Measurements from the CMS experiment at the LHC of dihadron correlations for charged particles produced in PbPb collisions at a nucleon-nucleon centre-of-mass energy of 2. 76 TeV are presented. The ...results are reported as a function of the particle transverse momenta (p T ) and collision centrality over a broad range in relative pseudorapidity (Δη) and the full range of relative azimuthal angle (Δ$\phi$). The observed two-dimensional correlation structure in Δη and Δ$\phi$ is characterised by a narrow peak at (Δη,Δ$\phi$)≈(0,0) from jet-like correlations and a long-range structure that persists up to at least |Δη|=4. An enhancement of the magnitude of the short-range jet peak is observed with increasing centrality, especially for particles of p T around 1-2 GeV/c. The long-range azimuthal dihadron correlations are extensively studied using a Fourier decomposition analysis. The extracted Fourier coefficients are found to factorise into a product of single-particle azimuthal anisotropies up to p T ≈3-3. 5 GeV/c for at least one particle from each pair, except for the second-order harmonics in the most central PbPb events. Various orders of the single-particle azimuthal anisotropy harmonics are extracted for associated particle p T of 1-3 GeV/c, as a function of the trigger particle p T up to 20 GeV/c and over the full centrality range.
A study of forward energy flow and central charged-particle multiplicity in events with W and Z bosons decaying into leptons is presented. The analysis uses a sample of 7 TeV pp collisions, ...corresponding to an integrated luminosity of 36 inverse picobarns, recorded by the CMS experiment at the LHC. The observed forward energy depositions, their correlations, and the central charged-particle multiplicities are not well described by the available non-diffractive soft-hadron production models. A study of about 300 events with no significant energy deposited in one of the forward calorimeters, corresponding to a pseudorapidity gap of at least 1.9 units, is also presented. An indication for a diffractive component in these events comes from the observation that the majority of the charged leptons from the (W/Z) decays are found in the hemisphere opposite to the gap. When fitting the signed lepton pseudorapidity distribution of these events with predicted distributions from an admixture of diffractive (POMPYT) and non-diffractive (PYTHIA) Monte Carlo simulations, the diffractive component is determined to be (50.0 +/- 9.3 (stat.) +/- 5.2 (syst.))%.
Measurements from the CMS experiment at the LHC of dihadron correlations for charged particles produced in PbPb collisions at a nucleon–nucleon centre-of-mass energy of 2.76 TeV are presented. The ...results are reported as a function of the particle transverse momenta (
p
T
) and collision centrality over a broad range in relative pseudorapidity (Δ
η
) and the full range of relative azimuthal angle (Δ
ϕ
). The observed two-dimensional correlation structure in Δ
η
and Δ
ϕ
is characterised by a narrow peak at (Δ
η
,Δ
ϕ
)≈(0,0) from jet-like correlations and a long-range structure that persists up to at least |Δ
η
|=4. An enhancement of the magnitude of the short-range jet peak is observed with increasing centrality, especially for particles of
p
T
around 1–2 GeV/
c
. The long-range azimuthal dihadron correlations are extensively studied using a Fourier decomposition analysis. The extracted Fourier coefficients are found to factorise into a product of single-particle azimuthal anisotropies up to
p
T
≈3–3.5 GeV/
c
for at least one particle from each pair, except for the second-order harmonics in the most central PbPb events. Various orders of the single-particle azimuthal anisotropy harmonics are extracted for associated particle
p
T
of 1–3 GeV/
c
, as a function of the trigger particle
p
T
up to 20 GeV/
c
and over the full centrality range.
The transverse momentum spectra of charged particles have been measured in pp and PbPb collisions at sqrt(sNN) = 2.76 TeV by the CMS experiment at the LHC. In the transverse momentum range pt = 5-10 ...GeV/c, the charged particle yield in the most central PbPb collisions is suppressed by up to a factor of 5 compared to the pp yield scaled by the number of incoherent nucleon-nucleon collisions. At higher pt, this suppression is significantly reduced, approaching roughly a factor of 2 for particles with pt in the range pt=40-100 GeV/c.
A study of forward energy flow and central charged-particle multiplicity in events with W and Z bosons decaying into leptons is presented. The analysis uses a sample of 7 TeV pp collisions, ...corresponding to an integrated luminosity of 36 pb
−1
, recorded by the CMS experiment at the LHC. The observed forward energy depositions, their correlations, and the central charged-particle multiplicities are not well described by the available non-diffractive soft-hadron production models. A study of about 300 events with no significant energy deposited in one of the forward calorimeters, corresponding to a pseudorapidity gap of at least 1.9 units, is also presented. An indication for a diffractive component in these events comes from the observation that the majority of the charged leptons from the W(Z) decays are found in the hemisphere opposite to the gap. When fitting the signed lepton pseudorapidity distribution of these events with predicted distributions from an admixture of diffractive (
pompyt
) and non-diffractive (
pythia
) Monte Carlo simulations, the diffractive component is determined to be (50.0 ± 9.3 (stat.) ± 5.2 (syst.))%.
A measurement is presented of the relative prompt production rate of
χ
c2
and
χ
c1
with 4.6 fb
−1
of data collected by the CMS experiment at the LHC in pp collisions at
. The two states are measured ...via their radiative decays
χ
c
→J/
ψ
+
γ
, with the photon converting into an e
+
e
−
pair for J/
ψ
rapidity |
y
(J/
ψ
)|<1.0 and photon transverse momentum
p
T
(
γ
)>0.5 GeV/
c
. The measurement is given for six intervals of
p
T
(J/
ψ
) between 7 and 25 GeV/
c
. The results are compared to theoretical predictions.
The transverse momentum spectra of charged particles have been measured in pp and PbPb collisions at
by the CMS experiment at the LHC. In the transverse momentum range
p
T
=5–10 GeV/
c
, the charged ...particle yield in the most central PbPb collisions is suppressed by up to a factor of 7 compared to the pp yield scaled by the number of incoherent nucleon–nucleon collisions. At higher
p
T
, this suppression is significantly reduced, approaching roughly a factor of 2 for particles with
p
T
in the range
p
T
=40–100 GeV/
c
.
A measurement is presented of the relative prompt production rate of
χ
c2
and
χ
c1
with 4.6 fb
−1
of data collected by the CMS experiment at the LHC in pp collisions at
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\begin{document}$\sqrt{s}= 7~\mathrm{TeV}$\end{document}
. The two states are measured via their radiative decays
χ
c
→J/
ψ
+
γ
, with the photon converting into an e
+
e
−
pair for J/
ψ
rapidity |
y
(J/
ψ
)|<1.0 and photon transverse momentum
p
T
(
γ
)>0.5 GeV/
c
. The measurement is given for six intervals of
p
T
(J/
ψ
) between 7 and 25 GeV/
c
. The results are compared to theoretical predictions.
The transverse momentum spectra of charged particles have been measured in pp and PbPb collisions at \(\sqrt{s_{NN}}=2.76~\mathrm{TeV}\) by the CMS experiment at the LHC. In the transverse momentum ...range pT=5–10 GeV/c, the charged particle yield in the most central PbPb collisions is suppressed by up to a factor of 7 compared to the pp yield scaled by the number of incoherent nucleon–nucleon collisions. At higher pT, this suppression is significantly reduced, approaching roughly a factor of 2 for particles with pT in the range pT=40–100 GeV/c.
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