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| CMS-HIN-11-008 ; CERN-PH-EP-2012-144 | ||
| Study of W boson production in PbPb and pp collisions at $\sqrt{s_{\mathrm{NN}}} =$ 2.76 TeV | ||
| CMS Collaboration | ||
| 29 May 2012 | ||
| Phys. Lett. B 715 (2012) 66-87 | ||
| Abstract: A measurement is presented of W-boson production in PbPb collisions carried out at a nucleon-nucleon (NN) centre-of-mass energy $\sqrt{s_{\mathrm{NN}}}$ of 2.76 TeV at the LHC using the CMS detector. In data corresponding to an integrated luminosity of 7.3 $\mu$b$^{-1}$, the number of $\mathrm{ W } \rightarrow \mu^- \bar{\nu}$ decays is extracted in the region of muon pseudorapidity $ | \eta^\mu | < $ 2.1 and transverse momentum $p_{\mathrm{T}}^{\mu^-} > $ 25 GeV/$c$. Yields of muons found per unit of pseudorapidity correspond to 159 $\pm$ 10 (stat) $\pm$ 12 (syst) $\times 10^{-8}$ $\mathrm{W}^+$ and 154 $\pm$ 10 (stat) $\pm$ 12 (syst) $\times 10^{-8}$ $\mathrm{W}^-$ bosons per minimum-bias PbPb collision. The dependence of W production on the centrality of PbPb collisions is consistent with a scaling of the yield by the number of incoherent NN collisions. The yield of W bosons is also studied in a sample of pp interactions at $\sqrt{s}=$ 2.76 TeV corresponding to an integrated luminosity of 231 nb$^{-1}$. The individual $\mathrm{W}^+$ and $\mathrm{W}^-$ yields in PbPb and pp collisions are found to agree, once the neutron and proton content in Pb nuclei is taken into account. Likewise, the difference observed in the dependence of the positive and negative muon production on pseudorapidity is consistent with next-to-leading-order perturbative QCD calculations. | ||
| Links: e-print arXiv:1205.6334 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; Public twiki page ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1-a:
(a) Single-muon transverse-momentum spectrum for $ {| \eta ^\mu | }<$ 2.1 in PbPb data (red-filled circles). Signal (green-hatched histogram) and background (blue-dashed histogram) contributions are fitted (black solid line) to the data. (b) Mean value of $p_{\mathrm{T}}$ for charged tracks as a function of centrality, before any event selection is applied on the muon-triggered data (black squares) and after it (red-filled circles), together with predictions from the PYTHIA+HYDJET samples (green triangles). (c) Transverse mass distribution for selected events in PbPb (red-filled circles) and pp (blue open squares) data, compared to simulation (green-hatched histogram). The error bars represent statistical uncertainties. (See the text for more details). |
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Figure 1-b:
(a) Single-muon transverse-momentum spectrum for $ {| \eta ^\mu | }<$ 2.1 in PbPb data (red-filled circles). Signal (green-hatched histogram) and background (blue-dashed histogram) contributions are fitted (black solid line) to the data. (b) Mean value of $p_{\mathrm{T}}$ for charged tracks as a function of centrality, before any event selection is applied on the muon-triggered data (black squares) and after it (red-filled circles), together with predictions from the PYTHIA+HYDJET samples (green triangles). (c) Transverse mass distribution for selected events in PbPb (red-filled circles) and pp (blue open squares) data, compared to simulation (green-hatched histogram). The error bars represent statistical uncertainties. (See the text for more details). |
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Figure 1-c:
(a) Single-muon transverse-momentum spectrum for $ {| \eta ^\mu | }<$ 2.1 in PbPb data (red-filled circles). Signal (green-hatched histogram) and background (blue-dashed histogram) contributions are fitted (black solid line) to the data. (b) Mean value of $p_{\mathrm{T}}$ for charged tracks as a function of centrality, before any event selection is applied on the muon-triggered data (black squares) and after it (red-filled circles), together with predictions from the PYTHIA+HYDJET samples (green triangles). (c) Transverse mass distribution for selected events in PbPb (red-filled circles) and pp (blue open squares) data, compared to simulation (green-hatched histogram). The error bars represent statistical uncertainties. (See the text for more details). |
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Figure 2:
Centrality dependence of normalised $\mathrm{ W } \rightarrow \mu \nu $ cross sections $(1/T_\mathrm {AA}) (N_\mathrm{ W } /\Delta \eta )$ in PbPb collisions, for all W candidates (red-filled points) and, separated by charge, $\mathrm{ W }^+$ (violet-filled squares) and $\mathrm{ W }^-$ (green-filled stars). The open symbols at $N_\text {part} \approx$ 120 represent the MB events. At $N_\text {part} = $ 2, the corresponding cross sections are displayed for pp collisions divided by $\Delta \eta $, for the same $\sqrt {s}$. For clarity, both $\mathrm{ W }^+$ and $\mathrm{ W }^-$ points are slightly shifted on the horizontal axis. The cross sections are given for the phase space region $|\eta ^\mu |<$ 2.1 and $ {p_{\mathrm {T}}} ^\mu > $ 25 GeV/$c$. The error bars represent the statistical, and the horizontal lines the systematic, uncertainties. |
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Figure 3:
Charge asymmetry $(N_{\mathrm{ W }^+ } - N_{\mathrm{ W }^- }) / (N_{\mathrm{ W }^+ } + N_{\mathrm{ W }^- })$ as a function of muon pseudorapidity for PbPb (red-filled circles) and pp (blue open squares) collisions at $\sqrt {s} =$ 2.76 TeV. Overlaid are predictions for pp collisions from MCFM calculations with MSTW2008 PDF (blue solid curve), as well as expectations for PbPb collisions from MCFM with MSTW2008 and EPS09 PDF, that include isospin and nuclear effects (red dashed curve). Each prediction has an additional uncertainty of 5%, estimated by the uncertainty from the choice of PDF. The experimental points have an additional 1.1% systematic uncertainty that is not shown in the figure. The dot-dashed horizontal line is drawn only for reference. |
| Tables | |
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Table 1:
Sources of systematic uncertainties in the analyses of PbPb and pp data. The last line of the table contains the uncertainty in luminosity for pp data, and the analogous uncertainty in the value of $T_{\mathrm {AA}} $ for MB events in PbPb data, as described in Section3. |
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Table 2:
The selected number of events with $\mu^{+} $ ($N_{\mu^{+} }^\text {sel}$) and $\mu^{-} $ ($N_{\mu^{-} }^\text {sel}$) and the normalised cross sections $(1/T_{\mathrm {AA} }) (N_{\mathrm{ W } }/\Delta \eta )$ as a function of centrality in PbPb data. The last row provides the selected number of events with $\mu^{+} $ ($N_{\mu^{+} }^{\text {sel-pp}}$) and $\mu^{-} $ ($N_{\mu^{-} }^{\text {sel-pp}}$) in pp data, and the cross sections derived from them (see text), divided by $\Delta \eta $. All values are reported for $|\eta ^\mu |<$ 2.1 and $ {p_{\mathrm {T}}} ^\mu >$ 25 GeV/$c$. |
| Summary |
|
The inclusive production of W bosons has been measured for $\mathrm{ W } \rightarrow \mu \bar{\nu}$ decays in PbPb and pp collisions at $\sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV. Decays of W bosons were identified by requiring a reconstructed muon with $|\eta^\mu|<$ 2.1 and $p_{\mathrm{T}}^{\mu}>$ 25 GeV/$c$ and transverse mass $m_{\mathrm{T}} >$ 40 GeV/$c^2$. The W yields for all PbPb collision centralities were found to be consistent with those measured in pp collisions scaled by the corresponding number of incoherent nucleon-nucleon interactions. The individual $\mathrm{W}^+$ and $\mathrm{W}^-$ boson yields are modified in PbPb compared to pp collisions due to the different proton and neutron content in the nuclear beams. The differences in the $\mu^+$ and $\mu^-$ yields from W decays (charge asymmetries) have been measured as a function of muon pseudorapidity both in PbPb and pp collisions. All measurements were found to be well reproduced by higher-order perturbative QCD predictions. The pp results combined with those obtained at $ \sqrt{s} = $ 7 TeV, exhibit the expected dependence of the charge asymmetry on the parton densities as probed at different collision energies. The results confirm the theoretical expectation that, in the probed ranges of parton fractional momentum and energy scale, further modifications of the nuclear parton distribution functions in the lead nucleus compared to the proton are small relative to the dominant isospin effect. These studies demonstrate the promise of W-boson measurements as powerful tools in the investigation of initial and final-state effects in nuclear collisions at the LHC. The charge asymmetry of W-boson yields in PbPb interactions provides unique sensitivity to the parton distribution functions for neutrons. Future analyses of larger data samples will yield enhanced constraints on the parton densities in nuclei and allow studies of W production in association with jets. |
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