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{{Short description|Hypothetical particle}}
'''Leptoquarks''' (LQs) are hypothetical particles that would interact with [[quarks]] and [[leptons]]. Leptoquarks are color-triplet [[boson]]s that carry both [[Lepton number|lepton]] and [[baryon number]]s. Their other [[quantum numbers]], like [[spin (physics)|spin]], (fractional) [[electric charge]] and [[weak isospin]] vary among theories. Leptoquarks are encountered in various extensions of the [[Standard Model]], such as [[technicolor (physics)|technicolor]] theories, theories of quark-lepton unification (e.g., [[Pati–Salam model]]), or [[Grand unification theory|GUT]]s based on [[Georgi–Glashow model|SU(5)]], [[SO(10)]], [[E6 (mathematics)|E<sub>6</sub>]], etc. Leptoquarks are currently searched for in experiments [[ATLAS experiment|ATLAS]] and [[CMS experiment|CMS]] at the [[Large Hadron Collider]] in [[CERN]].
'''Leptoquarks''' are hypothetical particles that would interact with [[quarks]] and [[leptons]]. Leptoquarks are color-triplet [[boson]]s that carry both [[Lepton number|lepton]] and [[baryon number]]s. Their other [[quantum numbers]], like [[Spin (physics)|spin]], (fractional) [[electric charge]] and [[weak isospin]] vary among models. Leptoquarks are encountered in various extensions of the [[Standard Model]], such as [[technicolor (physics)|technicolor]] theories, theories of quark–lepton unification (e.g., [[Pati–Salam model]]), or [[Grand unification theory|GUT]]s based on [[Georgi–Glashow model|SU(5)]], [[SO(10)]], [[E6 (mathematics)|E<sub>6</sub>]], etc. Leptoquarks are currently searched for in experiments [[ATLAS experiment|ATLAS]] and [[CMS experiment|CMS]] at the [[Large Hadron Collider]] in [[CERN]].


In March 2021, there were some reports to hint at the possible existence of leptoquarks as an unexpected difference in how [[Bottom quark|beauty quarks]] decay to create electrons or muons. The measurement has been made at a statistical significance of 3.1[[Standard deviation|σ]], which is well below the 5σ level that is usually considered a discovery.<ref>{{cite magazine| url= https://physicsworld.com/a/has-a-new-particle-called-a-leptoquark-been-spotted-at-cern/ | title = Has a new particle called a 'leptoquark' been spotted at CERN? | first=Hamish |last=Johnston | magazine=[[Physics World]] | date= 23 March 2021 | archive-url = https://web.archive.org/web/20210324054334/https://physicsworld.com/a/has-a-new-particle-called-a-leptoquark-been-spotted-at-cern/ | archive-date = 24 March 2021 }}</ref>
In March 2021, there were some reports to hint at the possible existence of leptoquarks as an unexpected difference in how [[bottom quark]]s decay to create electrons or muons. The measurement has been made at a statistical significance of 3.1[[Standard deviation|σ]], which is well below the 5σ level that is usually considered a discovery.<ref>{{cite magazine| url= https://physicsworld.com/a/has-a-new-particle-called-a-leptoquark-been-spotted-at-cern/ | title = Has a new particle called a 'leptoquark' been spotted at CERN? | first=Hamish |last=Johnston | magazine=[[Physics World]] | date= 23 March 2021 | archive-url = https://web.archive.org/web/20210324054334/https://physicsworld.com/a/has-a-new-particle-called-a-leptoquark-been-spotted-at-cern/ | archive-date = 24 March 2021 }}</ref>


==Overview==
== Overview ==
Leptoquarks, if they exist, must be heavier than all the currently known [[elementary particles]] otherwise they would have already been discovered. Current experimental lower limits on LQ mass (depending on their type) are around 1 [[TeV]]/c<sup>2</sup> (i.e., about 1000 times more than the [[proton]] mass).
Leptoquarks, if they exist, must be heavier than any of the currently known [[elementary particles]], otherwise they would have already been discovered. Current experimental lower limits on leptoquark mass (depending on their type) are around {{val|1|ul=TeV/c2}} (i.e., about 1000 times the [[proton]] mass).
By definition, leptoquarks [[particle decay|decay]] directly into a [[quark]] and a [[lepton]] or an antilepton. Like most of other [[elementary particle]]s they live for a very short time and are not present in ordinary matter.
By definition, leptoquarks [[particle decay|decay]] directly into a [[quark]] and a [[lepton]] or an antilepton. Like most of other [[elementary particle]]s, they live for a very short time and are not present in ordinary matter.
However, they might be produced in high energy particle collisions such as in particle [[collider]]s or from [[cosmic ray]]s hitting Earth's atmosphere.
However, they might be produced in high energy particle collisions such as in particle [[collider]]s or from [[cosmic ray]]s hitting the Earth's atmosphere.


Like quarks, leptoquarks must carry [[Color charge|color]] and therefore must also interact with [[gluon]]s. This [[strong interaction]] of theirs is important for their production in [[hadron]] [[collider]]s (such as [[Tevatron]] or [[LHC]]).
Like quarks, leptoquarks must carry [[Color charge|color]] and therefore must also interact with [[gluon]]s. This [[strong interaction]] of theirs is important for their production in [[hadron]] [[collider]]s (such as the [[Tevatron]] or [[LHC]]).


== Simplified typology (according to electric charge) ==
== Simplified typology (according to electric charge) ==
Several kinds of leptoquarks, depending on their [[electric charge]], can be considered:
Several kinds of leptoquarks, depending on their [[electric charge]], can be considered:
* Q=5/3: Such a LQ decays into up-type quarks ([[up quark|u<sup>2/3</sup>]], [[charm quark|c<sup>2/3</sup>]], [[top quark|t<sup>2/3</sup>]]) and charged leptons ([[electron|e<sup>−</sup>]], [[muon|μ<sup>−</sup>]], [[tau lepton|τ<sup>−</sup>]]).
* ''Q'' = {{frac|5|3}}: Such a leptoquark decays into up-type quarks ([[up quark|up]], [[charm quark|charm]], [[top quark|top]]) and charged [[antilepton]]s ([[positron|e<sup>+</sup>]], [[antimuon|μ<sup>+</sup>]], [[tau lepton|τ<sup>+</sup>]]).
* Q=2/3: LQ decays into up-type quarks and [[neutrino]]s (or antineutrinos), and/or to down-type quarks ([[down quark|d<sup>−1/3</sup>]], [[strange quark|s<sup>−1/3</sup>]], [[bottom quark|b<sup>−1/3</sup>]]) and charged leptons.
* ''Q'' = {{frac|2|3}}: Such a leptoquark decays into up-type quarks and [[neutrino]]s (or antineutrinos), and/or to down-type quarks ([[down quark|down]], [[strange quark|strange]], [[bottom quark|bottom]]) and charged antileptons.
* Q=−1/3: LQ decays into down-type quarks and (anti)neutrinos, and/or to up-type quark and a charged antilepton.
* ''Q'' = −{{frac|1|3}}: Such a leptoquark decays into down-type quarks and (anti)neutrinos, and/or to up-type quark and a charged lepton.
* Q=−4/3: LQ decays into down-type quarks and charged antileptons.
* ''Q'' = −{{frac|4|3}}: Such a leptoquark decays into down-type quarks and charged leptons.


If a LQ with a given charge exists, its [[antiparticle]] with an opposite charge and which would decay into [[charge conjugation|conjugated states]] to those listed above, must exist as well.
If a leptoquark with a given charge exists, its [[antiparticle]] with an opposite charge and which would decay into [[charge conjugation|conjugated states]] to those listed above, must exist as well.


A leptoquark with given electric charge may, in general, interact with any combination of a lepton and quark with given electric charges (this yields up to 3×3=9 distinct interactions of a single type of a LQ). However, experimental searches usually assume that only one of those "channels" is possible. Especially, a 2/3-charged leptoquark decaying into an [[electron]] and a [[d quark]] is called a "first [[Generation (particle physics)|generation]] LQ", a leptoquark decaying into [[s quark]] and [[muon]] is a "second-generation LQ" etc. Nevertheless, most theories do not bring much of a theoretical motivation to believe that LQs have only a single interaction and that the [[Generation (particle physics)|generation]] of the quark and lepton involved is the same.<ref>{{cite journal |doi=10.1007/JHEP10(2017)097 |journal=Journal of High Energy Physics |first1=B. |last1=Diaz |first2=M. |last2 = Schmaltz |first3=Y.-M.|last3=Zhong |title=The leptoquark hunter's guide: pair production |volume=97 |year=2017 |issue=10 |page=97 |arxiv=1706.05033|bibcode=2017JHEP...10..097D |s2cid=118894139 }}</ref>
A leptoquark with given electric charge may, in general, interact with any combination of a lepton and quark with given electric charges (this yields up to {{nowrap|1=3 × 3 = 9}} distinct interactions of a single type of a leptoquark). However, experimental searches usually assume that only one of those "channels" is possible. Especially, a ''Q''&nbsp;=&nbsp;{{frac|2|3}} charged leptoquark that decays into a [[positron]] and a [[down quark]] is called a "first [[Generation (particle physics)|generation]] leptoquark", a leptoquark that decays into [[strange quark]] and [[antimuon]] is a "second-generation leptoquark" etc. Nevertheless, most theories do not bring much of a theoretical motivation to believe that leptoquarks have only a single interaction and that the [[Generation (particle physics)|generation]] of the quark and lepton involved is the same.<ref>{{cite journal |doi=10.1007/JHEP10(2017)097 |journal=Journal of High Energy Physics |first1=B. |last1=Diaz |first2=M. |last2 = Schmaltz |first3=Y.-M.|last3=Zhong |title=The leptoquark hunter's guide: pair production |volume=97 |year=2017 |issue=10 |page=97 |arxiv=1706.05033|bibcode=2017JHEP...10..097D |s2cid=118894139 }}</ref>


==Leptoquarks and proton decay==
== Leptoquarks and proton decay ==
Existence of pure leptoquarks would not spoil the [[baryon number]] conservation. However, some theories allow (or require) the leptoquark to also have a diquark interaction vertex. For example, a Q=2/3 charged leptoquark might also decay into two d-type antiquarks. Existence of such a leptoquark-diquark would cause [[Proton decay|protons to decay]]. The current limits on proton lifetime are strong probes of existence of these leptoquark-diquarks. These fields emerge in [[Grand unification theory|Grand unification theories]]; for example, in the [[Georgi–Glashow model|Georgi–Glashow SU(5) model]], they are called [[X and Y bosons]].
Existence of pure leptoquarks would not spoil the [[baryon number]] conservation. However, some theories allow (or require) the leptoquark to also have a diquark interaction vertex. For example, a ''Q''&nbsp;=&nbsp;{{frac|2|3}} charged leptoquark might also decay into two down-type antiquarks. Existence of such a leptoquark-diquark would cause [[Proton decay|protons to decay]]. The current limits on proton lifetime are strong probes of existence of these leptoquark-diquarks. These fields emerge in [[Grand Unified Theory|grand unification theories]]; for example, in the [[Georgi–Glashow model|Georgi–Glashow SU(5) model]], they are called [[X and Y bosons]].


==Experimental searches==
== Experimental searches ==
In 1997, an excess of events at the HERA accelerator created a stir in the [[particle physics]] community, because one possible explanation of the excess was the involvement of leptoquarks.<ref>{{cite magazine |url=http://www.sciam.com/article.cfm?id=0004DE1C-609D-1C76-9B81809EC588EF21&page=2 |magazine=Scientific American |title=Leaping leptoquarks! Hints of "new physics" emerge from German accelerators |first=John |last=Horgan |date=24 March 1997}}</ref> However, later studies performed both at HERA and at the [[Tevatron]] with larger samples of data ruled out this possibility for masses of the leptoquark up to around 275–325&nbsp;GeV.<ref>{{cite journal |collaboration=H1 Collaboration |doi=10.1016/j.physletb.2005.09.048 |journal=Physics Letters B |last1=Andreev |first1=V. |display-authors=etal |title=Search for leptoquark bosons in e&nbsp;p collisions at HERA |volume=629 |pages=9–19 |year=2005 |issue=1 |arxiv=hep-ex/0506044 |bibcode=2005PhLB..629....9H|s2cid=119363170 }}</ref> Second generation leptoquarks were also looked for and not found.<ref>{{cite web |url=http://www-d0.fnal.gov/public/pubs/leptoquark_prl.html |title=The search for leptoquarks |publisher=Fermi National Accelerator Laboratory (Fermilab)}}</ref>
In 1997, an excess of events at the HERA accelerator created a stir in the [[particle physics]] community, because one possible explanation of the excess was the involvement of leptoquarks.<ref>{{cite magazine |url=http://www.sciam.com/article.cfm?id=0004DE1C-609D-1C76-9B81809EC588EF21&page=2 |magazine=Scientific American |title=Leaping leptoquarks! Hints of "new physics" emerge from German accelerators |first=John |last=Horgan |date=24 March 1997}}</ref> However, later studies performed both at HERA and at the [[Tevatron]] with larger samples of data ruled out this possibility for masses of the leptoquark up to around {{val|275|-|325|u=GeV/c2}}.<ref>{{cite journal |collaboration=H1 Collaboration |doi=10.1016/j.physletb.2005.09.048 |journal=Physics Letters B |last1=Andreev |first1=V. |display-authors=etal |title=Search for leptoquark bosons in e&nbsp;p collisions at HERA |volume=629 |pages=9–19 |year=2005 |issue=1 |arxiv=hep-ex/0506044 |bibcode=2005PhLB..629....9H|s2cid=119363170 }}</ref> Second generation leptoquarks were also looked for and not found.<ref>{{cite web |url=http://www-d0.fnal.gov/public/pubs/leptoquark_prl.html |title=The search for leptoquarks |publisher=Fermi National Accelerator Laboratory (Fermilab)}}</ref>


Current best limits on leptoquarks are set by [[LHC]], which has been searching for the first, second, and third generation of leptoquarks and some mixed-generation leptoquarks<ref>{{cite journal |url=http://pdg.lbl.gov/2018/reviews/rpp2018-rev-leptoquark-quantum-numbers.pdf |title=Review of Particle Physics: Leptoquark quantum numbers |year=2018 |first=M. |last=Tanabashi |display-authors=etal |collaboration=Particle Data Group |journal=Physical Review D |volume=98 |issue=3 |pages=030001 |doi=10.1103/PhysRevD.98.030001 |doi-access=free}}</ref>
Current best limits on leptoquarks are set by [[LHC]], which has been searching for the first, second, and third generation of leptoquarks and some mixed-generation leptoquarks<ref>{{cite journal |url=http://pdg.lbl.gov/2018/reviews/rpp2018-rev-leptoquark-quantum-numbers.pdf |title=Review of Particle Physics: Leptoquark quantum numbers |year=2018 |first=M. |last=Tanabashi |display-authors=etal |collaboration=Particle Data Group |journal=Physical Review D |volume=98 |issue=3 |pages=030001 |doi=10.1103/PhysRevD.98.030001 |doi-access=free}}</ref>
and have raised the lower mass limit to about 1&nbsp;TeV.<ref>{{cite web |url=http://pdg.lbl.gov/2016/reviews/rpp2016-rev-leptoquark-quantum-numbers.pdf |collaboration=Particle Data Group |title=Leptoquarks review |year=2016 |publisher=Lawrence Berkeley National Laboratory |place=Berkeley, California}}</ref> For leptoquarks coupling to a neutrino and a quark to be proven to exist, the missing energy in particle collisions attributed to neutrinos would have to be excessively energetic. It is likely that the creation of leptoquarks would mimic the creation of massive quarks.<ref>{{cite web |url=http://nicadd.niu.edu/~hedin/lq3.html |title=Search for third generation leptoquarks |author=Hedin, David |publisher=Northern Illinois University |place=DeKalb, IL |url-status=live |access-date=2020-03-05 |df=dmy-all}}</ref>
and have raised the lower mass limit to about {{val|1|u=TeV/c2}}.<ref>{{cite web |url=http://pdg.lbl.gov/2016/reviews/rpp2016-rev-leptoquark-quantum-numbers.pdf |collaboration=Particle Data Group |title=Leptoquarks review |year=2016 |publisher=Lawrence Berkeley National Laboratory |place=Berkeley, California}}</ref> For leptoquarks coupling to a neutrino and a quark to be proven to exist, the missing energy in particle collisions attributed to neutrinos would have to be excessively energetic. It is likely that the creation of leptoquarks would mimic the creation of massive quarks.<ref>{{cite web |url=http://nicadd.niu.edu/~hedin/lq3.html |title=Search for third generation leptoquarks |author=Hedin, David |publisher=Northern Illinois University |place=DeKalb, IL |access-date=2020-03-05 |df=dmy-all}}</ref>


For leptoquarks coupling to electrons and up or down quarks, experiments of atomic parity violation and parity-violating electron scattering set the best limits.
For leptoquarks coupling to electrons and up or down quarks, experiments of atomic parity violation and parity-violating electron scattering set the best limits.
Line 34: Line 35:
The [[LHeC]] project to add an electron ring to collide bunches with the existing [[Large Hadron Collider|LHC]] proton ring is proposed as a project to look for higher-generation leptoquarks.<ref>{{cite web |url=http://www.ep.ph.bham.ac.uk/exp/LHeC/ |title=Birmingham LHeC project page}}</ref>
The [[LHeC]] project to add an electron ring to collide bunches with the existing [[Large Hadron Collider|LHC]] proton ring is proposed as a project to look for higher-generation leptoquarks.<ref>{{cite web |url=http://www.ep.ph.bham.ac.uk/exp/LHeC/ |title=Birmingham LHeC project page}}</ref>


==See also==
== See also ==
* [[X and Y bosons]]
* [[X and Y bosons]]
* [[Quark–lepton complementarity]]
* [[Quark–lepton complementarity]]


==References==
== References ==
{{Reflist|30em}}
{{Reflist|30em}}


Line 46: Line 47:
[[Category:Hypothetical elementary particles]]
[[Category:Hypothetical elementary particles]]
[[Category:Grand Unified Theory]]
[[Category:Grand Unified Theory]]
[[Category:Gauge bosons]]

Latest revision as of 18:48, 7 November 2023

Leptoquarks are hypothetical particles that would interact with quarks and leptons. Leptoquarks are color-triplet bosons that carry both lepton and baryon numbers. Their other quantum numbers, like spin, (fractional) electric charge and weak isospin vary among models. Leptoquarks are encountered in various extensions of the Standard Model, such as technicolor theories, theories of quark–lepton unification (e.g., Pati–Salam model), or GUTs based on SU(5), SO(10), E6, etc. Leptoquarks are currently searched for in experiments ATLAS and CMS at the Large Hadron Collider in CERN.

In March 2021, there were some reports to hint at the possible existence of leptoquarks as an unexpected difference in how bottom quarks decay to create electrons or muons. The measurement has been made at a statistical significance of 3.1σ, which is well below the 5σ level that is usually considered a discovery.[1]

Overview

[edit]

Leptoquarks, if they exist, must be heavier than any of the currently known elementary particles, otherwise they would have already been discovered. Current experimental lower limits on leptoquark mass (depending on their type) are around 1 TeV/c2 (i.e., about 1000 times the proton mass). By definition, leptoquarks decay directly into a quark and a lepton or an antilepton. Like most of other elementary particles, they live for a very short time and are not present in ordinary matter. However, they might be produced in high energy particle collisions such as in particle colliders or from cosmic rays hitting the Earth's atmosphere.

Like quarks, leptoquarks must carry color and therefore must also interact with gluons. This strong interaction of theirs is important for their production in hadron colliders (such as the Tevatron or LHC).

Simplified typology (according to electric charge)

[edit]

Several kinds of leptoquarks, depending on their electric charge, can be considered:

  • Q = 5⁄3: Such a leptoquark decays into up-type quarks (up, charm, top) and charged antileptons (e+, μ+, τ+).
  • Q = 2⁄3: Such a leptoquark decays into up-type quarks and neutrinos (or antineutrinos), and/or to down-type quarks (down, strange, bottom) and charged antileptons.
  • Q = −1⁄3: Such a leptoquark decays into down-type quarks and (anti)neutrinos, and/or to up-type quark and a charged lepton.
  • Q = −4⁄3: Such a leptoquark decays into down-type quarks and charged leptons.

If a leptoquark with a given charge exists, its antiparticle with an opposite charge and which would decay into conjugated states to those listed above, must exist as well.

A leptoquark with given electric charge may, in general, interact with any combination of a lepton and quark with given electric charges (this yields up to 3 × 3 = 9 distinct interactions of a single type of a leptoquark). However, experimental searches usually assume that only one of those "channels" is possible. Especially, a Q = 2⁄3 charged leptoquark that decays into a positron and a down quark is called a "first generation leptoquark", a leptoquark that decays into strange quark and antimuon is a "second-generation leptoquark" etc. Nevertheless, most theories do not bring much of a theoretical motivation to believe that leptoquarks have only a single interaction and that the generation of the quark and lepton involved is the same.[2]

Leptoquarks and proton decay

[edit]

Existence of pure leptoquarks would not spoil the baryon number conservation. However, some theories allow (or require) the leptoquark to also have a diquark interaction vertex. For example, a Q = 2⁄3 charged leptoquark might also decay into two down-type antiquarks. Existence of such a leptoquark-diquark would cause protons to decay. The current limits on proton lifetime are strong probes of existence of these leptoquark-diquarks. These fields emerge in grand unification theories; for example, in the Georgi–Glashow SU(5) model, they are called X and Y bosons.

Experimental searches

[edit]

In 1997, an excess of events at the HERA accelerator created a stir in the particle physics community, because one possible explanation of the excess was the involvement of leptoquarks.[3] However, later studies performed both at HERA and at the Tevatron with larger samples of data ruled out this possibility for masses of the leptoquark up to around 275–325 GeV/c2.[4] Second generation leptoquarks were also looked for and not found.[5]

Current best limits on leptoquarks are set by LHC, which has been searching for the first, second, and third generation of leptoquarks and some mixed-generation leptoquarks[6] and have raised the lower mass limit to about 1 TeV/c2.[7] For leptoquarks coupling to a neutrino and a quark to be proven to exist, the missing energy in particle collisions attributed to neutrinos would have to be excessively energetic. It is likely that the creation of leptoquarks would mimic the creation of massive quarks.[8]

For leptoquarks coupling to electrons and up or down quarks, experiments of atomic parity violation and parity-violating electron scattering set the best limits.

The LHeC project to add an electron ring to collide bunches with the existing LHC proton ring is proposed as a project to look for higher-generation leptoquarks.[9]

See also

[edit]

References

[edit]
  1. ^ Johnston, Hamish (23 March 2021). "Has a new particle called a 'leptoquark' been spotted at CERN?". Physics World. Archived from the original on 24 March 2021.
  2. ^ Diaz, B.; Schmaltz, M.; Zhong, Y.-M. (2017). "The leptoquark hunter's guide: pair production". Journal of High Energy Physics. 97 (10): 97. arXiv:1706.05033. Bibcode:2017JHEP...10..097D. doi:10.1007/JHEP10(2017)097. S2CID 118894139.
  3. ^ Horgan, John (24 March 1997). "Leaping leptoquarks! Hints of "new physics" emerge from German accelerators". Scientific American.
  4. ^ Andreev, V.; et al. (H1 Collaboration) (2005). "Search for leptoquark bosons in e p collisions at HERA". Physics Letters B. 629 (1): 9–19. arXiv:hep-ex/0506044. Bibcode:2005PhLB..629....9H. doi:10.1016/j.physletb.2005.09.048. S2CID 119363170.
  5. ^ "The search for leptoquarks". Fermi National Accelerator Laboratory (Fermilab).
  6. ^ Tanabashi, M.; et al. (Particle Data Group) (2018). "Review of Particle Physics: Leptoquark quantum numbers" (PDF). Physical Review D. 98 (3): 030001. doi:10.1103/PhysRevD.98.030001.
  7. ^ "Leptoquarks review" (PDF). Berkeley, California: Lawrence Berkeley National Laboratory. 2016.
  8. ^ Hedin, David. "Search for third generation leptoquarks". DeKalb, IL: Northern Illinois University. Retrieved 5 March 2020.
  9. ^ "Birmingham LHeC project page".
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