Experimental Particle Physics 


The Compact Muon Solenoid (CMS) experiment is a particle physics experiment at the Large Hadron Collider (LHC) at CERN, Geneva. The main scientific goals of CMS are detailed investigations of particles and interactions at a new energy regime, understanding the origin of electroweak symmetry breaking (Higgs bosons), and search for direct or indirect signatures of new physics beyond the standard model of particle physics.

The first phase of LHC operation, so-called Run 1 (2010-2012), with proton-proton collisions at 7 and 8 TeV centre-of-mass energies, provided a total integrated luminosity of about 25 fb-1, and culminated in the discovery of a Higgs boson with a mass of about 125 GeV. The total number of papers submitted for publication by CMS on Run 1 collision data reached 462 by the end of 2015.

After the service break of the accelerator and the experiments in 2013-2014 the LHC was restarted in spring 2015, with a record centre-of-mass collision energy of 13 TeV. Four papers were submitted for publication with results obtained from the new Run 2 data. In addition, a number of preliminary results were produced. In particular, a small excess in the diphoton mass region of 750-760 GeV, which was seen in both ATLAS and CMS experiments, raised a lot of public attention. The signals are, however, not statistically significant and more data are needed to clarify the situation.

The CMS physics results are available at the public CMS physics results page. Researchers in University of Helsinki and Helsinki Institute of Physics (HIP) contributed in particular to Higgs analyses, to jet physics, and to B-physics analyses.

The final results on the search for charged Higgs bosons with Run 1 data were completed and accepted for publication in JHEP1 . The HIP Higgs group played a leading role in it by analyzing the most sensitive decay channel, H+ → τν in τ+jets final state, and by statistically combining the results for different decay channels. The world's most stringent limits on H+ production were set for mH+ < 400 GeV.

M. Voutilainen continued as the co-convener of the CMS Jets and Missing Transverse Energy Physics Object Group in 2015. MSc. (Tech.) Juska Pekkanen was awarded the 2015 Fundamental Physics Special Recognition Award by the CMS spokepersons Joseph Incandela and Tiziano Camporesi. The special recognition was due to his appreciated efforts in jet calibration studies and his major contributions to the dijet resonance search result with 13 TeV proton-proton collisions, the first public new physics search with the Run 2 data2.

 In the B physics sector, researchers in Helsinki were involved in the measurement of the weak mixing phase φs and effective lifetime analyses of the decay channel B0s→ J/Ψφ during 2015. The final measurement of the weak phase, comparable in accuracy with the previous world average, was submitted for publication in PLB3. P. Eerola was a member of the B-physics Publication Committee in 2014. She also continued as a member of the CMS Management Board, representing the small CERN member states.

1V. Khachatryan et al., the CMS collaboration, Search for a charged Higgs boson in pp collisions at √s = 8 TeV. arXiv:1508.07774 [hep-ex]; accepted for publication in JHEP.

2V. Khachatryan et al., Search for narrow resonances decaying to dijets in proton-proton collisions at √s = 13 TeV, arXiv:1512.01224 [hep-ex]; accepted for publication in PRL.

3V. Khachatryan et al., the CMS collaboration, Measurement of the CP-violating weak phase fs and the decay width difference DGs using the Bs to J/yf(1020) decay channel in pp collisions at √s = 8 TeV. arXiv:1507.07527 [hep-ex], submitted for publication in PLB.


Figure CMS1. Display of the event with the highest dijet mass (6.14 TeV) found in the analysis of 13 TeV LHC data. The transverse view with respect to the beam axis is shown. The kinematic quantities of the two wide jets are reported.

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Figure CMS2. The CMS Control room P5 during the first 13 TeV pilot collisions.


The TOTEM experiment at the Large Hadron Collider (LHC) is currently the leading forward physics experiment. Based on the Run I (2010-12) data, TOTEM has published several important physics results, especially the total proton-proton cross section measurements at √s = 7 and 8 TeV as well as the differential cross-section for elastic proton-proton scattering over a wide t-range. UH and HIP have had a key role in building and operating the GEM detector based T2 telescope, which has contributed significantly to the TOTEM measurements of the inelastic rate, the forward charged multiplicity and the cross-section of several diffractive processes.

In 2015, TOTEM published evidence that the |t| differential cross-section of elastic scattering deviates significantly from a pure exponentialrefTOTEM1, when ignoring electromagnetic interactions, in the so-called diffractive cone region, where hadronic interactions are expected to dominate, see Fig. TOTEM1. In a subsequent paperrefTOTEM2 using very low |t| data, TOTEM shows that the differential elastic cross-section indeed is non-purely exponential even when taking electromagnetic-hadronic interference effects into account and thus cannot be described by only one exchange process. At the same time, the ρ parameter was determined directly for the first time at the LHC, see Fig. TOTEM2. Also a new precise total cross-section measurement at √s = 8 TeV was presented, where for the first time at LHC no external input is used and electromagnetic-hadronic interference effects was explicitly taken into account.

In 2015, LHC Run II started at a new record collision energy, 13 TeV. In October, CMS and TOTEM successful took data together in special high β⋅ run with significantly increased luminosity (collected in total ~0.4 pb-1) with a special trigger dedicated for exclusive low mass resonance production. This allows to increase the statistics by about a factor 500 for the study of glueball candidates. In addition, this enabled to collect an unprecedented statistics (> 109) of elastic scattering events and thus allowing to measure the |t| differential cross-section up to 3.5 GeV2. Also a dedicated fill with the Roman Pots closer to the beam was taken to be able to measure the total proton-proton cross-section at 13 TeV.

The UH group is focusing on measuring inelastic and diffractive processes. The analysis of the single diffractive cross-section is well advanced and a measurement based on common CMS-TOTEM data classifying the inelastic events to non-diffractive, single and double diffractive using multivariate approaches has entered the first review stage. The UH group also actively participated in the TOTEM upgrade developing precise proton time-of-flight measurement using diamond sensors, see Fig. TOTEM3. The first diamond module was installed in November 2015, demonstrating the required time resolution in-situ. Three further modules will be built and installed during the first half of 2016 to be ready for a possible special run in the second half of 2016.

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Figure TOTEM1: The relative differential elastic cross-section at 8 TeV compared to a purely exponential reference distribution. The black dots represent data with statistical uncertainty bars. The yellow (hatched) band corresponds to the full systematic uncertainty (except the normalization). The purely exponential (Nb = 1) is excluded by more than 7σ, whereas quartic (Nb = 2) and cubic (Nb = 3) exponential in |t| fits the data well.



Figure TOTEM2: Energy dependence of the ρ parameter. The blue (green) triangles correspond to proton-(anti)proton measurements obtained from the Particle Data Group. The filled (hollow) red circle shows the (in)direct determination by TOTEM at LHC. The black curve gives the preferred proton-proton model by COMPETE [J. R. Cudell et al., Phys. Rev. Lett. 89 (2002) 201801].

Probing space-time in hadron interactions at the LHC

The Helsinki forward physics group concentrates on studies of the space-time structure of high energy hadron collisions. The ALICE experiment at CERN provides ideal framework for these studies based on the set of central and forward detectors with their excellent tracking, particle identification, rapidity and transverse momentum coverage. Moreover, during the normal high luminosity proton-proton runs at the LHC, ALICE can continue collecting precious forward physics data due to its special optics arrangement while the larger general purpose experiments, ATLAS and CMS, have to cope with large amounts of simultaneous collisions during the same bunch cross-overs (pile-up).

On the 17th of August 2015, the ALICE experiment published a precise measurement of the difference between ratios of the mass and electric charge of light nuclei and antinuclei. The result, published in Nature Physics, confirms a fundamental symmetry of nature to an unprecedented precision for light nuclei. The measurements are based on the ALICE experiment’s abilities to track and identify particles produced in high-energy heavy-ion collisions at the LHC. The new result, which comes exactly 50 years after the discovery of the antideuteron at CERN and in the US, improves on existing measurements by a factor of 10-100.

The group has actively developed novel particle detection techniques for the benefit of forward physics studies. Composite Scintillation Material (CSM) technologies were used to facilitate position sensitive minimum ionizing particle (MIP) detection in AD scintillators. In order to determine plastic scintillators modules (200x200x25 mm3 Saint Gobain BC-404 based elements) response on MIP interaction event at different points of scintillator the CSM based position sensitive detector was proposed (figure 2). Detector configuration includes CSM element that directly coupled, through a light concentrator to photosensor.

MoEDAL Experiment at the LHC -
looking for magnetic monopoles and dark matter particles at the LHC

The seventh LHC experiment: The Monopole and Exotics Detector at the LHC (MOEDAL) has begun the analysis of its first data collected during the year 2015. The prime motivation of MOEDAL is to search directly for the magnetic monopole – a hypothetical particle with a magnetic charge. The Helsinki group in MoEDAL concentrates on the potential production processes, Beyond the Standard Model (BSM), of exotic particles, including central exclusive production investigated in ALICE at lower energies.

The technical contributions of the Helsinki group in MoEDAL are based on the usage of the optical scanning facility at the Detector Laboratory. In 2016, a feasibility analysis of scanning MoEDAL's plastic Nuclear Track Detector (NTDs) elements is foreseen.


Figure Moedal: The layout of the MoEDAL experiment depicting an array of plastic Nuclear Track Detectors (NTDs) deployed around the ( Point-8 ) intersection region of the LHCb detector, in the VELO (VErtex LOcator) cavern.

CDF EXPERIMENT AT THE FERMILAB TEVATRON - analyzing 2 TeV proton-antiproton collisions

The Helsinki group has continued the physics analysis of the 2 TeV proton-antiproton data collected at the Fermilab Tevatron by Fall 2012. In many respects, this analysis has paved the way to the analysis approaches now used at the CERN LHC.

The Tevatron experiments, CDF and DZero, found evidence for a Higgs boson in 2012, looking at events in which two bottom-flavoured jets recoiled from a vector boson - either Z or a W. The challenge is to see to what extent the observed Higgs boson candidates follow the Standard model predictions, and whether the Higgses observed at the Tevatron represent a mixture of the same states that were in 2013 observed at the LHC.

In 2015, the CDF and DZero experiments joined combined their results, using the same techniques used in the SM Higgs search combinations. The signal strength of exotic Higgs bosons in the JP = 0- and 2+ states is no more than 0.36 times that predicted for the SM Higgs boson. Given a choice between the SM Higgs boson, which has JP = 0+, and one of the two exotic models replacing it with the same signal strength, the Tevatron data disfavours the exotic models with a significance of 5.0 standard deviations for 0- and 4.9 standard deviations for 2+.


The global CLIC collaboration hosted by CERN studies the Compact Linear Collider (CLIC) as an option for a future electron-positron linear collider for the post-Large Hadron Collider era. UH and HIP plays a leading role in the development of a physics model for the breakdown phenomena in the CLIC Accelerating Structures (AS) that are limiting the accelerating gradient. HIP and UH are also responsible for the assembly and integration of the prototype modules (about 2 m long repetitive parts of CLIC) including all necessary components required for operation, see Fig. CLIC1. In addition, UH develops a fast high precision manometer to measure outgassing in the CLIC AS during operation. Furthermore, UH develops a method to measure the internal shape of the CLIC AS with μm precision after the assembly of the disk stack based on Fourier Domain Short Coherence Interferometry (FDSCI). After obtaining a calibration for the FDSCI meterrefCLIC1, the method has been validated on a CLIC-like high precision machined Oxygen-Free Electronic (OFE) copper disk, see Fig CLIC2.

refCLIC1 R. Montonen, I. Kassamakov, E. Hæggström and K. Österberg, Calibration of Fourier domain short coherence interferometer for absolute distance measurement, Appl. Opt. 54 (2015) 4635.


Figure CLIC1: The first complete CLIC prototype module installed in the CLIC Test Facility 3 (CTF3) for tests with drive and probe beams to validate its functionality.


Figure CLIC2: Validation of the FDSCI method to measure the shape of high precision machined OFE copper used for the assembly of CLIC RF structures. A) Photograph of OFE Cu step sample, the dashed white lines indicate the path of the FDSCI scanning. B) A FDSCI step profile. The step heights determined in yellow boxed regions. The upper (lower) step heights correspond to the scan from right to left (left to right). C) 3D profile of the Cu step sample obtained with the FDSCI meter.