The thesis is devoted to the study of diffractive physics with the ATLAS detector at the LHC. In the last chapter, the possibility of the central diffractive W production measurement is investigated.
Soft Diffraction
Moreover, it is possible that the proton dissociates to a higher mass state during a diffractive interaction. You can see not only the contributions of the diverse processes, but also those of the non-diffractive processes.
Hard Diffraction
The double solid line represents the Pomeron exchange and the dots indicate the partonic structure of the Pomeron and the proton. Then ΦP(ξ, t) can be understood as a Pomeron flux and fP(x, µ2) describes the partonic structure of the Pomeron.
Exclusive Production
For example, Figure 1.14 shows the jet transverse energy dependence of the cross section as measured by the CDF experiment [18]. Predictions of the Monte Carlo generator (ExHuME) as well as of the theoretical calculation (KMR) are also presented.
Uncertainties of Exclusive Production Models 27
KMR and CHIDe Models
The soft (lower) scale is related to the transverse momentum of the gluons participating in the hard interaction. The hard part is obtained from the differentiation of standard, integrated gluon densities (GRV [43], MRS [44] and CTEQ [45]).
Implementation of Exclusive Production
The KMR model predicts a higher value of the cross section and a steeper dependence than the CHIDe model. The CDF Collaboration also published the exclusive jet cross section as a function of the jet mass, see Figure 2.9.
Sources of Uncertainty
The uncertainty of the FPMC-derived NMR model is also given for comparison. This is slightly different for the exclusive Higgs production. that the upper scale must be exactly equal to the mass of the Higgs boson.
Uncertainties at the LHC
We can see that this procedure leads to a significant reduction in model uncertainty on the Higgs cross section. This confirms the importance of LHC measurements to limit the uncertainty in the exclusive production.
Experimental Apparatus 49
The ATLAS Detector
The purpose of the inner detector is to record the trajectories (tracks) of the charged particles produced. Moreover, the positions of the interaction vertices can be reconstructed from the measured traces. The ATLAS calorimetry system [71, 72] is shown in Figure 3.6 and consists of the liquid argon (LAr) electromagnetic calorimeter (vessel + end cap), the LAr hadronic end caps, the tile calorimeter and the LAr forward calorimeter. (FCal).
The thickness of the entire calorimetric system is about 9.7 interaction lengths in the tube and 10.
ATLAS Trigger
The L1 trigger is hardware based and uses the reduced information coming from the calorimeters and the muon spectrometer. Technically, L1 defines the regions of interest (RoIs), which contain the position, type and energy of the observed objects. The final decision is made based on the combined information from all the RoIs and the accepted output event rate at L1 is limited to 75 kHz.
The L2 trigger accesses detailed information from the detector, but only in the vicinity of the RoIs provided by L1.
Data Processing
Additional identification can be carried out on the basis of more detailed information about the traces and shapes of the energy deposits in the calorimeters. It is based on the momentum conservation, especially the momentum components transverse to the direction of the colliding jets. However, for the interesting high-pT events, the transverse momentum of such particles is usually very small compared to the pT of the neutrino(s) produced.
That is why one cannot obtain information about the longitudinal momentum of the neutrino(s).
Diffractive Measurements with ALFA Detectors 67
Allowing different values of the azimuth angle leads to the elliptical shapes shown in Figure 4.3. Due to the fact that the ALFA detectors approach the beam in the vertical direction, the acceptance of the detectors under given conditions is to a first approximation independent of ∆E, while they are very sensitive to the pT value. This can be seen in Figure 4.4, where the geometric acceptance of the ALFA stations as a function of ∆E and pT is shown.
The plot shows the apparent cross section, required for both protons to be labeled in the ALFA detectors, as a function of the studied distance.
Measurement Using the ATLAS Central Detector
For the exclusive pion production process, the reconstruction efficiency is expected to be higher due to the simplicity of the arrangements containing only two tracks. The distribution of the pion energy is shown in Figure 4.12, where the shaded area shows the part of the distribution that can be accessed experimentally. Combining the visible cross sections from both calorimetric and tracker measurements leads to a total visible cross section of about 21µb.
The installation of the AFP420 detectors is technically difficult due to the LHC cryostat present at 420 meters and having to be redesigned and rebuilt to incorporate the proposed detectors.
Physics Motivation
The QCD program of AFP detectors consists of measuring the single and central diffractive production of jets, W and Z bosons. An example is the mass of the jet system Mjj and the missing mass MX calculated from AFP measurements (ξ1 and ξ2). Looking for an absolute difference value that is less than 0.075 results in a background reduction factor of about 20.
The time information can be compared with the longitudinal position of the vertex reconstructed in the ATLAS inner detector.
Detector System
Moreover, it must be possible to change the position of the detectors (their distance from the beam) according to the beam conditions. Second, the trajectories of the diffractively scattered protons are mainly bent in the horizontal direction. At 420 m from the ATLAS IP, the positions of the diffractive protons are on the other side of the beam pipe than at 210 m, see Figure 5.11.
The chosen solution for the AFP detectors assumes that a large part of the nozzle in which the detectors are located moves close to the beam when the beam is stable.
Proton Transport
The acceptance of AFP detectors discussed above requires a single proton labeled on one side of the interaction point. For dual-labeling events, in which both protons remain intact and are labeled on both sides of the IP, the important information is the detector acceptance as a function of the missing mass MX, i.e. For exclusive production, this mass is equal to the invariant mass of the resulting hard system (Higgs boson, two jets).
The AFP acceptance of double label events as a function of the mass of the centrally produced system is shown in Figure 5.17.
Scattered Proton Energy Unfolding
The axes of the graphs describe the trajectory that is measured in the AFP: x and y are the positions x′ and y′ are the angles. The last factor that can affect the reconstruction is the interaction of the proton with the first AFP station. At small values of ξ, the lack of knowledge of the transverse position of the vertex becomes the most important effect.
The leading contributions originate from the detector resolution and the unknown value of the vertex x0 position.
Central Diffractive W Charge Asymmetry Measurement 103
Central Diffractive W Production
The cross sections for W production can be calculated similarly to the non-diffractive case. The charge asymmetry in the case of central diffraction production can be considered not only as a function of y, but also as ξ1 and ξ2. Therefore, the asymmetry must be the same or very close to the asymmetry in the non-diffractive production.
Thus, the measurement of asymmetry can be used to test the mechanism of strong diffective interactions.
W/Z Cross Section Ratio
Measuring the W over Z cross-section ratio will allow the different contributions to be investigated. This can be seen in Figures 6.4 and 6.5, where the W/Z cross-sectional ratio is shown as a function of flavor composition. This leaves two independent variables that describe the flavor composition, namely the d/uands/uquark ratios (however, another choice would also be correct).
Therefore, measuring the cross section/Z ratio can lead to constraints on the quark diffraction PDFs.
ATLAS Simulation and Pile-up Treatment
During the default processing, forward proton information from all minimal bias interactions is not stored. Such a procedure has a major drawback - the correlation between the functions of the pile-up events and the forward protons is completely lost, i.e. The preselection is particularly useful for non-diffractive samples and small values of pile-up multiplicity.
In that case, normally very few events would meet the double tag criteria, leading to the loss of the majority of simulated events.
Monte Carlo Samples
It also has a big advantage: you can very easily create an example pre-selected by requesting a single or double tag in AFP, without losing events simulated in pt. ST and DT preselection, single diffiractive with NP and ST preselection and central difractive with NP preselection. The single diffractive samples consist of two subsamples with intact protons going in each direction.
The distributions of three non-diffractive and two individual diffractive samples are compatible, which confirms the correctness of the procedure.
Signal Selection
Figures 6.7, 6.8 and 6.9 present the distributions of few variables used in the W selection (muon transverse momentum, missing transverse energy and transverse mass, respectively) in successive stages of the W selection procedure. The second part of the selection aims to select the central diffractive signal from the non-divergent and single diffractive background. The boost will cause some of the charged particles to escape the tracer acceptance, reducing the observed charged multiplicity.
This is because the improvement on the signal over background ratio is smaller than the simultaneous loss of the signal statistics.
Results
This is due to the small difference in the magnet settings in beam 1 and beam 2 [124]. For the central diffraction in the Pomeron model, the asymmetry is zero, and the individual diffraction pattern has an asymmetry of 0.06 (obtained as in the ND case). The details of the single mechanism of diffractive production play an important role in differentiating between different models of central diffractive processes.
A smaller purity of the combined sample for μ = 3 results in a smaller sensitivity to the central diffractive production mechanism. The feasibility study of the central measurement of the diffractive production W is presented in the thesis in detail. Phojet generator predictions for non-diffractive, single diffractive, double diffractive, and central diffractive contributions are compared to the data.
Effect of varying the upper limit of the Sudakov form factor on ex-
Total uncertainty on the CHIDe model from the fit to the CDF mea-
Total uncertainty on the CHIDe model for exclusive Higgs production
Contributions to the total uncertainty on the CHIDe model for ex-
Observed (full line) and expected (dashed line) 95% CL combined
General scheme of the LHC accelerator (view from above). Beam 1
The general scheme of the CERN accelerator complex
General scheme of the ATLAS main detector
General scheme of the ATLAS inner detector
General scheme of the ATLAS calorimetry system
The general scheme of the ATLAS muon spectrometer
General scheme of an ALFA detector (one station)
Geometrical acceptance of the ALFA detector as a function of the pro-
Scheme of the measurement concept – pions are measured in the
