Angular distributions of the decay B 0 → K * 0 µ + µ − are studied using a sample of proton-proton collisions at √ s = 8 TeV collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 20.5 fb −1 . An angular analysis is performed to determine the P 1 and P 5 parameters, where the P 5 parameter is of particular interest because of recent measurements that indicate a potential discrepancy with the standard model predictions. Based on a sample of 1397 signal events, the P 1 and P 5 parameters are determined as a function of the dimuon invariant mass squared. The measurements are in agreement with predictions based on the standard model.

The cross section for coherent J/ψ photoproduction accompanied by at least one neutron on one side of the interaction point and no neutron activity on the other side, X n 0 n , is measured with the CMS experiment in ultra-peripheral PbPb collisions at √ s NN = 2.76 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 159 µb −1 , collected during the 2011 PbPb run. The J/ψ mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is dσ coh X n 0 n /dy(J/ψ) = 0.36 ± 0.04 (stat) ± 0.04 (syst) mb in the rapidity interval 1.8 < |y| < 2.3. Using a model for the relative rate of coherent photoproduction processes, this X n 0 n measurement gives a total coherent photoproduction cross section of dσ coh /dy(J/ψ) = 1.82 ± 0.22 (stat) ± 0.20 (syst) ± 0.19 (theo) mb. The data strongly disfavour the impulse approximation model prediction, indicating that nuclear effects are needed to describe coherent J/ψ photoproduction in γ + Pb interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing. IntroductionPhoton-induced reactions are dominant in Ultra-Peripheral Collisions (UPC) of heavy ions, which involve electromagnetic interactions at large impact parameters of the colliding nuclei. Because of the extremely high photon flux in ultra-peripheral heavy-ion collisions which is proportional to Z 2 , where Z is the charge of the nucleus, photon-nucleus collisions at the LHC are abundant [1][2][3]. Furthermore, in UPCs the LHC can reach unprecedented photon-lead and photon-proton center-of-mass energies.Vector meson photoproduction in UPCs has received recent interest [3]. Exclusive J/ψ photoproduction off protons is defined by the reaction γ + p → J/ψ + p, with the characteristic features that, apart from the vector meson in the final state, no other particles are produced and the vector meson has a mean transverse momentum significantly lower than in inclusive reactions. Another characteristic feature is that in exclusive photoproduction the quantum numbers of the final state can be studied unambiguously. The γ + p → J/ψ + p production process has been studied by H1 and ZEUS collaborations at the electron-proton collider HERA [4][5][6], by the CDF collaboration in proton-antiproton collisions at the Tevatron [7], and by the ALICE and LHCb collaborations at the LHC, in proton-lead [8] and proton-proton collisions [9], respectively. Since the cross section of photoproduced vector mesons such as J/ψ, ψ(2S), and Υ(nS), in leading order perturbative QCD, is proportional to the gluon density squared in the target [10,11], the study of such diffractive processes in high-energy collisions is expected to provide insights into the role played by gluons in hadronic matter. As an example, a J/ψ produced at rapidity y is sensitive to the gluon distribution at x = (M J/ψ / √ s)e ±y at hard scales Q 2 ∼ M 2 J/ψ /4, where M J/ψ is the J/ψ mass, The CMS detectorThe...

We present an investigation of the white noise plateau in residual phase noise measurement of an optical frequency comb for short and long external laser cavity lengths. OCIS codes: (140.5960) Semiconductor Lasers; (140.4050) Mode-locked lasers 1. Motivation Semiconductor harmonically mode-locked lasers have a number of features which make them attractive for use in communications, sampling, and metrology applications [1,2] For some of these applications, low timing jitter and narrow optical comb linewidth is required for good sampling resolution or increased stability. In semiconductor phase noise performance plots, we generally distinct characteristics at low, middling, and high offset frequencies relative to the carrier [3,4]. For good phase noise performance at high offset frequencies, we can implement a high finesse intra-cavity to suppress competing longitudinal mode-groups [5], and we can lower the shot noise floor by increasing photodetected optical power. In this submission, we address phase noise performance at middling offset frequencies corresponding to the white noise plateau. By decreasing optical linewidth, the corner, or "knee" frequency of the white noise plateau should also decrease proportionately [6], and we aim to hit the shot noise floor at lower offset frequencies and thereby reduce timing jitter. 2. Background It is well known that laser cavity length determines longitudinal mode linewidth [7]. It is also known that a mode-locked pulse train's temporal coherence is inversely related to longitudinal mode linewidth [8]. For pulses separated in time by more than the coherence time, we expect to see no coherence, and, importantly, no noise correlation. Thus the noise contribution at low offset frequencies is dominated by uncorrelated white noise. This result has been shown in a previous publication without the use of the etalon to suppress supermode contributions to phase noise [6], and here we will attempt to implement the concept into our existing laser architecture [9]. 3. Setup and Data The laser's setup is shown in Fig. 1 and a similar architecture is discussed in [9]. Here we perform measurements at the shortest manageable cavity length (22m, f0 = 9MHz) and for a longer cavity length (351 m, f0 = 570 kHz). Fig. 2 shows the change in knee location for the residual phase noise measurement when bypassing the etalon in the laser (not PDH-locked). Fig. 3 shows the residual phase noise measurement for the short cavity. As of the time of this submission, the PDH lock on the longer cavity is limited due to the higher required dynamic range for the fiber stretcher, but the dashed line in Fig. 3 shows where we expect to measure the new knee location once we have ability to stabilize the laser. For equal shot noise floor and noise power at 1 Hz offset, we expect a reduction in timing jitter between 10% and 20%. This represents a promising improvement in the timing jitter of our optical frequency comb sources. Fig. 1 Laser Setup Fig. 2 Residual phase noise of short and long etalon-less cavities Fig...

The first observation of the Z boson decaying to four leptons in proton-proton collisions is presented. The analyzed data set corresponds to an integrated luminosity of 5.02 fb −1 at √ s = 7 TeV collected by the CMS detector at the Large Hadron Collider. A pronounced resonance peak, with a statistical significance of 9.7 σ, is observed in the distribution of the invariant mass of four leptons (electrons and/or muons) with mass and width consistent with expectations for Z boson decays. The branching fraction and cross section reported here are defined by phase space restrictions on the leptons, namely, 80 < m 4 < 100 GeV, where m 4 is the invariant mass of the four leptons, and m > 4 GeV for all pairs of leptons, where m is the two-lepton invariant mass. The measured branching fraction is B(Z → 4 ) = 4.2