CMS closes major chapter of Higgs measurements

The data reveal that the particle discovered at CERN continues to behave just like the Standard Model predicts

Since the discovery of a Higgs boson by the CMS and ATLAS Collaborations in 2012, physicists at the LHC have been making intense efforts to measure this new particle’s properties. The Standard Model Higgs boson is the particle associated with an all-pervading field that is believed to impart mass to fundamental particles via the Brout-Englert-Higgs mechanism. Awaited for decades, the 2012 observation was a historical milestone for the LHC and led to the award of the 2013 Nobel Prize in Physics to Peter Higgs and François Englert. An open question arising from the discovery is whether the new particle is the one of the Standard Model -- or a different one, perhaps just one of many types of Higgs bosons waiting to be found. Since the particle’s discovery, physicists at the LHC have been making intense efforts to answer this question.

 

This week, at the 37th International Conference on High Energy Physics , a bi-annual major stage for particle physics, which in 2014 is held in Valencia, Spain, the CMS Collaboration is presenting a broad set of results from new studies of the Higgs boson. The new results are based on the full Run 1 data from pp collisions at centre-of-mass energies of 7 and 8 TeV. The analysis includes the final calibration and alignment constants and contains about 25 fb^-1 of data.

 

Decay to two photons

The Higgs boson is an ephemeral particle. It decays into pairs of lighter particles almost immediately after it is produced in LHC collisions. One such “decay channel” is the one in which the Higgs transforms into two photons. The latest CMS results in this decay channel show a peak in the data with a significance of 5σ, corresponding to a probability of less than one in 3,000,000 that the peak is caused by random fluctuations. Figure 1 shows the clear signal of the Higgs over the background in the data. CMS has also measured the mass of the Higgs boson with a precision of a few parts per thousand, with the systematic uncertainty of the measurement four times smaller than the previous preliminary value.

 

The precision of the new mass measurement - a few parts in a thousand - testifies to the inspired design and meticulous construction of the CMS detector, its efficient operation and calibration throughout Run 1 of the LHC, and the tireless efforts of the analysis teams in understanding all aspects of the detector performance.

 

 

Figure 1 - Combined diphoton mass spectrum illustrating the significance of the observed excess, where events are weighted by the expected signal-to-background ratio. The corresponding background-subtracted distribution is shown in the lower panel.

 

Combining decay channels, production modes

 

The two-photon analysis completes the set of Run 1 measurements with final calibration and alignment, covering the five primary decay modes of the Higgs boson [2,3,4,5]. This paves the way for a preliminary combination of all the decay channels observed thus far [6], to extract the maximum possible information on the properties of the new boson, including its couplings to the fundamental particles. The combined best-fit ratio of the signal strength observed to that expected in the standard model, is found to be 1.00 ±0.13, in square agreement with state-of-the-art standard model calculations. Furthermore, when the data are dissected into the separate production and decay properties of the Higgs boson, no significant deviations from the expectations for the standard model are found (Fig. 2). In addition to the coupling results, the preliminary combination includes a combined measurement of the Higgs boson mass from the two-photon and ZZ→4ℓ channels: m_H = 125.03 ± 0.30 GeV. Taken together, the results represent an impressive tour de force, the culmination of four years of painstaking effort that began with the first CMS searches for the Higgs boson in 2010.

 

 

Figure 2 - Compatibility of Higgs boson couplings to vector bosons (V) and fermions (f) with the standard model expectation (diamond). The allowed regions by each group of decay modes analyzed (colored) combine into a much smaller region (grey) that is fully compatible with the standard model.

 

Finally, the spin structure of the Higgs boson has been probed with unprecedented detail in a new set of CMS results searching for anomalous couplings to vector bosons. If the new particle is indeed a Higgs boson it should be a scalar, a particle with zero spin and positive parity. The analyses include separate investigations of the WW→2ℓ2ν [7] and ZZ→4ℓ [8] decay channels to test alternative spin-parity assignments against the expected scalar nature of the standard model Higgs boson. For the first time, the possibility that the particle is an admixture of different parity states is also investigated. Results are combined for the two channels and all alternative hypotheses studied are found to be significantly disfavored with respect to the standard model hypothesis.

 

Along with the recent CMS publication in Nature Physics demonstrating strong evidence for the Higgs boson decay to fermions [9], the new results presented in Valencia provide further striking signs of its standard model nature. With the wrapping up of Run 1 results, the CMS experiment is now intensely focused on preparations for Run 2, where the centre-of-mass energy of the LHC will be raised to up to 13 TeV and the luminosity will be much increased. With a more powerful accelerator and the upgraded CMS detector, the collaboration looks forward to the promise of new and exciting results on the Higgs boson in Run 2.

 

References

 

[1] The CMS Collaboration, “Observation of the diphoton decay of the Higgs boson and measurement of its properties”, arXiv: 1407.0558, submitted to Eur. Phys. J. C.

 

[2] CMS Collaboration, “Search for the standard model Higgs boson produced in association with a W or a Z boson and decaying to bottom quarks”, Phys. Rev. D 89 (2014) 012003, doi:10.1103/PhysRevD.89.012003.

 

[3] CMS Collaboration, “Measurement of Higgs boson production and properties in the WW decay channel with leptonic final states”, JHEP 01 (2014) 096, doi:10.1007/JHEP01(2014)096.

 

[4] CMS Collaboration, “Measurement of the properties of a Higgs boson in the four-lepton final state”, Phys. Rev. D 89 (2014) 092007, doi:10.1103/PhysRevD.89.092007.                            

                       

[5] CMS Collaboration, “Evidence for the 125 GeV Higgs boson decaying to a pair of τ leptons”, JHEP 05 (2014) 104, doi:10.1007/JHEP05(2014)104.

 

[6] The CMS Collaboration, “Precise determination of the mass of the Higgs boson and studies of the compatibility of its couplings with the standard model”, Physics Analysis Summary, CMS-PAS-HIG-14-009, http://cds.cern.ch/record/1728249?ln=en

 

 

[7] The CMS Collaboration, “Constraints on Anomalous HWW Interactions using Higgs boson decays to W+W- in the fully leptonic final state”, Physics Analysis Summary, CMS-PAS-HIG-14-012, http://cds.cern.ch/record/1728250?ln=en

 

 

[8] The CMS Collaboration, “Constraints on anomalous HVV interactions using H to 4l decays”, Physics Analysis Summary, CMS-PAS-HIG-14-014, http://cds.cern.ch/record/1728251?ln=en

 

 

[9] The CMS Collaboration, “Evidence for the direct decay of the 125 GeV Higgs boson to fermions”, Nature Physics advance online publication (2014), doi:10.1038/nphys3005.

 

 

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13 Gennaio 2014

Il gruppo di CMS Pisa e' in lutto per la scomparsa del professor Lorenzo Foa' 

Lorenzo e' stato un grande maestro di fisica per tutti noi, fondatore del nostro gruppo di ricerca

 

fisico e docente emerito della Scuola Normale Superiore. Professore ordinario di Fisica generale alla Normale dal 1985 e Vice Direttore dal 1999 al 2005, si era laureato in Fisica all’Università di Firenze e specializzato in fisica delle particelle elementari.

Professore ordinario a Trieste e Pisa, negli ultimi dieci anni è stato Direttore della Ricerca al CERN (Ginevra), membro delle “Scientific Policy Committees” dei maggiori laboratori mondiali e Presidente del European Committee for Future Accelerators (ECFA). E’ stato inoltre il primo Direttore del Dipartimento di Fisica dell’Università di Pisa e Preside della Classe di Scienze della Scuola Normale.

Vogliamo ricordare Lorenzo con questa bellissima intervista integrale che il prof. Lorenzo Foà rilasciò nel 2010 per Storie di Uomini e Quarks (editore SIF, Bologna, pp. 254)

 

 

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L'8 ottobre 2013 è stato assegnato il Premio Nobel per la Fisica 2013 a François Englert and Peter W. Higgs con la seguente motivazione:

"for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider"

Un altro giorno memorabile per l'esperimento CMS , per i laboratori del CERN e per tutta la comunità dei fisici dell'Istituto Nazionale di Fisica Nucleare.

 

 

  Peter W. Higgs                   Francois Englert

 

Il Large Hadron Collider (LHC) del CERN di Ginevra è un acceleratore di particelle, con una circonferenza di 27 Km, progettato per far collidere due fasci di protoni di alta intensità ad una energia massima di 14 TeV nel centro di massa.

CMS (Compact Muon Solenoid) è uno dei 4 esperimenti posti lungo il fascio di LHC ed ha come obiettivo principale la ricerca del bosone di Higgs e di fenomeni fisici non predetti dal Modello Standard ma da sue possibili estensioni, come ad esempio la Supersimmetria.

L'apparato sperimentale di CMS ha una struttura a “scatola cinese” e i suoi rivelatori sono immersi in un forte campo magnetico di 4T realizzato grazie al più grande solenoide superconduttore mai realizzato al mondo capace di immagazzinare un’energia di 2,4GJ. CMS è stato disegnato per ottenere massima efficienza e precisione nella rivelazione di fotoni, elettroni, muoni, leptoni tau, jet, jet originati da quark b ed energia mancante indice della presenza di particelle neutre elusive. L’ apparato è quanto più ermetico possibile e arriva a coprire in angolo fino a circa 1° dalla linea dei fasci. Se pure di grandi dimensioni rispetto ad esperimenti del passato (15 m di diametro per 22 m di lunghezza e un peso pari a 14500 t) CMS è un apparato elegantemente compatto. 

L’INFN è fortemente coinvolto in CMS, circa 250 fisici e ingegneri provenienti da 14 Sezioni e 2 Laboratori Nazionali lavorano all’esperimento. In particolare frazioni significative del sistema di rivelazione per muoni (sia DT che RPC), del calorimetro elettromagnetico e del tracciatore centrale a strisce di silicio sono stati realizzati in Italia. Una ampia ed esaustiva descrizione dell'esperimento è riportata nel menù posto alla sinistra e denominato "L'esperimento CMS".

 

 

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Gli scienzati degli esperimenti CMS e ATLAS al Large Hadron Collider (LHC) dei laboratori del CERN di Ginevra hanno annunciato di aver scoperto una nuova particella consistente con il bosone di Higgs con una massa di circa 125.3 GeV

Comunicato Stampa di CMS

Qui di seguito uno degli eventi candidati Higgs registrati da CMS.

 

 

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Il 1 ottobre 2012 l'esperimento CMS ha compiuto 20 anni.