Tag Archives: particle

Figure 1 (left) Exclusion limits for production of Higgsino production as a function of Higgsino mass and branching fraction. (right) Most sensitive search channel as a function of Higgsino mass and branching fraction. Credit: CERN

Recent results in the search for supersymmetry : CERN CMS

By Frank Wuerthwein, Keith Ulmer and Guillelmo Gomez Ceballos.

Among the leading candidates to describe physics beyond the standard model of particle physics is Supersymmetry, a new symmetry that posits the existence of a partner particle for each known particle in the standard model. Supersymmetry, or “SUSY” as it has come to be known, may help explain the nature of dark matter and the large difference in strength between the fundamental forces of nature. Each year, new experimental results and theoretical developments are reported in the “SUSY” conference series, with the 2014 edition (SUSY2014) happening this week in Manchester, England[1].

Figure 1 (left) Exclusion limits for production of Higgsino production as a function of Higgsino mass and branching fraction. (right) Most sensitive search channel as a function of Higgsino mass and branching fraction. Credit: CERN
Figure 1 (left) Exclusion limits for production of Higgsino production as a function of Higgsino mass and branching fraction. (right) Most sensitive search channel as a function of Higgsino mass and branching fraction. Credit: CERN

Experimental evidence for SUSY has been sought for many years at multiple colliders, including a vast array of search results from the CMS experiment at the Large Hadron Collider at CERN. With data from Run 1 of the LHC collected through the end of 2012, the full set of results thus far has not revealed any striking signs of physics beyond the standard model [2]. New searches presented at SUSY2014 have begun to probe increasingly complicated potential decay chains and to combine multiple searches to access more challenging new physics scenarios. Below we highlight some of the most recent results first presented this summer at SUSY14 and ICHEP 2014 [3].

Figure 2: Exclusion limits versus gluino and neutralino masses for a variety of gluino decay branching fractions from the “razor” search. Credit: CERN
Figure 2: Exclusion limits versus gluino and neutralino masses for a variety of gluino decay branching fractions from the “razor” search. Credit: CERN

Search for new physics in the final states hh, Zh, and ZZ plus MET

After its discovery only two years ago, the Higgs boson is already a powerful tool in the search for new physics. Earlier this year, CMS submitted for publication [4] a set of searches for associate production of W, Higgs, and missing transverse energy (“MET”, indicative of particles escaping the detector). At ICHEP this summer, CMS presented the first combined searches for hh, Zh, and ZZ plus MET. No excess above standard model backgrounds is observed. Figure 1 shows the interpretation of the results in terms of limits on higgsino pair production as a function of the higgsino mass and decay branching fraction. Within the framework of Gauge Mediated Supersymmetry Breaking (GMSB), the neutral higgsino decays to a gravitino and either a higgs or Z boson. The left plot in Figure 1 shows that CMS excludes higgsino production up to ~ 300GeV when the higgsino decays at equal rate to either of these two decays. The right plot in Figure 1 indicates that four different final states dominate the sensitivity in different parts of the 2D parameter space, clearly demonstrating that searches for new physics with one or two higgs bosons in the final state benefit greatly from combining many different decay channels.

Figure 3: Dilepton invariant mass distribution for “same flavor” events, compared to the background prediction from “opposite flavor” events. Credit:CERN
Figure 3: Dilepton invariant mass distribution for “same flavor” events, compared to the background prediction from “opposite flavor” events. Credit:CERN

Search for gluino pair production via the decays to top pairs, bottom pairs, or top and bottom plus MET

Up to now, CMS searches for gluino pair production inspired by “natural SUSY” (i.e. SUSY in which the masses of the SUSY partners are not much higher than those of the Higgs boson) have focused on final states with either four top or four b-quarks plus MET. In contrast, theoretically any combination of MET plus 4 quarks, top or bottom, is well justified. At ICHEP, CMS presented the first complete exploration of sensitivity across the full set of possible final states and branching fractions. Figure 2 shows the corresponding exclusion curves in the gluino vs neutralino mass plane. This search employs the so-called “razor” variables, and its sensitivity is dominated by all-hadronic final states. The more top quarks there are in the final state for a given gluino mass, the less momentum is left for all the decay products, and the harder it is thus to distinguish signal from background. Accordingly, the sensitivity decreases as the number of top quarks per event increases.

Figure 4: MSSMvsSM limit in the MSSM mmod+h scenario. At each mA - tanβpoint a Hypothesis test is performed testing the MSSM (A+H+h+BKG) hypothesis against the SM (hSM+BKG) hypothesis. Credit: CERN
Figure 4: MSSMvsSM limit in the MSSM mmod+h scenario. At each mA – tanβpoint a Hypothesis test is performed testing the MSSM (A+H+h+BKG) hypothesis against the SM (hSM+BKG) hypothesis. Credit: CERN

Searching for SUSY with an “Edge”

The dilepton invariant mass distribution for leptons from the decays χ20 to l+l- χ10, or similar decays via a slepton as an intermediate state, display the striking feature of a kinematic “edge” [5, 6]. As these decays conserve lepton flavor, this edge is present only in same-flavor events, i.e. ee and μμ, and is completely absent in the “opposite flavor” lepton sample, i.e. eμ events. In contrast, backgrounds for which each of the two leptons come from a different W decay, e.g. top pairs, WW, etc., will have identical dilepton distributions for same and opposite flavor. Thus, the eμ sample in data provides a perfect model of the background dilepton mass distribution – modulo effects from the trigger and lepton reconstruction. The kinematic edge is a sufficiently striking signature to reveal new physics even at relatively modest hadronic activity, HT and MET, i.e. in the presence of sizeable top and Z backgrounds.

CMS presented a search for such an “edge” in dilepton events with jets and MET at SUSY2014 using the full 8TeV data sample [7]. Figure 3 overlays the dilepton mass distribution in ee plus μμ (data points), with the corresponding one from eμ (pink histogram). The blue shaded region depicts the systematic error envelope for the background prediction. A small excess is visible below the Z peak. A signal region of 20GeV < mll < 70GeV was chosen before data taking. Within this region, 860 events are observed with an expected standard model background yield of 730 ± 40. The small excess is consistent with a 2.6 sigma fluctuation of the standard model background. For more details see [8].

Search for additional neutral MSSM Higgs bosons in the H→ττ decay channel

Another highlight among the CMS results presented at the SUSY2014 conference is the search for additional neutral Higgs bosons decaying to τ leptons, which is the most promising channel to search for such Higgs bosons in the context of the minimal SUSY extension of the standard model, the MSSM. Following the release of a preliminary result based on the full data set of the 2011/2012 data taking period [8], additional results based on a new interpretation of the data have been presented at this conference for the first time [9]. While the data selection has not changed, extensive work has set the ground for a new interpretation of the data in the context of modern benchmark models. In particular, the new models take into account the presence of the recently discovered Higgs boson with a mass of 125 GeV, as proposed in [10]. Also for the first time the model-dependent exclusion contours as a function of the mass of the CP-odd Higgs boson, A, and the ratio of the vacuum expectation values of the two SUSY Higgs doublets, tanβ, have been derived, taking the presence of the newly discovered Higgs boson properly into account in the test statistic. As recently demonstrated by CMS [11], all observations of the new boson are so far compatible with the SM expectation within ~10% accuracy, which justifies the standard model hypothesis to be the better choice for the test statistic. The hypothesis test now becomes a search based on a model with three Higgs bosons compared against the standard model with only one Higgs boson. Traditional limits, based on the test statistic excluding the Higgs boson from the standard model hypothesis have also been made public on the CMS web-pages [12]. Also made available to the public is an extended database of results based on a model-independent single-resonance search model, which will be extremely valuable to theorists engaged in model building. Figure 1 shows the exclusion contour in a modified mh,max scenario, also referred to as mh,mod+ exploiting the new statistical treatment for the statistical inference.

By Frank Wuerthwein, Keith Ulmer and Guillelmo Gomez Ceballos.

[1] http://www.susy2014.manchester.ac.uk

[2] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS

[3] http://ichep2014.es

[4] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS13006

[5] http://cds.cern.ch/record/1194507/files/SUS-09-002-pas.pdf

[6] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS11011

[7] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS12019

[8] CMS Collaboration, “Search for Neutral MSSM Higgs Bosons Decaying to Tau Pairs in pp Collisions”, (2013), CMS-PAS-HIG-13-021.

[9] CMS Collaboration, “Search for Neutral MSSM Higgs Bosons Decaying to Tau Pairs in pp Collisions”, to be submitted to JHEP.

[10] M. S. Carena et al, “MSSM Higgs boson searches at the Tevatron and at the LHC: Impact of different benchmark scenarios” Eur. Phy. J C 73, 2552 (2013) (arXiv:hep-ph/0511023).

[11] CMS Collaboration, “Precise determination of the mass of the Higgs boson and studies of the compatibility of its couplings with the standard model”, (2014), CMS-PAS-HIG-14-009.

[12] https://indico.hep.manchester.ac.uk/contributionDisplay.py?contribId=288….

Source: CERN CMS


Science, Economy and Peace: A study focusing Pakistan

Syed Faisal ur Rahman


 Abstract: A key difference between the first world and the third world is their progress in the fields of science and technology. Pakistan is mainly known as an agricultural economy but agriculture sector does not contribute much in shaping the modern global economy. We will analyze how science and technology helped in improving the lives of people but also will see its role in the economic development of countries. In the age of conflicts, war and economic rivalry, it is often hard to find common grounds for humanity to proceed for common goals. Fortunately, some big science projects have proved to be a beacon of hope for humanity in pursuing a better peaceful and prosperous future for this world.We will give an overview of some of the projects pursued by countries who are normally rivals at military and economic fronts, but for pursuing science goals they have to join hands, giving a better hope for peace and economic development. We will also see how Pakistan can learn from the experiences of other countries and regions to build a better future for it’s people.




Last century saw enormous developments in the field of science and technology, which also helped countries to rapidly develop their potential in industry, medical sciences, defense, space and many other sectors. Countries which made science and technology research and education as priority areas emerged as stronger nations as compared to those who merely relied on agriculture and the abundance of natural resources.

We can also see that big science projects, involving one or more than one country, have served our society through spin-off technologies, human resource development, boosting up economic activity and cooperation. Also, we will study the role of some big science projects in promoting peace and stability in the world.

Global Economy and Pakistan

According to Central Intelligence Agency (CIA) world factbook public data [14], global economy has a size of 71.3 trillion dollars if we look at Gross Domestic Product (GDP) based on official exchange rate and 83.12 trillion dollars based on GDP purchasing power parity (PPP).

The contribution of different sectors based on CIA world fact book 2012 estimates, is as:

Agriculture- 5.9%

Industry -30.2%

Services- 63.9%

Pakistan which comprises of ~2.5-2.7 (2011 World Bank Data) percent of world population, only has 230.5 billion dollars GDP (official exchange rate) and 514.6 billion dollars GDP (PPP) which makes it around 0.32 % of the world economy based on GDP (official exchange rate) and 0.62% based on GDP(PPP). This shows a serious gap in income scales of some of the developed countries of the world and a relatively poor economy like Pakistan. This high population and low GDP mean less money available to individuals living in the country. GDP per capita (PPP) of the world is 12,400 dollars based on CIA world factbook 2012 estimates and for Pakistan the figure is 2,900 dollars.

Pakistan is also relatively more dependent on the agricultural sector. Pakistan’s labor composition is estimated in 2012 CIA world fact book as:

Agriculture- 20.1%

Industry- 25.5%

Services- 54.4%

If we look at the labor distribution, then according to 2007 estimates, Pakistan’s ~45% population is involved in the agricultural sector, which is more than industry (~21%) and services (~34%).

 Science, Technology and Global Economy

Below is plot of World Bank 2011 data [13] for countries with highest Gross National Income (GNI) per capita:


Fig. 1: GNI per capita for 2011 based on World Bank Data

If we look at figure 1 then we can clearly see that most countries in top 20 GNI are knowledge based economies and some represent natural resource or energy based economies. In comparison with these economies, Pakistan’s GNI is 1,120 dollars based on the same criteria.

A more direct comparison can be given between GDP and science output is the table below showing top scientific and technical journal producers and their GDP rankings:

Rank(based on column 3) Country Scientific and Technical Journal Articles (2009, World Bank Data)[13] GDP Ranking ( based on 2011, World Bank Data) Human Development Index(HDI, based on 2012 UNDP Data) [11] Category
1 United States 208,601 1 Very High
2 China 74,019 2 Medium
3 Japan 49,627 3 Very High
4 United Kingdom 45,649 7 Very High
5 Germany 45,003 4 Very High
6 France 31,748 5 Very High
7 Canada 29,017 10 Very High
8 Italy 26,755 7 Very High
9 South Korea 22,271 14 Very High
10 Spain 21,543 11 Very High
11 India 19,917 8 Medium
12 Australia 18,923 12 Very High
13 Netherlands 14,866 16 Very High
14 Russia 14,016 9 High
15 Brazil 12,306 6 High
16 Sweden 9,478 20 Very High
17 Switzerland 9,469 18 Very High
18 Turkey 8,301 17 High
19 Poland 7,355 21 Very High
20 Belgium 7,218 22 Very High
46 Pakistan 1,043 45 Low

Table 1: Pakistan and the top 20 Sci-tech journal articles producing countries and their GDP rankings (based on the World Bank data). Also we have presented the Human Development Index (HDI) categories of these countries based on the 2012 United Nations Development Program’s HDI data.

Figures in table 1, clearly shows some relation between scientific output and the size of the overall economy. There are few exceptions like Saudi Arabia, which makes regularly into the top 20 economies and is not one of the top producers of scientific and technical journal articles. We can find such inconsistencies as there is more than one factor which contributes to the size of the economy like exploitation of energy resources, minerals, large size of populations and various other factors.

Also we can see that most sci-tech journal articles producing countries are in very high HDI countries with 3 in high and 2 in medium categories. We can see two medium category countries are two of the largest populations on earth i.e. China and India. HDI of a country depends on the access to health, income, access to education and living standard of the citizens of that country. This indicator provides a more realistic picture as compared to GDP for measuring quality of life as countries with large populations like China and India can have high GDP despite lower average income or can have a higher number of sci-tech publications or output despite not doing well in per person averages. In comparison to these countries, Pakistan is in the low HDI category which shows the low quality of life for the citizens of Pakistan.

Pakistan and comparison with India and China

We further narrow our comparison with countries having similar regional and economic history. For this we select India and China. India and China reside in the same region as Pakistan and got independence in the same time period of the late 40s. China has the largest population in the world and India has the second largest population having relatively high population density.

If we look at the historical comparisons after the separation of the East Pakistan from the federation, we can see we were well ahead of both China and India, in terms of GNI per capita and the economic freedom, for a good part of our history. Apart, from being relatively free market economy, Pakistan also did well in the development of techno-industry. Almost all major scientific organizations related to heavy industries, space, nuclear, agricultural and other areas developed in earlier decades of Pakistan. In later years, Pakistan was left behind in development by the two countries. One of the main reasons behind this is Pakistan’s lack of interest in the science and technology sectors and the inability to keep up with the pace of science and technology development in India and China. We can see historical GNI comparisons between Pakistan, China and India.

China adopted a focused techno-industrial development approach. According to Campbell, 2013 [3] paper, China developed its industrial base on Soviet lines till 1959 focusing on heavy industries. After that, till 1976 ideological domination of economic projects and economy didn’t progress much.  Then China adopted a more independent technology research policy with a relatively liberal economic agenda and in 2001 with further Chinese shift towards a market economy from a controlled economy, these policies started to give results as the involvement of private sector in such projects ensured the translation of technology research into commercial success.

Similarly, India focused strongly on science and technology from its early days and also started to initially focus on heavy industries on Soviet lines. Later, especially in early 1990s, with the liberalization of the economy and the policy shift towards more market economy, India started to promote small technology based industries. A good focus of India was on software industry which not only helped India in bringing more export revenues, but also helped improve corporate governance in India (Arora et al, 2002)[1]. This led to more productivity in many industries of India and with gradual shifts towards a market economy India also saw rapid economic growth.

Fig. 2: GNI comparison between Pakistan, China and India (World Bank 2013 Data)

Collaboration in Science and World Peace

Apart from economic development, science projects have also contributed in promoting peace and collaboration among many countries including many rival countries. The lead in promoting scientific collaboration for peace was taken by Europe. After the World War II, Europe learned to promote economic cooperation instead of unnecessary rivalry. This cooperation in economic areas grew further and expanded in other areas like science and technology. Launch of The European Organization for Nuclear Research, or CERN[4] in 1954 was a huge step in promoting scientific collaboration among European countries in post-World War II scenario. This spirit continued even in Cold War days (Gillies, 2011) [6] as the idea of exploring the nature of matter and energy proved to be bigger than the prejudices and blind nationalism.

This spirit continued further in other big sciences and we now see countries like USA, China, Russia, UK and others doing collaborations in space sciences, particle physics, astronomy, medicine and many other areas. Some of the examples in this regard are Square Kilometer Array (SKA), Synchrotron-Light for Experimental Science and Applications in the Middle East (SESAME), Search for Extra-terrestrial Intelligence (SETI), International Space Station (ISS) and other projects are forwarding such spirit.

Apart from this many countries are involved in other collaborative projects as well. These projects are always welcomed in civil society and the scientific community as a way to promote peace.

Pakistan is also involved in some of these projects like CERN and SESAME. Pakistan’s collaboration with CERN formally started in past two decades. Pakistan’s connection with CERN is even older than Pakistan’s formal entry in this collaboration. This connection was established through Pakistan’s Nobel Laureate, Dr. Abdus Salam. Still a lot is needed to be done by Pakistan to get the best out of these collaborations with CERN.

In SESAME, Pakistan played a key role by becoming a founding member. The idea is a brain child of Dr. Abdus Salam and Middle East based MESC (Middle East Scientific Cooperation) group headed by Sergio Fubini, a theoretician at CERN, who aspired for a synchrotron radiation source in the Middle East (Historical highlights, SESAME website) [10]. SESAME shares the same spirit of science for peace with CERN as it is helping to bridge the gap between historically rival nations and in improving people to people relations between countries like Pakistan, Iran, Israel, Palestinian Authority, Egypt, Turkey and others who are often involved in heated conflicts in the region. The project was shown full support by 45 Nobel Laureates in a joint declaration which also demanded friends of science and peace to support the project (Declaration, PETRA VI meeting, June 2008) [5].

Pakistan is still behind many countries of the world in space sciences despite being among the first few countries to launch a space rocket in the 1960s. Similarly, Pakistan has not played a significant role in any significant collaboration related to the promotion of astronomy. Our neighboring countries are playing key roles in projects like SKA (skatelescope.org, participating countries) [8] and are also expected to join ISS in the future (Spacenews, 2010) [9].

Big Science and Economic Development

Big science projects have not only played a crucial role in bringing peace or satisfying human curiosity to know more about the nature and origin of matter, energy and the universe, but the path to achieve such scale of science has led to many spin-off technology developments.

Development of World Wide Web (WWW) is a result of data sharing architecture designed for CERN (webfoundation.org, history of the web) [7], Wi-Fi is a result of CSIRO’s efforts to develop better techniques for radio astronomy (csiro.au, outcomes)[12], research in radio astronomy has also played a key role in developing techniques for locating cellular telephones, location for faulty transmitters (Bout, 1999)[2] and various other technologies.

The key here is to understand the importance of basic and fundamental sciences, and understanding the importance of adopting the right strategy for using the resulting science and technologies for economic and social development.

 Pakistan and Suggestions to Develop Science and Technology for Economic Development

The purpose of presenting various examples, data and figures is to show the necessity of developing a solid foundation for science and technology in Pakistan. We are a country with significant potential in minerals, energy and agricultural resources. Also, we have developed some advanced technology base in the defense sector. We also have a small but energetic Information Technology industry, which is growing well despite difficulties due to law and order situation, and electricity crisis in the country.

Below are some of the steps we can take to promote science and technology in Pakistan and then use it for developing Pakistan’s economy.

a) We need to improve basic science education in the country. The school level curriculum is way behind as compared to other parts of the world. We need to produce students who can think big and even if they do not pursue science as their career, they should be at least educated enough to appreciate the importance of fundamental research. Even if students end up pursuing management studies or end up as key decision makers in government or private sector offices then they will be better equipped to realize the importance of science and technology research in the progress of our country or to come up with business idea which will exploit scientific knowledge.

b) We need to promote research and development in the universities by encouraging industry-academia linkages by providing tax incentives for industries involved in promoting research and development in the universities of Pakistan.

c) We need to share the technology base developed in defense sector with the private sector so that it can be used for peaceful commercialization of technology.

d) We need to give tax and reward incentives to the private sector for contributing in fundamental sciences.

e) We need to promote collaboration between universities and strategic national organizations like SUPARCO and NESCOM.

f) The most important thing which is needed to be done is to give the leading role in policy making to the civilian scientists with sound academic and research background. Currently, institutions like SUPARCO, NESCOM and other institutions are under the direct or indirect control of military personnel who usually do not have enough academic and research background to make the right decisions and set the right priorities in the key areas of science and technology.

g) Another thing lacking in Pakistan is active inter-university and intra-university collaboration for science projects related to interdisciplinary sciences.

h) We also need to give priority to the science and technology collaboration in academic and fundamental research areas when planning our foreign policy. Currently, our foreign policy is security focused with no serious efforts to strengthen academic ties with other countries. Our embassies are needed to be run by people who understand how important it is to interact with the academia of the country they are serving in and how important it is to help our universities in making right relationships in foreign countries for scientific research. This will again be dependent on how good we will do in producing non-science graduates who understand the importance of science and technologies as most foreign office employees come from the arts departments, the business schools etc.

i) We finally need to start playing an active role in major areas of science and technology like particle   physics, astronomy, high performance computing, quantum computing, nano-technology and other areas where we have a potential to go ahead but lacking any serious progress due to lack of proper policy making and interest.

We also need to identify our strengths and weaknesses in various areas of technology and divide our science and technology base in:

a)      Commercial

In this category we can place technologies like information & communication, agricultural, pharmaceutical etc.

b)      Defense

Pakistan has done a significant investment over the past few decades in the development of nuclear, missile, fighter jets and other technologies. We can use these technologies for commercial purposes like producing energy or developing civil aeronautical industry.

c)       Strategic

Not all science and technology research produces immediate results but, their long term impact can be seen in other developed countries and some of them are mentioned above. In this category we can place big sciences like space, radio astronomy and high energy physics or even areas like quantum computing, geophysics etc.

d)      Fundamental or Basic

Fundamental or basic sciences help in creating the grounds for developments in other area mentioned previously. Physics is considered as the most fundamental science and in relative broader terms special sciences like chemistry and biology are also often made part of this category. In more liberal definitions, people also include mathematics, statistics and economics in this area as well. We need to improve research in this area and also we need to improve the teaching quality of these subjects in primary, secondary, higher secondary and tertiary level education systems.

This categorization will help Pakistan in better prioritizing the areas based on need and capacity.


We discussed the importance of science and technology in the economic development. We also presented a comparison between Pakistan and other countries, including neighboring China and India. We also discussed the role of science and technology in promoting peace and collaboration. We also discussed how big sciences can contribute to the economy through spin-off technologies. In the end, we also discussed some  suggestions for developing science and technology in Pakistan.


  1. Arora A. and Athreye A.,2002. The Software Industry and India’s Economic Development. Information Economics and Policy 14 (2002) 253-273.
  2. Bout P. V., April, 1999. Recent Examples of Technology Fostered by Radio Astronomy (Document).
  3.  Campbell J.R.,2013. Becoming a Techno-Industrial Power: Chinese Science and Technology Policy. Issues in Technology Innovation 23 (2013).
  4. CERN official website – http://home.web.cern.ch/
  5. Ely Wiesel Foundation Declaration, June, 2008. Declaration accepted by the Plenary Meeting of the Nobel Laureates at the PETRA IV Meeting on 19 June 2008 and released by Ely Wiesel Foundation.
  6. Gillies J., 2011, CERN can be model for global co-operation, http://www.publicserviceeurope.com/article/477/cern-can-be-model-for-global-co-operation
  7. History of web-Web foundation website http://www.webfoundation.org/vision/history-of-the-web/
  8. Participating Countries, SKA website- http://www.skatelescope.org/the-project/history-of-the-organisation/participating-countries-2/
  9. Seilding P.B. , Feb. 3, 2010, http://www.spacenews.com
  10. SESAME official website- www.sesame.org.jo
  11. United Nations Development Program (UNDP) HDI http://hdr.undp.org/en/statistics/hdi/
  12. Wireless LANs, CSIRO website- http://www.csiro.au/en/Outcomes/ICT-and-Services/People-and-businesses/wireless-LANs.aspx
  13. World Bank’s World Development Indicators (WDI) – http://data.worldbank.org/indicator
  14. World Fact Book, CIA-https://www.cia.gov/library/publications/the-world-factbook/‎


Computer-generated drawing of the Alpha Magnetic Spectrometer (AMS).

Credit: NASA

Particle detector finds hints of dark matter in space

Alpha Magnetic Spectrometer detects positrons in cosmic ray flux that hint at dark matter’s origin.

By Jennifer Chu

Researchers at MIT’s Laboratory for Nuclear Science have released new measurements that promise to shed light on the origin of dark matter.

Computer-generated drawing of the Alpha Magnetic Spectrometer (AMS). Credit: NASA
Computer-generated drawing of the Alpha Magnetic Spectrometer (AMS).
Credit: NASA

The MIT group leads an international collaboration of scientists that analyzed two and a half years’ worth of data taken by the Alpha Magnetic Spectrometer (AMS) — a large particle detector mounted on the exterior of the International Space Station — that captures incoming cosmic rays from all over the galaxy.

Among 41 billion cosmic ray events — instances of cosmic particles entering the detector — the researchers identified 10 million electrons and positrons, stable antiparticles of electrons. Positrons can exist in relatively small numbers within the cosmic ray flux.

An excess of these particles has been observed by previous experiments — suggesting that they may not originate from cosmic rays, but come instead from a new source. In 2013, the AMS collaboration, for the first time, accurately measured the onset of this excess.

The new AMS results may ultimately help scientists narrow in on the origin and features of dark matter — whose collisions may give rise to positrons.

The team reports the observed positron fraction — the ratio of the number of positrons to the combined number of positrons and electrons — within a wider energy range than previously reported. From the data, the researchers observed that this positron fraction increases quickly at low energies, after which it slows and eventually levels off at much higher energies.

The team reports that this is the first experimental observation of the positron fraction maximum — at 243 to 307 gigaelectronvolts (GeV) — after half a century of cosmic ray experiments.

“The new AMS results show unambiguously that a new source of positrons is active in the galaxy,” says Paolo Zuccon, an assistant professor of physics at MIT. “We do not know yet if these positrons are coming from dark matter collisions, or from astrophysical sources such as pulsars. But measurements are underway by AMS that may discriminate between the two hypotheses.”

The new measurements, Zuccon adds, are compatible with a dark matter particle with mass on the order of 1 teraelectronvolt (TeV) — about 1,000 times the mass of a proton.

Zuccon and his colleagues, including AMS’s principal investigator, Samuel Ting, the Thomas D. Cabot Professor of Physics at MIT, detail their results in two papers published today in the journal Physical Review Letters and in a third, forthcoming publication.

Catching a galactic stream

Nearly 85 percent of the universe is made of dark matter — matter that somehow does not emit or reflect light, and is therefore invisible to modern telescopes. For decades, astronomers have observed only the effects of dark matter, in the form of mysterious gravitational forces that seem to hold together clusters of galaxy that would otherwise fly apart. Such observations eventually led to the theory of an invisible, stabilizing source of gravitational mass, or dark matter.

The AMS experiment aboard the International Space Station aims to identify the origins of dark matter. The detector takes in a constant flux of cosmic rays, which Zuccon describes as “streams of the universe that bring with them everything they can catch around the galaxy.”

Presumably, this cosmic stream includes leftovers from the violent collisions between dark matter particles.

According to theoretical predictions, when two dark matter particles collide, they annihilate, releasing a certain amount of energy that depends on the mass of the original particles. When the particles annihilate, they produce ordinary particles that eventually decay into stable particles, including electrons, protons, antiprotons, and positrons.

As the visible matter in the universe consists of protons and electrons, the researchers reasoned that the contribution of these same particles from dark matter collisions would be negligible. However, positrons and antiprotons are much rarer in the universe; any detection of these particles above the very small expected background would likely come from a new source. The features of this excess — and in particular its onset, maximum position, and offset — will help scientists determine whether positrons arise from astrophysical sources such as pulsars, or from dark matter.

After continuously collecting data since 2011, the AMS team analyzed 41 billion incoming particles and identified 10 million positrons and electrons with energies ranging from 0.5 to 500 GeV — a wider energy range than previously measured.

The researchers studied the positron fraction versus energy, and found an excess of positrons starting at lower energies (8 GeV), suggesting a source for the particles other than the cosmic rays themselves. The positron fraction then slowed and peaked at 275 GeV, indicating that the data may be compatible with a dark matter source of positrons.

“Dark matter is there,” Zuccon says. “We just don’t know what it is. AMS has the possibility to shine a light on its features. We see some hint now, and it is within our possibility to say if that hint is true.”

If it turns out that the AMS results are due to dark matter, the experiment could establish that dark matter is a new kind of particle, says Barry Barish, a professor emeritus of physics and high-energy physics at the California Institute of Technology.

“The new phenomena could be evidence for the long-sought dark matter in the universe, or it could be due to some other equally exciting new science,” says Barish, who was not involved in the experiments. “In either case, the observation in itself is what is exciting; the scientific explanation will come with further experimentation.”

This research was funded in part by the U.S. Department of Energy.

Source: MIT News Office