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The TOTEM experiment

Total cross section, elastic scattering and diffraction dissociation measurement at the LHC

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Physics at TOTEM

Reconstructed tracks in T2 Telescope

Elastic candidate events in Roman Pots

TOTEM's potential resides in making some unique observations. In addition to the precise measurement of the proton-proton interaction cross-section, TOTEM's physics programme will focus on the in-depth study of the proton's structure by looking at elastic scattering over a large range of momentum transfer. Many details of the processes that are closely linked to proton structure and low-energy QCD remain poorly understood, so TOTEM will investigate a comprehensive menu of diffractive processes - the latter partly in co-operation with the CMS experiment, which is located at the same interaction point (IP5) on the LHC.

Early measurements at CERN's Intersecting Storage Rings (ISR) revealed that the proton-proton interaction probability increases with collider energy. However, the nature of the correct growth with energy remains a delicate and unresolved issue. A precise measurement of the total cross-section at the world's highest-energy collider should discriminate between the different theoretical models that describe the energy dependence. The value of the total cross-section at LHC energies is also important for the interpretation of cosmic-ray air showers. All of the LHC experiments will use TOTEM's measurement to calibrate their luminosity monitors, in order to calculate the probability of measuring rare events.

The TOTEM experiment

Total cross section, elastic scattering and diffraction dissociation measurement at the LHC

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Timeline in the TOTEM experiment

• Track multiplicities published

OCTOBER 2011

• First results published on total cross-section

SEPTEMBER 2011

• First measurement of the total cross-section
• T2 reconstruct tracks during the first LHC run of the year.
• Collisions are seen by Roman Pots as well.

December 2009

• Installation in the tunnel and commissioning of the stations of Roman Pots at 220m distance from the interaction point IP5.

October 2009

• Installation in the CMS cavern and commissioning of T2 (both telescopes).
• Assembly and test of one complete half (i.e. one telescope) of T1.

November 2008

• Roman Pots fully equipped with silicon detectors.
• Roman Pots tested with a beam coming from the PS in the test-beam area.
• T2 GEM chambers completely tested with a test-beam.

February 2008

• The 10 edgeless silicon detectors forming the heart of the experiment are assembled for final testing before they are installed in the special vacuum vessels called 'Roman Pots' at IP5 interaction point.

October 2007

• The Roman pots are installed in the LHC tunnel. The detectors housed in these eight specially designed vacuum chambers are connected to the beam pipes in the LHC, close to the collision point of the CMS experiment.
• The Roman Pots are assembled. Named after  the CERN group of Italian physicists from Rome, who invented these tools for use at the CERN's ISR (Intersecting Storage Rings, the world's first high-energy proton–proton collider in  the early 1970s to study similar physics), the Roman Pots will be located very close to the beam pipe  to capture particles that scatter at very small angles.
• The Gas Electron Multiplier (GEM) chambers are arranged in their  intended configuration for the first time. Of almost semicircular  shape, the GEMs will measure the forward particle flow in an annular region around the beam pipe. To avoid efficiency loss, the angular  coverage of each half plane is more than 180°.

November 2005

• The first prototype of Roman Pots is ready for testing. Since the experiment will detect particles produced very close to the LHC beams, it's detectors have to be housed in these specially designed vacuum chambers.
• Proposed in 1997, TOTEM is officially approved as the fifth LHC  experiment, studying forward particle production to focus on physics  not easily accessible to the general-purpose experiments. Among a range of studies it will determine the size of the proton by high  precision measurements of proton-proton interactions and scattering. Only one month later, the collaboration presents the first  prototype of its edgeless silicon detector.

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About TOTEM

Documentation About

TOTEM experiment: its journey and discovery

The TOTEM experiment is designed to explore the proton interactions when they survive intact after the collisions. On this respect, special detectors have been placed far from the interaction points to detect a small angle detection, due by elastic and inelastic interactions. This is why this physics is known as the 'forward' physics that inaccessible by the other LHC experiments.

As CERN's 'longest' experiment, TOTEM detectors were spread across almost half a kilometre around the CMS interaction point. TOTEM had almost 3,000 kg of equipment, including four particle 'telescopes' as well as 26 'Roman pot' detectors.

The 'telescopes' – T1 and T2 – were cathode-strip chambers and Gas Electron Multipliers (GEM) to track the particles emerging from collisions at the CMS interaction point. Meanwhile, 'Roman Pots', named for their shape and first use by physicists from Rome in the 1970s, were performing measurements of scattered protons.

TOTEM data have been used to measure precisely the proton-proton cross section at different energies, from 900 GeV up to 13,6 TeV, either total, elastic and inelastic, giving also evidence of the existence of a gluon compound, called Odderon. The existence of such type of gluon compound is hidden in the QCD theory but was never seen before.

With the forward telescopes TOTEM measured the track density at small angles giving a calibration for all the ultra-high energy cosmic rays experiments.

Nowadays, TOTEM experiment has finished its measurement period and part of the hardware has been inherited by the CMS experiment and became the PPS (Protons Precision Spectrometer) forward detectors. In the transition period, TOTEM contributed in searches for beyond the standard model physics, such as anomalous quartic coupling with photons and dark matter.

The open data are released under the Creative Commons CC0 waiver . Neither TOTEM nor CERN endorse any works, scientific or otherwise, produced using these data.

All released data samples will have a unique DOI that you are requested to cite in any applications or publications.

CERN Accelerating science

De-squeeze the beams: TOTEM and ATLAS/ALFA

A special proton-proton run with larger beam sizes at the interaction point is intended to probe the p-p elastic scattering regime at small angles

19 September, 2016

By Stefania Pandolfi

Nicola Turini, deputy spokesperson for TOTEM, in front of one of the experiment’s ‘Roman Pot’ detectors in the LHC tunnel. (Image: Maximilien Brice/CERN)

Usually, the motto of the LHC is “maximum luminosity”. But for a few days per year, the LHC ignores its motto to run at very low luminosity for the forward experiments. This week, the LHC will provide the TOTEM and ATLAS/ALFA experiments with data for a broad physics programme.

The  TOTEM experiment  at Point 5 and the  ATLAS/ALFA  experiment at Point 1 study the elastic scattering of protons, which are not observable in normal operation runs. In the elastic scattering process, the two protons survive their encounter intact and only change directions by exchanging momentum.

To allow this special run, the operators play with the so-called beta-star parameter. The higher the beta star, the more de-squeezed the beams are, and the more parallel the beams are when they arrive at the interaction point. For this special run, the beta-star had to be raised to 2.5 km (whereas in normal runs it is as small as 0.4 m).

Running with such a high beta-star parameter is an achievement: during Run 1, at 8 TeV, a value of 1 km was reached. But with a higher energy, the two incoming protons deviate by smaller angles, for equal transferred momentum. Since the TOTEM and ATLAS/ALFA  Roman Pot  detectors cannot be moved closer to the beams, the beta-star parameter must be raised to even higher values to provide acceptance for the smaller angles. “The effort that is required for the machine to deliver beams with such a high value of the beta-star parameter is extremely challenging,” says Simone Giani, spokesperson of the TOTEM Collaboration. “We are very thankful to the LHC team for having pushed the machine to such extreme settings,” adds Karlheinz Hiller, ALFA project leader.

The TOTEM physics programme foreseen for this special high beta-star run features many interesting measurements. In addition to the precise determination of the  total proton-proton interaction probability  (closely related to the “cross-section”) at 13 TeV, TOTEM will focus on a detailed study of the region of low transferred momentum of the elastic scattering, that is, when the two protons barely interact and the scattering angles are very small.

An in-depth study of this region is important for many different reasons. First of all, the interaction probability seems to diverge for very small transferred momenta, but as this should not be physically possible, a detailed study of that region will shed light on what is happening when the two protons almost don’t interact.

Secondly, in the same region, the contribution of the electromagnetic interaction (“Coulomb” scattering) interferes with the nuclear part of the elastic interaction. Studying this interference zone can shed light on the internal structure of the protons, and on which part of the protons (either the peripheral or the inner part) is actually responsible for the elastic scattering process.

Moreover, it is also possible to get information on the probability that two protons pass through each other without interfering, transparently. “This might appear awkward if you think of a proton as a billiard ball,” notes Simone Giani. “But the protons should be thought as multi-body quantum systems. ”

In other words, to use a metaphor, one can imagine the two scattering protons as two large “galaxies” (made internally of tiny moving particles) launched at high speed against each other: there is a finite probability that the two “galaxies” will pass through each other without the inner particles interacting significantly.

Finally, the TOTEM collaboration plans to conduct physics studies looking for evidence of special states formed by three gluons, which are theoretically predicted but for which the experimental evidence is still weak.

The physics goal of the ATLAS/ALFA experiment is also to perform a precision measurement of the proton-proton total cross section, but then to use this to determine the absolute LHC luminosity at Point 1 for the 2.5 km run.

For ATLAS/ALFA, the interesting part of the spectrum is at low values of transferred momentum, where Coulomb scattering is dominant: since the Coulomb scattering cross-section is theoretically known, its measurement gives an independent estimate of the absolute luminosity of the LHC. The luminosity measurements are otherwise normally done via Van der Meer scans, during the standard high-luminosity runs.

“With good statistics – such as 10 million good elastic events – we hope to be able to measure the absolute luminosity with a 3% precision,” says Patrick Fassnacht, deputy project leader of the ATLAS/ALFA project.

The latest results published by the TOTEM Collaboration include a first observation of deviations from a pure exponential form of the elastic cross-section at 8 TeV. More information on the  TOTEM website .

The latest result published by the ATLAS/ALFA Collaboration is the measurement of the total cross section from proton-proton elastic scattering at 7 and 8 TeV with the beta-star parameter at 90 m, whereas the data with beta-star at 1 km are still under analysis. More information on the  ATLAS/ALFA website .

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