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

GeV GEV may refer to: * ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games * Ashe County Airport, in North Carolina, United States * Gällivare Lapland Airport, in Sweden * Generalized extreme value distribution * Gev Sella, Israeli-Sou ...

). Its design was motivated by the investigation of charge-parity violation. BaBar is located at the

SLAC National Accelerator Laboratory SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratories, United States Department of Energy National Laboratory operated by Stanford Univers ...

, which is operated by

Stanford University Stanford University, officially Leland Stanford Junior University, is a private research university in Stanford, California. The campus occupies , among the largest in the United States, and enrolls over 17,000 students. Stanford is consider ...

for the

Department of Energy A Ministry of Energy or Department of Energy is a government department in some countries that typically oversees the production of fuel and electricity; in the United States, however, it manages nuclear weapons development and conducts energy-rel ...

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

Physics

BaBar was set up to understand the

disparity Disparity and disparities may refer to: in healthcare: * Health disparities in finance: * Income disparity between females and males. **Male–female income disparity in the United States **Income gender gap * Economic inequality * Income inequ ...

between the

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...

and

antimatter In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...

content of the universe by measuring Charge Parity violation.

CP symmetry In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge symmetry) and P-symmetry (parity symmetry). CP-symmetry states that the laws of physics should be the ...

is a combination of Charge-conjugation symmetry (C symmetry) and Parity symmetry (P symmetry), each of which are conserved separately except in

weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, ...

s. BaBar focuses on the study of CP violation in the

B meson In particle physics, B mesons are mesons composed of a bottom antiquark and either an up (), down (), strange () or charm quark (). The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark' ...

system. The name of the experiment is derived from the nomenclature for the B meson (symbol ) and its

antiparticle In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also known as an antie ...

(symbol , pronounced B bar). The experiment's

mascot A mascot is any human, animal, or object thought to bring luck, or anything used to represent a group with a common public identity, such as a school, professional sports team, society, military unit, or brand name. Mascots are also used as fi ...

was accordingly chosen to be

Babar the Elephant Babar the Elephant (, ; ) is an elephant character who first appeared in 1931 in the French children's book ''Histoire de Babar'' by Jean de Brunhoff. The book is based on a tale that Brunhoff's wife, Cécile, had invented for their children. ...

. If CP symmetry holds, the

decay rate Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

of B mesons and their

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 / meson-pair events, clearly showing the decay rates were not equal. Consistent results were found by the

Belle experiment The Belle experiment was a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers, at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibarak ...

at the KEK laboratory in Japan. CP violation was already predicted by the

Standard Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions - excluding gravity) in the universe and classifying a ...

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

, and well established in the neutral kaon system (/ meson pairs). The BaBar experiment has increased the accuracy to which this effect has been experimentally measured. Currently, results are consistent with the

, but further investigation of a greater variety of decay modes may reveal discrepancies in the future. The BaBar detector is a multilayer

particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing particles, such as those produced by nuc ...

. Its large

solid angle In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poi ...

coverage (near

hermetic Hermetic or related forms may refer to: * of or related to the ancient Greek Olympian god Hermes * of or related to Hermes Trismegistus, a legendary Hellenistic figure based on the Greek god Hermes and the Egyptian god Thoth ** , the ancient and m ...

), vertex location with precision on the order of 10

μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

(provided by a silicon vertex detector), good

pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...

–

kaon KAON (Karlsruhe ontology) is an ontology infrastructure developed by the University of Karlsruhe and the Research Center for Information Technologies in Karlsruhe. Its first incarnation was developed in 2002 and supported an enhanced version of ...

separation at multi-

momenta (provided by a novel 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. Studies of rare decays and searches for exotic particles and precision measurements of phenomena associated with mesons containing

bottom Bottom may refer to: Anatomy and sex * Bottom (BDSM), the partner in a BDSM who takes the passive, receiving, or obedient role, to that of the top or dominant * Bottom (sex), a term used by gay couples and BDSM * Buttocks or bottom, part of th ...

and

charm quark The charm quark, charmed quark or c quark (from its symbol, c) is the third-most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks. Examples of hadrons containin ...

s, as well as phenomena associated with

tau lepton The tau (), also called the tau lepton, tau particle, tauon or tau electron, is an elementary particle similar to the electron, with negative electric charge and a spin of . Like the electron, the muon, and the three neutrinos, the tau is a l ...

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

Detector description

The BaBar detector is cylindrical with the interaction region at the center. In the interaction region, 9

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s collide with 3.1 GeV antielectrons (sometimes called

positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...

s) to produce a center-of-mass collision energy of 10.58 GeV, corresponding to the (4S) resonance. The (4S) decays immediately into a pair of B mesons – half the time and half the time . 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 A wire chamber or multi-wire proportional chamber is a type of proportional counter that detects charged particles and photons and can give positional information on their trajectory, by tracking the trails of gaseous ionization. was located via ...

(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 (DIRC) : The DIRC is composed of 144

fused silica Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...

bars which radiate and focus Cherenkov radiation to differentiate between

s and

s. * Electromagnetic Calorimeter (EMC) : Made from 6580 CsI crystals, the EMC identifies electrons and antielectrons, which allows for the reconstruction of the particle tracks of photons (and thus of neutral pions ()) and of "long Kaons" (), which are also electrically neutral. *

Magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

: The Magnet produces a 1.5 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 T magnet, so it is mostly iron but there is also instrumentation to detect

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...

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 A Resistive plate chamber (RPC) is a particle detector widely used in high energy physics. They are used for detecting muons in most of the modern experiments including ATLAS, CMS, and BES III The Beijing Spectrometer III (BES III) is a particle phy ...

s (RPC), which were replaced in 2004 and 2006 with Limited Streamer Tubes (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 Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

just over delivered by the PEP-II positron-electron collider. 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

storage ring A storage ring is a type of circular particle accelerator in which a continuous or pulsed particle beam may be kept circulating typically for many hours. Storage of a particular particle depends upon the mass, momentum and usually the charge of th ...

at 1.73 A, paired with a record 2.94 A of

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." In 2008, BaBar physicists detected the lowest energy particle in the bottomonium quark family, η_b. 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." In May 2012 BaBar reported that their recently analyzed data may suggest deviations from predictions of the

of particle physics. The experiments see two particle decays,

B \to D^* \tau \nu

and

B \to D \tau \nu

, 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. 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 The LHCb (Large Hadron Collider beauty) experiment is one of eight particle physics detector experiments collecting data at the Large Hadron Collider at CERN. LHCb is a specialized b-physics experiment, designed primarily to measure the paramet ...

and the

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.2 Accelerators Find Particles That May Break Known Laws of Physics. Sept 2015
/ref>

Data record

Notes

External links

* {{cite web , title = BaBar Experiment Confirms Time Asymmetry , url = http://www6.slac.stanford.edu/news/2012-11-19-babar-trv.aspx
Official BaBar Website

BaBar Public Home Page

Report of 2001 announcement about detection of CP violation

YouTube Video of the BaBar Control Room
April 30, 2007 *Record fo
BaBar Experiment
on

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