students poster session

The production of b quark pairs in association with a Z/gamma* boson was studied at the CMS experiment using pp collision data collected in 2011. The normalized production cross section as function of the B hadron angular separation is compared with QCD predictions at tree-level and NLO accuracy. The identification of displaced secondary vertices is made with no use of jets, which allows to study B pair production at small angular separation.

A measurement of the Z/γ* + b(b) cross section is performed with the CMS detector in pp collissions at the LHC. The study has been done with a data sample from 2011 corresponding to an integrated luminosity of 2.1/fb. 
The Z+bb and Z+b production rates are presented together with some of the key kinematic variables.

TOTEM's recent results on differential elastic and total cross section of proton-proton scattering at the energy of 7 TeV.

Pseudorapidity gap distributions in pp collisions at sqrt(s) = 7 TeV are studied using a minimum bias data sample. We present the total inelastic cross section as a function of Delta eta F, the larger of the pseudorapidity regions extending to the limits of the ATLAS sensitivity, at eta = +-4.9, in which no final state particles are produced above a transverse momentum threshold pT cut. The measurements span the region 0 < Delta eta F < 8 for 200 < pT cut < 800 MeV. The measurements at larger gap sizes are dominated by contributions from diffractive dissociation processes. The large rapidity gap data are used to constrain the value of the Pomeron intercept appropriate to triple Regge models of soft diffraction. The cross section integrated over all gap sizes is compared with other LHC inelastic cross section measurements.

A brief summary of standard model jet measurements made in the ATLAS detector. The 2010 data, corresponding to an integrated luminosity of 37 pb−1, and 2011 data, corresponding to an integrated luminosity of 4.8 fb-1, were used to probe perturbative quantum-chromodynamics. Inclusive jet cross sections, dijet mass cross sections, dijet production with a veto on additional central jet activity, multi-jet cross sections, and dijet azimuthal decorrelations are presented.

The method for determining the inelastic rate for the luminosity independent cross section measurement will be presented. Main points include descriptions of corrections applied to the measured raw event rate as well as preliminary quantitative results.

CMS has performed several measurements on the phenomenon of parton energy loss in QGP. This poster combines the conclusion of the jet based measurements.

CMS has measured the nuclear modification factors of non-prompt J/psi (from b-hadron decays) and prompt J/psi in PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV. For prompt J/psi with relatively high pt (pt=6.5-30 GeV/c), a strong, centrality-dependent suppression is observed in PbPb collisions, compared to the yield in pp collisions scaled by the number of inelastic nucleon-nucleon collisions. In the same kinematic range, a suppression of non-prompt J/psi, which is sensitive to the in-medium b-quark energy loss, is measured for the first time.

We present the measurement of the production yields of J/psi in Pb-Pb collisions measured with their muon decays at forward rapidities in ALICE (2.5 < y < 4) in different collision centrality windows. The results are compared to the inclusive production cross-sections in p-p collisions and the nuclear modification factor is discussed.

Transverse momentum spectra of primary charged particles in Pb-Pb collisions measured with ALICE will be presented and compared to inelastic pp collisions in terms of RAA. The evolution of RAA as a function of collision centrality will be presented. Calculations from in-medium parton energy loss models will be compared to the measured RAA.

Electroweak boson production is an important benchmark process at hadron colliders. Precise measurements of W and Z production in heavy-ion collisions can help to constrain nuclear PDFs as well as serve as a standard candle of the initial state in PbPb collisions at the LHC energies. The analysis and selected results of W and Z production in PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV will presented.

We present the performance and unique capabilities at the LHC of the ALICE detector in the jet reconstruction for proton-proton collisions complemented with an outlook for jet measurements in high background environment of heavy-ion collisions

We present results on strangeness production in p+p collisions at different LHC energies (√s = 900GeV and 7 TeV). In particular, the rapidity and transverse momentum dependence of the antiLambda/Lambda, AntiXi+/Xi- and AntiOmega+/Omega- ratios will be shown

We will present the pt-differential cross section of Ds mesons in pp collisions at 7 TeV
measured in the central rapidity region of the ALICE experiment. 
The status of the Pb-Pb analysis at 2.76 TeV will also be discussed

Transverse energy measurements offer insight into the dynamics of heavy ion collisions and the total entropy created. At very forward pseudorapidities they may also be sensitive to the low momentum components of the nuclear wave-function. CMS has almost hermetic calorimetry coverage with fine granularity and excellent resolution. In this poster the transverse energy measurements from 2.76 TeV PbPb collisions as a function of pseudorapidity and centrality are shown. They are also compared to lower energy data.

The poster summarizes the measurements done by the CMS experiment on the various states of the Upsilon particle in PbPb collisions, compared to pp collisions. The results serve as a thermometer for the QCD matter created in the collisions, at extreme energy density.

We present the differential cross section of electrons from heavy-flavour hadron decays in pp collisions at √s = 7 TeV. Results are compared to FONLL pQCD calculations.

The ALICE experiment at the LHC has studied the J/psi production at mid-rapidity in pp collisions at sqrt(s) = 7 TeV through its electron pair decay. The fraction of J/psi from the decay of long-lived beauty hadrons was determined on a data sample  corresponding to an integrated luminosity Lint = 5.6 nb−1 for J/psi candidates with transverse momentum pt > 1.3 GeV/c and rapidity |y| < 0.9. The cross section for prompt J/psi mesons and for the production of b-hadron decaying to J/psi were also determined in the rapidity range |y| < 0.9. Using the shape of the pt and y distributions of b-quarks predicted by pQCD-based calculations to extrapolate the measured cross section, the mid-rapidity dsigma /dy and the total production cross section of b b-bar pairs were derived.

An observation of the chi_c and chi_b quarkonium states in proton-proton collision data at sqrt(s) = 7 TeV recorded by the ATLAS experiment is presented. The chi_c and chi_b states are reconstructed from their radiative decays to J/psi and Upsilon(1S,2S) respectively. In the charmonium sector, the chi_c1,2(1P) charmonium states are observed. This represents the first step towards more advanced measurements of the chi_c system with ATLAS. In the bottomonium sector, in addition to the known chi_b(1P) and  chi_b(2P) states, a new high mass structure is observed in radiative transitions to both Upsilon(1S) and Upsilon(2S). This observation is consistent with theoretical predictions for the chi_b(3P) system.

We present the measurement of direct CP asymmetry in the decay B^0->K* gamma with the full 2011 dataset in LHCb. A comparison with the SM prediction, as well as previous results is given.

An untagged, time-integrated analysis of the Bs -> phi phi decay has been performed with 1 fb-1 of pp collision data at centre-of-mass (COM) energy \sqrt{s} = 7 TeV taken using the LHCb detector. Optimised selections have yielded 801 Bs -> phi phi events at high signal to background ratio. This has allowed for measurements of polarisation amplitudes ($|A_0|^2$, $|A_\perp|^2$, $|A_\parallel|^2$) and strong phase difference (cos$\delta_\parallel$) to be performed. T-violating triple product asymmetries have yielded results A_U=-0.055 \pm 0.036(stat.) \pm 0.018(syst.) & A_V = 0.010 \pm 0.036(stat.) \pm 0.018(syst.).

The LHCb experiment is designed to perform flavour physics measurements at the Large Hadron Collider. Using a data sample corresponding to an integrated luminosity of 0.35 fb$^{-1}$ collected by LHCb in 2011, we report measurements of the direct CP asymmetries in the $B^0\to K^+\pi^-$ and $B_s^0\to \pi^+K^-$ decays. The results obtained represent the first evidence of CP violation in the decays of $B_s^0$ mesons to $K\pi$ pairs with a significance of $3.3\sigma$.
Furthermore, we report the first observation of CP violation in B0 decays at a hadron collider with a significance exceeding 6σ.

A model-independent search for direct CP violation in the Cabibbo suppressed decay $D^+ \to K^-K^+\pi^+$ in a sample of approximately 370,000 decays is described. The normalized Dalitz plot distributions for $D^+$ and $D^-$ are compared using four different binning schemes that are sensitive to different manifestations of CP violation. No evidence for CP asymmetry is found.

The determination of the CP-violating phase φs in B0s → J/ψφ decays is one of the key goals of the LHCb experiment. Its value is predicted to be very small in the Standard Model but can be significantly enhanced in many models of new physics. We present the world’s best measurement of φs and the first observation of a non-zero ∆Γs based upon 1fb−1 of data collected at LHCb during 2011.

Using an integrated luminosity of 0.69 fb-1 collected by LHCb in 2011, we report measurements of direct and mixing-induced CP violation in $B^0\to\pi^+\pi^-$ and $B_s^0\to K^+K^-$ decays. The measurements of the B0p+p- asymmetries are compatible with those from the B factories and yield 3.2s evidence of mixing-induced CP violation, whereas the Bs0K+K- asymmetries are measured for the first time ever.

Heavy quark production in 7 TeV pp collisions at the LHC need not be flavour symmetric. Here the production asymmetry, Ap , between Ds+ and Ds- mesons is measured using the phi pi decay mode. The difference between pi+ and pi- detection efficiencies is measured using the ratio of fully reconstructed to partially reconstructed D* decays. Using 1 fb^-1 of data collected with the LHCb detector, we find Ap = (-0.39+/-0.22+/-0.08)%.

The results of the LHCb opposite side flavour tagging optimisation and calibration on the 2011 dataset are presented.

The  Bs0 -> μμ  decay is a Flavour Changing Neutral Current process, highly suppressed in the Standard Model (SM). The small SM prediction of 3 x 10-9  might be substantially enhanced by coupling to new heavy particles. An analysis based on 2.4 fb−1 of integrated luminosity, using a multivariate, multi-bin selection method is presented. A limit of BR (Bs0 -> μμ) ≤ 2.2E-8 at 95% CL is obtained.

The first measurement of inclusive of J/psi polarization at the LHC was carried out in p-p collisions at √s = 7TeV, with the ALICE experiment. Reconstructing J/psi in the muon channel at forward rapidity (2.5 < y < 4), we have measured its polarization to be rather small, with a clearly finite value only for the lowest measured bin in transverse momentum, 2-3 GeV/c, in the helicity reference frame.

A search for the decay B+ to pi mu mu is presented using 1.0~fb$^{-1}$ of pp collision data integrated luminosity collected with the LHCb experiment at the Large Hadron Collider during 2011. 
This decay is observed for the first time with 5.2 $\sigma$ significance, at a branching fraction \BR(\pimumu) = (2.4~$\pm$~0.6~(stat)~$\pm$~0.2~(syst))$\times 10^{-8}$.

 A search for the decays Bs->4mu and Bd->4mu in a data sample of 1.0 inverse femtobarns collected with the LHCb detector in 2011 is discussed.  In the SM, the non-resonant \Bsmmmm branching fraction is expected to be <10^(-10)
  One signal candidate is observed in the $B_d$ channel, and no signal candidates are observed in the $B_s$ channel. These observed events are consistent with the expected backgrounds and upper limits on the branching fractions are set 
  at \\$\BR(Bs->4mu)<1.3 \times 10^{-8}$  and $\BR(Bd->4mu) < 5.4 \times 10^{-9}$ at 95\% CL.

A search for the rare decays Bs to mu+mu- and Bd to mu+mu- is presented, performed with the CMS detector with an integrated luminosity of 5 fb^-1.

We present an overview of J/psi, psi' and Upsilon production cross-section measurements in pp collisions at sqrt(s) = 7 TeV in the CMS experiment. First results from the 1P charmonium state and the exotic X(3872) are also discussed.

In this poster, we present the narrow J/ψφ resonances X(4140) and X(4274) search as well as the X(3872) mass and productions cross-section measurements. These analysis have been done using pp collisions data collected at LHCb. 

We present a measurement of the top quark mass mtop in the dilepton decay channel tt ̄ → (l+νlb) (l−ν ̄lb ̄) in pp collisions at sqrt{s}=7 TeV. The data sample of the measurement corresponds to an integrated luminosity of 2.3 fb−1 collected with the CMS detector at the LHC. Events are selected by requiring two leptons, at least two jets and missing transverse energy. The mass is reconstructed from the kinematic characteristics of the events with a full kinematic analysis. A set of templates is constructed from simulated samples and a likelihood fit is performed to derive the mass. The result yields a measurement of mtop = 173.3+/-1.2(stat.)+/-2.6(syst.) GeV/c2, and it is the single most precise measurement in this decay channel.

We present the recent measurement of the ttbar charge asymmetry in a dataset corresponding to an integrated luminosity of 4.7/fb, collected with the CMS detector at the LHC. In addition to the inclusive measurement, for the first time at the LHC also differential measurements are performed.

A measurement of normalized top quark pair differential cross-sections in pp collisions at sqrt(s)=7 TeV in 1.14 fb^-1 of data collected by the CMS detector is presented. The measurement is performed semileptonic and dileptonic final states, and provided as functions of the transverse momentum, (pseudo-)rapidity, and the invariant mass of the final state leptons, the top quarks, and of the top-antitop system.

We present a measurement of the single top t-channel inclusive cross section in proton-proton collisions at sqrt(s) = 7 TeV with the CMS detector at LHC considering decay channels with a muon or an electron in the final state, using data samples corresponding to an integrated luminosity of 1.14/1.51 inverse fb for the muon/electron channel. After a selection optimized for the t-channel, data-driven estimations of the main background processes from data were performed to reduce the assumptions on their modeling. The t-channel cross section was extracted through a fit to the pseudodorapidity distribution of the jet stemming from the light quark recoiling against the top, resulting in a measured cross section of 70.2  +/- 5.2(stat.)  +/- 10.4(syst.) +/- 3.4(lumi.) pb.

Jet flavour tagging is one of the key ingredients of the diverse physics program of the CMS experiment.
Various flavour-taggers based on the lifetime and semileptonic decay of the heavy quarks have been implemented. They use jets, tracks, leptons and/or vertices. We exploit the  \ttbar samples collected 
at $\sqrt {s}=7$ TeV running of the LHC during the first half of 2011, to extract the efficiencies for tagging the $b$-quark, and $c$-quark jets. Event samples identified as semileptonic and fully-leptonic decays of the  $t\overline t$ are used. Different techniques based on extraction of high purity $b$-flavored jets, or on requiring the consistency between the observed and expected number of tags in the events are used to extract the performance of the heavy flavor algorithms directly from data and measure the differences between data and simulation.

We present the results of the search for a Standard Model Higgs boson in the 'golden' H->ZZ(*)->4l channel. A total of 4.7 fb-1 of 2011 data collected by the CMS detector is analyzed. No evidence of deviations from the Standard Model prediction is found, therefore a limit on its production cross-section is set.

A search for the standard model Higgs boson decaying into tau leptons using CMS is described. The analysis is sensitive in a mass range 
from 110 to 145 GeV. A description of the measurement of the tau  identification efficiency for hadronic tau decays is given.

The search for a Higgs boson is one of the most important physics goals of the Large Hadron Collider program. The ATLAS experiment collected and analyzed the data corresponding to an integrated luminosity about 4.9 fb−1 for all Higgs boson search channels. A preliminary combination of Standard Model Higgs boson searches with the ATLAS experiment has been performed.

A search for the standard model Higgs boson decaying into two
photons using CMS is described. The analysis is sensitive in a mass range
from 110 to 150 GeV. Multivariate analysis techniques have been deployed to
optimize the search.

This poster presents a direct search of the Higgs boson using a data sample of L=4.6 fb-1, recorded by CMS detector in pp collision at LHC at sqrs(s) = 7TeV. The analysis searches the Higgs boson, produced in association with a Z boson, in its decay to a bottom quark pair. The Zdecays in lepton and neutrinos pairs are considered in the analysis. The expected 95% upper limits are derived on the production cross section in the Higgs mass range between 110 and 135.

This poster presents a direct search of the Higgs boson using a data sample of L = 4.6/fb, recorded by the CMS detector in pp collisions at the LHC with sqrt(s) = 7 TeV. The analysis searches for the Higgs boson, produced in association with a W boson, in its decay into a bottom quark pair. The expected 95% upper limits on the production cross section are evaluated in the Higgs mass range between 110 GeV and 135 GeV for the WH production mechanism and for the combined search merging WH and ZH decay modes.

A search for a neutral MSSM Higgs boson decaying into tau leptons using CMS is described. The analysis is sensitive in a large range of Higgs boson masses from 90 to 500 GeV.

Inclusive searches for neutral Higgs bosons h⁰/A⁰/H⁰→tau tau, based on
1.06 fb⁻¹, and for charged Higgs bosons H⁺→tau nu, based on 4.6 fb⁻¹ of
ATLAS pp-collision data, are discussed.
No excess is observed. Limits on production cross-section × branching
ratios are derived, and the findings are interpreted within the mhmax
scenario of the MSSM.

The search for the Standard Model Higgs boson via its decays into two Z bosons is presented, based on the 4.7-4.9 fb^{-1} of proton-proton collisions at a center-of-mass energy of 7 TeV, recorded in 2011 by the ATLAS experiment at the LHC.
The results obtained in the fully leptonic decay channel cover a wide range of Higgs boson mass hypotheses, between 110 GeV and 600 GeV. For Higgs boson masses above 200 GeV, the sensitivity is substantially enhanced by using channels in which one of the Z bosons decays into neutrinos or quarks. With the current integrated luminosity, a wide mass range is excluded at the 95% confidence level.

The results of a search for new physics in events with two same-sign isolated leptons (electrons, muons), hadronic jets, and missing transverse energy in the final state are presented. The analysis is based on a data sample corresponding to an integrated luminosity of 4.7  fb-1  collected by the CMS experiment at the LHC. The observed numbers of events agree with the Standard Model predictions and thus no evidence for new physics is found. The observations are used to set up- per limits on possible new physics contributions and to constrain supersymmetric models.

A search for pair production of supersymmetric gauginos decaying into final states with three leptons and missing transverse momentum is presented. The analysis uses 2.1 fb-1 of 7 TeV proton-proton collision data delivered by the LHC and recorded with the ATLAS detector in 2011.

A search for resonant production of ttbar pairs with data collected in 2011 by ATLAS. The analyses presented here concentrate on the lepton + jets and fully leptonic final states, with datasets corresponding to a total integrated luminosity of 2.05 and 1.04 fb-1 respectively. Limits are set on the production cross-section times branching ratio to top quark pairs of resonances predicted by key benchmark models. Prospects are also presented for an analysis tailored to the search for high mass resonances which decay to pairs of "boosted" top quarks with large transverse momenta.

A model-independent search is presented for the production of narrow heavy resonance with mass greater 1TeV decaying into top quark pairs in 7 TeV pp collisions at the LHC using 2011 data in the lepton+jets channel. In absence of evidence for a signal, we evaluate 95% C.L. upper limits on σ(pp → Z′ → t ̄t) · BR as a function of the invariant mass of the resonance.

A search is presented for new heavy particles using the razor variables. The search uses a mix of all-hadronic and leptonic samples to empirically model the various SM backgrounds. Constraints are placed on SUSY and exotic models.

A search for the decay of a light Higgs (120 - 140 GeV) to a pair of weakly-interacting, long-lived particles in 1.94/fb of proton-proton collisions at √s = 7 TeV recorded in 2011 by the ATLAS detector is presented.  The search strategy requires that both long-lived particles decay inside the muon spectrometer.

A search for supersymmetry using single lepton events at CMS is presented. A lepton-projection variable, based on W-polarization, is used to discriminate the dominant backgrounds arising from top and W production.

A search for physics beyond the Standard Model using events with at least three leptons is described. Numerous exclusive channels are studied using data-driven techniques to quantify the SM backgrounds

We present the results of a search for large extra spatial dimensions in events with dilepton final state using the 2/fb of data collected with the CMS detector at the LHC. The observed dilepton invariant mass distribution is found to be consistent with standard model expectation. Depending on the number of extra dimensions, limits are set on the onset of quantum gravity scale.

A search for supersymmetry using events with multijets and missing energy collected at CMS is presented. Backgrounds are evaluated using various data-driven techniques, and the results are interpreted within the context of the cMSSM as well as several simplified models.

During the 2010 PbPb run, some collisions took place at different vertex location with respect to the nominal one in all LHC experiments. A non-negligible number of those "satellite collisions" have been recorded by the ALICE experiment. Using a displaced vertex technique, the knowledge of the ALICE material budget and geometry have been improved.

This poster describes the performance of 2 parts of ATLAS:
- The Inner Detector which consists of 3 subdetectors: the Pixel detector, the SemiConductor Tracker (or SCT) and the Transition Radiation Tracker (or TRT).
Here, we report on Pixel detector and SCT performance over 2011.
- ALFA detector which will determine the absolute luminosity of the CERN LHC at the ATLAS Interaction Point (IP), and the total proton-proton cross section,
by tracking elastically scattered protons at very small angles in the limit of the Coulomb Nuclear interference region.

Muons are a key ingredient for many physics analyses in ATLAS. A measurement of the performance of the muon reconstruction and identification on LHC collisions recorded in 2011 is presented, with a particular focus on the effects of pile-up and a comparison with Monte-Carlo simulations.

With an average number of up to 23 pile-up interactions per bunch crossing (μ) in 2011 data taking the ATLAS Inner Detector (ID) already reached its design specifications. This poster presents studies in data and simulation demonstrating that the ID track reconstruction is well-prepared for the high pile-up expected in 2012.

The CMS experiment has developed a variety of algorithms that use the impact parameters of charged-particle tracks, the properties of reconstructed decay vertices, the presence of a lepton or combinations of these quantities to select samples of jets with different b purities.The performance of the algorithms in terms of efficiency and misidentification probability has been measured from multijet events.

Radiative Z decays to  pairs of muons, where one of the muons emits a Bremsstrahlung photon, can be used for numerous dedicated photon calibration and measurement purposes : they can be cleanly selected and their kinematics are well-constrained by the Z boson mass and the precision on the muon momenta. This poster presents three use cases, based on data recorded by the CMS experiment in 2010 and 2011.

During the 2011 LHC Data taking period the ATLAS Detector recorded 5.22 fb-1 which is 96.5% of the delivered data from proton-proton collisions. The recorded data with Muon Spectrometer was at a level of more than 99% good for physics analysis. This illustrates an excellent performance. Starting the 2012 data taking period the Muon Spectrometer will perform to more than 99% operational fraction at the Level 1 trigger and  at an average of more than 97.6% operational fraction of trigger and precision chambers. This poster presents the performance of the Muon Spectrometer trigger chambers as well as the precision chambers. In addition a combined Muon Spectrometer performance is presented.