ATLAS探测器在7 TeV pp碰撞中测得D介子生产截面
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"这篇论文是ATLAS合作组在2016年发表的，研究了在7TeV能量的质子-质子（pp）碰撞中，Dâ±、D±和Ds±介子的产生截面。利用了大型强子对撞机（LHC）上280nb^-1的综合光度数据进行分析，重建的介子具有3.5<pT(D)<100 GeV的横向动量和|η(D)|<2.1的伪快速度范围。论文中还讨论了这些介子的差分横截面，并与次级量子色动力学（QCD）预测进行了比较，同时提取了关于魅力夸克生产的相关物理量，如奇异抑制因子和D介子的产生总横截面等。"
本文详细介绍了在LHC的ATLAS探测器中进行的一项实验，目的是研究高能粒子碰撞中重味夸克，特别是带电魅力介子（Dâ±，D±和Ds±）的产生。在7TeV的质子-质子碰撞中，研究人员分析了大量数据，以重建介子的轨迹并测量其物理属性。所测量的横截面是粒子物理学中一个关键参数，它代表了特定条件下产生某种粒子的概率。
论文中提到的Dâ±、D±和Ds±介子是包含魅力夸克（c夸克）的复合粒子，它们的产生对于理解基本粒子间的相互作用，尤其是强相互作用，至关重要。研究人员通过测量介子的横向动量和伪快速度的分布，可以获取关于QCD过程的深入信息。这些测量结果与次级QCD预测进行了比较，尽管理论上的不确定性较大，但数据显示两者在可见的动量和伪快速度范围内基本一致。
此外，论文还涉及了奇异抑制因子的计算，这是描述在QCD过程中，相对于非奇异夸克，奇异夸克（例如s夸克）产生概率较低的现象。通过对可见横截面的分析和对全动量空间的外推，可以估算这一抑制因子的大小。同时，他们还确定了在7TeV能量下D介子的整体产生横截面，这有助于更全面地了解魅力夸克的产生机制。
这篇论文为理解高能粒子碰撞中的重味夸克产生提供了宝贵的实验数据，验证了QCD理论的预测，并且对魅力夸克的性质和宇宙射线中的粒子产生提供了新的见解。这些研究成果对于未来更精确的QCD计算和LHC的进一步实验有着重要的参考价值。
ATLAS Collaboration / Nuclear Physics B 907 (2016) 717–763 725
where x =|(m − m
0
)/σ |. This functional form, introduced to take into account the non-
Gaussian tails of resonant signals, describes both the data and MC signals well. The signal
position, m
0
, and width, σ , as well as the number of D
∗+
mesons are free parameters of the fit.
The threshold function has the form A ·(m −m
π
+
)
B
·exp[C ·(m −m
π
+
) +D ·(m −m
π
+
)
2
],
where m
π
+
is the pion mass and A, B, C and D are free parameters. The fitted D
∗±
yields are
N(D
∗±
) = 2140 ±120 (stat) and N(D
∗±
) = 732 ±34 (stat) for the low-p
T
and high-p
T
ranges,
respectively. Small admixtures (< 1%) to the reconstructed signals from the D
∗+
→ D
0
π
+
decays with D
0
decays to final states other than K
−
π
+
are taken into account in the accep-
tance correction procedure (Section 7). The combined value of the fitted mass differences is
145.47 ± 0.03 (stat) MeV, in agreement with the world average. The widths of the signals are
∼0.6MeV, in agreement with the MC expectations.
6.2. Reconstruction of D
+
mesons
The D
+
mesons are reconstructed from the decay D
+
→ K
−
π
+
π
+
. In each event, two
tracks from same-charge particles each with p
T
> 0.8 GeV are combined with a track from
the opposite-charge particle with p
T
> 1GeVto form a D
+
candidate. At least one of the two
particles with the same charge is required to have p
T
> 1GeV. Only three-track combinations
successfully fitted to a common vertex are kept. The pion mass is assigned to each of the two
tracks from same-charge particles and the kaon mass is assigned to the third track, after which
the candidate invariant mass, m(Kππ), is calculated using the track momenta refitted to the
common ve
rtex. To suppress combinatorial background the following requirements are used:
• χ
2
< 12, where χ
2
is the D
+
candidate vertex fit quality.
•|d
0
(D
+
)| < 0.15 mm.
•|z
0
(D
+
) sin θ(D
+
)| < 0.3mm.
• L
xy
(D
+
) > 1.2mm. The large value of the requirement on L
xy
(D
+
) is motivated by the
relatively large lifetime of the D
+
meson [47] and the large combinatorial background.
• cos θ
∗
(K) > −0.8, where θ
∗
(K) is the angle between the kaon in the Kππ rest frame and
the Kππ line of flight in the laboratory frame.
• cos θ
∗
(π) > −0.85, where θ
∗
(π) is the angle between the pion in the Kππ rest frame and
the Kππ line of flight in the laboratory frame.
To suppress background from D
∗+
decays, combinations with m(Kππ) − m(Kπ) <
153 MeV are removed. The background from D
+
s
→ φπ
+
, with φ → K
+
K
−
, is suppressed
by rejecting any three-track D
+
candidate comprised of a pair of tracks of oppositely charged
particles which, when assuming the kaon mass for both tracks, has a two-track invariant mass
within ±8 MeV of the world average φ mass [47]. MC studies indicate that the suppression
of the D
∗+
→ D
0
π
+
decays has a negligible effect on the D
+
signal, and the suppression of
the D
+
s
→ φπ
+
decays rejects less than 2% of the signal. The remaining small background
from D
+
s
→ K
+
K
−
π
+
decays is subtracted using the simulated reflection shape normalised to
the measured D
+
s
rate (Section 6.3). Smaller contributions, affecting mass ranges outside the
expected D
+
signal, from the decays D
+
s
→ π
+
π
−
π
+
, D
+
→ K
+
K
−
π
+
, D
+
→ π
+
π
−
π
+
and D
+
→ π
+
π
−
π
+
π
0
are subtracted using the simulated reflection shapes normalised to the
measured D
+
and D
+
s
rates.
Fig. 2 sho
ws the m(Kππ) distributions for low-p
T
and high-p
T
D
+
candidates after all
requirements. Sizeable signals are seen around the world average value of the D
+
mass,
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