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.

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Figure 1. a) The 68 % CL confidence regions for the signal strength σ/σSM versus the mass of the boson for the H → gg and H → ZZ final states, and their combination. The symbol σ/σSM denotes the production cross section times the relevant branching fractions, relative to the standard model expectation. Figure 1. b) Values of the best-fit σ/σSM for the overall combined analysis (solid vertical line) and separate combinations grouped by production mode tag, predominant decay mode, or both.

The LHC accelerator and the LHC experiments had a service break in 2013-2014, during which the equipment was maintained and repaired or replaced. The LHC Run 2 will start in spring 2015. The collision centre-of-mass energy will be raised to 13 TeV, and the luminosity will be further increased.

Physicists in CMS were nevertheless very busy during 2014 in completing the analyses of the Run I data.  The total number of papers submitted for publication by CMS on collision data reached 360 by the end of the year. The public CMS physics results are available at the CMS physics results twiki-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 Higgs boson properties were measured precisely1, and all measured properties were found to be consistent with the standard model predictions (Figure 1). New model-independent upper limits were established for the charged Higgs production cross sections times branching fraction in the charged Higgs mass range 80 to 160 GeV and 180 to 600 GeV2. HIP researchers were responsible for the charged Higgs results.

M. Voutilainen was the co-convener of the CMS Jets and Missing Transverse Energy Physics Object Group in 2014, and made significant contributions to jet energy calibrations, which are crucial for example to world-leading measurements of the top quark mass. Voutilainen and his students were responsible for a new inclusive jet cross section measurement at 7 TeV proton-proton collisions3. In addition they were involved in the preliminary jet cross section measurements at 8 TeV4 and 2.76 TeV.

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Figure 2. a) The The CMS ΔΓs and Φs measurement with 68%, 90% and 95% C.L. contours, together with the standard model expectation, and overlaid with the Heavy Flavor Averaging Group Spring 2014 combination. Errors are statistical only. Figure 2. b) Weighted distribution of the dimuon invariant mass. Superimposed on the data points in black are the combined fit (solid blue) and its components: the Bs (yellow shaded) and Bd (light-blue shaded) signal components; the combinatorial background (dash-dotted green); the sum of the semileptonic backgrounds (dotted salmon); and the peaking or both.

 In B physics, researchers in Helsinki produced the first CMS results on the weak CP  violation phase phis. The preliminary results5, comparable in accuracy with the previous world average (see Fig. 2 a), were presented at the ICHEP2014 conference in July 2014. CMS and LHCb combined their results on the rare decays Bs → μμ and Bd → μμ6 , yielding a 6.2 standard deviation observation of the Bs decay and a 3.2 standard deviation excess in the search for the Bd decay (see Fig. 2 b). 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, Precise determination of the mass of the Higgs boson and tests of compatibility of its couplings with the standard model predictions using proton collisions at 7 and 8 TeV. arXiv:1412.8662 ; HIG-14-009; submitted to Eur. Phys. J. C.; https://cds.cern.ch/record/1979247?ln=en

2V. Khachatryan et al., the CMS collaboration, Search for charged Higgs bosons with the H+ to tau nu decay channel in the fully hadronic final state at sqrt s = 8 TeV. CMS Physics Analysis Summary HIG-14-020; http://cds.cern.ch/record/1950346?ln=en; https://twiki.cern.ch/twiki/bin/view/CMSPublic/Hig14020TWiki

3V. Khachatryan et al., the CMS collaboration, Measurement of the ratio of inclusive jet cross sections using the anti-kt algorithm with radius parameters R = 0.5 and 0.7 in pp collisions at sqrt(s) = 7 TeV. arxiv:1406.0324; Phys. Rev. D 90 (2014) 072006;

4 V. Khachatryan et al., the CMS collaboration, Measurement of the double-differential inclusive jet cross section at sqrt(s) = 8 TeV with the CMS detector. CMS Physics Analysis Summary SMP-12-012; https://cdsweb.cern.ch/record/1547589; https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSMP12012

5 V. Khachatryan et al., the CMS collaboration, Measurement of the CP-violating weak phase phis and the decay width difference DeltaGammas using the Bs to J/psiphi(1020) decay channel. CMS Physics Analysis Summary BPH-13-012; http://cds.cern.ch/record/1744869?ln=enhttps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsBPH13012

6 V. Khachatryan et al., the CMS and LHCb collaborations, Observation of the rare Bs to μ+μ- decay from the combined analysis of CMS and LHCb data. arXiv:1411.4413 ; BPH-13-007; submitted to Nature; https://cds.cern.ch/record/1970675?ln=en;


Since 2011, the TOTEM experiment has published several important physics results, especially the first total proton-proton cross section measurement at LHC and the differential cross-section for elastic proton-proton scattering over a wide t-range. UH and HIP has had a key role in constructing 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 2014, TOTEM showed for the first time in proton-proton collisions (previously only shown in neutron-proton collisions) that the differential cross-section of elastic scattering deviates significantly from a pure exponential in the so-called diffractive cone region where nuclear interactions is expected to dominate, see Fig. 3. In addition, CMS and TOTEM published in 2014 their first joint publication: the measurement of the charged particle pseudorapidity distributions in proton–proton collisions at 8 TeV in the largest pseudorapidity range ever at a collider experiment (Eur. Phys. J. C74 (2014) 10, 3053, link: 10.1140/epjc/s10052-014-3053-6, see Fig. 4). The data was compared to several event generators and none of considered ones provided a consistent description of the measured distributions. The UH group focused on the measurements of inelastic and diffractive processes. The measurements of the single diffractive cross-sections are well advanced and an inelastic event classification analysis based on multivariate approaches is also in progress. In 2014, the UH group also launched an activity to develop silicon based precision timing sensors for the common CMS-TOTEM proton precision spectrometer upgrade. 

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Figure 3. The relative differential cross-section of elastic proton-proton scattering at 8 TeV compared to a purely exponential reference distribution. The data excludes a purely exponential distribution (Nb = 1, red curve) at more than 7σ.




Figure 4:Charged-particle pseudorapidity distributions in 8 TeV proton-proton collisions for an inclusive inelastic event sample (left), a non-single diffraction (NSD)-enhanced event sample (centre), and a single diffraction (SD)-enhanced event sample (right). The error bars represent the uncorrelated systematics between neighbouring bins and the bands show the combined systematic and statistical uncertainties. The data is compared to predictions of several event generators.


During Spring 2014, the Helsinki HIP-UH Forward physics group initiated analysis of diffractive proton-proton collision data collected by the ALICE experiment in Run 1 of the CERN LHC. Supported by the ALICE diffractive physics team, the group concentrates on the analysis of space-time structure of hadronic collisions in extreme kinematic configurations. To accomplish this, an upgrade program of the base line ALICE experiment was launched during the first long maintenance period of the LHC collider (LSS1). As the first step, the small angle coverage of the experiment was improved by adding 8+8 ADA/ADC detectors at ±20 meters on both sides of the proton-proton collision point IP2 (Figure 5).

After systematic beam tests at the CERN PS in October 2014, the new detectors, with their advanced optical read-out were installed in their final locations just before Christmas 2014. As the next step, the group now prepares for a further forward detector upgrade based on a novel approach to leading proton detection at ±110 meters around IP2.

Equipped with the present upgrade ADA/ADC forward spectrometers, ALICE will have unprecedented physics opportunities by efficiently triggering and analyzing hadronic interactions in kinematical configurations that are presently poorly understood. The newly installed forward ADA/ADC detectors enable the smallest diffractive masses to be detected, and allow detailed studies of matter in the region of extremely small relative quark/gluon momenta, xBj, xBj larger than 10-8.

AD module with two planes

Figure 5: A CAD drawing of an AD module with two planes of 4+4 detector quadrants assembled together with the wavelength shifter bars. The read-out is facilitated by optical fibers connected to the ends of the wavelength shifters indicated by purple coloring.

Low mass central exclusive process acts allows meson states with the selected ‘vacuum’ quantum numbers to be investigated (Figure 6). At higher central masses, the experiment turns the LHC into a gluon collider with a high yield of clean gluon jets for the benefit of detailed QCD studies.

Due to the special machine optics conditions at the ALICE interaction point, the experiment is able to continuously collect data during the nominal LHC pp/pA running periods.

For improved analysis of the forward physics processes, the Helsinki group continued to develop advanced multivariate techniques for soft classification of diffractive events to be used for the analysis of ALICE Run 1 and forthcoming Run 2 data. As part of the physics analysis preparations, the group installed and tested the DIME Monte Carlo package into ALICE analysis framework


Figure 6: The Central Exclusive Process (CEP) acts as a 'gluon filter' to enhance production of vacuum-like spin-parity states JPC = 0++, 2++ and gluon jets. Preliminary ALICE measurement of the low-mass p+p- system exhibiting enhanced production of f0(980) and f2(1270) resonances (red circles) as compared to the background (black).


The LHC is opening up a new energy regime for novel physics searches beyond the Standard Model. The search strategy for exotics - planned for the main LHC experiments - can be extended by dedicated experiments designed to complement and enhance the physics reach of the present base line detector designs. The CERN MoEDAL (Monopole and Exotics Detector at the LHC) project is such an experiment. The prime motivation of MoEDAL is to directly search for the Magnetic Monopole or Dyon and other highly ionizing Stable (or pseudo-stable) Massive Particles (SMPs) at the LHC7.

7 The Physics Programme of The MoEDAL Experiment at the LHC, MoEDAL Collaboration (B. Acharya et al.), May 29, 2014, Published in Int.J.Mod.Phys.A29(2014)1430050.

The Helsinki group plans to contribute in the MoEDAL experiment by using the special optical scanning techniques developed at the Helsinki Detector Laboratory8 for the search of highly ionizing particles in the emulsion sheets.

8 Timo Hildén, Quality Assurance of the Gas Electron Multiplier Detectors, A PhD thesis to be presented for public examnation in Spring 2015, Helsinki.


The global CTF3/CLIC collaboration hosted by CERN studies the Compact Linear Collider (CLIC) as an option for a future electron-positron linear collider for the post-LHC era. The University of Helsinki plays a leading role in the collaboration of the development of a physics model for the phenomena that causes breakdowns in the The global CTF3/CLIC collaboration hosted by CERN studies the Compact Linear Collider (CLIC) as an option for a future electron-positron linear collider for the post-LHC era. The University of Helsinki plays a leading role in the collaboration of the development of a physics model for the phenomena that causes breakdowns in the


Figure 7: Schematic (left) and photograph (centre) of setup for the verification of the micron precision of Fourier Domain Short Coherence Interferometry (FDSCI). Right: the measured FDSCI measurement bias (e) and the 2σ uncertainty for samples with different thicknesses. Inset: measured optical thickness (OTM) versus calibrated optical thickness (OTC).


For more than a decade, the Helsinki group has participated in a frontier high energy collider experiment, the CDF experiment at the Fermilab 2 TeV antiproton-proton collider. The Tevatron collider paved the way for the LHC physics programs, and still provides important complementary physics input to discovery physics such as the Higgs studies. As an example, the CDF analysis on Higgs decays into b-bbar pairs still provides superior sensitivity compared to the LHC experiments.

In addition to the highly popular Higgs studies, there are a number of physics analysis achievements with lesser publicity – still these represent significant new observations in high energy physics. The Helsinki CDF project has produced a record number of high impact Physical Review D and Physics Letters publications from the year 2012 on. In addition, a number of PhD and MSc theses were produced within the Helsinki group on Higgs and top quark studies. In 2012, the exclusive photon-photon process in high energy hadron collisions was observed for the first time and analyzed by the Helsinki group.