1,357 research outputs found
Triggering on hard probes in heavy ion collisions with CMS
We present a study of the CMS trigger system in heavy-ion collisions.
Concentrating on two physics channels, dimuons from decays of quarkonia and
single jets, we evaluate a possible trigger strategy for Pb+Pb running that
relies on event selection solely in the High-Level Trigger (HLT). The study is
based on measurements of the timing performance of the offline algorithms and
event-size distributions using full simulations. Using a trigger simulation
chain, we compare the physics reach for the jet and dimuon channels using
online selection in the HLT to minimum bias running. The results demonstrate
the crucial role the HLT will play for CMS heavy-ion physics.Comment: 4 pages, 4 fugures, contribution to QM'06 conferenc
Medium information from anisotropic flow and jet quenching in relativistic heavy ion collisions
Within a multiphase transport (AMPT) model, where the initial conditions are
obtained from the recently updated HIJING 2.0 model, the recent anisotropic
flow and suppression data for charged hadrons in Pb+Pb collisions at the LHC
center of mass energy of 2.76 TeV are explored to constrain the properties of
the partonic medium formed. In contrast to RHIC, the measured centrality
dependence of charged hadron multiplicity dN_ch/deta at LHC provides severe
constraint to the largely uncertain gluon shadowing parameter s_g. We find
final-state parton scatterings reduce considerably hadron yield at midrapidity
and enforces a smaller s_g to be consistent with dN_ch/deta data at LHC. With
the parton shadowing so constrained, hadron production and flow over a wide
transverse momenta range are investigated in AMPT. The model calculations for
the elliptic and triangular flow are found to be in excellent agreement with
the RHIC data, and predictions for the flow coefficients v_n(p_T, cent) at LHC
are given. The magnitude and pattern of suppression of the hadrons in AMPT are
found consistent with the measurements at RHIC. However, the suppression is
distinctly overpredicted in Pb+Pb collisions at the LHC energy. Reduction of
the QCD coupling constant alpha_s by ~30% in the higher temperature plasma
formed at LHC reproduces the measured hadron suppression.Comment: Talk given by Subrata Pal at the 11th International Conference on
Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1,
2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference
Series (JPCS
Search for a W\u27 boson decaying to a vector-like quark and a top or bottom quark in the all-jets final state at âs = 13 TeV
A search is presented for a heavy W0 boson resonance decaying to a B or T vector-like quark and a t or a b quark, respectively. The analysis is performed using protonproton collisions collected with the CMS detector at the LHC. The data correspond to an integrated luminosity of 138 fbâ1 at a center-of-mass energy of 13TeV. Both decay channels result in a signature with a t quark, a Higgs or Z boson, and a b quark, each produced with a significant Lorentz boost. The all-hadronic decays of the Higgs or Z boson and of the t quark are selected using jet substructure techniques to reduce standard model backgrounds, resulting in a distinct three-jet W\u27 boson decay signature. No significant deviation in data with respect to the standard model background prediction is observed. Upper limits are set at 95% confidence level on the product of the W0 boson cross section and the final state branching fraction. A W\u27 boson with a mass below 3.1TeV is excluded, given the benchmark model assumption of democratic branching fractions. In addition, limits are set based on generalizations of these assumptions. These are the most sensitive limits to date for this final state
Correlations in Proton-Proton Collisions at the LHC
Results on two-particle angular correlations for charged particles emitted in proton proton collisions at center-of-mass energies of 0.9, 2.36, and 7 TeV are presented, using data collected with the CMS detector over a broad range of pseudo-rapidity (h) and azimuthal angle (f). Short-range correlations in Dh, which are studied in minimum bias events, are characterized using a simple âindependent clusterâ parametrization in order to quantify their strength (cluster size) and their extent in h (cluster decay width). Long-range azimuthal correlations are studied differentially as a function of charged particle multiplicity and particle transverse momentum using a 980 nb^-1 data set at 7 TeV. In high multiplicity events, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate pT of 1â3 GeV/c, 2.0 \u3c jDhj \u3c 4.8 and Df _0. This is the first observation of such a long-range, near-side feature in two-particle correlation functions in pp or p anti-p collisions
Commissioning and Performance of the CMS Pixel Tracker with Cosmic Ray Muons
The pixel detector of the Compact Muon Solenoid experiment consists of three barrel layers and two disks for each endcap. The detector was installed in summer 2008, commissioned with charge injections, and operated in the 3.8 T magnetic field during cosmic ray data taking. This paper reports on the first running experience and presents results on the pixel tracker performance, which are found to be in line with the design specifications of this detector. The transverse impact parameter resolution measured in a sample of high momentum muons is 18 microns
Commissioning and Performance of the CMS Pixel Tracker with Cosmic Ray Muons
The pixel detector of the Compact Muon Solenoid experiment consists of three barrel layers and two disks for each endcap. The detector was installed in summer 2008, commissioned with charge injections, and operated in the 3.8 T magnetic field during cosmic ray data taking. This paper reports on the first running experience and presents results on the pixel tracker performance, which are found to be in line with the design specifications of this detector. The transverse impact parameter resolution measured in a sample of high momentum muons is 18 microns
Toward particle-level filtering of individual collision events at the Large Hadron Collider and beyond
Low-energy strong interactions are a major source of background at hadron colliders, and methods of subtracting the associated energy flow are well established in the field. Traditional approaches treat the contamination as diffuse, and estimate background energy levels either by averaging over large data sets or by restricting to given kinematic regions inside individual collision events. On the other hand, more recent techniques take into account the discrete nature of background, most notably by exploiting the presence of substructure inside hard jets, i.e. inside collections of particles originating from scattered hard quarks and gluons. However, none of the existing methods subtract background at the level of individual particles inside events. We illustrate the use of an algorithm that will allow particle-by-particle background discrimination at the Large Hadron Collider, and we envisage this as the basis for a novel event filtering procedure upstream of the official reconstruction chains. Our hope is that this new technique will improve physics analysis when used in combination with state-of-the-art algorithms in high-luminosity hadron collider environments
RooStatsCms: a tool for analyses modelling, combination and statistical studies
The RooStatsCms (RSC) software framework allows analysis modelling and
combination, statistical studies together with the access to sophisticated
graphics routines for results visualisation. The goal of the project is to
complement the existing analyses by means of their combination and accurate
statistical studies.Comment: Proceedings of the 11th Topical Seminar on Innovative Particle and
Radiation Detectors. 4 pages and 5 figure
Minimal Z' models: present bounds and early LHC reach
We consider `minimal' Z' models, whose phenomenology is controlled by only
three parameters beyond the Standard Model ones: the Z' mass and two effective
coupling constants. They encompass many popular models motivated by grand
unification, as well as many arising in other theoretical contexts. This
parameterization takes also into account both mass and kinetic mixing effects,
which we show to be sizable in some cases. After discussing the interplay
between the bounds from electroweak precision tests and recent direct searches
at the Tevatron, we extend our analysis to estimate the early LHC discovery
potential. We consider a center-of-mass energy from 7 towards 10 TeV and an
integrated luminosity from 50 to several hundred pb^-1, taking all existing
bounds into account. We find that the LHC will start exploring virgin land in
parameter space for M_Z' around 700 GeV, with lower masses still excluded by
the Tevatron and higher masses still excluded by electroweak precision tests.
Increasing the energy up to 10 TeV, the LHC will start probing a wider range of
Z' masses and couplings, although several hundred pb^-1 will be needed to
explore the regions of couplings favored by grand unification and to overcome
the Tevatron bounds in the mass region around 250 GeV.Comment: 25 pages. v2: small improvements and minor corrections, version
accepted for publication on JHE
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