Measurements are reported of differential cross sections for the production of a W boson, which decays into a muon and a neutrino, in association with jets, as a function of several variables, ...including the transverse momenta ( ) and pseudorapidities of the four leading jets, the scalar sum of jet transverse momenta ( ), and the difference in azimuthal angle between the directions of each jet and the muon. The data sample of pp collisions at a centre-of-mass energy of 7 TeV was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 5.0 fb super(-1). The measured cross sections are compared to predictions from Monte Carlo generators, MadGraph + pythia and sherpa, and to next-to-leading-order calculations from BlackHat + sherpa. The differential cross sections are found to be in agreement with the predictions, apart from the distributions of the leading jets at high values, the distributions of the at high- and low jet multiplicity, and the distribution of the difference in azimuthal angle between the leading jet and the muon at low values.
An updated search for heavy narrow resonances decaying to muon or electron pairs using the CMS detector is presented. Data samples from pp collisions at s = 7 TeV and 8 TeV at the LHC, with ...integrated luminosities of up to 5.3 and 4.1 fb - 1 , respectively, are combined. No evidence for a heavy narrow resonance is observed. The analysis of the combined data sets excludes, at 95% confidence level, a Sequential Standard Model Z SSM a2 resonance lighter than 2590 GeV, a superstring-inspired Z I a2 lighter than 2260 GeV, and KaluzaaKlein gravitons lighter than 2390 (2030) GeV, assuming that the coupling parameter k / M A= Pl is 0.10 (0.05). These are the most stringent limits to date.
LHC Forward Physics Akiba, K; Albrow, M; Avati, V ...
arXiv.org,
12/2017
Paper, Journal Article
Odprti dostop
The goal of this report is to give a comprehensive overview of the rich field of forward physics, with a special attention to the topics that can be studied at the LHC. The report starts presenting a ...selection of the Monte Carlo simulation tools currently available, chapter 2, then enters the rich phenomenology of QCD at low, chapter 3, and high, chapter 4, momentum transfer, while the unique scattering conditions of central exclusive production are analyzed in chapter 5. The last two experimental topics, Cosmic Ray and Heavy Ion physics are presented in the chapter 6 and 7 respectively. Chapter 8 is dedicated to the BFKL dynamics, multiparton interactions, and saturation. The report ends with an overview of the forward detectors at LHC. Each chapter is correlated with a comprehensive bibliography, attempting to provide to the interested reader with a wide opportunity for further studies.
Load balancing is one of the key components in many distributed systems as it heavily impacts performance and resource utilization. We consider a heterogeneous system where each server belongs to one ...of
K
classes and the speed of the server depends on its class. Two types of load balancing strategies are considered: arriving jobs are either immediately dispatched to a server class in a randomized manner, i.e., with probability
p
k
a job is assigned to class
k
, or are dispatched based on their size, i.e., jobs with a size in
T
k
-
1
,
T
k
)
are assigned to class
k
. Within each class a power of
d
choices rule is used to select the server that executes the job. For large systems and exponential job size durations the optimal probabilities
p
k
to minimize the mean response time can be determined easily via convex optimization. In this paper we develop a mean field model (validated by simulation) to investigate how the optimal probabilities
p
k
are affected by the higher moments and in particular by the variability of the job size distribution when the service discipline at each server is first-come-first-served. In addition, we make use of the cavity method to study the optimal thresholds
T
k
in case the dispatching is based on the job size.
Response times in large distributed systems can be reduced by exchanging jobs between idle servers and servers with pending jobs. When a pull strategy is deployed the initiative to exchange jobs is ...taken by the idle servers, while servers with pending jobs initiate the exchange when a push strategy is implemented. In this paper the performance of a class of rate-based pull and push strategies for large heterogeneous networks is studied using a mean field model. These strategies have the advantage that the rate at which servers probe other servers to initiate a job exchange can be controlled, allowing a fair comparison between pull and push strategies.
Based on two natural conjectures we derive a simple condition for the required probe rate to establish system stability when the system size becomes large and consists of two types of servers. In some specific cases we show that this condition coincides with the existence of a unique positive fixed point for which we also present an explicit expression. This fixed point is used to express the queue length distribution and mean response time in the system in explicit form. The accuracy of both the stability condition and mean queue lengths as predicted by the mean field model is validated using time-consuming simulation experiments. We end the paper with some numerical results that compare the performance of the rate-based pull and push strategies in a heterogeneous setting.
Normalized double-differential cross sections for top quark pair ( $\mathrm{t}\overline{\mathrm{t}}$ ) production are measured in pp collisions at a centre-of-mass energy of 8 $\,\text {TeV}$ with ...the CMS experiment at the LHC. The analyzed data correspond to an integrated luminosity of 19.7 $\,\text {fb}^{-1}$ . The measurement is performed in the dilepton $\mathrm {e}^{\pm }\mu ^{\mp }$ final state. The $\mathrm{t}\overline{\mathrm{t}}$ cross section is determined as a function of various pairs of observables characterizing the kinematics of the top quark and $\mathrm{t}\overline{\mathrm{t}}$ system. The data are compared to calculations using perturbative quantum chromodynamics at next-to-leading and approximate next-to-next-to-leading orders. They are also compared to predictions of Monte Carlo event generators that complement fixed-order computations with parton showers, hadronization, and multiple-parton interactions. Overall agreement is observed with the predictions, which is improved when the latest global sets of proton parton distribution functions are used. The inclusion of the measured $\mathrm{t}\overline{\mathrm{t}}$ cross sections in a fit of parametrized parton distribution functions is shown to have significant impact on the gluon distribution.
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