Proton reconstruction with the CMS-TOTEM Precision Proton Spectrometer

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

TOTEM

CERN-EP-2022-139

2023/09/12

CMS-PRO-21-001

TOTEM-2022-001

Proton reconstruction with the CMS-TOTEM Precision

Proton Spectrometer

The CMS and TOTEM Collaborations*

Abstract

The Precision Proton Spectrometer (PPS) of the CMS and TOTEM experiments col-

lected 107.7 fb−1in proton-proton (pp) collisions at the LHC at 13 TeV (Run 2). This

paper describes the key features of the PPS alignment and optics calibrations, the pro-

ton reconstruction procedure, as well as the detector efﬁciency and the performance of

the PPS simulation. The reconstruction and simulation are validated using a sample

of (semi)exclusive dilepton events. The performance of PPS has proven the feasibil-

ity of continuously operating a near-beam proton spectrometer at a high luminosity

hadron collider.

Published in the Journal of Instrumentation as doi:10.1088/1748-0221/18/09/P09009.

*See appendices A and B for lists of collaboration members.

arXiv:2210.05854v2 [hep-ex] 9 Sep 2023

Contents 1

Contents

1 Introduction........................................ 2

2 TheCMSdetectorandPPS ............................... 2

3 LHC optics and proton transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4 Datasets.......................................... 5

5 Alignment......................................... 6

5.1 Alignmentﬁll .................................. 7

5.2 Physicsﬁlls.................................... 8

5.3 TimingRPs.................................... 11

6 Optics model and calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6.1 Introduction ................................... 15

6.2 Calibration of the LHC optics . . . . . . . . . . . . . . . . . . . . . . . . . 16

6.3 Optics description and uncertainty model . . . . . . . . . . . . . . . . . . 21

7 Protonreconstruction .................................. 26

8 Apertureconstraints................................... 33

9 Protonsimulation..................................... 34

10 Uncertainties ....................................... 38

11 Validation with dimuon sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

12 PPStrackingefﬁciency.................................. 46

12.1 Silicon strip detector efﬁciency . . . . . . . . . . . . . . . . . . . . . . . . . 47

12.2 Pixel detector efﬁciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

12.3 Multi-RP efﬁciency factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

13 Timing........................................... 52

14 Summary ......................................... 54

A TheCMSCollaboration ................................. 61

B TheTOTEMCollaboration ............................... 78

1 Introduction

The Precision Proton Spectrometer (PPS) detector system has been installed and integrated into

the CMS experiment [1] during Run 2 of the LHC with 13 TeV proton-proton collisions. It is a

joint project of the CMS and TOTEM [2] Collaborations and measures protons scattered at very

small angles at high instantaneous luminosity [3]. The scattered protons that remain inside the

beam pipe, displaced from the central beam orbit, can be measured by detectors placed inside

movable beam pipe insertions, called Roman pots (RP), which approach the beam within a

few mm. The PPS detectors have collected data corresponding to an integrated luminosity of

107.7 fb−1during the LHC Run 2, which occurred between 2016 and 2018.

The physics motivation behind PPS is the study of central exclusive production (CEP), i.e. the

process pp →p(∗)+X+p(∗)mediated by color-singlet exchanges (e.g. photons, Pomerons,

Z bosons), by detecting at least one of the outgoing protons. In CEP, one or both protons may

dissociate into a low-mass state (p∗); dissociated protons do not produce a signal in PPS. The X

system is produced at central rapidities, and its kinematics can be fully reconstructed from the

4-momenta of the protons, thereby giving access to standard model (SM), or beyond SM (BSM)

ﬁnal states that are otherwise difﬁcult to observe in the CMS central detectors because of the

large pileup (multiple interactions per bunch crossing) at high luminosities. CEP provides

unique sensitivity to SM processes in events with Pomeron and/or photon exchange, and BSM

physics, e.g. via searches for anomalous quartic gauge couplings, axion-like particles, and new

resonances [4–8].

This paper is organized as follows. The CMS detector and PPS are described in Section 2. The

LHC optics and the concept of proton transport is presented in Section 3, followed in Section 4

by a description of the data sets used. Sections 5 and 6 describe the detector alignment proce-

dure and the LHC optics calibration. Section 7 details the proton reconstruction with the PPS

detectors. Sections 8 and 9 document the study of LHC aperture limitations and the simulation

of the proton transport and PPS detectors, and Section 10 describes the uncertainties affecting

the proton reconstruction. A validation of the reconstruction using a (semi)exclusive dimuon

sample is presented in Section 11. The measurement of the proton reconstruction efﬁciency

is discussed in Section 12. Section 13 describes a study of the performance of the proton ver-

tex matching criteria from time-of-arrival measurements. Finally, a summary is presented in

Section 14.

2 The CMS detector and PPS

The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diam-

eter, providing a magnetic ﬁeld of 3.8 T. Within the solenoid volume are a silicon pixel and

strip tracker, a lead tungstate crystal electromagnetic calorimeter, and a brass and scintillator

hadron calorimeter, each composed of a barrel and two endcap sections. Forward calorime-

ters extend the pseudorapidity coverage provided by the barrel and endcap detectors. Muons

are measured in gas-ionization detectors embedded in the steel ﬂux-return yoke outside the

solenoid.

Events of interest are selected using a two-tiered trigger system. The ﬁrst level (L1), composed

of custom hardware processors, uses information from the calorimeters and muon detectors to

select events at a rate of around 100 kHz within a ﬁxed latency of about 4 µs [9]. The second

level, known as the high-level trigger (HLT), consists of a farm of processors running a version

of the full event reconstruction software optimized for fast processing, and reduces the event

rate to around 1 kHz before data storage [10].

Figure 1: Schematic layout of the beam line between the interaction point and the RP locations

in LHC sector 56, corresponding to the negative zdirection in the CMS coordinate system and

the outgoing proton in the clockwise beam direction. The accelerator magnets are indicated in

grey and the collimator system elements in green. The horizontal RPs, which constitute PPS,

are marked in red. The vertical RPs are indicated in dark grey; they are part of the TOTEM ex-

periment. The vertical RPs are not used during high luminosity data taking; nevertheless, they

provide PPS with a reference measurement for the calibration and alignment of the detectors.

A more detailed description of the CMS detector, together with a deﬁnition of the coordinate

system used and the relevant kinematic variables, is reported in Ref. [1].

The PPS detectors

Figure 1 shows the layout of the RP system installed at around 200–220 m from the CMS inter-

action point (LHC interaction point 5 (IP5)), along the beam line in the LHC sector between the

interaction points 5 and 6, referred to as sector 56. A symmetric set of detectors is installed in

LHC sector 45. Some RPs approach the beam vertically from the top and bottom, some hori-

zontally. During standard machine operation, scattered protons undergo a large displacement

in the horizontal direction and a small vertical displacement at the RP positions. The horizontal

RPs are hence used. The vertical RPs are used in special conﬁgurations of the machine and in

low luminosity proton-proton ﬁlls for the calibration and alignment of the detectors.

Each detector arm consists of two RPs instrumented with silicon tracking detectors that measure

the transverse displacement of the protons with respect to the beam, and one RP with timing

detectors to measure their time-of-ﬂight. The tracking RP closer to the IP5 is referred to as

“near”, the other as “far”. Silicon strip sensors with a reduced insensitive region on the edge

facing the beam were initially used [11]. Each RP housed 10 silicon strip sensor planes, half at

a+45◦angle and half at a −45◦angle with respect to the bottom of the RP. These sensors could

not sustain a large radiation dose and could not identify multiple tracks in the same event.

For this reason they have been gradually replaced by new 3D silicon pixel sensors: one RP

(in each arm) during the 2017 data-taking run and all tracking RPs in 2018 were instrumented

with 3D pixel sensors. Each such RP hosts six 3D pixel sensor planes [3]. A summary of the RP

conﬁgurations used in 2016-2018 is provided in Table 1.

The difference between the proton arrival times in the detectors on both sides of the IP5 is

used to reject background events with protons from pileup interactions, or beam-halo particles.

Timing detectors were operational in 2017 and 2018, with four detector planes hosted in a

single RP. They consisted of single- and double-sided single crystal chemical vapor deposition

(scCVD) diamond sensor planes [12]; during 2017 data taking one of the four planes consisted

of ultra-fast silicon sensors [13] instead of diamond ones.

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EUROPEANORGANIZATIONFORNUCLEARRESEARCH(CERN)CERN-EP-2022-1392023/09/12CMS-PRO-21-001TOTEM-2022-001ProtonreconstructionwiththeCMS-TOTEMPrecisionProtonSpectrometerTheCMSandTOTEMCollaborations*AbstractThePrecisionProtonSpectrometer(PPS)oftheCMSandTOTEMexperimentscol-lected107.7fb−1inproton-proton(pp)co...

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Proton reconstruction with the CMS-TOTEM Precision Proton Spectrometer

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