Editing BaBar experiment

{{short description|Nuclear physics experiment}}
{{primary sources|date=June 2012}}
The '''BaBar experiment''', or simply '''BaBar''', is an international collaboration of more than 500 physicists and engineers studying the subatomic world at energies of approximately ten times the rest mass of a proton (~10&nbsp;[[Electronvolt|GeV]]). Its design was motivated by the investigation of [[CP symmetry|charge-parity violation]]. BaBar is located at the [[Stanford Linear Accelerator|SLAC National Accelerator Laboratory]], which is operated by [[Stanford University]] for the [[United States Department of Energy|Department of Energy]] in [[California]].

== Physics ==
BaBar was set up to understand the [[baryon asymmetry|disparity]] between the [[matter]] and [[antimatter]] content of the universe by measuring [[CP violation|Charge Parity violation]]. [[CP symmetry]] is a combination of [[C symmetry|'''C'''harge-conjugation]] symmetry (C symmetry) and [[P symmetry|'''P'''arity]] symmetry (P symmetry), each of which are conserved separately except in [[weak interaction]]s. BaBar focuses on the study of CP violation in the [[B meson]] system. The name of the experiment is derived from the nomenclature for the B meson (symbol '''{{SubatomicParticle|B}}''') and its [[antiparticle]] (symbol '''{{SubatomicParticle|AntiB}}''', pronounced '''B bar'''). The experiment's [[mascot]] was accordingly chosen to be [[Babar the Elephant]].

If CP symmetry holds, the [[decay rate]] of B mesons and their [[antiparticle]]s should be equal. Analysis of secondary particles produced in the BaBar detector showed this was not the case – in the summer of 2002, definitive results were published based on the analysis of 87 million {{SubatomicParticle|B}}/{{SubatomicParticle|AntiB}} meson-pair events, clearly showing the decay rates were not equal. Consistent results were found by the [[Belle experiment]] at the [[KEK laboratory]] in Japan.

CP violation was already predicted by the [[Standard Model]] of [[particle physics]], and well established in the [[Kaon#CP violation in neutral meson oscillations|neutral kaon system]] ({{SubatomicParticle|link=yes|Kaon}}/{{SubatomicParticle|link=yes|Antikaon}} meson pairs). The BaBar experiment has increased the accuracy to which this effect has been experimentally measured. Currently, results are consistent with the [[Standard Model]], but further investigation of a greater variety of decay modes may reveal discrepancies in the future.

The BaBar detector is a multilayer [[particle detector]]. Its large [[solid angle]] coverage (near [[Hermetic detector|hermetic]]), vertex location with precision on the order of 10&nbsp;[[micrometre|μm]] (provided by a silicon vertex detector), good [[pion]]–[[kaon]] separation at multi-[[GeV]] momenta (provided by a novel [[Cherenkov effect|Cherenkov]] detector), and few-percent precision electromagnetic calorimetry (CsI(Tl) scintillating crystals) allow a list of other scientific searches apart from CP violation in the B meson system.<ref>{{Cite journal | last1 = Aubert | first1 = B. | last2 = Bazan | first2 = A. | last3 = Boucham | first3 = A. | last4 = Boutigny | first4 = D. | last5 = De Bonis | first5 = I. | last6 = Favier | first6 = J. | last7 = Gaillard | first7 = J. -M. | last8 = Jeremie | first8 = A. | last9 = Karyotakis | first9 = Y. | last10 = Le Flour | first10 = T. | last11 = Lees | first11 = J. P. | last12 = Lieunard | first12 = S. | last13 = Petitpas | first13 = P. | last14 = Robbe | first14 = P. | last15 = Tisserand | first15 = V. | last16 = Zachariadou | first16 = K. | last17 = Palano | first17 = A. | last18 = Chen | first18 = G. P. | last19 = Chen | first19 = J. C. | last20 = Qi | first20 = N. D. | last21 = Rong | first21 = G. | last22 = Wang | first22 = P. | last23 = Zhu | first23 = Y. S. | last24 = Eigen | first24 = G. | last25 = Reinertsen | first25 = P. L. | last26 = Stugu | first26 = B. | last27 = Abbott | first27 = B. | last28 = Abrams | first28 = G. S. | last29 = Amerman | first29 = L. | last30 = Borgland | first30 = A. W. | title = The BABAR detector | doi = 10.1016/S0168-9002(01)02012-5 | journal = Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | volume = 479 | issue = 1 | pages = 1–116 | year = 2002 |arxiv = hep-ex/0105044 |bibcode = 2002NIMPA.479....1A | s2cid = 117579419 | display-authors = 29 }}</ref> Studies of rare decays and searches for exotic particles and precision measurements of phenomena associated with mesons containing [[bottom quark|bottom]] and [[charm quark]]s, as well as phenomena associated with [[tau lepton]]s are possible.

The BaBar detector ceased operation on 7 April 2008, but data analysis is ongoing.

== Detector description ==
[[File:Vertex Detector.svg|thumb|300px|alt=At the bottom of the image, two straight lines originate from a single point (the event origin), separate by an angle of 30 or so degrees. The two line cross two grids of squares (detector grids) placed on top of each other, separated by some distance. The grid squares crossed by the lines are highlighted in different color, corresponding to the detection of the particles which crossed them. |Principle of silicon vertex detectors: the particles' origin, where the event that created them occurred, can be found by extrapolating backwards from the charged regions (red) left on the sensors.]]

The BaBar detector is cylindrical with the interaction region at the center. In the interaction region, 9&nbsp;[[GeV]] [[electron]]s collide with 3.1&nbsp;GeV antielectrons (sometimes called [[positron]]s) to produce a [[center of mass|center-of-mass]] collision energy of 10.58&nbsp;GeV, corresponding to the [[Upsilon meson|{{SubatomicParticle|Upsilon}}(4S)]] resonance. The {{SubatomicParticle|Upsilon}}(4S) decays immediately into a pair of B mesons – half the time {{SubatomicParticle|B+}}{{SubatomicParticle|B-}} and half the time {{SubatomicParticle|B0}}{{SubatomicParticle|AntiB0}}. To detect the particles there are a series of subsystems arranged cylindrically around the interaction region. These subsystems are as follows, in order from inside to outside:

* '''[[Silicon Vertex Tracker]] (SVT)'''
: Made from 5 layers of double-sided silicon strips, the SVT records charged particle tracks very close to the interaction region inside BaBar.

* '''[[Drift chamber|Drift Chamber]] (DCH)'''
: Less expensive than silicon, the 40 layers of wires in this gas chamber detect charged particle tracks out to a much larger radius, providing a measurement of their momenta. In addition, the DCH also measures the energy loss of the particles as they pass through matter. See [[Bethe-Bloch formula]].

* '''[[Detector of Internally reflected Cherenkov light|Detector of Internally Reflected Cherenkov Light (DIRC)]]'''
: The DIRC is composed of 144 [[Fused quartz|fused silica]] bars which radiate and focus [[Cherenkov radiation]] to differentiate between [[kaon]]s and [[pion]]s.

* '''[[Calorimeter (particle physics)|Electromagnetic Calorimeter]] (EMC)'''
: Made from 6580 [[Caesium iodide|CsI]] crystals, the EMC identifies electrons and antielectrons, which allows for the reconstruction of the particle tracks of photons (and thus of neutral pions ({{SubatomicParticle|Pion0}})) and of "long Kaons" ({{SubatomicParticle|K-long}}), which are also electrically neutral.

* '''[[Magnet]]'''
: The Magnet produces a 1.5 [[tesla (unit)|T]] field inside the detector, which bends the tracks of charged particles allowing deduction of their momentum.

* '''Instrumented Flux Return (IFR)'''
: The IFR is designed to return the flux of the 1.5&nbsp;[[Tesla (unit)|T]] magnet, so it is mostly iron but there is also instrumentation to detect [[muon]]s and long kaons. The IFR is broken into 6 sextants and two endcaps. Each of the sextants has empty spaces which held the 19 layers of [[Resistive Plate Chamber]]s (RPC), which were replaced in 2004 and 2006 with [[Limited Streamer Tube]]s (LST) interleaved with brass. The brass is there to add mass for the interaction length since the LST modules are so much less massive than the RPCs. The LST system is designed to measure all three cylindrical coordinates of a track: which individual tube was hit gives the ''φ'' coordinate, which layer the hit was in gives the ''ρ'' coordinate, and finally the ''z''-planes atop the LSTs measure the ''z'' coordinate.

== Notable events ==

On 9 October 2005, BaBar recorded a record [[Luminosity (scattering theory)|luminosity]] just over {{nowrap|1 × 10<sup>34</sup> cm<sup>−2</sup>s<sup>−1</sup>}} delivered by the PEP-II [[particle accelerator|positron-electron collider]].<ref>[http://bbr-onlwww.slac.stanford.edu:8080/babarrc/RecLumPerDay-Run_5.gif Daily PEP-II-delivered and BaBar-recorded luminosities (bar chart)].{{dead link|date=May 2013}}  Accessed 11 October 2005.</ref>  This represents 330% of the luminosity that PEP-II was designed to deliver, and was produced along with a world record for stored current in an [[electron]] [[storage ring]] at 1.73&nbsp;[[Ampere|A]], paired with a record 2.94&nbsp;A of [[positron]]s. "For the BaBar experiment, higher luminosity means generating more collisions per second, which translates into more accurate results and the ability to find physics effects they otherwise couldn’t see."<ref>[http://www.slac.stanford.edu/front_page_continue.html Dynamic Performance from SLAC B-Factory]. Accessed 11 October 2005. {{webarchive |url=https://web.archive.org/web/20051016010355/http://www.slac.stanford.edu/front_page_continue.html |date=October 16, 2005 }}</ref>

In 2008, BaBar physicists detected the lowest energy particle in the bottomonium quark family, [[Bottom eta meson|η<sub>b</sub>]]. Spokesperson [[Hassan Jawahery]] said: "These results were highly sought after for over 30 years and will have an important impact on our understanding of the strong interactions."<ref>[http://www.physorg.com/news134923611.html Physicists Discover New Particle: the Bottom-most 'Bottomonium'] 2008-07-10, Accessed 2009-08-02</ref>

In May 2012 BaBar reported<ref>{{Cite journal|arxiv = 1205.5442|doi = 10.1103/PhysRevLett.109.101802|title = Evidence for an excess of {{overline|''B''}}→''D''<sup>(*)</sup>''τ''<sup>−</sup>{{overline|''ν''}}<sub>''τ''</sub> decays|year = 2012|last1 = Lees|first1 = J. P. |display-authors = etal |journal = Physical Review Letters|volume = 109|issue = 10|page = 101802|pmid = 23005279|s2cid = 20896961}}</ref> that their recently analyzed data may suggest deviations from predictions of the [[Standard Model]] of particle physics. The experiments see two particle decays, <math>B \to D^* \tau \nu</math> and <math>B \to D \tau \nu</math>, happen more often than the Standard Model predicts. In this type of decay, a B meson decays into a D or D* meson, a tau-lepton and an antineutrino.<ref>[http://esciencenews.com/articles/2012/06/18/babar.data.hint.cracks.standard.model BaBar data hint at cracks in the Standard Model (EScienceNews.com)].</ref>
While the significance of the excess (3.4 sigma) is not enough to claim a break from the Standard Model, the results are a potential sign of something amiss and are likely to impact existing theories. In 2015 results from [[LHCb]] and the [[Belle experiment]] strengthen the evidence (to 3.9 sigma) of possible physics beyond the Standard Model in these decay processes, but still not at the gold standard 5 sigma level of significance.<ref>[http://www.scientificamerican.com/article/2-accelerators-find-particles-that-may-break-known-laws-of-physics1/ 2 Accelerators Find Particles That May Break Known Laws of Physics. Sept 2015]</ref>

== Data record ==
{|class="wikitable" style="margin:1em auto; text-align:center"
! Run !! Period !! Integrated luminosity<ref>{{cite journal | author=BaBar Collaboration | title=Time-integrated luminosity recorded by the BABAR detector at the PEP-II e+e- collider | journal= Nuclear Instruments and Methods in Physics Research A| volume=726 | year=2013  | doi=10.1016/j.nima.2013.04.029 | pages=203–213| bibcode=2013NIMPA.726..203L | s2cid=33933422 | url=http://bura.brunel.ac.uk/handle/2438/8292 | hdl=10261/125266 | hdl-access=free }}</ref><br />([[barn (unit)|fb]]<sup>−1</sup>)
|-
| 1 || 22 October 1999 – 28 October 2000 || 22.93
|-
| 2 || 2 February 2001 – 30 June 2002 || 68.19
|-
| 3 || 8 December 2002 – 27 June 2003 || 34.72
|-
| 4 || 17 September 2003 – 31 July 2004 || 109.60
|-
| 5 || 16 April 2005 – 17 August 2006 || 146.61
|-
| 6 || 25 January 2007 – 4 September 2007 || 86.06
|-
| 7 || 13 December 2007 – 7 April 2008 || 45.60
|-
| Total || 22 October 1999 – 7 April 2008 || 513.70
|}

== See also ==
*[[B-factory]]
*[[B-Bbar oscillation]]

== Notes ==
<references />

== External links ==
* {{cite web | title = BaBar Experiment Confirms Time Asymmetry | url = http://www6.slac.stanford.edu/news/2012-11-19-babar-trv.aspx }}
* [http://www.slac.stanford.edu/BFROOT/ Official BaBar Website]
* [http://www-public.slac.stanford.edu/babar/ BaBar Public Home Page]
* [http://physicsweb.org/articles/world/14/8/9 Report of 2001 announcement about detection of CP violation]
* [https://www.youtube.com/watch?v=dj7gCZTEoq0 YouTube Video of the BaBar Control Room] April 30, 2007
*Record for [https://inspirehep.net/experiments/1108553 BaBar Experiment] on [[INSPIRE-HEP]]

[[Category:Particle experiments]]
[[Category:B physics]]