ATLAS探测器在13 TeV质子-质子碰撞中搜索光子+射流事件中新现象

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"这篇论文是ATLAS合作组在2016年发表的关于使用ATLAS探测器在质子-质子碰撞中寻找新物理现象的研究成果。在13TeV的质心能量下，分析了3.2fb^-1的数据，主要关注的是衰变为光子+射流的高能共振的产生。研究中选取了具有高横向动量的孤立光子和射流事件，以探索可能的新粒子或现象。" 在大型强子对撞机(Large Hadron Collider, LHC)上，ATLAS探测器进行了详细的分析，没有在光子+射流的组合中观察到与标准模型预测背景显著不同的信号。这使得研究人员能够设置限制在95%置信水平下，对于不同类型的超出标准模型的现象。他们考虑了两种特定的模型：一是带有矢量耦合的激发夸克(excited quarks)，这些夸克与标准模型粒子有类似矢量的相互作用；二是非热量子黑洞(non-thermal quantum black holes)，这是在额外空间维度理论中的现象。对于高斯形信号的共振，设置了可见截面的限制。如果假设的共振具有较小的宽度（高宽比为2%），则质量在1.5 TeV至5 TeV之间的最小排除可见截面从大约6 fb降低到约0.8 fb。另一方面，如果共振的宽度相对较大（高宽比为15%），则相同质量范围内的最小排除可见截面从50 fb降低到1.0 fb。此外，针对Randall-Sundrum模型（一种包含额外空间维度的理论，如 Arkani-Hamed-Dimopoulos-Dvali (ADD)模型），研究排除了质量低于4.4 TeV的激发夸克，以及质量低于3.8（6.2）TeV的非热量子黑洞，具体取决于额外维度的数量。这项工作对高能物理领域具有重要意义，因为它提供了在更高能量尺度上对标准模型的验证，同时也限制了新物理现象的存在范围。通过未观察到的信号，科学家们可以进一步改进理论模型，或者为未来的实验设计提供指导，以期在更广泛的参数空间内发现可能的新粒子或现象。

JHEP03(2016)041

are set to the photon transverse momentum. Jets of partons are reconstructed using the

anti-k

algorithm [41] with a radius parameter R = 0.4. The total transverse energy from

partons produced inside a cone of size ∆R =

(∆η)

+ (∆φ)

= 0.4 around the photon

is required to be lower than 2.45 GeV + 0.022 × p

, to match the selection requirement

described in the next section. Experimental eﬀects (detector reconstruction eﬃciencies and

resolution) as well as hadronization and pile-up are not taken into account in this sample.

4 Event selection

Each event is required to contain at least one primary vertex candidate with two or more

tracks with p

> 400 MeV. The tracks must satisfy quality requirements based on the

number of reconstructed intersections with the silicon pixel and strip detectors and the

track impact parameters with respect to the centre of the luminous region. The primary

vertex is deﬁned as the candidate with the largest sum of the p

of the tracks that are

considered to be associated to it, based on a requirement on a χ

variable calculated

between the estimated vertex position and the point of closest approach of the track to

the vertex.

Photons are reconstructed from energy deposits (clusters) found in the EM calorime-

ter by a sliding-window algorithm. The reconstruction algorithm looks for matches be-

tween energy clusters and tracks reconstructed in the inner detector and extrapolated to

the calorimeter. Well-reconstructed clusters matched to tracks are classiﬁed as electron

candidates while clusters without matching tracks are classiﬁed as unconverted photon

candidates. Clusters matched to pairs of tracks that are consistent with the hypothesis of

a γ → e

−

conversion process are classiﬁed as converted photon candidates. To maximize

the reconstruction eﬃciency for electrons and photons, clusters matched to single tracks

without hits in an active region of the innermost pixel layer are considered as electron

candidates and as converted photon candidates. Both unconverted and converted photon

candidate are used for the search presented in this paper.

The energies of the photon candidates are calibrated following the procedure described

in ref. [42]. The calibration algorithm, tuned using 13 TeV event simulation, accounts for

energy loss upstream of the EM calorimeter and for both lateral and longitudinal shower

leakage. Correction factors are extracted from 8 TeV Z → ee data and simulated events

reconstructed with the algorithms used in the 2015 data taking. Additional corrections

and systematic uncertainties take into account the diﬀerences between the 2012 and 2015

conﬁgurations.

To reduce backgrounds from hadrons, photon candidates are required to fulﬁl η-

dependent requirements consisting of nine independent selections, one on the hadronic

leakage and eight on shower shape variables measured with the ﬁrst two sampling layers

of the electromagnetic calorimeter [27]. The requirements were optimized for the 2015

data-taking conditions using simulated samples of photons and hadronic jets produced in

13 TeV pp collisions. The simulation is corrected for the diﬀerences between

√

s = 8 TeV

data and simulated events for each photon shower shape variable.

– 6 –

JHEP03(2016)041

Groups of contiguous calorimeter cells (topological clusters) are formed based on the

signiﬁcance of the ratio of deposited energy to calorimeter noise. To further reduce back-

ground photons from hadronic jets, the transverse isolation energy E

T,iso

of the photon

candidates is required to be less than 2.45 GeV + 0.022 × p

. This energy is computed

from the sum of the energies of all cells belonging to topological clusters and within a

cone of ∆R = 0.4 around the photon direction. The contributions from the underlying

event and the pile-up [43, 44], as well as from the photon itself, are subtracted. The iso-

lation requirement has a signal eﬃciency of about 98% over the whole photon transverse

momentum range relevant to this analysis.

Jets are reconstructed from topological clusters of calorimeter cells using the anti-k

algorithm with radius parameter R = 0.4. Jets aﬀected by noise or hardware problems in

the detector, or identiﬁed as arising from non-collision backgrounds, are discarded [45]. Jet

four-momenta are computed by summing over the topological clusters that constitute each

jet, treating each cluster as a four-vector with zero mass. To reduce the eﬀects of pile-up

on the jet momentum, an area-based subtraction method is employed [43, 44]. Jet energies

are then calibrated by using corrections from the simulation and scale factors determined

in various control samples (γ + jet, Z+jet and multijet events) in 8 TeV data [46] and

validated with early 2015 data [47]. These corrections are applied to 2015 data, after

taking into account in the simulation the changes in the detector and in the data-taking

conditions between 8 TeV and 13 TeV data, and propagating as systematic uncertainties

those related to this extrapolation procedure. Jets with p

< 20 GeV or within ∆R = 0.2

(0.4) of a well-identiﬁed and isolated electron (photon) with transverse momentum above

25 GeV are not considered.

Events are selected if they contain at least one photon candidate and at least one jet

candidate satisfying all the previous criteria and each having p

> 150 GeV. The photon

trigger has an eﬃciency of



99.9

+0.1

−1.3



% for these events. The trigger eﬃciency is measured

in data as the product of the eﬃciency of the high-level trigger computed from events

selected by the ﬁrst-level trigger and the eﬃciency of the ﬁrst-level trigger with respect to

oﬄine identiﬁcation [48].

Since t-channel γ+jet and dijet production rates increase while the rate of the s-channel

signal production decreases with the photon and jet absolute pseudorapidity, photons are

required to be in the barrel calorimeter, |η

| < 1.37. Moreover, as a consequence of

the diﬀerent production mechanisms for the signal and background, the pseudorapidity

separation ∆η between the photon and the jet candidates tend to be smaller for the signal

than for the background, particularly for large values of the photon-jet invariant mass. For

this reason, events with |∆η| > 1.6 are discarded.

In events in which more than one good photon or jet candidate is found, the highest-

candidate of each type is selected to form the resonant γ + jet candidate. Events in

which the angular separation between the photon and any jet with p

> 30 GeV (after

the jet-photon overlap removal) is ∆R < 0.8 are discarded. This requirement suppresses

background events from SM photon+jet production in which the photon is emitted at large

angles in the fragmentation of a quark or a gluon.

The total signal eﬃciency (including detector acceptance) depends on the resonant

mass of the hypothetical signal, and is described in the next section. The product of

– 7 –

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