Monthly Archives: July 2015

CMS presents first results with 13 TeV at 2015 EPS-HEP Conference

A 13TeV collision recorded by the CMS detector showing two high-energy particle jets with a collective mass of 5 TeV. Image: Thomas McCauley and Tai Sakuma
Figure 1: Measured charged-hadron production as a function of pseudorapidity (left), and the multiplicity in the central region compared to previous measurements at lower energies and theoretical models.
Figure 2: The di-muon invariant mass spectrum at 13 TeV
Figure 3: Di-jet invariant mass spectrum, showing the expected signal distribution from a hypothetical particle with 4.5 TeV transforming into two jets

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A technical description of the results (in English) along with a full list of presented analyses can be found here.


The CMS Collaboration at CERN is presenting a range of new physics results at the EPS-HEP conference in Vienna, Austria between 22 and 29 July 2015. The results will include the first analyses with the “LHC Run 2” data (13 TeV centre-of-mass energy, collected since June 2015) as well as more than 30 new analyses performed on the “Run 1” dataset (7 and 8 TeV, collected in 2011 and 2012 respectively). Below are summaries of some of the analyses shown at the conference.

A 13TeV collision recorded by the CMS detector showing two high-energy particle jets with a collective mass of 5 TeV. (Direct link to video) Animation: Thomas McCauley

1. Production of charged hadrons

The highlight for CMS is the first physics result from the LHC using 13TeV data: the measurement of the number and trajectories of charged hadrons produced in the proton collisions. This is usually one of the first measurements performed at hadron colliders at the start of exploration at a new energy regime. Because protons are not elementary – they are made of quarks and gluons – when two protons collide at high energies it is actually the quarks and gluons inside them that interact. So every proton collision produces a spray of charged hadrons, such as pions and kaons, flying in all directions. The number of these particles depends on the collision energy – the greater the energy, the higher the number of produced particles. It is therefore important to determine precisely how many charged hadrons are produced at the new collision energy of the LHC in order to make sure that the theoretical models used in simulations are accurate. The CMS Tracker is responsible for determining the trajectories of charged hadrons and is used to perform this measurement, which involved a few hundred thousand collisions recorded at zero magnetic field. The CMS measurement is well described by the theoretical models and will help accurately determine the “background” levels in the searches for new physics at 13 TeV.

DETAILS: CMS measured the differential multiplicity distribution of charged hadrons (dN/dη) for pseudorapidity less than 2, as shown in Figure 1. In particular, the measurement in the mid-rapidity range (|η|<0.5) was 5.49 ± 0.01 (stat.) ± 0.17 (syst.) per collision. The Letter with the result was submitted to Physics Letters B on 21 July and the pre-print can be found at https://cds.cern.ch/record/2036310/.

2. Re-discovering particles and testing the discovery potential

An important test of the CMS detector’s performance at 13 TeV lies in its ability to observe known particles. Figure 2 is a mass histogram of pairs of muons produced from proton collisions in the CMS detector, clearly showing peaks in the data corresponding to particles ranging from the omega meson (ω) to the Z boson. The particles in this spectrum were originally discovered over several decades but it took CMS just weeks to observe them all at 13 TeV. Details about the CMS performance studies can be found at https://twiki.cern.ch/twiki/bin/view/CMSPublic/PublicPlotsEPS2015.

Several processes in the 13TeV data have been studied in some detail. One highlight of this effort is a first look at the di-jet invariant-mass spectrum up to approximately 5 TeV (Figure 3), demonstrating the readiness of CMS for new physics at these high energies.

3. Wrapping up analysis of “Run 1” data

CMS continues to perform physics analysis on the “Run 1” data collected at 7 and 8 TeV, with more than 30 new results approved recently for the EPS-HEP conference. These include measurements of the two-photon production of W-boson pairs (FSQ-13-008), the production rates for particle jets at 2.76 TeV compared to 8 TeV (SMP-14-017), the production of two photons along with jets (SMP-14-021) and electroweak production of a W boson with two jets (SMP-13-012).

Discovered over two decades ago, the top quark continues to play a vital role in physics analysis for both measurements and searches. New CMS results with this fermion include measurements of the top-antitop production rates in the fully hadronic sample (TOP-14-018) and a measurement of the top-antitop+bottom-antibottom process in the lepton+jets channel (TOP-13-016). In addition, searches for signs of new physics continue, most recently in the process t→cH, where the Higgs boson transforms to photons (TOP-14-019).

Meanwhile, on the Higgs front itself, three new searches have been performed for non-Standard-Model Higgs bosons containing tau leptons in the decay products (HIG-14-029, HIG-14-033, HIG-14-034), while on the supersymmetry front, analyses have been performed looking for dark-matter candidates and other supersymmetric particles (SUS-13-023, SUS-14-003, SUS-14-015).

Heavy-ion results from Run 1, utilising proton-proton, proton-lead and lead-lead collisions, include Upsilon (Υ) polarisation as a function of charged-particle multiplicity in proton-proton collisions (HIN-15-003), Z-boson production (HIN-15-002), jet-fragmentation functions in proton-lead collisions (HIN-15-004), and nuclear modification of Upsilon (Υ) states in lead-lead collisions (HIN-15-001).


More information: http://cern.ch/cms, or contact: cms.outreach@cern.ch.

CMS is one of two general-purpose experiments at the LHC that have been built to search for new physics. It is designed to detect a wide range of particles and phenomena produced in the LHC's high-energy proton-proton and heavy-ion collisions and will help to answer questions such as: "What is the Universe really made of and what forces act within it?" It will also measure the properties of well-known particles with unprecedented precision and be on the lookout for completely new, unpredicted phenomena. Such research not only increases our understanding of the way the Universe works, but may eventually spark new technologies that change the world in which we live as has often been true in the past.

The conceptual design of the CMS experiment dates back to 1992. The construction of the gigantic detector (15 m diameter by nearly 29 m long with a weight of 14000 tonnes) took 16 years of effort from one of the largest international scientific collaborations ever assembled: CMS currently has around 2900 scientists (including nearly 1000 graduate students) plus over 1000 engineers and technicians, from 182 institutions and research laboratories distributed in 42 countries all over the world.

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CMS presents first 13TeV results

The latest results from the LHC experiments are presented in Vienna

Geneva/Vienna, 27 July 2015. The world particle-physics community has convened in Vienna for the 2015 European Physical Society Conference on High Energy Physics (EPS-HEP2015), where the latest results in the field are being presented and discussed. These include the first results from Run 2 of the Large Hadron Collider (LHC) at CERN1, which are being presented for the very first time, less than two months after the experiments started to take data at the unprecedented energy of 13 TeV, following a two-year long shutdown.

"It is much too early to expect any discovery, we will have to be patient,” said CERN Director General Rolf Heuer. “Nevertheless, the LHC experiments have already recorded 100 times more data for the summer conferences this year than they had around the same time after the LHC started up at 7 TeV in 2010. We can sense a fantastic pioneering spirit as the physicists are looking at completely new data at an unexplored energy.

As for any machine exploring a new energy frontier, operators at the LHC face many challenges on a daily basis. Since the start of Run 2, they have been gradually increasing the intensity of the LHC’s two beams, which travel in opposite directions around the 27-kilometre ring at almost the speed of light. The LHC has run at the record high energy with each beam containing up to 476 bunches of 100 billion protons, delivering collisions every 50 nanoseconds. In the coming days, the intensity should increase further with a new rhythm of 25 nanoseconds. After a planned technical stop in early September, the teams will also be able to increase the number of bunches with the goal of reaching more than 2000 bunches per beam by the end of 2015.

“During the hardware-commissioning phase, we have learnt to manage carefully the huge energy stored in the magnets. Now with beam commissioning we have to learn progressively how to store and handle the beam energy,” said CERN Director of Accelerators and Technology Frédérick Bordry. “Our goal for 2015 is to reach the nominal performance of the LHC at 13 TeV so as to exploit its potential from 2016 to 2018.”

The LHC has already delivered over 10 thousand billion collisions to the large experiments since the start of Run 2. This has allowed the LHC collaborations to measure a full suite of detector performance parameters that demonstrate the readiness of the experiments for discovery physics and precision measurements. The next step was to confirm the Standard Model at the new energy of 13 TeV. After only a few weeks of data taking, the experiments have now “rediscovered” all of the known fundamental particles, apart from the so-called Higgs boson, for which more data are still required. The collaborations are thus ready to test the Standard Model at 13 TeV and the hope is to find evidence of new physics beyond this well-established theory.

At the EPS-HEP2015 conference, the ATLAS and CMS collaborations presented the first measurements at 13 TeV on the production of charged strongly-interacting particles (hadrons). CMS has already submitted this result for publication – the first for the new energy region. Such measurements are important in understanding the basic production mechanism for hadrons.

The LHC experiments have also made the first measurements of cross-sections at 13 TeV. Cross-sections are quantities related to the probability for particles to interact, and their measurement is essential for identifying any new phenomena. For example, ATLAS has measured the cross-section for the production of pairs of top quarks and antiquarks, which is some three times higher at 13 TeV than at the energy of Run 1.

In addition, the conference is providing the opportunity for all of the LHC experiments to present many new or final results from the first run at the LHC. These include searches for dark matter, supersymmetric and other exotic particles, as well as new precision measurements of Standard Model processes.

In this respect, one highlight in Vienna is the presentation for the first time at an international conference of the recent discovery by the LHCb experiment of a new class of particles known as pentaquarks (see press release). LHCb also published today in Nature Physics a result confirming that a certain decay involving the weak force happens with beauty quarks having a “left-handed” spin. This result is consistent with the Standard Model, in contrast with previous measurements that allowed for a right-handed contribution.

In other highlights from Run 1, the ALICE and LHCb experiments have new results on long-range correlations in proton–lead collisions. The latest measurements show that the so-called “ridges” seen in the most violent collisions span across even larger longitudinal distances. In Run 2 data, ATLAS reported that the near-side ridge is seen in 13 TeV proton–proton collisions, with characteristics very similar to those observed by CMS in Run 1.

 

For more information:

Follow EPS HEPP Board on social media: Twitter and Facebook

The Press Conference of the 27th of July is available here.

ALICE Website

ATLAS Website

CMS Website

LHCb Website

 

Footnote(s)

1. CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. It has its headquarters in Geneva. At present, its member states are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, the Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Romania is a Candidate for Accession. Serbia is an Associate Member in the pre-stage to Membership. Turkey is an Associate Member. India, Japan, the Russian Federation, the United States of America, the European Union, JINR and UNESCO have observer status.

Physics Awards for the “Founding Fathers” of CMS

Tejinder Virdee (left) and Michel Della Negra in the CMS Control Room

Michel Della Negra and Tejinder (Jim) Virdee have been recognized for their work on CMS by their respective national physics institutes.

Michel has been awarded the 2014 prix André Lagarrigue for his “qualités exceptionelles de batisseur de dispositifs expérimentaux d’une très grande complexité, avec une compréhension profonde de la physique” or “exceptional quality in building experimental devices of great complexity, with a profound understanding of physics”. The full citation can be found at [PDF] http://prixlagarrigue.lal.in2p3.fr/wp-content/uploads/sites/2/2015/04/De...

In celebration of the award, a full day of seminars followed by a reception will take place at LAL, Orsay, Paris on 14 December.

Tejinder has been awarded the 2015 Glazebrook Medal and Prize, a Gold Medal of the UK Institute of Physics, for his “leadership of the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) where evidence for the Higgs boson was revealed after twenty years of research involving design, construction and data taking”. The full citation can be found at: http://www.iop.org/about/awards/gold/glazebrook/medallists/page_65852.html

The awards dinner ceremony will take place in London on 5 November.

Upon hearing of the awards Michel and Tejinder remarked, “It is especially satisfying to be recognized by fellow scientists. However, the long and arduous construction and data taking with CMS, the momentous discovery of the Higgs boson in 2012 and more, have only been possible through the dedicated and painstaking effort of CMS colleagues from over 40 countries. For us, it has been a pleasure and a privilege to work alongside them.”

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CERN’s LHCb experiment reports observation of exotic pentaquark particles

Geneva, 14 July 2015. Today, the LHCb experiment at CERN’s Large Hadron Collider has reported the discovery of a class of particles known as pentaquarks. The collaboration has submitted a paper reporting these findings to the journal Physical Review Letters.

“The pentaquark is not just any new particle,” said LHCb spokesperson Guy Wilkinson. It represents a way to aggregate quarks, namely the fundamental constituents of ordinary protons and neutrons, in a pattern that has never been observed before in over fifty years of experimental searches. Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted.”

Our understanding of the structure of matter was revolutionized in 1964 when American physicist, Murray Gell-Mann, proposed that a category of particles known as baryons, which includes protons and neutrons, are comprised of three fractionally charged objects called quarks, and that another category, mesons, are formed of quark-antiquark pairs. Gell-Mann was awarded the Nobel Prize in physics for this work in 1969. This quark model also allows the existence of other quark composite states, such as pentaquarks composed of four quarks and an antiquark. Until now, however, no conclusive evidence for pentaquarks had been seen.

LHCb researchers looked for pentaquark states by examining the decay of a baryon known as Λb (Lambda b) into three other particles, a J/ψ- (J-psi), a proton and a charged kaon. Studying the spectrum of masses of the J/ψ and the proton revealed that intermediate states were sometimes involved in their production. These have been named Pc(4450)+ and Pc(4380)+, the former being clearly visible as a peak in the data, with the latter being required to describe the data fully.

“Benefitting from the large data set provided by the LHC, and the excellent precision of our detector, we have examined all possibilities for these signals, and conclude that they can only be explained by pentaquark states”, says LHCb physicist Tomasz Skwarnicki of Syracuse University.

"More precisely the states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark.”

Earlier experiments that have searched for pentaquarks have proved inconclusive. Where the LHCb experiment differs is that it has been able to look for pentaquarks from many perspectives, with all pointing to the same conclusion. It’s as if the previous searches were looking for silhouettes in the dark, whereas LHCb conducted the search with the lights on, and from all angles. The next step in the analysis will be to study how the quarks are bound together within the pentaquarks.

 “The quarks could be tightly bound,” said LHCb physicist Liming Zhang of Tsinghua University, “or they could be loosely bound in a sort of meson-baryon molecule, in which the meson and baryon feel a residual strong force similar to the one binding protons and neutrons to form nuclei.”

More studies will be needed to distinguish between these possibilities, and to see what else pentaquarks can teach us. The new data that LHCb will collect in LHC run 2 will allow progress to be made on these questions.

 

For more information:

See paper on ArXiv

More on the LHCb collaboration website

 

 

 

The mass of J/ψ–proton (J/ψ p) combinations from Λb → J/ψpK-decays. The data are shown as red diamonds. The predicted contributions from the Pc(4380)+ and Pc(4450)+ states are indicated in the purple and black distributions, respectively. Inset: the mass of J/ψ p combinations for a restricted range of the K-p mass, where the contribution of the wider Pc(4380)+ state is more pronounced. (The other contributions from conventional hadrons, which are responsible for the remaining features in the data distributions, are not displayed.) © CERN / LHCb Collaboration

 

 

Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. The five quarks might be tightly bonded (left). They might also be assembled into a meson (one quark and one antiquark) and a baryon (three quarks), weakly bound together. © CERN

 

Footnote(s)

1. CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. It has its headquarters in Geneva. At present, its member states are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, the Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Romania is a Candidate for Accession. Serbia is an Associate Member in the pre-stage to Membership. Turkey is an Associate Member. India, Japan, the Russian Federation, the United States of America, the European Union, JINR and UNESCO have observer status.