NA60 results on thermal dimuons Arnaldi, R.; Banicz, K.; Borer, K. ...
European physical journal. C, Particles and fields,
06/2009, Letnik:
61, Številka:
4
Journal Article, Conference Proceeding
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Odprti dostop
The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158
A
GeV In–In collisions. A strong excess of pairs above the known sources is observed in the whole ...mass region 0.2<
M
<2.6 GeV. The mass spectrum for
M
<1 GeV is consistent with a dominant contribution from
π
+
π
−
→
ρ
→
μ
+
μ
−
annihilation. The associated
ρ
spectral function shows a strong broadening, but essentially no shift in mass. For
M
>1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell–Yan pairs. The slope parameter
T
eff
associated with the transverse momentum spectra rises with mass up to the
ρ
, followed by a sudden decline above. The rise for
M
<1 GeV is consistent with radial flow of a hadronic emission source. The seeming absence of significant flow for
M
>1 GeV and its relation to parton–hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck-like shape of the mass spectra at low
p
T
and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on
ω
in-medium effects.
Abstract The TOTEM collaboration has measured the elastic proton-proton differential cross section $$\mathrm{d}\sigma /\mathrm{d}t$$ d σ / d t at $$\sqrt{s}=13$$ s = 13 TeV LHC energy using ...dedicated $$\beta ^{*}=90$$ β ∗ = 90 m beam optics. The Roman Pot detectors were inserted to 10 $$\sigma $$ σ distance from the LHC beam, which allowed the measurement of the range 0.04 GeV $$^{2}$$ 2 ; 4 GeV $$^{2}$$ 2 $$$$ in four-momentum transfer squared | t |. The efficient data acquisition allowed to collect about 10 $$^{9}$$ 9 elastic events to precisely measure the differential cross-section including the diffractive minimum (dip), the subsequent maximum (bump) and the large-| t | tail. The average nuclear slope has been found to be $$B=(20.40 \pm 0.002^{\mathrm{stat}} \pm 0.01^{\mathrm{syst}})~$$ B = ( 20.40 ± 0 . 002 stat ± 0 . 01 syst ) GeV $$^{-2}$$ - 2 in the | t |-range 0.04–0.2 GeV $$^{2}$$ 2 . The dip position is $$|t_{\mathrm{dip}}|=(0.47 \pm 0.004^{\mathrm{stat}} \pm 0.01^{\mathrm{syst}})~$$ | t dip | = ( 0.47 ± 0 . 004 stat ± 0 . 01 syst ) GeV $$^{2}$$ 2 . The differential cross section ratio at the bump vs. at the dip $$R=1.77\pm 0.01^{\mathrm{stat}}$$ R = 1.77 ± 0 . 01 stat has been measured with high precision. The series of TOTEM elastic pp measurements show that the dip is a permanent feature of the pp differential cross-section at the TeV scale.
The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158 AGeV In-In collisions. A strong excess of pairs is observed above the yield expected from neutral meson decays. After ...subtraction of the decay sources, the shape of the resulting mass spectrum is largely consistent with a dominant contribution from
π
+
π
−
→
ρ
→
μ
+
μ
−
annihilation. The associated
ρ spectral function exhibits considerable broadening, but essentially no shift in mass. The acceptance-corrected
p
T
spectra have a shape atypical for radial flow. They also significantly depend on mass, pointing to different sources in different mass regions. Both mass and
p
T
spectra are compared to recent theoretical predictions.
TOTEM Roman Pot (RP) microstrip edgeless silicon detectors, fabricated with standard planar technology, reach full sensitivity within 50
μm from the cut edge and can operate with high bias voltage at ...room temperature. These detectors use a newly developed terminating structure, which prevents breakdown and surface current injection at high bias, while simultaneously providing extremely reduced dead zones at the edges. Moreover, radiation hardness studies indicate that when operated under moderate cooling, the detectors remain fully efficient up to a fluence of about 1.5×10
14
p
cm
−2. The mass production of these detectors for the TOTEM Experiment is being completed and their installation in the Roman Pots is ongoing. When the installation is complete and the LHC will be operational, these detectors will allow the TOTEM Experiment to detect leading protons at distance of ∼1
mm from the beam centre at the LHC. This work presented here is a survey of this recently developed device and its most up-to-date characterisations.
The TOTEM collaboration has measured the proton–proton total cross section at
s
=
13
TeV
with a luminosity-independent method. Using dedicated
β
∗
=
90
m
beam optics, the Roman Pots were inserted ...very close to the beam. The inelastic scattering rate has been measured by the T1 and T2 telescopes during the same LHC fill. After applying the optical theorem the total proton–proton cross section is
σ
tot
=
(
110.6
±
3.4
) mb, well in agreement with the extrapolation from lower energies. This method also allows one to derive the luminosity-independent elastic and inelastic cross sections:
σ
el
=
(
31.0
±
1.7
)
mb
and
σ
inel
=
(
79.5
±
1.8
)
mb
.
The new neutron time-of-flight facility (n_TOF) has been built at CERN and is now operational. The facility is intended for the measurement of neutron induced cross-sections of relevance to ...Accelerator Driven Systems (ADS) and to fundamental physics. Neutrons are produced by spallation of the
20
GeV/c
proton beam, delivered by the Proton Synchrotron (PS), on a massive target of pure lead. A measuring station is placed at
≈185
m
from the neutron producing target, allowing high-resolution measurements. The facility was successfully commissioned with two campaigns of measurements, in November 2000 and April 2001. The main interest was concentrated in the physical parameters of the installation (neutron fluence and resolution function), along with the target behavior and various safety-related aspects. These measurements confirmed the expectations from Monte Carlo simulations of the facility, thus allowing to initiate the foreseen physics program.
The TOTEM experiment at the LHC has performed the first measurement at
s
=
13
TeV
of the
ρ
parameter, the real to imaginary ratio of the nuclear elastic scattering amplitude at
t
=
0
, obtaining the ...following results:
ρ
=
0.09
±
0.01
and
ρ
=
0.10
±
0.01
, depending on different physics assumptions and mathematical modelling. The unprecedented precision of the
ρ
measurement, combined with the TOTEM total cross-section measurements in an energy range larger than
10
TeV
(from 2.76 to
13
TeV
), has implied the exclusion of all the models classified and published by COMPETE. The
ρ
results obtained by TOTEM are compatible with the predictions, from other theoretical models both in the Regge-like framework and in the QCD framework, of a crossing-odd colourless 3-gluon compound state exchange in the
t
-channel of the proton–proton elastic scattering. On the contrary, if shown that the crossing-odd 3-gluon compound state
t
-channel exchange is not of importance for the description of elastic scattering, the
ρ
value determined by TOTEM would represent a first evidence of a slowing down of the total cross-section growth at higher energies. The very low-|
t
| reach allowed also to determine the absolute normalisation using the Coulomb amplitude for the first time at the LHC and obtain a new total proton–proton cross-section measurement
σ
tot
=
(
110.3
±
3.5
)
mb
, completely independent from the previous TOTEM determination. Combining the two TOTEM results yields
σ
tot
=
(
110.5
±
2.4
)
mb
.
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