Measurement of the exclusive Z to mumu production cross-section in pp collisions

production cross-section in pp collisions at

√

s = 13 TeV

Danny Wernik

October 31, 2019

Studentnumber 10665978 Supervisor dr. W.D. Hulsbergen Institute Nikhef

Second assessor prof. dr. M.H.M. Merk 36 EC Master project

Abstract

An analysis on central exclusive production (CEP) of the Z boson in pp collisions at √s = 13 TeV is done. Used for this analysis is the 2016 magnet down data set collected by the LHCb experiment, which has an integrated luminosity of about 0.83/fb. The Z → µ+_µ− _{decay is used}

to measure the cross-section. Multiple selection criteria are applied in order to select the CEP events. No CEP events were found and therefore an upper limit has been set on the cross-section. The upper limit is set to 13 fb at a 95% confidence level and is in agreement with the theoretical predictions of this cross-section.

1 Introduction 1 2 The LHCb experiment 4 2.1 LHCb detector . . . 4 2.2 Tracking system . . . 5 2.3 Track types . . . 6 2.4 Particle identification . . . 6 2.4.1 Calorimeter system . . . 6 2.4.2 Muon stations . . . 7 3 Analysis 9 3.1 Event selection . . . 9 3.2 pT resolution . . . 12 4 Results 13 4.1 Cross-section measurement . . . 13 4.2 Efficiency . . . 14

4.3 Upper limit calculation . . . 16

5 Discussion 17

1 INTRODUCTION

1

Introduction

Central Exclusive Production (CEP), pp → pXp, is a process in which the initial protons keep intact and where part of their momentum is converted to create a system X [1]. In this analysis we look for the CEP of the Z boson at LHCb. The leading order Feynman diagram of this process can be seen in figure 1. The main decay mode for Z boson is the hadronic one, but this decay mode cannot be detected well due to the large background of QCD multi-jet production. Therefore, we study the leptonic Z boson decays, specifically the decay to two muons [2].

Figure 1: Leading order Feynman diagram of CEP of the Z boson. A photon (γ) couples to a quark-antiquark (q ¯q) pair and interacts with the (anti)proton through two gluon (g) exchanges and produces a Z boson. From [3].

The Standard Model predicts a total cross section of 11 fb at the LHC for the Z boson [2]. With the appropriate branching fraction of 0.034 for the muon decay mode, an event rate of 0.31 events per year is predicted, making it highly unlikely to detect these CEP events for the Z boson [4]. This prediction takes into account an integrated luminosity of 0.83/fb. The prediction is calculated using the whole rapidity range, while the LHCb detector does not cover the whole rapidity range. This means that the event rate will be even lower than 0.31 events per year at LHCb. Figure 2 shows the rapidity distribution for diffractive Z photoproduction. We see that the event rate will be lowered by a factor of about 3 or 4.

Figure 2: The rapidity distribution for diffractive Z photoproduction in pp collisions.

Some beyond the standard model (BSM) theories however may change the expected event rate. One example of a BSM theory can be found in [5]. The article discusses a contribution to the process seen in figure 1, via a coupling of the pomeron to the electroweak sector through a pair of color sextet quarks. These contributions will thus result in a higher event rate for this process, which will make it more likely to detect it.

LHCb in particular is suited for detecting exclusive events. Compared to other experiments, LHCb has a relatively large fraction of low primary vertex (PV) events. For the CEP events we require the event to be empty (apart from the decay products of the Z). This means that events with more than one inelastic collision are per definition excluded to be CEP events. Figure 3 shows the distribution of the number of PVs in an event.

0 2 4 6 8 10 # PVs 0 10000 20000 30000 40000 50000 60000 70000 Events / (1)

Figure 3: Distribution of the number of reconstructed primary vertices in Z → µ+_µ− _candidate

1 INTRODUCTION

Analyses on CEP of J/ψ, ψ(2S) and Υ are already done at LHCb [6, 7]. There are also anal-yses done on the cross-section calculations of the Z → µ+_µ− _{production [8]. However, almost no}

public papers on the CEP of the Z boson are available. There is one CDF paper from 2009 which reports their findings on the CEP of the Z boson [3]. No signal was found and an upper limit on the total exclusive Z cross-section in p¯p collisions at √s = 1.96 TeV was set at 960 fb at a 95% confidence level.

2

The LHCb experiment

The LHCb experiment is located at the Large Hadron Collider (LHC), which is a particle accel-erator in the accelaccel-erator complex at the European Organisation for Nuclear Research (CERN). Besides the LHCb experiment are three other experiments at the LHC. These are A Large Ion Col-lider Experiment (ALICE), A Toroidal LHC ApparatuS (ATLAS) and Compact Muon Solonoid (CMS). This chapter will give a brief overview of the detector and track reconstruction of the LHCb experiment. The next section provides a brief overview of the LHCb detector and the subdetectors important for this analysis.

2.1

LHCb detector

The LHCb detector is a single-arm forward spectrometer and covers the pseudorapidity range 2 < η < 5 [9], where the pseudorapidity is defined as η ≡ − lntan θ

2
. In the equation θ is the

angle between the particle three-momentum p and the positive direction of the beam axis. This means that a η of zero will result in a track perpendicular to the beam and as the pseudorapidity goes to infinity, the angle will be more and more parallel to the beam. At high energies b¯ b-hadrons tend to be boosted in one of the beam directions, hence the detector design choice. An overview of the detector can be seen in figure 4. The detector can be split in two different systems with their own set of subdetectors [10]. One system is the Tracking system, which consist of the Vertex Locator (VELO), Tracker Turicensis (TT), the dipole magnet and the tracking stations (T1-T3). The other system is the Particle IDentification (PID) system including the Ring Imaging Cherenkov detectors (RICH1 and RICH2), electromagnetic and hadronic calorimeters (ECAL and HCAL respectively) and the muon detector (M1-M5).

2 THE LHCB EXPERIMENT

Figure 4: Schematic overview of the LHCb Detector. From [11].

2.2

Tracking system

The tracking system takes care of the reconstruction of charged particles. The pp collisions take place around z = 0 in the VELO, which is a silicon micro-strip detector [12]. The VELO provides reconstruction of the position of the Primary Vertex (PV), b- and c-hadron vertices and detects tracks which do not originate from the PV. Downstream of the VELO is the TT. For charged particles the TT improves the momentum estimate and for long lived neutral particles it per-forms track measurements. The dipole magnet allows for a momentum reconstruction of charged particles. Charged particles get bent by the magnet and the curvature of the track is used to determine the momentum. The magnet has a bending power of 4 Tm and bends the particles in the xz-plane. It has two polarity configurations, up and down, to check for asymmetries in CP violation measurements. The T stations are placed downstream of the magnet. There are three T stations, each with an silicon Inner Tracker (IT) and a straw tube Outer Tracker (OT). The IT and OT are designed in such a way that the IT can deal with the highly occupied inner region, while the OT can deal with the less occupied outer region. More on the design of the IT and OT can be found in [13, 14].

2.3

Track types

Not all particles have hits in all the different subdetectors, therefore a distinction of different track types is made. Figure 5 shows a representation of the different track types. When a particle only has hits in the VELO it is labeled as a VELO track. A VELO track can be combined with hits in TT for an upstream track and to both TT and T hits for a long track. It can occur that there are no tracks in the VELO segment. The combination of TT and T-station hits is called a downstream track and only hits in the T-stations results in a T track [15, 16].

Figure 5: Schematic representation of different track types. From [16].

2.4

Particle identification

The Particle Identification (PID) system provides information to distinguish between pions, kaons protons, muons and electrons. The system consists of the two RICH detectors, ECAL, HCAL and the 5 muon stations. The muon stations will be explained in more detail in the next section, since they are more important in this analysis.

The RICH system can distinguish pions, kaons and protons using the emission angle of the Cherenkov radiation. It consists of two different subdetectors covering different momentum re-gions and with different angular acceptances. More on the RICH detectors can be found in [17]. The Calorimeter system (HCAL, ECAL, SPD/PS) identifies the hadrons, electrons and photons and provides the energies and position of the particles. The ECAL determines this for photons and electrons and the HCAL does this for heavier particles like protons, pions and neutrons. The SPD and PS are placed upstream of the calorimeters.

2.4.1 Calorimeter system

The main purpose of calorimeter system, consisting of the ECAL, HCAL, SPD and PS, is to identify hadrons, electrons and photons and to give their energies and positions. Figure 6 shows

2 THE LHCB EXPERIMENT

an overview of where the particles deposit their energy in the calorimeter system. The SPD separates charged from non-charged particles and allows for the distinction between electrons from photons and neutral pions. The Pre-Shower detector allows for the separation between charged pion background and electrons. The ECAL measures the energy and positions of the electromagnetic showers. The HCAL measures the energies and positions of the hadrons.

Figure 6: Overview of the locations of the energy deposits for hadrons electrons and photons in the calorimeter system. Note that the muons do not deposit anything in the system. From [10].

2.4.2 Muon stations

The muon system consists of 5 muon stations (M1-M5) and a side view of the Muon Detector can be seen in figure 7 [9, 10, 18]. The first muon station (M1) is placed between RICH2 and the calorimeters. It is used in the first level trigger to improve the pT measurement, but for the

high level triggers and offline it is less useful. Muon stations M2 to M5 are placed downstream the calorimeters and are interleaved with 80 cm thick iron absorbers. These are used to identify penetrating muons and are useful in both online and offline analysis.

Muon station hits from the first level trigger are used to give a quick pT estimate of the

muon candidate. Hits are required in all five muon stations and muon candidates are accepted if the estimated pT lies above a certain threshold. The muon stations M1-M3 have a high spatial

resolution along the x coordinate, which is the bending plane. These stations are used in the L0 trigger to calculate the pT of the candidate muon and to define the track direction to distinct

positive and negative candidates. M4 and M5 have limited spatial resolution and are useful to identify penetrating particles and tag false muon candidates.

3 ANALYSIS

3

Analysis

The Z boson can be identified through the two muons in which it decays. To identify the Z → µ+_µ−

decay multiple selection criteria are needed. In this chapter, we discuss and explain most of the used cuts used in this analysis. First we discuss some criteria which identify the Z boson and then we discuss the exclusivity cuts to obtain CEP events. We measure the production rate of both inelastic Z → µ+_µ− _{and CEP Z → µ}+_µ−_{, since we normalise with respect to the ordinary}

inelastic events. The next section will further elaborate on the different selection criteria that are used in this analysis.

3.1

Event selection

We have selection criteria for both the properties of the muons and the Z boson [19]. We first introduce the criteria to select Z bosons and then we introduce the criteria to select CEP events. The criteria to identify the Z boson require the muons to be of opposite charge. The tracks both have to be identified as muons by the IsMuon critera found in table 1. The tracks of the muons need to lie in the pseudorapidity range of 2 < η < 4.5, its relative momentum uncertainty has to be less than 10% and the probability χ2 of the track fit must be larger than 0.1%. The invariant mass of the dimuons is required to be in the range 60 < Mµ+µ− < 120 GeV/c2and both muons

need to have a transverse momentum higher than 20 GeV. A summary of these selection criteria can be found in table 2. These criteria are the same as found in [8].

Momentum range (GeV) Muon Detector Hits

1 < p < 6 M2 & M3

6 < p < 10 M2 & M3 & (M4 || M5) p > 10 M2 & M3 & M4 & M5

Table 1: IsMuon criteria. From [19].

Furthermore we need some exclusivity cuts to obtain our CEP events. We require only two opposite sign tracks in our event since the Z boson has neutral charge and we only want a event which contains one Z candidate. Furthermore, there should only be two velo tracks in the event. The pT of the Z boson should lie between 0 and 2 GeV/c. More on why we choose this criteria will

be discussed in section 3.2. Lastly we expect the number of SPDhits in an event to be lower than 20. In the CEP J/ψ, and ψ(2S) analysis a cut on the number of SPDhits is set on nSPDhits < 10 [6]. We choose a more conservative value of nSPDhits < 20. A summary of these CEP selection criteria can be found in table 3. The effect of the criteria on the number of events can be found in table 4.

Z criteria

Invariant mass: 60 < Mµ+_µ− < 120 GeV/c2

Muon pseudorapidity: 2 < η < 4.5

Muon transverse momentum: pT > 20 GeV

Opposite charge muons

Both muons identified by IsMuon criteria Relative momentum uncertainty:_σ(q/p)|q/p| > 10 Track fit χ2_{> 0.1%}

Table 2: Criteria to select all Z bosons

CEP criteria

Only two opposite sign tracks in an event Only two velo tracks in an event

Z boson transverse momentum: pT < 2 GeV

nSPDhits < 20

Table 3: Criteria to select all CEP events.

Selection Events MC up Events MC down Events Data down

Generated events 169066 164405

-Z candidates 49504 47813 183854

Z criteria 36236 34819 119020

CEP criteria 0 0 0

Table 4: Table showing the amount of generated events for both MC up and down. Z candidates are all events which end up in the ’XToMuMu’ branch of the tuple. We then apply extra cuts to obtain all Z bosons. Finally we apply exclusivity cuts to obtain the number of CEP events. In our case we are left with zero CEP-like events.

3 ANALYSIS

To show that we have Z bosons in our sample we look at the mass distributions of both Monte Carlo (MC) generated events and data. Note that the mass of the Z boson is measured to be 91.1876 ± 0.0021 GeV [20]. Figure 8 shows that we have events in the right mass range and that our signal indeed contains Z bosons.

60 80 100 120 [GeV] -µ + µ m 0 500 1000 1500 2000 2500 Events / (0.5 GeV) (a) 60 80 100 120 [GeV] -µ + µ m 0 1000 2000 3000 4000 5000 6000 7000 8000 Events / (0.5 GeV) (b)

Figure 8: Plots of the invariant mass distribution of the dimuons. In (a) and the mass distribution of MC down events is shown. (b) shows the distribution for data down. We see that both distributions look similar and we thus have Z bosons in our signal

All events that pass the selection criteria in simulation are considered background, since the event generator does not include Z → µ+µ− CEP events. As seen in table 4 no Z → µ+µ−events pass the selection in both MC up and MC down. This result is used later in section 4.3

3.2

p

T

resolution

One of the CEP criteria we used requires the pT of the Z boson to be lower than 2 GeV. As we

are dealing with CEP events we expect that the Z boson is produced with a low pT. Since no

model to generate CEP events was available, we are not able to set a good range for the Z boson pT cut. In the analysis at Tevatron the pT range is set to 0 < pT < 2 GeV and we will therefore

apply the same cut [3]. In the analysis of exclusive J/ψ and ψ(2S) production at LHCb the cut for p2_T is set at p2_T < 0.8 GeV, which implies that such a tight pT cut is, in terms of resolution,

achievable at LHCb [6]. We check that the used range is a reasonable one to choose by looking at the resolution of pT. This means we are looking at how good the reconstructed pT of the Z boson

holds up against its truth value. For these figures we require ptrue

T to be lower than 10 GeV, as we

are only interested in how good the resolution is for the low pT CEP events. Figure 9 shows the

resolution plot using MC down data. We obtain that around 85% of all events will fall between a pT difference of 2 GeV. Therefore we assume that the efficiency of the 2 GeV cut is about 85%.

For a more accurate determination of the efficiency a simulation with CEP events is needed.

Entries 21410 Mean 0.0585 Std Dev 0.621 Underflow 405 Overflow 2.29e+03 Integral 1.87e+04 2 − −1 0 1 2

[GeV]

true T

- p

rec T

p

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Events / (0.1 GeV)

Entries 21410 Mean 0.0585 Std Dev 0.621 Underflow 405 Overflow 2.29e+03 Integral 1.87e+04

Figure 9: Plot of prec

T − ptrueT . We see that about 85% of the events are within the pT range of -2

4 RESULTS

4

Results

4.1

Cross-section measurement

The per bin cross-section for the dimuon final state is defined as:

σµµ_Z (i) = 1 L· ρ µµ_{· f}µµ F SR(i) · f µµ unf(i) N_Zµµ(i) X j=1 1 (µ+_j, µ−_j ), (1) where L is the integrated luminosity. The purity is denoted by ρµµ _{The correction factors for}

the final-state radiation and resolution effects are given by f_{F SR}µµ (i) and f_unfµµ(i) respectively. The efficiency is denoted by (µ+_j, µ−_j) and takes into account reconstruction and detection efficiencies. The index i denotes the bin number and j is used for the candidates in an bin. We obtain the total cross-section by summing over all bins i.

σ_Zµµ=X

i

σ_Zµµ(i) (2)

The bins can be defined by multiple variables, for example pT and y of the Z boson. This

should be done since f_{F SR}µµ (i) and f_unfµµ (i) vary with the bin variables. This is also true for the purity, but this variation is not significant [8]. We, however, use a simplified version of equation 1, which suffices for the purpose of this analysis and looks as follows:

σ = N 

1

L, (3)

With use equation 3, the ratio of cross-sections σCEP

σZ

µµ and some rewriting we obtain:

σCEP = σZµµ· NCEP Nall · Z→µµ,all Z→µµ,CEP · 1 f (NP V = 1) · 1 (pT < 2) (4)

We will use equation 4 to calculate the cross-section of the Z → µ+_µ− _{CEP events. The factor} 1

f (NP V=1) is due to the luminosity ratio. In figure 3 we see that a significant fraction of events

contain more than one interaction. These extra interactions are called pile-up. In our selection criteria we implicitly required that the events did not contain any pile-up interactions, since the extra interactions would leave extra tracks in the event. We still need to correct for this effect by taking the fraction of events with only a single interaction. The fraction is extracted using figure 3 and we obtain that f (NP V = 1) = 0.394. The (pT < 2) term is a correction for the efficiency of

our pT cut and is equal to 0.85 (see section 3.2). For σZµµwe use the value obtained in [8], which

is equal to 198 ± 0.9 ± 4.7 ± 7.7 pb. The first uncertainty of this result is statistical, the second systematic and the third arises from the accuracy of the luminosity determination. NCEP and Nall

are obtained from the analysed data. For Nall we use all the Z → µ+µ− events which pass the Z

values can be found in table 4. We find that NCEP = 0 and Nall = 119020. The efficiency ratio

in the equation is determined using the ratio between the total number of generated (simulated) events and observed events and is further examined in the next section.

4.2

Efficiency

The efficiency for the Z → µ+_µ−_{process is calculated using MC generated events. We look at the}

amount of generated events (NGen) versus the amount of selected events (NSel). Calculating _NNSel

Gen

gives us the efficiency. We calculated this ratio as function of pT and nSPDhits, whose plots can

be seen in figure 10 and figure 11 respectively. The CEP events are expected to be in the lower pT

and nSPDhits region. The efficiency depends on two effects. One effect is due to the kinematics of the muons, which are dependent on the kinematics of the Z. We assume that CEP and non-CEP events have the same kinematic distributions, which implies that the efficiencies are identical. Looking at figure 10, we see that the average efficiency of the muons to pass the CEP selection does not depend on the ptof the Z. We also assume that the other kinematic distributions, except

for the rapidity distribution, remain the same. However further analysis is needed to confirm this. We therefore conclude that the efficiency is independent of this effect.

0 50 100 150 200 [GeV] T p 0 500 1000 1500 2000 2500 Events / (1 GeV) 0 50 100 150 200 [GeV] T p 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Events / (1 GeV) 0 50 100 150 200 [GeV] T p 0 1 2 3 4 gen / N sel Ratio N 0 20 40 60 80 [GeV] T p 0.2 0.25 0.3 0.35 0.4 gen / N sel Ratio N

Figure 10: Plots of the pT distribution for the selected Z (top left) and generated Z (top right).

The ratio Nsel

Ngen as function of ZpT is shown in the bottom plots, where the right one is a zoomed

4 RESULTS

The efficiency can also be affected by the ’fullness’ of events. Fuller events are expected to have a lower efficiency than empty events. This is due to tracks being harder to find in ’full’ events. We check if this effect is significant by looking at the nSPDhits distribution seen in figure 11. We see that this is effect does not influence the efficiency. Since both effects do not influence the efficiency significantly, the efficiency is set at 1 ± 0.1. The error comes from our pT dependence in figure 10.

We estimate the difference in the average efficiency and our efficiency in the limit pT → 0 to be

10%. 0 200 400 600 800 1000 nSPDhits 0 20 40 60 80 100 120 140 Events / (1) 0 200 400 600 800 1000 nSPDhits 0 100 200 300 400 500 600 Events / (1) 0 200 400 600 800 1000 nSPDhits 0.5 − 0 0.5 1 1.5 2 2.5 gen / N sel Ratio N 0 50 100 nSPDhits 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 gen / N sel Ratio N

Figure 11: Plots of the nSPDhits distribution for the selected Z (top left) and generated Z (top right). The ratio Nsel

Ngen as function of nSPDhits is shown in the bottom plots, where the right

one is a zoomed in version of the left one. Note that the ratio remains constant as a function of nSPDhits.

4.3

Upper limit calculation

From table 4 we see that we obtain no selected events both in the data sample and in the Z → µµ background MC sample. We use TROLKE, a fully frequentist model to set upper and lower lim-its, to set an upper limit on σCEP. The model used is TROLKE 2 which needs 5 variables (x,

y, em, tau and sde). Here, x is the number of observed events, y is the number of events in the background region, em is the estimate of the efficiency, tau equals the ratio between observed and simulated live-time and sde is the relative uncertainty of em. This model assumes Poisson distributed background. We set em = 1 and using the uncertainties on σZ

µµand the efficiency ratio

we obtain that sde = 0.167. We further acquired x = 0 and y = 0. With these inputs we find that the value returned by TROLKE does not depend on the value of tau. We do not expect this and question if TROLKE takes the background fluctuations correctly into account. We therefore compare the value of the upper limit for both y = 0 and y = 1 case. To do this we need the value of tau. It is obtained by calculating the ratio of selected Z events between data and MC. We use the row ’Z candidates’ from table 4 to get tau = 1.675. Furthermore we need to choose a confidence level, which is set to 0.95 (or 95%). Filling in these variables in TROLKE gives us an upper limit of 2.69 on NSel for y = 0 and 2.50 for y = 1.

We are now able to set a 95% upper limit on the cross section given in equation 4. Considering the case where y = 0 leads to the following result:

σCEP < 13f b (5)

Which is an upper-limit on the cross-section of exclusive Z → µ+µ− production. Note that this result only takes into account the decay of the Z boson to two muons and is not equal to the total cross-section.

5 DISCUSSION

5

Discussion

One important aspect to note is that the LHCb detector does not cover the full fiducial range. This means the prediction of 11 fb in the introduction must be downscaled in order to compare the prediction and the result from this analysis to one another. We do this by comparing the cross-section for the total Z → µ+_µ− _{of ATLAS to the cross-section obtained by LHCb . We}

use the results from and [21] and [8], which are 1998 pb and 198.0 pb for ATLAS and LHCb respectively. Therefore we assume that the factor that we need to scale down with is about 10. The predicted cross section will thus be 1.1 fb. This assumption, however, comes with some un-certainty. For example, the used mass window is different in both analyses. Therefore, we cannot compare them directly to one another. We also assume that ’normal’ and CEP events behave in the same way and that the scaling factor is the same for both cases. One other correction we need to make on the prediction is for the decay to muons. We will use the branching fraction of 0.034 for the decay of the Z boson into two muons. We finally obtain a predicted cross-section of 0.037 fb.

Our upper-limit of 13 fb, however, can be greatly improved on. For example, the biggest flaw in this analysis is the lack of a CEP event generator. This results in multiple assumptions and relatively large uncertainties in some calculations. One assumption we made was that the rapidity distributions for the exclusive and non-exclusive events behave similarly. Figures 12a and 12b show that this is not the case. We see that in the forward region the ratio between diffractive and CEP is larger than the ratio in the central region.

(a) From [22]. (b) From [2].

Figure 12: Rapidity distribution for both the diffractive (a) and central exclusive (b) Z production in pp collisions at LHC.

A larger data set can also be analysed to further improve on this upper limit. This includes the magnet up data from 2016 (0.83/fb), but also the data collected in 2017 (3.9/fb), 2018 (3.9/fb) and run-1 (2.2/fb). A higher integrated luminosity results in a better limit. How much the limit will improve depends on the background. With no background we expect that the upper-limit scales one-on-one with the integrated luminosity. With no background there should only be one signal event for a discovery. Thus, when the integrated luminosity is a factor of ten higher, the upper-limit is a factor of ten lower. With background events the upper-upper-limit scales with the number of background events (which scale with√N ). The expected improvement will be somewhere between

1 √

A and 1

A, due to the fact that sharper cuts will probably have to be made to obtain exactly one

background event. The term A is the improvement factor of the integrated luminosity. Using all the other data sets available we have an integrated luminosity larger by a factor of 11. This means we can at least improve on the upper limit by a factor √1

11 = 0.3 when analysing all the available

6 CONCLUSION

6

Conclusion

An analysis on the LHCb 2016 magnet down data set has been done to look for CEP Z → µ+_µ−

events in pp collisions at √s = 13 TeV. Zero CEP events have been found in this dataset and therefore an upper limit of 95% has been set on the cross-section of CEP of Z → µ+_µ−_{. This}

upper limit is set to σCEP < 13 fb. This result is heavily restricted due to the lack of a CEP MC.

However this means that there is room for improvement. As figure 12 suggests, the limit will be lower if the efficiency ratio is determined properly. The result is in agreement with the prediction and is an improvement of the limit set by CDF. Under the assumption that the cross-section for BSM processes scale with the beam energy in the same way as the SM process, we conclude that we are even more sensitive.

References

[1] LHCb collaboration. “Observation of charmonium pairs produced exclusively in pp colli-sions”. In: (July 2014). doi: 10.1088/0954-3899/41/11/115002. arXiv: 1407.5973. url: http://arxiv.org/abs/1407.5973.

[2] V. P. Goncalves and M. V. T. Machado. “Diffractive photoproduction of Zˆ0 bosons in coherent interactions at CERN-LHC”. In: (Oct. 2007). doi: 10.1140/epjc/s10052-009-1009-z10.1140/epjc/s10052-008-0633-3. arXiv: 0710.4287. url: http://arxiv.org/ abs/0710.4287.

[3] CDF Collaboration and T. Aaltonen. “Search for exclusive Z boson production and obser-vation of high mass p¯p → pγγ ¯p → pl+l−p events in p¯¯ p collisions at √s = 1.96 TeV”. In: (Feb. 2009). doi: 10 . 1103 / PhysRevLett . 102 . 222002. arXiv: 0902 . 2816. url: http : //arxiv.org/abs/0902.2816.

[4] L. Motyka and G. Watt. “Exclusive photoproduction at the Tevatron and LHC within the dipole picture”. In: (May 2008). doi: 10.1103/PhysRevD.78.014023. arXiv: 0805.2113. url: http://arxiv.org/abs/0805.2113.

[5] _{Alan. R. White. “The Physics of a Sextet Quark Sector”. In: (Dec. 2004). doi: 10.1103/} physrevd.72.036007. url: https://arxiv.org/abs/hep-ph/0412062.

[6] The LHCb Collaboration. “Central exclusive production of J/ψ and ψ(2S) mesons in pp collisions at√_{s = 13 TeV”. In: (2016). url: http://inspirehep.net/record/1482815.} [7] LHCb collaboration. “Measurement of the exclusive Υ production cross-section in pp

colli-sions at√s = 7 TeV and 8 TeV”. In: Journal of High Energy Physics 2015.9 (May 2015). issn: 10298479. doi: 10.1007/JHEP09(2015)084. arXiv: 1505.08139. url: http://arxiv. org/abs/1505.08139%20http://dx.doi.org/10.1007/JHEP09(2015)084.

[8] LHCb collaboration. “Measurement of the forward Z boson production cross-section in pp collisions at √_{s = 13TeV”. In: (2016). issn: 10298479. doi: 10.1007/JHEP09(2016)136.} arXiv: 1607.06495. url: http://arxiv.org/abs/1607.06495.

[9] LHCb collaboration. “The LHCb Detector at the LHC”. In: Journal of Instrumentation 3.08 (2008), S08005. url: http://stacks.iop.org/1748-0221/3/i=08/a=S08005.

[10] Siim Tolk. “Discovery Of Rare B decays.” PhD thesis. University of Groningen, 2016, p. 216. url: https://cds.cern.ch/record/2148631.

[11] LHCb collaboration. “LHCb Reoptimized Detector Design and Performance: Technical De-sign Report”. In: CERN-LHCC-2003-030 (2003).

[12] LHCb collaboration. LHCb VELO (VErtex LOcator) : Technical Design Report. CERN, 2001. url: http://cdsweb.cern.ch/record/504321.

REFERENCES

[13] LHCb collaboration. LHCb inner tracker: Technical Design Report. Technical Design Report LHCb. Geneva: CERN, 2002. url: http://cds.cern.ch/record/582793.

[14] LHCb collaboration. LHCb outer tracker: Technical Design Report. Technical Design Report LHCb. Geneva: CERN, 2001. url: https://cds.cern.ch/record/519146.

[15] Veerle Heijne. “Search for long-lived exotic particles at LHCb”. PhD thesis. Amsterdam: Vrije Universiteit Amsterdam, 2014, p. 194. url: https://cds.cern.ch/record/2005711. [16] F. M. Brochu. “Considerations on Xi- reconstruction in LHCb”. In: (2016). arXiv: 1610.

07825. url: http://arxiv.org/abs/1610.07825.

[17] The LHCb RICH group. “Performance of the LHCb RICH detector at the LHC”. In: (Nov. 2012). doi: 10.1140/epjc/s10052-013-2431-9. arXiv: 1211.6759. url: http://arxiv. org/abs/1211.6759.

[18] A A Alves et al. “Performance of the LHCb muon system”. In: Journal of Instrumentation 8.02 (Feb. 2013), P02022–P02022. doi: 10.1088/1748- 0221/8/02/p02022. url: https: //arxiv.org/abs/1211.1346.

[19] J. Anderson et al. “Measurement of the cross-section for Z → µ+µ− production with 1 fb−1 of pp collisions at√s = 7 TeV”. LHCb-ANA-2014-070.

[20] M. Tanabashi et al. “Review of Particle Physics”. In: Phys. Rev. D 98 (3 Aug. 2018), p. 030001. doi: 10.1103/PhysRevD.98.030001. url: https://link.aps.org/doi/10. 1103/PhysRevD.98.030001.

[21] The ATLAS collaboration. “Measurement of W± and Z-boson production cross sections in pp collisions at √s = 13 TeV with the ATLAS detector”. In: Physics Letters B 759 (Aug. 2016), pp. 601–621. doi: 10.1016/J.PHYSLETB.2016.06.023. url: https://www. sciencedirect.com/science/article/pii/S0370269316302763.

[22] Krzysztof Golec-Biernat et al. “Electroweak vector boson production at the LHC as a probe of mechanisms of diffraction”. In: Phys. Rev. D84 (2011), p. 114006. doi: 10.1103/ PhysRevD.84.114006. arXiv: 1110.1825 [hep-ph].