El CERN confirma una nueva partícula compuesta hallada por el LHC

El experimento CMS observa una nueva partícula en el LHC

 

 

El experimento CMS del Gran Colisionador de Hadrones (LHC) en el CERN ha confirmado la existencia de una nueva partícula compuesta.

 

The ≡b*0 particle shows a clear signal (blue) above the background level (red) (Image: CMS)

The Compact Muon Solenoid (CMS) experiment at CERN has submitted a paper for publication describing the first observation of a new particle, an excited beauty baryon called the ≡b*0 (≡b is pronounced "Csai - bee").

Baryons are subatomic particles whose mass is equal to or greater than that of a proton. The Standard Model of particle physics predicts the existence of ≡b baryons in charged, neutral or excited states. Though charged and neutral ≡b baryons have been seen in detectors before, this is the first time the an excited ?b beauty baryon has been observed. CMS measured the mass of the new particle to be 5945.0 ± 2.8 MeV.

CMS physicists found the ≡b*0 signal in a sample of about 530 trillion proton—proton collisions (an integrated luminosity of 5.3 inverse femtobarns) which were delivered by the Large Hadron Collider (LHC) operating at a centre-of-mass energy of 7 TeV in 2011.

The ≡b*0 adds to a growing list of discoveries at CERN in recent months. In December the ATLAS experiment announced the observation of a new "quarkonium state" containing a beauty quark bound with its antiquark, and in November the LHCb experiment reported a new effect in the decays of particles containing a charm quark (or antiquark).

With the LHC now running at 4TeV per beam, the collision number is set to increase, which enhances the machine's discovery potential considerably, and opens up new possibilities for searches for new and heavier particles.

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Tras analizar los datos de colisiones a 7 TeV durante 2011, unos 5,3 femtobarn inversos, CMS ha descubierto con una significación estadística de 5 sigmas una nueva partícula, un barión llamado Xi_b*^0. Los bariones son partículas compuestas por tres quarks, como el protón y el neutrón. Los quarks que componen esta nueva partícula son un quark up, un strange y un bottom.

El Xi_b*^0 es inestable y se desintegra inmediatamente, en el mismo punto de interacción de los protones que chocan. Esto quiere decir que no se puede observar directamente, sino que hay que reconstruir la cadena de desintegraciones desde los productos finales.

Según explica Ernest Aguiló, postdoc en la Universidad de Zürich y responsable del análisis, la cadena de desintegraciones del Xi_b*^0 es muy larga, con 4 estados intermedios. Lo que ha detectado son las trazas que provienen de los productos finales, y de ahí, paso a paso, se logró identificar el estado inicial.

De entre los miles de millones de colisiones registradas por CMS en 2011 se han encontrado solo 18 colisiones en que esta partícula se ha producido. Esto da una idea de la complejidad de los análisis de física realizados con la ingente cantidad de datos obtenida en el LHC.

Ernest Aguiló se doctoró en la Universidad de Barcelona trabajando en el experimento LHCb. Posteriormente estuvo de postdoc en DZero, uno de los dos experimentos de Tevatron (el acelerador de partículas estadounidense que dejó de funcionar el año pasado), donde participó en el descubrimiento del quark single top. Actualmente trabaja para la Universidad de Zürich en el experimento CMS.

http://www.fpa.csic.es

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Event display

Figure 1. Display of a typical event showing the reconstructed decay products of the expected signal.

 

 

 

Decay chain

Figure 2. schematic diagram of the decay chain explored in this analysis.

 

 

 

Xi_b mass plot

Figure 3. Invariant mass of the J/psi Xi- pairs showing a clear Xi_b signal

 

 
 

 

 

Q plot

Figure 4. Q mass-difference plot showing a clear signal for a new b baryon.

 

 

 

The CMS experiment has submitted a paper for publication describing the first observation of a new, excited beauty baryon known as the ?*b0, with a statistical significance of more than 5 standard deviations (5?) above the expected background. The mass is measured to be 5945.0 ± 2.8 MeV.

The observation was made in a data sample of proton—proton collisions delivered in 2011 by CERN's Large Hadron Collider (LHC) operating at a centre-of-mass energy of 7 TeV. The sample corresponds to an integrated luminosity of 5.3 fb-1[1].

 

Expectations of the Standard Model of Particle Physics

The well-established quark model predicts the existence of so-called ≡b baryons containing one beauty (b) quark, one strange (s) quark, and either an up (u) quark, which results in a neutral ?b0 baryon, or a down (d) quark, which results in a charged ?b- baryon. These may exist with various values of the quantum numbers for angular momentum (J) and parity (P). The ground-state, lowest-mass ?b baryons — both charged and neutral — have been previously observed.

None of the predicted excited states have ever been seen, including the ?*b0 state (with JP=3/2+), which is expected to break up rapidly in a cascade of decays to lower mass particles...[]

http://cms.web.cern.ch/news/observation-new-xib0-beauty-particle

 http://news.stanford.edu/news/2004/july21/femtobarn-721.html
 http://cms.web.cern.ch/news/should-you-get-excited-your-data-let-look-el...