Helen Caines
· Horace D. Taft (B.A. 1949) Professor of Physics / Director of Graduate StudiesVerifiedYale University · Department of Physics
Active 1993–2025
About
Helen Caines is a Professor of Physics and the Director of Graduate Studies in the Department of Physics at Yale University. Her research concentrates on understanding the behavior of nuclear matter under extremes of temperature and density. She investigates the formation and properties of Quark Gluon Plasma (QGP), a deconfined state of quarks and gluons that exists when energy densities exceed six times that of nuclear matter. Helen collaborates on the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven Laboratory and on the ALICE experiment at the Large Hadron Collider (LHC) at CERN. Her work involves measuring high momentum particles produced in heavy-ion collisions and studying their interactions with the QGP to gain insights into this new state of matter. She holds a Ph.D. from the University of Birmingham, earned in 1996, and has been recognized as a Fellow of the American Physical Society and the Institute of Physics, UK. Her contributions to experimental nuclear physics and her focus on understanding the properties of nuclear matter under extreme conditions are central to her scientific endeavors.
Research topics
- Particle physics
- Physics
- Nuclear physics
- Quantum mechanics
- Statistics
- Atomic physics
- Computer Science
- Astronomy
- Geometry
- Mathematics
- Mathematical analysis
Selected publications
Flavour-dependent chemical freeze-out of light nuclei in relativistic heavy-ion collisions
The European Physical Journal C · 2025-09-30
articleOpen accessSenior authorAbstract We study the production of light nuclei in Au+Au collisions at $$\sqrt{s_\textrm{NN}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:msub> <mml:mi>s</mml:mi> <mml:mtext>NN</mml:mtext> </mml:msub> </mml:msqrt> </mml:math> = 7.7–200 GeV and Pb+Pb collisions at $$\sqrt{s_\textrm{NN}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:msub> <mml:mi>s</mml:mi> <mml:mtext>NN</mml:mtext> </mml:msub> </mml:msqrt> </mml:math> = 2.76 and 5.02 TeV within a flavour-dependent chemical freeze-out scenario, assuming different flavoured hadrons undergo separate chemical freeze-out. Using the package, thermal parameters extracted from fits to various sets of hadron yields, including and excluding light nuclei, are used to calculate the ratios of the yields of light nuclei, namely, d / p , $$\bar{d}/\bar{p}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mover> <mml:mrow> <mml:mi>d</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> <mml:mo>/</mml:mo> <mml:mover> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> , t / p , t / d , $$^4\text {He}/^3\text {He}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>4</mml:mn> </mml:mmultiscripts> <mml:mtext>He</mml:mtext> <mml:msup> <mml:mo>/</mml:mo> <mml:mn>3</mml:mn> </mml:msup> <mml:mtext>He</mml:mtext> </mml:mrow> </mml:math> , and $$^3\text {H}_{\varLambda }/^3\text {He}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>3</mml:mn> </mml:mmultiscripts> <mml:msub> <mml:mtext>H</mml:mtext> <mml:mi>Λ</mml:mi> </mml:msub> <mml:msup> <mml:mo>/</mml:mo> <mml:mn>3</mml:mn> </mml:msup> <mml:mtext>He</mml:mtext> </mml:mrow> </mml:math> . A comparison with experimental data from the STAR and ALICE collaborations shows that a sequential freeze-out scenario provides a better description of light nuclei yield ratios than the traditional single freeze-out approach. These results suggest the flavour-dependent chemical freeze-out for final state light nuclei production persists in heavy-ion collisions at both RHIC and LHC energies.
Physical review. D/Physical review. D. · 2025-06-23
articleOpen accessWe report measurements of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi mathvariant="normal">ϒ</a:mi><a:mo stretchy="false">(</a:mo><a:mn>1</a:mn><a:mi>S</a:mi><a:mo stretchy="false">)</a:mo></a:mrow></a:math>, <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mi mathvariant="normal">ϒ</f:mi><f:mo stretchy="false">(</f:mo><f:mn>2</f:mn><f:mi>S</f:mi><f:mo stretchy="false">)</f:mo></f:math> and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi mathvariant="normal">ϒ</k:mi><k:mo stretchy="false">(</k:mo><k:mn>3</k:mn><k:mi>S</k:mi><k:mo stretchy="false">)</k:mo></k:math> production in <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mi>p</p:mi><p:mo>+</p:mo><p:mi>p</p:mi></p:math> collisions at <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:msqrt><r:mi>s</r:mi></r:msqrt><r:mo>=</r:mo><r:mn>500</r:mn><r:mtext> </r:mtext><r:mtext> </r:mtext><r:mi>GeV</r:mi></r:math> by the STAR experiment in year 2011, corresponding to an integrated luminosity <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:msub><t:mi mathvariant="script">L</t:mi><t:mrow><t:mi>int</t:mi></t:mrow></t:msub><t:mo>=</t:mo><t:mn>13</t:mn><t:mtext> </t:mtext><t:mtext> </t:mtext><t:msup><t:mi>pb</t:mi><t:mrow><t:mo>−</t:mo><t:mn>1</t:mn></t:mrow></t:msup></t:math>. The results provide precise cross sections, transverse momentum (<w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:msub><w:mi>p</w:mi><w:mi mathvariant="normal">T</w:mi></w:msub></w:math>) and rapidity (<z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:mi>y</z:mi></z:math>) spectra, as well as cross section ratios for <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"><bb:msub><bb:mi>p</bb:mi><bb:mi mathvariant="normal">T</bb:mi></bb:msub><bb:mo><</bb:mo><bb:mn>10</bb:mn><bb:mtext> </bb:mtext><bb:mtext> </bb:mtext><bb:mi>GeV</bb:mi><bb:mo>/</bb:mo><bb:mi mathvariant="normal">c</bb:mi></bb:math> and <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:mo stretchy="false">|</fb:mo><fb:mi>y</fb:mi><fb:mo stretchy="false">|</fb:mo><fb:mo><</fb:mo><fb:mn>1</fb:mn></fb:math>. The dependence of the <jb:math xmlns:jb="http://www.w3.org/1998/Math/MathML" display="inline"><jb:mi mathvariant="normal">ϒ</jb:mi></jb:math> yield on charged particle multiplicity has also been measured, offering new insights into the mechanisms of quarkonium production. The data are compared to various theoretical models: the color evaporation model (CEM) accurately describes the <mb:math xmlns:mb="http://www.w3.org/1998/Math/MathML" display="inline"><mb:mi mathvariant="normal">ϒ</mb:mi><mb:mo stretchy="false">(</mb:mo><mb:mn>1</mb:mn><mb:mi>S</mb:mi><mb:mo stretchy="false">)</mb:mo></mb:math> production, while the color glass <rb:math xmlns:rb="http://www.w3.org/1998/Math/MathML" display="inline"><rb:mrow><rb:mi>condensate</rb:mi><rb:mo>+</rb:mo><rb:mtext>nonrelativistic</rb:mtext></rb:mrow></rb:math> quantum chromodynamics (<tb:math xmlns:tb="http://www.w3.org/1998/Math/MathML" display="inline"><tb:mrow><tb:mi>CGC</tb:mi><tb:mo>+</tb:mo><tb:mi>NRQCD</tb:mi></tb:mrow></tb:math>) model overestimates the data, particularly at low <vb:math xmlns:vb="http://www.w3.org/1998/Math/MathML" display="inline"><vb:msub><vb:mi>p</vb:mi><vb:mi mathvariant="normal">T</vb:mi></vb:msub></vb:math>. Conversely, the color singlet model (CSM) underestimates the rapidity dependence. These discrepancies highlight the need for further development in understanding the production dynamics of heavy quarkonia in high-energy hadronic collisions. The trend in the multiplicity dependence is consistent with CGC/saturation and string percolation models or <yb:math xmlns:yb="http://www.w3.org/1998/Math/MathML" display="inline"><yb:mi mathvariant="normal">ϒ</yb:mi></yb:math> production happening in multiple parton interactions modeled by 8.
EPJ Web of Conferences · 2025-01-01
articleOpen accessSenior authorJet quenching is recognized as critical evidence for the existence of the quark-gluon plasma (QGP) and serves as an essential probe to study its transport properties. Measurements of hadron-triggered semi-inclusive recoil jets have gained popularity due to its capability to probe jets over an extended phase space at low transverse momenta ( p T ) and large radii. Recent ALICE measurements showed that the I AA , yield ratio of recoil jets between heavy-ion and p+p collisions, rises with jet p T and exceeds unity at high p T , contradicting conventional expectations that jet quenching should result in I AA values less than one. In this contribution, we re-examine the surface bias and study the effects of energy losses for both trigger hadrons and recoil jets on I AA , employing the Linear Boltzmann Transport (LBT) model to simulate jet-medium interactions. Our findings suggest that a large portion of hadrons used for the triggers undergoes substantial energy loss, despite surface bias. In particular, the energy loss of the trigger hadrons elevates the I AA baseline, corresponding to the case of no energy loss for recoil jets, to be greatly larger than unity. This enhancement of the baseline implies that the measured I AA values being larger than unity could still signal jet quenching.
Physical Review Letters · 2025-06-11 · 2 citations
articleThe STAR Collaboration presents measurements of the semi-inclusive distribution of charged-particle jets recoiling from energetic direct-photon (γ_{dir}) and neutral-pion (π^{0}) triggers in p+p and central Au+Au collisions at sqrt[s_{NN}]=200 GeV over a broad kinematic range, for jet resolution parameters R=0.2 and 0.5. Medium-induced jet yield suppression is observed to be larger for R=0.2 than for 0.5, reflecting the angular range of jet energy redistribution due to quenching. The predictions of model calculations incorporating jet quenching are not fully consistent with the observations. These results provide new insight into the physical origins of jet quenching.
Physical Review Letters · 2025-03-31 · 6 citations
erratumOpen accessThis corrects the article DOI: 10.1103/PhysRevLett.126.092301.
Electric-charge-dependent directed flow splitting of produced quarks in Au+Au collisions
Physics Letters B · 2025-01-17
articleOpen accessWe report directed flow ( v 1 ) of multistrange baryons (Ξ and Ω) and improved v 1 data for K − , p ¯ , Λ ¯ and ϕ in Au+Au collisions at s NN = 27 and 200 GeV from the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). We focus on particles whose constituent quarks are not transported from the incoming nuclei but instead are produced in the collisions. At intermediate impact parameters, we examine quark coalescence behavior for particle combinations with identical quark content, and search for any departure from this behavior (“splitting”) for combinations having non-identical quark content. Under the assumption of quark coalescence for produced quarks, the splitting strength appears to increase with the electric charge difference of the constituent quarks in the combinations, consistent with electromagnetic effect expectations.
Temperature measurement of Quark-Gluon plasma at different stages
Nature Communications · 2025-10-14 · 9 citations
articleOpen accessAbstract In a Quark-Gluon Plasma (QGP), the fundamental building blocks of matter, quarks and gluons, are under extreme conditions of temperature and density. A QGP could exist in the early stages of the Universe, and in various objects and events in the cosmos. The thermodynamic and hydrodynamic properties of the QGP are described by Quantum Chromodynamics (QCD) and can be studied in heavy-ion collisions. Despite being a key thermodynamic parameter, the QGP temperature is still poorly known. Thermal lepton pairs ( e + e − and μ + μ − ) are ideal penetrating probes of the true temperature of the emitting source, since their invariant-mass spectra suffer neither from strong final-state interactions nor from blue-shift effects due to rapid expansion. Here we measure the QGP temperature using thermal e + e − production at the Relativistic Heavy Ion Collider (RHIC). The average temperature from the low-mass region (in-medium ρ 0 vector-meson dominant) is (2.01 ± 0.23) × 10 12 K, consistent with the chemical freeze-out temperature from statistical models and the phase transition temperature from Lattice QCD. The average temperature from the intermediate mass region (above the ρ 0 mass, QGP dominant) is significantly higher at (3.25 ± 0.60) × 10 12 K. This work provides essential experimental thermodynamic measurements to map out the QCD phase diagram and understand the properties of matter under extreme conditions.
Physical review. C · 2024-04-19 · 3 citations
articleOpen accessWe measure triangular flow relative to the reaction plane at 3 GeV center-of-mass energy in $\text{Au}+\text{Au}$ collisions at the BNL Relativistic Heavy Ion Collider. A significant ${v}_{3}$ signal for protons is observed, which increases for higher rapidity, higher transverse momentum, and more peripheral collisions. The triangular flow is essentially rapidity-odd with a slope at midrapidity, $d{v}_{3}{/dy|}_{(y=0)}$, opposite in sign compared to the slope for directed flow. No significant ${v}_{3}$ signal is observed for charged pions and kaons. Comparisons with models suggest that a mean field potential is required to describe these results, and that the triangular shape of the participant nucleons is the result of stopping and nuclear geometry.
Physics Letters B · 2024-05-21 · 6 citations
articleOpen accessSenior authorCorrespondingIn relativistic heavy-ion collisions, a hot and dense state of matter, called the Quark-Gluon Plasma (QGP), is produced. Semi-inclusive jets recoiling from trigger hadrons of high transverse momenta (pT) can serve as an effective probe of the QGP properties, as they are expected to experience jet quenching when traversing the QGP. Recent experimental results on the ratio of recoil jet yields normalized by the trigger counts in heavy-ion collisions to that in p+p collisions (IAA) pose an unexpected challenge in its interpretation. It is observed that IAA rises with the jet pT and possibly exceeds unity at high pT, while traditionally it is expected that jet quenching would lead to IAA<1. To address this challenge, we utilize the Linear Boltzmann Transport (LBT) model to simulate jet transport in the QGP, and study the effect of jet quenching for high-pT triggers and recoil jets separately on IAA. We find that the quenching of the colored triggers alone is responsible for the rising trend and larger-than-unity value observed experimentally.
Physical review. C · 2024-04-09 · 2 citations
articleOpen accessAngular distributions of charged particles relative to jet axes are studied in $\sqrt{{s}_{NN}}=200$ GeV Au+Au collisions as a function of the jet orientation with respect to the event plane. This differential study tests the expected path-length dependence of energy loss experienced by a hard-scattered parton as it traverses the hot and dense medium formed in heavy-ion collisions. A second-order event plane is used in the analysis as an experimental estimate of the reaction plane formed by the collision impact parameter and the beam direction. Charged-particle jets with $15<{p}_{\mathrm{T},\mathrm{jet}}<20$ and $20<{p}_{\mathrm{T},\mathrm{jet}}<40\phantom{\rule{4pt}{0ex}}\mathrm{GeV}/c$ were reconstructed with the anti-${k}_{\mathrm{T}}$ algorithm with radius parameter setting of $R=0.4$ in the 20--50% centrality bin to maximize the initial-state eccentricity of the interaction region. The reaction plane fit method is implemented to remove the flow-modulated background with better precision than prior methods. Yields and widths of jet-associated charged-hadron distributions are extracted in three angular bins between the jet axis and the event plane. The event-plane (EP) dependence is further quantified by ratios of the associated yields in different EP bins. No dependence on orientation of the jet axis with respect to the event plane is seen within the uncertainties in the kinematic regime studied. This finding is consistent with a similar experimental observation by ALICE in $\sqrt{{s}_{NN}}$ = 2.76 TeV Pb-Pb collision data.
Frequent coauthors
- 701 shared
Z. Tang
Brookhaven National Laboratory
- 680 shared
N. Xu
Lawrence Berkeley National Laboratory
- 644 shared
Y. G.
- 588 shared
B. Mohanty
National Institute of Science Education and Research
- 559 shared
V. Okorokov
- 534 shared
L. Kumar
- 527 shared
A. Kisiel
- 511 shared
A. Bhasin
Argonne National Laboratory
Awards & honors
- Fellow of the American Physical Society (APS) (2019)
- APS CSWP Woman Physicist of the Month (Jan 2012)
- Elected Fellow of the Institute of Physics, UK (2008)
- Enseignant Invite (Invited Professor) Fellowship, Strasbourg…
- Engineering and Physical Sciences Research Council Advanced…
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